Sedimentary characteristics and environmental evolution of Da-kangpu layer of Early Cretaceous Yixian Formation in western Liaoning

Widely distributed in Yixian Basin in western Liaoning, Dakangpu layer of the Yixian Formation belongs to Lower Creta-ceou and is an important horizon of "Jehol Biota". A typical section was measured in detail for sedimentology and 11 lithofacies types were recognized, such as matrix supported conglomerate facies, tuffaceous medium-coarse grained sandstone facies and medium-fine grained sandstone facies with parallel bedding. Dakangpu layer comprises mainly lacustrine sediments including shore shallow lake fa-cies and deep and semi-deep lake facies. Based on analyzing the changes of vertical sequence characteristics and varves characteristics of Dakangpu layer, the authors divided it into four depositional stages. At the initial stage, the lake was just formed with a large num-ber of shallow water fossils. After that it entered into the middle-range stage with deep and semi-deep water in the relatively closed and semi-closed state, during which the salinity increased. In the late depositional stage, the lake became shallow with higher hydrodynamic condition and low salinity. At the end of the deposition the lake had deep and semi-deep water, and the varves were devel-oped which indicated poor terrigenous clast and little precipitation. The evolution of the lake was controlled by paleoclimate obviously.