ABSTRACT

Using an area of recent lacustrine sedimentation as a reference model, several methods of treating grain-size distribution of sands were evaluated for their ability to discriminate between, or identify, depositional environments.

The techniques of Passega (1957), Mason and Folk (1958), Friedman (1961), Sahu (1964), and Klovan (1966) were applied to sediment samples from known depositional settings. When analyzed as though from unknown environments, none of the methods could reliably identify the true depositional environment of the samples as established on the basis of hydrographic, topographic, geographic and sedimentologic criteria. Of the methods, only factor analysis (Klovan, 1966) produced mappable results consistent with known energy conditions at the depositional sites.

Differences in energy conditions which may exist between lacustrine and marine environments may be such that the techniques might not be expected to be environmentally sensitive. However, the ultimate goal of all such "grain-size-environment" studies is to predict depositional environments of ancient sand bodies for which little or no information concerning the depositional setting is available. Because factor analysis was able to produce useable results even under the restricting conditions of a lacustrine setting it may be expected that it will perform even better in the marine realm.

The lack of success of the methods may result from any or all of the following factors: (1) Standard bivariate plots cannot adequately express the complex processes producing particular grain-size distributions. (2) Grain-size parameters are merely descriptors of grain-size distributions. As such they may not contain all available information, nor perhaps of the right kind, to enable environmental determination. (3) Combinations of amount and type of kinetic energy (which may be primarily responsible for the grain-size distribution of a sand) may produce similar distributions in different environments.

The hypothesis that grain-size distributions can identify depositional environments is probably not universally valid.

It is concluded that grain-size distributions reflect depositional processes rather than environment and that the two need not be the same.