DEPOSITIONAL FACIES AND ENVIRONMENTS OF EOCENE EVAPORITES OF THE HETAOYUAN FORMATION (THE ANPENG DEPOSITS), BIYANG DEPRESSION , NANYANG BASIN, CHINA

The Eocene Hetaoyuan Formation of the Biyang depression in central China is composed of three depositional members, the middle being a 900m-thick evaporite-dominated sequence (Anpeng deposits) characterized by laminated oil shale, dolomite and abundant sodium carbonate minerals nahcolite (NaHCO₃) and trona (Na₂CO₃). Nahcolite, an unusual evaporite mineral formed under high atmospheric pCO₂ levels, occurs in beds over 40 m in thickness in the Anpeng deposits, which are strikingly similar in age, mineralogy inferred depositional environment to evaporite deposits of the Eocene Green River Formation, Piceance Creek Basin, Colorado, USA. The Green River Formation is well known through extensive research and publication, but much less is known about the Hetaoyuan Formation. The objectives of this study are to review and report on the depositional features and sedimentary facies along with the petrography, mineralogy, and geochemistry of the Hetaoyuan evaporite-bearing strata in order to infer the depositional environment and climate regime under which they were deposited.

Analyses of three new drilling cores show the lithologies of the middle member of the Hetaoyuan Formation are dominated by trona-nahcolite, oil shale, dolomite, dolomitic mudstones, mudstone, analcime, siltstone and sandstones. Evaporites include thick successions of trona, nahcolite and a considerable accumulation of shortite, with minor amounts of calcite. The evaporite sequence is represented by trona-nahcolite layers that are interbedded with dolomite and oil shale layers. The sedimentary structures observed include mudcracks, ripple marks small-scale cross-bedding, and rip-up clasts, indicate the strata accumulated in in ephemeral lake with frequent dry periods, occasional significant currents and episodes of resedimentation. These indicate a saline playa environment for the deposition of the Hetaoyuan Formation.