Tallahatta Sandstone: Sedimentology
The Tallahatta Formation consists of several types of siliciclastic sedimentary rocks. The dominant lithology is micaceous sandy claystone. It is thick-bedded, massive, and contains abundant trace fossils and burrows. A study by Schroeder and Harris (2004) in Mississippi determined that the claystone contained significant amounts of opal-CT (essentially non-crystalline quartz) and the zeolite mineral clinoptilolite. The presence of both of these minerals are significant as they are thought to be alteration productions derived from volcanic ash. Indeed, Schroeder and Harris (2004) imply that much of the fine sediment in the Tallahatta Formation in Mississippi was derived from a volcanic source.
Excellent exposure of poorly-cemented Tallahatta Formation. Cliff exposures such as these are common adjacent to river incisions.
They also suggested that opal-rich claystone in Alabama might have been derived from more biogenic sources as they contain fewer zeolite minerals and, as observed in this study, more marine indicators (e.g., burrows). Other researchers (e.g., Counts and Savrda, 2004) have suggested that diatoms might have been the source of much of this biogenic silica.
Sandstone and occasionally pebbly sandstone intervals also occur in the Tallahatta Formation and are particularly important for archaeological reasons as they provided the materials for lithic tools in south Alabama. The sandstone is technically classified as quartz arenite (Folk, 1954), as the majority of the grains (>95% of total particles) are composed of quartz. Most quartz grains are moderately well-sorted, sub-angular to sub-rounded in shape, and fine to medium in size. Accessory minerals (those present in trace quantities) include glauconite, muscovite, magnetite, ilmenite, garnet and glaucophane. Occasional petrified wood fragments (Counts and Savrda, 2004) and carbonaceous material (fossil wood fragments) are also found in the Tallahatta Formation (especially in lower portions). Some sandstones and claystones also contain marine shells (e.g., Discocyclina advena, Protoscutella mississippiensis, and Ostrea spp.; Copeland, 1968), and trace fossils of shallow marine origin (e.g., Ophiomorpha nodosa). The most likely depositional environment for the Tallahatta Formation was beach-shelf. Fine-grained lithologies (claystone) were deposited on the shelf, whereas coarse-grained lithologies (sandstone) were deposited at or near the shoreline. As a general rule, beach sediment is usually well-rounded and well-sorted sand. The Meridian Member fits this requirement and was likely deposited on a beach; however, much of the sandstone elsewhere in the Tallahatta Formation is composed of sub-angular grains. The degree of rounding generally reflects the amount of sediment transport, but it is less effective in resolving depositional environment (Pettijohn, 1957). It is possible that the sub-angular nature of Tallahatta quartz grains resulted from wind-generated impacts in an aeolian dune environment. If so, this would mean that the overall Tallahatta Formation was deposited in a coastal dune, shoreline-shallow marine environment very much like that near Gulf Shores and Orange Beach, AL today.
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The University of South Alabama
Updated: 9/5/2004