Abstract: Five seams of mudstone from the 4 th interval (T3xj⁴) of Xujiahe formation in Southern Sichuan were geochemically and petrologically researched in detail.Results have shown that the TONSTEINS are all synsedimentary acid volcanic ash,which were turned to relatively pure kaolinitic mudstone through hydrolysis and diagenesis in peat moor environment.For these mudstones the types of lithological texture are relatively simple,dominated by compact or crumby-compact textures.The types of lithological texture,the content of lithophile elements and their composite relations are stable in large area.They are very important marked beds for regional coal-seam correlation and for chronostratigraphic framework establishment.