Indication origin and type of organic matter and its relation to depositional conditions using molecular composition and geochemical elements: the Jurassic Amran sediments from Samae area in the Taiz Governorate, Southwestern Yemen

Abstract

This study is the first attempt which provides information regarding the biomarker characteristics and geochemical elements of the Jurassic Amran sediments from Samae area in the Taiz Governorate, Southern Yemen. The geochemical results helped us to investigate the origin and type of organic matter and its relation to environment conditions during deposition of the sediments. The Amran sediments have the highest concentration of terrestrial detritus elements (e.g., SiO2, Al2O3) with relatively high carbonate element (CaO), indicating that these sediments are limestones with argillaceous and clastic contents that are deposited in a shallow marine environment. This is supported by low total sulfur (TS) contents and Sr/Ba ratios and biomarker environment indicators. Their biomarkers provide also evidence for a mixed organic matter by aquatic and terrigenous organic matter contributions. This is confirmed by bimodal distribution of normal alkanes, relation of acyclic isoprenoids to normal alkanes, relatively high tricyclic terpanes compared to tetracyclic terpanes, and high proportion of C27 (36.1–44.4 %) and C29 (33.4–41.1 %) compared to C28 (21.9–23.8 %) regular steranes. Moreover, the suboxic bottom water conditions are evidenced in the Jurassic Amran argillaceous limestone sediments, as identified from V/Ni, and pristane/phytane (Pr/Ph) ratios in the range of 1.53–2.54 and 1.09–1.38, respectively. Therefore, suboxic conditions contribute to organic matter (OM) preservation during deposition of the Amran sediments.