The Noyon Uul syncline, southern Mongolia, is an east-west trending structure which provides excellent exposure of Permian through lowermost Jurassic non-marine strata. In the summer of 1997, field work was conducted to evaluate the character of basin sedimentation and its tectonic implications. Five stratigraphic sections, each about 3 kilometers thick, were measured to document stratigraphy, paleoflow and provenance of Mesozoic strata exposed along the flanks of the syncline.

Stratigraphy of the Noyon Uul syncline is characterized in terms of five facies related to depositional environment. Coarse-grained, braided fluvial facies consists of coarse sand and pebble to cobble conglomerate and makes up 25-30% of the Noyon Uul Triassic-Jurassic record. Large bar sets, up to 10 meters in relief, and large, braid plain intervals are common. The meandering fluvial facies is composed of channelized, thick, cross-stratified sand and interbedded shale with local caliche and redbed horizons and comprises 35-40% of the total measured section. The shallow lacustrine facies consists of green and gray shale with thin, cm to dm scale interbedded sand and massive carbonate layers with meter-scale thickness. Lacustrine deltaic facies includes dm to m scale lenticular, cross-stratified, channelized sandstone with abundant soft sediment deformation interbedded with planar-laminated shale. The deep lacustrine facies consists of organic-rich, laminated carbonate.

Felsic to intermediate composition volcanic detritus is the dominant clastic component of sandstone and conglomerate at the base of the section, generally comprising over 50% of total clast content. An increasing component of granite clasts and K-feldspar grains upsection suggests unroofing of granitic source regions. Paleocurrent measurements indicate predominantly west-directed flow, consistent with an axial drainage pattern in front of an east-west trending highland source region.

The character of depositional environments, paleodispersal patterns and thicknesses of Noyon Uul Mesozoic strata is consistent with the interpretation of a syntectonically developing foreland basin in front of a north-verging thrust system. Such a Mesozoic contractile deformation system has been reported from northern China, 100 km south of Noyon Uul.

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