Morphology and Sedimentology of Ambergris Ooid Shoal, Caicos Platform

Eugene C. Rankey¹, Thiago B. Correa¹, and Stacy L. Reeder^2
1RSMAS/University of Miami, Miami, FL
²Schlumberger-Doll Research, Cambridge, MA

Ooid shoals have a range of morphologies and controls. Ambergris ooid shoal, on the southeastern Caicos platform, is an elongate, asymmetric (steeper to the north), 20 km long spit-like feature westward of Ambergris Cays. The purpose of this study is to describe the sedimentology and morphology of the Ambergris complex, the ‘type example’ of a wind-influenced ooid shoal.

The Ambergris shoal crest is bare sand, partly exposed at low tide, ornamented by low-amplitude sand waves with variable, but systematically changing orientations. The crest smoothly varies along strike from ~200 to 700 m in width. Sediments on the shoal crest are dominated by well-sorted coarse sand-sized ooids (mean grain size ~600 μm), which most commonly include abundant laminae. Flanking the shoal crest on the north side is a ~2 m deep rubbly shoulder of Holocene hardground with a thin sediment veneer. Outward from the crest and rubbly flank on the north flank is deeper (~3m), sandy, rippled bare sand (oolitic-peloidal, mean grain size ~520 μm; many partly micritized ooids), passing sharply to gradationally to a moderately sorted, medium sandy flat to burrowed bare sandy bottom (>4 m) with scattered reefs. The southern flank passes from the shoal crest to a slightly deeper (2-4 m) flat to undulatory bare sandy bottom (>30 % ooids, common clasts) but with only rare ripples, before passing to the burrowed sandy facies.

The shoal is influenced by tidal and wave energy. Our on-going work continues to evaluate and quantify the role of these processes and compare them with other Bahamian examples.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas