Shallow-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 691

context, and early-diagenetic dolomitization indicate an origin in the lower
intertidal zone.
Important trace fossils of the Trypanites Ichnofacies are Caulostrepsis ,
Entobia , Gastrochaenolites , Gnathichnus , Maeandropolydora , Rogerella , and
Trypanites , as well as numerous microborings.
4. CASE STUDIES
4.1 Ordovician Cool-Water Carbonates:
St. Petersburg Region, Russia
During the Ordovician, the Baltic paleocontinent migrated from a subpolar to a
subequatorial position within the southern hemisphere ( Cocks and Torsvik,
2005 ). This is reflected in the succession of facies from predominantly silici-
clastics (sand and black shale) in the Tremadocian, through cool-water bioclas-
tic wackestones in the Floian to Sandbian, to tropical sabkha dolomites and
pelmicrites in the Katian to Hirnantian ( Dronov and Rozhnov, 2007 ). The Ordo-
vician carbonates of the St. Petersburg region (Floian to Katian) reach a thick-
ness of 20-160 m and belong to the relatively shallow-water part of the
epicontinental basin of Baltoscandia. The rocks are typically rich in bryozoans,
brachiopods, trilobites, ostracods, echinoderms, and conodonts. Shells of gas-
tropods, bivalves, cephalopods, and sponge spicules are locally abundant. In
this chapter, cool-water carbonates of Volkhovian (Dapingian) age are des-
cribed ( Fig. 2 ), which are best exposed and most extensively studied in natural
outcrops and quarries in the St. Petersburg region.
Basic ichnological features of these carbonates were described by Dronov
et al. (2002) and Dronov and Mikul´ˇ (2010) . In general, the ichnodiversity
of the Dapingian ramp is relatively low but due to the condensed character
of the section, the trace-fossil density can be very high at some levels. The main
ichnogenera are Balanoglossites , Bergaueria , Chondrites , Gastrochaenolites ,
Palaeophycus , Thalassinoides , and Trypanites . Major portions of the succes-
sion belong to the Cruziana Ichnofacies with numerous omission surfaces
demarcated by the Glossifungites and Trypanites ichnofacies.
The well-bedded Dapingian limestone has been extensively quarried for
building purposes since the foundation of the town of St. Petersburg in 1703
( Fig. 3 ). The basements and staircases of most buildings in the historical part
of the city are made from certain beds of the “Glauconite Limestone”. There
is an old tradition among the local quarrymen to give names to distinctive beds
and bedsets as well as to some bedding surfaces ( Fig. 2 ). Regional conformity of
this nomenclature reflects the lateral persistence of these lithological units.
Objectively, this informal terminology is related to some distinctive lithological
and ichnological features, such as hardness and homogeneity of the rock, mode
of intercalation, distribution of colors as well as intensity of bioturbation, and
specifically the ichnotaxa content of each bed. This informal terminology has