Two new stratigraphic units of the Upper Devonian in the northwest border of the Junggar Basin, Xinjiang

Abstract: The Xar Burd Mountain and adjacent areas of Junggar Basin in Xinjiang constitute one of the major representative districts for the study of the Devonian and Lower Carboniferous strata in the Junggar-Hinggan stratigraphic region, and hence have aroused much interest among geologists. In this paper, major strata concerned are those below and above the Upper Devonian Hongguleleng Formation. The Haer Member (new name) is named to represent the basal part of the original Hongguleleng or “Hebukehe” Formation established by the Regional Geological Survey in the 1970’s, which is characterized by a large suite of conglomerate-sandstone-siltstone. A thick interval of conglomerates features the lower part of the Haer Member, which is distinctly different from the underlying original Zhulumute Formation; the contact between the two units is undulatory, probably representing a minor hiatus. The member is variously colored, including grayish green, purple-red, dark gray, etc. It yields plant fossils only, with its age assignable to the Late Devonian based on the plant Leptophloeum rhombicum, which is consistent with the original Zhulumute Formation in age, and probably also in the common nature of terrestrial facies due to the absence of marine fossils. The lower part of the Heishantou Formation is named the Namu Member (new name), which is characterized by dark gray dense sandy limestone or calcareous/siliceous siltstone with some ferruginous shelly coquinas and dark or gray, grayish green siliceous mudstone, siltstone, and (fine) sandstones. The overlying Carboniferous middle part of the Heishantou Formation is distinguished from the Namu Member by its large suite of gray-black siltstone-mudstone. The underlying Chasi Member (Hongguleleng Formation) is distinct from the Namu Member in its weathering (brown) yellow color of the rock. The Namu Member yields the ammonoids Cymaclymenia spp. and Mimimitoceras sp. and the trilobite Omegops, which suggests that the member still belongs to the Devonian. In addition, the new brachiopod Syringothyris-Spirifer fauna is present in the Namu Member, which is different from the cyrtospiriferid fauna of the underlying Chasi Member.