鄂西南奥陶系-志留系交界地层研究兼论宜昌上升

ON THE ORDOVICIAN-SILURIAN BOUNDARY STRATA IN SOUTHWESTERN HUBEI,AND THE YICHANG UPLIFT

对鄂西南四县（五峰、鹤峰、宜都和长阳）8条奥陶系-志留系交界剖面的野外调查发现，奥陶系与志留系之间普遍存在沉积间断，这是宜昌上升的直接证据。这一间断多发生在P．pacificus带与C．cyphus带之间，缺失至少4个笔石带（N．extraordinarius，N．persculptus，A．ascensu5和P．acuminatus），个别剖面甚至还缺失了奥陶系凯迪阶上部的2～3个笔石带（P．pacificus，D．complexus和部分D．complanatus），缺失量最大的在五峰小河村，从D．complanatus带上部到D．convolutus带共缺失10个半笔石带。两系间的接触关系类型有3种：A型——五峰组与龙马溪组呈假整合接触，B型——临湘组与龙马溪组呈假整合接触，C型——奥陶系一志留系交界处发育古风化壳。根据研究，宜昌上升的起始时间在P．pacifiCUS带上部（五峰组上部，大致在446Ma前后）；结束时间各地并不一致，最早结束于C．vesiculosus带（宜都潘家湾）（-442Ma），多数地区结束于C．cyphus带（-441Ma），最晚在D．convolutus带（五峰小河村）（-439Ma），也是目前上扬子区宜昌上升已知的最晚结束时间；推测该次运动延续的时间大约4-7Ma。依据古风化壳中高岭石矿物和五峰组地层残留物，认为：宜昌上升导致研究区曾经上升为陆地（即露出海面的岛屿），接受风化剥蚀。从区域构造演变来看，“华夏古陆”不断向西及西北扩展，进而产生由东南向西北的水平动力，这种水平动力是导致宜昌上升的主控因素，基于不同地区所受动力强弱、古地理和地势等因素不尽相同，宜昌上升在所涉及地区的表现各异。

Investigation on eight Ordovician-Silurian boundary sections in Wufeng, Hefeng, Yidu and Changyang counties, southwestern Hubei Province, shows that there is a hiatus between the Upper Ordo- vician and the lower Silurian in this region, which is solid evidence of the Yichang Uplift. This hiatus oc- curred between the late Ordovician Paraorthograptus pacificus Zone and the early Silurian Coronograptus cyphus Zone, missing at least four graptolitic biozones, including the Normalograptus extraordinarius Zone, Normalograptus persculptus Zone, Akidograptus ascensus Zone and Parakidograptus acuminatus Zone. At some sections, the top two to three biozones of upper Katian （Late Ordovician） are also missing including the P. pacificus Zone, the Dicellograptus complexus Zone and part of the D. complanatus Zone. The longest hiatus at the Xiaohe Village, Wufeng County, where the middle Aronian Demirastrites convolutus Zone directly and disconformably overlies the lower-middle Katian D. complanatus Zone with more than 10 graptolitic biozones missing. There are three kinds of contacts between the Ordovician and the Silurian in the study area. In Type A contact, a disconformity is developed between the Wufeng and Lungmachi formations. In Type B contact the Lungmachi Formation disconformably overlies the Linhsiang Formation. In Type C contact, a weathering crust occurs at the boundary between the Ordovician and Silu- rian. Based on our detailed investigation conducted in southwestern Hubei, the Yichang Uplift commenced at a time roughly corresponding to the upper P. pacificus Zone （the upper Wufeng Formation, -446 Ma）, and ended diachronously. At Panjiawan of Yidu County, the Yichang Uplift ended in the Cystograp- tus vesiculosus Zone （-442 Ma）, while at most other localities it ended in the C. cyphus Zone. The Xiao- he village of Wufeng County is the only locality where the Yichang Uplift lasted till the end of the D. conv- olutus Zone （-439 Ma）. So, the Yichang Uplift in southwestern Hubei lasted about 4-7 Ma. Analysis of kaolinite minerals and weathering residual of the Wufeng Formation in the ancient weathering shows that the Yichang Uplift caused subaerial emergence and weathering in the study area. The Uplift was probably connected with the western and northwestern movement of the Cathaysian Oldland during the Late Ordovi- cian and early Silurian. The movement was a major force driving the Yichang Uplift which had different manifestations （such as magnitude and duration） at different localities owing to the different paleogeo- graphic conditions.