Geological Cross-section Through Part of the Southern Appalachian Orogen Inner Piedmont to Valley and Ridge (North Carolina, Tennessee and Virginia) July 20-26, 1989 Field Trip Guidebook T365 Leaders: Steven A. Goldberg ]. Robert Butler American Geophysical Union, Washington, D.C. Copyright 1989 American Geophysical Union 2000 Florida Ave., N.W., Washington, D.C. 20009 ISBN: 0-87590-602-8 Printed in the United States of America T T COVER Top figure entitled "Types ofThrusts", from Bailey Willis, 1893, The mechanics ofAppalachian structure, U.S. Geological Survey, 13th Annual Report of the Director, 1891-1892, Part 2. Lower photographofBakersville dike intrudingPrecambrian gneisses, Loafers Glory, North Carolina. . TABLE OF CONTENTS Page Part 1: GeologicalDescriptionoftheRegion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Purpose oftheFieldTrip 1 GeneralPhysiography andGeography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 Overviewofthe RegionalGeology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 HistoricalSketchoftheRegion. . . . . . . . .. . . . . . . . . . . . . . . . ". . . . . . . . . . .3 OrganizationofGuidebookandFieldTrip. . . . . . . . . . . . . . . . .. . . .. . . .. . . .. . . .. 5 RegionalGeology 6 Valley andRidge Province. . . . . . . . . . .. . . . . . .. . . . . . . . . " "" .. . . . ". . . . . .6 BlueRidgeProvince. . . . . .. . . . .. . . . . . . . . . . . . . . . . . . . . " " . . . . . "". . . 7 Introduction andGeologic Setting. . . . .. . . .. .. . . . . .. . . .. . . . . . . . " .. . . .. . . .. 7 General Structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . "". . . . . . . . . .. . . 7 BlueRidgeThrustComplex.. . . .. . . .. . . .. . . . . .. .. . . .. . . . . . . ". .. . . .. . . .. . .8 GrandfatherMountainWindow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . .8 BrevardZone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ,8 Piedmont 9 GrenvilleBasement: ThePre-IapetanSupercontinent 10 Nature andAge oftheBasementRocks 10 LateProterozoicRifting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 Rift-RelatedSedimentation .11 Ocoee Supergroup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . .11 MountRogers Formation. . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . .11 GrandfatherMountainFormation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 Rift-RelatedMagmatism 12 BakersvilleIntrusiveSuite " 12 BeechandCrossnoreIntrusive Suites " .12 LinvilleMetadiabase 14 MontezumaMember, GrandfatherMountain Formation " 14 MountRogersFormation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . .14 OpeningofIapetus: PassiveMargin Sedimentation 14 ChilhoweeGroup 15 Taconic Orogeny: FirstPhaseoftheAppalachianEvents. . . . . . . . . . . . . . . .. . . . . . .15 StratigraphicEvidence 15 TaconicThrusts 15 Metamorphism 15 Alleghanian Orogeny: Climaxofthe AppalachianEvents 16 StratigraphicEvidence 16 ThrustSheets andThrustFaults 16 TheEndoftheAppalachianWilson Cycle. . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 AsheMetamorphic Suite: A Far-TraveledThrustSheet(?). . . . . . . . . . . . . . . . . . . . . J7 Metamorphic Historyofthe BlueRidge. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .18 GeneralMetamorphic Grade. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .18 PrecambrianMetamorphism. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . .. . . .. . . .19 PaleozoicMetamorphism. . . . . . . . . . . . . .. . . . . . .. . .. .. . . .. . . .. . . .. . . . . . . .19 ConditionsofMetamorphism 19 StructuralHistoryoftheBlueRidge. . . .. . . . . . . . .. . . . . . . . . . . . .. . . . . . . . . . . .20 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .20 StructuralAnalysisoftheBasement-Ashe Boundaryin theBlueRidgeThrustComplex, 21 Interpretationsofthe Ashe-basementboundary " . " ".21 Presentstudy " " 21 Mylonite " 21 Mesoscopic structures 21 Discussion. . .. . . . . .. .. . . . . .. . . .. . . . .. . . .. . . .. . . . . . . . . . . . . .. . . . . . 24 v DeformationMechanisms andKinematic Analysis ofMylonitesfromtheBlueRidgeThrustComplex. . . . . . . . . . . . . . . . . . . . . . 29 Introduction. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29 Microtextures 32 Kinematic indicators. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36 Part2: RoadLogfortheFieldTrip. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38 Day 1(7/20): Raleigh toJefferson, NC 38 Day 2 (7/21): ToMountRogers area, returntoJefferson 39 Day 3 (7/22): Jefferson toGrandfatherMountainwindow toBannerElk 41 Day 4 (7/23): BannerElktoRoanMtn., return to BannerElk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43 Day 5 (7/24): BannerElktoNewlandarea andBeechMtn., return toBannerElk. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44 Day 6 (7/25): BannerElkto SprucePine andBakersville. . . . . . . . . . . . . . . . . . . . . . .46 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .48 vi Leaders: Steven A. Goldberg and J. Robert Butler Department of Geology University of North Carolina Chapel Hill, NC 27599 vii IGC FIELDTRIPT365: THE SOUTHERN APPALACHIAN OROGEN INNORTHWESTERN NORTH CAROLINAAND ADJACENT STATES Steven A. Goldberg, J. RobertButler,JonathanW. Mies, andCharlesH. Trope DepartmentofGeology, UniversityofNorthCarolina,ChapelHill INTRODUCTION relief are generally lower than those in the Blue Ridge. Purposeofthe FieldTrip Drainage in the Piedmont is directed south and southeast to the Atlantic Ocean. Most of the Blue This field trip is intended to demonstrate the Ridge and alloftheValley and Ridge drain north and major lithologic sequences and the main tectonic and northwestto the OhioandTennesseeRivers. Thesein metamorphic features in a segmentofthe Blue Ridge tum flow into the Mississippi River and eventually geologicprovince innorthwestern North Carolina and intotheGulfofMexico. TheAtlanticOcean-Gulfof adjacent parts of Tennessee and Virginia. Many of Mexico drainage divide (or Eastern Continental the field locations chosen for the six-day field trip Divide) typically follows the eastern edge ofthe Blue emphasizegeologicalcharacteristicscommonto much Ridge and trends northeast- southwest. The Blue ofthe southernand centralAppalachians. Mostofthe Ridge Parkway, which is a scenic roadway with localities to be visited are in the Blue Ridge·, with adjacent narrow strips of National Park land, some in the Valley and Ridge province to the generally follows the divide through most of the area northwest and the Brevard zone and Inner Piedmont describedinthis guidebook. tothe southeast. Thetrip provides an overviewofthe The Blue Ridge - Piedmont boundary is a anatomy and evolution of terranes subjected to the prominent escarpment, along which the land surface following tectonic episodes: Late Proterozoic rifting, drops as much as 600m across a distanceofafewkIn the development of oceanic lithosphere, the from the Blue Ridge upland down to the level of the subsequent closure of this ocean, and further Piedmont. This escarpmentis called the "Blue Ridge modification by multiple episodes of Paleozoic front" and discussions of its origin have generated deformation and metamorphism. Two geological much controversy [White, 1950; Hack, 1982]. At themes will be emphasized: the nature ofthe tectonic several places along the Blue Ridge front, boundaries and the evidence for Late Proterozoic southeastward-flowing streams have cut into the rifting. Participants will have an opportunity to upland and captured streams of the higher surface, examine classical geologic features such as the resulting in spectacularwaterfalls and gorges such as Grandfather Mountain window and to evaluate field LinvilleFallsandLinvilleGorge. evidenceforrecentinterpretationsofregionalhistory. The present drainage system in the southern Appalachainshad its beginnings in the highlands General Physiographyand Geography formed by the Appalachian orogenies. By the end of the Paleozoic, the region was entirely emergent and The field trip makes an east-west traverse across was part of the Pangaea supercontinent. Drainage parts of three physiographic provinces (Fig. 1): (1) across what is now the southern Appalachians was The Piedmont province is a surface of generally low westerly. Judson [1975] postulated a broad uplift reliefand rolling topography, with local development affecting much of eastern North America during the of inseIbergs (isolated residual hills) and low Triassic. Judson interpreted the Triassic basins to be mountain ranges such as the Sauratown Mountains. locally downdropped parts of the uplift. The crest of TheelevationofthePiedmontrangesforthemostpart the uplift was probably in the eastern Piedmont in from 120 m (400 feet) to 360 m (1200 feet), with North Carolina. When the Atlantic Ocean began to some peaks as high as 820 m (2700 feet). (2) The open in the Jurassic, there was a westward migration Blue Ridge province is more mountainous than the of the drainage divide. The migration of the divide Piedmont, with both greater relief and higher has resulted in capture ofwesterly-flowing streams in elevations. Elevations in most of the Blue Ridge are the Blue Ridge upland by easterly-flowing streams above 600 m. Reliefis locallyas greatas 1000m, but along theBlueRidgefront. is more commonly 200 to 400 m. Mount Mitchell, which is the highest peak in the eastern U. S., has an OverviewoftheRegional Geology elevation of 2038 m (6,684 feet). (3) The Valley and Ridge province is characterized by northeast-trending Raleigh-Durham Airport and surrounding areas valleys separated by elongate ridges; elevations and are located in the Piedmont physiographic province. T365: area of figure 2........ TN FIGURE 1 Major lithotectonic belts ofNorth Carolina and adjacent Virginia, South Carolina, and Tennessee. RB: Raleigh belt; CSB: Carolina slate belt; CB: Charlotte belt; KMB: Kings Mountain belt; SMA: Sauratown Mountains anticlinorium; SRA: Smith River allochthon; GMW: Grandfather Mountain window; BFZ: Brevard faultzone. This terrain is characterized by low relief and lush (Fig. 1) andits northeastextension, theBowensCreek vegetation. Drivingwestwardonthefirstday, wewill fault zone. TheBlueRidgephysiographicfront trends traverse several geologic provinces (Triassic basin, in a more northerly direction than the geologic Carolinaslate belt, Charlotte belt, and Inner Piedmont boundary, so thattheBrevardzoneandBowensCreek belt), but there is little geology to be seen from the fault zone lie within the Piedmont physiographic interstate highways. West of Winston-Salem, there provincein northern North CarolinaandVirginia. are more inselbergs and clusters of low mountains Mostofthis field trip will be spent atoutcrops in rising above the Piedmont surface than farther east. the Blue Ridge province. Fig. 2 shows the major In this region, the Sauratown Mountains are one tectonic features intheregionofinterest. Table 1lists example of isolated mountains, which can be seen to the major lithologic units and the sequence of the north ofour route, and the Brushy Mountains are deformational and tectonic events. Until a few years anotherexampletothesouth. ago, the Blue Ridge was considered to consist of one At the Blue Ridge front, the roads climb steeply or two main thrust sheets with tectonically lower up the front, cross theEasternContinentalDivide, and rocks exposed in the Grandfather Mountain window. pass into the mountainous upland. The southeastern The Grandfather Mountain window, which is one of boundaryoftheBlue Ridgephysiographic provinceis thelargest tectonic windows inthe world,exposes the the Blue Ridge front, but it does not coincide Table Rock thrust sheet below the Blue Ridge thrust everywhere with the boundary of the Blue Ridge complex, and parautochthonous rocks or a lower geologic province. The southeastern boundary ofthe thrust complex in the main part of the window. Blue Ridge geologic province is the Brevard zone Recent mapping and tectonic analyses, which are T365: 2 lEIPaleozoic metasedimentary rocks 1::::::::::::::::1~lt~l~i~:~~~:f theCrossnore I:::::::J Mount Rogers Fm. Crystallinethrust sheets ~ Pumpkin Patchthrust sheet ................................. ................................. ................................. ................................. ................................. ................................. ................................. 4000000 mN I·::.::.::.::·1 AlligatorBack Fm. G Ashe Fm. o 20 km 10000 meter grid (UTM) FIGURE2 TheBlueRidge thrustcomplexinthevicinityoftheGrandfatherMountainwindow, showingthrust' sheets and lithotectonic units discussed in text. Boundaries shown as bold black lines are thrust faults. Questionmarksindicateuncertainboundarylocations. PP =PardeePoint. described below, now indicate that the Blue Ridge Grenville age in the basement may be difficult to includes multiple thrust sheets, probably as many as distinguish from those of Paleozoic age. We have six along someeast-westtraverses. usedthe lateProterozoic Bakersvilledike swann (Rb Along the northwestern part of the Blue Ridge, Srage =734±25 Ma) to help distinguish Proterozoic the geologic and physiographic boundaries generally from Paleozoic metamorphic events. coincide. The boundary is placed where thrust sheets Along the western boundary of the Blue Ridge carrying crystalline "basement" are emplaced province, there is a prominent but irregular northwestward over sedimentary rocks of the Valley escarpment that developed as a result of the and Ridge province. The term "basement" as used differential erosion between crystalline and here refers to crystalline rocks, mainly granitic to sedimentary rocks. In the eastern Valley and Ridge quartz monzonitic gneiss with locally abundant andwestern BlueRidge, theMountainCitywindowis migmatite, paragneiss, and pelitic schist. These a second large-scaletectonic window. Thewindow is typically have radiometric ages of about one billion framed on the southeast and southwest mainly by years, and were metamorphosed to amphibolite or BlueRidgecrystallinerocks. granulite facies during the Proterozoic Grenville orogeny. Where the Paleozoic metamorphic and Historical Sketchofthe Region tectonic overprint is mild, Grenville-age amphibolite to granulite facies assemblages in the basementrocks Thefrrst humans to enterthe southeastern United are easily recognized. Where the Paleozoic overprint States probably arrived about 12,000 years ago is more intense, metamorphic assemblages of [Purrington, 1983]. Evidence for the arrival of these T365: 3 SPRUCEPINETHRUSTSHEET Mainlithologies: Biotitegneiss andschist, amphibolite. Grenvillebasement: Notrecognized. Precambrianmetamorphism: None(?). BakersvilleSuite: Notpresent. BeechSuite: Notpresent. Paleozoicmetamorphism: Amphibolitefacies- kyanite and sillimanitegrade. Paleozoicdefonnation: Intense; multipleepisodes. Specialfeatures: Ultramafic andSprucePinepegmatite bodies occuronlyin this sheet. PUMPKINPATCHTHRUSTSHEET Mainlithologies: Pyroxene andhornblendegneiss, granitic gneiss. Grenvillebasement: Majorunits, including 1.8Gaold. Precambrianmetamorphism: Granuliteandamphibolitefacies. BakersvilleSuite: Abundantdikes andmetagabbrobodies. BeechSuite: Notpresent. Paleozoicmetamorphism: Greenschist, amphibolite, and granulitefacies. Paleozoicdefonnation: Minor, notpervasive. BEECHMOUNTAINTHRUSTSHEET Mainlithologies: Granitic gneiss; biotitegneiss and schist. Grenvillebasement: Majorunits. Precambrianmetamorphism: Amphibolitefacies. BakersvilleSuite: Dikespresent, butnotabundant. BeechSuite: Numerousintrusions, associatedwith Cranberry-typemagnetite bodies. Paleozoic metamorphism: Greenschistfacies, biotite (andgarnet?) grade; locallyepidote-amphibolitefacies. Paleozoicdefonnation: Penetrativefoliation. PARDEEPOINTTHRUSTSHEET Mainlithologies: Granitic gneiss, biotitegneiss. Grenvillebasement: Majorunits. Precambrianmetamorphism: Amphibolitefacies. BakersvilleSuite: Dikesandsmallmetagabbrobodies arecommon. BeechSuite: Probably absent; scarcerhyolitedikesmay belongto suite. Paleozoic metamorphism: greenschist-faciesmineralsin.much ofthrustsheet; locallyrimsofblueamphibole. Paleozoicdefonnation: None. TABLE 1 Subdivisions of the Blue Ridge thrust complex in the vicinity of the Grandfather Mountain window. The thrust sheets are listed in order from structurally highest to lowest. "Paleozoic deformation" refers to penetrative defonnation within the thrust sheets and excludes late Paleozoic shear zones, which bound each thrust sheet and locally occur within them. T365: 4