Braided-river Delta Deposition of the Quanji Quaternary Qigequan Formation in Sanhu Depression of Qaidam Basin

It was found out that a braided-river systems prograded into an inland lake in the Quanji Quaternary Qigequan Formation in Sanhu Depression of the Qaidam Basin,in which it formed a braided-river delta depositional system.It included probraided-river delta subfacies and braided-river delta front subfacies and 5 microfacies. The characters of braided-river delta are distinctly different from the fan deltas and meandering-river deltas. The deposits of braided-river deltas are mainly composed of pebbled sandstone and medium-coarse sandstone. The bars of the subaqueous distributary channels are the main bodies of deltas and they take the form of lenticular bodies. A single lenticular channel sand body has a thickness of 0.5~2.5m. Many,finger-upward lenticular channel sand bodies pile up vertically each other. Cross-stratifications are abundant,but lateral accretion cross-beddings resulting from lateral accretion of channel sand bodies are the most principal sedimentary structures and it is one of the most distinct characters of braided-river deltas.