Evidence for a Late Jurassic felsic igneous province beneath the southeastern Georgia (U.S.) Coastal Plain: Constraints from geophysical, petrological, and geochronological data

Legacy petroleum exploration well data from the southeastern Georgia Coastal Plain document the presence of an extensive subsurface felsic igneous province (~14,000 km² in area) in the absence of most other basement lithologies. Previously interpreted as Precambrian or Early Paleozoic in age, a published U-Pb zircon age of 159 +/- 3 Ma (2 σ) on granitic rocks within this complex implies these igneous rocks are Late Jurassic (Oxfordian) in origin. Log suites from a total of 17 wells, along with seismic reflection, potential field, petrological, geochemical, and geochronologic data, are used to evaluate the context of the felsic igneous units (granites, rhyolite flows, and vitric/crystal felsic tuffs) in relation to other subsurface geologic features and units including (1) the Brunswick Suture Zone, (2) the Gondwanan Suwannee Basin stratigraphic sequence, (3) Triassic rift basins of the South Georgia Rift, (4) the Red Beds of Hazlehurst (RbH), and (5) the basal Coastal Plain unconformity (CPU). Current work suggests the rhyolite and volcanoclastic flows sit stratigraphically above the early- to mid-Paleozoic Suwannee Basin stratigraphic sequence, disproving a Precambrian or Early Paleozoic origin for these rocks. The locus of felsic igneous activity appears to coincide with a structurally complex NE-SW-striking zone within the Suwannee Basin sequence, as well as a previously identified transfer zone within the Triassic South Georgia Rift basins. The granite bodies appear at the paleo land surface as the youngest dated unit below the Coastal Plain unconformity in southeastern North America. Tuffaceous sandstone, interpreted as part of the Redbeds of Hazlehurst, sits stratigraphically above felsic tuff, and provides a potentially new and important age constraint for these deposits. The widespread abundance of thick sections of felsic tuff (up to 1 km in thickness) are consistent with a caldera origin for this large igneous complex, which may be reflected in potential field signatures from gravity and magnetic data. A Late Jurassic age for this suite of rocks below the Coastal Plain Unconformity implies either (1) seafloor spreading in the Central Atlantic is as much as 40 Ma younger than previously interpreted or (2) the transition from rift-to-drift in North America is not represented by the Coastal Plain unconformity.