ABSTRACT

Criteria for the recognition of the depositional environment of sandstones are important in the reconstruction of paleoenvironments. Petrographic characteristics of recent sands from dune, beach, and river environments have been studied to determine if there are mineralogical or textural characteristics which will permit diagnosing the environment of deposition. At present satisfactory depositional petrographic criteria are nonexistent. Since near-shore sands are shifted from one environment to the other, it is necessary to relate the petrographic characteristics to the terminal environment.

The mineralogy of clastic sediments for the most part seems to reflect the nature of the source rock, whereas textural parameters are chiefly related to the mode of transportation and the energy conditions of the transporting medium. Grain-size distribution analyses represent a plot of abundance or frequency against grain-size. Dune sands commonly can be distinguished from beach sands on the basis of such plots. The distinction between the sand types can be numerically stated by computing the third moment (skewness) of the distribution curve. On the phi scale the third moment (skewness) for dune sands is generally positive, whereas that of beach sands is generally negative. This seems to hold whether the dune samples are from barrier islands, coasts, lakes, rivers, or deserts. Beach s nds of positive skewness occur on Padre Island, Texas, near the delta of the Rio Grande River, and on Horn Island, Mississippi. Sporadic positively-skewed beach sands have been found elsewhere, but for medium-to fine-and very fine-grained sands these appear to be relatively uncommon. Within the widely scattered samples a number of dune samples with slight negative skewness (-0.28 or less) have also been found. A plot of mean grain-size against third moment (skewness) results in an almost complete separation of the fields representing dune sands and beach sands. The sign of the skewness is not affected by the mineralogy of the sample. Sands of quartz, carbonate, gypsum, and olivine all follow the same general rule.

The distribution curves of river sands like those of dune sands are generally positively skewed, but a number of exceptions to this rule have been noted. Within limitations, medium-to fine-and very fine-grained river sands can be distinguished from beach sands on the basis of plots of third moment (skewness) against standard deviation (sorting). The third moment (skewness) of coarse-grained sand is inconclusive as an indicator of depositional environment. Dune sands can commonly be distinguished from river sands by their sorting characteristics; dune sands tend to be better sorted than river sands. Since dune and river sands are skewed in the same direction, a further criterion is needed for distinguishing river from dune sands which have overlapping sorting characteristics. This has een found by separating the light mineral grains from those of the heavy mineral and determining the mean grain-size ratio of quartz and that of a specific heavy mineral in the same sand, such as garnet or magnetite. The ratio of the radius of quartz to that of a specific heavy mineral is usually larger for river sands than for dune sands. For one Mid-Continent drainage system (Arkansas River and tributaries), plots of ratio of mean grain-size of heavy to light minerals (using the phi system) against the ratio of sorting (standard deviation) of heavy to light minerals show points representing river sands to lie in a different area of the graph from those for dunes.

Transportation of dune and river sands represents, for the most part, unidirectional flow. The upper size range of grains carried in suspension or by saltation is governed by the competency of the transporting medium. Such limitation does not affect fine particles in transport. The result of this limiting competency is reflected at the coarse-grained end of the frequency distribution curve by the lack of a "tail" usually present in a normal curve, resulting in positive skewness.

In beach sands the fine-grained particles of sand are removed by winnowing. The distribution curve of a winnowed sand appears to have a "chopped off" tail at the fine-grained end in comparison to a normal curve, thus indicating negative skewness.