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Further Paleomagnetic Evidence for Oroclinal Rotation in the Central Folded Appalachians from the Bloomsburg and the Mauch Chunk Formations

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Further Paleomagnetic Evidence for Oroclinal Rotation in the Central Folded Appalachians from the Bloomsburg and the Mauch Chunk Formations TECTONICS, VOL. 7, NO. 4, PAGES 749-759, AUGUST 1988 FURTHER PALEOMAGNETIC EVIDENCE FOR OROCLINAL ROTATION IN THE CENTRAL FOLDED APPALACHIANS FROM THE BLOOMSBURG AND THE MAUCH CHUNK FORMATIONS Dennis V. Kent Lamont-DohertyGeological Observatory and Departmentof GeologicalSciences ColumbiaUniversity, Palisades, New York Abstract.Renewed paleomagnetic investigations of red fromthe Bloomsburg, Mauch Chunk, and revised results bedsof theUpper Silurian Bloomsburg and the Lower recentlyreported for theUpper Devonian Catskill Formation Carboniferous Mauch Chunk Formations were undertaken togetherindicate 22.8•>+_11.9 oof relativerotation, accounting with theobjective of obtainingevidence regarding the for approximatelyhalf thepresent change in structuraltrend possibilityof oroclinalbending as contributing to thearcuate aroundthe Pennsylvania salient. The oroclinalrotation can be structuraltrend of thePennsylvania salient. These formations regardedas a tightenS.*'.3 o/'a lessarcuate depositional package cropout on both limbs of thesalient and earlier, but less thatdeveloped across a basementreentrant, to achievea definitivepaleomagnetic studies on these units indicate that curvaturecloser to that of the earlierzigzag continental margin earlyacquired magnetizations can be recovered. Oriented outline. sampleswere obtained from nine sites on the southern limb of thesalient and eight sites from the northern limb in the INTRODUCTION Bloomsburg.The naturalremanent magnetizations are multivectorial,dominated by a component(B) with a A testof the oroclinehypothesis [Carey, 1958] wasa major distributedspectrum of unblockingtemperatures ranging up to impetusof earlypaleomagnetic studies in theAppalachians. 670øC,and a component(C) with a higherand very discrete Irvingand Opdyke [1965] foundthat the mean declination in distributionof unblockingtemperatures. The B componentis theUpper Silurian Bloomsburg Formation, sampled at a uniformlyof reversepolarity, shows a statisticallysignificant localityon the northernlimb of thePennsylvanian salient, was synfoldingcharacter, and represents a Late Paleozoic 30ø moreclockwise than the declinationdirection reported by remagnetization.The C componentpasses fold tests with Graham [1949] from the Middle SilurianRose Hill Formation normaland reverse polarity site means. The C component sampledat a localityon thesouthern limb. The subsequent, directionsfrom the southernlimb (345.1'>/-31.6ø) andthe morecomprehensive study of theBloomsburg by Roy et al. northernlimb (359.3o/-29.7ø) aresignificantly different in [ 1967] soughtto obtainpaleomagnetic data in tshesame rock declination(14.2ø+_10.4 ø) but not in inclination(1.9ø+9ø). unitfrom samplingsites distributed around the salient. The Sampleswere also analyzed from seven additional sites in the four additionalsites (A, B, C, D) from the northernlimb Mauch Chunk on the southem limb of the salient. Inclusion of confirmedthe directionobtained from the one site (P) studied thesenew data gives a revisedestimate of thedifference by Irvingand Opdyke (Declination/Inclination = 005ø/-30% between southern and northem limb mean directions of a95=10ø, N=5). However,it wasrecognized that most of the prefoldingmagnetizations in the Mauch Chunk of 23.3ø+_12.5 sitesfrom the southernlimb wereseverely overprinted by in declinationand 4.8'+_11 ø in inclination.Paleomagnetic data Permianmagnetizations, and only one sitefrom thisarea gave whatwas tentatively interpreted as a Siluriandirection (336ø/ -37ø) after thermal demagnetization to 550øC. Thedifference Copyright1988 in paleomagneticdeclination of 29ø betweenthe southern and by the AmericanGeophysical Union. northernlimb wasagain compatible with at leastpartial Papernumber 8T0214. bendingaround the salient, but no strongconclusion could be 0278-7407/88/008T-0214510.00 reached because of the limited data from the southern limb. 7 5 0 Kent: AppalachianOroclinal Rotation Shortlythereafter, the paleomagnetic study of theLower 79 ø 78 ø 77 ø 76 ø CarboniferousMauch Chunk Formation by Knowlesand Opdyke[1968] provided what has long appeared to be strong evidenceagainst an orocline origin for thePennsylvania salient.Paleomagnetic vectors of apparentlyprefolding origin wereisolated in 23 samplingsites distributed around the salient,and these directions were found to be virtuallythe sameon bothlimbs. The generalconclusion of thisstudy was morerecently amplified [Schwartz and Van derVoo, 1983]by a combinedstatistical analysis of thenavailable paleomagnetic datafrom four other Appalachian rock units (Upper Ordovician JuniataFormation, the Silurian Rose Hill and Bloomsburgformations, and the UpperDevonian Catskill Formation).Schwartz and Van derVoo [1983]and Eldredge etal. [1985] concludedthat thesedata are consistentwith a primaryorigin for the largearcuate structural trends in the Appalachiansand that the orocline hypothesis must be abandoned for this mountain belt. In responseto mountingcontroversy regarding the Paleozoicreference paleopoles for cratonicNorth America [Royand Morris, 1983;Irving and Strong, 1984] we have undertakenrenewed paleomagnetic investigations of several criticalAppalachian rock units. It hasnow been demonstrated N thatthe paleomagnetic directions previously reported from the ß..• South limb CatskillFormation [Kent and Opdyke, 1978; Van derVoo et al., 1979]and in particular,the Mauch Chunk Formation .. [Knowlesand Opdyke, 1968] are seriously contaminated by secondarycomponents acquired during or afterthe Alleghenianorogeny in theLate Carboniferousand Permian / \ .or, [ 'n• •' trend [Kentand Opdyke, 1985; Miller and Kent, 1986a, b]. The pre- ! : : '•LB • i I (O) Alleghenianfolding magnetizations now isolated in theMauch Chunkand Catskill moreover reveal a discrepancyin [Sitebedding declinationof 15ø-25ø acrossthe Pennsylvania salient, and thesenew data reopen the case for oroclinalbending as a contributingcause for thislarge-scale structural feature. An objectiveof thepresent study was to seeif supportive evidencefor rotationaround the Pennsylvania salient could be obtainedin a renewedpaleomagnetic study of the Bloomsburg Fm. Bloomsburg.Samples from additional sites in thesouthern Fig. 1. (a) Map showingpaleomagnetic sampling sites of limb of exposureof theMauch Chunk were also taken and BloomsburgFormation (outcroptrace from Hoskins[ 1961]). analyzed,to refine the statisticalconfidence limits of the data Someof the presentsampling sites are at the samelocalities reportedby Kent andOpdyke [1985]. sampledby Roy etal. [1967], specifically,sites A, B, C correspondto localityK; siteD to localityI; sitesE andF to BLOOMS BURG FORMATION localityH; sitesG and H to localityG; siteP to localityD; site Q to localityC; andsites R andS to localityB. (b) Polesto GeologicSetting and Sampling beddingfor Bloomsburgsampling sites. At its typelocality in centralPennsylvania the Bloomsburg is largely red shalebut includessandstones to the southwest whereit attainsa maximumthickness of 2000 feet (600 m) fromsites M andN forreconnaissance). The sites represented [Hoskins,1961]. The Bloomsburgred bedsgrade into marine bedsof variousdips to permit a foldtest. The mean bedding depositsof Niagaran(Middle Silurian) and Cayugan (Upper strike,calculated from a great-circlefit topoles to bedding,is Silurian)age to the westand southwest,losing their red color N80øEfor thenorthern sites and N36øE for thesouthern sites, in the Virginias [Dennison,1982]. thedifference (44 ø) reflecting the change in structuraltrend The samplingstrategy was similar to thatemployed by aroundthe salient(Figure lb). Roy etal. [1967], and in fact manyof their siteswere reoccupied(Figure la). Orienteddrill coresamples (three to PaleomagneticResults sevenper site)were obtained from 17 sites,nine sites(A-I) from the southernlimb of the salient,and eight sites from the The naturalremanent magnetization (NRM) of the northernlimb (J-S,excluding the singlehand samples taken Bloomsburgwas anticipated to be multivectorial,and mostof Kent:Appalachian Oroclinal Rotation 7 51 a) b) 620ø 660 ø 670 ø W, UP W, UP 5O( 640 ø 400 ø • N 300 ø 05•'"'""---O-.• 675ø 0.1 A/m 0.1 Aim •0 o Q-7As85o200•665o 100 ø S-1 NRM NI:WI• E, DN c) d) W, UP W, UP F-4 N __ 625j 67øøI 350o 500• 0.1A/m t - -0.01 Aim ••,•00 ø 100 o NRM / 675ø670ø •) NI:WI E, DN E,DN Fig. 2. RepresentativeZijderveld [1967] diagrams of progressivethermal demagnetization of NRM of Bloomsburg red bedsfrom (a)-(b) northernand (c)-(d) southernlimbs of salient. Open (solid)symbols plotted on vertical (horizontal)planes in geographiccoordinates; treatment levels in Celsius. the sampleswere subjectedto progressivethermal observationsof Irving and Opdyke[1965]. In contrast, demagnetizationat a minimumof 8 to as manyas 30 stepsto samplesfrom the southernsites tend to havea reversed isolatecomponents of NRM. Alternatingfield polarityC component,with an appreciableto dominant demagnetization(to 100 mT) provedineffective because of contributionfrom the B component(Figure 2c) as previously very high coercivitiesassociated with hematite. Chemical observedby Roy et al. [1967]. Two sites(B andF), however, demagnetization(one sample per sitein 8N to 10N HC1 for up do showa normalpolarity C component(Figure 2d). to 1000 hours)showed poor evidence for component Leastsquares fits to the lineardemagnetization trajectories separationas observedpreviously in someother Appalachian for the B and C componentsin eachsample were madeby red beds [Miller and Kent, 1986a]. principalcomponent analysis [Kirschvink, 1980]. Only siteK Aside from an initial
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