鄂尔多斯盆地西北部下白垩统物源定量分析研究

Quantitative analysis of provenance in the Lower Cretaceous of the northwestern Ordos Basin

下白垩统是鄂尔多斯盆地晚中生代的一个重要构造层,同时也是近年来新发现砂岩型铀矿的重要层位。恢复该时期的源汇系统对认识燕山运动晚期在华北克拉通西部的构造沉积响应、古气候转变和盆地内砂岩铀矿的富集规律均有较大的帮助。本文通过对鄂尔多斯盆地西北部下白垩统进行古流向测量、不同层位碎屑锆石U-Pb测年,优化周缘潜在物源区锆石U-Pb年龄数据集的汇编方案,定量计算出各物源区对盆内的碎屑贡献比例,恢复了鄂尔多斯盆地西北部早白垩世的源汇系统。研究结果表明,鄂尔多斯盆地西北部下白垩统中碎屑锆石具有1.79~1.64 Ga、2.02~1.82 Ga和2.58~2.45 Ga 3个稳定的前寒武纪年龄峰值,以及336~257 Ma和476~425 Ma两个稳定的显生宙年龄峰值。定量的物源分析结果表明,阴山、狼山、桌子山贺兰山和阿拉善地区以不同的贡献比例为研究区提供物源,具多物源同时供给的特征。阴山在整个早白垩世一直是主要的物源供给区,与狼山共同向盆地内部提供河流相沉积。阿拉善地区则通过原地风蚀将物质从桌子山北部和贺兰山顶以风沙搬运的方式带入鄂尔多斯盆地,在洛河罗汉洞沉积期共同构成了研究区风成与水成交替的沉积特征。

The Lower Cretaceous represents a significant structural layer in the Ordos Basin,particularly notable for the emergence of sandstone-type uranium deposits in recent years.Understanding the source-sink dynamics during this period offers valuable insights into the tectonic-sedimentary processes triggered by the late Yanshanian orogeny in the western North China Craton,the paleoclimate transition,and the enrichment patterns of sandstone uranium deposits within the Ordos Basin.In this study,we conducted paleocurrent azimuth measurements and detrital zircon U-Pb dating across various strata of the Lower Cretaceous in the northwestern Ordos Basin.Furthermore,we refined the compilation approach for the zircon U-Pb age dataset in the potential provenance area surrounding the basin.Utilizing quantitative analyses,we assessed the detrital contributions from each provenance to the basin.Subsequently,the source-sink system of the Lower Cretaceous in the northwestern Ordos Basin was reconstructed.The results delineate the U-Pb ages of detrital zircons from the Lower Cretaceous into three primary groups:1.791.64 Ga,2.021.82 Ga,and 2.582.45 Ga,corresponding to Precambrian ages.Additionally,two major age groups of 336257 Ma and 476425 Ma signify Phanerozoic ages.Through quantitative provenance analysis,we identified the Yinshan,Langshan,Zhuozishan-Helanshan,and Alxa regions as collective sediment contributors to the study area,each with varying contribution ratios,indicating a diverse provenance supply.Our study suggests that during the Early Cretaceous,the Yinshan and Langshan regions primarily acted as provenance areas,supplying fluvial deposits to the basin s interior.In contrast,the Alashan area transported sediments from the northern part of Zhuozishan and the Helanshan peak into the Ordos Basin through aeolian processes.This phenomenon led to the deposition of alternating eolian and fluvial formations in the study area during the Luohe-Lohandong period in the Early Cretaceous.