Platform-Margin, Slope, and Basinal Carbonate Depositional Environments

Platform-Margin, Slope, and Basinal Carbonate Depositional Environments

Robert G. Loucks, Charles Kerans, Xavier Janson
Bureau of Economic Geology

Design Objectives

Platform-Margin Sand-Shoal Depositional Environments

The platform-margin environment is an area of strong wave, wind, and tidal-current energy. If reefs do not form in this area, sand shoals can form. These sand shoals are noticeable on the leeward side of isolated platforms, such as on the Great Bahama Bank. The shoals are commonly formed of ooids or hardened peloids. The primary depositional structure is well-developed crossbedding.
The sand shoals will have tidal channels with associated flood-tidal deltas. The landward side of the sand bodies can have spillover lobes and shelfal-directed splays of sand produced by intense storms (hurricanes or typhoons).

Wave-Dominated Platform-Margin Sand Shoals (Ball, 1967; Halley et al., 1983)

Effects of wave or tidal energy varies relative to different platform-margin characteristics. The Cat Cay sand shoals on the west side of the Great Bahama Bank are affected strongly by storm waves, whereas Lily Bank on the north side of Little Bahama Bank is strongly affected by storms and tidal currents. These sand bodies are parallel or slightly oblique to the platform margin.

Tide-Dominated Platform-Margin Sand Shoals (Ball, 1967; Halley et al., 1983)

South end of Tongue of the Ocean on the Great Bahama Bank platform margin is dominated by tides. The resulting ooid-sand ridges are perpendicular to the margin rather than parallel.

Rock Types

The equivalent rock type of platform-margin sand shoals is well-sorted, crossbedded grainstone.

High-energy, crossbedded, ooid grainstone from the ramp-crest shoaling environment of the Jurassic Smackover Formation of the Gulf of Mexico.

Thin section of crossbedded ramp-crest grainstone from the Jurassic Smackover Formation of the Gulf of Mexico. It is a well-sorted ooid grainstone with a few fossil fragments. Strong waves and tidal currents have washed out any lime mud.

Crossbedded ramp-crest dolograinstone from the Permian San Andres Formation in West Texas.

Thin section of crossbedded ramp-crest dolograinstone from the Permian San Andres Formation in West Texas. Grains are highly dolomitized; however, some of the grains appear to have been ooids.

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