SEQUENCE STRATIGRAPHIC FRAMEWORK, OCEANIC ANOXIC EVENTS, AND STABLE CARBON ISOTOPE SIGNAL OF ALBIAN ADRIATIC PLATFORM, CROATIA

Albian oceanic anoxic events (OAE) are marked by three separate events of organic carbon deposition. The first of these events (OAE1b) is global, and has been documented from numerous platform and basinal settings; OAE1c and OAE1d are less well known, especially their expression within shallow-marine Tethyan platforms. This study focuses on establishing the sequence stratigraphic framework for the Albian Adriatic platform, evaluating the stable carbon isotope signal, and documenting the platform response to Albian OAEs. The study is based on ~400 m thick succession of shallow-marine, platform-interior carbonates, coupled by carbon-isotope analysis of bulk lime mudstone matrix of samples collected at 1-meter intervals.

The Albian supersequence has 4 third-order depositional sequences bounded by breccias at sequence boundaries: Alb1 (~120 m thick), Alb2 (~115 m), Alb3 (~135 m), and Alb4 (~65 m). All four sequences are characterized by cyclic peritidal carbonates. The abundance of laminites capping parasequences suggests that climate was mainly semi-arid. The δ¹³C of lime mudstone matrix varies from -2.3 to +2.2 ‰ VPDB (average +0.4‰). The stable carbon isotope profiles document the presence of OAE1b and OAE1c, and enable correlation of the shallow-marine Adriatic platform with pelagic sections, whereas OAE1d is not evident in dolomite. Stable carbon-oxygen cross-plots are similar to trends of pelagic facies suggesting that semi-arid climate inhibited meteoric diagenesis, and primary calcite muds of the platform were not greatly modified by burial diagenesis. The study shows that the Adriatic platform remained shallow and there was no incursion of basinal waters onto the platform during Albian OAEs, implying significant oceanic stratification.