Grain Size Distributions and Depositional Processes

ABSTRACT Extensive textural study of both modern and ancient sands has provided the basis for a genetic interpretation of sand texture. Analysis is based on recognizing sub-populations within individual log-normal grain size distributions. Each log-normal sub-population may be related to a different mode of sediment transport and deposition, thus providing a measure of their importance in the genesis of a sand unit. The three modes of transport reflected are: (1) suspension; (2) saltation; and (3) surface creep or rolling. Each of these is developed as a separate sub-population within a grain size distribution. The number, amount, size-range, mixing, and sorting of these populations vary systematically in relation to provenance, sedimentary process, and sedimentary dynamics. The analysis of th se parameters is the basis for determining the process-response characteristics of individual sand units. A number of processes are uniquely reflected in log-probability curves of grain size distributions of sands and sandstones. These include: (1) current; (2) swash and backwash; (3) wave; (4) tidal channel; (5) fallout from suspension; (6) turbidity current; and (7) aeolian dune. The combination of two or more of these processes also produce characteristic log-probability curve shapes. Ancient sands show some differences from their modern analogues, but these are usually minor. Log-probability plots of ancient sands are directly comparable to those from modern sands. The principal limitation of this study is in comparing sands formed under comparable conditions and obtaining an independent determination of the processes of formation of ancient sands.