High resolution continuous sedimentary records of Upper Cretaceous obtained from the continental drilling (SK-1) borehole in Songliao Basin: Sifangtai and Mingshui Formations

The Sifangtai and Mingshui formations were continuously cored in the SK-1n borehole (China Cretaceous Continental Scientific Drilling–SongKe1-the north borehole). The core is 767.96 m long, and the recovery is 94.7%. The ages of the formations range from middle Campanian to Danian. The sequence and process of lithology-lithofacies and cyclic stratigraphy were described in detail. Eight litho-types compose the Sifangtai Formation, and 15 litho-types compose the Mingshui Formation. Deposition was predominantly in meandering river and lacustrine environments, including 10 microfacies in the Sifangtai Formation and 15 microfacies in the Mingshui Formation. The complete sequence is composed of 535 m-scale cycles (sixth-order cycle), 152 fifth-order cycles, 42 fourth-order cycles and five third-order cycles. The centimeter-scale description of the section revealed some previously unknown horizons such as a special type of mudstone, marl, volcanic ash and favorable sand reservoirs in the formations. The new-found evidence is very important for the interpretation of the evolution of the basin, conditions such as lake oxic events, the K/Pg boundary, tectonism in the late sag basin stage, and the reservoir-cap rock assemblages in the shallow stratigraphy.