黔东北志留系远岸印江型香树园组的岩相和生物相

LITHO-AND BIOFACIES OF THE OFF-SHORE “YINJIANG TYPE” XIANGSHUYUAN FORMATION(SILURIAN) IN NORTHEASTERN GUIZHOU

上扬子区黔中古陆以北的陆表海底地貌、深度和浑浊度变化控制志留纪岩相和生物相的差异性。香树园组为黔东北志留纪兰多维列世鲁丹期—埃隆中期碳酸盐岩台地相地层单元,在近岸"白沙型"香树园组浅水灰岩地层中含有丰富的后生动物礁滩序列;同期远滨带是以印江合水小安土剖面为代表的"印江型"香树园组灰岩,其沉积环境处于最大的浪基面之下,岩性组份中粉砂质和泥质含量较高,粒度偏细,后生动物大化石含量很低,松软底质上存在潜穴生物栖居,但不是类似龙马溪组笔石页岩的缺氧带。印江型沉积区水深偏大和水温偏低的环境指标抑制了近岸"白沙型"后生动物礁滩分布向广海区的空间延伸。在黔东北地区,"印江型"香树园组正好位于"白沙型"礁滩相香树园组与缺氧底质的龙马溪组黑色笔石页岩之间的过渡带。

Ocean floor topography, water depth, and turbidity were principal factors controlling Silurian bio-and lithofacies distribution in the epicontinental sea of the Upper Yangtz Platform, South China Block. The Xiangshuyuan Formation consisting of carbonate platform sequences is assigned to the Rhudda- nian to middle Aeronian Stage of the early Silurian Llandovery Series. The ＂Baisha-type＂ Xiangshuyuan Formation was deposited in shallow-water and near-shoal environment around the Qianzhong Land and is composed of reef and bank facies rich in metazoan fossils. Coeval limestones at the Xiaoantu section, Hes- hui, Yinjiang is a representative of the ＂Yinjiang-type＂ Xiangshuyuan Formation, which was deposited in an off-shore setting below the storm wave base and contains a greater abundance of silty and muddy sedi- ments, a lower abundance of metazoan macrofossils, and a moderate abundance of burrows in find-- grained sediments. Compared with the ＂Baisha type＂, the ＂Yinjiang-type＂ Xiangshuyuan Formation was deposited in a deeper and colder environment which was not conducive for the development of metazoan reefs and banks. The ＂Yinjiang type＂ Xiangshuyuan Formation represents a transitional environment be- tween the reef complex of the ＂Baisha type＂ Xiangshuyuan Formation and the anoxic marine--floor of the graptolitic black shales of the Lungmachi Formation.