Sedimentary Characteristics of the Cretaceous Desert Facies in the Wushan Area, Northern Margin of the Western Qinling Mountains

Key words: 
• western Qinling Mountains /
• Upper Cretaceous /
• desert facies /
• aeolian cross-beddings /
• surface structure of quartz sands

Abstract: The red beds of late Cretaceous are widely and sporadically distributed in the western Qinling Mts. At late Cretaceous, the tectono-geomorphological features in the area are critical in elucidating the structural and geomorphological situation at the end of Mesozoic and determining the beginning time of the uplifting of the northeastern Qinghai-Tibet Plateau. That is, the red beds deposited in the independent intermontane basins or unified flat basin in the intra-continental orogenesis. In this paper, a suite of brick-red sandstones of the red beds of the upper Cretaceous located in the Wushan area, northern margin of the western Qinling Mts. are studied on the sediments sequence, structure and grain size analyses. Combining the mineral compositions, microscopic and SEM surface textures of quartz of the sandstones, the rocks have been discerned to have the following features: 1) developing macroscopic eolian cross-beddings; 2) the medium-fine grained, extremely fine-grained components dominate, but the siltstone-clay components are absent, with even sizes and well-sorting; 3) quartz is well-rounded and the surface develops typical eolian-origin disc-and crescent-type pits, erosion pits, and siliceous films. Therefore, we put forward that the brick-red sandstones are the typical desert facies deposits. According to the desert deposits symbols, the sediments are further subdivided into sand dune and inter-dune deposits. It is well known that the desert deposits have two essentials, the torrid-arid climate and relatively flat topography. Combining the determination of the desert facies sediments at the Wushan area, northern margin of the western Qinling Mts. similar desert facies sediments on the mount tops at Hadapu, Xijiang of Minxian county in the interior of the western Qinling Mts., it is proposed that the western Qinling Mts. were in torrid-arid climate and flat topography in late Cretaceous. That is the large scale, flat and unified basin developed in the area at that period. This deduction is critical for re-constructing the paleo-topography and climate of the western Qinling Mts. in late Cretaceous, and for geological constraining the geological background before the uplifting of the western Qinling Mts. since the Cenozoic.