宣龙坳陷青白口系长龙山组碎屑岩地球化学特征

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

宣龙坳陷青白口系长龙山组为一套滨浅海碎屑岩沉积,可划分为滨岸和浅海陆棚2个相区以及3种不同的相带。在对样品测试结果进行分析的基础上,系统论述了微量元素的富集规律及其所蕴含的环境意义,并利用相关性分析对各氧化物之间的关系进行了探讨。研究表明长龙山组沉积期海平面逐渐上升,多处古河流的汇入不仅为碎屑岩沉积提供了充足的陆源补给,而且使沉积水体淡化、盐度较低;同时也反映出该区地球化学特征与原始沉积环境有着极为密切的关系:无论是微量元素还是氧化物,其含量大多数都由前滨→临滨→滨外陆棚呈明显增加的趋势,特别是微量元素、P2O5、MgO、CaO、MnO含量及M值(M=100×MgO/Al2O3)随水体变化特征显著,都可作为沉积环境判别的有效标志。此外,解释了陆源组分Al2O3、K2O、Fe2O3在水体较深的临滨及滨外陆棚环境中含量增高的主要原因。上述特征不仅表明该区地球化学特征可作为沉积相分析的重要标志,也说明野外地层和沉积相划分的准确度较高。

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.