Analysis of high-frequency sequences and sea-level change of the Middle Jurassic Matuo Formation in the Yanshiping area, northern Tibet

Abstract：The Middle Jurassic late Bajocian-early Bathonian Matuo Formation in Yanshiping, northern Tibet, is represented by mixed continental shelf deposits composed mainly of sandstone and mudstone with minor limestone. The condensed interval with shells, downlap and ravinement surface constitute the critical boundaries of the systems tracts of the Matuo Formation. The systems tracts are characterized by a binary structure, i.e. transgressive-highstand systems tracts, and the sedimentary thickness of TST is greater than that of HST. The parasequences are divided into three types, i.e. parasequences dominated by sandstones, parasequences dominated by mudstones, and upward-shallowing parasequences dominated by the tidal flat system. The prograding sequence and retrograding parasequence set is formed by parasequence stacking. A curve of relative sea-level change of the Matuo Formation has been established in the study area by using the depositional system method, and the constraints on the sea-level change in the late Bajocian-early Bathonian Matuo Formation in the study area have been studied based on a comparative study of the curve of sea-level changes in southern Tibet and the globe, combined with the data of oxygen and carbon isotopes and magnetic susceptibility. The eustatic sea-level changes control the early Late Bajocian sea-level change in the Yanshiping area, while the volume change of the regional oceanic basin caused by northward subduction of the Bangong Co-Nujiang suture is the primary factor responsible for the sea-level change during the latest Bajocian-early Bajthonian.