Dark, laminated and organic-rich mud rocks, usually referred as “black shales”, are very typical of the Early Cretaceous rock record. They correspond to the so-called “Oceanic Anoxic Events” (OAE), and reflect abrupt... [ view full abstract ]
Dark, laminated and organic-rich mud rocks, usually referred as “black shales”, are very typical of the Early Cretaceous rock record. They correspond to the so-called “Oceanic Anoxic Events” (OAE), and reflect abrupt changes in global carbon cycling. The Aptian Oceanic Anoxic Event (OAE 1a, 120 Ma), represents one of the best examples of these events, with a global distribution recorded in all the main ocean basins, associated to massive burial of organic matter in marine sediments. OAE 1a is concomitant with the “nannoconid crisis”, which represents an episode of major biotic turnover, and is coincident with a widespread demise of carbonate platforms. Much research has been done on OAE 1a from different sections across the world over the last two decades, since the definition of the C-isotope stratigraphy of the event. Notwithstanding, high-resolution studies across the entire event will be crucial to elucidate the precise timing and rates of the different environmental and biotic changes involved.
Here, we present a new recently drilled-core, the Cau core, spanning the lower to basal upper Aptian interval. The Cau section is located in NE of the Alicante province, in the Prebetic Zone (Betic Cordillera), and records the deposits of the distal parts of a shallow carbonate ramp. The lower Aptian of the Cau section is represented by an hemipelagic unit (Almadich Formation, ca. 200 m thick), deposited in a highly subsiding sector of a tilted block, located in the distal parts of the Prebetic Platform. Previous works on this section have focused on the stratigraphy, bioevents, C-isotope stratigraphy, and organic and elemental geochemistry, pointing out an expanded nature of the succession, along with very well-preserved palaeontological and geochemical records, that have led to the recognition of a thick level of black marls corresponding to the OAE 1a in the lower part of the Schackoina cabri planktonic foraminifer biozone, H. irregularis nannofossil biozone.
Here we present preliminary results from the sedimentological analysis of the core. This study reveals a cyclic vertical evolution of the succession, with alternations of laminated, massive and burrowed marlstones. These vertical facies changes are concurrent with color variations mostly related to the TOC content and lithology. Main results of this sedimentological approach point to cyclic variations in the redox conditions across OAE 1a. These results will be checked against subsequent geochemical and quantitative paleontological analyses.
Acknowledgements: This work is a contribution of the research project CGL2014-55274-P (Gov. of Spain).
