Stratigraphy, correlation, depositional environments, and cyclicity of the Early Cretaceous Yixian and ?Jurassic-Cretaceous Tuchengzi formations in the Sihetun area (NE China) based on three continuous cores

摘要

Three continuous cores acquired in the Sihetun area of Liaoning Province, famous for feathered non-avian dinosaurs of the Jehol Biota, reveal the stratigraphic and facies relationships of the divisions of the Early Cretaceous Yixian and underlying ?Jurassic-Early Cretaceous Tuchengzi formations. Based on these cores, we propose a modified lithostratigraphic nomenclature in the Sihetun Sub-basin in which the Yixian Formation is divided into three members: the Lujiatun Member, a fluvial and alluvial volcaniclastic sandstone and conglomerate, unconformably overlying the Tuchengzi Formation, with articulated tetrapods plausibly preserved in burrows; the Xiatulaigou Member, an extrusive basaltic flow and breccia comprising the only true lavas in the area; and the Jianshangou Member, a cyclical lacustrine mudstone containing a taphocoenosis of articulated compression fossils with soft tissue preservation including non-avian dinosaurs, birds, pterosaurs, mammals, fish, crustaceans, insects, mollusks, and plants. The latter, is divided into 4 units: the basal Dajianshanzi Bed, a shallow-water lacustrine unit with abundant mollusks and rooted zones; the Anjiagou Bed, the main tetrapod-bearing unit, consisting of microlaminated dark gray mudstones with many interbedded airfall ashes deposited in a meromictic lake; the Hengdaozi Bed comprised of thin and rhythmically to medium bedded light gray to tan mudstones and sandstones with few discrete ashes, famous for its diverse insects and plants, including early angiosperms deposited in alternating dysoxic and oxygenated bottom water; and the Huangbanjigou Bed, the most obviously cyclical of the units, with dark gray microlaminated intervals with articulated vertebrates alternating with more massive lighter colored mudstones and volcaniclastic sandstones deposited in lakes that oscillated in depth. Cyclicity in the Jianshangou Member is dominated by meter-scale cycles that may be paced by orbital variations. Unambiguous, relatively rapid, lateral fades changes within the Yixian Formation are suggestive of local relief, not the bottom of a giant flat-bottomed basin. (C) 2016 Elsevier B.V. All rights reserved.