Study Of Permain Tectonic Palaeogeography In The Middle Yangtze Region

Based on the measured Permian section in the middle Yangtze region, according to stratigraphic, lithofacies and sedimentary facies analysis, petrology, sedimentary and geochemical and other methods, microfacies analysis as a basic means of the measured profile, combination of measured profile and microfacies analysis, Permian in the Yangtze region characteristics, lithology, sedimentary characteristics and palaeogeographic have been systemly studied. Evolution of sedimentary environments and deposition models have been founded and the relationship beween sedimentary and tectonic evolution has been dephly discussed.Lower Permian is missing in a part of study area, only exposed in the southeastern and western Hubei, northwestern Hunan. The thickness of Middle Permian is gradually thinning from south to north, lithofacies show a gradient relations from south to north by the shallow marine limestone to the bathyal and deep-sea siliceous limestone, siliceous rock, depth of sea gradually becomes deeper from south to north. Upper Permian mainly formed by the carbonate rocks, Longtan and Wujiaping formation, Changxing and Dalong formation is same phase different facies, which reflecting the deposition of the Late Permian differentiation.According to the sedimentary environment analysis, three primary kinds of sedimentary environments were identified, which include ramp, platform and basin. Lower and Upper Permian have mainly deposited on the carbonate platform, the Middle Permian have mainly deposited on the ramp and basin. Types of gravity-flow deposits have been found in Maokou Formation Middle Permian, western Hubei, from top to bottom slump deposits, debris flow deposits. The sedimentary evolution indicates that two sedimentary cycles of transgression and regression occurred in the Permian:Lower-Middle Permian and Middle-Upper Permian. By use of measured section and microfacies analysis, 12 kinds of microfacies can be recognized:bioclastic grainstone, mudstone, micrite calcisiltite, bcalcirud, silica and etc. Microfacies analysis shows widely developed calcirudite in the study area is allochthonous instead of differential compaction causes.Early Permian, the sea has not yet fully penetrated the study area, part of study area is still land, southeast of the study area is restricted platform - open platform deposition. Qixiaian suffers extensive transgression in the Middle Permian, in the northern margin of the Middle Yangtze is the outer ramp– deep water basin, in the main Middle Yangtze craton is ramp deposition, some area is carbonate platform facies, western Hubei trough is outer ramp; Maokouan transgression is aggravated, the northern margin of the Middle Yangtze is deep basin and the compensation is extended. Ramp remains in the main Middle Yangtze area, western Hubei becomes to deep water basin. platform-ramp–deep water basin from south to north build up a carbonate ramp depositional system in the study area, and is consistent with the regional tectonic framework. Syndepositional fault activity is a certain control element of the deposition in the study area, deposition of the northern margin of Middle Yangtze basin is by the constraints of the South Qinling sea and under its control.Late Permian mainly developed open platform, shelf and basin, Wuchiapingian developed shallow marine sediments, deep-water basin range becomes small, Changxing developed platform and basin facies, deep-water basin range is expanding. Sedimentary facies of the Late Permian is controlled by the activities of the South Qinling sea, an obvious sedimentary facies differentiation occurred during Late Permian, which is controlled by basin evolution. Palaeogeographic evolution shows that the northern margin of the Middle Yangtze Wujiapingian is deep basin, in the main Middle Yangtze craton open platform developed, western Hubei trough is an isolated basin; Changhsingian transgression intensified and reach a climax in Late Changxingian, the northern margin of the Middle Yangtze is deep basin and the scope further expanding, the Yangtze craton come into craton margin basin.Carbonate facies analysis shows that diagenesis of Daxiakou section is consist of metasomatism, cementation, compaction pressure solution, dissolution and recrystallization, with characteristics of carbon and oxygen isotope, the diagenetic environment could be distinguished marine-atmospheric fresh water-burial environment,correlation of porosity and diagenetic phasis has been dissised. Paleotemperature and pale salinity fromδ¹³C andδ¹⁸O value suggests that study area is stable open sea and warm climate in Permian.