Variations in Carbonate Shelf Cycles in Response to Appalachian Tectonism
Thomas J. Algeo
Shelf facies strata of the Upper Mississippian Bangor Limestone in northwest Georgia and southeast Tennessee comprise asymmetric regressive cycles that are similar to shallowing-upward cycles described in many ancient and modern shallow marine carbonate sequences. Typical Bangor cycles consist of a lower 0.6-m transgressive hemicycle of poorly sorted intraclast-oolite grainstones, and an upper 15-m regressive hemicycle that grades vertically from open-marine fossil wackestone and packstone through barrier-bar oolite grainstone, to burrowed lagoonal wackestone and laminated fenestral tidal-flat mudstone and dolostone.
Lateral variations in the number, thickness, and facies composition of cycles were controlled by the position of each Bangor section relative to the Mississippian shoreline and shelf margin, and by localized shelf downwarping in response to Appalachian foreland basin evolution. To the northeast, at Monteagle, Tennessee, evaporitic tidal flats flanked the low-lying Nashville dome. There, laminated fenestral mudstone and dolostone dominate a thin (58-m) Bangor section, with only one major marine transgression reaching this area. At Raccoon Mountain, Tennessee, in the midshelf area, syndepositional downwarping of the Raccoon Mountain trough controlled sedimentation and deposited a thick (120-m) Bangor section containing seven cycles of highly variable thickness and facies composition. To the southeast, at Pigeon Mountain, Georgia, the outer shelf was increasingly influenced by foreland basin sedimentation during the late Bangor. There, the lower part of a thin (52-m) Bangor section contains two normal regressive cycles, but abundant thin shale laminae and frequent facies shifts in the upper 15 m document increasing clastic influx and tectonic instability in source areas to the southeast.
Although 1-2 transgressive-regressive events affected the entire Bangor shelf and are either tectonic or eustatic in origin, localization of rapid shelf subsidence and repeated sea level fluctuations at Raccoon Mountain are linked to the southwestward extension of the Appalachian foreland basin. Regional stratigraphic relations support this hypothesis: the Raccoon Mountain section is located along strike with the Lookout Mountain and Cahaba synclines, and the rapidly subsiding Late Mississippian troughs, in northeast Alabama; also, high sedimentation rates persisted at Raccoon Mountain during deposition of the type Raccoon Mountain Formation in the Early Pennsylvanian.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.