Sand-Shoal Depositional Environments

Where the fetch of the wind creates wave action or where other currents cross a shelf, sand shoals can develop, especially in areas of positive topographic relief. Sands are composed of local biota winnowed from the adjacent deeper, subtidal environment or are formed by the chemical precipitation of ooids under higher energy shoaling conditions. Sands are commonly well sorted and strongly crossbedded. The surface will have sand waves and ripples. Sand shoals can be as small as a few tens of meters wide and tens of meters long or areally extensive, with dimensions of tens of square kilometers.

Large Sand Field on the Great Bahama Bank

Platform-interior sand shoals near Eleuthera Island on the Great Bahama Bank. Distinct sand ridges (dunes) have developed by wave and tidal currents moving across the bank. Area of photograph is 28.2 by 46.1 km. NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team (http://asterweb.jpl.nasa.gov/gallery/gallery.htm?name=Tarpum).

Ooid Sand Shoal on the Caicos Platform

Caicos platform showing several areas of platform-interior shoals. A prominent linear ooid-sand shoal extends east-west in the lee of Ambergris Cays. A close-up photograph (box) displays the surface features of the shoal. Image courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center (http://eol.jsc.nasa.gov). Photograph ID = ESC_large_ISS004_ISS004-E-13320.JPG.

Sand Sheet on the South Florida Platform behind the Barrier Reef

Behind the Florida reef tract are a series of skeletal sand shoals. Image by Scientific Visualization Studio, NASA Goddard Space Flight Center; data courtesy Landsat Project. Click on red square to see detailed example.

Rock Types

In the ancient record, this environment is represented by burrowed to crossbedded grainstones.

Trough crossbedding in a platform-interior Lower Cretaceous shoal composed of ooids and hardened pellets (grainstone). Upper shoreface facies where daily tidal and wave currents reworked the sands.

Bioturbated ooid/hardened pellet sand, with larger organisms composed of bivalves, oysters, and gastropods (grainstone). Lower shoreface facies (deeper water), where daily tidal and wave currents were less strong, allowing organisms to live in the sand. These sediments were, however, strongly reworked by storms.

Platform-interior shoal grainstone composed of ooids, peloids, and intraclasts.

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