STRUCTURAL CONTROLS ON ORDOVICIAN AND DEVONIAN STRATIGRAPHY: APPALACHIAN VALLEY AND RIDGE PROVINCE NEAR PIEDMONT ALABAMA

The Devonian Frog Mountain Sandstone thickens abruptly eastward across the Eastern Coosa thrust fault from <12 m on the west to >60 m on the east. The thin Frog Mountain on the west unconformably overlies the Cambrian-Ordovician Knox Group. The upper third of the Frog Mountain is mostly sandstone, and the lower part is mostly shale. The Frog Mountain is overlain by the Mississippian Maury Shale (~1 m thick) and Fort Payne Chert (~50 m thick). The thick Frog Mountain on the east rests on the Middle Ordovician Athens Shale, a black shale >150 m thick. The Athens overlies the Knox Group. The thick Frog Mountain is nearly all sandstone, and is overlain by Fort Payne Chert, which apparently is only a few meters thick.

The two contrasting successions are separated structurally by the Eastern Coosa thrust fault. The footwall of Eastern Coosa fault contains narrow, north-northeast-striking, linear outcrops of the western stratigraphic succession. Smaller folds plunge almost perpendicular to strike. Beds in the hanging wall of the Eastern Coosa fault strike east-northeast; smaller folds plunge nearly perpendicular to the predominant strike. An upper-level, younger-over-older thrust fault with thick Frog Mountain in the hanging wall cuts more than 290 m down-section from Athens to lower Knox in the footwall. The upper-level Frog Mountain thrust sheet crosses over the Eastern Coosa fault, and truncates folds in the Eastern Coosa footwall, documenting late-stage thrusting that is consistent with a break-back history on some large-scale faults.

The proximity of two contrasting stratigraphic successions suggests either thrust telescoping of a regional stratigraphic gradient or older tectonic controls on local abrupt thickness variations. The limited extent of Athens Shale and abrupt variations in the Frog Mountain Sandstone suggest basement fault controls on the distribution and thickness of Ordovician and Devonian strata.