湘鄂地区晚寒武世末至早奥陶世特马豆克期层序地层与岩相古地理

从寒武纪末至早奥陶世特马豆克期（牙形类Cordylodus intermedius 带至Serratognathus 带），地处扬子陆块的湘鄂地区分为三个沉积区：浅水碳酸盐台地、台地东南缘碳酸盐岩、泥质岩混合沉积区和较深水细碎屑岩沉积区。前二个沉积区的层序地层分为4个正层序，在同一陆块内不同沉积区这四个三级层序相互可对比，代表了四次三级海侵、海退旋回。其中牙形类Paltodus deltifer 带中、下部是特马豆克期最大的海侵时期。特马豆克早期（Glyptoconus quadraplicatus 带）和晚期（Serratognathus带中、上部）分别发生过大的海退事件。各正层序的体系域岩相古地理格局能更客观地反映该地区特马豆克期古地理演化特征。特马豆克初期（C.lindstromi 带至C.angulatus带下部）自鄂西北至湘中地区依次为潮坪、潮间泻湖白云岩相、开阔碳酸盐台地、台地边缘浅滩、碳酸盐岩台地边缘斜坡、黑色碳质页岩盆地的古地理格局。特马豆克早期（Glyptoconus quadraplicatus 带）缓慢海退时期海水普遍变浅，自北向南依次为潮坪、潮间泻湖白云岩相、局限台地相、台地边缘浅滩相和较深水碳酸盐外陆架。特马豆克中期海进时期，生屑灰岩和页岩沉积向北超覆，扩展至青峰襄广断裂。湘鄂浅水碳酸盐沉积区为开阔碳酸盐台地，混合沉积区为较深水碳酸盐外陆架，湘中地区为深水泥质下外陆架。特马豆克晚期缓慢海退时期海水再度变浅，在湖北、湘西北开阔碳酸盐台地上，出现台内生屑浅滩、台地边缘生屑浅滩，浅滩上发育有海绵、苔藓虫礁丘。

SEQUENCE STRATIGRAPHY AND LITHOFACIES PALAEOGEOGRAPHY OF THE TREMADOCIAN OF THE LATEST CAMBRIAN TO EARLY ORDOVICIAN IN THE HUNAN AND HUBEIREGION

 Four third-order orthosequences have been recognised in the latest Cambrian to Early Ordovician Tremadocian ( from the Cordylodus intermedus to Serratognathus zones) successions in the Hunan and Hubei parts of the Yangtze platform. These four orthosequences could be well subcorrelated with each other in different depositional settings of the Yangtze platform, indicating four third-order transgression-regression cycles. Among them , the transgression systems tract in sequence 3 ( the lower and middle Paltodus deltifer Zone ) represents the maximum transgression event of the lower-order cycle of the Tremadocian, and the major regression events occurred during early Tremadocian Glyptoconus quadraplicatus Zone and late Tremadocian Serratognathus Zone ( middle and upper part ), respectively.Integrated sequence stratigraphy and facies analysis has improved our understanding of lithofacies palaeogeographical evolution in the region. At the beginning of the early Tremadocian ( C.lindstromi to lower C. angulatus Zone) the depositional facies from the northwestern Hubei to central Hunan change successively from tidal flat, tidal lagoon dolomite facies through restricted carbonate platform, open carbonate platform , platform margin shoal complex, to platform marginal slope and the black carbonaceous shale basin . During the early stage of Tremadocian (Glyptoconus quadraplicatus Zone ), relative sea level fall caused gradual regression from north to south, resulting in depositional environments varying from tidal flat, tidal lagoon dolomite facies, restricted carbonate platform, to platform margin shoal and outer shelf. During the middle Tremadocian, a new transgression resulted in onlapping of the bioclastic limestone and shale towards the Qingfeng-Xiangguang fault zone to the north. At this stage, the shallow-water carbonate domain evolved into an open carbonate platform, the mixed carbonate and siliciclastic zone evolved into a deep-water outer shelf, and the central Hunan region became a lower outer shelf. During the late Tremadocian, another relative sea level fall caused a further slow regression, and the open carbonate platform in Hubei and northwestern Hunan was developed with the bioclastic shoal of inner shelf and platform margin shoal complex. On these platform margin bioclastic shoals, sponge and bryozoan reefs were well developed. The mixed sedimentary realm evoled into an upper outer shelf, and the central Hunan area became a deep muddy outer shelf.