The thick-bedded coarse clastic rocks at the bottom of the Mesozoic erathem were widely developed in the Yingen-Ejin Banner Basin, northern China, whose age was determined in combination with their sedimentary charac-teristics in this study. The coarse clastic rocks deposited in a fan sedimentary environment developed at the initial stage of the sags, and the sedimentary facies were fan delta and underwater fan etc.. The coarse clastic rocks were characterized by "extensive development and locally increased thickness" and deposited during the stable and contin-uous subsidence of the basin with stable depositional rate. The deposition time of the coarse clastic rocks was 115.6-137.0 Ma determined by zircon U-Pb chronology, corresponding to the age of early Cretaceous period. The sporopollen assemblages of these rocks were in a combination of Classopollis-Protoconiferus-Perinopollenites, and the Ostracod fos-sils were mainly composited of single rib Actress, Metacypris, Lycopterocypris, etc.. The fossil characteristics were similar to that of the Early Cretaceous established by predecessors in the Yingen-Ejin Banner and peripheral basins. Thus, the age of the coarse clastic rocks should be early Cretaceous period, and their bottom boundary could be de-fined as the regional marker layer of the early Cretaceous strata. These results do not only solve the problems including the lack of evidence on stratigraphic division, shortage of unified stratigraphic classification scheme but also provide information about the Mesozoic sags forming era and potential tectonic evolution of the Triassic to Jurassic period with-out other stratigraphic records.%银根—额济纳旗盆地(简称银额盆地)中生界中下部广泛发育一套厚层粗碎屑岩,结合沉积特征,利用U-Pb定年、孢粉和介形虫资料对其沉积年代进行的厘定结果显示,这些粗碎屑岩可能是凹陷形成初期扇三角洲、水下扇等环境接收的沉积,相对稳定的湖盆沉降和沉积速率形成了银额盆地沉积地层"广泛发育、局部巨厚"的分布特征.锆石U-Pb测年厘定的粗碎屑岩沉积时间为115.6～137.0 Ma,对应年代为早白垩世.粗碎屑岩段的孢粉以Classopollis-Protoconiferus-Perinopollenites组合为特征,介形虫化石主要有单肋女星介、圆星介、狼星介等.这些古生物特征与前人建立的银额盆地及其周缘盆地早白垩世化石特征相似.因此,该粗碎屑岩的地层年代应该为早白垩世,其底界应该是下白垩统底界,可以作为区域性标志层.这样一来,不仅解决了银额盆地中生界下白垩统底界划分依据不明确、划分方案不统一的问题,也能为中生界断陷湖盆形成时期的确定、该区地层普遍缺失的三叠纪—侏罗纪时段的构造演化特征研究等提供依据.
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