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The AAPG/Datapages Combined Publications Database
Wyoming Geological Association
Experimental and Field Studies on the Genesis of Sandstones
With few exceptions, sedimentologists have neglected experimental studies of sediment transport and deposition in favour of field and petrologic studies of Recent and ancient sediments. The advantages of experimental laboratory studies include the following: i) A small part of the problem may be isolated and studied under closely controlled conditions, ii) It is possible to study the physical processes directly. Even in Recent sediment studies, processes must commonly be inferred because field conditions make it impossible to observe them directly.
Two types of laboratory study may be distinguished: in the first, there is an attempt to reproduce, usually in a qualitative fashion, some phenomenon observed in sedimentary rocks. Little attention is given to quantitative measurements or scaling problems. Examples are Kuenen's experiments on turbidity currents, Rucklin's and Dzulynski and Walton's studies of sole markings (flutes and other structures), and McKee's experiments on ripples and cross-bedding. Such studies are very valuable and are immediately appreciated by the geologist working with ancient rocks, but they may fail to give sufficient insight into the physical processes which cause the phenomenon being investigated.
In the second type of laboratory study, the experimenter proceeds by "the method of radical oversimplification" to study one aspect of the problem only. Artificial materials (e.g. beads rather than sand) may be used, and an attempt is made to obtain quantitative data for most of the pertinent physical variables. Attention is paid to scaling problems in an attempt to permit inferences to be drawn safely from small scale to large scale phenomena. The results are analysed in terms of physical theory, including modern fluid mechanics. This type of study appears to many field geologists to be too far removed from nature to be worthwhile but the concepts developed by such studies may eventually have far-reaching significance for geology. Examples are the many studies made on factors affecting the settling velocity of particles, studies by Hjulstrum and Shields on the conditions necessary for the beginning of particle movement on the bed of a stream, studies by Gilbert and by Simons and Richardson on the regimes of flow in streams with sand beds, studies by Bagnold of particle interactions at high concentrations, and recent detailed studies by Jopling on the origin of cross-bedding.
Scaling problems can be considered by using dimensional analysis. The important dimensionless variables which should be identical in the model and in the prototype (nature) are in the Froude number (Fr = u/√gh), the Reynolds number (Re = uh/n) and the grain or boundary Reynolds number (Re' = u*d/n) where u is the average velocity, u* is a measure of the velocity near the bed, called the shear velocity, h is the depth of flow, d is a measure of the particle size or of the size of the roughness elements on the bottom (ripples, dunes), n is the kinematic viscosity of the fluid, and g is the acceleration due to gravity. Of these numbers, only the Froude number can be properly scaled in the laboratory. It is thus impossible to produce laboratory models of large scale natural sedimentation phenomena which are proper scale models: the most that can be expected from small scale models is that certain aspects of the model will correspond to nature.
The concepts discussed above will be further illustrated by a description of the author's own laboratory experiments on the formation of graded bedding by turbidity currents.
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