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D. S. Snipes and Others
Figure 3. Photomicrograph of a thin section of volcanic rock recovered from the Hilton Head Test Well #1. Albite phenocrysts are
enclosed in a fine-grained groundmass which consists of quartz, albite, and sanidine. Both the phenocrysts and the groundmass have
been slightly-to-moderately sericitized.
the southeastern Georgia subsurface. Neoproterozoic basaltic
andesites through rhyolites have also been recovered
from drill holes in northeast Florida and southeast Georgia
Figure 4. Ternary plot of feldspar compositions in the Hilton
Head rhyolitic rock sample.
(Barnett, 1975; Chowns and Williams, 1983; and Heatherington
and Mueller, 1995, in press). Heatherington and
Mueller (1995, in press) argue that all of these volcanics
belong to a single association which they call the “North
Florida Volcanic Series”.
Horton and others (1989) and Heatherington and Mueller
(1995, in press) suggested that the North Florida Volcanic
Series was part of the Suwannee terrane, which includes
Paleozoic sediments as well as early Paleozoic-late Precambrian
plutonic rocks. Moreover, they suggested that this terrane
was coeval with the Carolina terrane. Kish (1992)
published a geochemical and isotopic study of an altered
granite recovered from Well C-10, situated north of the early
Mesozoic South Georgia Rift/Summerville Basin shown in
Figure 2. He reported a 87Sr/86Sr isotope model age of 440
to 550 Ma because most granites in the Piedmont have relatively
low 87St/86Sr initial ratios. Furthermore, he suggested
that the age of crystallization of the C-10 granite was coeval
with the Carolina and Suwannee terranes.
In summary, the altered rhyolite from the Hilton Head
well is younger than 1094 Ma (Heatherington and Mueller,
written communication, 1995) and older than 402 Ma (Hops,
written communication, 1995). Thus, it is late Precambrian
to early Paleozoic in age, and it is probably coeval with the
felsic volcanic rocks reported by Chowns and Williams
(1983) and Heatherington and Mueller (written communication,
1995).
Based on similarities in composition, absolute age dates,
and proximity, we conclude that the rhyolitic volcanic recovered
from the Hilton Head Test Well #1 belongs to the North
Florida Volcanic Series as defined by Heatherington and
Mueller (written communication, 1995).
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