GeologyofYanceyCounty.docx

GeologyofYanceyCounty.docx

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GeologyofYanceyCounty

GEOLOGYOFYANCEYCOUNTY

GEOLOGICNOTENO.5

BY

CarlE.Merschat

NORTHCAROLINAGEOLOGICALSURVEY

DIVISIONOFLANDRESOURCES

CharlesH.Gardner,DirectorandStateGeologist

RALEIGH

1997

（ReleasedfortheInternetinMarch2004）

TABLEOFCONTENTS

Page

Preface3

Physiography3

GeneralGeology6

StructuralSetting6

Metamorphism7

RockDescriptions7

Pegmatite7

SprucePineSchist9

SprucePineAlaskite9

ChilhoweeGroup–UnicoiFormation9

Meta-UltramaficRock10

SnowbirdGroup10

AlligatorBackFormation10

AsheMetamorphicSuite11

BakersvilleMetagabbro12

GranodioriticGneiss12

BiotiteGraniticGneiss13

MigmatiticBiotite-HornblendeGneiss13

MineralResources14

Acknowledgements16

Glossary16

References18

Appendices20

TABLES

Table18

ILLUSTRATIONS

Plate1（seecontentsforlink）

Figure1………….4

Figure2………….5

GEOLOGYOFYANCEYCOUNTY

PREFACE

Thispublicationisintendedtopresentabriefintroductiontothephysiography,geology,andmineralresourcesofYanceyCounty.Italsointroducesadigital,geologicmapofthecounty.

Thetextcontainsmanygeologicterms;sometermsaredefinedwithinthetext,whileothersareinboldanditalicizedprintandaredefinedintheglossary.Mineralnamesareusedthroughoutthetext,butarenotdefinedbecausetheycanbefoundinmostelementary,mineralogytextbooks.Figure1servesasalocationmapformanyofthefamiliargeographicplacesandfeaturesreferredtointhisreport.Inaddition,itcontainsthelocationoftheactiveminesandafewofthemanyinactivemineswithinthecounty.Figure2showsthephysiographicprovincesandgeologicbeltsofNorthCarolina.AbriefelaborationoftheseprovincesandbeltsisincludedinappendicesAandB.

PHYSIOGRAPHY

YanceyCountyislocatedintheBlueRidgeMountainsPhysiographicProvinceoftheAppalachianMountainsystem.Thecountyconsistsprimarilyofmountainrangesandintermontanevalleys.ItsmountainsarethehighestintheeasternUnitedStates,withMountMitchellat6,684feetthetallestpeak.Themostprominentintermontanevalleyisthestraight,narrow,steepsided,east-westtrendingLaurelCreektrenchvalley.LaurelCreek,forwhichthetrenchvalleyisnamed,liesinneighboringMadisonCounty,butthefeatureextendsacrossallofYanceyCounty.ThevalleysofBaldCreekandLittleCrabtreeandseverallowdrainagedividesformtheLaurelCreektrenchvalleyinYanceyCounty.U.S.Highway19Eisbuiltalongthisfeature.

Arock’sresistancetoweatheringanderosioncontrolstopography,thegeneralconfigurationofthelandsurface.Forexample,quartzite（ametamorphosedquartzsandstone）isthemostresistantrocktypefoundinthecounty,andcapstheruggedFlatTopMountainarea.Metagraywackeisthesecondmostresistantrocktypethroughouttheremainderofthecounty,followedbygranitoidsandotherfelsicrocks.TheBlackMountainrangewhichcontainsMountMitchellowesitsprominencetothenumerous,massive,metagraywackelayersthatunderlieit.Maficandultramaficrocksaretheleastresistant,butlocallyanyrocktypemayformahightopographicarea.

Inadditiontorocktype,jointing,faulting,andthicknessoflayeringcontributetorockresistance.Abundantandcloselyspacedjoints,fractures,andfaultsincreasetheexposedsurfaceareaofarock.Thissubsequentlycausestherocktobecomelessresistant,toweatheranderodemoreeasilyandtoformtopographiclows.TheLaurelCreektrenchvalleyisprobablyaresultofjointing,fracturing,andfaultingacrossdifferentrocktypes.

Similarly,thethickerandmoremassivelayersareusuallymoreresistantthanthinlylayeredrocks.MassivelylayeredgranitoidsbeneaththeBald,Cane,andGreenMountainsformthehighpeaks,whilethemorethinlylayeredgranitoidsthroughthesameareasunderliethevalleys.

Figure1:

LocationMap

Figure2:

NorthCarolinaPhysiographicProvincesandGeologicBelts

GENERALGEOLOGY

Geologicmapsarescaledrepresentationsofthegeologyoftheearthatitssurface.Theyarethefundamentaldatabasefortheearthsciencesandareusedforengineering,environmental,hydrologic,mineralandenergyresources,andotherpertinentscientificandsocietalissues.Plate1isthegeologicmapofYanceyCounty.Itshowsthedistributionofthedifferentrocktypes,theirstructure,relativeage,metamorphismandmineralresources.Metamorphicrockis,byfar,themostabundantclassofrockinthecounty.Igneousrockisalsopresent,buttruesedimentaryrockisnot.Metamorphosedsedimentaryrocksarequitecommon,however.Manyhavebeenalteredsolittlethattheirsedimentarycharacterisstillquiteobvious.Mostoftherocksrangeinagefrom600millionyearstooverabillionyears（figure3）.Theyaremultiplymetamorphosed,folded,andfaulted.Onlytheyoungestigneousintrusives,the390-million-year-oldalaskitesandpegmatites,haveescapedmetamorphism.Theoldermetamorphosedintrusiverocksvarywidelyinageandcomposition.MostoftheintrusiverocksofYanceyCountyhavebeeneconomicallyimportantassourcesoffeldspar,mica,andolivine.

STRUCTURALSETTING

YanceyCountyislocatedintheBlueRidgegeologicbelt.Large,structuralfeaturescontroltheregionalrockoutcroppatternshownonthegeologicmap.Historically,theoverallpatternwasinterpretedasalargeanticlinorium.Today,manyofthecontactsseparatingthemajorrocksequencesaremappedasancientthrustfaults,whicharelowanglebreaksintherockalongwhichtherehasbeenmovement.

Theregionalmappatternisnowdepictedasapileofstackedthrustsheets.YanceyCountyispartofalarge,transportedareathathasbeenmovednorthwestwardalongmajorthrustfaultsatleast30milesandprobablymorethan125miles.

ThreemajorthrustfaultscontrolthemappatterninYanceyCounty.

Theyoungest–andmostnorthwesternthrustfault–istheStoneMountainfault（plate1）.Thisfaultoccurredabout240to290millionyearsago.ItisthesoutheasternboundaryofthelatePrecambrianageSnowbirdandCambrianageChilhoweerocksthatunderlieFlatTopMountain.

EastoftheStoneMountainfaultistheForkRidgefault（plate1）.ItstraceextendsfromnorthwestofLittleBaldMountainthroughSioux.About300to370millionyearsago,thisthrustcarriedbiotitegraniticgneissandmigmatiticbiotite-hornblendegneissunitsoverandontothegranodioritegneissunit.

TheHollandMountainthrustfault,theeastern-mostthrustfault（plate1）,formedjustbeforeorduringthesecondmetamorphismabout475millionyearsago.ThisfaultistheoldestandleastobviousofthethreemajorfaultsinYanceyCounty.ItcarriedtheAsheMetamorphicSuiteandAlligatorBackFormationovertheoldermigmatiticbiotite-hornblendegneissunit.ItstraceismappedinalinefromjustwestofPaintGaptoBurnsvilletotheToeRiver.

IneasternYanceyCounty,alaskiteintrusivesmodifytheoutcroppatternoftherocks.Thesealaskitesaretheonlyintrusivebodieslargeenoughtosignificantlymodifytheoutcroppatternshownonthemap.

Attheoutcropscale,foldsarethemostrecognizablestructuralfeature.ThestyleoffoldsvisibleinYanceyCountyindicatesthatatonetimetheserockswereveryhotandbehavedquiteplastically.

METAMORPHISM

ThemetamorphichistoryoftherocksofYanceyCountyisquitecomplex,formanyhavebeenmetamorphosedtwoorthreetimes.Thefirstmetamorphismoccurredoveronebillionyearsagoandaffectedrocksthatcropoutinthenorthwesternhalfofthecounty.Theserocksincludethemigmatiticbiotite-hornblendegneissunit,thebiotitegraniticgneissunitand,possibly,thegranodioritegneissunit.Veryhightemperaturesandpressurescharacterizedthismetamorphism.Evidenceforthismetamorphismisthepresenceofthemineralhypersthene.Metamorphicrockscontaininghypersthenearecalledgranulite.

Asecondmetamorphismoccurredabout475millionyearsagoandaffectedalloftherocksinYanceyCounty,exceptfortheintrusivealaskiteandpegmatitethatwereemplacedmuchlater,about390millionyearsago.Thiseventwasaretrogressiveeventonthethreemapunitsaffectedbytheearlierhighergrade,Precambrianmetamorphism.

Thelastmetamorphismoccurredabout300millionyearsago–thiswasaverylow-gradeevent,anditseffectsarenotverypronounced.Itisbestevidencedbyrarechloritereplacingbiotite,hornblende,andgarnetinsomeoftherockunitsinthenorthwesternpartofthecounty.

ROCKDESCRIPTIONS

Thefollowingrockdescriptionsareanelaborationofthemapunitsdescribedonthegeologicmap（plate1）.TheyareintendedtomorethoroughlydescribetherocksofYanceyCountyandhelpexplainsomeofthelocalvariationsobservedthroughoutthecounty,becauseageologicmapisonlyascaledrepresentationoftheearth’ssurface.Arepresentativeoutcropofeachmapunitisdesignatedonthegeologicmap（plate1）.Atthislocationeachmapunitmaybemorecloselyexamined.Theoutcropisselectedforitsaccessibilityandeaseofparkingformorethanonevehicle.Nonetheless,mostarelocatedalongnarrow,curvystatehighwaysandroads.Ahighlevelofcautionshouldbeusedwhenexaminingtherocks.

PEGMATITE

Pegmatiteisanexceptionallycoarselygrained,igneousrock,usuallycomposedoffeldspar,quartz,andmica.Itcommonlyoccursasirregulardikesandsillsatthemarginsofverylarge,intrusiveigneousrockbodies.

ThousandsofsmallbodiesofpegmatiterockoccurthroughoutmuchofYanceyCounty.Theyexistinallmapunits,excepttheChilhoweeandtheSnowbirdGroups.

TheSprucePinepegmatiteintheMicavilleareaismineralogicallysimilarandclosely