湘鄂西部地区晚奥陶世三叶虫相和古地理演化

对湘鄂西地区 8个晚奥陶世剖面的 3个时段 (Caradoc中期 ,晚期及 Ashgill早期 )地层作了详细采集 ,获得了丰富的三叶虫材料 ,根据动物群的组成、分异度和各组成分子的相对丰度 ,识别出外陆棚—棚外深水区的 9种三叶虫相 ,它们在时空分布上具有逐渐过渡更替的特征。根据水中浮游类型三叶虫 (圆尾虫类 )的分异度、相对丰度以及与它们共生的底栖三叶虫生态特征 ,对各种三叶虫相所指示的水体深度作了推测。三叶虫相研究表明 ,区内Caradoc中期处于海侵高峰 ,古地理样式显示了以拉张为主的区域构造背景 ;Caradoc晚期开始发生持续海退 ,同时扬子陆块南缘受到构造挤压 ,广大外陆棚区地形变得起伏不平。晚奥陶世古地理变迁证明华夏和扬子陆块的会聚事件始于 Caradoc晚期

Late Ordovician Trilobite Biofacies and Palaeogeographical Development, Western Hubei-Hunan

On the basis of representative collections made from eight measured sections in Western Hubei Hunan, nine Late Ordovician trilobite biofacies are recognized in relation to an onshelf offshelf environment gradient. Depth ranges of different biofacies are inferred largely according to the diversity and relative abundance of mesopelagic cyclopygids. Briefly outlined in figures 1 and 12 are the distribution of lithofacies and biofacies belts and the palaeogeography of this region at the middle, late Caradoc and early Ashgill (intervals respectively of H. europaeus Zone, lower part of P. insculptus Zone and Nankinolithus Zone, see Table 1). The occurrence of offshelf trilobite biofacies in the middle Caradoc marks the climax of Ordovician transgressions. However, from the late Caradoc on, the region mainly underwent a persistent regression as indicated by the seaward expansion of outer shelf into the previous offshelf area. At the same time interval, as part of the southern margin of the Yangtze Block, the region was also subjected to compression due to the initial collision of the Cathaysia and Yangtze Blocks. This led to the formation of a rather rolling topography of the vast outer shelf, as revealed by the replacement of trilobite biofacies, with the Yichang area strongly subsided and the Liujiachang Cili Changde area relatively elevated.