GEOLOGICA CARPATHICA, OCTOBER 2006, 57, 5, 415—422
www.geologicacarpathica.sk
Introduction
Trace fossils represent a specific type of geological “mem-
ory record” which possesses numerous applications, for
example sedimentological, paleobiological and strati-
graphic (cf. Frey 1975). The value of a particular assem-
blage of trace fossils for interpretations is very variable.
Whether or not a trace fossil assemblage can give answers
to questions of the geological history of the host sediment
also depends on completeness and punctuality of the field
documentation, and on the possibility of combining ich-
nological and non-ichnological knowledge.
Lower and Middle Ordovician platform sediments of
the St Petersburg region represent a classical area of pale-
ontological, stratigraphical and lithological studies (e.g.
Lamansky 1905); more detailed and complete ichnologi-
cal research, however, has only recently started. Besides
the studies directed to the ichnofabric of the lower Middle
Ordovician succession (Dronov et al. 2002), unique ichno-
logical structures were also discovered in the Lower Or-
dovician sediments during the studies focused on
sedimentological, stratigraphic and tectonic features of
the Baltic-Ladoga Glint, a several hundred of kilometer
long linear structure with numerous natural and artificial
outcrops (Fedorov & Ershova 2004; Ershova 2005). With
the present state of knowledge of the basin, the impor-
tance of these trace fossils are the following: 1 – they in-
dicate (better than other kinds of evidence) the degree of
hardening of the bottom; 2 – material filling the trace
fossils may contain a high percentage of phosphorite,
which is in contrast to the composition of surrounding
Trace fossils on and above the transgressive surface:
substrate consistency and phosphogenesis (Lower Ordovician,
St Petersburg region, Russia)
VICTORIA B. ERSHOVA
1
, PETER V. FEDOROV
1
and RADEK MIKULÁŠ
2
1
Department of Geology, St Petersburg State University, Universitetskaya Emb. 7/9, St Petersburg, Russia;
vika-ershova@yandex.ru; fedorov@gg2686.spb.edu
2
Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 269, 165 00 Praha 6, Czech Republic;
mikulas@gli.cas.cz
(Manuscript received November 25, 2005; accepted in revised form March 16, 2006)
Abstract: The basal layers of the Leetse Formation (Lower Ordovician, St Petersburg region, Russia) are characterized
ichnologically by: 1 – local occurrence of the ichnogenera Conichnus, Bergaueria and ?Thalassinoides at the base.
These traces originated by burrowing in consolidated substrate of the underlying black, clayey Dictyonema Shale and
were buried by the basal bed of the Leetse Formation; 2 – the layer with camerate burrows (Amphorichnus div. isp.),
filled with phosphatic substrates, ca 15 cm above the base. Considering the position of the localities of the phosphatized
burrows in the basin, we presume that burrows on tops of tectonically uplifted blocks were filled by cryptocrystalline
phosphorite matter. Rapid phosphate mineralization points to conditions of shallow burial, high concentration of
dissolved pore water phosphate, and suitable redox interval caused by decomposition of organic matter of burrow
dweller tissues.
Key words: Lower Ordovician, Russia, St Petersburg region, firmgrounds, platform sediments, trace fossils, phosphate.
rocks. Therefore, this circumstance opens the question of
the origin and distribution of phosphates in the basin.
The aim of the present paper is to describe the trace fos-
sil assemblages and to discuss the two above-mentioned
questions of bottom consistency and phosphatization.
Previous work and geological setting
Mainly on the basis of outcrop data, the Ordovician suc-
cession of Baltoscandia has been subdivided into ten major
depositional sequences (Dronov & Holmer 1999). From the
base to the top they are as follows: (1) Pakerort, (2) Latorp,
(3) Volkhov, (4) Kunda, (5) Tallinn, (6) Kegel, (7) Wesen-
berg, (8) Fjaka, (9) Jonstorp, and (10) Tommarp sequence.
The studied material comes from the Latorp sequence,
which corresponds in the studied area to the Leetse Forma-
tion. Its lower part is informally called Glauconitic Sand-
stone (Schmidt 1881) and consists of rather thin-bedded,
chiefly fine- to medium-grained quartz-glauconite sand-
stones, often with an admixture of clay and carbonate; up-
wards, they pass into sandy marls with clay intercalations.
The overall thickness of the Glauconitic Sandstone varies
from 30 to 185 cm in the St Petersburg region.
The underlying Pakerort sequence coincides with the
Pakerort Regional Stage. In the St Petersburg region, it
comprises shallow-water, cross-bedded quartz sands of the
Tosna Formation, overlain by relatively deep water black
graptolitic shales of the Kopor’e Formation. The sands and
sandstones, which are informally known as the Obolus
Sandstone, represent a lowstand system tract, while the
416
ERSHOVA, FEDOROV and MIKULÁŠ
black Dictyonema Shale is interpreted as a transgressive
system tract (Dronov & Holmer 1999).
Until recently, Lower and Middle Ordovician trace fos-
sils of the St Petersburg region have attracted little atten-
tion from researchers. Two types of borings represent a
notable exception: 1 – Trypanites-like borings first de-
scribed from the region by Vishnjakov & Hecker (1937),
and 2 – the so-called “amphora-like” borings (Orviku
1940; Männil 1968). The latter structure has been reported
from all over the Baltoscandia (Andersson 1896; Laman-
sky 1905; Vishnjakov & Hecker 1937; Orviku 1940,
1960; Hecker 1960; Jaanusson 1961; Lindstrom 1963,
1979). Recently, a similar structure was described as Gas-
trochaenolites oelandicus by Ekdale & Bromley (2001).
Except for these two types of borings, numerous other
trace fossils, especially Skolithos, Thalassinoides, Ber-
gaueria, and Chondrites attracted attention only recently
(Dronov et al. 2002; Mikuláš & Dronov 2005). For the
Glauconitic Sandstone itself, a preliminary account of its
trace fossil content was published by Fedorov & Ershova
(2004) and Ershova (2005).
Material and methods
The described material was collected on natural expo-
sures. Because of the horizontal position of the beds in the
studied area, the outcrops at the river banks (Tosna and
Sablinka rivers at Sablino) usually perfectly expose verti-
cal structures of the rock, and the horizontal aspect is not
easily observable. Vice versa, planar exposures on the val-
ley bottoms often show upper bedding planes of sand-
stone or limestone beds. This is the case at the locality at
the Syas River (30 km NW from the town of Volkhov,
Fig. 1), where the biogenic structures were found on the
lower bedding plane of the well exposed basal bed of the
Glauconitic Sandstone. Besides the documentation in situ,
about 10 rock samples from Syas and 150 specimens
(mostly fills of chambers of trace fossils) from Sablino (To-
sna and Sablinka Rivers; Fig. 1) were collected and
housed in the collections of the Department of Historical
Geology, St Petersburg State University. The laboratory
research also included the study of polished and thin sec-
tions and X-ray analyses of trace fossil fills and the host
rocks.
Synopsis of ichnotaxa
The ichnogenus Amphorichnus Männil, 1966, was
erected for large (several centimeters in diameter), regular,
“vase-like” chambers with extremities (i.e. “papillae”) at
their bases. It remains an incompletely understood ichno-
taxon as a modern revision of Amphorichnus does not ex-
ist yet. We conclude, in analogy to the treatment of similar
ichnotaxa, and especially Gastrochaenolites Leymerie,
Fig. 1. Location map of localities containing phosphorite-rich sediments near the base of the Glauconitic Sandstone. From Ershova (2005).
Abbreviations used: AR—PR – Archaean to Proterozoic; V – Vendian; E
1
– Lower Cambrian; E
2
– Middle Cambrian; E
3
– Upper
Cambrian; O
1
– Lower Ordovician; O
2
– Middle Ordovician; O
3
– Upper Ordovician; D
2
– Middle Devonian; D
3
– Upper Devonian;
C
– Carboniferous; Q – Quaternary. For detailed explanation of the geological map see Ershova (2005).
417
TRACE FOSSILS ON AND ABOVE THE TRANSGRESSIVE SURFACE (RUSSIA)
that the diagnosis of Amphorichnus should be broadened,
and should include all drop-like, bulbous or vase-like bur-
rows in soft substrates. Amphorichnus papillatus Männil,
1966 is so far the only ichnospecies attributed to the ich-
nogenus. Bulbous and “torpedo-like” forms have not been
given ichnospecific names yet. Traces attributable to Am-
phorichnus ispp. were found in the basal beds of the Leet-
se Formation in the valleys of Tosna and Sablinka Rivers
(Fedorov & Ershova 2004).
The ichnogenus Bergaueria Prantl, 1945 is represented
by shallow, hemispherical to cylindrical solitary burrows
(convex hyporeliefs or full reliefs) circular in section, per-
pendicular to bedding planes. Their diameter is mostly
10—20 mm; the ratio depth/diameter varies in most cases
from 0.5 to 2.0. The base of burrows is hemispherical, rare-
ly flat or conical. Its surface is smooth and a wall lining is
absent. Trace fossils of the above-described morphology
occur on the basal bedding plane of the Leetse Formation
in the Syas River Valley (Ershova 2005).
The ichnogenus Conichnus Männil, 1966 consists of
conical, deep holes (more often preserved as their fills in
lower bedding planes). The base of the cone is not sharp
but finger-shaped; the depth of the trace is 1.5 to 2 high-
er than its diameter; its wall unlined, sometimes bearing
an irregular radial ornament (modified after Pemberton et
al. 1988). Similar to Bergaueria, Conichnus occurs locally
at the base of the basal beds of the Leetse Formation in the
Syas River Valley. It probably represents the dwelling bur-
rows of anemones or similar organisms.
Thalassinoides
Ehrenberg, 1944 consists of three-di-
mensional burrow systems consisting predominantly of
smooth-walled cylindrical tunnels. They branch more or
less systematically; branchings are Y- to T-shaped. Tun-
nels may be enlarged at bifurcation points. Each system
usually has essentially a horizontal component (subsur-
face tunnel network) and vertical shafts joining the tun-
nels with the bottom surface. At the described sites, no
complete networks of Thalassinoides have been found.
Segments of tunnels connected to the chamber-like traces
at and above the base of the Leetse Formation, however,
can be attributed with some reservation to the ichnogenus.
The ichnogenus Gastrochaenolites Leymerie, 1842 is
one of the most frequent boring structures in the fossil
record. It consists of drop-like chambers of circular, ellipti-
cal, almond-shaped or nut-shaped cross-section; the cross-
section of the neck region may differ from that of the
lower part of the chamber. Well-known drop-like struc-
tures found in hardgrounds of the Volkhov sequence have
been placed in Gastrochaenolites by Ekdale & Bromley
(2001) under the name G. oelandicus. However, the situa-
tion is complicated both by the presumed variability of
substrates, and by the variability of the trace itself, which
are not only drop-like, but also spherical, pencil-like or
conical. In the Volkhov sequence, it is evident by cross-
cutting of large bioclasts that at least some of these struc-
tures are real borings into a hard substrate. In the basal
layers of the Leetse Formation, no such evidence was
found. However, the morphological similarity of Am-
phorichnus isp. (div. isp.) from the Latorp sequence and
Gastrochaenolites ex gr. oelandicus from the Volkhov se-
quences is notable.
Ichnofabrics
In the basal beds of the Leetse Formation in the Tosna
and Sablinka River Valleys, the burrows are up to 10 cm
long and 1—4 cm wide and are concentrated on the top of
the homogeneous bed of quartz sand, which are 8—18 cm
thick and directly overlie black shales (Dictyonema Shales)
of the Kopor’e Formation (Fig. 2). The burrows are usually
vertical, vase-like, heart-shaped, pumpkin-like or amphora-
like. Most of them occur solitarily but locally amalgam-
ation of 2—3 burrows can also be found. The apertures of the
trace fossils coincide with the top surface of the basal bed.
Fill of the burrows is hard, and the individual specimens
can be easily extracted from the host sand/sandstone. Petro-
Fig. 2. Schematic drawing of the geological profile at the right
bank of the Tosna River, ca. 200 m N of the Sablino Waterfall.
The layer rich in phosphatized burrows is marked by arrow.
418
ERSHOVA, FEDOROV and MIKULÁŠ
Fig. 3. A – Photograph of the lower part of the profile as shown in Fig. 2, Tosna River at Sablino; B – in situ preserved Amphorich-
nus isp. The bracket shows the stratigraphic position of the finding in the profile in Fig. 3A; C—L – Amphorichnus ispp. Phosphatized
burrows showing the variety of shapes. The same locality and layer as Fig. 3B.
419
TRACE FOSSILS ON AND ABOVE THE TRANSGRESSIVE SURFACE (RUSSIA)
Fig. 4. A—I – Amphorichnus ispp. Phosphatized burrows; the same locality and layer as Fig. 3B.
420
ERSHOVA, FEDOROV and MIKULÁŠ
graphically, the fill is represented by cryptocrystalline
phosphorite and quartz grains in variable proportion, but
phosphorite mostly prevails, having scattered quartz grains.
Only occasionally, nearly pure phosphorite, or vice versa,
sandstone with phosphorite cement, may form the burrow
fill. The central parts of the burrows are often broken by sy-
neresis cracks. Some of the burrows resemble the amphora-
shaped ichnospecies Amphorichnus papillatus Männil,
1966, as the characteristic “papilla” at the bottom of the
trace fossil can be recognized. Other burrows are best under-
stood as potentially new ichnospecies of Amphorichnus
(see the Synopsis of Ichnotaxa) (Figs. 3,4).
Basal beds of the Leetse Formation in the valley of the
Syas River preserve biogenic structures which extend
down into the consolidated substrate of the underlying
Dictyonema Shale. Here, a dense population consisting of
Conichnus, Bergaueria and Thalassinoides producers
made an untypical complex pattern on the basal surface of
the Glauconitic Sandstone (Fig. 5).
Substrate consistency
The Dictyonema Shale usually shows no discernible
biogenic sedimentary structures, in accordance with most
of the classical “black shales”. Only partial colonization
windows, which can usually be interpreted as short epi-
sodes of peturbation of anoxia or dysoxia in the bottom
waters can be distinguished occasionally; they are marked
by the ichnogenus Chondrites (cf. Mikuláš 1992 for an
example from the Lower Paleozoic). Thin tunnels of
Chondrites (usually few millimeters in diameter) could
have originated and be preserved most probably in a
somewhat consolidated substrate, not in “soupgrounds”
(this type of substrate consistency is presumed, e.g. for the
black Posidonia Shale of the Jurassic of Germany, cf. Mar-
till 1993). The trace fossils, preserved at the bottom of the
Glauconitic Sandstone at the Syas River, point to the even
more persistent substrate than of a usual softground: both
the conical and drop-like structures undoubtedly represent
domichnia, namely chambers used for a relatively long
Fig. 5. A – Conichnus isp., base of the Dictyonema Shale, Syas Riv-
er Valley. B – Conichnus isp. and Bergaueria isp., base of the Dic-
tyonema Shale, Syas River Valley. Scale bar s = 1 cm.
time. These were not lined, nonetheless they did not col-
lapse after the death of the tracemaker as there are evident-
ly several generations of burrows on the surface, and they
sustained even the erosion of the whole surface joined
with truncation of the upper parts.
Fig. 6. Proposed localization of transitory deposition of phosphorite on the tops of the uplifted blocks during the late Hunnebergian—
early Billingenian transgression. From Ershova (2005).
421
TRACE FOSSILS ON AND ABOVE THE TRANSGRESSIVE SURFACE (RUSSIA)
The presence of a firmground on the top of the unit of
the Dictyonema Shale has not been suggested previously.
The fact that only one locality provided this fossil record
is explained by the limited area of firmground develop-
ment, which might be influenced by the local tectonic and
the resulting sedimentological regime, through which the
bottom of a subsiding block was protected from current
erosion. Doubtless this substrate was inhabited by differ-
ent dwellers which loosened the host substrate. Reworked
firmground was easily removed by current erosion, and
only rare parts might not be eroded. Probably we investi-
gated the first of such a scrap at the Syas River.
The occurrence of firmgrounds in the Glauconitic Sand-
stone is not surprising in the context of the previous re-
search of the unit (Dronov et al. 2002), but it has not yet
been documented so straightforwardly as by the trace fos-
sils in the Sablino area.
Phosphogenesis
The phosphatic filling of the ichofossils in the lower lay-
ers of the Glauconitic Sandstone is not relevant to the ich-
notaxonomical treatment of these trace fossils. Therefore, it
is not pertinent for the assessment of the environment dur-
ing the colonization of the substrate. This, however, does
not restrain the importance of the fill composition to the
study of the sedimentation regime of the basin in the period
which followed immediately after the death of chamber pro-
ducers. The formation of phosphorite requires elevated
phosphate ion concentrations, which commonly result in
marine pore-water from the degradation of organic matter in
areas of upwelling (e.g. Compton et al. 2002).
According to our research at the base of the Leetse For-
mation, phosphate-rich rocks are known in three different
forms: 1 – phosphatic fillings of the trace fossils Am-
phorichnus ispp. at the Tosna and Sablinka River Valleys
SE of St Petersburg; 2 – phosphatized body fossils of
hexactinellid sponges at Izhora River SE of St Petersburg;
3 – “Dictyonema Shales” cemented with phosphorite in
the vicinity of Kingisepp town west of St Petersburg
(Kingisepp Quarries). The distribution of the phosphatized
rocks corresponds well with the position of tectonic
blocks recognized in the Baltic paleobasin, namely the
basin of the epeiric sea, which covered the St Petersburg
area during the Early Paleozoic (Fig. 6). Phosphatization
took place on the tops of tectonically uplifted blocks dur-
ing the transgression of the late Hunnebergian—early Bill-
ingenian; the proposed source of phosphate may have
been located in relatively deeper water over the subsiding
blocks. Phosphate-enriched water was brought on the top
of elevated blocks by local semi-permanent upwellings.
Phosphorus, probably assimilated by microorganisms, pre-
cipitated on the bottom surface. Dispersed organic matter
was decomposed. This process elevated phosphate ion
concentration in the porewater.
Relatively large dead bodies of burrow-dwellers could
provide a suitable substrate for phosphorite (apatite) precip-
itation from dissolved porewater phosphate, because of a
high redox interval. Alternatively, the burrows could be left
by the burrowers and infilled by porous, unconsolidated
sediment. After that, the phosphogensis started, being limit-
ed to the burrow fills (whereas the initial sediment was al-
ready cemented and less accessible to phosphogenesis).
Conclusions
1 – Sediments of the Dictyonema Shale forming the
bottom of the Ordovician sea of the St Petersburg region
before the late Hunnebergian—early Billingenian trans-
gression were locally consolidated in some places to form
a substrate which was close to a firmground. It is demon-
strated by the suite of trace fossils (Conichnus, Bergaue-
ria, Thalassinoides) found in the Syas River Valley;
2 – Glauconitic Sandstones (Leetse Formation; late
Hunnebergian to Billingenian) have had long coloniza-
tion windows on firm substrates, as proved by trace fossils
Amphorichnus ispp.;
3 – The burrows on the top of uplifted blocks were
filled by phosphatic substrate. The phosphate was proba-
bly delivered by semi-permanent currents from the bottom
of the troughs in the basin.
Acknowledgments: The research has been supported by
the Grant No. 205/04/0151 of the Grant Agency of the
Czech Republic and Grant No. UR 09.01.349 of the Uni-
versity of Russia Grant Agency. We thank the official re-
viewers, Karl Föllmi (Neuchâtel) and Alfred Uchman
(Kraków) for constructive and helpful comments.
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