Jan 10, 1992 - JOHN A. TARDUNO 1 , WILLIAM LOWRIE 2, WILLIAM V. SLITER 3,. TIMOTHY J. BRALOWER 4 AND FRIEDRICH HELLER 2. Paleomagnefic and ..... R. ticinensis at 50.05 m identifies the base of the R. ticinensis. Zone. Calcareous ..... Interval III, Intermediate NRM Unblocking Temperatures. As in some ...
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. B1, PAGES 241-271, JANUARY 10, 1992
ReversedPolarity CharacteristicMagnetizationsin the Albian Contessa Section,Umbrian Apennines,Italy: Implications for the Existence of a Mid-CretaceousMixed Polarity Interval JOHNA. TARDUNO 1, WILLIAMLOWRIE 2, WILLIAMV. SLITER 3, TIMOTHY J. BRALOWER4 AND FRIEDRICH HELLER2 Paleomagneficand paleontologicdata from the Valle della Contessa(Umbrian Apennines,Italy) spana 14 m.y. gap in previousmagnetostratigraphic sectionsand reveal severalintervals of reversed
characteristic remanentmagnetization (ChRM) in limestones andmarlsof theAlbian Marnea Fucøidi. Rock magneticdata indicatethat the reversedChRM is carriedby hematitehaving high unblocking temperatures (,.•625-690øC).Hematitewith theseUnblockingtemperatures alsocarriesa normalpolarity ChRM in otherintervals.A potentialbiastowardsthe identificationof normalpolarityis presentsinceat every horizonmeasureda normalpolarity component,carriedby magnetiteand hematite,is isolatedat lower unblockingtemperatures (,.•300-600øC).Bulk magneticpropertiesvary throughoutthe measured section. In the portion containingmost of the reversedChRM intervals, hematiteis the dominant carrierof naturalremanentmagnetization, while magnetiteis the dominantcarrierstratigraphically above and below. These changescorrespondapproximatelywith lithology. The zone of dominanthematite magnetization is markedby reddishbands(pigmentaryhematite)which can be correlatedthoughout Umbriaand are thoughtto recordchangesin the oxidationstateof the seafloorduringdeposition.Two modelsmayaccountfor theobserved magnetization directions.The reversedmagnetizations mayrecord a remagnetization whichoccurredat least18 m.y. afterdeposition, while thebedswerestill flat-lying,in reversedpolaritychron33R (83-79 Ma) or later. If thereversedChRM represents a remagnetization, the processis potentiallyof greatimportancesinceit can producemagnetizationpatternswhich resemble polarityintervals. Alternatively,the reversedmagnetizations may have been acquiredduringintense
seafloor oxidation episodes ruing themid-Albian(107-104Ma) andmayrecordunrecognized intervals of reversedgeomagneticfield polarity. Milankovitch-likebeddingcyclicitycan be usedto tune ttie sedimentation rate and obtainestimatesof the durationof the potentialreversedintervals.Using these estimates,two of the potentialreversedpolarityintervalsare of sufficientduration(> 100 kyr) to be recognizablein bothdetailedstratigraphic sectionsand marinemagneticanomalysurveys.A primary magnetizationmodel predictsthat severalintervalsof reversedmagnetizationshouldbe found near the boundaryof the Biticinella breggiensisand Ticinella primula foraminiferal zones and within the Prediscosphaeracretacea nannofossilzone on a worldwide basis.
INTRODUCTION
as a gap of approximately 14 m.y. interrupts the Cismon section[Charmellet al., 1979a]removingupperAptian-upper Two magnetostratigraphic
Our knowledgeof the mid-Cretaceousgeomagneticreversal Albian sediments(Figure 2).
chronology, originally derived fromtheinterpretation of sections fromtheUmbrian Apennines andonefromthe
marinemagneticanomalies[Larsonand Pitman, 1972; Larson
SouthernAlps partiallyfill this gap. When paleomagnetic andHilde, 1975],hasbeencontinnedby magnetostratigraphic sampling hasbeendense(i.e., 1 specimen/50 cm or greater), study of uplifted Tethyanmarine sequences[Lowrie et al., brief reversedpolarityintervalshavesometimes beenfound, 1980a,b; Lowrie and Alvarez, 1977].
These studies have
definedtwo reversedpolarity zones,corresponding to chrons 33R and M0, boundingan ,-,34-m.y.-longinterval of normal polarity,the CretaceousNormal Polarity Superchron(K-N). The lack of reversalswithin the K-N Superchronlimits our attemptsto correlatethe strikinggeologicalcharacteristics of these mid-Cretaceoussections,such as periods of oceanic anoxia, which have been of broad interest to the geologic community. One of the most thoroughmagnetostratigraphies defining the mid-Cretaceous time scale is that derived from the Gubbio-
reversedintervalswhich are not includedin most geomagnetic time scale syntheses[e.g., Hadand et al., 1982; Kent and Gradstein, 1985, Hadand et al., 1990].
Paleomagneticsampling of the Valdorbia section [Vandenberget al., 1978; Lowde et al., 1980b] of the Umbrian Apenninescovered,-•2 m.y. of the lower part of the gap (upper Aptian) and definedone brief reversedpolarity interval.
Recently thiszone,theISEA reversed polarityinterx;al, has been shown to correlate on a worldwide basis [Tarduno et al.,
1989; Tarduno,1990]. Only one magnetostratigraphic section
in theApennines covers themajority of themid-Cretaceous
Cismoncompositesection(Figure 1) [Lowrie et al., 1980b]. samplinggap (that of middle-lateAlbian age), the Poggio le Yet this compositesectionhas its limitationsin coverage Guainesection[Lowrieet al., 1980a]. Sampling,however,
wasextremely sparse (sevensamples in 25 m) sincethe
Scripps Institution of Oceanography, La Jolla,California.
2Institut fiirGeophysik, ETH-H6nggerberg, Ziirich,Switzerland. emphasisin that study was on Early Cretaceousmagnetostratigraphy. Paleomagnetic samplingof the Cerro Veronese 3U.S.Geological Survey, MenloPark,California. 4Department ofGeology, University ofNorth Carolina, Chapel Hill. sectionfrom the SouthernAlps (northernItaly) [Charmellet al., 1979a] covered a 7-m interval of the gap with 14 hand
samples, butmiddleAlbian-upper Albiansedihaents werenot
Copyright 1992 by the American GeophysicalUnion.
sampled. Two reversedpolarity intervals were found in the Valle del Mis sectionof the SouthernAlps [VandenBerg and
Papernumber91JB02257. 0148-0227/92/91 JB-02257505.00 241
242
TARDUNOET AL.: REVERSEDPOLAmTY MAGNETIZATIONS, ALmAN CONTESSA
12* 33'
have occurredalong the southernmarginof the Tethyson a
12* 34'
promontoryof the African Plate known as Adria [Channell
et al., 1979b]. Subsequentclosure of the Tethys and deformationrelatedto the formationof the Apennineshave exposed a sequence of pelagicrockshavingidealproperties for magnetostratigraphic analysis[Lowrie and Heller, 1982]. A magnetostratigraphic type sectionfor the Upper CretaceousPaleocenehas been establishedin this sequencein the Bottaccionegorge near Gubbio [Alvarezet al., 1977]. In
43*23'
ALBIAN
CONTESSA:
SECTION
Umbria, the middle to late Albian is represented by the middle to upperportionof the Marne a Fucoidi (alsoknown as the Scistia Fucoidi or FucoidMarls), a seriesof marls and marly limestones punctuated by occasional blackshales.The
43* 2
marls take their name from "Fucoidi", the local name of the
ichnofossil Chondriteswhichis abundantin theformation[de
BoerandWonders, •1984;Tornaghiet al., 1989]. The marls gradeupwardsinto theupperAlbianScagliaBianca,a white '0 [• 43'21'
to grey limestone.
1 km Quaternary
:r-•'•Marnoso Schlier & arenacea, Bisciaro ß.[-'•Scaglacmerea •] Scagliarossa& bianca
5•-• Fucoid marls
r•r3•Majolica
/
Perugia •/ Osteria delGatto
TheMarnea Fucoidi,whichrestsonlowerAptianMaiolica Limestone, hasbeendividedinto thefollowingsix members [Coccioni et al., 1987,1989,1990]: (1)the Greenish-Grey ChertyMember; (2) the Lower ReddishMember; (3) the BrownishClayeyMember;(4) the Greenish Marly Member; (5) the UpperReddishMarly Member;and (6) the Whitish Marly LimestoneMember. The spectacular changesin sedimentcolordisplayedby the Marne a Fucoidihave been
Fig. 1. Locationmap, Albian Contessasection,UmbrianApennines, interpretedas an indicatorof the oxidationstateof the seafloor Italy. duringdeposition [ArthurandFischer,1977;Tornaghiet al.,
1989]. Locally,detachment thrusts havebeendeveloped in the Marnea Fucoidi[Alvarezet al., 1978]. Togetherwith weathering, thesefaultsrendermanyoutcrops unsuitable for
Wonders, 1980] from siliceouslimestonesthought to be of middle-late Albian age. Recent reexaminationof this section, magnetostratigraphic analysis. however,has suggestedthat most of theselimestonesmay be In July1989,52 paleomagnetic coresand20 handsamples much older (E. Erba, personalcommunication,1990). were collectedin a sectionthrough the Marne a Fucoidi In additionto the lack of a thoroughmagnetostratigraphicin the Valle della Contessa, approximately 2 km northwest section in mid-Albian Tethyan sediments,other data sets of the Bottaccione gorge[Tarduno,1989] (Figure1). The indicate that our picture of mid-Cretaceousgeomagnetic section wasmeasured in thesouthern cement quarrywherean polarity may be incomplete. Paleomagneticdata from unusually freshfacewasexposed by thequarrying operation. sediment cores recovered by Deep Sea Drilling Project
The face sampledis apparentlythe sameillustratedby
(DSDP) leg 26 in the Indian Ocean were interpretedby Coccioniet al. [1989, pp. 572-573, illustrationD] in their
Green and Brecher [1974] and Jarrard [1974] as evidence
studyof theSelliLevel,a prominent earlyAptianblackshale
for severalbrief intervalsof reversedpolarity in the Albian. interval which has recentlybeen shown to correlatewith Based on this interpretation, van Hinte [1976] defined a similardeposits in the Pacific[Sliter, 1989; Tardunoet al., "site 263" mixed polarity interval in the mid-Cretaceous. 1989]. In the ContessaQuarry,the Selli intervalis involved Hailwood et al. [ 1980] interpretedpaleomagnetic data from in a zoneof local structural complexity whichmay have the Bay of Biscay (DSDP leg 40) as representingseveral formedby backthrusting or slumping[Coccioniet al., 1989]. "short-periodreversal events" in the Albian and correlated Our section wasmeasured fromthebaseof twoprominent these with the "site 263" mixed polarity interval. More thickerbedded marls(labeled20 m) stratigraphically above recently,Urrutia-Fucugachi[ 1988] has interpreteddata from this disturbed interval. The sectionextendsthroughan a preliminary magnetostratigraphic study of the Morelos interval of reddish marls which can be further subdivided into Formation(southernMexico) as recordinga reversedpolarity tworeddish bandsapproximately 1-1.5m thick,separated by "event"
in the Late Albian.
4-5 m of greenish marls.The rocksgenerally become more
To fill the samplinggap in the Tethyansectionsand to induratedupsection.The sectionendsin the transitionfrom evaluatethe existenceof a mid-Cretaceous mixed polarity the Marne a Fucoidito the ScagliaBiancaLimestone.The interval, new paleomagneticand paleontologicspecimens intervalsampledcorresponds to members4 to 6 of Coccioni were collected from the Valle della Contessa of the Umbrian
et al. [ 1990].
Apennines(Figure 1). This locality is well known for its Many of themarlsweredifficultor impossible to collect lithostratigraphy [e.g., Coccioniet al., 1989], cyclic sedi- by drilling.Therefore, ourcollection preferentially sampled mentation[Schwarzacher and Fischer, 1982] and Paleogene the most indurated marl or limestone beds. The section was magnetostratigraphy [Lowrie et al., 1982]. recollected in October 1989to improve thesampling density. LITHOsTRATIGRAPHY AND PALEoMAGNETIC SAMPLING
In addition, samples werecollected in thesamestratigraphic horizonto checkthe lateralpersistence of directions.For
Depositionof the Cretaceousstratigraphic sequenceof the muchof the intervalof interest, the stratigraphic sampling Umbrian Apennines[Bortolottiet al., 1970] is thoughtto intervalis roughly1 sampleper 30 cm.
TARDUNOET AL.: REVERSEDPOLARtrYMAGNETI7JkTIONS, ALBIAN CONTESSA
CRETACEOUS
AGE
MAGNETIC
Mßy.
POLARITY•o 30
-
31•
•
75--' 80
66.5
•
< 31
60
• I
' Z 27
33•
i 34
-
i
84•
•
-1O0 --
Abathomphalus mayaroens•s
LA.mayaroens•s
F A.mayaroens•s
Globotruncana aegyphaca Globotruncanella havanens•s
FG.ganssen
F G.aegypt•aca I.G. calcarata
Globotruncan•ta calcarata
FG.calcarata
Globotruncamta elevata
LD. asymetrica
Dicannella asymetnca
Dicarinella concavata
FD.asymetnca
::D 21
Helvetoglobotruncana helvetica
L H.helvehca
•
Rotalipora cushmam I
88.5"'•'" 22
Mar•t,notruncana s,.½al,
FD.concavata
91•z 20 Wbtem•,lla archaeocretacea FH.helvetica < b D•cannella algenana LR.cushmani 19
< 718 z•
I
-
24
87.5r,r',,•23
•
-
25
I
I•
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DATUM MARKERS
F G. ventncosa
I
90--
ZONES - SUBZONES
z• 26 Globotruncanaventncosa •
•
95--
KS
z
< 29 < 28 745 :•
-
-
c•
• 30 Gansser, nagansser,
32•
-
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BIOSTRATIGRAPHY
PLANKTONIC FORAMINIFERS
•:
•
_
-
AND
z•
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70--
SCALE
•> LU
IN
-
TIME
243
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I I •
975 ro 17 16 15
I
Rotahpora re•cheh a
.
F D. algenana
Rotahpora greenhornensis R.cushmani F(R / LR.reicheli greenhornensis F R. re•cheh
Rotahpora brotzem Rotahpora appenmmca Rotahpora hcmens•s
FR.brotzen• FR.appennmica
-_:i:i:i:!:i:i:i:!:i: I i:i:!:i:i:i:i:i:i:i: J:!::::::::::: b[:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: Rotahporasubt, c,nens,s F onens,s :i:!:!:!:i: :i:!f:Ji!:!: i:!:i:i:i:i'"•i•i:&,½•'r."'.:•'•i•i:i::::: FR.t, R. subt, onens,s
- i:i:i:i:i:i:i:i:i:i:i:i:i:i:!:i:i:i:i:i:i:!.•. :i:i :i:!:i:i:!:i:i :i•i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:!:!:i:!:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:]:!:i:i:!:i:i:i:i:i:i:!:i:i:i:i:i:i:!:!:!:i:i:!:i:i:i:!:!:i T.praet, c,nens,s _ :::::::::::::::::::
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110
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• MO[•1
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-
•
119
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. II 5
rr
I
4
L L.cobh FL.cabri
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F H. s•m•hs
MI•
- M3• I 124 ;575øC) of reversedpolaritywere as that carrying the Brunhes field direction. In the rest observed fromsomehorizons nearintervalII. As opposed to of the section, the "Matuyama" componentis isolated at generallybetween150ø and intervalII, this reversedcomponent appearsas a muchless higherunblockingtemperatures, intensecomponent compared to thenormalpolaritycompo- 300øC. Fine-grainedhematiteis postulatedas the mineral nent. As in intervalII, thisreversed polaritycomponent was carrierand the origin of magnetizationis tentativelyidentified clearlydefined(i.e., demagnetized) only afterthe removalof as VRM. If the "Matuyama" componentis also carried by a normalpolaritycomponent whichextendedto unblocking higher unblockingtemperaturehematite, as suggestedby a temperatures as high as 575øC. In contrastto intervalII, we few samples(e.g., Figure 10d, 36.52 m, 325-400øC), an
remanent magnetization is morelikely. believethis high unblocking temperature reversedpolarity originby chemical componentis carriedby hematite. SUMMARY OF PALEOMAGNETICAND ROCK MAGNETIC DATA
TABLE 2. Matuyama Overprint Directions in Geographic Coordinates
Brunhes Field Direction
All threestratigraphic intervalswithin the sectioncontain Sample(m) a low unblockingtemperaturecomponentof NRM with ' MF(27.65) MF(30.40) in situ directioncloseto the Brunheschron (0.73-0 Ma) MF(33.45) field (Figure16). Detailedthermaldemagnetizations (25øC MF(49.55) steps)were appliedto only selectsamplesin the lowest MF(50.05)
MF(48.25) MF(47.35) the component removedduringthisdetaileddemagnetization MF(39.10)
unblockingtemperaturerange (0-200øC). The inclinationof
wasfit by leastsquares linefits.Thefactthatthiscomponent is oftensteeperthanthe Brunhesfield perhapsindicates its
See Table
Te•nperature Range,øC n 75.0-125.0 3
D, deg I, deg 207.5 -45.7
75.0-125.0 75.0-125.0 150.0-250.0 150.0-300.0 200.0-300.0 200.0-300.0 175.0-250.0
191.4 198.9 183.1 179.9 210.9 210.9 212.3
3 3 3 4 3 3 4
-36.7 -36.2 -62.0 -59.5 -62.5 -67.7 -51.4
1 footnotes.
Fig. 15. Rockmagnetic datafromcharacteristic samples of rockmagnetic intervalIII (38.20-50.05 m) of theAlbian Contessa section.(a) Progressive acquisition of an isothermal remanent magnetization in fieldsto 5 T. (b) Thermal demagnetization of a composite IRM acquired alongtwoo•xhogonal directions in fieldsof 0.15T and5 T. (c) Volume magnetic susceptibility versus thermal demagnetization temperature forspecimens usedin IRM acquisition experiments
(Figure15b). (d) Orthogonal vector plotsof progressive thermal demagnetization of natural remanent magnetization shown in geographic (insitu)coordinates. Circles, inclination; crosses, declination. Demagnetization steps (in degrees Celsius) shown: 45.65m, 0, 100,150,200,250,300,350,375,400,425,450,475,500,525,550;47.05m, 0, 100,150,
200, 250, 300, 350, 375, 400, 425, 450, 475, 500, 525, 550.
256
TARDUNO ET AL.: REVERSEDPOLARtrY MAaNETt7_•T•ONS,ALBIAN CONTESSA
A
Contessa
TABLE 3. Normal Characteristic Directions in Geographic Coordinates
Sample(m)
Temperatm'e Range,øC n
D, deg I, deg
MF(20.14) MF(21.00) MF(22.77) MF(23.40) MF(23.90)
300.0-350.0+0 250.0-350.0+0 200.0-400.0+0 250.0-350.0+0 300.0-400.0+0
4 5 8 5 6
279.2 275.2 279.8 279.1 279.4
21.1 30.8 32.3 23.0 24.9
MF(24.60)
300.0-350.0+0
4
278.4
34.2
MF(25.43) MF(25.97) MF(26.66) MF(27.65) MF(30.40) MF(30.85) MF(30.97) MF(31.95)
200.0-300.0+0 250.0-550.0+0 200.0-400.0 200.0-425.0+0 225.0-325.0+0 250.0-350.0+0 250.0-350.0+0 275.0-625.0+0
4 13 7 10 6 5 6 16
274.3 285.4 287.6 276.0 282.3 280.6 283.5 279.4
28.6 27.1 36.2 32.7 23.9 24.0 28.0 33.2
MF(32.20) MF(32.40)
2oo.o-55o.o 275.0-575.0+0
13 14
263.6 274.6
1.4 38.8
MF(32.75)•':[: 300.0-565.0 MF(35.35)}$ 250.0-350.0 MF(35.36)f• 25o.o-35o.o
12 267.5 20.3 4 279.3 32.0 4 287.7 19.2
MF(36.32) MF(36.63) MF(36.92) MF(37.40) MF(37.50) MF(37.75) MF(37.90) MF(38.14) MF(38.52)
18 15 19 12 18 19 20 13 14
266.7 263.7 283.0 277.7 266.4 274.8 265.9 291.3 274.5
19.9 8.3 25.8 7.1 24.5 20.4 21.9 23.9 23.0
MF(39.10)• 300.0-400.0 MF(39.36)• 300.0-400.0 MF(39.48):[:325.0-450.0 MF(39.88):[:325.0-450.0
5 5 6 6
259.7 287.7 286.1 269.3
38.3 34.8 37.7 44.5
MF(40.23) MF(40.33) MF(40.45) MF(40.57) MF(40.77)
325.0-550.0+0 300.0-550.0+0 325.0-425.0+0 300.0-500.0+O 250.0-350.0+0
270.7 275.4 278.7 272.0 281.1
40.6 36.8 31.6 30.9 33.6
MF(41.10) MF(41.30) MF(41.50) MF(42.50) MF(42.85) MF(43.25) MF(43.75) MF(44.15) MF(44.65) MF(45.05) MF(45.30) MF(45.65) MF(46.00) Fig. 17. Stereographic projectionof normalpolaritycomponents isoMF(46.25) lated from the Albian Contessasectionshownin geographic(in situ) eoordinates.(a) Normalpolaritycharacteristic remanentmagnetiza- MF(46.65) MF(47.05) tions(ChRM) from samplescontaining only a normalpolarityChRM (N=61,Tabl•3):'(b)Normal polarity directions obtained fromsamples MF(47.35) containing a reversedpolarityChRM isolatedat unblocking tempera- MF(47.65) tures above 600øC (Table 7). MF(48.25) MF(48.65) MF(49.15) MF(49.55) Normal Polarity CharacteristicDirection MF(50.05)
325 0-425.0 250.0-375.0 350.0-400.0+O 300.0-550.0 250.0-400.0 300.0-550.0+0 250.0-350.0 250.0-350.0 250.0-425.0 250.0-375.0 300.0-550.0+O 300.0-550.0+0 300.0-550.0+O 350.0-550.0 300.0-525.0+O 250.0-375.0+0 350.0-550.0+O 350.0-550.0+0 350.0-525.0+0 350.0-525.0 350.0-500.0+0 300.0-550.0+0 300.0-550.0
11 12 6 10 5 5 5 4 10 5 11 3 3 6 4 11 11 11 9 10 5 10 10 9 8 8 12 11
277.4 285.9 282.9 272.6 290.0 274.8 283.0 296.5 286.8 297.8 279.7 280.0 276.8 281.2 295.2 269.8 281.1 278.1 268.3 257.8 268.4 275.9 262.5
27.1 43.1 31.8 3.3 14.7 25.5 25.3 13.4 20.8 9.5 14.0 37.3 25.6 20.9 22.2 17.0 18.4 36.8 38.2 46.2 47.9 24.8 21.5
MF(36.30) 300.0-600.0
B
-(- ;.?.
Contessa
+ t t I
300.0-670.0 300.0-625.0 250.0-650.0+0 300.0-575.0 300.0-660.0 300.0-660.0 200.0-660.0 300.0-600.0 300.0-625.0
1• 274.5 23.9
All samplescollectedyieldeda normalpolaritycomponent See Table 1 footnotes. O, origin used in fit. of magnetization with unblocking temperaturesexceeding I component doesnottrendtowards origin. thoseof the inferredlate Tertiary-recentcomponents discussed • sample excluded in finalpolaritycolumn. above.After tectoniccorrection(strike=297.9 ø, dip=27.2øN),
this componerit is similarto the Cretaceous normalpolarity characteristic direction obtained throughout the Umbrian Apennines (Figure 17) (Tables 3-7). This component, however,is carriedin differentunblockingtemperatureranges and by differentmineralsthroughoutthe section. In interval
I the component is isolatedat temperatures between150ø and 450øC and is carried by magnetite. In interval II the component is carriedbetween150øCandan unblocking temperature rangingfrom 350ø to 680øC, dependingon
TARDUNO ETAL.: REVERSED POLAmTYMAGNETIZATIONS, ALSrnNCONTESSA
257
TABLE 4. Bed A ReversedCharacteristicDirectionsand Normal Overprintsin GeographicCoordinates Reversed
Normal Overprint Sample A5.0 MF36.48 A8.23 A9.04 A10.00
ChRM
Meter
Temperature Range, øC
n
/9, deg
1, deg
Temperature Range, øC
n
D, deg
i, deg
5.00 5.45 8.23 9.04 10.00
300.0-425.0 300.0-350.0 300.0-425.0 300.0-425.0 300.0-375.0
6 3 6 6 4
251.2 276.1 263.2 263.1 266.8
6.6 23.0 21.8 11.3 7.4
600.0-650.0-t-O 575.0-650.0-t-O 600.0-660.0-t-O 600.0-650.0-t-O 600.0-680.0-t-O
5 5 6 5 8
152.4 120.0 146.8 133.8 152.1
-35.3 -38.4 -35.4 -55.8 -46.1
See Table 1 footnotes. O, origin used in fit.
(N=61).Thelatterfail•hecolatitude testbutpass the the particulai' sample;the dominant carrierappears to be polarity
hematite. Alternatingfield demagnetization experiments, longitudetest,suggestinga departurefrom Fisherianbehavior however,suggestthat magnetitealso carriesthe normal different from the causeof the nonantipodalcharacterof the polaritycharacteristic direction.In intervalIII, thecomponentnormaland reversedpolarity directions.
is mainlyisolatedbetween300ø and580øCandis carried mainly by magnetite.
An UnbiasedPolarity Column
To examinethe stratigraphicsignatureof the intervalscarrying the reversedpolarityChRM, we haveplottedinclination and declinationof the highestunblockingtemperaturecomReversed polaritycharacteristic directions were foundat ponent isolatedversus stratigraphicposition for the Albian 24 horizonsthroughoutthe sectionand were isolated(i.e., Contessasection(Figure 19; this sequenceis derived from demagnetized) only after applyingthermaldemagnetization Tables3 and 4). Caution is advised,however,in interpreting above575øC (Figure 18, Table 6). We believe that all this plot (Figure 19) as a reliable sequenceof ChRM polarity of these directions are carried by hematite which has a zones. Becausea normal polarity magnetizationis carried relativelydiscreteset of unblocking temperatures, generally between 625ø and 690øC. Most intervals having reversed by each horizon measured,a potential bias exists in the recognitionof intervals carrying reversed polarity ChRM. polaritycharacteristic magnetizations arefoundin intervalII, but one horizon was found within interval I. In addition, Specifically,a weak reversedpolarity ChRM may be masked by a strongnormal polarity component. Therefore, some of interval III containsnumeroushorizonsclose to the boundary the changesin ChRM polarity between 30 and 40 m may be with intervalII whichrevealedreversedpolaritycharacteristic artifacts.
ReversedPolarity CharacteristicDirection
directions.
Statistical The
Treatment
isolated
directions
were
examined
with
colatitude and longitude tests of Fisher et al.
the formal
[1987] to
determine whether the distributions were Fisherian [Fisher,
If a reversedpolarity ChRM is being maskedby a normal polarity component,upon demagnetizationthe normalpolarity componentshoulddivergefrom the origin. Samplesfrom two zonesbetween 30 and 40 m (32.75-35.36 m and 39.10-39.88 m) displayedsuch behavior. Unfortunately,samplesfrom thesezonesexhibiteddirectionalinstability at relatively low unblockingtemperatureslikely related to chemicalchanges in the rocks due to heating. It is difficult to determine in such sampleswhether a reversedpolarity ChRM carried by hematite is present. There[ore, to limit any bias in our polarity column, we have marked these intervals as of uncertainpolarity (see Table 3). We have excluded these samplesto obtain our final polarity column (Figure 20; this sequenceis derived from Figure 19 by deleting samples from 32.75-35.36 m and 39.10-39.88 m). If some of these uncertainintervalsare actually of normal polarity, there are more polarity intervals than indicatedin the final polarity column (Figure 20).
1953] (Table 11). These tests reveal that the hypothesis that the reversedpolarity distribution(N=24) and the total normalpolaritydistribution(N=85) are Fisheriancan not be rejectedat the 95% confidencelevel. The meansof these distributions,however,are not antipodal(A= 152ø) (Tables 8 and 9) (Figure 18). The invertedinclinationsare similar (61 = 4ø), but the inverted declinationsdiffer substantially (6D = 34ø). One directionappearsto be transitionalbetween the two polaritygroupsas discussed furtherbelow. When examinedseparately, the hypothesis that the normal polarity"overprints"on the sampleshavinga reversedpolarity characteristicremanent magnetization(N=24) are Fisherian Thepolaritycolumnshowstwomajorintervalsof reversed alsocannot be rejectedat the 95% confidencelevel (Tables 10 and 11), but the hypothesiscan be rejected for the polarityalongwith severalshorterintervals.Each reversed distribution derived from samples displaying only normal interval,with the exceptionof VC-1R (29.25 m), is defined
TABLE 5. Bed B Reversed Characteristic Directions and Normal Overprints in Geoga'aphicCoordinates
Normal Overprint Sample B5.0
MF35.86 B6.27 B6.75 B7.77 B12.30
ReversedChRM
Meter
TemperatureRange,øC
n
L), deg
1, deg
TemperatureRange,øC
n
L), deg
1, deg
5.00 5.48 6.27 6.75 7.77 12.30
300.0-425.0 300.0-475.0 300.0-425.0 300.0-425.0 300.0-425.0 300.0-425.0
6 8 6 6 6 6
267.2 251.2 272.2 279.2 265.6 270.6
33.5 15.9 29.2 27.3 15.1 12.9
600.0-650.0-t-O 600.0-625.0F 625.0-650.0F 600.0-660.0-t-O 600.0-680.0-t-O 625.0-680.0-t-O
5 3 3 6 8 7
138.2 123.2 148.8 131.0 134.4 129.5
-40.8 -56.3 -35.2 -29.1 -46.1 -49.6
See Table 1 footnotes. O, origin used in fit; F, Fishe•ian average.
258
TARDUNO ETAL.:REVERSED POLARtrY MAONETIZAT•ONS, ALB•d'•CONTESS^
TABLE 6. ReversedCharacteristicDirectionsin Geographic Coordinates
Saxnple (m) MF(29.35)
Temperature Range, øC' n 525.0-575.0+0 4
MF(31.54) MF(31.60) MF(31.70) MF(33.45) MF(34.40) MF(34.55) MF(35.75)
625.0-680.0+0 600.0-625.0+0 600.0-670.0+0 575.0-650.0+0 615.0-650.0+O 600.0-650.0+0 600.0-640.0F
MF(31.23)
600.0-650.0F
4
BEDB(35.86)t 600.0-680.0+0
6
BEDA(36.48) • 575.0-680.0+0
5
MF(36.05) MF(36.13)
600.0-640.0+0 600.0-640.0F
6 3 7 5 5 5 3 5 3
MF(36.52) MF(37.05) MF(37.30) MF(38.20) MF(38.42) MF(38.69) MF(38.94) MF(39.00) MF(39.62) MF(39.70)
600.0-670.0+0 575.0-650.0 600.0-655.0F 600.0-660.0+0 600.0-670.0 600.0-670.0 600.0-650.0 600.0-670.0 575.0-600.0+0 600.0-640.0+0
7 4 6 7 6 6 4 6 3 4
,MF(40.03)
600.0-640.0
3
SeeTable 1 footnotes. O, origin usedin fit; F, age.
netizationshiftedtowardhigherunblockingtemperatures as a
reversalboundarywasstratigraphically approached. Second, the highestunblockingtemperaturecomponentcarried in D, deg I, deg hematiterecordeda fieldreversalat stratigraphic levelsbelow 129.6 -67.9 thecorresponding reversalrecorded by magnetite.Basedon 41.0 -43.8 theseobservations,Channellet al. [1982] developeda model 121.1 -40.6 of hematitemagnetization in theUmbrian 123.7 -46.9 for the acquisition 125.1 -46.2 limestones.Hematitewas interpretedas a diageneticproduct 118.7 -54.7 growingfroma goethite precursor. Thenucleation of hematite 126.1 -57.2 wasviewedas a continualprocess,penetratingdownto some 134.8 -47.6 conditions no longer 130.2 -43.3 depthat whichthe localenvironmental 134.9 -43.1 permittedthe formationof new hematite.In any collection 117.4 -20.7 of hematitegrains,the largestgrainswould be the oldest. 119.6 -34.1 Sincein single-domain hematite,grainsizeis directlyrelated 141.4 -42.9 to unblockingtemperature,the largestgrainsare also those 152.5 -48.6 grainshavingthehighestunblocking temperatures. Hematite 107.1 -23.8 acquire 145.4 -36.5 grainshaving very high unblockingtemperatures very early, earlier than the acquisition 114.7 -52.5 their magnetizations 146.0 -53.8 of a postdepositional remanentmagnetization by magnetite. 135.9 -46.3 Thesevery high unblockingtemperature grains,therefore, 130.2 -48.5 are the best representation of a "primary"magnetization in 103.2 -59.3 the Umbrian rocks. Progressively smaller hematite grain 126.1 -24.5 97.8 -28.3 with correspondingly lower unblochngtemperatures carry a 132.3 -33.3 progressively later magnetization. Basedon the datafrom the Scaglialimestones of the Gubbiosection,hematitecan Fisherianavernucleatedownto a depthasgreatas60 cm (aftercompaction)
t Average ofspecimens frombedB (Table5). $ Average ofspecimens frombedA (Table4).
andtherefore cancarrya magnetization asmuchas105years
olderthanthe "primary"high-temperature hematitesignal. Followingthe modelproposedby Channellet al. [1982] (we emphasizethat here, and in the followingdiscussions, by morethanonesa•nple.One sample(31.23m) appears we assumethe grainsare single domain)the high unblockto recorda transitionaldirectionon the boundaryof VC-2R. ing temperature hematitethat carriesthe reversedpolarity Faultsin the sectioncould have producedan unrecognized magnetizations in the Albian Contessasectioncouldbe the overlapin the section,and thereforeone or more of the oldestandpossiblythemostreliableindicatorof geomagnetic shorterintervalscouldbe a samplingartifact. It is unlikely that the two longerintervals,VC-3R and VC-7R, are the duplicationof a singlezone. TABLE 7. Normal Overprint Directions in Geographic Coordinates
TIMING OF MAGNETIZATION
Sasnple(m)
The observedzones of reversedpolarity characteristic MF(29.35) magnetizations arelaterallypersistent, suggesting thattheyare MF(31.23)
recordings of reversedgeomagnetic polarityintervalsduring deposition of theContessa section.Sincethereversed polarity magnetization may be carriedsolelyin hematite,however,a remagnetization modelmay alsoexplainthe observed data. Recentwork in North America has focusedon the pervasive
nature of Paleozoicremagnetizations now thoughtto be
MF(31.54) MF(31.60) MF(31.70) MF(33.45) MF(34.40) MF(34.55) MF(35.75)
TemperatureRange,øC
n
D, deg
I, deg
250.0-400.0
7
275.3
33.1
200.0-350.0 200.0-350.0 300.0-350.0 200.0-350.0 225.0-300.0 300.0-350.0 250.0-350.0 300.0-425.0
5 5 3 5 4 3 4 6
292.1 274.6 291.2 262.6 270.2 267.1 281.0 270.9
38.3 13.0 13.7 21.0 32.2 30.1 22.4 45.1
the consequence of fluid migrationdue to tectonicactivity. Could the reversedpolaritymagnetizations observedin the
MF(35.86)B300.0-475.0
65 267.5 22.5
Contessasection be the result of remagnetization? Below
MF(36.05) MF(36.13)
4 5
MF(36.q8)A300.0-350.0
we assess thesetwo hypotheses (primarymagnetization and MF(36.52) remagnetization) for the originof the reversed polarity MF(37.05) characteristic remanent magnetizations in theAlbianContessa MF(37.30)
MF(38.20) MF(38.42) MF(38.69) Model1: PrimaryReversed PolarityMagnetizations MF(38.94) MF(39.00) Channellet al. [1982] examinedthe relativetiming of hematiteand magnetitemagnetizations in the Gubbiosec- MF(39.62) MF(39.70) tion, located2 km SE of the Contessa section. Samples MF(40.03) section.
werecollectedcloseto previouslydetermined polarityzone
boundariesin Paleocene and CretaceousScaglia limestone.
Channellet al. [ 1982]madetwoprincipalobservations. First, theunblocking temperatures carrying a reversed polaritymag-
See Table
25O.O-35O.O 300.0-400.0 300.0-350.0 300.0-500.0 250.0-350.0 250.0-375.0 250.0-375.0 250.0-350.0 250.0-400.0 300.0-450.0 300.0-425.0 325.0-475.0 300.0-475.0
269.3 279.4
17.3 33.6
55 263.9 14.1 3 9 4 5 5 4 6 7 6 7 8
253.3 276.1 288.3 282.4 281.8 287.8 274.3 271.3 273.1 272.0 282.6
1 footnotes.
• Fisherian average ofspecimens frombedB (Table5). • FisherJan average ofspecimens frombedA (Table 4).
22.7 23.9 18.9 26.4 25.3 26.8 17.4 34.2 28.8 33.3 30.6
TARDUNOET AL.: REVERSEDPOI•ARrrv MAGNETIZA•ONS, At3tAN CONTESSA
A
259
TABLE 8. Averaged Directions for Geographic Coordinates
Contessa
Average
n
I, deg
D, deg
k
c•9s
1 Brunhes ove•l•rint 2 Matuyama overprint
8 8
66.4 -53.3
358.6 199.2
81.9 32.7
5.5 8.7
3 Normal
61
26.9
277.8
40.5
2.8
4 Normal overprint
ChRM
24
26.3
275.3
46.8
4.2
5 Normal total (3+4)
85
26.8
277.0
41.3
2.4
6 Reversed
24
-45.4
123.7
18.3
6.7
ChHM
ß
See Table
ß ßvV vVVv
thermaldemagnetization between500ø and 575øC (interval II) is observed,suggestingthe removal of multiple (normal andreversedpolarity?)components (Figures10, 12, and 13). There are reasons,however,to expectdifferencesbetween
ß 4-
ß
B
1 footnotes.
Contessa
the Gubbio Scaglia limestoneand the Marne a Fucoidi. The high unblockingtemperaturehematite magnetizationsin the Contessasectioncoincideroughly,althoughnot exactly,with two reddishbandsin the outcrop(Figure 21). These reddish marls can be correlateddirectly to other sectionsin Umbria suchas that recoveredby drilling nearPiobicco[PremoliSilva et al., 1983; Pratt and King, 1986; Herbert et al., 1986; Erba, 1988]. The reddish color is likely a pigmentaryhematite, having small grain sizes and reflected in our magnetic measuresashematitewith low unblockingtemperatureranges (i.e., that hematite in the coercivity fraction magnetizedat 0.15 T in interval II). This pigmentaryhematitemay reflect the oxidationstateof the seafloorat the time of deposition. Interbedded
within
the reddish
marls
are occasional
black
shales[Tornaghiet al., 1989], indicatinga rapid shift between highly oxidizing and highly reducingconditions.In contrast, the Scaglia limestone studied by Charmell et al. [1982] mostlikely recordsdepositionundermore uniform oxidation conditions.
We hypothesizethat the reversedChRM at Contessamay be carriedby hematiteacquiredduringtwo intenseoxidation eventsas evidencedby the two reddishmarl units. Magnetite andfine-grainedhematitemay recordmagnetizations acquired slightly after deposition(i.e., normal polarity overprintson sampleswith reversedChRM). This hypothesisalso implies that the hematitecould have grown down to a depth of at least60 cm [Charmellet al., 1982] during the later oxidation event, possiblyoverprintinga portionof interval II (Figures 19 and 21). We note that many greenish-grey bedscarry a reversedpolarity magnetizationcarriedby hematite. The only otherreversedpolarityzone identifiedin Umbria strata depositedduring the K-N Superchronis the ISEA et al., 1978]. This Fig. 18. Stereographic projectionof normal- (Tables3 and 7) and interval of late Aptian age [VandenBerg reversed-polarity(Table 6) directionsfrom the Albian Contessasec- interval is also associatedwith reddish marls, the "couche tion. (a) Geographic(in situ) coordinates.(b) Stratigraphiccoordi- rouge", and has a magnetization dominatedby hematiteas nates(after lilt correclion). Solid circlesrepresent95% confidence denoted by IRM acquisition data [Lowrie et al., 1980a]. intervals. Also shownis the invertednormalpolarity confidenceinterval for comparisonwith the confidenceinterval for the reversed The primary magnetizationmodel outlined above fails to accountfor this coincidence,other than by suggestingthat polarity directions.
hematite bearing zones are more reliable recordersof short
polarityintervalsthan are highly reducedpelagicfacies(i.e., field polarity duringinitial deposition.Importantly,this high unblocking temperaturehematite carries both normal and reversedpolarity magnetizations. Unlike the data from the Scaglia limestone of the Gubbio section [Charmell et al., 1982], our data from the Albian
Contessasection do not exhibit a clear shift toward higher unblockingtemperatures as a reversalboundaryis approached. Instead, complex directional behavior during progressive
TABLE 9. AveragedDirectionsfor Stratigraphic Coordinates Average
n
I, deg
D, deg
k
61
32.5
293.3
40.5
2.8
4 Normal overprint
24
33.1
290.7
46.9
4.2
5 Normal total 6 Reversed ChRM
85 24
32.6 -36.6
292.6 146.7
42.3 18.3
2.3 6.7
3 Normal
See Table
ChRM
1 footnotes.
c•9s
260
TARDUNOET AL.: REVERSEDPOLARtrY MAGNETIZATIONS, ALB/AN CONTESSA
TABLE 10. Lateral Consistency Test, Geographic Coordinates Average Bed A normal overprint Bed A reversed
ChRM
n
Mu
Pass? ME
Pass? I, deg
D, deg
k
c•95
5
0.883 0.867 0.815 1.122
yes yes yes yes
yes yes yes yes
263.9 141.4 267.5 134.9
48.3 35.4 43.5 47.8
9.0 10.5 8.7 8.3
5
Bed B normal overprint
6
Bed B reversed
6
ChRM
0.566 0.728 0.838 0.642
14.1 -42.9 22.5 -43.1
See Table 1 footnotes. M•r, ME after Fisher et al. [1987]. Yes, hypothesisthat distribution is Fisherima ca,mot be rejected at 95% confidence.
magnetitemay be produced as an authigeneticmineral in reducedfacies and hencerecord a later magnetization).There is not a one-to-onecorrelationbetweenhematiteand polarity in the Albian Contessasection(nor, apparently,in the couche rouge); hematite carries both normal and reversedpolarity magnetizations.
associatedwith specificpathwaysfor fluid flow and specific diageneticevents.
How applicableare the studiesin Paleozoic carbonates to our new paleomagnetic resultsfrom the Albian Contessa section? First, we should note that while the Paleozoic
examplesdiscussedare carbonates,thereare greatdifferences betw•n these rocks and the Umbrian pelagic limestone Model H: RemagnetizedReversedPolarity Magnetizations sequence.The Paleozoicexamplesare from shallowmarine The possibilitythat hematiteis the sole carrierof reversed facies (e.g., subtidal,intertidal,and supratidallimestoneand polarity characteristicmagnetizationssuggeststhat modelsfor dolomite), while the rocks from the Umbrian sequencethat results are wellthe precipitation and magnetizationof hematite long after have yielded reliable magnetostratigraphic cemented pelagic limestones. The general difference betw•n depositioncould also explain thesedata from the Valle della the two rock types is notable since conduits for fluid flow Contessa. An analogy for such a remagnetizationscenario might be found in the numerousstudiesof North America might be more restrictedin the micritic Umbrian rocks. While there do semn to be important differences in Paleozoiccarbonateswhich documentoverprintsacquiredup carriedby to 150 m.y. after depositionduringthe Permo-Carboniferousgrosslithology,the mechanismof remagnetization hematite in the North American Paleozoic carbonates may be ReversedPolarity Superchron(PC-R or Kiaman). While the magnetizationin many of these carbonatesappearsto applicableto the Contessasection. Basedon the analogy we can developthe following be carried in two distinct grain size fractionsof magnetite to Kiaman remagnetizations, [Jackson, 1989], examples of remagnetizationdirections scenario. Oxidizing fluids flowed through the Marne a Fucoidi at Contessaduring a reversedpolarity chron, at carriedby hematitehave alsob•n noted(Table12). In manyof thesestudieswherehematitecarriestheremag- least 18 m.y. after deposition,causingthe precipitationof netization,texturalrelationshave providedstrongsupporting hematitewhich acquireda reversedpolarity magnetization. evidence for the creation of a CRM by precipitationof Fluid flow affectedonly a portion of the measuredsection hematite during fluid flow. For example, Elmore et al. and occurred along specific beds while the section was [1985] found a Kiaman remagnetizationcarriedby hematite flat-lying. If acquiredduringreversedpolaritychron33R, the wouldhave occurredat a subbottomdepthof whichtheysuggest precipitated dueto infiltrationof oxidizing remagnetization fluids that caused dedolomitization. In a review of Kiaman at least 200 m, basedon the compactedoverburdencalculated remagnetizations, McCabeand Elmore[1989] concluded that from the Moria section [Alvarez and Lowrie, 1978]. The remagnetizationscenariois appealingsince it seemthe fluids related to hematite precipitationcould have b•n betweenhematiteand basinalor meteoricand that when present,remagnetizations ingly explains(1) the correspondence carriedby hematiteare not necessarilypervasive.Remagne- reversedpolarity magnetizationsand (2) the nonantipodal tized rocksare sometimesfoundproximalto nonremagnetized characteristicsof the normal and reversedpolarity ChRM rocks with differencesperhapsrelated to the state of ce- sincethe two polaritiescould be acquiredin differenttime menration of the rock [McCabe and Elmore, 1989]. Rather intervals. In fact, the reversedpolarity ChRM data agree than being pervasive,the secondaryhematiteappearsto be more closelywith data from polarity chronsyoungerthan the
TABLE
Average
1l.
Tests for Fisheran
Distribution
n
M t•
Pass?
ME
Pass?
Brtmhes
8
Matuyama
8
Normal ChRM geographic Normal overprint geographic Normal total geographic Reversed ChRM geographic Normal ChRM stratigraphic Normal overprint stratigraphic Normal total stratigraphic reversed ChRM stratigraphic
61 24 85 24 61 24 85 24
1.307 0.840 1.056
no yes yes
1.000 0.847 1.425
yes yes no
0.779 1.095
yes yes yes yes yes yes yes
0.730 1.197
yes no yes no yes yes yes
1.183 1.052 0.777 1.070 1.191
0.915 1.413 0.722 1.005 0.913
See Table 1 footnotes. Mu, ME after Fisher et al. [1987]. Yes, hypothesisthat distribution is FisherJan cannot be rejected at 95% confidence. No, hypothesis that distribution is FisherJancan be rejected at 95% confidence.
TARDUNOET AL.: REVERSEDPOLARtrY MAGNETIZATIONS, ALmAN CONTESSA
A
261
B
5O
45
•
30
25
2o
-80
-40
0
40
80
200 100 Declination ø
Inclination ø
300
300
400
500
600
700
Max. Temperature øC
Fig. 19. Normal-andreversed-polarity (a) inclinations and(b) declinations versusmeterlevelin theAlbianContessa
section. (c)Themaximum unblocking temperature used toobtain thedata(Figures 19aand19b).
A
B
C
5O
45
•
40
VC-R7
E
VC-R6 VC-R5 VC-R4
VC-R3
VC-R2
VC-R1
25
2O I
-80
-40
0
40
Inclination o
80
100
I
I
200
Declination
I
300
o
Fig. 20. Unbiasedpolaritycolumnfor the Albian Contessa sectionobtainedby omittingsamplesfrom rock magnetic intervalsII andIII havingundefined directions at highunblocking temperatures (seetext andTable3). Stippledregionis of uncertainpolarity due to lack of samples.
262
TARDUNOET AL.: REVERSEDPOLARITYMAGNETIZATIONS, ALBIAN CONTESSA
Green-greymarlswith black shales
Key:
Reddish
marls
• Grey-whitemarlsandlms [:.'.v:::.':.'l with blackshales
Fig.21. Interpreted polarity versus stage, gross lithology, andage.Absolute ages afterSliter[1991].
TABLE 12. Kiaman RemagnetizationCarried by Hematite in PaleozoicCarbonates Unit
Age
Unblocking Range
Reference
oc
St. George Group.
O
300-530
Taum
C
285-500
KindbladeFormationt
O
194-609
Peerless Fm.
C
670
Sauk Limestone
Deutsch and Prasad [1987], Figure 6 Dunn and Elm ore [1985]Figure 7 Elmore et al. [1985]Figure 3 Peck et al. [1986]Figure 3
'[Alsocontains a component ofunkown orginatunblocking temperatures above 609ø(2; age abbreviations' C, (2ambrian;O, Ordovician.
TARDUNO ETAL.:REVERSED POLARITY ]V[AGNETIZATIONS, ALBIANCONTESSA 360
263
If a remagnetizationoccurredlong after depositionwhen the field had a different directionand proceededlong enoughto includea changein polarity,the normalpolaritydata should be bimodalin declination.Insteadit is azimuthallyuniform.
/• 340 •
320 300
I '•1 IContess• NI I I 34SB
34SR
33R
33
32R
32
31R
I 31
I 280 30R
Chron
DISCUSSION
Both hypotheses (primarymagnetization andremagnetization) fail to account completelyfor the propertiesof the reversedpolarity characteristic magnetizations.The principal deficiencyof both modelsis in explainingthe magnetization of only the largesthematitegrains. In light of thesediscrep-
ancies, several areas merit further discussion. If the zones Fig. 22. Comparisonof declinationvalues from Moria (solid triof reversedpolarity ChRM are primary magnetizationsand angles),Gubbio (solid circles) [Alvafez and Lowrio, 1978] and the are of sufficientduration(>100 kyr), corresponding zones Albian Contessasection(solidlines, this work) shownversuspolarity chron(34SB, averagefrom the ScagliaBiancaof the K-N Superchron; should be found on a worldwide basis. Below we review the 34SR, averagefrom the ScagliaRossa,K-N Superchron).Data from durationof the potentialreversedpolarityintervalsand their adjacentpolarityzonesare not antipodal.Normalpolaritymeandireccorrelationto previousstudies. Assignmentof the reversed tion from the Contessasectionis closeto the K-N Superchronaverage polarity characteristic magnetizations to a post-K-N Superfrom Gubbio,while reversedpolaritymeanagreesbetterwith averages chron magnetizationrelies on the accuracyof the isolated from youngerchrons. Contessa directions
K-N Supercln'on as definedin the Gubbiosection(Figure22). When examinedcritically,however,the model has several deficiencies. The model fails to explain the discrete unblockingrange of hematite carrying the reversedpolarity magnetizations.The resultsfrom Contessadiffer from studies of Paleozoic rocks in that the reversedpolarity ChRM is carried by very high unblockingtemperaturehematite. In most examples of remagnetization,a greater unblocking
and the reference
directions.
Below
we
also review the reliability of the differencesbetween the mean normal and reversedpolarity directionsand Umbrian reference
directions.
Duration of PotentialReversedPolarityInterval
If the zones of reversedpolarity ChRM in the Albian Contessa sectionarerecordings of reversed polarityintervals, we would like to obtain some estimate of the duration
temperature range is affected (Table 12). The model also fails to explain the occurrenceof both normal and
of theseintervals. The reversedpolarity ChRM intervals can be tied to geologicalstageand absoluteage usingthe reversedpolarity magnetizationscarried by the very high micropaleontological data(Figure21). The only complete unblockingtemperature hematiteother than by coincidence: biozonerepresented is theBiticinellabreggiensis foraminiferal someintervalsmustcarrya primarydirectionof only normal zone. The first appearance datum(FAD) of B. breggiensis polarity at very-highunblockingtemperatures,while others (36.48 m) has been estimatedto occur at 105 Ma [Sliter, carry a remagnetization of only reversedpolarityat the same 1991], while the FAD of Rotaliporaticinensis(50.05 m), very high unblockingtemperatures. which identifies the R. ticinensisZone, has been estimated Oneway to explainthe normalmagnetizations at very high at 101 Ma. Theseagesrely on the absoluteage calibration unblockingtemperatures is to suggestthat the remagnetization of biozonesgiven in the Harland et al. [1982] time scale. occurredover sufficienttime to encompass a polaritychange. Therevisedtimescaleof Harlandet al. [1990]incorporates When this refinementis added,however,the remagnetization only minor changesin the relevantportionof the Albian. model can not accountfor the nonantipodalcharacterof the The ages discussedabove suggesta sedimentation rate of normal and reversedpolarity directionsas outlined above. approximately3.4 rn/m.y. for the Albian Contessasection. For example,duringthe growthof the secondaryhematite, Thesesedimentation ratessuggest durations of the polarity the field is of reversedpolarity. Hematitegrowthcontinues intervals recorded bythereversed polarityChRMwhichrange so that the largestgrainsare the oldestand carry a reversed from 973 to 59 kyr (Table 13). polaritymagnetization.The remagnetization occurslong after A secondmethodcan be employedto estimatethe duinitial depositionso the direction of the inverted reversed rationof the Contessa reversed polarityintervals.Cyclic
ChRM differsfromtheprimarynormalpolaritymagnetizationsedimentation has long been recognizedin the Valle della (acquiredduringthe K-N Superchron).At somepoint the Contessa [Schwarzacher andFischer,i982, Figure7]. Anal(remagnetization) field revertsto normalpolarity. In certain yses in the Scaglia Bianca and Maiolica Limestoneshave bedsthehematitegrowthstartsonly afterthe polaritychange recognizedgroupsof four to six beds, termed"bundles", but must proceedlong enoughso that very large grains whichare thoughtto coincidewith the Earth's100-kyr havingvery high unblockingtemperatures are precipitated. eccentricity cycle[Schwarzacher andFischer,1982;Fischer But hematitegrowthshouldbe a continualprocess with small and Schwarzacher,1984; Arthur et al., 1984; de Boer and hematitegrains constantlyforming. Futhermore,from our Wonders, 1984].Thesebundles canalsoberecognized in the rock magneticdata we can infer that a significantamount Marne a Fucoidi(Figure23). of hematitehavingsmallergrainsizesis present.Yet none The most straightforwardestimateof durationof the reof the smaller hematitegrains (having lower unblocking versed polarity intervals recordedin the Contessasection temperatures) has a magnetization predictedby this scenario: wouldbe the numberof bundles per polarityzone. Unfora normal polarity magnetizationwith a declinationlike that tunately, an accurate countwasnot possible throughout the
exhibitedby the invertextreversedpolarityChRM data set. entiresectiondue to small-scale faultingand difficultiesin
264
TARDUNOET AL.: REVERSEDPOLARITYMAGNETI7•TIONS,ALBIAN CONTESSA
TABLE 13. Duration of Potential Reversed Polarity Intervals Event
Range, m
Thickness, m
Duration, kyr Estimate
VC- 1R VC-2R VC-3R VC-4R VC-SR VC-6R VC- 7R
29.35 31.1-31.8 32.9-36.2 36.4-36.6 37.0-37.35 38.17-38.47 38.61- 40.10
indeterminate .7 3.3 .2 .35 .3 1.49
1
Estimate
indeterminate 20t3 973 59 103 88 439
Estimate 1 is basedon paleontological data after Sliter [1991]. Milankovitch-likebeddingcyclicity. access.Instead,we rely on a lesssatisfactoryapproach;estimating a mean sedimentationrate. Slightly over five bundles can be identified per 2-m interval (Figure 23), suggesting a sedimentationrate of approximately4 m/m.y. This rate suggestsdurationsof the Contessareversedintervalsranging from 825 to 50 kyr (Table 13). We urge caution in the applicationof thesedurationsfor
2
indeterminate 175 825 50 88 75 373
Estimate
2 uses
declinationcontrol and thosebasedon inclination-onlydata. The former include most field studies, while the latter in-
clude most studiesof cores recoveredby the DSDP and its successor,the Ocean Drilling Program. Reports of reversed polarity magnetizationsfrom DSDP Albian sedimentsare reviewedin termsof their reliability and possiblecorrelation with intervals of reversed polarity ChRM from the Albian several reasons. There are substantial uncertanties in the Contessasequencein the appendix(Table 14). absoluteagesused for the paleontologicaldata. In addition, In summary, the presence of an Albian mixed polarity minor faults are present and changesin sedimentationare interval based solely on DSDP examples is tenuous and likely within the section. Nevertheless,it appearsthat some explainswhy suchan interval has not been adoptedin recent studies[e.g., Harland et al., 1982; Kent and Gradstein, 1986; of the zonesrepresenttime intervalsgreaterthan 100 kyr. If thesereversedpolarityzonesare primaryrecordersof reversed Harland et al., 1990]. All examplescould be artifacts due geomagneticfield polarity, they would representsubchrons to accidentallyinverted core pieces, core sections,or entire (as opposedto short polarity eventsor "tiny wiggles") not cores. An ^lbian mixed polarity interval has previouslybeen suggestedby van Hinte [1976] using data from DSDP site previouslyrecognizedwithin the K-N Superchron. 263 in the Indian Ocean [Green and Brecher, 1974; Jarrard,
Correlatior• ofPotential Reversed Polarity Intervals Reversedpolaritymagnetizations have beenreportedpreviously from a number of Albian sequences. These data fall into two categories:those having both inclination and
1974]. If real, the "site 263" mixed polarity interval occurs at a slightly later time than those recordedat the Contessa section and records reversed polarity intervals of shorter duration.
Possible
field reversals
recorded
at DSDP
site 400
in the North Atlantic fall close to the Contessaevents(Figure .
Fig. 23. Detail of beddingin theAlbian Contessasection.Groupsof four to six beds,called"bundles",thoughtto reflect the Earth's 100-kyr eccentricitycycle [Schwarzacherand Fischer, 1982], are labeled (black arrows). Slightly over five bundlesare identifiedper 2-m interval (ruler shown).
TARDUNOETAL.: REVERSED POLARITYMAGNETIZATIONS, ALBIAN CONTESSA TABLE
265
14. Possible Brief Reversed Polarity Intervals: K-N Superchron
Event ' Class Stage
Age, Ma
Location
Reference
ISEA 463 402-2R 317 400-5R 402-1R
A A B B B B
Aptian Aptian Aptian AptJan AptJan Aptian-Albian
115 115 115 115 1157 1137
Valdorbia, Italy
400-1R 400-2R 400-3R 400-4R 260-1R 263-1R 263-2R 263-3R 263-4R ZC
B B B B B B B B B A
Albian Albian Albian Albian Albian Albian Albian Albian Albian Albian
98-1047 98-1047 104-1077 104-1077 104-1077 98-1047 98-1047 98-1047 104-1077 98-1047
Lowrio ctal. [1980a] Tarduno [1990] Tarduno [1990] Tarduno [1990] Hailwood[1979] Tarduno [1990] Hailwood [1979] Hailwood [1979] Hailwood [1979] Hailwood[1979] Jarrard [1974] Jarrard [1974] Jarrard [1974] Jarrard [1974] Jarrard [1974] Urrutia-Fucugauchi[1988]
VC-1R
A
Albian
107.0
VC-2R
A
Albian
106.5
VC-3R VC-4R VC-5R VC-6R VC-7R
A A A A A
Albian Albian Albian Albian Albian
105.6 105.0 104.8 104.5 104.2
DSDP
site 463
DSDP
site 402
DSDP
site 317
DSDP
site 400
DSDP DSDP
site 402 site 400
DSDP DSDP DSDP
site 400 site 400 site 400
DSDP DSDP DSDP
site 260 site 263 site 263
DSDP DSDP
site 263 site 263
Zopilote Canyon Contessa. Italy Contessa. Italy Contessa. Italy Contessa Italy Contessa. Italy Contessa Italy Contessa. Italy
this
work this work this
work
this
work this work this
work
this
work
24). Nevertheless, few deep-seasectionshaverecoveredthe From our data and a review of the previousinvestigations biozones represented at Contessa. At present, we areawareof it is not possible to deftnatively ascribe the nonantipodal no paleomagneticstudy(reportingmixed or constantnormal polaritymagnetizations) of a DSDP or ODP site (or on-land section)that has been analyzedin sufficientdetail to serve as a rigoroustestby correlation of theprimarynatureof the Contessa reversed polarityintervals.
natureof the normal and reversedpolarity directionsto any single cause. Unfortunately, both the Contessadirections and the referencedirections(i.e., Gubbio and Moria) may be biased by unremovedoverprints. At present, however, it is reasonable
to assume
that the differences
between
the
normal and reversedpolarity ChRM directionsat Contessa NonantipodalDistributions are exaggerated by unseparated components such as the Theevaluation of whether reversed andnormalpolarity "Matuyama" field overprint.
magnetizations are antipodalis commonlytakenas a consistency testfor the reliabilityof paleomagnetic directions
SUMMARY
AND CONCLUSIONS
[e.g.,Gordon et al., 1984].If suchdataarenotantipodal, it The two models (primary magnetizationand remagnetizais concluded thatan unremoved overprint is present.With tion) proposedto explain the intervalsof reversedpolarity both Brunhesand Matuyamafield overprints identifiedin ChRM isolated in the Albian Contessa section can be assessed the Contessa section,oftenwithina singlespecimen, it is by the following observations:(1) the reversedpolaritymagreasonable to questionthe accuracyof our representationnetizationsare isolated(i.e., demagnetized)only at very high of theCretaeous geomagnetic fielddirection by the thermal unblockingtemperatures;(2) very high unblockingtemperademagnetization data. An argument against thepresence of ture hematitealso carriesnormal polarity magnetizations;(3) unremoved overprints is the lack of noticeable streaking in the reversedpolarity magnetizationsare laterally continuous thenormalandreverseddatasets.Nevertheless, curvature of and the normal and reversed polarity magnetizationsform the demagnetization pathsare sometimesobserved.This is stratigraphic zones;(4) the normalandreversedpolaritydata especiallyapparentin the declinationvaluesusedto define setsare not antipodal(A= 152ø);and (5) the reversedpolarity thenormalpolaritycomponent (Figures 9-14). It is possibleChRM (large grainedhematite)correspondsapproximately thatthermal demagnetization hasnotcompletely removed the with two stratigraphicbands of reddishmarls (pigmentary
effectsof the"Matuyama" fieldoverprint. The questionof how well Cretaceousdata conformto a
hematite).
If the reversedpolarity ChRM is a remagnetization,the
modelof antipodal reversals wasaddressed by Alvarezand processwould have to be one that couldaffectonly a portion Lowrie [1978] in their studyof the Moria section. Most of the stratigraphicsection,alongspecificbedsbeforetilting. of the normalpolarityintervalsstudiedwere foundnot to By analogywith Kiaman remagnetizations,fluids could have
be antipodal to adjacent reversed polarityintervals (Figure flowed through the section long after depositionresulting 22). Demagnetization treatments werenot as rigourous as in the growth of hematitethat acquireda reversedpolarity thoseapplied in thisnewwork(i.e.,onlyAF demagnetization magnetizationonly in those beds which acted as pathways was applied)whichmay explainwhy the Moria data are for the fluids. A candidatefor the time of remagnetization not Fisherian[AlvarezandLowrie, 1978]. In addition, is reversedpolarity chron33R (83-79 Ma). In this scenario, complications maybepresent dueto likelytectonic rotations the reversedpolarity ChRM is not antipodalto the normal duringdeposition [LowrieandAlvarez,1975].Theredoseem polarity directionbecauseit was acquiredat a much later to be trends in the declination data seen both in the Moria
andGubbiosection[Alvarezand Lowrie, 1978].
time.
Scrutiny reveals several flaws in this remagnetization
266
TARDUNO ETAL.'REVERSED POLARITY MAGNETIZATIONS, ALBIAN CONTESSA
N •øa•ø'•a 'd I I
•,
I
-04
I I !løJJ!ø$!a'ml '3
I I
I I I I
o
I
I
ß
!F)JJ!:)s!xrnl 'H
•-qdst. tmld 'H /ulntu.ud'œ-!qos!.• 'H
\
I ,
N
!F)JJ!as.uml ':•
•Inu•.•d 'œ
•
TARDUNOET AL..' REVERSEDPOLARITY MAGNETIZATIONS,ALBIAN CONTESSA
model. If secondaryhematitegrew as described,we should expecta rangeof grainsizes,with the largesthavinggrownthe longest,rangingdown to very small grainswhichjust started to nucleate when fluid flow ceased. The reversed polarity magnetizations,however, are isolated only at very high
267
75
unblockingtemperatures, above600øC,indicatingthat these magnetizationsare carriedonly by the largesthematitegrains. The remagnetization model alsofails to explainthe presence of high unblocking temperaturehematite carrying normal polaritymagnetizations.If a remagnetization,the mechanism must be one previously unappreciatedbut neverthelessof great importancesinceit can producemagnetizationpatterns which resemble reliable recordingsof polarity intervals and can affect only a restrictedrangeof hematitegrain sizes. Alternatively,the reversedpolarity ChRM may have been acquired during intense seafloor oxidation episodesin the Albian (107-104 Ma) evidencedby the reddishsedimentsand may recordunrecognizedbrief geomagneticreversedpolarity intervals. This model is consistentwith the largesthematite grains carrying the most accuratemeasureof field polarity since they are the first to nucleate, as has been observed in detailed studies near polarity transitions [Charmell et al., 1982]. The nonantipodalcharacter of the reversed and normal polarity directionsmay be due to unremoved components,especiallya "Matuyama"field overprint. If a primary magnetization,the reversalsrecordedin the Contessa section imply a substantialrevision of the mid-Cretaceous geomagneticchronology(Figure 25). Some of the reversed intervals appear to be of short duration (100 kyr or less) which
would
account for their lack of resolution
Campanian 80
33R
Santonian
85
Coniacian Turonian
90
Cenomanian
95
in marine
magneticanomaly records. Two intervals are longer ()100 kyr), however, and should be identifiablein both detailed stratigraphicsection and marine magnetic anomaly surveys. A candidate in the seafloor record is a marine magnetic anomalylabeled35R identifiedby Ladd [1974] in the South
100
Albian
Atlantic.
An obviousdeficiencyof the primary magnetizationmodel is the apparentcorrelationof reversedpolarity ChRM with hematite. Therefore, a remagnetization scenario can not be excluded
until
the reversals
identified
at Contessa
are
identified elsewhere. The primary magnetization model predicts that several intervals of reversed polarity magnetization shouldbe found near the boundaryof the Biticinella breggiensisand Ticinella prirnula foraminiferal zones, within the Prediscosphaeracretaceanannofossilzone of mid-Albian (107-104 Ma) age. Note added in proof. Recently a paper by Hambach and Krurnsiek [1989] has become available describing a series of reversedpolarity intervalsof Albian and Cenomanianage from the study of drill cores in the Ruhr area (western Germany).Unlike the stratigraphicrange studiedat Contessa (middle Albian), the Cenomanianhas been sampledin detail in severalsectionsfrom the UmbrianApenninesand Southern Alps [e.g. Alvarez and Lowrie, 1978; Charmellet al., 1979a].
Contessa R ChRM Series
105
110
Aptian
ISEA
115
The report of Hambachand Krurnsiek[1989] is surprising since in these previous studies no intervals of reversed polarity have been identified in Cenomanian-agesediments.
M0
The only exampleof Cenomanianreversedpolarityintervals Barremian 120 comesfrom the Quero-Schievenin section[Vandenberg and Wonders,1980], but this sectionmay have complications due to slumping (H. Thierstein, personal communication, Fig. 25. PossibleMid-Cretaceous PolarityTime Scale.Absoluteages 1989). Interestingly,Hambach and Krurnsiek[1989] describe after Hadand et al. [1982]. ISEA after Tarduno [1990]. Contessa the mineralcarrierin their studyas having a high coercivity reversed ChRM series, this work.
268
TARDUNO ET AL.: REVERSEDPOLARITYMAGNETIZATIONS,ALBIAN CONTESSA
and low unblocking temperature. Such low unblocking temperaturesin the Albian Contessasectionwere found to record an alternationof normal "Brunhes"field overprints and reversed"Matuyama"field overprints.Basedon the lack of substantialreversedpolarity intervalsin otherCenomanian sections and by analogy with the overprintsobservedin the Contessasection, we suggestthat the reversedpolarity intervals
observed
from
the
Ruhr
area
are
artifacts
of
overprintingin a Matuyama (or late Tertiary)field. We invite further studiesto clarify this issue. APPENDIX
The magnetostratigraphic studyof Cretaceous DSDP and ODP sectionsencountersseveral disadvantages when comparedto the studyof their on-landcounterparts. Full recovery is rare for indurated Cretaceousdeep-sea sediments. In recordingdepthsduring drilling, it is conventionto group the unrecovereddepth of a core at the baseof the recovered portion. The "unrecoveredportion", however, could have accumulatedat many gaps within the total depthpenetrated. Therefore, a misleading picture of the relative widths of paleontologicaland polarity zones can result when recovery is not complete. This problem can be acute when brief polarity eventsrepresentedby one meter or lessare the focus of study. In addition, most CretaceousDSDP and ODP sectionsobtainedby rotary drilling are broken into pieces duringdrilling. Accidentallyinvertedpiecesproducethe false impressionthat reversalshave occurred. In rare instances, entire DSDP sectionshave been accidentallyinverted [see Tarduno, 1990]. Occasionally,an overprint is presenton DSDP cores, such as that related to the present-dayfield. Demagnetizationof such an overprintcan be used to define a proxy for declination(A ø of Tardunoet al. [1989]) to
the assignment of nannofossil zones which are too old
for Cretaceousage sediments,and this best explainsthe discrepancyof reported ages [Bukry, 1974; Proto Decima, 1974].
Bukry [1974] suggestsa pre-Aptianage for the section below core 260-12 basedon presence-absence criteria in the low-diversityassemblage. Proto Decima [1974], however, placed the section in the Prediscosphaera cretacea Zone (CC8) of middle Albian age basedon a singleoccurrence of Prediscosphaera cretacea in core 260-12. In addition, assemblages from the deepercoresare similar to both those of core 260-12 and middle Albian assemblages at DSDP site 259 in the PerthAbyssalPlain. Benthicforaminifersfrom core 260-12 and adjacentcores suggesta late AptJan to early Albian age [Scheibnerova,
1974]. The foraminiferalassemblageconsistsmostly of solutionresistantcalcareous andagglutinated speciesof broad stratigraphicrange. Nevertheless,the benthic assemblages are clearly distinguishablefrom pre-Barremianassemblages and supportthe middle Albian age for core 260-17. This age interpretationplaces the possiblesite 260 core 260-17 reversedpolarity zone within the stratigraphicrange of the
potentialreversedpolaritysequenceat the ContessaQuarry. Using the above age interpretations,the sedimentation rate duringthe Albian at site 260 is at least2-3 timesgreaterthan that recordedat the ContessaQuarry. Similarintervalsof reversedpolaritymagnetization (263-1: core 263-20-3, 26 cm to 263-3, 32 cm, 483.26-483.32 mbsf; 263-2: core 263-21-4, 6 cm to 263-21-4 11 cm, 519.56519.61 mbsf; 263-3: core 263-23-2, 103 cm to 263-23-2 113 cm; 596.53-596.63 mbsf, 263-4: core 263-29-2, 99-101 cm;
765.99-766.01mbsf) were foundfrom claystones recovered at DSDP site 263 in the Cuvier AbyssalPlain [Greenand
distinguish actual reversed magnetizations from the above Brecher, 1974; Jarrard, 1974].
artifacts.If suchan overprintis not present,it is not possible to make this distinction.
The three uppermostintervalsin cores 263-20 to 263-23 (263-1R, 263-1R, 263-3R) are dated as late Albian basedon
Some deep-sea sections have certain advantagesover the occurrenceof the nannoplankton Eiffellithusturriseiffeli on-land sections such as higher sedimentationrates, high [Proto Decima, 1974]. Macrofossils in cores 263-26 and rates of recovery in soft intervals, and excellectmicrofossil 263-18 supportthe late Albian age [Stevens,1974]. The preservation.If many sitescontainsuchevidencefor reversed presenceof thesepossiblereversedpolarityzonesled van polarity magnetizationsin a limited paleontologicalrange, Hinte [1976] to definea mid-Albianmixedpolarityinterval. rate for site 263 is thismay be takenas a consistency testfor the actualexistence As at site 260, the implied sedimentation at least 2-3 times greater than that from the Italian sections of corresponding reversedpolarity intervals. Reversedpolarity magnetizationshave been inferred in (Figure 24). Anotherinterval of reversedpolarity magnetizations was Albian sedimentsfrom two principal DSDP legs: leg 27 in the Indian Ocean and leg 48 in the Bay of Biscay. Below, alsonotedin core29-2 (99-101cm) andmay be mid-Albian these studiesare reviewed in light of our new observations in age basedon the presence of the nannofossil Vagalapilla from the Albian Contessa section. matalosain the overlying core [ProtoDecima, 1974]. If correct, this event would correlate with the Contessa reversal
DSDP Leg 27 (Sites 260 and 263), Indian Ocean
sequence.
At DSDP site 260 in the GascoyneAbyssalPlain,reversed polarity magnetizations basedon inclination-onlydata were DSDP Leg 48 (Sites400 and402), Bay of Biscay reportedin nannoradiolarianooze of core 260-17-1 (121-140 Paleomagnetic analysesof azimuthallyunorientedcores cm; 311.21-311.40 m below seafloor (bsf')) even though recovered by DSDP leg 48 in theBay of Biscaynotedseveral only minimum levels of alternating field demagnetization intervalsof reversedpolaritymagnetization [Hailwood,1979]. (5 mT) were applied. The possibilitythat thesereversed At site400, five shortzoneswere definedin Aptian-Albian magnetizationsare an artifact due to accidentallyinverted sediments(400-1R: core 400-62-3, 22-24 cm to 400-62-3, core pieces can not be excluded from the available data. 105-107cm, 657.23-658.06mbsf;400-2R: core400-62-CC, Dating this possiblereversedpolarity zone is difficultdue to 663.50 mbsf; 400-3R: core 400-63-2, 107-109 cm, 666.08 the poor preservationand patchy distributionof Cretaceous mbsf;400-4R: core400-63-4, 120-122cm, 669.21mbsf;400fossils.Calcareous nannofossils are bothselectivelydissolved 5R: core400-72-4,4648 cm, 753.97mbsf),although four and reworked. Both processes shouldproducea bias toward of these"zones"aredefinedby only a singlespecimen each
TARDUNOET AL.: REVERSEDPOLARITYMAGNETIZATIONS,ALBIAN CONTESSA
(400-2R,400-3R, 400-4R, and 400-5R). Althougheachof
bundle
identifications
used to estimate
269 sedimentation
rate.
We are
theseintervalswas taken as indicatingreversedgeomagnetic gratefulto Bob Butler, Mark Leckie, JohnBarronandBenitaMurchey for thoughtfulreviewsof the manuscript.This researchwas supported fieldpolarity[Hailwoodet al., 1979, 1980],a well-defined by National ScienceFoundationgrantsINT 88-11466 and EAR 90present-day overprint is absent andthepossibility of inverted 04457 (to Tarduno).
corepiecescannot be dismissed. Hailwoodet al. [1979],
REFERENCES
however,noted the coincidenceof the Albian intervalswith
those of site 263. The two lower Albian intervals (400-3R Alvarez,W., andW. Lowfie,UpperCretaceous paleomagnetic stratigand 400-4R) correlatewith the Contessaintervals. The two raphyat Moria (UmbriaApennines,Italy): Verificationof the Gubupperintervals(400-1R and400-2R) are containedwithin the bio section,Geophys.J. R. Astron. Soc., 55, 1-17, 1978. Eiffellithus turriseiffeli Zone and do not correlatewith those Alvarez,W., M.A. Arthur,A.G. Fischer,W. Lowrie, G. Napoleone, at Contessa.
I. Premoli Silva, and W. Roggenthen,Late Cretaceous-Paleocene
Althoughrare, planktic foraminfersyield the best age for the lowermost interval at site 400 (400-5). This interval was placed within the Globigerinelloides ferreolensis Zone [Hailwood, 1979] by the recognitionof three specimensof the nominatespeciesin the core-catchersamplefrom core 400-72 which containsinterval 440-5 [Dupeuble, 1979]. The core catcher sample of the overlying core (sample 400-71-CC), however,containedrare specimensof both Globigerinelloides algerianus and Hedbergella trocoidea [Dupeuble, 1979] indicatingthat, if present,the G. algerianusZone lies somewhere within core 400-72. Therefore,interval 400-5 may correlate with the ISEA reversedpolarityinterval(upperAptian) which falls within the G. algerianus Zone [Tarduno, 1990]. Two zones of reversed polarity were defined through paleomagneticanalysesof Aptian-Albian sedimentsat site 402 [Hailwood, 1979]. These zones (402-1R: 413.29-413.74 mbsf; 402-2R: 441.91-442.47 mbsf) have been comq. rmed and are discussedby Tarduno[1990]. Since a well-defined present-dayoverprintwas not definedduringdemagnetization, the possibilityof accidentallyinvertedcore sectionscan not be excluded. The lowermostzone (402-2R), however,may correlatewith the ISEA reversedpolarity interval [Tarduno, 1990]. The upperinterval(402-1R), falls within the Ticinella bejaouansisZone [Dupeuble,1979] which rangesfrom late Aptian to early Albian in age.
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Channell,J.E.T.,R. Freeman,F. Heller,andW. Lowrie,Timingof Summary
diagenetic haematitegrowthin redpelagiclimestones fromGubbio (Italy), Earth Planet. Sci. Lett., 58, 189-201, 1982.
Coccioni,R., O. Nesci,M. Tramontana, F.C. Wezel,andE. Moretti, Descrizione di un Livello-Guida "Radiolaritico-BituminosoTaking the publishedpaleomagneticinterpretations at face Ittiolitico"alla basedelleMarneFucoidiNell'Appennnino Umbrovalue, the "reversed" polarity zones used to define the Marchigiano,Boll. Soc. Geol. It., 106, 183-192, 1987. "site 263" mixed polarity interval do not correlate with Coccioni,R., R. Franchi,O. Nesci,F.C. Wezel, F. Battistini,and P. the potential reversed polarity zones at Contessa. The Pallecchi,Stratigraphy andmineralogy of the SelliLevel(Early high implied sedimentationrates for site 263 are consistent AptJan)at the baseof the Marne a Fucoidiin the Umbro-Marchean Apennines (Italy),in Cretaceous of theWestern Tethys, Proceedings with the paleoenvironmental interpretationcalling for the 3rd International Cretaceous Symposium, Tiibingen1987, edited influencesof a large river [Veeverset al., 1974]. The high byJ. Wiedmann, pp. 563-584,Schweizerbart'sche, Stuttgart, Gersedimentation rates and the small stratigraphicthicknesses many, 1989. of the site 263 "reversed"polarity zonesimpliesthat these Coccioni,R., R. Franchi,O. Nesci,N. Perilli,F.C. Wezel,andF. Batzonesare artifacts,recordsof higher-frequency geomagnetic tistini,Stmtigrafia, micropaleontologia e mineralogia delleMarnea Fucoididellesezioni di Poggio le Guainee delFiumeBosso (Apfluctuationsthan thosepotentiallyrecordedat Contessa,or penninoumbro-marchigiano), in Atti 2ø Convegno Internazionale that sediment accumulation was not uniform. "Fossili, Evoluzione, Ambiente" Pergola,25-30ottobre, pp. 163The single reversedpolarity zone at site 260 could be 201, 1990. correlatedwith the Contessasequence.If real the site 260 de Boer,P.L.,andA.A.H.Wonders, Astronomically induced rhythzone would best correlate with one of the thinner zones at micbedding in Cretaceous pelagicsediments nearMoria(Italy),in andClimate, Part1, editedbyA.L. Bergeret al.,pp. Contessa.The two lowerreversedpolarityzonesinterpreted Milankovitch
at site 400 can also be correlated with those at Contessa.
If due to inverted cores, it is a curious coincidence that such mistakes should occur so close to the Contessa reversal
sequenceand not fartherdown the core (Figure24).
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270
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TARDUNOET AL.: REVERSEDPOLARITYMAGNETLZATIONS, ALBIAN CONTESSA
T. J. Bralower, Department of Geology, University of NorthCar-
olina,ChapelHill, NorthCarolina27599-3315,
J. A. Tarduno,GeologicalResearchDivision,ScrippsInstitutionof Oceanography, La Jolla,CA 92093-0215.
F.HellerandW.Lowfie,Institut flitGeophysik, ETH-H6nggerberg,
Z'tirich,SwitzerlandCH-8093,
W. V. Sliter,U.S. Geological Survey,345 Middlefield Road,MS 915, Menlo Park, CA 94025,
271
(ReceivedMay 23, 1991; revisedAugust 12, 1991; acceptedAugust 27, 1991.)
