The depositional sequence and prototype basin for Lower Triassic Hongshuichuan Formation in the eastern segment of East Kunlun Mountains

Based on a study of typical sedimentary structure and sedimentary system of Hongshuichuan Formation, the authors analyzed the prototype basin. The Hongshuichuan Formation, from the bottom to the top, is divided into six rock members. The first rock member is mainly composed of grayish green gravel-bearing coarse conglomerate and purple fine clastic rock; the second rock member is mainly grayish green pebble-bearing coarse sandstone, coarse sandstone locally interbedded with conglomerate layer combinations; the third rock member is mainly composed of dark gray-grayish black thin-medium-thin-bedded limestone; the fourth rock member is mainly composed of light gray-gray medium-thin-bedded siltstone, medium-fine-grained sandstone; the fifth rock member is mainly the turbidity combination, composed of many sections of regular rhythm combinations in Bouma sequence; the sixth rock member is mainly composed of gray-gray and black medium-thin-bedded fine conglomerate, fine gravel-bearing sandstone, fine sandstone, interbedded with grayish black thin-bedded calcareous siltstone and a small amount of limestone as well as marl. The analysis of the rock assemblages and depositional structure shows that the first and second members belong to the fan-delta depositional system, the third and fourth rock members belong to the shallow diamictic shelf deposition system, the fifth rock member belongs to bathyal-deep-sea turbidity current depositional system and the sixth rock member belongs to the littoral (shallow sea) depositional system. The depositional sequence of Hongshuichuan Formation is similar to the forearc basin sedimentary sequence, which records the consecutive deposition process of initial deposition, normal deposition and late deposition in the forearc basin. Based on previous researches on the paleocurrent, arc magmatism and ophiolite mélange, the authors consider that Hongshuichuan Formation formed in the forearc basin, which recorded the process of formation of forarc basin and the subduction activity.