Earth-Science Reviews 151 (2015) 176–226
Contents lists available at ScienceDirect
Earth-Science Reviews
journal homepage: www.elsevier.com/locate/earscirev
Invited review
Petrography and environmental controls on the formation of
Phanerozoic marine carbonate hardgrounds
Nicolas Christ a,b,⁎, Adrian Immenhauser b, Rachel A. Wood c, Khadija Darwich b, Andrea Niedermayr b
a
b
c
Institute for Geosciences, University of Potsdam, K.-Liebknecht-Str. 24, 14476 Potsdam-Golm, Germany
Institute for Geology, Mineralogy, and Geophysics, Ruhr-Universität Bochum, Universitätsstraße 150, D-44801 Bochum, Germany
School of GeoSciences, Grant Institute, University of Edinburg, Kings Building, West Main Road, Edinburg EH9, 3JW, United Kingdom
a r t i c l e
i n f o
Article history:
Received 31 March 2015
Received in revised form 2 September 2015
Accepted 5 October 2015
Available online 17 October 2015
Keywords:
Carbonate hardgrounds
Seafloor lithification
Early marine diagenesis
Phanerozoic
Carbonate precipitation
Petrography
Paleoenvironmental archives
a b s t r a c t
Early marine seafloor lithification of carbonate sediments leads to the formation of hardgrounds and is known
from rocks as old as the Proterozoic. Hardground surfaces, however, are more commonly recorded in the Phanerozoic sedimentological archive. While ecological studies of hardground biota abound, the environmental and
physico-chemical parameters leading to the development of seafloor lithification remain, in many cases, poorly
understood and documented. This paper reviews published evidence on the petrography, mineralogy and geochemistry of Phanerozoic carbonate hardgrounds within a process-oriented context of their environmental controls. Hardgrounds typically develop in warm and shallow, agitated tropical waters that are saturated with
respect to CaCO3, but are also reported from hemi-pelagic to bathyal realms and from cool-water, temperate settings. A range of early marine cement types are documented from present-day hardgrounds whereas (preserved)
cement fabrics and related mineralogies are less diverse in early Phanerozoic hardgrounds. Carbonate
hardgrounds are widespread during calcite sea periods (e.g., Ordovician and Cretaceous), as opposed to some
(preservation bias?) ancient aragonite seas (e.g., Permian and Triassic). Obviously, the relation between seawater
chemistry and hardground abundance and attributes is not an easy one. Here, the concept of aragonite versus calcite seas serves as a first-order template for the sake of a structured discussion whereas modern aragonite mode
oceans document a significant diversity in spatial and bathymetric seawater properties. This spatio-temporal
complexity is perhaps reflected in the scarce record of calcite sea hardgrounds from the Devonian or Paleogene
rock record contrasted by the abundance in aragonite Holocene and Recent seas. Holocene hardgrounds allow for
the direct assessment of rates of, and reasons for, early subaquatic lithification and—in most cases—escaped subsequent non-marine diagenetic overprint. Conversely, studies of ancient hardgrounds are often biased by early
biological and mechanical erosion, the degree of diagenetic overprint, and tectonic to orogenic processes. The formation, distribution and physical properties of hardgrounds depend on the global climatic context (greenhouse/
icehouse modes), on sea-level changes, on ocean stratification, on the spatial extent of epicontinental seas and
carbonate depositional environments, and on the spatially different mineralogies and rates of carbonate production. The fabrics and mineralogies of early hardgrounds cement seem to substantially hinge on variations in atmospheric CO2. Thus, the application of lessons from modern to ancient hardground case examples is not
straightforward. A holistic model explaining the full variability of controls on submarine hardground formation
and their cement petrography throughout the Phanerozoic is as yet lacking. A more critical view on secular
changes in hardground formation patterns is clearly needed. Future work should rely on the combination of sedimentological, stratigraphic, palaeoecological, petrographic, and geochemical approaches. Specific emphasis
should be given to time intervals (Triassic, Carboniferous, Cambrian etc.) where the bulk of reported case studies
is at best scarce.
© 2015 Elsevier B.V. All rights reserved.
Contents
1.
2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Processes of carbonate precipitation at hardgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
⁎ Corresponding author at: Institute for Geosciences, University of Potsdam, K.-Liebknecht-Str. 24, 14476 Potsdam-Golm, Germany.
E-mail address: christ@geo.uni-potsdam.de (N. Christ).
http://dx.doi.org/10.1016/j.earscirev.2015.10.002
0012-8252/© 2015 Elsevier B.V. All rights reserved.
177
179
180
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
3.1.
Physico-chemical and (micro-)biological parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.1.
CaCO3 saturation of seawater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.2.
Carbonate precipitation rates and mineralogy of early marine cements in hardgrounds . . . . . . . . . . . . . . . .
3.1.3.
Organic material degradation and microbial activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.
Hydrodynamic level, facies type, and sediment accumulation rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1.
Hydrodynamic level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.2.
Relation of pore space and permeability to facies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.3.
Sediment accumulation rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.
Global sea-level change and the significance of hardgrounds as stratigraphic surfaces . . . . . . . . . . . . . . . . . . . . .
3.3.1.
Regressive-marine hardgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.2.
Maximum-flooding surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.
Petrography, petrophysics, and geochemistry of hardground cements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.
Cement petrography in hardgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.1.
Acicular, fibrous and bladed cements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.2.
Radiaxial fibrous cements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.3.
Microcrystalline/cryptocrystalline cements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.4.
Equant spars, syntaxial overgrowth, scalenohedral calcite and peloidal fabrics . . . . . . . . . . . . . . . . . . . .
4.1.5.
Non-carbonate phases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.
Petrophysical properties and fluid flow modelling of hardgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.
Isotope geochemistry of marine carbonate hardgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.1.
Carbon and oxygen isotope signatures of recent and sub-recent hardgrounds . . . . . . . . . . . . . . . . . . . .
4.3.2.
Carbon and oxygen isotope signatures of ancient hardgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.
Patterns in Phanerozoic hardgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.
Aragonite sea III . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.1.
Holocene and Pleistocene neritic hardgrounds: The epeiric tropical realm . . . . . . . . . . . . . . . . . . . . . .
5.1.2.
Holocene and Pleistocene neritic hardgrounds: The temperate realm. . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.3.
Oligocene to Pliocene tropical hardgrounds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.4.
Oligocene and Miocene cool-water and warm-temperate hardgrounds from heterozoan-dominated carbonate platforms
5.1.5.
Deep neritic to bathyal hardgrounds in the open oceanic realm. . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.6.
Bathyal hardgrounds in the semi-enclosed Red Sea and Mediterranean basins . . . . . . . . . . . . . . . . . . . .
5.2.
Calcite sea II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1.
Paleocene–Eocene hardgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.2.
Upper Cretaceous chalk sea hardgrounds of northern Europe. . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.3.
Lower Cretaceous neritic hardgrounds of the Tethyan Realm . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.4.
Jurassic neritic hardgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.5.
Jurassic and Cretaceous drowning-related hardgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.
Aragonite sea II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.1.
Permian and Triassic hardgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.2.
Carboniferous hardgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.
Calcite sea I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.1.
Silurian and Devonian hardgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.2.
Ordovician hardgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.3.
Cambrian hardgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.
Aragonite I (Earliest Cambrian) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.
Discussion and synopsis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.
Hardground preservation through time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.
Hardground formation environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3.
Hardground distribution and petrography in aragonite and calcite seas . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4.
Impact of atmospheric pCO2 and global temperatures on hardground mineralogy . . . . . . . . . . . . . . . . . . . . . .
6.5.
Greenhouse versus icehouse mode hardgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.6.
Relation of epeiric sea extent and hardground frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
Hardgrounds are stratigraphic horizons that are cemented syndepositionally at, or near the seafloor. They are recorded from periods
as old as the earliest Paleoproterozoic (Rasmussen et al., 2015) but
commonly are known from the Phanerozoic (Taylor and Wilson,
2003). Marine hardgrounds, mostly resulting from the lithification of
a carbonate seabed, may represent short-lived to very prolonged hiatal
intervals of non-deposition and erosion. Geological information on the
time gap, if at all preserved, is contained in the morphology, mineralogy, geochemistry and palaeo-ecology of hardground surfaces and the
directly underlying rock interval (Bromley, 1978; Fürsich, 1979;
177
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Sadler, 1981; Bromley et al., 1990; Fürsich et al., 1992; Strasser et al.,
1999; Cachão et al., 2009; Rameil et al., 2012; Godet, 2013). Therefore,
an insufficient understanding of hiatal surfaces represents a significant
obstacle for those concerned with the time resolution and interpretation of Earth's ancient carbonate archives. Moreover, marine
hardgrounds may represent the physical expression of pivotal time intervals in Earth's climatic and evolutionary history (Late Devonian extinction: Bond and Wignall, 2008; Permian-Triassic: Hips and Haas,
2009; Triassic/Jurassic boundary: Wignall, 2001; Early/Late Cretaceous:
Eren and Tasli, 2002; Cenomanian-Turonian: Reitner et al., 1995;
Turonian/Coniacian: Olszewska-Nejbert, 2004; Cretaceous/Paleogene:
Chacón and Martín-Chivelet, 2008) and serve for correlation purposes
178
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
Table 1
List of literature used in the context of this review. Refer to Figs. 1 and 2 for locations.
Label
Locality
Period
Study
Qs1
Qs2
Qs3
Qs4
Qs5
Qs6
Qs7
Qs8
Qs9
Qs10
Qs11
Qs12
Qs13
Qs14
Qs15
Qs16
Qd1
Qd2
Qd3
Qd4
Qd5
Qd6
Qd7
Qd8
Qd9
Qd10
Qd11
C1
C2
C3
C4
C5
C6
C7
M1
M2
M3
M4
M5
M6
M7
M8
M9
M10
M11
M12
M13
M14
M15a
M15b
M16
M17
M18
M19
M20
M21
M22
M23
P1
P2
P3
Bahamas—Exuma Cays
Bahamas—Eleuthera Bank
Bahamas—Yellow Bank
Bahamas—Joulters Cays
Bermudas
Persian Gulf
Persian Gulf—Kuwait Bay
Persian Gulf—Qatar Peninsula
Gulf of Mexico
USA—Florida
Southern Australia
Delaware Bay (USA)
Denmark
Western Canada—British Columbia
Mediterranean Sea—several places
SW Australia
Bahamas—Tongue of the Ocean
Bahamas—Great Bahamas Bank
Atlantic and Pacific seamounts
Atlantic—Mid-Atlantic Ridge
North Eastern Atlantic
Eastern Mediterranean Sea
Eastern Mediterranean Sea
Eastern Mediterranean Sea
Eastern and Western Mediterranean Sea
Red Sea
Red Sea
Bahamas—Great Bahamas Bank
Italia
Malta
Malta
Southern Australia and New Zealand
Southern Australia
South Eastern Australia
Northern France
Eastern England
The Netherlands and Belgium
Belgium
Southern England
South Western England
Eastern Turkey
Oman
Oman
Oman
France and Switzerland
UAE—Abu Dhabi
Greece
England, France and Poland
France
France and Algeria
Morocco
USA—Utah
USA—Wyoming
France
England
Southern Spain
Southern Spain
Poland
South Korea
Canada—Ontario
Canada—Southern Ontario
Pleistocene–Holocene
Pleistocene–Holocene
Pleistocene–Holocene
Pleistocene–Holocene
Pleistocene–Holocene
Holocene
Holocene
Holocene
Pleistocene–Holocene
Pleistocene–Holocene
Pleistocene–Holocene
Pleistocene–Holocene
Pleistocene–Holocene
Holocene
Pleistocene–Holocene
Pleistocene–Holocene
Pleistocene–Holocene
Pleistocene–Holocene
Pleistocene
Pleistocene–Holocene
Pleistocene–Holocene
Pleistocene–Holocene
Pleistocene–Holocene
Pleistocene–Holocene
Pleistocene–Holocene
Pleistocene–Holocene
Pleistocene–Holocene
Pliocene
Miocene
Miocene
Miocene
Oligocene–Miocene
Oligocene–Miocene
Oligocene–Miocene
Late Cretaceous
Late Cretaceous
Late Cretaceous
Late Cretaceous
Late Cretaceous
Late Cretaceous
Mid-Cretaceous
Early Cretaceous
Early Cretaceous
Early Cretaceous
Early Cretaceous
Lower Cretaceous
K/T boundary
Middle-Late Jurassic
Middle Jurassic
Middle Jurassic
Middle Jurassic
Middle Jurassic
Middle Jurassic
Middle Jurassic
Middle Jurassic
Middle Jurassic
Late Jurassic
Late Jurassic
Early Ordovician
Middle Ordovician
Middle Ordovician
P4
P5
P6
P7a
P7b
P7c
P7d
P8
P9
P10
P11
Canada—Newfoundland
Norway
USA—Utah
USA—Iowa
USA—Ohio
Russia
USA—Utah-Nevada
Sweden
USA—Upper Mississippi Valley
Canada—South-Central Ontario
South Western Europe and North Western Africa
Late Ordovician
Middle Ordovician
Early Ordovician
Middle Ordovician
Late Ordovician
Early Ordovician
Cambrian–Ordovician
Early Ordovician
Late Ordovician
Middle Ordovician
Middle Cambrian
Whittle et al. (1993)
Dravis (1979)
Taft et al. (1968)
Harris (1978)
Vollbrecht (1990)
Shinn (1969)
Khalaf et al. (1987)
Taylor and Illing (1969)
Poppe et al. (1990)
Obrochta et al. (2003)
Rivers et al. (2008)
Allen et al. (1969)
Jørgensen (1976)
Garrison et al. (1969)
Alexandersson (1969)
James et al. (1999)
Schlager and James (1978)
Malone et al. (2001)
Milliman (1966)
Schroeder et al. (2002)
Noé et al. (2006)
Aghib et al. (1991)
McKenzie and Bernoulli (1982)
Milliman and Müller (1973)
Allouc (1990)
Gevirtz and Friedman (1966)
Milliman et al. (1969)
Beach (1993)
Mutti and Bernoulli (2003)
Pedley and Bennett (1985)
Gruszczyński et al. (2008)
Nelson and James (2000)
James and Bone (1992)
Nicolaides and Wallace (1997)
Jarvis (1980)
Jeans (1980)
Molenaar and Zijlstra (1997)
Richard et al. (2005)
Kennedy and Garrison (1975)
Garrison et al. (1987)
Eren and Tasli (2002)
Rameil et al. (2012)
Immenhauser et al. (2004); Sattler et al. (2005)
Immenhauser et al. (1999, 2000a, b)
Hillgärtner (1998)
Dickson et al. (2008)
Pomoni-Papaioannou and Solakius (1991); Pomoni-Papaioannou (1994)
Fürsich (1979) and references therein
Purser (1969)
Aissaoui and Purser (1983)
Christ et al. (2012a) and the present work
Wilson and Palmer (1992, 1994)
Wilkinson et al. (1985)
Brigaud et al. (2009a, 2009b)
Marshall and Ashton (1980)
Reolid et al. (2010); Reolid and Nieto (2010)
Coimbra et al. (2009)
Gruszczyński (1986)
Kim and Lee (1996)
Wilkinson et al. (1982)
Brett and Brookfield (1984); Brookfield and Brett (1988); Brookfield (1988);
Palmer and Wilson (2004)
Hender and Dix (2008)
Möller and Kvingan (1988)
Benner et al. (2004)
Palmer and Palmer (1977); Palmer (1978); Palmer and Wilson (2004)
Palmer and Wilson (2004)
Rozhnov and Palmer (1996); Palmer and Wilson (2004)
Hintze (1973); Ross et al. (1989); Palmer and Wilson (2004)
Ekdale and Bromley (2001)
Delgado (1980, 1983)
Mancini (2011)
Zamora et al. (2010)
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
179
Table 1 (continued)
Label
Locality
Period
Study
P12
P13
P14
P15
France
Iran
USA—Southern Appalachians
USA—Indiana
Early Cambrian
Middle-Late Cambrian
Late Cambrian
Carboniferous (Mississippian)
Alvaro and Clausen (2010)
Kruse and Zhuravlev (2008)
Glumac and Walker (2002)
Dodd and Nelson (1998)
on basin scale and beyond (Sattler et al., 2005; Rais et al., 2007; Godet
et al., 2013).
Sequence stratigraphers approach hardgrounds both from a conceptual and an environmental perspective (Sarg, 1988; Handford and
Loucks, 1993; El-Ghali, 2005; Rais et al., 2007; Catuneanu et al., 2009),
whilst applied geologists focus on their role in compartmentalization
of reservoir rocks and the consequences for fluid flow in the subsurface
(Read and Horburry, 1994; Cander, 1995; Amour et al., 2012; Christ
et al., 2012a; Shekhar et al., 2014; Agada et al., 2014). In this sense, it
is perhaps remarkable that comparably little attention has been paid
to the petrology of ancient and modern hardgrounds (Shinn, 1969;
Purser, 1969; Whittle et al., 1993). Often, hardground fabrics and mineralogies are described in a circumstantial manner (e.g., Kennedy and
Garrison, 1975; Brett and Brookfield, 1984). Similarly, a systematic review of environmental parameters leading to early marine seafloor cementation is as yet lacking.
Nucleation and precipitation of early marine carbonate cements per
se does not necessarily lead to hardground formation. Examples for
non-hardground marine cementation include ‘reciprocal cements’ as
ooid formation (Davies et al., 1978; Simone, 1980; Tucker, 1985;
Whittle et al., 1993), volumetrically significant inorganic cements in
reefal frameworks (Krebs, 1969; Friedman et al., 1974; Macintyre,
1977; Davies, 1977; James et al., 1982; Harris, 1993; Emmerich et al.,
2005), in deeper slope settings (van der Kooij et al., 2009, 2010), or in
intertidal zones (Beach, 1993). The latter include cements in tepee
belts (Shinn, 1969; Assereto and Kendall, 1971; Kendall and Warren,
1987; Chafetz et al., 2008; Christ et al., 2012b), or marine blocky calcite
occluding bird's eyes in ancient tidal flat environments (Scholle and
Kinsman, 1974; Christ et al., 2012b).
Throughout the Phanerozoic, early marine lithification of carbonate
seafloors seems more common during calcite than during aragonite-sea
modes (Taylor and Wilson, 2003; Palmer and Wilson, 2004). Examples
include the Ordovician, Jurassic or Cretaceous periods, when inferred
early low-Mg calcite (b4 mol% MgCO3) cementation is often pervasive
(Palmer and Wilson, 2004). The apparent dominance of early diagenetic
subaquatic lithification during ancient calcite-sea intervals contrasts
with the abundant examples of seafloor lithification reported from the
present-day aragonite mode oceans (Gevirtz and Friedman, 1966; Taft
et al., 1968; Shinn, 1969; Milliman et al., 1969; Dravis, 1979; Videtich
and Matthews, 1980; Khalaf et al., 1987; Poppe et al., 1990; Allouc,
1990; Aghib et al., 1991; Malone et al., 2001; Obrochta et al., 2003;
Wang et al., 2006; Noé et al., 2006; Immenhauser, 2009). Having said
that, care must be taken to not draw oversimplified conclusions from
first-order models of Phanerozoic sea-water chemistry. As documented
in Steuber (2002) for portions of the Cretaceous, the concept of aragonite versus calcite seas is at best a coarse approximation. Ocean properties might have changed significantly also during time intervals
commonly assigned to calcite and aragonite modes. Moreover, seawater
chemistry changes both latitudinally, spatially and with bathymetry
(Feely et al., 2004; Bijl et al., 2009).
The chemical, physical, and microbial processes leading to (in)organic carbonate precipitation and dissolution in modern oceans form
the topic of an extensive literature (Bathurst, 1974; Mucci and Morse,
1984; Given and Wilkinson, 1985; Burton and Walter, 1990; Burton,
1993; Zuddas and Mucci, 1998; Grammer et al., 1999; Sanders, 2003;
Gussone et al., 2005; Morse et al., 2007; Stanley et al., 2010;
Ruiz-Agudo et al., 2011 amongst many others). Early marine lithification is common in tropical and subtropical shallow-water carbonate
environments and particularly so in neritic domains characterized by
seawater saturation state Ω N 1 with respect to calcite and/or aragonite
(Shinn, 1969; Purser, 1969; Kennedy and Garrison, 1975; Dravis, 1979;
Fürsich, 1979; Marshall and Ashton, 1980; Garrison et al., 1987;
Vollbrecht, 1990; Whittle et al., 1993; Immenhauser et al., 2000a;
Immenhauser et al., 2004; Sattler et al., 2005; Dickson et al., 2008;
Rameil et al., 2012; Christ et al., 2012a). Independent of seawater saturation state with respect to calcite and aragonite, inorganic carbonate
precipitation in tropical, shallow-marine settings is often closely associated with elevated hydrodynamic levels (Dravis, 1979; Lighty, 1985)
and sediment by-pass, low sediment accumulation rates or periods of
non-deposition and winnowing (Shinn, 1969; Kennedy and Garrison,
1975).
Processes related to microbial activity might also play a significant
role in the formation of what is commonly referred to as “abiogenic” cements (Kandianis et al., 2008). Microbial or bacterial metabolic end
products lower thermodynamic boundaries for cement nucleation
(Dupraz et al., 2009) and increase (or decrease) rates of physicochemical processes involved; microbes may simply offer charged
surfaces for organogenic micrite precipitation that commonly takes
place within microbial “mucus” (EPS; Pentecost, 1985; Arp et al.,
1999; Riding, 2000; Bissett et al., 2008; Shiraishi et al., 2008).
The above considerations form a strong motivation for this review of
marine cementation of carbonate seafloors in a process-related context
aiming at compiling the evidence for the intricate combination of factors
leading to the formation of submarine carbonate hardgrounds throughout the Phanerozoic. This contribution is not intended as a comprehensive review of all aspects of marine hardgrounds, rather the focus is on
hardground cement petrography and the environmental factors that
lead to the nucleation and precipitation of specific carbonate phases
through the Phanerozoic. Conversely, this paper does not addresses
the issue of hardground ecology, a topic covered by a series of landmark
papers (Goldring and Kazmierczak, 1974; Palmer and Fürsich, 1974;
Fürsich and Palmer, 1975; Brett and Brookfield, 1984; Brett, 1988;
Akpan, 1991; Ekdale and Bromley, 2001, 2003; Gingras et al., 2001;
Droser et al., 2002; Taylor and Wilson, 2003; Benner et al., 2004; Brett
et al., 2007). Moreover, this paper does not serve as a compendium of
geochemical proxies and methods or as a “how-to” guide on palaeoenvironmental analysis. Rather, it represents a broad, non-specialist
and relatively jargon-free compilation and characterization of marine
hardground through time with an emphasis on what is already known
and where we need to learn more. Along these conceptual lines, the following apparently simple questions are explored: (i) Why and when do
marine carbonate hardgrounds form? (ii) Which carbonate (and noncarbonate) phases contribute predominantly to the early marine lithification of the seabed? (iii) Why seem hardgrounds more common during some time intervals and less so during others? The data base used
here combines evidence from more than hundred published studies
dealing with Phanerozoic carbonate hardgrounds (Table 1; Figs. 1
and 2). We do not discuss Precambrian case examples because the
Precambrian biosphere-atmosphere-hydrosphere system was, in
many aspects, very different from the Phanerozoic one (Dobretsov
et al., 2008; Eriksson et al., 2013).
2. Terminology
The term “hardground” was first introduced by Murray and Renard
(1891) as a descriptive term for rocky seafloors in a report on deep sea
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deposits collected during the voyage of HMS Challenger. It was not until
the 1960s, however, that the first detailed studies of syn-sedimentary
seafloor lithification—from deep and shallow marine settings and in
both ancient and modern environments, respectively—were published
(Jaanuson, 1961; Gevirtz and Friedman, 1966; Milliman, 1966; Taft
et al., 1968; Milliman et al., 1969; Shinn, 1969; Purser, 1969; Kennedy
and Garrison, 1975; Fürsich, 1979; Dravis, 1979). In the cited work as
well as in subsequent papers, a variety of in part contrasting definitions
for the term “hardground” have been proposed. Most of these definitions refer to field or at least macroscopic evidence whilst petrographic
approaches, dealing with the recognition of ancient marine
hardgrounds are less common in the literature. Refer to Table 2 and
Figs. 3 through 5 for list definitions and classifications of marine
hardgrounds in comparison to other types of discontinuities.
Hardground formation represents a gradual process and progresses
via “firmgrounds” and “incipient hardgrounds” to “fully lithified
hardgrounds” (Fig. 3). In contrast, ichnofacies features, such as borings
by litophage bivalves (Gastrochaenolites) or sponges are often related
to incipient or fully lithified marine hardgrounds but must not be seen
as a sole line of evidence for synsedimentary cementation. According
to Bathurst (1975; page 395), a lithified carbonate seafloor is a
hardground if “…its upper surface has been bored, corroded or eroded
(by abrasion), if encrusting or other sessile organisms are attached
to the surface, or if pebbles derived from the bed occur in the
overlying sediment…”. Wilkinson et al. (1985), later re-defined
hardgrounds as lithified surfaces cemented at or within a few
centimetres of the sediment-water interface (Fig. 4) being distinct
from “sub-hardgrounds” in the sense of Molenaar (1990); Molenaar
and Zijlstra (1997) and Mišík and Aubrecht (2004). These authors
refer to a hardground sensu Wilkinson et al. (1985) either as “incipient
hardground” (Molenaar, 1990), because these features may lack evidence of formation at the sediment/water interface (Molenaar and
Zijlstra, 1997), or as surfaces lithified in the shallow burial domain successively exhumed by erosion (Mišík and Aubrecht, 2004).
The interpretation of a lithified surface as an exhumed subhardground is sometimes brought forward where the lack of palaeoecological evidence, more precisely the absence of borings and
encrusting organisms renders the interpretation of these features difficult. The lack of encrusting organisms or borings also led Hart et al.
(2005) to suggest that some case examples of syn-sedimentary lithified
surfaces represent “…not true hardgrounds…”. Along the same lines,
Bathurst (1975) and Bromley (1975) also defined hardgrounds based
on genetic, petrographic, and palaeo-ecological evidence (see also
Riggs et al., 1996). Brett and Brookfield (1984) implicitly raised the
issue of missing ecological imprints amongst hardgrounds, by mentioning that the scarcity or lack of colonization in deep and near-shoal settings may be related to short seafloor exposure duration combined
with strong seafloor abrasion and erosion.
In summary and with reference to the Phanerozoic world, traces
related to boring/etching or encrusting organisms, on and directly beneath ancient marine hardground surfaces may aid in their identification (Fig. 5) whereas their absence must not necessarily imply that
the surface under consideration is not early marine-diagenetic in origin.
Indeed, evidence pointing to a lithified seafloor is also present in the
case of “rock grounds”, defined as “… older layers of rock which through
erosion or submersion formed the seafloor …” (Fürsich, 1979; page 103
in Clari et al., 1995). Furthermore, beach rock, defined as “… hard coastal sedimentary formations consisting of various beach sediments, lithified through the precipitation of carbonate cements …” (Vousdoukas
et al., 2007; page 23) or reefal complexes (Krebs, 1969; Riding, 2002;
Perry and Hepbum, 2008) may share palaeo-ecological, morphological,
and petrographic similarities with marine hardgrounds. Beach rocks
are a specific group of hardgrounds, as they generally share the
synsedimentary cementation of carbonate particles whilst dissolution
is equally significant and cementation from mixed marine and meteoric
fluids is very common.
In an attempt to simplify the in part bewildering terminology related
to marine hardgrounds we propose the following pragmatic definition:
Hardgrounds are features in the sediment record that present evidence for
significant early marine cementation of the seafloor itself and/or portions of
the near-seafloor sediment column. The advantage of this definition is
that it remains non-interpretative with respect to the type of evidence
pointing to seafloor lithification and incorporates Precambrian lithified
ocean floors lacking evidence for boring or encrusting organisms. Deviations from the above definition, either based on the rock coherence at
the time of formation (such as the firmness of a patchily indurated
“firmground”, Table 2 and Figs. 3 and 5), the nature of cements lithifying
the sediment (such as meteoric cements in “beach rocks”, Table 2 and
Fig. 3), or evidence for the exhumation of significantly older sedimentary, metamorphic or magmatic rock intervals (such as in the case of “rock
grounds”, Table 2 and Fig. 3) do not, in the view of the authors, qualify
the resulting features as hardgrounds in the sense discussed here.
3. Processes of carbonate precipitation at hardgrounds
In the following, we provide a condensed overview on environmental and physico-chemical parameters relevant for marine hardground
formation. Refer to cited work for further detail.
3.1. Physico-chemical and (micro-)biological parameters
3.1.1. CaCO3 saturation of seawater
Secular change in the chemistry of the world's oceans (Burke et al.,
1982; Sandberg, 1983; Opdyke and Wilkinson, 1990; Stanley and
Hardie, 1998; Veizer et al., 1999; Webb, 2001; Royer et al., 2001;
Steuber and Veizer, 2002; Horita et al., 2002; Ries, 2004; Miller et al.,
2005; Riding and Liang, 2005; Farkas et al., 2007; Berger, 2011) constitute the background of marine calcium carbonate precipitation. At more
local scales, seawater and porewater saturation states are important and
variable parameters (Table 3; Given and Wilkinson, 1985; Morse and
Wang, 1997; Stanley and Hardie, 1999). Moreover, pCO2 and water
temperature (James and Choquette, 1983; Burton and Walter, 1991;
Morse and Wang, 1997) are factors controlling ocean and porewater
chemistry. Both vary with latitude and bathymetry and considerably influence local changes in CaCO3 saturation states.
In modern subtropical or tropical shallow water areas, seawater supersaturated with respect to aragonite and calcite is the most common
source for abiotic aragonite and high-Mg calcite (N 4 mol% MgCO3) cements (Bathurst, 1975; James and Choquette, 1983; Fig. 6). Cooler seawater temperatures and a decrease in CaCO3 saturation state typify
higher latitude ocean surface waters. These settings are prone to reduced marine carbonate cementation and low carbonate production
(Nicolaides and Wallace, 1997; Nelson and James, 2000). Marine cement precipitation in pore space beneath the sediment/water interface
at mid-latitudes is often composed of low-Mg calcite (Fig. 6) and marine
dissolution of aragonitic biogenic parts is common (James et al., 2005;
Knoerich and Mutti, 2006a; Rivers et al., 2008; Fig. 6).
Polar or sub-polar settings are generally undersaturated with respect to calcium carbonates and are dominated by dissolution processes
and/or lack of inorganic carbonate production whilst carbonate secreting organism are commonly present (Fig. 6). A limited number of ancient (Rao, 1991; Rogala et al., 2007; James et al., 2009) and modern
Fig. 1. Stratigraphic distribution of hardgrounds. Height of vertical black bars indicating ages of hardground surfaces are exaggerated. Stratigraphic time scale after Gradstein et al. (2004).
See Table 1 and Fig. 2 for details.
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
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Fig. 2. Location of hardgrounds reviewed in the context of this study. See Table 1 and Fig. 1 for more details.
examples (Rao et al., 1998; Didie et al., 2002) refer to carbonate production and accumulation at these high latitudes. Hardground formation at
polar or sub-polar latitudes is uncommon particularly during glacial intervals, when the equator-to-pole gradient of seawater surface temperature is steep (Kobashi et al., 2004; Bijl et al., 2009) and cold surface
seawaters are present at the poles. Portions of the seafloor around
Antarctica, constantly swept by the circum-Antarctic current might represent exceptions to this rule.
Independent of latitude, seawater temperature is usually linked to
bathymetry with colder, denser water masses found in deeper settings
and warmer, lighter water masses characterizing near surface environments. With the noteworthy exception of up- or down-welling current
settings (Chierici and Fransson, 2009; Mathis et al., 2012), seawater
temperature, water depth and saturation state are interrelated. With increasing water depth, CaCO3 saturation state and temperature decreases (Fig. 6; James and Choquette, 1983). The saturation state—and
consequently the stability—of carbonate minerals diminishes gradually
with increasing bathymetry until calcite compensation depth (CCD;
Fig. 6), under which the dissolution of calcite is nearly complete
(James and Choquette, 1983). The depth of the CCD varies spatially
across a given oceanic basin, between different oceanic basins and
through Earth's history (Van Andel, 1975; James and Choquette,
1983). Seawater is supersaturated, on average, to depths of ~ 1400 m
for aragonite and ~ 3800 m for calcite but these values fluctuate both
in time and space (Chen et al., 2006).
3.1.2. Carbonate precipitation rates and mineralogy of early marine
cements in hardgrounds
The precipitation rate of carbonate cements—in most cases determined by the rate of carbonate ion supply to crystal surfaces—is an
important control on the nature of abiotic carbonate phases in marine
environments (Given and Wilkinson, 1985; Niedermayr et al., 2013).
The availability of CO2−
on crystal surfaces depends on fluid chemistry,
3
temperature, and flow intensity controlling the rates of fluid exchange
at the site of precipitation (Given and Wilkinson, 1985). An increase in
solution Mg/Ca molar ratios lowers calcite precipitation rates (Zhang
and Dawe, 2000) or inhibits calcite precipitation and favours thereby
aragonite precipitation (Folk, 1974a) Precipitation rates of aragonite,
formed from seawater, increase with increasing water temperature relative to calcite (Burton and Walter, 1987). On the other hand organic
substances like polyaspartic acid (as it occurs e.g. in bivalve shells) can
inhibit aragonite precipitation and in consequence calcite precipitates,
also at elevated Mg/Ca ratios, if elevated saturation states are achieved
(Niedermayr et al., 2013). Thus, sea- or porewater carbonate saturation
state does not, at least not as a single dominant factor, govern the mineralogy of inorganic carbonate phases (aragonite, high-Mg calcite or calcite; Burton and Walter, 1987). Experimental work is thereby essential
to understand the influence of thespecific chemical (e.g. organic and inorganic inhibitors) and physical parameters (e.g. temperature) on formation of various polymorphs.
Sulphate concentration also influences the precipitation of CaCO3
polymorphs (Fernández-Díaz et al., 2010; Bots et al., 2011; Table 3).
Bots et al. (2011) have reported that an increase in dissolved SO4 lowers
the Mg/Ca ratio needed to destabilize calcite minerals. Therefore, periods of the Phanerozoic with elevated concentrations of dissolved SO4
coincided with more common precipitation of aragonite (Bots et al.,
2011).
Absolute CaCO3 precipitation rates, and therefore the time required
to lithify sediments at or near the seafloor, can generally be obtained
from direct observations in modern environments (Table 4). Results
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
183
Table 2
Definition scheme for condensed, firm or lithified surfaces.
Main Group
Type of
surface
Non-hardbottoms Soupgrounds
Hardbottoms
Definition (at the time of deposition)
Rock
coherence
Additional definition (based on nature, type and extent
of lithification)
Water-saturated muds
No
coherence
Soft
Loose
Stiff
Firm
Incipiently
lithified
Fully
lithified
−/(also defined using ichnofacies association)
Softgrounds
Loosegrounds
Stiffgrounds
Firmgrounds
Hardgrounds
Partially dewatered muddy sediments
Partially stabilized sediments
Stabilized sediments
Firm, dewatered, compacted sediments
Hard, synsedimentary marine lithified sea floors
Rockgrounds
Hard, inherited, exhumed lithified horizons
Beachrocks
Hard, coastal sedimentary surfaces cemented by marine,
meteoric or mixed cements
Fully
lithified
Fully
lithified
−/(also defined using ichnofacies association)
−/(also defined using ichnofacies association)
−/(also defined using ichnofacies association)
−/(also defined using ichnofacies association)
Nodular/“Concretional”
Patchy
Sub-hardgrounds (beneath sediment/water interface)
Simple hardground (single phased)
Multi-phased hardgrounds (more than one marine
cementation phase/sea level oscillations)
Composite hardground (at least a subaerially emergent phase)
–
–
−/:no additional definition.
from present-day shallow, wave agitated marine tropical and subtropical environments point to fast marine cementation rates on the order of
a few mm per 100 years (Shinn, 1969; Taylor and Illing, 1969; Dravis,
1979; Grammer et al., 1999; Table 4). Cement precipitation at depths
below effective wave base may still be considerable (lithification in
b105 years) but clearly lags behind near surface settings (Schlager and
James, 1978; Table 4). Conversely, precipitation rates cannot be quantified directly for fossil marine settings. Nevertheless, several authors
assessed the formation of hardgrounds to occur between 10 and
1000 years (e.g., Kennedy and Garrison, 1975; McLaughlin et al.,
2008), durations that are similar to those given for marine cementation
of modern seabeds (Table 4). Time involved in the lithification of ancient seafloors, however, cannot be determined in an absolute manner.
Indeed, loose estimations on the duration of the hiatuses are given, rather than on the time required for the initial cementation of the seafloor
(e.g., the 10 Ma unconformity studied by Rameil et al., 2012).
3.1.3. Organic material degradation and microbial activity
Organic material may inhibit precipitation even from supersaturated
seawater (Berner et al., 1978). Indeed, acids that are released through
the decomposition of organic matter within surface sediment may result in undersaturated pore waters with respect to CaCO3 (Berner
et al., 1978). Such waters, deprived in regard to CaCO3 might therefore
also been found in shallow marine tropical setting, where seawater is
generally supersaturated with respect to all carbonate minerals
(Morse et al., 1985; Walter and Burton, 1990).
Morse et al. (2007) underlined the role of bacteria and other microbes, prevailing over the contribution of ocean chemistry, in favouring
or inhibiting marine carbonate cement precipitation. Because of changes in the microenvironment by microbially-induced mechanisms, dissolution of carbonate minerals can occur in seemingly supersaturated
waters or, conversely, precipitate from apparently undersaturated seawater (Alexandersson, 1974; Emerson and Hedges, 2003; Morse et al.,
2007; Shiraishi et al., 2008; Konhauser and Riding, 2012).
As for purely abiotic processes, carbonate precipitation associated
with microbial processes is strongly dependent on the “alkalinity engine” (Dupraz et al., 2009) of a depositional system. Indeed, when carbonate alkalinity in seawater increases through specific microbial
metabolic activities, CaCO3 precipitation is generally promoted
(Riding, 2000; Dupraz et al., 2004; Dupraz et al., 2009; Glunk et al.,
2011). Conversely, other varieties of microbial metabolic mechanisms
enhance the concentration of dissolved inorganic carbon (DIC) or
drive the production of organic acids, both processes leading to a
decrease in pH and favouring carbonate dissolution (Wenzhofer et al.,
2001; Dupraz et al., 2009). Dupraz et al. (2009) documented that several
hundreds of microbial species may coexist within a few cm2 of seafloor
in natural habitats. All these are subdivided in 5–7 groups (“guilds”) of
microbes having similar metabolisms. Some of these promote carbonate
precipitation (e.g., cyanobacteria, sulphate reducers), some others
favouring dissolution (e.g., aerobic heterotrophs, fermenters (Visscher
and Stolz, 2005; Dupraz et al., 2009). Dupraz et al. (2009) concluded
that the balance of all of these microbial metabolic activities strongly influences carbonate precipitation or, conversely, dissolution. How
microbially-induced carbonate precipitation impacts or modifies seafloor lithification in the geological record remains, however, poorly understood and requires further experimental and field investigations.
3.2. Hydrodynamic level, facies type, and sediment accumulation rates
3.2.1. Hydrodynamic level
Seawater and pore water circulation in the uppermost sediment column as induced by waves and currents (here cumulatively referred to as
“hydrodynamic level”) are parameters strongly affecting marine seafloor lithification (Shinn, 1969; Milliman, 1974; Seeling et al., 2005;
Sattler et al., 2005; van der Kooij et al., 2010; Christ et al., 2012a). Enhanced hydrodynamic levels result in higher rates of water circulation
in the porous uppermost sediments below the sediment-water interface, providing calcium carbonate ions for early cementation and favour
seafloor lithification (Shinn, 1969; Marshall and Ashton, 1980; Coimbra
et al., 2009).
Relatively constant and elevated hydrodynamic levels particularly
refer to carbonate seafloors that are within the reach of the fairweather wave base or wave-induced current activity (Shinn, 1969;
Dravis, 1979; Whittle et al., 1993; Sattler et al., 2005; Immenhauser,
2009; Christ et al., 2012a). Beneath wave depth, pore-water pumping
may be triggered by (i) tidal or wave-induced currents or internal
waves (Pomar et al., 2012 and references therein), (ii) evaporation of
seawater on the platform top or inner ramp settings and the resulting
downslope cascading of more saline, heavier water masses (Wilson
and Robert, 1992; Suga et al., 2000; Shumilin et al., 2002) and (iii) geothermal circulation (Jones et al., 2000; Rougerie and Wauthy, 1993; van
der Kooij et al., 2010). In even deeper settings, i.e. beneath the shelfedge, processes associated with early marine cementation and reduced
sediment accumulation rates include coastal upwelling and tidal
pumping (van der Kooij et al., 2010), contour currents (Noé et al.,
2006), or strong thermohaline currents (Aghib et al., 1991).
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N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
Fig. 3. Schematic diagram showing different stages of submarine chalk firm- to hardgrounds. Modified after Kennedy and Garrison (1975). (A) Firmground; (B) Incipient (nodular)
hardground; (C) Fully lithified marine hardgrounds. Terminology (A1–A3; B1–B3; C1–C3) according to Kennedy and Garrison (1975) is mainly based on biological activity and colonization of surfaces. See text for discussion.
In the domain of wave-induced water circulation, two main hydrodynamic base levels are relevant for near-bottom water circulation
and hence pore-water circulation in the uppermost sediment column:
(i) the fair-weather (and swell) and (ii) the storm-wave base (cf.
Immenhauser, 2009 and references therein). The bathymetric depth
of any type of wave base varies in relation to wind direction, the
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
185
Fig. 4. Schematic sketch of features commonly characterizing early marine seafloor lithification.
geometry of the basin and hence the fetch, but also due to seasonal
weather patterns, to platform morphology, or the presence and absence of coastal barrier islands or shoals, seafloor morphology and
the type of sediment. In recent depositional environments, the fairweather wave-base extends to water depths of up to 50 m whilst
storm wave base activity was found down to water depths of
250 m (Immenhauser, 2009) depending on the coast studied. During
storm events, unconsolidated sediments are entrained and removed
from the seafloor.
Winnowing of the seafloor may be permanent above the fairweather wave base (Immenhauser, 2009). Low sedimentation rates
associated with hardgrounds and erosion above or at the fairweather wave-base are reported from several studies (Shinn, 1969;
Purser, 1969; Whittle et al., 1993; Immenhauser et al., 2000b; Christ
et al., 2012a). Below the fair-weather wave base, but above stormwave base, sediment entrainment is rather punctuated, but may
have a considerable impact on the fine-grained sediment commonly
accumulated at these depths (Immenhauser, 2009). Examples of
hardground formation related to storm-induced waves and currents
are documented and discussed in the literature (Kim and Lee, 1996;
Reolid et al., 2010).
3.2.2. Relation of pore space and permeability to facies
According to the limestone classifications of Folk (1959) and
Dunham (1962), two conceptual end member groups of hydrologically
or gravitationally accumulated carbonate sediments and sedimentary
rocks can be broadly identified: (1) fine-grained, mud-dominated facies
(e.g., carbonate ooze and its lithified counterpart, mudstone) and
(2) coarse-grained, mud-lean or mud-free substrata (e.g., ooidal sediments and corresponding grain- or rudstones). Permeability in carbonates, and hence the ability to circulate pore waters, mainly depends on
grain size and porosity. The permeability of unconsolidated sediments
is generally higher in sand-sized grainy carbonates than in muddominated carbonates (e.g., Franseen et al., 2003).
Commonly, in carbonate seafloor environments, the thickness of the
lithified interval increases with increasing grain size. In finer-grained
sediments, the on average lower permeability limits marine cementation to the first centimetres beneath seafloor as pores are clogged
rapidly (e.g., Shinn, 1969). Sediments above the fair-weather wave
base are generally winnowed and hence mud-lean. With depth, the
mean hydrodynamic level decreases and the mud content increases
(“mud line”; in Immenhauser, 2009). Thus, permeability of sediments
and the potential for early marine cementation is high above the fairweather wave-base and decreases with increasing depth. Therefore, in
wave-swept neritic domains, hardground development is significantly
favoured by winnowing promoting higher cementation combined
with reduced sedimentation rates. An overview of the thickness of
early marine lithification intervals of some recent and ancient
hardgrounds is given in Table 5 in combination with proposed controls
on hardground formation.
3.2.3. Sediment accumulation rates
Sediment accumulation rates result from the balance of the rate of
sediment production and the rate of erosion. Consequently, low sediment accumulation rates may result from either reduced sediment production, for instance during transgressive pulses (Meyers and Sageman,
2004), or from sediment removal under increased hydrodynamic levels
(Dravis, 1979; Christ et al., 2012a). Alternatively and with regard to consolidated or lithified surfaces, such as firm- or hardgrounds, low sediment and/or rock preservation may result from mainly physicochemical erosion and bioerosion including both burrowing or boring
(Taylor and Wilson, 2003; Carmona et al., 2007; Kulkarni et al., 2008).
The formation of hardgrounds is in most cases linked to periods of
suspended or reduced sedimentation, i.e., condensation (Shinn, 1969;
Purser, 1969; Kennedy and Garrison, 1975; Schlager and James, 1978;
Bromley and Allouc, 1992; Savrda, 1995; Obrochta et al., 2003;
Roberts and Boyd, 2004; McLaughlin et al., 2008).
Reduced sediment accumulation rates are also typical for midlatitudes and result in a positive cement/sediment volume ratio
(Nicolaides and Wallace, 1997). Often, hardground formation in such
cool-water settings is less common relative to that in tropical settings.
Nevertheless, where relative sea-level fall, reduced sedimentation
rates and wave abrasion of soft sediments coincide with enhanced
pore-water circulation, relatively thick, lithified intervals form in coolwater settings too (James et al., 1994; Nicolaides and Wallace, 1997;
Table 5). In deeper marine settings, where seawater is less saturated
186
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Fig. 5. Features of modern and ancient hardgrounds. (A–B) Borings cutting early marine cements and grains beneath Middle Jurassic hardgrounds (Bajocian, Paris Basin, France);
(C) Persian Gulf; underwater image of pottery embedded into quartz sandstone cemented by carbonate phase. (D) Photomicrograph of thin-section of 20 year old rock (at the time
when the photo was taken) from the very shallow waters of the Persian Gulf showing acicular aragonite growing on pottery. (E) Photomicrograph of encrusting oyster on ooidal
hardground of a Middle Jurassic carbonate ramp (High-Atlas, Morocco); (F) Close-up of E, showing truncation of grains and early marine cement. Photos (A–B) and (C–D) are reprinted
from Purser (1969) and Shinn (1969) with the permission of Wiley-Blackwell.
with respect to CaCO3, the formation of hardgrounds is often related to
long-term sediment starvation also due to sediment entrainment and
transport (Galloway, 1989; Bromley and Allouc, 1992; Bryan, 1992;
Remia et al., 2004). Sediment starvation, however, is also known from
shallow marine settings (e.g., Obrochta et al., 2003; McLaughlin et al.,
2008).
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
187
Table 3
Overview of the main cementation inhibition, precipitation and dissolution parameters and related parameters. After van der Kooij et al. (2010).
Parameters
CO2−
CaCO3
pH Seawater
3
supply saturation
Mg/Ca ratio
state (Ω)
Inhibition
−
effect
Cementation +
−
−
+
+
−
−
−
Dissolution
+ low-Mg
calcite
+ aragonite
determine
mineralogy
– high-Mg
calcite
+ low-Mg
calcite
T° Salinity
Alkalinity
SO2−
4
PO3−
Corg
4
Bacteria/microbes
0
– if low alkalinity,
low saturation
+
+
+
+ poisoning of crystal
lattice, aerobic oxidation
+ carbonate source
0
−
0
+ in oxic environment
0
0
+ +
increase
Ω
− 0
+ reduction in anoxic 0
environment
0
Mediating/
catalysing
+: positive correlation (enhancing); −: inverse correlation; 0: no dominant effect or no reported effect.
3.3. Global sea-level change and the significance of hardgrounds as
stratigraphic surfaces
referred to as either regressive (or regression) surfaces or (maximum)
flooding surfaces (cf. definitions in Catuneanu et al., 2009; Fig. 7).
Hardgrounds generally represent hiatal intervals, the duration of
which may range from 10s of years up to several millions of years
(e.g., the ca. 10 Myr hiatal gap of the Aptian top Shu'aiba Formation unconformity; Rameil et al., 2012). These discontinuities often formed due
to significant relative sea-level change such as substantial tectonoeustatic sea-level fall and/or rise (Kendall and Schlager, 1981; Eberli
et al., 2010; Strasser et al., 2012; Christ et al., 2012a). Other hardgrounds
represent pivotal events in global climate dynamics or result from environmental stress resulting in a forced adaptation of carbonate factories
(Wignall, 2001; Bond and Wignall, 2008). The resulting surfaces are
usually prominent in outcrops or recognized in the subsurface
from seismic profiles and can be traced platform- and/or basin-wide
and beyond. Generally, these hardgrounds are of significance in sequence stratigraphy where they commonly represent the base or the
top of cycles or sequences, allowing for regional (or beyond) correlation.
Hardgrounds forming on neritic carbonate systems can be generally
3.3.1. Regressive-marine hardgrounds
Stratigraphic markers represented by hardgrounds are often
(maximum) regressive surfaces (Fig. 7). Prominent examples of
regressive-surface hardgrounds were reported from a Middle Jurassic
ramp of the High-Atlas, Morocco (Christ et al., 2012a). These discontinuities develop due to wave action at shallow water depths, resulting in
sediment entrainment and non-deposition (Christ et al., 2012a).
More complex case examples come from the Albian Nahr Umr Formation in Oman. There, hardgrounds formed during regression phases
of transgressive–regressive cycles (Immenhauser et al., 1999). These
marine hardground surfaces were subsequently overprinted by a brief
subaerial exposure stage at the peak of relative sea-level fall and again
superimposed by a second hardground phase as sea level rose again.
As a consequence, ‘composite surfaces’ formed, sharing attributes of
marine hardgrounds and subaerial exposure surfaces. Similar composite
discontinuities pointing to regression were reported from other Lower
Cretaceous platforms of Oman (Sattler et al., 2005) or hardgrounds at
the Triassic–Jurassic boundary of SW England (Wignall, 2001).
3.3.2. Maximum-flooding surfaces
Maximum-flooding surfaces constitute a further common type of
stratigraphic surface associated with submarine lithification in
shallow-marine environments (Fig. 7). Unlike regressive-surface
hardgrounds, hardgrounds formed during flooding pulses reflect the
deepest associated facies and are often underlain by condensed and
strongly bioturbated facies. Examples include platforms of the Upper Jurassic and lowermost Cretaceous exposed in Switzerland and France.
Some of these hardgrounds point to a drowning of—or shift in—the
carbonate factory due to a rapid relative sea-level rise (Pasquier
and Strasser, 1997; Pittet and Strasser, 1998; Hillgärtner, 1998). Often,
these surfaces cap orbitally-controlled cycles of 4th to 6th
(parasequences) order but may also reflect maximum-flooding stages
in 3rd order sequences (Hillgärtner, 1998).
4. Petrography, petrophysics, and geochemistry of
hardground cements
The detailed understanding of the petrographic, petrophysical and
geochemical properties of hardgrounds is a crucial element in the interpretation of processes that governed their formation and alteration.
4.1. Cement petrography in hardgrounds
Fig. 6. Three-dimensional schemes representing the predicted qualitative (and semiquantitative) distribution of primary carbonate minerals precipitating in seawater relative
to the variation of the CaCO3 saturation state of the oceans. CaCO3 saturation state is principally a function of water depth and latitude and seawater Mg/Ca ratios. See text for discussion. ACD: Aragonite Compensation Depth; CCD: Calcite Compensation Depth.
With regard to Phanerozoic marine hardgrounds, the most commonly observed inorganic calcitic and aragonitic cements include
(i) acicular, fibrous and bladed, (ii) radiaxial fibrous and fascicularoptic, (iii) microcrystalline/cryptocrystalline, (iv) equant sparry,
syntaxial overgrowth, scalenohedral, peloidal and (v) non-carbonate
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N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
Table 4
Rates of early marine sea floor cementation for some modern and ancient hardgrounds.
Source cited
Geological period/geographic location
Bathymetric setting
Time involved in sea floor lithification/Dating method
Shinn (1969)
Taylor and Illing (1969)
Dravis (1979)
Pigott and Land (1986)
Grammer et al. (1999)
Schlager and James (1978)
Kennedy and Garrison (1975)
McLaughlin et al. (2008)
Holocene—Persian Gulf
Holocene—Persian Gulf
Recent—Bahamas
Recent—Jamaica
Recent—Bahamas
Recent—Bahamas
Cretaceous—England
Middle Palaeozoic
Neritic
Neritic
Neritic
Neritic
Deeper platform
Deep–Periplatform
Epeiric–Hemipelagic
Neritic
b20 years (observations—growth on pottery) (*)
b4500 years (14C)
~0.1–1 years (observations)
b130–140 years (14C) (**)
b1 year
105's years
~101–102 years (estimates)
~102-103 years (estimates)
(*) See Fig. 4C, D; (**) Reef rocks.
fabrics. Refer to Fig. 8 for an overview of different hardground cement
types.
4.1.1. Acicular, fibrous and bladed cements
The most common group of marine carbonate fabrics lithifying
hardground intervals are isopachous, acicular, fibrous, and bladed cements (Table 6). These fabrics commonly fringe sediment particles occluding intra-particle porosity (Flügel, 2004; Figs. 8 and 9). At the time
of deposition cement mineralogy is commonly metastable aragonite
and high-Mg calcite (Figs. 8 and 9), the latter phases subsequently
transforme to low-Mg calcite. Evidence for the secondary nature of
these calcite cements is often found in their patchy luminescence
under cathodoluminescence (e.g., Knoerich and Mutti, 2006b; Rameil
et al., 2012).
A remarkable feature is that acicular, fibrous, and bladed cements
are common in hardgrounds from the Mesozoic onwards but rarely
Table 5
Depth affected by lithification beneath sea floor given for modern and ancient
hardgrounds and relationship with carbonate substrate, hydrodynamic levels or submarine setting and/or stratigraphic implications.
Study
Lithified interval
beneath
hardgrounds
Nature of lithification/stratigraphic
significance of lithified interval
Wilkinson et al.
(1982)
Shinn (1969)
~10 cm
Purser (1969)
5–10 cm
Texture of the carbonate substratum
(Coarse-grained (biosparite1))
Texture of the carbonate substratum
(Coarse-grained)
Texture of the carbonate substratum
(Coarse-grained)
Texture of the carbonate substratum
(Coarse-grained)
Texture of the carbonate substratum
(Medium to coarse-grained)
Time involved (low-frequency cycles?)⁎
Texture of the carbonate substratum
(Fine-grained (mudstone2))
Time involved (high-frequency cycles?)⁎
Texture of the carbonate
substratum/hydrodynamic levels—Mud
dominated (lagoon)
Hydrodynamic levels—Semi-protected
setting
Texture of the carbonate
substratum/hydrodynamic levels –
Coarse-grained (oolithic)
Texture of the carbonate substratum
(Lime-mud)
Time involved (eustasy)/low
sedimentation rates—Top of 3rd-order
cycles
Time involved (eustasy)/low sedimentation
rates—Top of megasequences
10–15 cm
Mutti and Bernoulli Up to 25–30 cm
(2003)
Christ et al.
Up to 30 cm
(2012a)
b2 cm
Marshall and
Ashton (1980)
Superficial
3–5 cm
~20 cm
1–3 cm
Nicolaides and
Wallace (1997)
5–20 cm
Up to 1 m
1
Limestone classification after Folk (1959).
Limestone classification after Dunham (1962).
⁎ Relative comparison between both types of lithified intervals described from Christ
et al. (2012a).
2
documented in Paleozoic tropical and subtropical depositional environments (Tables 6 and 12). The reasons for this pattern are not well understood but several hypotheses have been suggested. Most of the
Paleozoic Era (mid-Cambrian to mid-Carboniferous; Stanley and
Hardie, 1998) was characterized by a geochemical composition of seawater that did not favour aragonite or high-Mg calcite cement precipitation. Environmental factors probably included low seawater Mg/Ca
ratios, high CaCO3 saturation states and high atmospheric and seawater
pCO2 (Sandberg, 1983; Stanley and Hardie, 1998; Royer et al., 2001,
2004; Riding and Liang, 2005). Conversely, fibrous, bladed, and rare
acicular high-Mg calcite cements have been described from Miocene,
Cretaceous and Jurassic case examples (Figs. 8 and 9; Tables 7 and 11;
Fürsich, 1979; James and Bone, 1992; Mutti and Bernoulli, 2003; Rameil
et al., 2012).
Numerous previous workers suggested a systematic relation of fluid
geochemistry and cement morphology (e.g., Folk, 1974b; Flügel, 2004).
Generally, high fluid Mg/Ca ratios promote acicular and fibrous fabrics
whereas low Mg/Ca fluid ratios preferentially induce low-Mg equant
calcites. Nevertheless, in natural carbonate depositional environments,
the relation between fluid chemistry and carbonate cement mineralogy
and fabric is not straightforward. In this sense, previous authors reported that carbonate fabrics and corresponding mineralogies favoured by
low fluid Mg/Ca ratios may coexist with carbonate phases commonly
precipitating in waters with elevated Mg/Ca ratios (Wilkinson et al.,
1985). Reasons for this may include a series of interrelated factors
(kinetic versus thermodynamic effects, fluid chemistry, biologic or substrate effects, etc.) that may account for apparently unexpected patterns
in cement fabrics and mineralogy (Table 6; Burton, 1993). Finally, an in
part significant preservation bias of ancient carbonate fabrics must be
taken into consideration. This implies that specific cementation fabrics
as presently observed in ancient carbonates must not correspond to
those originally precipitated in the marine diagenetic realm (Swart,
2015).
4.1.2. Radiaxial fibrous cements
Radial, radiaxial, and fascicular-optic fibrous calcites, termed here
“radiaxial calcites” (Richter et al., 2011), are common fabrics of Paleozoic and Mesozoic marine carbonate facies. However, radiaxial calcites are
volumetrically of subordinate importance in hardground intervals compared to other carbonate fabrics. With respect to early marine diagenetic lithification of carbonate seafloors, radiaxial fibrous fabrics have been
described from a limited number of Cenozoic and Mesozoic case examples only (Table 6; Middle Triassic Latemar platform; Dunn, 1991;
Oligo-Miocene Torquay Basin; Nicolaides and Wallace, 1997; Jurassic
and Cretaceous Western Carpathians; Mišík and Aubrecht, 2004).
Where present, radiaxial calcites form comparably thick (many
mm's to several cm's) fringes precipitated on the walls of pore space
in (sub)tropical marine carbonate environments. Precipitation is expectedly related to elevated water circulation into the near-surface
pore space (Immenhauser et al., 1999; van der Kooij et al., 2010) and
seawater supersaturation for CaCO3. As it is the case for acicular, fibrous
and bladed cements, the primary mineralogy of ancient radiaxial cement is often considered to be Mg-calcite (Gray and Adams, 1995;
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
189
Fig. 7. Timing of the formation of most common stratigraphic surfaces (maximum flooding and maximum regressive surfaces) expressed by hardgrounds in shallow-marine carbonate
systems. Modified after Catuneanu et al. (2009).
Wilson and Dickson, 1996; Tobin and Walker, 1996) since in some
cases, secondary micro-dolomites are present in the crystal lattice of ancient case example (Lohmann and Meyers, 1977; Muchez et al., 1991;
Bruckschen et al., 1992; Lavoie and Bourque, 1993; Mazzullo, 1994).
4.1.3. Microcrystalline/cryptocrystalline cements
Microcrystalline and cryptocrystalline fabrics are often present in
micrite or micrite-sized carbonate facies (Flügel, 2004). Microcrystalline
and cryptocrystalline fabrics are defined as crystals with diameters of a
few microns or less, respectively, but this definition is not applied by all
workers (Folk, 1959; Kropacheva et al., 1976; Stow, 2005). Microcrystalline cement may fringe grains forming a thin isopachous rim and
shares similarities with bladed, fibrous or acicular cements. Despite
the small size of these crystals, micro-, and cryptocrystalline fabrics
might play an important role in seafloor lithification as some ancient
hardground intervals are completely micritized (Table 6). As a consequence, syn-depositional or early diagenetic features are obscured and
often difficult to recognize in thin sections. Examples include lithophage
borings that are recognized due to subtle differences in the colour of the
host carbonate and the sediment infilling the boring. In some cases,
fine-grained skeletal material within these fillings is also considered diagnostic for micritized bored hardgrounds (Fig. 10).
Microcrystalline and cryptocrystalline fabrics represent the main cement type in hardground intervals in a variety of marine depositional
environments (Table 6). These include shallow marine tropical
(Table 6; Holocene Bahamas; Whittle et al., 1993; Cenozoic Central
Basin of Iran; Okhravi, 1998; Cretaceous eastern Arabian Peninsula;
Dickson et al., 2008; Jurassic High-Atlas ramp; Christ et al., 2012a; Ordovician Dumugol Formation; Kim and Lee, 1996) or temperate depositional environments (Table 6; Holocene Delaware Bay; Allen et al.,
1969; Cenozoic Maiella platform; Mutti and Bernoulli, 2003). Microcrystalline and cryptocrystalline fabrics are also reported from lithified
hemi-pelagic and pelagic seafloor intervals (Table 6; Red Sea;
Milliman et al., 1969; Mediterranean Sea; Milliman and Müller, 1973;
Atlantic Ocean; Noé et al., 2006).
The interpretation of micritic hardgrounds is not straightforward, as
an assessment of the nature (sedimentary, erosional, microbiallyinduced or diagenetic) and paragenetic stages is often difficult due to
the small size of the crystals and their poorly constrained diagenetic environment (Reid et al., 1990; Kazmierczak et al., 1996). Cryptocrystalline high-Mg calcite is a common feature observed in intertidal
beach sediments and is generally thought to have precipitated from
pore water of marine origin (Folk, 1974a). Given that all the micritic
fabrics of different nature are morphologically similar, and often near
indistinguishable to detrital micrite, misinterpretations are likely
when classifying and interpreting these carbonate rocks (Friedman,
1985; Reid et al., 1990). Along similar lines of reasoning, Reid et al.
(1990) reported that micritic intervals in lithified layers may not reflect the cemented carbonate oozes, but rather be the result of recrystallization of depositional micritic grains in the shallow burial
diagenetic domain. Similarly, Reid and Macintyre (1998) pointed
out, that micritic grains could also form due to the early diagenetic
recrystallization of carbonates in shallow marine settings. Concluding,
the interpretation of microcrystalline calcite-cemented hardgrounds
remains challenging. Potential ways forward include detailed petrographic and geochemical analyses of mechanically disintegrated and
separated fractions of these fine-grained carbonates (Turpin et al.,
2008, 2011 and 2012).
4.1.4. Equant spars, syntaxial overgrowth, scalenohedral calcite and
peloidal fabrics
Equant spars, syntaxial overgrowth (mainly on echinoderm skeletal
fragments), scalenohedral calcite and peloidal fabrics have been reported to form early marine fabrics in hardgrounds (Fig. 8; Table 6). Equant
spars and particularly syntaxial overgrowth cements on echinoderms
dominate Lower Paleozoic hardgrounds but are also reported from Cretaceous and Jurassic case examples (Table 6). The nature of peloidal fabrics in hardgrounds is, similarly to that of micritic grains, under
discussion (Macintyre, 1985; Kazmierczak et al., 1996). With the noteworthy exceptions of peloidal cements (Malone et al., 2001) and equant
spars (Jørgensen, 1976), fabrics such as scalenohedral calcite and
syntaxial overgrowths are absent (or at least not reported) in presentday hardgrounds (Table 6).
In ancient case examples, low-Mg equant spars, scalenohedral calcites and syntaxial overgrowths often typify calcite sea periods and
form during an early marine diagenetic stage (Fig. 8). The comparison
of some (seemingly) decisive periods of hardground development
(Pleistocene–Holocene, Oligocene–Miocene, Jurassic–Cretaceous and
Cambrian–Ordovician) documents that the distribution and nature of
early marine cement mineralogy and fabrics shows important variances
over time (Fig. 11). Specifically, the temporal distribution pattern of
hardground-related low-Mg equant spars, scalenohedral calcite and
syntaxial overgrowths versus that of acicular and fibrous (and micro/
cryptocrystalline)—mostly aragonite and/or high-Mg calcite—cements
190
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
Fig. 8. Marine cement fabrics in hardgrounds. Cement types after Flügel (2004). Thin section images: (1) Acicular aragonite (Holocene Persian Gulf; Shinn, 1969). (2) Fibrous calcite (Jurassic; Purser, 1969). (3) SEM photograph of bladed calcite (Holocene NE Atlantic; Noé et al., 2006). (4) Microcrystalline calcite (Jurassic; High-Atlas); (5) Radiaxial fibrous calcite (Triassic,
Latemar); (6) Botryoidal cement (Triassic, Latemar); (7) Syntaxial overgrowth (Jurassic, High-Atlas); (8) Pore-filling equant calcite (Triassic, Latemar); (9) Scalenohedral cement (Triassic;
Latemar); (10) Gravitational cements (Triassic, Latemar). Images 1 and 2 are reprinted with permission of Wiley-Blackwell and image 3 with permission of Springer.
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
191
Table 6
Types of early marine cement fabrics lithifying hardgrounds
Cement fabric
Period (marine chemistry)
Setting (climatic zone)
References cited
Isopachous acicular
Modern–Recent (aragonite sea)
Modern–Recent (aragonite sea)
Jurassic (calcite sea)
Shinn (1969); Obrochta et al. (2003)
Milliman et al. (1969); Bernoulli and McKenzie (1981)
Fürsich (1979); Wilkinson et al. (1985)
Isopachous fibrous-bladed
Modern–Recent (aragonite sea)
Shallow marine (tropical)
Deep marine
Shallow marine
(tropical/subtropical)
Shallow marine
(tropical/subtropical)
Shallow marine (temperate)
Deep marine
Shallow marine
(tropical/subtropical)
Shallow marine (temperate)
Shallow marine
(tropical/subtropical)
Shallow marine
(tropical/subtropical)
Shallow marine
(tropical/subtropical)
Shallow marine (temperate?)
Shallow marine (temperate)
Shallow marine
(tropical/subtropical?)
Shallow marine (tropical)
Shallow marine (tropical)
Shallow marine (tropical)
Shallow marine (temperate)
Deep marine
Modern–Recent (aragonite sea)
Modern–Recent (aragonite sea)
Cenozoic (aragonite sea)
Cenozoic (aragonite sea)
Cenozoic (calcite sea)
Cretaceous (calcite sea)
Jurassic (calcite sea)
Pore-filling radiaxial fibrous
calcite
Crypto- and microcrystalline
(isopachous or pore-filling)
Ordovician (calcite sea)
Cenozoic (aragonite sea)
Jurassic–Cretaceous (calcite sea)
Triassic (aragonite sea)
Modern–Recent (aragonite sea)
Modern–Recent (aragonite sea)
Modern–Recent (aragonite sea)
Modern–Recent (aragonite sea)
Cenozoic (aragonite sea)
Cenozoic (aragonite sea)
Cretaceous (calcite sea)
Crypto- and microcrystalline
(isopachous or pore-filling)
Jurassic (calcite sea)
Carboniferous–Mississippian
(calcite sea)
Ordovician (calcite sea)
Peloidal
Modern–Recent (aragonite sea)
Jurassic (calcite sea)
Equant (sparry, blocky)
Modern–Recent (aragonite Sea)
Cretaceous (calcite sea)
Jurassic (calcite sea)
Ordovician (calcite sea)
Syntaxial overgrowth (on
echinoderms)
Cambrian (calcite sea)
Cretaceous (calcite sea)
Shallow marine
(tropical/subtropical)
Deep marine
Shallow marine
(tropical/subtropical)
Shallow marine (temperate)
Shallow marine
(tropical/subtropical)
Shallow marine
(tropical/subtropical)
Shallow marine (temperate?)
Ordovician (calcite sea)
Cambrian (calcite sea)
Shallow marine (temperate?)
Cretaceous (calcite sea)
Shallow marine
(tropical/subtropical)
Shallow marine
(tropical/subtropical)
Shallow marine
(tropical/subtropical)
Ordovician (calcite sea)
Jurassic (calcite sea)
Microstalactitic
Shallow and relatively deep
marine (tropical/subtropical)
Shallow marine
(tropical/subtropical)
Shallow marine (tropical)
Shallow marine (temperate?)
Shallow marine
(tropical/subtropical)
Shallow marine
(tropical/subtropical)
Shallow marine
(tropical/subtropical)
Shallow marine (temperate?)
Jurassic (calcite sea)
Scalenohedral (dog-tooth)
Shallow marine
(tropical/subtropical)
Shallow marine (temperate)
Jurassic (calcite sea)
shows in part opposing trends (Fig. 11). This observation may reflect
distinct patterns in seawater chemistry during the lithification of these
hardground surfaces.
Dravis (1979); Videtich and Matthews (1980); Khalaf et al.
(1987); Vollbrecht (1990); Whittle et al. (1993); Stentoft (1994)
Allen et al. (1969); Garrison et al. (1969)
Milliman (1966); Milliman et al. (1969); Malone et al. (2001)
Beach (1993); Okhravi (1998)
James and Bone (1992); Mutti and Bernoulli (2003)
Molenaar (1990)
Hillgärtner (1998); Immenhauser et al. (2000a, 2000b); Eren
and Tasli (2002)
Purser (1969); Aissaoui and Purser (1983); Wilkinson et al.
(1985)
Brett and Brookfield (1984); Möller and Kvingan (1988)
Nicolaides and Wallace (1997)
Mišík and Aubrecht (2004)
Dunn (1991)
Pigott and Land (1986); Whittle et al. (1993)
Poppe et al. (1990) – dolomite
Allen et al. (1969)
Milliman et al. (1969); Milliman and Müller (1973); Schlager
and James (1978); Allouc (1990); Malone et al. (2001); Noé
et al. (2006)
Okhravi (1998)
Mutti and Bernoulli (2003); Gruszczyński et al. (2008); James
et al. (2011)
Reitner et al. (1995); Dickson et al. (2008)
Gruszczyński (1986); Fürsich et al. (1992); Christ et al. (2012a)
Dodd and Nelson (1998)
Kim and Lee (1996)
Malone et al. (2001)
Wilson and Palmer (1992, 1994)
Jørgensen (1976); James et al. (1999)
Garrison et al. (1987); Eren and Tasli (2002); Rameil et al.
(2012)
Wilkinson et al. (1985); Reolid et al. (2010); Reolid and Nieto
(2010)
Wilkinson et al. (1982); Brett and Brookfield (1984); Brookfield
and Brett (1988); Palmer and Wilson (2004); Hender and Dix
(2008); Mancini (2011)
Kruse and Zhuravlev (2008)
Jarvis (1980); Garrison et al. (1987); Molenaar and Zijlstra
(1997); Eren and Tasli, 2002; Richard et al. (2005)
Purser, 1969; Wilkinson et al. (1985); Brigaud et al. (2009a,
2009b); Christ et al. (this study)
Kim and Lee (1996)
Wilkinson et al. (1982); Brett and Brookfield (1984); Brookfield
and Brett (1988); Ekdale and Bromley (2001); Palmer and
Wilson (2004); Hender and Dix (2008)
Glumac and Walker (2002); Kruse and Zhuravlev (2008)?;
Zamora et al. (2010)
Garrison et al. (1987); Molenaar and Zijlstra (1997)
Wilson and Palmer (1992, 1994); Brigaud et al. (2009a, 2009b)
Purser (1969); Brigaud et al. (2009a, 2009b)
4.1.5. Non-carbonate phases
A number of non-carbonate phases are present in many hardground
intervals. Numerous publications reported on early diagenetic phosphate
192
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
Fig. 9. Common early marine cement fabrics in neritic tropical settings (1–3 in Fig. 8). Modified after Flügel (2004).
or glauconite cements in hardground intervals (Pomoni-Papaioannou,
1994; Rehfeld and Janssen, 1995; Föllmi, 1996; Caplan and Bustin,
1998; Ludvigson et al., 2004; Jarvis, 2006; Bodin et al., 2006; Tarawneh
and Moumani, 2006; Pasava et al., 2008; Özgüner and Varol, 2009;
Alvaro and Clausen, 2010; Föllmi et al., 2011; Yilmaz et al., 2012;
Ribbert and Piecha, 2014). Similarly, Loucks and Ruppel (2007)
described phosphate- and pyrite-bearing hardgrounds from a hemipelagic Mississippian shale-gas system characterized by reducing conditions below the oxygen minimum zone.
Phosphates and glauconites—and to some extent iron and manganese minerals—are present in hardgrounds corresponding to longlasting hiatal or drowning surfaces (Clari et al., 1995; Godet, 2013).
Phosphogenesis and glauconitization often follow significant environmental changes of local to supra-regional significance such as relative
sea-level rise or continental weathering pulses driven by patterns in
the hydrological cycle (Föllmi, 1996). Following previous work, specific
pattern in environmental change may lead to an increase of the primary
marine productivity that in turn is coupled to the deterioration of
phototrophic reefal platform biota eventually leading to relative
deepening and hardground formation (Godet, 2013). Hardgrounds
representing the demise of carbonate factories and drowning are common in the Jurassic and particularly so in the Cretaceous periods
(Jenkyns, 2010; Godet, 2013).
Phosphatic hardgrounds develop through the pumping, often under
elevated hydrodynamic levels, of dissolved sulphate into pore space
close to the sediment/water interface. This process is affected by
phosphogenesis involving the bacterial degradation of buried organic
matter (Föllmi, 1996). Glauconite and other Fe–Mn oxides associated
with phosphates on hardgrounds commonly involve microbial breakdown of organic matter (Alvaro et al., 2010; Reolid, 2011; Banerjee
et al., 2012). Jimenez-Millan and Nieto (2008) documented ferromanganese crusts of different Jurassic hardgrounds on pelagic swells and attributed the formation of these surfaces to principally inorganic
processes. Bathymetric changes related to relative sea-level change,
modifications in redox conditions and the input of detrital Fe, Mn, as
well as other metallic elements are other factors commonly attributed
to the development of impregnated hardground intervals. In a rigorous
review, Konhauser (1998) argued that Mn-reducing bacteria are often
the main drivers of the formation of manganese crusts above seafloor.
Summing up, non-carbonate phases are common in hardgrounds
throughout the Phanerozoic and from all climate zones and in some
cases may surpass carbonate phases in volumetric significance. The precipitation of the different minerals (glauconite and other Fe–Mn-oxides,
phosphate, pyrite etc.) takes place during and after hardground formation (burial stage) and the driving processes involve inorganic and microbial processes.
Table 7
Petrophysical properties of three hardgrounds, minor discontinuities and underlying non-hardground carbonate intervals of a Middle Jurassic carbonate ramp (n.d. = no data).
Major surface
Sample number
Type of sample/interval
Stratigraphic position relative to surface (cm)
Density (cm3 g−1)
Porosity (%)
Permeability (mD)
Hardground DS9
GC1-13
GC1-12
C2
C2 subsample-1
C2 subsample-2
GC-C8
GC-C7
GC-C6
GC-C5
GC-C4
GC-C3
GC-C2
GC-A6
GC-A3
Major hardground
No hardground
Major hardground
Major hardground
Major hardground
Major hardground
No hardground
Hardground
Firmground
No hardground
Incipient hardground
No hardground
Major hardground
No hardground
0
−25
0
0
0
0
−15
−20
−35
−65
−75
−135
0
−45
2.70
2.67
2.68
2.66
2.58
2.67
2.67
2.68
2.70
2.69
2.69
2.68
2.69
2.65
0.3
1.4
1.2
0.1
3.6
1.6
1.6
1.3
0.5
0.7
0.7
1.0
0.9
2.3
n.d.
n.d.
n.d.
0.017
0.170
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
Hardground DS5
Hardground DS4
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Fig. 10. (A) Thin section image of boring into micritic carbonates beneath a hardground surface (Middle Jurassic, High-Atlas, Morocco). Crystals filling the burrow are calcite pseudomorphs
of primary gypsum minerals; (B) Close-up of (A). Note barely visible (white hatched line) interface between micrite-filled boring and host micrite.
Fig. 11. (A) Mineralogy and (B) types of early marine cements in hardground intervals plotted for corresponding time intervals. Data provided from Tables 1, 6, 8, 9, 10, 11 and 12.
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4.2. Petrophysical properties and fluid flow modelling of hardgrounds
Early marine cementation of carbonate seafloors leading to
hardground formation is intrinsically linked to changes in the porosity
and permeability of the lithified interval. Moreover, the thickness of
the lithified interval is directly related to the pore-water flow and
hence the supply of Ca and CO3 ions to the site of cement precipitation
(Kennedy and Garrison, 1975; Allouc, 1990; Swart, 2015). With increasing cementation, the permeability of the carbonate facies diminishes
close to the seafloor, impeding fluid circulation and therefore hampering and limiting downward lithification. Therefore, hardground formation often will reach a lower limit in the sediment column. Once
sediment accumulation above the hardground continues, lithified intervals will be subjected to burial diagenesis and shallow to deeper burial
cements will occlude remaining pore space. Questions of interest in
this context are: (i) to which degree early marine cementation reduces
pore space and permeability, or (ii) vice versa, how much open pore
space is still present when hardground intervals enter the burial domain? These questions have both fundamental and applied relevance.
Indeed, spatially more extensive hardground surfaces potentially represent tight, low-permeability rock intervals and may form barriers to vertical flow and thus result in reservoir compartmentalization (Cander,
1995; Read and Horburry, 1994; Wagner et al., 1995; Amour et al.,
2012; Christ et al., 2012a). Despite the significance of these considerations, only few studies deal with the petrophysics of ancient
hardground intervals and/or their impact on subsurface fluid flow
(Eberli et al., 2003; Shekhar et al., 2014).
The initial porosity of a given ancient carbonate facies and the remaining porosity after its early marine cementation are controlled by
a series of parameters. Porosity and its change with time is affected by
many factors, including sediment type and carbonate texture, mineralogy, the degree of initial compaction, extent, timing, and type of early
marine cementation and alteration of metastable carbonate mineralogies as well as the duration of hardground formation. With regard to
the Cretaceous Chalk rocks, Scholle et al. (1998) studied porosity reduction trends resulting from early marine cementation and hardground
formation. These data were then compared to under- and overlying
chalk beds and their relative behaviour through burial diagenesis.
Hardground porosity was more significantly reduced by early marine
cementation relative to that of unconsolidated chalk beds. Along similar
lines, Richard et al. (2005; Cretaceous Chalk hardgrounds) document a
porosity decrease from initially between 32 and 39% to between 19
and 28% due to early marine lithification. Subsequently, hardground
and non-hardground intervals were affected during burial diagenesis.
Scholle and Kennedy (1974) described a reduction of between 45–70%
in pore space of chalk hardgrounds relative to uncompacted Cretaceous
chalks. A similar pattern, albeit much less pronounced, was observed by
Richard et al. (2005). Nevertheless, primary differences in porosity between hardground and non-hardground intervals were still visible
after burial overprint. Specifically, hardground intervals subjected to
early marine and subsequent burial diagenesis represented the most
lithified, i.e. the least porous, intervals of the succession studied
(Scholle et al., 1998). Hence, early marine hardground cementation
causes incipient lithification but substantial portions of the initial pore
network may remain open when the hardground interval enters the
burial domain.
Given the scarcity of published data and the focus on Cretaceous
chalk hardgrounds, we compiled a petrophysical data set on Jurassic
carbonate ramp hardground intervals from Morocco (Fig. 12). The aim
was to contrast and compare the petrophysical properties of
hardground intervals versus that of non-hardground limestones beneath. The Bajocian (Middle Jurassic) Assoul Formation corresponds to
a ca. 220-m-thick stratigraphic succession of carbonate ramp deposits
located in the High-Atlas of Morocco (Christ et al., 2012a). Within this
stratigraphic interval, around 80 marine hardground surfaces ranging
from minor condensed horizons to true hardgrounds have been
identified. Judging from field and thin section work, individual
hardground intervals reach thicknesses ranging from ca. 5 to 30 cm.
Some of the major hardground surfaces could be traced over distances
of several kilometres (Christ et al., 2012a). Most of the spatially extensive hardgrounds are overlain by transgressive mudstones or marls
(Table 7; Figs. 12 and 13). Combined with the effects of the underlying
hardgrounds, these fine-grained intervals represent tight sedimentary
rock packages in the stratigraphic succession and impact subsurface
fluid flow patterns (Amour et al., 2012; Shekhar et al., 2014).
Geophysical properties of carbonate hardground intervals and—for
comparison—the limestones directly beneath—were determined for
twelve hand specimens and for several drilled cylindrical cores (for relative position of samples refer to Figs. 12 and 13). Small cylindrical cores
were diamond-drilled from three of these rock samples parallel and orthogonal to the sedimentary layering. Density was derived using Archimedes' method for all samples, including the irregularly shaped hand
specimens, and additionally relying on a geometrical volume determination for the cylindrical cores. The hand specimens exhibited volumes
of 40 to 280 cm3 and the cylindrical cores were dissected in order to assess heterogeneity on the scale of 5 to 10 cm3, i.e., an order of magnitude
smaller than the initial specimens. Propagation velocities of ultrasonic
P-waves were determined from transmission parallel and perpendicular to the layering using transducers with a central frequency of
500 kHz. Two cylindrical cores were used for permeability measurements by simple flow-through experiments at a confining pressure of
8 MPa using Argon as pore fluid at 0.4 to 2.0 MPa inlet pressure and
an outlet at ambient conditions.
All investigated hand specimens represent dense calcite aggregates exhibiting P-wave velocities close to theoretical values indicating that they contain isometric pores nor large aspect ratio
(micro)fractures. The porosity of the investigated limestone samples
(ρcc = 2712 kg/m3) ranges between 0 and 4%. Hardground and
non-hardground samples can neither be distinguished by their
density-velocity relations (Fig. 14A) nor do they show significant systematics in their anisotropy (Fig. 14B). Either group exhibits modest
anisotropy within the range associated with preferred orientation of
calcite minerals; a dominance of a fast direction relative to layering
is however not found, a feature that is typical for coarse-grained carbonate facies (Immenhauser et al., 2004).
The properties of the drilled cores generally agree with the
ones of the hand specimens but yield densities that in cases are so
high that minerals heavier than calcite are needed to explain them
(e.g., dolomite or iron oxides) a feature that agrees with thin section observations. The heterogeneity of hardground and non-hardground samples on the scale of several cm3 does not differ significantly. The
measurements did not reveal a systematic density variation with distance to hardground top; the variability in density parallel to bedding
is actually on the same level as perpendicular to it. Two hardground
samples cored perpendicular to layering yield permeability values of
10−16 to 10−17 m2 in agreement with the inference from density and
P-wave velocity that samples contain modest porosity and are devoid
of (micro)fractures. Concluding, burial cementation occluded all
remaining pore space resulting in limestones with petrophysical properties that are near-identical to inorganic calcite crystals. Porosity remaining after initial marine cementation could not be assessed. The
collected data are of interest as they significantly contrast data from
the Cretaceous chalk hardgrounds that lack extensive burial diagenesis
(Scholle and Kennedy, 1974; Scholle et al., 1998; Richard et al., 2005;
Fig. 15). No case example was found in the literature that describes
the quantitative evolution of hardground porosity for more grainy,
(bio)clastic sediments.
The question remains, if the initial marine cementation of a spatially
limited rock interval impacts fluid flow in the subsurface? In order to
answer this question, a fluid flow simulation model was compiled
(Agada et al., 2014; Shekhar et al., 2014; Fitch et al., 2014). The model
is based on field data and combined with geological facies modelling
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Fig. 12. Stratigraphic and sedimentologic features of the Jurassic carbonate ramp setting, Central High-Atlas, Morocco with indication of marine hardground surfaces (red).
(A) Stratigraphic section of the lower portion of ramp succession. Hardground surfaces labelled DS4 and DS5 were analysed for their petrophysical properties (green stars (B) Outcrop
image with laterally significant hardgrounds indicated by white and green lines (DS1 through DS7). Note recessive marl-dominated facies above hardground surfaces corresponding to
transgressive pulses. (C) Field image of hardground surface DS9 with litophage bivalve borings and Fe-mineralization.
(Amour et al., 2012; Christ et al., 2012a). The focus was on major, spatially extensive hardgrounds representing early diagenetic lithification
intervals of approximately 5 to 30 cm in the Moroccan field area. The
model output suggests that along a modelled burial path, the early lithified hardground intervals did not affect fluid flow in a significant manner prior to entering pervasive burial diagenesis arguably because these
intervals are crosscut by numerous fractures providing vertical pathways of fluid flow in the subsurface (Fitch et al., 2014). Nevertheless,
the effects of hardground intervals combined with the effect of the
stratigraphically thick (~2–3 m), low porosity transgressive shale facies
overlying the hardgrounds controlled fluid flow in a predicable manner
(Shekhar et al., 2014).
A series of preliminary implications are drawn from this admittedly
limited data set and equally limited previous work: (i) from the viewpoint of porosity and permeability evolution, different carbonate facies
(fine-grained chalk versus coarse-grained ramp limestones) respond
significantly different to early marine and subsequent burial diagenesis.
(ii) Early marine hardground cementation is sufficient to bind sediment
particles and to create a wave-resistant, lithified carbonate seafloor. The
degree of cementation is a function of omission time, pore water circulation and saturation. Substantial amounts of the initial pore framework
may remain open, when hardground intervals enter the burial domain.
(iii) Partially (marine) lithified hardground intervals might escape
compactional destruction of initial pore space during initial burial due
to their higher mechanical stiffness. In the absence of pervasive burial
cementation, these intervals might in fact represent high-porosity levels.
(iv) Medium-scale (104–105 yr.'s) marine hardgrounds represent comparably insignificant obstacles to subsurface fluid flow due to fracturing
and faulting forming pathways for vertical fluid migration. (v) These
conclusions might not hold true for regionally important, fully lithified
omission surfaces that represent long-term hiatal intervals. These
features clearly impact (compartmentalize) reservoir flow behaviour.
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Fig. 13. Schematic erosional profiles of sections sampled for petrophysical investigations. Position of samples taken from carbonate hardground intervals, firmgrounds and underlying carbonates are marked. Major facies changes across hardground surfaces are indicated. (A) refers to hardground DS9 in Fig. 11A; (B) refers to hardground DS4 and (C) refers to hardground
DS5.
4.3. Isotope geochemistry of marine carbonate hardgrounds
Carbon and oxygen isotopes have been widely applied to identify the signatures of subaerial exposure and related meteoric diagenesis of ancient carbonate seafloors (Allan and Matthews, 1977,
1982; Beier, 1987; Goldstein et al., 1990; Goldstein, 1991; Driese
et al., 1992; Joachimski, 1994; Immenhauser et al., 2002; Railsback
et al., 2003; Sattler et al., 2005; Theiling et al., 2007; Christ et al.,
2012b and many others). Conversely, the application of light stable
isotopes to submarine hardground cements has been less systematic. Nevertheless, Nevertheless, analyses of isotope ratios
of early diagenetic cements in hardgrounds were published along
with the growing interest in seafloor lithification at the end of
the 1960's and onwards (Choquette, 1968; Shinn, 1969; Videtich
and Matthews, 1980; Marshall and Ashton, 1980; Allouc, 1990;
Immenhauser et al., 2000a; Mutti and Bernoulli, 2003; Sattler
et al., 2005; Dickson et al., 2008; Rameil et al., 2012; Christ et al.,
2012a). Commonly, these authors aimed to extract the δ 13 C and
δ18O signatures of the marine porewater during cement precipitation and the separation of marine, meteoric, early marine burial
and late burial phases. With respect to carbon isotopes, several
authors noted the significance of microbial activity in the context
of hardground diagenesis.
4.3.1. Carbon and oxygen isotope signatures of recent and sub-recent
hardgrounds
In a case study of a recent deep water hardground in the Eastern
Mediterranean Sea, Aghib et al. (1991; Fig. 16) document increasing
δ13C and δ18O values with increasing degree of lithification. Specifically,
marine, hardground-related cements have δ13C and δ18O values that are
enriched by 2.5 and 4‰, respectively, relative to the unlithified coccolith
carbonate oozes underlying lithified intervals. According to Aghib et al.
(1991), elevated δ18O of hardground cement reflects cold basinal
porewater temperatures, while depleted coccolith ooze δ18O reflects
the warm sea surface temperatures of the habitat of these algae. Moderately elevated hardground cement δ13C were interpreted to document
the involvement of 13C-enriched methanogenetic carbonate in pore
fluids (Aghib et al., 1991).
Similarly, McKenzie and Bernoulli (1982) and Allouc (1990) reported on Mediterranean lithified deep-water carbonate crusts corresponding to early stages of hardground development. Bulk carbonate carbon
and oxygen isotope values increase with increasing degree of early marine cementation (Fig. 17). Moreover, higher bulk magnesium contents
tend to correlate with increasing levels of lithification (Fig. 17). Most
likely, this pattern reflects a shift in the relative proportion of low-Mg
coccolith calcite and magnesian calcites (Allouc, 1990; Fig. 17)
representing the hardground cement phases.
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domain. Shinn (1969) reported 18O-enriched signatures from shallowwater hardgrounds of the Persian Gulf and interpreted these patterns
as reflecting evaporative conditions (Fig. 18). Carbon and oxygen isotope analyses were carried out using bulk carbonates, different skeletal
components and abiotic cements. While all carbonate materials reflect
the shallow marine domain, δ13C and δ18O values exhibit intersample
variability of up to 2.3 and 1.5‰, respectively (Shinn, 1969). It seems
likely, that this pattern reflects differential fractionation between seawater/porewater (e.g., Dickson et al., 1991), aragonitic, and (Mg-)calcitic mineralogies (Turner, 1982).
Fig. 14. Cross-plot of ultrasonic P-wave velocity and density for hand specimens. Circles and
squares represent results for velocity measurements parallel and perpendicular to the sedimentary layering, respectively. Grey and orange symbols indicate non-hardground and
hardground samples, respectively. Error bars give absolute uncertainties; the relative
error in density corresponds approximately with symbol width. Theoretical relations
(HS+: upper Hashin-Shtrikman bound; Voigt: Voigt bound) for porous polycrystalline calcite aggregates are given for reference; (B) Anisotropy factor, i.e., ratio between P-wave velocities measured parallel and perpendicular to the sedimentary layering. Within the
hardground versus non-hardground classification, blocks are organized according to
order of increasing anisotropy factor.
Whilst the geochemical signatures of surficial, relative to basinal,
water masses are well exemplified in deep-water hardground cements,
this difference is far less pronounced in the shallow-water neritic
Fig. 15. Porosity range given for hardground and non-hardground intervals from various
Mesozoic carbonates. (a) Middle Jurassic ramp of the High-Atlas, Morocco (this study);
(b) Late Cretaceous chalk of western Europe (Scholle and Kennedy, 1974). (c) Late Cretaceous (Campanian) chalk of Belgium (Richard et al., 2005).
4.3.2. Carbon and oxygen isotope signatures of ancient hardgrounds
Under favourable conditions, diagenetically stable, early marine
hardground cements potentially record the geochemical signatures of
the marine porewaters from which these cements precipitated. Controls
of diagenetic resetting of these fabrics include: (i) alteration of metastable high-Mg calcite or aragonite mineralogies; (ii) subaerial exposure of
hardground surfaces involving meteoric diagenesis; and (iii) deep burial diagenesis. A series of case studies dealing with the geochemistry of
ancient hardground bulk and cement samples have been published
and are briefly reviewed below.
James and Bone (1992) investigated Oligo-Miocene hardgrounds
from the cool-water carbonate Eucla platform in Southern Australia.
The dominant hardground cement phase consists of high-Mg fibrous
calcite. Three types of carbonate materials from depths of less than
150 m were analysed for their isotopic signatures (Fig. 19): (i) Poorly
lithified host limestones (bulk “friable calcarenites”); (ii) hardground
carbonates (bulk “well lithified calcarenites”); and (iii) coeval brachiopod valves. Bulk hardground δ13C and δ18O values are significantly
lower than those of brachiopods, whilst they are enriched relative to
bulk host calcarenites (Fig. 19). James and Bone (1992) discussed continental freshwater input in the mixing zone affecting both calcarenites
and hardground intervals. Due to the reduced permeability of
hardground intervals, the effects of the meteoric signature were subdued relative to the more porous host calcarenites.
The observation that hardground intervals record several diagenetic
stages is rather common and for example reported from Lower Miocene
outer ramp case examples in Italy (Mutti and Bernoulli, 2003). The early
marine cements lithifying the ramp hardgrounds are high-Mg fibrous
calcites, predating marine diagenetic microcrystalline calcites
(“micrite”). The bulk δ13C and δ18O values of the 20 cm-thick
hardground interval and of carbonate rocks located 10 m up- and
downsection were analysed. Bulk δ18O values are enriched relative to
over- and underlying intervals (Fig. 20). Hardground data cluster in
two fields, each with a distinct isotopic signature. One cluster, characterized by enriched δ18O values, corresponds to intervals of the
hardground that are rich in fibrous cement. The second cluster, characterized by relatively depleted δ18O and δ13C signatures, refers to the microcrystalline, cement-rich, upper 2 cm of the hardground interval. The
concept of early marine fibrous cements precipitated from cool pore
waters supports these 18O-enriched values (A in Fig. 20). The more depleted δ13Cbulk values of the second cluster (B in Fig. 20) were attributed
to bacterially mediated precipitation (Irwin et al., 1977) at the sediment/water interface (Mutti and Bernoulli, 2003). Similar conclusions
were drawn for Miocene–Pliocene hardgrounds from the Great Bahama
Bank (Swart and Melim, 2000) and from a Lower Cretaceous carbonate
ramp (Dickson et al., 2008) involving bacterial oxidation or bacterial
sulphate-reduction.
Similarly to recent hardgrounds, Marshall and Ashton (1980) observed a relationship between the degree of lithification and bulk
carbonate carbon and oxygen isotope ratios of shallow neritic
hardgrounds from the Middle Jurassic of England. Describing three
basic surface types with increasing omission time, those characterized
by the most advanced degree of lithification displayed the most
enriched bulk δ13C and δ18O values (Fig. 21). Oxygen-isotope ratios of
hardground subsamples are enriched by up to 3‰ relative to those
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Fig. 16. Carbon and oxygen isotope composition of unlithified carbonate ooze, lithified
chalk and hardground carbonates from Bannock Basin (Eastern Mediterranean Sea).
After Aghib et al. (1991).
obtained from low-Mg calcite oysters, the latter arguably reflecting marine values. Marshall and Ashton (1980) also observed that δ13C and
δ18O ratios gradually decrease from hardgrounds to the underlying carbonates (Fig. 21). Gradually decreasing isotope ratios are in agreement
with a gradual decrease in the volume of fibrous calcites, forming the
main hardground cement phase. Isotope ratios of each hardground
type as described in Marshall and Ashton (1980), however, cluster in
distinct isotopic fields (Fig. 21). Obviously, this pattern reflects the full
complexity of early marine diagenetic isotope signatures affected by
subsequent diagenetic pathways (e.g., van der Kooij et al., 2010; Christ
et al., 2012b). As so often, the question remains to which degree early
marine porewater signatures are preserved? Fully lithified hardgrounds
developed in high-energy tidal channels (Marshall and Ashton, 1980)
correspond to 18O-enriched isotope signatures and might reflect the
contribution of colder, less stagnant water masses resulting from vigorous, diurnal water circulation and pervasive marine cementation.
Hardgrounds showing intermediated degrees of cementation, typical in more protected, shallow depositional settings, were in many
cases subjected to subaerial exposure and meteoric diagenesis
(Marshall and Ashton, 1980). It is at least conceivable that the on average 18O-depleted values of bulk carbonate samples from these
hardgrounds (Fig. 21) correspond to depleted meteoric oxygen.
Hardgrounds showing patchy lithification are characterized by the
Fig. 18. Carbon and oxygen isotope ratios of various shoalwater carbonate materials sampled from Persian Gulf Holocene hardgrounds. Data from Shinn (1969).
most depleted bulk micrite isotope values reported in the literature
(Marshall and Ashton, 1980). This overall pattern is similar to that of
sub-recent deep water hardgrounds from the Mediterranean (Allouc,
1990; Aghib et al., 1991). Concluding on the above discussion, marine
hardgrounds may form reasonable but non-trivial archives of past seawater, or rather marine porewater, properties. The detailed investigation of marine hardground and subsequent diagenetic pathways must
form the foundation of the assessment of these geochemical data. To
the knowledge of the authors, this task has not yet been performed in
a systematic manner.
5. Patterns in Phanerozoic hardgrounds
Hardgrounds lithify principally due to marine carbonate (and noncarbonate) cement precipitation. The fabric, mineralogy and volumetric
Fig. 17. (A) Trends in δ13C and δ18O ratios from seafloor carbonates with increasing degrees of lithification from the present-day eastern Mediterranean basin; (B) Pattern in the mineralogy
of the carbonates under increasing degrees of lithification. Note correlation between δ13C and δ18O values (A) and magnesium content (B), along with an increase in seafloor lithification.
Modified after Allouc (1990).
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Fig. 19. Carbon and oxygen isotope ratios of hardgrounds and weakly lithified limestones from three sampling locations in Oligo-Miocene carbonate shelf (Southern Australia). Note that
both hardgrounds and friable limestone intervals have δ13C and δ18O values that are more depleted than coeval brachiopod δ13C and δ18O ratios. Modified after James and Bone (1992).
Fig. 20. (A) Carbon and oxygen isotope ratios from Miocene hardground and under- and overlying carbonate facies, Maiella ramp (Italy). Labels B and C in Fig. 18A refer to images to the
right providing conceptual models for two consecutive stages in hardground evolution. (B/C) Conceptual models summarizing two phase sedimentary processes and diagenetic products
within the hardground. Modified from Mutti and Bernoulli (2003).
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and Wallace, 1997; Nelson and James, 2000; Mutti and Bernoulli, 2003).
The Oligocene–Miocene epochs correspond to a continual cooling of the
earth system following the Eocene–Oligocene transition (~ 33.9 Ma;
Gradstein et al., 2004). This critical time interval marks an abrupt
change from greenhouse to icehouse conditions with the onset of permanent continental ice-sheet formation in Antarctica (Crowley and
North, 1988; Wade et al., 2012). Shortly before that time, the world's
oceans witnessed a geologically rapid change from a calcite to an aragonite sea mode (Sandberg, 1983; Stanley and Hardie, 1998; Gothmann
et al., 2015). The frequency of reported hardgrounds during aragonite
III time interval is similar to that of reported hardgrounds during previous calcite sea periods (Palmer and Wilson, 2004). Case examples also
include pelagic to bathyal open oceanic settings (Atlantic and Pacific)
or saline, warm, semi-enclosed seas (Mediterranean and Red Sea).
Fig. 21. Carbon and oxygen isotope ratios of carbonates from three Jurassic shallow-water
hardgrounds in England. Samples from stratigraphically thick, fully lithified hardground
are significantly enriched in 18O. Adapted from Marshall and Ashton (1980). Grey field delineates estimated isotopic composition of Middle Jurassic seawater (after Veizer et al.,
1999).
abundance of these cements are strongly influenced by CaCO3 saturation (and also SO4 contents) and subordinate Mg/Ca ratio of the ambient
sea- and pore water. While marine Mg/Ca ratios are directly related to
(changes in) mid-ocean spreading rates and remain rather homogeneous over extended time periods, CaCO3 saturation largely depends
on water temperature and hence, local bathymetry and current/wave
patterns. This dependence implies that any discussion of Phanerozoic
hardground must include secular changes in environmental (and biological) factors. Here we consider: (i) seawater modes, i.e., aragonite
versus calcite seas; (ii) climate modes, i.e., icehouse versus greenhouse/hothouse periods; (iii) bathymetry; (iv) latitude; and (v) direct
observation versus circumstantial evidence.
For the sake of structure, this chapter follows a reverse chronological
path from the modern back into the ancient using the aragonite sea (I, II
and III)/calcite sea (I and II) modes of the Phanerozoic Era as redefined
by Stanley and Hardie (1998). We acknowledge that the general notion
of calcite versus aragonite seas as redefined by Stanley and Hardie's
(1998) model represents an oversimplification and is debated
(Rowley, 2002; Steuber, 2002; Balthasar and Cusack, 2015; Gothmann
et al., 2015). In essence, ocean water chemistry differs not only in time
but also in space and particularly with latitude (Fig. 6). Moreover, aragonitic cements are reported from calcite seas and vice versa. Having said
this, overall secular patterns between hardground mineralogy and seawater chemistry have been proposed before (Taylor and Wilson, 2003;
Palmer and Wilson, 2004; Fig. 22)). Here, a more detailed and critical
exploration of this relation is attempted.
5.1. Aragonite sea III
Pleistocene–Holocene and present-day occurrences of marine
hardgrounds are reported from a variety of geographical areas and
bathymetric ranges (Fig. 2; Tables 8 and 9). These zones include epicontinental and neritic settings such as the warm tropical to sub-tropical
areas of the western Atlantic (e.g., the carbonate banks of the
Bahamas or the shelves of the Gulf of Mexico and Florida; Dravis,
1979; Poppe et al., 1990) or the Persian Gulf (Shinn, 1969; Khalaf
et al., 1987). Other case examples include temperate and cool-water
carbonate shelves such as those of Southern Australia (James et al.,
1999; Rivers et al., 2008) or the Mediterranean Sea (Alexandersson,
1969). Most of the published work dealing with cool-water carbonate
hardgrounds is from the pre-Pleistocene (~33.9 to ~2.6 Ma) and particularly so the Oligocene–Miocene (Pedley and Bennett, 1985; Nicolaides
5.1.1. Holocene and Pleistocene neritic hardgrounds: The epeiric tropical
realm
The carbonate banks of the Bahamas and the Persian Gulf are shallow, sub-tropical to tropical depositional environments locally characterized by recent and sub-recent ocean floor lithification (Table 1;
Fig. 2). A substantial amount of literature on these hardgrounds has
been published (e.g., Taft et al., 1968; Shinn, 1969; Dravis, 1979;
Khalaf et al., 1987), with hardgrounds forming on Bahamian carbonate
banks being probably the best studied ones (Taft et al., 1968; Harris,
1978; Dravis, 1979; Whittle et al., 1993). Nevertheless, hardgrounds in
the western Atlantic are also reported from several other Caribbean carbonate platforms (Videtich and Matthews, 1980), the Gulf of Mexico
(Poppe et al., 1990; Obrochta et al., 2003) and the Bermudas
(Vollbrecht, 1990).
As expected in the warm, shallow waters of these tropical to subtropical regions, aragonite and high-Mg calcite cements are dominant
(Fig. 6). According to a detailed study of Bahamian Yellow Bank carbonates (Taft et al., 1968), aragonite and high-Mg calcite cements represent
up to 99% of all fabrics present. These are generally acicular to fibrous,
forming isopachous fringes around grains (Table 8). Microcrystalline
(Harris, 1978) or cryptocrystalline (Whittle et al., 1993) cements were
reported too. Dependent on interparticle pore size, cementation is
more pronounced within coarse-grained (e.g., ooidal shoals) mud-lean
carbonate sediments relative to fine-grained lime muds (Beach, 1993).
In relation to grain size and thus initial porosity, the type of fabrics
may vary spatially and the thickness of the lithified hardground intervals ranges from the seafloor a few mm's to several tens of cm's into
the sediment column (Table 8).
Significant pore water circulation is commonly associated with the
formation of oolitic hardgrounds on Eleuthera Bank, Bahamas (Dravis,
1979). Following Shinn (1969), lithification in sand-sized sediment is
highest close to the sediment/water interface and pores are generally
plugged downsection to a depth of 10 to 15 cm below the seafloor, precluding further downward cementation of the underlying sediments.
Nevertheless, some of the early marine lithified intervals described in
Dravis (1979) exceed the threshold thickness of 10 to 15 cm as suggested by Shinn (1969) for Persian Gulf hardgrounds (Table 8). These
metre-thick cemented intervals may imply that the rate of cementation
in high-energy tropical platforms, such as Eleuthera Bank, may periodically overwhelm even high sedimentation accumulation rates, allowing
thicker intervals to be lithified. Moreover, it is conceivable that very
thick hardground intervals (up to 3 m) actually form by continuously
adding and rapidly lithifying sediment on top of a pre-existing
hardground surface. In this sense, hardground formation might be cyclical with intervals of sediment accumulation alternating with intervals of
marine cementation (Taft et al., 1968; Khalaf et al., 1987; Table 8).
Both, on the Great Bahama Bank and in the Persian Gulf, seafloor
lithification occurs at very shallow water depths of 0 to 20 m under variable hydrodynamic levels (Table 8). For example, Whittle et al. (1993)
reported hardground formation from five areas in the western part of
Great Bahama Bank that in total expand over 30 km2 (Fig. 23). All of
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
201
Table 8
Depositional environment and petrology of some modern and recent neritic marine hardground (n.d. = no data).
Locality
Bathymetry
Tropical and subtropical setting (Aragonite III period)
Bahamas carbonate banks
b10 m
Sediment type
Mineralogy
(% rock volume)
Cement type (size)
Lithified
interval
Reference cited
Lime muds to coarsegrained shoals
Aragonite
High-Mg calcite/
(Low-Mg calcite)
Aragonite
High-Mg calcite
Fibrous and
cryptocrystalline
Cryptocrystalline
Fibrous
Microcrystalline
n.d.
Whittle et al., 1993
Dravis, 1979
Aragonite
Aragonite
High-Mg calcite
Acicular (?)
Acicular and
microcrystalline
Microcrystalline
Fibrous
Crypto- to
microcrystalline
Acicular and fibrous
Microcrystalline
Acicular
Several cm
to
10's cm
Up to ~1 m
mm to 10's
of cm
n.d.
Vollbrecht, 1990
10–15 cm
Shinn, 1969
10's of cm
to 3 m
Khalaf et al., 1987
Up to ~1 m
n.d.
Taylor and Illing,
1969
Poppe et al., 1990
Microcrystalline
(b10 μm)
Fibrous and
cryptocrystalline
Fine equant spars
(20–25 μm)
Acicular? (needles)
Fibrous
n.d.
Rivers et al., 2008
n.d.
Allen et al., 1969
20–30 cm
Jørgensen, 1976
n.d.
Microcrystalline
5–20 cm
Microcrystalline
Micropeloidal
Finely crystalline
n.d.
Garrison et al.,
1969
Alexandersson,
1969
James et al., 1999
Bahamas carbonate banks
1–11 m
Ooid shoals
Bahamas carbonate banks
Bahamas carbonate banks
b6 m
Ooid-peloid shoals
Skeletal and ooidalpeloidal sediments
Bermuda Islands
0–20 m
Persian Gulf
0–20 m
Ooid shoals
Persian Gulf (Kuwait Bay)
0–10 m
Ooid shoals
Aragonite
High-Mg calcite
Aragonite
Persian Gulf (Qatar Peninsula)
0–30 m (cores)
Ooid shoals
High-Mg calcite
Aragonite
Aragonite
High-Mg calcite
Shelf-edge of the Gulf of Mexico 90–110 m
Dolomite
Warm- and cool-water temperate settings (Aragonite III period)
Carbonate shelf of Southern
b120 m
Skeletal (biofragments
Australia
and intraclasts)
Outer continental shelf
75 m
Sandstone
of W USA
Nearshore of the Eastern
Few m below sea
Mixed siliciclastic-carbonate
Denmark
level
sediments
High-Mg calcite
Aragonite
Frasier River Delta, W Canada
Close to sea-level
Siliciclastic sediments
Low-Mg calcite
Nearshore settings
(E and W Mediterranean Sea)
Continental shelf
(SW Australia)
0–20 m
Algal framework
b40 to N100 m
(inner to outer
shelf)
Pelagic grainstones to
packstones
High-Mg calcite
(13–15 mol% MgCO3)
High-Mg calcite
(6–16 mol% MgCO3)
these hardgrounds formed in water depths of less than 10 m. The depositional environments and the corresponding hydrodynamic levels are
diverse and range from high-energy ooidal shoals and tidal channels
to low-energy lagoonal settings (Fig. 23). The diversity of hardground
types and formation environments in these comparably small observational area documents the significance of local mechanisms including
hydrodynamic parameters.
The petrography of hardgrounds and their early diagenesis also depends on the nature of the carbonate substrate, i.e. the texture of the
sediments at the seafloor. Several hardground surfaces form on ooidal
shoals and are spatially restricted to these sediment bodies and the
local, high-energy water conditions prevailing (e.g., Dravis, 1979;
Khalaf et al., 1987; Table 8). The spatial extent of specific hardground
surfaces may significantly vary from a few metres to several kilometres
(Harris, 1978), depending on the dimensions of the shoals. Some of
these ooid shoals reach 17 km in length (Dravis, 1979) allowing for
the formation of laterally extensive lithified intervals.
5.1.2. Holocene and Pleistocene neritic hardgrounds: The temperate realm
Recent and sub-recent hardgrounds forming within the temperate
climate belts have been reported from various locations worldwide
(Allen et al., 1969; Garrison et al., 1969; Alexandersson, 1969;
Jørgensen, 1976; James et al., 1999; Table 8). Allen et al. (1969) described a mixed carbonate-siliciclastic outer shelf in the eastern USA
that underwent sub-recent early marine diagenetic lithification by cryptocrystalline and fibrous aragonite cements (Table 8). At water depths
of 75 m and the prevailing low salinity cool water masses, this cement
type should rather be uncommon in the Holocene marine record of
High-Mg Calcite/mol%
MgCO3
Aragonite
Microcrystalline
Acicular and
cryptocrystalline
Microcrystalline
Taft et al., 1968
Harris, 1978
the eastern USA (Allen et al., 1969). The main phase of cementation
has been estimated to take place 4 kyr ago in a marine environment. Following the Last Glacial Maximum, sea level rose by about 120 m but stabilized around 3 to 2 ka BP (Gehrels, 2010). In the view of methanederived depleted carbon isotope ratios, Allen et al. (1969) suggested
that the 4 ka cementation event reflects a regional pattern affecting
the eastern shelf of the United States. Another similar case example of
sub-recent to recent shallow marine hardground surfaces off the Danish
coastline has been reported by Jørgensen (1976; Table 8). There, the
seafloor is lithified by aragonite and high-Mg calcite cements. Similar
to Allen et al. (1969); Jørgensen (1976) speculates on the involvement
of contemporaneous oxidised methane triggering cementation of the
ocean floor. More specifically, cementation took place through mixing
of sea- with meteoric water enriched in bicarbonate due to the oxidation of methane.
Contrasting with above mentioned examples of hardgrounds
forming exclusively in the marine realm, Garrison et al. (1969) reported
on patchy syn-sedimentary low-Mg calcite fibrous cementation affecting sandstones in a coastal river mouth (Table 8). According to the authors, lithification is likely not controlled by the chemistry of seawater
or river freshwater but rather attributed to specific processes occurring
in the pores. Aragonitic shells indeed are well preserved, indicating that
dissolution providing local supersaturation of CaCO3 and permitting cementation was not active. Garrison et al. (1969) suggested that factors
controlling early lithification were principally the decay of organic matter inducing carbonate dissolution, or the exchange of cations from clays
lowering the Mg/Ca ratio in the pore water and triggering low-Mg calcite cementation.
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N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
Table 9
Formation environment and petrology of some modern and recent open marine hardgrounds.
Locality
Water
depth
Lithified sediment
Deep-water oceanic hardgrounds (Aragonite III period)
Bahamas periplatform setting, 700–1100 m Oozes (pteropods)
Atlantic Ocean (Tongue of
the Ocean)
Bahamas periplatform setting, 300–450 m
Oozes
Atlantic Ocean
600–2000 m Oozes (planktonic
Atlantic and Pacific Oceans
foraminifera)
guyots and seamounts
(flanks)
Atlantic Ocean, Mid-Atlantic
700–2600 m Oozes (pteropods,
Ridge
planktonic foraminifera)
NE Atlantic Ocean
600–700 m
Oozes (foraminifera and
nannoplankton)
Deep-water hardgrounds in semi-enclosed seas (Aragonite III period)
Eastern Mediterranean Sea
2900–3200 m Oozes (coccoliths)
(Bannock Basin)
Eastern Mediterranean Sea
1600–3000 m Oozes (coccoliths)
(Hellenic Trench region)
Eastern Mediterranean Sea
850–4000 m Oozes (coccoliths and
planktonic foraminifera)
100 s-3000 m Oozes (coccoliths)
Eastern and Western
Mediterranean
Sea (several places)
Red Sea
400–1700 m Oozes (pteropods)
Red Sea
800–2700 m
Oozes (pteropods)
Mineralogy
(% rock volume)
Cement type (size)
Low-Mg calcite/
Microcrystalline (2–4 μm)
3.5–5 mol% MgCO3
Lithified
interval
Reference cited
n.d.
Schlager and James (1978)
High-Mg calcite/
~ 14 mol% MgCO3
High-Mg calcite
Microcrystalline and peloidal n.d.
(10–60 μm)
Microcrystalline (?)
n.d.
Aragonite
Microcrystalline
Low-Mg calcite
Bladed and microcrystalline
(2–6 μm)
High-Mg calcite
(50–70%)
High-Mg calcite
(up to 100%)
High-Mg calcite/
8–12 mol% MgCO3
High-Mg calcite/
9–13 mol% MgCO3
Microcrystalline
1–3 cm
Aghib et al. (1991)
Microcrystalline (b 5 μm)
n.d.
McKenzie and Bernoulli (1982)
Microcrystalline (b10 μm)
5–10 cm (?)
Milliman and Müller (1973)
Aragonite
High-Mg calcite/
9–12 mol% MgCO3
Aragonite
High-Mg calcite
Several occurrences of hardgrounds are also reported from the shallow warm-temperate waters of the Mediterranean Sea (Alexandersson,
1969; Tables 1 and 8; Fig. 2). While details on the origin of hardground
are not given, Alexandersson (1969) reported that hardground micritic
and bladed cements are of high-Mg calcite mineralogy as opposed to the
low-Mg calcite mineralogy of the host sediments (Table 8). In a more recent publication dealing with modern temperate carbonates deposited
on a South Australian shelf, James et al. (1999) also identified high-Mg
calcite and aragonite cements as the agents responsible for the lithification of hardgrounds (Table 8).
Despite the limited number of studies documenting the petrology of
hardgrounds, it seems that the formation of modern temperate or coolwater hardgrounds often responds to local factors (e.g., organic matter
degradation) inducing change in sea- or pore water carbonate saturation. Controls on hardground formation and petrography are diverse.
Aragonite cements precipitate despite the fact that seawater temperature is generally low enough to decrease seawater saturation regarding
CaCO3. Thus, in terms of their formation mechanisms, early marine aragonite cements in modern cool-water hardgrounds seem to differ from
such in tropical settings but more work is required to explore the mechanisms involved here.
5.1.3. Oligocene to Pliocene tropical hardgrounds
Whereas numerous papers document hardground formation in
the modern tropical realm, similar studies remain surprisingly rare
for the Neogene and the Oligocene (Beach, 1993; Wigley and
Compton, 2013). Hardground surfaces on the Pliocene portions of
Great Bahamas Bank (Beach, 1993) share important similarities with
Pleistocene-Holocene ones and particularly so in terms of their petrography, dimension, morphology and reconstructed hydrodynamic levels
(Table 10).
Wigley and Compton (2013) reported syn-sedimentary seafloor
lithification from a subtropical Miocene shelf off South Africa. The
hardgrounds were interpreted to have formed during periods of
maximum flooding and reduced sediment accumulation. In terms of
Malone et al. (2001)
Milliman (1966)
15 cm to
Schroeder et al. (2002)
several metres
n.d.
Noé et al. (2006)
Microcrystalline (0.2–10 μm) 0.1–8 cm
Allouc, 1990
Fibrous and syntaxial fibrous
Microcrystalline
0.5 to
several cm
Gevirtz and Friedman (1966)
Fibrous and drusy
Cryptocrystalline (0.1–1 μm)
n.d.
Milliman et al. (1969)
the hardground petrography, iron oxides, phosphogenesis and
glauconitization have been reported and related to upwelling currents
and the deposition of organic-rich mud (Wigley and Compton, 2013).
Evidence on hardground carbonate cements is lacking.
5.1.4. Oligocene and Miocene cool-water and warm-temperate
hardgrounds from heterozoan-dominated carbonate platforms
Published data on cool-water carbonate hardgrounds are derived
from numerous Cenozoic case examples. Most of these come from the
Oligocene and Miocene (33.9 to 5.3 Ma; Gradstein et al., 2004). Two regions with abundant hardground formation have been studied in some
detail: (i) the cool-water carbonate depositional environments of the
Southern Australian and New Zealand shelves (Nicolaides and
Wallace, 1997; Nelson and James, 2000; Table 10) and (ii) the warmtemperate carbonate ramps of the Mediterranean Sea (Pedley and
Bennett, 1985; Mutti and Bernoulli, 2003; Table 10). Both depositional
settings are dominated by a heterozoan carbonate factory (James,
1997).
During the Oligocene and Miocene periods, Southern Australian and
New Zealand were located further south at latitudes of 45 to 60°S. Despite this southern position, their local Oligocene and Miocene climates
were similar to the present-day ones, perhaps slightly cooler in the Oligocene and slightly warmer in the Miocene (Scotese, 2001). High-Mg
calcite forms the main mineralogy of hardground cements on South
Australian and New Zealand shelves despite cool seawater temperatures (14 to 18 °C; James and Bone, 1992; 10–20 °C; Nicolaides and
Wallace, 1997; Table 10). Another interesting feature includes
isopachous fringes of fibrous to bladed crystals that form the early cement phase in these hardgrounds. These fabrics are not intuitively expected as they are typical for tropical settings with high cement
precipitation such as the present-day Persian Gulf (Shinn, 1969) or
the Bahamas (Dravis, 1979; Whittle et al., 1993). The latter observation
is of significance, as cool-water carbonate factories are often characterized by destructive diagenesis of seafloor carbonate sediments, i.e. dissolution may locally outpace precipitation (James, 1997; Nelson and
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
203
Table 10
Formation environment and petrology of Cenozoic marine hardgrounds in neritic carbonate domains. n.d. = no data.
Locality
Bathymetry
Rock texture beneath
surface
Tropical and subtropical settings (Aragonite III period)
Bahamas carbonate banks (platform
N10 m
Skeletal and peloidal
interior)—Pliocene
packstone1
Warm- and cool-water temperate settings (Aragonite III period)
n.d. (outer
Coarse-grained2
Carbonate ramp (Maiella,
shelf)
Italy)–Mediterranean
Sea—Miocene
n.d. (outer
Fine grained2
Shelf of the Maltese
shelf)
Islands–Mediterranean
Sea—Miocene
n.d., ?above Fine-grained calcareous
Shelf of the Maltese
silt2
storm
Islands–Mediterranean
wave-base
Sea—Miocene
Skeletal calcarenites to
Cool-water carbonate shelf (Southern n.d.
calcirudites3
Australia and New Zealand)—Late
Eocene to Middle Miocene
Cool-water carbonate shelf (Southern N50 m
Calcarenites3
Australia)—Oligo-Miocene
Coarse- to fine-grained
Cool-water carbonate shelf (Southern n.d., below
skeletal grainstone1, 2
Australia—Oligo-Miocene
storm
wave-base
Mineralogy
(% rock volume)
Cement type (size)
Thickness of
lithified
interval
References
cited
Aragonite and/or
High-Mg calcite
Fibrous to bladed
0.3–8.4 m
Beach (1993)
High-Mg calcite
Fibrous (15–30 μm)
Microcrystalline (micrite)
Up to 25 cm
Mutti and Bernoulli
(2003)
?Aragonite
(dissolution)
?Microcrystalline (rare fringe
cements observed)
Up to 75 cm
Pedley and Bennett
(1985)
?High-Mg Calcite
Microcrystalline
b30 cm
Gruszczyński et al.
(2008)
High-Mg calcite
(8–12 mol% MgCO3)
Fibrous to bladed (5–20 μm)
Crypto- to microcrystalline
20 cm and up
to ?2 m
Nelson and James
(2000)
High-Mg calcite
(8–12 mol% MgCO3)
High-Mg calcite
Fibrous
Up to 10 cm
James and Bone (1992)
Radiaxial fibrous
Few cm to 1 m
Nicolaides and
Wallace (1997)
James, 2000; James et al., 2005; Knoerich and Mutti, 2006b; Cherns and
Wright, 2009).
Southern Australian and New Zealand shallow-marine hardground
intervals reach a few cms to 20 cm in thickness (Nicolaides and
Wallace, 1997; Nelson and James, 2000; Table 10). The thicknesses of
these lithification intervals are directly comparable to modern ones in
the Persian Gulf (Shinn, 1969). Nicolaides and Wallace (1997) and
Nelson and James (2000) reported lithified hardground intervals as
thick as 1 to 2 m from the early Miocene of an Australian shelf
(Table 10). Metre-thick hardground intervals might represent several
cycles of lithification and sediment deposition with subsequently younger hardground stages forming on top of older ones.
Nicolaides and Wallace (1997) described four hardground surfaces
from the mid-Cenozoic of the SE Australian shelf that formed at shallow
depths under elevated hydrodynamic levels (Fig. 24). Enhanced current
activity and related sediment entrainment reduced sediment accumulation and enhanced porewater circulation favouring extensive early marine seafloor cementation. Lithification at the sediment/water interface
is initially patchy in nature as suggested by the presence of carbonate
nodules with diameters of 5–20 cm (Fig. 24b). The combined effects of
elevated hydrodynamic levels and the reduced erosion potential due
to incipient lithification allows for further lateral growth of nodules
(Fig. 24c). Eventually, nodules amalgamated to form thick and continuous submarine hardgrounds (Fig. 24d). Amongst the four hardgrounds
described by Nicolaides and Wallace (1997), only one reached a mature
lithification stage.
The above-described hardground patterns in cool-water settings require an explanation. Periods of relative sea-level fall during the overall
cool climate of the Mid-Cenozoic are commonly related to an overall increase in mean sea-water temperature of water masses on the increasingly shallow shelf areas. Supposedly, these shallow water masses
respond more rapidly to seasonal warming and are hence characterized
by enhanced carbonate saturation states. Under falling sea level, swellwave action and tidal currents preclude sediment accumulation on the
seafloor. Hydrodynamic levels strongly increase the circulation of relatively warm waters through the pore space of near-seafloor sediment
column hence promoting cementation by high-Mg fibrous calcite. During relative sea-level lowstand, upwelling of cold, nutrient-rich deeper
water masses results in mixing with warmer shelf waters (Nelson
and James, 2000). Mixing of water masses releases CO2 favouring/
maintaining elevated CaCO3 saturation states. Given that interstitial waters are undersaturated with respect to aragonite, the contemporaneous
dissolution of skeletal aragonite is of significance and further enhances
the saturation state with respect to calcite in pore waters.
In summary, extensive Mid-Cenozoic seafloor cementation of the
South Australian and New Zealand shelves is the consequence of several
controls acting at different scales. At a global scale, a reorganization of
global oceanic circulation patterns characterizes the early Oligocene.
The main driver is the continuous segregation of the Australian plate
from Antarctica and the build-up of an Antarctic ice cap. Highamplitude sea-level fluctuations (Nelson and James, 2000) are driven
by plate tectonic reorganization and continental ice formation. The
early cementation of the carbonate seafloor during sea-level lowstand
was induced by local, spatio-temporal controls such as increasing temperatures of shallow shelf water masses.
Upper Oligocene to Miocene carbonate platforms from the Mediterranean Sea are generally referred to as heterozoan carbonate factories,
similar to those on the mid-Cenozoic or present-day South Australian
shelves. On average, surface water temperatures are somewhat higher
(14 to 23 °C for the Miocene of Malta; Betzler et al., 1997; Knoerich
and Mutti, 2003) compared to the above-reported southern hemisphere
case examples (10 to 20 °C; James and Bone, 1992; Nicolaides and
Wallace, 1997). Consequently, the mid-Tertiary Mediterranean ramps
are sometimes referred to as “warm-temperate” (Vecsei and Sanders,
1999; Mutti and Bernoulli, 2003). Mediterranean Miocene hardground
surfaces were reported from the Maltese Archipelago (Pedley and
Bennett, 1985; Carbone et al., 1987; Rehfeld and Janssen, 1995;
Gruszczyński et al., 2008) and from Italian ramp settings (Mutti and
Bernoulli, 2003).
Similar to cool-water case examples from the southern hemisphere,
isopachous fibrous calcites and/or microcrystalline cements typically
lithify warm-temperate Mediterranean seafloors. Similar to the coeval
Australian examples, high-Mg calcite dominates the petrography of
Mediterranean hardgrounds (Mutti and Bernoulli, 2003; Gruszczyński
et al., 2008; Table 10). The processes leading to submarine lithification
were probably similar to those cementing seafloors off Australia, although the geographic setting and bathymetry differ. On the Miocene
Maiella platform of Central Italy, hardground formation was initiated
during sea-level lowstand and probably also related to increased hydrodynamic levels (Mutti and Bernoulli, 2003). Upwelling of cold, nutrient-
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rich waters increased pore water circulation. Cementation of up to
30 cm-thick intervals beneath the sediment/water interface was facilitated by the relatively coarse-grained carbonate sediment of the outer
shelf setting. Seafloor lithification was further strengthened by the continuation of upwelling, increasing productivity and fluxes of organic
matter to the seafloor. Degradation of organic matter by microbes at,
and beneath, the seafloor promoted the precipitation of microcrystalline calcite (micrite) and enhanced hardground lithification (Fig. 20).
Furthermore, Mutti and Bernoulli (2003) tentatively related submarine
lithification to major changes in Mediterranean Sea circulation patterns
in the early Miocene. A modification in oceanic circulation mechanisms
may result from the collision between Arabian and Eurasian plates and
related far-field effects.
A palaeoceanographic implication for seafloor lithification mechanism (Mutti and Bernoulli, 2003) seems unlikely in the case of the
hardgrounds in the Miocene Globigerina Limestone Formation of
Malta (Gruszczyński et al., 2008). Instead, these authors interpreted
hardground development to continuously occur independently of
changes in relative sea level and plate tectonic reconfiguration. This
may document the significance of local factors including the stormdominated nature of these environments. Nevertheless, Maltese
hardgrounds do share similarities with the Maiella platform examples.
In both cases, bacterial consortia driving organic matter degradation
and micrite precipitation might have been instrumental.
5.1.5. Deep neritic to bathyal hardgrounds in the open oceanic realm
The mechanisms leading to the hardground formation at greater
water depths are complex and may significantly differ from those involved in shallow neritic seafloor lithification. Consequently, the nature
of deep neritic to bathyal hardgrounds is rather variable. Controls include different bathymetric domains each with different physicochemical properties of related water masses and different geographical
settings.
Despite occasional reports on bathyal hardground surfaces from the
Plio-Pleistocene and the Holocene of the Pacific (Milliman, 1966;
Malfait and Van Andel, 1980) and Indian Realm (Sharma et al., 1997),
case examples from the Atlantic Ocean dominate the literature by far
(Table 9). Examples include hardgrounds in neritic peri-platform settings of the Bahamian platform (Schlager and James, 1978; Malone
et al., 2001) and such from guyots and seamounts (Milliman, 1966;
Wisshak et al., 2010). Moreover, carbonate crusts have been reported
from deep-water carbonate mounds in the north-eastern Atlantic
(Noé et al., 2006; Wienberg et al., 2008; van der Land et al., 2010). Topographic highs, such as the Mid-Atlantic Ridge (Thompson et al., 1968;
Schroeder et al., 2002) and the Pacific Carnegie Ridge (Malfait and Van
Andel, 1980) represent favourable sites for early diagenetic seafloor
cementation.
Malone et al. (2001) documented Pleistocene hardgrounds from a
Bahamian peri-platform slope at reconstructed water depths of 300 to
400 m (Table 9). In the view of these authors, seafloor lithification is
principally related to glacial intervals. Reduced concentrations of total
dissolved inorganic carbon in bottom water masses, caused by a change
of vertical oceanic circulation, might have promoted seafloor cementation during glacial lowstand periods. Slow sedimentation rates allowed
for the complete oxidation of organic matter and thus the preservation
of a relatively elevated CaCO3 saturation state, significantly contributing
to early ocean seafloor lithification.
In another Bahamian carbonate peri-platform case study (Schlager
and James, 1978; Table 9), lithification of the seafloor was—and
is—favoured by sediment bypass and non-deposition due to the steepness of the seafloor forming cliffs. At water depths in excess of 700 m,
early diagenetic marine cementation of the Pleistocene and Holocene
seafloor takes place due to rapid aragonite solution and Mg-loss and
low-Mg calcite cements form. Schlager and James (1978) suggested
that aragonite dissolution locally enhanced pore water supersaturation
and thus favoured cementation by low-Mg calcite. These processes do
not apply to hardgrounds forming at comparable depths of about
700 m in the north-eastern Atlantic (Noé et al., 2006; Table 9). There,
carbonate oozes undergoing lithification are mainly composed of lowMg calcite foraminifera and nanoplankton skeletal material. In these
settings, chalk seafloor cementation mainly results from high seawater
alkalinity, but above all, from strong bottom currents resulting in vigorous porewater circulation (Noé et al., 2006).
While in most cases early marine lithification at or just beneath the
carbonate seafloor is promoted by pore water pumping, cementation
can also arise from the upward percolation of serpentinization fluids
through the sediment column. This phenomenon is reported from the
Mid-Atlantic Ridge, where near-seafloor calcareous oozes are lithified
at water depths of 700 to 2600 m (Schroeder et al., 2002; Fig. 25). The
bacterial oxidation of the methane (plumes), arising from the diffuse
venting of serpentinization fluids along mid-ocean ridges, releases bicarbonate that favours carbonate precipitation. Vertical faults in some
areas increase hydrothermal sub-surface circulation of pore-water and
carbonate seafloor lithification (Fig. 25). The possible significance of
serpentinization fluids was also brought forward for lithified seafloors
in the Plio-Pleistocene of Pacific oceanic ridges (Thompson et al.,
1968). On the level of a working hypothesis, it is suggested that the relation between serpentinization fluids and seafloor lithification is more
common than previously assumed in bathyal settings.
5.1.6. Bathyal hardgrounds in the semi-enclosed Red Sea and
Mediterranean basins
A considerable amount of literature deals with recent and subrecent marine hardgrounds in semi-enclosed seas, such as the tropical
Red Sea and the subtropical-temperate Mediterranean Sea (Gevirtz
and Friedman, 1966; Milliman et al., 1969; Allouc, 1990; Aghib et al.,
1991). Both of these basins share a significant level of continentality
and are largely isolated from the major global oceanic circulation patterns. The Strait of Gibraltar, connecting the Mediterranean with the Atlantic Ocean, is between 300 and 900 m deep (Sannino et al., 2002) and
the Gulf of Aden, forming the gate between the Red Sea and the Indian
Ocean, is as deep as 125 m (Milliman et al., 1969).
In the context of Plio-Pleistocene glaciations, for example during
the Last Glacial Maximum, sea level was lower by around 120 to
130 m relative to its present day position (Lambeck et al., 2002). During periods of sea-level lowstand, the Red Sea became isolated from
the Indian Ocean and the water exchange between the Atlantic and
Mediterranean water masses was severely restricted. Additionally,
sea-level fall also virtually cut off the water circulation between the
Western and the Eastern Mediterranean basins as the Strait of Sicilia
experienced significant shallowing (Milliman et al., 1969). Thereby,
the negative water balance of these basins likely resulted in increasing seawater salinity during glacial periods (Locke and Thunell,
1988; Paul et al., 2001; Mikolajewicz, 2011) with values even beyond
the more-than-average present-day salinity of the Red Sea (b 41;
Locke and Thunell, 1988) and in the Mediterranean Sea (38–39;
Marret et al., 2009). Specifically, during the Last Glacial Maximum,
surface water salinities in the Red Sea probably reached peak values
up to 47 (Locke and Thunell, 1988).
In the Mediterranean and the Red Sea, microcrystalline cements
(Table 9) form the main fabric cementing carbonate seafloors similar
to hardgrounds from the Atlantic Ocean. The basinal, fine-grained
coccolith ooze facies (Table 9) and the limited corresponding
pore space of near seafloor sediments may explain the predominance
of these cements. Whereas the microcrystalline fabrics of these
hardgrounds share important similarities with those in deep-oceanic
hardgrounds, aragonite and high-Mg calcite are the dominant mineralogies in Mediterranean and Red Sea basins as opposed to the more calcitic hardground mineralogy of the Atlantic (Table 9; Fig. 26). Aragonite
and high-Mg calcite are rather uncommon at water depths of several
thousand metres and Mg elemental abundances in hardground calcite
cements from Atlantic hardgrounds decrease systematically with
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
increasing water depth (Schlager and James, 1978). This pattern is consistent with the decrease in carbonate saturation state with depth and
temperature. In contrast, the calcitic cements from the Mediterranean
and Red Sea basins still yield elevated Mg contents of ~ 12% mol
MgCO3 at water depths of about 3000 m (Fig. 26). Elevated Mg concentrations of basinal waters are assumed to reflect their relatively elevated
temperatures (Schlager and James, 1978) and thus higher saturation
states with respect to calcite.
Pleistocene-Holocene hardground formation in the basinal Red Sea
at depths of 800 to 2300 m (Gevirtz and Friedman, 1966; Milliman
et al., 1969; Table 9) is probably related to glacial-interglacial intervals. Specifically, hardgrounds lithified by aragonite cements developed between 20 and 11 ka BP (Fig. 27). At that time sea level
was considerably lowered relative to its present-day position and
bottom water salinities were elevated. Former aragonite-cemented
hardgrounds, developed during Pleistocene glacial periods, are characterized by a present-day high-Mg calcite mineralogy. Milliman
et al. (1969) provided an interpretation of this pattern and suggested
that the high-Mg calcite cements reflect an intermediate alteration
product of a former aragonitic mineralogy. The same authors reported
on primary high-Mg calcite hardground fabrics and specifically so
from hardgrounds formed after the termination of the last glaciation
(~10.7 ka BP; Fig. 27).
The correlation between secular fluctuations in Red Sea salinity concentrations and the presence and absence of aragonite- or high-Mg
calcite-cemented hardgrounds is not obvious (Milliman et al., 1969).
Particularly because variations in bottom water salinity are expected
to have only a moderate effect on the mineralogy and precipitation
rates of marine cements (Zhong and Mucci, 1989). Conversely, higher
Red Sea basinal water temperatures (Locke and Thunell, 1988) were
probably the dominant control inducing aragonite and high-Mg calcite
precipitation at the seafloor.
Hardground cements in the Pleistocene–Holocene deep western
(Gamberi et al., 2006) and eastern Mediterranean Sea (Milliman and
Müller, 1973; Allouc, 1990; Aghib et al., 1991) precipitated from basinal
water masses that are intermediate in nature between the warm and saline deep waters of the Red Sea and the cold, normal saline bottom waters of the Atlantic ocean (Milliman and Müller, 1973; Table 11).
Mediterranean hardgrounds lack extensive cementation by aragonite,
albeit sporadically present, while several thick, successive high-Mg calcite cemented intervals are present at water depths of 4000 m
(Milliman and Müller, 1973). Compared to the widespread seafloor lithification in the Red Sea, however, hardgrounds in the eastern Mediterranean Sea are laterally less continuous. Depending on the studied
settings, Milliman and Müller (1973) attributed the formation of Mediterranean hardgrounds to either glacial or interglacial periods and suggested an important contribution of the thermohaline circulation.
According to these authors, the decreasing abundance of high-Mg calcite cements in hardgrounds 7 to 9 ka ago reflects a transition stage between the Last Glacial Maximum (~20 ka BP; Clark et al., 2009) and the
present-day interglacial period (Fig. 27). Particularly, the melting of
Table 11
Temperature and salinity ranges in subtropical to temperate oceans, Mediterranean Sea
and Red Sea. After Milliman and Müller (1973).
Seawater
temperature (°C)
Salinity range
(part per thousands)
Sea surface
Subtropical to temperate ocean water
Mediterranean Sea
Red Sea
15–25
15–25
24–30
34–36
37–38.8
38–40
Below 2000 m
Subtropical to temperate ocean water
Mediterranean Sea
Red Sea
1.5–5
13–14
22
34–36
38.4–38.7
40.5–41
205
European continental ice masses resulted in increased freshwater runoff
combined with rising sea levels. Melt water runoff lowered the Mgcalcite saturation state of western Mediterranean deep water masses
and increased water column stratification whilst circulation patterns
decreased reducing the overall potential for seafloor lithification
(Milliman and Müller, 1973).
The question if hardgrounds in the deep eastern Mediterranean Sea
formed from warmer (Milliman and Müller, 1973; Müller and Staesche,
1973; Müller and Fabricius, 1974) or colder (Hellenic Trench region;
Bernoulli and McKenzie, 1981; McKenzie and Bernoulli, 1982) water
masses is debated. General agreement, however, exists regarding the
enhancement of eastern Mediterranean bottom water circulation by
seafloor topography, providing CaCO3 ions for seafloor cementation
whilst reducing sediment accumulation rates (Allouc, 1990; Aghib
et al., 1991).
5.2. Calcite sea II
5.2.1. Paleocene–Eocene hardgrounds
The Paleocene–Eocene age is dominated by greenhouse conditions
(Scotese, 2001; Jenkyns, 2003), a calcite sea mode (Stanley and
Hardie, 1998), an overall high sea level (Miller et al., 2005; Sluijs et al.,
2008) and flooded continental shelves (Blakey, 2011) forming wide
epeiric seas. Despite the fact that these controls favour widespread seafloor lithification, the number of case studies on Paleocene–Eocene
hardgrounds is limited and their nature and petrography are often not
well documented.
Detailed case studies include the Middle Eocene cool-water
hardgrounds from South Australian shelves (James and Bone, 2000),
the Lower Eocene phosphatized hardgrounds from a subtropical,
mixed carbonate-siliciclastic carbonate platform in the SE United
States (Coffey and Read, 2004) and hardgrounds formed on mixed
carbonate-siliciclastic sandstones in shallow-marine subtropical to
tropical shelf environment exposed in the Pyrenees (Molenaar et al.,
1988; Molenaar, 1990; Molenaar and Martinius, 1990; Martinius and
Molenaar, 1991). Reduced clastic sedimentation combined with the
abundance of carbonate grains in these mixed sediments in the Pyrenean case examples triggered carbonate-dominated seafloor lithification.
Carbonate clasts provided substrata for the nucleation of cements in
hardgrounds.
To the knowledge of the authors, no published case study documents tropical Lower Cenozoic hardgrounds. At present, the impression
is gained that the scarcity of Paleocene–Eocene hardgrounds is rather
due of a lack of published work dealing with these surfaces than
reflecting a genuine feature of the geological record.
5.2.2. Upper Cretaceous chalk sea hardgrounds of northern Europe
Owing to the globally high sea level during the Late Cretaceous,
much of what is now northern Europe was covered by an epicontinental
sea (Voigt and Hilbrecht, 1997; Miller et al., 2005). High sea levels and
arid climate limited continental erosion and terrigenous fluxes in
these shallow seas (Hancock, 1975; Anderskouv and Surlyk, 2011 and
references therein). Chalky low-Mg calcitic limestones, predominantly
formed by coccolith ooze, represent the main marine deposits
(Kennedy and Garrison, 1975; Hancock, 1975) at water depths of between 50 and 600 m (Kennedy and Garrison, 1975; Hancock, 1975;
Molenaar and Zijlstra, 1997).
Kennedy and Garrison (1975) defined chalk hardgrounds as “… the
terminal products of a sequence of depositional and early diagenetic
events associated with interruptions in deposition …”. The lithification
of Cretaceous chalky seafloors is often due to low-Mg calcite cement
precipitation in accordance with the calcite chemistry of Cretaceous
seas and the low-Mg calcite host sediment (Table 12; Jarvis, 1980;
Molenaar and Zijlstra, 1997; Richard et al., 2005; Stanley and Hardie,
1998; Palmer and Wilson, 2004). High-Mg calcite and even aragonite
cements, however, have been described from some of these
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N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
Table 12
Environment and petrology of Mesozoic marine hardgrounds in neritic and/or epeiric carbonate domains (n.d. = no data).
Locality—Age
Water depth
Rock texture beneath
surface
Mineralogy
(% rock volume)
Cement type (size)
Thickness of
lithified
interval
Study
Coarse-grained
Low-Mg calcite
?Low- or High-Mg
calcite
N45 cm (up to
90 cm)
Up to 50 cm
Jarvis (1980)
Fine-grained chalk
Overgrowth of carbonate
grains
n.d.
Fine- to
medium-grained
chalk
Low-Mg calcite
Cm to
dm-thick
Molenaar and Zijlstra
(1997)
n.d., shallow basin
Chalk
Low-Mg calcite
~ 35 cm
Richard et al. (2005)
Between 50 and
200–300 m
Chalk
Supposed high-Mg
calcite (low-Mg calcite
and/or aragonite not
excluded)
Supposed high-Mg
calcite (perhaps also
aragonite)
Syntaxial overgrowths of
echinoderms, scalenohedral
crystals and syntaxial bladed
crystals
Syntaxial overgrowth
(microcrystalline size)
?Microcrystalline
20–100 cm
(interval given
for nodular
horizons)
Dm-thick
(N20 cm)
Kennedy and Garrison
(1975)
Cretaceous Chalk hardgrounds (Calcite II period)
Anglo-Paris Basin (N France)— n.d.
Late Cretaceous
b10 to ~110 m
Continental shelf—Red
Chalk and Lower Chalk
(E England)—MidCretaceous
n.d., ?outer shelf
Continental shelf (The
Netherlands and
Belgium)—Late Cretaceous
Mons Basin (Belgium)—Late
Cretaceous
Anglo-Paris Basin
(S England)—Late
Cretaceous
Anglo-Paris Basin (SW
n.d.
England)—Mid-Cretaceous
Chalk
Tethyan marine Cretaceous hardgrounds (Calcite II period)
? b 20 m
Continental margin of the
Peloidal grainstone1
northern Neo-Tethys
(Turkey) – Mid-Cretaceous
Continental shelf (Oman) –
Early Cretaceous
(Aptian-Albian)
Continental shelf
(Oman)—Early Cretaceous
(Barremian–Aptian)
Continental shelf,
intracratonic basin
(Oman)—Early Cretaceous
(Aptian–Albian)
Carbonate platform (France
and Switzerland)—Early
Cretaceous (Berriasian,
Valanginian)
Carbonate ramp (United
Arab Emirates)—Early
Cretaceous
Continental shelf
(Greece)—Late Cretaceous
(~ K/T boundary)
Supposed above
fwwb
Tidal flat(close to
sea-level) to outer
platform (below
swb)
Above fwwb
(b10–20 m)
Below and above
fwwb (lagoon to
shoals)
Peloidal to bioclastic
grainstone to
rudstone1
Mudstones,
wackestones to
fine-grained pack- to
grainstones1
Coarse-grained
High-Mg calcite
?Mudstones to
grainstones1
Fine-grained
?Below fwwb
(mentioned
low-energy setting)
n.d., shallow
Chalk
pelagic swell
Jurassic hardgrounds (Calcite II period)
Shoals (also lagoon and open Mainly above
shelf) (England, France and fair-weather wave
base
Poland)—Middle-Late
Jurassic
Epeiric sea (France)—Middle Above
Jurassic
fair-weather
wave-base, also
intertidal
Carbonate ramp
(Morocco)—Middle
Jurassic
Above
fair-weather
wave-base
Sheltered lagoon (Utah,
USA)—Middle Jurassic
n.d.
Mixed siliciclastic-carbonate
(?platform) setting
(Wyoming, USA)—Late
Jurassic
n.d.
n.d. (supposed calcite)
Syntaxial overgrowth
Equant (sparry)
Rare isopachous fibrous
calcite
Isopachous bladed
Garrison et al. (1987)
n.d.
Eren and Tasli (2002)
?cm-thick
Rameil et al. (2012)
n.d.
Immenhauser et al.
(2004); Sattler et al.
(2005)
High-Mg calcite
n.d.
Equant (blocky)
?Microcrystalline (inferred
from the fine-grained texture of
the underlying limestones)
n.d.
Isopachous bladed
n.d.
Immenhauser et al.
(1999, 2000a, 2000b)
Aragonite
Isopachous fibrous
n.d.
Hillgärtner (1998)
Calcite
Microcrystalline
1–10 cm
Dickson et al. (2008)
Apatite (primary or
secondary or both?)
Crypto- and microcrystalline
n.d.
Pomoni-Papaioannou
and Solakius (1991);
Pomoni-Papaioannou
(1994)
Acicular
?Microcrystalline (internal
micrite)
10–30 cm
Fürsich (1979 and
references therein)
Fibrous to bladed
5–10 cm to
?more
Purser (1969);
Aissaoui and Purser
(1983)
1–30 cm
Christ et al. (2012a);
this study
n.d.
Wilson and Palmer
(1992)
n.d.
Wilkinson et al. (1985)
Fine to coarse-grained ?Aragonite and/or
high-Mg calcite
(inferred from cement
type)
Relatively
Aragonite and/or
coarse-grained
Mg-calcite (inferred
from the cement type)
High-Mg (or low-Mg)
calcite (inferred from
the cement type)
?high-Mg calcite
1
Mud- to grainstone
High-Mg calcite
Skeletal packstone
Silty micrite/ooid
grainstone1
Grainstones1
Syntaxial overgrowth of
echinoderms, scalenohedral,
fibrous, microcrystalline,
equant (sparry)
Jeans (1980)
High-Mg calcite
(inferred from cement
types)
Aragonite and/or
High-Mg and/or
low-Mg calcite
Syntaxial overgrowth on
echinoderms
Microcrystalline
Microcrystalline (main
cement)
Syntaxial overgrowth
Fibrous to bladed
Microcrystalline (peloidal
micrite) and scalenohedral
Acicular-fibrous
Syntaxial overgrowth
Equant
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
207
Table 12 (continued)
Locality—Age
Water depth
Rock texture beneath
surface
Mineralogy
(% rock volume)
Cement type (size)
Thickness of
lithified
interval
Study
Carbonate ramp
(France)—Middle Jurassic
Inner (b30 m) to
middle ramp
(30–50 m)
Mainly grainstone,
wacke- to packstone1
High-Mg calcite
Fibrous and syntaxial
overgrowth
~20 cm
Brigaud et al. (2009a,
2009b)
Low-Mg calcite
Microstalactitic, meniscus,
inclusion-free bladed,
scalenohedral and syntaxial
Marshall and Ashton
Acicular, bladed, radial fibrous Up to N20 cm
(higher energy (1980)
setting)
Carbonate barrier-island
complex (England)—
Middle Jurassic
? b 30 m (tidal flat, Mudstones to ooidal
on-barrier, lagoon) grainstones1
Pelagic swells (S
Spain)—Middle Jurassic
50–100 m
Ammonitico Rosso
Pelagic swells (S spain)—
Late Jurassic
N Tethys Carbonate shelf
(S Poland)—Late Jurassic
b200 m
Ammonitico Rosso
10–15 m
Grainstones1 (shoals)
S Tethys continental shelf
(W India)—Late Jurassic
n.d., below
fair-weather
wave-base
High-Mg calcite mainly
(maybe low- or
intermediate-Mg
calcite?)
Low-Mg calcite
(neomorphised)
High-Mg and/or
low-Mg calcite
Aragonite
Equant (sparry), (also
microcrystalline?)
n.d.
Microcrystalline
n.d. (incipient
hardground)
5–10 cm
Acicular to bladed
High- or low-Mg calcite Microcrystalline
Wackstone-packstone n.d.
?Microcrystalline (incipient
1
to fine grainstone
phase)
hardgrounds (Kennedy and Garrison, 1975; Garrison et al., 1987;
Table 12). Given the generally fine-grained texture of chalks, microcrystalline cements (Garrison et al., 1987) and microcrystalline syntaxial
overgrowths form the dominant cement fabrics (Jarvis, 1980; Garrison
et al., 1987; Molenaar and Zijlstra, 1997; Richard et al., 2005;
Table 12). Other early marine cements include scalenohedral calcites
(Garrison et al., 1987; Molenaar and Zijlstra, 1997; syntaxial bladed calcites (Molenaar and Zijlstra, 1997) and fibrous and equant calcites
(Garrison et al., 1987).
According to Kennedy and Garrison (1975), omission surfaces, nodular limestones and intra-formational conglomerates refer to, or describe, successive, intermediate steps towards complete seafloor
lithification. Omission surfaces representing the initial, least-lithified
type are characterised by burrows—as opposed to lithophage
borings—and correspond to a stage when near-seafloor sediments
dewatered and gained initial firmness (Fig. 3). In case of prolonged exposure of the seafloor, cementation was initiated in the form of small
lithified patches (nodules) at very shallow burial depths below the sediment/water interface (Fig. 3). This process leads the formation of widespread “nodular limestones” in the sense of Kennedy and Garrison
(1975). Under prolonged hiatal duration, prograding early diagenetic
lithification resulted in the cementation of a cm to dm-thick interval,
eventually forming a chalk hardground (Fig. 3).
Reports on the thickness of the lithified hardground intervals remain
inconsistent. Molenaar and Zijlstra (1997), for instance, described a few
cm-thick Campanian–Maastrichtian hardground intervals whilst
Kennedy and Garrison (1975) and Garrison et al. (1987) identified submarine lithified layers as thick as 20 cm. Exceptionally extensive chalk
hardgrounds reaching ca. one metre in thickness have been reported
too (Kennedy and Garrison, 1975; Jarvis, 1980; Table 12). For the authors, it seems likely that these features represent successions of amalgamated hardgrounds.
Garrison et al. (1987) described intermediate stages of seafloor lithification, including nodular limestones, intra-formational conglomerates, and eventually hardground formation from other mid-Cretaceous
chalky limestones of NW Europe. Based on these papers, it seems likely
that the alternation of progressive seafloor lithification and episodes of
coccolith ooze deposition is a characteristic feature of NW European
Cretaceous Chalk Seas. The latter has been implicitly suggested by
Kennedy and Garrison (1975), who observed a rhythmical occurrence
of nodular limestones and unlithified chalks. Essentially, chalk
hardground formation is considered a rapid process (10 to 102 yrs;
Cm-thick
Reolid et al. (2010);
Reolid and Nieto
(2010)
Coimbra et al. (2009)
Gruszczyński (1986)
Fürsich et al. (1992)
Table 4). Surprisingly, most authors seem to agree that vigorous bottom
currents did not play a significant role (Kennedy and Garrison, 1975).
According to the classical work of Kennedy and Garrison (1975), cementation of chalk hardgrounds is mainly an inorganic process. Conversely, Jeans (1980) suggested an alternative model proposing the
significance of bacterially-induced cement nucleation and growth involving ammonification and sulphate-reduction. These processes enhanced porewater alkalinity through the release of Ca2 + and NH3
(Jeans, 1980). Bacterial processes supposedly induced both the anaerobic precipitation of early Fe-rich calcite, responsible for seafloor lithification, and the precipitation of glauconite within the 1 to 2 cm-thick
sediment interval directly beneath the seafloor that was dominated by
aerobic conditions (Fig. 28). Acknowledging the significance of
microbially-induced reactions in the near-seafloor porewater domain,
environmental processes including seawater saturation state, and temperature as well as the omission time of specific hardground surfaces
were clearly of significance too.
Several authors proposed that Cretaceous chalk hardgrounds
formed regionally significant surfaces expanding over N 1500 km2
(Kennedy and Juignet, 1974). Others (Kennedy and Garrison, 1975),
suggested spatially more limited chalk hardgrounds with lateral dimensions of some 100s of metres only. Apparently, both case studies are
supported by solid field evidence and it is concluded that chalk
hardground seafloors of variable spatial extent exist.
5.2.3. Lower Cretaceous neritic hardgrounds of the Tethyan Realm
The extensive tropical neritic seas of the Cretaceous are
characterised by overall warm sea surface temperatures (Pearson
et al., 2001) and high CaCO3 saturation states (Stanley and Hardie,
1998; Fig. 29). Both of these factors favour extensive seafloor cementation (Taylor and Wilson, 2003; Palmer and Wilson, 2004). Elevated hydrodynamic levels are related to the often shallow, platform top realm
where these hardgrounds form (Hillgärtner, 1998; Immenhauser
et al., 1999, 2000b; Eren and Tasli, 2002; Sattler et al., 2005; Rameil
et al., 2012; Table 12). Besides shallow shoal settings, hardgrounds
from lagoonal and more open platform domains have been reported
too (Hillgärtner, 1998; Sattler et al., 2005; Dickson et al., 2008).
Only a limited number of case studies document the mineralogy of
the early hardground cement phases. Where present, mainly calcitic
(Dickson et al., 2008; Rameil et al., 2012) and aragonitic (Hillgärtner,
1998) cements were reported (Table 12). Accordingly, Tethyan
hardgrounds differ from Chalk Sea (predominantly) low-Mg calcite
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hardgrounds (Table 12). Hauterivian hardgrounds offshore Abu Dhabi
described by Dickson et al. (2008) reach 1 to 10 cm in thickness but
have regional correlation significance and form mainly in open neritic
micritic carbonates during regressive stages. Many of these surfaces
are mineralized and stained either black/golden due to pyrite or red
due to hematite. Hardground micrites have little intervening porosity
and pores were presumably filled with (Mg-?)calcite cements. The distinction between the original sedimentary particles and the microcrystalline cement is, as so often, difficult (Dickson et al., 2008).
Several studies reported on the difficulty to pinpoint early marine
hardground cements due to several superimposed marine and meteoric
diagenetic stages (Rameil et al., 2012) resulting in what was referred to
as “polygenic discontinuity surfaces” (Immenhauser et al., 2000a) or
“composite hardgrounds or surfaces” (Immenhauser et al., 2000b;
Sattler et al., 2005). In case examples from the Albian epeiric carbonate
platform deposits in Oman, Immenhauser et al. (2000b) identified at
least two marine hardground stages—separated by a subaerial exposure
stage—within a single discontinuity. The first hardground stage was the
consequence of a relative sea-level fall and the lowered effective wave
base resulted in sediment entrainment and erosion. The latter induced
porewater circulation beneath the sediment/water interface and enhanced early marine cementation. Under ongoing sea-level fall,
hardground surfaces emerged, resulting in the superimposition of meteoric diagenetic features on marine ones. The subsequent sea-level
rise and related wave action removed much, but not all, of the meteoric/pedogenic features and a second hardground stage including seafloor
winnowing, boring, and encrusting took place at the same surface.
These surfaces, when traced towards more distal settings, change in nature and display evidence for hardground stages only and then finally
pinch out basin-wards (Immenhauser et al., 2000b).
5.2.4. Jurassic neritic hardgrounds
The break-up of supercontinent Pangaea, initiated during the Late
Triassic, continued throughout much of the Jurassic (Bortolotti and
Principi, 2005). As Pangaea rifting proceeded, global sea level rose
(Miller et al., 2005) and vast epeiric seas formed, predominantly in the
western Tethyan domain. The extent of these shallow epeiric domains
further increased with the opening of the Central Atlantic and resulted
in the formation of a shallow seaway between the Tethys and the Pacific
Ocean by Early-Middle Jurassic times (Bertrand and Villeneuve, 1989;
Aberhan, 2001). Additionally, global oceans changed from aragonite to
calcite sea mode around the Early-Middle Jurassic (Sandberg, 1983;
Stanley and Hardie, 1998; Fig. 29). Mean global Jurassic atmospheric
temperatures were warm, but perhaps not as warm as those prevailing
during Triassic and Cretaceous hothouse/greenhouse climates (Royer
et al., 2001).
Case studies from Middle-Upper Jurassic hardgrounds come mainly
from tropical to subtropical depositional environments (Table 12).
Hardground surfaces are often, but not exclusively, associated with
the proximal portions of the large carbonate ramp systems typical of
this time, most of them related to ooid-dominated facies (Purser,
1969; Marshall and Ashton, 1980; Brigaud et al., 2009a; Christ et al.,
2012a; Table 12). Consequently, early marine lithification of Jurassic
seafloors is often associated with shoal environments and elevated hydrodynamics above the fair-weather wave base (Purser, 1969; Fürsich,
1979; Marshall and Ashton, 1980; Aissaoui and Purser, 1983;
Gruszczyński, 1986; Brigaud et al., 2009a; Christ et al., 2012a).
Hardgrounds display a broad spectrum of early diagenetic cement fabrics. Nearly all types of marine fabrics (Table 11) are found, albeit not
all of these in a single hardground interval. Carbonate mineralogies
range from aragonite to calcite, with a noticeable prevalence of aragonite and high-Mg calcite (Table 12).
The thicknesses of lithified intervals range between 0 and 30 cm
similar to many modern neritic hardgrounds (e.g., Shinn, 1969). Given
the shallow bathymetric domain (b 20 m), the comparable thickness
of the lithified interval and the similarities in cement fabrics and
mineralogies of these surfaces, the fundamental mechanisms that
caused Jurassic neritic seafloor lithification were probably similar to Holocene to Recent ones (Shinn, 1969; Table 12). Along these lines, Purser
(1969) noted that the middle Jurassic carbonate hardgrounds forming
in the Paris Basin share similarities with present-day ones formed in
the shallow epicontinental Persian Gulf. Paris Basin Jurassic
hardgrounds likely developed on ooid shoals (Purser, 1969), a feature
shared, albeit not exclusively, with modern Persian Gulf lithified surfaces (Khalaf et al., 1987).
A major difference between Middle Jurassic hardgrounds and recent
ones lies in their stratigraphic significance. Persian Gulf hardgrounds
developed in the last 10 kyr (Shinn, 1969; Khalaf et al., 1987), conversely, those from the Paris Basin represent considerably longer hiatal intervals. Hiatal surfaces often correspond to stratigraphic limits between
successive stages of the Middle Jurassic such as the Alenian–Bathonian
to Callovian–Oxfordian boundaries (Purser, 1969). Hence, while Holocene hardgrounds in the Persian Gulf are best explained by local environmental parameters, the ones from the Jurassic of the Paris Basin
clearly have large-scale and long-term environmental drivers.
Nevertheless, middle to Upper Bajocian hardgrounds (Pierre, 2006)
described from tropical carbonate ramp settings in Morocco, represent
comparably short omission phases thus share potentially even more
similarities with Holocene ones. Nearly 80 surfaces ranging from incipient condensed surfaces to fully lithified hardgrounds were documented
along proximal-to-distal transects (Christ et al., 2012a). Most of these
features are interpreted as maximum-regressive features representing
laterally continuous regional or semi-regional hardgrounds pinching
out basin-wards. Pulses of sea-level fall resulted in a lowered wave
base in the middle to mid-inner ramp domain and wave-action caused
seafloor sediment erosion and lithification. Probably due to high regional subsidence rates, low-amplitude sea-level fall was insufficient to subaerially expose the lithified carbonates seafloor (Christ et al., 2012a).
Locally, hardgrounds of limited spatial significance are localized upon
and within ooidal shoal bodies, usually indicating former shoal surfaces
that were later covered. The mineralogy of the micro-crystalline
hardground cements within these fine-grained carbonates is difficult
to characterize (see also Dickson et al., 2008), as detrital micrites and
micritic or microcrystalline cements inducing seafloor lithification are
difficult to differentiate (Fig. 10).
5.2.5. Jurassic and Cretaceous drowning-related hardgrounds
Drowning-related hardgrounds are rather common features of ancient carbonate platforms. These surfaces represent the inability of
carbonate-secreting marine reefal organisms, and thus carbonate platforms, to keep up with rising sea level (Godet, 2013). As a consequence,
the platform top subsides below the upper photic zones resulting in a
demise of light-dependent sessile organisms. Reasons for the demise
of the carbonate factories are commonly environmental stress leading
to reduced carbonate sedimentation rates or, less often, exceptionally
fast sea-level rise (Schlager, 1993, 1998, 1999). In many, but not all
cases, drowning is initiated by relative sea-level fall and subaerial exposure of platform tops (Sattler et al., 2009; Godet, 2013) resulting in the
transient shutdown of the carbonate factory followed by renewed sealevel rise. Drowning unconformities have been assigned to type 3 sequence boundaries sensu Schlager (2005). While documented from
many time intervals, drowning-related hardgrounds seem to be specifically common during the Jurassic and Cretaceous periods.
Papers dealing with drowning-related hardgrounds include case examples from the Oligo-Miocene (Mutti and Bernoulli, 2003), but the
probably best documented ones are from the Jurassic and Cretaceous
periods (Everts et al., 1995; Ilyin, 1997; Fourcade et al., 1999; Eren
and Tasli, 2002; Olszewska-Nejbert, 2004; Smuc and Gorican, 2005;
Yilmaz et al., 2012; Leonide et al., 2012; Godet et al., 2013; Godet,
2013). Reasons for this abundance of major, drowning-related
hardgrounds during the Mesozoic might include punctuated periods
of atmosphere-hydrosphere reorganization including Oceanic Anoxic
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
Events often related to environmental crises and changes in coastal marine ecosystems (Jenkyns, 2010; Rameil et al., 2010; Godet, 2013).
Often, drowning-related hardgrounds represent significant hiatal intervals (several 105 to 107 years; Schlager, 2005; Godet, 2013; Table 4).
Drowning surfaces are commonly overlain by fine-grained pelagic
sediments, such as silty limestones with radiolaria, dark, argillaceous
shales, or mudstones (Sattler et al., 2009; Yilmaz et al., 2012; Godet,
2013). Very often, drowning hardgrounds are related to condensed intervals with sedimentary rocks being mineralized by phosphatic and/
or glauconitic phases (Ilyin, 1997; Olszewska-Nejbert, 2004; Yilmaz
et al., 2012; Godet et al., 2013). Moreover, crusts of ferromanganese
mineralization have also been reported (Smuc and Gorican, 2005;
Yilmaz et al., 2012). Frequent carbonate hardground fabrics include
isopachous fibrous cements (Godet et al., 2013). Omission-related
early diagenetic biogenic silicification in the form of opal-A and
cristobalite/tridymite as well as quartz cements were reported from
chalk-chert-phosphorite hardgrounds of a Coniacian-Campanian ramp
from Jordan (Powell and Moh'd, 2012). Along similar lines, Leonide
et al. (2012) reported early marine, ferroan calcite from an Early Jurassic, drowning-related hardground surfaces of SE France.
Phosphate minerals and glauconite are indicative for sediment starvation and condensation. A well-documented case example is reported
from the Early Cretaceous of the Helvetic Alps (Godet et al., 2013). These
authors describe platform drowning-related phosphogenesis spanning
2.8 Myr. As such, drowning hardgrounds significantly contrast with
209
most of the submarine lithified surfaces that are often described as sequence boundaries. The time interval represented by drowning discontinuities might include several sea-level cycle (Godet et al., 2013).
However, in some cases (e.g., Yilmaz et al., 2012), the drowning event
of an Upper Hauterivian platform of Turkey is recorded by multiple
hardground surfaces, pointing to intermediate sedimentation/omission
pulses. This pattern was also observed by Leonide et al. (2012), who
state that the Toarcian Oceanic Anoxic Event (T-OAE) is preceded by a
multi-phased unconformity witnessing a major palaeoenvironmental
change during the Early Jurassic.
5.3. Aragonite sea II
5.3.1. Permian and Triassic hardgrounds
Reports on marine hardgrounds of the Triassic hothouse world
(Preto et al., 2010), are infrequent compared to those from the Cretaceous and Jurassic periods (Wilson, 1997; Bertling, 1999). Triassic
hardgrounds and microbialites are often related (Cozzi, 2002; Woods
and Baud, 2008), with the former feature representing the substratum
for this peculiar facies. Detailed case studies of the petrography of Triassic hardgrounds are exceptionally scarce (Fig. 22), while most published
work has a clear focus on hardground palaeoecology (Runnegar, 1979;
Toomey et al., 1988; Slowakiewicz and Mikolajewski, 2009).
The scarcity of reported hardgrounds in the Late Permian and
throughout the Triassic remains poorly understood. Reasons for this
Fig. 22. Total versus relative number of publications dealing with marine hardground formation throughout the Phanerozoic (compiled in 2011 by Prof. M.A. Wilson, College of Wooster,
USA) placed against reconstructed Phanerozoic aragonite/calcite sea intervals (a and c black and grey intervals, respectively, after Stanley and Hardie, 1998): A. including Pleistocene-Holocene periods; B. omitting Pleistocene-Holocene periods.
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Fig. 23. Satellite image of Exuma Cays (Bahamas) with indication of different types of early marine lithified surfaces. Different marine hardground cement lithologies are indicated.
(A-E). (Based on Whittle et al., 1993); (A) Low-energy leeward shelf, Normans Pond Cay (3–4 m water depth); (B) Low-energy channel (Salt Pond Channel); (C) Low-energy lagoonal setting; (D) High-energy shoals (back sides of channel ooid levees; 1–2 m water depth); (E) High-energy channel (4–8 m water depth).
lack of reported case studies may or may not include the fact that the
Late Permian and much of the Triassic is characterized by both greenhouse (hothouse) conditions and aragonite sea chemistry (Stanley
and Hardie, 1998; Fig. 29). Overall, Mg/Ca seawater ratio is high and
so is the ocean saturation state with respect to calcite relative to the
Fig. 24. Conceptual model for Oligo-Miocene hardground formation on a cool-water temperate shallow-marine shelf, Southern Australia. Localized patches of lithified carbonate
sediment coalesce from A to D. Modified after Nicolaides and Wallace (1997).
Fig. 25. Alternative model for seafloor lithification due to serpentinization fluids rising
through sediment column. (A) Schematic cross-section through central area Atlantis
Massif, Mid-Atlantic Ridge. In the absence of major fault system, the depth of hydrothermal circulation and serpentinization is limited to the upper stratigraphic levels.
(B) Schematic cross section through southern Atlantis Massif. Here, abundant, deepreaching fault systems enhance hydrothermal circulation and facilitates pervasive
serpentinization, formation of well-indurated carbonates at seafloor and focused venting.
Modified after Schroeder et al. (2002).
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
211
hardground-colonizing organisms, following the Permo-Triassic mass
extinction. The recovery of hardground ecosystems was slow and
progressed only stepwise during the Triassic (Crasquin-Soleau et al.,
2007; Tong et al., 2007). According to Bertling (1999), Triassic
hardgrounds are sparsely colonized, at least in the few cases where
these features are documented. In contrast, the Ordovician and Jurassic
periods were characterized by a diverse and rich hardground fauna
(Palmer, 1982). Hardgrounds, often recognized as such on the basis of
ichnofacies, may have thus been overlooked in the case of the Triassic
low faunal diversity.
Summing up, the Permo-Triassic world, characterized by limited
epeiric sea extent, is generally less favourable for widespread neritic
hardground formation. Given that pre-Jurassic oceanic lithosphere
was destroyed in subduction zones, except where thrust onto continental margins (ophiolites), evidence of deep-water hardgrounds will be
equally scarce. Finally, the predictably aragonitic mineralogy of early
marine hardground cements combined with an overall low diversity
hardground fauna might render the recognition of Permo-Triassic
hardgrounds difficult.
Fig. 26. Compilation of bathymetry versus mol% MgCO3 contents of early marine cements
in pelagic hardground intervals in the Atlantic and Pacific placed against the Mediterranean and the Red Sea. Modified after Schlager and James (1978).
modern world or to Lower Jurassic seas for comparison (Stanley and
Hardie, 1998; Fig. 29). Aragonite cemented hardgrounds are prone to
subsequent diagenetic alteration, a process that reduces the fossilization
potential of these features.
A second line of reasoning might include the fact that most ancient
hardgrounds formed in neritic water depths of less than 100 to 200 m.
Permian and Triassic epicontinental seas, due to an overall low global
sea level, were much reduced in size (Chumakov and Zharkov, 2002;
Miller et al., 2005). In comparison, much of the Jurassic and Cretaceous
periods are characterized by high sea levels and the corresponding vast
epeiric seas, following the break-up of Pangaea (Miller et al., 2005). A
comparable pattern is perhaps found in the Middle-Late Cambrian and
the Ordovician break-up of the supercontinent Pannotia resulting in
global sea-level rise and the flooding of vast continental areas (Miller
et al., 2005). The resulting epeiric seas favoured widespread hardground
development in the extensive shallow marine settings.
A third line of reasoning for the scarcity of reports on Permo-Triassic
hardground features is perhaps found in the absence of abundant
5.3.2. Carboniferous hardgrounds
Carboniferous hardgrounds are rarely discussed in the literature
(Wilson and Palmer, 1992; Taylor and Wilson, 2003). The transition between the Calcite I and Aragonite II global seawater stages takes place
during the mid-Mississippian (Stanley and Hardie, 1998; Stanley,
2006). For the sake of simplicity, we here refer to the Carboniferous as
Aragonite II seawater mode as aragonite and high-Mg calcite cementation is typical for much of the Carboniferous. Amongst the limited studies dealing with Carboniferous hardgrounds (Suchy and West, 1988;
Dodd and Nelson, 1998; Rankey, 2003; Zhang et al., 2009, 2010), only
one deals, to the knowledge of the authors, with the controls and petrography of marine hardgrounds (Dodd and Nelson, 1998) whilst papers dealing with hardground ecology are more common (Loope,
1994; Wilson and Palmer, 1998; Sumrall, 2001).
In an attempt to explain the scarcity of well-documented
hardgrounds (and associated biota) during the Carboniferous, Zhang
et al. (2009) suggested that soft-sediment dwelling, benthic marine organisms might have been favoured over hard-substrate taxa due to specific environmental stressors. Similar to the Permo-Triassic case
examples, Zhang et al. (2009) furthermore suggest that the knowledge
about Carboniferous hard substrate communities, assumed to be of low
diversity, is perfectible. Moreover, Zhang et al. (2009) argued that Carboniferous oceans are mostly dominated by high Mg/Ca seawater ratios,
favouring the precipitation of aragonite and high-Mg calcite over low-
Fig. 27. Comparison of the relative proportion of carbonate cement mineralogies in deep oceanic basins and semi-enclosed deep basins over the last 20 kyr; (A) Deep oceanic basins;
(B) Mediterranean Sea; (C) Red Sea. LGM: Last Glacial Maximum.
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Fig. 28. Schematic diagram summarizing mechanism leading to precipitation of early ferroan calcite cement by sulphate-reducing bacteria. Glauconite is formed in the uppermost oxic
layer of chalky sediments. Modified after Jeans (1980).
Mg calcite and thus limiting the fossilization potential of these features.
Similar to the discussion for the Permo-Triassic, the scarcity and nonsystematic nature of studies on Carboniferous hardground intervals represents a significant obstacle to a sufficient comprehension of their formation processes and forms a strong motivation for future research.
5.4. Calcite sea I
5.4.1. Silurian and Devonian hardgrounds
Devonian hardgrounds are frequently described in the literature
(Wendt, 1988; Dix, 1990; Fejer and Narbonne, 1992; Curran and
Hurley, 1992; Chow and Longstaffe, 1995; Oliver et al., 1996; Cornell
et al., 2003; Lubeseder et al., 2010; Aboussalam and Becker, 2011) but
published evidence dealing with the petrography of these features is
once more limited. Some of the Upper Devonian hardgrounds were
interpreted to represent long-lasting hiatal or drowning surfaces linked
to the Late Devonian biological crisis (Caplan and Bustin, 1998; Bond
and Wignall, 2008). Case studies from NW Thailand report low sedimentation rates and surfaces impregnated by Fe/Mn crusts (Königshof
et al., 2012). Similarly, Hüneke (2013) described phosphoritic
hardgrounds from Morocco.
The situation for the Silurian is not much different from the one for
the Devonian. Hardgrounds are reported in many studies (Tucker,
1971; Cherns, 1980; Cherns, 1982; Kershaw et al., 2006; Vinn and
Wilson, 2010; Copper et al., 2012), but these authors focus on
hardground biota and their palaeo-ecology and detailed descriptions
of the hardground petrography are lacking.
5.4.2. Ordovician hardgrounds
Greenhouse conditions prevailed during much of the Ordovician period (Stanley and Hardie, 1998). Nevertheless, a long-term cooling
trend, initiated during the Middle Ordovician and terminating in the
Hirnantian glaciation, was accompanied by one of the most severe biological crisis of the Phanerozoic (Marshall et al., 1997; Delabroye and
Vecoli, 2010). High sea levels are reconstructed for much of the Ordovician greenhouse (Miller et al., 2005) and resulted in vast epeiric seas
flooding continental shelves both in the tropical belt and in temperate
regions (Scotese, 2001).
A series of case studies dealing with Lower to Upper Ordovician
hardgrounds have been published (Wilkinson et al., 1982; Delgado,
1983; Brett and Brookfield, 1984; Kim and Lee, 1996; Palmer and
Wilson, 2004; Benner et al., 2004; Fig. 22). According to some of these
authors, the mineralogy of hardground cements is generally calcitic
with early marine low-Mg calcite phases being dominant (Table 13).
Most studies, however, remain rather unspecific with respect to the primary nature of the cement phases lithifying hardgrounds (Möller and
Kvingan, 1988; Ekdale and Bromley, 2001; Benner et al., 2004; Hender
and Dix, 2008; Mancini, 2011). Descriptions of hardground petrography
are biased towards fabrics and their analogy with present-day case settings (Wilkinson et al., 1982). This analysis is in agreement with the
high seawater saturation state with respect to CaCO3 that typifies the
Ordovician oceans (Stanley and Hardie, 1998).
Despite the dominance of low-Mg calcites, some case examples of
magnesian calcite cements have been reported from marine
hardgrounds too (Delgado, 1980; Brett and Brookfield, 1984;
Brookfield and Brett, 1988; Brookfield, 1988; Kim and Lee, 1996;
Palmer and Wilson, 2004; Table 13). Conversely, most workers seem
to agree that Ordovician hardgrounds were largely devoid of early aragonitic cements. Nevertheless, aragonite precipitated in Ordovician
shallow seas either as inorganic phase in reefal frameworks or as invertebrate exoskeletons (Rao, 1991; Yoo and Lee, 1993; Gao et al., 1996;
Balthasar et al., 2011). Palmer and Wilson (2004) suggested that metastable aragonite skeletal fragments in carbonate sediments provide an
important porewater source of CaCO3 ions for the formation of calcitic
cements (Fig. 30). Amongst the carbonate fabrics of Ordovician
hardgrounds, syntaxial overgrowths on skeletal grains (mainly on echinoderm fragments) are perhaps the most widely documented ones
(Table 13). Previous work suggested that the dominance of syntaxial
overgrowths in Ordovician hardgrounds reflects reduced competition
of nucleation sites due to the lack of aragonite cements (Kim and Lee,
1996).
In an extensive description of hardgrounds from an Ordovician
shoal-to-basin transect, Brett and Brookfield (1984) identified a series
of hardgrounds changing in nature and extent with their relative position along the transect. The hardgrounds all share early marine cement
fringes of bladed to equant calcite and syntaxial overgrowths on echinoderms. Brett and Brookfield (1984) identified omission surfaces
(firmgrounds; Fig. 3A) and nodular limestones (incipient hardgrounds;
Fig. 3B) as the early stages of seafloor lithification. Fully developed
hardground surfaces formed in high-energy tidal settings and are similar to those reported from many other time intervals. Taylor and Wilson
(2003 and references therein) assume a mutual relationship between
an enhancement in the temporal and spatial distribution of
hardgrounds in the Early Ordovician and the evolution of sessile echinoderms. Echinoderms are thought to promote rapid seafloor lithification
(Taylor and Wilson, 2003), thus potentially accounting for the high record of hardgrounds in the Ordovician (Fig. 22).
5.4.3. Cambrian hardgrounds
Published work on the petrology of Cambrian hardgrounds is scarce.
Where present, case studies reported that hardground cementation is
mainly due to calcitic syntaxial overgrowths on echinoderms but local
phosphogenesis is described as well (Glumac and Walker, 2002;
Zamora et al., 2010; Kolata et al., 2001; Kruse and Zhuravlev, 2008;
Table 13). Rozhnov (2001) argued that Cambrian hardgrounds
from the Cambrian–Ordovician transition in the Baltic basin, in analogy
to Ordovician ones, generally result from the abundance of echinoderm
ossicles (Sprinkle and Guensburg, 1995) forming the substratum
of early marine diagenetic overgrowth fabrics (“self reproducing”
hardgrounds).
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
213
Fig. 29. Publications dealing with marine hardground formation (compiled in 2011 by Prof. M.A. Wilson, College of Wooster, USA) placed against reconstructed Phanerozoic seawater saturation (Ω, orange curve, Riding and Liang, 2005) and Mg/Ca ratio (grey curve, after Stanley and Hardie, 1998). Reconstructed atmospheric CO2 levels (dark blue hatched, Royer et al.,
2004). Reconstructed mean global temperatures (light green hatched, Royer et al., 2004). Estimation of mean global carbonate platform coverage (red curve, Kiessling et al., 2003); Estimation of flooded continental areas (light blue hatched, Hay et al., 2006); Preserved carbonates (dark green, Mackenzie and Morse, 1992) Note correlation of published studies on
hardgrounds and calcite sea modes. The abundance of hardgrounds in the Quaternary aragonite sea suggests that the scarcity of reported case studies in Palaeozoic aragonite seas is a
preservation artefact.
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Fig. 30. Conceptual sketch of petrographic evidence pointing to early diagenetic dissolution of skeletal hardparts at Ordovician seafloors. (A) Shell hardparts at post-mortem deposition
prior to dissolution. (B) Aragonite is dissolved leading to secondary early marine lithification of host sediments. After Palmer and Wilson (2004).
5.5. Aragonite I (Earliest Cambrian)
6. Discussion and synopsis
During the early onset of metazoan biomineralization (Late
Ediacarian to earliest Cambrian; Zhuravlev and Wood, 2008, 2009),
world oceans were dominated by an aragonite seawater chemistry
(Stanley and Hardie, 1998) and icehouse conditions (Brasier and
Sukhov, 1998). The earliest Cambrian aragonite sea I mode refers
to a very brief—albeit crucial—phase of the earliest Phanerozoic. Biostratigraphic time control is not well constrained for this important period. Earliest Cambrian inorganic marine cements and ooids were
mainly aragonitic (Tucker, 1992; Zhuravlev and Wood, 2008). We speculate that corresponding hardground cements were aragonitic and
magnesian calcitic in nature and these metastable mineralogies limited
the preservation – and hence the recognition - of hardgrounds in the
field or subsurface material. Summing up, published data set on marine
hardgrounds of the Calcite I and Aragonite I seas are scattered and detailed petrographic studies remain at present scarce. Clearly, it seems
important that future work aims at closing this important knowledge
gap.
6.1. Hardground preservation through time
The formation of marine carbonate hardgrounds results from a complex interplay of abiogenic and biogenic controls and environmental parameters. Recent to sub-Recent hardgrounds are physically accessible
and allow for the direct observation of related controls leading to a reasonable understanding on how the features form and evolve over time
(e.g., Shinn, 1969). Conversely, our understanding of fossil hardgrounds
results from the observation and description of field, subsurface, and
laboratory data. Nevertheless, the identification of pre-Neogene
hardgrounds is not straightforward (Fig. 31). The latter implies that
the contrast-comparison of recent and ancient marine hardgrounds is,
as so often in geology, inherently biased.
Specifically, a series of physical and diagenetic controls are likely to
obscure or fully remove fossil hardground features. These include factors that might act at an early stage such as the erosional truncation
or karsting of shallow neritic hardgrounds due to sea-level fall and
Fig. 31. Types of biases affecting the recognition of hardground surfaces throughout the geological record.
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
215
Table 13
Formation environment and petrology of Palaeozoic marine hardgrounds in neritic domains (n.d. = no data).
Locality
Bathymetry
Carboniferous hardgrounds (Calcite I period)
Shallow platform to ramp
Very shallow
(Indiana, USA)—Mississippian
Ordovician hardgrounds (Calcite I period)
b100 m,
Storm-influenced carbonate
above storm
ramp (Dumugol Formation,
wave-base
South Korea)—Early
Ordovician
?Shelf (Ontario,
Canada)—Middle Ordovician
n.d.
Rock texture beneath
surface
Mineralogy
(% rock
volume)
Cement type (size)
Thickness of
lithified
interval
Reference cited
n.d. (?mudstones)
High-Mg
calcite
Microcrystalline, fibrous
rinds and ?syntaxial
overgrowth
n.d.
Dodd and Nelson (1998)
Mudstones
Low-Mg
calcite (also
high-Mg
calcite
Microcrystalline
Few cm
Kim and Lee (1996)
~10 cm
Wilkinson et al. (1982)
1 to few cm
Brett and Brookfield (1984); Palmer
and Wilson (2004); Brookfield and
Brett (1988); Brookfield (1988)
n.d.
Hender and Dix (2008)
10–15 cm
Möller and Kvingan (1988)
N4 cm (depth
of borings)
n.d.
Benner et al. (2004)
Biomicrites and
biosparites
Low-Mg
calcite
Low-Mg
calcite
Syntaxial overgrowth on
echinoderms
Syntaxial overgrowth of
echinoderms
Equant cement
Low-Mg
calcite
?Low-Mg
calcite
Continental shelf (Southern
Ontario, Canada)—Middle
Ordovician
b20–50 m
Mixed carbonate-siliciclastic
ramp (Newfoundland,
Canada)—Late Ordovician
0 m to above
the storm
wave-base
Packstones–grainstones
(to rudstones)
Shelf of an epicontinental
basin (Norway)—Middle
Ordovician
?~50 m,
below
storm-wave
base
n.d.
Mudstones–packstones
Syntaxial overgrowth on
?Low-Mg or echinoderms
high-Mg
calcite
Calcite (low- Bladed
or high-Mg) Syntaxial overgrowth on
echinoderms
Inclusion-rich equant
cement
Calcite
Fibrous
Grainstones (mostly)
Calcite
n.d.
n.d.
Grainstones (mostly)
Low-Mg
calcite
Fibrous and bladed
Syntaxial overgrowth on
echinoderms
n.d.
Mudstones–wackestones Calcite
(“micritic and
fossiliferous”)
Mudstones–wackestones High-Mg
calcite
Low-Mg
calcite
Dolomite
Fine- to coarse-grained
Calcite
peloidal grainstones
Shelf (Utah, USA)—Early
Ordovician
?Shelfs
(Utah–Nevada–Ohio–Iowa,
USA and Russia)—Late Cambrian to Late Ordovician
Epicontinental setting
(Sweden)—Early Ordovician
Epeiric sea (Upper Mississippi
Valley, USA) – Late
Ordovician
Below
fair-weather
wave base
Epicontinental sea
(South-central Ontario,
Canada)—Middle Ordovician
n.d.
Biomicrites,
biomicrosparites and
biosparites
Cambrian hardgrounds (Calcite I period)
n.d. (?above
Shallow high-energy settings
Wackstone–packstone
fwwb)
(SW Europe and NW
Africa)—Middle Cambrian
Shelf-edge (France)—Early
n.d.
n.d.
Cambrian
Outer shelf (Iran)—
Close to swb Grainestones–rudstones
Middle-Late Cambrian
(tempestites)
Shelf of a passive margin
(S Appalachians, USA)—Late
Cambrian
Close to
sea-level
n.d. (?mudstones)
Bladed to equant cement
Palmer and Wilson (2004 and
references therein)
Epitaxial
N6 cm (depth
of borings)
Ekdale and Bromley, 2001
Microcrystalline
Equant (sparry)
Finely crystalline
Cm-thick
Delgado, 1980, 1983
Microsparitic
Sparry crystals
n.d.
Mancini, 2011
Calcite
Syntaxial overgrowth on
echinoderms
n.d.
Zamora et al., 2010
Fluorapatite
n.d
n.d.
Alvaro and Clausen, 2010
?dm-thick
(thickness of
tempestite
beds)
n.d.
Kruse and Zhuravlev, 2008
Calcite (low- Microsparitic and ?also
or high-Mg) syntaxial (mentioned
echinoderms)
High-Mg
and low-Mg
calcite
subaerial exposure and related meteoric diagenesis (Immenhauser
et al., 2000a, 2000b). Alternatively, the subaquatic erosion of
hardgrounds along coast lines in the context of wave-abraded terraces
is perhaps more common than assumed (Rameil et al., 2012). Moreover,
coastal settings are also the site of intense biogenic erosion of limestones by etching and boring organisms.
Microcrystalline, fibrous to
bladed rinds and syntaxial
overgrowth on echinoderms
Glumac and Walker, 2002
Processes that are likely to overprint, rather than remove, ancient
hardground surfaces involve the burial alteration of metastable aragonitic and high-Mg calcitic marine hardground cements to secondary calcite phases (e.g., Malone et al., 2001; Swart, 2015). Dissolution of
metastable phases, recrystallization and partial or pervasive dolomitization may remove most of the primary hardground evidence.
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Moreover, carbonate preservation rate is expected to decrease gradually towards the early Phanerozoic (Berner and Mackenzie, 2011) and
hardground preservation is expected to decline accordingly. The latter
preservation depends, however, on the ratio between the volume of
carbonates that accumulated for a given period and the percentage of
this volume that was preserved (Mackenzie and Morse, 1992). A clear
relationship, however, between carbonate production and survival,
and hardground preservation and record, is not evidenced (Fig. 29).
Moreover, most of the pre-Jurassic oceanic lithosphere has been
destroyed in subduction zones (Berner and Mackenzie, 2011; Picotti
et al., 2014) implying that pre-Jurassic oceanic hardgrounds are generally not preserved unless portions of the lithosphere have been
obducted onto continental margins, but not metamorphosed in orogenic belts In contrast, hardgrounds that formed in the extensive epicontinental basins during periods of sea-level highstand have a higher
fossilization potential as important portions of the ancient epicontinental rock archives is preserved to the present day in surface outcrops or in the subsurface (Berner and Mackenzie, 2011). Hardgrounds
draped by facies with a low porosity and permeability, i.e. clays, are expected to have a high preservation potential. These intervals may act as
barriers for vertical fluid flows and therefore reduce burial rock-fluid interaction (Christ et al., 2012a; Shekhar et al., 2014).
Bioturbation might promote early marine cementation by creating a
significant pore network below seafloor, where seawater saturated in
CaCO3 circulates. Periods of intense bioturbation may therefore also correspond to enhanced hardground formation (e.g., Ordovician), and thus
increase their formation and preservation potential. The latter indicates
that evolutionary patterns are of significance as faunal activity might
both enhances—and destroys—hardgrounds. Along similar lines, major
biological crises and extinction events in Earth history are of significance due to their impact on carbonate producing ecosystems and
hardground biota (e.g., Bertling, 1999). Indirectly, the lack of typical
hardground fauna perhaps hinders the recognition of ancient lithified
surfaces. Examples include some hardgrounds from the earliest Silurian,
Carboniferous, Jurassic and Palaeogene, time intervals directly following
major Phanerozoic biologic crises (Bambach, 2006; Kiessling et al.,
2008). Most prominent case examples of hardgrounds lacking a typical
faunal expression come from Triassic surfaces, a period marked by the
low recovery following the Permian–Triassic Mass Extinction
(Crasquin-Soleau et al., 2007; Tong et al., 2007). The above-discussed
pattern, however, offers no explanation for the scarcity of reported
Permian hardground case examples (Figs. 22 and 29), i.e. from a time interval preceding the extinction event. Clearly the latter remark evidences the complex interplay of numerous factors impacting the
formation and preservation of these features.
6.2. Hardground formation environments
In present-day warm, shallow-marine tropical seas characterized by
elevated CaCO3 saturation, early cementation of the ocean floor and
thus hardground formation are common. Nevertheless, apparently
subtle differences in bathymetry and topography, seafloor substrate,
wave and current hydrodynamics, and the balance between sedimentation and erosion result in spatially differential hardground
properties. These controls are well exemplified in the variable
hardground types as described by Whittle et al. (1993) in the Exuma
Cays, Bahamas.
Early cement fabrics from modern tropical hardground are often
isopachous acicular or fibrous and their mineralogy often aragonitic or
high-Mg calcitic. Very similar fabrics are equally recognized in many ancient tropical neritic hardgrounds. Nevertheless, the variety in
hardground surface types, such as those identified in recent settings,
has only been reported from a limited number of ancient case examples
(Sattler et al., 2005; Rameil et al., 2012). In studies dealing with the fossil
hardground record, variability exists between hardgrounds described
from the same outcrop belt (Hillgärtner, 1998) but often, the
isochronous nature of these hardgrounds is not documented. This implies that the observed variability might be both spatial and temporal
(Immenhauser et al., 1999). Conversely, ancient hardgrounds often represent key stratigraphic surfaces such as maximum regression surfaces
or drowning unconformities (Föllmi et al., 2011; Yilmaz et al., 2012), a
feature that commonly reflects global or relative sea-level changes or
pronounced changes in climate dynamics, the latter of which often results in changes in carbonate production and/or accumulation (Godet,
2013; Brigaud et al., 2014).
In cool-water carbonate platforms, referred to as “destructive” systems (James, 1997) due to an often reduced carbonate production potential (Schlager, 2005), hardground formation contrasts that in
tropical settings (James et al., 1999; Rivers et al., 2008). The midCenozoic case examples of the Australia and New-Zealand shelves
(Nicolaides and Wallace, 1997; Nelson and James, 2000) and the
Oligo-Miocene heterozoan platforms of the Mediterranean (Pedley
and Bennett, 1985; Mutti and Bernoulli, 2003) have contributed most
to our understanding of cool-water hardgrounds. In these settings, seawater is generally undersaturated with respect to CaCO3 favouring dissolution. Where cementation exceeds dissolution hardgrounds may
form. Often, local patterns in seawater saturation are taken as explanation for this pattern (Allen et al., 1969; Garrison et al., 1969). Other factors include the oxidation of methane in near-seafloor sediments, the
decay of organic matter or exchange with clay minerals releasing Mg
ions thus increasing porewater Mg/Ca ratios (Garrison et al., 1969). All
these processes locally increase the CaCO3 saturation and the potential
for early seafloor cementation. Unlike modern cool-water hardgrounds,
the formation of Mid-Cenozoic case examples is apparently governed by
local and global environmental patterns. Examples include the Eocene/
Oligocene buildup of permanent ice shields on Antarctica leading to significant glacio-eustacy and modifications in oceanic circulation patterns
(e.g., Nelson and James, 2000). Similarly, hardgrounds described by
Nelson and James (2000) might be related to a rise in shelf seawater
temperatures during periods of sea-level lowstand. The argument
brought forward includes the shallower shallower, and hence volumetrically reduced, water masses responding more rapidly to warming
(“shaved shelf model”; James et al., 1994).
An important difference between tropical and cool-water platforms
lies in the on average higher stratigraphic and spatial frequency patterns
of hardgrounds in (sub)tropical carbonate platforms, ramps or mounds
(Nicolaides and Wallace, 1997; Nelson and James, 2000; Gruszczyński
et al., 2008). Quantitative examples of the stratigraphic frequency distribution of hardgrounds in Mesozoic carbonate platforms or ramps are
given in Hillgärtner (1998; 15 hardground surfaces in 30 section
metre); Sattler et al. (2005; 7 hardground surfaces in 60 section
metre) or Christ et al. (2012a 80 hardground surfaces in 220 section
metre).
Pelagic and bathyal hardgrounds are well known from the open oceanic realm and from land-locked seas such as the Mediterranean and
the Red Sea (Milliman et al., 1969; Aghib et al., 1991). Conversely, pelagic hardgrounds are far less commonly described from the fossil rock record. The closest ancient analogues to modern pelagic or hemi-pelagic
hardgrounds are found in Cretaceous chalk hardgrounds formed at
palaeo-water depths of up to 200 to 300 m (Kennedy and Garrison,
1975). While cementation in Cretaceous chalk sediments seems to be
related to starvation combined with low sediment accumulation, cementation was considered rapid (10 to 102 years; Kennedy and
Garrison, 1975), perhaps due to significant water circulation.
Early marine carbonate fabrics are generally related to temperature
and seawater CaCO3 saturation and thus to their bathymetric context.
Hardground cements are generally of high- or low-Mg calcite mineralogy in open oceanic settings. Conversely, the mineralogy of hardground
cements is either high-Mg calcitic or aragonitic in narrow, land-locked
basins where the thermocline is lowered due to isolation and related
to enhanced salinity levels. The cement fabrics are mostly micro- to
cryptocrystalline.
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
6.3. Hardground distribution and petrography in aragonite and calcite seas
Throughout the Phanerozoic, seawater chemistry in mainly the
(sub)tropical portions of the world's oceans oscillated between an aragonite mode (Mg/Ca N 1) and a calcite seawater mode (Mg/Ca b 1;
Sandberg, 1983; Stanley and Hardie, 1998; Fig. 29). The authors acknowledge that the concept of calcite versus aragonite seas is at best
an approximation and spatial as well as bathymetric variability of seawater chemistry severely complicates this issue. Moreover, as documented in Steuber (2002) for the case example of the middle to Late
Cretaceous, higher order changes in seawater chemistry were
superimposed on calcite sea modes and the same might account for aragonite seas. Within the limitations of this general concept, a number of
petrographic patterns merit discussion. The more common occurrence
of hardgrounds in tropical calcite sea carbonates (Fig. 29) contrasts
the rather limited number of reported hardground occurrences during
aragonite sea II (Carboniferous to Early Jurassic). In contrast,
hardgrounds are abundantly documented from carbonates deposited
during the aragonite III seawater period (Oligocene to present) and
case studies reach a maximum in modern settings. Along similar lines,
the Carboniferous and Early Jurassic calcite sea II hardgrounds are
more frequently reported than those that formed during the Silurian
to Cambrian calcite sea I.
Early marine cements documented from calcite I intervals are principally (low-Mg) calcitic and their fabrics mostly of pore-filling nature
(microcrystalline, equant to drusy, and syntaxial). Only rarely,
isopachous fibrous or bladed fabrics are found and acicular cements
have not been reported at all (Table 13). Calcite is the dominant mineralogy documented in early marine cements of calcite sea II hardgrounds
while high-Mg calcite cements are much more frequent and even aragonite is relatively common. The main difference between calcite seas I
and II is that hardgrounds cement fabrics in the Jurassic to Eocene calcite
sea II cover the full spectrum of fabrics known from marine
hardgrounds. A similar diversity in cement fabrics is not present in calcite sea I case examples. Calcite sea II hardground fabrics include sparry,
microcrystalline, and syntaxial overgrowths and are commonly of (lowand high-Mg) calcitic mineralogy whereas aragonitic, high-Mg calcitic
acicular, and fibrous cements are equally common.
With regard to the Miocene to Recent aragonite sea III period
(Fig. 29), aragonite and high-Mg calcite form the dominant mineralogies of present-day and Neogene mainly (sub)tropical hardground cements. In contrast, hardly any petrographic evidence of hardground
cements has been reported from aragonite I and II periods limiting a direct comparison to the aragonite sea III interval. Along these lines,
Stanley and Hardie (1998) suggested that early marine cements in aragonite mode oceans are metastable magnesian calcites and aragonites
that have a poor preservation potential. It seems likely, that this preservation bias is largely responsible for the scarcity of well documented
pre-Jurassic aragonite mode hardgrounds.
An interesting case is found in the temporal distribution of syntaxial
overgrowth cements in marine hardgrounds. To our knowledge,
syntaxial overgrowth cements have not been described from aragonite
III sea deposits but seem to be exclusively limited to carbonates from
calcite mode oceans. The post-Mesozoic absence of syntaxial overgrowth may partly find its origin in a loss of platform building potential
for echinoderms after the Jurassic (Kiessling et al., 2003). If this holds
true, the relation between evolutionary trends in marine biota, biogenic
calcite mineralogies and related marine hardground cement types
might be more direct than perhaps assumed.
Though less commonly, low-Mg calcite is also found as early marine
cement in modern aragonite sea hardgrounds (Fig. 11). Nevertheless,
case examples come mainly from cool-water and deep marine carbonate settings (Garrison et al., 1969; Noé et al., 2006). A broad spectrum
of cement fabrics has been described from actualistic hardgrounds but
evidence that one specific carbonate cement fabric is dominant in one
specific hardground type or environment is lacking.
217
6.4. Impact of atmospheric pCO2 and global temperatures on hardground
mineralogy
A gradual replacement of aragonite and high-Mg calcite skeletal
mineralogy over low-Mg calcite shell secretors is observed throughout
the Phanerozoic (Wilkinson, 1979; Zhuravlev and Wood, 2009). According to Zhuravlev and Wood (2009), this pattern is driven by a gradual pCO2 decrease towards the Neogene and was overprinted by a series
of mass extinctions. The cited authors suggest that patterns in pCO2
dominate over seawater Mg/Ca ratios as a first-order control on
biomineralogy. Similarly, marine inorganic carbonate cements in
hardgrounds from the early Paleozoic are described as principally
being of low-Mg calcite mineralogy (Fig. 11; Table 13). The latter is in
agreement with the model of a low-Mg mineralogy associated to a calcite sea mode (Hardie, 1996). On the level of a working hypothesis, it is
worth considering that low-Mg calcite precipitation at the beginning of
the Phanerozoic has gradually given way to prevailing high-Mg calcite
and aragonite mineralogy due to the significant decrease in atmospheric
pCO2 (Zhuravlev and Wood, 2009). Cenozoic and modern hardgrounds,
formed in a world that is characterized by—in comparison to for example the Paleozoic—(still) low atmospheric pCO2, are in agreement with
this concept. Jurassic and Cretaceous hardgrounds perhaps represent
the best arguments for a relation between pCO2 and hardground carbonate cement mineralogy through time. Although the latter periods
are predominantly characterized by greenhouse climate and a calcite
sea mode, early marine cements in hardgrounds are not limited to
low-Mg calcite, but also include aragonite and high-Mg calcite mineralogies. While still relatively elevated (Berner and Kothavala, 2001; Royer
et al., 2004), atmospheric CO2 is significantly reduced in the Jurassic and
Cretaceous atmosphere when compared to early Paleozoic pCO2 concentrations and the corresponding calcite sea mode. The latter pattern
may explain an increase in aragonite and high-Mg calcite cements during the Mesozoic and the scarcity of early marine diagenetic low-Mg calcite in modern hardgrounds.
6.5. Greenhouse versus icehouse mode hardgrounds
Most workers agree that Phanerozoic hothouse/greenhouse/icehouse periods influenced seawater structure and chemistry, oceanic circulation patterns and related to this, carbonate production and platform
architecture. With the exception of comparably short-lived cold snaps
during the Mesozoic (Jenkyns, 2003; Dera et al., 2011; Krencker et al.,
2014), three major icehouse periods are identified over the last
550 Myr. These include (i) the Late Ordovician glaciation (Loi et al.,
2010), (ii) the Late Paleozoic icehouse, ranging from the Carboniferous
to the Late Permian (Isbell et al., 2012) and (iii) the recent glacial
mode initiated by the establishment of a permanent ice sheet over
Antarctica at the Eocene–Oligocene transition ca 34 Myr ago (Katz
et al., 2008; Lear et al., 2008; Liu et al., 2009).
In general, (sub)tropical hardgrounds are most commonly formed
during greenhouse intervals (Fig. 29). This pattern is evidenced by studies from Cambrian (Kruse and Zhuravlev, 2008), Ordovician (Brett and
Brookfield, 1984; Ekdale and Bromley, 2001), Devonian (Königshof
et al., 2012), Jurassic and Cretaceous carbonates (Hillgärtner, 1998;
Sattler et al., 2005; Christ et al., 2012a). Conversely, only few
hardgrounds are described from the Paleocene–Eocene periods
(Molenaar and Martinius, 1990) and even less from the Triassic
(Figs. 22 and 29), referred to as a “hothouse” world (Preto et al.,
2010). The latter notion is, at first glance, surprising given that warm
Triassic seawater temperatures (Preto et al., 2010) should allow for
near global carbonate precipitation. Arguments explaining the scarcity
of Triassic hardgrounds may include (i) the lack of hardground colonizing biota, (ii) the globally low sea level resulting in limited epicontinental sea extent, and (iii) the destruction of Triassic oceanic lithosphere in
subduction zones.
218
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
Icehouse periods, even in the (sub)tropical climate zone, lack abundant evidence for seafloor lithification. Published data refer to Carboniferous and Permian hardgrounds but are limited (Dodd and Nelson,
1998; Thomka and Brett, 2014; Figs. 22 and 29). Conversely, upper Jurassic–Lower Cretaceous hardground surfaces, representing a coolhouse
climate mode, have been frequently reported (Hillgärtner, 1998; Reolid
and Nieto, 2010; Schwarz and Buatois, 2012; Godet et al., 2013). Similarly, published data on Neogene hardgrounds are abundant and particularly so with reference to cool-water carbonate hardgrounds (cf.
Nelson and James, 2000; Mutti and Bernoulli, 2003). Specifically, the
frequency of Neogene hardground documents that hardground formation might be widespread during icehouse modes. Nevertheless, most
of these marine surfaces are described from the Oligocene–Miocene periods (James and Bone, 1992; Gruszczyński et al., 2008) whereas they
are scarce in Pliocene carbonate depositional environments (Beach,
1993). The fact that the recent (aragonite) icehouse world is also the
site of abundant hardground formation suggests that the relation between long-term climate mode and hardground abundance is not
straightforward.
6.6. Relation of epeiric sea extent and hardground frequency
The extent of epicontinental seas and the nature, amplitude, and frequency of relative sea-level change most likely significantly influenced
patterns in Phanerozoic hardground formation. Many of the regionally
extensive Phanerozoic carbonate platforms nucleated and prograded
in epeiric-neritic seas and then formed the site of widespread
hardground formation (Hay et al., 2006; Harries, 2009). Carbonate platform coverage ranged from a minimum of 6.2 ∗ 106 km2 in the Permian
to a maximum of 1.3 ∗ 107 km2 in the Ordovician (Kiessling et al., 2003;
Fig. 29; mean = 9.5 ∗ 106 km2). With the noteworthy exception of the
Paleogene, time intervals with the most widespread global carbonate
platform expansion, such as the Cretaceous and Ordovician (Kiessling
et al., 2003), also represent periods with the highest number of reported
hardground occurrences. Conversely, the Carboniferous, Permian, and
Triassic—representing periods of minimal global platform expansion
(Kiessling et al., 2003; Fig. 29)—coincide with minima in reported carbonate hardground distribution. Periods of widespread epicontinental
seas related to high sea levels, such as the Ordovician or the Cretaceous
(Hay et al., 2006; Harries, 2009), tend to coincide with abundant
hardground formation (Fig. 29). This pattern does not hold true for Devonian and Carboniferous periods characterized by a limited number of
reported hardground surfaces despite the wide extent of shallow epeiric
seas (Hay et al., 2006; Harries, 2009; Fig. 29). Similarly, mean global
Neogene carbonate platform surface area is in the order of the mean
Phanerozoic coverage but is characterized by a frequency distribution
of hardground surfaces that is above average. Finally, the extent of recent epeiric seas is exceptionally low whereas the abundance of marine
hardgrounds is high.
Concluding, the relation of hardground frequency, and equally important, the hardground preservation potential, versus temporal changes in seawater chemistry is not an easy one. Clearly, our understanding
of past hardground mineralogies is severely limited by the preservation
potential of related fabrics and obscured during diagenetic pathways.
Moreover, as detailed below, patterns in atmospheric CO2 composition,
ocean stratification, eustatic sea-level changes, plate tectonics and evolutionary trends of marine organisms add significant complications to
this matter. Independent of these problems, a critical and more detailed
look into secular changes in hardground fabrics is encouraged.
7. Conclusions
The present paper reviews published literature dealing with the
petrographical, bio-geochemical and environmental parameters leading
to Phanerozoic marine hardground formation. The following simplified
definition is proposed: Marine hardgrounds are features in the
sediment record that present evidence for significant early marine cementation of the seafloor itself and/or portions of the near-seafloor sediment column.
A specific focus is on the characterization of the early diagenetic carbonate and non-carbonate hardground cement fabrics. The following
main cement types are recognized: (i) acicular, fibrous and bladed
fabrics; (ii) radiaxial fibrous and fascicular-optic fabrics; (iii) microand cryptocrystalline fabrics; (iv) equant spar, syntaxial overgrowth
fabrics, scalenohedral calcites, peloidal fabrics and (v) non-carbonates,
i.e., phosphate and glauconite cements. Incipient early marine cementation leads to a reduction of pore-space in near-seafloor sediments but
often, marine hardground intervals enter the burial realm with a significant proportion of open pore space later occluded by late cements.
Petrophysical and petrological data suggest that most of the hardground cementation is related to the late burial stage. Hardgrounds
representing moderate hiatal durations (104 to 105 yrs) have a limited
impact on subsurface fluid flow, a characteristic that may contrast
with long-term, fully cemented unconformities (several Ma) representing
regionally important low-porosity intervals.
Published work regarding the petrology of hardgrounds and the
controls leading to their formation is substantial but suffers from a significant data, regional, and preservation bias. Reported case examples
of marine hardgrounds are, in terms of the number of published studies,
dominated by such from the Recent aragonite sea mode, followed by
studies dealing with Cretaceous, Late Jurassic and Ordovician calcite
sea mode hardgrounds. Controls on hardground formation and environments are complex and variable, and reflect the interplay of a multitude
of parameters that changed significantly and repeatedly throughout the
last 545 Myr. Moreover, mechanical or chemical erosion and variable
levels of diagenetic overprint removed or masked characteristic features
of many ancient hardgrounds making their recognition and interpretation difficult. Particularly, fossil hardgrounds cemented by formerly aragonitic or magnesian calcites have a significantly lower fossilization
potential relative to those that are lithified by more stable, marine calcite cements. Thus, the scarcity of studies dealing with pre-Neogene aragonite sea mode hardgrounds is probably due to a preservation bias
rather than representing a genuine feature of these oceans. Having
said this, the concept of aragonite versus calcite seas is accepted on
the level of a first-order simplified approach. Spatial and bathymetric
variability in recent - and most likely also in ancient oceans - is significant and the relations between hardground and global seawater properties are less than trivial.
Characteristic palaeo-ecological patterns change significantly
though the Phanerozoic and the scarcity of hardground biota during
specific periods, i.e. following the mass extinction at the PermoTriassic boundary, renders the recognition of ancient hardgrounds difficult. In some cases, hardgrounds reflect local to regional parameters, in
other cases, these represent the expression of globally significant environmental or biotic perturbations. Regionally important hardgrounds
may represent hiatal intervals ranging from several hundreds to many
millions of years and are important surfaces in sequence stratigraphic
interpretations.
In terms of their formation environment, hardgrounds are most
common in the tropical and sub-tropical shallow, neritic-epeiric realm
with water depths from the sea surface down to about 200 m. Here,
hardground formation is mainly triggered by vigorous seawater circulation and over-saturation with respect to CaCO3. Hardgrounds from
cool-water or heterozoan carbonate settings are often related to
the oxidation of methane, the decay of organic matter in nearseafloor sediments or clay-porewater reactions leading to localized
CaCO3 super-saturation and cementation. Hemi-pelagic hardgrounds
in water depths of many 100s to 1000s of metres are cemented by microcrystalline fabrics and may include aragonite and magnesian calcites
(e.g., Mediterranean Sea) or calcitic mineralogies (Atlantic domain).
These deep-water hardgrounds are often attributed to sediment starvation combined with current-driven water circulation but may also
N. Christ et al. / Earth-Science Reviews 151 (2015) 176–226
involve serpentinizing fluids from oceanic lithosphere. Carbon and
oxygen isotope data from well-preserved hardground cements have
potential as archives of past (marine) porewater geochemistry but
might as well be affected by bacterial oxidation or sulphate reduction
inducing non-marine isotope values. The δ13C and δ18O signatures
of hardground carbonates often differ from that of non-hardground
carbonates under and overlying the early lithification interval.
Future work including sedimentological, stratigraphic, palaeoecological, petrographic and geochemical approaches should focus on
time intervals where published evidence from marine hardgrounds is
at best scarce. These include mainly the Cambrian and the Permian
but the Silurian, Carboniferous and Triassic data base is clearly insufficient, too. All of these studies must be performed in a rigorous,
process-oriented context.
Acknowledgements
The authors acknowledge M. Wilson (College of Wooster, Ohio) for
providing a bibliography of hardgrounds and lithified surfaces as used
for the Fig. 22 in this paper. We are especially grateful to J. Renner
(Ruhr-Universität Bochum, Germany) who provided numerous valuable comments on an earlier version of this paper. B.H. Wilkinson and
T. Dickson acted as reviewers for ESR and provided critical and important comments that significantly improved this paper. We acknowledge
the editorial guidance of P. Wilson.
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