Carbonate Platform Record of Mid-Cretaceous Oceanic Anoxic Events (OAE), Croatia

Abstract

Abstract

Mid-Cretaceous Aptian and Albian oceanic anoxic events (OAE) are marked by four separate events of organic carbon deposition. The first two of these events (OAE1a and OAE1b) are global, and have 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 evaluates the record of these events and their effects on a shallow-marine, Bahama-like Adriatic platform of Croatia. The study is based on detailed logging of ∼500 m thick succession of platform-interior shallow-marine carbonates, coupled with stable carbon isotope analysis of bulk lime mudstone matrix (sampled at 1-meter intervals).

The Aptian-Albian supersequence has 8 third-order depositional sequences bounded by breccia sequence boundaries. Sequence Ad1 (latest Barremian-Early Aptian) is dominated by shallow subtidal, commonly amalgamated carbonate parasequences and late highstand, sequence boundary zone breccias. Sequence Ad2 (Early Aptian to earliest Late Aptian) has a significant flooding event associated with OAE 1a, marked by deposition of relatively shallow water, but dysaerobic laminated carbonates. Sequences Ad2, Ad3 and Ad4 are dominated by peritidal carbonate parasequences updip, especially in the early TST and late HST, and subtidal parasequences elsewhere. All four Albian sequences (Alb1 through Alb4) are characterized by cyclic peritidal carbonates. The abundance of laminite caps to parasequences suggests that climate was mainly semi-arid. The stable carbon isotope profiles document presence of OAE1a, OAE1b and OAE1c, and enable correlation of shallow-marine Adriatic platform with pelagic sections. Stable carbon-oxygen cross-plots are similar to pelagic facies suggesting that the 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 Aptian OAE1a resulted in short term incipient drowning, but the Adriatic Platform remained shallow and not affected by incursion of basinal waters during Albian OAEs, implying significant stratification of ocean waters.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016