A successful rock magnetic technique for ... - Wiley Online Library

3 downloads 0 Views 1MB Size Report
Mar 10, 1997 - tensity and hence inclination shallowing in a rock. Jackson et al.'s [1991] .... demagnetized at 30 mT, were chosen because they did not con-.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. B3, PAGES 5193-5205, MARCH 10, 1997

A successfulrock magnetictechniquefor correctng paleomagnetic inclination shallowing:Case study of the Nacimiento Formation, New Mexico K. P. Kodama Department of EarthandEnvironmental Sciences, LehighUniversity,Bethlehem, Pennsylvania

Abstract. The accuracy of aninclination shallowing correction technique wastestedon the remanence of thePaleocene NacimientoFormation.The correction assumes thatchanges in paleo-

magnetic inclination duringdeformation will bedirectlyrelatedtochanges in remanence anisotropy. Theremanence anisotropy is alsoa functionof themagnetic anisotropy of theindividualmagneticgrains.TheNacimiemo Formation wasselected for thistestbecause its inclinationis shallowby 7øto 8øandprevious paleomagnetic studies indicatethatit hasa primary remanence with a well-constrained ageandthatmagneticoverprints areminimal.Tectonics arean unlikelyexplanation foritsshallow inclination. Theseconditions allowtheinclination-corrected remanence to becompared to a well-defined paleomagneti½ poleto determine theaccuracy of the technique. Thecharacteristic remanence from20 Nacimiento siteswasisolated byalternating field andthermaldemagnetization andwascorrected usingtheanisotropy of anhysteretic remanence measured for threeto five samples persite.Individualparticleanisotropy wasdetermined by dryinga magnetic separate/epoxy mixture in a 35-50mTmagnetic field.Laboratory compaction of disaggregated Nacimiento materialwasalsousedtoyieldaneffective individual magnetic particle anisotropy. Thetwotechniques resulted in similarvalues forindividual particleanisotropy andsimilarinclinationshallowingcorrections. Bothcorrections weresuccessful, andthe inclination-corrected formationmeandirectionwasindistinguishable fromthedirectionpredicted

byNorthAmerica's Paleocene paleopole. Theinclination shallowing correction technique reported hereshotfidbeappliedroutinelyin paleomagneti½ studies, provided thattheremanence is a primarydepositional remanence thathasbeenaffected onlybysyndepositional orearly postdepositional processes (e.g.,compaction). Introduction

The paleomagnetic inclinationmeasuredin sediments and

Recently,workershaveattemptedto developrock magnetic techniques to identifyand correctfor inclinationshallowing in rocksand sediments. Jacksonet al. [1991] cardedouta pio-

rocks is crucial to tectonic interpretationsof paleomagnetic data, sinceinclinationis the primaryrecordof paleolatitude.It is also importantto paleosecularvariationand polarity transition studiesof the Earth'smagneticfield becauseanomalously low inclinationsare usedas evidencefor geomagneticexcursionsor to characterizefield behaviorduringexcursionsor po-

neeringstudyin which they deriveda theoreticalrelationship betweenthe intensityof anisotropyof anhystereticremanence (AAR) and inclination shallowing.This relationshipdemonstratesthat the magneticanisotropyof the individual magneticparticlesis a major factorcontrollingmagneticfabricintensityand henceinclinationshallowingin a rock.Jacksonet larity transitions.Previousexperimentalwork has attempted al.'s [1991] work was basedon a theoreticaldevelopmentreto delineatethe probability,magnitude,and mechanism of in- lating remanenceanisotropyto susceptibilityanisotropyin a clinationshallowingcausedby a laboratoryanalogof burial rock [Stephensonet al., 1986]. Both of these studiesassume compactionin syntheticsediments.Studiesby Anson and that the magnetic grain orientationdistributionin a rock or Kodarna [1987], Dearner and Kodarna[1990], and Sun and sedimentis initially nearly uniformand that anisotropy(and Kodama [1992] have indicatedthe importanceof clay content inclination shallowing) developmentfollows grain orientain causingabout 10ø of inclinationshallowingduringa vol- tion distributiondevelopmentas the rock is deformed.Kodama ume loss of about 50%. At least four studies of natural marine

sedimentshave observed inclination shallowing in clay-rich sediments(>50% clay) [Morgan, 1979; Kent and $pariosu, 1982; Arason and Levi, 1990; Tarduno, 1990], indicatingthat

and Davi [1995] derived curves from Jackson et al.'s theoretical relationships that relate the amount of inclination shal-

lowingto the anisotropydegree(Kmax/Kmin, the maximumand minimum principal axes of the anisotropyellipse) in a rock compaction-causedinclination shallowingdoes occur in na- through the individual particle anisotropy, a (Figure 1). ture. At least one other study of marine sedimentsindicates Jacksonet al. [1991] determineindividual magneticparticle that clay contentmay not be the only factor controllingincli- anisotropy by measurementsof a magneticextract from a rock. This information is then used to correct for inclination nation shallowing[Celaya and Clement,1988]. shallowing. To derive their theoretical relationship,Jackson et al. Copyright1997by theAmericanGeophysical Union. [1991] assumethat the magneticparticledistributionin a sedimentwill developa preferredorientationin a regularway durPapernumber96JB03833. ing increasingcompaction.Kodama and Sun [1992] testedthis 0148-0227/97/96JB-03833509.00 5193

5194

KODAMA: CORREL•ING

=4/3

PALEOMAGNEFIC

INCLINATION

SHALLOWING

transportyet carriesan anomalouslyshallowinclination.The unit which fits this descriptionis the PaleoceneNacimiento

/

Formationof northwestern New Mexico.We will applyboth Jacksonet al.'s [1991] rockmagneticapproachand Kodama and Davi's [1995] compaction experimentapproachto obtain an inclinationshallowingcorrection.Sincethe correctincli-

a=3

nationmaybedetermined fromtheapparent polarwanderpath for NorthAmerica,we mayobjectively evaluate the accuracy of thesetechniques.

Paleomagnetism of the Naclmiento Formation o o

0.1

0.2:

0.3

0.4

0.5

0.6

0.?

ln(Kmax/Kmin)

The palcomagnetism and the magneticmineralogy of the NacimientoFormationhavebeenextensively studied[Butler et aL, 1977;Lindsayet aL, 1978; Taylor and Butler, 1980; Lindsay et al., 1981; Butler and Lindsay, 1985]. The

Figure 1. Theoreticalcurvesrelatingremanenceanisotropy Nacimiento Formation lies in the San Juan Basin of north(Kmax/Kmin) to inclinationshallowing(ln(tanlo/tanlc))derived by Kodama and Davi [1995] from Jacksonet aL's [1991] theory. Different curves result from different individual particle anisotropies,a.

westernNew Mexico(Figure2). It consists of flat-lying,terrestrialclaystones, shales,siltstones, andsandstones exposed in badlands and cliffs. It has well-documented vertebrate fau-

nas, indicatingit is early and middle Paleocenein age. assumption using laboratory compaction experiments of a synthetic clay-rich sediment.These experimentsindicate that the assumptionmay not hold in the very early stagesof compaction, when void ratios are high. For this reason, Kodama and Davi [1995] correctedthe anomalouslylow inclinationof the CretaceousPigeon Point Formation turbiditesof coastal California by disaggregatingand compactingmaterial from the Pigeon Point Formation in the laboratory. Inclination shallowingand AAR were measuredduringthe progression of the compactionexperiments,and the experimentallyderived relationshipbetweenthe two was usedto correctthe characteristic remanences of samplesbasedon theirAAR intensity.The resultsof this study suggestthat almost half of the 25ø inclination shallowingobservedfor thesesedimentaryrocksis due to burial compaction and the remainder is due to tectonic transport. The compaction experiment approach used by Kodarnaand Davi [1995] is quite time consuming,and most workers correctingfor inclination shallowing have used the simplerAAR measurementstrategyof Jacksonet al. [1991]. Hodych and Bijaksana [1993] have used AAR intensitiesto study inclination shallowing in marine limestones, and Hodychand Buchan[1994] haveusedanisotropyof isothermal remanentmagnetizations(AIR) to correctanomalouslylow inclinations

in

Silurian

redbeds

from

Althoughthe Nacimientocropsout in variouslocalitiesin the San JuanBasin,we have chosento samplethe unit at Kutz

Canyon,wherethemostcomplete sectionis exposed. A series of studies hasestablished a detailedmagnetostratigraphy for the San Juan Basin sedimentaryrocks, including the NacimientoFormation[Butleret al., 1977;Lindsayet al., 1978; Taylor and Butler, 1980;Lindsayet al., 1981;Butler

ß Farmington Kutz Oanyon

I

I



25km

KutzCanyon

Newfoundland.

Stamatakoset al. [1994] have criticizedHodychand Buchan's approach,sincethey believe that AIR inadequatelydetermines the magneticfabric in all the remanence-carryinghematite grains.Stamatakoset al. [1995] have subsequentlyprovided detailed evidence that the anomalouslyshallow inclinationsin the Silurianredbedsfrom Newfoundlandare due to magnetic overprintsdominatingthe remanencein coarse-grainedsand-

Angel Peak

stones.

The objectiveof this studyis to derivea reliablerock magnetic techniquethat can identify and correct for inclination shallowingin rocksand sediments.Moreover,it is important that this techniquebe relativelyeasy to conduct,so that it will become a routine reliability test for palcomagneticstudies. Our approachwill be to study a formationwhich is well-constrainedin age, has been demonstratedto have minimal magnetic overprinting, and has no geologic evidence for tectonic

,pling locality

Route44

I

3kin

I

Figure 2. (top)Map indicatinglocationof Kutz Canyonin northwestern New Mexico. (bottom) Location map of sampling localities in the Nacimiento Formation. Each locality includes more than one site. The southernmost

sampling localityis neartheradioantennae tower(seetext)

KODAMA:

CORRECTING

PALEOMAGNETIC

25

Kutz Canyon --700

--600

•60

my

---65

my

--500

--400

Nacimiento --300

Format

ion

INCHNATION

SHALLOWING

5195

Nacimiento for a paleopoledetermination.These sites, each consistingof threespecimenswhich were alternatingfield (af) demagnetized at 30 mT, werechosenbecausetheydid not contain directionsof intermediatepolarity (more than 40ø from the mean normal or reverseddirections)or did not havec•95 greaterthan40ø. Forty two of the 104 siteswere normalpolarity, 62 were reversed.The reversedand normal polarity meandirectionsare within 1.5øof beingantipodal,eachmean being well within the 95% cone of confidenceof the other mean (normalct95=5.4ø;reversedct95=3.6ø). Thesestriking resultsindicatethat magneticoverprintsdo not greatlyaffect the primaryremanencein theserocksand thataf demagnetization has donea goodjob removingany magneticoverprints that do exist. Since the age of the NacimientoFormationhas been well-constrainedby magnetostratigraphy, its mean direction(I=51.3ø, D=343.9ø, K=21.9, c%5=3.0 ø) may be compared with confidence to the direction calculated from the

--200

-lOO

--70

my

Cande

and -o

Kent

meters

CompositeSan

Juan

(1992)

Basin

Paleocenepaleopolefor NorthAmericadetermined by Diehl et al. [1983] from Late Cretaceous-early Tertiary igneouscomplexes in the north-central Montana alkalic province (I=58.1ø, D=351.1ø for a site at 36.5øN,108øW).Theseigneousrocks have been well-datedby both K-At and fission trackdatingand give an age of 60-69 m.y. for the paleopole [Diehl et al., 1983]. The Nacimiento-igneous complexpaleopole comparison indicates that the inclination of the

NacimientoFormationis anomalouslyshallowby about7ø.

Although possible tectonic rotationsof the Colorado Plateau Figure 3. Butler and Linsday's[1985] magnetostratigraphy [Hamilton, 1981, 1988; Bryan and Gordon, 1986, 1990; for the NacimientoFormationshowingits correlationto their Steiner, 1986, 1990; Kent and Witte, 1993] could affect the composite San Juan Basin magnetostratigraphyand the declinationof the NacimientoFormation,theycouldnot affect GeomagneticPolarityTimescale[Candeand Kent,1992]. Tick its inclination. Tectonics cannot be an explanation for the marks indicate the stratigraphiclocationof the 10 sites colshallowinclination,sinceit is unlikelythat this interiorporlected in the Nacimiento which produced characteristic tion of North Americanhas movednearly800 km northward remanencesfrom both af and thermaldemagnetization. since the Paleocene.

These paleomagneticresults indicate that the Nacimiento Formationis the ideal unit for testingrock magnetictechand Lindsay, 1985]. This magnetostratigraphy indicatesthat niquesthatcouldbe usedto correctfor inclinationshallowing. studywas conductedon the the NacimientoFormationexposedin Kutz Canyonmay be First, a small-scalepaleomagnetic Nacimiento to reproduce the mean directiondocumented by correlatedwith anomalies27 and 28 of the Geomagnetic previouswork [Butlerand Taylor, 1978]. Both thermaland af PolarityTimescale[Lindsayet al., 1981;Butlerand Lindsay, demagnetization were used,as well as modernprincipalcom1985; Cande and Kent, 1992] (Figure 3), indicatingthat it is ponentanalysistechniquesfor demagnetization data analysis from 62 to 64 m.y. in age. The magnetostratigraphy devel[Kirschvink, 1980], sincethe previousNacimiento Formation oped for the NacimientoFormationnot only establishesa data. well-constrained age for the unit,so thatits remanence may be paleopolewas only basedon blanketaf demagnetization The AAR of af demagnetized samples was measured, and the inaccuratelycomparedto remanence directionscalculatedfrom the appropriatepart of the North American apparent polar dividual magneticparticleanisotropieswere determinedeither of magneticseparates[Jacksonet al., 1991] wanderpath,it alsoindicatesthattheremanence mustbea pri- by measurement or by compaction experimentswith disaggregated Nacimiento mary, most likely, depositional remanence. As part of their

magnetostratigraphic studiesof the NacimientoFormation, Butlerand Lindsay[1985]alsodetermined themagnetic mineralogyof the unit.Derailedstrong-field thermomagnetic behavior,microprobeexamination, andX ray analysisof magneticseparates indicatethattheremanence is carriedby a detrital titanohematitewith compositionsbetween 0.45