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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 52 (1968)

Issue: 11. (November)

First Page: 2123

Last Page: 2136

Title: Contemporaneous Normal Faults on Flanks of Birmingham Anticlinorium, Central Alabama

Author(s): William A. Thomas (2)


Contemporaneous normal faults in Late Mississippian sandstone on the flanks of the Birmingham anticlinorium in the southernmost Appalachians are approximately parallel with structural strike and are downthrown in the direction of dip away from the crest of the anticlinorium. Dip of the faults decreases from 70° to 40° with depth. Throw of the faults decreases upward from a maximum of less than 60 ft. Part of the sequence is thicker on the downthrown block of each fault than on the upthrown block, and the faults are buried beneath younger, unfaulted beds. Dip of the downthrown beds toward the fault locally results in anticlinal structure on the downthrown block. Sediment-flow structures are associated with the faults. Except in maximum displacement, the contempor neous normal faults along the Birmingham anticlinorium appear to be structurally similar to those in the subsurface Tertiary of the Gulf coastal plain.

Faulting was initiated by downdip mass flow of hydroplastic clay and unconsolidated sand on an unstable initial slope. Normal gravity-slide faulting at the updip edge of the moving mass produced an irregular surface that permitted accumulation of greater thicknesses of sand in the depressed blocks. Perhaps sand loading enhanced movement of the already active faults. Faulting ceased once a stable angle of slope had been built.

Initial dip slopes of Late Mississippian sedimentary strata coincide with present structural dip of the flanks of the Birmingham anticlinorium. Evidently, Appalachian structural deformation was active in the Birmingham anticlinorium as early as Late Mississippian time.

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