We report the results of recent field work in southern Mongolia, where we have documented a thick sequence of fluvial-lacustrine strata related to a large Triassic lake system. The Noyon Uul and Tost Uul regions of southern Mongolia contain approximately 3 km of braided fluvial, meandering fluvial, lacustrine deltaic, and open lacustrine facies. Gilbert delta style bar sets, locally with >15m of relief, are common, suggesting that rivers supplying conglomeratic detritus to the lake margins had appreciable size. The bulk of Triassic strata in the Noyon and Tost Uul areas comprise meandering fluvial and lacustrine deltaic facies, characterized by well developed channel and crevasse splay sandstones (commonly soft-sediment deformed) and abundant overbank/avulsion siltstones. Open lacustrine strata are present in the core of the Noyon syncline in the form of a 200 m thick sequence of organic-rich, laminated dolomitic shale. Paleocurrent analysis throughout the Noyon Uul/Tost Uul sections indicates consistent transport to the west. West of the Noyon Uul/Tost Uul region at the Toroyt locality, ostensibly equivalent strata consist of fine sandstone, shale, and local carbonate. Based on the finer overall grain-size and the development of channelized geometries and lateral accretion surfaces in coarser lithologies, we interpret most of the Toroyt section to have been deposited in a distal lacustrine deltaic setting. The presence of carbonate intraclastic grainstones and abundant phosphatic debris (primarily fish scales?) suggests episodic lake expansion/deepening.

Recent reports of a regional, north-verging fold-thrust belt ~100km south of the southern Mongolian field area are consistent with our interpretation that a large lake occupied a collisional foreland-style depocenter during Triassic time. Given the occurrence of source prospective organic-rich shales in the Noyon area, the widespread nature of this Triassic lacustrine system (possibly extending west along tectonic strike into the Turpan or Junggar basins), may significantly increase the prospectivity of southern Mongolian basins, as well as those of western China.

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