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

61 Pages·2016·4.66 MB·English

by  Steven A. Goldberg

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