SEDIMENTOLOGY, STRATIGRAPHY AND PALEOGEOGRAPHY OF TRIASSIC STRATA, NORTHWEST ORDOS, NORTH-CENTRAL CHINA

4 km of Triassic nonmarine strata are exposed in the Helan Shan and Zhuozi Shan, along the western Ordos basin. This thick succession unconformably overlies Permian rocks and is succeeded, unconformably, by Jurassic strata. Better documentation of the genetic stratigraphy of the Triassic is needed in order to establish the early Mesozoic paleogeography and tectonics of the region.

In the Helan Shan, the Triassic section consists of 2-3 km of fluvial sandstone, overlain by more than 1 km of lacustrine strata. Fluvial systems are characterized by lenticular trough cross-stratified sandstone with minor pebble conglomerate lenses, rare mudstone interbeds, and south or east-directed paleocurrents. Lacustrine strata are observed only in the Helan Shan and include both low-energy deepwater and marginal settings. These are characterized by dark shale and laminated siltstone beds, and tabular fine rippled sandstone beds. Thick (up to 200 m), laterally extensive packages of fluvial sandstone occasionally interrupt lacustrine facies. Conglomerate intervals occur within the lacustrine section along the western Helan Shan.

The Zhuozi Shan, to the east, contains an entirely fluvial Triassic section that is less than 2 km thick. These fluvial strata contain laterally-extensive intervals of lenticular trough cross-stratified sandstone and pebble conglomerate, separated by red mudstone. Paleocurrents in the Zhuozi Shan are west-directed.

Facies distribution and differences in thickness suggests that the Triassic basin along the western margin of Ordos subsided rapidly to form an asymmetric trough, thickening to the west. Fluvial systems filled the basin from both sides with steeper-gradient systems on the western side. Ultimately, lacustrine systems occupied the part of the basin now exposed in the Helan Shan. We interpret this distribution of facies to result from the evolution of a previously hypothesized Triassic extensional basin, bounded by a master fault system, now concealed west of the Helan Shan.