东濮凹陷古近系沙三中亚段泥页岩地球化学特征及湖盆演化分析

GEOCHEMICAL CHARACTERISTICS AND LAKE-BASIN EVOLUTION OF MUDSTONE AND SHALE OF THE MIDDLE SECTION OF THE THIRD MEM-BER OF SHA HEJIE FORMATION IN DONGPU DEPR ESSION

依据岩相、泥页岩元素地球化学特征开展东濮凹陷沙三中亚段古地理环境分析。通过主微量以及稀土元素的配比,在四个高精度等时地层格架内开展湖盆的古氧化还原条件、古气候、古盐度、岸线距离以及古水深分析。结果表明:样品的Th/U在0.37~5.86之间,平均值为3.68;V/(V+Ni)在0.67~0.87之间,平均值为0.78;V/Cr比值在0.97~3.14之间,平均值为1.68;V/Sc比值在6.56~17.63之间,平均值为8.00;Ce/Ce*比值在0.79~1.08之间,平均值为0.98;Ceanom比值在-0.18~-0.03之间,平均值为-0.08;Sr/Cu比值在5.05~97.31之间,平均值为29.14;Mg/Ca值整体偏低,平均值为0.4;Lan/Ybn值为0.74~1.87,均值为1.54;Rb/Sr比值在0.02~1.26之间,平均值为0.43;Fe/Mn比值在8.09~147.82之间,平均值为63.51;Sr/Ba比值在0.05~6.92之间,平均值为1.40;Rb/K比值在4.40~5.88之间,平均值为4.96。整体看来,研究区沙三中亚段气候整体干热,湖盆水体为强还原半咸水-咸水。湖盆经历了四次干热与温湿交替,对应了四次湖盆扩张与收缩,湖盆扩张期水体矿化度较低,萎缩期显著升高,尤其上部两个层序湖盆萎缩期膏盐岩发育。高矿化度流体以浓缩成因为主,伴有间歇性的热液补给。在湖盆扩张期,湖盆中心以深湖相暗色泥岩为主,岸线附近发育滨浅湖、三角洲沉积;萎缩期湖盆中心向岸线方向依次发育厚层膏盐岩夹薄层泥岩沉积、三角洲前缘-滩坝沉积、红色基调的间歇暴露三角洲平原沉积。

Analysis of the paleogeographic environment of the middle section of the third member of the Shahejie Formation in Dongpu Depression based on petrographic and elemental geochemical characteristics of mud shale.The ratio of major,trace and rare earth elements are used to analyze the paleoredox conditions,paleoclimate,paleosalinity,paleoshoreline and paleowater depth of the lake basin,and four highresolution isochronous stratigraphic frameworks are established.The results show that Th/U of the samples ranges from 0.37 to 5.86,with an average value of 3.68;V/(V+Ni)ranges from 0.67 to 0.87,with an average value of 0.78;V/Cr ratio ranges from 0.97 to 3.14,with an average value of 1.68;V/Sc ratio ranges from 6.56 to 17.63,with an average value of 8.00;Ce/Ce*ratio ranges from 0.79 to 1.08,with an average value of 0.98;Ceanom ratio ranges from-0.18 to-0.03,with an average value of-0.08;Sr/Cu ratio ranges from 5.05 to 97.31,with an average value of 29.14;Mg/Ca value is generally low,with an average value of 0.4;Lan/Yb.values ranges from 0.74 to 1.87,with an average value of 1.54.Rb/Sr ratio ranges from 0.02 to 1.26,with an average value of 0.43.The Fe/Mn ratio ranges from 8.09 to 147.82,with an average of 63.51.The Sr/Ba ratio ranges from 0.05 to 6.92,with a mean value of 1.40;Rb/K ratio ranges from 4.40 to 5.88,with an average of 4.96.Overall,it is recognized that the climate of the middle section of Sha-3 in the study area is dry and hot,and the water body of the lake basin is strongly reductive brackish water.The lake basin has experienced four alternations of dry heat and warm humidity,corresponding to four times of lake-basin expansion and contraction.The water salinity was low in the expansion period,and significantly increased in the shrinking period,especially in the shrinking period of the lake basin in the upper two sequences.Highly mineralized fluid is mainly concentrated and accompanied by intermittent hydrothermal recharge.In the expanding stage of the lake basin,the center of the lake basin was dominated by deep lacustrine dark mudstone,while shallow lake and delta deposits were developed near the shoreline.During the shrinking period,thick layer of salt-rock with thin layer of mudstone,delta-front and beach-bar deposits,and intermittenly exposed delta-plain deposits with red tone developed successively from the center of the lake basin to the shoreline.