Sedimentary Characterristics Of The Upper Triassic Riganpeicuo Formation At Renacuo Area, Gaize, Tibet

This dissertation gets the support of China Geological Survey project: FourRegional Geological Surveys of Renacuo, Tibet, and central university fundamentalresearch fund: Upper Triassic Organic Reef of Renacuo District in Gaize, Tibet andIts Petroleum Geological Significance. Through detailed analyses of field geologicalinvestigation, indoor rock thin-section analysis, isotope geochemistry of the carbonate,and element geochemistry, sedimentary characteristics of Riganpeicuo formation ofthe Upper Triassic of Renacuo District in Gaize, Tibet had been studied systematicallyand the main results and conclusions were as follows:1. The carbonate was identified11kinds of major carbonate rocks: marlite,bioclastic mudstone, bioclastic wackestone, bioclastic packstone, intraclast packstone,intraclast grainstone, algal grainstone, multiple types of oolitic grainstone, a singletype of oolitic grainstone, pellets packstone and biohermal limestone. According tothe characteristic of rocks and combination type it can be divided into5sedimentaryfacies: debris littoral facies,restrcted platform facies, open platform facies, platformedge shallow facies and platform edge reef facies.2. Multi-layer Oolitic beaches develop in the Upper Triassic RiganpeicuoFormation, and according to sedimentary environment, it can be further devided intoopen platform and marginal platform shoal. Microfacies analysis shows thatintraplatform shoal oolitic is formed by mixing and accumulation of in situ andallochthonous oolitic, and platform edge oolitic is mainly in situ oolitic; vertically,platform edge oolitic transformes from omicrite to radial ooides, from radial toconcentric ooides, and the grain size of ooides inceased and deposited thickness gotthicker markedly upward. This represents that, in the depositional period, sea levelchanges from rapid transgression to slow regression.3. Organic reefs of the upper Triassic in Gaize，Tibet were well developed. Itshows that the reef-building organisms consist of hexacorallia，calcispongiae andcalcareous algae. Reef-inserted organisms consist of foraminifera，brachiopods，bivalves，gastropods，echinoderms and bryozoans. The development of platformmargin reef generally includes three cycles，each of which has the same structuralcomposition and can be divided into reef base，reef core and reef cap from the bottomup. The cycle I and II were in a large scale，and have complicated construction，but the cycle III is small and simple comparatively. All of the three cycles forms thesecondary prograde sequence under a transgressive background.4. Sedimentary environment characteristics of Riganpeicuo formation inRenacuo district were discussed by analyses of isotope geochemistry and elementgeochemistry, which further proofed Riganpeicuo formation was developed in thecarbonate platform facies.5. According to the analysis of cross-sectional structure of the phase sequence, itcan be identified four transgressive-regressive cycles in Riganpeicuo formation,which shows the overall trend of sea deepening. Trough contrast with Riganpricuoformation in Rongma and regional data, it discussed the distribution rules and itscharacteristics of Riganpeicuo formation in the transverse direction.