BALANCED CROSS SECTIONS BASED ON SEISMIC INTERPRETATION OF THE SOUTHEASTERN APPALACHIANS IN ALABAMA

Seismic reflection data from the southeastern Appalachians in northeastern Alabama and the central Black Warrior basin of western Alabama and eastern Mississippi are used to develop balanced cross sections that focus on the relationships between Alleghanian compressional structures, older basement extensional structures, and Alleghanian normal faults in the Black Warrior basin. The key to developing the balanced cross sections is accurate depth conversion of the seismic reflection data. Stacking velocities are used to determine interval velocities at selected positions on each of the seismic profiles. Interval velocities are then time-averaged to produce stepwise continuous velocity profiles that are correlated with major stratigraphic horizons (typically, the tops of the basement, Conasauga Fm., Knox Fm., and Chickamauga Fm.). The interval velocities are then spatially averaged to obtain either a single interval velocity for each stratigraphic unit or, in areas which indicate systematic velocity changes along the profile, a uniformly varying velocity. Two-way seismic travel time through each unit is then converted to stratigraphic thickness, and a true depth cross section is constructed and verified by comparison to outcrop and well data. The seismic interpretation and depth sections are iteratively refined during cross section balancing until a final integrated geological and geophysical interpretation is obtained.

A profile crossing the Wills Valley and Peavine anticlines in northeastern Alabama tightly constrains the Alleghanian structures and their relation to older basement normal faults on the western edge of the Birmingham graben. The Birmingham graben is a westward dipping half graben. The basal Alleghanian detachment rides over the graben on a nearly horizontal plane, ramping up slightly beneath the Peavine Anticline at its western edge. A duplex of the Rome and Conasauga forms the core of the anticline. The Knox formation shows no structural repetition and only moderate fold shortening. The basal detachment cuts up section westward, reaching the top of the Conasauga beneath the Wills Valley anticline. Basement normal faults beneath the Wills Valley thrust appear to have been active through Conasauga time but were not reactivated during the Alleghanian orogeny.