Clastic rock geochemical features of Changlongshan Formation in Qingbaikou System within Xuanlong Depression

Abstract：The Changlongshan Formation of the Qingbaikou System in Xuanlong Depression is dominated by relatively high-maturity littoral-neritic clastic sediments and can be roughly divided into barrierless coast and neritic shelf facies zones as well as into three different facies tracts. Based on an analysis of the testing result of clastic rock samples, the authors systemically discuss the enrichment mechanism of trace elements and its environmental significance and, using correlation analysis, deal with the correlativity between oxides of clastic rocks. The results show that, due to rising sea level during the deposition of Changlongshan Formation, the inflow of ancient rivers not only provided clastic sediments with sufficient terrigenous materials but also caused low salinity of sedimentary water body, suggesting that geochemical characteristics were closely related to the initial sedimentary environment in the study area: the content of both trace elements and oxides significantly increased from foreshore through nearshore to offshore shelf; the content of trace elements, P2O5, MgO, CaO, MnO and the values of M(M=100×MgO/Al2O3) changed especially significantly with the increasing water depth, which can be used as effective marks for the sedimentary environment. This paper also clarifies the major causes for the increase of the content of the terrigenous components (such as Al2O3, K2O, Fe2O3) in deeper water of the nearshore and offshore shelf environment. These features indicate that the geochemical characteristics of the area can be used as important marks for sedimentary facies analysis and that the accuracy of field division of strata and sedimentary facies is relatively high.