SIGNALS OF TWO-STAGE ACCRETION OF THE LATE PLEISTOCENE CORAL REEF DEPOSITS IN TREASURE BEACH AND RIO BUENO, JAMAICA

Late Pleistocene fossil reefs provide a unique insight into accretionary history, regional tectonics, and global sea level change during the last interglacial warm period. These reef exposures are well preserved and accessible in southwestern (Treasure Beach) and north central (Rio Bueno) Jamaica.

Six representative sections (4 in Treasure Beach and 2 in Rio Bueno) were documented using the line transect method. Additionally, slab analysis was performed on 37 coral samples (142 slabs) collected at both localities to quantify bioerosion, encrustation and sediment occlusion, and to observe internal taphonomic and growth features. Field observation and slab analysis were used to interpret reef growth characteristics and the relative time spent in the taphonomically active zone at each section.

Field observations of coral biofacies and growth fabric show two distinct reef units in both localities. Treasure Beach exposures show a sharp vertical shift from biofacies associated with high turbulence in the lower unit to relatively high sedimentation in the upper unit. Rio Bueno exposures, however, show a shift from a lower unit of 1.3-1.5 m tall patch reefs dominated by Orbicella annularis capped by Agaricia agaricites to a 3 m thick upper unit composed primarily of branched Porites sp.

The two reef units in both localities represent a possible jump in sea level during the last interglacial period resulting in two-stage accretion consistent with previous observations in Yucatan and Barbados. However, accretionary response and facies geometry vary significantly between Treasure Beach and Rio Bueno. Treasure Beach shows a progradational response indicated by the replacement of crest facies with sediment tolerant back reef and lagoonal facies due to a seaward shift of the reef crest. Rio Bueno indicates an initial “catch-up” growth phase shown by pronounced vertical growth and over-roofing of O. annularis corals. Vertical changes in growth fabric and coral species composition at Rio Bueno suggest a switch from superstratal to constratal growth driven by an increase in lagoon sedimentation. This was likely as a result of rapid sea level rise. Differences in reef response and facies geometry between both localities appear to be a function of the accommodation space available for accretion at the time of the jump in sea level.