Discussion on Criterion for Recognizing Sandy Mass-transport Deposits: Deep-lacustrine Massive Sandstone—A Case Study of the Yanchang Formation (Upper Triassic), Ordos Basin

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, no accepted criteria exist to identify the... [ view full 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, no accepted criteria exist to identify the deposits of subaqueous mass-transport processes. After the rigorous examination of several outcrops in the Ordos lacustrine basin in central China, we propose that the “mud-coated structures” 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 structures” are a newly discovered special depositional phenomenon in the massive clean sandstones in China’s lacustrine basins. A mud-coated structure 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, we called them “mud-coated structures” in the present study. The depositional features suggest that the sediments that contain these mud-coated structures were characterized by en masse emplacement and were supported by the strength of the plastic medium at all times. Accordingly, mud-coated structures 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.