Characteristics of Cenomanian–Turonian Oceanic Anoxic Event 2 and its Impact on Organic Carbon Accumulation in the Abu Roash (F) Organic-Rich Carbonate Rocks in the Abu Gharadig Field, Egypt

Abstract

The Cenomanian–Turonian was a period of climatic perturbations associated with extraordinary marine production of organic carbon during the redox conditions in the southern Neo-Tethys Ocean. The Abu Roash-F carbonates in the northwestern desert of Egypt were deposited during this period. This work revealed some organic and inorganic geochemical results combined with microscopic examinations of the carbonate rocks of the mid-Cretaceous Abu Roash-F Member from two exploration wells in the Abu Gharadig Basin, Egypt. This work was designed to understand the paleoenvironmental conditions during Cenomanian–Turonian oceanic anoxic event 2 (OAE2) and its impact on the organic carbon-rich accumulation in Abu Roash (F) carbonates. Geochemical proxies indicated that AR-F contains total organic matter and sulfur contents in the ranges of 0.89–3.32 and 0.30–2.73 wt %, respectively, confirming the marine environment for the studied samples. The presence of a marine environment, ranging from deep sea to slope and deep shelf, was also inferred by the large quantities of planktic forams and other marine skeletal assemblages, including bivalves and echinoderms, with rare ostracods. The Ni and V geochemical proxies, together with their ratios and enrichment factors, confirmed that these facies were mainly found in low oxygen environments, ranging from dysoxic to anoxic nonsulfidic conditions, thereby contributing to the preservation of organic matter. An Rb/Sr ratio of less than 0.1 indicated that the facies were accumulated in warm and humid climates, thereby resulting in an intensity of subaerial weathering. The dominant presence of warm-water plankton species (i.e., Globigerinoides sp.) together with a large number of primary producers (i.e., algae and dinocysts) in the Cenomanian–Turonian AR-F carbonate facies further suggested that warm climatic conditions occurred during OAE2. These warm climatic conditions can be the source of a mass influx of nutrients into the basins, which could be attributed to increased productivity of the water column. The high productivity was further supported by the presence of the shelf nannofossil taxon Zeugrhabdotus erectus. Such conditions resulted in high productivity and preservation of organic matter during OAE2, and thus, organic carbon-rich accumulation in the carbonate-rich sediments within the AR-F Member.