Cerdanya basin geometry and its implication on the Neocene evolution of the Eastern Pyrenees

Tectonophysics,

129 (1986) 355-365

Elsevier Science Publishers

CERDANYA

355

B.V., Amsterdam

- Printed

BASIN GEOMETRY

ON THE NEOGENE

J. POUS l.*, R. JULIA

in The Netherlands

AND ITS IMPLICATION

EVOLUTION

OF THE EASTERN PYRENEES

3 and L. SOLE SUGRANES

*.’

I Depurtement de Fisicu de la Terra i el Cosmos, Unwersitut de Burcelonu. Diugonul 645, 08018 Burcelonu (Spuin) ’ Institut d’Estudis Cutuluns, Lahorutori de Geofrsicu “Eduurd Fontserk”. Curme 87, 08001 Barcelonu (Spuin) -’ Institute Jaume Almeru, Murti Frunqub (Received

s/n,

Consejo Superior de Inoestiguciones

Cientificus,

08028 Barcelonu (Spuin)

June 18, 1985; revised version

accepted

December

23, 1985)

ABSTRACT

Pow,

J., Julia,

Neogene

R. and Sole Sugranes,

evolution

Geological

Evolution

Cerdanya

is a Neogene

Axial Zone. Subvertical Neogene

sediments

Vertical

electrical

toward

MacroRossello,

half-graben

soundings

motion

Tectonophysics,

overlap

and

Palaeozoic

deposits

and its implication

SM.

Wickham

on the

(Editors),

The

129: 355-365.

the half-graben

rocks of the Eastern

at the southern

and southeastern

rocks in the northern

show that the Miocene

lacustrine

Basin geometry

E. Banda

basin that lies over the Palaeozoic

fault scarps bound

may directly

In:

sequence

from the central

Pyrenean

margins,

but

margin.

reaches

a maximum

thickness

part of the basin grade to coarser

of over fluvial

the margins.

and micro-structural Conflent,

Capcir

data analyzed

in Cerdanya

and Seu d’Urgel1, suggest

along an old northeastern-trending

east-trending

Pyrenees.

of the Pyrenees.

700 m and that fine-g&red deposits

L.. 1986. Cerdanya

of the Eastern

and other

that they formed

fault. The stress situation

related

Neogene

as a consequence

favoured

sinistral

basins,

such as

of transtensile movements

along

faults,

INTRODUCTION

The role of northeast-trending structures that cut the main eastward trends of the Pyrenees obliquely have been the subject of abundant discussions over the last few years. One of these structures is referred to as the Catalunya fault and it cuts across the Eastern Pyrenean Axial Zone from Perpinya to La Seu d’Urgel1. Neogene basins, such as Rossellb, Conflent, Capcir, Cerdanya and Seu d’Urgel1, lie along it and their origin must be related to the last movements of the blocks along this fault zone (Fig. 1). OO40-1951/86/$03.50

0 1986 Elsevier Science Publishers

B.V.

Post- Pyrenean molasse

basement

Polaeozo~c

Pyreneon

cover

@

Neogene

~___
:

4- Conf lent l- Seu d’urgell 2-Cerdanya 3- Capchr Fig. 1 Neogene

Cerdanya

~-LO

Selva

basins in the Eastern

Pyeneeh

is one of the largest

century it has been the subject Neogene age of the lacustrine

of these basins

and

of several geological and fluvial deposits

since

the end of the last

studies that pointed out the in the basin, as well as its

asymmetry. The basin has a sharp southern border, formed by subvertical faults with suggested vertical offsets of over 2000 m, and an irregular northern limit where the Neogene sediments may directly overlap the Palaeozoic basement. However, these previous studies bring little insight on the geometry of the basin and Neogene rocks, and a dynamic model relating all the Neogene basins of this part of the Pyrenees is lacking. Boissevain (1934) Sole Sabaris (1970) Julivert et al. (1974) among

others,

related

this set of basins

to a common

fault zone that lately

was

referred to as the Segre fault (Garrido and Rios, 1972) or as the C’atalunya fault (Souquet and Mediavilla, 1976). All these authors agree about the sinistral movement of the fault, but there is no agreement about its significance and its role as a plate boundary. Sole Sugrafies (1978) suggested that the Segre fault is a late erogenic transcurrent fault concomitant with the deposition of the Oligocene molasse in the eastern Southern Pyrenees. But Garrido and Rios (1972) and Souquet and Mediavilla (1976) suggested that the fault was an active palaeogeographic limit during the Mesozoic sedimentation in the Pyrenean trough. Souquet et al. (1977) defined it as the limit between two tectonic and palaeogeographically different Pyrenean sectors: the Central Pyrenees or “tronqon navarro-languedocien” and the Eastern Pyrenees or

357

“tronqon Catalan”. According to them, this fault would have played a significant role in the Pyrenean orogeny controlIing the movements between the Iberian and the European plates. Julia (1984) suggested that the Cerdanya could have developed as a pull-apart basin related to the Neogene sinistral movements of the Segre fault, or it could have been formed by stress relaxation behind an older overthrust. In order to test some disagreements about the proposed models, we have started a geological and geophysical survey of these basins. In the Cerdanya basin, field work on the geometry of the faults and sedimentary bodies has been verified with data from more than twenty deep vertical electrical soundings. The data we have analysed up to now suggest that Cerdanya is a rhomb-shaped half-graben related to the Segre fault, but the tension vector that caused the basin to subside must have had a trend close to north-northwest, not compatible with a sinistral movement along the fault. GEOLOGICAL

SETTING

OF CERDANYA

BASIN

The Cerdanya (Fig. 2) Basin extends about 35 km in the NE-SW direction, yet hardly exceeds 5 km in width. The mean altitude of the Cerdanya plain is about 1100 m above sea level, while the mean altitude of the mountains around the basin is over 2500 m. It is filled by Neogene and Quaternary sediments that are completely surrounded by Palaeozoic rocks of the Pyrenean Axial Zone. The actual shape shows a rhomboidal half-graben with a linear, scarped southern margin and more irregular northern limits. The western half of the basin is bounded

DEEP

LACUSTRINE

PALAEOZOIC ?t

COAL

Fig. 2. Geological

DEPOSIT

BASE

PITS

map of the Cerdanya

Basin without

Quaternary

deposits

by an almost

continuous

Basin. The other

E-W

fault and it is referred

half may be referred

Basin and it is bounded

mainly

to as the western or Bellver or Puigcerd&-Sanabastre

to as the eastern

by northeast-trending

faults.

The Neogene deposits A Late Miocene age for the Neogene rocks was first suggested by DCperet and RCrolle (1885) on the basis of ~~ppurjo~ gracik and Z~t~t~eriu~ found at Sanabastre and surroundings. A Late Miocene age (Vallesian). based on mammals and micromammals, was also reported by Crusafont and Golpe (1974) and Agusti et al. (1979) for similar sediments in the Seu d’Urgel1 Basin, suggesting that the sedimentation had been concomitant in both basins. In Cerdanya, a mamma1 fauna was found in the uppermost detrital levels of the Miocene sequence, but the Miocene sedimentation continued for at least two million years according to the lacustrine sedimentary rates (Margalef and MarrasC. 1985) and the total thickness of the lacustrine beds. After this, we can assume that the subsidence of the Cerdanya Basin must have started not later than the Middle Miocene. An upper conglomerates deposits

sequence of Pliocene and/or Quaternary overlies the Miocene sediment unconformably.

outcrop

mainly

thickness. The Miocene sequence environment sedimentary

Pli0

basin

margins

and

may

attain

basement

stratigraphic

about

250 m in

may be divided into three units formed under different conditions: alluvial fans. lacustrine and fluvial-deitaic.

- Quaternory

Palaeozotc Fig. 3. Idealized

along

coarse sandstones and These Plio-Quaternary

section

from the central

part of the western basin.

359

These units may grade laterally one into the other and a clear sequence of the three units occurs only in the central part of the basin. An idealized section of this central part (Fig. 3) shows a lower elastic unit that corresponds to the alluvial fan facies and which is formed of pale-brown yellowish feldspathic quartz gravels, sands and arkoses, with thin interbedded silty layers. The middle unit is formed by very fine blue-grey to dark-grey mudstones and diatomites which correspond to deep-water lacustrine facies (Margalef, 1957). A concentration layer of phosphor&es near the top of the lacustrine sequence suggests a sudden change (climatic or tectonic) in the sedimentary conditions and the fine lacustrine sediments were overlain by alluvial and shallow lacustrine deposits. Margalef and MarrasC (1985) suggest that the diatomites were deposited at a depth of about 500 m with a sedimentary rate of 0.16 mm/yr. The estimation of the depth of the lake is in good agreement with the thickness of the Miocene sequence that outcrops south of Bellver and with data from geoelectrical soundings in the eastern basin (see below). These deep lacustrine facies only occur in the central part of the basin and grade laterally to coarser alluvial-fluvial and shallow lacustrine sediments. The upper elastic unit is formed by sands, silts and lignites deposited in a shallow lacustrine and alluvial plain environment. Neogene tectonics

The sharp morphologial limits of the southern border of the basin approximately coincide with subvertical fault scarps or narrow fault zones, but faults are hardly visible on the northern border. Neogene strata directly overlap the Palaeozoic rocks of the basement in many places and this has led some geologists to negate or minimize the existence of a northern structural border (Birot, 1936). Short faults may, however, locally underline this limit (SolC Sabaris, 1970). Geoelectrical profiles (see above) show that small local faults exist close to this border, but Neogene strata usually overlap them. Blocks of Palaeozoic and Mesozoic rocks occur close to the southern border. Some of the blocks could have glided into the basin, but most of them form a megatectonic breccia that in some places (south of Bellver) is more than 100 m thick. Miocene sediments are tilted toward the south and locally faulted and folded. Some faults and folds may be related to gravitational gliding in the central part of the basin, but close to the southern border, Miocene beds dip vertically. Also the Pliocene sequence close to this border is disturbed by late faults and unconformities, suggesting that they were active throughout the Neogene. In Sanabastre coal pit and close to the southern border of the basin, Miocene beds are folded around an axis trending N150°. Fold axes plunge gently to the southeast in the central part of the basin and to the northwest in the southern margin. Fault planes that cut Devonian rocks near the southern limit show superimposed slickensides that prove the existence of at least two sets of movements along these

; E

0

500

2 1.000 L

1500

1.500

-

NW

NW

PROFILE

PROFILE

II

I SE

361

planes. The oldest slickensides are always near-parallel to the fault-plane trend and suggest a sinistral offset along northeast-trending faults. The pitch of the younger generation of shckensides is steeper and corresponds to normal faults with a moderate strike-slip motion. The pole movements defined by these slickensides have been analyzed in three stations (two close to the southern border and one in the central part of the basin). The plot of tension gashes, fold axis and pole of movements defined by the younger generation of slickensides suggests a strain system with an intermediate strain axis plunging about 70°, maximum elongation oriented NNW-SSE (X axis trend to N150°) and maximum shortening oriented ENE-WSW (Z axis trend to N58O). The vertical offset of the main faults that limit the Cerdanya basin must have been about 2000 m from the Oligocene, since the trough developed over a quasipeneplained area partially covered by Oligocene molasse remnants of which now lie on the top of Serra de1 Cadi, 2000 m above the basin floor.

GEOELECTRICAL

PROFILES

Twenty-one vertical electrical soundings (VES) have been carried out in the Cerdanya Basin with a Schlumberger array. VESs were distributed so as to be grouped into several longitudinal and cross profiles, but severe relief conditions constricted the actual distribution, as shown in Fig. 4. The maximum distance between the electrodes was 3500 m and the power lines were oriented parallel to the structural trends (NE-SW and E-W). A minimum distance between the borders of the basin and the power lines equivalent to the half distance between the electrodes was kept to avoid the border influence on the soundings.

Electrode

Fig. 5. Typical

vertical

electrical

spacing

sounding

( AB/Z ) m curves with data points.

A high resistivity contrast was expected between the rocks of the Palaeozoic basement and the Neogene sediments. A11 apparent resistivity curves (ARC) show an

end

tail

of H-type

that

conductor

and the resistant

irregular

due to the variable

corresponds Palaeozoic.

to the contact

between

The first sections

distribution

the

of the curves

of the Quaternary

sediments.

Neogene are more Figure

5

shows typical ARCS. An automatic interpretation of the vertical electrical soundings has been done using the Pous et al. (1984) algorithm, and a set of equivalent models was found for each solution. Finally, one of these equivalent models was retained as the most compatible among all the possible equivalent solutions interpretation was grouped into four geoelectrical profiles

The basement

for each sounding. in Fig. 4.

This

unit

The Palaeozoic basement shows the highest levels of resistivity (over 300 am), but it was not possible to evaluate its true resistivity because the ARCS do not reach the final asymptotic tendency. A less resistent correspond to graphitic slates. Cross-sections

layer in VESs 18 and 19 (Fig. 4) could (Fig. 4, profiles I and II) clearly show

the asymmetry of the basin: while the northwestern margins dip gently (about 10-15’) to the southeast, the southeastern margins are sharp and steep. The central part of the basement floor is relatively flat, but a stepwise discontinuity occurs near the northern margin between VESs 4 and 21 (Fig. 4, profile I) and VESs 3 and 7 (Fig. 4, profile II). Between VESs 3 and 7, the basement depth increases by more than 350 m. The maximum thickness of Neogene deposits was found in the soundings near the southern border (more than 700 m in sounding 7). Longitudinal profile IV, parallel to the structural trends of the eastern shows a sudden

increase

of the depth

of the basement

basin,

floor from Coil de Saig to

Sanabastre. In the western basin, the relief conditions made the setting up of soundings near the southern margin difficult and only a longitudinal profile was carried out (Fig. 4, profile III). Along this profile, the basement floor is shallower than in the eastern basin but its depth must increase towards the southern margin to allow for more than 600 m in thickness of Miocene deposits that outcrop south of Bellver. Upfilling units Miocene rock resistivities range from 15 to 100 Om and can basically be grouped into three geoelectrical units. Two of these units can be clearly differentiated in the central part of the eastern basin and a third unit occurs mainly on the northern and eastern borders of the basin. In the central part of the eastern basin. the lower geoelectrical unit is relatively uniform, with resistivities lower than 25 Inm, while the upper unit is formed by interbedded thin conductive and resistive layers (Fig. 4).

363

Both units grade toward whose resistivity In the central uniform

vertical

correlated

the northern

margins

of the basin

into a unique

third unit

ranges from 30 to 45 Grn (VESs 1, 3, 4, 5, and 15). part of the western distribution

basin,

the geoelectrical

of the resistivities

to the lower geoelectrical

(Fig.

unit in the eastern

soundings

4, profile

show a more

III);

this can be

basin, but they also grade to

higher resistivity levels toward the basin margins. The correlation between geoelectrical and lithostratigraphic units lished on the basis that the lowest resistivities must correspond

may be estabto the finest

sediments, while the highest resistivities must correspond to the coarsest elastic rocks. Thus, based on this principle and the knowledge of the rocks that outcrop at the site of each vertical sounding, a tentative correlation between geoelectrical and lithostratigraphical units has been established. The lower geoelectrical unit may be correlated with the middle lithostratigraphic unit (Fig. 3), formed by mudstones and diatomites, whereas the upper geoelectrical unit may correspond to the upper elastic unit formed by interbedded gravels, sands, silts and lignites. Mudstones and diatomites reach a maximum thickness near the steeper margin (about 400 m in VES 6). The third geoelectrical unit may be correlated with the coarse rocks that form the aluvial fans on the borders of the basin. In the longitudinal profile IV in Fig. 4, the more resistive VES 7) overlie

the conductive

fine lacustrine

rocks (300 m thick in

facies in the eastern

basin.

They are

correlated to sand, gravels, silts and lignites which outcrop at the Sanabastre coal pit. These rocks have not been detected in the western basin, because most of the electrical soundings there start in the fine lacustrine layers, missing the elastic upper unit. The lower elastic lithostratigraphic unit is not conspicuous in all ARCS. A layer about 100 m thick and with a resistivity of under 100 &?m could exist according to the ARCS shape, but such a layer of resistivity the diatomites

and the basement

The first sections overlie the Miocene

half way between

will be masked

of some ARCS correspond sequence

unconformably.

by suppression

the resistivities (Kunetz,

to the Plio-Quaternary The resistivity

of the Tartera

DISCUSSION

IN THE EASTERN

THE ORIGIN

OF THE NEOGENE

BASINS

rocks

that

of the Segre terraces

over 100 am and rises to over 700 S2m in the “caliches” ABOUT

of

1966).

alluvial

is

fan.

PYRENEES

The Rossell6, Conflent, Cerdanya and Seu d’Urgel1 basins lie along a fault zone that extends for at least 120 km from Perpinya to La Seu and their origin must be related to Neogene movements along it. All these basins are filled with Miocene and Pliocene sediments and show a similar rhomb-graben shape, with steep southern margins. The fault zone along which the basins lie can be considered as a part of the Catalunya fault defined by Souquet et al. (1977) or as a northward extension of the Segre fault. Anyway, the most conspicuous motion along the fault is an anticlock-

364

wise strike-slip

offset of the Pyrenean

rocks of the Southern the Segre fault. have

taken

Pyrenees

After

place

Mesozoic

allochthonous

Sole Sugraiies

during

the

(197X). the greater

Oligocene.

This

compatible

with an approximately

N-S

northward

drifting

plate during

of the Iberian

sedimentary

cover. The C’retaceous

units are offset by about 40 km along part

sinistral

shortening

of this motion

slip

along

of the Pyrenean

the Paleogene

the

must

fault

belt caused

(Choukroune

is by

et al..

1973). Grimaud et al. (1982) suggest a change of the relative plate motion at the end of the Oligocene. From then on the North Pyrenean fault would have undergone a dextral change

strike-slip of motion

motion resulting in a NNW-SSE shortening. However. this does not agree with the opening of the Neogene basins on the

Pyrenean Axial Zone. The subsidence of the basins

started

at the end of the Oligocene

in the Rossello

and continued throughout the Miocene and Pliocene. The subsidence extended progressively from Rossello to the west, toward the inner basins. the Seu d’urgell being the youngest basin. The tension vector computed from the motion of the faults during the Neogene and from the folds that bend the Miocene beds, trends to the north-northwest and will coincide with the shortening vector if the change of motion suggested by Grimaud et al. (1982) had taken place. However a progressive shift from a N-S to a NE-SW compression will result in a transtension situation along the Segre fault zone, favouring normal faults with a sinistral slip component along east and northeast-trending fault planes. The opening of the basins associated with short left-lateral offsets along east-oriented faults will result in the present stepwise appearance of the Catalunya fault. Durand-Delga (1982) points out the difficulties in extending the Segre fault through main fault, since segments of northeast-trending

Cerdanya and Rossello in a single faults are offset by sinistral E-W

faults such as occurs in Cerdanya. The fault motions suggested in our model are valid for the Eastern Pyrenees during the Neogene, but the actual geometry of the faults and sedimentary

basins results from offset accumulation

conditions

the Pyrenean

throughout

under variable

stress

orogeny.

ACKNOWLEDGMENTS

This research

was supported

(field work) and CSIC program

by IEC Laboratori 23105.03.

‘ Eduard

de Geofisica

Fontsere’

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