Abstract

Massive red sands assigned to the middle Eocene Lisbon Formation (Claibornian Stage) cap the higher hills in Washington, Clarke, and Choctaw Counties in southwestern Alabama and closely resemble, in mineralogy and texture, sediments of the upper Pliocene-lower Pleistocene Citronelle Sand, the Miocene Ecor Rouge Sand and Pleistocene terrace deposits of the Tombigbee River. Because of this similarity, and the fact that these sands are nowhere overlain by stratigraphically higher units, questions have been raised as to whether they do, in fact, represent a fluvial phase of the marine Lisbon Formation or whether some might actually belong to one of the other mentioned units. Multivariate classification techniques were applied to the mineral data from samples from each of the units to determine which formation the sands most closely resemble. Cluster analyses were able to effect a segregation of the Ecor Rouge and Citronelle Formations but did not effectively isolate the red sands with any other single unit. Discriminant analyses proved more efficient and consistently grouped samples from the red sands with either the Lisbon Formation, or more commonly, with terrace deposits of the Tombigbee River. Field checks on localities where the samples had indicated a "Lisbon similarity" disclosed that at most of these sites the red sands were in an apparent erosional contact with the laminated clays of the Lisbon Formation. This would indicate that, rather than being a shoreward facies of the Lisbon, these sands represent material reworked from the Lisbon Formation and are separated from it by either a diastem or major unconformity. Only a few locations were found where the red sands unequivocally belonged to the Lisbon, hence it was concluded that the blanket assignment of most of the red sands to the Lisbon Formation in southwestern Alabama is unjustified. Most represent high terrace deposits associated with the Tombigbee River whereas the remainder possess a Lisbon "character" resulting from reworking of the Lisbon marine sediments.

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