Controls and timing of Cenomanian-Turonian organic enrichment and relationship to the OAE2 event in Morocco, North Africa

Abstract

The Cenomanian-Turonian Oceanic Anoxic Event (OAE2) is a significant global event that has been linked to major source rock development in marine environments. The aim of this study is to integrate biostratigraphic, sedimentological and geochemical data (including stable carbon isotopes and trace elements) to document the timing and controls of organic matter enrichment across the Cenomanian-Turonian boundary.Morocco has excellent exposures of Tethyan carbonate platform and basinal facies and this paper details a high-resolution analysis of organic-rich Late Cretaceous intervals exposed in the Errachidia-Goulmima Basin, to investigate their relationship to the OAE2. Integrating planktic foraminiferal and ammonite biostratigraphy with carbon isotope correlations indicate that organic-rich black mudstones identified were deposited from latest Cenomanian to early Turonian, suggesting a late-OAE2 to post-OAE2 age.The sedimentology, redox water conditions and palaeoproductivity of the palaeoenvironments were studied to characterise the lithofacies and geochemical signatures, and to analyse the controlling factors for organic–enrichment. In the study area during the lower OAE2 interval, a shallow water mid-ramp environment prevailed, with predominantly bivalve-rich limestone deposition. Subsequent organic-rich mudstone development can be correlated with the latest Cenomanian (late OAE2 interval) and early Turonian (post-OAE interval) transgression, which allowed development of anoxic to euxinic conditions in the outer-ramp environment, characterized by high TOC values, from 2.7 to 17 wt%. Shallow platform locations continued to experience limestone deposition with no organic enrichment.Trace and major element data from the C/T black mudstones show an extremely high concentration in palaeoproductivity-sensitive (P, Zn, Ni and Cu) and redox-sensitive elements (V, Mo, and U). The redox environments evolved from dysoxia to anoxia and sulfidic euxinia, which is interpreted to have provided optimum conditions for organic carbon preservation in the Errachidia area.The results highlight the important interplay of local paleogeography and eustatic sea-level (the late Cenomanian/early Turonian transgressions) as the main controls on organic mudstone deposition, rather than simply global OAE2 control. The development of organic-rich mudstones reflects both increased productivity at this time and enhanced organic matter preservation due to the change in redox bottom water conditions.