The Meter-scale Sedimentary Cycles And Its Forming Mechanisms Of Middle Jurassic Xiali Formation In Eastern Qiangtang Basin, Tibet

Jurassic is a very important geological period in our country, however, Jurassic marine strata are lost in many regions of China for the impact of Indosinian orogeny. Typical sections to research the Jurassic strata are developmental in the North Tibet, as an important component of global Paleotethys area, which preserves more complete Jurassic marine strata .The Tanggula Mountains northern slope in the north of Tibet is known as the most developmental region of Jurassic marine sediments. Particularly Yanshiping Group, as the representative of the littoral-neritic facies, is always paid close attention by most scholars for its most sensitive to the fluctuations of sea level. Among it, Xiali Formation is the most perfect strata to study the Jurassic cyclostratigraphy, for which is characterized by centimeter to meter-scale interbedding of sandstone and mudstone, less clastic rocks and carbonate group, pregnant with a rich structure, sea-level change and the role of sediment accumulation information.In this paper, based on cleaning up lithostrata, biostrata, chronostrata materials of Qiangtang Basin, combining field survey and previous data of the study area, the author have studied the Xiali Formation systematically in petrology, sedimentology and lithofacies paleogeography. Yanshiping section was chosen as the study target for its rich fossils, continuous strata, clear lithology and sedimentary structures and high studied level. Genetic type of meter-scale cycles in Xiali Formation is divided by systemizing cm-meter scale lithological characters, for example particle size, bedding and color in the scope of the naked eye. The Fischer plots is attempted to clarify the sea-level change curve and its formation mechanism in the depositional period of Xiali Formation.At the same time, the use of frequency analysis and filter analysis technology divides the levels of the high-frequency cycle.After cleaning up strata, Jurassic in the Qiangtang Basin is divided into Quse Formation, Quemocuo Formation, Buqu Formation, Suowa Formation and Xueshan Formation from the bottom to the top surface. Sedimentary features, as well as the results of the analysis of palaeogeography in the depositional period of Xiali Formation show that Qiangtang Basin remained the characteristics of"two depressions and one swelling", and developed 5 facies, including the land source erosion, the alluvial coastal plain, tidal flat, lagoon and shelf facies.85 meter-scale cyclic sequences are identified in Xiali Formation of the Yanshiping section, including clastic and clastic- carbonate cyclic sequences which can be divided into 13 subclasses. The common characters they have are that grains vary into fine one upward coinciding with the changs of the shallowing depositional environment, which characterises the positive graded bedding formed by tide.They are vertically composed by 40 level-6, 13 level-5, and 5 level-4 cyclic sequences. A level-6 cycle is mostly superposed by meter-scale cycle of 2:1, fewer3:1; level-5 cycle mainly by level-6 of 2:1, fewer 4:1 ; and level-4 mainly by level-5 of 3:1 or 4:1,fewer 2:1.