Editorial

Abstract

In advocating a rational theory for delta formation, Bates (1953) suggested three types of river-mouth sediment plumes: (1) hypopycnal plume for floating river water that has lower density than basin water, (2) homopycnal plume for mixing river water that has equal density as basin water, and (3) hyperpycnal plume for sinking river water that has higher density than basin water. Although Middleton and Hampton (1973) did not classify hyperpycnal flows as a sediment-gravity flow in their classification, these flows are indeed sediment-gravity flows. A global evaluation of density plumes, based on 45 case studies [e.g., Yellow River, Yangtze River, Copper River, Hugli River (Ganges), Guadalquivir River, Río de la Plata Estuary, Zambezi River, among others], suggests a complex variability in nature. Real-world examples show that density plumes (1) occur in six different environments (i.e., marine, lacustrine, estuarine, lagoon, bay, and reef), (2) are composed of six different compositional materials (e.g., siliciclastic, calciclastic, planktonic, etc.), (3) derive material from 11 different sources (e.g., river flood, tidal estuary, subglacial, etc.), (4) are subjected to 22 different external controls (e.g., tidal shear fronts, ocean currents, cyclones, tsunamis, etc.), and (5) exhibit 24 configurations (e.g., lobate, coalescing, linear, swirly, U-Turn, anastomosing, etc.). The major problem is that there are at least 16 types of hyperpycnal flows. Therefore the simplistic facies models of hyperpycnites, intrabasinal turbidites, and extrabasinal turbidites are obsolete.