Shallow Carbonate-Bank-Margin Growth and Structure, Little Bahama Bank, Bahamas

Abstract

Extensive high-resolution seismic profiling and bottom sampling of the shallow northern margin of Little Bahama Bank have revealed a complexity and diversity in structure, facies, and growth history that cannot be encompassed within any single model. Cores, aerial photographs, LANDSAT imagery, and SCUBA diving observations provided supplemental data. Five bank-margin types have been identified. Counterclockwise from Walker's Cay on the northeast to Grand Bahama Island on the south they are: a windward, reef/oolite shoal complex; a terraced, rock shoulder; and three leeward margins that exhibit discontinuous reefs in various stages of development. On leeward margins unprotected from the rear the reefs now are buried; on ridge- and island-protected margins the reefs, although small, are still growing. The dominant processes responsible for the variability of these bank-margin types are: (1) physical processes, i.e., the direction, magnitude, and duration of tidal currents, waves, and storms; (2) the nature and history of sea-level fluctuations; (3) the antecedent (usually bed rock) topography; (4) biogenic barriers (both reefs and sea-grass zones); and (5) sediment production, type, and texture. Physical processes are the most important, as they in turn control biologic and chemical responses and set up feedback relations with both the antecedent and modern topography. The original pre-Triassic continental(?) basement of the Bahamas has subsided approximately 10 km. All of the margin-forming processes act within a larger tectonic framework which imparts an important long-term influ nce on growth and development of the entire bank. However, the present shallow margin of the Bahamian-type bank seems to be determined mainly by environmental processes acting within the most recent cycles of sea-level change. Relations mapped on this bank-margin area contradict some commonly held generalizations. Oolite shoals and reefs are present together and not at the expense of one another. Talus debris can be thin or absent seaward of active reefs and instead can develop as thick wedges on the bank side in the lee of deep, drowned reefs. Distance as well as topography can act as an energy barrier. Also, the site and history of reef development can depend on the direction of sediment flux. Reefs not only require wave energy from offshore, but protection against burial from behind by offbank sediment transport. Complexities such as these, coupled with significant variations in structure and sediment facies along a relatively short segment of the total Bahamian bank margin, show that more factors need to be considered in any attempt at bank-margin modeling. No one generalization as yet should be held up against particular ancient paleogeographic and paleoecologic settings.