Title: Geology of the Dian-Qian-Gui foldbelt, Southwest China
Abstract: This report on the geology of the Dian-Qian-Gui foldbelt, Southwest China, is based on sparsely available literature.The region is confined generally in latitudes 23°50 T N to 25°00 T N and longitudes 103°30 T E to 107°00 T E and covers the border areas of southeastern and eastern Yunnan, western and south-central Guizhou, and western Guangxi Provinces.The Nanpanjiang depression and the Baishai (Baise) basin are known to contain oil and gas deposits and are located in the central part and the southeastern part of the study region, respectively.This foldbelt is one of the integral tectonic units of the South China fold system and borders the southern margin of the Yangzi (Yangtze) (Chang Jiang) paraplatform, which was formed during the Upper Proterozoic Yangzian (Yangtzeian) orogeny.By the end of the Caledonian orogeny, the South China fold system was well established.In the study region, the initial depositional framework of Middle and Upper Proterozoic marine and continental sedimentary sequences was formed during the Lower Proterozoic Zhongtiaoian (Chungtiaoian) orogeny.Throughout most of the region, the Lower Paleozoic sedimentary sequences are incompletely preserved, but the stratigraphic record shows that the Upper Paleozoic marine platform deposits were well developed and extensive.These strata have been chosen to represent the type sections for South China from the Devonian through the Permian systems.In Lower Mesozoic time, the widespread Tethys Sea covered most of South China, and it retreated from this region during the Upper Triassic.In the southern part of the study region, Tertiary sea transgressions are recorded locally.Coal deposits occur chiefly in the Lower Carboniferous, Lower and Upper Permian, and Upper Triassic sedimentary sequences.Locally, coal beds are mined.Oil and gas deposits occur in the Upper Paleozoic and Lower Mesozoic carbonate and shale facies of shoreline and shallow shelf environments in the northeasterly-trending Nanpanjiang depression.Oil and gas deposits also occur in the Lower Tertiary detrital facies in the northwesterly-trending Baishai basin.The initial depositional framework of both the Nanpanjiang depression and the Baishai basin were formed during the Caledonian orogeny and were expanded, especially the Nanpanjiang depression, during the Variscan and Indosinian orogenies; later the Baishai was greatly expanded by rejuvenated faulting along the basin border during the Lower Tertiary Yanshanian (Yenshanian) orogeny.Subsequently, the Nanpanjiang depression was filled with very thick marine sequences of Devonian, Carboniferous, Permian, and Triassic reef and shale facies, in which the bioherms and carbonaceous shale are the favorable source rocks.The Baishai basin, however, was filled with thin marine sequences of Upper Paleozoic carbonate and shale facies, and thick continental and marine sequences of the Lower Tertiary fine detrital facies, in which the Lower Tertiary carbonaceous clayey detritus is considered to be the source rocks.In this study region, dome and syncline structures, and carbonate platform margins and algal banks, are believed to be generally favorable for oil and gas prospecting.2b