Rational Theory of Delta Formation

TLDR

The study develops a rational framework for predicting how sediments are deposited in lakes or oceans at specific locations and times. The authors employ a modified jet theory that treats the ocean as a static, rotating basin with a hydraulic head at the river mouth, and illustrate the approach with examples from the Mississippi Delta. The model identifies three deltaic deposit types—hyperpycnal, homopycnal, and hypopycnal—based on density contrasts, and applies them to explain the sedimentary features of the Mississippi and Lake Ponchatoula deltas.

Abstract

From the approach of the geological oceanographer, this paper attempts to develop rational lines of reasoning for hindcasting how nature disposed of sediments brought into a lake or ocean at a particular place and time. By assuming the ocean to be a currentless, tideless, wave-free basin, jet theory as developed by Tollmien and subsequent workers is utilized in determining the magnitude and interrelations of limnological and oceanographic phenomena observed off mouths of sediment-laden rivers. To do this, the basic theory of jets is modified by introducing a rotating fluid system and postulating a hydraulic head at the orifice. If a delta is defined as a sedimentary deposit built by jet flow into or within a permanent body of water, these modifications allow for the existence of three basic types of deltaic deposits, depending on the density contrast between entraining and entrained fluids. Examples of these different types of inflow are cited from the Mississippi Delta region. In the case of hyperpycnal inflow (inflow more dense), it is suggested that turbidity currents carrying coarse-grained material during glacial stages eroded the Mississippi Submarine Canyon and supplied the material for the submarine delta off the canyon's mouth. Homopycnal inflow (inflow equally dense), which gives rise to the deltaic deposit characterized by classical top-, fore-, and bottom-set beds, is described for the Lake Ponch rtrain, Louisiana, area when the Bonnet Carre Floodway is in operation and for crevasses in the Lower Mississippi River. Hypopycnal inflow (inflow less dense) is discussed in some detail, including such aspects as flocculation effects, lunate bar development at river mouths, development of distributaries and flank depressions, the seaward movement of river water upon leaving the river's mouths, and effects of the earth's rotation. Modifying effects of wind, waves, and tides on deltaic deposits are discussed briefly.