Analysis Of Middle Jurassic Sedimentary Facies And Stratigraphic Sequence Of Matuo Formation In Yanshiping, Northern Tibet

The Qiantang Basin, having the most complete and extensive marine sedimentary strata during Jurassic, is the second larger Mesozoic marine sedimentary basin. Although many achievements are obtained in recent years, the research are still only involved in second- or third-order of Mesozoic sequence. On the base of sedimentation facies analysis of Matuo Formation, high frequency sequence are studied.Through the correlation of NWW-SEE and SN, the thickness and limestone intervals of Matuo Formation gradually increase from west to east. This represent the center of sedimentation is possible located in east of the region. The Matuo Formation thickness gradually increase and then reduce from north to south. Meantime , limestone intervals and fossils gradually increase as well. The Paleogeographic pattern is shoreface-carbonate platform- slope circumstance. The Middle Jurassic (late Bajocian-early Bathonian ) Matuo Formation in Yanshiping, northern Tibet, is a mixed siliclasstic-carbonate shell deposits that are dominated by alternating sandstone, siltstones, mudstones, and minor shelled intervals. According to lithology, color, size of grain, sedimentary structure , and fossil types , the Matuo Formation vertical lithofacies are grouped into four types, represented tidal flat circumstance dominated by tide and shoreface circumstance dominated by wave, respectively. The lithofacies can be grouped into three types: structure upward deeping of tidal flat-shoreface dominated by tidal; structure upward deeping of tidal flat-shoreface dominated by sandstone; structure upward shallowing and then deeping of tidal flat-shoreface.It is condensed section with shells, downlap surface and ravinement surface that constitute the critical boundaries in dividing the system tracts of Matuo Formation. By means of the identification of these key boundaries, it has been recognized that twenty-eight 5th -order, five 4th -order and two 3rd -order sequences from the lithological sequence of Pm profile. Moreover, the system tracts in sequence are characterized by dualistic structure, namely transgressive-highstand system tracts, and TST>HST. The parasequences are divided into three types, which is respectively dominated by sandstones , mudstones, and shallowing-upward tidal flat system. And prograding sequence and retrograding sequence are composed of stacking parasequence set.The total varying trend of Jurassic sea-level is gradually ascending, and then the sea-level is at the static stage without obvious falling. By the adoption of method of deposiontal systems analysis, the thesis has preliminarily established the curve of relative sea-level changes of Matuo Formation in studied area, which is correlated with the curve of sea-level changes in southern Tibet and the global, and eventually come to the conclusion that the tectonism is the primary factor that resulted in the long-term sea-level change of Tibet Tethys region, and the climate is the secondary factors, furthermore, the difference of high-frequence sea-level change of various districts in Tibet Tethys region is principally induced by paleotopography, sediment supply and regional tectonism.