Oceanic Anoxic Events and Stable Carbon Isotope Records of Albian Adriatic platform, Croatia

Abstract

Aptian-Albian oceanic anoxic event (OAE1) is marked by four separate events of Corg deposition. The first two of these events (OAE1a and OAE1b) are global, and have been recorded from numerous platform and basinal settings. Much less is known about the Albian OAE1c and OAE1d, especially their expression within shallow-marine Tethyan platforms. This study focuses on the Albian OAE and stable carbon isotope record of the Croatian Adriatic platform, and is based on detailed bed-by-bed analysis of ~460-m-thick thick platform-interior sequence, coupled by δ13C analysis of bulk carbonate matrix of samples collected at 1-meter intervals. The Albian supersequence has 4 third-order depositional sequences bounded by breccia sequence boundaries; each sequence is characterized by cyclic carbonates. Fenestral caps to cycles predominate in the lower part of the succession. This suggests that moderately humid conditions initially inhibited development of microbial laminites, which become more abundant near the top of the Albian with drying. This is marked by increase in dolomitization and abundance of microbial laminite caps to parasequences. The mean values for δ13C are +0.55‰ VPDB. The base of the Albian sequence Alb1 corresponds to the OAE1b and is characterized by a stepwise negative excursion of δ13C, reaching a minimum value of -6.52‰ VPDB. Sequence Alb2 begins with a positive δ13C excursion, followed by δ13C values varying within the range of -6.52‰ to +2.19 ‰ VPDB; Sequence Alb3 represents a period of low δ13C values, with the minimum value of -5.54 ‰ PDB. Sequence Alb4 encompasses both the highest and lowest values of δ13C within the Upper Albian part of the supersequence, +2.19‰ and -4.77‰ PDB, respectively. The stable carbon isotope profiles of shallow-marine carbonates from the Croatian Adriatic platform show only a vague similarity with those from Tethyan pelagic sections, suggesting diagenetic overprint. This is particularly evident for subaerially exposed parts of the succession and massive crystalline dolomites, where carbonate-isotope values show highly depleted values. There is a general trend to lighter δ13C values towards the end of Albian, as evident by preliminary correlation of OAE1d. Carbon has larger excursions from the mean during the OAE1b in the Early Albian and again in the Late Albian post-OAE1d event. The study shows that Albian OAEs exhibit no characteristic sedimentary record within the platform-interior succession.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015