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DAVIES, DAVID J., and STEVEN J. PARKER, Geological Survey of Alabama, Tuscaloosa, AL

ABSTRACT: Stratigraphic Framework of Inner Shelf Storm-Dominated Sand Ridges, Alabama EEZ: Implications for Sequence Stratigraphy, Global Climate Change, and Petroleum Exploration

The Alabama exclusive economic zone (EEZ) contains an abundance of orthoquartzitic shelf sand ridges elongate northwest-southeast diagonally from the shoreline. The ridges commonly are found in water depths of less than 15 m, with many being shoreface attached, although ridges are found in all water depths on the inner Alabama shelf. Ridges are abundant on the Alabama eastern shelf, but rare on the western shelf due to high muddy sediment flux from Mobile Bay. They average 6 km in length, and average 1.6 km in width.

Soft-sediment peels from 59 Vibracores(tm) from the Alabama inner shelf permit detailed description of sand ridge sedimentary structures, fabrics, and eight sea-floor sediment types. These overlie the pre-Holocene sequence boundary and lower Holocene transgressive sediments. The ridges and inter-ridge troughs are embedded in a blanket shelf sand sheet representing widespread deposition of reworked palimpsest orthoquartzites following Holocene transgression.

In general, the ridges are capped by coarse stacked graded shelly sands, echinoid sands, and clean sands deposited well above storm wave base. The graded shelly sand microfacies, the most common sediment type, is inferred to represent shelf storm deposits because of its graded nature, sharp base, and variable thickness (0.1 to 4 m). Commonly, graded shelly sands are found as stacked storm deposits on upper ridge flanks, with only the basal, graded shelly parts preserved. The orthoquartzite facies also is found on the ridge crests, upper flanks, and on the sand sheet proper. The echinoid sand microfacies is a background, agitated water deposit formed on sand-ridge crests and low-relief parts of the sand sheets, and is compositionally similar to the orthoquartzite microfacies; however, t contains recently dead parautochthonous echinoid hash, which restricts it to surficial deposits in sandy areas of high concentrations of echinoid colonies.

The inter-ridge troughs receive quieter water and fine-grained sediment between storms, but during storms receive thin stacked shelly washovers. Where correlative from ridge to swale, clean sands commonly thin into the swales. Therefore, the ridges contain thicker sequences of coarse, reworked Holocene sediments than do the surrounding swales, representing positive build-ups of Holocene sediment above the pre-Holocene surface.

Considerable patchiness of facies is found on a single sand ridge, even with close (1 km) core spacing. Commonly, muddy vs. sandy sediment can be correlated seaward (i.e., parallel to ridge axis) in a general manner; however, specific muddy or sandy microfacies grade laterally. The internal facies pattern does not indicate lateral migration of the ridges, and no obvious differences in facies patterns are found on Gulf-facing versus shoreline-facing ridge flanks. The facies patchiness may result from the interplay between relict sediment distribution, present hydrodynamics (especially helical storm flow parallel to the ridge axes), and local differences in preserved shell content. Sand ridge distribution is not controlled by the paleotopography of the pre-Holocene unconformity; ridges pparently are Holocene in origin, with their location, morphology, and internal facies geometry controlled by the present hydrodynamic regime. Due to the microtidal regime of the Alabama EEZ and the prevalence of the graded sands on the ridge crests, the ridges are interpreted to be dominantly storm-wave in origin. This type of coarse, clean sandy deposit is a poorly studied yet important possible model for many shelf-sand petroleum reservoirs.

AAPG Search and Discovery Article #90989©1993 GCAGS and Gulf Coast SEPM 43rd Annual Meeting, Shreveport, Louisiana, October 20-22, 1993.