Shale Layers in the Alabama Smackover Formation and Their Implications for the Relative Sea-Level Change and Regional Correlation

Several organic-rich, siliciclastic shale layers, ranging in thickness from 0.5 ft to 50 ft were discovered in the Smackover Formation during the recent drilling in Conecuh, Covington, and Escambia counties of Alabama. Shale layers are readily correlative across the eastern arm of the Conecuh Embayment and appear to pinch out along the rims of the basin. In the basin-ward direction, these shales grade into muddy outer ramp carbonates of a normal marine Smackover sequence.

Shale beds are black, laminated, slightly limey, and nearly devoid of marine fauna. In some cases, they host graded silt layers. Most importantly, shales contain abundant terrestrially derived, herbaceous organic matter. Powdered x-ray diffraction analyses of the clay sized fraction reveals an assemblage of chlorite, illite, and kaolinite.

We suggest that the deposition of these shales occurred during sea-level falls when siliciclastics and terrestrial organic matter were delivered into restricted basinal lows of the region via run off. The clay content of the shale closely approximates the mineralogy seen in the low-grade metamorphic slates, phyllites, and graywackes described in drill penetrations along the Pensacola Arch.

The occurrences of these shale layers may point to as many as three sea-level falls during the deposition of the Smackover Formation in Alabama. It is also likely that the shale intervals are correlative with exposure surfaces previously described from the Smackover Formation in the up-dip areas of the Eastern Gulf Coast region. Therefore, it can be inferred that the observed Alabama Smackover depositional events closely correspond to the deposition of the Smackover “C”, “B”, and “A” cycles in Mississippi, Arkansas, and Louisiana.