Milankovitch cyclicity and rock-magnetic signatures of ...studied in detail. Rock-magnetic results from Cismon and Pra da Stua are presented. The cyclostratigraphy of the Cismon section

Cretaceous Research (1999) 20, 189–214Article No. cres.1999.0145, available online at http://www.idealibrary.com on

Milankovitch cyclicity and rock-magneticsignatures of palaeoclimatic change in the EarlyCretaceous Biancone Formation of the SouthernAlps, Italy

Helmut Mayer1 and Erwin Appel

Institut fur Geologie und Palaontologie, Abteilung Geophysik, Eberhard-Karls-Universitat Tubingen, Sigwartstr. 10,72076 Tubingen, Germany. 1Present address: Institute of Arctic and Alpine Research, University of Colorado,Boulder, CO 80309-0450, USA; email: [email protected]; also at: Geomathematik, Fachbereich VIGeographie/Geowissenschaften, Universitat Trier, 54286 Trier, Germany

Revised manuscript accepted 21 October 1998

Detailed cyclostratigraphic analyses of the Valanginian to Hauterivian part of the Biancone Formation, a pelagic nannofossillimestone in the Southern Alps of Italy, were carried out. The Cismon section in the Belluno Trough near Feltre and the Prada Stua section on the Trento Plateau near Avio were studied. Carbonate content, magnetic susceptibility and naturalremanent magnetization were measured on densely spaced samples from Cismon. The first two properties vary in a cyclicfashion in this pelagic limestone section and are almost perfectly negatively correlated, while cyclicity in natural remanentmagnetization is only vaguely indicated. Quantitative time-series analysis is critical in cyclic stratigraphy. The geostatisticalmethod of cova functions (a generalization of the cross-variogram) which has proven to be the most versatile and robusttime-series-analysis method is applied. Cova functions can be calculated from unevenly and non-correspondingly spaced timeseries without any preprocessing. This method also retains relatively more of the signal when noise and extreme outliersobscure the picture. The periodicities detected in the Cismon time series fall in the range of Milankovitch cycles. Cycleperiods of 45 cm, 80 cm and 180 cm likely correspond to dominant precession, obliquity and eccentricity cycles. Owing to theinaccuracy of the Cretaceous time scale, periods cannot be matched exactly, but cycle ratios are extremely close to expectedratios so that Milankovitch climate cycles could be positively identified in this Early Cretaceous section. In the Pra da Stuasection bedding thickness was measured and analyzed quantitatively. A cycle period of 55 cm is dominant in this data set,while periods of 115 cm and 170 cm are only vaguely indicated, although bedding in the sampled interval visually appearscyclic and even hierarchically structured. It can be expected that densely spaced measurements of sedimentary propertiessuch as susceptibility and carbonate content will reveal the cyclicity much better. This identification of Milankovitch cyclicityin the pelagic Biancone Formation has important consequences for our understanding of the climate system in the past.These results demonstrate that orbital forcing was effective enough to create palaeoclimatic cycles even in the Cretaceouswarm, equable, ice-free climate state. Magnetic susceptibility proved to be a reliable proxy for carbonate content reflectingpalaeoproductivity cycles in this pelagic setting. ? 1999 Academic Press

K W: Milankovitch cycles; rock magnetism; palaeoclimate; carbonate content; susceptibility; Biancone Formation;Valanginian; Hauterivian; Southern Alps.

1. Introduction

The focus of this study is on sedimentary-parametervariations through the sections studied, their analysisas stratigraphic time-series and their palaeoclimaticinterpretation. The role of rock-magnetic parametersin this context is emphasized. The study area wasselected based on the following criteria: continuity ofsedimentation, uniformity of facies, lack of tectonicand metamorphic overprint, low degree of diageneticalteration and quality of exposure. In all these respects

0195–6671/99/020189+26 $30.00/0

the Mesozoic sequence of the Southern Alps offeredmost favourable conditions. The Valanginian toHauterivian portion of the Cismon section wasstudied in detail. Rock-magnetic results from Cismonand Pra da Stua are presented. The cyclostratigraphyof the Cismon section is investigated utilizing suscep-tibility and carbonate-content fluctuations. For thePra da Stua section bedding-thickness measurementsare analyzed. Quantitative time-series-analysis isapplied to the evaluation of these geologic time series.

? 1999 Academic Press

190 H. Mayer and E. Appel

The palaeoclimatic significance of Milankovitch cyclesand rock-magnetic parameters in the Cretaceous isdiscussed. This paper contains overview sections toprovide some background information about thevarious fields of research tied together here.

2. Geological setting

Evolution of the Southern Alps

The sections studied are located in the SouthernAlps of northern Italy (Figure 1), which represent atectonically inverted, former passive continentalmargin.

Extensional tectonism began in the Early Jurassicwith the rifting and opening of the Piemonte-LigurianTethys Ocean. On its southern end a block-faultedextended continental margin formed, whose palaeo-relief is clearly reflected in Jurassic facies distributionsin the Southern Alps (Aubouin, 1963; Bernoulli &Jenkyns, 1974; Gaetani, 1975; Winterer & Bosellini,1981). From west to east, the alternating palaeogeo-graphic basins and swells are the Lombardian Basin,Trento Plateau, Belluno Trough and Friuli Shelf(Figure 1).

Jurassic breccias and slump deposits as well asthickness contrasts along the boundaries betweenthese blocks are evidence for synsedimentary normalfaulting (Bernoulli, 1964; Bosellini et al., 1981). Dur-ing the latest Jurassic and Early Cretaceous the entirerealm deepened, and differential movements betweenthe blocks diminished so that a blanket of pelagicnannofossil-lime ooze draped over the pre-existingrelief which was gradually levelled out. This nanno-

fossil ooze was deposited at the basins in depths ofseveral thousand metres (Bosellini & Winterer, 1975).The Trento Plateau also received a reduced thicknessof this pelagic sediment. The resulting limestone isgenerally known as the Maiolica Formation or locallyas the Biancone Formation. In the Belluno Trough,where the Cismon section is situated, the BianconeFormation extends stratigraphically from latestTithonian/Berriasian to Aptian (Weissert, 1981). Ingeneral the Maiolica/Biancone is characterized byabundant slumps and similar synsedimentary defor-mation features (Weissert, 1981). Pelagic conditionsprevailed through the Cretaceous into the Eocenewhen terrigenous flysch was deposited in response tothe onset of Alpidic deformation. The tectonic defor-mation of the Southern Alps during the Alpidic orog-eny produced gentle large-scale folds, thrust faultsand transcurrent faults (e.g., van Bemmelen, 1966;Doglioni & Bosellini, 1987). During the Neogene theVenetian Alps in particular, i.e., that part of theSouthern Alps where the Cismon section is located,have been deformed into a fold-and-thrust belt ofcoherent thrust blocks with little internal deformation(cf., Doglioni, 1992). Overall, the Alpidic defor-mation of the Southern Alps was relatively mildcompared to that of the Western and Eastern Alps.

Figure 1. Location map for Cismon and Pra da Stuasections in northern Italy. Palaeogeographic domains ofthe Southern Alps are shown: diagonal ruling—Lombardian Basin; horizontal ruling—Trento Plateau;stippled pattern—Belluno Trough; vertical ruling—Friuli Shelf (boundaries after Gaetani, 1975, andWeissert, 1981).

Stratigraphic framework

The development of the stratigraphic sequence of theSouthern Alps (Figure 2) since the Jurassic was asfollows. On the Trento Plateau peritidal conditionsprevailed through the Liassic. During the MiddleJurassic a thin layer of red nodular limestone (RossoAmmonitico Inferiore) covered the platform reflectingits drowning by suddenly increased subsidence.Pelagic conditions continued with the deposition ofcherty aptychus limestones (Oxfordian FonzasoFormation) and the Kimmeridgian Rosso Ammo-nitico Superiore (Bosellini et al., 1981). Starting withthe Tithonian the white nannofossil-lime ooze of theBiancone Formation covered the area and accumu-lated slowly through the Early Cretaceous andCenomanian, when it was replaced by the ScagliaRossa, reddish pelagic limestones and marls extendingacross the Cretaceous/Tertiary boundary (PremoliSilva & Luterbacher, 1966). Near the western marginof the Trento Plateau the varicoloured marls andmarly limestones of the Scaglia Variegata are inter-calated between Biancone and Scaglia Rossa, similarto the situation in the Lombardian Basin (Cita, 1964).In the Eocene the Scaglia Rossa grades into the marlsof the Scaglia Cinerea. Volcanic layers are common inthese Palaeogene beds. However, during the Eocene a

Milankovitch cyclicity and rock-magnetic signatures of palaeoclimatic change 191

Figure 2. Schematic stratigraphic succession of Mesozoicstrata in the Southern Alps (Dolomites/Venetian Alpssector). Special facies on the western Trento Plateau onthe left (after Gwinner, 1971, and Bosellini et al.,1981).

drastic change in the depositional conditions occurredwith the onset of flysch sedimentation which wasfollowed by molasse from the Miocene onwards. Theneighbouring Belluno Trough features a very similarbut thicker and more complete sequence (Praturlon &Sirna, 1975). The main differences are in the Lowerand Middle Jurassic. The plateau margin migratedsuccessively west during the Early and Middle Jurassicas individual fault blocks subsided (Sarti et al., 1992).The Oxfordian Fonzaso Formation, which occursonly in patches on the Trento Plateau, is coherentlydeveloped in the Belluno Trough. Starting with theRosso Ammonitico Superiore in the Kimmeridgianthe relief between the former swell and trough hadbeen greatly reduced. The Biancone and the remain-der of the sequence are almost identically developed inboth areas apart from thickness differences and thelack of volcanism in the Belluno Trough.

This study is restricted to the Lower CretaceousBiancone Formation of the Trento Plateau and theBelluno Trough, which corresponds to the Maiolica ofthe Lombardian Basin to the west and to the Maiolicaof the Umbrian-Marchean Basin in the Apennines.Sedimentologic and lithostratigraphic investigationsof the South Alpine Maiolica/Biancone Formationwere carried out by Weissert (1981), Barberis et al.(1992) and Bersezio (1993).

Since the chronostratigraphy, biostratigraphy,magnetostratigraphy and geochronology of the EarlyCretaceous stages covered are still active fields ofresearch with many unresolved problems, a numberof conflicting correlations are in use. The chronostra-tigraphy of the Early Cretaceous is, continuing fromthe Jurassic, based on ammonite zones. These arewell studied in stratotypes in southeastern France(Cotillon et al., 1984; Bulot et al., 1992). However, asammonites are rather rare in most portions of thepelagic sequence of the South Alpine and Apenninicbasins, microfossils and nannofossils have become themost useful groups for biostratigraphy there.

In the Upper Tithonian to Lower Aptian Maiolicaof the Apennines, Micarelli et al. (1977) established amicrofossil zonation based on nannofossils, calpionel-lids and foraminifera. Similar efforts were undertakenin the Southern Alps by Channell et al. (1979) whoincluded magnetostratigraphy. The foraminiferalzonation goes back to Sigal (1977), while the calpi-onellid zonation was established by Allemann et al.(1971). The first calcareous-nannofossil zonation ofThierstein (1971) was continuously refined (cf.,Bralower, 1987; Bralower et al., 1989; Bergen, 1994).Nevertheless the correlation to stage boundaries andmagnetic-polarity chrons is fraught with problems(cf., Rawson, 1983; Cooper, 1984; Remane, 1986;Marton, 1986). A number of investigations in theCretaceous of the Southern Alps have contributed toan improvement of these correlations (e.g., Channellet al., 1979, 1987; Channell & Medizza, 1981;Bralower, 1987; Channell & Grandesso, 1987; Erba &Quadrio, 1987; Channell & Erba, 1992).

A new definition of Cretaceous stage boundaries iscurrently being developed. The boundaries of interestto this study have recently been proposed as followsat the Second International Symposium on Creta-ceous Stage Boundaries, Bruxelles, 8–16 September,1995 (J. Erbacher, personal communication 1995;Mutterlose, 1995): the Berriasian/Valanginian bound-ary at the base of the Calpionellites darderi Zone (verypreliminary), the Valanginian/Hauterivian boundaryat the first occurrence of the ammonite genus Acanth-odiscus corresponding to the last occurrence of the


nannofossil Tubodiscus verenae and a positive carbon-isotope (ä13C) excursion, and the Hauterivian/Barremian boundary at the first occurrence of theammonite species Spitidiscus hugii. Recently Ceccaet al. (1994) and Channell et al. (1995a) proposedrevisions to the correlations between biostratigraphiczones, magnetic chrons and the Hauterivian andBarremian stage boundaries based on new ammonitefinds. Earlier, Channell et al. (1993) had attributedreduced intensities of magnetization in the upperValanginian, where the positive ä13C excursion oc-curs, to increased magnetite dissolution by reductionwhich in turn was induced by increased carbon burial.According to the new set of magnetobiostratigraphiccorrelations proposed by Channell et al. (1995a) andby Erba et al. (1995) the Valanginian would comprisemagnetic chrons CM15N through CM11R, while theHauterivian would extend from CM11N throughCM4. This means that the sections describedhere would belong entirely to the Hauterivian, sincethey could be correlated to chrons CM10N-CM8(Cismon) and CM10-CM5 (Pra da Stua), respect-ively (Mayer, 1996, 1997). However, until these rec-ommendations are formally accepted, it is preferred todesignate the sections of this study as ‘Valanginian toHauterivian’, which is also consistent with the lowmagnetization intensities encountered (see below).

Sedimentary cyclicity

Apparent stratigraphic cycles in sedimentary rocksequences are common throughout the stratigraphicrecord, occur in a variety of depositional environ-ments and have caught the attention of geologistsearly on (see Gilbert, 1895, 1900). Weller (1930)recognized the widespread repetition of characteristicsedimentary cycles in the Pennsylvanian (UpperCarboniferous) of the North American midconti-nent consisting of a given sequence of lithologies.From then on research on sedimentary cyclicityconcentrated on the transgressive/regressive cyclesin the coal-bearing Carboniferous deposits in NorthAmerica (Weller, 1966) as well as in Britain(Robertson, 1952). Starting with Schwarzacher’s(1947, 1954) pioneering observations on periodicchanges in bedding thickness of the Dachsteinkalk ofLofer in the Triassic of the Northern Calcareous Alps,Austria (in particular the hierarchical structure withbundling of five smaller cycles in one large cycle),continued by Fischer’s (1966) sophisticated elabor-ation on the Lofer cyclothems, this type of cyclicity insedimentary rocks began to attract more and moreinterest of the geologic community. This increasedinterest became manifest in the publication of a

landmark volume titled ‘‘Symposium on CyclicSedimentation’’ (Merriam, 1966) by the KansasGeological Survey.

Duff & Walton (1962) made an effort to assess thecyclicity of sedimentation quantitatively by the appli-cation of statistical methods, which was laterexpanded by Duff et al. (1967). Indeed, this is theonly way to document true cyclicity convincingly,since geologists tend to see more order in nature thanthere actually may be, as Zeller (1966) suggested inhis provocative article ‘Cycles and psychology’.

Various types of cyclic succession have been recog-nized. In a succession of more than two lithologies(e.g., a, b, c) cycles can be symmetric (a-b-c-b-a-b-c. . .) or asymmetric (a-b-c-a-b-c. . .). Such cyclesare common in clastic or mixed carbonate-clasticsequences, marine, lacustrine or fluvial as in theCarboniferous coal measures where cyclic sedimen-tation was studied early on. They also occur inshallow-water carbonate platforms with subtidal, in-tertidal and supratidal beds (e.g., Read et al., 1986).In hemipelagic and pelagic limestone-marl alter-nations, where the focus of research in cyclic sedimen-tation has shifted to since the 1970s (e.g., Dean et al.,1978; Volat et al., 1980; Einsele, 1982; Cotillon,1984; Einsele & Ricken, 1991; Huang et al., 1993),generally only two lithologies are involved (a-b-a-b-a-b. . .), but redox cycles may be superimposed on thecarbonate cycles resulting in the occurrence of inter-vening black-shale beds. In many cases the limestonemember of the couplet is thicker than the correspond-ing marl or shale member, but the opposite situationalso occurs (Einsele, 1982). Cyclic fluctuations ofcarbonate content can even be detected in mon-otonous limestone sequences, thus providing a con-tinuously defined lithologic parameter which lendsitself to direct numerical analysis.

In the case of clastic cycles and of most platform-carbonate cycles their primary depositional origin isobvious and goes unchallenged. This is not the casefor limestone-marl alternations and carbonate cyclesin limestones. A purely diagenetic origin or a domi-nant diagenetic modification has been postulated forthis type of cycle by several authors (e.g., Sujkowski,1958; Hallam, 1964, 1986, 1987; Eder, 1982;Ricken, 1985, 1986; Ricken & Eder, 1991; Thierstein& Roth, 1991). On the other hand, many studiesdemonstrated that rhythmic diagenetic unmixingfrom a perfectly homogeneous primary marl sedimentis extremely implausible (cf., Arthur et al., 1984;Fischer, 1986; Ogg et al., 1987; Weedon, 1987) andthat at least a small primary difference in carbonatecontent or structure must have existed which mayhave been more or less modified by diagenesis.


Bioturbation is another factor perturbing primarysedimentary signals (e.g., Guinasso & Schink, 1975)and could potentially obliterate cyclicity. Forexample, Berger & Heath (1968) demonstrated thatthe effective vertical displacement of particles is 9 cmfor a typical homogeneous-layer thickness of 4 cm indeep-sea settings. This is a considerable aberration ifwe look for first or last appearances of microfossils forbiostratigraphic purposes. However, if we look forcyclicity using quasi-continuous physical or chemicalproperties, the effect of bioturbation is much lesssevere, it acts like a moving-window averaging filterand thus reduces the highest attainable resolution(e.g., Trauth, 1995). It does not distort periodicitiesin the Milankovitch band at typical pelagic sedimen-tation rates, but may decrease the amplitudes of peaksin power spectra (Dalfes et al., 1984; Pestiaux &Berger, 1984).

Additionally, the effect of variable sedimentationrates on cyclic signals in the sedimentary record needsto be considered. The magnitude of sedimentationrate seems to have little effect except for the obvioussituation of high-amplitude high-frequency signals attoo low a sedimentation rate. In this case the signalcannot be resolved in the sedimentary record (Morley& Shackleton, 1984). Very strong changes of sedimen-tation rate through one section will obviouslyobliterate a time-periodic signal, so that it may beunrecoverable in extreme cases. However, minor fluc-tuations of sedimentation rate do not obliterate theperiodicities severely, so that the resulting distortionsmay be eliminated by mathematical procedures(Schiffelbein & Dorman, 1986; Park & Herbert, 1987;Trauth, 1995).

Another subject of controversy regarding sedimen-tary cycles is the question whether the cycles arecreated by environmental oscillations within the depo-sitional system entirely driven by sedimentation itself(‘autocycles’) or whether the cyclic sedimentation isdriven by oscillations of some driving force outside thedepositional system (‘allocycles’), possibly of globalscale. Note the slightly generalized usage of the terms‘autocycle’ and ‘allocycle’ compared to Beerbower’s(1966) original definition; it portrays the widelyadopted usage of these terms more exactly (cf.,Ginsburg, 1971; Fischer, 1986; Osleger, 1991).Schwarzacher (1993b) demonstrated with the help ofnumerical simulations that the influential autocyclemodel of Ginsburg (1971) for carbonate platformscannot work.

In the following study the apparent cyclicity ofthe limestone-marl type in the Lower CretaceousBiancone Formation of the Southern Alps isinvestigated.

Section description

Two sections in the Lower Cretaceous nannofossillimestone of the Biancone Formation, Cismon andPra da Stua, were studied.

Cismon. Geographically, the Cismon section is locatedin the valley of the river Cismon. Palaeogeographi-cally, it belongs to the western part of the BellunoTrough (Figure 1). The Valanginian to Hauterivianpart of the section, which was sampled, is exposed inthe steep walls along the old road around the tunnel‘‘Pala della Lerla’’ (46.1518)N/11.9063)E), 15 kmnorthwest of the city of Feltre on the road (SS 50)leading to Passo Rolle (Figure 3a).

An almost continuous succession through theCretaceous and into the Tertiary is accessible inroadcuts and cliffs along the river. The BianconeFormation extends here from the higher Tithonianinto the lowermost Aptian. Above that a varicolouredmarly interval follows, the Scaglia Variegata whichresembles the Umbrian Scisti a Fucoidi and extendsthrough the Cenomanian. A hiatus is present in theAptian to Albian (Channell et al., 1979). The remain-der of the sequence consists of Scaglia Rossa. Previousstudies dealt with the sedimentology (Weissert, 1981),the magnetic stratigraphy at lower resolution andbiostratigraphy of the Hauterivian to Campanian por-tion (Channell et al., 1979) and Maastrichtian toEocene portion (Channell & Medizza, 1981), andwith the stable-isotope stratigraphy of the Aptian/Albian portion (Weissert et al., 1985). Claps et al.(1991) performed cyclostratigraphic time-seriesanalysis on the Cenomanian Scaglia Variegata atCismon independent of our cyclostratigraphy in theBiancone (Mayer, 1993, 1994). A new high-resolution magnetostratigraphy was establishedrecently (Mayer, 1995, 1996, 1997).

The lower part of the section is particularly wellsuited for stratigraphic studies, because sedimentationwas continuous, the succession is not disturbedby slumping, and faulting produced only small off-sets which are readily restored. Unlike higher up inthe section bedding is not so clearly developedin the Valanginian part as to allow unambigu-ous bedding thickness measurements which havemostly been used to date for cyclostratigraphicstudies in Cretaceous pelagic carbonates (e.g.,Fischer, 1980; Schwarzacher & Fischer, 1982; Fischer& Schwarzacher, 1984; Herbert, 1992). This lessclearcut bedding is due to only small-amplitudevariations of an overall high carbonate content ofc. 90%. At Cismon, the Biancone Formation consistsof white to light grey pelagic nannofossil limestones


with occasional bands of dark grey to black chertnodules (Figure 3a).

Bedding planes recur every 5 to 20 cm and aregrouped in major units of c. 80 to 100 cm thickness(see Figure 3a). Bedding is generally well developed,but somewhat undulatory and often paralleled bystylolitic seams resembling ‘‘pseudobedding’’ in thesense of Alvarez et al. (1985). But slight lithologicchanges up-section prove that the observed bedding isparallel to the primary sedimentary stratification. Thesampled section is free of slumps. The general bed-ding attitude is 110)/10)NE (strike/dip) in the lowerpart of the section and 85)/17)NW in the upperpart. Several minor normal faults disrupt the section.

Additional deformation is restricted to gentle warping,monoclinal folding and dragfolding next to faults.These normal faults form a graben and some staircaseblocks in the outcrop. The offsets are on the order ofone to two metres and were restored by matchingcharacteristic bed sequences across faults in order topiece together a continuous stratigraphic section. Thepossibility of undetected hiatuses cannot absolutely beruled out, but the absence of physical indications andbiostratigraphic evidence for hiatuses minimizes theprobability of their presence. For the reasons givenabove, thickness measurements were not suited forquantitative cyclostratigraphic analysis. Therefore,31 m of the section were sampled with a hand-heldgasoline-powered rock drill to obtain cores forpalaeomagnetic work and material for the analysis ofphysical properties and geochemistry.

For the cyclostratigraphic investigation the lowerpart of the section was sampled in great detail at anaverage interval of 5.6 cm (151 oriented cores in8.45 m). Based on field observations, this portion wasselected as representative for the entire 145 m ofBiancone at Cismon.

Pra da Stua. The section Pra da Stua is located on theeast flank of Monte Baldo along the road betweenAvio and San Valentino just above the reservoir Pra daStua (45.8883)N/11.2058)E, 11 km from Avio)(Figure 1) and was called ‘‘Valle Aviana’’ by Cita(1964) and by Weissert et al. (1985). Palaeogeo-graphically, this section is situated on the TrentoPlateau, near its western margin (Figure 1). TheBiancone Formation gradually develops through anarrow transitional zone from the underlying RossoAmmonitico Superiore. It extends from the UpperTithonian into the Barremian and is unconformablyoverlain by the Albian to Cenomanian Scaglia Vari-egata. The Biancone here consists of white to drablimestones which are well bedded, with intercalationsof brown to black chert nodules in average intervals of50 cm (Figure 3b). Numerous joints, mostly perpen-dicular to bedding, some curved, dissect the rock.Stylolites in various orientations are common. Thebeds generally strike 20) and dip 15) to the NW. Onlyone normal fault produces an offset of 80 cm in theentire section. Slumping is absent except for a narrowinterval with phacoidal structures at metre 18. Pre-vious work on this section was restricted to biostratig-raphy (Cita, 1964) and to oxygen- and carbon-isotopestratigraphy (Weissert et al., 1985). Weissert et al.(1985, p. 537) also mention an unsuccessful attemptat magnetostratigraphy. A preliminary magneto-stratigraphy correlating the Biancone Formation atPra da Stua to polarity chrons CM10 to CM5 was

Figure 3. Outcrop of Early Cretaceous Biancone For-mation. (a) Around road tunnel Pala della Lerla, 15 kmNW of Feltre along SS 50 in the Cismon valley. Normalfaults display only small offsets. Bedding planes arespaced from 5 to 20 cm and bundled into major units80 to 100 cm thick. (b) Opposite the reservoir Pra daStua, 11 km W of Avio on the east flank of MonteBaldo, level 2600 cm to 2800 cm (Scale with centi-metre subdivisions on the left and inch subdivisions onthe right is located in lower centre of photograph).

a

b


published recently (Mayer, 1996, 1997). For cyclo-stratigraphic analysis bedding thickness was measuredin the outcrop in an interval where bedding appearedparticularly cyclic and even hierarchically structured(level 2500 cm to level 2900 cm, CM5, Hauterivian;Figure 3b).

3. Rock magnetism

In the course of magnetostratigraphic studies of theCismon and Pra da Stua sections (Mayer, 1995,1996, 1997) rock-magnetic experiments were carriedout to obtain some information on the magnetic-mineral content of the rocks studied. Previous studieshad revealed that the dominant magnetic mineral inthe Maiolica/Biancone limestone is magnetite in theSouthern Alps (Channell et al., 1979, 1992, 1993;Channell & Erba, 1992) as well as in the UmbrianApennines (Lowrie et al., 1980a, b; Lowrie & Alvarez,1984; Lowrie & Channell, 1984). The results ofrock-magnetic experiments performed on samplesfrom the Cismon and Pra da Stua sections are inagreement with these earlier studies. During acqui-sition experiments of isothermal remanent magnetiz-ation (IRM) using an ASC Scientific IM-10 impulsemagnetizer Biancone samples from Cismon and Prada Stua almost reach saturation around 0.2–0.3 T(Figure 4), which indicates the dominance of magnet-ite, while high-coercivity minerals like haematitecontribute only little to the remanence.

Hysteresis loops were obtained from ten samplesdistributed through the entire lower Cismonsection by experiments on an Alternating GradientForce Magnetometer (Princeton MeasurementsCorporation) (Figure 5). The majority of thesesamples showed a dominant diamagnetic behaviourwith a small ferrimagnetic component; some have astronger ferrimagnetic character with some diamag-netic contribution. The dominant diamagnetism is dueto the low content of ferrimagnetic and paramagneticminerals of the samples and not surprising for rockconsisting of 90% calcite. Only one sample was domi-nated by paramagnetism with a small ferrimagneticcomponent, while another sample had a stronger ferri-magnetic component with a paramagnetic contri-bution. The paramagnetism must be attributed to adifferent clay-mineral content, since the non-carbonatefraction is not higher in these samples compared todiamagnetic samples. These same two samples alsohave by far the highest initial susceptibility values of allsamples. Furthermore, these high susceptibility valuesdo not correspond to low carbonate content, whereasin the remainder of the section these two variables arevery tightly negatively correlated.

After correction for the dominant diamagnetic orparamagnetic slope, hysteresis properties of theferrimagnetic component were evaluated. Ratios of

Figure 4. Acquisition curves of isothermal remanent mag-netization (IRM) for typical Biancone samples. (a)Cismon specimens ca109, ca150.5, ca248.5 and ca365.The dominant remanence carrier is magnetite, with asmall contribution of a high-coercivity mineral, prob-ably haematite. (b) Pra da Stua specimens ps000B,ps800B, ps900B and ps1800B. The dominant rema-nence carrier is magnetite, with a small contribution ofa high-coercivity mineral, probably haemtite.


saturation remanent magnetism over saturation mag-netization (Mr/Ms) and of remanent coercive forceover coercive force (Hcr/Hc) were computed and plot-ted in a diagram of Mr/Ms versus Hcr/Hc (after Dayet al., 1977) (Figure 5e) which allows for the assess-ment of the domain state of the magnetic carriermineral, if only one phase in a narrow grain-size rangeis present. Coercivity ratios are consistent with mag-

netite of pseudo-single-domain state. The CismonBiancone samples do not have ‘wasp-waisted’ hyster-esis loops as are characteristic for remagnetizedlimestones (Jackson, 1990). In a comparative studyof hysteresis parameters of unremagnetized and re-magnetized limestones Channell & McCabe (1994)examined a large number of pelagic limestone samplesfrom various Mesozoic formations of Italy. All whiteMaiolica samples fell in the pseudo-single-domainfield and had normal hysteresis loops, whereas pinkand red limestone samples from other formations felloutside the pseudo-single-domain field and had‘wasp-waisted’ hysteresis loops, characterizing themas remagnetized. The results of hysteresis-loop analy-sis of Cismon Biancone samples presented herestrongly suggest that these rocks carry a ‘primary’magnetization.

A sudden increase in susceptibility and intensityobserved during thermal demagnetization at tempera-tures between 350) C and 500) C (Mayer, 1996,1997) may be attributed to the experiment-inducedoxidation of pyrite or other iron sulfides to magnetiteand maghemite or to other mineralogical changes inCismon and Pra da Stua samples. This change isstronger and more common in rocks from Cismonthan in rocks from Pra da Stua. Volume-susceptibilityvalues range from 5#10"6 to 30#10"6 (SI units) atCismon with the exception of the aforementioned twosamples reaching about 60#10"6. At Pra da Stuasusceptibility values are between 11#10"6 and87#10"6 (SI units).

4. Cyclostratigraphy

Figure 5. Hysteresis analysis of some Cismon samples.(a)–(d): Hysteresis loops for: (a) Sample ca239 display-ing dominant diamagnetism at high fields with a smallferrimagnetic component. (b) Same as (a) corrected fordiamagnetic slope. (c) Sample ca438 displaying domi-nant paramagnetism at high fields with a small fer-rimagnetic component. (d) Sample ca446.5 dominatedby ferrimagnetic component (small paramagnetic sloperemoved). (e) Hysteresis-properties diagram (afterDay et al., 1977) for ten samples distributed throughthe lower Cismon section. SD=single-domain field;PSD=pseudo-single-domain field; MD=multi-domainfield (see text for explanation).

Introduction

During the late 1970s and early 1980s renewed inter-est in the causes of the Pleistocene ice ages led to afirm verification of the Milankovitch theory of palaeo-climates (e.g., Hays et al., 1976; Berger, 1977, 1978a;Imbrie et al., 1984; see below). Stimulated by this

e


success, more and more stratigraphers and sedimen-tologists concentrated their efforts on the detection ofMilankovitch cycles in the older record, so that cyclicstratigraphy or cyclostratigraphy (Fischer et al., 1990)emerged as a distinct sub-discipline of geology incor-porating a variety of techniques from related fields anddeveloping new methodologies of its own. This devel-opment over the last decade can be traced in thevolumes edited or introduced by Einsele & Seilacher(1982), Berger et al. (1984), Arthur & Garrison(1986), Smith (1989), Einsele et al. (1991), Fischer &Bottjer (1991) and de Boer & Smith (1994). Anincreasingly important aspect of Milankovitch cycles istheir usefulness for geochronology, not for absolutedating, but for accurate estimation of the time spansrepresented by cyclic sequences (House, 1985), in par-ticular when combined with magnetostratigraphy in thesame section (Mayer, 1994, 1996, 1997, in press).

A central role in cyclostratigraphy is played byquantitative time-series-analysis methods, their appli-cability and value for the detection of characteristiccycle periods. Important contributions to this fieldwere made by Schwarzacher (1964, 1975, 1987a, b,1989, 1991; Schwarzacher & Fischer, 1982), Wigley(1976), Weedon (1986, 1989, 1991), Park & Herbert(1987), and Hinnov & Goldhammer (1991).

In the following, cyclic fluctuations of magneticsusceptibility and carbonate content through theCismon section will be described and analyzed quan-titatively. To that extent, the powerful geostatisticaltime-series-analysis method of Herzfeld (1990) will beapplied in order to gain information about thepotential cyclicity of the time series.

Variations in magnetic susceptibility and carbonate content

The magnetic susceptibility of the samples fromCismon was measured on a Kappabridge KLY-2(Agico, Brno, Czech Republic; formerly GeofyzikaBrno). Each sample was measured five times, and themean of these values is reported after volume normal-ization. The Biancone limestone is characterized byvery low magnetic susceptibility (5 to 30#10"6 di-mensionless SI units, with the exception of twosamples reaching c. 60#10"6). The susceptibilityvariations through the section appear to follow a cyclicpattern (Figure 6) the analysis of which is presentedfurther down.

Through the entire lower section (8.45 m) a split ofeach drilled sample has been analyzed for carbonatecontent by coulometry. The amplitudes of the vari-ations in carbonate content are relatively small, allsamples fall between 87 and 97% CaCO3. However,the curve of carbonate content through the section

clearly contains rhythmic oscillations (Figure 6) whichare almost perfectly negatively correlated with theoscillations in magnetic susceptibility, with the excep-tion of two susceptibility spikes at c. 450 and 610 cm,which do not correspond to carbonate lows.

The numerical correlation is strongly affected bythese mismatches in two very narrow intervals: thecorrelation coefficient jumps from "0.36, when allpoints are included, to "0.90, when only those fourpoints are excluded which form the major susceptibil-ity spike around 450 cm. When only one more point isremoved (the one constituting the minor spike at610 cm), the correlation coefficient becomes "0.93(Figure 7). This excellent correlation between carbon-ate content and susceptibility indicates that magneticsusceptibility can be used as an approximate measureof variations in carbonate content, if care is taken thatextreme values not correlated to opposite extremes incarbonate content are excluded (Mayer, 1993; Mayer& Appel, 1995).

Time-series analysis

With the increasing degree of quantification sought ingeology today more and more geologists apply various

Figure 6. Variations of carbonate content (left) and mag-netic susceptibility (right) in the lower Cismon section.Cyclic fluctuations are well developed in both variables,but in opposite directions. The negative correlationbetween the two is very tight except for two narrowintervals at level 460 cm and level 610 cm, whereuncorrelated susceptibility spikes (indicated by thedashed line) attributed to magnetic alteration occur.


Figure 7. Cross-correlograms of the carbonate and suscep-tibility series from the Cismon section. Dotted line:carbonate and complete susceptibility series. Dashedline: carbonate series and susceptibility series with mainspike at level 460 cm removed. Solid line: carbonateseries and susceptibility series with main spike at level460 cm and minor spike at level 610 cm removed.

time-series-analysis methods to geological data (seeMerriam, 1967). In particular, time-series analysisseems to be suited for the study of stratigraphic data,because the stratigraphic succession of sedimentaryrocks basically records the passing of (geologic) time.Ideally, stratigraphic thickness can be converteddirectly to length of time. The validity of this approachdepends on the following condition: well-dated layersat the bottom and top of the section must be present.In order to measure time by thickness in subsectionsbetween two well-dated layers the additional con-ditions of continuous sedimentation and constantsedimentation rate must be fulfilled. Unfortunately,these conditions are very rarely exactly met anywherein the sedimentary record. Kominz & Bond (1990)therefore developed an alternative method which con-siders different accumulation rates for the differentlithologies comprising a cycle. But we can find, on theother hand, sedimentary sections where the ideal caseis approximated to a high degree, so that time-series-analysis methods can be applied to stratigraphic datasets with limited uncertainty in such cases. Theseconditions are met in the selected interval of theCismon section. The sedimentary time series obtainedwill be described in detail. The main focus of anearlier paper was on the value of time-series analysis incyclic stratigraphy and its application (Mayer, 1993).For this matter, a detailed comparison of three differ-ent time-series-analysis methods was carried out: onepopular spectral technique (adaptive multitaper) wascompared with a basic statistical method (auto-/cross-correlation) and a versatile geostatistical approach(cova functions). The results of this analysis will be

summarized here, because they form the basis for theensuing interpretations.

The detection of periodicities in stratigraphicsequences requires the application of quantitativetime-series-analysis methods. These methods, how-ever, differ in their approach to the problem and intheir suitability for different types of data sets. Theadaptive-multitaper technique requires evenly spacedsamples. So, the data have to be interpolated first ifthey are unevenly spaced, as in our example and inmost geologic applications. Several filters are appliedand two inversions from the time domain to thefrequency domain and back are involved. All thesesteps may introduce rounding errors and thus leadaway from the original data. Correlation analysis alsorequires evenly spaced data in the time series. Linearinterpolation and cubic-spline interpolation werecompared in this case and were found to give verysimilar results. Linear interpolation was preferred,because it did not even out the magnitudes of thepeaks in the correlograms as much as cubic-splineinterpolation. Cova functions can be applied directlyto the raw data, because they can accomodateunevenly and non-correspondingly spaced series. Thisis a considerable advantage over other methods, sincethere is less room for introducing errors throughpreprocessing steps. Therefore, cova functions wereselected as the analysis method for this study. Beforethe results are presented, a short overview of themethod is given.

Based on the cross-variogram which allows thequantification of the covariation of two series sampledin unevenly spaced corresponding locations, Herzfeld(1990) formulated the generalized case, extending theapplicability to unevenly and non-correspondinglyspaced data series, and termed it cova function.

The variogram is the structure function commonlyapplied in geostatistics, the theory of regionalizedvariables (Journel & Huijbregts, 1978). For a stochas-tic process Z(x), the variogram (sometimes also calledsemivariogram) is defined as

where E denotes the expectation and h the distancebetween two points. The (auto-)covariance function isalso a function defined in the lag domain, whereas thepower-spectral density is defined in the frequencydomain. If the stochastic process Z(x) is stationary,then the variogram is related to the (auto-)covariancefunction, denoted cov(h), by


(cf. Journel & Huijbregts, 1978, p. 40f), and therelationship to the power-spectral density is stated bythe Wiener-Kinchin Theorem: the power spectrumequals the Fourier transform of the (auto-)covariancefunction (cf. Herzfeld, 1992). Although it is theoreti-cally possible to transform the variogram and thecovariance function into the spectrum and back, thereare differences in practice. Variogram estimation hasproven useful in the geosciences because of its robust-ness and simplicity as a function in the lag domain.Noise in geophysical data series, for instance, andsmall errors in the spacing of sampling positions affectthe power spectrum much more than a function in thelag domain. Uneven spacing of experimental data isusually handled by calculating the covariance functionand the variogram in distance classes, that is, a pair ofpoints (x1,x2) is used in the multiplication in theexperimental formula

if the distance D(x1,x2) is in a distance class [h+ä,h"ä], where 2ä is the size of the distance class. Anexperimental spectrum can be calculated in distanceclasses too, but the usual technique is to use aninterpolation and resample at evenly spaced locations.Such an intermediate step may distort the signal andis not needed in lag functions. Furthermore, thevariogram exists in situations where the covariancefunction and the spectrum may not exist because oftrends in the data. To compare two data sets thecorresponding bivariate functions are used. For abivariate stochastic process Z(x)=[Z1(x), Z2(x)] thecross-variogram is defined as

Relationships to the cross-covariance function andthe spectrum hold, similar to the one-dimensionalcase. Uneven spacing of experimental data is handledby calculation in distance classes, analogous to theone-dimensional case. Two geophysical data sets,however, may not be spaced correspondingly. Withdata from different survey or measurement techniquesthe corresponding spacing (assumed in most process-ing software) is rarely the case. To date, the usualpractical approach to compare two variably spaceddata sets is to preprocess at least one of them inorder to match the spacing, commonly by linear orcubic-spline interpolation. Although there is a largenumber of software packages for comparing two timeseries, most routines are restricted to equally spaceddata. Valuable information is lost in the required

preprocessing steps. The cova function defined inHerzfeld (1990) avoids all preprocessing and stillpermits the investigation of the variability of two dataseries that may be spaced unevenly and non-correspondingly. The cova function is based on theconcept of the cross-variogram (cf. Eqn. 4).

To handle unevenly spaced samples the calculationof cova functions is carried out in distance classeswhich are multiples of a unit lag or step size. Analysisof non-correspondingly sampled series presents theadditional problem that matching pairs of points inthe two series need to be found. This is achieved byintroducing a variable tolerance: given a pair (y11, y12)in time series 1 in a particular distance class, a pair(y21, y22) in time series 2 in the same distance class isused in the multiplication in the formula correspond-ing to (Eqn. 4), if y11 and y21 are at most the value oftolerance apart in time (distance). Both step size andtolerance can be chosen by the user to fit particulardata sets. Herzfeld (1990) provides an extensive set ofsimulated example time series and resulting covafunctions which demonstrate the robustness and gen-eral applicability of the method. The particular prop-erties that characterize cova functions in contrast to allspectral methods are: their direct applicability to theoriginal data without any preprocessing and the factthat residual cova functions are defined for non-stationary time series. Of course, the method is alsoapplicable to evenly and correspondingly spaced timeseries. In the case of correspondingly spaced series thecova function reduces to the cross-variogram if toler-ance is set equal zero. Two series of different variablescan be compared (cova function) or one series withitself (auto-cova function). To make this possible twocopies of the same series are concatenated in order toprovide long enough overlapping sequences. Spectral-analysis methods and correlation analysis requireequally spaced data points, whereas cova functionsand cross-variograms are not subject to this limi-tation. The advantage of cova functions over cross-variograms in our case is that bad data can be left outin one series (susceptibility) while all the data can beretained in the other series (carbonate), becausecova functions are defined for non-correspondinglyspaced series.

In the following, the results of the application ofcova-function analysis to the carbonate and suscepti-bility series obtained from the Cismon section arepresented. At first, only thickness periods areanalyzed. How these might be related to timeperiods is discussed later together with associatedtime-scale-accuracy and -resolution problems.

Auto-cova functions were calculated for the Cismoncarbonate series (150 samples, Figure 8a) and the


Cismon susceptibility series (144 samples, Figure 8b).Residual and ordinary (auto-)cova functions werecalculated in each case. Cova functions were com-puted comparing the carbonate curve with the suscep-tibility curve (Figure 8c). Cova values close to zeroindicate the highest degrees of covariation. Therefore,one has to look for minima of the absolute cova value inplots of cova value versus distance in order to detectpotential cycle periods. If we look at the calculatedcova functions in this way, we find that major minimaoccur in the auto-cova function of the Cismon car-bonate series at distances of 80 cm, 160 cm and265 cm. Minor minima occur at 120 cm and 175 cm(Figure 8a). These numbers are valid for a step size of5.0 cm and a tolerance of 0.0 cm. A step size close tothe average spacing of the data points in the timeseries gives the best results (Mayer, 1993). In the caseof the carbonate series (average spacing 5.63 cm)5.0 cm is chosen as the preferred step size.

The auto-cova function for the susceptibility series(Figure 8b) looks much like the one for carbonate(Figure 8a): the three most prominent minima are at85 cm, 160 cm and 265 cm. Secondary minima arelocated at 45 cm and 180 cm. If we now analyze thecovariation of the two variables carbonate content andsusceptibility, we get negative cova values because thetwo variables are negatively correlated. So, in this plotwe need to look for actual maxima (being minima ofthe absolute value) to identify cycle periods. We findthat again we get three prominent minima (of theabsolute value) at 85 cm, 160 cm and 265 cm. Sec-ondary minima (of the absolute value) are found atdistances of 45 cm, 120 cm and 175 cm (Figure 8c).

For all the series mentioned, residual cova functionsas well as ordinary cova functions were calculated, andit was found that these are essentially the same foreach case except for a small deviation at large lags.This demonstrates that the time series do not underlya pronounced trend. However, a small trend to highercarbonate values and lower susceptibility valuesup-section can be detected in the original curves (seeFigure 6), which is confirmed by the small deviationbetween ordinary and residual cova functions at largelags.

Cova functions produce essentially the same majorcycle period for the Cismon carbonate and suscepti-bility profiles and their cross-comparison at a thick-ness period of 80 cm. The prominent periods of160 cm and 265 cm must be interpreted as multiples.Cova functions reveal additional significant periods at45 cm and 180 cm. The resolution is limited by thesample spacing in the original time series and theaccordingly chosen step size (here, 5.0 cm). In synop-sis, we can conclude that apparent cycle periods in the

Cismon section can be confirmed at thicknesses ofapproximately 45 cm, 80 cm, and 180 cm.

Natural remanent magnetization (NRM) was ana-lyzed as another sedimentary parameter in the section(Figure 9). This time series shows some potentiallycyclic undulations similar to the susceptibility series aswe would expect. However, the regularity and struc-ture of variations is much less clear in this parameterand is further obscured by several large spikes. Cova-function analysis of the NRM series indicates poten-tial cyclicity at periods of 50 cm and 105 cm (Figure9b). However, these data are not good enough forconclusive extraction of cycle periods. They can onlybe interpreted together with the susceptibility andcarbonate-content data from the same section.

In the Pra da Stua section we measured beddingthickness in an interval that visually appeared cyclicand even hierarchically structured (level 2500 cm tolevel 2900 cm; see Figure 3b). Bedding thickness wasmeasured in the outcrop and then resampled in regu-lar intervals of 5 cm to produce a stratigraphic series(Figure 10a). Visual inspection of this time seriessuggests the presence of a subtle cyclicity on the orderof half a metre. Cova-function analysis reveals perio-dicities of 55 cm, 115 cm and 170 cm (Figure 10b).However, only the 55-cm cycle can confidently beextracted, while the others are only vaguely indicated.In the following, we concentrate on the susceptibilityand carbonate series from Cismon which producedunequivocal evidence for periodic cyclicity.

Once thickness periods have been established, theproblem arises as to how they convert to time periods.To approach this problem the correlation of the newCismon magnetic-polarity sequence (Mayer, 1996,1997) with the marine magnetic anomalies of theM-sequence in the version of Harland et al. (1982)was used to estimate time (Figure 11). The geo-chronologic scales are rather poorly constrained in thistime interval. There are only two widely acceptedreliable radiometric calibration points, namely at theOxfordian/Kimmeridgian boundary [156 millionyears ago (Ma)] and the Barremian/Aptian boundary(119 Ma) (Kent & Gradstein, 1985; Ogg, 1995). Inbetween these tie points all ages for chronostrati-graphic levels and magnetic reversals are based onlinear interpolation assuming constant sedimentationrate and constant seafloor-spreading rate, in ourexample over a time span of 37 million years (m.y.).As these assumptions have to be regarded as ratherunrealistic, in particular for time periods that long,there is considerable potential for improvement of thetime scale by new methods and approaches, such ascyclostratigraphic calibration (Mayer, 1994, in press).Other radiometric time scales (e.g., Odin et al., 1982;


Figure 8. Cova functions for the carbonate and suscepti-bility series from the Cismon section. Step size is5.0 cm, tolerance is zero. (a) Auto-cova functions for thecarbonate series. (b) Auto-cova functions for the sus-ceptibility series. (c) Cova functions for the carbonateand susceptibility series.

a

b

c

Figure 9. Variations of natural-remanent-magnetization(NRM) intensity through the Cismon section. (a)Stratigraphic plot. (b) Auto-cova functions. Step size is5.0 cm, tolerance is zero.

a

b

Harland et al., 1982, 1990; Hallam et al., 1985) haveincorporated more radiometric dates in this timeinterval, but these dates are discussed quite controver-sially (see Harland, 1983; Hallam et al., 1985; Odin,1985). Not surprisingly a compilation of a number ofdifferent time scales revealed substantial disagreement


Figure 10. Variations of bedding thickness through the Prada Stua section. (a) Stratigraphic plot. (b) Auto-covafunctions. Step size is 5.0 cm, tolerance is zero.

a

b

Figure 11. Correlation of the Cismon-polarity sequence tothe geomagnetic-polarity-time scale of Harland et al.(1982) which is based on the Hawaiian marine-magnetic-anomaly lineations of Larson & Hilde (1975).Biostratigraphic information is taken from Channellet al. (1979).

on the duration of the magnetic-polarity chrons ident-ified at Cismon ranging from a minimal 0.704 m.y. toa maximal 1.583 m.y. (Table 1). The 8.79 m ofcumulative thickness from the Cismon section whichwere correlated with polarity subchrons were dividedby the respective duration from each of the time scalesused in order to obtain average sedimentation rates. Inthis calculation those subchrons only partially repre-sented in the section and the sampling gap are ex-cluded. The resulting rates range from 5.55 m/m.y. to12.49 m/m.y. with a mean of 7.72 m/m.y. (Table 1).

These rates appear a little low if compared to typicalaccumulation rates for calcareous ooze in deep-seaenvironments amounting to tens of metres per millionyears (Berger, 1974), but are within the possiblerange, albeit at the very low end. Compaction canonly account for a small fraction of the discrepancy.Nevertheless, using these sedimentation rates the


Table 1. Comparison of sedimentation rates calculated from the cumulative thickness of magneticsubchrons identified at Cismon (partially represented subchrons and sampling gap not included) andtheir cumulative duration according to different time scales.

Thickness[m]

Duration[m.y.]

Sedimentation rate[m/m.y.] Time scale

8.79 0.704 12.49 van Hinte (1976)1.017 8.64 Lowrie (1982)1.583 5.55 Harland et al. (1982)1.556 5.65 Hallam et al. (1985)1.390 6.32 Kent & Gradstein (1985)1.170 7.51 Lowrie & Ogg (1986)1.038 8.47 Haq et al. (1988)1.100 7.99 Harland et al. (1990)1.248 7.04 Gradstein et al. (1994)1.160 7.58 Channell et al. (1995b)

7.72 mean

Table 2. Conversion of observed thickness periods atCismon to time periods for cycle analysis using selectedsedimentation rates from Table 1.

Thickness period[cm]

Corresponding time period [kyr]using sedimentation rate

min. mean max.

180 324 233 14480 144 104 6445 81 58 36

Table 3. Milankovitch periods for selected times in geologic history (after Berger et al., 1992).

Cycle periods [kyr]

Time Precession 1 Precession 2 Average of p.1 and p.2 Obliquity Eccentricity

Present 19 23 21 41 95100 Ma 18.5 22.3 20.4 38.8 95125 Ma* 18.35 22.1 20.23 38.25 95150 Ma 18.2 21.9 20.05 37.7 95

*Values for 125 Ma were obtained by linear interpolation between values for 100 Ma and 150 Ma.

thickness periods from the time-series analyses wereconverted to time periods. The resulting cycle periodsvary considerably depending on the time scale used,but all the periods calculated fall in the range expectedfor Milankovitch cycles (Table 2), although they seemtoo long for the expected Milankovitch cycle periodsof 19 thousand years (kyr) and 23 kyr (precession),41 kyr (obliquity) and 95 kyr (eccentricity). Consider-ing the poorly constrained time scales in this timeinterval which leave room for large errors, we can only

say that the periods are of the right order of magni-tude, but we cannot get any closer to the true cycleperiods by this conventional method. In this situation,a different approach is needed. Even if we consider thecase that the numerical ages are far off, the ratiosbetween the sedimentary cycles at Cismon still matchthe expected ratios for present-day Milankovitchcycles very closely. The same is true if we considerperiods of these astronomical parameters as calculatedback for the mid-Cretaceous by Berger et al. (1992)(18.5 kyr and 22.3 kyr for precession, 38.8 kyr forobliquity and an unchanged 95 kyr for eccentricity at100 Ma) (see Tables 3–5). Based on this extremelyclose match of period ratios we can confidently as-sume that the observed cycles represent Milankovitchcycles. Then we can use the known periods of thelatter to convert the observed thickness periods ofthe former into time periods. In order to explore theeffects of the uncertainty in the observed cycle periodson the match, ratios were calculated for the possiblerange of thickness periods determined by time-seriesanalysis (Table 5). Milankovitch-cycle periods for thepresent, 100 Ma and 150 Ma were taken from Berger


et al. (1992), and values for 125 Ma, which is aboutthe time of sediment deposition at the studied section,were obtained by linear interpolation (Table 3).Ratios between these Milankovitch-cycle periods werealso computed (Table 4). Comparing Table 5 withTable 4, we can see that the best match is obtainedusing cycles of 45 cm, 80 cm and 180 cm and present-day Milankovitch periods. This set of sedimentarycycles also emerged as the best solution after appli-cation of different time-series-analysis methods(Mayer, 1993). These cycles appear thick (almosttwice as thick) compared to Milankovitch cycles inthe Cretaceous of the Apennines. This implies that theaccumulation rate is correspondingly higher in theBiancone, which is supported by lithologic evidence.In the Apennines limestone-marl cycles were studiedwith a much greater variation in carbonate contentand thus accumulation rate between members of thecouplets. In the lower Biancone of the Southern Alps,on the other hand, we found carbonate variations of amuch lower amplitude at a generally high carbonatecontent (87 to 97%) resulting in a higher overallaccumulation rate. This also explains the more con-sistent cycle and bundle pattern we found. It issurprising that the match with Milankovitch periodsas calculated for the Early Cretaceous is not quite asgood. The reason for this is unclear, but the differenceis actually very small and the match is still extremely

good. This cyclostratigraphically derived time infor-mation was recently applied to the estimation of theduration of Early Cretaceous magnetic-polarity zonesand to an improvement of the time scale (Mayer,1994, 1996, 1997, in press).

5. Palaeoclimatic significance

The stratigraphic, sedimentologic and magnetic dataobtained in the course of this study lend themselves tofurther interpretation in terms of palaeoclimatic sig-nificance. In this respect, the present investigationcontributes to a number of palaeoclimatic issues,in particular to the question of orbital forcing inpre-Pleistocene times, climatic variability in theEarly Cretaceous and magnetic records of climaticchange.

Table 4. Ratios between Milankovitch cycle periods for selected times in geologic historycalculated from values in Table 3.

Cycle ratios

Time Avg. prec.:Obliquity Avg. prec.:Eccentricity Obliquity:Eccentricity

Present 0.512 0.221 0.432100 Ma 0.526 0.215 0.408125 Ma 0.529 0.213 0.403150 Ma 0.532 0.211 0.397

Table 5. Ratios between sedimentary cycle periodsdetected at Cismon encompassing the uncertainty range(see text).

Cycle seta:b:c[cm]

ratioa:b

ratioa:c

ratiob:c

45:80:175 0.563 0.257 0.45745:85:180 0.529 0.25 0.47245:85:175 0.529 0.257 0.48645:80:180 0.563 0.25 0.444

Cretaceous climate

The Cretaceous is generally considered a time periodof much warmer than present temperatures and amuch reduced equator-to-pole temperature contrast(Frakes, 1979; Barron, 1983; Crowley, 1983; Hallam,1985); its climate is thus characterized as warm andequable (Barron, 1983). This has been inferred fromtraditional palaeontological and geological datasuch as the geographic distribution of certain palaeo-ecologically significant biological communities andpalaeoclimatically significant sedimentary deposits(e.g., evaporites, coal, tillites, desert sandstones) aswell as from stable-isotope temperature proxies. How-ever, new oxygen-isotope data from the Cenomanianhave been interpreted recently in terms of tempera-tures not significantly warmer or colder than atpresent near the equator, but still warmer near thepoles (Sellwood et al., 1994). These new data andtheir interpretation have been received with somescepticism regarding their global significance (Barron,1994). Earlier, Kemper (1983) had argued for sub-stantial climatic changes during the Cretaceous with


two major cold periods in the Valanginian and lateAptian to early Albian bracketing a pronounced warmperiod. On the other hand, Weissert (1991) has de-duced veritable greenhouse conditions for the Val-anginian and Aptian from sediment-distributionpatterns in the Tethyan and North Atlantic realmsand from positive carbon-isotope excursions. TheAptian ä13C peak is split by a trough, which isinterpreted as representing a cold period or even apossible glacial period by Weissert & Lini (1991). Thelate Valanginian to early Hauterivian ä13C peak re-flects accelerated global carbon cycling attributed toan elevated atmospheric CO2 level and thus repre-sents a first episode of Cretaceous greenhouse climate(Lini et al., 1992). A global cooling trend at the end ofthe Cretaceous is well documented (cf. Crowley,1983). However, except for some indications of minorglaciation, solid evidence for massive glacial condi-tions at any time during the Cretaceous or the entireMesozoic, for that matter, has never been presented.It is therefore interesting to investigate whether orbitalforcing was effective enough to be transmitted intothe stratigraphic record through a climate system sodifferent from the Quaternary glacial regime.

Milankovitch theory of palaeoclimates

The astronomical theory of palaeoclimates afterMilankovitch (1930, 1941) relates climatic change tovariations of the amount of solar energy received bythe Earth as a consequence of quasi-periodic changesin celestial geometry (see Imbrie & Imbrie, 1979, andBerger, 1988, for reviews). Of particular importanceare variations in the eccentricity of the Earth’s orbit(with dominant periods of roughly 400 kyr and100 kyr), in the obliquity of the Earth’s rotationalaxis relative to the orbital plane (with characteristicperiods of 54 kyr and 41 kyr) and in the precessionof the equinox or more exactly the eccentricity-modulated precession expressed by the precessionalindex of Berger (1976, 1978b) (with periods of 23 kyrand 19 kyr). The obliquity cycle is more effective inhigh latitudes, whereas the low latitudes are domi-nated by the precessional cycle.

The invocation of astronomical variations as causalfactors of climate change, in particular for the Pleisto-cene ice ages, goes back to Adhemar (1842; fideImbrie & Imbrie, 1979). Croll (1875) developed theseideas into a coherent astronomical climate theory. Theexact mathematical formulation of orbital variationsand the calculation of an insolation curve throughtime which could be matched to the Pleistoceneglacial-interglacial cycles (Koppen & Wegener, 1924)was achieved by Milankovitch (1930, 1941). The

theory was not readily accepted by many, because itsverification was impossible at the time due to theabsence of suitable dating and correlation methodsand because the insolation variations seemed toosmall to produce such a dramatic climate change asbetween glacials and interglacials. This holds particu-larly for the 100 kyr eccentricity cycle (Ruddiman &Wright, 1987). It was not until the mid-1960s thatnew evidence for the validity of the Milankovitchtheory started to increase (Emiliani & Geiss, 1959;Emiliani, 1966; van den Heuvel, 1966; Broecker et al.,1968; Mesolella et al., 1969; Broecker & van Donk,1970), although Emiliani (1955) had broken theground much earlier by developing the oxygen-isotopetechnique for determination of palaeotemperatures.The rapid development in the 1960s and 1970s wasdue to improvements in oxygen-isotope stratigraphy,magnetostratigraphy and radiometric dating. A decis-ive breakthrough towards verification of the astro-nomical theory was achieved when Hays et al. (1976)matched the oxygen-isotope variations in a firstPleistocene Pacific sediment core to the theoreticallypredicted insolation curve. This work was later cor-roborated by new data from a number of additionalcores (Imbrie et al., 1984). It became apparent thatthe subtle changes in insolation can be greatly en-hanced within the complex climate system withits global energy redistribution by atmospheric andoceanic circulation. A number of models have beenadvanced postulating nonlinear responses (Imbrie &Imbrie, 1980; Berger & Guiot, 1981; Imbrie et al.,1993; Liu, 1995), feedback mechanisms involving thealbedo of growing polar ice caps (Suarez & Held,1976; Weertman, 1976; Le Treut & Ghil, 1983;Pollard, 1983) or the effects of oceanic circulation(Ruddiman & McIntyre, 1981) and atmospheric CO2

content (Shackleton & Pisias, 1985).For the obliquity and precession cycles Imbrie et al.

(1992) established a direct linear link between insol-ation and climate fluctuations. The strength of the100 kyr eccentricity cycle in the records, however, isstill somewhat enigmatic and has to be explainedby nonlinear response models (Imbrie et al., 1993).Although the exact mechanisms are still not com-pletely resolved, the role of orbital variations as thedriving force of glaciation cycles is now generallyaccepted (Berger, 1992; Imbrie, 1992). It is much lessclear whether orbital forcing was as effective as in theQuaternary during earlier periods of geologic history,particularly during times of ice-free climate conditionslike the Cretaceous. However, since the astronomicalcycles have been stable through the Phanerozoicexcept for a small gradual lengthening of some periods(obliquity and climatic precession) owing to the


changing Earth-Moon distance (Berger, 1989; Bergeret al., 1989, 1992; Berger & Loutre, 1994), it is notunreasonable to look for a record of Milankovitchcycles from pre-Pleistocene periods (cf. Fischer, 1986;Kelly & Cubitt, 1993; Schwarzacher, 1993a).

Several studies reported evidence for Milankovitchcyclicity also in older sedimentary rocks: in theTertiary (e.g., Schwarzacher, 1987c; Hilgen, 1991),Upper Cretaceous (e.g., Schwarzacher & Fischer,1982; Hart, 1987; Cottle, 1990), Jurassic (e.g.,House, 1985; Weedon, 1986), Triassic (e.g.,Goldhammer et al., 1987) and even in the Pre-cambrian (e.g., Grotzinger, 1986). These examplesindicate that climate changes probably followed thesame astronomical cycles through the Earth’s historyeven at times of different climate states.

Milankovitch cycles can be expected to be recordedin certain sensitive depositional systems, where peri-odic climatic changes result in environmental changes,which in turn are reflected in the composition or otherproperties of the sediments deposited. Climaticchanges attributable to variations in insolation caninclude the growth and decay of continental ice sheetsas in the Quaternary, the latitudinal migration ofhumid-arid belts, varying precipitation volumes due toa changing land-sea temperature contrast (Barronet al., 1985), and switches between monsoonal andzonal wind systems (Prell & Kutzbach, 1987). A mostimportant effect of insolation-driven variations in lati-tudinal temperature contrast are changes betweenvigorous and sluggish oceanic-circulation regimes.Vigorous circulation leads to intensified upwellingwhich triggers increased productivity (e.g., Berger,1974). Sluggish circulation, on the other hand, pro-motes stratification of the water mass leading toreduced bottom oxygenation and productivity. Allthese changes are potentially recognizable in thesedimentary record. Of course, oceanic circulation isalso influenced by changes in the configuration ofcontinents and in submarine morphology. However,the plate-tectonic processes responsible for thesechanges are not cyclic (reversible) in this sense andoperate on longer terms than orbitally driven climatechange.

Magnetic records of climatic change

It has been established mainly during the last twodecades that magnetic properties of sediments andsedimentary rocks can provide a detailed record ofpalaeoclimatic change in stratigraphic sections anddrill cores (King & Channell, 1991; Verosub &Roberts, 1995; Reynolds & King, 1995). Variations inthe concentration, type and grain size of magnetic

particles can sensitively reflect environmental changesin the depositional realm or its catchment (Thompson& Oldfield, 1986) which are in many cases attribu-table to climate. The best results of this new approachto palaeoclimatology have been achieved in aeolian,lacustrine and pelagic settings (Reynolds & King,1995). It has to be pointed out here that rock-magnetic parameters by themselves have little palaeo-climatic meaning. But if calibrated with the help ofpalaeontological or isotopic climate indicators for thespecific environment studied, they provide the long,continuous and areally extended records neededfor useful palaeoclimatic reconstruction, which tra-ditional climate indicators cannot provide becausethey are generally not continuously distributedthrough sections and their determination is much moretime-consuming. Thus, the value of environmental-magnetic methods lies in the fact that they are in mostcases fast, easily applicable, non-destructive and in-expensive. Parameters indicative of the concentrationof magnetic particles as well as those indicative ofmagnetic mineralogy and grain size have proven useful.Of the first group, susceptibility and intensity of naturalremanent magnetization are the most important. Hintson the magnetic mineralogy and grain size of rocks areusually obtained from parameters derived from de-magnetization behaviour, from the acquisition of iso-thermal remanent magnetization, from the temperatureand frequency dependence of susceptibility and fromhysteresis properties, among others.

In the Biancone Formation of the Cismon sectionwe have no drastic lithologic changes which couldindicate fundamental climatic changes. Accordingly,the rock-magnetic parameters indicative of magneticmineralogy do not show significant variations throughthe section. IRM-acquisition curves (Figure 4) ands-ratios (IRM acquired at backfield of 0.3 T relative tothe IRM acquired at 1.2 T) (Figure 12) indicate thatthe dominant carrier of remanence is magnetitethroughout the section with a small contribution fromhaematite. The same is true for the Pra da Stuasection. However, at three levels in the Cismon sec-tion (Figure 12) lower s-ratios occur, which must beattributed to a higher concentration of high-coercivityminerals, probably haematite. A very subtle trend todecreasing s-ratios up-section is recognizable, accen-tuated by the three abnormally low values, which alsobecome lower the higher the level examined in thesection. These features may reflect a trend to slightlymore oxidized conditions, but this is not sufficientlydocumented by these data. In this study a detailedrecord of susceptibility variations through the Cismonsection was obtained which is discussed in thefollowing section.


Evaluation of sedimentary cycles

Much more conclusive regarding palaeoclimaticinterpretation than the other rock-magnetic par-ameters are the variations in susceptibility which areclearly cyclical through the section. A hierarchyof cycles could be identified and matched toMilankovitch cycles. The excellent negative corre-lation between susceptibility and carbonate contentverifies that the susceptibility signal reflects theconcentration of the non-carbonate fraction, hence aprimary depositional feature (Figure 6). The NRMcurve also shows some undulation through thesection, but does not reveal the cycles so clearly(Figure 9).

The dominant initial-susceptibility signal in marinelimestones is carried by the (detrital, authigenic orbiogenic) non-carbonate fraction of the rock (cf.Lowrie & Heller, 1982). To explain cyclic variationsof carbonate content, granted they are primary, threevariables have to be considered (Einsele, 1982;

Fischer, 1986; Ricken, 1986): The variations canoccur in biogenic carbonate production, carbonatedissolution and/or input of detrital material. The endmembers of this system are referred to as productivitycycles, dissolution cycles and dilution cycles, respect-ively. Of course, any combination of these variablescan occur to complicate the record. In practice, avariety of palaeontological and geochemical methodsis required to track down the dominant variable for aparticular formation. However, at the root of all thesedifferent cycles there are climatic variations (Fischer& Schwarzacher, 1984). So, if we are interested inperiodicities of climate change, the exact mechanismof recording that change is only of secondary interestas long as it can be established that stratigraphic cyclesare primary sedimentary and not purely diageneticfeatures.

No geochemical analyses other than carbonate con-tent have been performed on samples from the lowerCismon section. However, comparison with otherCretaceous sections in the Southern Alps andUmbrian Apennines (Weissert et al., 1985; Herbert &Fischer, 1986; Herbert et al., 1986; Premoli Silvaet al., 1989; Bersezio, 1993) demonstrates the domi-nance of variations in primary productivity in this partof the Tethys Ocean during Early to mid-Cretaceoustimes (Mayer, 1992). For the Cretaceous of theWestern Interior of North America, where previouslydilution had been favoured (cf. Gilbert, 1895; Fischer,1980; Arthur et al., 1984; Bottjer et al., 1986), Eicher& Diner (1991) also made a case for the prevalence ofproductivity cycles. A small but recognizable trendup-section is apparent in both sedimentary-parametercurves. Susceptibility tends to decrease, whereas car-bonate content tends to increase (Figure 6). After theforegoing demonstration of Milankovitch cyclicityin this record, the trend may be the expression oflong-term climatic change.

Figure 12. Variations of the s-ratio (isothermal remanentmagnetization (IRM) acquired at backfield of 0.3 Trelative to the IRM acquired at 1.2 T) indicate vari-ations in magnetic mineralogy through the Cismonsection.

6. Conclusions

Magnetic susceptibility and carbonate content weremeasured on densely spaced samples from theValanginian to Hauterivian part of the Cismon sectionand analyzed as geologic time series. The variations ofboth parameters through the section are truly cyclic asrevealed by quantitative time-series analysis. A hier-archy of cycles with periods at c. 45 cm, 80 cm and180 cm was calculated. The geostatistical cova func-tions proved to be the most versatile and robust of thecompared methods for the analysis of this kind ofgeological data. Natural remanent magnetization insamples from Cismon and bedding-thickness vari-ations in the Pra da Stua section were also analyzed.


Cyclicity of the same periods is indicated in theseparameters, but not nearly as clearly and conclusivelyas in the susceptibility and carbonate-content vari-ations. Susceptibility is tightly correlated negativelyto carbonate content so that it can be utilized as aproxy for the latter in this palaeoenvironmental set-ting. The carbonate cycles represent productivitycycles which can be attributed to climatically drivenshifts in oceanic circulation. Susceptibility here pro-vides a reliable record of palaeoclimatic change. Theratios between the periods of the thickness cyclesalmost exactly match the ratios between the dominantorbital periods of precession, obliquity and eccen-tricity expected for Milankovitch cycles. The evidencefor Milankovitch cycles in the Early Cretaceous pre-sented here suggests that orbital forcing was effectivethen in a similar way as it has been during theQuaternary, although the climatic boundary con-ditions were quite different with warm, equable andice-free conditions in contrast to the Quaternaryice-age climate. Apparently, orbitally forced climaticchange was strong enough even in the Cretaceousclimate state to leave a clear mark in the sensitivepelagic record, although polar ice caps, which play amajor role in climate models for the Quaternary, didnot exist.

Acknowledgements

This investigation was carried out in the palaeo-magnetic laboratories of the Institut fur Geologieund Palaontologie, Eberhard-Karls-Universitat,Tubingen, Germany; Scripps Institution of Ocean-ography, University of California San Diego, La Jolla,California, USA; and United States GeologicalSurvey, Denver, Colorado, USA. We thank LisaTauxe (La Jolla), Gary S. Calderone, Richard L.Reynolds and Joseph G. Rosenbaum (all Denver) forgranting access to facilities and equipment as well asfor valuable discussions. We also thank Ute Herzfeldfor introducing us to cova functions and for helpfuldiscussions. The thorough reviews of journal refereesAlfred G. Fischer and Reinhard Gaupp, which ledto improvements in the manuscript, are gratefullyacknowledged. However, the interpretations, con-clusions and potential errors contained remain theauthors’. This work was supported by, and formsa contribution to, Sonderforschungsbereich 275‘‘Klimagekoppelte Prozesse in meso- und kano-zoischen Geookosystemen’’ (Climate-coupled pro-cesses in Mesozoic and Cenozoic geoecosystems),Eberhard-Karls-Universitat Tubingen, funded bythe Deutsche Forschungsgemeinschaft (GermanResearch Council).

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Documents

Milankovitch cyclicity and rock-magnetic signatures of ...studied in detail. Rock-magnetic results from Cismon and Pra da Stua are presented. The cyclostratigraphy of the Cismon section