湘西北王村地区寒武系花桥组是“钙屑浊积岩”吗?

Is "Calciturbidites" the Cambrian Huaqiao Formation in the Wangcun Area, Northwestern Hunan

扬子陆块东南缘的寒武系以各种类型的"碳酸盐重力流沉积物"发育为显著特征,前人据此认为存在一个连续的"台缘斜坡相带"。"古丈阶"国际层型剖面和点位(GSSP)——湘西北王村剖面的寒武系"花桥组"曾被认为由大量"钙屑浊积岩"组成。详细的沉积岩石学研究表明,王村剖面花桥组并非典型的"钙屑浊积岩",而是以浪基面之下沉积的低能碳酸盐岩为主。其"浊积序列A段"中的所谓砾屑和底冲刷构造等,其实是由成岩期压切作用形成的大量灰岩透镜体和条带组成;"B段"中的所谓"波状层理"实际为压实变形的水平层理和类似风暴成因的丘—洼状交错层理,加之本次新发现的两层鲕粒灰岩,指示出受间歇性动荡水流影响的深水缓坡环境。结合区域沉积记录,对湘西北地区相应时期古地理格局提出新认识,认为存在未形成典型"台缘斜坡"的碳酸盐缓坡。

Cambrian sequences in the southeast margin of Yangtze Platform are characterized by various types of ＂carbonate gravity flow deposits＂. Such cases have been viewed as the proxies of ＂platform margin slope facies＂. The ＂Huaqiao Formation＂ at Wangcun section, northwestern Hunan, which is the ＂GSSP＂ of the ＂Guzhangian Stage＂, has been considered to ＂calciturbidites＂ lithologically. The detailed lithological and sedimentary study illustrates that the Huaqiao Formation is not typical ＂calciturbidites＂, but normal carbonate-dominating sediments from the low-energy depositional environments at depth below the wave base. Section A of the ＂typical turbidite sequence＂, which were considered to develop in the Huaqiao Formation, is composed of numerous limestone lens and lumps formed in diagenesis compaction. ＂Undulate bedding＂ developed in section B of the ＂turbidite sequence＂ should be deformed horizontal bedding and hummocky cross bedding, coupled with discovery of two layers of oolitic limestone. These suggest a deep-water environment intermittently effected by oscillatory currents. In comparison with regional sedimentary records, this study suggests new understanding for the paleogeographic pattern in corresponding period and considers existence of a carbonate shelf （ramp） without typical platform-edge slope developed in northwestern Hunan.