--> Abstract: Reef Facies Distribution Patterns, Pleistocene (125 Ka) Falmouth Formation, Rio Bueno, Jamaica, W.I., by W. F. Precht and W. H. Hoyt; #91004 (1991)

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Reef Facies Distribution Patterns, Pleistocene (125 Ka) Falmouth Formation, Rio Bueno, Jamaica, W.I.

PRECHT, WILLIAM F., Reef Resources, Miami, FL, and WILLIAM H. HOYT, University of Northern Colorado, Greeley, CO

Detailed paleoecologic and sedimentologic studies of the well-exposed, Pleistocene (125 Ka) Falmouth Formation from Rio Bueno, Jamaica, were undertaken to define both temporal and spatial changes in reef architecture. Analyses of samples reveal an overall shallowing-upwards motif and a distinct lateral zonation of reefal facies similar to those observed in Recent fringing-reef and bank-barrier reef complexes from the eastern and western sides of Discovery Bay, Jamaica, respectively.

The Falmouth Formation that crops out on the eastern shore of Rio Bueno Harbor is continually exposed (north-south) for approximately 0.5 km in length. The top of the reef exposure is approximately +4.5 m above MSL. This height correlates directly with sea level maxima for the Sangamon in Jamaica based upon a wave-cut

notch in the Hopegate Formation at the top of the Falmouth onlap surface. Interpretation of these deposits indicates that no lagoon or back-reef facies were present and that there was a gradation of energy regimes from high-to-low, north-to-south within this true fringing-reef complex. This is most apparently recognized in the distribution of corals along this gradient with Acropora palmata and Acropora cervicornis being the most abundant species present in the north, A. cervicornis, Diploria sp. and Montastrea spp. in the middle, and Porites furcata and Siderastrea radians in the south. A most conspicuous feature throughout the length of this exposure is a coralline red algal crust and rhodolith zone. This feature is interpreted as a disturbance generated zone and subsequent reef rea tivation surface. Reef growth above this surface is marked by colonization of sediment tolerant hemispherical, in-situ colonies of S. radians which in turn are capped by large coppices of branching P. furcata. This overall sequence is capped by small, encrusting, shallow-water morphotypes of S. radians.

Interpretation of the Falmouth Formation on the western shore of Rio Bueno Harbor consists of well-developed back-reef facies indicative of the rear-zone of a bank-barrier reef. In-situ corals consist of abundant S. radians, S. siderea, P. furcata, and Manicina areolata with large transported and overturned fore-reef debris including A. palmata, M. annularis, and Colpophyllia natans. A coralline-algal dominated zone is also present but is locally capped by bioturbated (Callianasa burrows) back-reef skeletal sands and circumrotary growth forms of S. radians and small patches of P. furcata in growth position.

Detailed sedimentologic analysis of thin-sections from all the above lithologies confirms the aforementioned paleoenvironmental interpretations. This study emphasizes the usefulness of modern counterparts in Pleistocene reef analysis and interpretation, and allows for an understanding of temporal (vertical) and spatial (horizontal) variations due to both physical disturbance and local sea level history that are preserved in these reefal deposits.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)