July 10 , 2016

Highlights

The three members of the Yixian Formation represent historical phases, not facies equivalents.
Basal Lujiatun Member houses tetrapod fossils preserved in 3 dimensions in fluvial and volcaniclastic rock.
Xiatuliagou Member lavas and breccias are the only true flow units in the area.
Jianshangou Member laminites, air-falls, and tuffaceous rocks house the Jehol Biota.
The extraordinary preservation of the Jehol biota is a result of deep lake anoxia.

Abstract

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 facies changes within the Yixian Formation are suggestive of local relief, not the bottom of a flat-bottomed basin.