Drainage Analysis in Geologic Interpretation: A Summation

Abstract

Drainage analysis is useful in structural interpretation, particularly in areas of low relief. Analysis includes consideration of drainage patterns, drainage texture, individual stream patterns, and drainage anomalies. Drainage patterns generally are subdivided into basic and modified basic. To these might be added pattern varieties. A basic pattern is one whose gross characteristics readily distinguish it from other basic patterns. Modified basic patterns differ from the type patterns in some fairly obvious regional aspect as, for example, a tendency toward parallelism of the larger tributaries in a dendritic pattern. Thus many modified patterns are transitional in character between basic patterns, and the naming of such patterns may be a matter of judgment. Pattern varieties are characterized by internal details, commonly obscure. In a broad sense, the basic patterns, the modified basic patterns, and the pattern varieties are analogous to the genera, species, and varieties of the zoological classi ication. A complex pattern consists of two contemporaneous patterns adjacent to each other; a compound pattern consists of two unlike superimposed patterns. The palimpsest pattern consists of two superimposed patterns, but one is a paleopattern. Drainage texture depends on a variety of factors. In any one small area where all other factors are constant, drainage texture may provide information on underlying materials and indirectly on structure. Individual stream patterns may display characteristics similar to those of the gross drainage pattern and may be referred to by the same name. Thus individual patterns may be referred to by such terms as rectangular, angulate, or contorted. Other stream patterns include irregular, rectilinear, meandering, braided, misfit, and beaded. Drainage anomalies are local deviations from drainage and stream patterns which elsewhere accord with the known regional geology and/or topography. The expectable pattern is regarded as the norm; the anomalies indicate departures from the regional geologic or topographic controls. Analysis of drainage anomalies has revealed structural data in some flatland regions where other methods of investigation have been unsatisfactory.