combined magnetostratigraphic, palaeomagnetic and calpionellid investigations across...

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Cretaceous Research 25 (2004) 771–785

Combined magnetostratigraphic, palaeomagnetic and calpionellidinvestigations across Jurassic/Cretaceous boundary strata

in the Bosso Valley, Umbria, central Italy

V. Housaa, M. Krsa, O. Mana, P. Prunera,*, D. Venhodovaa, F. Ceccab,G. Nardic, M. Piscitelloc

aInstitute of Geology, Academy of Sciences of the Czech Republic, Rozvojova 135, 165 02, Praha 6 – Lysolaje, Czech RepublicbUniversite ‘‘Pierre et Marie Curie’’, Laboratorie de Micropaleontologie, Case 104, 4, place Jussien, F-75252 Paris Cedex 05, FrancecUniversita degli Studi di Napoli – Federico II, Dipartimento di Scienze della Terra, Largo San Marcellino, 10, 80138 Napoli, Italy

Received 6 May 2003; accepted in revised form 2 July 2004

Available online 27 August 2004

Abstract

The principal aim of a detailed magnetostratigraphic and micropalaeontological investigation of the Jurassic/Cretaceous (J/K)boundary limestones in the basal portion (39 m) of the Bosso Valley section in Umbria, central Italy, was to determine precisely the

boundaries of magnetozones and narrow reverse subzones, and to find a correlation between magnetostratigraphic data (reflectingglobal events) and calpionellid zonation. Two reverse subzones were detected in magnetozones M20n and M19n. These are in thesame positions relative to the magnetozones above and below as the reverse subzones at the recently studied locality of Brodno, near

Zilina, West Slovakia. Both the Brodno and Bosso sections so far represent the only magnetostratigraphic profiles across J/Kboundary strata in continent-based outcrops in the Tethyan realm displaying both reverse subzones, which correlate well withanalogous subzones in the M-sequence of marine magnetic anomalies. The samples of the upper Tithonian and lower Berriasian

limestones studied are characterized by a three- or even four-component remanence, with the carrier of palaeomagnetic directionsbeing the C-component, separated by multi-component analysis after progressive thermal demagnetization in the interval of ca.400 �C to the magnetite unblocking temperature (around 550 �C). Other aspects of the use of magnetization changes in limestonesfor possible correlation are also discussed. In addition, the existence of a prominent post-tectonic component of remanent

magnetization is indicated. This imprint, recorded in the Bosso Valley and elsewhere in the Tethyan realm, dates most probably tothe Neogene and is worthy of further investigation.� 2004 Elsevier Ltd. All rights reserved.

Keywords: Central Italy; Bosso Valley; J/K magnetostratigraphy; Calpionellid biostratigraphy; Biozones; Magnetozones

1. Introduction

This paper presents the results of a continuation ofa joint geophysical and palaeontological project focusedon detailed magnetostratigraphic and palaeontologicalstudies of Jurassic/Cretaceous (J/K) boundary strata

* Corresponding author.

E-mail address: [email protected] (P. Pruner).

0195-6671/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.cretres.2004.07.001

with the objective of establishing a correlation of thebiozones around this boundary in the Tethyan andBoreal realms using global palaeomagnetic events.Localities in the Tethyan realm were selected, witha planned extension to at least one locality in the Borealrealm in the future. The sites selected in the Tethyanrealm include two localities in the Western Carpathians(Stramberk, North Moravia and Brodno, near Zilina,West Slovakia), two localities in Spain (Rıo Argos,

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772 V. Housa et al. / Cretaceous Research 25 (2004) 771–785

south-east Spain and Puerto Escano, southern Spain)and one locality in Italy (the Bosso Valley, Umbria).The two localities in the Western Carpathians havealready been investigated. The Brodno locality providedhigh-resolution data around the J/K boundary (Housaet al., 1999a). Two reverse subzones were detected inmagnetozones M20n and M19n (Housa et al., 1999b).The Rıo Argos section proved unsuitable for theinterpretation of magnetostratigraphic data: the lime-stones studied showed syn- and post-tectonic compo-nents of remanent magnetization (Hoedemaeker et al.,1998). Therefore, a new section was searched for inSpain; the Puerto Escano section, now under investiga-tion, was found to be suitable. Its study has been aidedby an abundant faunal content, both ammonites andcalpionellids, permitting correlation of palaeomagneticzonation with the calpionellid and ammonite zonationsof J/K boundary strata. This section provided limestonesamples with exceptionally favourable physical proper-ties for the inference of palaeomagnetic directions.

In this paper we present the results of magnetostrati-graphic and micropalaeontological analyses of a 39-m-thick portion of the Bosso Valley section comprisingTithonian and lower Berriasian limestones. This localityhas been already subjected to a synoptic study, and theessential pattern of magnetozone distribution wasreported by Lowrie and Channell (1983). The stratigra-phy of the individual parts of the section used by theseauthors was based on the calpionellid zonation ofMicarelli et al. (1977). Accordingly, the importantCrassicollaria Zone/Calpionella Zone boundary (thepresent J/K boundary) is located within magnetozoneM19r, contrary to the findings at other localities at thattime. The positions of the nannoplankton zones relativeto magnetozones were also different from those at otherlocalities (Lowrie and Channell, 1983). For this reason,J. Kirschvink, the reviewer of the study, attached a note‘‘that the marine biological changes were probably notsynchronous at the Jurassic-Cretaceous transition’’(Lowrie and Channell, 1983, p. 47). However, the baseof the Calpionella Zone sensu Micarelli et al. (1977) isdefined by the appearance of Calpionella and coincideswith the base of Remane’s A2 Subzone (see Micarelliet al., 1977, p. 70, fig. 9). Therefore, the Calpionella Zoneof Micarelli et al. (1977) is not equivalent to thestandard Calpionella Zone of Allemann et al. (1971;see Remane et al., 1986). This explains the apparentcontradiction (Cecca et al., 1990), Lowrie and Channell(1983) having been misled by a zonation that used thesame names as the standard zones but which actuallycorresponded to different chronostratigraphic units.

In the Tethyan realm, the J/K boundary coincideswith the boundary between the standard Crassicollariaand Calpionella zones that was agreed after the‘‘Colloque sur la limite Jurassique Cretace’’, Lyon –Neuchatel, in 1973.

Bearing this in mind, we selected the Bosso Valleysection for a detailed study in order either to prove ordisprove this determination. Our data allowed us tospecify the individual magnetozones in more detail, ledto a precise localization of the two reverse submagne-tozones in M19n and M20n, and provided a basis fora more precise calpionellid zonation. It was found thata relationship between the calpionellid zones andmagnetozones in the Bosso Valley section is identicalto those at other known localities and displays noanomalies. The boundary between standard calpionellidCrassicollaria and Calpionella zones (Remane et al.,1986) lies in magnetozone M19n, at practically the sameposition as the boundary at Brodno. Other importantcalpionellid events in the section are also in the sameposition in the magnetostratigraphic scale as at otherlocalities. Therefore, there is no evidence for a possiblediachroneity of biostratigraphic events at the J/Kboundary (as for the calpionellids).

2. Geological setting

The study area was a part of the MediterraneanTethys, particularly of the so-called Adria promontory(Channell et al., 1979) or Apulian block (sensu Dercourtet al., 1985), which is described as a northern promon-tory of the African continent. The vast Late Triassicperitidal carbonate–evaporite platform was rifted duringthe early Liassic, producing numerous isolated shallow,Bahamian-type carbonate platforms surrounded bypelagic basins (Bernoulli, 1972). The Umbria–MarcheApennines correspond to one of these basins. After themain rifting phase, however, numerous small, isolated,shallow carbonate platforms were still productive untilthe middle Liassic, at which time they were drowned,becoming pelagic swells (Farinacci et al., 1981). Thelatter, which are currently called pelagic carbonateplatforms (PCP) according to Santantonio (1993, 1994),were surrounded by deep troughs characterized bydifferent facies and thicker successions.

The successions exposed in the Bosso Valley showthat this area was part of a trough located on thesouthern margin of the large Monte Nerone PCP area(Cecca et al., 1990; Cecca, 1993, fig. 28; Santantonio,1993).

The development of distinct PCP and troughsuccessions virtually ceased in Umbria–Marche duringthe latest Tithonian with Maiolica deposition nearby(Centamore et al., 1971; Farinacci et al., 1981).However, topographic gradients on the sea bottom,though less pronounced than in the Jurassic, persisted inthe Early Cretaceous as demonstrated by Lowrie andAlvarez (1984). In fact, lithological and palaeontologicaldifferences exist between the Maiolica deposited abovethe Jurassic PCPs and troughs (Micarelli et al., 1977).

773V. Housa et al. / Cretaceous Research 25 (2004) 771–785

The section sampled for our paper belongs to theMaiolica Formation, which spans the uppermostTithonian–lowermost Aptian. Its lithological character-istics correspond to the Maiolica deposited in theJurassic troughs. It consists mainly of thickly beddedwhite micrites with interbedded chert nodules andlayers, and thin, dark, pelitic interbeds whose frequencyand thickness increase markedly towards the contactwith the overlying Marne a Fucoidi Formation (Aptian–upper Albian); slumps and, less frequently, pebblymudstones occur at some levels.

The section sampled (Fig. 1) lies on the northern sideof the road between the villages of Secchiano andPianello di Cagli, between km 9.2 and 9.6 of the roadnumbering. It begins in the uppermost part of the‘‘Calcari a Saccocoma ed Aptici’’ Formation (Cecca,1993; the basal part of the section studied is shown hisfig. 8) and passes to the basal part of the MaiolicaFormation, which spans the uppermost Tithonian–lowermost Aptian. The vertical scale of the section usedin this paper follows older numbering marked in red onthe rock. The vertical scale used by Micarelli et al.(1977) lies at about the same level as its zero point, andmay be identical to the red numbering on the outcrop.This is also suggested by the distribution of somecalpionellid taxa. The part of the section we studiedextends deeper than that examined by Micarelli et al.(1977), our lowermost sample lying 9 m below the zeropoint of the adopted vertical scale. Our uppermostsample is 30 m above the zero point. Hence, thethickness of the part of the section discussed herein is

39 m. Of this, the basal 10.5 m are referable to theCalcari a Saccocoma ed Aptici Formation (see below),and the overlying section to the Maiolica Formation.

A different vertical scale was used by Lowrie andChannell (1983). Our section encompasses the intervalof ca. 300–330 m in the vertical scale of Lowrie andChannell, but extends several metres below it. A newdefinition and numbering of the individual beds wasadopted for the purpose of this paper because noauthors have previously achieved such a high resolution.

3. Analysis of magnetic and palaeomagnetic properties

Remanent magnetization in its natural state and afteralternating field (AF) and thermal demagnetization aswell as volume magnetic susceptibility of limestonesamples were measured using a JR-5 spinner magne-tometer and a KLY-2 Kappabridge (Jelınek, 1966,1973). AF demagnetization was applied to several pilotsamples only, using a Schonstedt GSD-1 demagnetizer.Progressive thermal demagnetization employing a MA-VACS demagnetizer (Prıhoda et al., 1989) proved to bevery effective. In the first stage of our laboratory studies,pilot samples were subjected to an analysis of IRMacquisition and AF demagnetization curves with the aimof establishing magnetic hardness of the magneticallyactive minerals contained in the limestones. Other pilotsamples were isothermally magnetized under direct fieldsof up to 900 mT and then progressively demagnetized inten thermal fields to determine unblocking temperatures

Fig. 1. Location map of the Bosso Valley, Umbria, central Italy (4Z 43 �31#10$N; lZ 12 �34#16$E).


Table 1

Basic magnetic parameters of limestone samples from the Bosso Valley section

Age Magnetozone Number of samples Modulus of natural

remanent magnetization M [10�6 A/m]

Volume magnetic

susceptibility k [10�6 SI]

Mean value Standard deviation Mean value Standard deviation

Early Berriasian normal 61 324 211 �1.0 2.5

reverse 44 521 512 �2.1 2.4

Late Tithonian normal 124 1052 1449 16.4 18.8

reverse 27 325 325 10.0 6.4

with adequate precision. In the second phase, allsamples used for inferring a magnetostratigraphic pro-file were subjected to progressive thermal demagnetiza-tion using the MAVACS demagnetizer in 10–11 thermalfields up to the unblocking temperatures of mineral-carriers of palaeomagnetization. The measurement datawere subjected to a multi-component analysis of rema-nence (Kirschvink, 1980).

3.1. Basic magnetic properties

Values of volume magnetic susceptibility of samplesin natural state (k) decrease from older to youngerrocks, as in the section of J/K boundary strata atBrodno (Housa et al., 1999a). Berriasian rocks rangingbetween magnetozones M18 and M17 show negative kvalues; here, diamagnetism of the limestone materialprevails over ferrimagnetism of mineral-carriers ofremanent magnetization (M ). Values of M moduli alsodisplay a decrease from older to younger rocks; a similardependence was recorded at Brodno. Although therocks show relatively low M values, remanent magne-tization could be always measured even after thermaldemagnetization reached values close to the measure-ment noise of the spinner magnetometers employed (ca.5! 10�6 Am�1). Basic magnetic properties of all of thelimestone samples studied are summarized in Table 1.

3.2. Palaeomagnetic properties of pilot samples

Altogether eight pilot samples were subjected toprocessing of IRM acquisition up to the field of 900 mTand to subsequent AF demagnetization. The results ofthese measurements, demonstrated for four samples inFig. 2, show that some of the magnetic particles displayhigh magnetic hardness. Similar properties are alsodisplayed by samples from Brodno, Puerto Escano, andother localities in the Tethyan realm (e.g., Cirilli et al.,1984; Galbrun, 1985). The curves of IRM acquisitionimply that remanent magnetization approximates thestate of saturation in the field of 900 mT. In the nextstep, the isothermally magnetized samples with rema-nent magnetization Ms (0.9 T) were subjected to pro-gressive thermal demagnetization up to the unblockingtemperature characteristic of magnetite (ca. 550 �C).This method was applied to eight samples in total;measurement results for four of them are given in Fig. 3as examples. The figure also shows values of volumemagnetic susceptibility versus laboratory temperature.Although increases in kT values occurred in severalsamples at temperatures above 400 �C with respect tohigh magnetic vacuum in the MAVACS demagnetizer,this phase change was not manifested in the diagram ofnormalized Ms,T/Ms,0 values, and the unblocking tem-peratures were determined with the same precision as inmagnetically stable samples.

0 20 40 60 80 100Peak field [mT]

0.0

0.5

1.0

IRM/IRMmax116.0 mA/m

0 20 40 60 80 100Peak field [mT]

0.0

0.5

1.0

IRM/IRMmax53.2 mA/m

A B

Fig. 2. Examples of IRM acquisition and AF demagnetization curves, limestone samples.


0 100 200 300 400 500 T [°C]0.0

0.5

1.0Ms,T /Ms,0 Ms,T /Ms,0

116 mA/m

0.0

0.001

0.002

-d[Ms,T /Ms,0]/dT -d[Ms,T /Ms,0 ]/dT

0 100 200 300 400 500 T [°C]0

50kT [10-6

SI] kT [10-6 SI]

0 100 200 300 400 500 T [°C]0.0

0.5

1.0 51.0 mA/m

0.0

0.001

0.002

0 100 200 300 400 500 T [°C ]0

10

A B

Fig. 3. Results of progressive thermal demagnetization of IRM for limestone samples. Prior to demagnetization, the sample was magnetized to the

saturated state whose remanent magnetic moment was Ms,0; the same magnetic moment after the demagnetization at temperature T is denoted by

Ms,T. The observed values of this quantity (circles) were interpolated by a smoothing spline (thick line), whose derivative with respect to temperature

T is denoted by d(Ms,T/Ms,0)/dT and indicated by the thin line. The volume magnetic susceptibility of the sample demagnetized at temperature T is

denoted by kT.

As in limestone samples from other localities in theTethyan realm, progressive thermal demagnetizationemploying the MAVACS demagnetizer proved to beexceptionally efficient. The samples were progressivelydemagnetized in 10–11 thermal fields at temperatures of100, 150, 200, 250, 300, 350, 400, 450, 500, 540 (550),and in some cases up to 580 �C. Typical examples ofthe results of thermal demagnetization of limestonesamples from different parts of the section studied arerepresented by four samples with different values ofmoduli of natural remanent magnetization M, includingtwo samples with normal palaeomagnetic directions(Fig. 4) and two with reverse directions (Fig. 5). Thesamples display a well-defined 3–4-component rema-nence: the A-component is undoubtedly of viscousorigin, being separable in the temperature range of 20–100 (150) �C, the B-component in the range of 100(150)–(200) 250 �C, and the C-component is demagnet-izable in the range of 400–540 (580, 590) �C. Anadditional component at the transition between the B-and C-components, referred to as the B1-component,occurs in many samples in the range of (200) 250–350(400) �C. For example, the B1-component is clearlyvisible in Figs. 4 and 5. The origin of this componentwas examined by statistical processing of a large set ofdata and is discussed below (Section 4).

The high magnetic vacuum generated by the MA-VACS apparatus during progressive thermal demagne-tization provided effective demagnetization even forweakly magnetic samples (down to values of 10�2 of

the original M modulus) and more strongly magneticsamples (down to values of 10�3). Directions ofremanent magnetization were precisely determined evenfor samples demagnetized close to the unblockingtemperature. The limestone samples show physicalproperties favourable for inferring palaeomagneticdirections, as pointed out previously by Lowrie andChannell (1983), who employed different laboratorytechniques.

4. Multi-component analysis of remanence,

palaeomagnetic directions and virtual

pole positions

Altogether 260 oriented hand samples were collectedand subjected to progressive thermal demagnetizationusing the MAVACS demagnetizer and subsequentmulti-component analysis of remanence (Kirschvink,1980). All were also subjected to a study of mineralog-ical stability by investigating the dependence of mag-netic susceptibility on temperature. Most (237) yieldedcomponents of remanence in Zijderveld diagrams ofa quality similar to that displayed in Figs. 4 and 5.

The A-component of remanence constitutes a signif-icant portion of the natural remanent magnetizationmodulus (M ), frequently reaching 40–60% of the Mmodulus. This component is entirely unstable and wasseparated from all samples within the temperature rangeof 20–100 (150) �C.


-0.8 -0.6 -0.4 -0.2 0.2 0.4 0.6 0.8Wup

EdownmA/m

0.2

0.4

0.6

NmA/mNS NS

XYXZ

100

150

200

250

300350 400 500

540s

100

150

200

250

300350450

0 100 200 300 400 5000.0

0.5

1.0MT /M0 1.34 mA/m

T [°C]

N30

60

E120

150S

210

240

W30

0

330

NS

downup

540

150100

200250300

500350

0 100 200 300 400 500 T [°C]0

10

20kT [10-6 SI]

0 100 200 300 400 500 T [°C]0.0

0.5

1.0MT /M0 0.209 mA/m

-0.1 0.1Wup

E

SdownmA/m

0.1

NmA/mNS NS

XYXZ

100 100150 150

200 200250 250

300 300350 350

400 400450 450

500 500

N30

60

E120

150S

210

240

W30

0

330

NS

downup

100150

200 250300

350

450

500 400

T [°C]0 100 200 300 400 5000

2

4

6kT [10-6 SI ]

B

A

Two other components, B and B1, generally charac-terized by a single polarity, are of secondary origin. Theset of B-components is dominated by vertical directionswhen related to limestone strata corrected for tectonicdip. These components were separated within the rangeof 100 (150)–(200) 250 �C from some samples (seeFig. 6). Mean B-component directions of the two setsof rocks, upper Tithonian and lower Berriasian, areidentical within the limits of data dispersion, with theBerriasian rocks displaying a somewhat higher disper-sion (see Table 2). Inclination of the field of theoreticalco-axial geocentric magnetic dipole for the Bosso Valleyis 62.2 �; this is very close to the mean inclination(64.3 �), but of the reverse polarity of B-components notcorrected for the dip of strata (in situ directions). Thesecomponents were undoubtedly imprinted in the recentpast but during reverse polarity of the Earth’s magneticfield, most probably in the Neogene after the rocks hadbeen folded. Analogous post-tectonic components havealso been detected at other localities in the Tethyanrealm (Pares and Roca, 1996; Villalaın et al., 1996;Hoedemaeker et al., 1998).

B1-components of remanence, also of secondaryorigin, were separated from some samples in the rangeof (200) 250–350 (400) �C. These, like the B-components,are most prominently manifested in older strata of thelimestones studied, and the intensity of their effectdecreases towards younger strata. Directions of B1-components for Tithonian rocks are given in Table 2;those for lower Berriasian rocks show only a minoreffect, and their mean direction was therefore inferredwith a wider scatter. The palaeomagnetic pole positioncalculated from mean directions after structural tiltcorrection for Tithonian and Berriasian rocks is 54 �N,254 �E. This pole position is close to that for Paleogenesediments of the Northern Apennines (60 �N, 258 �E;Marton et al., 1988).

The C-components were reliably inferred for most ofthe limestone samples studied after thermal demagneti-zation in the range of (350) 400–540 (580, 590) �C.

Fig. 4. Results of progressive thermal demagnetization of two

limestone samples (A, Bo 0.85; B, Bo 15.00) with normal palae-

omagnetic polarity. Zijderveld diagram: solid circles represent pro-

jection on the horizontal plane (XY); open circles represent projections

on the north–south vertical plane (XZ); numbers refer to the

temperatures of the demagnetizing fields [ �C]; NS, natural state. A

graph of normalized values of the remanent magnetic moments versus

thermal demagnetizing fields; MT, modulus of the remanent magnetic

moment of a sample subjected to thermal demagnetization; MO,

modulus of the remanent magnetic moment of a sample in the natural

state. A graph of the normalized values of volume magnetic

susceptibility versus thermal demagnetizing fields; kT, value of volume

magnetic susceptibility of a sample subjected to thermal demagneti-

zation. A stereographic projection of the remanent magnetization of

a sample in the natural state (NS) and after progressive thermal

demagnetisation; numbers refer to the temperatures of the demagne-

tization fields [ �C].


0 100 200 300 400 500 T [°C]0.0

0.5

1.0MT /M0

MT /M0

0.153 mA/m

-0.1

Wup

EdownmA/m

EdownmA/m

0.1

S

N

mA/m

NS NS

XYXZ

100

150

200

250

300350

400450

500

550

100

150

200

250

300350

400450

500

W

N30

60

E120

150

S210

240

300

330 NS

downup

100150

200 250

300

450

500

550

T [°C]0 100 200 300 400 5000

10

20kT [10-6 SI]

0 100 200 300 400 500 T [°C]0.0

0.5

1.00.122 mA/m

-0.04 -0.02 0.02 0.04 0.06 0.08

Wup

0.02

0.04

0.06

S

NmA/mNS NS

XYXZ

150 100 100 150200

250

300

350400

450

250200

300

350400

450

500

540580

N30

60

E120

150

S210

240

W30

0

330

NS

downup

100

150

200

250

300

350400

450

500

540

580

0 100 200 300 400 500 T [°C]0

10

20

kT [10-6 SI]

B

A

Similarly, as at other localities of Mesozoic limestones inthe Tethyan realm (provided the limestones were nottotally remagnetized), the directions of C-componentscorrespond to palaeomagnetic directions and can beused for inferring a magnetostratigraphic profile (cf.Lowrie and Channell, 1983; Cirilli et al., 1984; Ogget al., 1984, 1988; Galbrun, 1985; Housa et al., 1999a,b).Directions of C-components for upper Tithonian andlower Berriasian limestones are shown in Figs. 7 and 8(see also Table 2). Limestone beds in the Bosso Valleydisplay uniform bedding-plane orientation, with a meanstrike of 103.2 � G 12.7 � and a mean dip angle of27.3 � G 5 � SSW. The uniform bedding-plane orienta-tion is also responsible for the equal dispersions of themean directions of remanence components in situ (notcorrected for tectonic dip) and those corrected fortectonic dip.

Table 3 summarizes the virtual pole positionscalculated for the upper Tithonian and lower Berriasianlimestones, in which directions with reverse polaritywere transformed into directions with normal polarity.The table also gives pole positions calculated for allsamples studied; reverse palaeomagnetic directions wereagain transformed into normal directions. Palaeomag-netic directions as well as the calculated virtual polepositions indicate a counter-clockwise palaeotectonicrotation. An analogous rotation has been reportedfor Cretaceous and Jurassic rocks in a broader region ofthe Northern Apennines (Irving et al., 1976; Channell,1977; Lowrie and Alvarez, 1977; Channell et al., 1978;

Fig. 5. Results of progressive thermal demagnetization of two

limestone samples (A, Bo 6.20; B, Bo 19.15) with reverse palae-

omagnetic polarity: see caption for Fig. 4.

A BN

30

60

E120

150S

210

240

W30

0

330N

30

60

E120

150S

210

240

W30

0

330

Fig. 6. Upper Tithonian and lower Berriasian limestone, directions of

B-components of remanence corrected (A) and not corrected (B) for

dip of strata. Stereographic projection; full (open) small circles

represent projection onto the lower (upper) hemisphere; the mean

directions calculated according to Fisher (1953) are marked by a small

square; the confidence circle at the 95% probability level is circum-

scribed about the mean direction; the star shows the present dipole

field.


Table 2

Mean directions of the components of remanence corrected and not corrected for structural tilt

Structural tilt correctionNo structural tilt correction(in-situ directions)

Mean directions Mean directions

.Decl.[o]

210.0

292.2

282.7

321.5

317.9

320.6

296.8

107.0

289.7

100.1

285.8

291.6

295.1

Age of rocks

Late Tithonian

Early Berriasian

L. Tith.+ E. Berr.

Late Tithonian

Early Berriasian

L. Tith.+ E. Berr.

Late Tithonian

Late Tithonian

Early Berriasian

Early Berriasian

Early Berriasian

L. Tith.+ E. Berr.

Late Tithonian

Com

pone

nt o

fre

man

ence

B

B

B

B1

B1

B1

C

C

C

C

C

C

C

α95[o]

4.3

12.0

5.9

11.1

28.1

13.2

4.0

7.8

5.6

10.0

5.2

3.0

3.6

k

29.0

4.8

8.5

6.2

1.9

2.9

11.5

14.8

12.8

6.1

8.7

10.0

11.8

Decl.[o]

191.8

203.8

197.2

327.9

336.6

330.5

309.8

120.0

311.8

119.0

306.5

307.5

308.0

Incl.[o]

-64.7

-63.6

-64.3

9.3

33.8

17.4

24.4

-26.2

34.3

-32.0

33.5

28.2

24.8

α95[o]

4.4

11.4

5.7

11.2

29.5

13.4

4.1

7.4

5.5

9.2

5.0

3.0

3.6

k

27.7

5.3

9.1

6.2

1.8

2.9

11.1

16.2

13.4

7.0

9.5

10.4

11.6

n

40

37

77

32

28

60

117

25

55

40

95

237

142

See

Figu

re

-

-

7

6

-

-

9A

9B

8B

8A

-

-

-

Pola

rity

R

R

R

N

N

N

N

R

N

R

N*)

N*)

N*)

Incl.[o]

-89.2

-83.7

-87.2

27.3

55.6

36.9

34.2

-32.3

42.7

-39.0

41.3

36.9

33.9

N, R, normal, reverse polarity; N*, set of samples with N polarity (reverse polarities were transferred into normal polarities); C-components, data in

shaded fields correspond to mean palaeomagnetic directions.

Van den Berg et al., 1978: for palaeogeographicreconstructions, see Krs et al., 1992). This rotation hasobviously no influence on the pattern of normal andreverse magnetozones and submagnetozones in thesection studied. The palaeolatitude calculated frompalaeomagnetic data for the section is 19.8 �N, whichcorresponds to a latitudinal drift of 23.7 � (G2 �) northfrom Tithonian–Berriasian times to the present.

A BN30

60

E120

150S

210

240

W30

0

330N

30

60

E120

150S

210

240

W30

0

330

Fig. 7. Upper Tithonian limestone, directions of C-components of

remanence corrected (A) and not corrected (B) for dip of strata;

normal directions are marked by small circles; reverse directions are

represent by small triangles in which directions were transformed into

normal polarity: see caption for Fig. 6.

5. Magnetostratigraphic profile

As noted above, the Bosso Valley section was studiedin a general way by Lowrie and Channell (1983).By contrast, our analysis concentrated on a detailedinvestigation of the basal 39 m of the section, onthe limestone strata around the J/K boundary, todetermine as precisely as possible the boundaries of

A BN30

60

E

150S

210

240

W30

0

330N

30

60

E120

150S

210

240

W30

0

330

Fig. 8. Lower Berriasian limestone, directions of C-components of

remanence corrected (A) and not corrected (B) for dip of strata: see

captions for Figs. 6 and 7.


Table 3

Palaeomagnetic data for the Bosso Valley locality (upper Tithonian and lower Berriasian limestones)

Age Location Mean palaeomagnetic

directions

a95 [�] k n Palaeomagnetic pole

positions

Ovals of

confidence

Lat.f

[ �] N

Long.l

[ �] E

Decl.

[ �]

Incl.

[ �]

Palaeolat.fp

[ �] N

Palaeolong.lp[ �] W

dm[ �]

dp[ �]

Late Tithonian 43.52 12.57 295.1 33.9 3.6 11.8 142 30.7 80.5 4.1 2.4

Early Berriasian 43.52 12.57 285.8 41.3 5.2 8.7 95 27.2 69.7 6.3 3.9

L. Tithonian–

E. Berriasian

43.52 12.57 291.6 36.9 3.0 10.0 237 29.5 76.3 3.5 2.1

Pole positions were calculated for samples with normal polarity, reverse-polarized samples were transformed into normal-polarized samples.

submagnetozones expected in normal portions ofmagnetozones M19 and M20. The average samplingdensity for the whole section was around five samplesper 1 m of true thickness of limestone strata, and 30 andmore samples per 1 m in critical portions of the section.

The resulting magnetostratigraphic profile is shown inFig. 9. It displays the modulus of natural remanentmagnetization (M ); the volume magnetic susceptibilityof samples in a natural state (k); co-ordinates of thevirtual pole position (VP longitude and VP latitude); anda discriminant function defining the polarity. Both thelast of these and the virtual pole position are functions ofthe direction of the remanence C-component, havingbeen inferred by means of a multi-component analysis.The latter is given by the inner product of the directionand the principal eigenvector of the orientation matrix,whose positive or negative values imply normal orreverse polarity, respectively. This procedure will betreated comprehensively in another paper.

Older limestones in the section studied, mostly lateTithonian in age, display higher values of M and kmoduli. A similar decrease in magnetization from olderto younger rocks was observed in the Brodno sectionand in the section through the J/K boundary limestonesat Puerto Escano. As indicated in Fig. 9, normal (N) andreverse (R) magnetozones and submagnetozones areclearly manifested in the interpreted values of VPlongitude and VP latitude and a discriminant functiondefining the polarity. Samples Bo 1.55 and Bo 1.68yielded directions with insufficient confidence (tendencyto reverse directions?); therefore, this portion of theprofile is indicated by a question mark. Both sampleswere collected from places where limestone strata havebeen affected by tectonic deformation.

The basal interval of the Bosso Valley sectionprovides evidence of the presence of magnetozonesranging from M20n to M17r, thus confirming thesynoptic picture presented by Lowrie and Channell(1983). The principal aim of our study was met: twonarrow reverse subzones were detected in M20n andM19n, precisely defined and correlated with analogoussubzones in the Brodno section (Housa et al., 1999b),and subzones detected in the M-sequence of marinemagnetic anomalies.

The repeated collection of samples and subsequentaccumulation of palaeomagnetic data resulted in theprecise detection of two reverse polarity subzones. TheKysuca Subzone detected in normal magnetozone M20nis only 17 cm thick (see Figs. 9, 10). By analogy with theBrodno profile (Housa et al., 1999a), it is situated abovethe middle of normal magnetozone M20n. Anotherreverse subzone, the Brodno Subzone, detected in theupper part of normal magnetozone M19n, is 77 cm thick(see Figs. 9, 11). Both the Kysuca and Brodno subzoneswere marked on the outcrop in the Bosso Valley sectionby aluminium cylinders 25 mm in diameter stamped‘‘Kysuca’’ and ‘‘Brodno’’ for subsequent reference in thefield (see Figs. 12, 13).

6. Calpionellids

As noted above, the basal part of the section is in theCalcari a Saccocoma ed Aptici Formation (Cecca, 1993).The lowest 8.2 m (Beds 2–27) fall within the lower part ofmagnetozone M20n. Samples from this interval do notcontain any calpionellids that are not reworked. AtBrodno, the calpionellidChitinoidella Zone begins withinmagnetozone M20r (see Housa et al., 1999a). In theoldest part of the Bosso section, indeterminable remainsof Chitinoidella were occasionally found in intraclasts.The only well-preserved specimen of Chitinoidellaslovenica Borza was found in a clast in Sample 9/96from the middle part of Bed 15. The assignment of thisbasal portion of the section to the Chitinoidella Zone isconfirmed by the presence of cysts of calcareousdinoflagellates referable to species that indicate theColomisphaera tenuis Zone (lower Tithonian). In theWestern Carpathians this zone corresponds to the upperpart of the Chitinoidella Zone (Rehakova, 2000a,b).

No calpionellids were found (if occurrences in intra-clasts and disputable remains are omitted from consid-eration) in Bed 28 (0.3 m thick) containing the Kysucamagnetozone (0.2 m thick). The lower section of thePostkysuca part of magnetozone M20n (Beds 29–45,4.3 m thick) is also devoid of them. Their first appearancewas recorded in Bed 46 (Sample BO-97/16: Praetintin-nopsella andrusovi) and is linked with a more pronounced


M [mA/m] k [10-6 SI] VP longitude [°] VP latitude [°] Normal

Reverse

Pola

rity

Scal

e [m

]

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

0 2 4 6 8 0 20 40 60 80 0 90 180 270 360 -90 0 90 -1 0 1

Dis criminant function

M20

nM

20n

M20

n.1r

M19

nM

19n.

1rM

19r

M18

nM

17r

M18

r

Fig. 9. Magnetostratigraphic profile across the Bosso Valley J/K boundary strata, summary results of magnetic, palaeomagnetic, lithostratigraphic

and calpionellid data.M, remanent magnetization in the natural state; k, value of volume magnetic susceptibility in the natural state; VP, virtual pole;

1, micritic limestone; 2, chert layers; 3, clayey limestone; 4, pink bioclastic limestone; 5, laminated bioclastic limestone.


Normal

Reverse

Pola

rity

Scal

e [m

]138

136

132

130

128

126

124

123

121

119

116115

102

98

94

9290

88

86

78

747372

71

676665

63

61

55

51

383736343332302928

26

23

22

15

129

2

Chi

tinoi

della

slov

enic

aC

hitin

oide

llasp.

Chi

tinoi

della

bone

ti

prim

itiv

eca

lpio

nell

idA

prim

itiv

eca

lpio

nell

idB

Prae

tintin

nops

ella

andr

usov

i

Cra

ssic

olla

ria

sp.

Cra

ssic

olla

ria

inte

rmed

iaC

rass

icol

lari

am

assu

tinia

naC

rass

icol

lari

abr

evis

Cra

ssic

olla

ria

parv

ula

Tin

tinno

psel

laca

rpat

hica

Cal

pion

ella

gran

dalp

ina

Cal

pion

ella

alpi

na

Cal

pion

ella

elli p

talp

ina

Cal

pion

ella

sp.B

Cal

pion

ella

llipt

ica

aff.

eC

alpi

onel

lam

inut

a

Tin

tinno

psel

lado

lipho

rmis

Rem

anie

llafe

rasi

niR

eman

iella

fera

sini

aff.

A3

A2

A1

Cr2

Cr1

CH2

CH1

B

B

δ

δ

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

KJ

1

2

3

4

5

M20

nM

20n

M20

n.1r

M19

nM

19n.

1rM

19r

M18

nM

17r

M18

r

Lithologiccolumn

Bednumbers

Calpionellidzones

Calpionellid distribution

Fig. 9. (continued)


VP longitude [°] VP latitude [°] Normal

Reverse

pola

rity

scal

e [m

]

0 90 180 270 360 -90 0 90 -1 0 1

discriminant function

M20

nM

20n

-1.5

-1.0

-0.5

Kys

uca

Subz

one

Fig. 10. The Kysuca Subzone (M20n.1r) with reverse palaeomagnetic polarity. VP, virtual pole (derived from C-components of remanence).

onset of light-coloured limestones. From this levelupwards, calpionellid occurrences are common (Fig. 9).

The uppermost part of magnetozone M20n (Beds46–54, 2.0 m thick) is assigned to the uppermost

Chitinoidella Zone (CH2), with Praetintinnopsella andru-sovi (Beds 46–49), and the lowermost CrassicollariaZone, i.e. the ‘‘Tintinnopsella’’ remanei Subzone (Cr1)(Beds 50–54). The first appearance of Calpionella

VP longitude [°] VP latitude [°] Normal

Reverse

pola

rity

scal

e [m

]

0 90 180 270 360 -90 0 90 -1 0 1discriminant function

M19

nM

19n

Bro

dno

Subz

one

18.0

18.5

19.0

19.5

Fig. 11. The Brodno Subzone (M19n.1r) with reverse palaeomagnetic polarity: see caption for Fig. 10.


grandalpina in Bed 55 (Sample BO-97/23) marks thebase of the Crassicolaria intermedia Subzone (Cr2). Thisfirst appearance is still within the uppermost part ofmagnetozone M20n (15 cm below the base of magneto-zone M19r), i.e., at the same level as in other sections,such as that at Brodno.

The top of magnetozone M20n (in Bed 55) and thewhole of M19r (upper part of Bed 55 to lower part ofBed 63) fall within the C. intermedia Subzone, as doesthe lower part of M19n (upper part of Bed 63 to Bed 77).This interval is characterized by abundant Tithonianspecies of Crassicollaria (e.g., C. intermedia, C. brevisand C. massutiniana). Magnetozone M19r was recordedthrough a thickness of 1.8 m.

The position of the boundary between the standardCrassicollaria and Calpionella zones (Allemann et al.,1971) is defined as the base of the first appearance ofabundant minute, globular Calpionella alpina. At Brod-no, this appearance is very pronounced, being associatedwith the extinction of large species of Crassicollaria andCalpionella (C. grandalpina and C. elliptalpina). In theBosso Valley section, the onset of Calpionella alpina is

Fig. 12. The reverse Kysuca Subzone (M20n.1r), the width of which is

marked by two aluminium cylinders (1 inch in diameter bearing the

name Kysuca) cemented into drill holes.

Fig. 13. The reverse Brodno Subzone (M19n.1r), the width of which is

marked by two aluminium cylinders (1 inch in diameter bearing the

name Brodno) cemented into drill holes.

rather gradual, as is the disappearance of species ofCrassicollaria and of Calpionella grandalpina. Thissignificant ‘‘event’’ is located between Beds 76 and 79(ca. 60 cm), i.e., between 13.1 and 13.7 m of the section.Calpionella alpina becomes rather abundant in Bed 76(Samples 48/96 and 97/1) but a major increase in itsabundance occurs in Bed 78 (Sample 97/2) and in theoverlying beds. The base of the standard C. alpinaSubzone, hence also the J/K boundary, is thereforeplaced at the boundary between Beds 77 and 78. Thehighest occurrences of Tithonian species ofCrassicollariaare below this level: the last representatives ofC. intermedia were found in Bed 74, and of C. brevis inBed 75; C. massutiniana was recorded from Bed 76and several specimens were found in Sample BO-49from the basal part of Bed 78, which is in the lower-most Calpionella Zone (i.e., the lowermost C. alpinaSubzone).

In relation to the magnetostratigraphic scale, the J/Kboundary in the Bosso Valley section lies approximatelyin the lower one-third (35G 2% of the local thickness)of magnetozone M19n. This is in good agreement withthe position of this boundary in other sections.

In the relatively monotonous assemblage of calpio-nellids of the oldest Berriasian (C. alpina Subzone), abrief peak in abundance of Crassicollaria parvula occursin a characteristic horizon in the basal Berriasian. In theBosso Valley section, it was recorded in Sample 56/96from the youngest part of Bed 88 (at 16.0 m). It occursin the basal part of the upper half of magnetozoneM19n, approximately half-way between the J/K bound-ary and the basal boundary of the Brodno submagne-tozone, which corresponds to the position of thishorizon in other sections.

The first appearance of the large Berriasian form ofCalpionella alpina, provisionally designated as Calpio-nella sp. B, was recorded at the level of the Brodnosubmagnetozone (M19r.1r). It occurs at the same levelas in the Brodno section (Housa et al., 1999a,b).

The base of magnetozone M18r is not characterizedby any ‘‘event’’ in the calpionellid assemblage. Withinthis magnetozone, first representatives of Tintinnopselladoliphormis appear, a phyletic precursor of Remaniella.The first specimens of Remaniella (R. ferasini) wererecorded only in the upper part of magnetozone M18nin Bosso Valley section.

The reason for the absence of calpionellids from thebasal part of the section (Beds 1–45) is unknown, butthis is a frequent phenomenon in basinal Maiolicasuccessions of the Umbria–Marche Apennines (Micar-elli et al., 1977). Calcareous sediments with abundantfragments of calcitic skeletal remains of Saccocoma andrelatively well-preserved cysts of calcareous dinoflagel-lates indicate that the depositional environment wasprobably favourable for the preservation of micro-granular tests of primitive calpionellids. The fact that


calpionellids are not found suggests that they may nothave been present during this period of deposition.

7. Discussion of the main results

The detailed magnetostratigraphic and micropa-laeontological investigation of the basal, 39-m-thickportion of the magnetostratigraphic profile previouslystudied (Lowrie and Channell, 1983), the measurementof basic magnetic properties, and the results of a multi-component analysis of remanence in the J/K boundarylimestones has yielded new information on the correla-tion of palaeomagnetic events with biozones and eventsin the Tethyan realm, as follows:

1. Magnetozones and submagnetozones from M20nto M17r in the Bosso Valley section and M21r to M18rin the Brodno section have been defined. In the former,two reverse submagnetozones were precisely detected inmagnetozones M20n and M19n at relative positionsanalogous to those in the Brodno section. Thesesubzones were named the ‘‘Kysuca Subzone’’ and the‘‘Brodno Subzone’’, respectively (Housa et al., 1999b).In order to define accurately the boundaries of thesesubzones in the Bosso Valley section, sampling densityreached more than 30 samples per 1 m of true thicknessof limestone strata owing to repeated sampling in thecritical intervals. The Brodno and Bosso Valley sectionsrepresent the only sections in pelagic limestones acrossthe J/K boundary in continent-based outcrops whereboth reverse subzones have been proved. Both lie at thesame position relative to the surrounding magnetozones,and correlate well with analogous subzones in the M-sequence of marine magnetic anomalies.

2. The M and k values in the two sections and in thesection across the J/K boundary limestones at PuertoEscano, provide additional evidence of correlation.Magnetization intensities of the limestones show a de-creasing trend from older to younger rocks. As a de-crease in M moduli is accompanied by a decrease in kvalues, the upward-decreasing magnetization from up-per Tithonian to lower Berriasian limestones is mostprobably of primary origin.

3. Results of the multi-component analysis ofremanence permitted the interpretation of palaeomag-netic directions for the majority of the samples: 237 ofa total of 260 samples collected yielded reliabledirections. The inference of palaeomagnetic directionsfrom samples of Mesozoic limestones generally appearsto require a separation of remanence components fromsamples progressively thermally demagnetized in thetemperature range of ca. (350) 400 �C to the unblockingtemperature of magnetite (around 550 �C), which is thecarrier of palaeomagnetism.

4. Post-tectonic components of remanence, mostprobably of Neogene age, were inferred from some of

the samples, particularly the upper Tithonian lime-stones. Analogous components have been reported fromother sections in Mesozoic limestones in the Tethyanrealm (Pares and Roca, 1996; Villalaın et al., 1996;Hoedemaeker et al., 1998). The event responsible for theNeogene components of remagnetization, or even forthe total remagnetization of limestones (such as in theRıo Argos section; Hoedemaeker et al., 1998), isundoubtedly worthy of further study.

5. Three prominent events of calpionellid biostratig-raphy are applicable to precise correlation of theBrodno and Bosso Valley sections: (1) the suddenappearance of Calpionella grandalpina (Z base of thecalpionellid Subzone A2 – Crassicollaria intermedia); (2)a pronounced increase in abundance of Calpionellaalpina (Z base of the calpionellid Zone B – Calpionella,i.e. the J/K boundary); (3) a brief acme of Crassicolariaparvula in the early Berriasian. In both sections, event 1slightly preceded (i.e., lies several cm below) thebeginning of M19r. No prominent palaeomagneticevents occur in close proximity to events 2 and 3. Inboth sections, the levels of these two events are withinM19n at approximately the same level. The J/Kboundary is at around 35G 2% of the local thicknessof M19n. The acme of Crassicollaria parvula is situatedhalf-way between the J/K boundary and the base of theBrodno Subzone.

Acknowledgements

We thank Prof. Dr. A.E.M. Nairn and Dr. J.Adamovic for their reviews and for improving ourEnglish. We are grateful to Prof. D.J. Batten for hisscientific review of the manuscript and much editorialeffort. The constructive comments of Dr. G. Muttoniand one anonymous referee are much appreciated.The location map was drafted by Alexandre Lethiers(Universite Pierre et Marie Curie, Paris VI). Theresearch was supported by the research project of theInstitute of Geology AS CR No. Z3 013 912, and grantprojects of the Grant Agency of the Czech RepublicNos. 205/97/0063 and 205/02/1576.

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