Characteristics, Genesis, and Sedimentary Environment of Duplex-Like Structures in the Jurassic Sediments of Western Qaidam Basin, China

A particular type of soft-sediment deformation structure, similar to imbricate structure, is developed in the Jurassic strata at Honggouzi, western Qaidam Basin, China. We refer to this structure as a duplex-like 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. The layers of duplex-like structure crop out in the southeastern limb of the core of a medium-sized anticline. The average dip direction of the duplex-like structure is 301.2° and the average dip angle is 54.7°. Duplex-like deformed laminations are composed mainly of weakly metamorphosed, extremely poorly sorted, feldspathic lithic graywacke. Sericite can be observed along bedding planes. The duplex-like structure occurs within a sequence of river-channel fine conglomerate, interchannel carbonaceous mudstone (shale), shallow-water delta sand-shale, shallow-lake calcareous mudstone, olistostromes, and a turbidite. At the bottom of the sequence, we found brownish-red shallow-lake calcareous mudstone and carbonaceous mudstone and at the top olistostromes whose genesis is related to the slip and drag of a slumped body of submarine sediment. In combination with other symbiotic and associated structures, it is considered that the duplex-like structure was formed by the slumping and subsequent movement and traction of delta-front semiconsolidated sediments over the bottom sediments of a shore or shallow-lake sedimentary environment, probably triggered by an earthquake. The attitude of the duplex-like structure indicates that the direction of gliding was from NW301.2° toward SE121°, which is consistent with the current location of the Altun Mountains, indicating that the Altun Mountains existed in the Jurassic. 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.