E - Revistas Científicas Complutenses

Magnetic Reversal Stratigraphy in the Late
Oligocene succession of the Ebro Basin,
near Fraga, Province of Huesca, Northern
Spain
P. F. FRIEND (1); 5. A. BRAZIER (2); L. CABRERA (3); K. W. A.
FEISTNER (1) and J. SHAW (4)
(1) Deparímen! of Earlh Sejences, University of Cambridge, Downing St.,
Cambridge CB2 3EQ (England)
(2) Briíish Geological Survey (J—Iydrocarhon and Marine Surveys
Direclorate), «Souíhpark», 19 Grange Terrace,
Edinburgh EH9 2LF ~‘Scolland,)
(3,) Departamento de Geología Dinámica, Geofísica y Paleontología.
Universidad de Barcelona. Fac. Geología, 08028 Barcelona (Spain)
(4) Departmení of Geological Sciences, Universily of Liverpool, P. O. Box
147, Brownlow Sí., LiverpooL L69 3BX <‘England,)
SUMMARY
A palaeomagnetic investigation of an outcrop section in Late Oligocene, non-marine mudstones, sandstones and limestones is described. Tite
section was chosen as representative of tite outcrops of tite more central
parts of tite Ebro basin, nortitern Spain. It turns out to be impossible to
measure an earl>’ remanence in tite limestones that are an important rock
t>’pe of the basin centre. However distinct magnetic components, titought
to be early, can be measured in man>’ of tite mudrock samples. Seven reversal events occur wititin tite 145 m titick section, and define six magnetic polarit>’ intervals. Assuming a relativel>’ stead>’ stratigrapitic accumulation rate, tite titicknesses of tite magnetic intervals correspond to the
time durations of tite (global) magnetic polarit>’ time-seale. The sequence
of thickness intervals in our section is not long or distinctive enough to
allow us to suggest an unique correlation witit tite global sequence, but
we suggest two possible alternative correlations. Tite alternatives impí>’
stratigrapitic accumulation rates of 0.04 and 007m/ 1.000 years.
STRATIGRAPHIC INTRODUCTION AND OBJECTS
Tite Ebro basin is a foreland basin titat developed in response to crustal loading during ihe Tertiar>’ evolution of tite P>’renees (Fig. 1). Tite
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.-Vlagneuic Reversal Stratigraphy in the Late Oligocene...
123
main evidence for titis is the general northward increase in tite titickness
of tite basin-filling sedimenis, from about 1 km in tite central areas (including tite Fraga area reported here), to between 3 and 4 km below the
southern P>’renean thrust front (Riba et al., 1987).
Tite Ebro basin arose as a weIl defined entity during tite Oligocene,
when tite southern titrust sheets of tite Pyrenees started to emerge to form
its nortitern margin, and its southern margin also became active tectonicalI>’ (Puigdefábregas et al., 1986).
Tite surface outcrops across most of the basin are of sediments of Ohgocene and Miocene age, formed in river and lake environments (Fig. 1).
A number of river systems carried sediment from different parts of
tite basin margins towards its centre. Tite Late Eocene-Early Ohigocene
Scala Dei S>’stem built out from tite south-west end of tite Catatan Coastal Range; the Oligocene-Miocene Matarranya-Guadalupe System extenda] from tite linking zone between tite Catalan CoastaJ Range and tite Iberian Range in the south, and Oligocene-Miocene s>’stems built from tite
P>’renees in tite north (Cabrera et al., 1985; I-Iirst and Nicitols, 1986; Nichols, 1987). In alí these cases, conglomeratic alluvial fan sequences accumulated along tite basin margins, and passed distalí>’ into distributive
fluvial facies. Titese facies consist of titick mudstone dominated sequences
with interbedded conglomeratic and sandstone bodies from 1 m up to
10 m thick. Both vertical and lateral aceretion are recorded in titese channel deposits which show ribiton and sheet geometries.
Titese river s>’stems interdigitated, towards tite basin centre witit a central zone where playa-lake muds, carbonates, g>’psums and coals formed
(Cabrera, 1983; Cabrera et al., 1985; Friend, et aL, 1986; Nichols, 1987;
Cabrera and Sáez, 1987). In tite SE Ebro Basin, titese facies make up sequences up to 600 m thick (Los Monegros lacustrine system) (Cabrera,
1983; Cabrera and Sáez, 1987). Titis stud>’ was carried out in the transitional area between one of tite river systems coming from tite P>’rcnees
and tite Los Monegros lacustrine s>’stem of the central zone (Fig. 1).
Tite dating of tite non-marine, Oligocene and Miocene deposits depends largel>’ on tite vertebrate fossils tite>’ contain, particularí>’ tite mammal microfossils. Titese are of sufficiently great biostratigrapitie interest,
in terms of tite evolution of European terrestrial faunas, that tite Ramblian and Aragonian nonmarine stages itave been defined in nortit-central Spain (Daams et al., 1987). More specificall>’, in tite study area, some
twenty mammal microfossil localities itave been found and titis has suggested a Late Oligocene-Early Miocene biozonation, and tite tentative proposal of a mammal stage stratotype (Agusti et al., 1 988b). However it has
proved difficult to correlate titese faunas with otiter successions, parlicularí>’ titose witit globalí>’ established marine biostratigrapity.
124
P. F. triená, 5. A. Brazier, L. Cabrera. K. lE A, Eeistner and J. Shaw
Our objects in working on tite magnetie reversal stratigraph>’ in tite
Ebro basin itave been
1) to establish local rates of stratigraphic accumulation as a means of
anal>’sing the basin growth,
2) to investigate tite relationsitips between these rates and tite processes of river sedimentation,
3) to establish correlation between tite terrestrial and tite marine biostratigraph>’.
Titis paper wiU focus on tite first aim.
THE SECTION AND ITS SEDIMENT TYPES
The section we report on itere starts some 1.5 km soutit-east of the oíd
bridge across tite river Cinca, in the town of Fraga, Province of Huesca,
Spain (Fig. 1). Map references and latitude and longitude of tite base of
the section are BF 797 998 and 0 20 E, 41 31 N. A detail sedimentar>’ log
of the section forms part of a study (Cabrera, 1983) of the basin-centre
sediments in this pan of tite Ebro basin.
Tite 1 45m section consists largel>’ (63 0/o) of mudstone, witit varyig colour (brown or red), and content of silt. Titis mudstone was tite main sediment íype that we sampled as oriented blocks, and most of our succesful measurements were of titis material. Brown fine to medium-grained
sandstones make up about 33 % of tite section, and sometimes provided
successful measurements. Titese sandstones outcrop as titin (a few cm)
sheets, or as larger sandstone bodies up to 3 m titick.
Limestones, sorne 0.20 m to 3 m thick, formed on~y 4 O/~ of our Fraga
section, and are mainly present in its top 30 m. However similar limestones are tite dominant sediment t>’pes exposed in some of tite otiter basin centre sections that we sampled extensivel>’ within 20 km of Fraga.
Wc sampled them wide]>’ because they tend to outcrop strongly, provide
good material for detaching oriented blocks, and itad shown measurable
natural remanence during our earl>’ explorator>’ work. Wc were titerefore
disappointed to find, on using the thermal demagnetisation routines described below, titat tite limestones generalí>’ lack a component of remanence that is stable. We itave concluded that tite limestone remanence resides in tite brown oxide patcites and veins titat are t>’pical of most of tite
samples, and we suggest that titis brown material results from late stage
oxidation of tite pyrite witich is often abundant. We have titerefore abandoned tite measurement of limestones from these sections.
Magnetic Reversal Stratigraphy in ihe Late Ql/gocen e...
125
MEASUREMENT ROUTINE AND ANALYSIS
Oriented blocks were collected at «vertical» stratigraphic intervals
averaging about 2 m. Titis sonietimes involved excavation beJow the surface.
Tite blocks were chosen large enough to provide at lcast titree cores
witicit are 9 mm in diameter, and at least 1 cm in length. Oriented cores
for measurement were prepared in the workshop in Cambridge by dr>’ drilling using a slowly rotating coring bit. Titis was formed from a stainless
steel tube, to avoid magnetic contamination, with teeth individualí>’ hand
cut and set to acitieve maximum self-clearing of tite cylindrical incision.
Ah tite measurements were made using tite eryogenic magnetometer
designed and run in tite Department of Geolog>’, lJniversity of Wales at
Cardiff. We found titat thermal demagnetisation of tite samples produced
relativel>’ clear separation of components of tite remanence, whereas alternating ficíd demagnetisation proved to be ineffective. We used a measurement routine involving steps of 100’ C, from 0~ C to 5000 C. A lower maximum temperature would not itave resulted in complete demagnetisation, and larger steps would itave resulted in loss of component information. Each core took about 3 hours to measure.
We routinel>’ plotted tite movement of tite magnetie vectors that occurred during titermal demagnetisation, using both stereographic (direetion), and vector (direction and intensit>’-Zijderveld, 1967) plots. Tite stable magnetic directions were then estimated from titese plots (eg. Fig. 2).
Our statistical treatment of the titree or more stable cores from each site,
used the method adopted in other recent studies of reversal stratigrapities
(cg. Johnson and others, 1985), whereb>’ sites were classified as class A,
class B, or rejected, depending on tite degree of agreement between tite directions from the different cores.
RESIJLTING MAGNETOSTRATIGRAPHY
Wc measured cores from 34 sites scattered through the 145 m thick
Fraga section (Figs. 3 and 4). Our stereographic and vector results >‘ield
exactí>’ similar polarity intervals, even titough tite actual directions var>’
in detail. Here we present tite vector data, consisting of 33 class A sites,
and 1 class B site. Samples from 9 other sites were measured but showed
no stable component during demagnetisation from one temperature to
another, and were therefore rejected.
Fig. 3 sitows tite declination and inclination averaged for eaeit of tite
34 acceptable sites. Wc also present tite VGP (Vidual Geomagnetie Folarity) latitude, witich is tite present latitude of the pole calculated from
tite remanence at eacit site. Our data locate seven reversal events in tite
126
1’. 1. Friená, 5. A. Brazict,., L. (labrera. 1<. W A. h’isflwr aná JShaw
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or normal) vector of remanence, decreasing steadily when demagnetised by heating in 100
C steps from 0’ C to 500’ C. Sample FR 3e shows two components of remanence, under
te same demagnetisation treatment, and the higher temperature component is reversed
(south-up).
section, and titese define six polarit>’ intervals. Ifwe assume that tite rate
of stratigraphic accumulation of tite section is relativel>’ steady, we can
then examine tite thickness of our sequence ofpolarity intervals, and compare them with thc (global) magnetic polarity timescale, as, for instance,
reviewed by Harland and others (1982). The vertebrate biostratigraph>’ of
the area (Agusti et al., 1988) suggests that tite section lies somewhere in tite
Late Oligocene, and it is immediately apparent, titat our sequence of po-
127
A’Iagnetic Reversal Síratigraphy in <he Late Oligocene...
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Fig. 3. Magnetie directional data for sites sampled at intervals isp Ihe Fraga section, located atad described in this paper. Magnetic componenis have been selected by examination of vector plots (see Fig. 2) of measurements made during thermal demagnetisation. Columns represent 1) declination; 2) inclination; 3) vidual geomagnetic pole latitudes based
on 1) atad 2); 4)intervals between magnetic reversals with normal intervals shaded, and reversed inlervals unshaded; 5) atad 6) alternative correlations witb global magnetie seale as
outlined and labelled by ¡-larlatad and others (1982). Ages from the latter publication are
placed approxiniateiy, beside coluintas 5) and 6).
128
P. F. Friená, 5. A. Brazier, L. Cabrera, K
1V. .4. Feisiner and J? Shaw
N
w
E
s
LOWER HEMISPMERE
,á Class A
A Class E
e
Mean normal/reversed direction
UPPER MEMISPHERE
u Class A
Fig. 4. Stereographic projection of meatas of each of 34 sites ita Fraga section. Class A and
Class B sites, upper hemisphere and lower hemispliere directions are distinguished. Means
of normal and reversed directions are shown, and are very nearly diametrically opposite.
larity intervais is not long enough, nr distinetive enough to
víncing correlation. Wc therefore suggest two alternative
Fig. 3.
provide a conpossibilites in
Magnetie Reversal Stratigraphy in <he Late Oligocene. -.
129
ESTIMATES OF RATES OF STRATIGRAPHIC ACCUMULATION
Our Fig. 3 sitows two possible correlations of our polarity intervals
using tlw chron terminolog>’ of Harland and others (1982). Using also tite
ages for these citrons suggested b>’ Harland and others (1982), we arrive
at average stratigrapitic accumulation rates for tite witole of our section
of 0.07 m/L000 years, and 004 m/l.000 years for tite two correlations.
These rates are at tite 10w end of the ranges quoted from otiter foreland
basins (Johnson and otiters, 1985; Johnson and others, 1986; Homewood
et aL, 1986), and titis shows titat the Fraga area was one of oní>’ moderate
subsidence. In tite introduction to titis paper we used basin thickness totais to argue that the Fraga area subsided moderatel>’ compared with tite
area of tite P>’renean front.
Acknowle dgeme nts
We offer our sincere thanks to tite United Kingdom Natural Environment Research Council wito supported Sarah Brazier and Karl Feistner
while they sampled and measured tite material, and to Dr. C. Puigdefábregas, and tite Servei Geolégic de Catalunya for fxeld support, during
this investigation of Ebro basin palaeomagnetism. AlI staff and colleagues
at Cardiff contributed to tite excellent running of tite measurement laboratory. Professor Oriol Riba itas provided a base for tite stratigrapitic work
on the Ebro basin, in his Department of tite Universit>’ of Barcelona.
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Recé ved 10 FQP. 1988.
Accepted 13 Juno /988.