Facies association analysis of a Toarcian siliciclastic‑carbonate lacustrine system, Sichuan Basin, China

Abstract

Globally, considerable coverage has been devoted to marine environment affected by the Jenkyns Event, but little attention has been given to lacustrine system. Here, siliciclastic‑carbonate samples of the Da'anzhai Member (J1dn) were examined for petrology and geochemistry in order to evaluate lithofacies characteristics, depositional processes and environmental changes of the palaeo-Sichuan lacustrine basin contemporaneous with the Jenkyns Event. Unique sedimentological and petrological features were used to identify five principal facies associations (FA) and depositional environments, including the distal delta front, accretionary bioclastic shoals, proximal bioclastic shoal ramp, distal bioclastic shoal ramp and (semi-)deep lacustrine environment. The vertical transitions of FA reveal that J1dn forms a complete lacustrine transgressive-regressive sedimentary system during the Toarcian, which can be further divided into 3 units in combination with standard gamma logging. The lower unit is characterised by frequent intermixing of multiple FA interpreted as frequent lake-level fluctuations. The middle unit is characterised by the deposition of clay-rich mudstone and the demise of bioclastic shoals interpreted as intensive lacustrine transgression that were at times affected by storm-induced waves or oscillatory currents coinciding with the initiation of the Jenkyns Event. The upper unit is characterised by the thicker and higher proportion of biogenic carbonate-rich lithofacies interpreted as the lake regression, contemporaneous with the end of the Jenkyns Event. Overall, clay-rich lithofacies from suspension settling, turbidity current or slumping triggered by storm events were deposited during a high relative lacustrine-level, accompanied by organic carbon-rich deposition and preservation. In contrast, carbonate-rich lithofacies were mainly controlled by biogenic carbonate sedimentation, associated with a low relative lacustrine-level unconducive to organic matter accumulation. Finally, the proposed depositional evolution model at the lake-level highstand stage and lowstand stage of the palaeo-Sichuan lacustrine basin provides new insights and understanding on the changing palaeolimnological history of the Toarcian lacustrine system.