Probability Cumulative Grain Size Curves in the Paleogene Clastic Sediments and Environmental Significance in Maxian Region of Northern Qaidam Basin

Based on core observation and detailed lithofacies analyses in the Cenozoic section,using plenty of analytical data of grain size,a large number of probability cumulative grain size curves in the main terrigenous depositional environments of the northern Qaidam Basin were summarized.The results show that the study area mainly developed alluvial fan deposit in the Lulehe Formaition(E1+2),braided river in the Lower Xiaganchaigou Formation(E13)and braided delta in the Upper Xiaganchaigou Formation(E23).Each sedimentary facies is divided into several subfacies and microfacies again.The characteristic of probability cumulative grain size curves in different sedimentary facies are as follows:The probability cumulative grain size curves of alluvial fan include typical "bi-segment" pattern,i.e.,"one suspension" and "uphill arched arc" pattern;The braided river facies have "bi-segment","high slope one bouncing population-one suspension population-transitional zone" pattern and "tri-segment" pattern;The braided delta facies contain typical "bi-segment","tri-segment","one bouncing population-1suspension population-transitional zone","high slope one bouncing population-1suspension population-transitional zone","low slope multi-bouncing population-one suspension population".Each sedimentary subfacies has its own peculiar probability cumulative grain size curves.