SEDIMENTATION HISTORY AND PROVENANCE ANALYSIS OF A LATE MESOZOIC RIFTING EVENT AT TAVAN HAR, EAST GOBI, MONGOLIA

The East Gobi basin in southeastern Mongolia is one of several basins in eastern China and Mongolia that was formed by intracontinental rifting during the late Mesozoic. The understanding of intracontinental deformation and basin-forming processes in the region is constrained by a lack of detailed structural, sedimentary and paleogeographic data. This study investigated the sedimentology and provenance of pre-, syn- and postrift sequences at Tavan Har in the northern East Gobi Basin in order to reconstruct the rifting event. The prerift (Early to Middle Jurassic) Khamarkhoovor Formation is exposed in the southeast part of the field area containing 10-15 m of conglomerate and sandstone beds. The early synrift (Late Jurassic) Sharilyn Formation, found ~40 km to the north of Tavan Har, shows thick sequences of fine-grained sandstones. The synrift (Lower Cretaceous) Tsagantsav and Hukhteg Formations are the units found most extensively at Tavan Har. Several measured sequences record basal conglomerate beds that grade into fine-grained lacustrine deposits. The postrift (Upper Cretaceous) Bayanshire Formation unconformably overlies the youngest synrift deposits, and contains more mature conglomerate along with fine-grained fluvial and lacustrine beds. Sandstone compositions from throughout the pre- and synrift sequences are lithic-rich and are followed by more quartz-rich postrift sediments. Commonly used ternary diagrams interpret the lithic-rich samples to indicate a magmatic arc provenance. However, this provenance is understood to be a result of remnant arc sequences that make up much of the basement rock in the area rather than contemporaneous arc magmatism. The initiation of late Mesozoic rifting at Tavan Har resulted in normal faulting and further uplift of already exposed basement rock, which led to episodic, coarse, alluvial deposition. This was followed by the development of fluvial and lacustrine systems in the area. In the mid-Cretaceous, the synrift sediments were deformed by tectonic inversion of faults in response to compressional stress. More quartz-rich postrift sediments were deposited in alluvial and fluvial systems on shallower slopes. This study shows that detailed field studies can add significantly to the understanding of late Mesozoic deformation in northern Asia.