MOLLUSCS AS STORM-DEPOSITION INDICATORS IN A MADRACIS MIRABILIS LAYER, CAÑADA HONDA, DOMINICAN REPUBLIC

Exceptional Holocene exposures around Lago Enriquillo, in the SW Dominican Republic, present an opportunity to examine the interior of a well-preserved coral reef. Five facies within the outcrop range from shallow-branching to deep-platy corals, and match modern Caribbean reef environments. A layer, up to a meter thick and dominated by the branching coral Madracis mirabilis, separates two massive-coral layers along the seaward end of the outcrop. A study was conducted to determine whether the layer reflects sudden storm deposition or more subtle change in environmental conditions. The 125-m layer was divided into five equal sections. Within the first 10 meters of continuously exposed Madracis in each section, transects were established at the top and bottom of the layer. Molluscs occurring at 10-cm marks along these transects were collected (n = 345), identified (usually to species level), and evaluated for taphonomic change (fragmentation, wear, biont cover, borings, and color preservation). At least 28 mollusc species were identified in the Madracis layer, including 7 species not found in the massive-coral facies immediately below. The massive facies contained 31 species, of which 12 were unique. Madracis samples were also collected for taphonomic analysis. Chama congregata, an epifaunal cementing bivalve, and the most abundant mollusc in both collections, shows a greater degree of attachment and less wear in the massive facies (35% with minimal wear; 3% with heavy wear) than in the Madracis layer (15% minimal wear; 20% heavy wear). In addition, C. congregata in the Madracis are relatively unadorned and smooth, while those in the massive facies display a more ornamented, “leafy” growth form, which is characteristic of some members of the genus in calm water environments (as occurs in the deeper, massive-coral zone). It is proposed here that the Madracis mirabilis layer reflects a storm event or series of events that interrupts a single massive-coral unit. This preserves a unique time surface through the massive facies that provides important information about reef morphology, post-disturbance recovery, and changes in community structure at given water depths. The layer may be able to provide information about the frequency and intensity of storms, and will help to describe local climate variability within the late Holocene.