ABSTRACT

Clastic grains exhibit approximately lognormal size distributions in most sedimentary rocks. These lognormal distributions are essentially random ones and reflect the fact that most processes of erosion, transportation, and deposition affect only the medians and sorting coefficients, but not the fundamental shapes, of the size-frequency curves of sedimentary materials. Thus, normal sedimentary transportation and deposition con be considered as "non-selective" with regard to the fundamental shape of size distribution curves.

Non-normal distributions con be detected most easily by plotting cumulative frequencies on logarithmic probability paper, which shows deviations from lognormality even in some sediments for which standard quartile measurements indicate a skewness of 1. Non-normal distributions can be caused by a variety of processes, all of which may be classed generally as "selective." These processes include post-depositional elutriation of fine materials and rapid deposition of freshly weathered, little transported debris.

Size distributions of sedimentary rocks can be determined readily by sieving (for the bulk sediment) or by bromoform separation and grain counting (for the heavy minerals). Sediments which show non-normal distributions can generally be presumed to have been affected by some special, selective process. The identification of such processes is of great importance in analyzing the geologic history of a sediment and determining its mode of formation.