Keywords: 
• Yanyuan Basin /
• grain size /
• wavelet transform /
• basin evolution

Abstract: The Cenozoic India-Eurasia collision and uplift of the Tibetan Plateau remain hot topics. The Yanyuan Ba-sin in southwest Sichuan province is located in the Sichuan-Yunnan block,which is between the Red River and Xianshuihe-Xiaojiang fault zones. It is a tectonic escape basin that formed during the continental collision,and its evolution process is valuable in revealing the history of the continental collision and uplift of the Tibetan Plateau. In this paper,detailed grain size measurements were carried out on the Pliocene-Early Pleistocene sediments in the Yanyuan Basin,and the grain size data were processed using wavelet transform technology. The results indicate that wavelet transform data can clearly show potential cycles of grainsize variations and,therefore,can provide an effec-tive way to divide the sedimentary stratum interface quantitatively. According to the wavelet transform grain-size anal-ysis,the sedimentary stratum in the Yanyuan Basin can be divided into two level-1,six level-2,and six level-3 muta-tions,which reflect the tectonic and sedimentary environment changes on different time scales. Further analysis of the grain-size distributions within the representative samples for different periods prove that this sequence is mainly com-posed of sediments from lake and river facies. Combined with the grainsize level-1 mutation of the wavelet transform data,the formation and evolution of the Yanyuan Basin can be divided into three stages:5.4-4.7 Ma is the formation phase of the basin,the sedimentary type is mainly a shallow lake subfacies,and the wavelet transform curve shows a high amplitude shift feature;4.6-3.6 Ma is the expansion phase of the basin,the sedimentary types are mainly deep and half-deep lake subfacies,and there is a low amplitude shift feature in the wavelet transform grainsize curve; 3.6-1.8 Ma corresponds to the basin contraction and filling phase,and the features of sedimentary facies and wavelet transform curves are similar to those of the basin formation phase. The formation of the Yanyuan Basin in the Early Pliocene (5.4-4.7 Ma) is related to an India-Eurasia collisional event,while the sedimentary environment change that occurred following 3.6 Ma corresponds to an important uplift event of the Tibetan Plateau.