An analysis of the source and the sedimentary system of the Carboniferous Benxi Formation in Yanchang area of Ordos Basin

Abstract：By investigating data extracted from well cores, field outcrop samples, thin sections, cathodoluminescence images and the laboratory tests, the authors conducted a comprehensive study of the Carboniferous Benxi Formation in Yanchang area of Ordos Basin and then deduced its source directions from the angles of the mineral composition, genetic type, paleocurrent features, and enrichment regularities of rare earth elements. On such a basis, the authors analyzed the sedimentary facies distribution and evolution. It is revealed that from north to south in the study area, the sandstone grains decrease in size and increase in psephicity; the quartz cluster cathodoluminescence images are mainly fuscous, light-dark brown in color, indicating metamorphic genesis. The debris component is chiefly metamorphic debris, which is up to 85.1%. The paleocurrent of field outcrop moved northward, from northwest southward and southeastward. the distribution of the rare earth elements is quite similar to the Daqingshan old land at the northeast edge of the basin. It is thus concluded that the Benxi Formation primarily originated from Yinshan-Alxa in the north. The second stage of Benxi Formation in late Carboniferous epoch constituted a major turning point in the Upper Palaeozoic evolutionary history of Ordos. Due to the invasion of the slowly-subsiding sea water from the east, the study area formed a system composed of shallow argillaceous shelf, barrier island-lagoon and lagoon. The first stage was similar to the second stage in general features, but had larger argillaceous shelf and lagoon because of the intense water invasion, which had a good performance in the sand body continuity in parallel with the coastline.