The Impact of Ordovician Climate Change on the Carbonate Facies of the East Baltic (Estonia and northwest Russia)
The initial siliciclastic phase (Pakerort to mid-Billingen Stages) is marked by numerous unconformities, long hiatues, and thin and laterally discontinuous units. Carbonate deposits are absent and fauna sparse. The sediment accumulation rate is low (2.3m/Ma). Cool-water carbonates (Mid-Billingen to Volkhov Stages) were initially dominated by chemical processes such as hardground formation, mineralization and corrosion that overprint very slow (1.1 m/Ma) accumulation of bioclastics and clay materials. Upper Volkhov strata consist of argillaceous bioclastic carbonates with firmgrounds, common glauconite and minor iron ooids that accumulated at a rate of 6.9 m/Ma. Temperate carbonates (Kunda to Haljala Stages) are characterized in their lower part by rare glauconite, common to abundant iron ooids, sandy carbonates in updip sections, and an accumulate rate of 6.7 m/Ma. In the upper part, carbonates facies are slightly muddier and include kukersite beds; pronounced progradational stratal geometries are associated with increased sediment accumulation rates (11.1 m/Ma). Tropical carbonates (Keila to Porkuni Stages) contain normal tropical constituents including carbonate ooids, corals, and numerous small reef bodies (mud mounds). Progradational geometries occur at several levels associated with the upper parts of sequences or prograding sets of sequences. The sediment accumulate rate was 16.4 M/Ma.
The carbonate facies document the shift in major constituents, sea-floor diagenesis, and accumulation rates associated with the climate transition. Less predictable findings are the predominance of hardground and corrosion surfaces in the cool-water facies, and the sequential changes of carbonate constituents. Well-developed progradational geometries in the Kukruse and later intervals are associated with higher sediment accumulation rates.