中扬子及邻区层序地层与原型盆地演化

中扬子及邻区原型盆地可划分为 9个大层序,其中,1～6大层序由中、古生代海相地层组成;第 7大层序为早、中侏罗纪的陆相沉积;第 8大层序为晚侏罗世的前渊沉积;第 9层序为晚白垩世至第三纪的伸展盆地。震旦纪,中部地区为台地相环境,向南为浅海陆棚环境,向北以陆表海为主;寒武纪至早奥陶世为典型的大陆边缘盆地;中、晚奥陶世至志留纪为裂谷盆地发展阶段;泥盆纪至石炭纪该区发生广泛海侵,与华南海域和太平洋海域多处相连;二叠纪至早、中三叠世为原型盆地的重要转型时期,随着分隔秦岭微板块与扬子板块的勉略古洋盆向东打开,扬子北缘由前期的区域性隆起转变为向北加深的大陆边缘盆地;晚三叠世经历了两次幕式旋回,巴东组沉积时期盆地沉降缓慢,九里岗组沉积时期则较快;早、中侏罗世原型盆地早幕 (早侏罗世)沉积缓慢,晚幕 (中侏罗世)快速沉降,以水下沉积作用为主;晚侏罗世可划分为两大构造演化阶段,早期构造活动相对平静,晚期强烈;晚白垩世至老第三纪,盆地范围缩小,以湖泊、河流体系为主。由此,中扬子及邻区在地质历史时期经历了 4种不同性质的原型盆地,具有幕式充填特征。

STRATIGRAPHIC SEQUENCES IN THE MIDDLE YANGTZE & ADJACENT REGION AND EVOLUTION OF PROTOTYPE BASINS

Prototype basins in the middle Yangtze and its adjacent region can be divided into nine mega- sequences. Sequence I-VI are composed of Mesozoic-Palaeozoic marine beds; Sequence VⅡ is made up of the early-middle Jurassic terrestrial deposits; Sequence VⅢ consists of Jurassic foredeep deposits; Sequence IX is characterized by the late Cretaceous and Tertiary rift basin deposits. During the Sinian Period, the central region was in the environment of platform facies while the south region was in the environment of shallow marine shelf and the north region was mainly in the environment of epicontinent sea. The Cambrian to the early Ordovician is characterized by typical continent marginal basins. Rift basins were well developed during the middle-late Ordovician-Silurian; Extensive transgression events occurred during the Devonian and Carboniferous which connected with several places of the South China Sea waters and the Pacific Ocean sea waters; From the Permian to the middle Triassic, the prototype basins were in the stage of transformation with Mianlue palaeo-oceanic basin(which separated Qinling micro-plates from the Yangtze plate)opening towards east and the former regional uplift in the northern margin of the Yangtze was transformed into the continent marginal basins of deepening northward. During the late Triassic, two episodic depositional cycles occurred in the region where Badong Formation is characterized by slow subsidence deposits and Jiuligang Formation by rapid subsidence deposits. The prototype basins experienced slow deposition in the early Jurassic and rapid deposition dominated by underwater sedimentation in the middle Jurassic. Two tectonic evolution stages can be classified in the late Jurassic, tectonic activities in the early stage was relatively quiet and in the late stage was quite strong. During the late Cretaceous-early Tertiary, the basins became smaller in size dominated by lacustrine and fluvial deposits. Therefore,the middle Yangtze and its adjacent region have experienced in geological history four types of prototype basins with the characteristics of episodic sedimentation.