Skewness as a Paleoenvironmental Indicator

ABSTRACT Because it has been suggested by many sedimentologists that skewness characteristics of recent sediments can be applied to the interpretation of paleoenvironments of unconsolidated and diagenetically unaffected sediments, we have compared skewness to both other textural features and compositional variations of the unconsolidated, uppermost Miocene sediments of central Delaware. Previous studies have indicated a lagoonal or bay environment for these silts and sands. Analysis of samples from 22 wells revealed a lignite-bearing, poorly sorted, dark gray, silty central region almost surrounded by shell fragment-bearing, poorly sorted, relatively heavy mineral-rich, gray, silty, subangular quartz sands. In the central silty region, clay (up to 20 percent) is more abundant to the north where a terrestrial source area was located. In the surrounding sandy region, the best sorted sediments, containing frosted quartz grains, occur to the south, indicating a possible transition into an offshore barrier island complex with windblown sands carried northward into the oceanward margin of the lagoon. Plotting the skewness values revealed a nearly identical central region with negative skewness almost surrounded by a marginal region of positive skewness, mimicking the "bulls-eye" distributional patterns of the other textural and compositional data. Comparison with recent environmental studies of skewness suggests that the central region probably represents proximity to a river mouth or tidal estuary where winnowing may have dominated over deposition, whereas the surrounding region may represent sands deposited in an adjacent shallow, sheltered lagoonal area dominated by deposition. This study supports the suggestion that the sign of skewness, coupled with lithologic and other textural analyses, is potentially useful in identifying ancient sedimentary environments. However, further work is necessary to fully evaluate its usefulness.