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AAPG Bulletin

Abstract



Volume: 65 (1981)

Issue: 2. (February)

First Page: 261

Last Page: 290

Title: Carbonate Sand Bodies Along Contrasting Shallow Bank Margins Facing Open Seaways in Northern Bahamas

Author(s): Albert C. Hine (2), R. Jude Wilber (3), A. Conrad Neumann (3)

Abstract:

High-resolution seismic profiling, bottom sampling, and aerial and space imagery show that widely contrasting carbonate sand bodies are present along the shallow windward, leeward, and tide-dominated margins of Little and Great Bahama Banks which face open seaways--the Northwest Providence Channel and the northern Straits of Florida.

This high diversity in sand bodies and their associated bank margins is due to variations in the level, duration, and magnitude of the physical energy flux across the edges of the platforms. In the Bahamas, the dominant winds, waves, and currents are from the east; hence easterly facing margins are windward, westerly facing margins are leeward. Where strong tidal currents are present (because of adjacent basin shape) the windward and leeward effects are diminished. Thus, bank-margin orientation relative to the climatic energy patterns appears to assert the major controls upon sedimentation.

Antecedent topography in the form of islands or subtidal rock ridges creates energy barriers which can control the direction and volume of sediment flux on or off the banks. Along windward margins, energy barriers such as large islands not only block the bankward transport of sands but appear to augment offbank transport by setting up seaward flows. In contrast, along leeward margins, energy barriers prevent the vigorous offbank sand transport that normally occurs when these margins are open and water flow is unrestricted.

In addition, sand-body geometry and grain type are controlled by bank-margin orientation. Along windward, open margins, skeletal sands accumulate in the lee of low-relief, mostly relict reefs. These margins are generally sediment-barren, having only a sporadic, thin veneer of sand. Along leeward, open margins a broad (10 × 10 km), thick (12 m), bankward-thinning blanket of sand rich in nonskeletal components (peloids, composite grains) lies on top of rock ridges and reefs. Large, asymmetric sand waves superimposed on this sand body indicate net offbank sand transport. Along the tide-dominated margin, large, broad, oolitic sand lobes have migrated onto adjacent lagoonal muddy sands and indicate net bankward transport.

We suggest the following classification of modern, shallow, carbonate bank margins: (1) windward open; (2) windward protected; (3) leeward open; (4) leeward protected; (5) tide dominated.

This classification seems to fit many ancient analogs and may be useful in paleoenvironmental reconstructions.

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