Ordos Cretaceous Luohe - Central River Huachi Group Sedimentary Characteristics

Based on the synthetic research of field outcrops, drilling section, well log and office compilation, combined with analyses of the geological background, lithological association, sedimentary structure and palaeontological characteristic, the cretaceous Luohe formation to Huanhe-Huachi formation can be divided into five kind of sedimentary facies such as lakes, rivers alluvial fans, deltas and aeolian deposit. Furthermore, fifteen kind of sedimentary facies are identified, namely shore and saucer fan, half-depth lake, underwater sedimentary fan, delta plain, delta front, meander riverbed, braided riverbed, flood plain, top fan, middle fan, foot fan, sand dune, sand lake, gravel, and wadi.There are quite different principal sedimentary facies between the cretaceous Luohe formation and Huanhe Huachi formation of Ordos basin and the distribution of sand bodies are obviously controlled by the sedimentary facies belts. The south, northwest and southwest of the downstream of Luohe formation are nearby material alluvial deposits, debris flow and weathered deposits. Luohe formation mainly develops aeolian deposit, alluvial fan and braided deposit. The area of basins become more narrow entering the Huanhe Huachi formation, the sedimentary facies of the verge of the basin mainly are alluvial fan and braided river and the center of it are lakes and deltas. The distribution of sandstone of Luohe formation is wide and thick. The existence of weathered sand rock and the distributing region can be furtherly verified according to the component of the sandstone, the characteristics of the quartz surface and the sedimentary structure.The distribution of sandstone of basins greatly influenced by the facies control, the Luohe formation is mainly composed of aeolian sandstone with the quite thicker sediment and great variety in the horizontal level. Huanhe formation is mainly composed of mud rock deposits and the sandstone has the characteristics of being thin in the east and thickness in the west and gradually becoming thicker from the northeast to the southwest though the land stratum is thicker but the sandstone doesn't develop.The 3^rd depression of Ordos basin came about under the Yanshan movement after the Trias Jura & Jurassic and the center gradually move to the west, which lead to a huge anisomerous syncline of widely slow eastern side and precipitously narrow west. Luohe formation is still weathered sedimentary stratum because of basin structure and climate. However the continuous depression of the lakebed of Huanhe Huachi formation and the widening erosion lead to the close shore river deltas lake sedimentary stratum, the obvious law of the plane variety of the sedimentary facies is as the following ,the gradrually transitional evolution from the verge of diluvial facies (debris flow) river facies sand lake river lake facies(sand facies) of the center.