Depositional environment and organic matter accumulation of Lower Jurassic nonmarine fine-grained deposits in the Yuanba Area, Sichuan Basin, SW China

This study investigated the lithological, sedimentological and geochemical characteristics of Lower Jurassic strata (Ziliujing Formation) in the Sichuan Basin to identify the lithofacies and understand the depositional environments and differences in organic matter accumulation of these nonmarine fine-grained sediments. Based on mineralogy and petrology, rock types were recognized, with mudstones, siltstones, and limestones being the most common. Rock types, sedimentary structures, biota, and well log data were used to define and characterize 17 lithofacies and the following five facies associations of the fine-grained sediments: (1) delta front; (2) clastic lakeshore; (3) clastic shallow lake; (4) clastic semi-deep lake; and (5) carbonate lake. The content of total organic carbon ranges between 0.04% and 3.84%, with an average of 0.74%. The organic matter primarily consists of Type Ⅲ kerogen. Major processes inferred from the lithofacies analyses include suspension, biogenic, and storm deposition. On the basis of facies associations, redox, climate and salinity conditions, biotic productivity, and terrigenous input, a total of six depositional environments are proposed: (I) high-productivity low-oxygen swamp (II) low-oxygen semi-deep lake (III) high-oxygen semi-deep lake (IV) high-oxygen shallow lake (V) deep-water low-oxygen semi-deep lake and (VI) high-salinity low-oxygen shallow lake. The nonmarine environments changed frequently, unlike more stable environments. The development of bivalves in the shallow lake is suggested to be related to high salinity caused by dry climate, together with limited terrigenous input. Organic-rich nonmarine fine-grained sediments in the Ziliujing Formation formed in swamp and semi-deep lake with settings under reducing conditions, in a humid climate, with relatively high terrigenous input. Results of the study may help us to understand the deposition of nonmarine organic-rich fine-grained Lower Jurassic sediments in southwest, China.