Blue Ridge Anticlinorium & Valley-and-Ridge Province (MD-VA-WV)


The Blue Ridge anticlinorium in northern Virginia (and adjoining Maryland and West Virginia) is a first-order basement-cored ramp anticline that plunges northward; it is underlain by the Blue Ridge thrust (which is asymptotic to a basal decollement) along which the Blue Ridge thrust sheet has been transported ~50-80 km. The core of the anticline is made up of imbricated slices of variously deformed Mesoproterozoic (1.2 – 1.0 Ga) crystalline basement rocks and Neoproterozoic (~700-730 Ma) granitic intrusives; the older rocks show evidence of a Grenville (980-1020 Ma) thermal imprint. Both limbs of the anticline and a synclinal in-fold along its crest expose Neoproterozoic (~680-730 Ma) clastic metasedimentary rocks (Swift Run, Meechums River, and Lynchburg Formations) that unconformably overlie the crystalline basement. These are overlain by ~575 Ma metabasalts (and some metarhyolites) of the Catoctin Formation, which in turn are overlain by Cambrian clastics (Chilhowee Group) and carbonates. Cambo-Ordovician carbonates and various clastic sequences, ranging in age up to the Pennsylvanian, make up the deformed rocks of the Great valley and the Valley-and-Ridge that lie west of the Blue Ridge.


The rocks of the Blue Ridge anticline show evidence for older deformation events (mainly Grenville and some Taconic-age features) but the dominant deformation features were produced during the Alleghanian (Pennsylvanian-Permian) orogeny. The earlier Taconic (Ordovician) and Acadian (Devonian) orogenies are mainly represented by synorogenic clastic sequences of the Martinsburg-Juniata-Tuscarora and Jennings-Catskill Formations exposed in the Valley-and-Ridge province. During the Alleghanian orogeny, the Blue Ridge thrust sheet served as a strong basement wedge that was emplaced along the sole thrust; this, in turn, drove thrusting on to the foreland, forming a lower taper thin-skinned fold-thrust belt (namely the Valley-and-Ridge province). The folds of the Valley-and-Ridge are thrust related and are underlain by detachment surfaces at different stratigraphic levels.


Here are some pictures of typical Blue Ridge and Valley-and-Ridge structures that we see on our field trips to the Appalachians.



Geology


Gently dipping layering and steeper cleavage in Catoctin basalts on Catoctin Mountain in east limb of Blue Ridge anticline (at Gambrill State Park, MD)

Columnar jointing in Catoctin basalts (loose blocks) on west limb of Blue Ridge anticline (in Shenandoah National Park, VA at Stony Man Trail parking)

Deformed pillows in Catoctin basalts on west limb of Blue Ridge anticline (in Shenandoah National Park, VA near Stony Man Trail parking)

Catoctin diabase feeder dike cutting through Grenville crystalline basement at Mary’s Rock Tunnel, Shenandoah National Park, VA

Sigmoidal veins in Cambrian Weverton quartzites along the Potomac river at Harpers Ferry, WV

Gently dipping folded cleavage and steeply dipping bedding (brown zones) in Cambrian Harpers formation at Harpers Ferry National Monument, WV

Boudinage in Ordovician Stones River limestone at Forsyth quarry, near Williamsport, MD

Recumbent folding in Ordovician Edinburgh limestone at Ruffner quarry along Shenandoah river, west of Luray, VA

Upright fold showing fanning of cleavage (suggesting early layer parallel shortening, LPS) and cleavage refraction from bed to bed at Roundtop, MD

Wedging (formed during early LPS) at hinge of fold at Roundtop, west of Hancock, MD

Fibrous slickenside on bedding surface in fold limb indicating flexural slip folding, at Roundtop, MD

Small fault propagation fold modified by translation over fault bend, at Roundtop, MD


People


On east limb of Blue Ridge anticline at Gambrill State Park, MD

Looking for veins in Weverton quartzite along the Potomac river at Harpers Ferry, WV

Studying Blue Ridge crystalline basement rocks near Madison, VA

End of the 2007 Spring field trip west of Luray, VA