Paleoredox Conditions, Paleoproductivity, and Terrigenous Sediment Influx of the Lower-Middle Cenomanian Strata in the Abu Gharadig Basin, Northern Egypt

Abstract

During the Late Cretaceous (Cenomanian), significant disruptions in the carbon cycle, global warming, and episodes of oceanic anoxia occurred, leading to the deposition of organic carbon-rich sediments. In well BED2-3, located in the BED2 gas field within the Abu Gharadig Basin (north Western Desert, Egypt), the lower-to-middle Cenomanian Bahariya Formation displays thick alternating layers of sandstones, siltstones, and shales. Detailed geochemical analyses were conducted on thirty-three cutting samples from the Bahariya Formation, focusing on total organic carbon (TOC), whole-rock elemental geochemistry, and carbonate content. These geochemical measurements provided valuable information regarding paleoredox conditions, marine biological productivity, terrigenous sediment influx, weathering and paleoclimate conditions, and mechanisms influencing organic matter accumulation. The enrichment factors (EF) of redox-sensitive trace elements were utilized to infer oxygenation conditions and marine biological productivity during the deposition of the Bahariya Formation. The stratigraphic distribution of redox-sensitive elements allowed for the Bahariya Formation to be categorized into lower and middle-upper intervals. The results revealed that the lower interval exhibited strong-to-enriched EF values of redox-sensitive elements and fair-to-rich TOC content, indicating a prevalent anoxic setting during deposition. In contrast, the middle-upper interval displayed weakly-to-slightly enriched EF values with poor-to-fair TOC content, suggesting deposition under oxic-suboxic redox conditions. By examining Al-normalized redox-sensitive ratios and their correlations with TOC content, significant relationships were observed in the lower interval, indicating a coupling between the enrichment of redox-sensitive elements and organic matter. This suggests enhanced biological productivity during deposition of the lower interval compared to the relatively low productivity during deposition of the middle-upper interval of the formation. These conditions controlled the production and preservation of organic matter in the lower interval, while the middle-upper interval suffered from organic matter dilution and destruction due to an increased influx of terrigenous material and lower biological productivity. Geochemical proxies related to detrital materials provided evidence of alternating terrigenous sediment flux, consistent with shifts between coarse- and fine-grained fractions and related facies of sandstones, siltstones, and shales. These findings align with active continental weathering in the source terrane and deposition under enhanced warm-humid climatic conditions, with intermittent arid-to-semi-arid phases. These conclusions are further supported by the palynomorph assemblages and clay mineralogy within the Bahariya Formation.