湘西花垣排碧寒武系花桥组上段—车夫组沉积环境的探讨

A Discussion on Sedimentary Environment of the Upper Member of Huaqiao Formation—Chefu Formation of Cambrian in Paibi, Huayuan, Western Hunan

湘西排碧位于中上扬子陆块的东南缘,前人认为在寒武纪第三世时期该地区位于以各种类型的"碳酸盐重力流沉积"发育为显著特征的"台缘斜坡相带"内,其主要岩石类型为泥质条带灰岩及砾屑灰岩。本文通过对花桥组上段—车夫组典型沉积物的岩石组分及沉积构造进行详细研究,发现条带灰岩并非泥质条带灰岩,而是由灰岩条带——颗粒灰岩、泥晶灰岩与粉砂质条带——粉砂质灰岩、纹层状含炭质粉砂质灰岩互层组成,表现出内源碳酸盐沉积物与陆源碎屑沉积物混积以及陆棚环境沉积物的典型特征。砾屑灰岩及伴生岩石组合发育丘状(洼状)交错层理等众多风暴成因的沉积构造,为具有不同风暴沉积序列的风暴沉积物。此外,花桥组上段—车夫组沉积物中含各种藻类等浅水生物及大量三叶虫骨刺。因此,认为花桥组上段—车夫组的沉积环境应为正常浪基面以下、风暴浪基面以上,受周期性强风浪作用影响的混积陆棚环境。

Most of the previous researches showed that, Paibi Section was located in the ＂platform margin slope＂ of southeast margin of the Middle--Upper Yangtze Block, with various types of ＂ carbonate gravity flow deposits＂ as the significant feature in the third series of the Cambrian, and the main sediments of the Upper Member of the Huaqiao Formation--Chefu Formation were argillaceous limestone and calcirudite. On the basis of the detailed study on the rock constituents and sedimentary structure of the typical sediments, this paper found that the banded limestone with the unstable thickness often appeared lenticular or veined was not the argillaceous limestone but the silty banded limestone, which was composed of the limestone bands - grain limestone, micrite and silty limestone bands - silty limestone, laminated carbonaceous silty limestone. So the banded limestones were the mixed sediments with endogenous carbonate sediments and epicontinental clastic sediments, and showed the typical feature of the shelf sediments formed in the mixed shelf. The sedimentary structures caused by storms, such as hummocky （hollow） cross-stratification, ripple mark, wave-ripple bedding, and underside erosion structure, largely developed in calcirudite and its associated rock association, which composed of the tempestite with different sedimentary sequences. In addition, there were a number of algaes, arthropods, other shallow - dwellers fossils and bony spurs of trilobites in the sediments of the Upper Member of Huaqiao Formation--Chefu Formation. Therefore, the depositional environment of the Upper Member of Huaqiao Formation--Chefu Formation was the mixed shelf environment influenced by periodic strong waves or storms, where were below the normal wave base and above the storm wave base in the Third Series of the Cambrian.