INVESTIGATING STRUCTURAL AND STRATIGRAPHIC RELATIONSHIPS WITH DETRITAL HEAVY MINERAL GEOCHRONOLOGY FOR THE DEVONIAN FROG MOUNTAIN SANDSTONE IN ALABAMA, SOUTHERN APPALACHIANS

The Devonian Frog Mountain Sandstone is exposed in the Coosa deformed belt of the southern Appalachian thrust belt, and varies widely in lithology, thickness, as well as stratigraphic and structural position throughout. The unit is distributed sensu lato in two distinctive outcrop areas across central and northeastern Alabama.

The more laterally continuous of these is characterized by a relatively thin succession with proportionately less sand, which is located northwest of the Pell City fault and pinches out to the northwest. In one such area southwest of the Reads Mill Offset, the Frog Mountain Sandstone varies in thickness from 0 to about 55 meters and consists of an upper sandstone unit, middle mudstone and shale unit, and a lower, interbedded mudstone and chert unit. The formation unconformably overlies the Ordovician Newala Limestone in Frontal Tier and Angel Block thrust sheets, the Ordovician Little Oak Limestone in Intermediate Tier thrust sheets, is absent in thrust sheets within the Leading Part of the Interior Tier, and the Ordovician Athens Shale in the Trailing Part of the Interior Tier.

The second outcrop area consists of mostly thick, sand-dominated successions in a band of scattered outliers–presumed to be klippen–near the Georgia state line. Further dissimilarities in the rocks among the individual, often adjacent, klippen suggest isolated, emplaced blocks rather than remnants of a continuous thrust sheet.

Sediment provenance of the Frog Mountain Sandstone, which would greatly expand understanding of the formation, remains uncertain because of the lack of detrital geochronology conducted. Initial results from one sample (n=44), structurally located within the Frontal Tier of the Coosa deformed belt exhibits a detrital-zircon age spectrum with multiple populations spanning the Paleo- and Meso-Proterozoic, as well as a Neo-Archean population.

We report detrital zircon, apatite, and rutile U-Pb ages from the Devonian Frog Mountain Sandstone to integrate structural and stratigraphic field relationships with sediment provenance. The primary goal is to advance regional paleo-geographic reconstructions and, thus, better resolve the structural framework that controls the distribution of the formation and address the various stratigraphic and depositional differences.