新疆克拉玛依红山湖剖面白垩系清水河组结核成因及其意义OA

This study focuses on sandstone"concretions"in the Cretaceous Qingshuihe Formation at the Hongshanhu Section.It systematically investigates the lithological characteristics of concretion-bearing strata,as well as the external morphology,internal structure,material composition,formation environ-ment and genesis of the concretions.The Cretaceous Qingshuihe Formation at the Hongshanhu Section mainly develops a set of shallow-water delta facies sandstones with large-scale cross-bedding.Spherical,hemispherical,ellipsoidal,and other shaped sandstone"concretions"are widely distributed in the bottom sandstones of distributary channels of different stages.Currently,there are three viewpoints on the genesis of these concretions:(1)spherical weathering genesis;(2)diagenetic genesis;(3)intraclast genesis.To clarify their genesis,this paper systematically studies the macro occurrence,microtexture,material composition of the concretions,and their genetic relationship with host sandstones through field geological section measurement and petrographic thin section observation.The results show that the concretions are not intraclasts,but products of"concretionary"diagenetic cementation of sandstones during the syn-genetic-diagenetic stage,followed by differential weathering after uplift to the surface.Carbonized plant detritus is commonly developed in the center of the"concretions",with ferruginous calcite and calcite cement zonations sequentially formed outward around the plant detritus.The bedding structure of the"concretions"is consistent with that of the host sandstones without later tectonic disturbance.Due to dense cementation,the sandstone"concretions"have significantly stronger weathering resistance than the surrounding uncemented or weakly cemented sandstones,presenting spherical,hemispherical,ellipsoidal,and other irregular shapes after epigenetic weathering.Combined with the analysis of regional sedimentary background and diagenetic environment,it is believed that the formation of such diagenetic"concretions"indicates strong hydrodynamic forces in distributary channels,abundant organic matter input during the shallow-water delta deposition of the Qingshuihe Formation,and weakly alkaline-neutral pore water medium during the early diagenetic stage,which provided favorable chemical conditions for cement precipitation.The research results not only clarify the controversy over the genesis of sandstone"concretions"in the Qingshuihe Formation,but also provide key geological evidence for the restoration of the Cretaceous shoreline shallow-water delta sedimentary environment and the study of diagenetic evolution in the Junggar Basin,and have reference value for the heterogeneity research of oil and gas reservoirs and oil and gas development.