Subaqueous sandy mass-transport deposits in lacustrine facies of the Upper Triassic Yanchang Formation, Ordos Basin, Central China

Subaqueous sandy mass-transport deposits (SMTDs), which are a type of sedimentary deposit formed in deep water environments, have attracted increased attention in recent years. Based on the analysis of outcrops, cores and thin sections using X-ray diffraction and scanning electron microscopy, numerous sandy mass-transport deposits or sandy debrites have been identified in the delta-front and deep lake environments recorded by the Chang 6-7 Member of the Yanchang Formation in the Ordos Basin; thus, a mass-transport model has been established in this paper. These sandy mass-transport deposits are characterized by two distinct features: first, sandstones are characterized by a low matrix content and are defined as massive clean sands; second, massive sandstones are characterized by the widespread development of a rimed chlorite cement with a two-layer structure, which is identical to that present in distributary channel sands of the delta-front environment of the Yanchang Formation. The occurrence of and components of clay minerals indicates that the isopachous rimed cements in the inner layer are related to the original sedimentary environment. Rimed chlorite cements are formed from isopachous clay films that originally adhered to the surfaces of the detrital particles and represent syn-depositional products. Further studies suggest that during the process of retransporting delta-front sediments and forming the sandy debris flows, this material, which was adhered to the surfaces of clastic particles as isopachous clay coatings, together with the small amount of clay-water matrix present in the flow, acted as an "adhesive agent" between particles during the subsequent remobilization of the deposits and the formation of sandy debris flows. The adhesion strength and local matrix support strength created by the clay-water matrix (gels), together with the frictional strength of the sediments, provided support strength for the subaqueous mass transport of the Yanchang Formation. The adhesion formed by the isopachous clay rim or the clay-water matrix is especially dominant and is considered to be the root cause holding the sandy debris flows together during their mass transport. This research may have global implications for better understanding analogous subaqueous mass-transport processes and the distribution of sandstone reservoirs in other locations.