Mud-coated intraclasts: A criterion for recognizing sandy mass-transport deposits – Deep-lacustrine massive sandstone of the Upper Triassic Yanchang Formation, Ordos Basin, Central China

Abstract Numerous examples of sandy mass-transport deposits (SMTDs) in deep-marine and deep-lacustrine environments have been documented by researchers worldwide in recent years. However, it is an enormous challenge that how to distinguish the subaqueous mass-transport processes from the depositional records. After the rigorous examination of several outcrops in the Ordos lacustrine basin in central China, we propose that the “mud-coated intraclasts” in the deep-water thick massive sandstone of the Yanchang Formation are a viable criterion to recognize deposition from sandy debris flows in delta front environments. These so-called “mud-coated intraclasts” are a newly discovered special depositional phenomenon in the massive clean sandstones in China’s lacustrine basins. A mud-coated intraclast generally consists of two components: an inner core and a mud coating. The inner core is generally composed of muddy blobs or sandy lumps. The mud coatings are generally composed of thin shales or mud-rich fine-grained sediments. The inner core, which is generally coated by nearly concentric ring-shaped mud coatings, is embedded in massive deep-water sandstones. Because these features have never been documented in the geological literature, the present authors called them “mud-coated intraclasts” in the present study. The depositional features suggest that the sediments that contain these mud-coated intraclasts were characterized by en masse emplacement (i.e., a Bingham plastic behavior) and were supported by the strength of the plastic medium at all times (i.e., no flow transformations occurred during their transport and deposition). Accordingly, mud-coated intraclasts are one of the most significant criteria that indicate sandy debrites as the origin of the deep-water thick massive sandstones in the Yanchang Formation. This diagnostic feature may be useful to identify analogous SMTDs in other locations.