Juarssic Sedimentary Facies And Organic Geochemistry Of Black Shales In Northern Tibet

The Qiangtang Basin, having the most complete and extensive marine sedimentary strata during Jurassic, is the second larger Mesozoic marine sedimentary basin. Although many achievements were obtained during recent years, researching areas and contents are still limited and uncompleted and there is a debate about Jurassic stratigraphic framework and sedimentary environments in Yanshiping area and its southern places. During early Jurassic the palaeo-oceanographical changes was an event influencing global geologic evolution. Its most obvious sedimentary record is that extensive organic-rich sediments distribute in main ocean basin. Many researchers described those special sediments as black shales. Their characteristics of geochemistry, stable carbon isotopic excursion, genetic model are still hot topics to geological field. Regarding above-mentioned questions, this paper chooses the Jurassic stratigraphy in northern Tibet as studying object, focusing on the Jurassic sedimentary facies and organic geochemistry of two sets black shales.For long time, to the Jurassic stratigraphic framework of the studied area and its adjacent places, different workers adopted different Group, Formation and chronostratigraphic units. On the basis of twice fieldworks of 2001 and 2002 in northern Tibet, combining formers' research results, the Jurassic stratigraphical system are builded from lower to upper as follows: Quse Fm (Pliensbacnian-Toarcian), Quemoco Fm. (Bajocian), Matuo Fm. (Bajocian-Bathonian), Wenquan Fm. (Bathonian), Xiali Fm. (Callovian), Suowa Fm. (Oxifordian-Kimmeridgian), Xueshan Fm.(Tithonian-Berriasian).According to field investigation, thin section identification and many other methods of sedimentary petrology, rock types of Jurassic strata include carbonates, argillaceous rocks and clastic rocks. Especially in Shuanghu area and Amdo 114 station, the sedimentary rocks is organic-rich, mostly consist of black shales, marls and mudstones. From these characteristics of biostratigraphy, lithostratigraphy and chronostratigraphy, the oil shales in Shuanghu area are similar to those in early Toarcian Europe. On the basis of sratigraphic reorganization, paleontology and petrologic markers, Jurassic sedimentary facies of the studing area are divided into the transitional facies, clastic rock marine facies and the carbonate facies, representing three different deposite environments. The transitional facies can also be divided as estuarine, tidal flat and lagoon; The depositional envirnononents of clastic rock marine facies include Offshore, shallow water continental shelf, deep water continental shelf, Slop as well as the deep water basin; The carbonate facies can also be divided into a carbonate platform and ramp. Through the analysis of the stratigraphy and sedimentary facies of the research areas, the description model between the stratigraphic framework and deposite environment can be set up firstly by the transition from the "three horizons sandstone with two intercalated limestone horizons'Mn Yanshiping area to shale and marl in Amdo-Zigetang Co , and to oil shale in Bilong Co, then by the transition of the shallow water platform facies to the deep water continental shelf, slope, and to the deep water basin.The oil shale deposit is controlled by the paleo-oceanographical condition. It is known from the information and data available that there are two sets black shales, representing the deep water deposit in Jurassic of Northern Tibet. Their organic matter contents are very high . Except for plenty of ammonitefossiles, abundant coccotithophorid is also found but simple species variation. Therefore, We put forward that black shale deposition, especially the oil shales may have the relationship with the bloom of coccotithophorid of palaeo-oceanographical phytoplankton. This is similar to the synchronous deposition of black shale in Europe. During the Lower Toarcian, coincident with the peak of the lower Jurassic transgression, highly bituminous shales were deposited in extensive parts of the European e...