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and the geomagnetic polarity record

W. LOwRIE Institul lar Geophysik, ETH-Hónggerberg, 8093 ZUrich (Switzerland.)

ABSTRACT

The global nature of geomagnetie polarity reversals makes magnetic polarity a powerful method of correlating one stratigraphic section with another, especially when carried out in conjunction with a biostratigraphic study. Evaluated together, the two data sets provide a more reliable and finer correlation of stratigraphic sections with each other —or with a magnetic polarity scale— than is possible with one data set alone. An interesting and potentially valuable application of inte- grated magneto- and studies is in sedimentary basin analy- sis, for example in determining sedimentation rates, or as the time base for analysis of cyclical patterns in sedimentation. Where different paleon- tological dating schemes have been used to date sections, the magnetie stratigraphy allows these to be compared and synchronised with each other. An important result of magnetostratigraphic research has been the correlation and dating of the polarity obtained from oceanic mag- nctic anomalies. The interpretation of lineated marine magnetic anomalies has provi- ded a continuous record ofgeomagnetic polarity since the onset ofthe pre- sent phase of sea-floor spreading, following the break-up of Pangea in the early Jurassic. The dominant characteristics of this time interval are two sequences of alternating geomagnetie polarity, the younger of which has lasted from the Late Cretaceous to the . The older M-sequence falís in the Late Jurassic and Early Cretaceous, and is separated from the youn- ger sequence by the Cretaceous quiet interval, lasting about 35 Myr, in which no magnetie reversals took place. Almost alí the geomagnetic po-

lnslitut flir Geophysik. ETH-Zúrich. Contribution Nr. 567 96 11¼Lowrie larity intervals in these reversal sequences have been corroborated and da- ted by coordinated magnetostratigraphic and biostratigraphic research in pelagie sedimentary sections. Magnetie polarity scales for the late Mesozoic and the adopt the oceanie magnetie polarity sequen- ce as basis, as it is the best continuous rccord available for this time pe- riod. Several versions of the magnetic polarity time seale have been pro- posed. They differ from each other mainly in the number of tie-leveis used and the optimum ages accepted for the tie-levels, and reflect different cri- tical evaluations of the accepíable radiometric data base.

INTRODUCTiON

Although the causes are not understood, the fact that the dipole com- ponent of the earth’s main magnetic fleld of internal origin has changed polarity in the geological is well documented in the remanent mag- netizations of rocks. Early in the present century, Brunhes (1906) found volcanie rocks that had magnetization directions opposite to that of the present fleld, and Matuyama (1929) showed that the polarities of oppo- sitely magnetized groups of lavas were dependent on their stratigraphic position relative to one another. I-Iowever, self-reversal of the thermore- manent magnetization by a magnetomineralogical mcchanism was the fa- vored explanation of many rock magnetists for the reversed magnetiza- tions in these igneous rocks. Laboratory experiments demonstrated thai, during cooling, certain igneous rocks acquired a magnetization which was antiparallel to the ambient fleld. With the development of a precise ra- diometrie dating technique and its application to lavas of known magne- tization polarity, the history of geomagnetic polarity reversals within the last few million was convincingly documented. The specter of self- reversal as the cause of alí observed reversed polarities was finally laid to resí by the discovery of the same polarity sequence in modern deep sea sedimcnts (Opdyke eZ aL, 1966). A complctely different, isothermal pro- cess is responsible for the depositional remanent magnetization of sedi- ments and thermally activated self-reversal can be ruled out. This does not discount the possibility that reversed magnetizations in some igneous rocks may in fact be due to a self-reversal mechanism.

MAGNETIC FIELD BEI-IAVIOR DURINO A POLARITY REVERSAL

The behavior of the earth’s magnetie field during a polarity change has been studied in igneous and sedimentary roeks. The change of paleo- field direction to an antipodal value is observed as simultaneous transi- tions of the inclination and declination. Paleointensity results from mo- Magnetostratigraphy and the geomagnetic polarity record 97 dem deep-sea sediments and slowly cooled igneous intrusives indicate that the intensity of the magnetic field decreases to a 10w value during the transition (Opdyke et aL, 1973; Dodson a aL, 1978; Prévot et aL, 1985). This is interpreted as evidence for disappearance of the dominant dipole component of the geomagnetie ficíd. Although the dipole fleld decays during a reversal, the non-dipole field evidently does not and a standing non-dipole fleld component remains during the reversal (Hillhouse and Cox, 1976). The morphology of the transitional fields is assumed to be axisymmetric. Current research is ai- med at establishing whether the transitional field configuration is contro- lled by the quadrupole component or the octupole component of the non- dipole fleld (Fuller et aL, 1979). From transition studies in oceanic sedimenis with constant sedimen- tation rates, the duration of a directional transition has been estimated to last about 4,000-5,000 yr; the intensity change takes about twice as long as the directional change (Opdyke et aL, 1973). For magnetostratigraphy these are rapid changes. In a pelagic limestone section with sedimenta- tion rate 10 m/Myr the transition is represented by a mere 4-5 cm of sec- tion. The average length of a polarity interval in ihe Cenozoic was about 450,000 yr (Heirtzler et aL, 1968). Thus each transitional interval is only about 1 % as long as the average polarity interval. Accordingly, about 1 ~/o of the paleomagnetie samples from a magnetostratigraphic section may be expected to have intermediate directions.

MAGNETOSTRATIGRAPHY

Fleid Sampíing

The usual purpose of a magnetostratigraphic study is to establish the record of magnetie polarity in a geological formation of known age. Alt- hough it is possible to piece together the record from a of short out- crops, this procedure is usually not precise enough. It is desirable that the formation be exposed for sampling in as long and continuous sections as possible. The criterion of continuity of sedimentation is extremely impor- tant. To be sure that there are no gaps in sedimentation, as well as to date the reversals found, a magnetostratigraphic study must be accompanied by a thorough paleontological study. The combination of magnetostrati- graphy and biostratigraphy in the same section provides mutually suppor- tive data, which result in a more finely resolved stratigraphic evaluation and a larger number of reliable tie-levels for correlationsbetween sections. The sampling interval of a magnetostratigraphic section is determined by the required resolution. As can be inferred from the description of field 98 W, Lowrie behavior during a polarity transition, the shortest possible magnetic po- larity interval must last longer than tite time for a double transition, and is therefore around 10,000 yr. Tite shortest known, well documented in- tervals of constant magnetie polarity are known from the oceanic magne- tic record to have lasted about 20,000 yr. A rough calculation shows that titis is about the optimum resolution to be expected. The maximum mag- netic anomaly y over the middle of a narrow block of oceanic crust with vertical magnetization contrast M, thickness t and width 2d, at a depth h below tite magnetometer is approximately given by V = 4MJ (0~ — 02). where ~ = arctan{d/hI and 02 = arctanjd/(h+t)}. Although a significant pro- portion of the oceanie magnetie anomaly signal arises in the gabbroic seis- mic Layer 2B of tite oceanic crust (Kent et aL, 1978), the observed signal is generated predominantly in tite layer of basaltic lavas that form Layer 2A, which has a thickness of about Oi km and an average basalt magneti- zation of 5 A/m (Lowrie, 1979). Assuming a water depth of 5 km and a shipboard magnetometer resolution of around 50 nT, Ihe resolvable width of a magnetized block is about 1 km, a figure reached by Cox (1969) from other considerations. On an anomaly with a fast spreading rate (5 cm/yr) titis corresponds to a time interval of about 20,000 yr. This is approximately the maximum resolution of the oceanic magnetic reversal record. The goal of a magnetostratigraphic study is usually to match titis re- solution. Tite desire to locate and define aecurately tite shortest magne- tozones in tite section must be balanced against the effort of sampling, measurement and data processing. When the sampling density is so high that tite number of samples taken is much greater than tite number of mag- netozones expected in tite section, the manner in which thc samples is dis- tributed stratigraphically is not important. If the goal of tite study is to define the magnetic polarity zonation completely but economically with as few samples as necessary, tite geometry of tite sampling seheme beco- mes important and optimum sampling is achieved with uniform síratí- graphic spacing (Johnson and McGee, 1983). In many sections of pelagic limestone, tite sedimentation rate is typically around 10 m/Myr. In order to achieve a resolution of 20,000 yr, a sampling interval of 20 cm would be needed, which in a 100 m thick section requires t.aking 500 samples. Every sample must be processed and remeasured many times, and con- sequently the amount of effort required for this resolution quickly beco- mes disproportionate to tite goal. A suitable compromise consists of ta- king samples at 1 m intervals, corresponding to 100,000 yr resolution. Af- ter comparison of tite magnctostratigraphy with tite polarity sequence in tite oceanic record, tite section can be resampled in more detail where shorter intervals are expected. This practical comprornise emphasises the confirmation of known reversals at tite expense of discovering short evcnts missing in tite oceanic record. vlagnetosrrarigraphy and

Laboratory and Analyticaí Procedures

The samples from a magnetostratigraphic section must be subjected to Uit same rigorous progressive demagnetization procedures as samples from a conventional paleomagnetic investigation. Only in titis way can the stable characteristic remanent magnetization direction be determined properly, preferably as tite straight line leading to the origin of a vector diagram (As and Zijderveld, 1958). Less rigorous metitods are inadequa- te and can lead to false interpretation. For example, in a magnetostrati- graphic study of Oligocene sections in tite Western U.S.A., Protitero et aL (1982) classifled many samples with positive inclinations as reversed be- cause tite directions sitowed tendencies towards tite negative polarity he- mispitere during progressive demagnetization. Tite fact that stable direc- tions were not established implies that the demagnetization technique was inadequate or tite samples were unstable. As a result, the positions of re- versal boundaries in the interpreted polarity sequence were wrongly iden- tified- Absolute dates for ash flows in tite magnetostratigrapitie section were wrongly correlated with tite polarity sequence, and incorporated sub- sequently as incorrect datum points in tite magnetie time seale of Berg- gren et aL (1985). Tite history of geomagnetic polarity is usually portrayed as a binary sequence of normal and reversed polarities, intermediate directions being regarded as not representative of a stable state. Tite polarity data are cus- tomarily shown as a column of alternately black (conventionally used for normal polarity) and white (for negative polarity) magnetozones. Tite in- terpretation of tite limits of magnetozones can be made from plots of in- clination and declination against stratigraphic position (Fig. 1). The indi- vidual points in each plot should be connected by straigitt lines to aid the viewer, as it is otiterwise difficult to obtain a correct visual impression of the quality of tite data. Witen the data points are unconnected, it is easier to overlook incompatible directions corresponding to disturbed parts of the section, and sitort intervals of opposite polarity to tite dominating po- larity of a magnetozone are more easily «lost». A convenient way of com- bining inclination and declination in a single polarity parameter is to cal- culate tite latitude of tite geomagnetie pole when the formation magneti- zation was acquired. For this calculation, declinations must be corrected for any post-depositional rotation and tite paleolatitude of tite site, calcu- lated from the mean inclination, must be used instead of tite present la- titude. The boundaries for tite magnetozones are located at tite zero-cros- sings of the plot of pelar latitude (sometimes misleadingly labelled VGP latitude), or of tite inclination, or at tite corresponding intersections of de- clinations with a value 90’ away from tite vector mean declination (Fig. 1). Ideally each magnetozone should be represented by several samples. However, in many sections individual samples are found with polarity an- J00 1V. Lowrie

PR ESAL E VGR Ma9netic lncl¡ pat ion Dectinoton Lati lude Potarity o so 30 210 20 SON

263< 2621

u, e, 2 5~ ‘1, 243,4 E ¡ ~Iji 238.3 e o t u 231.0 u, o-o u fi -c a 211 5 O

O, 203.2 230 u 200.1 O .198.8 1 954 09 93.9

r 8 7) 16 3.0 ¡ 180< Fig. 1. Stratigraphic variations of inclination, declination and the latitude of the virtual geomagnetic pole in the Presale section of the Lower Cretaceous Maiolica liniestone in Um- bria, Italy (Lowrie and Alvarez, 1984). tipodal to that of their neighbours. Tite anomalous directions may be due to unstahle magnetizations, or to disturbance of tite section. Tbey may have a geomagnetic origin and represent only temporary excursions of the magnetic pole away from a stable position, or they may in fact re- present very short polarity chrons which are too short to be resolved in tite conventional oceanic magnetie record. A simple but satisfactory met- hod of designating single-sample magnetozones in a polarity column is to draw them only half tite width of tite column (see, for example, Ogg et aL, 1984). Extra, short magnetozones found in a stratigraphic section which have no counterpart in tite oceanie record may be real, and titeir existen- ce is extremely important, but unless they are verified in other sections, titeir reality is suspect. Magnetosrratigraphv and

Magnetostrat¡graphy and Sedimentary History

Identífication and Correlation of Magnetic Polarity Zones

To identify individual magnetic polarity chrons in a magnetie time scale, it is necessary to use a suitable nomenclature. Cox (1982) proposed a simple and logical convention which is employed with minor modifica- tion itere. Tite chrons are numbered sequentially backwards in time from tite present, in correspondence with tite number of the oceanic magnetic anomaly from witich each polarity citron is interpreted. Each citron con- sists of a normal polarity subchron and a negative polarity subeitron, which are distinguished by attaching tite letter n or r to tite citron num- ber. Titus tite present interval ofnormal geomagnetie polarity corresponds to citron 1 n, while tite Cretaceous- Tertiary boundary is correlated with tite upper part of chron 29r (Alvarez a at., 1977). Titis nomenclature will be used in tite rest of titis paper. Tite most eommon metitod of identifying a sequence of magnetozo- nes is to find tite equivalent pattern in tite oceanie record. Ifa distinctive «flngerprint» is not present, an unambiguous correlation or identification may not be possible. Tite pattern recognition is usually done visually, but an alternative, quantitative method is by tite use of correlograms (Lange- reis et al., ¡984). This involves computing tite cross-correlation function of polarity interval lengths at successive matching positions as one pola- rity sequence is moved progressively past tite otiter (only alternate mat- ches in which intervals of like polarity are matehed need be considered). Lowrie and Alvarez (1984) used correlograms (Fig. 2) to correlate tite se- quences of magnetozones in Italian magnetostratigrapitic sections of Late Jurassic and Early Cretaceous age to tite M-sequence polarity record (Fig. 3). Tite correlations of tite magnetozone record in tite Bosso section with chrons Ml 3-M19 is the only one that is statistically signiflcant. Titree significant correlations are possible in tite Gorgo a Cerbara section, and four are possible in the Presale section. Tite oldest correlations can be ru- led out as they would imply tite age of tite section to be Jurassic, witereas it is known to be Early Cretaceous. Lithological arguments favor tite youn- gest signifscant correlation in each seetion. Tite correlation of sections by means of their magnetostratigraphy ne- cessitates that they itave tite same history of sedimentation rate. For corre- lation with tite magnetie reversal record derived from interpretation of oceanic magnetic anomalies, tite sedimentation rate and sea-floor sprea- ding rate must both be constant over the time interval investigated. Va- riable sedimentation rate distorts tite thicknesses of magnetozones and makes recognition of a distinctive pattern difficult or even impossible. Channelí and Grandesso (1987) found a poor visual match between the 102 1V. Lowrie

GORGO A CERBARA 1.0

o

a ¡gn¡t¡cence

-1~0 60 12 20 30 40 50

4-, FRESAL E ci a 1.0 (-3

4- 2 M15-M21 M2!-M24 a) MC-Me - 1?1~i 1 oo o c o -1-> u a)

o 10 - o 10 20 30 L.0 50 50

BOLSO 10

o

10 11 20 30 40 50 60 Match Ros¡tion Fig. 2. Correlograms for the possible correlahions of the Bosso, ¡‘resale and upper Gorgo a Cerbara sections with the M-sequence magnetie reversal record (Lowrie and Alvarez, 1984).

) Magnetostraíigraphy and the geomagnetic polarity record 103

Oceon¡c Moqnetic Gorgo o Anomolies Cer boro Fresate Frantoje

MO

Ml M2

M3

Mt ME Mt M7 MB M9 MIO

51 ION

Nl II

512

5413

54 it

51 5

54 16

5417

54 It

1419

Fig. 3. Correlations of Lower Cretaceous M-sequence magnetostratigraphic sections in fimbria and the Southern Alps, Italy. Descriptions of sections: Umbrias Bosso (Lowrie and Channell, 1 984a), Gorgo a Cerbara, Presale, Frontale (Lowrie and Alvarez, 1984), Fon- te del Giordano (Cirilli el al?, 1984); Southern Alps: Capriolo (Channel el al., 1987). 104 1V. Lowrie marine polarity record and the magnetie in the Xausa and Frisoni sections in the Italian Southern Alps, which titey postulated was due to changing sedimentation rate in titese sections, or to variation in sea-floor spreading rate during formation of tite corresponding M-sequen- ce anomalies. Tite close agreement of Ihe coeval magnetie stratigraphy in the Umbrian Bosso section with the marine polarity record favored the ínterpretation of changing sedimentation rates in tite Southern Alps see- tions and allowed titeir magnetostratigrapities to be corrected correspon- dingly.

Magnetostratigraphy and Basin Analysis

The ability to provide information about sedimentary history makes magnetostratigraphy a powerful tool for basin analysis. Once a section itas been correlated with tite magnetic time scale, tite sedimentation rate can be estimated. Tite tie-lines of the correlations also dramatize minor fluc- tuations of sedimentation rate within individual sections. Magnetostrati- graphic correlations in the Maiolica limestone in tite Italian Appenines (Hg. 3) give valuable information about tite Early Cretaceous develop- ment of tite Umbria-Marches sedimentary basin. Contemporaneous dif- ferences in sedimentation rate within tite basin are seen by comparing the Presale section, which has a sedimentation rate of 7.1 m/Myr, with tite up- per part of tite Gorgo a Cerbara section in which tite sedimentation rate (9.5 m/Myr) is more titan 30 % greater. Tite difference reflects tite posi- tions of the sections on opposite sides of a listric normal fauh that was active during sedimentation. Tite Presale section represents deposition on a seamount on tite upthrown side, while tite Gorgo a Cerbara section was deposited in tite deeper basin on tite downthrown side (Lowrie and Alva- rez, 1984). Furtiter scrutiny shows a marked variation in sedimentation rate during Maiolica deposition. The sedimentation rate in tite Bosso see- tion, witich spans tite .lurassic/Cretaceous boundary, is 9.6 m/Myr. Hig- her sedimentation rates are found in the coeval lower parts of tite Gorgo a Cerbara and Frontale sections (1 5 m/Myr and 19 m/Myr, respectively), altitough titese sections are about 50 km apart. Tite increase in sedimen- tation rate is accompanied by a marked decrease of the intensity of mag- netization due to dilution of tite principal magnetie mineral (magnetite) in the sections. Titis dilution results in deterioration of the paleomagne- tic signal, witich becomes too weak to measure reliably. Polarity chrons MIl r to Ml3n have not been found in Umbrian magnetostratigraphic re- search, but they have been securely located in tite Capriolo section in tite Southern Mps (Channelí eta?., 1987). ¡VIagndosl ratígraphy and he geomagnelic polaritv record tos

COORDINATION OF MAGNETOSTRATIGRAPHY WITH BIOSTRATIGRAPHY

Mercanton (1926) first recognized that a paleomagnetie reversal is a globally synchronous ; the record of polarity is independent of geo- graphic location. One of tite major interests of magnetostratigrapitic re- search itas been in tite correlation and dating of tite polarity itistory ob- tained from oceanie magnetic anomalies, and its most valuable applica- tion itas invariably been in conjunction with a detailed biostratigrapitic study. Tite two disciplines reinforce eacit other and increase tite number of usable tie-levels for correlation. The location of paleontological boundaries of known radiometrie age relative to the magnetic polarity re- corcl provides invaluable tie-points for tite development of magnetic po- larity time scales. In turn, titis enables the estimate of numerie ages for paleontological zones, first appearance datum (PAL» and last appearance datum (LAD) levels. An interesting example of tite combined use of magnetostratigraphy and biostratigrapity is seen in the results from pelagic carbonate sections in tite Contessa valley in Umbria, Italy (Lowrie el aL, 1982) and site 522/522A of tite Deep Sea Drilling Project (DSDP) in tite South Atlantic (Poore el aL, 1982). Tite polarity sequences in both sections are excellent, unambiguous records. They agree with the polarity sequence derived from marine magnetie anomalies and expressed in the form of a magnetic po- larity time scale (LaBrecque et aL, 1977). Both sections were dated using foraminifera and calcareous nannofossils, and tite stratigraphic positions of key LAD and FAD levels were determined and located relative to tite magnetic polarity time seale (Fig. 4). The numeric ages of appearances and extinctions of the same species in tite Tethys (Contessa section) and in tite South Atlantic (DSDP 522) were calculated. Tite average age of FAD’s from tite Tethys was 0.45 (±0.39)Myr younger titan in tite South Atlantic, and tite average LAD age in tite Tetitys was 0.57 (±0.28)Myr older titan in tite Soutit Atlantic (standard errors in parentitesis). Tite sam- píes are small and tite interpretation is open to speculation. The differen- ces may be a real expTession of evolutionary rates in tite Tethys and South Atíantie. or they may express systematic discrepancies in tite dating sche- mes used by different paleontologists (LaBrecque el aL, 1983). Tite exam- píe demonstrates two valuable applications for magnetostratigraphy, in permitting direct comparison of paleontologically dated sections with a common independent framework, and in determining the absolute ages to be associated with important paleontological events. 1V. Losvrie 106

[-A ¡ CALCAREGUS NANNOFOSSILS FORAMINIFERA ‘ ‘ -A o ea ‘ Contessa DSDP Coptesga DSDP e ‘2 ‘- - Ouarry 522/5224 Quaery 522/5224

EaÓy OB Mio- u O. abiseotus & sc Retico leí eres Sr a Ola bi gente oides Op cl ¡ can go [it hus bisecta 01 nbi ge,inteid es ab’ sedes ph, tel it HAs

lii u [2 u Cgo tebigenina Sp benetit hes airplla perlerA

h Disceasíer Oleborotal a Hanlb crin a Di sotaste, ‘E ~ O. cerroazAlensts 5 arbadíersis bat ba di ers i 5 1 C,rrsaAASeSlstS [si> It [s.l.> u Isthnrnti titos Olebi genintatheka o ~ <1 hhantkeeirta no Oteb¡ seninatheka ‘2 u

Type Number Age Dilference of event of events (DSDP - Tethys)

FAO 6 + 0.45±0.39 Myr

LAD 8 — 0.57±0.28 Myr

Fig. 4. Magnetostratigraphic positions of key LAD and FAO Jevela in the Conleasa quarry section and DSDP site 522 (Lowrie, 1986; after Forre el al?, 1982), using the magnetie Po- larity time seale of Laflrecque el al? (1977). Magnetostratigraphy and

GEOMAGNETIC POLARITY HISTORY Tite history of geomagnetie polarity is now well established for Ceno- zoic and late Mesozoic time and quite accurate paleomagnetic polarity time seales have been developed on tite basis of tite dated reversal sequen- ces. Tite detailed reeord of geomagnetie polarity from tite late Middle Ju- rassic to the present is derived from the interpretation of lineated ocea- nic magnetic anomalies generated by tite sea-floor spreading process (Heirtzler el aL, 1968). Titis reversal record is tite longest continuous his- tory of geomagnetic polarity avaitable, and is tite standard of comparison for alí magnetostratigraphic studies in titis range of geological ages. Tite reversal record can be divided broadly into titree parts: a mixed polarity interval from tite present back to tite Late Cretaceous, a quiet interval of apparently constant normal polarity in the Middle Cretaceous, and a mixed polarity interval in the Early Cretaceous and Late Jurassic.

Neogene Magnetie Poíarity History Direct determination of radiometric ages gives a magnetic polarity time seale for chrons younger titan 5 Myr (Mankinen and Dahymple, 1979). Tite same polarity sequence itas been confirmed also in modern deep sea sediments (Opdyke et aL, 1966; Foster and Opdyke, 1970; Opdy- ke, 1972). Tite precision of tite KIAr metitod is not adequate to resolve older sitort citrons by direct dating. However, it itas been possible to ex- tend absolute dating back to about 10 Myr in thick lava sequenees by in- corporating tite stratigraphic relationships of tite paleomagnetic sites (McDougall el aL, 1976). Tite polarity chrons of tite uppermost Miocene were confirmed in a detailed magnetostratigrapitic study of paleontologi- cally dated marine clay sections on Crete (Langereis el aL, 1984). Alt- itough several studies of Pliocene and polarity history have been carried out in very long, overlapping piston cores of unconsolidated deep-sea sediments (e.g. Foster and Opdyke, 1970; Theyer and Ham- mond, 1974), the correlations with tite magnetie time seale are someti- mes ambiguous and some of tite correlations itave recently been questio- ned (Berggren el al., 1985). Due to poor recovery in DSDP and ODP co- res, and the scarcity of suitable exposures on land, most of tite Miocene geomagnetic polarity history has not been verified and dated by indepen- dent magnetostratigraphic investigations.

Paleogene and Late Cretaceous Magnetostratigraphy

Virtualiy ah ihe magnetie polarity citrons oftite Ohigocene, Eocene, Pa- leocene, and Late Cretaceous have been confirmed independently in mag- 108 It? Lovírie netostratigraphic studies of pelagic sediment sections in tite Tethyan realm (Fig. 5), and in DSDP cores. Detailed paleontological dating of the magnetostratigraphic sections has tied tite positions of major paleontolo- gical stage boundaries to tite geomagnetie polarity sequence. Tite associa- tion of absolute ages with titese tie-levels itas enabled tite construction of magnetic polarity time scales, which in turn allow biozonations to be re- lated to a framework of absolute ages, and provide dates for important pa- leontological events. Among the important stage boundary correlations in titis time are tite Oligocene/Miocene boundary near the chron 6Cn/6Cr boundary, the association of tite Eocene/Oligocene boundary with chron 1 3r (Lowrie el al., 1982), tite position of tite Cretaceous/Tertiary boun- dary near to the young margin of chron 29r, and the end of tite Creta- ceous quiet interval very close to tite oíd edge of citron 33r (Alvarez el aL, 1977; Lowrie and Alvarez, 1977).

Early Cretaceous and Late Jurassic Magnetostratigraphy

Tite period from early Aptian to tite end of the Campanian is charac- terised by constant normal polarity. Ocassional reports of negative pola- rity zones wititin tite Cretaceous quiet interval have not been verified by repetition in otiter magnetostratigrapitie sections. The start of titis quiet interval, after the youngest polarity chron of tite M-sequence (MO), has been established as very early Aptian, just younger titan the Aptian-Harre- mian boundary, in pelagie carbonate sections in tite Southern Alps (Chan- nelí el aL, 1979) and in Umbria (Lowrie el aL, 1980). Most of the Lower Cretaceous polarity chrons were confirmed in magnetie stratigraphy in- vestigations in Umbria (Lowrie and Alvarez, 1983; Cirilli el aL, 1984), with the exception of citrons Ml Ir to MI3n where the limestone magne- tization was too weak to be measureable (Fig. 3). Recent studies in tite Italian Southern Alps itave independently confir- med and dated tite polarity chrons from MS to M23, including tite inter- val MIl r-M 1 3n (Citannelí et aL, 1987). The magnetic stratigraphy of thc type section (Cjalbrun and Rasplus, 1984) was difficult to corre- late unambiguously witit tite polarity record. The base of the section did not reach tite Jurassic/Cretaceous boundary. the location and correlation of tite iurassic/Cretaceous boundary to tite magnetie polarity time seale illustrates a furtiter application of mag- netostratigrapity to the syncitronisation of different paleontological da- ting schemes. Titere is no unanimous agreement among paleontologists on tite definition of this boundary, due in part to tite difficulty of corre- laíing one paleontological dating seheme with another. Rcsults from Um- bria, tite Soutitern Alps and DSDP sites (Fig. 6) give different relative po- sitions of tite boundary, depending on which paleontological scheme is Magnetos

[o

23

vM el A2M

3A

5r T NS

3 e ¡ ¡ Iii É Fig. 5. Summary of magnetostratigraphic results from sections of Cenozoic, Late and Middle Cretaceous age in Umbria and the Southern Alps, and correlation with the oceanie magnetie polarity record (Lowrie and Alvarez, 1981). lío W. Lowrie

MiO M17 Mli M19

FessiI Type L oca Iiiy ca ¡plena ¡uds 1 N. Apenemos Nannocanus colomil — S. AIps dinollagellates —— fj site 534 N. ceíoni¡¡ site 534 calp¡onellids site 534 BERRIASIAN

Fig. 6. Comparison of different locauions of Use iusassicfCresaeeous boundary based un different dating sehemes (Lowrie and Channelí, 1984b). used (Ogg, 1984). Most investigations place tite boundary near reversed polarity citron Ml8r, but it has been correlated as young as the oíd edge of citron Ml 7r, on the basis of calpionellids in tite Umbrian lvlaiolica Ii- mestone (Lowrie and Channelí, 1984a), and as oídas the middle of citron Ml 9n, based on nannofossils, dinoglagellates and calpionellids in DSDP site 534 (Rotit, 1983). Hased on calpionellid biozonations, tite Juras- sic/Cretaceous boundary was placed in the base of citron Ml 7r in tite Umbrian Bosso section (Lowrie and Channelí, 1 984a). Citannelí and Grandesso (1987) reinterpreted tite Umbrian calpionellid zonation and suggested a correlation wititin Mi 8n. Depending on how the Jurassic/Cre- taceous boundary is defined in terms of ammonite zonation and tied to tite calpionellid zonation, titey found alternative correlations in Southern Alps sections witit the base of Ml7r or the top of chron Ml9n. Ogg and Lowrie (1986) have suggested resolving the uncertainty of paleontological definitions of the Jurassic/Cretaceous boundary by defining it at tite base of chron MlSr, a readily indentifiable event, and using this defmition to calibrate tite different paleontological dating schemes against each otiter. A position of the Jurassic/Cretaceous boundary consistent with this defi- n~tton has been fnuind in magnetostratigrapitie -sections--in-tite-South-em Alps (Channelí a aL, 1987). Thc M-sequencc magnetie polarity history for tite Late Jurassic (Fig. 7) itas been confirmed in the Foza and San Giorgio magnetostratigraphic sections in nortitern Italy (Ogg, 1981) and in the Carcabuey and Sierra Gorda sections in southern Spain (Ogg et aL, 1984). Although several mag- Valle Magneíos

Lowrie & Ogg 1986 *del 13

135-

San Giorgio 140- 1¡

145

Fig. 7. Summary of magnetostratigraphic results from sections of Early Cretaceous and Late Jurassic age in Umbria, the Southern Alps and Spain, and correlation with the M-se- quence magnetie polarity time seale (Lowrie and Ogg, 1986). Descriptions of sections: Um- bria: Bosso (Lowrie and Channell, 1 984a); Sou

Middíe and Earíy Jurassic Magnetostrat¡graphy

For tite early Jurassic polarity history before tite Jurassic quiet interval titere is no oceanic record. Wititout titis guide, the correlation between mag- netostratigrapitic sections becomes much more difflcult. Magnetie pola- rity sequences, dated with ammonites in some cases, have been reported for early Jurassic sections in Italy, Hungary and Spain but coeval polarity re- cords do not agree well with eacit other (Citannelí el al., 1982). Contem- poraneous sections from different sedimentary environments might not be expected to show the same polarity sequence if tite sections have dif- ferent of sedimentation rate. However, even overlapping sections from nearby localities show poor internal consistency in Umbria (Chan- nelí el aL, 1982) and in northern Spain (Stciner el aL, 1985). The magne- tostratigrapitie studies were carried out with tecitniques comparable to titose used in successful correlations of younger sections to the known ma- rine record. Although the magnetic stratigrapities contain securely defi- ned intervals of normal and reversed polarity, many of tite observed mag- netozones are defined only by single samples. This may imply inadequate sampling intervals, or that the frequency of reversals in tite early .lurassic was itigh. Alternatively, tite breakdown of consensus between early Juras- sic sections may most simply be ascribed to variation of sedimentation rate wititin and between the investigated magnetostratigraphic sections. Possibly tite depositional basins in tite Tetitys following tite brcak-up of Pangea were smali and characterised by inconstaní sedimentation. Ihis Magneíos

MAGNETIC POLARITY TIME SCALES

Magnetie polarity time seales for tite last 5 Myr can be formed by di- rect of lavas, for which tite K/Ar metitod is accurate enough to resolve individual polarity chrons lasting about 100,000 yr or longer. Tite older polarity chrons in time scales cannot be dated directly. Titeir ages must be estimated by linear interpolation between known tie- points such as, for example, tite paleontological stage boundaries witich have been correlated by magnetostratigrapity to tite polarity sequence. Within each time seale ages are calculated to tite nearest 0.01 Myr (10,000 yr), which is close to tite achievable resolution. This relative ac- curacy is about 100 times better titan the absolute accuracy with witich tie-point ages are known. The first magnetic polarity time scale for tite Late Cretaceous and Ce- nozoic was constructed by Heirtzler el aL, (1968), primarily on the basis of a very long magnetic anomaly profile in tite South Atlantic. Tite ocean ridge anomalies younger than 335 Myr were correlated directly to the ra- diometrically dated polarity time scale, and older anomalies were dated by extrapolation from this very short base. The time seale of LaBrecque el aL (1977) incorporated tite following improvements: (1) the sequence of sitort reversais originally named anomaly 14 was dropped, as it was ab- sent in most marine magnetic anomalies; (2) a revision of tite relative lengths of tite older Late Cretaceous chrons 29 to 34 was included; and (3) tite magnetostratigraphic correlation of tite Cretaceous/Tertiary boun- dary with reversed polarity citron 29r (Lowrie and Alvarez, 1977) was adopted as a tie-point, and associated witit an age of 65 Myr. Tite ages of intervening polarity citron boundaries were calculated by interpolation. The sequence of polarity in the LaBrecque el al., (1977) time seale itas been used in alí subsequent versions, stretched and contracted in linear segments between tie-points. Tite major stage and sub-stage boundaries of the Upper Cretaceous and Paleogene were correlated to tite geomagnetic polarity sequence as a re- sult of extensive magnetostratographic research in pelagic marine carbo- nate sections on land (Alvarez el aL, 1977; Lowrie el aL, 1982) or sam- pled in the Deep Sea Drilling Project (LaBrecque el al., 1983). Lowrie and Alvarez (1981) developed a magnetic polarity time seale for the Upper Cretaceous and Cenozoic based upon interpolations between II correla- ted tie-points, for which revised absolute ages had been calculated. Some of titese ages were poorly chosen, and resulted in apparent sudden citan- ges in sea-floor spreading rate. To minimize Ihese effects while retaining 114 1V. Lowrie as many of the magnetostratigraphic correlations as possible, Cox (1982) dropped tite sub-stage boundaries as tie-points, and adj usted the absolute ages of tie-points within titeir error limits so as to obtain a time-scalc which was consistent with a model of nearly constant sea-floor spreading. A more radical polarity time scale for the Cenozoic was prepared by Berggren el aL (1985), who disregarded the magnetostratigraphic correla- tions of stage boundaries. They substituted a handful of selected radio- metric age dates which met titeir own criteria. At least two of titeir tic- points are wrong, because they derive from the misinterpreted Oligocene magnetostratigrapity of Protitero el al. (1982). On balance, the most appropriate magnetie polarity time-seale for the Late Cretaceous and Cenozoic is probably titat of Cox (1982). It makes optimum use of available magnetostratigraphic correlations, witile mini- mizing drastic citanges in sea-floor spreading rate. The construction of a magnetie polarity time scale for 0w M-sequen- ce polarity citrons is itandicapped by tite absence of well dated tie-levels. Although an adequate number of tite Lower Cretaceous and Upper Jurassic stage boundaries itave been correlated satisfactorily to tite M-sequence, the absolute ages for titese boundaries are not well known. The polarity sequence is basically that obtained by Larson and Hilde (1975) from tite interpretation of M-sequence magnetic anomalies in the North Pacific and Atlantic oceans. Tite original calibration of these anomalies was ba- sed upon the paleontological ages of tite sediment in contact with igneous basement in DSDP boles drilled near tite young end and the oíd end of the M-sequence. Assuming approximate absolute ages for tite ends, tite in- tervening polarity citrons were again dated by linear interpolation. Tite same metitod was used by Vogt and Einwicit (1979), who introduced an additional early Valanginian calibration point for DSDP site 387 within the M-sequence. It is often uncertain whether a DSDP hole has indeed reached igneous basement, or whether it itas merely encountered an in- trusion. Also, tite length of time between formation of tite lava and de- position of the first datable sedimenis is not known. Finally, tite paleon- tological dates often itave large errors, amounting to an entire stage or more, and ages determined witit different fossil types can differ appre- ciably. As a result of tite magnetostratigrapitie investigations in pelagic limes- tone sequences in Italy chron MO was correlated with the ver>’ earliest Ap- tian, close to tite boundary (Citannelí el aL, 1979; Lowrie el aL, 1980). Titis was incorporated as one of two tie-points in a magnetic polarity time-scale for tite M-sequence by Cox (1982); however, the older tie-point was a paleontological age from a DSDP itole. Ogg el al. (1984) correlated tite Oxfordian/Kimmeridgian boundary with the M-sequence just younger titan M25 in magnetostratigraphic sections in Spain (Fig. 7). Kent and Gradstein (1985) derived a magnetic polarity time seale for tite Magne

M-sequence, using titese magnetostratigraphic ties for MO and M25 and the age estimates by Harland el aL (1982) for the Oxfordian/Kimmerid- gian and Barremian/Aptian stage boundaries. They titen compared titeir new time scate with the magnetostratigraphic correlationsof tite otiter Lo- wer Cretaceous and Upper Jurassic stage boundaries (Lowrie and Citan- nelí, 1984a; Lowrie and Alvarez, 1984; Ogg el aL, 1984) and estimated the absolute ages of titese boundaries. Tite Jurassic/Cretaceous boundary is not clearí>’ defined paleontologi- calI>’ (Fig. 6), and Ogg and Lowrie (1986) proposed titat an optímum correlation is with tite base of polarity citron Ml8r. Lowrie and Ogg (1986) added this tie-point to the MO and M25 correlations. They disre- garded tite age estimates of Harland el aL (1982) for titese titree tie points as ínvalid, and adopted tite values given by Hallam el aL (1986). A mag- netic polarity time scale for the M-sequence polarity chrons was titen constructed by linear interpolation on tite two segments MO-M 18 and M19-M25; ages for M25-M29 were estimated by extrapolation of tite Ml 9-M25 segment. Ages were also estimated for the otiter Lower Creta- ceous and Upper Jurassic stage boundaries from their correlations with tite polarity time seale. Because of the additional Jurassic/Cretaceous tie point and the different ages assumed for tite three calibration points, the ages of alí polarity chrons in the time seale of Lowrie and Ogg (1986) are 5-10 Myr younger than in the time seale of Kent and Gradstein (1985), although tite agree quite closely with earlier estimates of titese stage boun- daries by Van Hinte (1976a, 1976b). Tite uncertainties in absolute ages of calibration points result in large differences between M-sequence time scales (Fig. 8).

Dl SCU SS ION

Tite record of geomagnetic polarity derived from analysis of oceanic magnetic anomalies is dominated by two sequences of mixed geomagne- tic polarity, tite younger extending from tite present until the Late Creta- ceous, and tite older spanning the Early Cretaceous and Late Jurassic. They are separated it>’ an interval of constant normal polarity —tite Cre- taceous Quiet Interval. As a result of oceanic magnetie anomaly surveys tite polarity sequences have become securely established. They itave been largel>’ confirmed independentí>’ in magnetostratigraphic studies. Coordi- nated magnetostratigrapitie and biostratigrapitic investigations in pelagic sedimentar>’ rocks have dated titis histor>’ of geomagnetie polarity. Tite C’retaceous Quict Interval lasted about 35 Myr in tite middlc Cretaceous, from tite earliest Aptian until the Santonian/Campanian boundary. Apart from titis interval —in which magnetostratigrapitic correlations are not 116 1V. Lowrie

0dm Var> Hinte Lowrie & Ogg Harland e> aL & 1982 1976a, 1976b 1986 1982 Orado tein 985 110— Ap —110 Ea Apt¡an Ap

Ha Bar rernian Sa AP Ap 120— —120 Ve Ea Ea Haojteriviar, Ha

Ha Ha Bar Va la ngin¡an Va 30— -130 Po Berr¡asiarrt Ser Va Va Portland an Ki 4 Ti thonian —ita LO— Ser Sor Os K¡mmer¡dgian Osfordian Ki Os Ti Ti 150- —150 Ki Kl

Os Ox lOO— —160 Fig. 8. Comparison of different time seales for the Early Cretaceous and Late Jurassic.

possible— most of tite major paleontological stage boundaries from the Oxfordian/Kimmeridgian to tite Oligocene/Miocene have now been corre- lated reasonabí>’ satisfactorily to tite oceanic polarity sequence. Titese correlations have fostered tite construction of magnetic polarity time sca- les covering tite Late Jurassic, Cretaceous and entire Cenozoic. In turn, undated paleontological stage boundaries, as well as paleontological zo- nes and events, have been dated by titeir correlations to titese time scales. Different versions of tite same polarity time seale reflect successive im- provements in tite defmition of tite polarity sequence, tite number of correlated tie-levels used, and tite optimum ages accepted for tite tie-le- veIs. Tite>’ also reflect different critical evaluations of tite radiometric age data base. Discrepancies between different versions of tite Late Creta- ceous and Cenozoic polarity time scale are small (Table 1); tite differen- ces for tite M-sequence polarity chrons are larger (Fig. 8). In order to re- solve titem and produce definitive versions of eititer time scale, improve- ments in tite number and quality of absolute age estimates for the tie-le- veIs are needed. Magne

TABLE 1

LaBrecque Lowrie and Cox Berggren sTAcIE et aL, 1977 Alvarez 1981 1982 el aL, 1985 Cliron Age Cliron Age Age Age Miocene 6Br 22.5 6Cr 24.6 24.5 23.7 Oligocene 15r 38 13r 38 38 36.6 Eocene 23 55 24r 54.9 55 57.8 29r 65 29r 66.7 65 66.4 top 33 72.5 top 33 72.3 73 75 Campanian 33r-34 80 33r-34 84.1 83 84 Santonian

Table 1. Ages (in Myr) of Late Cretaceous and Paleogene stage boundaries in different po- lariíy time scales. In the time seale of LaBrecque el al? (1977) the correlations of stage boun- daries are inferred from their assumed ages, except for the Cretaceous/Tertiary boundary and in the Late Cretaceous. The magnetostratigraphic correlations of stage boundaries with the polarity chrons in the time scale of Lowrie and Alvarez (1981) are also used in the time seales of Cox (1982) and Berggren el al? (1985).

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