Magnetostratigraphic characterization of a thick Lower Pleistocene lacustrine sequence from the Baza Basin (Betic Chain, Southern Spain)

PHYSICS OFTHE EARTH AND PLAN ETA RY INTERIORS

ELSEVIER

Physics of the Earth and Planetary Interiors 85 (1994) 173—180

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Magnetostratigraphic characterization of a thick Lower Pleistocene lacustrine sequence from the Baza Basin (Betic Chain, Southern Spain) 0. Oms a

a,b,* M. Garcés a J.M. Pares a J• AgustI b P. Anadón a R. Julia a Institute of Earth Sciences J. Almera (CSIC), c / MartI i Franquès s / n, 08028, Barcelona, Spain b Institute of Paleontology M. Crusafont, c / Escola Industrial 23, 08201 Sabadell, Spain Received 6 September 1993; revision accepted 5 January 1994

Abstract The magnetic polarity stratigraphy from a lacustrine sequence of the Baza Basin (Betic Chain, Southern Spain) has been interpreted as having a Lower Pleistocene age. Fifty-nine paleomagnetic sites have been obtained and a set of 179 specimens has been demagnetized with both thermal and alternating field procedures. Several magnetic parameters, that depend on the lithology, have been obtained. The characteristic remanent magnetization polarity has been unambiguously recovered over the whole sequence. The materials studied span chron lr.2, located in the upper Matuyama epoch. This allows the placement of the faunas studied, ranging from the Late Villanyian to the Late Biharian, in the magnetic polarity time scale (MPTS). This work also proves that magnetostratigraphy can be established in lacustrine sediments even with extremely low magnetic intensities.

1. Introduction In Western Europe, biostratigraphy of terrestrial Neogene and Quaternary faunas is widely known but it lacks absolute dating. This leads to major ambiguities and makes any correlation between marine stages and events within continental stages difficult. Magnetostratigraphic studies of continental sediments are scarce when cornpared with marine ones. There are several reasons why non-marine sediments are generally

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*

Corresponding author,

more complicated to study than marine sequences: non-continuous sedimentation, successive weathering during deposition, etc. In the Iberian Peninsula, only a few studies of Neogene and Quaternary non-marine magnetostratigraphy exist (Dijksman, 1977; Opdyke et al., 1989; Garcés, 1993). Magnetic polarity of non-marine Tertiary and Pleistocene sequences has been established by several authors around the world, as summarized by Opdyke (1990). The study reported here attempts to place some biostratigraphic data into the magnetic polarity time scale (MPTS), thus forwarding an approach to determining absolute age durations for the biozones recognized.

0031-9201/94/$07.00 © 1994 Elsevier Science By. All rights reserved SSDI 0031-9201(94)05055-3

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0. Oms et al. /Physics of the Earth and Planetary Interiors 85 (1994) 1 73—180

The Baza Basin is a useful site for establishing the magnetostratigraphy of Neogene continental stages owing to the occurrence of continuous outcrops that furnish abundant paleontological sites.

2. Geological setting The Betic Chain is an Alpine fold and thrust belt which extends along the southern Iberian Peninsula. On a large scale, it can be considered as the westernmost edge of the Alpine European fold belts. In detail, the Betic Chain has a complicated structure of folds and large thrusts which is believed to have originated by the westward emplacement of the Alboran microplate on to the South Iberian passive margin during the Neogene (Andrieux et al., 1971; Balanyá and GarciaDueñas, 1987). This complex geodynamic history has led to a division of the Betic Chain into the Internal and the External zones. The External

Zone is made up of cover materials which mainly consist of Mesozoic and Cenozoic rocks. The Internal Zone is broadly formed by Paleozoic and Triassic thrust sheet basement units. The Guadix—Baza Depression (Fig. 1) is located in the contact between the External and Internal zones and its origin is related to a set of faults oriented northwest—southeast and northnortheast—south-southwest that fonned a depression where Neogene sediments accumulated (Sanz de Galdeano and Vera, 1992). This depression embraces several basins such as the Baza Basin. The Baza Basin fill sequence provides continuous outcrop with plenty of paleontological sites that provide continental micromammal faunas. Agusti (1986) proposes a local micromammal biozonation that spans from the Late Miocene to the

Middle Pleistocene for the Baza Basin. Several environments have been distinguished in the Baza Basin that allow the separation of two main domains in the basin (Vera 1970). A marginal domain, mainly alluvial, represented by the Guadix Formation and a distal domain, de-

MAIN BETICS INTRAMONTANE BASINS,~~

HI

~ MEDITERRANEAN SEA

ST DYAREAS~°T~~

~

ZA~L~”’~ ~“~‘~‘

GUADI

Other Neogene deposits

Tnassic evapontes Mesozoic carbonates

—

—

F~~] Pleistocene Baza Fm [~]

0

20 km

Metamorphic rocks

Fig. 1. Geological map of the Guadix—Baza Depression and its location in the main Neogene basins of the Betics (modified from Anadón et al., 1986).

0. Oms et a!. /Physics of the Earth and Planetary Interiors 85 (1994) 173—180

posited in the center of the basin and represented by several formations, of which the Baza Formation is the most widespread. In the northeastern part of the Baza Basin (Orce area), Vera et al. (1985) defined two members: a mainly alluvial ‘red detrital member’, overlain by a mainly lacustrine ‘upper siltycalcareous member’. Anadón et al. (1986) interpreted this upper member as the deposits of a slightly saline lake. The same authors stated that the deposits in the Orce region show lateral fa-

cies transitions to evaporitic gypsiferous sequences towards the center of the basin. Similar changes occur in the Cortes de Baza area.

3. The section studied: biostratigraphy and absolute dating The section studied, located in the surroundings of the village of Cortes de Baza, corresponds to the western area of the Baza Formation out-

SUSCEPTIBILITY (x1O~S.l.)

NRM INTENSITY (5106 Nm)

VGP LATITUDE

H

__

______

175

CB 88

H

_

J*CB1 0ni’.’—’~~’’

:1:1

LEGEND ~

-2.5 0

2.5 5 7.5 10

1

dark mudstones and silty mudstones massive limestones and limestones with gypsum

1O~

10

-90°

.450

00

silts and silty sandstones

*

fossiliferous level

Fig. 2. Lithological log of the Cortes de Baza section with susceptibility, NRM intensity and VGP latitudes obtained from the paleomagnetic sites. All sites display clear reversed polarities although VGP values are not well aligned.

176

0. Oms et al. /Physics of the Earth and Planetary Interiors 85 (1994) 173—180

CORTES DE BAZA SECTION top older 17 (U,Thf}-CB88~

netic gypsum that cross the marls only contained 0.1 ppm of 238U and showed isotopic equilibrium (234U/238U = 1.07 ±0.04 and 230Th/234U = 0.98

&

~

Mm Q

2 ostramosensis Mm 0, pliocaenicus MN1617b Mimomys poloflicus MN Mimomys haJnackensis MN 16 a

LCB 1~7M.

tomens,s

~

.

Alloph. pliocaenicus M. ~.

tween 126 and 161 kyear. The crystals of diage-

BIOZONES ~

~

~/~ 8 ~ _~

Fig. 3. Biostratigraphic setting of the Cortes de Baza section according to the Plio—Pleistocene micromammals biozona-

crops. The 135 m thick Cortes de Baza sequence (see Fig. 2) is made up of mudstones, silty mudstones and limestones alternating with sandy layers and carbonaceous dark mudstones. Lenticular gypsum beds also occur. Pefla et al. (1977) reported a micromammal site near Cortes de Baza. They attributed the micromammal fauna to the Lower Pleistocene. This micromammal site is correlated with the lower part of the section studied in the present paper. The lower part of the Cortes de Baza section is characterized by the presence of Mimomys cf. tornensis (CB-1 level, Fig. 2). This level can be tentatively placed in the Mimomys ostramosensis zone of AgustI (1986) or MmQ-1 (M ostramosensis zone) of Agust~et al. (1986), although its attribution to an older age (M pliocaenicus zone) cannot be excluded (Agusti, unpublished work). Therefore, the base of the Cortes de Baza section may be biostratigraphically ascribed to the Upper Villanian (Upper Pliocene) or Lowermost Pleistocene (Fig. 3). An evolved Microtus (with Microtus enamel differentiation) appears at the highest micromammal site (level CB-88, Figs. 2 and 3). This level may be ascribed to the MmQ-3 zone (M savini zone; AgustI et al., 1986), although a younger age cannot be discounted. Uranium series analyses of the uppermost lacustrine deposits were carried out in order to obtain a calibrated age. Three different materials were analyzed: marls, large crystals of diagenetic gypsum, and ostracod valves, by standard alpha spectrometry methods. The marl samples, reaching 3.3 ppm of 238U, yielded U/Th dates be-

±0.03). Therefore, an age over 300 kyear is ob234U/ tamed for the diagenetic gypsum. The isotopic composition ostracod valves showed a 238U ratio of of0.90 that suggests the possible mi.

gration of uranium. The isotopic results prove that these lacustrine deposits were open to the migration of younger radioisotopes and incongruous dates have been obtained. Considering that the diagenetic gypsum (the latest mineral phase) shows isotopic composition in equilibrium, the age of the uppermost lacustrine deposits outcropping in the Cortes de Baza area must be older than 300 kyear.

4. Methods 4.1. Sampling Paleomagnetic sites were spaced as close as possible, with a mean of 1 site every 2.4 m throughout a section which is 135 m thick (see Fig. 2). Fifty-nine levels were sampled but only 43 provided stable demagnetization plots. Most of the samples were taken from finegrained lithologies, thus avoiding the sampling of sandstones and coarser materials because such sediments were unconsolidated and heavily weathered. When sampling mudstones, an important volume of weathered sediment was removed in order to obtain fresh outcrops. More than three cores per site were collected and were carefully oriented. Cores were obtained with an electrically powered drilling machine but sometimes a manual sampling device (Lerbekmo, 1990) was needed. 4.2. Laboratory Magnetic cleaning was carried out by both thermal (TH) and alternating field (AF) procedures. Concerning the TH cleaning, a thermal specimen demagnetizer, model TSD-1, was used, and AF demagnetization was carried out with a

0. Oms et al. /Physics of the Earth and Planetary Interiors 85 (1994) 173—180

GSD-5 tumbling-specimen demagnetizer (both of the Schonstedt Instrument Company). The samples were measured with a GM-400 three-axes cryogenic magnetometer (Cryogenic Consultants Ltd). During TH cleaning, susceptibility was checked with a KLY-2 bridge (Geofyzika AS) in order to monitor mineralogical alteration of the samples during heating. At least three specimens per site were demagnetized with both AF and TH methods. Pilot suites, containing specimens of every site, were demagnetized in very short steps. Thermal cleaning was carried out in very close steps especially where unblocking temperatures become critical, which depends on the lithological type. Usually this procedure involved up to 17 steps of demagnetization. Later on, the bulk of the samples were treated with a typical demagnetization program consisting of wider spaced steps, depending on the lithologies.

177

With AF cleaning, the steps were of 2.5 mT from NRM to 15 mT. The rest was carried out with steps of 5 mT until the intensity was very low or until 100 mT.

5. Results Initial intensity and susceptibility appeared to be related to different kinds of lithologies. Five groups of lithologies have been established and are shown in Table 1. Demagnetization artifacts resulting from the overlap of the coercivity spectra of two components have been observed. This made thermal demagnetization the only fully reliable method to elucidate the structure of the magnetic components as sometimes with AF only a secondary magnetization was demagnetized. Regardless, some sites provided a stable demagnetization be-

up, w 2500C

~

THERMAL DEMAGNETIZATION

• horizontal o vertical

NRM:8.O1O~Nm

Fig. 4. Standard Zijderveld plot of a limestone thermally demagnetized. Note how it displays a very simple demagnetization behavior with no more than two components, and how a secondary antiparallel remagnetization is successfully demagnetized. The ChRM can be easily established.

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0. Oms et al. /Physics of the Earth and Planetary Interiors 85 (1994) 173—180

havior with both thermal and AF demagnetization. The whole set of samples provided a relatively stable demagnetization behavior. The resulting Zijderveld plots (Zijderveld, 1967) were quite simple, showing no more than two components. Commonly, in all sites, a secondary magnetization resulting from the present-day field was demagnetized successfully above 120°C,and the primary characteristic remanent magnetization (ChRM) was then identified easily (see Fig. 4). The extremely low intensity of the NRM in some samples created some noise in the ChRM. This led to the recovery of slightly different values for a single site. Regardless, the polarity could be established unambiguously (Fig. 5).

up, N 2500C

O

vertical

•

horizontal

THERMAL DEMAGNETIZATION 3500C

NR

A/m

The thermally demagnetized specimens could be subdivided into two groups according to the maximum unblocking temperatures (TBmaz) of the characteristic remanent magnetization: one group with TBmax around 350°C and the other group with TBmax over 500°C. These groups, however, are not related to a specific lithology. Above these maximum unblocking temperatures, susceptibility increased suddenly and the magnetization became unstable as shown by random end vectors on the orthogonal plots. During AF cleaning, samples were fully demagnetized when the applied field was 25 or 30 mT. Occasionally, some samples became demagnetized at higher fields.

Table 1 Maximum, minimum and mean of the susceptibility and initial intensity Lithology NRM intensity

Mean

Maximum

Minimum

(x iO~ A rn’)

Dark mudstones Silty mudstones Poorly consolidated silts

17.3 15.5

27.2 65.3

11,1

1.40

6.24

12.5

2.45

4.12 4.60

9.71 14.2

0.55 1.83

62.57 46.69

86.65 87.65

21.85 —2.40

Poorly consolidated silts

32.57

71.35

14.50

Limestones with gypsum Massive limestones

18.18 15.33

31.00 42.95

10.50 48.00

Limestones with gypsum Massive limestones Susceptibility (x 10 Dark mudstones Silty mudstones

-~

E

Fig. 5. Example of an orthogonal demagnetization plot of a silty limestone. This specimen was demagnetized thermally and had a very low 5 A natural m1. Although remanent the magnetization demagnetization (NRM)is intensity: 2.4X i0 not very stable, polarity can be unambiguously established.

One fact inducing instability during demagnetization was the occurrence of gypsum. Such a mineral, that is in fact diamagnetic in a pure state, produced some magnetic minerals that in turn produced instability in the rock and even erased the ChRM. Finally, the virtual geomagnetic pole (VGP) plot of each sample was calculated and plotted, displaying a completely reversed span for all the section as shown in Fig. 2.

SI)

________________________________________

6. Discussion and conclusions Reversed characteristic magnetizations are found throughout the section although the VGP

0. Oms et al. /Physics of the Earth and Planetary Interiors 85 (1994) 173—180

data are scattered appreciably. This scatter is attributed to the low intensity of some samples that make demagnetization plots of the same site slightly different although always with the same polarity. However, the complete set of samples provides clear polarities. Any consideration of a completely reversed remagnetization is also almost impossible owing to the position of the section in the magnetic polarity time scale as will be discussed below. Regarding the correlation of the magnetic sequence with the mammal biostratigraphic scale, two micromammal zones have been distinguished in the studied section: the M ostramosensis zone, which can be correlated with the Late Villanyian, and the M savini zone, that belongs to the Late Biharian. Late Villanyian faunas equivalent to those of the Cortes de Baza sequence, have been placed within the Late Pliocene (Mein et al., 1978). Nevertheless, the section studied is fully reversed and therefore the uppermost Villanyian has to be placed inside the Lower Pleistocene. According to Steininger et al. (1990), some faunas of the MN-17 zone may be correlated with the Olduvai event. In particular, mammal faunas attributed to MN-17 are underlain by basalt flows with reversed magnetization dated at 1.9 Myear in Chillac, France (Azzaroli et al. (1988) and Boeuf (1983) in Steininger et al. (1990)). This datum is congruent with the results obtained in other sections of the Baza Basin, like the Orce and Galera sections (AgustI et al., 1986; Garcés et al., 1992; Garcés, 1993). According to these data, the M ostramosensis zone, (MmQ-1 of Agusti et al., 1986) defined above the MN-17, should probably be correlated with the postOlduvai reverse chron ir (Cande and Kent, 1992). According to these data, the fully reversed Sequence studied must be correlated with the lr.2 chron Therefore, the MmQ-1 (M ostramosensis zone) has to be placed, at least in its upper part, in the Lower Pleistocene according to the Pliocene— Pleistocene boundary established in the Vrica section by Aguirre and Pasini (1985). In addition, the MmQ-3 (M savini zone) keeps its position in the Lower Pleistocene.

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Acknowledgments This work has been partially supported by projects DGICYTtoPB9O-0575 and PB91-0096. We are indebted Llorenç Planagumà and Juan Peña for field assistance. We are also indebted to two anonymous referees for the helpful suggestions and comments in their reviews.

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