鄂尔多斯地块西北缘贺兰山中段长城系沉积环境研究OA

This study focuses on the lithological composition,sedimentary features,and paleoenvironmental conditions of the Huangqikou Formation within the Mesoproterozoic Changcheng System in the middle part of the Helan Mountains.The research further examines the provenance signatures and tectonic-depositional setting of Changcheng System clastic rocks,offering critical geological constraints for lithofacies paleogeographic reconstruction and petroleum potential assessment in this stratigraphic unit.Integrated field outcrop observations and petrographic analyses were performed on the Huangqikou Formation clastic rocks in the middle part of the Helan Mountains,combined with systematic geochemical characterization of major,trace,and rare earth elements(REE).These investigations elucidate the sedimentary environment,depositional evolution,and sedimentary model.The Huangqikou Formation of the Changcheng System is predominantly composed of grayish-white to purplish-red,medium-to thick-bedded(locally thin-bedded)medium-to fine-grained quartz sandstones and quartzitic sandstones.These clastic rocks exhibit good sorting and rounding,indicating high textural maturity.Sedimentary structures including wave ripples and tidal bedding are well developed.The formation is interpreted to have been deposited in a brackish to semi-saline shoreface environment under warm and humid climatic conditions,characterized by:shallow water,strong hydrodynamic energy,predominantly oxidizing conditions,with localized transitional zones of oxic to weakly reducing environments.Provenance analysis reveals that the detrital components were overwhelmingly derived from syn-collisional,S-type granites in the northern Helan Mountains orogenic belt.During the early stage of the Changchengian Period(Early Mesoproterozoic),a marine transgression commenced,resulting in the deposition of the lower member of the Huangqikou Formation unconformably over the Paleoproterozoic Zhaochigou Group.This basal succession is dominated by shoreface facies clastic rocks.With the progressive marine transgression,the middle member of the Huangqikou Formation developed,characterized by dark fine-grained tidal flat deposits.The Upper Huangqikou Formation was deposited during sustained marine transgression,consisting of a low-energy sequence dominated by greyish-white quartzitic sandstone interbedded with greyish-white,pale pinkish-red dolomite and argillaceous dolomite.These deposits are overlain by the Jixian System,which represents the shallow-marine carbonate platform deposits.