CORAL MORPHOLOGY AS AN INDICATOR OF SEDIMENTATION RATE: CAÑADA HONDA, DOMINICAN REPUBLIC

Coral distribution, abundance and morphology were measured in a subaerially exposed Holocene reef in Cañada Honda, in the western Dominican Republic. Five coral facies (branching, mixed, and three distinct massive-coral beds) were identified. The two lowermost massive-coral facies (M1, M2) are focused upon in this study. Six vertical transects through the M1 and the overlying M2 facies were sampled at 20-cm intervals. Twenty corals at each transect were also described in terms of morphology, degree of bioerosion, and vertical position within a facies. Samples of ‘typical’ corals were collected for lab analysis.

Facies M1 is distinguished by the prominence of Montastrea spp., while Siderastrea spp. dominates M2. Differences in colony morphology also distinguish the facies: M1 has more corals with ‘pancake’-like morphologies (i.e., draping of successive layers) while M2 shows a more diverse range of shapes, including domes, inverted cones and some ‘pancakes’. All shapes can be associated with multiple coral species, but Montastrea spp. strongly tends toward the ‘pancake’ morphology.

The excellent exposure at Cañada Honda allows for the correlation of growth form with sedimentation rate and the degree of bioerosion. Overall, conical forms tend to be more bioeroded, regardless of species or facies. They either encountered slower sedimentation or grew faster, resulting in colonies with longer exposure times and, thus, more bioerosion. High and variable sediment rates lead to the periodic swamping of pancake-shaped colonies, which then overgrew the sediment as deposition slowed. Rapid burial lead to less bioerosion. Annual growth bands revealed in X-radiographs are being used to quantify this relationship. Comparisons of coral-growth rate in the various forms may allow us to assign absolute values to terms such as “fast” or “slow” with respect to sedimentation in fossil reefs.