New Stratigraphical and Sedimentological Data on the Lower Cretaceous Rocks in Section Kalotina, Western Bulgaria

Abstract

Biostratigraphic, microfacies and C–O isotope data from the lowermost Cretaceous deposits in section Kalotina reveal the development of a shallow carbonate platform (Slivnitsa Formation) in a photozoan mode under moderate to high-energy conditions during the Berriasian. Around the Berriasian/Valanginian boundary the carbonate production switched to a heterozoan-type suggesting a biological crisis. After a subaerial exposure in the earliest Valanginian and subsequent karst formation the platform was drowned during the latest Valanginian. Deposition of carbonate-argillaceous sediments (Salash Formation) occurred in a deeper, open-marine environment under mesotrophic to eutrophic, low-energy conditions with periodical formation of crinoid-rich packstones probably by mass gravity flows. Limestone samples from the Slivnitsa Formation yield higher δ13C values compared to coeval open-marine deposits which can be explained by an elevated amount of aragonite in the photozoan carbonate production and an enrichment in δ13C of the shallow platform waters due to high levels of photosynthesis. The most depleted δ13C value in the uppermost part of the unit most likely reflects a change from oligotrophic to mesotrophic conditions. The C-isotope signatures in limestones of the Salash Formation indicate correspondence to the third, smooth decrease phase in the δ13C record of the global Weissert Event (latest Valanginian and Early Hauterivian) where values tend to recover to pre-excursion values.