摘要
从寒武纪末至早奥陶世特马道克期 (牙形类Cordylodusintermedius带至Serratognathus带 ) ,地处扬子陆块的湘鄂地区分为 3个沉积区 :浅水碳酸盐台地沉积区、台地东南缘碳酸盐岩与泥质岩混合沉积区和较深水细碎屑岩沉积区。前两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 4个三级层序相互可对比 ,代表了 4次三级海侵、海退旋回。其中牙形类Paltodusdeltifer带中、下部是特马道克期最大的海侵时期。特马道克早期 (Glyptoconusquadraplicatus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道克期古地理演化特征。特马道克初期 (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。特马道克早期 (Glyptoconusquadraplicatus带 )缓慢海退时期海水普遍变浅 ,自北向南依次为潮坪、潮间湖白云岩相、局限台地相、台地边缘浅滩相和较深水碳酸盐外陆架。特马道克中期海进时期 ,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 。
Four third order orthosequences have been recognised in the latest Cambrian to Early Ordovician Tremadocian ( from the Cordylodus intermedius to Serratognathus zones) successions in the Hunan and Hubei parts of the Yangtze platform. These four orthosequences could be well correlated 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 during Tremadocian, and the major regression events occurred during the 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 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 the 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 mixedsedimentary realm evolved into an upper outer shelf, and the central Hunan area became a deep muddy outer shelf.
出处
《古地理学报》
CAS
CSCD
2000年第3期15-26,共12页
Journal of Palaeogeography：Chinese Edition
基金
国家自然科学基金会资助项目 (No .49772 0 87)成果之一。
国家自然科学基金会"资助留学人员短期回国工作讲学专项基金项目 (4 9810 76
关键词
湘鄂地区
寒武世
奥陶世
岩相古地理演化
层序地层
Hunan and Hubei region, Early Ordovician Tremadocian, lithofacies palaeogeographical evolution, sequence stratigraphy
