STRATIGRAPHIC AND DEPOSITIONAL FRAMEWORK OF DEVONIAN SHALE IN THE BLACK WARRIOR BASIN AND APPALACHIAN THRUST BELT OF ALABAMA

Shale gas exploration is providing new insight into the characteristics and origin of Devonian strata in the Black Warrior basin and the Appalachian thrust belt. The Black Warrior basin was a cratonic ramp that dipped south-southwest. Lower to Middle Devonian limestone and chert form an unconformity-bound wedge that thickens southward. This wedge is onapped by the organic-rich Chattanooga Shale, which is regionally extensive and is typically 1 to 12 m thick. The shale thickens to 30 m in a depocenter at the southeast basin margin, which foreshadows a major Pennsylvanian depocenter.

The northeastern part of the Alabama thrust belt includes an Iapetan basin that was inverted during the Taconian orogeny. Above the inverted basin fill, the Devonian section is thin or absent, indicating limited sediment accommodation for a prolonged period following inversion. Downplunge of the Iapetan basin, by contrast, an exceptionally thick (> 500 m) Devonian section is preserved in the Greene-Hale synclinorium. Here, the Lower-Middle Devonian wedge passes into black micrite and shale and is onlapped by numerous radioactive shale beds that are largely equivalent to the Chattanooga Shale. Accordingly, the Greene-Hale synclinorium is interpreted to contain a remnant of an Acadian basin.

Although Devonian shale may have been deposited in an euxinic basin, that basin was highly dynamic. Bioturbation is common in most organic-rich shale, indicating at least temporary dysoxia. Sedimentary structures provide evidence for episodic currents, erosion, and resedimentation of mud. Radiolarian layers reflect episodes of high nutrient availability associated partly with upwelling. Resuspension of organic-rich mud, moreover, may have facilitated oxidation of nitrite to nitrate, which in turn could have stimulated phytoplankton blooms. Penecontemporaneous deformation structures include folds and tilt-block structures, indicating that shale deposition occurred on unstable slopes, particularly in the Black Warrior depocenter and the Greene-Hale synclinorium.