| The Qiangtang basin is the largest Late Paleozoic-Mesozoic marine basin of Chinese mainland. Basement property and Tectonic evolution of the basin has been disputed for a long time. Besides, it has suffered intensive structural deformation and uplift effect since the Cenozoic. So it has caused controversies about whether considerable oil-gas resources are found in the basin, or not. Investigating these long-standing scientific questions will not only contribute to improving understanding of the north Tibet Plateau, which was researched lowly, but also contribute to the oil and gas exploration work. There is a nearly EW-trending mountain in the center of the basin, which divided it into southern and northern Qiangtang basins. Recently, at the northwest of a hot-spring in Rong Ma Township, Nyima County, the north part of the southern Qiangtang basin, some sedimentary rocks have been found such as the Tashishan formation of middle and late Ordovician is parallel unconformity on thick-bedded low-grade metamorphic quartz sandstones which has been interbeded by thin layer of marl. More than 300 detrital zircon dating results indicate the middle and late Ordovician quartz sandstone which cover the unconformity yield age probability peaks in the range of 470~520,580~800 and 950~1100Ma. Among them, peaks of 500, 600, 800 and 985 Ma are most obvious, the most reliable and younger detrital zircon age is 471±6Ma, indicate that the maximum sedimentary age of quartz sandstone at the bottom of Tashishan formation is 471±6 Ma, consistent with the age of the fossil. Nearly 600 detrital zircon of the metaquartzite which under the unconformity yield age probability peaks in the range of 525~700,885~1100 and 2350~2580Ma, peaks of 615, 980 and 2500 Ma are most obvious among them. The most reliable and younger detrital zircon age is 525 Ma, indicate that the maximum sedimentary age of the low-grade metamorphic quartz-sandstone is middle and late Cambrian. The sedimentary age gap between the quartz sandstones above the unconformity and the underlying low-grade metamorphic quartz sandstones is up to 56 Ma, indicative of an obvious depositional hiatus between the two quartz sandstones. Another independent evidence is that the metamorphic quartz sandstone is intruded by granite in the age of 475 Ma. This further confirms that the unconformity between the two quartz sandstones is Late Cambrian-Early Ordovician. The sedimentary rock underlying the unconformity is, therefore, named as Rongma formation and classified in upper Cambrian to lower Ordovician era in this study. The cathodoluminescence images and geochronology research indicate that the metamorphic complexes formed in the late period of the Pan-African movement had pulled back on the surface in early Ordovician, then it suffered denudation and provided material source for the Ordovician quartz sandstone. The discovery of the parallel unconformity of Ordovician reflects that the south Qiangtang basin has Pan-African basement and it is one portion of the Gondwana supercontinent, as Himalayan and Lhasa terranes. The distinctive difference in Paleozoic strata between south Qiangtang and north Qiangtang basins shows that there is an ancient ocean basin between the two basins in the Early Ordovician at least, and the two basins evolved independently.The southern Qiangtang basin northward subducted beneath the northern Qiangtang in the late Triassic, while led to the formation of organic-rich foreland basin at the northern margin of the southern Qiangtang basin. Following the continuous expansion of Bangong-Nujiang Tethyan Ocean, Jurassic to Early Cretaceous shallow marine strata, which is characterized by organic-rich reef-limestone, was widely deposited in southern and northern Qiangtang basins. From Late Cretaceous to Paleocene times, the Bangong-Nujiang Tethyan Ocean was gradually disappeared due to continuous northward movement of Lhasa terrane, eventually collided with the southern Qiangtang block, then the southern Qiangtang basin transformed from marine basin situated on passive continental margin into a continental sedimentary basin which is filled with thick terrigenous clastic rocks, unconformably overlie the early marine sedimentary rocks. During the ongoing collision between India and Asian continents, the southern Qiangtang basin was southward juxtaposed over the Lhasa terrane. Several large-scale thrust faults were formed in southern Qiangtang basin, which led to much Upper Triassic, Jurassic and Cretaceous organic-rich marine Reef-limestone buried deeply in the basin again. Structural deformation is extremely weak between these large thrust faults, broad and gentle synclines are widely developed in this basin and Cenozoic continental strata is distributed in the syncline core, so we speculate that the organic-rich marine strata is still exist under the Cenozoic continental strata. Therefore, it indicates huge amounts of oil-gas potential resources in the southern Qiangtang basin. We should look for oil-gas resources in the footwall of thrust faults and broad, gentle syncline core region within the southern Qiangtang basin. |
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