Paper No. 6
Presentation Time: 10:15 AM
SUBSURFACE CRYSTALLINE TERRANES OF SOUTH CAROLINA
DENNIS, Allen J., Biology and Geology, University of South Carolina Aiken, Aiken, SC 29801-6309, SHERVAIS, John W., Geology Dept, Utah State Univ, Logan, UT 84322-4505 and MAHER Jr., Harmon, Department of Geography/Geology, University of Nebraska at Omaha, Omaha, NE 68182, dennis@sc.edu
The geology of the crystalline rocks of the US Dept of Energy Savannah River Site in west central South Carolina has been characterized. Basement cores aggregating more than 10,000 m have been recovered from 57 deep wells at SRS. Cores from these wells, along with structural trends defined by aeromag lineaments, define four distinct units within the basement beneath the Coastal Plain: (1) Crackerneck Metavolcanic Complex (greenschist facies greenstones and felsic tuffs), (2) Deep Rock Metaigneous Complex (lower amphibolite facies mafic to felsic volcanic and plutonic rocks), (3) Pen Branch Metaigneous Complex (granulite facies amphibolites, garnet amphibolites, garnet-biotite schists, and gneiss), (4) Triassic Dunbarton Basin Group, a sedimentary unit filling a NE-trending graben beneath rocks of the Atlantic Coastal Plain. Dextral reverse motion along the Four Mile Branch fault emplaced Pen Branch MIC above the Deep Rock MIC in the Upper Penn. Triassic Andersonian reactivation of the Four Mile Branch fault as the Dunbarton basin border fault was coeval with basin deposition. Reactivated Late Paleozoic mylonites acted as conduits for metasomatic fluids rich in silica and Gr I cations. Later reverse motion (Pen Branch fault) on the border fault offset Cretaceous and younger sedimentary rocks. Major drainages crossing SRS are localized along Cenozoic and younger reactivation of Alleghanian basement faults.
All of the metaigneous rocks have calc-alkaline fractionation trends, consistent with formation in subduction-related arc terranes at convergent margins. Zircon U-Pb xlln ages of ca. 626 Ma to 619 Ma show that Deep Rock and Pen Branch complexes do not correlate with younger Carolina terrane (570–535 Ma) or Suwannee terrane (ca. 550 Ma). Deep Rock and Pen Branch MICs may be a continuation of Proterozoic basement that forms the older “Hyco” infrastructure of Carolinia. We interpret that the meta-igneous rocks southeast of the Modoc zone, including the Uchee belt in AL and GA, are variably Alleghanian-remobilized ca. 620-630 Ma basement translated well south along the Eastern Piedmont Fault System. The contact between the Crackerneck MVC (= Persimmon Fork Fm?) and the Deep Rock and Pen Branch MICs may be equivalent to the angular unconformity between the Uwharrie Fm and the Virgilina sequence observed in NC.
