GEOLOGY OF THE APPOMATTOX COURT HOUSE NATIONAL HISTORICAL PARK, VIRGINIA PIEDMONT: EDIACARAN MAGMATISM ALONG THE LAURENTIAN MARGIN

Appomattox Court House National Historical Park (ACHP) is in the western Piedmont of south-central Virginia. In April 1865, the Union and Confederate armies fought an engagement here that concluded with the surrender of the Confederate Army of Northern Virginia effectively ending the American Civil War. Although the park’s historic significance is well known, its underlying geology is poorly understood. We conducted detailed bedrock and surficial mapping in and near the ACHP to better understand the geology of the area. The ACHP is in the enigmatic Smith River Allochthon (SRA), a possible suspect terrane interpreted as a regional-scale thrust sheet of metaclastic and meta-igneous rocks inferred to have been emplaced over lower grade greenschist facies assemblages. In the northern SRA, a suite of poorly understood meta-volcanic rocks (Oakville Suite) occurs in association with meta-pelites of the Fork Mountain Formation.

Rocks in the ACHP include hornblende gneiss, fine-grained epidote gneiss, and fine-grained quartzofeldspathic schist all interpreted to be part of the Oakville Suite. Some rocks preserve vestiges of primary igneous structures including quartz and feldspar phenocrysts in a fine-grained recrystallized matrix as well as flattened epidote amygdules in mafic rocks. The dominant foliation in these metamorphic rocks is folded into gentle to open upright folds with northeast-southwest trending axes. These open folds refold a set of earlier tight to isoclinal structures. Two steeply dipping joint sets (east-west and northeast-southwest) cut the metamorphic units. Surficial deposits include abundant saprolite plus fine-grained alluvium along the major streams. Cutbank exposures reveal ~1-meter of post-settlement deposition on the floodplain.

Whole-rock geochemistry indicates that the Oakville Suite records bimodal magmatism (basalt & rhyolite). Zircon U-Pb geochronology on four samples of felsic schist/meta-rhyolite from the Oakville Suite yield ages of 540 to 570 Ma. One sample of mafic schist, originally a volcanoclastic rock, yield detrital zircons with predominantly Grenvillian ages (1.0 to 1.2 Ga) . We interpret the Oakville Suite as the product of rift-related magmatism on a stretched block of Laurentian crust during the Ediacaran.