Sedimentary characteristics of turbidite of Chang 7 member in southwestern Ordos Basin

Based on outcrops, cores and grain size analyses, the authors studied systematically sedimentary characteristics of turbidite of Chang 7 member in southwestern Ordos Basin. The results show that turbidites are sub-angular lithic feldspar sandstone with low compositional maturity and textural maturity. The turbidites exhibit typical turbidite particle size distribution. The sedimentary structures include graded bedding, flute cast, delve mold, gravity cast and collapse, and the common Bouma of turbidite sequences includes AB, ABC, ADE and AE. Hackberry facies can be divided into three sedimentary subfacies, i.e., the upper fan, mid fan, outer fan as well as relative sedimentary microfacies such as main groove and braided groove, and the upper fan and mid fan are main bodies of the turbidite with a wide distribution range. During the evolution of paleogeography, turbidites were small during Chang 7₃ stage of Chang 7 member. With the weakening of lake invasion and the retreat of the deep lake line, turbidites were developed gradually from Chang7₂-Chang7₁ and were distributed along Huachi-Qingcheng-Heshui area. Topography of the lake basin, source supply and tectogenesis seem to have been the important factors influencing formation and distribution of turbidites.