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Geological Institute, Academy of Sciences of the Czech Republic, Rozvojová 135,

165 02 Praha 6-Lysolaje, Czech Republic

(Manuscript received February 25, 1998; accepted in revised form September 1, 1998)

Abstract: A short reverse polarity magnetosubzone, herein defined as the Brodno Subzone, was detected in the upper

part of the magnetozone M19n at the locality of Brodno near Žilina (Western Carpathians) using high-resolution

magnetostratigraphy. An analogous short reverse polarity magnetosubzone, herein defined as the Kysuca Subzone

occurs in the middle part of the magnetozone M20n. Both the magnetosubzones are known from marine profiles but

have been detected only sporadically and documented insufficiently in continental outcrops. These two subzones have

not yet been detected together in one and the same continental section. Their stratigraphic position in the Brodno

section is defined and their interpretation in the other studied section at Štramberk is inferred. In the Brodno section, the

Kysuca Subzone represents the basal part of the calpionellid Remanei Subzone of the Crassicollaria Standard Zone

(early late Tithonian), its base lies at the level of 55 % of the local thickness of the magnetozone M20n. The Brodno

Subzone lies within the calpionellid Alpina Subzone of the Calpionella Standard Zone (earliest Berriasian) and its base

in the Brodno section lies at the level of 82 % of the local thickness of the magnetozone M19n. In both the studied

sections, the Jurassic/Cretaceous boundary based on calpionellids (base of the Calpionella Standard Zone) lies ap-

proximately at the end of the lowermost third of the magnetozone M19n (at the level of 34 % of the local thickness of

the magnetozone M19n in the Brodno section). Magnetostratigraphic calibration of calpionellid events proved their

isochronous character in the localities of Brodno and Štramberk. The interval of ca. ±5000 years, during which a

transition occurred from normal (reverse) to reverse (normal) polarity of magnetic field of the co-axial geocentric

dipole of the Earth, can be determined from an analysis of paleomagnetic directions inferred from samples with inter-

mediate polarity collected from normally and reversely polarized boundary strata at the locality of Brodno and with

respect to the sedimentation rate. This value represents the relative accuracy of possible correlations of the boundaries

of the detected magnetosubzones with boundaries of analogous subzones at other localities on the Earth using the above

given synchronous global event.

Key words: Jurassic/Cretaceous boundary strata, Tethyan Realm, Brodno and Štramberk sections, high-resolution

magnetostratigraphy, two magnetosubzones, calpionellid and magnetostratigraphic correlation.

units in the two realms using indirect methods. The most

precise of these methods is magnetostratigraphy. For the

Tethyan Realm, magnetostratigraphic profiles of the J/K

boundary strata were worked out for a number of localities,

however with imprecise or no correlation with the biostratig-

raphy of these sections. Only a single magnetostratigraphic

profile was published from the Boreal Realm (Ogg et al.

1991), unfortunately containing numerous hiatuses and pro-

viding insufficient correlation with the biostratigraphic zo-


Imprecise and indefinite taxonomic and biostratigraphic

interpretations of fossil calpionellid associations recorded in

magnetostratigraphically analysed sections in the Tethyan

Realm resulted in the placing of the biostratigraphic J/K

boundary into different magnetozones by different authors at

different localities. The variation was also caused by chang-

es in the general position of the J/K boundary in biostrati-

graphic scales (see below). Most frequently, the J/K bound-

ary was placed in the M19n (e.g. Channell & Grandesso


The possibilities of biostratigraphic correlations of chronos-

tratigraphic units of the Jurassic/Cretaceous (J/K) boundary

strata between the Tethyan and Boreal realms are very limited

due to the practically complete divergence between their fossil

associations. This fact hinders the establishment of a J/K

boundary acceptable on a global scale, although the biostrati-

graphic zonation of the J/K boundary strata is elaborated in

much detail both in the Tethyan and Boreal realms. Biostrati-

graphic zones between the Tethyan and Boreal realms unfor-

tunately cannot be correlated directly as they share no com-

mon taxa at specific and even generic levels. Consequently,

provisional boundaries defined at the boundaries of regional

stages are used in both realms, independently of each other.

These provisional J/K boundaries in the Tethyan and Boreal

realms are not isochronous.

Nevertheless, there are several possibilities to correlate the

biostratigraphic scales, and hence also chronostratigraphic

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126                                                                HOUŠA, KRS, MAN, PRUNER and VENHODOVÁ

1987: Fig. 12), on the base of the M18r (e.g. Lowrie & Ogg

1986, p. 342; Manivit et al. 1986, p. 117; Galbrun et al.

1990: Fig. 8), in the M19r (e.g. Channel & Grandesso 1987:

Figs. 4, 17), but also in the M17r (e.g. Cirilli et al. 1984: Fig.

8) or in the M16 (e.g. Márton 1982: Fig. 9) magnetozones.

 However, as magnetostratigraphic events represent chro-

nologically very precise correlation horizons, the biostrati-

graphic events — if expected to have relevant correlation

significance — should not occupy different positions rela-

tive to magnetostratigraphic events at different localities. J.

Kirschvink (in Lowrie & Channell 1984, p. 47) noted that

“the marine biological changes were probably not synchro-

nous at the Jurassic-Cretaceous transition”. This fact was the

main motive of the present authors for an attempt to cali-

brate the biostratigraphic scales of the J/K boundary strata

using magnetostratigraphy, preferably high-resolution

magnetostratigraphy, first in the Tethyan Realm and later

also in the Boreal Realm. This would allow their precise cor-

relation and test the degree to which isochronous biostrati-

graphic events are used for chronostratigraphic purposes.

First, it is necessary to define the herein applied bio-

stratigraphic, taxonomic and methodological criteria. The J/K

boundary based on ammonites was defined in Lyon–Neuchâ-

tel in 1973 (published in 1975) as the base of the Jacobi-

Grandis Zone. This decision was confirmed on all the

following meetings of the Working group on J/K boundary

(Munich 1982; Moscow 1984; Sümeg 1984 and others) and

has recently been universally accepted. However, the precise

determination of this boundary by means of ammonites in

the whole Tethyan Realm is possible at several localities

only. It is due to the presence of stratigraphically significant

species of ammonites being strictly limited only to sublit-

toral facies. Unfortunately, the sublittoral facies are also

characterized by frequent hiatuses. Moreover, ammonite fau-

nas do not allow determination of the J/K boundary with the

precision required here (down to several centimetres, see be-

low). On sections without ammonites (and in the Tethyan

Realm such sections are more than 99 %), the J/K boundary

can be determined only if some other group of organisms is

used, which, in the majority of cases, are calpionellids. In con-

tradiction to ammonites, calpionellids are common in the J/K

boundary strata in the whole Tethyan Realm and their associa-

tions occur not only in sublittoral facies but are especially

abundant in deeper basinal facies with less frequent hiatuses.

The position of the J/K boundary based on calpionellids was

agreed upon in Sümeg in 1984 (Remane et al. 1986) as the

base of the Calpionella Standard Zone. In sections, the posi-

tion of this boundary defined on calpionellids can be deter-

mined very precisely, usually within several centimetres.

Therefore, we use the J/K boundary defined on calpionellids,

basic distinguishing characteristics of this boundary along

with criteria used for its precise determination were published

in detail elsewhere (Houša et al. 1996b, p. 137).

According to Remane et al. (1986, p. 10), the “…boundary

at the base of the Jacobi-Grandis Zone is practically identi-

cal with the base of the Calpionella Standard Zone”. Howev-

er, Tavera et al. (1994) proved that in the Puerto Esca


o sec-

tion (S Spain), both limits are different and that the

ammonite J/K boundary is slightly older than the calpionel-

lid one. In our opinion, the J/K boundary defined by calpi-

onellids is more precise, well determinable and much more

universally usable because of the presence of calpionellids

in the majority of outcrops, than the boundary defined by a

group, which occurs in sufficient composition at only a few

localities in the whole Tethyan Realm (ammonites).

With respect to taxonomy, we prefer to use the denomina-

tion of the big Tithonian variety of Calpionella alpina sensu

lato as Calpionella grandalpina Nagy 1986 and the prolon-

gated one as Calpionella elliptalpina Nagy 1986 (see Houša

1990, p. 362). Below the denomination of Calpionella alpi-

na Lorenz 1902 we understand a short, spherical, “middle-

size” form only, which is characteristic for the “explosion”

of the species on the base of the Calpionella Zone (see

Houša 1990, p. 361). This “explosion” must be distin-

guished from the increase in the abundance of Calpionella

by the end of the Tithonian (see Houša et al. 1996b, p. 138).

From the methodological point of view, extremely dense

sampling was carried out both in paleomagnetic study

(“high-resolution magnetostratigraphy”) and in biostrati-

graphic study, particularly in intervals with important bound-

aries. The positions of magnetostratigraphic boundaries, i.e.

horizons of change of the paleomagnetic polarity in rocks,

were only provisionally determined through interpolation

between two closest samples with different polarity (as is a

common practice so far) and localized with maximum possi-

ble precision by means of additional sampling of the section

(generally 1–2 cm; the boundary was sometimes lying direct-

ly in the measured sample showing intermediate polarity of

remanence). In sampling, precise mutual positions of bios-

tratigraphic and magnetostratigraphic samples were recorded

as well as their exact positions in lithological sections. The

sampling points were marked in the sections to allow verifi-

cation sampling or additional sampling aimed at higher sam-

Fig. 1. Location map of the Brodno locality.

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ple density any time in the future. Such precision was ap-

plied to magnetostratigraphic study at the locality of Brodno

near Žilina (Fig. 1). Magnetostratigraphic study in a similar

detail was carried out at the locality of the Bosso Valley,

Umbria, central Italy. The present study also refers to magne-

tostratigraphic profile at the locality of Štramberk, northern

Moravia, which was subjected to a synoptic study with no

demands for high resolution.


In the terminology of magnetostratigraphic polarity units,

we follow the International Stratigraphic Guide (Salvador

1994, p. 69–75). For the “classic” magnetostratigraphic units

we use common informal numerical designation (e.g. M19;

the numbers are given from a certain arbitrary level on the

Barremian/Aptian boundary in the order from the youngest

unit to the oldest one). Every such numbered “classic” mag-

netostratigraphic unit has two parts, the older (lower) part

with reverse paleomagnetic direction, and the younger (up-

per) one with normal paleomagnetic direction. For each part

we use the term “zone” (we follow the more advanced no-

menclature of Quaternary magnetostratigraphic units — see

l.c., p. 75, Fig. 12). Magnetostratigraphic polarity zone

(magnetozone) is “the basic formal unit in the classification

of magnetostratigraphic polarity units” (l.c., p. 71, paragraph

C). “Magnetostratigraphic polarity zones may consist of (1)

rock bodies with a single polarity of paleomagnetization

throughout, (2) an intricate alternation of normal and reverse

units (mixed polarity), or (3) an interval of dominantly either

normal or reverse polarity, containing minor subdivisions of

the opposite polarity. (Thus, a zone of dominantly normal

polarity may include lesser-rank units of reverse polarity.)”

(l.c., p. 71). Examples of the magnetozones are M19n, or

M19r, etc. So, every classic magnetostratigraphic unit has

two magnetozones, reverse (older) and normal (younger). If

a magnetozone includes a short part with the opposite polari-

ty, we designate it with the term magnetostratigraphic polari-

ty subzone (magnetosubzone, subzone). By “magnetostrati-

graphic polarity subzone” (magnetosubzone, subzone), we

understand a rock body with a single polarity of paleomag-

netization throughout and within a magnetozone with domi-

nantly opposite paleomagnetic polarity.

 Magnetozone or magnetosubzone are terms of magneto-

stratigraphic classification. In chronostratigraphic terminolo-

gy every magnetozone (magnetosubzone, respectively) cor-

responds to a chronozone (subchronozone), in geo-

chronological terminology it corresponds to a chron (sub-

chron), see also Ogg & Lowrie (1986). Chrons and subchrons

are time units and their reflection in rocks are zones. Speaking

about rock units delimited by their paleomagnetic polarity, we

consider it correct to use the terms magnetozone and magneto-

subzone, instead of incorrect magnetochron (or magnetosub-

chron), which must be conserved for designation of the time

unit with certain paleomagnetic polarity only.

In the terminology of magnetostratigraphic polarity units

during the last 3.5 million years of the Earth’s history, every

magnetozone and every magnetosubzone is named (see l.c.,

Fig. 12) in accordance with the general rules for naming

stratigraphic units (l.c., section 3.B.3). In Mesozoic

magnetozones, we prefer to respect their numerical designa-

tion combined with the letter “n” or “r” according to their

polarity (e.g. M20n). (“This system is firmly entrenched in

the literature and is being usefully employed.” — l.c., p.73).

However, for naming magnetosubzones (subzones) we pre-

fer to avoid numbers and letters and we use the general rules

valid for the naming of stratigraphic units (see l.c., section

7.H, last paragraph) because the numerical designation of

magnetosubzones is more complicated and inappropriate for

practical use. Short reverse magnetosubzone in M19n was

designated by Ogg et al. (1991) M19n-1. The use of this des-

ignation e.g. in linguistic expressions is unnecessarily com-

plicated or difficult and it does not solve the denomination

of parts of a magnetozone divided by a magnetosubzone.

Therefore we propose naming magnetozones with simple

geographically derived names as more practical. For this rea-

son, we use the individual geographically derived names

with a clearly designated standard (name-bearing standard)

for recognition of the unit named.

Standards in magnetostratigraphy fulfil a different role in

comparison to standards in chronostratigraphy. Every chro-

nostratigraphic unit is an artificial one, it does not exist in re-

ality, it must be defined (by means of a standard, which is its

type section) and delimited (by its boundary stratotypes).

This is not the case with magnetostratigraphic polarity units.

The pattern of polarity reversals preserved in sea-floor-

spreading anomalies or in the sequential record of reversals

in rocks everywhere on the continent, reflect the real history

of the Earth´s magnetic field. Its units really exist indepen-

dently of an observer who studies them. If an observer

names one such unit, he must only determine exactly which

unit he named, nothing more. He can do it simply on an out-

crop, where the named unit is preserved, by a permanent ar-

tificial marker. Such a stratotype is the standard of the name

applied, not the standard of the magnetostratigraphic unit,

because the unit needs no stratotype for its exact determina-

tion and delimitation.

Magnetostratigraphic studies in the Tethyan


Most of the magnetostratigraphic studies of J/K boundary

strata were carried out with the aim of setting out a synoptic

scheme of normally and reversely polarized magnetozones or

possibly magnetosubzones not aspiring to their detailed de-

limitation. Synoptic sampling was naturally insufficient for

determination of the so-called polarity transition zones where

rocks are classified as having intermediate polarity. The J/K

boundary was placed at different levels in different magneto-

stratigraphic studies, ranging between the magnetozones M19

and M17. A synoptic magnetostratigraphic profile of Upper

Mesozoic rocks was published from northern Tunisia (Nairn

et al. 1981); however, whereas late Jurassic limestones were

mostly suitable for paleomagnetic study, Cretaceous rocks

generally displayed secondary components of remanence.

Magnetostratigraphic studies of the early Cretaceous Maiolica

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128                                                                HOUŠA, KRS, MAN, PRUNER and VENHODOVÁ

Fm. pelagic limestones from the Bosso Valley, Umbria, cen-

tral Italy, resulted in the detection of the magnetozones M20

to M14, and probably to M13 (Lowrie & Channell 1984). In

the last mentioned study, the J/K boundary was placed within

the lowermost part of the magnetozone M17. Pelagic white

limestones rich in ammonites from southern Spain were mag-

netostratigraphically studied in two sections: in Carcabuey

and Sierra Gorda. The detected magnetozones ranging from

M15 to M19 were well correlable with magnetic marine M-

anomalies and, in a narrower range, also with magnetozones

at the locality of Foza, northern Italy. The presence of a re-

verse subzone was also detected in the normal part of the

magnetozone M20 (Ogg et al. 1984). The Umbrian Maiolica

Formation was studied by combined biostratigraphic and pale-

omagnetic methods using samples of white pelagic limestones

collected from the locality of Fonte del Giordano (Cirilli et al.

1984). The detected magnetozones were correlated with the

magnetic marine M-anomalies M19 to M14 for a lower calpi-

onellid section and — above a hiatus — also for an upper ra-

diolarian section. The critical section around the magnetozone

M19 could not be studied due to the occurrence of hiatuses.

Determination of the J/K boundary based on the correlation of

magnetozones and calpionellid zones was discussed by Már-

ton (1986) who proposed placing this boundary within the

magnetozone M17. Lowrie & Channel (1984) placed this

boundary close to the base of M17, while the authors of earli-

er papers placed it above the magnetozone M19. Further mag-

netostratigraphic studies of pelagic limestones of the Berria-

sian stratotype in Ardéche, France (Galbrun 1985), and of

Berriasian/Valanginian boundary strata in Cehegín, southern

Spain, province Murcia (Ogg et al. 1988), also indicate appli-

cability of this method to global correlation. Pelagic lime-

stones in all the above mentioned studies proved to have re-

corded the paleomagnetic field. However, in other localities of

pelagic limestones, paleomagnetic directions could not be de-

termined; samples of Mesozoic limestones displayed syn-tec-

tonic and post-tectonic components of remanence (Villalaín et

al. 1996; Parés & Roca 1996; Hoedemaeker et al. 1998). The

importance and interpretation aspects of magnetostratigraphy

of the J/K boundary interval in the Tethyan and Boreal realms

were discussed in the paper of Ogg et al. (1991).

Pilot samples of the Tithonian-Berriasian limestones were

magneto-mineralogically and paleomagnetically studied

originally at five localities in the Western Carpathians, out of

which two lie in northern Moravia and three in western Slo-

vakia. In the first stage, a synoptic magnetostratigraphic

study was done at the localities of Štramberk, N. Moravia,

and Brodno near Žilina, W. Slovakia (Houša et al. 1996a). A

detailed sampling at Brodno followed by paleomagnetic and

micropaleontological study resulted in high-resolution mag-

netostratigraphy (Houša et al. 1996b, 1997). Interpretation

of data including the results from samples collected in 1997

are presented in the submitted paper.

Short reverse polarity magnetosubzones

Dense sampling for paleomagnetic studies allowed detec-

tion and precise delimitation of two short reverse magneto-

subzones in the sections studied. One of them lies in the up-

per part of the magnetozone M19n, the other one lies imme-

diately above the middle (i.e. in the upper) part of the mag-

netozone M20n. Both these reverse magnetosubzones were

previously known from marine profiles (see Ogg et al. 1991)

and one of these magnetosubzones was found in fossil sec-

tions in two cases (see Ogg et al. 1984; Lowrie & Channell

1984). However, both of these magnetosubzones have never

been found in a single section yet, except in the Brodno sec-

tion, described in this paper.

Ogg et al. (1991) designate these magnetosubzones with

symbols derived from the symbols of the magnetozones in

which they are located, such as M19n-1 and M20n-1. This no-

menclature is, however, considered impractical by the present

authors. Instead, one-word nomenclature is herein proposed

for these magnetosubzones, following the guidelines set out

by the International Stratigraphic Guide for the nomenclature

of stratigraphic units. The reverse magnetosubzone in the up-

per part of the magnetozone M19n is designated as “Brodno”;

the name is derived from the name of a village, in the vicinity

of which the thoroughly studied section containing the name-

bearing type of this subzone is located (Fig. 2). The reverse

magnetosubzone in the middle part of the magnetozone M20n

is designated as “Kysuca”; the name is derived from the name

of a river in the valley of which the Brodno locality containing

the name-bearing type of this magnetosubzone is situated

(Fig. 3). A detailed delimitation including the required formal

specifications related to the establishment of these names are

given in the text below.

The geographical names of “Kysuca” and “Brodno” have al-

ready been used by other authors in the past for the designation

of lithostratigraphic units: Brodno Member (Scheibner 1967,

Aptian–Albian) and Kysuca Member (Scheibner & Scheibner-

ová 1958, Cenomanian–Turonian). None of these units occur at

the stratotype of the described magnetosubzones (Brodno Quar-

ry). With respect to the fact that no other suitable geographical

names usable in the international scale are available (i.e. simple

names easily pronounced in world languages), both of the

above mentioned names are herein used for the designation of a

different kind of formal stratigraphic units than those they have

been used as there is no risk of any misunderstanding.

The presence of a magnetosubzone in a magnetozone, di-

vides this magnetozone into three parts, i.e. into the magne-

tosubzone proper and parts of the magnetozone before (be-

low) and after (above) the magnetosubzone. For example,

the Kysuca reverse magnetosubzone divides the normal

magnetozone M20n into (1) the older (lower) part of the nor-

mal zone, (2) the Kysuca reverse magnetosubzone and (3)

the younger (upper) part of the normal zone. We prefer to de-

rive the informal designation of both parts of the normal mag-

netozone from the designation of the reverse magneto-

subzone, by prefix “pre-” (for the older part of the normal

magnetozone) and “post-” (for the younger part of the normal

magnetozone). So, the magnetozone M20n is divided into

three parts: the pre-Kysuca part (the older normal part), the

Kysuca reverse magnetosubzone and the younger normal

post-Kysuca part. Analogically, the M19n magnetozone is di-

vided by presence of the Brodno magnetosubzone into the

normal pre-Brodno part, the Brodno reverse magnetosubzone

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and the normal post-Brodno part. This nomenclature can be

effective until both parts of the normal magnetozones receive

their individual designations.

Brodno near Žilina, W. Slovakia

Basic information

The locality of Brodno near Žilina (Western Carpathians,

NW Slovakia, Fig. 1; — see Michalík et al. 1990; Houša et al.

1996a,b) was selected for a detailed magnetostratigraphic study

of the Tithonian-Berriasian limestone strata among

five previously considered localities (Houša et al.

1996a; Fig. 1) for its (1) favourable geological set-

ting (relatively continuous sedimentation in a quiet

basinal environment, favourable lithology), (2)

favourable physical properties of the rocks en-

abling us to infer primary paleomagnetic directions

with a high degree of reliability, using multi-com-

ponent remanence analysis combined with fold

tests, and (3) rich calpionellid associations. With

respect to the relatively low sedimentation rate of

the limestones, the original collecting of orientated

samples was realized with short sampling intervals

and the inferred data were related to limestone stra-

ta numbered by Michalík et al. (1990). The in-

ferred magnetozones M21r to M17r could be cor-

related with analogous sections in the Tethyan

Realm (Foza, Bosso, Štramberk) and with marine

M (Mesozoic) anomalies. A narrow subzone with

reverse polarity was first detected in the upper part

of the magnetozone M19n. This state of knowl-

edge has been published by Houša et al. (1996a).

Later, the Brodno section was labelled with new,

more detailed numbering in order to detect another

expected reverse subzone within the magnetozone

M20n and to meet the needs of high-resolution

magnetostratigraphy, particularly to specify more

exactly the positions of the determined magneto-

stratigraphic and biostratigraphic boundaries

(Houša et al. 1996b). The older, synoptic number-

ing was also preserved.

In 1996 and 1997, very dense (locally even

continuous) collecting of orientated paleomagnet-

ic samples was performed in several consecutive

phases at this locality. Therefore, the profile can

be characterized as a high-resolution one. Rela-

tively extensive laboratory paleomagnetic, petro-

magnetic and micropaleontological analyses were

realized due to the financial support of the Grant

Agency of the Academy of Sciences of the CR in

Prague and of the Dionýz Štúr Geological Insti-

tute in Bratislava. Detailed sampling of the sec-

tion (averaging 20 to 35 orientated samples per 1

m of true thickness) allowed a more precise iden-

tification of boundaries of the individual magne-

tozones and of both reverse subzones within the

magnetozones M19 and M20 (Houša et al. 1997).

Fig. 3.  The Kysuca Subzone, the width of which is marked by two aluminium

cylinders (of 1 inch diameter) cemented into the drill holes. The two aluminium

cylinders bear the name Kysuca.

Fig. 2.  The Brodno Subzone, the width of which is marked by two aluminium

cylinders (of 1 inch diameter) cemented into the drill holes. The two aluminium

cylinders bear the name Brodno.

In 1997, collection of additional samples was aimed primarily

at identification of the boundaries of both reverse polarity

subzones. In consequence, these subzones are defined with a

high precision today. A new procedure in magnetozone and

subzone interpretation was also proposed during the detailed

processing of magnetostratigraphic data from the Brodno lo-

cality: it is based on analysis of the angle deviation of the sep-

arated fossil component of remanence from the most probable

paleomagnetic direction considered for the whole studied sec-

tion. A procedure providing estimated mean values as well as

standard deviations of the smoothed interpolated course of the

given quantities was also applied.

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130                                                                HOUŠA, KRS, MAN, PRUNER and VENHODOVÁ


Altogether 360 orientated hand samples were collected for

the construction of a high-resolution profile with the maximum

sampling density between the base of the magnetozone M21r

and the base of the magnetozone M18r. In geological cross-sec-

tion, this interval represents only 10 metres of the true thickness

of strata.

The volume magnetic susceptibility k and the remanent

magnetization J of samples were measured by means of the

KLY-2 A.C. bridge and the JR-5 spinner magnetometer

(Jelínek 1973, 1966), respectively. A part of the set of samples

was subjected to demagnetization by alternating field using

the Schonstedt GSD-1 apparatus. Demagnetization in thermal

fields generally proved to be more effective; consequently,

each sample of the whole set was subjected to progressive

thermal demagnetization up to 590 


C in eleven to thirteen

thermal fields on average using the MAVACS apparatus

(Pøíhoda et al. 1989). The measured values of remanent mag-

netization of thermally demagnetized samples were subjected

to multi-component analysis of remanence following the

method of Kirschvink (1980), paleomagnetic directions were

subjected to fold tests and, after correction for dip of strata,

used for construction of the magnetostratigraphic profile. In

addition, diagrams of normalized values of M




 vs. demag-

netizing temperature t [


C] were constructed for all samples,



 being the modulus of the moment of remanent magnetiza-

tion of the thermally treated sample after cooling, M



the modulus of the moment of remanent magnetization of the

sample in its natural state. These diagrams were used for esti-

mation of the values of unblocking temperatures in all sam-

ples from the given set. A more precise determination of un-

blocking temperatures was derived on pilot samples following

the methods described in Houša et al. (1996a, p. 186–188).

All the samples, with no exception, displayed large compo-

nents of secondary magnetization, corresponding to the vis-

cous component and to chemo-remanent magnetization condi-

tioned by weathering. The stable component of remanence

was separated with an unblocking temperature of 520 to



C linked with the content of magnetite as a carrier of the

primary paleomagnetic directions. These results are in accor-

dance with the results of combined magneto-mineralogical

and X-ray diffraction analyses of the pilot samples. Diagrams

showing the correlation of normalized values of volume mag-

netic susceptibility k




 vs. temperature were constructed for

all the studied samples to assess the influence of possible

phase changes of magnetically active minerals during thermal

treatment of the samples (Krs & Pruner 1997).

The studied limestones are ranked among medium to weakly

magnetic rocks. The scatter of J


 and k


 values is relatively

wide, with a marked decrease in magnetization from older to

younger rocks. Statistics for the quantities J


 and k


 for both

medium magnetic late Tithonian and weakly magnetic early

Berriasian limestones are given in Table 1. The table also im-

plies that the paleomagnetic polarity of the samples is not re-

flected in the changes of basic magnetic parameters.

The magnetostratigraphic profile shows the values of

moduli of natural remanent magnetization J


 in [10



units, the values of volume magnetic susceptibility of sam-

ples in natural state k


 in [10


SI] units, paleomagnetic dec-

lination  D


 and inclination  I


 in degrees and the so-called

discrimination function first introduced into the interpretation

of magnetostratigraphic data (Figs. 5 and 9).

A newly proposed procedure for evaluating

magnetostratigraphic data

An innovation to the hitherto used method of data process-

ing and graphic presentation of results (cf. Houša et al. 1996a,

1997) was applied to the herein submitted processing of mag-

netostratigraphic data from the locality of Brodno near Žilina.

This innovation (by O.M.) employed some of the procedures

described in the monograph of Fisher et al. (1987).

The essential purpose of magnetostratigraphy is to continu-

ously, if possible, subdivide the studied stratigraphic section

into intervals corresponding to normal (N) and reverse (R) po-

larity of the paleomagnetic field. Accordingly, data processing

comprises two steps: the first step is the construction of a dis-

crimination function, the direction of remanent magnetization

being its independent variable. On the basis of the discrimina-

tion function, the detected direction can be classified, i.e.

placed into one of two classes — N or R. The second step in-

cludes the interpolation and smoothing of the detected direc-

tions, and the constructed discrimination function as well,

along the magnetostratigraphic profile. The applied procedure

provides continuous estimates of both the mean value and

standard deviation of a studied quantity thereby providing the

required subdivision of the section or, where appropriate, the

designation of intervals where the quality of input data does

not allow a reliable classification. Both these steps will be dis-

cussed separately in the two paragraphs below.



of samples

Normal  (N)

Reverse (R)


Modulus of natural

remanent magnetization






Volume magnetic susceptibility










































Table l: Brodno near  Žilina, basic magnetic parameters of limestone samples.

background image


Construction of the discrimination function

The directions


,   i = 1, ..., n                                                             (1)

of remanent magnetization of all the samples from the mag-

netostratigraphic profile are plotted using the Lambert equal-

area projection in Fig. 4. This sample of directions corre-

sponds to a hitherto unknown distribution, which should be

(in an ideal case only the paleomagnetic component of rema-

nent magnetization, which originated at the time of sedimen-

tation is involved):

1) bimodal, with modes corresponding to opposite directions,

a normal mode and a reverse mode, herein referred to as s



and s



2) isotropic with respect to the axis intersecting both


The above idea may be confronted with the qualitative

features of the data set (1), enhanced by non-parametric esti-

mate of the true probability density f (s) giving rise to the

data. The estimate, being a modification of Parzen estimate

(Parzen 1962), has the form (Fisher et al. 1987):

$( )

( , )







s s




,                                                         (2)












( , )

/ (


) exp(

cos( , ))

s s

s s




and C

is a parameter, whose value C

= 28.5 was found using

the maximum probability method. The estimated density, be-

ing displayed in Fig. 4, is not in evident contradiction to the

presumed properties of the distribution. The estimate implies

the values of s


 (inclination 37


, declination 263


) and



 (inclination –41


, declination 65


), which are in a rath-

er good agreement with each other.

Although the decision on the assignment of a direction to

the class N or R may be based immediately on the quantities

of declination and inclination, such an approach is not the

best one. In order to avoid ambiguity, the classification

should be based on a single scalar quantity — discrimination

function d(s


). Providing that the distribution has the above

given properties, an optimum choice for this function is the

angle between directions s


 and s


. Then, inequality d(s




/2 or d(s


) > 


/2 implies the classification of direction s


to class N or R, respectively.

After the classification, polarity in the class R may be re-

versed and the two classes may be grouped together again to

estimate the mean direction of magnetization regardless of

its polarity. In this way, mean direction s





, declination 250.9


) was found. Providing that the dis-

tribution has the above given properties, this direction can

be regarded as a better approximation of s


 than that pre-

viously derived from the estimate of probability density.

The graphic presentation of results includes the diagrams of

declination and inclination of the paleomagnetic directions for

the individual samples. The values s


, and s


 may be

seen in the diagrams, too. The values of the discrimination

function computed for individual samples are also plotted.

Interpolation and smoothing

The samples are obviously not distributed continuously

along the section. Besides, the paleomagnetic directions of

remanent magnetization show a relatively high dispersion

even in the same stratum. This is understandable as the stud-

ied component of remanent magnetization is usually very

low if compared with natural remanent magnetization, the

values of which are typically of the order of 1 mA/m. These

two reasons suggest a need for interpolation and smoothing

of the detected direction or the discrimination function de-

rived from it. Several approaches to the solution of this prob-

lem were tested, mostly mentioned in the monograph of Fish-

er et al. (1987), such as the use of smoothing splines (Reinsch

1967). Among these, a relatively simple technique of moving

average seems to be the most advantageous. The algorithm is

described below.

Each sample is characterized by the coordinate t


, cor-

responding to its position normal to stratification or to the

time of sedimentation, and by the direction described by a

unit vector s



i) A weight function w(t), e.g.,


an integer value constant c, e.g. c = 6, and a real parameter

step are chosen.

ii) The following operations are performed for a chosen

coordinate t = t


: the weights



= w ((t

- t


)/h),  i = 1,.., n,

where the parameter h is chosen so as to meet the condition









,                                                              (6)

are assigned to individual samples. Then the weighted mean

direction s

is calculated using the formulas






















,                        (7)

and so is the angular standard deviation of direction



 = arrccos (r / c) .                                                     (8)

The quantities s




 are assigned to the coordinate t


iii) The coordinate t

is substituted by t


 + step and the

process is repeated from step ii).

The essence of the algorithm can be described very simply.

A window whose shape, position, and width are given by the

function w(t), the coordinate t


, and the parameter h, respec-


1 for 





  ½ ,

w(t) = 

   0 for 








w(t) = exp (- ½ t

)  for arbitrary real t,


$f (s)

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132                                                                HOUŠA, KRS, MAN, PRUNER and VENHODOVÁ

Fig. 5. Brodno locality. Kysuca Reverse Polarity Subzone. D


 — paleomagnetic declination; I


 — paleomagnetic inclination; discrimina-

tion function expressing total angular deviation of paleomagnetic direction; normal, reverse — normal (N), reverse (R) polarity of paleo-

magnetic direction for respective parts of the magnetozone or magnetosubzone.

Fig. 4. Brodno locality. Paleomagnetic directions and hence derived probability density function, Lambert equal-area projection.

background image


tively, is moving along the magnetostratigraphic profile as-

signing a weight w


 to the individual samples. While its shape

is kept firm, its width varies according to the local density of

samples, so that the sum of all weights is constant. The

weighted mean direction and its angular standard deviation

are found for each window position. The discriminating func-

tion introduced in the preceding paragraph may be treated in a

similar way.

In the graphic presentation, the diagram of a studied quanti-

ty, e.g. declination, inclination, or discriminating function,

shows both the estimate of its mean value (depicted by full

line) and the zone

mean value ± standard deviation

(see Figs. 5 and 9). For declination, the standard deviation is

defined by expression 



Definition of the Kysuca and Brodno


The Kysuca reverse magnetosubzone is situated above the

middle of the normal magnetozone M20n. The reverse paleo-

magnetic direction of this magnetosubzone at locality Brodno

is represented by limestone bed No. 99, only 15 cm in total

true thickness, i.e. 6 % of the total thickness of the normal

zone M20n (2.37 m, covering the pre- and post-Kysuca nor-

mal parts as well as the Kysuca reverse magnetosubzone), see

Figs. 5 and 6. The base of the Kysuca reverse magnetosub-

zone in Brodno lies at a level of 55 % of the local thickness of

the magnetozone M20n. Samples collected from the Kysuca

magnetosubzone were subjected to progressive thermal de-

magnetization in the fields of 100


, 150


, 200


, 250


, 300





, 400


, 450


, 500


, (520


), (540


) up to 590 



Figures 7 and 8 show Zijderveld diagrams of samples from

the Kysuca magnetosubzone indicating moduli of natural re-

manent magnetization (NRM) and of remanent magnetization

after the final step of thermal demagnetization (RM). Graphs

showing dependence of k




 vs. temperature of demagnetiza-

tion field are drawn below the Zijderveld diagrams. Symbols

N or R indicated with each of the Zijderveld diagrams denote

normal (N) or reverse (R) polarity of the primary paleomag-

netic component of remanence. This component was inferred

using multi-component analysis (Kirschvink 1980) and sub-

jected to combination with fold test. The proportion of the in-

tensity of secondary components is high in all samples, reach-

ing from 80 to 90 % of J


. Unblocking temperature of

minerals — carriers of primary components of remanence —

vary between 560


 and 590 


C thus indicating the presence of

magnetite. The results of the multi-component analysis have

proved that J


 consists of three components: The A-compo-

nent of remanence was inferred in the temperature interval of



C, undoubtedly being of viscous origin; the B-com-

ponent of secondary origin was inferred in the temperature in-

terval of ca. 100–350 


C, whereas the C-component corre-

sponding to the primary (paleomagnetic) component of

remanence was determined in the temperature interval of ca.



C (350 


C) to 500 


C (590 


C), cf. Houša et al. (1996a).

The results of thermal demagnetization are presented in this

paper as examples only for some samples on the basis of

which the Kysuca reverse subzone was interpreted. In sample

No. 7550, the C-component displays both normal polarity (in

the temperature interval of 350–500 


C) and reverse polarity

(in the temperature interval of 520–590 


C). This sample

comes from the boundary interval separating the uppermost

part of the Kysuca reverse subzone from the post-Kysuca part

of the magnetozone M20n. With respect to the thickness of

the sample (2 cm) having two polarities of primary compo-

nents of remanence and to the presumed low value of the sedi-

mentation rate of the boundary interval claystones, it can be

concluded that the transition from reverse to normal polarity

of the geocentric co-axial magnetic dipole of the Earth oc-

curred within a time span of ca. ±5000 years (cf. also Butler

1992, p. 191). The boundary sample No. 7554 seems to dis-

play intermediate polarity direction.

The Brodno reverse magnetosubzone was detected in the

upper (late) part of the normal magnetozone M19n and con-

stitutes the uppermost part (8 cm) of the bed 24A, the whole

bed 24B and also the whole overlying bed 24C. Its complete

thickness is 24 cm. The Brodno reverse magnetosubzone

represents only 8 % of the total thickness of the normal mag-

netozone M19n (3.13 m, covering the pre- and post-Brodno

normal parts as well as the Brodno reverse magnetosub-

zone), see Figs. 9 and 10. The base of the Brodno reverse

magnetosubzone in Brodno lies at a level of 82 % of the local

thickness of the magnetozone M19n. This subzone was de-

fined on the basis of an analysis of paleomagnetic parame-

ters carried out in the same manner as for the preceding sub-

zone. Analogically, J


 consists of three components of

remanence, the C-component corresponding to the primary

(paleomagnetic) component of remanence was determined

in temperature interval of ca. 300 


C (400 


C) to 500 





C) in the process of progressive laboratory thermal de-

magnetization. The results of thermal demagnetization of

only 12 samples from the Brodno magnetosubzone and its

vicinity are shown as examples in Figs. 11 and 12, out of

which six have normal (N) paleomagnetic polarity and six

have reverse (R) paleomagnetic polarity. The high propor-

tion of secondary components of remanent magnetization

frequently reaching 90 % of J


 is visible in figures again. A

transition between reverse (normal) and normal (reverse) po-

larity of the magnetic field of the dipole of the Earth was not

detected in this subzone.

Correlation of paleomagnetic events

and calpionellid biostratigraphy

The oldest calpionellids, i.e. the first species of genus

Chitinoidella (Ch. slovenica Borza, Ch. colomi Borza, Ch.

dobeni Borza) characterizing the oldest calpionellid Dobeni

Subzone of the Chitinoidella Zone, were found in high num-

bers in the late part of the magnetozone M20r. The first rep-

resentatives of these species appear in the bed 84 and the last

ones were recorded in the uppermost part of the bed 86, i.e.

at the very base of the overlying magnetozone M20n.

The base of the pre-Kysuca part of the magnetozone M20n

lies in the upper part of the bed 86. The earliest portion of

the pre-Kysuca part still belongs to the calpionellid Dobeni

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134                                                                HOUŠA, KRS, MAN, PRUNER and VENHODOVÁ

Fig. 6. Brodno locality. Kysuca Reverse Polarity Subzone. Paleomagnetic sample Nos, strata Nos and palentological sample Nos. Ch. —

Chitinoidella; Pr. — Praetintinnopsella; Cr. — Crassicollaria; T.  — Tintinnopsella.

background image


Subzone; the Boneti Subzone starts in the bed 87 (i.e. in ap-

prox. one tenth of the local thickness of the pre-Kysuca

part). The acme of the species Ch. boneti Doben was record-

ed in the late portion of the pre-Kysuca part. The top of the

Boneti Subzone, hence also the top of the Chitinoidella

Zone, is defined by the first appearance of small Tintinnop-

sella (Remane et al. 1986) first recorded at Brodno in the

bed 98, i.e. below the Kysuca magnetosubzone. Thus, the

pre-Kysuca part of M20n comprises the late portion of the

calpionellid Dobeni Subzone, the whole calpionellid Boneti

Subzone and the earliest portion of the Remanei Subzone of

the Crassicollaria Standard Zone (the bed 98).

In terms of calpionellid biostratigraphy, the Kysuca Polari-

ty Subzone is situated in the section at Brodno at the very

base of the Crassicollaria Standard Zone. The lowermost

subzone of the Crassicollaria Standard Zone, i.e. the calpi-

onellid Remanei Subzone, in the Brodno section has a thick-

ness of 100 cm, stretching from the topmost bed of the pre-

Kysuca part (the bed 98) across the Kysuca magnetosubzone

(i.e. the bed 99) and almost the whole overlying post-Kysuca

part (except for its latest portion — the bed 4B; bed numbers

see Houša et al. 1996b: Fig. 12). The top of the Remanei

Subzone is marked by a major event representing the base of

the overlying calpionellid Intermedia Subzone. This event is

the appearance of species Calpionella grandalpina Nagy.

The first (oldest) representatives of this species in the Brod-

no section were recorded in the middle of the limestone bed

4B, i.e. immediately below the top of the post-Kysuca part

(lying between the beds 4B and 5 and in fact representing

the boundary between the magnetozones M20n and M19r).

Thus the base of the calpionellid Intermedia Subzone coin-

cides with the latest portion of the post-Kysuca part.

The whole magnetozone M19r is constituted by the calpi-

onellid Intermedia Subzone. This subzone also extends to

the overlying magnetozone M19n. Here, its top is defined as

the base of the Calpionella Standard Zone, herein considered

as the J/K boundary. In the studied section, the J/K boundary

lies at the level of 40 % of the thickness of the pre-Brodno

part, i.e. at approx. 35 % of thickness of the whole magneto-

zone M19n. The last Tithonian calpionellid Intermedia Sub-

zone therefore starts in the topmost part of the post-Kysuca

part and corresponds to the whole magnetozone M19r and

approximately the lowermost one-third of the magnetozone

M19n; the rest of the magnetozone M19n is included into

the Calpionella Standard Zone, i.e. the basal part of the Ber-


This implies that the boundary between the Crassicollaria

and Calpionella Standard Zones (i.e. the J/K boundary in the

present concept as recognized in the sections studied) lies

within the pre-Brodno part of the magnetozone M19n. No

magnetoevents lie in the immediate proximity of this boundary.

There is another event important for the verification of the

position of the J/K boundary based on calpionellids: a short

acme of species Cr. parvula Remane lying in the earliest part

of the Calpionella Zone. This acme is well defined in the

section at Brodno, being confined to the beds 20 and 21.

This calpionellid event also lies within the pre-Brodno part,

at approx. one half of the interval between the J/K boundary

and the base of the Brodno reverse magnetosubzone.

The whole Brodno magnetosubzone lies within the Calpi-

onella Standard Zone (Alpina Subzone). The interval occu-

pied by this magnetosubzone in the section at Brodno is in-

cluded in the monotonous part of the calpionellid Alpina

Subzone and so is the whole overlying post-Brodno part.

The boundary between the magnetozones M19n and M18r

lies between the limestone beds 25B and 26A. In the opinion

of Michalík et al. (1990), this level corresponds to the top of

the calpionellid Alpina Subzone (i.e. the base of the calpi-

onellid Cadischiana Subzone), but it has not proved possible

for the present authors to confirm this with the required de-

gree of accuracy. Accordingly, the whole magnetozone M18r

should be included in the calpionellid Cadischiana Subzone.

Definition of the Jurassic/Cretaceous boundary

according to calpionellids

According to calpionellids, the J/K boundary (i.e. the Titho-

nian/Berriasian boundary) is placed at the base of the Calpi-

onella Standard Zone as defined by Remane et al. (1986). The

basic diagnostic features for the identification of the base of

the calpionellid zone Calpionella were already defined by Re-

mane (1964). The problem of the position of J/K boundary in

the Brodno section was discussed in more detail by Houša et

al. (1996b, p. 137–139) who placed this boundary in the

Brodno section between the beds 15A (the latest Tithonian)

and 15B (the earliest Berriasian). The reasons for the errone-

ous placement of this boundary in the Brodno section at a dif-

ferent level by other authors (to a stratigraphically lower level

— approx. to the level of the upper portion of the bed 8 in the

present, more detailed numbering) were also explained.

The base of the Calpionella Standard Zone represents one

of the most prominent events in the relatively short history of

calpionellid evolution. A great advantage of the Brodno sec-

tion is that no hiatuses, slumps or washouts occur either at the

level of this event or in its close proximity. According to all

indicators, the limestone sedimentation at this horizon and in

its close proximity at Brodno was quiet, relatively slow and

continuous, and characterized by conditions very favourable

for the fossilization of calpionellid loricae. Gradual changes

associated with this event can be, therefore, studied in consid-

erable detail (see Houša et al. 1996b; Fig. 6).

Štramberk, northern Moravia

Basic information

The Štramberk Limestone represents a complex of peri-reef

accumulations of fine or coarser organic debris with an almost

complete absence of terrigenous admixture. In some intervals,

grain-sized particles disappear and the rock passes into finer

varieties, to micritic limestones. This fact probably reflects

sea-level fluctuations but may also result from the position of

the given site of sedimentation with respect to the main axes

of detrital material transport in a debris talus around Tithonian-

Berriasian reefs. Sedimentation rates in the Štramberk peri-reef

accumulation must have been variable in space and in time as

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136                                                                HOUŠA, KRS, MAN, PRUNER and VENHODOVÁ

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Fig. 7. Brodno locality. Kysuca Reverse Polarity Subzone. Results

of progressive thermal demagnetization of samples by means of the

MAVACS apparatus. Only selected samples  are demonstrated to

show typical examples, see Fig. 6. R (reverse), N (normal) polarity

of the paleomagnetic  remanence component derived by multi-com-

ponent analysis is indicated for respective samples. The Zijderveld

diagrams represent orthogonal projection onto the horizontal  X,Y

plane (full circles) and the vertical X,Z plane (empty circles). NRM

— natural remanent magnetization. Beneath the Zijderveld dia-

grams, the normalized values of  k

/ k


 in relation to temperature t



C] are plotted; k


 is the volume magnetic susceptibility of the

sample demagnetized at temperature t and cooled to room tempera-

ture; k


  is the volume magnetic susceptibility of the sample in its

natural state (prior to thermal treatment).

well. The occurrence of washouts must also be considered

probable in such shallow-water depositional environments.

Larger blocks of limestones (several metres in size), represent

olistoliths in detrital material, they are probably derived from

eroded reef bodies emerged during temporary eustatic sea-lev-

el falls.

The studied section was therefore chosen outside the coarse

to blocky facies, in a deeper part of the original peri-reef accu-

mulation farther from the source of materials, where a lower

incidence of hiatuses, washouts or secondary olistoliths can

be anticipated. The section was situated on the 6th level of the

Kotouè Quarry where rather finer varieties of biofragmental

limestones occur in a suitable position, at some levels passing

into micritic limestones several metres thick.

The studied section begins at the edge of the limestone body

of the Homole Hill and stretches along the 6th level northern

wall to the central part of the Kotouè Quarry, where it ends

on the opposite side of the body of the Homole Hill (close

to the Mendocino Fault). The section is 620 m long, being in-

tersected by no major fault. Stratigraphically, it covers approx-

imately the same time interval (between the magnetozones

M21n and M18n) as the above discussed part of the Brodno

section, which is only 11 m thick (extending between the mag-

netozones M21r and M18r).


Magnetostratigraphic study of the J/K boundary limestone

strata at the locality of Štramberk was started in 1992 in two

sections. Priority was given to the section on the 6th level of

the Kotouè Quarry. Altogether 342 orientated drill samples

were collected from the northern wall of the 6th level. The

limestone samples are exceptionally weakly magnetic with

moduli of J


 ranging between several tens to several hundred



 A/m]. The values of k


 are mostly negative, dia-

magnetism of the limestone mass prevails over weak para-

magnetism and ferrimagnetism. Tithonian limestones were

measured from 94 samples in the first stage. The mean value

of 78.1 µA/m and standard deviation 72.4 µA/m were ob-

tained for J

of samples with normal polarity, while the mean

value of 56.0 µA/m and standard deviation 45.2 µA/m were

obtained for J


of samples with reverse polarity. The mean

value of –12.7




 SI and standard deviation 2.6





were obtained for k

(Houša et al. 1992, 1993). The procedure

described for the Brodno locality was used for the precise de-

termination of unblocking temperatures and for the X-ray dif-

fraction determination of ferrimagnetic minerals in exception-

ally weakly magnetic limestones. Unblocking temperatures of



C corresponding to magnetite were determined.

Analogous unblocking temperatures were determined in all

samples used for construction of the magnetostratigraphic

profile. Magnetite content determined in pilot samples is ap-

prox. 0.3 g.t


. Irregular, less commonly isometric and

spherolitic magnetite particles range between 3 and 20 µm in


All samples collected were subjected to progressive thermal

demagnetization using the MAVACS apparatus. The results

clearly demonstrate that, in spite of the very weak magnetiza-

tion of the limestones studied and a higher proportion of sec-

ondary components of remanence, the samples are suitable for

inferring paleomagnetic directions (see Houša et al. 1996a,b).

A magnetostratigraphic profile constructed on the basis of

samples collected in 1992, indicated the basic positions of

magnetozones, but proved to be rather complicated in some

intervals due to tectonic deformations and the generally dy-

namic sedimentation of limestones deposited in the peri-reef

zone. In 1993 and 1994, additional sampling was carried out

to reach a sampling point density of ca. 3 samples per 10 m

of true thickness and — in other intervals — of ca. 8 sam-

ples per 10 m. Documentation of paleomagnetic samples is

included in the report of Houša et al. (1994) as well as a

magnetostratigraphic profile with values of J

, k

, D

, I


and with interpreted normal and reverse magnetozones and


In the submitted study, the essential results from the Štram-

berk section are shown only in the form of a comparative

scheme of hitherto studied magnetostratigraphic profiles for

the localities of Brodno and Štramberk in Fig. 13. The basic

magnetozones were proved in the Štramberk section, howev-

er, the reverse magnetozone M19r and the Kysuca reverse

magnetosubzone were indicated with a lesser degree of con-

clusiveness. Two reverse subzones were recorded in the late

part of the normal magnetozone M19n at the level of the

Brodno reverse subzone. The above mentioned shortcomings

of the magnetostratigraphic profile at Štramberk may be

caused by the complicated tectonic setting and possibly also

by the extremely dynamic sedimentation. This section should,

therefore, be regarded as an orientational one, not reaching the

accuracy and reliability of the section at the Brodno locality.

Calpionellid associations

Considering the character of sedimentation in a peri-reef ta-

lus of calcareous detritus and its close vicinity (see above), the

preservation of calpionellid associations itself in the Štram-

berk Limestone is remarkable. Loricae of calpionellids were

most probably transported by water flow into interstices

among detrital particles of calcareous organic remains along

with other allochthonous material forming the matrix of the

rock. Calpionellids are generally less abundant in bio-


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138                                                                HOUŠA, KRS, MAN, PRUNER and VENHODOVÁ

Fig. 8. Brodno locality. Kysuca Reverse Polarity Subzone. See caption to Fig. 7.

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Fig. 9. Brodno locality. Brodno Reverse Polarity Subzone. See caption to Fig. 5.

Fig. 10. Brodno locality. Brodno Reverse Polarity Subzone. See

caption to Fig. 6.

fragmental varieties of the Štramberk-type limestones, which

most probably originated close to the source of the biofrag-

mental material, i.e. probably in shallow marine conditions

not far from the reefs as such. In contrast, calpionellids are

more abundant in finer, micritic Štramberk-type limestones,

which probably represent a more distal, deeper-water environ-

ment possibly originating during periods of sea-level rise. The

abundance of calpionellids in the Štramberk Limestone is

generally low and only exceptionally (e.g., in thin sections of

micrite fills of ammonite shells) comparable with the calpi-

onellid abundances in limestones from basinal localities (such

as Brodno).

No material sufficient for the definition of the oldest

calpionellid Dobeni Subzone was obtained anywhere at

Štramberk. Occasional finds of species of the Dobeni Sub-

zone association are absolutely insufficient for delimitation

of the Subzone. On the contrary, the following calpionellid

Boneti Subzone was recorded in all larger bodies of the

Štramberk-type limestones. The occurrence of the species

Chitinoidella boneti Doben in the Štramberk Limestone in

fact corresponds to the interval of its maximum abundance

(acme). In the studied section, this species was found in

samples from the late portion of the pre-Kysuca part of the

magnetozone M20n. Ch. boneti thus occurs at the same

stratigraphic position here as does the acme of this species at

Brodno. The last occurrence of Ch. boneti at Štramberk co-

incides with the first appearance of calpionellids with hya-

line lorica walls. This event lies immediately below the top

of the pre-Kysuca part in the studied section.

The pre-Kysuca part in the section at Štramberk is 62 m

thick (sic!, only 90 cm at Brodno) and the interval of occur-

rence of Ch. boneti is 20 m thick here (acme of this species is

restricted to ca. 50 cm at Brodno).

The base of the Crassicollaria Standard Zone in the studied

section was localized at the very top of the pre-Kysuca part.

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140                                                                HOUŠA, KRS, MAN, PRUNER and VENHODOVÁ

Fig. 11. Brodno locality. Brodno Reverse Polarity Subzone. See caption to Fig. 7.

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Fig. 12. Brodno locality. Brodno Reverse Polarity Subzone. See caption to Fig. 7.

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142                                                                HOUŠA, KRS, MAN, PRUNER and VENHODOVÁ

Fig. 13. Resultant magnetostratigraphic profiles across the Tithonian/

Berriasian boundary strata at Brodno and Štramberk.

This situation exactly corresponds to that at Brodno. The

Crassicollaria Zone (its lowermost calpionellid Remanei Sub-

zone) also includes the overlying Kysuca magnetosubzone,

the thickness of which is still difficult to assess precisely (a

thickness of 3 m can be estimated from interpolation). The

above given lowermost subzone of the Crassicollaria Standard

Zone also includes the overlying post-Kysuca part having a

thickness of some 30 m here (as opposed to 90 cm at Brodno).

The base of the calpionellid Intermedia Subzone should coin-

cide with the base of the magnetozone M19r, however, this

magnetozone is only insufficiently documented up to now. Its

thickness was assessed at only 3.5 m by interpolation, which

may be caused by primary reduction of sedimentary record at

this level. Datable calpionellid samples from this level are

also missing up to now and, consequently, the first specimens

of  Calpionella grandalpina Nagy (base of the Intermedia

Subzone) are known from the earliest part of the magnetozone


The most important calpionellid event — the base of the

Calpionella Standard Zone (i.e. the J/K boundary) is well de-

fined in the Štramberk section and its position was determined

more precisely on the basis of a denser sampling. It lies within

the magnetozone M19n, 22 m above its base, i.e. approxi-

mately at 30 % of the local thickness of the whole magneto-

zone (at 35 % at Brodno). This implies that the last Tithonian

calpionellid subzone — Intermedia Subzone — also corre-

sponds here approximately to the lowermost one-third of the

magnetozone M19n (the pertinence of the magnetozone M19r

to this calpionellid subzone in Štramberk has still not been

shown by any fossiliferous sample).

An interesting point about the Štramberk section is the

presence of two reverse magnetosubzones in the late part of

the magnetozone M19n. The Brodno magnetosubzone corre-

lates either to one or to both of them. This cannot be decided

on the basis of biostratigraphic criteria, as the Brodno mag-

netosubzone lies in the monotonous part of the Alpina Sub-

zone of the Calpionella Standard Zone. The position of the

short acme of species Cr. parvula has still not been deter-

mined more precisely within the Štramberk section, due to

sparse sampling.

Discussion of results

The positions of the Kysuca and Brodno magnetosubzones

are also confirmed by our preliminary results obtained from

the section at Bosso (Italy). Lowrie & Channell (1984, p. 45)

have speculated that “A single reversed sample at the base of

the section in the top of the more slowly deposited Calcari Di-

asprigni may represent the short reversed interval between

M19 and M20”, i.e. the herein described Kysuca Subzone.

This occurrence of reverse magnetization was not confirmed

by detailed sampling at the level of 304.15 m or in its vicinity.

The equivalent of the Kysuca Subzone itself was recorded in

the Bosso section rather at the level of 299.2 to 299.55 m, i.e.

4.5 m lower. It is represented by bed 28 in our numbering.

Calpionellids are unfortunately completely absent from this

basal interval of the Bosso section.

The only equivalent of the magnetosubzones in M19n, i.e.

the Brodno Subzone, in the Bosso section corresponds to the

level of 318.90–319.55 m (it is represented by the beds 100–

103 of our numbering). Its base lies at the level of 80 % and

its top at the level of 85 % of the local thickness of the magne-

tozone M19n. As at Brodno, it lies within the monotonous

part of the calpionellid Alpina Subzone of the Calpionella

Standard Zone.

From the geophysical point of view, the magnetostratigraphic

profile at the locality of Brodno near Žilina can be considered

absolutely unique among all the hitherto studied sections across

the J/K boundary strata in the Tethyan Realm. This is the first

section on the continent, where two reverse subzones were very

precisely detected within the magnetozones M20n and M19n at

positions corresponding to marine M (Mesozoic) anomalies.

Although the paleomagnetic components of remanence are very

low in comparison with natural remanence, they were easily in-

ferred with the use of progressive thermal demagnetization by

the MAVACS apparatus and subsequent multi-component anal-

ysis of remanence. Samples with intermediate polarity were de-

tected at the boundaries of the Kysuca Subzone localized within

background image


the magnetozone M20n in the zones of transition from N to R

and R to N polarities. Time interval within the limits of ca.

±5000 years can be assumed for a transition from normal (re-

verse) to reverse (normal) polarity of magnetic field of the co-

axial geocentric dipole of the Earth with respect to the thickness

of the samples (2 cm) and the assumed sedimentation rate (ca.

2 mm/ka). This figure depends on the estimation of the sedi-

mentation rate of the studied pelagic sediments, but is in agree-

ment with data obtained from other localities (Butler 1992). A

similar sedimentation rate (2.27 mm/ka) may be obtained

from the Brodno profile if the magnetozones of total thickness

from the base of M21 to the base of M18 (10 m) and the cor-

responding time interval (4.4 Ma) are considered. An analo-

gous figure was also obtained from the Miocene sediments of

the Sokolov Basin, western Bohemia (Krs et al. 1991).

Ackowledgements: The authors wish to thank Dr. J. Michalík,

Dr. D. Reháková and two anonymous referees for reviewing the

paper and some helpful suggestions.


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