Preliminary analysis of the depositional environments of the Lower Cretaceous Yixian Formation in the Sihetun area, western Liaoning, China

The Lower Cretaceous Yixian Formation in the Sihetun area consists of, in ascending order, the Lujiatun, Lower Lava, Jianshangou and Upper Lava units. Twelve lithofacies are recognized within the sedimentary Lujiatun and Jianshangou units. These form two facies associations: a volcaniclastic apron association in the Lujiatun Unit and a lake association in the Jianshangou Unit. The former is distributed in a linear belt along the northern margin of the depositional basin, and is composed of debris flow, hyper-concentrated flood-flow, stream-flood, sheet-flood and lahar deposits. It can be further subdivided into syneruption and inter-eruption assemblages. The syneruption assemblage is composed of debris flow, hyper-concentrated flood-flow and lahar deposits, whereas the inter-eruption assemblage consists of stream-flood and sheet-flood deposits. Well-developed clast imbrications of clast-supported conglomerates indicate that the ancient flow direction of the volcaniclastic apron was from east to west and south (170–310°) along the margin of the depositional basin. The lake association is widely distributed in the middle and southern part of the basin and composed of wave activity, suspension, hyper-concentrated and concentrated density flow and turbidity-current deposits. It includes three assemblages: a beach-nearshore assemblage distributed in a linear belt along the western margin of the lake deposits, and made up of wave activity, hyper-concentrated density flow, suspension and turbidity current deposits; a lake floor assemblage composed of suspension, hyper-concentrated density flow and turbidity-current deposits; a fan delta assemblage distributed along the eastern margin of the lake deposits, consisting of alternating hyper-concentrated and concentrated density flow with turbidity-current deposits.