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Petrology and Stratigraphy of the Alabama Miocene
Wayne C. Isphording (1)
Miocene sediments crop out almost continuously from Texas eastward to the Florida peninsula and reflect deposition that took place in the Gulf Coast Geosyncline at a rate second only to the present. These strata consist, largely, of fluvial and transitional marine sediments in Texas, Louisiana and Mississippi, and marine and transitional marine materials in Alabama and Florida. The Alabama sequence, though the least investigated of Miocene units in the Gulf Coast, is important because it represents sediments that were apparently deposited on the eastern margin of the rapidly subsiding Gulf Coast depositional basin. Though carried for years in Alabama State reports as "undifferentiated Miocene," two lithologically distinct, mappable units can be identified in surface exposures. Formal formational status for the two is now proposed, with the lower clays designated as the "Mobile Clay" and the upper fluvial sands named the "Ecor Rouge Sand".
The mineralogy and thickness of Miocene sediments in the northern Gulf Coast clearly indicates a multiple source area for their provenance. The thick sequence found in Texas, Louisiana and western Mississippi resulted from weathering of source rocks in the Western Interior, the Big Bend region of Texas and, to a lesser extent, other source areas drained by the ancestral Mississippi River. Miocene units in eastern Mississippi and Alabama lack the volcanic-derived materials found in contemporaneous units to the west and were derived chiefly from re-working of older Coastal Plain formations, Paleozoic rocks of the Cumberland Plateau and from the southern Appalachians and Piedmont. The Florida Miocene sediments consist of unusual palygorskite-sepiolite clays that formed by direct crystallization and also contain a detrital suite that was derived largely from weathering of metamorphic rocks in the southeastern Piedmont.
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