Abstract: Response of Active Oolite Sand Shoal to Its Physical Environment and Holocene Sea-Level Rise

Albert C. Hine

High-resolution seismic profiling, bottom sampling, coring, tidal-current and bed-form measurements, and aerial photography and ERTS-imagery analysis of an active oolite sand shoal (Lily Bank) along the open, windward, northeastern margin of Little Bahama Bank have provided a detailed sea-level history reconstruction and explanation of the tidal and storm processes affecting this complex, bank-margin sand belt.

Lily Bank, independent of underlying bedrock topography and lying on a 2-m-thick pellet-aggregate/lagoonal sediment unit, has developed directly bankward of two large bank-margin reentrants which allow a relatively unrestricted water flow on and off the bank. With sea-level rise, the reentrants accelerated and channeled tidal/storm flow bankward which generated large (3 to 4 km long), linear, parallel-to-flow, sand ridges (200 to 500 m spacing, 2 to 4 m high). A linear belt of sand waves (20 to 70 m spacing, 1 to 2 m high), oriented transverse to flow, developed simultaneously as flow decreased farther in on the bank top. With continuing sea-level rise, oolite-sand generation in the sand-wave field eventually produced a 4-m-thick section on top of the 2-m lagoonal sediment unit, which was deposited during initial bank-top submergence. However, with current velocity decreasing as the reentr nt channeling effect decreased, the seaward, linear sand ridges became less active and eventually were covered with marine grasses. The adjacent sand-wave field continued to migrate slowly onto the shelf lagoon in response to the dominant bankward energy flux while maintaining itself vertically near the sea surface.

The present geometry and sand movement on the oolite shoal results from an interplay between tidal-current and storm-generated flow. Storm-generated currents have developed channels which cross the sand body and terminate in flood- or ebb-oriented spillover lobes. Between the lobes are wide, flood-dominant zones mantled with flood-oriented sand waves protected from ebb-tidal flow by topographically higher shield areas covered with symmetrical sand waves. During storms, the shield areas are flattened and sand is transported bankward onto the shelf lagoon when the dominant energy flux is in that direction. Tidal currents reestablish shield areas during poststorm recovery allowing the shielded zones to develop as well.

The preserved record of primary sedimentary structures would show a dominance of bankward-oriented cross-laminations developed during high-energy periods. However, study of normal conditions reveals that the sand body is maintained by tidal currents which provide a constantly moving substrate necessary for ooid growth and the prevention of stabilization by marine-grass growth or subsea cementation.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA