The Sedimentary Characteristics And Depositional Model Of The Cambrian In The Central And Southern Hunan Province

The study of this paper was preceded and accomplished under the guiding ideology of the "the tectonic controls basins, basins control sedimentary facies, and sedimentary facies control the basic geological conditions of oil and gas". Aiming at solving these problems including the stratigraphic division, sedimentary characteristics, sedimentary facies, depositional models of the Cambrian in the Central and Southern Hunan province, the research method of the field work and laboratory study and macro-and micro-combination all were used in this paper. On the basis of the careful observation of field profiles and rock slices identification for the Cambrian, combining the analysis and comparison of geochemical data including major elements, trace elements and rare-earth element, thus the petrologic characteristics, the characteristics of sedimentary structures and sedimentary facies of the Cambrian in researched area have been got the in-depth study. Under the base of above researches, the planar distribution characteristics of the sedimentary face zones were analyzed and related depositional models had been set up, and4lithofacies paleogeography had been drawn up. Form the results of these researches, the basic geological conditions of the oil and gas of the Cambrian could be defined and the favorable depositional area of the Cambrian hydrocarbon source rock in the Central and Southern Hunan province could be scoped. Meanwhile some related sedimentology evidence about the tectonic and sedimentary pattern between the Yangtze Block and the Cathaysia block and whether "the South China Ocean" was existing in the Cambrian. All of the above have been full of significance.1.The comparative study of petrologic characteristics, the characteristics of sedimentary structures, geochemical characteristics and paleocurrent characteristic of the Cambrian of the both sides of the Pingxiang-Chenzhou fault (the boundary between the Yangtze block and Cathaysia block), which shows that the sediments of the Cambrian of the both sides of the Pingxiang-Chenzhou fault had been with a high degree of similarity, and formed the same sedimentary environment, represented as the shelf margin slope facies. And the simple analysis results of provenance nature and the paleocurrent characteristic also indicated that material source of the sediments had been from the same provenance-the southeast or east side of the Cathaysia block, the provenance nature showed up the nature of the Andean-type active continental margin.2.0n the basis of the detailed study on the rock constituents and sedimentary structure of the typical sediments-banded limestone and calcirudite of Series3in the southeastern Hunan, it 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, 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 shallow-dwellers fossils and bony spurs of trilobites in these sediments. Therefore, the depositional environment of the Series3was the mixed shelf environment influenced by periodic strong waves or storms, where was below the normal wave base and above the storm wave base in the third Series of Cambrian, but not the "platform margin slope".3. On the basis of outcrop data of the field profiles, through careful researches of the macroscopic characteristics, petrologic characteristics, the characteristics of sedimentary structures and geochemical characteristics for the various stages of the Cambrian, it concluded that there were mainly2types of sedimentary facies in the Cambrian in the Central and Southern Hunan province:the shelf facies, basin facies. And also the shelf facies could be divided into3subfacies:the mixed shelf facies, deep-water shelf facies, and clastics shelf facies.4. The planar distribution of sedimentary facies of the4ages of the Cambrian in the Central and Southern area of Hunan province have been summed up, and also each lithofacies paleography maps had been drawn. And the4ages of the Cambrian were defined as the Terreneuvian-early Series2, the later Series2, the Series3and the Lotus series. It could be discovered that there were no significant differences and jump facies phenomenon between the Yangtze block and Cathaysia block but very similar, and the sedimentary facies zones took on a continuous and complete planar distribution pattern.5. From the depositional mode of the Cambrian in the central and southern area of Hunan province which established in this paper, we could discover that there was always a deep area in the Central area of Hunan. This deep area was a deep-water basin developing onto the shelf. And the different sediments of the Epeiric Sea developed the east and west of the basin; a stable sedimentary pattern which was deepening from the two sides of the Yangtze block and Cathaysia block to the middle formed in the Cambrian.6. The deep-water basin developed in the Central area of Hunan, and the stable black sediments which were consisted of the black shales, carbonaceous shales intercalated with silicolites, carbonaceous siliceous shales, silty carbonaceous shales and some less stonecoal deposited in the basin. These black sediments were the source rocks with a good potential oil-generating capacity. These sedimentary characteristics indicated that the Central area of Hunan was a favorable sedimentary area of hydrocarbon source rocks in the Cambrian, and also the carbonaceous sediments laid material conditions for hydrocarbon accumulation.7. The continuous and complete planar distribution of the sedimentary facies and depositional pattern of the Cambrian in the Central and Southern area of Hunan province showed that the Yangtze block and Cathaysian blocks had collided together and formed the identical sedimentary environment of the neritic sea. There was no more "the South China Ocean" between the Yangtze block and Cathaysian blocks. The deepest area of the sedimentary environment of the neritic sea was just the deep-water basin onto the shelf.