14-6 EDIACARAN RIFT BASINS, THE GREAT UNCONFORMITY, AND PALEOZOIC TECTONIC INVERSION IN THE CENTRAL VIRGINIA BLUE RIDGE

Session: New Research in the Appalachian-Ouachita Orogen: Integrated studies from the Foreland to the Hinterland

Presenting Author:

Authors:

Abstract:

The Ediacaran period (635-539 Ma) was a dynamic time in Earth’s history, and rocks of this age exposed in the southern Appalachians formed during the rifting of Rodinia.  In the Blue Ridge Mountains of central Virginia, the Swift Run and Catoctin formations unconformably overlie 1.0 to 1.2 Ga granitic basement rocks along the ‘Great Unconformity’.  In the Sherando 7.5’ quadrangle our detailed geologic mapping reveal two separate Ediacaran rift basins.  These basins are framed to the NW by a set of steeply-dipping ENE-striking faults. The Little Stoney Creek basin includes >300 meters of clastic metasedimentary rocks and metabasaltic greenstone in the hanging wall of a steeply-dipping ENE-striking fault.  The cover sequence rocks in the Little Stoney Creek basin are strongly folded and well-foliated.  The bounding fault forms a narrow zone (<30 m thick) of mylonitic rocks derived from the granitic basement complex.  Petrographic and kinematic analysis indicates that the mylonitic rocks developed under mid-greenschist facies conditions and record significant NW-SE contraction, consistent with the strain geometry preserved in the cover rocks. We infer that the bounding fault in the Little Stoney Creek basin originated as an Ediacaran normal fault and was then re-activated, partially inverted, and served as a buttress during mid-Paleozoic crustal shortening in the Blue Ridge.  The Pond Hollow Fault is an ~8-km long, steeply-dipping, ENE-striking fault that bounds the Crawford Knob basin.  In contrast to the Little Stoney Creek basin, there is less overall shortening in the cover sequence rocks of the Crawford Knob basin, and the cover units are notably thicker to the SE in this basin. Although the modern-day geological map pattern indicates two separate basins, they likely were a contiguous rift basin originally.  The ‘Great Unconformity’ is exposed at multiple locations in the Sherando 7.5 quadrangle and includes a wide range of lithologies immediately above the contact.  Structural contour analysis of the ‘Great Unconformity’ indicates that it is locally is a planar surface with little original topographic relief.  Petrographic evidence from the basement granitoids exposed below the contact is consistent with significant subaerial chemical weathering at the time the unconformity developed.

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