AN ANOMALOUS SILURO-DEVONIAN SUCCESSION IN THE SUBSURFACE OF WEST-CENTRAL ALABAMA, SOUTHERN APPALACHIANS

Sporadic oil and gas exploration between the early 1980s and the late 2000s targeted conventional overthrust traps and, subsequently, unconventional shale gas resources in west-central Alabama. This drilling led to the discovery of an unnamed, anomalously thick Siluro-Devonian succession of mostly black shale, chert, limestone, and sandstone. The complex relationship of this thickened stratigraphic section and the lithostratigraphic architecture of the Valley and Ridge (including those portions of the Appalachians beneath Cretaceous strata of the coastal plain) warrants a detailed examination.

Deep drilling in Greene, Hale, and Bibb Counties is sparse and unevenly distributed; however, the areal extent of the northeastward-trending, southeastward-thickening succession is inferred to cover at least 700 square miles. The interval abruptly thickens to more than 1,700 feet in the study area. In well logs, the succession is bracketed by an underlying Ordovician “Chickamauga” Limestone and an overlying Mississippian Fort Payne Chert; the geophysical signatures for the units below and above are relatively consistent across the region.

The southeastward extent of this thickened succession is apparently truncated by the Talladega fault. To the southwest, the succession appears to be truncated by late Paleozoic thrust structures near the Mississippi-Alabama state line. In one area on the northeast, the succession is truncated abruptly by the Helena fault between two closely spaced wells. No evidence of the thickened section is recognized in the sparse well control farther to the northeast.

The lithologic composition of the succession grades from predominantly black shale (gamma ray >150 API) and other dark-colored shales in the southwest to cherts and limestones interbedded with minor amounts of similar shales and some sandstones in the northeast. The more northeastern facies evidently correlate laterally with Ordovician Athens Shale and Devonian Frog Mountain Sandstone mapped at the surface near Montevallo.

We synthesize well data and a suite of cross sections to illustrate the composition and extent of this anomalous succession. Our goal is to improve resolution of depositional framework and potential structure-related controls along this part of the Siluro-Devonian margin of Alabama.