30 Applications of Igneous Petrology, Geochemistry, and Geochronology to Understanding Earth and Other Planetary Systems

This session invites contributions that explore igneous systems through traditional and nontraditional methods. Studies that link crustal evolution, planetary formation, resource potential, and comparative planetology across temporal and spatial scales are especially encouraged.

Tuesday, 10 March 2026
1:30 PM - 3:30 PM
RCC, 104

The Waycross Caldera Complex: Late Jurassic Magmatism in the Southeastern U.S. During Continental Breakup

Paper number: 30-1
Program: Technical Sessions
Time: 1:45 PM – 2:00 PM

Authors

Abstract

The Waycross Caldera Complex (WCC) is an extensive (>14,000 km2), roughly circular (>150 km diameter), intrusive and eruptive center of Late Jurassic age buried beneath the Coastal Plain of southeastern Georgia, U.S. Legacy petroleum exploration data (geophysical and lithology logs) from 17 wells spanning 1938-1988 document the presence of vitric/crystal felsic tuffs, porphyritic rhyolite flows, granophyric granite, and tuffaceous sandstone subcropping at the Coastal Plain Unconformity within this footprint, to the exclusion of all other lithologies (except one well with amygdaloidal basalt). Felsic tuff deposits, ranging up to 1 km in thickness, occur in eight wells and surround much of the periphery of the known extent of the WCC. Porphyritic rhyolites are likewise peripheral, occurring in three wells. Granophyric granite occurs in five wells along a WNW-ESE linear trend through the northeastern third of the complex.

Previously identified as a felsic igneous province of a co-genetic origin (Applin, 1951; Chowns and Williams, 1983), these rocks have historically been interpreted as Precambrian or Early Paleozoic in age. Since the last treatment of this province in the literature in 1983, two additional exploration wells were drilled (GGS-3466 and GGS-3633), a U-Pb zircon date (159 +/- 3 Ma; Heatherington et al, 1999) was determined on one of the granophyric granites (GGS-120), and deep seismic reflection data (COCORP) were collected across the region (e.g. McBride and Nelson, 1988). These collected observations lead to the following conclusions: (1) these rocks appear to be of a co-genetic origin based on their spatial coincidence, extent, and lithology, (2) the predominance and thickness of vitric and crystal tuff suggests a caldera setting, (3) a Late Jurassic (Oxfordian) age is indicated by the U-Pb crystallization age of the granite, (4) the volcanic deposits sit with angular unconformity above deformed strata of the Paleozoic Suwannee Basin sequence, and (5) the Redbeds of Hazlehurst appear to be coeval with the WCC. Jurassic magmatism is seemingly rare in the rifting process of the U.S. Atlantic margin, but these formations represent the youngest yet identified beneath the Coastal Plain unconformity. Either seafloor spreading in the Central Atlantic is much younger than previously interpreted or the transition from rift-to-drift in North America is not represented by the Coastal Plain unconformity.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1

doi: 10.1130/abs/2026SE-411581

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