GEOLOGIC MAPPING IN NORTHWESTERN ALABAMA REVEALS EVIDENCE FOR ANCIENT MOUNTAIN BUILDING, BASEMENT REACTIVATION, AND BEDROCK CONTROL OF DRAINAGE DEVELOPMENT

Geologic mapping at the 1:24,000-scale provides the basic framework for an area, especially for applied earth science investigations. We present new geologic mapping efforts along the Natchez Trace Parkway in northwestern Alabama, supported by the National Park Service. The Natchez Trace Parkway traverses the Highland Rim and Coastal Plain physiographic provinces of Tennessee, Alabama, and Mississippi and is underlain by Paleozoic strata between Nashville, TN and Cherokee, AL. Geologic mapping in the Alabama sector of the Natchez Trace Parkway reveals the presence of (1) NW-SE-trending sand injectites and soft-sediment deformation features in Middle Mississippian strata, (2) two, previously unmapped, gentle (interlimb angle >20°) anticline-syncline pairs, and (3) evidence of Cretaceous infilling of northwest-southeast trending paleotopography. We interpret the presence of sand injectites in Mississippian strata to reflect paleo-seismicity associated with the onset of the Ouachita orogeny in nearby central Mississippi and west-central Alabama. Cretaceous infilling of NW-SE paleotopography trends suggests development of a small-scale, “Valley and Ridge” style topography in NW Alabama associated with distal Ouachita foreland deformation. Gravity models display a distinct discontinuity that may reflect a crustal-scale basement fault, which may have partitioned stress at this location in the foreland basin. The modern course of the middle Tennessee River runs parallel to the NW-SE trend; therefore, we postulate that the middle Tennessee River is largely controlled by subtle fold-trends in the Paleozoic bedrock. With the parallel orientation of Cretaceous infilling, it is possible this NW-SE trend hosted an ancient drainage system that currently hosts the modern Tennessee River.