PLIO-PLEISTOCENE TURNOVER ON SILICLASTIC DOMINATED REEFS FROM JAMAICA

A major turnover of Caribbean coral species in the Plio-Pleistocene re-organized reef community composition with species richness rising through the late Miocene, peaking in the Pliocene and falling dramatically after the early Pleistocene. While these patterns are well-documented, there is little information about how these changes affect the paleoecology of particular reef systems. Recent mapping in south east Jamaica has revealed very distinctive fringing reefs developed in coarse siliciclastics ranging from the late Miocene through late Pleistocene. Excellent exposures of coral in growth position throughout this section allow a detailed examination of how Plio-Pleistocene diversity changes play out in these unusual siliciclastic dominated reefs.

Coarse conglomeratic units in the late Miocene August Town Formation, the early Pleistocene Old Pera Formation and the late Pleistocene Port Morant Formation of Jamaica contain reefs and associated lithofacies that are remarkably similar. Each show coral colonization of cobbles and boulders capped by coral growth fabric in a coarse conglomeratic matrix. These fringing systems are up to 2-3 m thick and can extend across >50m of outcrop.

Comprehensive sampling and transect methods were used to document richness and relative abundance across exposures with coral growth fabric from multiple sites from each unit. In each formation, transects were placed across approximate bedding plane exposures as well as through vertical sections. Ordination was used to examine ecology and reef zonation within units and to compare patterns between formations.

Coral species richness sampled here is 15-20 spp. for each unit and does not appear to parallel patterns for the Neogene-Recent Caribbean as a whole. This suggests that richness in these clastic-stressed environments is not governed by the size of the overall Caribbean species pool but by the paleoecology of the individual species present. Turnover of species occurs between each of the units, though some dominant species, such as Stylophora monticulosa, range through more than one unit. These key species generally appear in the same parts of reefs and with same abundance throughout their stratigraphic range showing a marked stability in their ecology in the face of major changes in reef composition.