debris flows

deepwater fans

hypopycnal flow

homopycnal flow

mass flows

turbidites

The proximal portion of a deltaic system collects sediments carried in suspension from the adjacent river. The distances of transportation vary from inshore to distal portions of the adjacent shelf, and down slope into the basin to even collect on deepwater fans. Bates (1953) explained this transportation and accumulation of sediments on the basis of the properties of the decelerating flows of different mixed water and sediment plumes. Basically he tied sediment accumulation to the density of the suspension load of the river, its velocity at the river mouth, its interaction with wave and current energy in the depositional setting, and the density of the adjacent sea water.

The density of sediment may match that of the sea water, be less than it, or more. These density flows are:

• hypopycnal flows in which density of the suspended sediment flow is less than that of the water
• homopycnal flows in which density of the suspended sediment flow is equal to that of the water
• hyperpycnal flows in which density of the suspended sediment flow is more than that of the water

The diagram above captures the role of hypopycnal, homopycnal, and hyperpycnal flows and the Bedload sediment, and fluvial discharge.

In the upper portion "A" of the figure Orton & Reading (1993) show how hypopycnal flow of suspended sediment can be detached from the Bedload and extend some distance seaward, buoyantly entrained in the sea water as are fine grained sediments associated with the Mississippi delta. In contrast mixed load channels like those of the middle portion "B" of the above figure produce a friction dominated river mouths with bifurcating channels. Finally gravelly Bedload or mass-flow dominated channels of the lower portion "C" of the above figure produce an inertia dominated river mouth with a propensity for over steepening of the basin margin and the development of mass flows.

Van Wagoner,et al, (2003) have recently expanded on this concept to develop depositional models that are based on the properties of the decelerating flows. They use them to explain most clastic most deposits, including those associated with fluvial systems, deltas, beach profiles and deepwater fans.

References Cited
Bates, C. C., 1953, "Rational theory of delta formation": Bulletin American Association of Petroleum Geologists, v. 37, p. 2119-2162
Orton, G.J., and H.G. Reading (1993): "Variability of deltaic process in terms of sediment supply, with particular emphasis on grain size": Sedimentology, vol. 40, p. 475-512.
Van Wagoner, John C., R. T. Beaubouef, D. C. J. D. Hoyal, P. A. Dunn, N. L. Adair, V. Abreu, D. Li, R. W. Wellner, D. N. Awwiller, and T. Sun., 2003, "Energy Dissipation and the Fundamental Shape of Siliciclastic Sedimentary Bodies"., Abstract AAPG/SEPM: Siliciclastic sequence stratigraphy - Going Beyond parasequences; Technical Program AAPG Annual Meeting 2003