Permian to Cretaceous Evolution of the Piedmont Along the Alabama - Georgia Coastal Plain Unconformity

Abstract

The Piedmont of Alabama and Georgia contains the southernmost exposure of crystalline rocks related to the culmination of accreted terranes associated with the formation of the Appalachian Mountain system. This area is critical for tectonic interpretation of Alleghanian metamorphic timing and exhumational history due to being overlain by Cretaceous Coastal Plain sedimentary rocks and it being composed of demonstrably exotic terranes. Samples were collected along a ~190 km segment along the Coastal Plain onlap from Notasulga, Alabama, to Macon, Georgia, in the Piedmont and Uchee terrane of Alabama and Georgia. 40Ar/39Ar analyses of hornblende, muscovite, and biotite from exposure of a migmatite in north Columbus (Barin Quarry), Georgia, are dated at 292 ± 0.5 Ma, 276 ± 3.1 Ma, and 284.8 ± 1.0 Ma, respectively. (U-Th)/He analyses date zircon and apatite at 231.3 – 162.7 and 175.3 – 118.5 Ma, respectively, across the transect. Thermobarometry techniques based on microprobe analyses were used to characterize peak metamorphic temperature and pressure conditions at the Barin quarry study area of ~686 °C and ~5.4 kb, inferring middle to upper amphibolite-facies conditions. A pressure-temperature-time model is constructed for the Barin quarry area based on thermochronometers and peak-metamorphic conditions. The model demonstrates peak metamorphic conditions following the Alleghanian orogeny at ~310 Ma with a subsequent period of rapid cooling and erosional uplift (25°/m.y. cooling with uplift at 0.85 mm/yr). Timing of this rapid unroofing corresponds well to the Pottsville Formation and sediment accumulation in the Black Warrior, Cahaba, and Coosa foreland basins in west and central Alabama. Depressurization, evident in the model from rapid uplift, correlates well to the onset of Late Carboniferous - Permian plutonism in the Piedmont of central Georgia. Apatite ages that young to the east are interpreted as evidence of increased heat flow due to crustal rifting associated with the breakup of Pangea along the eastern margin of Laurentia.