TYPICALLY SOFT DUPLEX STRUCTURES IN JURASSIC SYSTEM IN WESTERN CHAIDAM BASIN, CHINA

A particular type of soft-sediment deformation structure, similar to imbricate structure, is developed in the Jurassic strata at Honggouzi, western Chaidam basin, China. We refer to this structure as a duplex deformation structure, because it is inferred to have been formed by the action of fast-moving, submarine gravity current sediments gliding across a pre-existing semiconsolidated sedimentary layer. They are most often viewed in cross section, where they are observed to have a inbricate occurrences and have sharp tops and, usually, gradational bases. The geometric features are as follows: (I) inbrication-like, with about 20°-45°dipping angle; (II) 1-2cm in thickness of the inbricate sheets and mostly about 10cm in width, in ES direction, up to 40-50cm; (III) the sheet being mostly upright or slightly curve, convex-upward and sometimes with uneven curvature; (IV) sometimes, the tail of sheets being bifurcate and like swallow tail, or knotted and more irregular; (V)laterally, soft duplex structures may terminate rapidly and sudden breaks are characteristic of soft duplex structure layer; (VI)often，accompanied by other soft deformation sedimentary structures, such as convolution bedding, sand dike, small flute marks and soft deformation fold and so on. They develop mainly in yellowish silt sandstone, seldom in reddish brown fine silt sandstone interbedded with extremely thin limestone. The layer containing a typically soft duplex structure is overlain by a medium-thick, sand-rich turbidites with pebbles and boudinaged to balled-up masses of sandstone.

The attitude of the duplex-like structure indicates that the direction of gliding was from NW toward SE, this are interpreted to be result of up-lifting of Altyn mountain. The orogenesis of these mountains likely involved seismic activity, and an earthquake was the triggering factor in the formation of the duplex-like structure by causing the slumping/gliding of the olistostrome sediments.