Ecozonation and Sediment Distribution of Three Reef Areas on St John, US Virgin Islands

Feazel, Charles

Compared substrate grain size and composition (coral fragments & volcanic rock fragments) with ecological zones to test the theory that ecological zones can be defined by the substrate. Study took place off St. John, US Virgin Islands at the Lagoon Point Reef, Mary Creek Reef and Reef Bay. Includes sections on the process of sand formation, sediment production rates, coral skeleton durability, physical transport of sediments, biological modification of physical transport (e.g. seagrass beds), and diagenetic alteration (physical processes which may alter the original form of particles). Examination of grain size and constituent composition in the vicinity of standing populations of skeletal reef organisms suggests that the zonation patterns of life assemblages may be reconstructed fram the sedimentary facies distribution. Three reefs on the coast of St. John (18°20'N; 64°44'W) were mapped in detail, and ecozonation contrasted with sediment distribution patterns. Sediment size at a given location on the reef is dependent on standing crop turnover rates and skeletal breakdown processes. Estimates of potential sediment production for the St. John reefs indicate that the majority of sediment finer than 250 microns generated on the reef flat is swept seaward into deeper water: coarser detritus tends to move landward. The ratio of coral: Halimeda fragments and the relative abundance of Homotrema rubrum fragments in the sediments indicate proximity to the reef crest. The relative contribution of rock fragments indicates proximity to the volcanic shoreline. Energy-responsive phenomena (grain size and stable-substrate community structure) are symmetrically distributed about the reef crest. The final lithified product of a skeletal community is dependent on the interactions of the following factors: skeletal growth rates, standing crop turnover rates, skeletal structure, disarticulation and durability of skeletal particles, physical transport processes and their biological modifications, and postdepositional alteration through biological disruption or diagenesis. The findings of this study are applicable to facies analysis in older reef deposits, with modifications to compensate for the changes in reef community structure through time.