COASTAL SEDIMENTARY EVOLUTION OF ST. THOMAS AND ST. JOHN, US VIRGIN ISLANDS: A FUNCTION OF DRAINAGE BASIN DIVERSITY

A suite of sediment cores collected from thirteen coastal environments surrounding St. Thomas and St. John, U S Virgin Islands, recorded the natural coastal development of the islands during the last sea-level rise, culminating in a phase of anthropogenically enhanced infilling from terrestrial sources. A wide variety of sediment textures and compositions were detected that are interpreted to reflect the unique nature of each individual drainage basin. This wide variety of sediment types is likely due to a combination of many factors, including: a) variable basin geology (i.e., mineralogy and rock type); b) influence of the marine environment; c) vegetation; d) degree and duration of inundation; and e) degree of anthropogenic development within the drainage basin.

The record of anthropogenic activities is most evident in surface sediments of heavily developed areas such as along the west coast of St. John and east and southeast coasts of St. Thomas. The anthropogenic signal manifested in surface sediments is highly variable in both magnitude and type (e.g., coarsening, fining, and/or changes in sediment composition) due to the type of activity, as well as the natural character of each basin. Although the suite of sediment textures and compositions is highly variable, all record a similar history of coastal evolution: a) terrigenous deposition during the last sea-level low stand; b) carbonate deposition reflecting open marine conditions during initial flooding associated with the last sea-level rise; c) paralic deposition as sites became isolated by mangrove construction and/or reef growth; and, d) infilling by terrigenous input, which is often anthropogenically enhanced. Consequently, the final stages include the natural infilling of coastal depocenters by island runoff, but anthropogenic activities appear to be accelerating this process, possibly causing the premature destruction of island coastal wetlands.