Shallow-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 697

A proximal-to-distal trend is clearly recognizable in these storm deposits. The
most distal facies are closer to the shore than the red-colored tempestites of the
Zheltiaki Limestone. The individual units of the Frizy Limestone contain, in
specific colonization horizons, Thalassinoides (various patterns and sizes of
networks and boxworks), Palaeophycus (simple, rarely branched, lined
tunnels), Bergaueria ( Fig. 6 H), and rarely Gastrochaenolites . Inside the upper
unit of intercalation, a “patchy hardground”, composed of hardened Thalassi-
noides tunnels, is bioeroded with Trypanites . The top of the Koroba Bed is
marked by a prominent hardground surface pitted with Trypanites and is inter-
preted as the upper sequence boundary.

4.1.5 Sea-Level Fluctuations
The sediments of the Dapingian succession in the St. Petersburg region were
deposited on a cool-water carbonate ramp within a shallow-marine, storm-
dominated environment ( Dronov, 1998 ). These conditions reflect short-term
sea-level fluctuations where even minor changes in depth can cause an abrupt
shift of facies. About eight lithofacies can be discriminated based on their litho-
logy and ichnological content and can be arranged according to the relative
depth of their deposition on the carbonate ramp. A sea-level curve has been
reconstructed on the basis of the shift of these facies along the storm-dominated
ramp profile ( Dronov, 1999 ).
Major rises of relative sea level occurred at the following levels (with ref-
erence to the traditional bed nomenclature): (1) Krasnenky, (2) Butina, and
(3) Krasnota. All of these events are marked by the appearance of red-colored
deposits accumulated in the central part of the basin under relatively deep-
water conditions. Important relative sea-level drops occurred at the following
levels: (1) “Steklo” surface (base of Volkhov), (2) Butok, (3) Tolstenky, and
(4) Koroba. Overall, the sea-level curve is comparable to the one constructed
by Nielsen (2004) for the Komstad Limestone in Scania, except for the Middle
Volkhov interval, where the data from Russia support the interpretation of a
greater water depth than in the Lower Volkhov. As a consequence, the B II a

/

B II b boundary in the shallow-water model ( Dronov, 1999 ) is interpreted as a

deepening (transgressive) event, whereas the same boundary in the deep-
water model ( Nielsen, 2004 ) is interpreted as a shallowing (regressive) event.
This discrepancy is related to a different interpretation of facies and stratig-
raphy; further ichnological studies have the potential to assess the interpreta-
tion of these sea-level changes.

B. triadicus burrows/borings, Putilovo quarry. (F) Bedding plane within the Pereplet Bed with a
dense network of B. triadicus , Putilovo quarry. (G) Zheltenky Bed of the Zheltiaky unit with Chon-
drites isp., Putilovo quarry. (H) Lower bedding surface from the upper member of intercalations of
the Frizy unit with glauconitic Bergaueria isp. penetrating arcuate burrows cf. Phycodes , Putilovo
quarry.