Implications of loess and fluvial deposits on paleoclimatic conditions during an icehouse–hothouse transition, Capitanian upper Quanzijie low-order cycle, Bogda Mountains, NW China

Abstract

Permian terrestrial sedimentary records provide important clues on paleoclimatic conditions and changes in a vegetated planet during the last icehouse–hothouse transition. Seven stratigraphic sections of Middle Permian upper Quanzijie low-order cycle (QZJ LC) in Bogda Mountains, NW China, are studied to interpret the depositional environments and paleoclimatic conditions within a multi-order stratigraphic framework. The upper QZJ LC contains distinct laterally persistent, thick, and massive red mudrocks interbedded with laterally discontinuous conglomerates. The mudrocks are interpreted as loess deposits on the basis of their massive structure, uniform multi-modal grain size distribution, lateral extent, mineral composition, and uniform detrital zircon ages. Thin conglomerates with a low-relief erosional base are interpreted as ephemeral braided stream deposits, whereas thick conglomerates with a high-relief base that display a clear fining-upward trend as meandering stream deposits. In Tarlong–Taodonggou area in the south, common Protosols in loess deposits indicate dust accumulation under arid to semiarid conditions. In Dalongkou 70km to the north, argillic Protosols indicate subhumid conditions alternating with arid–semiarid episodes. Gleysols in the upper part in both areas indicate a change to subhumid–humid conditions at the end of QZJ time. The results strongly suggest that persistent aridity, eolian processes, and dust trapping mechanisms had existed in NE Pangea during Capitanian time. The gradual but dramatic climatic change from arid–semiarid to subhumid–humid conditions at the end of Capitanian correlates to and is the result of the global demise of the late Paleozoic Ice Age.