A REVISED STRATIGRAPHY FOR THE NEOPROTEROZOIC COVER SEQUENCE IN THE NORTHERN & CENTRAL VIRGINIA BLUE RIDGE

Neoproterozoic cover rocks are exposed above the Grenvillian basement on both the northwestern and southeastern limbs of the Blue Ridge anticlinorium in central and northern Virginia. These rocks are regionally significant because they provide evidence of climatic, environmental, and tectonic conditions during the Ediacaran and Cyrogenian. The nomenclature for the cover sequence is complicated by the inconsistent application of regional and local unit names. Precise age control and regional correlation is further hampered by the lack of datable volcanic rocks in this sequence. We propose a revised (and simplified) nomenclature for this sequence.

In the western Blue Ridge, the Neoproterozoic cover sequence includes the <1 km thick Swift Run and Catoctin formations deposited during active rifting in the Ediacaran (570 - 550 Ma). In Shenandoah National Park, a few remnants of older (~700 Ma) units are present below the Ediacaran sequence providing evidence for significant regional denudation prior to the onset of successful Iapetan rifting.

The Mechum River Formation is exposed in an elongate structural inlier near the center of the anticlinorium. Recently identified volcanic ash beds near the base of the unit yield U-Pb zircon ages compatible with Cyrogenian deposition during the Sturtian glacial interval.

In the southeastern Blue Ridge, the Catoctin Formation forms a marker unit exposed from northernmost Virginia southward into central Virginia. Neoproterozoic strata beneath the Catoctin Formation comprise the Lynchburg Group, a ~1 to >6 km thick clastic sequence. In central Virginia, Lynchburg Group nomenclature has precedence over the Alligator Back and Ashe formations, which may (or may not) be correlative to these southern Appalachian stratigraphic units. The Lynchburg Group consists of marine and non-marine strata that include a number of local formations separated by unconformities. The basal units in the Lynchburg Group were deposited in erosional troughs cut into the basement complex. The upper contact of the Lynchburg Group with the Catoctin Formation is a regionally significant unconformity, although it is locally faulted. Arkosic marine strata in the upper Lynchburg Group are not correlative with Swift Run Formation which is younger and limited to the western Blue Ridge.