Shallow-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 694

is interpreted as a highstand systems tract of the underlying Latorp sequence. The
upper part of theDikari Limestone represents a lowstand systems tract,whereas the
Zheltiaki Limestone and the Frizy Limestone represent transgressive and high-
stand systems tracts, respectively ( Dronov andHolmer, 1999; Dronov et al., 2002 ).
4.1.1 Highstand Systems Tract of the Underlying Latorp Sequence
The lowermost part of the Dikari Limestone is represented by four distinctive
beds ( Fig. 2 ). The succession of the beds demonstrates a shallowing-upward
trend and is interpreted as a highstand systems tract. On top of the Beloglaz
Bed (bed 4 in Fig. 2 ) rests the Zeliony Bed (bed 5) with a prominent flat hard-
ground surface covered by a thin glauconite veneer. It marks the base of the
Middle Ordovician and is interpreted as a transgressive surface and sequence
boundary ( Fig. 5 ). The surface is pitted by so-called Amphora-like burrows
and/or borings ( Orviku, 1960; Vishnjakov and Hecker, 1937 ), corresponding
to Gastrochaenolites oelandicus Ekdale and Bromley, 2001 ( Fig. 6 A and B).
The hardground surface at the base of the Volkhov depositional sequence
occurs not only in the Syas River area (“Steklo” surface) but also in Estonia
(“Pustakkiht”), on the island of ¨ land in Sweden “Blommiga Bladet”, and in
FIGURE 5 Development of the “Steklo” transgressive surface mainly bioeroded with Gastrochae-
nolites oelandicus . Modified after Posamentier and Allen (1999 ).