High Frequency Cyclostratigraphy Of Shangyoushashan Formation And Xiayoushashan Formation In The Ganchaigou Area, Western Qaidam Basin

Lacustrine sediments is too better than other natural records in reconstructing short-scaled climatic and environmental evolution, for its rich information, continuous depositions, high-deposition rate and high-resolution time, to be replaced.The Shang and Xia Youshashan Formation in Ganchaigou area of western Qaidam Basin is researched in this paper, which fully recorded Eocene-Pliocene sedimentary evolutional history. Cyclostratigraph based on astronomical period views and sedimentology are used to analyse the changes of Youshashan Formation sedimentary facies. Meanwhile meter-scale cyclic sequences and high frequency cyclic sequences are further studied.The Shang Youshashan Formation and Xia Youshashan Formation sedimentary facies and depositional environment can be divided into delta facies, shore of lake facies, shallow lake facies, and shoreline beach bar facies, showing that sedimentary facies changed frequently for near sources and rapid deposition. Lake-level showed one descent after the first increase, coinciding with the trend of heat and drought.By the principles of meter-scale cycles, we divide the aimed formations into 9 kinds meter-scale cycles. Then the structures types and distribution pattern of meter-scale cycles are established. Using the frequency analysis techniques to research the periodicity of the aimed formation, the researcher find that large quantity of recorded Milankovitch cycles information are contained in the Youshashan Formation, which coincide with astronomical periods well since 65Ma. So the Youshashan Formation is divided into 3 astronomical periods: 23ka annual precession, 98.6ka and 123.46ka short eccentricity, 400ka long eccentricity. As the result, short eccentricity period is the main controlling factors of cyclic sequences coinciding with level-5 cyclic sequences.Graded structure divides the researched formation into 42 level-4 cyclic sequences controlling by long eccentricity, most of which are composed of 2~6 level-5 cyclic sequences that coincide with the 1:4 rate of short eccentricity and long eccentricity of Milankovitch cycles. In three-demension, every level-4 cyclic sequence from bottom to the top shows thick-thin in depth of stratum, which represent the process of changes from depth to shallow of the lake level. It is showed that Fischer plots illuminates the characters of the changes high-frequency lake level and the trend of the changes of the long periodic lake level, which can increase us the recognition ability to the closed inland lakes. To the controlling factors of high-frequency changes of the lake level, the climate changes dominating by Milankovitch cyclic period is reckoned as the main controlling factors of the cyclic sequence in Youshashan Formation.