ANALYSIS OF SEDIMENTARY FACIES, METER-SCALE SEDIMENTARY CYCLES AND CLIMATE CHANGE OF THE YANSHIPING AREA OF THE QIANGTANG BASIN IN MIDDLE JURASSIC

Qiangtang Basin is located in the north of the Tibetan Plateau, whose tectonic structures lie in the eastern section of the giant Tethys - Himalayan tectonic region, and is clamped among the Gangdise & Nyainqentanglha and Hoh Xil & Bayan Har. Based on the first-hand data of the Yanshiping profile Xiali formations sratigraphy, lithology, sedimentary facies and description of the Xiali formations meter-scale sedimentary cycles characteristics, as well as the formation mechanism and results of the climate analysis, the author draws the conclusions as follows:

1. Through the study of Xiali formations stratigraphy, characteristics of sedimentary petrology, and characteristics of paleontology, the author believes that Xiali formations all are the tidal flat environment. Based on the features of stratigraphic sequence to carry on divide of tidal flat environment for the Xiali formations, the subphases separately are: supralittoral zone subphase, intertidal subphase and subtidal zone subphase.

2. The Xiali formations formed 152 meter-scale cycles, which can be divided into clastic rock type and clastic–carbonatite rock type. The clastic rock type combination of them totally are 12 types, and the clastic-carbonatite rock type is also divided into six types, the common features they both have are that particles upward gradually tapering, depositional environment overall upward become deeper, with the character of tides "normal graded bed sequence”.

3. Through the climate change analysis of the CaCO₃ and SO₄^2-, it reasons out that the climate has changed gradually from the semi-arid to arid environment, during the Middle Jurassic period.

4. Through the spectrum analysis of spectrogram of the CaCO₃ , we know that the frequency of the diagram includes 0.0081, 0.027, 0.063, and 0.214 which is corresponding to the wavelength of 123, 37.04, 16.13, 4.76(1: 0.301: 0.129: 0.039) and the ratio is similar to the ratio among of eccentricity cycles 400ka and123ka , tilt ratio cycles 50.6ka , and the precession cycle 17.6ka (1: 0.308: 0.127: 0.044), which indicates that the ratio among them has a good corresponding relationship between the ratio of the Milankovitch cycle ratio, It demonstates that Milankovitch cycles that control the development of climate change.