Paleomagnetism of the Upper Ordovician Juniata Formation of the Central Appalachians Revisited Again
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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].