Date Thesis Awarded

4-2018

Document Type

Honors Thesis

Degree Name

Bachelors of Science (BS)

Department

Geology

Advisor

Christopher M. Bailey

Committee Members

Rebecca Jiron

Martin Gallivan

Abstract

Beneath Rockfish Gap, one of the lowest elevations along the crest of the Blue Ridge Mountains in Virginia, sits the historic 1858 Blue Ridge Tunnel (BRT). The BRT cuts directly through foliated metabasalts and metasedimentary rocks of the Ediacaran Catoctin Formation and provides a rare 3-dimensional exposure of the Blue Ridge cover sequence on the western limb of the Blue Ridge Anticlinorium. The purpose of this study is to characterize brittle and ductile deformation features in the Catoctin Formation in the BRT to determine their kinematic history. The Catoctin Formation includes a thick sequence of metabasaltic greenstone with thin layers of meta-arkose, sandstone, phyllite and conglomerates that formed between 570-550 Ma. In the arkoses and conglomerates, quartz and perthitic feldspars are the dominant clasts and are typically surrounded by a sericite matrix. Foliation strikes to the NE and dips moderately to the SE with down dip chlorite elongation lineations. Large (1 - 10 m) epidote-rich sandstone boudins typically display top-to-the NW asymmetry. Small, localized folds also occur in the sedimentary units and are tight, overturned NW-verging folds, consistent with NW-directed Neoacadian ductile deformation. Foliation in the greenstone is cut by low-angle top-to-the west shear zones that likely represent the later stages of deformation. Petrographic analysis reveals microstructures formed from dissolution, mass transfer, and volume loss processes under lower greenschist facies conditions. Mineral assemblages indicate that the Catoctin Formation at Rockfish Gap experienced temperatures between 300ºC - 400ºC. The Catoctin Formation is cut by two dominant fracture sets, a WNW-ESE set and NE-SW set that formed from two separate brittle deformation events; the former from the late Alleghanian Orogeny, and the latter from Atlantic rifting in the early Mesozoic.

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