No sedimentary records indicating southerly flow of the paleo-Upper Yangtze River from the First Bend in southeastern Tibet

It is a long-debated issue whether the Upper Yangtze River once flowed southward from its First Bend as a major tributary of the Red River. The Yangbi valley was assumed as a trough carved by the paleo-Upper Yangtze River. This argument, however, has not been substantiated because of the uncertainty of what is beneath the Quaternary sediments of the valley. We conducted a drilling project in an attempt of resolving this outstanding puzzle. We also studied the Upper Eocene-Lower Oligocene Baoxiangsi and Jinsichang Formations distributed widely to the west of the Yangbi valley because sandstone of the Baoxiangsi Formation were previously regarded as remnants of the south-flowing paleo-Upper Yangtze River. Two drilled holes reveal that the Yangbi valley is entirely filled with Quaternary alluvial-fan and bog deposits and contains no typical fluvial sandstone. The base of the valley is the well-consolidated breccia of the Baoxiangsi Formation. The sedimentary analysis shows that the Baoxiangsi and Jinsichang successions are composed mostly of alluvial-fan, lacustrine, fan-delta and braided-stream facies, and interpreted to have formed in an intermontane rift basin. Facies analysis in conjunction with paleocurrent restoration further shows that the bulk of the Baoxiangsi and Jinsichang sediments were shed from local highlands and debouched to the basin mainly from the south and west Collectively, our sedimentary investigations of both boreholes and outcrops lend no support of the long-held view that the paleo-Upper Yangtze River flowed to the south through the region south of the First Bend. It, however, remains an unresolved problem how the ancestral Upper Yangtze River evolved during the Tertiary. (C) 2015 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.