扬子台地西南缘奥陶纪达瑞威尔期—桑比期地层划分对比与古地理

DARRIWILIAN-SANDBIAN(ORDOVICIAN) STRATIGRAPHY AND PALEOGEOGRAPHY OF THE SOUTHWESTERN YANGTZE PLATFORM, SOUTH CHINA

扬子台地西缘(今昆明—西昌—成都之间)奥陶纪达瑞威尔期—桑比期地层以往多被笼统地归入“巧家组”,其空间分布情况不详,划分与对比仍存争议。综合最新研究成果与前人资料,本文认为“巧家组”仅限于台地西南缘(今川滇交界南部与滇中),代表康滇古陆与滇黔桂古陆之间的浅水海湾沉积,与台地西缘其他地区相当层位岩相差别明显,宜专文优先详述。在分布区内,“巧家组”两分明显:下部以白云岩为主,广布于滇中至川滇交界南部,向古地理上远岸一侧相变为灰岩;上部以一套碎屑-碳酸盐混积岩为特色,底部常发育铁鲕层,其分布限于川滇交界南部,亦向远岸一侧过渡为灰岩沉积。以此认识为基础,按照现代命名规范的目标和要求,本文提出“巧家组”划分命名修订方案,以最大限度地确保研究区内地层单位名称的稳定与统一,即:维持“巧家组”原义,提升其为巧家群;恢复使用诺多组和华弹组分别指称“巧家组”的下部和上部;将华弹组非正式划分为下、中、上三段;鉴于华弹组底部铁鲕层在上扬子区分布较为广泛,具有重要的地层对比价值,创名西瑶层称之。同时,本文认为诺多组和华弹组的时代分属于达瑞威尔早期和达瑞威尔晚期—桑比期。在新的地层框架下,本研究系统汇总了现有的资料,进而讨论了研究区达瑞威尔期—桑比期的古地理变迁。

The Darriwilian-Sandbian(Ordovician) strata on the western Yangtze Platform of South China(present-day SW Sichuan and central-northeastern Yunnan) are known as “the Chiaochia Formation”, and their stratigraphy and spatial pattern have still been the subject of considerable debate. Our recent investigations indicate that the sedimentation of the typical “Chiaochia Formation” was restricted within an elongate gulf of the northwestern Dian-Qian-Gui Oldland. The succession can be subdivided into two lithologically distinct units, with the lower chiefly consisting of dolomites or dolomitic limestones, and the upper characterized by mixed silisiclasitic-carbonate sediments, therefore necessitating a stratigraphic revision of this interval in the region. The “Chiaochia Formation” is upgraded to the group rank, and the Nuoduo and Huadan formations are revised to refer to its lower and upper parts of early Darriwilian and late Darriwilian-Sandbian ages, respectively. The Huadan Formation is subdivided into lower,member and upper members. A new lithostratigraphic unit, the Xiyao Bed, is proposed for the iron-oolitic sediments consistently occurring in the basal part of the lower member of the Huadan Formation. The new data permit refined paleogeographic reconstructions of the southwestern Yangtze Platform during the Darriwilian-Sandbian interval.