宁夏六盘山盆地早白垩世六盘山群原型盆地特征

摘要

Liupanshan Basin is a Mesozoic-Cesozoic rift sedimentary basin which is located in the southern North China Plate and the east extension part of Hexi Corridor and lies along Qinling-Qilian Caledonian fold belt. In this paper, the authors analyzed con⁃glomerate gravel characteristics and palaeocurrent of Liupanshan Basinand explored provenance characteristics of Lower Cretaceous strata. Outcrop observations show that separation and grinding roundness of conglomerate and sandstone of Sanqiao Group are poor , reflecting middle-low structure maturity and nearby source. At the beginning of the Early Cretaceous ,basin boundary was affected by strong compressive tectonic background of Late Jurassic,a series of highland internal basins and peripheral mountains suffered from weathering denudation, and the southwestern margin was mainly characterized by coarse clastic sedimentary piedmont pluvial fan ac⁃cumulation and deposited piedmont pluvial fan of predominantly coarse fragments,which was gradually transitional to braided river towards the interior of the basin. In late Early Cretaceous,the basin boundary was expanded towards the west. Accompanied by alter⁃nate drought-humid climate, there developed alluvial fan, braided river, delta, shallow lake and saline lake facies.%六盘山盆地是近南北向狭长的中新生代断陷沉积盆地，位于秦岭、祁连加里东褶皱带与鄂尔多斯盆地西缘断褶带之间，华北古板块南部边缘，为河西走廊东延部分。通过六盘山盆地六盘山群底部砾岩砾石特征、古水流等分析，探讨下白垩统沉积时期的物源特征。从露头观察结果看，三桥组沉积的粗粒砂砾岩具有分选性和磨圆度差的特点，反映结构成熟度低及离物源近的特点。早白垩世初期，六盘山盆地受晚侏罗世挤压构造的影响，盆地周缘山体为剥蚀区，西南缘沉积了一套以粗碎屑为主的冲积扇，向盆地内部过渡为辫状河；早白垩世晚期，受伸展构造的影响，盆地边界向外扩展，坳陷一并向西部扩展，随着气候的干旱-湿润交替出现，发育冲积扇、河流、三角洲及浅湖、咸化湖沉积。