湘中下石炭统风暴岩的研究

本文在对湘中各地发现的下石炭统风暴岩的一般特征的讨论的基础上,区分出浅缓坡近源型、过渡带远源型以及深缓坡末梢型等风暴岩类型,并认为这些风暴岩的发育受当时全球古地理控制。早石炭世北方大陆和南方大陆之间存在的古地中海,造成了赤道附近飓风的形成条件。当时华南板块位于赤道以南接近赤道的地方,必然形成了大量风暴沉积。

STUDY ON TEMPESTITES OF LOWER CARBONIFEROUS IN CENTRAL HUNAN, SOUTH CHINA

The Lower Carboniferous in Central Hunan, South China comprises the Shao-dong, Menggongao, Liujiatang, Shidengzi, Ceshui and Zimenqiao Formations. The Shaodong and Ceshui Formations consist mainly of clastic rocks and the other four formations mainly of carbonate rocks. The tempestites developed well in the Lower Carboniferous in Central Hunan, which include carbonate tempestites and clastic tem pestites. In this research the carbonate tempestites have been investigated in detail.The carbonate tempestites are mainly distributed in the Menggongao, Liujiatang, and Shidengzi Formations which were formed in the ramp (offshore shelf) deposi-tional system. Three types of carbonate tempestites are recognized: shallow ramp proximal tempestites, transitional ramp distal tempestites, and deep ramp distal tempestites.Shallow ramp proximal tempestites are represented by the echinoderm debris grainstones in Menggongao and Shidengzi Formations along the Xiandong-Shuang-jiangkou region. They are characterized by the coarse-grained bioclastic grainstone and packstones, erosional surface with gutter casts and escaped structures, gradded bedding, and hummocky.cross stratification, and the associated fair-weather sediments contain the strong bioturbation structures and also recognizible Zoophycos and Thallassinoides trace fossils. This tempestite is believed to be formed at the shallow ramp near the organic mounds and bioclastic shoals where the abundant supply of the bioclasts and strong storm disturbing favor the forming of the tempestites with thick-bedded coarse grainstones and the composited or amalgamated beds.Transitional ramp distal tempestites existed in Meggongao and Liujiatang (lower part) Formations, which are characterized by thin-bedded (5-8 cm) calcaren-itic and bioclastic grainstone and packstones with erosional surface, typical graded bedding and shelter porosity, with which the very thin-bedcd calcareous shales of fair weather are intercalated. The bioturbation is less common but occasionally Con-drites trace fossils can be found.Deep ramp distal tempestites are only restricted in the upper part of Liujiatang Formation, which are preserved in the thin-bedded limestones (about 10-30 cm thick) intercalated within the thick-bedded calcareous shales. The tempestites consist of shelly packstone and wackestone and the shell beds are just 2-5 cm thick. They are characterized by the erosional surface, gutter cast, graded beding, and shelter porosity, etc. This tempestue is not conmon in the shale-dominated sequence because the deeper water environment, perhaps just below the storm weather wave base, is not benefit the forming of storm deposits.The palaeogegraphical significance and global palaeogeographical controls of these tempestites are also discussed. It is believed that the development of these storm deposits was controlled by global palaeogeography. During early Carboniferous the proto-tethys existed between the north continents and south Gondwanaland, which produced the condition of equator hurricane. The South China plate was-situated in the south hemisphere near the equator and the storm deposites must be developed well.