GEOLOGICA CARPATHICA, 49, 1, BRATISLAVA, FEBRUARY 1998
THE NEOGENE REMAGNETIZATION AND PETROMAGNETIC
STUDY OF THE EARLY CRETACEOUS LIMESTONE BEDS FROM
THE RÍO ARGOS (CARAVACA, PROVINCE OF MURCIA, SE SPAIN)
PHILIP J. HOEDEMAEKER
, MIROSLAV KRS
, OTAKAR MAN
, JOSEP M. PARÉS
and DANIELA VENHODOVÁ
National Museum of Natural History, Postbus 9517, 2300 RA Leiden, Netherlands
Geological Institute, Academy of Sciences of the Czech Republic, Rozvojová 135, 165 00 Prague 6 Lysolaje, Czech Republic
Institute of Earth Sciences, Jaume Almera, Lluís Solé i Sabarís s/n, 08028 Barcelona, Spain
(Manuscript received March 18, 1997; accepted in revised form December 11, 1997)
Abstract: The Early Cretaceous limestone strata around the Río Argos (Caravaca Region, Province Murcia, SE Spain)
were selected for magnetostratigraphic investigations. This section was chosen due to its importance, detailed geo-
logical and paleontological documentation and good outcrops of individual strata. Altogether 361 oriented hand samples
were collected covering the Berriasian, Valanginian, Hauterivian, Barremian and the Early Aptian sequence strata.
The laboratory specimens were subjected to both the alternating-field and thermal demagnetization procedures, while
the thermal treatment by means of the MAVACS apparatus was carried out at relatively dense temperature steps up to
C, in many cases up to 690
C. Twenty pilot samples were tested for the anisotropy of magnetic susceptibility.
Fifteen samples were selected for detailed analysis with the aim of determining the unblocking temperatures with
higher precision and verifying the possible occurrence of self-reversal phenomena of remanence during laboratory
thermal treatments. All the 361 collected samples were subjected to systematic thermal or combined demagnetization
procedures. Multi-component analysis was applied to separation of respective remanent magnetization components,
s (1953) statistics were used for the calculation of the separated remanence components combined with fold
tests. Few samples were found totally weathered, these are characterized by low unblocking temperatures (below
C), and their magnetic susceptibility is markedly lower. The vast majority of samples showed three components
of remanence, A-, B- and C-components. It was clearly proved that the studied un-weathered limestones can be
divided into two groups of rocks, the first group with syn-tectonic magnetization, and the second group of limestones
totally remagnetized in the Neogene. This way, the Early Cretaceous limestones from the Río Argos were found
unsuitable for derivation of a magnetostratigraphic scale. Apart from totally weathered limestones, magnetite with a
well defined unblocking temperature (around 540
C) was found as the carrier of remanent magnetization in the
majority of massive and fresh-looking limestone samples. From the study of the anisotropy of magnetic susceptibility
it could be concluded that the fabric of the limestones in both the groups of totally and partially remagnetized samples
showed the same features. It is of interest that the limestones under study display no signs of thermal, hydrothermal,
chemical, dynamometamorphic or other alterations. The principal aim of the paper is to demonstrate typical case
histories aimed at methodological problems since similar rocks may be selected for magnetostratigraphic studies in
other regions of the Tethyan realm and similar remagnetization phenomena may be encountered as already described
in the papers by Villalaín et al. (1996), Parés & Roca (1996).
Key words: SE Spain, Province Murcia, the Río Argos area, Early Cretaceous limestones, petromagnetism and
magnetomineralogy, anisotropy of magnetic susceptibility, remagnetization.
The global pattern of normal and reverse magnetozones
may serve as an important correlation criterion for the defi-
nition of the Jurassic/Cretaceous boundary. It allows us to
overcome the problems with biostratigraphic scale correla-
tions in the Tethyan and Boreal realms. Magnetostratigraph-
ic results hitherto obtained from samples from the Jurassic/
Cretaceous boundary strata in the Tethyan realm (cf. Lowrie
& Channell 1983; Ogg et al. 1984, 1988, 1991; Márton
1986; Zeiss 1986) along with the results recently inferred
from the localities of Brodno near ilina, Slovakia, and
tramberk, Moravia (Houa et al. 1996a,b) stimulated de-
tailed magnetostratigraphic research, particularly working
out of the high-resolution magnetostratigraphic scales in
other regions of the Tethyan realm. Such a scale has already
been worked out for the locality of Brodno near ilina,
where two narrow reverse subzones were detected very pre-
cisely within the normal magnetozones M19 and M20, in
addition to the normal and reverse magnetozones M17
Two additional sections in the Tethyan realm were selected
for high-resolution magnetostratigraphic research, i.e. sec-
tions in the Bosso Valley (Umbria, central Italy) and in the
Río Argos (Subbetics, Spain). The studies in the Bosso Val-
ley follow the already completed basic magnetostratigraphic
research which indicated clearly defined magnetozones suit-
able for correlation with magnetic anomalies M19 through
M14, or probably through M13 of the marine Mesozoic se-
quence M (Lowrie & Channell 1983). Suitable physical prop-
16 HOEDEMAEKER, KRS, MAN, PARÉS, PRUNER and VENHODOVÁ
erties of rocks as well as an adequate geological setting com-
prise the fundamental premise for the working out of a high-
resolution magnetostratigraphic scale at this locality. Another
goal was to work out a magnetostratigraphic scale for a lime-
stone-dominated Lower Cretaceous succession in the Río Ar-
gos (Caravaca) area, documented in much detail both geolog-
ically and paleontologically (Hoedemaeker & Leereveld
1995). Much attention was paid to the Río Argos section with
respect to its high importance, good outcrops of the individu-
al strata and their clear numbering in the field. However, it
was at this section that serious problems resulting from re-
magnetization of limestones were encountered.
Progressive thermal demagnetization using the MAVACS
apparatus (Pøíhoda et al. 1989) proved to be the most suit-
able tool for the inference of remanent magnetization com-
ponents. The measured data were tested using multi-compo-
nent remanence analysis and the separated remanence
components combined with fold tests were statistically
evaluated. The values of the volume magnetic susceptibility
of samples subjected to thermal demagnetization were also
registered so that the possible phase and mineralogical
changes of magnetically active minerals could be deter-
mined at each step of the thermal demagnetization process.
Magneto-mineralogical analysis and magnetic susceptibility
anisotropy study of pilot samples were carried out. The sta-
tistically rich material (361 oriented hand samples from re-
spective limestone beds) has unambiguously proved that the
absolute majority of rock samples display post-tectonic and
syn-tectonic magnetization with a large portion of the rocks
having been totally remagnetized in the Neogene. It was
also possible to determine the situation of the epicentrum of
processes resulting in the maximum remagnetization during
the reverse polarity of paleomagnetic field in the Neogene.
Primary paleomagnetic directions were practically com-
pletely destroyed, although they are well reproducable at
other or nearby localities with rocks of analogous age and
composition (e.g. Cehegin, Carcabuey). The syn-tectonic
magnetization was inferred on the basis of the study of the
precision parameter k or of the semi-vertical angle of the
(Fisher 1953) depending upon gradual
changes in the dip of strata (ranging between maximum dip
angles and the horizontal position). The components of syn-
tectonic magnetization indicate a clockwise paleotectonic
rotation. Analogous methodological and paleotectonic con-
clusions were reported from other localities in Spain, e.g.
from the Betic orogen (Villalaín et al. 1996; Parés & Roca
1996). The aim of the presented study is to briefly highlight
the main geophysical and paleotectonic conclusions as anal-
ogous problems may appear at other localities in the Tethy-
A brief outline of the geology of the studied region
The Lower Cretaceous Río Argos succession is situated
in the frontal parts of the Subbetic Zone of the Betic Cordil-
leras (SE Spain) and crops out in several sections along the
River Argos and its tributaries west of Caravaca. It is the
most complete and best preserved Lower Cretaceous suc-
cession in the Betic Cordilleras. Detailed logs of the entire
succession were constructed and all beds were numbered.
Lithology of the Río Argos Succession
The part of the succession incorporated in this study is
1500 m thick and consists of a rather monotonous cyclic al-
ternation of olive grey marly limestone beds and dark grey,
shaly marlstone interbeds. In the micritic limestone beds
and marlstone interbeds, clay is the main siliciclastic frac-
tion. From the middle Valanginian part upwards, up to 1 %
silt-sized quartz grains occur. In the upper Barremian and
Aptian proximal sandstone turbidites are frequent. The lime
fraction (for the upper Berriasian ranging between 58 and
83 % with a mean value of 72.5 %) consists almost entirely
of coccolith and Nannoconus tests and their fragments,
slightly encrusted by sparry calcite.
Diagenetic overprinting has been indicated: apart from
mechanical compaction (8090 % for the marlstone inter-
beds and 4060 % for the limestone beds), differential dis-
solution and cementation has occurred giving rise to en-
hancement of the lithologic contrast of bedding and to
nodular limestone beds.
The entire Lower Cretaceous Río Argos Succession was
deposited in pelagic environments. Several paleo-ecological
arguments (Hoedemaeker & Leereveld 1995) suggest that
deposition occurred at a depth estimated to be in the order
of 300400 m. Many megafossils were pyritized and later
Fossils and stratigraphy
The Río Argos Succession is 1500 m thick and comprises
the Berriasian up to the lower Aptian stages. 99 % of the
megafossils consist of ammonites, either preserved as calcar-
eous moulds or as limonitized steinkerns. Other megafossils
are echinoids, brachiopods, belemnites, a few bivalves and a
few gasteropods. The microfossils are foraminifers, di-
noflagellate cysts, calpionellids, radiolarians, nannoconids
The chronostratigraphy, biostratigraphy and sequence
stratigraphy of the Río Argos Succession has been de-
scribed in Hoedemaeker & Leereveld (1995). A cyclostrati-
graphic analysis of the Berriasian has been done by ten Kate
& Sprenger (1989) and Sprenger & ten Kate (1992).
Geological setting and tectonic evolution
The geology and tectonics of the region have been studied
by Van Ween (1969) and Hoedemaeker (1974). They pre-
sented a detailed description of the geology of the region
and geological maps. For the tectonic units of the Moratal-
laCaravaca region see Fig. 1. The Lower Cretaceous Río
Argos Succession forms part of the allochthonous Subbetic
Zone, which mainly consists of pelagic deposits (with the
exception of the Triassic and Lower Jurassic rocks). In Ser-
THE NEOGENE REMAGNETIZATION AND PETROMAGNETIC STUDY 17
Fig. 1. Tectonic units of the MoratallaCaravaca region.
18 HOEDEMAEKER, KRS, MAN, PARÉS, PRUNER and VENHODOVÁ
ravallian time (1014 Ma) the frontal parts of the Subbetic
Zone thrusted northwestward over the autochthonous Prebet-
ic Zone, which is mainly composed of rocks deposited in a
neritic environment. The tectonic evolution of the Subbetic
Zone of the Caravaca region can be characterized as a décol-
lement process leading, through diapiric action of the
gypsiferous Triassic rocks, to concentric folding of the Sub-
betic rocks above a detachment plane into box-folds that
evolved either into diapiric folds (flower structures) or, when
asymmetrical, into fold-overthrusts. The cores of the folds
consist of Triassic rocks. The detachment plane is situated in
the upper gypsiferous part of the parautochthonous Triassic
rocks. The Paleozoic basement rocks did not take part in the
thrusting. The primordial boxfolds started growing at the be-
ginning of the Lutetian age, some 50 Ma ago.
The Río Argos Succession is situated in the frontal fold-
overthrust (the Buitre Unit) of the Subbetic Zone where Ju-
rassic carbonate rocks have thrusted at least 5 kilometers
nothwestward over the deep, autochthonous basin-shaped
syncline of Moratalla, which forms part of the Prebetic
Zone. The Río Argos Succession forms the southeastern
flank of the overthrusted fold (Fig. 2).
The deeper steep part of the overthrust fault is probably sit-
uated vertically below the Río Argos, along which the Lower
Cretaceous succession is exposed. This fault may have some
relation to a basement structure because of the presence of a
small diabase intrusive body found 100 m north of the Cortijo
de la Puerta piercing through the overturned part of the Preb-
etic Zone close to the overthrust fault (Hoedemaeker 1974, p.
96, enclosure 5). The age of this diabase is unknown but can-
not be older than late Ypresian. This diabase has nothing to
do with the diabase intrusives commonly found in the Trias-
sic rocks, which are generally considered to be of Triassic age
(Van Veen 1969, p. 104), but might be related to the late Neo-
gene volcanic phase in SE Spain, during which small volca-
noes were formed near Calasparra, 15 km to the northeast.
This basement structure may approximately be parallel to the
northeast-southwest strike of the overthrust fault. This base-
ment structure would be vertically below the Lower Creta-
ceous outcrops along the Río Argos.
The Palaeozoic basement is divided into blocks bounded
by northeast-southwest trending normal faults. These faults
already existed in Berriasian time, and became active again
in early Albian times (Hoedemaeker 1974, p. 187). From the
detailed investigations of the stratigraphy of the Río Argos
Succession by Hoedemaeker (field work from 1973 on-
ward), it became clear that there is a gradual thinning fol-
lowed by an abrupt thickening of the stratigraphic units
when following them from west to east along the Río Argos.
From this variation in thicknesses it can be concluded that
there existed a roughly north-south trending submarine fault
escarpment separating an eastern from a western Early Cre-
taceous sub-basin. This fault escarpment is tentatively inter-
preted as a reflection of a northwest-southeast trending
basement fault, which would cross the basement structure
that runs parallel to the strike of the overthrust fault.
Collection of oriented rock samples,
Altogether 361 oriented hand samples were collected
from well defined limestone beds covering the epochs of the
Berriasian, Valanginian, Hauterivian, Barremian and the
Early Aptian. The individual intervals of the section across
limestone beds are marked by Z, Y, M, N, P, A, Q2, Q
V2 and U as described by Hoedemaeker & Leereveld
(1995). Field sampling was done under the supervision of
Ph.J. Hoedemaeker and the individual oriented hand sam-
ples were numbered 1 to 361 from the Lower Berriasian to
the Lower Aptian beds. In some cases, two samples were
collected from a single bed (but from a different sampling
site) for verification purposes. The repeated samples show
these numbers: 94 (from bed Y 90), 98 (bed Y 102), 104
(bed Y 125), 110 (bed Y 148), 119 (bed Y 182), 123 (bed Y
197), 138 (bed Y 258), 139 (bed Y 261), 156 (bed Y 319),
174 (bed M 275), 177 (bed M 290), 211 (bed P 19), 216
(bed P 26), 220 (bed P 41), 236 (bed A 9), 237 (bed A 14),
246 (bed A 51), 266 (bed A 145) and 335 (bed V2 56). Time
overlap is present in one part of the section, i.e., the samples
were collected from two different parts of beds but of the
same age: samples of Nos. 144 to 155 corresponding to
beds 276 C to 319 of the Y section chronologically coincide
with samples of Nos. 157 to 167 corresponding to beds 200
to 248 of the M section (see Fig. 3). For study purposes,
seven oriented samples numbered 355 to 361 were taken
from the bed A 154 (the uppermost Hauterivian). Basic
magnetic data related to remanence components are given
in Figs. 3 to 6 for the individual numbered samples. Num-
bers of studied strata are described in detail in the paper by
Hoedemaeker & Leereveld (1995).
Laboratory procedures were selected to allow the separa-
tion of the respective remanence components and the deter-
mination of their geological-historical origin. Therefore,
moduli and directions of natural remanent magnetization (J
and directions of the separated remanence components were
measured. Zijderveld diagrams as well as graphs of normal-
ized remanent magnetization and magnetic susceptibility val-
ues in relation to the progressive demagnetization tempera-
ture were also plotted for each of the samples. Minerals
acting as carriers of the respective remanence components
were determined from the inferred unblocking temperatures.
Fig. 2. Diagrammatic cross-section showing the tectonic structure
of the relevant Jurassic strata in the MoratallaCaravaca region.
The Río Argos Succession is situated in the southern part of the
Buitre Unit just north of the Egea Subunit, which is the frontal part
of the Loma de Solana Unit.
THE NEOGENE REMAGNETIZATION AND PETROMAGNETIC STUDY 19
Fig. 3. The Río Argos section, Early Cretaceous limestone strata, samples Nos. 1 to 180. J
modulus of natural remanent magnetiza-
volume magnetic susceptibility of samples in natural state; temperature range temperature interval in which the B-compo-
nent of remanence was derived by multi-component analysis; D
declination, inclination of B-component remanence; polarity
polarity of B-component remanence. Samples totally remagnetized are denoted by small full squares (overprint).
20 HOEDEMAEKER, KRS, MAN, PARÉS, PRUNER and VENHODOVÁ
Fig. 4. The Río Argos section, Early Cretaceous limestone strata, samples Nos. 180 to 360. See legend to Fig. 3.
THE NEOGENE REMAGNETIZATION AND PETROMAGNETIC STUDY 21
Fig. 5. The Río Argos section, Early Cretaceous limestone strata, samples Nos. 1 to 180. J
modulus of natural remanent magnetization;
volume magnetic susceptibility of samples in natural state; temperature range temperature interval in which the C-component of
remanence was derived by multi-component analysis; D
declination, inclination of C-component remanence; polarity polarity of
C-component remanence. Samples totally remagnetized are denoted by small full squares (overprint).
22 HOEDEMAEKER, KRS, MAN, PARÉS, PRUNER and VENHODOVÁ
Fig. 6. The Río Argos section, Early Cretaceous limestone strata, samples Nos. 180 to 360. See legend to Fig. 5.
THE NEOGENE REMAGNETIZATION AND PETROMAGNETIC STUDY 23
Cube-shaped laboratory specimens 20
20 mm in size
were prepared from the oriented hand samples and mea-
sured by the JR-5 spinner magnetometer (Jelínek 1966).
The specimens were subjected to progressive thermal de-
magnetization with the use of the MAVACS apparatus
(Magnetic Vacuum Control System) ensuring the generation
of a high magnetic vacuum in the environment of thermally
demagnetized specimens (Pøíhoda et al. 1989).
Several specimens were also experimentally subjected to
demagnetization by alternating field (A.F. procedures) using
the Schonstedt GSD-1 apparatus. It was found suitable for
the removal of viscous magnetization components but gen-
erally less effective than the MAVACS apparatus.
Phase and mineralogical changes frequently take place
during thermal demagnetization, especially at higher temper-
atures. Therefore, the dependence of magnetic susceptibility
on temperature was also measured. The values of volume
magnetic susceptibility were measured on the KLY-2 kappa-
bridge (Jelínek 1973).
Fifteen specimens were selected for detailed analyses
with the aim of determining unblocking temperatures with a
high precision and to verify the possible existence of a self-
reversal phenomenon of remanence. These specimens were
subjected to isothermal magnetization using a direct mag-
netic field to the state of saturation at the maximum magne-
tizing field intensity of 900 mT (9000 Oe). Specimens with
saturated remanent magnetization were subjected to pro-
gressive thermal demagnetization.
Separation of the respective remanent magnetization
components was done using multi-component analysis (Kir-
schvink 1980). Fishers (1953) statistics were used for the
calculation of the mean directions of J
and of the directions
of the separated remanence components combined with fold
tests. Temperature intervals in which the respective rema-
nence components were inferred are also given in graphs in
Figs. 3 to 6.
The values of magnetic susceptibility
and remanent magnetization
The values of the moduli of J
of the studied rocks in their
natural state are exceptionally low, largely depending on the
origin of magnetization. The values of volume magnetic sus-
ceptibility are also low but mostly show a smaller scatter than
values. The samples of the analysed rocks can be classified
into three categories according to the values of J
i) Exceptionally low magnetization values are indicated
for samples of Nos. 23 to 45 (with the exception of anoma-
lous samples of Nos. 33 and 41): mean values of J
= 0.26 ±
= 24 ± 5
[SI], n = 21 (n being the
number of samples). These samples were collected from
weathered rocks, as also indicated by the low unblocking
temperature values, see Figs. 3 and 5. Rocks of this type are
not suitable for the multi-component remanence analysis.
ii) The second category of samples shows increased val-
ues of J
. The mean values of J
= 6.13 ± 6.36
= 55 ± 12
[SI], n = 85. This set includes samples of
Nos. 46 to 156 and some other samples. These samples
were totally remagnetized in the Neogene; they are marked
by the overprint symbol (small full squares) in Figs. 3 to 6.
iii) The third category of samples shows lower values of
, although the values of susceptibility are identical with
those of the preceding category. This set includes samples
of Nos. 1 to 22 and 157 to 361 (exceptions are marked by
the overprint symbols in Figs. 3 to 6). The mean values of J
= 1.34 ± 1.35
= 53 ± 21
[SI], n = 224.
As we shall see later, this group of samples contains syn-
Two samples of totally remagnetized limestone (Nos. 120
and 121) with reverse and normal polarities of remanent
magnetization are shown in Figs. 7 and 8 as examples.
Whereas the unblocking temperature ranges between 540
C in the vast majority of samples, samples of Nos.
120 and 121 show a somewhat higher unblocking tempera-
ture. The viscous remanence component can be removed by
a 20 mT alternating field or by thermal demagnetization to
C. The remagnetization components prevail in the tem-
perature interval of 100610
A typical example of a limestone sample with syn-tecton-
ic magnetization is shown in Fig. 9. Remanent magnetiza-
tion is of a relatively low value, showing three different
components: the A-component corresponds to viscous mag-
netization, the B-component is a normal one (in interval of
C) and the C-component is a reverse one (in tem-
perature interval of 540580
C). The C-component has a
very low amplitude; it was inferred with a low confidence
level in many cases.
The C-component was inferred at higher demagnetization
temperature intervals, above 400 to 500
C and is largely re-
versely polarized. To verify the possible origin of a self-re-
versal of remanence and to determine the unblocking tem-
peratures more precisely, 15 pilot samples representing both
totally remagnetized samples and samples with syn-tectonic
magnetization were subjected to the following tests: the se-
lected samples were progressively isothermally magnetized
by a direct field of intensities of 5, 10, 20, 100 and 900 mT.
The values of saturated remanent magnetization (J
high values several tens to hundreds of 10
malized values of saturated remanent magnetization and of
magnetic susceptibility for four samples are given as exam-
ples in Fig. 10. The absence of self-reversal of remanence
was proved for all fifteen pilot samples and magnetite was
determined as the principal carrier of remanent magnetiza-
tion on the basis of unblocking temperatures. A higher un-
blocking temperature above 600
C was determined in only
one sample (totally remagnetized), indicating an admixture
of other minerals (
Directions of J
(NRM) and of separated B- and
C-components of remanent magnetization
The distribution of J
directions suggests that the studied
limestone beds were either totally or partially remagnetized.
Fig. 11 shows the J
directions of limestone beds not cor-
rected for dip. Three sets of J
directions are very similar but
the set in Fig. 11c shows no reverse directions of J
24 HOEDEMAEKER, KRS, MAN, PARÉS, PRUNER and VENHODOVÁ
declination around 180
. This set includes samples with
somewhat higher values of declination (D). As it is obvious
from the text below, these differences result from a lower
degree of remagnetization of rocks shown in Fig. 11c.
All the collected samples (n = 361) were subjected to pro-
gressive demagnetization at relatively dense steps. The
Schonstedt GSD-1 apparatus was used for alternating field
demagnetization and the MAVACS apparatus was used for
thermal demagnetization (Pøíhoda et al. 1989). At the begin-
ning of laboratory experiments, a relatively small set of
samples was subjected to thermal demagnetization at tem-
peratures of 60, 90, 120, 160, 200, 240, 280, 320, 360, 400,
450, 500, 540, 560 and 590
C. A larger portion of samples
was subjected to a combined demagnetization by alternating
field of 50, 100, 150, 200 mT and by thermal field at tem-
peratures of 100, (150), 200, (250), 300, 360, 400, 440, 480,
520, 560, 580 or 590
C, (610 or 620, 650, 680 or 690
The remaining samples were demagnetized only thermally.
Temperatures applied to some selected sets of samples are
given in brackets.
The directions of remanent magnetization inferred by the
above given procedures were tested using a multi-compo-
nent analysis (Kirschvink 1980). Generally, the samples
showed three remanence components: A, B and C. The A-
components are mostly of viscous or chemoremanent
(weathering) origin. They can be removed by an alternating
field to 20 mT or by a thermal field to 100
C. For a better
understanding of the remagnetization problem, the tempera-
ture intervals at which the directions of the B- and C-com-
ponents were separated are plotted in Figs. 3 to 6.
On the basis of the B- and C-components direction analy-
sis, the studied rocks can be divided into two groups of
rocks of post-tectonically totally remagnetized and syn-tec-
The normal and reverse B-component directions of the to-
tally remagnetized samples (marked by small full squares in
Figs. 3 and 4) form two well-defined sets of samples with
fisherian distribution. The directions not corrected for the
dip of strata (in situ directions) are shown in Fig. 12, and the
mean directions calculated after Fisher (1953) for the 95 %
probability level along with the corresponding paleomag-
netic pole position are summarized in Table 1, see Fig. 13.
The data imply that the rocks of the studied samples were
totally remagnetized in the Neogene. The situation of the
epicentrum of processes causing the overprint of B-compo-
nents is located in the area of collection of samples of Nos.
46 to 156. The analysis of C-components further revealed
that numerous samples were totally remagnetized during a
Fig. 7. A limestone sample No. 120 with reverse polarity and totally remagnetized during the Neogene, results of combined (A.F. and
thermal) demagnetization. Upper part of the figure: M
remanent magnetic moment of a sample demagnetized by alternating field
(H, mT), at temperature t (
remanent magnetic moment of a sample in natural state (NS); M
malized values of remanent magnetic moment and of volume magnetic susceptibility, respectively. Lower part of the figure: Zijderveld
diagram, solid circles represent projections on the horizontal plane and open circles represent those on the N-S vertical plane. Stereo-
graphic projection, NS sample in natural state (NRM), open (solid) circles represent projection on the upper (lower) hemisphere.
THE NEOGENE REMAGNETIZATION AND PETROMAGNETIC STUDY 25
Fig. 9. A limestone sample No. 90 with syn-tectonic magnetization. See legend to Fig. 7.
Fig. 8. A limestone sample No. 121 with normal polarity and totally remagnetized during the Neogene. See legend to Fig. 7.
26 HOEDEMAEKER, KRS, MAN, PARÉS, PRUNER and VENHODOVÁ
Fig. 10. Results of the thermal demagnetization of four pilot samples (Nos. 78/2; 88/2; 90/2; 103/2) subjected to isothermal magnetization
up to the saturation state prior to thermal treatment. M
remanent saturation magnetic moment demagnetized at temperature t (
remanent saturation magnetic moment at room temperature (20
are normalized values of remanent magnetic mo-
ment and of volume magnetic susceptibility, respectively.
THE NEOGENE REMAGNETIZATION AND PETROMAGNETIC STUDY 27
Fig. 12. Directions of B-components of remanence of limestone
samples totally remagnetized during the Neogene, not corrected
for dip of rocks (in situ directions). These samples are denoted by
small full squares in Figs. 3 and 4.
Fig. 11. Directions of natural remanent magnetization (J
) not corrected for dip of rocks (in situ directions): a) set of samples Nos. 1114;
b) set of samples Nos. 115229; c) set of samples of Nos. 231355. Stereographic projections of directions onto lower (upper) hemi-
sphere are denoted by small full (open) circles.
reverse polarity of the paleomagnetic field in the Neogene.
The epicentrum of processes resulting in the overprint of C-
components is located in the area of collection of samples of
Nos. 81 to 154, which further restricts the area of processes
leading to total rock remagnetization. Table 2 parallels Ta-
ble 1 but the data summarized relate to the C-components;
the calculated results confirm the Neogene age of the over-
print. In both tables, the reverse directions were transformed
into normal directions due to the calculation of the paleo-
magnetic pole position.
The directions of the B-components of remanence of the
remaining samples were also statistically evaluated (Fisher
1953) with the exclusion of totally weathered samples (of
samples with unblocking temperature below 100
C) and of
totally remagnetized samples (shown on Fig. 12). The fol-
lowing results were obtained:
; k = 6.2; n = 176
28 HOEDEMAEKER, KRS, MAN, PARÉS, PRUNER and VENHODOVÁ
Fig. 13. Mean directions of B-components of samples described in
Fig. 12. The mean directions denoted by solid or open crossed cir-
cles and circumscribed by circles of confidence were calculated
according to Fisher (1953) at the 95 % probability level.
for samples not corrected for the dip of strata (in situ direc-
; k = 6.7; n = 176 for samples corrected
for the dip of strata. The differences in the calculated values
are statistically not significant. Table 3 summarizes the
mean values of declination (D), inclination (I) of rema-
and k for samples not corrected for the dip of
strata (correction 0 %) and for samples corrected for the dip
of strata (correction 100 %) as well as for transitional dip
corrections at 10 to 90 %. The presented results indicate a
syn-tectonic origin of B-components of remanence in this
sample set. It is worth mentioning that with the exception of
three samples the B-components are exclusively normally
polarized, see Figs. 14 and 15. This a priori excludes the
syn-sedimentary origin of the B-components of remanence.
Study of anisotropy of magnetic susceptibility
The magnetic susceptibility of a rock is the ratio of induced
magnetization to applied magnetic field. The anisotropy of
magnetic susceptibility (AMS) can be described as a second-
order tensor, which defines the susceptibility ellipsoid, with
maximum (K1), intermediate (K2) and minimum (K3) princi-
pal axes defined by their magnitude and direction.
AMS in rocks can basically reflect: a) shape anisotropy,
caused by the alignment of elongated grains; b) crystalline
anisotropy, arising from the alignment of crystal axes. At
low magnetic fields (0.1 mT), the susceptibility anisotropy
of magnetite is controlled by the shape of the grain as long as
the grains are not interacting. Hematite, like the paramagnetic
(micas, hornblende, chlorite) and the diamagnetic (plagioclase,
Oval of confidence
Table 1: The Río Argos. Mean paleomagnetic direction and pole position calculated from B-components of totally remagnetized lime-
Table 2: The Río Argos. Mean paleomagnetic direction and pole position calculated from C-components of totally remagnetized lime-
Oval of confidence
Table 3: The Río Argos. Mean directions of B-components of re-
manence of samples with syntectonic magnetization.
THE NEOGENE REMAGNETIZATION AND PETROMAGNETIC STUDY 29
Fig. 15. Directions of B-components of samples described in Fig. 14, but corrected for dip of rocks.
Fig. 14. The directions of B-components of remanence of partially remagnetized samples (with syn-tectonic remanence), not corrected
for dip of rocks (in situ directions). a) From the set of samples Nos. 1229. b) From the set of samples Nos. 231335.
quartz, calcite) rock-forming minerals, yields a component of
magnetic fabric to the rock that is influenced primarily by the
crystallographic alignment of the anisotropic grains.
Ten samples of totally remagnetized limestones and ten
samples of partially remagnetized limestones (with syn-tec-
tonic magnetization) were selected for study of the anisotro-
py of magnetic susceptibility using the method of Jelínek
(1977). The Anisotropy of Magnetic Susceptibility (AMS)
was measured using the Kappabridge KLY2.02 (Geofyzika
Brno), which is based on measuring the so-called direction-
al susceptibilities, corresponding to certain directions in the
rock specimen. It would be sufficient to measure directional
susceptibilities in six suitably chosen directions since the
susceptibility tensor is symmetrical and thus has six inde-
pendent components. Nevertheless, Jelínek (1977) devel-
oped a method according to which the measurement is per-
30 HOEDEMAEKER, KRS, MAN, PARÉS, PRUNER and VENHODOVÁ
formed in fifteen different positions and the elements of the
susceptibility tensor are then determined by computer using
a least squares method. An ellipsoid defined by the maxi-
mum, intermediate and minimum susceptibilities (K1 > K2
> K3) can be associated to the symmetrical tensor. The aim
of this study was to verify whether the samples from both
groups of limestones show similar or different parameters
related to the anisotropy of magnetic susceptibility.
The fabric of limestones in both groups of samples shows
the same features: the axes of minimum anisotropy are verti-
cal (normal to the bedding) and the axes of maximum and in-
termediate anisotropy are contained within the bedding plane
(Fig. 16). The maximum axes are roughly grouped, along
WNWESE directions. This could reflect either an original
paleocurrent or an extension direction due to the Tertiary
compression that affected the region under study. In any case,
the fabric is dominantly sedimentary: the maximum axes are
vertical and normal to bedding. Another important feature is
that the magnetic ellipsoids for all the samples are oblate.
This means that foliation dominates over lineation, which is
very common for sedimentary-type fabrics.
The values of bulk susceptibility (see Figs. 3 to 6) are
very low, indicating both the influence of the diamagnetism
component (calcite) and the low concentration of ferro- and
Discussion to possible geological causes
of the remagnetization
The décollement tectonics and the diapiric folding of the
Buitre Unit do not involve large stresses. The stresses involved
were mainly transmitted through the competent Jurassic car-
bonate rocks and not through the incompetent Cretaceous
marls that were riding piggy-back upon the Jurassic carbon-
ates. This also applies for the adjacent Loma de Solana Unit a
few kilometers to the south, where the Cretaceous marls are
not remagnetized. This means that extreme stresses cannot be
regarded as the cause of the remagnetization of the rocks.
Thermal causes for the remagnetization also cannot be
taken into consideration, because the delicate dinoflagellate
cysts are still nicely preserved. They would immediately be
destroyed by the slightest heating and by weathering.
If the interpretations of the basement structures in the Paleo-
zoic basement are correct, it would imply that there would be a
crossing of basement faults approximately below the frontal
Buitre Unit of the Subbetic Zone. This may perhaps be the site
of the remagnetization described in this paper.
The Early Cretaceous strata of limestones at the locality
of the Río Argos were subjected to relatively extensive lab-
oratory tests aimed at the inference of magnetostratigraphic
data. However, the obtained results clearly show that the
limestones were either syn-tectonically remagnetized or to-
tally post-tectonically remagnetized in the Neogene. With
respect to growing activities in magnetostratigraphy in other
regions of the Tethyan realm, it is worth mentioning the
main methodological results:
i) The vast majority of the studied limestone samples
show three components of remanence A, B and C. The A-
components are of viscous or chemoremanent origin (ef-
fects of weathering) and were inferred in the temperature in-
Fig. 16. Equal area projection of AMS principal axes for all measured
samples: a) samples of limestone strata totally remagnetized in the
Neogene (with post-tectonic magnetization); b) samples of limestone
strata with syn-tectonic magnetization. Ellipses indicate the confi-
dence level at 95 %. Squares maximum principal axes. Triangles
intermediate principal axes. Dots minimum principal axes.
THE NEOGENE REMAGNETIZATION AND PETROMAGNETIC STUDY 31
tervals below 100
C. The B-components were mostly in-
ferred in temperature intervals of 100 to 400
C, see Figs. 3
and 4. The C-components of remanence could be inferred
for a considerable number of samples in temperature inter-
vals of 400 to 580
C, see Figs. 5 and 6. The C-components
of weakly magnetic, syn-tectonically remagnetized lime-
stone samples show a large scatter and, therefore, could not
be used for a reliable interpretation. However, the C-compo-
nents of more strongly magnetic, totally remagnetized sam-
ples could be used for interpretation, see Table 2.
ii) A relatively small number of samples proved to be in-
tensely weathered. These samples were not used for the
multi-component analysis. The samples of intensely weath-
ered limestones are characterized by unblocking tempera-
ture values below 100
C, see samples of Nos. 23 through
45 (except for samples of Nos. 33 and 41) in Fig. 3. The
magnetic susceptibility of these samples is also markedly
lower than in all other samples.
iii) Samples Nos. 46 to 156 and several other samples
marked by small full squares in Figs. 3 to 6 represent sec-
tions of limestone beds totally remagnetized in the Neo-
gene. The most intense remagnetization occurred at a re-
verse polarity of paleomagnetic field. The epicentrum of
processes resulting in total remagnetization of limestone
beds is located in the area between samples Nos. 46 to 156,
i.e., between the beds Z 212 and Y 319.
iv) Apart from the samples listed under ii) and iii), the
limestone samples from the whole Río Argos section indi-
cate a syn-tectonic remanent magnetization. This magneti-
zation was inferred from the study of precision parameter k
or of the semi-vertical angle of the confidence cone
(Fisher 1953) in dependence upon different dip angles of
strata, see Table 3. The B-components of syn-tectonic rema-
nent magnetization indicate a clockwise paleotectonic rota-
tion, see Figs. 14 and 15.
v) Magnetite with a well-defined unblocking temperature
proved to be the carrier of remanent magnetization in the vast
majority of massive limestone samples from the Río Argos
locality. Higher unblocking temperatures above 600
recorded in only a few limestone samples with increased re-
manent magnetization totally remagnetized in the Neogene.
These temperatures indicate a possible admixture of some
other minerals (
?). Similar magnetic properties
are shown by Mesozoic limestones with magnetite admixture
in the Tethyan realm, which are mostly suitable for magneto-
stratigraphic studies. Petromagnetic and paleomagnetic anal-
yses of limestones from the Río Argos locality represent a
typical case history: magnetite-carrying fresh and massive
limestones were remagnetized to such a degree that they
completely lost the components of remanent magnetization
syngenetic with the rock. These limestones macroscopically
display no signs of thermal, hydrothermal, chemical, dyna-
mo-metamorphic or other alterations. Similar examples of re-
magnetization of Mesozoic limestones were also pointed out
by other authors (e.g. Villalaín et al. 1996; Parés & Roca
1996) and analogous cases could undoubtedly pose a certain
danger during routine magnetostratigraphic studies. The geo-
logical cause of the remagnetization of limestones at the Río
Argos locality remains unexplained.
Acknowledgements: The authors of this Report wish to
thank the Netherlands Oil Company for financial support.
J.M. Parés is grateful to the Spanish authorities for support
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