Organic-rich carbonate characteristics and depositional settings of the Late Jurassic Lam Member in the Shabwah depression, southeastern Yemen

Organic-rich sediments of the Late Jurassic Lam Member (LM) in the Shabwah depression, southeastern Yemen were investigated using organic geochemical, biomarker and carbon isotope compositions with microscopic observations to assess the organic matter input and evaluate the sedimentary depositional conditions, as well as their role in controlling organic carbon accumulation. The LM samples are characterized by elevated organic matter (OM) and sulfur, reaching maximum values of 8.40 wt% and 18.85 wt%, respectively, indicating marine environment under anoxic and sulfidic conditions, promoting effective preservation of OM during deposition. This interpretation is supported by redox-related biomarker proxies, particularly low Pr/Ph ratio of less than 1.5, and higher abundance of sulfide mineral (FeS) in small size framboidal pyrite form, which it is formed due to sulphidation of iron under poor-oxygenated conditions. The source-related biomarker parameters, along with carbon isotope data also provide evidences of mainly marine (i.e., planktonic-bacterial) organic matter, with low amounts of land-plant input, reflecting mainly hydrogen-rich kerogen as original OM during deposition of the LM sediments. The predominance of marine-derived OM is evidenced by the abundance of alginite, including telalginite and lamalginite, together with diverse bituminite organic matter. The elevated warm-water climate event during the Late Jurassic time associated with anoxic conditions, promoting greater bioproductivity in the photic zone, and confirming a great impact on OM preservation and accumulation during deposition.