MAGNETOSTRATIGRAPHY OF THE TOARCIAN AMMONITiCO ROSSO. LIMESTONE AT IZNALLOZ, SPAIN. Bruno Galbrun 1, Francois Baudin 1, Eric Fourcade ...
GEOPHYSICAL RESEARCH LETTERS, VOL.17,NO.12,PAGES 2441-2444, DECEMBER 1990
MAGNETOSTRATIGRAPHY OF THE TOARCIANAMMONITiCOROSSO LIMESTONE AT IZNALLOZ, SPAIN
Bruno Galbrun 1, Francois Baudin 1, EricFourcade 1, Pascual Rivas 2 Abstract. TheIznalloz section (Beticcordillera, Spain) provides a continuous outcrop(20 m thick)of Toarcian
TheToarcian, thelaststageof theLias,extends from
about180to !87 Ma on the timescaleof Odin(in press) andcould correspond toa highreversal frequency period(Courtillot and Besse,1987).Despiteseveral
ammonitico rosso limestones with all the ammonite zones defined in the Boric realm. The characteristic
magnetization direction foreachsample wasobtained by
magnetostratigraphic studiesof Toarcianagesediments
detailedthermal demagnetizationand was stablefrom
(Homer and Heller, 1983;Channe!let al., 1984; Ga!brun et al.,1988a,b)themagnetic polarity sequence forthisstageisnotwellestablished. Thisis duemostly
about450 to 560øC.The magnetic polaritysequence extendsfrom the Polymorphumto the Aalensis
ammonite zone,and coversthe wholeToareianstage.
to difficulties of correlation between the different
sections studied. Thusthissectionpresentsa goodpatternof the reversal succession duringToarciantimes.Considering hiatuses We present in this paper the first of sampling andexcluding single-sample magnetozones, magnetostratigraphic resultsobtainedfrom the Toarcian theredoesnot seemto be anyimportantchangein the of BeticCordillera (Spain)in theTethyanrealm.The reversalrate duringthis period. results obtained provide an excellent record of the geomagnetic reversal historyduringthe Toarcian. Introduction
Geological settingandstratigraphy
Magnetostratigraphy has become an essential correlationand dating tool, especiallyfor upper
The studiedsectionis located in the Betic Cordillera
Cretaceous andCenozoic sediments. This toolis notyet availablefor Jurassicand older sediments,becausethe magneticpolaritysequenceof theseperiodsis not well known.The Jurassicand Triassicpolarity succession mustbe established by magnetostratigraphic studiesof sedimentarysections,since there is no oceaniccrust older than middle Jurassic.
3 km west of the village of !znallozalong the Granada-Moreda railway(37.39 ø N, 3.49ø W). Tertiarytectonics createdfolding,lateralshearand terranos;thus the beddingin the Iznalloz sectionis
overturned. Detailedammonitebiostratigraphy in the
Toarcian of the Boric Cordillera has been established
previously (Jimenoz andRivas,1979).All theammonite
On the other handstudiesof magneticreversalrates throughgeologictime allowedsomegeodynamic models to be proposed.For exemp!ethe controversial modelfor couplingof core and mantle convectionsuggests t•hat
zones defined for the Toarcian are to be found in the
lznallozsection. A detaileddescription of thissection hasbeenpublished by Bragaeta!. (1981) andwe will
onlypresent a briefsummary of the!ithostratigraphy of
times of increasingreversalrates correspond with periodsof fast true polar wandering(Courtillot and Besse,1987).Thismodelrequiresan increasein reversal frequencyduringthe upper Triassicand earlyJurassic times.
The magnetostratigraphic dataavailablefor the lower Jurassicare scarce(Mlfrton •t al., 1980; Horner and
Heller,1983;Charmell et al., 1984;Galbrunet al., !988
a, b) anddonotprovide a complete magnetic polarity
the Toarcianwhichcomprises 2,0m of ammonitico rosso limestones (seelithologic log in Figure4). The lowerpart of the Polymorphum ammonitezone
is a pro•minent greyrnicritic limestone; theupperpartof thisPolymorphum zoneandtheSerpentinus andBifrons zonesare composed of red or greymarlylimestones and red marls. The Gradata, Failaciosurn, Re•esi and Aalensis zones are mainly red nodular limestone interbeddedwith some marl layers. The nodular
sequence for thisperiod,nor a precisecorrelation with
limestonesrepresenta lower sedimentationrate than the
the ammonitebiozonations developed in different biogeographic realms. Thisobiective requires thestudy of manysedimentary sectio•giventhe difficulties of m.agnetostratigraphic studies (inconsistent preservation of
rossolimestones, whichare goodrecorders of magnetic
pnmary remanent magnetizationin relatively old
sedimentary rocks,difficulty of collecting samples in
marlyfacies, hiatuses in outcrops, frequent lackof fauna and difficulties in correlating ammonitebiozonations fromdifferent realms).
1 Universit6ParisVI 2 Universidad de Granada
marls.
Thus, a completeToarcian sectionof ammonitico polarity reversals(Charmell et al., 1982; Lo•e and Heller, 1982;Channe!let al., !984; Freeman,198.6)is presentat thislocality. Magneticproperties
The sectionwas intensivelysampledMth a mean samplingspacingof 13 cm, exceptin the marlsof •,e lower part of the sectionwhereonly a few samples couldbe •red. A total of I0! stratigraphic levelswere sampled from the 20 m of this •,o.n. All measurements
•were made
on
a
three-axis
RS4)I
(LETI/CEA) etyoge•e.•magnetometer; the no•e level is suchth,atmagneti•tionintemities of lessthan2x!O 4 A/m cannot be precisely measured.The natural
Copyright 1990 by the American Geophysical Union.
reinanentmagnetization (NR•M) of t• ,,IznaH,oz limesmaes .ranges .from3.3x10 ø to 2.6x!:0 'a A/re. 'The
Paper number 90GL02548
0094-8 534/ 90/ 90GL-02548503 . 0'0 2441
2442
Galbrunet al.' Magnetostratigraphy of Toarcian Limestones
lowerpartof thesection exhibits weaker magnetization, theNRM intensity beingabout0.2-5x10 '4A/m,whilethe upperpartshows an increase of NRM intensity towards
the top of the sectionup to 2.6x10 '3 A/m in the
A NRM = 8.1x 10'4 A/m
:ooo
UP/N
IZ 99.1
uppermost bed.
MostNRMdirections areofnormal polarity (before
any tilt correction) and grouptowardsthe NW. The
•00ø
ammordtico rosso limestones usually havea magnetic mineralogy dominated by magnetite (whichis most probably detritalin origin),andauthigenic haematite,
3'00ø • 4000'•X -.-.
grownverysoonafterdeposition (Charmell et al., 1982;
Lowde and Heller, 1982;Freeman,1986).The acquisition ofisothermal reinanent magnetization (IRM) isa convenient wayto characterize themagnetic carriers
-- E/E
ofpelagiclimestones (LowfieandHeller,1982;Galbrun and Butler, 1986). IRM acquisition curvesfrom the
ammonitico rossolimestones of Iznalloz(Figure1) indicate a rapidincrease in applied fieldupto0.1or0.5 T anda subsequent slowincrease, saturation being unattained at 1.6T. Progressive thermal demagnetization of themaximum IRM indicates a blocking temperature of 580øC(Figure1, sampleIZ 7A) and a small
persistent component above600øCwhichis clearly evidentin sampleIZ 9A. Thesecharacteristics showthat
thepredominant low-coercivity mineralis magnetite and the highcoercivities are dueto haematite withhigh blockingtemperature.
© v
OH
Fig.2.Orthogonal projections of thermaldemagnetization
datafortwotypical samples. Thedemagnetization range
is givenin centigrades, the horizontal(H) andvertical (V) pro•ectionsare indicated. All directionsare correctedfor tilt (see explanations in text).
Twenty pilotsamples representative of eachlithology
underwent progressive thermaldemagnetization in 50øC
incrementsfrom 100 to 500øCand in 20øCincrements above500øC.The directionsof the principalcomponents of magnetizationin each samplewere determinedby least squares analysisusing the software of Torsvik (1986). Two typicalthermal demagnetization plots are shownin Figure2, theyboth indicatetwo distinguishable components.A first, soft, componentis removedby heating to 150-200øC.It has the present earth'sfield direction(beforetilt correction)and thus is probablya viscousremanent magnetization(VRM). Above these temperatures a stable component is progressively removed.This has a blockingtemperaturespectrum rangingfrom 300øCto more than 560øC and is likely due to the magnetite as shown up by the IRM measurements. This componentcanbe either of reversed polarity (sample 34-1) or of normal polarity (sample 99-1) andthusis mostprobablyof primaryorigin.Above 560øC the reinanent magnetizationis too weak to be measured and mineralogical transformationsoccur probably because the samples exhibit considerable magneticviscosity.
Given theseresults,the remainingsamples fromthe Iznalloz section were measured at a selected set of
about 4 or 5 demagnetizationsteps between450 and 560øC. The characteristicremanent magnetization (ChRM) directionfor eachsamplewasaveraged by a three-dimensional least squarestechnique(Kirschvia• 1980). These ChRM vectorsform a nearlyantipodal group of normal and reverse olarity directions,with some samplesexhibiting•ntermed•atepolarity (Figure 3). The largescatterbetweendirections is probablydue to inadequatebeddingcorrections in places.As a matter of fact the sectionis sometimes disturbedandthedipof beds is difficultto estimate,especiallyin marly fades. Mean
directions
were
calculated
for both the normal
and reverse polarity groups, and for all samples combined after exclusion of intermediate polarity samples(Table 1). The normal and reversedmeansare o
270
90
•80
Ida,}eric field (D
re,'r•c•t•e 'C
ß lower
Hemisphere
o upper
Fig.1.(A) IRM acquisition curvesfor two samplesand (B) continuous thermaldemagnetization of themaximum IRM acquiredin a field of !.6 T (see explanationin
Fig.3.Stereographic plotof highblocking temperatur. e
text).
correction).
componentmagnetizationdirections(after tectornc
Galbrun etaI.:Magnetostratigraphy ofToarcian Limestones TABLE 1. Characteristic meanmagnetization directions(after tilt correction).
Samples
N
D
I
normal reversed combined
33 47 80
272.9 84.9 269
43.5 -62.1 54.6
a9s k 7 5.1 4.5
12.1 16 12.!
statistically different; this is probablydue to an incomplete separation of components, andmayreflecta partialoverprint by a late secondary component witha highblocking temperature. The Iznallozsectionis of overturned beddingand thus the mean palcomagnetic direction has no
geodynamic significance. Magneticstratigraphy- Conclusion The declination and inclination of characteristic
magnetizationdirections plotted as functionsof
stratigraphic position,give a clear magneticpolarity
sequence (Figure4). Most of the polarityintervalsare documented by severalsamples, thosedefinedby single samples alwayspresenta goodpa!eomagnetic direction withoutany doubtaboutthe polarity,exceptonesample at the top of the Fallaciosumzone. The magnetic polaritysequenceestablishedat Iznalloz, which covers all of the Toarcian apart from a few interbedsin the
middlepart of the sectionin marlyfacies,is probably oneof the mostrepresentative polaritysuccessions for
2443
theToarciantime.We maytakenoteof thefollowing
with regardto this:
(1) Magnetostratigraphic studies of the Toarcian type
sections of Thouars andAirvault(France)yieldedtwo reliablemagnetic polaritysequences (Galbrunet al., 1988b), but somelacunaeoccurat the base of these
condensed typesections (6.60m thickfor theToarcian)
and the magneticpolaritysequences established are probablynot complete.
(2)TheToarcian oftheSierraPalomera section (Iberian Cordillera, Spain) isthick(75m oflimestones andmar!y limestones), butsome stratigraphic orsampling gapsand difficulties in separating primarymagnetization from secondary magnetizations hinder the establishment of a
complete recordof themagnetic polarityzones(Galbrun et al., 1988 a).
(3) The Valdorbiasection(CentralItaly) has poor faunalcontentanda majorhiatusis suggested in this section(Guex,!975;G6czy,1984).Thusthe magnetic polaritysuccession proposed for thissection (Charmell et al., 1984)is probably not complete. It is difficultto correlatethe Iznallozpolari• sequenceto otherreliable sequences suchas the one
established for the BreggiaGorgesection(southern Switzerland) (HomerandHeller,1983).Thisrequires establishment of good correlationsbetweenammon/te
biozonations in the differentbiogeographic re.alms and study of historiesof sedimentationrate in different
sections. Sucha workneedslongdiscussion and could not be presented in the presentpaper.Meanwhileit seems that there are more reversals in the Iznalloz
section,especially in the upperToarcian.This section,
l
Fig.4.Ammonite zonatio•lithologica! logandmagnetostratigraphy (incl'mado.n anddec!•tion of thestablecharacteristic magnetization, magnetic polity)for the!ma!!ozscion.
2444
Galbrunet al.: Magnetostratigraphy of ToarcianLimestones
which certainlyhas a goodstratigraphic completeness, presentsprobablythe best record of the geomagnetic reversalsuccession duringthe Toarcian.Excludingsingle sample magnetozonesin the uppermost Toarcian (Aalensiszone),it doesnot seemthat thereis important variation in reversalrate duringthe Toarcian.This rate averages4 reversalsper ammonit.ezone. Meanwhilethe samplingintervalis lessclosein the lower part of the section(in part becauseof the mar!yfacies)and other shorterpolarity zonesmay exist. During Jurassictimes high reversalrates (5 to 10 reversals per ammonite zone) are known for the Bajocian and Bathonian (Steiner et al., 1987), and Pliensbachian (Horner and Heller, 1983). The magnetic polarity time scale for the lowermost Jurassic (Sinemurianand Hettangian) is not well documented but the few data available indicate a low reversal rate
(Haq eta!., 1987).The Aalenianof the BreggiaGorge section also showsa low reversal rate (Homer and Heller, 1983). The Callovian and Oxfordianpolarity patternsare poorlydocumented, but the availableresults alsoindicatea moderaterate of magneticreversals(Ogg and Steiner, 1988). We showedin this paper and previouswork that the Toarcianhasa moderatereversal rate. Thus the Jurassicexhibitsperiodsof high reversal rate (Pliensbachian, Bajocian-Bathonian) alternatingwith times of low reversal rate (Hettangian-Sinemurian, Toarcian-Aalenian,Callovian-Oxfordian).
pr61iminaires surle Toarciende la SierraPalomera
(Chaine ib6rique, Espagne), Bull.Soc.G6ol.Franca
•
_1,193-198, 1988a.
Galbmn, B. andR.F.Butler, Curietemperature analysis of UpperJurassic andLowerCretaceous pelagic limestones, Geophys, J. R• Astr,Soc,,86, 885-892, 1986.
Galbmn, B., J. Gabilly, and L. Rasplus, Magnetostratigraphy of the Toarcianstratotype sectionsat Thouarsand Airvault(Deux-S•vres, .France),Earth Planet.Sci, Le.tt, 87, 453-462,1988b.
G6czy,B.,Provincialism ofJurassic ammonites; examples from Hungarianfaunas,Acta Geol. Hung•,.22, 379-389, 1984.
Guex, J., Descriptionbiostratigraphique du Toarcien
sup6rieur de la borduresuddesCausses (France), Eclog. geol. H½!.v,., f• 97- 129, 1975.
Haq, B.U., J. HardenboI,andP.R. Vail, Chronology of fluctuatingsealevelssincethe Triassic,• 1156-1167, 1987. Homer, F., and
F.
Heller,
Lower
Jurassic
magnetostratigraphy at the BreggiaGorge (Ticino,
Switzerland) andAlpeTurati(Como,Italy),Geophy% J. R, Asl;r. $0C,, 73, 705-718, 1983. Jimenez, A.P., and P. Rivas, E1 Toarciense en la zorn Subbetica, Chad. Geol., I__Q, 397-411, 1979.
Kirschvink,J.L., The least-squareline and planeandthe analysisof palaeomagneticdata, Geophys.J. R, astr, Soc., 62, 699-718, 1980.
A•knowle•lgement$. The authorswish to thank Dr L. Daly who made availablethe facilitiesof Laboratoirede G6omagn6tisme(Universit6 Paris VI) for the IRM measurements. We appreciatereviewsprovidedby Dr E. Hailwood and an anonymousreviewer.We are grateful to John Thompsonfor revisionof the English. This researchwas fundedby the Centre National de la Recherche Scientifique (UA 1315) and the "GroupememScientifiqueT6thys".
Comas, F.
171-192, 1982. M,5.rton, E., P. M,Srton, and F. Heller, Reinanent
magnetizationof a Pliensbachian limestonesequence at Bakonycsernye (Hungary),Earth Planet.Sci.Lett., 48, 218-226, 1980. Odin, G.S., The Jurassic Time Scale in 1988, in The
Girourn-Pacific Jurassic, Edited byG.E.G. Westerman, PergamonPress,in press. Ogg, J.G., and M.B. Steiner,Magnetostratigraphy of the Callovian
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(ReceivedJuly9, 1990; revised October 22, 1990;
acceptedOctober22, 1990)
