TIME VERSUS TECTONIC SIGNIFICANCE: THE DISCONFORMITY AT THE BASE OF THE CHILHOWEE GROUP AND THE NEOPROTEROZOIC-CAMBRIAN BOUNDARY

In the Blue Ridge province, fluvial to shallow marine strata of the Lower Cambrian Chilhowee Group overlie Mesoproterozoic granitoids and Neoproterozoic metavolcanic rocks that range in age from 1149 to 562 Ma. The base of the Chilhowee Group has been variously considered to be conformable, disconformable, and nonconformable, and the Neoproterozoic-Cambrian boundary. The Chilhowee rocks are exposed on the limbs of the Blue Ridge-South Mountain anticlinorium and are different along and across strike. From northeast to southwest, the coarse basal rocks on the west limb (Loudoun, Weverton, Unicoi, and Cochran Formations) were subaerial deposits. On the east limb, fine-grained basal rocks (Loudoun, Weverton, True Blue, and Candler Formations) were subaqueous deposits above pillow basalts of the Catoctin Formation.

From PA to northern VA, the Loudoun Formation at the base of the Chilhowee Group consists of discontinuous layers of phyllite and conglomerate. The phyllitic rocks are volcanogenic and are considered part of the Catoctin Formation. The polymictic conglomerate most logically belongs with the overlying Weverton Formation. At two locations the Weverton Formation disconformably overlies Mesoproterozoic granitoid, and the Catoctin Formation and Loudoun phyllite thin toward the nonconformity. New SHRIMP II U-Pb zircon geochronology of metarhyolite of the uppermost Catoctin Formation in PA yields a Neoproterozoic age of 562±5 Ma. Rounded cobbles of metarhyolite within the Loudoun conglomerate suggest a hiatus before its deposition.

Although the Catoctin Formation is absent in southern VA, layers of metabasalt in the lower part of the Unicoi Formation may be coeval with Catoctin volcanism. Further southwest in TN, the Cochran Formation disconformably overlies the Sandsuck Formation (Walden Creek Group) which contains xenotime overgrowths (923 ± 54 Ma) on detrital zircons (1100 Ma).

The Cambrian fossils Rusophycus (upper Unicoi in southern VA) and fragments of the trilobite Olenellus (Antietam Formation in southern PA) require that most of the Chilhowee Group is Early Cambrian. Thus the disconformable base of the Chilhowee Group could be, but does not have to be, the Neoproterozoic-Cambrian boundary (542 Ma). A hiatus with erosion and change in provenance occurred in the Neoproterozoic prior to sedimentation of the Cambrian Coastal Plain.