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Middle and Upper Ordovician deposits occupy a significant position in the Paleozoic sequence in the southern Appalachians, since they represent a transition from passive margin carbonate to active margin clastic deposition. In the Alabama Valley and Ridge these Middle and Upper Ordovician deposits are exposed in two northeast-southwest trending outcrop belts separated by the Helena fault. Units west of the fault are essentially autochthonous, while those east of the Helena have been displaced some distance to the west by late Paleozoic thrusting.
Middle Ordovician units show a transition from shallow-water deposits in the west to deeper water basinal deposits in the east. West of the Helena fault the Middle Ordovician is represented by peritidal to shallow subtidal lithologic characteristics of the Chickamauga Limestone. East of the Helena these shallow-water deposits are replaced by deeper water carbonates of the Lenoir and Little Oak Limestones and graptolitic shales of the Athens Formation. As this deep-water basin filled during the late Middle Ordovician, tectonic uplift generated clastic sediments which prograded into the basin from the east. Red-green mudrocks of the Greensport Formation were deposited in shallow-shelf to tidal-flat environments and were in turn overlain by quartz arenites of the Colvin Mountain Sandston , deposited as part of a shallow-barrier system.
With continued uplift during the Late Ordovician, additional clastics prograded westward over the filled basin. Early Late Ordovician shallow-shelf to tidal-flat mudrocks of the Sequatchie Formation grade westward into shallow-water carbonates of the Inman and Leipers Formations. With continued input, Sequatchie clastics prograded westward and overrode the westerly carbonates. A relative sea-level rise during the late Late Ordovician was accompanied by deposition of open-marine shelf, bio-clastic
limestones of the Fernvale facies of the Sequatchie throughout much of the western Valley and Ridge.
The petroleum potential of the Middle and Upper Ordovician sequence in the Alabama Appalachians appears to range from marginal to moderate. West of the Helena fault the Chickamauga Limestone appears to have the best potential, though both source rock and reservoir potential are limited. Source rock potential is better east of the Helena, particularly in the Athens Formation, but reservoir potential is limited again. The existence of significant reservoirs in this area appears dependent upon the development of fracture porosity associated with Appalachian structures.
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