鄂尔多斯盆地奥陶纪层序岩相古地理

Sequence lithofacies paleogeography of the Ordovician in Ordos Basin,China

通过大量露头与钻井沉积相观察,系统编制了三级层序地层格架下的鄂尔多斯盆地奥陶纪岩相古地理图。研究认为,早奥陶世发育近SN走向的伊盟—中央古陆将盆地分为东、西2部分碳酸盐台地沉积区,其沉积古地理分别受控于华北海及祁连海;由早奥陶世至中奥陶世盆地总体呈现出逐渐海侵及伊盟—中央古陆逐渐缩小的过程,伊盟—中央古陆在中奥陶世大部分被海水侵没并已分化为南、北相隔的2个古陆即庆阳古陆和伊盟古陆,至中奥陶世晚期即达瑞威尔期(马家沟组六段沉积期)海侵达到最大范围;晚奥陶世发生海退,造成现今鄂尔多斯盆地范围内大部分表现为古陆剥蚀区,仅在盆地西缘及南缘发育了较窄的开阔镶边台地及其斜坡相沉积,至晚奥陶世中晚期(凯特期—赫南特期)几乎全部隆升为陆。自早奥陶世晚期(弗洛期)至中奥陶世中期(达瑞威尔初期)即马家沟组一段至五段沉积期,盆地中东部发育大面积的局限蒸发环境,形成局限潟湖甚至膏盐潟湖。盆地西部与南部受贺兰拗拉槽与秦祁海槽发展演化控制,主要发育大陆边缘沉积体系,由早奥陶世—中奥陶世早期的被动陆缘碳酸盐缓坡台地向中奥陶世晚期—晚奥陶世的主动陆缘弱镶边碳酸盐台地演化。

Through the sedimentological observations of a large number of outcrops and wells, this study prepared the lithofacies paleogeographie maps of the Ordovician in Ordos Basin in a 3rd-order sequence stratigraphic framework. In the Lower Ordovician, the Ordos Basin was divided into two sedimentary areas （i. e. , eastern and western carbonate platforms） by the south-north Yimeng- Central Palaeouplift, where the sedimentary paleogeography was controlled by the North China Sea and the Qilian Sea, respectively. In the Lower-to-Middle Ordovician, the transgression occurred and Yimeng-Central Palaeouplift gradually reduced in the basin. In the Middle Ordovician, the Yimeng-Central Palaeouplift was largely submerged by seawater and divided into two parts, i. e. , Qingyang oldland in the south and Yimeng oldland in the north. In the late Middle Ordovician, i. e. , the Darriwilian Age （deposition period of the 6th Member of Majiagou Formation）, the transgression occurred in the maximum scale. In the Upper Ordovician, the regression occurred, resulting in the vast denuded zones of oldland in the present Ordos Basin with narrow open rimmed platforms and slope facies in the western and southern margins of the basin. In the middle-to-late Upper Ordovician （Katian-Himantian Age）, the Ordos Basin almost entirely uplifted to the continent. From Floian Age （the late Lower Ordovician） to Darriwilian Age （the middle stage of Middle Ordovician）, i. e. , deposition period of the 1st-5th Members of Majiagou Formation, an evaporation environment was formed in vast areas of the eastern basin, leading to the formation of localized lagoon and even gypsum salt lagoon. The western and southern basin was controlled by the development and evolution of Helan Aulacogen and Qinqi Troughs and mainly developed continental margin sediments, which evolved from the carbonate ramps of passive continental margin in the Lower and early Middle Ordovician to the rimmed platform of active continental system in the late Middle and Upper Ordovician.