Unconformity-bounded Upper Paleozoic megasequences in the Beishan Region (NW China) and implications for the timing of the Paleo-Asian Ocean closure

摘要

The Upper Paleozoic sedimentary sequence in the Beishan Region, northwest China, provides a complete record of the complex amalgamation and extension history of the Central Asian Orogenic Belt (CAOB) for this part of China. Integrating newly obtained stratigraphic, paleontological and radiometric data, this paper reviews the Upper Paleozoic stratigraphic sequences of the Beishan Region. The Upper Paleozoic strata here are divided into three megasequences (MS). They are, in ascending order: Devonian, Mississippian-Guadalupian and Lopingian megasequences (MS1, MS2, and MS3), separated by two major unconformities each indicative of a huge sedimentary hiatus. MS1 is made up of continental and epicontinental marine deposits in the pre-rift stage; MS2 of transgressive marine and volcanic strata in the syn-rift stage; and MS3 of continental sediments in the inversion stage. The major unconformity between MS1 and MS2 is interpreted to represent the regional uplift and erosion caused by the final amalgamation of CAOB in the Beishan Region. The syn-rift MS2 represented by transgressive marine and volcanic strata in the Beishan Region can be well-correlated with Pennsylvanian-Guadalupian megasequences in Inner Mongolia, where they rest on a major interregional unconformity. This correlation suggests the closure of the Paleo-Asian Ocean in Beishan and Inner Mongolia before the Carboniferous, followed by the development of a synchronous marine rift megasequence in the southern part of the CAOB.