Caicos Platform Shoals: Formation and Erosion, Turks and Caicos Islands

John A. Dickson² and Arthur Saller^1
1Chevron, Houston, TX
²Department of Earth Sciences, Cambridge University, Cambridge, United Kingdom

The Ambergris shoal at the southeast end of the Caicos platform is approximately 25 km long and composed of ooids. Ooids with nuclei of quartz grains with rutile inclusions were found in one sample. The coatings are up to 0.05 mm thick. Quartz does not occur naturally on the surface in Caicos. Therefore, coatings on quartz nuclei probably formed in the last 500 years when humans might have brought in quartz sand. Some of the quartz is in composite grains. The quartz-cored ooids have radially-oriented crystals in their cortex, in contrast to tangentially-oriented needles in other ooids on the shoal.

An “island” in the Caicos mid-platform shoals is approximately 500 m long by 100 m wide and is now at sea level. Small eolian dunes extend above sea level in one small part of the “island”. Lithified grainstone beds along the margins of the “island” dip away from the middle of the “island” similar to beach deposits. Those dipping beds are truncated at sea level, and have fibrous cements similar to beachrock and other marine cements. The middle of the island also contains lithified grainstones at and slightly below sea level, however those grainstones have equant calcite cements supporting precipitation from freshwater. The freshwater cements suggest that eolian dunes covered the entire island in the past. A large storm apparently removed poorly lithified eolian sands above the water table, leaving meteoric-cemented rocks in the middle and marine-cemented rocks around the margins of the island.

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