GeolCarp_Vol57_No2_79

GEOLOGICA CARPATHICA, APRIL 2006, 57, 2, 79—90

www.geologicacarpathica.sk

Ichnofabric and substrate consistency in Upper Turonian

carbonates of the Bohemian Cretaceous Basin (Czech Republic)

RADEK MIKULÁŠ

Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 135, 165 00 Praha 6, Czech Republic;

mikulas@gli.cas.cz

(Manuscript received March 1, 2005; accepted in revised form January 6, 2006)

Abstract: The basal bed of the Upper Turonian sediments at the Úpohlavy Quarry, corresponding to clayey limestone in its
character, bears obvious signs of rapid lithification from softground to firmground (Ophiomorpha—Thalassinoides—
Spongeliomorpha). Immediately before reaching the firmground stage, the substrate was colonized by tracemakers of
chemichnia (Chondrites). Biogenic reworking of the overlying, irregularly rhythmically bedded limestone/marlstone beds
does not fit the idea that the differences between the limestone and marlstone ichnofabrics are due to primary fluctuations in
oxygen content in water and in sediments. Documentation of a section of approximately 10—13 m above the base of the
succession revealed that bioturbation down to the depth of ca. 20 cm is more intensive in beds with higher calcium carbonate
content (Thalassinoides often passively filled with clay-richer substance and subsequently colonized by Chondrites tracemakers).
Beds with lower calcium carbonate content are usually dominated by Chondrites but Thalassinoides, Palaeophycus
and Phycodes are also present. These differences in ichnofabrics rather indicate a different succession of coloniza-
tion (marls – Chondrites followed by Thalassinoides; calcareous beds – Thalassinoides followed by Chondrites
filling the Thalassinoides burrows). Therefore, the consistency of the substrate and its increase through time after deposi-
tional events, and the content of primary organic matter widely used by the “homogenizers” of marly substrates were the
decisive factors. After a partial compaction, the substrate was utilized by less demanding r-strategic substrate feeders (tracemakers
of ichnogenus Phycosiphon) and lastly by chemosymbionts, which also made successful use of the trapping potential of
abandoned domichnia. This situation (i.e. high concentration of fluids in abandoned tunnels) is also evidence of a considerable
compaction and rapid diagenesis of limestones, allowing even surfaces of limestone beds to function as firmgrounds.

Key words: Cretaceous, Czech Republic, Ichnofossils, shallow-marine settings, firmground, softground.

Introduction

The active quarry at Úpohlavy located 70 km NW of the
centre of Prague is one of the most significant exposures of
Upper Cretaceous sediments in the northwestern Bohemian
Cretaceous Basin. The mining area includes extensive areal
exposures suitable for collection of macrofauna as well as
fresh and easily accessible vertical sections. This makes the
quarry area a favourable excursion (Čech et al. 1996) and
collection site, a subject of sequence-stratigraphic analysis
(Svobodová et al. 2002; Laurin, pers. comm. 2003) and an
important finding place of unique fauna (plesiosaurid teeth;
Ekrt et al. 2001).

Bioturbation structures (effect of sediment mixing due to

biogenic activity) and ichnofossils (individualized biogen-
ic structures) are very common. All aspects of the fossil
record formed by bioturbation and bioerosion in all scales
(i.e. ichnofabric) are directly related to the ongoing se-
quence stratigraphic studies (Laurin, pers. comm. 2003) as
well as to the paleontological and paleobiological research
(e.g. Ekrt et al. 2001). The knowledge of this topic is frag-
mentary; however, lists of ichnotaxa in the Coprolite Beds
and the overlying beds together with elementary spatial re-
lations and descriptions of complex ichnofabrics in marl-
stone beds were provided by Laurin (1996). The study of
ichnofabrics should respect the succession of substrate col-
onization, thus avoiding the principal pitfalls of the

“Seilacherian” ichnofacies analysis; the problem of ichno-
fabric interpretation lies, however, in the fact that most ich-
notaxa can be hardly determined from vertical sections (cf.
Uchman & Mikuláš 2002). The gap in the understanding of
the locality thus lies in the absence of “classic” systematic
ichnological studies, which would not only improve the in-
terpretation of ichnofabrics but also serve as material for
comprehensive paleobiological studies. The significance of
the Úpohlavy locality is given by the number of specific
situations observed, suitable for fabrication of general
schemes. Moreover, these situations are framed by the com-
pleted or culminating sequence stratigraphic and paleozoo-
logical studies. The aim of the paper is to display the
interaction between bottom colonization and substrate con-
sistency, which can be considered a relevant contribution to
the integration of the ichnofacies studies based primarily on
substrate quality (cf. Buatois et al. 1998) and ichnofabric
study based on the succession of the individual episodes of
bioturbation and bioerosion (Bromley 1996).

Description of the locality

The Úpohlavy Quarry displays Upper Turonian sedi-

ments (lower part of the Teplice Formation) in a thickness
of ca. 25 m. The lowermost beds are exposed primarily in a
system of drainage ditches and retention pools. They are

MIKULÁŠ

formed by dark grey marlstones with scarce fossils. The up-
permost part of this member contains two highly fossiliferous
beds with increased calcium carbonate contents. These beds
can be petrologically characterized as limestones with a high
admixture of clay and silt, and frequent primary and di-
agenetic inhomogeneities (phosphorite-rich coprolites;
ichnofabric-controlled concretionary structures). They
are traditionally termed the Lower and Upper Coprolite
Beds. Higher above, clayey limestones close to classic
“opukas” alternate with light grey to dark grey marlstones;
both these lithotypes are exploited for cement production.
The first 8 meters above the Upper Coprolite Bed are strongly
dominated by limestones. In the following 8 m, the two litho-
types alternate rhythmically, with limestone beds from 0.4 to
2 m thick and marlstone beds from 0.2 to 0.8 m thick. The top
part of the preserved sedimentary succession (approx. 5 m,
see Fig. 6.1) is dominated by marlstones (4 beds, 0.5—1.3 m in
thickness) prevailing over limestones (4 beds, 0.3, 0.2, 0.4
and 0.8 m thick, respectively) (Čech et al. 1996; Ekrt et al.
2001; Svobodová et al. 2002; Laurin pers. comm. 2003).

A review of previous ichnological research

Laurin (1996) provided a survey of ichnological data from

the section and summarized them in the paper of Čech et al.
(1996). In the Lower Coprolite Bed, he recognized macrobor-
ings of clionid sponges within thick prismatic layers of inoc-
eramid bivalve shells; phosphate particles are bored, for
example by Gastrochaenolites isp. The lower contact of the
bed is densely penetrated by a monotypic omission surface
suite of burrows (Thalassinoides  ?suevicus). Infills of the bur-
rows are rich in skeletal as well as non-skeletal coarse-grained
detritus. In the interval between the coprolite beds, relics of
primary stratification are recognizable; Chondrites,  Plano-
lites,  Trichichnus and ?Helminthopsis constitute a low-abun-
dance trace fossil assemblage. Laurin (1996) also noted
complex ichnofabrics with generally moderate to high ich-
nodiversities in the succession of rhythmically bedded cal-
careous mudstones and limestones; the most common
ichnotaxa include Thalassinoides,  Planolites,  Chondrites,
Palaeophycus, ?Asterosoma and Taenidium. A channel in-
cised into slightly deformed units described above bears fre-
quent skeletal particles at its base, some of them having
traces of macro- and microbioerosion; at the base, an omis-
sion suite of burrows is well developed. The channel fill,
formed mostly by calcareous mudstone to wackestone, re-
vealed sparse ichnofabric dominated by Anconichnus  hori-
zontalis, ?Muensteria isp., and Bergaueria isp. It should be
noted that the “ichnogenus” Muensteria is not recommended
for further use by Mikuláš & Uchman (1996) for the hetero-
geneity of the type material, and that Laurin (1996) designat-
ed as ?Muensteria isp. most probably Taenidium-like traces.

Systematic ichnology

The ichnotaxa described below are grouped into infor-

mal groups; the groups are ordered according to the fre-

Fig. 1. A sketch map of the Czech Republic showing the extent of the
Bohemian Cretaceous Basin (area bordered by the continuous line)
and the location of the Úpohlavy Quarry (marked by an asterisk).

Fig. 2. Lithofacies of the Upper Turonian in the Úpohlavy Quarry
(after Koš ák et al. 2003).

ICHNOFABRIC CONSISTENCY IN UPPER TURONIAN CARBONATES (CZECH REPUBLIC)

quency of their representatives and importance for the in-
terpretation of the ichnofabrics.

The depicted material is, at present, housed in the In-

stitute of Geology, AS CR Prague. It will be subsequent-
ly delivered to the National Museum in Prague. A few
specimens are in private collections as indicated in the
figure captions.

Branched structures related to the Ophiomorpha group

Spongeliomorpha de Saporta, 1887

Spongeliomorpha isp.

Fig. 5.1,4,8

M a t e r i a l :

Ten fillings of simple tunnels or segments of

branched systems, either collected or documented in situ
from the Upper Coprolite Bed.

D e s c r i p t i o n :

Horizontal to inclined, cylindrical tun-

nels 14 to 20 mm in diameter, locally branching, with no
lining of walls and sharp, thin ridges and grooves on walls.
These sculptures are usually subparallel and meet at very
acute angles. They are roughly parallel (Fig. 5.1), diagonal
to almost perpendicular (Fig. 5.8) to the tunnel axes.

R e m a r k s :

A factually and historically complicated

discussion is being continued on the validity or, more
exactly, synonymy of ichnogenera Spongeliomorpha de
Saporta, 1887, Thalassinoides Ehrenberg, 1944 and
Ophiomorpha Lundgren, 1891. Schlirf (2000) advocated
the synonymy of all the above mentioned genera point-
ing out that transitional forms are often found, and mor-
phological differences depend exclusively on the
character of the tunnel wall (Spongeliomorpha: ridges
and grooves after exploitation of firmground mud; Ophi-
omorpha: knobby lining in loose substrates; Thalassi-
noides: lined or unlined tunnels in softgrounds). Most
reactions to this proposal are, however, reserved, as in
most cases the tracemaker (usually whole populations of
decapods) created tunnels in sediments of more or less
uniform consistency. Regardless of the ichnotaxonomic
interpretation, however, the study of these traces gives a
good chance to track down the changes in substrate con-
sistency in time and space. For example, Fig. 5.8 shows a
thin cylinder of Spongeliomorpha isp. adjoining to a
larger cylinder of Thalassinoides isp. The succession of
trace fossils well documents the rapidly increasing hard-
ness of the substrate.

Fig. 3. Ichnological, sedimentation/erosion and hardening story of the Upper Coprolite Bed reconstructed according to the exposure in
the SW part of the Úpohlavy Quarry. Overall thickness of the figured sediment is approximately 50 cm; not to the scale. 1 – coloniza-
tion of marlstones by the tracemaker of Chondrites; 2 – deposition of calcareous siltstone with bioclasts; mixing the upper layer; origin
of lined burrows of the ichnogenus Ophiomorpha; 3 – erosion of the mixed layer and appearance of firmground; colonization window
functioned for bioclasts (Entobia) as well as for the firmground (Thalassinoides); 4 – deposition of clayey limestone with bored pieces
of wood; mixing its uppermost part; origin of lined burrows of the ichnogenus Ophiomorpha; 5 – erosion of the mixed layer; the sub-
strate remains soft; origin of softground/shiftground burrows Ophiomorpha/Thalassinoides; 6 – compaction and early diagenesis of the
substrate; re-elaborating the existing burrows in the firmground (appearance of Spongeliomorpha isp.).

MIKULÁŠ

Ophiomorpha Lundgren, 1891

Ophiomorpha isp.

Fig. 5.2,3

M a t e r i a l :

Six tunnel fillings or segments of branched

systems, collected or documented in situ from the Upper
Coprolite Bed.

D e s c r i p t i o n :

Fragments of fillings of simple vertical

shafts or systems of vertical shafts and horizontal tunnels
having circular cross-sections 18—20 mm in diameter.
Walls lined by oval granules 1—2 mm in size, produced by
agglutination of the ambient substrate.

R e m a r k s :

See remarks to ichnogenus Spongeliomorpha.

Thalassinoides Ehrenberg, 1944

Thalassinoides isp.

Figs. 5.5,6,8,10; 6.2,4,5,7; 7.1,4,5,7—9.

M a t e r i a l :

Tens to hundreds of tunnels and tunnel sys-

tems, collected or documented in situ or in blocks of rock on
spoil tips in all units and lithotypes of the Úpohlavy Quarry.

D e s c r i p t i o n :

Horizontal or vertical, more rarely in-

clined, filled tunnels and shafts forming commonly
branched systems. Tunnel diameters are not constant within

the whole systems in the Upper Coprolite Bed; some tun-
nels have diameters several times larger than the others
(Fig. 5.5—6; usually 1.5—4 cm in diameter) and often
broaden in the proximity of branching. Tunnels are
smooth and unlined. Their fill is homogeneous, struc-
tureless; the tunnels may differ from the ambient rock in
their petrological composition (higher carbonate content,
higher degree of lithification, diagenetic effects – forma-
tion of concretions). Transitions to ichnogenera Ophio-
morpha (remnants after pelletal imprints in walls) or
Spongeliomorpha (scratches) are common. Vertical range
of some systems exceeds 50 cm. The system was probably
functioning for a number of generations of tracemakers
and maybe even hosted different biotaxa at different
times. It was rebuilt many times, as evidenced by the
crossing of tunnels bearing typical features of all three
participating ichnotaxa.

In the limestones and marlstones of the lower part of

the Teplice Formation, traces classified as Thalassi-
noides are present with different frequencies as densely
branched systems including horizontal tunnels (ca.
80 %) and inclined or vertical shafts (ca. 20 %). Tunnel
diameters are usually constant in the whole system
(roughly 1—1.5 cm), exceptionally larger near the branching.

Fig. 4. Ichnological, sedimentation/erosion and hardening history of the rhythmically alternating beds ca. 10—13 m above the Upper Co-
prolite Bed. Reconstructed on the basis of exposures in the SW part of the Úpohlavy Quarry. Overall thickness of the figured sediment is
approximately 90 cm; not to the scale. 1—3 – colonization of marlstone; 4 – truncation; 5 – deposition of silty limestone; 6 – coloniza-
tion of resulting firmground; 7 – deposition of clayey layer and passive filling of the open burrows; 8 – re-colonization of the filled bur-
rows and inner space of nautiloid shells; 9 – origin of Trichichnus isp.

MIKULÁŠ

Phycodes Richter, 1850

cf. Phycodes isp.

Fig. 6.6

M a t e r i a l :

The only find from the bed of light grey

marlstone some 10 m above the Upper Coprolite Bed.

D e s c r i p t i o n :

Horizontal system of smooth tunnel

fillings having oval cross-sections, preserved in full relief.
The specimen is composed of the main tunnel (“runway”
sensu Uchman & Mikuláš 2002) and three tunnels branch-
ing in a fan-like manner. All tunnels are lined by trough-
like structures (spreiten); different magnitudes of mutual
displacement of the “troughs” and their different numbers
in each tunnel result in different widths of the individual
branches: 9 mm; 12 mm; 22 mm.

R e m a r k s :

In its general construction plan, the de-

scribed trace fossil equals the ichnogenus Phycodes (see,
e.g. Fillion & Pickerill 1990); it cannot be, however, ex-
cluded – due to the uniqueness of the find – that it is a
morphologically abnormal segment of Thalassinoides.

Borings

Teredolites Leymerie, 1842

Teredolites clavatus Leymerie, 1842

Fig. 5.9

M a t e r i a l :

A singular find of coalified wood with bor-

ings, found in clayey limestones some 2 m above the Up-
per Coprolite Bed.

D e s c r i p t i o n :

Borings in wood, teardrop to spherical

in shape, 3—6 mm in maximum diameter.

R e m a r k s :

Borings of bivalves which serve dwelling

purposes (i.e. not traces after feeding on wood substrate).
For ethological and ichnotaxonomic classification see
Kelly & Bromley (1984).

Teredolites longissimus Kelly et Bromley, 1984

M a t e r i a l :

A singular find of coalified wood with a

boring in clayey limestones some 2 m above the Upper
Coprolite Bed.

D e s c r i p t i o n :

Borings in wood, elongate, cylindrical in

shape, ca. 5 mm in maximum diameter, with calcite linings.

R e m a r k s :

For classification see Kelly & Bromley

(1984).

Entobia Bronn, 1837

Entobia cf. cretacea (Portlock, 1843)

M a t e r i a l :

Two finds of shell fragments of bivalve Inoc-

eramus sp. with the described trace fossil from the Upper
Coprolite Bed.

D e s c r i p t i o n :

Filled chambers of roughly oval, egg-

shaped or irregular nodular shape, with smooth surface,
interconnected with all neighbouring chambers by thin
tunnels. The whole trace fossil thus forms a network devel-
oped several mm below the surface of the shell frag-
ment. The chambers are 3—6 mm in diameter, the tunnels

ca. 0.3 mm in diameter; the removed material represents
approx. 50 % of the original mass of the shell.

R e m a r k s :

Clionid sponges were mostly bored by

tracemakers of the ichnogenus Entobia. Entobia isp. is
very frequent approximately from the mid-Mesozoic at all
places where hard carbonate substrates in marine settings
(rocky coasts, lithoclasts or bioclasts) are exposed to high
physical-energy conditions for at least the life cycle of the
tracemakers (i.e. months and years on average); deep- and
quiet-water representatives have also been recorded but
they are not so diverse and frequent (cf. Bromley 1994).

Spreiten-structures

Phycosiphon Fischer-Ooster, 1858

Phycosiphon incertum Fischer-Ooster, 1858

Fig. 7.6

M a t e r i a l :

Several finds in a block of light grey marl-

stone from a broken bed several meters above the Upper
Coprolite Bed.

D e s c r i p t i o n :

Flat lamellae of darker reworked materi-

al indistinctly laminated due to biogenic activity (spreite),
bounded by meandering lines which may represent poorly
preserved marginal tunnels. Width of lobes 5—10 mm.

R e m a r k s :

Traces of complex feeding strategy, com-

prising sediment feeding and maybe also its modification
(loosening) permitting repeated effective feeding on the
same portions (cf. Wetzel & Bromley 1994).

Simple structures

Palaeophycus Hall, 1847

Palaeophycus tubularis Hall, 1847

Fig. 7.3

M a t e r i a l :

Approximately 10 specimens collected or

documented in situ in a limestone bed ca. 16 m above
the Upper Coprolite Bed and in limestone clasts on spoil
banks of the quarry.

D e s c r i p t i o n :

Straight or slightly curved horizontal

tunnels with a markedly smooth surface and a distinct
lining. The tunnels are 3—4 mm in diameter, the traced
segments are several cm to several tens of cm long.

R e m a r k s :

For taxonomy of Palaeophycus see Pem-

berton & Frey (1982). Palaeophycus is usually interpreted
as dwelling burrows of predators.

Planolites Nicholson, 1873

Planolites beverleyensis (Billings, 1862)

Figs. 5.7 (partim), 7.2, 7.7 (partim)

M a t e r i a l :

Several tens of observations in situ or on

spoil banks, or collected specimens from the Upper Copro-
lite Bed and from different beds of light grey to dark grey
marlstones of the Teplice Formation.

D e s c r i p t i o n :

Straight or curved, horizontal or in-

clined, mostly unbranched, smooth, filled tunnels, with no
wall lining. The infill of the tunnels usually differs from

MIKULÁŠ

Branched structures of the Chondrites group

Pilichnus Uchman, 1999

Pilichnus dichotomus Uchman, 1999

Fig. 6.3

M a t e r i a l :

Several tens of observations in situ or on

spoil banks, or collected specimens from different beds of
light grey to dark grey marlstones of the Teplice Formation.

D e s c r i p t i o n :

A system of horizontal, straight or

slightly curved, irregularly branching thin tunnels, usually
ca. 1 mm wide, largely filled with material contrasting
with the ambient substrate by its darkness. Branching
takes place at angles close to 90° or at acute angles. Any
indications of concentric structure are missing.

R e m a r k s :

Trace fossils of similar morphology from

different (mostly pelitic formations) were described as
Chondrites isp. by many authors. Uchman (1999) revealed
the groundlessness of this interpretation and erected
ichnogenus Pilichnus, which was also identified by
Mikuláš (2003) in the Ordovician Šárka Formation of the
Barrandian. The existing evidence suggests that these are
“deep-tier” trace fossils, similarly like Chondrites, having
a high potential for preservation.

Chondrites Sternberg, 1833

Chondrites isp.

Figs. 5.8 (partim), 5.10 (partim), 6.4 (partim),

6.7 (partim), 7.4 (partim), 7.5 (partim), 7.7 (partim),

7.8 (partim), 7.9 (partim)

M a t e r i a l :

Hundreds observations in the field and

dozens of collected specimens.

D e s c r i p t i o n :

Systems of narrow, inclined or subhori-

zontal, radially branching tunnels having circular cross-sec-
tion, typically 1—2 mm in diameter, filling 1/5 to 1/20 of the
substrate volume. More completely preserved systems have
a fan-like appearance. The infill of the tunnels differs from
the ambient rock in its composition – it is usually darker.

R e m a r k s : Chondrites

is one of the most common ich-

nofossils in fine-grained marine sediments. Its tracemaker
was very abundant in conditions posing ecological stress
for the rest of the infauna (lack of oxygen near the bottom
and in the sediment, relatively deeply buried or already
partly lithified substrates; see, e.g. Wetzel & Uchman
2001). From the ethological point of view, it represents
probably most often “chemichnia”, structures developed
to absorb methane and hydrogen sulphide from the sedi-
ment (see, e.g. Mikuláš 1997). As a result, the tracemaker
of ichnogenus Chondrites was attracted by abandoned and
passively filled tunnels of ichnotaxon Thalassinoides isp.
In this case, once filled trace fossils were reworked repeat-
edly (see, e.g. Fig. 7.5). The limited space did not allow
the Chondrites tracemakers to build classic radial struc-
tures, which generated a major problem for ichnotaxono-
my (cf. Bertling et al. 2004). In such case, the analogy
with borings is respected here (isomorphic and xenomor-
phic borings, cf. Bromley & D’Alessandro 1987): even
highly bizarre forms (see Fig. 6.4) are attributed to ichno-

genus Chondrites because the tracemaker and the etholog-
ical purpose were probably identical with the “typical”
Chondrites. More information on aberrant Chondrites can
be found in the paper by Uchman & Wetzel (1999).

Trace fossils preserved on surfaces of internal moulds of
nautiloids

“Chondrites” isp.

Fig. 8.3,4

M a t e r i a l :

Three finds of the fossil Euthrepoceras sp.

with the described trace fossil from the limestones of the
Teplice Formation (coll. J. Valíček and J. Filous).

D e s c r i p t i o n :

Filled tunnels resembling overlapping

systems of the ichnogenus Chondrites, restricted exclusively
to the surface of internal moulds of the nautiloid Euthrepo-
ceras sp. They were probably formed in a soft fill of the
shell of the dead nautiloid before its diagenetic dissolution.

R e m a r k s :

See remarks to the ichnotaxon “Urohelm-

inthoida” isp.

“Cochlichnus” isp.

Fig. 8.6

M a t e r i a l :

A singular find of the fossil Euthrepoceras

sp. with the described trace fossil from limestones of the
Teplice Formation (coll. Mr. Filous).

D e s c r i p t i o n :

Meandering, thin, shallow groove resem-

bling the ichnogenus Cochlichnus, exclusively restricted to
the surface of internal mould of the nautiloid Euthrepo-
ceras sp. It probably originated in the soft fill of the shell
of a dead nautiloid before its diagenetic dissolution.

R e m a r k s :

See remarks to the ichnotaxon “Urohelm-

inthoida” isp.

“Megagrapton” isp.

Fig. 8.1,2,5

M a t e r i a l :

Three finds of the fossil Euthrepoceras sp.

with the described trace fossil from limestones of the Tep-
lice Formation (coll. J. Valíček and J. Filous).

D e s c r i p t i o n :

Systems of thin filled tunnels branch-

ing at angles close to 90° thus resembling the ichnogenus
Megagrapton, exclusively restricted to the surfaces of in-
ternal moulds of the nautiloid Euthrepoceras sp. They
probably originated in the soft fill of the shell of a dead
nautiloid before its diagenetic dissolution.

R e m a r k s :

See remarks to the ichnotaxon “Urohelm-

inthoida” isp.

“Urohelminthoida” isp.

Fig. 8.5

M a t e r i a l :

A singular find of the fossil Euthrepoceras

sp. with the described trace fossil from limestones of the
Teplice Formation (coll. J. Valíček).

D e s c r i p t i o n :

Systems of thin filled tunnels turning

at acute angles, with short extensions behind the turns

ICHNOFABRIC CONSISTENCY IN UPPER TURONIAN CARBONATES (CZECH REPUBLIC)

(Bromley 1996) represented by the ichnogenera Chondrites
or possibly Pilichnus.

Less frequent are ichnotaxa participating in direct sub-

strate-feeding (Phycosiphon, Planolites, Phycodes), which,
moreover, concentrate to specific horizons. Domichnia are
characteristic for both main lithotypes (limestones and
marlstones), while chemichnia are restricted almost exclu-
sively to marlstones with the exception of Coprolite Beds,
but also to marl-dominated infills of trace fossils in lime-
stones and to partly closed enclaves limited by nautiloid
shells. The last mentioned occurrence indicates that the
paucity of chemichnia in the limestones was not due to un-
favourable substrate consistency but to the lack of fluids
suitable for chemosymbiosis; these were, in contrast,
present in sufficient amounts in the cephalopod shells.
Transported fragments of wood substrates and lithoclasts
(shells of Inoceramus sp.) contain borings. Whereas borings
in wood were obviously transported from pelagic settings,
borings of clionid sponges in bioclasts of the Upper Copro-
lite Bed are probably autochthonous, corresponding to the
presumed shallowing and high physical energy of the envi-
ronment (Ekrt et al. 2001).

Intensity of bioturbation, character of ichnofabric

and environmental parameters

The intensity of substrate mixing in upper tiers of sedi-

ment is difficult to assess because primary sedimentary
structures in pelitic and carbonate rocks are generally poor-
ly visible. However, weakly re-bioturbated synsedimentary
structures were found in the Upper Coprolite Bed: the hy-
porelief of Planolites cf. beverleyensis is such a structure; it
can be, therefore, speculated, that substrate mixing was not
absolutely pervasive, or that the homogenized layer (cf.
Uchman 1999) did not reach too deep. The Coprolite Beds
show obvious signs of rapid hardening from softground
to firmground (Ophiomorpha—Thalassinoides—Spongeliomor-
pha), with substrate colonization by tracemakers of chemich-
nia immediately before reaching the firmground level
(Fig. 5.8). A more complicated problem from the viewpoint
of ichnofabrics is the alteration of limestone and marlstone
beds. The idea that the differences between limestone and
marlstone ichnofabrics were primarily controlled by fluctu-
ations in the oxygen content of the water and sediment is
questionable. As shown by the partial documentation of the
interval of 10—13 m above the Upper Coprolite Bed, more
intensive bioturbation in CaCO

-rich beds reaches to a

depth of ca. 20 cm (Thalassinoides often passively filled
with clay-richer substance and secondarily colonized by
Chondrites tracemakers). CaCO

-poor beds are usually

dominated by Chondrites, but Thalassinoides, Palaeophy-
cus and Phycodes are also present. Differences in ichnofab-
rics thus rather point to a different colonization succession
(marlstones – Chondrites followed by Thalassinoides;
CaCO

-rich beds – Thalassinoides followed by Chon-

drites in the infills of the Thalassinoides burrows). The con-
trolling factors therefore included substrate consistency and
its increase through time following depositional events, as

well as the content of primary organic matter widely used
by “homogenizers” of marly substrates, later – after a par-
tial compaction – by less demanding r-strategic substrate-
feeders (tracemakers of the ichnogenus Phycosiphon), and
in the last stage by chemosymbionts also successfully using
the trapping potential of abandoned domichnia. This situa-
tion of high fluid concentration in abandoned tunnels indi-
cates, among other things, a significant compaction of
limestones and early diagenesis; then, the surfaces of lime-
stone beds could have also functioned as firmgrounds.

As an analogy, the Volkhov sequence, as old as the Ear-

ly Ordovician, can be mentioned. Dronov et al. (2002) de-
scribed rich Thalassinoides—Chondrites ichnofabrics with
numerous varieties of cross-cutting relationships; borings
and epibionts document that the carbonate substrate was
capable of obtaining the hardground consistency.

On the other hand, Chondrites might even in this situa-

tion function as an indicator of oxygenation. It occurs
preferentially in marly substrates, including the marly fill-
ings of Thalassinoides tunnels in the limestone beds.
Marlstones are less porous than limestones and clay min-
erals diminish fluid exchange. As a result, trapping of flu-
ids for chemosymbiosis is easier in marly beds than in
limestones (cf. Kedzierski & Uchman 2001).

Ichnoassemblages attributable to firmgrounds in the up-

per part of the quarry show occasional signs of bioclast con-
centration. This condensation was probably not induced by
increased flow intensity but by the absence of sediment
(starvation s.s.). In these cases, the trace fossils are filled
with bioclasts (scales, plant material), probably biologically
sorted – transported inside by the activity of the tracemak-
er, as opposed to the passive infill typical, for example, of
the Upper Coprolite Bed (cf. J. Laurin, pers. comm. 2002).

Acknowledgments:

The study was as a comparative sub-

project of the grants No 205/06/0842, and No 205/04/0151
of the Grant Agency of the CR. I was encouraged to study
the ichnofossils at the locality by M. Koš ák (Faculty of Sci-
ence, Charles Univ., Praha), who provided me with valuable
field data, an introduction to the local stratigraphic and
zoopaleontological conditions, and financially support-
ed my field research from the grant project GAUK
167B/Geo/1998. M. Mazuch (Faculty of Science, Charles
Univ., Praha) contributed with field comments and techni-
cal assistance in the preparation of the drawings. I am also
indebted to J. Laurin (Geophysical Institute AS CR Praha)
for consultation, to P. Bosák and J. Adamovič (Institute of
Geology, AS CR, Praha) for conceptual, language and sedi-
mentological comments, to J. Vedral (Praha), J. Sklenář, J.
Valíček (Most), M. Alferi (Bílina), R. Vodrážka (Praha) and
J. Filous (Dlažkovice) for their help in field research and for
providing their finds for study.

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