EARLY-MIDDLE MISSISSIPPIAN FLEXURAL PARTITIONING OF THE APPALACHIAN FORELAND-BASIN SYSTEM

Regional isopach trends in the Kinderhookian-Meramecian strata of the Appalachian basin define the flexural boundaries of the Early-Middle Mississippian foreland-basin system. Lower-Middle Mississippian rocks reach maximum thicknesses of between 200-800 m along an orogen-parallel trend between western Virginia and northeast Pennsylvania. This area of increased accommodation represents the location of the foredeep depozone. West of the foredeep, similar aged rocks thin to <100 m along a similar NE-SW trend before thickening to >200 m in central Ohio. These areas of thinning and thickening strata represent the forebulge and foredeep depozones, respectively. Collectively, stratal thickness trends define a foredeep >100 km wide and a forebulge ~200 km wide. Comparing the position of Early-Middle Mississippian foreland-basin system depozones with those for the Late Devonian, there is a southeast migration of the forebulge of >300 km. This orogen-directed forebulge migration has been interpreted to represent viscoelastic-relaxation of the foreland lithosphere following Acadian thrust-load emplacement (Quinlan and Beaumont, 1984). Alternatively, the interpreted flexural-partitioning of the Appalachian foreland may be in response to changes in the spatial position of topographic loads in the Appalachian orogen during Early-Middle Mississippian time.