Southeastern Section - 67th Annual Meeting - 2018

Paper No. 25-3
Presentation Time: 8:40 AM


THOMAS, William A.1, MUELLER, Paul A.2, OSBORNE, W. Edward3 and VOLK, Natalie Ann2, (1)Emeritus University of Kentucky, Lexington, KY 40506-0053; Geological Survey of Alabama, P.O. Box 869999, Tuscaloosa, AL 35486-6999, (2)Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, FL 32611, (3)Geological Survey of Alabama, P.O. Box 869999, Tuscaloosa, AL 35486-6999

The Greater Black Warrior foreland basin dips SW beneath the Ouachita thrust front in Mississippi and is imbricated in the Appalachian thrust belt on the SE in Alabama. A SW-thickening Mississippian-Pennsylvanian synorogenic clastic wedge progrades NE. The Upper Mississippian clastic succession includes delta-front parasequences (Parkwood Formation) that downlap NE onto a SW-deepening and thinning carbonate ramp. The Lower Pennsylvanian (Pottsville Formation) succession includes a lower part, in which deltaic and back-barrier parasequences pass NE into barrier-island sandstones, and an upper coal-bearing part. The sandstones include sublitharenites and litharenites with volcanic and low-grade metasedimentary rock fragments. Distributions of facies and thickness, along with sandstone petrography, indicate NE dispersal of sediment from an arc and subduction complex. Superimposed on the pattern of NE dispersal, a polymictic conglomerate in the upper Pottsville has paleocurrents indicating NW dispersal in a braid-plain; clasts include various sedimentary, plutonic, and volcanic rocks. The largest clasts are limited to the SE in the Appalachian thrust belt, but some pebbly beds extend NW into the foreland.

U-Pb ages of detrital zircons from the middle and upper Parkwood, the lower Pottsville, and the upper Pottsville all have dominant peaks at 1070–1010 Ma, consistent with Grenville-age crust; with some exceptions, the samples have only a few younger and older grains. Although most of the samples have narrow peaks between 1250 and 950 Ma, the lower Parkwood sample has a broader peak between 1500 and 950 Ma, and the upper Parkwood sample has a secondary peak at 1400 Ma, indicating pre-Grenville sources. Unlike all others, the lower Parkwood sample has a strong secondary peak at 700–500, suggesting derivation from an accreted Gondwanan terrane; the upper Parkwood has a subdued peak at 520 Ma. The southeastern lower Pottsville sample is similar to the other samples, but the northwestern lower Pottsville sample has two exceptional peaks at 1635 and at 435 Ma. The upper Pottsville sample from the polymictic conglomerate has secondary peaks at 2715 Ma and at 335 Ma. The variations between samples indicate variations in details of provenance through time and variations in local dispersal systems.