Stratification, Bed Forms, and Flow Phenomena (With an Example From the Rio Grande)
J. C. Harms, R. K. Fahnestock, 1965. "Stratification, Bed Forms, and Flow Phenomena (With an Example From the Rio Grande)", Primary Sedimentary Structures and Their Hydrodynamic Interpretation, Gerard V. Middleton
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Flow in alluvial channels is controlled by many variables, most of which are interdependent and adjust mutually. Bed form, for example, adjusts in response to changes in depth and slope, to changes in diameter, density and shape of particles, and to changes in viscosity and density of the sediment-water mixture. Because stratification is the product of migrating bed forms, it too is complexly related to many variables. The concept of flow regime allows a grouping of the combined effects of these variables.
Five distinct stratification types were recognized in shallow trenches in the Rio Grande bed near El Paso, Texas. Four of these stratification types are products of specific bed forms observed in this section of the river during the preceding irrigation season, when discharge, velocity, depth, channel width, temperature, and sediment concentration had been measured and bed forms mapped at the trench areas. The five stratification types that can be related in a general way to flow phenomena follow: (1) Large-scale trough cross-stratification (sets 0.2 to 2 feet thick), volumetrically the most important sedimentary structure, forms by dune migration in the upper part of lower–flow regime where water depths exceed one foot. (2) Small–scale trough cross–stratification (sets 0.1 foot thick) commonly veneers the river bed and forms by ripple migration in the lower part of the lower–flow regime. (3) Tabular cross–stratification (sets 0.2 to 2 feet thick) forms by migration of bars or of terrace–like features in the lower–flow regime. (4) Horizontal stratification is the product of plane–bed transport achieved in the upper–fiow regime, and is preserved in thin sheet–like sets on bar surfaces. (5) Parallel stratification, represented in a thin silt and clay layer mantling the forms on the emerged river bed, is deposited by settling of suspended material as flow slackens.
Stratification is potentially one of the most useful indicators of flow environment. However, stratification is the product of many complexly interrelated variables, some of which leave no discrete geologic record. Stratig–raphers have traditionally interpreted stratification in terms of velocity, depth, and/or slope. Such interpretations are incomplete considerations of the problem and are commonly incorrect. It is more correct and useful to state environment simply in terms of flow regime, the integrated resultant of all variables.
Assignment of sedimentary units to flow regime categories allows comparative, although generalized, interpretation of depositional flow environment. The regime classification is interpretively useful in any setting with unidirectional currents, whether fluvial or marine. For example, flow regime, as recorded by stratification, gives important clues about stream type or, in the case of marine turbidity currents, the erosional and transport capacity of currents. These interpretations of stratification, used with other kinds of geologic evidence, should lead to improved understanding of many sediments.