Lily Bank, Bahamas: History of an Active Oolite Sand Shoal

Abstract

ABSTRACT Study of the response of an active oolite sand shoal (Lily Bank), located along the northeastern margin of Little Bahama Bank, to its physical environment and the Holocene sea-level rise has provided a developmental sequence from initial formation to present maintenance for this marine sand belt. Lily Bank is oriented parallel to the bank margin and is independent of underlying bedrock topography. It has developed immediately bankward of two large, intrareef bank-margin reentrants which allow a relatively unrestricted flow of tidal and storm-generated currents on and off the bank top. Subsequent to the initial sea-level rise over the bank approximately 5,700 yr BP, these reentrants funnelled bankward-directed flow which resulted in two zones of bedforms: (1) an outer zone of linear, p rallel-to-flow sand ridges (200-500-m spacing, 3-4 km long, 2-3 m in height) oriented in a radiating pattern away from the reentrant throats; (2) an inner belt of sand waves oriented transverse to flow (90-300-m spacing, 1-2 m in height). These now-relict bedforms developed approximately 4,000 yr BP. As sea level continued to rise, bankward flow velocities decreased as the reentrant channeling effect decreased. The linear sand ridges, which require a higher flow velocity for development and maintenance, became less active and eventually were stabilized by marine grasses. The adjacent sand-wave field developed rapidly during the deceleration in sea-level rise which occurred around 3,500 yr BP. A portion of this sand-wave field continues to migrate slowly into the shelf lagoon in response o the dominant bankward energy flux and maintains itself in pace with the slowed sea-level rise. The present geometry and sand-transport regime on the oolite shoal appear to result from interplay between storm-generated and tidal currents. 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-dominated zones 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 the shield areas during poststorm recovery. Tidal bar belts oriented perpendicular to bank margins, and marine sand belts, parallel to bank margins, are not necessarily mutually exclusive, and occur together along this same bank edge. The now inactive linear sand ridges seaward of Lily Bank may be analogous to active tidal bar belts found elsewhere in the Bahamas today. Paleoenvironmental ramifications are that, despite differences in sand-body and sedimentary structures, both types of sand body may exist along the same bank margin. The tidal bar field is located near the bank edge and the marine sand belt is adjacent to it but farther bankward. Because reentrants within the bank margin are common, they have played an important role in localizing oolite shoals which then may serve as foundations for future island development, perhaps during a later sea-level fluctuation. Some of the large Pleistocene islands in the Bahamas may be situated upon reentrant-controlled oolitic sand shoals.