Research paperDepositional environment of the Lower Silurian Akkas hot shales in the Western Desert of Iraq: Results from an organic geochemical study

•We investigate the depositional environment of the Lower Silurian hot shales.•These hot shales were deposited under euxinic conditions.•These hot shales were deposited in stratified water column.

The lower Silurian hot shales in the lower part of Akkas Formation in Akkas field, western Iraq were studied to determine their depositional environments. For this purpose, organic and inorganic proxies were used. These include total organic carbon content (TOC), sulfur, pyrite, and carbonate content measurements, as well as the saturated and aromatic biomarkers. Organic carbon content (TOC), pyrite, and carbonate contents were high and pristane/phytane ratio (Pr/Ph) was low in these hot shales, indicating deposition under anoxic marine conditions. The presence of gammacerane suggests water column stratification; the presence of aryl isoprenoids (originating from aromatic carotenoids of green photosynthetic sulfur bacteria), indicates deposition under euxinic conditions, extending from the photic zone to the sediment-water interface. The conditions became more oxygenated upward, which led to the deposition of organic-poor gray-green shales in the upper part of Akkas Formation, with low pyrite and carbonate contents and high Pr/Ph ratios.