Journal of
Sedimentary
Research
Journal of Sedimentary Research, 2005, v. 75, 665–678
DOI: 10.2110/jsr.2005.054
GENESIS AND DISPERSAL OF CARBONATE MUD RELATIVE TO LATE QUATERNARY SEA-LEVEL
CHANGE ALONG A DISTALLY-STEEPENED CARBONATE RAMP
(NORTHWESTERN SHELF, WESTERN AUSTRALIA)
GEORGE R. DIX,1 NOEL P. JAMES, 2 T. KURTIS KYSER, 2 YVONNE BONE,3 AND LINDSAY B. COLLINS4
1
Department of Earth Sciences, Carleton University, and Ottawa Carleton Geoscience Centre, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada
e-mail: gdix@ccs.carleton.ca
2 Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canada
3 Geology and Geophysics (School of Earth and Environmental Sciences), University of Adelaide, Adelaide, South Australia 5005, Australia
4 Department of Applied Geology, Curtin University, Perth, Western Australia 6102, Australia
ABSTRACT: Accumulation of carbonate mud is minor on the Northwest Shelf (Western Australia), a broad (, 200–300 km wide), distally
steepened, tropical carbonate ramp. It is negligible along most of the
inner ramp (0 to 50 m water depth) owing to siliciclastic (riverine)
input. It is also negligible along the mid (from 50 to 120 m) ramp, and
where the entire ramp narrows to , 100 km, owing to constant reworking by storm waves and ocean swell. In these areas, carbonate
mud is derived mostly from mechanical degradation of skeletal fragments, but this does not include calcified remains of calcareous green
algae (e.g., Halimeda) although these plants grow locally. Only along
the outer (120 to 200 m water depth) ramp and slope is there any
relatively abundant accumulation of carbonate mud. Here the sediment
consists of modern pelagic (foraminifer, pteropod, and nannoplantkon)
ooze and aragonitic needle-rich (, 2 mm) micrite, the latter having
an age (based on AMS 14C) of ; 19 ka. Distribution of aragonite
micrite defines an extensive (5.0 3 104 km2), yet relatively narrow (,
40 km), tract running ; 600 km along the platform margin. Crystal
morphology, mineralogy, and isotope (C, O) composition identify the
micrite to be a seawater precipitate, its production and seaward export
having occurred during the Last Glacial Maximum, when lowered sea
level transformed the otherwise broad platform into a narrow epicontinental shelf margin, now the modern distal slope. Today, this sediment body lies stranded in deep water, about 200–300 km seaward of
the shallow inner ramp. The volume of micrite produced appears equal
to the present-day production potential of the Great Bahama Bank.
Rapid postglacial rise in sea level, and change in platform geometry,
climate, and oceanography, were very influential factors, in addition
to water depth and area of submergence, in the production and distribution of aragonite micrite along the Northwest Shelf. In general:
(1) epicontinental carbonate platforms can lack substantial micrite accumulation when oceanic current and wave energies sweep unimpeded
across a platform, and coastal clastics and absence of shoal barriers
prevent development of protective lagoons and carbonate tidal flats;
(2) not all highstand carbonate systems are associated with significant
amounts of micrite in shallow water environments, nor do they necessarily export large volumes of micrite to the periplatform realm; and
(3) some lowstand systems can produce and export volumes of carbonate mud that rival highstand systems.
INTRODUCTION
Micrite or clay-size carbonate is an important depositional constituent of
most modern and ancient tropical, shallow-water (0 to 20 m) carbonate
platforms (Bathurst 1975; Wilson 1975). Dynamic facies models illustrate
inner shelf or ramp mud as an important facies in each system (lowstand,
transgressive, and highstand) tract associated with a sea-level cycle (Sarg
1988; Schlager 1991; Burchette and Wright 1992; Handford and Loucks
1993; Read 1995). The origins of this carbonate mud include skeletal production and precipitation from seawater (Bathurst 1975; Shinn et al. 1989;
Macintyre and Reid 1992; Robbins and Blackwelder 1992; Milliman et al.
1993). These sources are usually highly sensitive to environmental controls
(Bathurst 1975; Robbins et al. 1997). As such, temporal variation in abundance of shelf-derived mud along carbonate slopes is a critical proxy for
platform submergence (Schlager et al. 1994).
Late Quaternary sedimentation along the Northwest Shelf, Western Australia (Fig. 1), illustrates a more complicated potential for micrite production; that is, there is a less direct response between production/accumulation
potential and sea-level change. Along this platform margin, abundant shallow-water carbonate mud is associated with recent glacial lowstand conditions, not the modern highstand setting. Influential factors prior to and
during postglacial (, 18 ka) rise in sea level include rate of sea-level rise,
oceanography, climate, and platform geometry and size.
SETTING
The area examined in this study is that part of the Northwest Shelf,
Australia, seaward of a broad continental embayment between Dampier and
Broome (Figs. 1, 2). This extensive carbonate platform (6.4 3 105 km 2)
is a tropical, distally steepened, oceanic ramp (Read 1985; James et al.
2004).
Bathymetry.—The ramp has a gentle (0.058) seaward gradient extending
to a distal slope positioned ; 200 to 300 km offshore in water depths of
about 200 m (Fig. 2). The continental embayment is expressed mostly by
middle-ramp bathymetry: a drowned paleoshoreline lies along the base of
an escarpment of ; 30 m relief in water depths of ; 125 m, immediately
seaward of the 100 meter isobath (Fig. 2). The escarpment is the result of
intertidal erosion during the sea-level lowstand (2125 m) at the end of the
Last Glacial Maximum (; 19 ka; Yokoyama et al. 2001). The orientation
of the 100 meter isobath suggests that the paleoshoreline bordered four
shallow-water embayments (I–IV; Fig. 2) along the inner paleoplatform.
Oceanography and Climate.—The Northwest Shelf is a very high-energy environment because of cyclones (Lourensz 1981), storm and strong
fair-weather waves (Jones 1973; James et al. 2004), and strong tidal currents. There is a northward increase in tidal range from microtidal (, 2
Copyright q 2005, SEPM (Society for Sedimentary Geology) 1527-1404/05/075-665/$03.00
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FIG. 1.—Oblique view (satellite image,
looking southwest) of the nearly 1000-km-long
coastline bordering Northwest Shelf (Western
Australia), southwest of Broome (B). The coastal
promontory of the De Grey River delta (D) has
visible plumes of terrigenous mud along its
seaward margin (arrows), and lies northwest of
the Dampier Archipelago (DA) and Exmouth
Gulf (E). Photo courtesy of NASA: http://
www.eol.jsc.nasa.gov/sseop.
m) southwest of Barrow Island to extreme macrotidal (101 m) at Broome
(Holloway 1983).
Data on seawater salinity and temperature collected during fieldwork in
late winter (James et al. 2004) indicate that water temperatures in the upper
100 m were ; 268C across most of the ramp, and 27–288C in the near
(, 20 km) coastal area. Salinity was ; 34–35‰ in surface waters to water
depths of 150 m across most of the ramp, and slightly elevated (35–36‰)
in coastal waters. The salinity at water depths between 150 and 250 m was
similar to that of coastal waters. For summer months, satellite images (CSIRO, unpublished data) indicate sea surface temperatures of . 358C in some
coastal areas, and ; 28–30oC across the ramp. Evaporation is intense, ;
3200 mm/y, and rainfall and river flow are mostly limited to summer
months but are torrential when associated with cyclones (Commonwealth
Bureau of Meteorology 1972).
Onshore Terrestrial Environment.—The adjacent continental landscape has low relief (Fig. 1). Late Quaternary shore sediments are predominantly siliciclastic, with a rocky, hilly terrain in the Dampier region. Eighty
Mile Beach is an extensive beach–dune system with back-beach marshlands
along the seaward margin of the Great Sandy Desert (Fig. 3A; Wyrwoll et
al. 1986). The De Grey River delta is a subdued sandy promontory that
marks the northern limit of major ephemeral rivers in the region (Fig. 3A).
Ramp Surface Sand-Size Sediments.—The seaward boundaries of the
inner and middle ramps are in water depths of about 50 m and 120 m (Fig.
2), which approximate fair-weather and storm wave bases, respectively
(James et al. 2004). These boundaries divide the ramp according to regional
patterns of sediment texture (Fig. 3A).
Inner-ramp sands contain mixtures of modern and relict skeletal fragments, as well as lithoclasts (laterite, Pleistocene bedrock). Fragments of
calcareous green algae (e.g., Halimeda) are not found in the sediments.
Coral fragments are most abundant southwest of Dampier, where patch
reefs and fringing reefs form a veneer on Pleistocene bedrock. South of
Barrow Island, near the coast, subtidal shoals consist of skeletal and lithoclastic sands and gravels (Dix 1989).
Middle-ramp sands define a regionally uniform high-energy environment, with strong reworking of seafloor sediment by waves and tidal cur-
rents. Sediments contain a similar population of skeletal fragments as
above, with an increase (30–80%) in relict grains (mainly foraminifers),
intraclasts of Holocene age, and nonskeletal grains (microbored ooids and
peloids). Fragments of stranded (early Holocene; James et al. 2004) Halimeda, coral, and gastropods occur at water depths between 40 and 70 m.
Dredging along the outer ramp, in water depths from 120 to 200 m,
reveals sand and mud fractions of Holocene pelagic (foraminifer, nannofossil, pteropod) carbonate. Dredge hauls also contained stranded carbonate
sand and mud fractions, including skeletal fragments of shallow-water benthic (bivalve, gastropod, bryozoan) biota, planktic foraminifers, and large
(, 4 cm 3 15 cm) calcareous tubes (James et al. 2004). A Holocene
sediment ridge (Fig. 2) of fine carbonate muddy sand, consisting of pteropods and planktic foraminifers, runs parallel to the 200 m isobath, and rises
up through water depths from ; 200 m to 140 m. The ridge likely reflects
near-surface productivity with startup of the modern Leeuwin Current, ;10
ka (James et al. 2004).
METHODOLOGY
Mud samples were collected during cruise FR04/99 (June–July) of CSIRO R.V. Franklin (James et al. 2004). Seafloor sampling involved use of
seafloor dredge and grab devices, such that the majority of samples are a
mixture of surface and near (, 10 cm) subsurface material. A total of 113
sample sites are used in this study (Fig. 2). Mud-size material was collected
by removing an aliquot (100 ml) of mud slurry during wet sieving of dredge
and grab samples on board the ship. In the laboratory, wet sieving of this
aliquot further removed any remaining sand fraction.
An aliquot of bulk sediment was removed for archive. The remaining
material was washed in distilled water, then, using a centrifuge, separated
into silt and clay size fractions. These separates were subdivided for the
following analyses. For secondary electron microscopy (SEM), a fraction
of the archived bulk sediment as well as clay and silt fractions were mixed
with distilled water, pipetted as a dilute slurry onto aluminum stubs, airdried, and Au–Pd coated. Secondary and back-scattered electron imaging
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FIG. 2.—The study area along the Northwest Shelf, Australia (see box in inset map), illustrating bathymetry (meters), ramp divisions, transects of sample sites (numbered
black dots), and general hinterland geology. The positions of a Holocene pelagic ridge (thick black line) and four paleoembayments at 19 ka are indicated (I–IV). Modified
from James et al. (2004). Inset map shows location of Bonaparte Gulf (B) referred to in the text.
used a JEOL 6400 SEM, set with a working distance of 15–17 mm, and
accelerating voltages of 15 and 20 kV.
Samples for stable (C, O) isotope analysis and X-ray diffractometry
(XRD) were mixed with dilute hydrogen peroxide to remove organic material, then washed in distilled water and dried. For XRD analysis (Cu Ka),
powdered samples were scanned from 4 to 64 degrees 2u to identify peak
types, then stepwise analysis of individual carbonate peak areas (from 25
to 32 degrees 2u) for weight percent calculation (Milliman 1974). A standard error of 1 5% was determined for multiple repeats. The process for
carbon and oxygen isotope analysis followed McCrea (1950). Samples were
run on a MAT 252 at Queen’s University, and values are reported relative
to V-PDB. Three bulk samples of the clay fraction, clean of organic detritus, were sent to Isotrace Laboratories (Toronto) for AMS 14C age-date
determination. Results were calibrated to calendar years using Bard et al.’s
(1998) curves, and subtracting 400 years for the ocean reservoir effect.
RESULTS
Distribution and Composition of Mud
Regional Distribution.—Carbonate mud is a significant part of surficial
sediments only along the outer ramp, seaward of the 120 m isobath. In this
region, modern and late Pleistocene sediment fractions (see below) are
present. Moving landward across the middle-ramp and inner-ramp areas,
the abundance of siliciclastics increases (Fig. 3A). Carbonate or siliciclastic
mud is negligible along the middle ramp and across the entire ramp southwest of Dampier, where the platform narrows to , 100 km (Fig. 2). In
this latter region, mud-size sediment is preferentially trapped in the lee of
reefs and rock platforms protected from strong tidal currents. Between
Dampier and Broome, mud of mixed carbonate and terrigenous origin occurs along the shoreline (Fig. 4A). From about Dampier to east of the De
Grey River delta (Fig. 2), the mud fraction is predominantly terrigenous
with abundant clay-size sediment (Fig. 3B). This is qualitatively reflected
by an increased reddening of bulk sediment samples moving toward the
shoreline, especially about the De Grey delta (Fig. 3A). There are small
plumes of reddish mud-laden water around this delta (Fig. 1); in coastal
waters south of Barrow Island, similar plumes migrate up and down the
coast daily because of strong tidal currents (Dix 1989).
Silt-Size Sediment.—On the inner ramp, silt-size sediment consists of
30% carbonate and 70% siliciclastic sediment. Benthic skeletal fragments
include mostly bivalves, gastropods, sponge spicules, foraminifers, and
bryozoans. Diatoms are present locally, and coral sediment occurs southwest of Dampier. The ubiquity of planktic foraminifers illustrates the lack
of an oceanic barrier along the ramp. The siliciclastic fraction of inner ramp
mud consists of quartz, pyroxene, amphibole, feldspar, and rock fragments,
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recognized by XRD and SEM, all of which possess a variable Fe-oxide
coating that gives these sediments a reddish color (Fig. 3A). On the outer
ramp and upper slope, the silt-size fraction of Holocene mud contains fragmented and whole pelagic foraminifers and pteropods.
Clay-Size Sediment.—Where the siliciclastic fraction is most abundant
along the inner ramp from about Dampier to east of the De Grey River
delta (Fig. 3B), the clay-size fraction consists mostly of illite, but kaolinite
is also present. Clay-size carbonate consists of rare to locally common
nannofossils, and trace amounts of carbonate needles. There is submicron
(200–600 nm) detritus of indeterminate carbonate and siliciclastic origins
present in the higher energy ramp facies southwest of Dampier and near
Broome.
At middle-ramp sites, sparse clay-size sediment consists of illite and
kaolinite, highly abraded and broken coccoliths, and carbonate needles (Fig.
4B). The abundance of needles increases (, 40%) inboard of the outerramp boundary. This is of particular note in areas where the sand facies
contains ooid and peloid facies (Fig. 3B), left stranded in deep water owing
to sea level rise (James et al. 2004).
Along the outer ramp, the clay-size fraction of Holocene mud consists
mostly of nannofossil fragments. However, older sediment (see below) is
dominated (. 60–80%) by carbonate needles (Fig. 4C), skeletal carbonate
fragments of uncertain origin, and indeterminate carbonate particles (,
10%) submicrometer in size (Fig. 4C, right). The few sample sites located
farther seaward along the distal slope, in water depths of 200 to 500 m,
indicate that needle abundance decreases seaward to trace amounts. There,
clay-size sediment contains diatoms, nannoplankton (which become dominant seaward of 500 m), carbonate fragments of uncertain origin, and
locally abundant illite.
Carbonate Needles.—The distribution of needle-rich (. 40%) mud
along the outer ramp defines a large area, ; 5.0 3 104 km 2, but a narrow
tract (; 40 km) running 6001 km along the ramp margin (Fig. 3B).
Needle abundance is greatly reduced (, 10%) where the ramp narrows
southwest of Dampier, and needles are admixed with other material (Fig.
3B).
Two types of needles are recognized. The most abundant type consists
of small (; 1 mm long) acicular crystals, several hundred nanometers in
width (Fig. 4C, left). The crystals are doubly terminated; some have
smooth, rounded surfaces whereas others are platy and more euhedral. This
texture is similar to crystals in cortices of stranded aragonitic ooids and
peloids (Fig. 4D) distributed along the middle ramp (Fig. 3B; James et al.
2004). Nuclei of some ooids are lithoclasts that define a crystalline texture
similar to needle-rich mud and peloids (Fig. 4E). The second needle type
is larger (. 2 mm long), also acicular, but with squarish cross section (Fig.
4C, middle). These crystals are rare, and are found locally along the middle
ramp.
Mineralogy
The regional distribution of clay-size aragonite is patchily developed
(Fig. 5A; site-specific values are presented in Table 1). In contrast, variation
in abundance of high-magnesium calcite, which constitutes 20–40% of inner ramp mud, reveals a regional landward increase in abundance (Fig.
5B).
Along the inner ramp and middle ramp, locally elevated abundances of
aragonite appear to occur in areas where there is a greater abundance of
sand-size molluscan and coral detritus. This suggests that clay-size aragonite may be derived locally from reworking of these skeletal particles.
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Along the outer ramp, a narrow (, 50 km) area of elevated (. 40%)
aragonite percentages (Fig. 5A) overlaps the region of needle-rich mud
(Fig. 3B). Needle-rich muds along the outer ramp contain more aragonite
and less magnesian calcite than skeletal-bearing sediment along the inner
ramp (Fig. 5A, inset).
Carbon and Oxygen Isotope Variation
Carbon and oxygen isotope values from clay-size bulk samples decrease
landward across the ramp (Fig. 6; site-specific values are presented in Table
1). Each dataset defines a narrow (20–40 km) region along the outer ramp
in which values are most elevated (d13C 5 2–3‰; d18O 5 . 0‰). This
outer-ramp region is also underlain by mud with the highest percentages
of aragonite and needle-rich mud (Figs. 3B, 5A). Because the samples are
mixtures of various carbonate sources, a regression line (r 2 5 0.75)
through a plot of d13Ccarbonate versus aragonite (Fig. 7) provides an estimate
of the d13C value for a sample of 100% aragonite. This value, ; 4‰ (Fig.
7), occurs in the range (3.5 to 4.5‰) for stranded aragonite ooids in the
middle ramp (James et al. 2004).
14
C (AMS) Age Dates
Two bulk samples (NW-79, NW-48; Table 2) of needle-rich (. 60%)
mud were chosen from the outer ramp, at present-day water depths of ;
200 m (Fig. 8). The determined ages for both are similar, ; 18 ky 14C yr,
equivalent to ; 19 ka (Table 2), and demonstrably older (by ; 5 ky) than
ooids stranded in water depths of ; 120 m (Fig. 8; James et al. 2004).
DISCUSSION
Late Quaternary micrite production and its accumulation on the Northwest Shelf identifies substantial differences between the character of the
lowstand (19 ka) and highstand (modern) carbonate factory. The differences
are not those expected from a simple association with changing water depth
and platform submergence. Instead, they reveal interactions among platform geometry, climate, and oceanography during postglacial rise in sea
level.
Micrite Production, 19 ka
At the end of the Last Glacial Maximum, the position of sea level along
the northwest Australian continental margin was about 125 m lower than
that of today (Yokoyama et al. 2001). A prominent escarpment along the
middle ramp seaward of the present 100-m isobath (Fig. 1) likely approximates the paleoshoreline. The paleogeography and platform geometry
along this part of the Australian continent was much different than found
today (Fig. 9A): most of what is now the modern ramp lay exposed; there
was a relatively narrow (5–10 km) shallow-water shelf with associated
broader paleoembayments (Fig. 9), and a relatively steep paleoslope was
present, now occupied by the distal slope (; 200 to 500 m; Fig. 2).
The age of needle-rich micrite identifies that significant carbonate production occurred toward the end of the glacial maximum. The texture (Fig.
4) and mineralogy (Fig. 5A) of the micrite suggests that the needles are
most likely aragonite, as is also supported by the estimated d13C value for
a sample of 100% aragonite (Fig. 7). The landward decrease in oxygen and
carbon isotope signatures of this mud (Fig. 6) reflects a likely mixing trend
with increased abundance of skeletal sources.
The more abundant and smaller (1 mm) type of aragonite needle is tex-
←
FIG. 3.—A) Distribution of general colors of mud across the platform, and location of prominent accumulation of carbonate mud along the outer ramp (light gray) and
terrigenous mud along the inner ramp (dark gray). B) Distribution of prominent carbonate components of mud-size sediment shown relative to distribution of stranded
ooids and peloids in the accompanying sand-size sediment. Bathymetric contours in meters.
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turally similar to those known or interpreted to have precipitated directly
from shallow tropical seawater (Macintyre and Reid 1992; Robbins and
Blackwelder 1992; Milliman et al. 1993). The needles are also texturally
similar to aragonite crystals of ooids and peloids found today along the
middle ramp of the Northwest Shelf (Fig. 4C, D, E). Likewise, the texture
is also similar to modern but active ooids from other Holocene platforms
(Tucker and Wright 1990). The estimated d13C value for micrite consisting
entirely of needles (Fig. 7) not only is similar to the composition of the
ooids from the middle-ramp and outer-ramp regions (Fig. 4) but also falls
within the field for Bahaman ‘‘whitings’’ (Bathurst 1975; Shinn et al.
1989).
The rare and larger acicular crystals with squarish cross sections (Fig.
4C, middle) are also likely aragonite. They have a texture similar to that
of crystals contained within the skeletal structure of codiacean algae (Wray
1977; Macintyre and Reid 1992). On the Northwest Shelf, these crystals
are abundant only in the middle ramp, where mechanical reworking of now
stranded Halimeda fragments may have produced clay-size sediment.
In summary, the present area of the platform margin underlain by needlerich micrite defines a narrow (5–10 km) shallow-water paleoshelf adjacent
to a steep slope, the former once positioned within the optimum zone (,
20 m) of shallow-water carbonate production (James 1979). Glacioeustatic
lowering of sea level to 2125 m by 19 ka had transformed a once distal
slope into this shallow-water setting. Yet, despite lowstand conditions, the
area available for shallow-water (, 20 m) production of micrite remained
significant, ; 2.2 3 104 km 2, because of the vast size of the Northwest
Shelf. The area (; 5.0 x 104 km 2) of the platform underlain by needlerich micrite extends for at least 600 km along this paleomargin (Fig. 9A).
The depositional framework for micrite production appears to have been
similar to that of many other flat-topped platforms or shelves (Schlager et
al. 1994), but defines lowstand shedding of carbonate mud in contrast to
negligible production of similar sediment in the inner ramp under modern
highstand conditions.
Sea-Level Rise and Oceanography
The late Quaternary sedimentary record of nearby Bonaparte Gulf (inset
Fig. 2), identifies a rapid (; 15 m) postglacial rise in sea level in the first
500 years (Yokoyama et al. 2001). The similar ages of the 19 ka micrite
and termination of the glacial maximum suggests that the observed landward decrease in micrite abundance across the Northwest Shelf reflects
either reworking of this source during early transgression or a continued
but diminishing production level with rise in sea level. Textural similarity
between the aragonite mud and intraclastic nuclei of some ooids or peloidcrystal textures (Fig. 4C, D, E) supports the role of sediment reworking.
One might discount an association between diminishing sediment production and concurrent increase in area of platform submergence, given that
it is at odds with evidence from other carbonate platforms for elevated
seaward export of shelf-derived sediment during late Quaternary transgression (e.g., Droxler et al. 1983; Boardman et al. 1986; Schlager et al. 1994;
Dunbar and Dickens 2003). Yet, this is what occurred on the Northwest
Shelf.
Over the first few thousand years of transgression (; 18 ka to ; 13
ka), the rate of sea-level rise remained high along this continental margin,
about 5 m kyr21 (Yokoyama et al. 2001). Ooid shoals and minor Halimeda
sediment along the middle ramp (Fig. 9; James et al. 2004) demonstrate
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that late Pleistocene–early Holocene shallow-water carbonate production
was active. Yet these deposits were left stranded in relatively deep water,
with production clearly insufficient to keep pace with sea-level rise. Dominguez et al. (1988) considered a lesser rate (4 m kyr21) to be sufficient
for what they called ‘‘incipient drowning,’’ or suppression of carbonate
production, during the early Holocene along the small Cat Island platform
margin in the Bahamas. A similar response appears possible for immense
platforms.
Factors other than just water depth were influential along the Northwest
Shelf. There were changes in oceanographic and climatic conditions, both
of which worked in tandem with sea-level rise to strongly influence carbonate production. First, under lowstand conditions, the absence of significant input of siliciclastics along the 19 ka paleoshelf, when compared to
its abundance today, appears to support a cooler, drier climate (with less
fluvial input) along the northwest Australian continent (COHMAP 1988).
With greater aridity, there was likely an increased amount of evaporation
of seawater along the paleomargin; this may have influenced salinity and
seawater precipitation of micrite along the coast (Bathurst 1975; Shinn et
al. 1989; Robbins and Blackwelder 1992; Milliman et al. 1993). Unlike
today, the Leeuwin Current was not active at this time (James et al. 2004).
With postglacial sea-level rise, initial rapid submergence quickly transformed the narrow paleoshelf into a distally steepened ramp. For the period
12 to 6 ka, climate simulations have proposed a strengthening of summer
rains for this region, a trend somewhat reversed after 6 ka (COHMAP
1988). By about 12 ka, therefore, there may have been increased fluvial
input (and siliciclastic input) coincident with sea-level rise. Furthermore,
the Leeuwin Current became active soon after that time (James et al. 2004).
The timing of these climatic and oceanographic factors coincide with suppression of shallow-water carbonate production that created the ooid, peloid, and calcareous algal sediments now stranded in deep water along the
middle ramp. The apparent absence of any significant algal biomineralization along the inner ramp today (Dix 1989; James et al. 2004) is striking
in comparison to calcification of early Holocene forms along the Northwest
Shelf (James et al. 2004).
Today, carbonate production is prominent only along the inner ramp
southwest of Dampier, where coarse-grained sediments form a sedimentary
veneer on coral reef platforms (Dix 1989). Oceanographic controls continue
to influence the potential for micrite production and its accumulation. For
example, without an oceanic barrier, strong tidal currents establish an effective cross-platform oceanic exchange of outer-ramp and inner-ramp waters. This greatly reduces residence time of waters on the platform. As
illustrated from the Great Bahama Bank, inorganic precipitation of aragonite (needle) micrite from whitings requires a relatively protected water
mass (e.g., leeward of Andros Island, Bahamas) allowing elevated salinities
to develop (; 38–401 %) through evaporation arising from long residence
times (Bathurst 1975; Shinn et al. 1989; Robbins and Blackwelder 1992;
Milliman et al. 1993). Today, the Northwest Shelf offers little potential for
this process. With intense evaporation during the summer months, the most
saline surface water likely is restricted to the coastal zone, which is overwhelmed by siliciclastic input. Tidal movement of reddish turbid water
laden with siliciclastic mud in this region (Dix 1989) must create a hostile
environment, greatly reducing the potential for development of whitings.
NASA satellite imagery (unpublished data) refer to ‘‘algal blooms’’ off the
De Grey River delta. Other than mud plumes (Fig. 1), these probably rep-
←
FIG. 4.—Secondary electron images (SEI) of mud and relict nonskeletal grains. All scale bars 5 1 micron. A) Typical composition of inner-ramp mud northeast of
Dampier, showing clay particles, lithoclast (a), and skeletal detritus. B) Increased abundance of relict aragonite needles from middle-ramp sediment, and submicron platy
detritus (a) of siliciclastic composition. C) Three images of aragonite needles from the outer ramp: (left) typical morphology showing smooth rounded to platy forms, both
with pointed terminations; (middle) rare platy acicular needles with squarish cross sections, and well defined crystal faces; (right) rounded, sub-micron carbonate detritus.
D) Cross section through ooid shows texture and fabric of aragonite crystals in laminae (a) and cortex (b), the dashed line defining the boundary. E) Cross section of
peloid shows texture and fabric of aragonite crystals.
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TABLE 1.—Mineral (%, with ; 5% std. error) and stable carbon and oxygen (‰,
6 0.1‰) values for sites (Fig. 1) used in this study.
NW02
NW03
NW04
NW05
NW06
NW07
NW08
NW08 (duplicate analysis)
NW09
NW10
NW11
NW12
NW13
NW14
NW16
NW17
NW18
NW19
NW21
NW22
NW23
NW24
NW25
NW26
NW27
NW28
NW29
NW30
NW31
NW32
NW33
NW34
NW35
NW36
NW37
NW38
NW39
NW40
NW41
NW42
NW43
NW45
NW46
NW48
NW49
NW50
NW51
NW53
NW54
NW56
NW57
NW58
ARAG
LMC
HMC
d13CPDB
d18OPDB
4
80
33
29
21
4
31
92
16
46
25
40
69
45
4
4
21
46
40
27
24
1.4
3.6
21.4
0.0
3
2
29
20
21
37
40
34
19
0
31
30
41
35
30
16
37
31
29
35
33
37
34
29
32
33
20
37
3
20
32
22
32
37
41
42
35
35
42
34
35
30
40
37
66
60
50
49
62
43
51
59
76
100
54
51
53
48
53
68
46
48
41
55
47
51
47
57
58
50
43
36
65
61
41
50
38
41
47
51
49
51
56
50
41
47
33
38
31
38
21
31
17
20
9
7
5
0
15
19
6
17
17
16
17
21
30
10
20
12
19
14
10
17
37
27
32
19
27
28
30
22
12
7
16
14
2
16
24
23
27
25
1.0
0.9
1.0
1.0
21.4
21.0
21.0
21.2
1.0
21.1
1.2
21.3
2.2
1.9
1.4
1.4
2.0
2.5
1.8
1.7
1.3
1.6
20.2
20.1
20.6
20.8
20.7
0.5
20.8
20.8
21.1
20.8
1.8
2.0
1.8
20.1
20.5
20.6
1.7
20.2
1.2
20.7
2.0
20.5
1.8
1.9
2.2
1.7
20.1
20.1
0.4
20.8
1.3
20.8
resent diatom productivity, a biologic constituent present in inner-ramp
sediments. Absence of obvious algal biomineralization along the inner ramp
may indicate a strong influence of prominent seasonal oceanographic gradients—e.g., large temperature fluctuations create seasonal differences in
the dominant non-calcareous benthic flora (Semeniuk 1993)—as well as
the longer term influence of the Leeuwin Current.
Significance for Carbonate Sequence Stratigraphy
Dynamic facies models for carbonate platforms require a distillation of
known variation in sedimentation and stratigraphic patterns. The Northwest
Shelf offers new breadth in understanding processes influential on carbonate production and mud distribution. First, epicontinental platforms (and
673
TABLE 1.—Continued.
NW59
NW60
NW62
NW63
NW64
NW64 (duplicate analysis)
NW65
NW66
NW67
NW70
NW71
NW72
NW73
NW74
NW75
NW76
NW77
NW78
NW79
NW81
NW82
NW83
NW83A
NW84
NW85
NW86
NW87
NW88
NW91
NW92
NW93
NW94
NW95
NW96
NW97
NW98
NW99A
NW99C
NW99D
NW101
NW101
NW103
NW104
NW105
NW106
NW107
NW108
NW109
NW110
NW112
NW113
ARAG
LMC
HMC
d13CPDB
d18OPDB
44
33
32
37
32
31
38
52
48
57
25
29
16
15
11
1.0
21.4
46
61
46
30
22
38
22
37
18
33
40
43
68
55
70
57
31
33
20
30
29
33
32
14
32
14
30
19
30
32
40
27
35
30
27
37
37
31
32
19
33
42
32
18
30
36
23
40
47
40
31
47
37
53
43
44
46
30
40
22
41
65
50
47
49
43
41
37
52
43
52
42
44
42
38
42
49
36
32
47
46
38
42
41
56
39
29
48
41
39
18
16
14
23
38
31
31
26
29
24
16
11
2
5
8
2
4
17
33
21
28
26
31
34
25
34
28
37
28
30
18
24
29
38
26
18
25
27
27
25
28
29
20
41
31
1.9
1.8
2.0
2.8
1.7
0.0
20.1
0.1
20.4
20.8
1.0
22.0
1.4
0.8
1.9
21.4
21.6
20.7
2.0
20.5
2.3
20.3
2.7
0.6
2.0
20.5
1.3
0.8
21.4
21.9
1.0
21.2
1.5
1.3
21.1
21.3
1.8
1.4
20.7
20.6
1.2
1.1
0.9
20.7
20.8
21.8
1.2
21.6
Abbreviations: ARAG, aragonite; LMC, low-magnesium calcite; , 4 mol% MgCO3; HMC, high Mgcalcite; . 4 mol% MgCO3.
ramps especially) can lack substantial micrite accumulation when ocean
current and wave energies sweep unimpeded across the platform, and when
coastal clastics and absence of shoal barriers prevent lagoons and carbonate
tidal flats from developing. Second, not all highstand carbonate systems
need be associated with significant amounts of micrite in shallow-water
environments, nor export large volumes of micrite to the adjacent periplatform (slope) realm. Third, according to platform geometry, some lowstand carbonate systems can produce large amounts of carbonate mud during lowered sea level, a volume seemingly on a par with that produced by
some highstand systems.
On the basis of a strong association between sea-level change, area of
platform submergence, and carbonate production potential, published facies
models generally predict an equal potential for production of mud along
←
FIG. 5.—Regional abundance and distribution of A) aragonite and B) high-magnesium calcite (HMC) percentages in clay-size carbonate sediment. The inset ternary plot
shows mineral distribution (A, aragonite; LMC, low-magnesian calcite; HMC, high-magnesian calcite) of inner ramp (dark pattern) versus outer ramp (lighter pattern), and
distinguishes sediments containing . 40% needles (white circles) or less (open circles). Bathymetric contours in meters.
674
G.R. DIX ET AL.
FIG. 6.—Regional distribution of A) d13CPDB and B) d18OPDB values in clay-size carbonate sediment. Bathymetric contours in meters.
JSR
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LATE QUATERNARY CARBONATE MUD, NORTHWEST SHELF
675
TABLE 2.—Radiocarbon dates* and stable isotopes** of mud, Northwest Shelf.
Sample
Location
Depth (m)
NW-48
NW-79
18807.79S, 118851.99W
19805.69S, 117803.99W
211
200
14
C age, Yr BP
Cal Yr
18,500 6 120
17,970 6 110
19,379
18,768
d13CPDB, d18OPDB,
‰
‰
2.35
3.72
0.06
0.93
* Isotrace Laboratories; ** Queen’s University.
FIG. 7.—Comparison of d13CPDB values and aragonite percentages in needle-rich
(. 40%) mud and skeletal-rich mud. Extrapolation of the distribution of needlerich mud to 100% aragonite shows a d13CPDB end-member composition within the
range of relict ooids from the middle ramp (from James et al. 2004).
an inner shelf or ramp within successive systems tracts of a complete sealevel cycle. The Northwest Shelf reveals that other factors have profound
influence on mud production and its accumulation.
By at least 19 ka, sea-level variation typical of icehouse conditions (Read
1995) moved a distally steepened slope into the optimum zone of tropical
carbonate production. Sampling suggests that the needle-rich mud likely
thins seaward, thereby defining a thin lowstand sedimentary wedge along
the paleomargin. The volume of this sediment body is similar to that forming Holocene highstand sedimentary wedges described from Bahamian
platform margins (Wilbur et al. 1990; Glaser and Droxler 1991). Indeed,
the timing for influx of large volumes of shelf-derived micrite along the
Northwest Shelf is reverse with respect to sea-level change to that defined
along Bahamian platforms (Droxler et al. 1983; Mullins 1983; Droxler and
Schlager 1985; Haak and Schlager 1989; Droxler et al. 1990; Schlager et
al. 1994).
The reason is that even during lowstand conditions the available area for
shallow water micrite production on the Australian paleoshelf remained
large, ; 5.0 3 104 km 2, up to ten times greater than the area associated
with the modern Great Bahama Bank (; 3.3 3 103 km 2; Robbins et al.
1997). If lowstand deposition at 19 ka was part of a longer (e.g., thirdorder) sea-level cycle, margin progradation associated with increased sedimentation along the slope could rival what is normally considered typical
only of highstand shedding (Schlager et al. 1994). Sarg (1988) documented
examples of lowstand ‘‘autochthonous wedges,’’ but these are generally
associated with significant shale or sandstone shelves. Along the Northwest
Shelf during the recent sea-level lowstand, a drier climate and a still vast
area for shallow-water production conditions ensured that a significant volume of aragonite mud could be generated with little terrigenous input.
For small sea-level falls along carbonate ramps, facies models (Burchette
and Wright 1992; Handford and Loucks 1993) propose that shallow-water
muddy facies keep pace with sea-level change such that these deposits
become stratigraphically connected through time. Such a linkage is broken
if factors other than sea-level change influence sedimentation production
(see above) or with large sea level variation as occurs during icehouse
periods (Burchette and Wright 1992; Read 1995). As shown by the late
Quaternary history of the Northwest Shelf, the lowstand carbonate mud is
now likely buried by a thin pelagic drape, and separated by some 200–300
km from modern (highstand) inner-ramp sediments. With oceanographic
factors reducing the potential for highstand micrite production or its accumulation, the lowstand carbonate sediments form a stranded (now, in
deep water) shallow-water deposit of regional stratigraphic extent.
The diagenetic potential of such a regional (600 km long) stratigraphic
body with elevated abundance of aragonite, now in deep water, is significant. Near-surface lithification and porosity development may occur in the
presence of marine-derived pore waters when compared to the lesser potential of the overlying pelagic carbonate (Schlager and James 1978; Dix
and Mullins 1988). This style of diagenesis, in combination with the scale
of deposit along a platform margin, offers a reservoir potential similar to
that of diagenetic traps of allochthonous deep-water deposits of ancient
shelves (e.g., Hobson et al. 1985; Enos 1985). These attributes are distinct
from those considered typical of ramps (Burchette and Wright 1992), yet
they might occur along ancient ramp margins during icehouse climates.
CONCLUSIONS
The late Quaternary history of shallow-water production of aragoniterich micrite and its accumulation on the Northwest Shelf demonstrates the
impact of changing oceanography, climate, and platform geometry and size
in relation to variation in sea-level and platform submergence. These influences engineered a reversal in the predicted timing of such production,
under lowstand (19 ka) rather than current highstand conditions.
Today, an insignificant amount of carbonate mud is accumulating across
most of this distally steepened ramp because of oceanographic factors:
siliciclastic input in the coastal region, high-energy waves and currents that
are unimpeded by coastal or shelfal barriers, and an absence of obvious
biomineralization by green calcareous algae. Along the outer ramp (in water depths of 1201 m), pelagic ooze, representing current highstand sedimentation, appears to overlie needle-rich (. 40%) aragonite mud that accumulated toward the end of the Last Glacial Maximum, and forms a regional (; 600 km long) stratigraphic body along the paleomargin. The
needles (texturally, compositionally) identify seawater precipitation with
minor import of sediment from mechanical degradation of calcareous algae.
A considerable volume of this sediment was produced under oceanographic
676
JSR
G.R. DIX ET AL.
FIG. 8.—Present bathymetric position, age (14C ky), and areas of needle-rich (. 40%) mud (dark gray area) and earliest age of ooids (light gray area) (from James et
al. 2004). The profiles (from James et al. 2004) show the transposed position of ooid sites along each transect. For bathymetric profiles A–A9 and B–B9: P, pelagic ridge
(shaded); E, escarpment. Bathymetric contours in meters.
and climatic conditions different from those found today, and the vast size
of the Northwest Shelf ensured a still significant shallow subtidal realm
even with lowered sea level. The aragonite-rich mud now lies stranded in
deep water, spatially divorced (; 200 km) from shallow-water inner ramp
facies, and possesses a higher diagenetic potential than the modern pelagic
ooze.
The late Quaternary history of the Northwest Shelf broadens current
understanding of the dynamics of carbonate mud production on ancient
platforms. Namely: not all highstand carbonate systems are associated with
significant amounts of micrite in shallow-water environments, nor do they
export large volumes of micrite to the periplatform realm, and some low-
stand systems can produce and export volumes of carbonate mud that rival
highstand systems.
ACKNOWLEDGEMENTS
This research was supported by the Natural Sciences and Engineering Research
Council of Canada (GRD, NPJ, TKK) and the Australian Research Council (YB,
LBC). We thank CSIRO (Division of Oceanography) and the captain and crew of
the R.V. Franklin for their help and support during the research cruise. Shipboard
assistance was provided by K. Glenn and C. Rancourt. We also thank journal reviewers R. Garber, A.E. Saller, and E. Shinn.
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LATE QUATERNARY CARBONATE MUD, NORTHWEST SHELF
677
FIG. 9.—Summary of events. A) A narrow paleoshelf developed seaward of 2125 m during the Last Glacial Maximum, and broadened locally into four paleoembayments
(I–IV). Seaward export (arrows) and area of micrite distribution (light gray) are indicated relative to four paleoembayments (dark gray). B) Earliest postglacial sea-level
rise initiated ooid and peloid growth, with possible continuation of micrite production and/or reworking of existing mud by strong currents and waves. C) Modern
conditions: a relatively narrow inner-ramp carbonate factory occurs only in areas not dominated by terrigenous input; the high-energy mid-ramp lies between the fairweather (FWB) and storm (SWB) wave bases, yielding negligible mud accumulation; pelagic ooze accumulates on the outer ramp and slope.
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