Abstract

Reef-rimmed margins on isolated carbonate platforms are heterogeneous, and studying Holocene platforms provides a valuable means for characterizing the spectrum of facies variability and understanding the processes behind facies changes. In this study, insights from petrographic data, granulometric measurements, bottom observations, and remote sensing and subbottom profiler data from the northwestern part of Caicos Platform make this one of the more intensely studied shallow-marine carbonate systems in the Caribbean. Collectively, analyses of the thicknesses and heterogeneity of shallow-water Holocene carbonates and the platform margin reveal details of the considerable geomorphic variability present on this part of the margin, on which the orientation changes markedly.

In this area, the shelf margin arcs from northwest- to north- to northeast-facing, accompanied by changes in morphology as well as the distribution, thickness and type of sediments. The NW-facing margin has discontinuous reefs near the margin, flanked platformward by coarse sands in poorly developed aprons that transition to a deeper rocky bottom (interpreted as Pleistocene) with patches of sand to fine gravel sized sediment. Locally, lobate-to-arcuate shoals with medium sands (< 500 μm mean size; peloids and skeletal grains) form discontinuous nearshore sediment wedges. Above the gently seaward-dipping surface of the interpreted top-Pleistocene surface on this part of the margin, the most pronounced bathymetric changes (greatest relief ~ 3 m) occur around reefs and tidal deltas. In contrast, the NE-facing margin includes a continuous aggraded reef with just two passages to the open ocean (Sellar's Cut and Wheeland Cut). Just behind the reef, landward-fining coarse skeletal-peloidal sands (> 500 μm mean size) form a reef apron that is continuous along strike and over 1000 m wide. Because the interpreted top-Pleistocene surface is gently dipping seaward, Holocene sediment thicknesses vary with changes in bathymetry and generally thin away from the reefs. Sediments in most areas are between 1.5 and 3 m thick but can reach over 6 m locally. Although the interpretation of the base of the Holocene can be ambiguous under shelf-margin reefs, subbottom profiles illustrate that backreef shelf patch reefs do not necessarily nucleate on Pleistocene bedrock highs.

Facies trends are interpreted to reflect a combination of physical oceanographic, sedimentologic, and biologic processes acting within the framework of Pleistocene bedrock configuration. The NE-facing margin, with the continuous reef and expansive sand apron, is interpreted to be more wave-dominated, influenced by swells from the open Atlantic, and by locally derived wind waves. Comparison with Bahamian platforms suggests that this trend is not unique; margins facing open Atlantic swells from the north–northeast have the best-developed reefs, not east-facing (windward) margins. In contrast, the beaches and tidal deltas on the NW-facing margin suggest tidal currents and longshore transport are more pronounced here, consistent with the nature of geomorphic changes through time. The enhanced tidal influence reflects the inlets open to the open bank interior, a factor which also may discourage growth of reefs on this part of the margin. Nonetheless, other Bahamian platforms with fewer tidal passes have comparable discontinuous reefs, suggesting that island continuity is not the most influential parameter on leeward reef development. Collectively, these results illustrate the nature, scale, and causes of along-strike heterogeneity and provide a conceptual model for ancient carbonate platform margins, some of which may be equally complex in their facies architecture.

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