CHRONOSTRATIGRAPHIC SUBDIVISION AND CORRELATION OF THE UPPER MIOCENE OF THE LINXIA BASIN IN GANSU,CHINA

Terrestrial strata of the upper Miocene are well developed in China,among which the widely distributed red clays in northern China contain very rich mammalian fossils of the Hipparionfauna.The upper Miocene of the Linxia Basin in Gansu Province consists of a sequence of red clays of the Liushu Formation,in which many fossil localities were found.The fossil assemblages were subdivided into four mammalian faunas from the bottom to the top according to their horizons in the sections,and respectively named as the Guonigou,Dashengou,Yangjiashan,and Qingbushan faunas based on the representative localities.Paleomagnetic dating shows that the Liushu Formation covers the entire upper Miocene,including the lower upper Miocene Bahean Stage and the upper upper Miocene Baodean Stage.The Guonigou,Dashengou and Yangjiashan faunas belong to the Bahean,while the Qingbushan fauna belongs to the Baodean.If the lower boundary of the Bahean Stage correlated with that of the marine Tortonian Stage(lower boundary of Chron C5r.2n),it occurs in the sedimentary sequence in the Linxia Basin,with an age of11.6Ma,and the first appearance datum of Hipparionis slightly higher than this boundary,with an age of 11.1Ma.If the lower boundary of the Baodean Stage is correlated with that of the marine Messinian Stage,its position can be accurately determined in the Liushu Formation and also in the Woma Formation of Gyirong,Tibet.Because the Baodean was derived from Baode County,Shanxi Province,the lower boundary stratotype of this stage should be based on the Jijiagou section of Yaozhuang Township in this county,within Chron C3Br.1rnear the base of the red clays in the Baode Formation and with an age of 7.25Ma,and the first appearance datum of Hipparion forstenae can be regarded as a biostratigraphic marker.