GeolCarp_Vol49_No1_15

GEOLOGICA CARPATHICA, 49, 1, BRATISLAVA, FEBRUARY 1998

1532

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

PETR PRUNER

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

590

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,

Fisher

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

100

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.

Introduction

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

through M21.

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

confidence cone

(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-

an realm.

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

limonitized.

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

and coccoliths.

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-

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,

laboratory procedures

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

, S2,

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 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

and k

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 ±

0.09

-4

[A/m], k

= 24 ± 5

-6

[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

-4

[A/m],

= 55 ± 12

-6

[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

-4

[A/m], k

= 53 ± 21

-6

[SI], n = 224.

As we shall see later, this group of samples contains syn-

tectonic magnetization.

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

and 580

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

100

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

100520

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

) reached

high values several tens to hundreds of 10

-3

[A/m]. Nor-

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 (

- or

-Fe

?).

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

with

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

C).

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-

tonically remagnetized.

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

, M

remanent magnetic moment of a sample demagnetized by alternating field

(H, mT), at temperature t (

C); M

remanent magnetic moment of a sample in natural state (NS); M

, M

and k

, k

are nor-

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.

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-

tions) and

= 4.5

; 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-

nence,

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,

Mean geogr.

coordinates

Mean direction

Pole position

Oval of confidence

Lat.

[

Long.

[

Decl.

[

]

Incl.

[

]

[

]

Paleo-lat.

Paleo-long.

[

]

[

]

38.1

358.1

359.3

56.2

2.8

31.8

88.6

200.8

4.0

2.9

Table 1: The Río Argos. Mean paleomagnetic direction and pole position calculated from B-components of totally remagnetized lime-

stone samples.

Table 2: The Río Argos. Mean paleomagnetic direction and pole position calculated from C-components of totally remagnetized lime-

stone samples.

Mean geogr.

coordinates

Mean direction

Pole position

Oval of confidence

Lat.

[

Long.

[

Decl.

[

]

Incl.

[

]

[

]

Paleo-lat.

Paleo-long.

[

]

[

]

38.1

358.1

2.8

60.2

4.1

15.8

86.3

32.7

6.2

4.7

Table 3: The Río Argos. Mean directions of B-components of re-

manence of samples with syntectonic magnetization.

Corr. for

dip

Mean directions

[%]

Decl. [

]

Incl. [

]

[

]

100

38.9

49.2

4.46

6.67

176

36.5

46.7

4.45

6.69

176

34.3

44.2

4.45

6.70

176

32.2

41.5

4.45

6.69

176

30.3

38.7

4.46

6.67

176

28.5

35.8

4.48

6.62

176

26.9

32.9

4.50

6.56

176

25.3

30.0

4.53

6.48

176

23.9

27.1

4.57

6.39

176

22.6

24.3

4.61

6.29

176

21.4

21.5

4.66

6.19

176

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

paramagnetic minerals.

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.

Principal results

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

C were

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 (

- or

-Fe

?). 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

within the E.C. Project No. CI1-CT94-0114.

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