鄂尔多斯盆地西缘中-晚奥陶世沉积演化特征及古环境分析

SEDIMENTARY EVOLUTION CHARACTERISTICS AND PALEOENVIRONMENTAL ANALYSIS OF THE ORDOVICIAN IN THE WESTERN MARGIN OF THEORDOSBASIN

鄂尔多斯盆地西缘中-晚奥陶世的沉积体系的研究一直存在争议,尚未达到广泛的共识,严重制约了盆地的勘探与开发。本次研究基于野外露头剖面、钻井岩心和薄片鉴定等资料,结合碳氧同位素、锶同位素以及微量元素多种地球化学手段,对鄂尔多斯盆地西缘中-晚奥陶世沉积环境和沉积特征进行了研究。结果表明,在中-晚奥陶世,鄂尔多斯盆地西缘地区自西向东发育台地(未镶边)-斜坡-盆地相沉积,沉积相带呈南北带状分布,东西分异的特征。三道坎至乌拉力克沉积期,台地规模逐渐消减,陆棚和盆地相沉积区逐渐增大。地球化学指标显示,中-晚奥陶世研究区海平面总体呈现上升的趋势,其中又包含有两期次级海平面的升降,在乌拉力克沉积期达到最大海泛面,δ^(13)C值达到最大。三道坎至桌子山沉积期,Sr/Cu比值平均250.62,V/(V+Ni)平均值为0.31,δCe平均值为0.10,表明研究区气候相对干热,处于氧化-弱氧化环境;克里摩里-乌拉力克沉积期,Sr/Cu比值平均2.69,V/(V+Ni)平均值为0.61,δCe平均值为1.93,表明该时期气候温暖湿润,沉积环境逐渐转变为了弱氧化-还原。海平面和古气候条件的变化,影响了鄂尔多盆地西缘中-晚奥陶世沉积物的分布和沉积相带的展布。

The research on the sedimentary system of the Middle Late Ordovician in the western margin of the Ordos Basin has been controversial and has not yet reached a widespread consensus,which seriously restricts the exploration and development of the basin.This article is based on field outcrop profiles,drill-ing cores,and thin section identification data,combined with various geochemical methods such as carbon and oxygen isotopes,strontium isotopes,and trace elements,to study the sedimentary environment and characteristics of the Middle Late Ordovician in the western edge of the Ordos Basin.The results indicate that during the Middle to Late Ordovician,the western edge of the Ordos Basin developed platform(un bordered)slope basin sedimentary facies from west to east,with sedimentary facies belts distributed in a north-south belt and characterized by east-west differentiation.During the sedimentation period from Sandaokan to Wulalike,the scale of the plateau gradually decreased,and the sedimentary areas of the con-tinental shelf and basin facies gradually increased.Geochemical indicators show that the overall sea level in the Middle Late Ordovician study area showed an upward trend,including two periods of secondary sea level rise and fall,reaching the maximum flooding level during the Wulalike sedimentation period,the av-erage a13C value has reached its maximum.During the sedimentation period from Sandaokan to Zhuozis-han,the average Sr/Cu ratio was 250.62,and the average V/(V+Ni)value was 0.3l,the average Ce value is O.10,indicating that the climate in the study area is relatively dry and hot,and is in an oxidizing weakly oxidizing environment;During the Kelimoli to Wulalike sedimentation period,the average Sr/Cu ratio was 2.69,and the average V/(V+Ni)value was 0.61,the average Ce value is 1.93,indicating that the climate was warm and humid during this period,and the sedimentary environment gradually trans-formed into weak oxidation-reduction.The changes in sea level and paleoclimatic conditions have affected the distribution of Middle Late Ordovician sediments and the distribution of sedimentary facies belts in the western edge of the Ordos Basin.