Organic-rich Permian lacustrine facies source oils in the giant Karamay field and in several other areas of the Junggar and southern Turpan basins. Recent field investigations have focused on nonmarine facies in outcrop localities surrounding the Turpan-Hami basins. These new observations, combined with previous work, extend the known distribution of Permian lacustrine facies to an area of at least 800 km by 200 km, indicating that widespread Permian lakes represented a major paleogeographic feature of central Asia.

The Lower Permian Yierxitu Formation of the Turpan and Hami basins consists of a series of transgressive-regressive cycles 1-3 m thick that reflect lake-level fluctuations. A typical cycle grades upward from stromatolitic limestone into profundal, laminated organic-rich mudstone, and finally into lake-marginal siltstone and sandstone. The overlying Upper Permian Daheyan Formation is dominated by imbricated alluvial and fluvial conglomerate which, at one locality, contains silicified tree trunks up to 0.50 m in diameter. This unit grades up into the Tarlong Formation which varies from profundal lacustrine facies containing well-preserved fish fossils, to cyclic lake-marginal facies. Heterogeneous volcanic rocks underlie, and locally are interbedded with, the lower nonmarine deposits

Permian lakes occupied a basin lying north of the present Tian Shan range, into which coarse elastic sediments were periodically shed from an ancestral Tian Shan uplift. Up to 5 km of total subsidence are recorded, yet the tectonic significance of this basin (or basins) remains controversial. Structural relations beneath a sub-Middle Triassic angular unconformity adjacent to the far western Turpan basin suggest regional compression immediately following deposition of Upper Permian lacustrine units.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah