ABSTRACT

Detailed correlations in the subsurface and outcrop of northern Alabama document that Mississippian clastic rocks between the Tuscumbia Limestone and the Bangor Limestone are thickest along a band across the northern and eastern parts of the Black Warrior basin. The interval thins markedly southeastward across a northeast-trending line in Monroe County, Mississippi, and Lamar County, Alabama from more than 350 ft to less than 150 ft. The line of abrupt thinning has a right-angle bend in strike in southern Marion and northern Fayette Counties, Alabama, to a northwest trend; and along a northwest-trending line in western Alabama, the interval thins dramatically southwestward from more than 350 ft to less than 100 ft. The area of persistently thin stratigraphic section is termed herein the Lowndes-Pickens block.

The interval between the Tuscumbia Limestone and the Bangor Limestone is predominantly mudstone but includes four sandstone units and minor amounts of limestone. This interval is divided into four formats. The upper three formats pinch out abruptly southeastward in Mississippi and southwestward in Alabama. Facies and thickness variations across this area suggest a local uplift-downwarp pair that was active during deposition of the upper three formats in the interval between the Tuscumbia and Bangor Limestones.

The thickness distribution suggests synsedimentary differential subsidence of crustal blocks. The northeast-trending block boundary in the Black Warrior basin nearly parallels an interpreted northeast-trending late Precambrian rift segment farther southeast. The northwest-striking boundary closely parallels an interpreted northwest-trending transform fault farther southwest. The block boundaries are interpreted as basement faults that originated during late Precambrian rifting. Subsequently, the older faults were reactivated by convergence during the Mississippian, simultaneously with the initial dispersal of clastic sediment into the Black Warrior foreland basin.