JOURNALOF GEOPHYSICALRESEARCH, VOL. 94, NO. B2, PAGES1843-1849, FEBRUARY10, 1989

PALEOMAGNETISM OF THE UPPER ORDOVICIAN JUNIATA FORMATION OF THE CENTRAL APPALACHIANS REVISITED AGAIN

JohnD. Miller1 andDennis V. Kent Lamont-DohertyGeological Observatol 3, and Department of GeologicalSciences, ColumbiaUniversity, Palisades, New York

Abstract.Two componentsof magnetizationwere isolated demagnetizationin thestudy of red beds.During this time in theUpper Ordovician Juniata Formation sampled in the area periodall of themajor Appalachian red beds were studied or of the Pennsylvaniasalient. The thermallydistributed, restudiedusing modem paleomagnetic techniques. The revised reversedpolarity B componentwas most likely acquired resultsfrom the Juniatawere reported by Van der Voo and duringAlleghenian deformation, and although it is poorly French[ 1977], andwere incorporatedinto the analysisof grouped,it is similarto otherAppalachian synfolding Schwartzand Van der Voo [ 1983], which concludedthat there magnetizations.The pre-Alleghenianage C magnetizationis was no oroclinal rotation involved in the formation of the entirelyof normalpolarity and showsa differencein Pennsylvaniasalient, a majorstructural feature of thecentral declinationsbetween the meanmagnetizations isolated on the Appalachians. northern and southern limbs of the salient of 24 ø _+23 ø. This Recentcontroversy regarding the Paleozoic reference poles anomalyis consistentwith the senseand magnitude of for North Americaand their tectonic implications [Kent and declinationanomalies observed in pre-Alleghenian Opdyke,1978; Van der Voo et al., 1979;Irving, 1979;Roy magnetizationsisolated in otherthroughgoing Appalachian red andMorris, 1983; Irving andStrong, 1984] sparkedanother bedsof Silurian,Devonian, and early Carboniferousage. The roundof studyinto thepaleomagnetism of the Appalachianred meaninclination of-44.7 ø suggestsa paleolatitude of about beds.Although late Paleozoicremagnetization of Appalachian 26øSfor the centralAppalachians in the Late Ordovician.This red bedshas long beendocumented [Roy et al., 1967], the paleolatitudefits a trendof southwardmotion of North new studiesbrought to light the previouslyunsuspected Americafrom the Ordovicianto the Early Devonian,followed complicationthat the Kiamanremagnetization was by northwarddrift throughthe remainderof the Paleozoic. synchronouswith the Allegheniandeformation. Thus although the previousresults from the Mauch Chunk[Knowles and Introduction Opdyke,1968] andCatskill [Van derVoo et al., 1979] had reporteddual polarities and positive fold tests,they were in The Paleozoicsedimentary sequence of the North fact seriouslycontaminated by the synfoldingremagnetization AmericanAppalachians contains several clastic wedges within [Kent and Opdyke, 1985; Miller andKent, 1986a,b]. whichred bedsoccur, principally the Upper Ordovician The revised results from the Mauch Chunk and Catskill, as Juniata,Middie SilurianRose Hill, Upper Silurian well as new datafrom the BloomsburgFormation [Kent, Bloomsburg,Upper DevonianCatskill, lower Carboniferous 1988], reveal a declinationanomaly in the pre-Alleghenianage Mauch Chunk,and the upperCarboniferous/Lower Permian magnetizationsbetween the northernand southern limbs of the Dunkardformations [Thomas, 1977]. This sedimentpackage Pennsylvaniasalient which couldindicate oroclinal rotation. wasdeformed in the Penno-CarboniferousAlleghenian In addition, the recent results from the Lower Devonian orogeny[Dennison, 1982]. Andreasred beds [Miller and Kent, 1988a] and from the The relativelysimple structure of theValley andRidge Bloomsburg[Kent, 1988] both showa bestgrouping of the Provinceand the easily measured, high stability higheststability component at lessthan full tilt correction, magnetizationsof thered bedsmade these rocks prime perhapsindicative of someremagnetization and folding candidatesfor paleomagneticstudy. Indeed, these units were associatedwith the DevonianAcadian orogeny even though someof the firstrocks to be sampledfor paleomagneticstudy structural evidence for Acadian deformation in the central in North America,with oneof the firstpositive fold tests Appalachiansis ambiguous. havingbeen recorded in samplestaken from the Rose Hill by The major PaleozoicAppalachian red bedswhich havenot Graham[1949]. The Juniatared bedswere first sampledfor beenrestudied since the recognitionof synfoldingKiaman paleomagneticstudy some 30 yearsago [Collinsonand remagnetizationsare the Juniata,Rose Hill, and Dunkard Runcorn,1960]. Data from this early study were used to help formations.The purposeof the currentstudy is to reportnew bolster the first-order observation of continental drift between resultsfor the Juniata.In the studyof Van der Voo and French NorthAmerica and Europe. These original data represented the [ 1977], the Juniatawas sampledprimarily in the southernlimb totalnatural remanent magnetization (NRM) of the samplesas of the Pennsylvaniasalient. Here samplingwas obtained from no demagnetizationwas doneon the collection. both the southern and northern limbs so as to allow for better In the middle to late 1960sand in the 1970s,recognition of documentation of evidence for oroclinal rotation. primaryand secondary magnetizations in Appalachian rocks The Juniataand underlyingBald Eagle fom•ationsof the suchas the Bloomsburg[Irving andOpdyke, 1965; Roy et al., centralAppalachians lack a distinctivefossil assemblage but 1967] clearlyshowed the value of employingthermal areconstrained to be Late Ordoviciansince they are underlaid by the AshgillianReedsville shale and overlain by the LlandoverianTuscarora sandstone [Thompson, 1970a, b]. Sampleswere collectedand dataanalyzed using standard paleomagnetictechniques [see Miller andKent, 1986a]. Orientedsamples from a totalof 21 siteswere drilled in the 1Now at AmocoProd. Co, Houston, TX JuniataFormation with eightsites in thenorthern limb of the salient,three sites in the hingezone, and 10 sitesin the Copyright1989 by the AmericanGeophysical Union southernlimb. We reoccupiedthe samelocation as site 13 of Van der Voo and French [ 1977] as sitesE,F,G and their site Papernumber 88JB03637 14 as sitesL,M,N. Site locationsare givenin Table 1 and 0148-0227/89/88 JB-3637 $05.00 Figure 1.

1843 Miller andKent: Upper Ordovician Juniata Formation

TABLE 1. JuniataFormation Site Data andPaleomagnetic Directions.

B Component C Component

S L s STK/DP LAT/LON n k a95, GD, GI, n k a95, GD, GI, deg deg deg deg deg deg

A N 5 256/77 40.30/77.13 2* - - 286.7 14.6 5 16 19.5 340.2 38.7 B N 6 255/90 40.30/77.13 1' - - 122.6 72.3 5 17 19.3 341.4 37.5 C N 5 250/54 40.28/77.27 0* .... 5 20 17.7 347.4 8.7 D N 5 259/54 40.28/77.27 2* - - 155.9 -26.6 3 4 77.1 4.1 -1.9 E S 5 213/96 40.01/78.48 4 19 21.5 206.4 -19.8 3 7 50.2 337.4 53.7 F S 4 215/100 40.01/78.48 4 10 31.4 210.4 -20.1 1' - - 24.5 48.8 G S 5 205/94 40.01/78.48 5 51 10.8 199.0 -35.4 0* - - - . H S 5 26/45 39.88/78.50 5 5 36.3 145.5 37.3 3 13 36.0 21.3 -60.7 I S 5 26/45 39.88/78.50 4 4 57.0 167.4 37.0 0* - - - _ J S 6 193/42 40.17/78.50 1' - - 209.8 -18.2 3 21 27.4 337.7 -22.4 K S 7 192/43 40.17/78.50 5 7 31.7 225.5 16.9 5 11 24.0 319.7 -3.9 L S 5 14/52 40.16/78.37 5 32 13.8 160.5 6.9 3 17 31.1 86.9 -64.5 M S 5 12/58 40.16/78.37 5 46 11.5 156.2 .7 0* - - - . N S 5 12/57 40.16/78.37 5 48 11.2 159.5 3.9 1' - - 27.5 -4.0 O H 8 236/90 40.95/77.75 2* - - 242.5 -52.6 2* - - 344.4 53.3 P H 5 53/36 41.00/77.50 5 17 19.1 167.8 -25.1 5 38 12.5 67.9 -67.9 Q H 5 53/36 41.00/77.50 5 12 22.8 154.7 -37.9 3 13 35.2 10.3 -65.4 R N 7 96/12 40.92/77.50 7 32 10.9 179.1 -8.9 0* - - - _ S N 7 250/37 40.92/77.50 4 4 57.8 194.3 1.7 6 6 30.2 3.4 -13.7 T N 6 74/80 40.85/77.17 6 14 18.7 152.4 38.8 0* - - - . U N 6 251/35 40.85/77.17 3 6 53.7 194.3 -35.5 5 6 35.1 359.7 -13.0

Means:

North Limb 0%TC 4/8 5 44.0 181.2 -1.3 6/8 10 22 353.6 9.3 50%TC 4/8 12 28.3 180.0 -5.0 100%TC 4/8 5 43.5 181.1 -9.4 " 44 10.2 357.4 -47.8 South Limb 0%TC 9/10 5 26.3 181.0 3.4 5/10 2 73 346.9 -27.6 80%TC 9/10 8 19.2 174.4 -5.9 100%TC 9/10 8 19.9 176.1 -7.2 " 19 18.1 333.0 -43.0 Hinge 0%TC 2/3 - - 161.7 -31.7 2/3 - - 37.5 -69.3 100%TC 2/3 - - 181.6 -64.0 " - - 351.6 -44.0

S is letterdesignation for site;L denotessalient limb sitewas located on (N,H,S = north,hinge, south); s is numberof samplestaken at site;STK/DP are strike (90 ø counterclockwisefrom direction of dip)and dip of bedding; LAT/LON are latitude(øN) andlongitude (øE) of site;n is thenumber of samplesused in sitecalculations, or for means,the ratio of sitesused in meancalculation to totalsites (not used with *); k is Fisher'sprecision parameter; a95 is semi-axisof radiusof confidence;GD andGI aredeclination and inclination in geographiccoordinates; TC is tilt correction.

DemagnetizationBehavior of the NRM is cardedby two ancientcomponents of magnetization,one of whichis thermallydistributed and is Thermal demagnetizationbehavior of the sampleswas unblockedat demagnetizationtemperatures ranging from much as describedby Van der Voo and French[ 1977]. 300øC to about660øC (labelled "B") and the otherwhich is Samplesrarely contain a low unblockingtemperature (<300øC) thermallydiscrete and mostly unblocked above 660øC (labelled componentwhich canbe attributedto recentacquisition. Most "C").The qualityof thedemagnetization data was generally low with many sampleshaving unstable magnetizations and/or sufferingalteration at high demagnetizationtemperatures, manifestedby large increasesin susceptibility.Petrographic analysisshows magnetite to be rare to absentin the Juniatabut LIMB • hematiteand hemo-ilmenite are commonlyobserved • RIK.(.IT..) PENNSYLVANIANY • [Thompson,1970b]. The high unblockingtemperatures of boththe B and C componentssignify hematite as the carderof 80•1 ) CA-D) 50• ,• remanence. Only about35% of the sampleswere well behavedand .41ø 60• HINGE •.O] • allowedisolation of bothB and C componentsin the same specimenby principalcomponent analysis (Figure 2a). Another35% of the sampleshad well-defined B components o • _. • _ with lineardemagnetization trajectories clearly not trendingto

•M• the origin,but alterationof thesesamples at hightemperatures of demagnetizationprohibited isolation of thehigh unblocking 80 ø ) temperatureC component(Figures 2b-2e). In contrast,15% of W(BG)V•l 79ø 78ø 77ø MD 76ø DEL the samples,characterized by northerlydirections and limited removalof NRM below 660øC,possessed only the C Fig. 1. Samplinglocalities of JuniataFormation (dots) and the component(Figure 2f). The remainderof the samplecollection distributionof beddingstrikes (inset). Outlinedarea is traceof hadno straightline segmentsrevealed in thedemagnetograms Ordovicianage outcrop. andgave only "spaghetti"patterns that could not be Miller andKent: Upper OrdovicianJuniata Formation 1845

OJP5A A OJE1 A OJF 1A W,UP B N,UP C N, UP NRM"'• ..... 660

660 400 '-,,, 'f ,© 660 "',,,;..•,•.. 'o

1 mA/m

A/m

OJB 5A OJM 3 N UP NRM F N UP 400 W,UP D OJN 3A /•400 630•) 675 675

lm A/m-

675

Fig. 2. RepresentativeZijderveld [1967] demagnetograms in tilt-corrected coordinates. Open (solid)symbols are projections on vertical(horizontal) plane. Thermal demagnetization temperaturesin øC. a) Samplein whichboth B andC componentscould be isolated.b, c, d, ande) Samplesillustrating removal of B componentwith final northerlycomponent not isolated(dotted lines are spuriousmagnetizations at highdemagnetization temperatures. f) Samplewith only C component.

interpreted.Only the sitemean directionsfor both the B and C which had interpretablemagnetizations shared a common componentsfrom siteswhere the componentof interestwas strikeand dip. The in situ directionfor the B component isolatedin at leasthalf of the samplesare incorporatedinto the isolatedin the hinge zoneis 161.7ø/-31.7ø (N = 2). formation mean directions discussed below. The C sitemean componentsare directedto the north and have negativeinclinations after correctionfor the beddingtilt MagnetizationDirections (Figure 4). The C componentwas directlyisolated in slightly over half of the sitessampled, nevertheless, the data from the The meandirections of ancientcomponents isolated in the northern and southern limbs of the salient show a marked Mauch Chunk, Catskill, and Bloomsburgformations show improvementin groupingwith full tilt correction(Figures 4 significantdifferences between the northernand southern and 5). The tilt correctedmean magnetization for the datafrom limbs of the salient(the differencesin the secondaryB the northernlimb is 357.40/-47.8ø (a95 = 10.2ø for N = 6 magnetizationsare interpretedto be due to differentages of sites),from the southernlimb data is 333.00/-43.0ø (a95 = magnetization[Miller and Kent, 1988b], whereasdifferences 18.1ø for N = 5 sites),and for the hingezone is 351.6ø/-44.0ø in pre-AlleghenianC componentsare due to oroclinal (N = 2; Table 1). No reversedpolarity C componentswere rotation). Thereforewe reportthe JuniataB and C isolatedin this studyand the sampledemagnetization componentsfrom the limbs of the salientseparately. trajectorieswhich bypassedthe originwere clearlytrending The B componentmagnetizations are generallydirected to into the northernhemisphere (similar to the behaviorillustrated the south-southeastwith low to moderatepositive and negative in Figure 4 of Van der Voo and French, [ 1977]), consistent inclinations(Figure 3). The B componentsfrom both the with the unresolved,final componenthaving normal polarity northernand southernlimbs of the salientare poorly grouped (Figures2b-2e). with a maximumprecision parameter (k) value for the northern sitesof 11.5 and only 8.2 for the southernsites. The north Interpretation limb B componentis bestgrouped at 50% tilt correctionwith a B Component mean declination/inclinationof 180.00/-5.0ø (a95 = 28.3, for N = 4 sites;Table 1). The southlimb B componentobtains its As notedabove, the timing of theremagnetization appears bestgrouping at 80% tilt correctionwith a mean directionof to vary with geographicposition along the Appalachians 174.4ø/-5.9ø (a95 = 19.2ø for N = 9 sites). No fold testwas [Miller and Kent, 1988b],but the poor groupingof the Juniata possiblefor the sitesfrom the hingezone, as the two sites B componentmakes the mean directionsfrom the northernand 18q6 Miller andKent: Upper Ordovician Juniata Formation

N N 15

- B Component _ B Component _ _

- Sou•Limb NorthLimb - _

- _

- _ 10 9 sites _ - 4 sites North Limb i i i i i i i i •- i i i i -- i i i i -l- i i i i i i i l N=4 - _Hinge- o -

Geographic - 2 sites_ o - _ - N=15 _

_ - ß ß o - ß o - 5, Limb Cb - - _ - --13• ]•ZZ .... ß ß ......

i i i i i 80%TC •- O, ._o

• C Component '- 413. m i m ! .-- Bedding

30' O North Limb N=6

Fig.3. Sitemean B componentdirections. Open (solid) 10' symbolsare plotted on upper(lower) hemisphere of equal-area projection.VF is positionof meanin situoverprint N=5 componentreported by Van der Voo and French[ 1977]. 0 0 20 40 60 80 100

Percent Tilt Correction southernlimbs of the salientstatistically indistinguishable. The formationmean (hinge zone included), best grouped B Fig. 5. Fisher'sprecision parameter (k) versusincremental tilt magnetizationis 175.3ø/-11.3ø, a95 = 15ø for N = 15 sitesat correctionfor the B andC components.Dashed lines are 60% tilt correction.This magnetizationis similarto the minimumk valuesrelative to peakk valuesrequired for fold secondaryin situdirection reported by Van derVoo and testto be significantat the 95% confidencelevel. French [1977] of 161.3ø/-1.8ø (a95 = 7.5 ø, k = 7.6 for n = 53 samples)suggesting that we haveisolated the same component.The pole positionassociated with the best groupedformation mean B componentis 106øE,56øN (A95 = consistentwith the suppositionthat the remagnetization is 12.5ø, k = 10.4calculated from N = 15 sitemean pole Alleghenianage. positions).This poleplots on thePermian portion of the Althoughthe directionand unblocking temperature spectra NorthAmerican apparent polar wander path (Figure 6), of the B componentare similarto the Kiamanoverprints commonlyobserved in otherAppalachian red beds(referenced above),it is somewhatdisturbing that this B component

N N shouldbe so poorlygrouped, both in our sampleset and that • C Component• of Van der Voo andFrench [ 1977]. Indeedeven the pre- AlleghenianC componentis bettergrouped than is theB componentin theJuniata (Table 2). The precisionparameters (k values)observed for otherKiaman remagnetizations isolatedin Appalachianred bedstabulated by Miller andKent [ 1988b]range from 29 to 168 with a meanof 82, compared with value of less than 10 in the Juniata. Part of the reasonfor thispoor overall grouping of the B componentcould be becauseof thecomplicated relative timing of the remagnetizationand folding of theJuniata. For example, for sitesR andS the bestgrouping is obtainedat 0% tilt correction, the data from sites E,F,G,H, and I are best 1,5,sit,e,s, ,-,, ; ...... •o ! / ...... :,,, ?s,it,os, , groupedonly at 100% tilt correction,while datafrom sitesT andU reachbest grouping at 70% tilt correction,even though the overallbest grouping (highest k value) is obtainedat 60% tilt correction.Moreover, the JuniataB componentis not as well definedin the sampledemagnetization data as are the B componentsobserved in mostother Appalachian red beds. Geochemicaland petrographic evidence suggests that the underlyingBald EagleFormation was at onepoint red but that Fig. 4. Site meanC componentdirections. Open (solid) thered pigmentwas subsequentlyleached from thisunit symbolsare plotted on upper(lower) hemisphere of equal-area [Thompson,1970b]. The poordefinition on demagnetograms projection.VF is positionof meanprefolding component of theJuniata B componentmay reflect inhibition of the reportedby Van der Voo and French[ 1977]. formation, or removal, of the hematite that carriesthe B Miller andKent: Upper Ordovician Juniata Formation 18•47

red beds [Miller and Kent, 1988a] and the Silurian BloomsburgFormation [Kent, 1988]. It was speculatedthat the enhanced(although not significantat the 95% confidence level) groupingof the Andreasand Bloomsburg magnetizationsat slightlyless than full tilt correctionmight be dueto remagnetizationduring or afterwide-scale minor foldingassociated with theDevonian Acadian orogeny in the centralAppalachians. While we cannot exclude the possibility thatthe Juniata C componentrepresents a postdepositional but pre-Alleghenianfolding remagnetization, we seeno evidence of anintervening folding event in ourresults from the Juniata. The meanC componentdirection (333.00/-43.0 ø, a95 =' 18.1o) we find from the southlimb of the Pennsylvaniasalient is steeperand more northwesterly than the site mean prefoldingmagnetization reported for theJuniata by Van der Voo andFrench [1977] of 348.6ø/-31.8ø, a95= 4.9ø, even Fig. 6. SelectedPaleozoic poles and alternative APW paths thoughnot differentat the 95% confidencelevel accordingto for North America. Line with arrowis APW path takenfrom the test of McFadden and Lowes [ 1981]. Therefore, it is not Van der Voo (1988). Brokenline representssouthern precludedthat thesetwo groupmeans were drawnfrom the extensionof pathsuggested by Andreasresult [Miller and samemagnetization population. However, a majordifference Kent, 1988a]. Paleomagneticpole positionsfor Juniataare between the two studies does exist in that Van der Voo and shownfrom previous (OJ) studyby Van der Voo andFrench Frenchreported prefolding reversed polarity samples from 11 [ 1977] andour data (NJ-C is prefolding,NJ-B is secondary out of 17 sites(six sitesgave all normalpolarity), while in our with circleof confidence).Plotted for referenceare poles from studyno convincingevidence for reversedpolarity prefolding Moccasin-Bays(MB [Wattsand Van der Voo, 1979]), componentswas found. We believethat this discrepancy Cordovasecondary (Co [Dunlopand Stifling, 1985]), Wabash betweenthe two studiescan be explainedin lightof therecent (Wa [McCabe and others,1985]), RoseHill (RH [Frenchand documentationof widespreadsynfolding Kiaman overprinting Van der Voo, 1979]), Catskill (Ca [Miller and Kent, 1986b]), of Appalachianred bedsdiscussed above. AlthoughVan der Andreas(An [Miller andKent, 1988]), Bloomsburgand Voo andFrench were well awareof possibleKiaman Mauch Chunk(B1,MC [Kent, 1988]), Deer Lake (DL [Irving remagnetization,at the time of their studythe remagnetization and Strong,1984]), and Peel Sound(PS [Dankers,1982]) hadnot yet beencharacterized as synfolding. units. Open(solid) circles are poles from unitssampled in the For sites 8, 13, and 16 from Van der Voo and French southern(northern) limb of the Pennsylvaniasalient; open [1977], all samplemagnetizations were reported of reversed squaresare poles from unitssampled on thecraton of North polarity(southerly declination) but the sitemean directions America. 240, 270 and300 are agesin Ma of meanpoles passthrough the horizontal at partialtilt correction,consistent from the North AmericanAPW pathof Irving andIrving with (re)interpretationof these magnetizations assynfolding. [1982]. Kiaman,rather than of prefoldingorigin. Our sitesE,F,and G arefrom the samelocality as site 13 of Van der Voo and French,and in ourcollection the magnetization of these sites was stronglydominated by the synfoldingremagnetization componentduring the chemical event which resulted in the (Figures2b and 2c). Sites2,5,6,9,14,15, and 17 of Van der morecomplete leaching of theBald Eagle. Voo andFrench have both normal and reversed polarity samplemagnetizations, but from the publisheddata we cannot C Comoonent determineto what extent,if any,contamination by Kiaman magnetizationscontributed to thesesite mean directions. One The fourfold and ninefold increases in k for the C reversedpolarity sample direction was reported from site 14, componentsfrom the northernand southernlimbs of the the locationof which correspondsto our sitesL,M,and N, salient,respectively, are eachindicative of positivefold testsat wherewe coulduncover no convincingevidence for a betterthan the 95% confidencelevel [Watson, 1956]. The C prefoldingreversed polarity component in our samplesfrom componentsfrom bothsalient limbs are thereforeconstrained thesesites (Figures 2d and 2e). Site 1 of Van der Voo and to predatethe Alleghenianage folding and may dateto the time Frenchis an interestinganomaly in thatall of the sample of depositionin the Late Ordovician.There is no possibility magnetizationswere reversedpolarity, but the sitemean for a fold testfrom the two sitesfrom the hingezone. The observation that the maximum k for the Juniata C magnetizationdirection is nevershallower than 28 ø at any stage of tilt correction.Site 1 couldtherefore possibly represent a componentis only reachedat full tilt correctionis in contrastto pre-Alleghenianreversed polarity magnetization. the bestgrouping at 80% to 90% tilt correctionof thepre- This leaves sites 3,4,7,10,11, and 12 which are most Alleghenianmagnetizations from theLower DevonianAndreas likely freeof Kiamanremagnetizations, since all samplesfrom thesesites were reported to havenormal polarity magnetizations.This subsetof datafrom Van der Voo and TABLE 2. JuniataFormation Paleomagnetic Pole Positions Frenchpasses the fold testat the 95% confidencelevel and yieldsa tilt correctedmean of 347.60/-38.6ø (k = 77.0, a95 = Component Limb LON LAT K A95 N 7.7 ø, N = 6 sites)that is much closerto the mean for our data setfrom the southernlimb. In the currentstudy, 62% of the B (60%TC) N,H,S 106 56 10 13 15 siteshave been accepted as reliable records of theprefolding C(100%TC) N 105 21 29 13 6 magnetization.Our reanalysisof the dataof Van der Voo and C(100%TC) H 110 22 -- - 2 C(100%TC) S 128 19 14 21 5 French[1977] resulted in 35% siteacceptance. Full accessto the previousdata setwould probablyresult in similar Limb denotessalient limb site was locatedon (N,H,S = north,hinge, acceptancerates for bothstudies. south);LON, LAT are the longitude(øE) and latitude (øN) of the Discussion paleopole;K is theprecision parameter; A95 is theradius (in degrees)of 95% confidencecircle; N is numberof sitemean virtual geomagnetic The C componentsfrom the northernand southern limbs polesused to calculatepaleopole; TC is tilt correction. of the salientdiffer in declinationby 24 ø _+23 ø andin Miller andKent: Upper Ordovician Juniata Formation inclinationby 4.8 ø + 21o (95% confidence limits calculated lO accordingto Demarest[1983]). The senseand magnitude of o N 270 the between-limbdeclination anomaly (northern limb being o more clockwise)are consistentwith the averageanomaly CO BL CA (22.8ø + 11.9ø) which hasbeen documented in the Bloomsburg,Catskill, and Mauch Chunkformations [Kent, H 1988] andascribed to partialoroclinal rotation involved in the -20 formationof the Pennsylvaniasalient. Documentation of oroclinal rotation in the formation of the -30 Pennsylvaniasalient complicates construction of a Paleozoic APW path for North Americaconsiderably, as many of the AN DL key Paleozoicpole positionscome from rock unitswhich -40 JU outcropin the salient. Sincewe do not yet know therotational historyof the limbs of the salientwith respectto the eraton,we -50 OrdovicianIS•lurian[)evonian I Carboniferous IPermian ß cannotcorrect for thepotential error of 20ø or morein the 500 400 •00 measureddeclinations and the correspondinguncertainty in the RockAge (Ma) pole positions.Poles determined for rock unitsfrom the Figure7. Palcolatitudeprogression calculated for eastern undisturbederaton are not affectedby suchrotation but North America (40.5ø N, 78ø W) from APW path. generallylack field stabilitytests, and therefore the ageof Abbreviationssame as Figure6 with additionof Lower thesemagnetizations is oftenpoorly constrained. OrdovicianOneota Formation. (ON [Jacksonand Van der Nevertheless,the few availablepoles from the undisturbed Voo, 1985]). Note thatBloomsburg paleolatitude fits bestif cratonare vital pinningpoints; even if the magnetizationage is themagnetization is Devonianand that Juniata, Cordova, and not the sameas the rock (or thermochronometric)age, the path PeelSound paleolatitudes could also fit asremagnetizations in musthave passed through that paleopoleposition at sometime the Devonian. since the rock formed. The apparentpolar wander path shown in Figure6 is equatorialpaleolatitude (6øS) in theMiddle Ordovician based essentiallythat of Van der Voo [1988], drawnusing poles on the Moccasin-BaysFormation result to some25øS by the with hisquality index greater than 5. The pathpasses Middle Silurianas indicatedby the WabashFormation data southwestfrom the pole from theMiddle Ordovician (Figure7). The resultfrom the Andreasred beds[Miller and Moccasin-Bays(which although not from the Pennsylvania Kent, 1988a] suggeststhat North Americacontinued to salientmay havesuffered some thrust sheet rotation in the migratesouth to a paleolatitudeof about35øS in the Early southernAppalachians) to reachthe Middle Silurian pole from Devonian. North Americathen moved northward during the theWabash Formation. The paththen loops out to the westto Devonianto a paleolatitudeof about16øS in the Late Devonian passthrough the MDL polefrom the Lower Devonian Peel and Early Carboniferous,as suggestedby the revisedresults SoundFormation and returns east to passthrough the early from the Catskill and Mauch Chunk, with continued CarboniferousDeer Lakepole; the APW paththen joins the northwarddrift duringthe Carboniferousand Permian. relativelywell-defined Late Carboniferous through Permian Adjustmentof the Juniatasite mean directions we obtained trend. from the northlimb andhinge zone to the meandeclination of As discussedby Miller andKent [ 1988a],the new the site means from the southern limb of the salient allows Andreasresult implies that the Peel Sound MDL magnetization calculationof a formationmean inclinationof-44.7 ø (a95 = is Middle to Late Devonianand thatthe loop shouldbe 6.9ø). Thesenew dataindicate a more southerlypaleolatitude broadenedas shownby the brokenline. Expansionof the (26øS+ 12ø) thanwas reportedby Van der Voo andFrench loopis required,since oroclinal rotation of theAndreas (17øS+ 5ø) for the Juniataoutcrop area in the Late Ordovician. magnetizationwould not displace the Andreas pole northward, The 26øSindicated by the new Juniataresult could fit into the towardthe Peel SoundMDL pole. A Late Devonianage for latitudinaldrift patterneither during the southwarddrift from the northernpart of thisexpanded loop is suggestedby this the Middle Ordovician to the Middle Silurian, or as a modification. remagnetizationduring the northwarddrift duringthe The Cordovasecondary magnetization is potentially Devonian (this possibilityis, of course,also open for any of importantto the discussionof the Juniatamagnetization, since the availabledata from Ordovicianthrough Lower Devonian it hasbeen interpreted by Dunlopand Sterling [ 1985] asbeing rock unitsdue to the broadage constraintson the associatedwith a plagioclaseAr40/39 plateauage of 436 Ma, magnetizations).The observationthat the JuniataC or Late Ordovicianaccording to the DNAG [Palmer,1983] componentpole from thenorthern limb of the Pennsylvania time scale. If we foregothe possibilityof a Late Ordovician salientfalls to the westof the Wabasheratonic pole could loop in the APW path,the positionof the Cordovasecondary suggesta Late Silurianto Middle Devonianage for the Juniata pole next to the Peel Soundpole, andaway from our revised C component.However, the similarityof the RoseHill pole Juniataresults, suggests that the Cordovamagnetization is from the southern limb of the salient and the Wabash cratonic more likely to be of Middle to Late Devonianage. The poleimplies that the southernlimb rotatedless with respectto interpretationthat the Cordovasecondary magnetization, the eraton than the northern limb. If this is so, then the whichis part of a rathercomplex total magnetization, is agreementwithin the confidencelimits of the southernlimb Devonianwould imply that it hasnot beencompletely isolated JuniataC componentpole with the Wabashpole, combined and/orthat the observedmagnetization continued to havebeen with the observationthat the bestgrouping of the JuniataC acquiredfor a considerableperiod of time afterthe isotopic componentis obtainedat full tilt correction,support the blockingin the Cordovaplagioclase grains. We notethat both interpretationthat the JuniataC componentis Ordovicianin the locationof the pole positionand the paleolatitudeinferred age. from the BloomsburgC componentsuggest that this magnetizationmay alsobe a Devonianremagnetization. Acknowledgments.We appreciatethe constructive Oroclinalrotation of the Pennsylvaniasalient would not commentson the manuscriptprovided by N. Christie-Blick, affectthe paleolatitudes derived from the inclinations of rock S. Haldegahl,D. Walker, W. Witte, and the Journal unitsthat outcropon the salient. Translationof thepolar reviewers. Researchsupported by the NationalScience wandercurve into a paleolatitudeprogression calculated for the Foundation,Earth Science Division grantsEAR85-07046 and centralAppalachians (40.5øN, 78øW) showsthat this part of EAR88-03814. Lamont-DohertyGeological Observatory North America should have drifted south from a near contribution 4389. Miller andKent: Upper Ordovician Juniata Formation 1849

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