Differentiation of Unfossiliferous Clastic Sediments: Solutions from the Southern Portion of the Alabama-Mississippi Costal Plain

Abstract

The scarcity of fossils in most non-marine Coastal Plain sediments of Tertiary age has surrounded these strata with a multitude of controversies. Not only is it difficult to correlate units believed on the basis of stratigraphic position to be of the same age, but it often poses a similar challenge to differentiate deeply weathered sands and gravels of markedly different ages when they are present in the same general area. As a consequence of this, some units simply have been "lumped" together and terms such as "upper Tertiary coarse elastics" and "undifferentiated Miocene" appear too often on state geologic maps. Further, this same lack of fossils also has led to major disagreements with respect to deposits laid down in more recent times: specifically, the age and number of identifiable terraces that developed by still-stands of the Pleistocene seas and the origin of these terraces (marine vs. non-marine'?, erosional, depositional, or tectonic?). Multivariate statistical analysis has proved useful in dealing with the identity of deeply weathered "red sands" in the Alabama Coastal Plain and strongly indicates that some units previously assigned to the Eocene Lisbon Formation are more likely recent terrace deposits. Similarly, detailed examination of sedimentary structures mineral suites, and gravels has shown that the Miocene sediments in Alabama can be assigned to two mappable units (the Mobile Clay and the Ecor Rouge Sand). Criteria also exist that allow the differentiation of these units from the overlying Citronelle Formation. Recent work on a major vertebrate site, disconformably underlying the Citronelle Formation in Mobile County, Alabama, has also provided new information on the Citronelle age problem but, for several reasons, mis-use of the term "Citronelle" will undoubtedly continue. Similarly, though the controversy involving the number of Pleistocene terraces (and their origin) is far from solved, evidence is present in southern Mobile County, Alabama that clearly shows the deposition of marine features did take place during the Pleistocene and that this deposition took place at elevations in excess of 10 meters above present mean sea level.