Detailed geologic mapping geodatabase for the Blue Ridge Parkway in Virginia

The US Geological Survey, in cooperation with the National Park Service, mapped 35 7.5-minute quadrangles, within a 2-mile-wide+ corridor centered on the Parkway, from BLRI (Blue Ridge Parkway) Mile Post (MP) 0 near Afton, Virginia southward to MP 218 at Cumberland Knob, approximately 1.3 km south of the Virginia – North Carolina State Line.
Detailed bedrock geologic mapping for this project was conducted at 1:24,000-scale by systematically traversing roads, trails, creeks, and ridges within and adjacent to the 2-mile-wide+ corridor along the 216.9-mile length of the BLRI in Virginia. Geologic data at more than 23,000 station points were collected during this project (September 2009 – February 2014), with approximately 19,500 included in the accompanying database. Station point geologic data collected included lithology, structural measurements (bedding, foliations, folds, lineations, etc), mineral resource information, and other important geologic observations. Station points at the start of this project (September 2009) were located in the field using topographic reckoning; after May 2012 stations were located using Topo Maps (latest version 1.12.1) for Apple IPad 2, model MC744LL/A. Since the start of the project, station point geologic data and locational metadata were recorded both in analog (field notebook and topographic field sheets) and digitally in ESRI ArcGIS (latest version ArcMAP 10.1).
Station point geologic data were used to identify major map units, construct contact lines between map units, identify the nature of those contacts (igneous, stratigraphic or structural), determine contact convention control (exact – located in field to within 15 meters; approximate – located to within 60 meters; inferred – located greater than 60 meters), trace structural elements (faults, fold axes, etc) across the project area, and determine fault orientation and kinematics. Geologic line work was initially drafted in the field during the course of systematic detailed mapping; line editing occurred during office compilation in Adobe Illustrator (latest version CS 4). Final editing occurred during conversion and compilation of Illustrator line work into the ArcGIS database, where it was merged with station point geologic data.
Station point geologic data, contacts and faults from previous work in the BLRI corridor were evaluated for compilation and synthesis in the BLRI mapping project. Station point geologic data compiled from previous work are referenced and marked with a “C” in the database. Compiled line work is also clearly tagged and referenced.
The BLRI cuts at an oblique angle nearly the entire width of the Blue Ridge Geologic Province in Virginia. Thus, the geology varies significantly along it’s along its 216-mile traverse. North of Roanoke (BLRI MP 115), the Blue Ridge is defined as an orogen-scale, northwest-vergent, northeast-plunging reclined anticlinorium, and from its start at MP 0 near Afton, Virginia, southward to Roanoke, the BLRI traverses the western limb of this structure. Here, rocks range in age from Mesoproterozoic to Cambrian: Mesoproterozoic orthogneisses and metamorphosed granitoid rocks of the Shenandoah massif comprise “basement” to Neoproterozoic to Cambrian mildy-  to non-metamorphosed to sedimentary “cover” rocks; the BLRI crisscrosses in many places the contact between cover and basement.  Mesoproterozoic basement rocks in the Shenandoah massif represent the original crust of the Laurentian (ancestral North American) continent; sedimentary cover rocks were deposited directly on this crust during extension and breakup of the Rodinian supercontinent in the Neoproterozoic to earliest Cambrian. Very locally, diabase dikes of earliest Jurassic age intrude older basement and cover sequences. These dikes were emplaced in the Blue Ridge during continental extension (rifting) and the opening of the Atlantic Ocean in the Mesozoic Era.
From MP 103.3 to MP 110.3 near Roanoke, the BLRI crosses into and out of a part of the Valley and Ridge Geologic Province. Unmetamorphosed sedimentary rocks of Cambrian to Ordovician age – mostly shale, siltstone and carbonate – occur here. These rocks were deposited in a terrestrial to shallow marine environment on the Laurentian continental margin, after extensional breakup of Rodinian supercontinent in the Neoproterozoic and earliest Cambrian, but before mid- to late-Paleozoic orogenesis.
South of Roanoke, the Blue Ridge Geologic Province quickly transitions from an anticlinorium to a stack of imbricated thrust sheets. After crossing the southern end of the Shenandoah Mesoproterozoic basement massif (MP 124.1 to MP 144.4), the BLRI enters the eastern Blue Ridge province, a fault-bounded geologic terrane comprised of high-metamorphic-grade sedimentary and volcanic rocks deposited east of the Laurentian continental margin from the Neoproterozoic to early Paleozoic. These rocks were significantly metamorphosed, deformed, and transported westward onto the Laurentian margin along major orogenic faults during Paleozoic orogenesis.
Sixty bedrock map units underlie the BLRI in Virginia. These units consist of one or more distinguishing lithologies (rock types), and are grouped into formal and informal hierarchal frameworks based on age, stratigraphy (formations-groups), and tectonogenesis. Many of these units exhibit characteristics and field relationships that are critical to our understanding of Appalachian orogenesis. Most of these units are named based on the dominant occurring lithology; other units follow formal nomenclature, some of which was developed and has been used for more than 100 years.
Oldest rocks occurring along the BLRI corridor are Mesoproterozoic orthopyroxene-bearing basement rocks of the Shenandoah massif, in the core of the Blue Ridge anticlinorium. Preliminary SHRIMP U-Pb zircon geochronology (J. N. Aleinikoff, this study) shows that these rocks can be grouped based on crystallization ages: Group I (~1.2 to 1.14 Ga) are strongly foliated orthogneisses and Group II (~1.06 to 1.0 Ga) are less deformed metagranitoids. Group I orthogneisses, which occur discontinuously from near Irish Gap (MP 37) to Cahas Overlook (MP 139), comprise 10 map units: leucogranitic gneiss (Yllg); megacrystic quartz-monzonitic gneiss (Yqg); granitic gneiss (Yg); lineated granitoid gneiss (Ylgg); garnetiferous leucogneiss (Yglg); Sandy Creek gneiss (Ysg); porphyroblastic garnet-biotite leucogranitic gneiss (Ygtg); dioritic gneiss (Ydg); Pilot gneiss (Ypg); and megacrystic granodioritic gneiss (Ygg). Group II metagranitoids, which are first encountered along the BLRI at Reeds Gap (MP 14) and occur discontinuously to Roanoke River Overlook (MP 115), comprise 8 map units: megacrystic meta-quartz monzonitoid (Yqm); massive metagranitoid (Ymgm); megacrystic metagranitoid (Ypgm); mesocratic porphyritic metagranitoid (Ygpm); metagranodioritoid (Ygdm); Vesuvius megaporphyritic metagranitoid (Yvm); quartz-feldspar leucogranitoid (Yqfm); and Peaks of Otter metagranitoid (Ypom). An additional relatively undeformed metagranitoid with a preliminary SHRIMP U-Pb zircon age of ~1.12 Ga is assigned to the Bottom Creek Suite (Ybcm), and well layered migmatitic gneiss (Ymg) near Irish Gap (MP 37) has a a preliminary SHRIMP U-Pb zircon age of ~1.05 Ga. Other rocks of Mesoproterozoic age include orthogneisses in the Fries thrust sheet between MP 139 and MP 144.5 that range in age from ~1.19 to
~1.07 Ga: biotite-muscovite leucogneiss (Ymlg); biotite granitic augen gneiss (Ybgg); blue-quartz gneiss (Ybqg); and biotite leucogneiss (Yblg). Latest Mesoproterozoic rocks include paragneiss and pegmatite (Yprg) near Porters Mountain Overlook (MP 90), and a suite of igneous intrusive nelsonites and jotunites (Yjn).
Two units, foliated metagreenstone (Zdm) and foliated metagranitoid (Zgm), locally intrude older Mesoproterozoic rocks in the core of the Blue Ridge anticlinorium. Metagreenstone is fine-grained and mafic in composition, and occur as narrow dikes and sills; metagranitoid is medium-grained and generally felsic in composition, and intrude basement rocks as small plutons, stocks, and a few narrow dikes.
On the west limb of the Blue Ridge anticlinorium, metamorphosed sedimentary and volcanic rocks of Neoproterozoic to Cambrian age crop out discontinuously along the BLRI from near Afton (MP 0) to MP 103.3, in the vicinity Roanoke Mountain (MP 120 to MP 124), to near Adney Gap (MP 136). These rocks are assigned to a formal stratigraphic sequence: Swift Run Formation; Catoctin Formation; Chilhowee Group.
Metasedimentary and meta-igneous rocks of lower Paleozoic (?) to Neoproterozoic age are assigned to the Alligator Back Formation, Lynchburg Group, and Ashe Formation. These units crop out southeast of the Red Valley fault from MP 144.5 southwestward to the North Carolina–Virginia State Line at Mile Post 216.9.  Rocks assigned to the Alligator Back crop out in the Blue Ridge Parkway corridor from Mile Post 174.5 southward to the North Carolina–Virginia State Line: compositional-layered biotite-muscovite gneiss (abg); garnet-biotite-muscovite-quartz schist (abs); quartzite and quartz-rich metasandstone (abq); and marble (abm).  The following lithologic map units along the BLRI corridor are correlated with Lynchburg Group formations: graphitic schist (lgs), muscovite-biotite metagraywacke (lmg), and graphite-muscovite-quartz metasandstone (lms). These rocks crop out between the Red Valley fault (Mile Post 144.5) and the Rock Castle Creek fault (Mile Post 174.5).  Coarse-grained- to conglomeratic metagraywacke (acm), underlying Lynchburg Group rocks west of the Rock Castle Creek fault in the vicinity of Rakes Millpond (MP 162.3) and Rocky Knob Visitors Center (MP 169), are considered to be the lower metamorphic grade-equivalent of the higher metamorphic-grade Ashe Formation at its type section in northwestern North Carolina. Five meta-igneous lithologic map units occur as mappable bodies interlayered or intrusive into metasedimentary rocks of the Alligator Back Formation and Lynchburg Group. Altered ultramafic and ultrabasic rocks (um) occur mostly within the Lynchburg Group, but few thin bodies also occur within laminated amphibolite gneiss (aml) locally (laminated amphibolite gneiss is associated with rocks of the Alligator Back Formation). Amphibolites of the Lynchburg Group include biotite-bearing amphibolite (amb), clinozoisite-bearing amphibolite (amc) and quartz-layered amphibolite (amq).
Map units that occur beneath the Blue Ridge fault are assigned to the Valley and Ridge Province. In the BLRI corridor, these rocks are first exposed near Bearwallow Gap (MP 91) and continue southward beyond Roanoke near MP 125. Oldest formations are Shady Dolomite (Cs) and Waynesboro Formation (Cw), which regionally can be mapped in stratigraphic continuity with cover and basement rocks of the Blue Ridge anticlinorium. Conversely, nowhere in the Appalachians can rocks of the Elbrook Formation (Ce) or Conococheague Formation (Cco) be mapped in direct stratigraphic continuity with Mesoproterozoic basement rocks in the core of the Blue Ridge anticlinorium; however, they do stratigraphically overlie older Waynesboro Formation and Shady Dolomite within imbricates of the Blue Ridge fault system. Ordovician Martinsburg Formation (Omb) occurs beneath the Pulaski fault north of Roanoke River, about 0.85 km northwest of MP 108.
Paleozoic mylonite (Pzmy) is a tectonic rock unit that that occurs in high-strain fault zones in the core of the Blue Ridge anticlinorium from near Afton (MP 0) to Adney Gap (MP 136). Jurassic diabase (Jd) occurs in narrow dikes that intrude older map units, just south of Roanoke River near MP 118 and MP 125, north of James River near MP 56, and in the vicinity of Afton near MP 3 and north of MP 0.

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Organization: Department of the Interior