Granitic rocks of the Robertson River Pluton intrude Precambrian Y gneisses in the core of the Catoctin-Blue Ridge Anticlinorium in northern Virginia. Cobbles and boulders of lithologically similar granite are present in the overlying Mechum River and Fauquier (Lynchburg) formations. Dikes of metabasalt, texturally and lithologically similar to the flows of the Catoctin Formation cut the Robertson River Pluton. The Catoctin metavolcanic rocks themselves were extruded following deposition of the Fauquier and correlative clastic sedimentary rocks. Zircons from four samples of the Robertson River Pluton yield a discordant U-Pb age of 730 m.y., indicating that the Catoctin Formation is considerably younger than the 820 m.y. previously assigned.

Abstract The Blue Ridge province in north-central Virginia forms a large basement massif at the hinterland edge of the central Appalachian fold-and-thrust belt. Rocks and structures exposed in the Blue Ridge record a long tectonic history that encompasses the Mesoproterozoic Grenvillian orogen, Neoproterozoic Iapetan crustal extension, multiple Paleozoic collisional events, and Mesozoic tectonism. The purpose of this field trip is to provide an overview of Blue Ridge tectonics, highlight the findings of recent studies, and discuss the outstanding questions that remain unanswered in Blue Ridge geology. The trip will traverse the Blue Ridge from southeast to northwest and includes stops along the Skyline Drive in Shenandoah National Park and the Shenandoah Valley.

Abstract The Blue Ridge and Piedmont provinces in the central Virginia Appalachians are underlain by Proterozoic and Paleozoic rocks that record multiple episodes of continental collision and rifting. This trip focuses on rocks and structures formed at the southeastern margin of Laurentia during: (1) the Mesoproterozoic assembly of Rodinia, (2) the Cryogenian to Ediacaran rifting that ultimately created the Iapetus Ocean, and (3) the Paleozoic deformation and metamorphism associated with the closure of the Iapetus Ocean and Appalachian orogenesis. A Neoproterozoic to Early Cambrian cover sequence records the transition from continental rifting to a passive margin, but the character of this sequence is vastly different on the eastern and western limbs of the Blue Ridge anticlinorium, reflecting spatial differences in both the timing and tectonics of the Iapetan rift. Blue Ridge rocks experienced NW-directed contractional deformation during the Neo-Acadian (355-330 Ma), whereas low-grade metasedimentary rocks in the western Piedmont were deformed and cooled prior to ca. 400 Ma. In central Virginia, the boundary between the eastern Blue Ridge and western Piedmont is a 3- to 5-km-wide zone of distributed dextral transpression.

Abstract Detailed geologic mapping and new SHRIMP (sensitive high-resolution ion microprobe) U-Pb zircon, Ar/Ar, Lu-Hf, 14 C, luminescence (optically stimulated), thermochronology (fission-track), and palynology reveal the complex Mesoproterozoic to Quaternary geology along the ~350 km length of the Blue Ridge Parkway in Virginia. Traversing the boundary of the central and southern Appalachians, rocks along the parkway showcase the transition from the para-autochthonous Blue Ridge anticlinorium of northern and central Virginia to the allochthonous eastern Blue Ridge in southern Virginia. From mile post (MP) 0 near Waynesboro, Virginia, to ~MP 124 at Roanoke, the parkway crosses the unconformable to faulted boundary between Mesoproterozoic basement in the core of the Blue Ridge anticlinorium and Neoproterozoic to Cambrian metasedimentary and metavolcanic cover rocks on the western limb of the structure. Mesoproterozoic basement rocks comprise two groups based on SHRIMP U-Pb zircon geochronology: Group I rocks (1.2-1.14 Ga) are strongly foliated orthogneisses, and Group II rocks (1.08-1.00 Ga) are granitoids that mostly lack obvious Mesoproterozoic deformational features. Neoproterozoic to Cambrian cover rocks on the west limb of the anticlinorium include the Swift Run and Catoctin Formations, and constituent formations of the Chilhowee Group. These rocks unconformably overlie basement, or abut basement along steep reverse faults. Rocks of the Chilhowee Group are juxtaposed against Cambrian rocks of the Valley and Ridge province along southeast- and northwest-dipping, high-angle reverse faults. South of the James River (MP 64), Chilhowee Group and basement rocks occupy the hanging wall of the nearly flat-lying Blue Ridge thrust fault and associated splays. South of the Red Valley high-strain zone (MP 144.5), the parkway crosses into the wholly allochthonous eastern Blue Ridge, comprising metasedimentary and meta-igneous rocks assigned to the Wills Ridge, Ashe, and Alligator Back Formations. These rocks are bound by numerous faults, including the Rock Castle Creek fault that separates Ashe Formation rocks from Alligator Back Formation rocks in the core of the Ararat River synclinorium. The lack of unequivocal paleontologic or geochronologic ages for any of these rock sequences, combined with fundamental and conflicting differences in tectonogenetic models, compound the problem of regional correlation with Blue Ridge cover rocks to the north. The geologic transition from the central to southern Appalachians is also marked by a profound change in landscape and surficial deposits. In central Virginia, the Blue Ridge consists of narrow ridges that are held up by resistant but contrasting basement and cover lithologies. These ridges have shed eroded material from their crests to the base of the mountain fronts in the form of talus slopes, debris flows, and alluvial-colluvial fans for perhaps 10 m.y. South of Roanoke, however, ridges transition into a broad hilly plateau, flanked on the east by the Blue Ridge escarpment and the eastern Continental Divide. Here, deposits of rounded pebbles, cobbles, and boulders preserve remnants of ancestral west-flowing drainage systems. Both bedrock and surficial geologic processes provide an array of economic deposits along the length of the Blue Ridge Parkway corridor in Virginia, including base and precious metals and industrial minerals. However, common stone was the most important commodity for creating the Blue Ridge Parkway, which yielded building stone for overlooks and tunnels, or crushed stone for road base and pavement.