Radek Mikuláš, Jindřich Hladil
Radek Mikuláš, Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová
269, 16502 Praha 6, Czech Republic; e-mail: mikulas@gli.cas.cz
Jindřich Hladil, Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová
269, 16502 Praha 6, Czech Republic; e-mail: hladil@gli.cas.cz
Large trace fossils in nodular limestones (Devonian, Czech Republic): sedimentological
consequences, ethology and taphonomy
Running title: Trace fossils in nodular limestones
Abstract. Large star-like trace fossil was found on the upper bedding plane of nodular
limestone of the Praha Formation (Pragian, Devonian) at Praha. It was tentatively placed to
the ichnogenus Capodistria. The trace fossil partly intersects nodules that cover surfaces of
most bedding planes of the Praha Formation, demonstrating that the nodules formed during
the earliest stages of diagenesis.
In general, the size of individual marine invertebrate burrows rarely exceeds few decimetres,
except uninterrupted locomotion traces. Exceptions from the norm deserved a special study
(Bromley et al., 1975). Large size does not automatically mean that the trace fossils are
among the first recognized during field documentation: when working on small outcrops,
debris and drill cores, a complete specimen cannot be seen. A description of such a large trace
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from the Lower Devonian of the Barrandian area (Czech Republic) is the subject of the
present contribution.
Calciturbiditic, nodular packstone beds characterize the Praha Formation. These are
deposited in lower ramp and slope settings (e.g., Hladil et al., 1996; Hladil et al., 2010;
Koptíková, 2011). The specific environment of deposition and the mode of formation of the
nodules is still a matter of a debate in terms of the Barrandian (e.g., Fürsich, 1973; Kukal,
1975; Bednarczyk, 1991; Holcová, 2004; Flügel, 2010; Koptíková et al., 2010; Poul and
Melichar, 2010). This debate centres on the relative importance of bioturbation and/or early
diagenetic concretionary cementation prior to early burial pressure solution. The effects of
primary heterogeneity and self-organizing processes along with shear-stress are considered as
potential factors in the formation or enhancement of nodular limestone fabric. An assessment
of the relationships between the formation (and preservation) of a large trace and nodular
fabric of the limestone can, therefore, provide also an improved understanding of the
sedimentary and diagenetic processes acting in the Praha Formation.
Despite the long tradition of research, no complex ichnologic research has been done
for the facies; minor works include only the contributions by Wiessenbach (1930) and Prantl
(1943).
Geological settings
The Barrandian area lies in the central and western Bohemia, Czech Republic, and
extends in subsurface to eastern Bohemia. The Neoproterozoic and Lower Palaeozoic rocks
belong to the Teplá-Barrandian Unit (TBU) of the Bohemian Massif (Máška and Zoubek
1961) or alternatively, a similarly defined Bohemicum Unit (Malkovský 1979). The
uppermost units of the TBU (Ordovician to Lower Devonian) form a complex elongated
synform. The Lower Devonian limestone outcrops are mainly in the inner parts of the Prague
2
Basin (Havlíček, 1981; Mikuláš, 1999), alternatively known as the Prague Synform or
Synclinorium (Jaroš, 1984; Melichar, 2004). The Devonian occurrences in the Bohemian
Massif are described by Chlupáč (1993) and Chlupáč et al. (1998). The regional stratigraphy
has been documented and discussed recently by Cháb et al. (2010).
The well exposed limestones are rich in fosils, and are lying concordantly on the
Silurian; several stratotypes and parastratotypes (namely Silurian/Devonian boundary and
base of the Pragian Stage) were defined herein (Chlupáč et al., 1998). Most of the Lower
Devonian in the study area consists of open marine limestones with rare argillaceous and
submarine tuffitic interbeds. The stratigraphic succession and palaeontology is similar to that
of the Carnic Alps, Sardinia, and other peri-Gondwanan areas (e.g., the Ibero-Maghrebian
domain; Chlupáč et al., 1998; Plusquellec and Hladil, 2001; Slavík, 2004).
One of the prominent facies of the Praha Formation is the Dvorce – Prokop facies
(Chlupáč, 1962; Chlupáč et al., 1998), and is of Pragian–Emsian age (cf. Hladil et al., 2010;
Hladil et al. 2011). It consists of grey, rhythmically bedded packstones, mostly with knobbly
bedding surfaces. These nodular limestones are considered to have been deposited in
hemipelagic and calciturbiditic regimes with periods of sediment starvation and submarine
lithification evidenced by submarine corrosion/erosion marks (e.g., Hladil et al., 1996; Suchý
et al., 1996; Hladil and Kalvoda 1997; Koptíková et al., 2011). The nodular and wavy
limestone beds of the Dvorce sub-type are typically developed at the locality Branická skála,
geographically on the periphery of Prague, between the settlements of Dvorce and Braník (No
1 on Fig. 1). This site provided the stellate trace documented herein. The Loděnice facies of
the Praha Formation is composed of variegated platy limestones with rare to absent
nodulation and local concentrations of coarse-grained organic detritus. This facies is poorly
represented on the Cikánka site (No 2 on Fig. 1), where it also yielded a find of large stellate
trace fossil (Mikuláš, 1998).
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The localities
Branická skála (The Braník Rock; No 1 on Figure 1): Large abandoned quarry
between co-ordinates 50°2'26.12"N, 14°24'37.90"E and 50°2'34.68"N, 14°24'53.38"E, in the
southern part of the city of Prague. The quarry contains medium- to thick-bedded limestones
(packstones) of the Praha Formation (Pragian Stage). Neither the lower nor upper boundary of
the Praha Formation is exposed in the quarry, and the estimated thickness of the exposed
portion is 140 m (Kříž, 1999). On the opposite bank of the Vltava River, on better exposed
sites at distance of ca 1 km to NW, the whole thickness of the Praha Formation is 180 m.
The profile studied in detail (Fig. 2) belongs to the middle part of the Praha Formation
and is adjacent to a planar outcrop (50°2'26.30"N, 14°24'39.85"E) with strata association with
the horizon that yielded the star-like trace.
The Braník Rock is lithologically monotonous, consisting of a well-bedded greyish
packstone in (5)10-100 cm thick layers. Beds are nodular or undulose. The average/typical
size and shape of nodules are laterally uniform and characteristic of each bed; typical size of
nodules is 5-15 cm in length/diameter. Some limestone layers are separated by thin
intercalations of dark-grey calcareous shale with Chondrites isp. The limestones contain
rather rare but diverse macrofauna, including trilobites Odonotchile sp. and Phacops sp.,
bivalve Kralovna sp., minute brachiopods, columnals of crinoids, fragments of corals. During
the present fieldwork, probable hardgrounds are recognized by clusters of Trypanites isp.
(Fig. 2).
Cikánka (No 2 on Fig. 1; between 49°59'58.08"N, 14°19'17.52"E and 49°59'56.17"N,
14°19'45.88"E) is a quarry in operation on the SW margin of Prague. The quarry and its close
vicinity are important as a source of the Slivenec Marble that was used in numerous historical
buildings, chiefly in Prague (Chlupáč, 1993). The Praha Formation (Pragian) is there
4
composed mainly of the thick-bedded, coarse-grained, biodetritic to sparitic, pink Slivenec
Limestone (Chlupáč et al., 1998). The Slivenec facies is overlain by a thin layer of variegated
limestones with poorly developed nodules on bedding planes; these are similar to the facies of
the Loděnice Limestone (Chlupáč, 1998; Mikuláš, 1998) which is otherwise not developed at
Cikánka. In these layers, a giant star-like trace fossil similar to the find from the Braník Rock
was found and collected in the past but subsequently the specimen was lost (Mikuláš 1998).
These layers are overlain by reddish-brown, fine-grained, nodular limestones of the Řeporyje
Facies (4-5 m thick) and the remaining portion of the Praha Formation is composed of the
Dvorce-Prokop limestones as described on the previous locality.
Systematic ichnology
Ichnogenus Capodistria Vialov, 1968
Diagnosis: A stellate structure preserved in hypichnial semi-relief, composed of
relatively few narrow radial ridges and a central knob or knobs (modified after Uchman,
1995).
? Capodistria isp.
Figure 3, 4, 5
Material: Two specimens, one lost, the second one collected.
Description: Large ? Capodistria composed of approx. 10 radiating surface grooves
(convex hyporelief or concave epirelief). The rays are moderately bent such that the whole
structure is bilaterally symmetrical. The first specimen (the collected one, coming from the
5
Braník Rock) is a star-shaped structure, roughly circular in plan view. Its long axis is 60 cm,
the short one is 45 cm. The trace consists of 10 pronounced rays plus one poorly preserved
ray. Neighbouring rays diverge mostly at angle of 30°, in two cases 60°. Well preserved rays
are moderately bent, 18-45 cm long, and are 22-25 mm wide. The rays are preserved as
concave epireliefs, up to 10 mm deep. Bending of the rays varies from negligible to distinct;
the bending is systematic, so the whole structure is bilaterally symmetrical. Opposing rays
show equal curvature. An irregular, diagenetically deformed depression, oval in outline, ca 3
x 6 cm in size, occurs where the rays would intersect if extrapolated. The rays terminate
abruptly, the terminations are rounded and the depth of the ray usually decreases slightly near
the termination.
The trace fossil is preserved in nodular limestone (packstone). The nodules have
preferred orientation, and in some cases the rays intersect the nodules; hence, the biogenous
activity occurred after the appearance of the basic ground plan of nodules. The rays are partly
filled with softer calcareous mud without observable internal structure.
The second, lost specimen was described as ?Phoebichnus isp. (Mikuláš, 1998). It was
a star-like trace fossil, approx. 50 cm in diameter, consisting of 8 – 10 straight or moderately
bent radial rays, about 3 cm wide. The trace was preserved as a convex hyporelief. As in the
previously described specimen, the nodularity of limestone was partly influenced by the shape
of rays and vice versa. The fill of the rays was homogeneous and probably passive. The
description was recorded by R. Horný, I. Chlupáč and A. Galle (personal communications
1996-1997) and it was subsequently supplemented by I. Chlupáč (pers. comm. 2000).
Remarks and relations: The first (lost) specimen of the described ichnofossil was
determined by Mikuláš (1998) as ?Phoebichnus isp., basically because of similar size.
According to criteria complied by Bertling et al. (2006), however, size should not be
6
ordinarily used as ichnotaxobase; therefore, the argumentation is not admissible under the
present state of knowledge.
The find of another specimen at Branická skála allows full characterization of the trace
fossil. Most importantly, it is clear from that specimen that the rays originally formed as
surface grooves, not full-relief tunnels as originally presumed. We can therefore exclude the
affiliation of the trace to some radiating ichnogenera, namely: Phoebichnus Bromley and
Asgaard, Dactyloidites Hall, Stelloglyphus Vyalov, Asterichnus Bandel. Also the possible
resemblance of the trace to the “deep-tier” ichnogenus Skolichnus Uchman can be questioned.
Resemblance to Glockerichnus Pickerill is doubted by the simple character of the rays; in
Glockerichnus, the rays are bifurcated (Uchman, 1998).
Resemblance of the described trace fossil to Capodistria is questioned by the length
and shape of the rays. As stated by Uchman (1998), Capodistria is composed of “short,
simple hypichnial ridges”. It can be argued how far the adjective “short” refers to the size of
the trace fossil (which is generally not a valid ichnotaxobase; Bertling et al., 2006) and how
much it characterizes the width/length ratio. If we regard the second possibility, then the find
form the Braník Rock does not fit well to the diagnosis by Uchman (1998). As the clear
identifying with any of the existing ichnogenera is not possible and the erection of a new
ichnotaxon based of the very limited material is not advisable, the find is kept in the open
nomenclature.
Interpretation of the structure, substrate consistency, sedimentological consequences
The find of ? Capodistria isp. may provide a unique key for the relative dating of the
formation of nodularity in the limestones. The large and deep radial rays partly intersect
individual “half-relief” nodules of the carbonate, and some nodules partly intersect or deform
margins of the trace fossil, that are otherwise of constant width. It is, therefore, evident that
7
the biogenic activity was contemporaneous with the active formation of the undulatory
surfaces on the sea floor, not during deeper burial (e.g., Kukal, 1975; Bathurst, 1987) or
tectonic deformation (e.g., Gründel and Rösler, 1963; Poul and Melichar, 2010). According to
material from the Braník Rock, the late diagenetic processes (and those due to regional shear
and cleavage) modified the geometry of nodules very slightly, rather with a few degrees, by
tilting of nodules, or millimetre to centimetre lateral shifts of them. The branches of the large
stellate trace in the upper part of the bed remained almost unchanged with the further
development of nodular objects and provide an excellent piece of evidence about the earliest
lithification and nodule formation of a thin calciturbidite bed (and its very thin hemipelagite
coating). Nodule formation was during a period of sedimentary starvation, before the
sedimentation of the overlying bed, i.e. practically in contact with precipitation-dissolution
processes on the sea floor.
The oriented slabs of the Braník trace-fossil hosting bed show lamination and
imbrications of minute clasts that suggests that sediment intermittently was deposited from a
low to medium-density (hypopycnal) suspension. The majority of the bed is inferred to be
deposited, from a low-velocity settling of a high-density lime-mud suspension. In its lower
part, imbricated clasts of trilobite carapaces and cephalopod shells occur, but in the upper
part, an inclined, double-cone orientation pattern of dacryoconarid shells predominate (see
Hladil et al., 1996). A few isolated, large but low density bioclasts were rotated to subvertical
positions. The laminated (or gently rippled) turbidite to hemipelagite in the uppermost part of
the beds are poorly preserved. Infill of depressions in relief are a few millimetres thick.
Bioturbate structures in millimetric sizes are in present (Fig. 6).
To resist erosion by turbidite current, it is inferred that the surface was lithified to
hardground after creation of the soft-sediment trace fossil ? Capodistria. No similar borings
are known. Moreover, deformation of walls of trace fossils through continuing growth of
8
nodules would be excluded in the case of borings, which would rather cross-cut nodules.
When examined in further detail, the upper part of the relevant calciturbitite beds bear many
signs of sediment starvation and gradual surface hardening, e.g., local recrystallization of the
surface, truncation of grains, and both fissures and burrows filled by a different sediment than
either the surrounding or the overlying rocks have. In addition, some holes and fissures are
filled with iron-oxide rich sediment. Diagenesis enhanced nodular relief, but formation of
other additional patterns in sculpted rock surfaces was possible (cf. Jamtveit and Hammer,
2012). Similar carbonate hardgrounds are known on recent slopes, in the depths of several
hundreds to thousands of meters (e.g., Coniglio and Dix, 1992; Wilber and Neumann, 1993;
Messing, 2004; Schroeder, 2007; JAMSTEC, 2013).
In the Branická skála locality, hardground trace fossils Trypanites isp. were found in
two layers occurring few decimetres below the surface that yielded the find of ? Capodistria.
Trypanites colonization surfaces are much less undulose than other limestone bedding planes.
The details of hardgrounds have not been described so far either from the Branická skála
locality or from the Praha Formation, though indications of their occurrence exist (Mikuláš,
1993 – ichnology, Cikánka site; Hladil et al., 2010 – sedimentary petrology).
To summarize, both firmgrounds and hardgrounds (which variety was developed
depending probably on the time to be at the disposal for the process) were subsequently
exposed to slow mud sedimentation; calcareous mud filled preferably depressions in the
substrate, both those resulting from geochemical self-organized processes and biogenic
activity of the tracemaker. This is the present material of the trace fossil fill. Further
sedimentation event was most probably an episodic gravity (? turbidity) current that created
the next limestone bed; surface of the gravity-transported material then again became selforganized by precipitation processes (cf. Hammer, 2008) and colonized by benthos. The
9
development only occasionally attained the hardground consistency of the substrate as
evidenced by the low number of layers with Trypanites.
Ethologic sense of the stellate trace
In general, radial/stellate trace fossils pose, despite the coincidence of the basic ground
plan, are considered to have several quite different ethologic purposes (e.g., Häntzschel, 1970,
Seilacher, 2007, p. 136). Long-time known, well studied and relatively easy to interpret are
resting traces (cubichnia) of ophiuroids and asteroids, which reflect substantially body shape
of the tracemaker (the ichnogenus Asteriacites von Schlotheim). Radial traces adjacent to a
vertical structure (that, however, can be preserved incompletely or not preserved at all), can
be interpreted mostly as combined feeding-dwelling structures; e.g., an informal group of
asterosdomis as introduced by Seilacher (2007). Yet another important ethologic variant of a
star-like trace is the structure that resulted from a utilization of a superficial film of algae or a
detritus-rich lamina on the sediment surface (gyrophyllitids sensu Seilacher, 2007). Last but
not least, stellate forms can be found among typical “fucoids” (in-faunal feeding traces or
chemichnial structures, e.g., Phymatoderma; Fu 1991) and among graphoglyptids (Lorenzinia
da Gabelli, Sublorenzinia Książkiewicz).
Therefore, there is a considerably wide scope of possible analogues of ? Capodistria
isp. We can infer that it is a feeding trace comparable to gyrophyllitids (rather than a
cubichnion or domichnion), that are inferred to collect detritus trapped to the grooves/rays; 2,
trace of active surface deposit feeding/predation for in-faunal prey, analogously with the
assumption of Geister (2001) expressed for groove-like traces.
Conclusions
10
1) The large, star-shaped ichnofossil found on the upper bedding plane of nodular
limestone of the Praha Formation (Pragian, Devonian) can be best compared to the
ichnogenus Capodistria Vialov, 1968. An older find of a similar structure, tentatively placed
by Mikuláš (1998) to the ichnogenus Phoebichnus, is synonymised.
2) ? Capodistria isp. from the Praha Formation is probably a feeding trace; it could
function as a trap for detritus particles, or it is the record of active searching for in-faunal
prey.
3) Relatively large and deep radial rays of the ichnofossil partly intersect individual
semirelief carbonate nodules widespread on the bedding planes of the Praha Formation.
Conversely, some nodules intersect or deform margins of the otherwise constant-width
groove-like rays. It is, therefore, evident that the biogenic activity was contemporaneous with
the active formation of undulated surfaces, indicating that the nodules formed during the
earliest diagenesis.
Acknowledgement: Authors gratefully acknowledge the financial support provided by Czech
Science Foundation (GA14-18183S) and Academy of Sciences of the Czech Republic (RVO
67985831). The field and laboratory work resulting to the present study was supported by the
Research Program of the Institute of Geology, Academy of Sciences of the Czech Republic,
Prague (AV0Z 3013 0516).
11
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Explanation of figures
Fig. 1. Location map; localities: 1 – Branická skála (50°2'26.12"N, 14°24'37.90"E); 2 –
Cikánka (49°59'58.08"N, 14°19'17.52"E).
Fig. 2. Branická skála locality, geological profile at 50°2'26.184"N, 14°24'38.656"E, beds of
nodular packstone. Scale = 50 cm. G – type layer of Glockerichnus amissus isp. n.; T – layer
with the smooth upper surfaces in which numerous specimens of Trypanites isp,. were found.
Fig. 3. Glockerichnus amissus isp. n., holotype. Branická skála locality. A photograph of a
fallen block prior its collection. Scale bar = 30 cm.
Fig. 4. Glockerichnus amissus isp. n., holotype, the specimen after its collection. Scale bar =
30 cm.
Fig. 5. Glockerichnus amissus isp. n., holotype, an interpretative drawing done prior the
breaking and collection of the limestone slab.
Fig. 6. Thin-section images of a limestone bed at Branická skála; samples taken form the type
layer with ? Capodistria isp. (A) Vertical section showing a typical rock composition, i.e.
diversely-oriented, 100-200 micrometres long, lamellar or bent shell fragments floating in
mixture of fine- to coarse sized calcisiltite particles. The primary porosity of settled slurry bed
was low and lithification was achieved rather due to recrystallization than cementation. Clasts
of fine-sand size, or bigger, also occur, but these are hydrodynamically light (e.g., the crinoid
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brachial fragment in the lower left corner). (B) Other detailed view; vertical section; a
dacryoconarid shell is embedded in medium sized calcisiltite fraction, having a typical apexobliquely-upward position, showing a geopetal infill surface. A small bright object left of the
shell is a sponge spicule replaced by calcite. (C to F) Illustrations of more complex fabrics
found in a mostly monotonous bed deposited from a slow moving and spreading slurry-flow;
C - Vertical section across inhomogeneities which originated during settling of the bed and
soon after the consolidation; the oval section left of the centre is possibly a Chondrites
burrow; D - Horizontal section, structures in the upper third of the bed were modified by
bioturbation during the gradual stiffening of the substrate. Trichichnus? and diminutive
Rosselia? are possibly present here. E - Vertical section showing the stratification of the bed.
Relatively coarser sediment occurs at the base of the bed and contains imbricated shell
fragments of millimetre size together with a sub-vertically rotated crinoid pluricolumnal; the
fabrics can be related to deposition from a medium to high-density turbulent flows with
decreasing velocity. In upper part of the bed, more heterogeneous material was deposited;
admixture of very fine particles is in contrast with occurrence of up to centimetre sized
fragments of trilobite carapaces and cephalopod shells, often bearing microborings. The shell
fragments embedded in uppermost parts of the bed were often rotated to steep or sub-vertical
positions. F - Another vertical section shows a local reduction of the main, calcisiltite slurry
mass. The upper part of the bed containing the microbored shells is relatively thick and
apparently bioturbated. The uneven, possibly truncated-corroded surface of the bed was, after
a period of sedimentary starvation, covered by a few millimetres thick layers of
calciturbidites; however, this upper contact of the bed was also slightly modified by pressure
solution sutures and zones from shallow burial environments.
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