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Calibration of the late Pliocene-early Pleistocene transition in the continental beds of the Guadix-Baza basin (southeastern Spain)
Pergamon PII: S10404182(%)OtK~S-1
QuaternaryInternational,Vol. 40, pp. 93-100,1997. Q 1997 INQUA/ Elsevier Science Ltd All rightsreserved.Printedin GreatBritain. 1040-6182/97 $32.00
CALIBRATION OF THE LATE PLIOCENE-EARLY PLEISTOCENE TRANSITION IN THE CONTINENTAL BEDS OF THE GUADIX-BAZA BASIN (SOUTHEASTERN SPAIN) Jorge Agusti,* Oriol Oms,t Miguel Gamest
and Josep M. Par&t
*Institute of Palaeontology ‘A4 Crusafont’, 08201 Sabadell, Spain tlnstitute of Earth Sciences ‘J. Almera’, CSIC, 08028 Barcelona, Spain
In this paper a survey of the correlationbetween magnetic polarity and the mammahan succession in the Plio-Pleistocene sequence of the Guadix-Baxa basin is presented.In this basin a complete continental sequence from the early Pliocene to the Middle Pleistocene is present, allowing the correlationof mammalian bioxones with the geomagnetic polarity scale. Two sections of the Baxa sector of the basin have been analyzed in orderto calibrate the Pliocene-Pleistocene transition:the Galera (middle Pliocene to early Pleistocene) and the Grce (late Pliocene to early Pleistocene). It is concluded that the boundarybetween the MN 15 (late Ruscinian) and the MN 16 (early Villanyian) is placed between the 2An. 3n chron and the 2An. In sub&ton. The late Villanyian (MN 17) extends up to the upperpartof chron 2r. Jr. The entry of the Pleistocene markerAllophuiomys pliocaenicus can be placed within subchron2n (Olduvai). Q 1997 INQUA/ Elsevier Science Ltd. All rights reserved.
INTRODUCTION
the middle member consists of sediments deposited on an alluvial plain. On the basis of its faunal content and geochemical composition (Anadon et al., 1987), the Upper Member was deposited in a slightly saline lake. Within the Baza Formation the Galera and Orce sections were chosen for a detailed calibration of the PlioPleistocene transition in the basin.
Most problems of Plio-Pleistocene continental biostratigraphy arise from the fact that mammalian biochronology has been based mainly on isolated localities which lack a good stratigraphic context. This applies particularly to terms such as ‘Villafranchian’ or ‘Villanyian’, commonly used by mammalian palaeontologists. In an attempt to resolve these problems, a joint programme of the Institute of Pa&ontology ‘M. Crusafont’ (Sabadell) and the Institute of Earth Sciences ‘J. Almera’ (CSICBarcelona) has been established in recent years. This programme has involved intensive palaeontological and palaeomagnetic sampling of long continental sections which contain a number of fossiliferous levels, instead of attempting correlations with well-known classical localities lacking a consistent stratigraphical context. The Guadix-Baza basin in southern Spain was chosen for study, since this depression provides large continuous outcrops of horizontal sediments ranging in age from early Pliocene to late Pleistocene. The basin is an intramontane depression located at the transition between the Internal and External Zones of the Alpine Betic Chain, and covers an area of 3000 km2 (Fig. 1). Its infilling occurs at about 1000 m a.s.l., and two thick lithostratigraphic units displaying different facies associations have been distinguished (Vera et al., 1985): a marginal unit, constituting the Guadix Formation alluvial sediments, with a large proportion of conglomerate, and a distal unit represented mainly by the lacustrine Baza Formation. Within the Baza Formation three lithostratigraphic units have been distinguished (Vera et al., 1985): a Lower Calcareous Member, a Middle Red Detrital Member and an Upper Silty Calcareous Member. The lower and upper members were characterized as lacustrine deposits, while
METHODOLOGY In order to achieve these goals, intensive palaeontological and palaeomagnetic sampling were carried out in the Galera and Orce sections. A standard sample of 5070 kg was taken from each potentially fossiliferous level and a mean of 1000 kg was sieved from each level which gave positive results. In the palaeomagnetic analysis, two measurements were made initially on each sample: natural remanent magnetism (NRM) and initial susceptibility. A GM-400 triaxial cryogenic magnetometer (Cryogenic Consultants Ltd.) was used to measure natural remanent magnetization as well as further remanent magnetization during progressive demagnetization. Bulk susceptibility was analyzed with a KLY-2 bridge (Geofyzika). Demagnetization was carried out using both the thermal method and alternating fields. Thermal cleaning was performed with a TSD-1 thermal specimen demagnetizer, while alternating field demagnetization was carried out on a GSD-5 tumbling-specimen demagnetizer (both from the Schonstedt Instrument Co.). During tbermal cleaning the KLY2 bridge was used to monitor any new mineral formation due to heating. The demagnetization strategy involved the characterization of the demagnetization behaviour for each lithology from a pilot suite. Where possible, sampling was focus& on fine grained lithologies and 93
94
J. Agusti et al.
FIG. 1. Geological map of the Guadix-Baza basin and location of the studied sections (modified from Anadh et al., 1987).
weakly weathered sediments. Some of the sediments could not be sampled because they were totally unconsolidated and highly weathered. In the Grce section, and similarly in the Galera section, the pilot suite of thermally cleaned specimens was treated using very small temperature increments, with 12 steps from 0 to 550°C. The alternating field pilot demagnetization suite involved up to 16 steps of 5 or 10 mT, from Natural Remanent Magnetization to 100 mT. A minimum of three specimens per site was demagnetized using both thermal and alternating field methods.
RESULTS
The Galera section The Galera section is a hundred meter thick sequence located in the eastern part of the basin where carbonates, evaporites and marls are dominant. Towards the top, this section is dominated by thinly bedded sands, marls and gypsum, culminating in conglomerates. In this section four fossiliferous levels have provided abundant micromammal faunas which allow correlation with the general mammalian sequence of the Neogene-Quatemary of Europe (Mein, 1975). The lowermost, Galera lC, occurs in the Lower Carbonatic Member of Vera et al. (1985). This level includes the following rodent fauna: Dolomys adroveri, Mimomys occitanus, Stephanomys donnezani, Apodemus dominans, Raghapodemus, sp., Castillomys crusafonti, Occitanomys brailloni, Eliomys intennedius and Pliopetaurista bressana. This association indicates a late Ruscinian age (MN 15 mammal unit of Mein, 1975). The most characteristic arvicolid species, Dolomys adroveri, has been found in several localities of this age in the Iberian peninsula, such as Grrios 3, Grrios 7 and Poblado Ib&ico in the Teruel basin (Fejfar et al., 1990), Laina (prov. Soria, NE Spain) and Asta Regia 3, in the Jerez basin (Aguirre et al., 1995; Castillo and Agusti, in
press). In terms of their morphology and degree of hypsodonty, the specimens from Galera 1C are very similar to those from Grrios 3 (with a moderately undulated linea sinuosa, see Fig. 2). The specimens of Stephanomys donnezani and Castillomys crusafonti from Galera 1C fall within the range of variability of the sample from the fissure infilling of Laina. The remaining faunal levels of the Galera section occur within the Upper Calcareous Member of Vera et al. (1985). Galera 1G has provided a small mammal association including the following rodent species: Mimomys medasensis, Mimomys cf. tomensis, Apoaknus dominans and CastiZZomyscrusafonti spp. This association indicates a late Villanyan age, similar to that of the Spanish locality of Islas Medas (Michaux, 1971). Some meters above this locality, Galera 2 has produced an abundant rodent fauna including Kislangia gusii, Mimomys medasensis, Occitanomys sp., Stephanomys cf. thaleri, Castillomys crusafonti spp. and Eliomys intermedius. This assemblage is closely comparable to those found in other late Villanyian localities in Spain, such as Valdeganga 34 and Ahnenara 1. Kishutgia gusii. from Galera 2 (Fig. 2) displays features very similar to the same species from the fissure infilling of Almenara 1 (Agusti et al., 1993). The few specimens recovered of Mimomys mea!asensis(Fig. 2) also fall within the range of variability of the population from Almenara 1 (EstebanAenlle and Lopez-Martinez, 1987). The uppermost fossiliferous level in the Galera section, Galera lH, contains a poorly diversified fauna in which most of the murids from previous levels, such as Stephanomys, are absent and only Castillomys crusafonti spp. and Apodemus dominans persist. Mimomys cf. reidi, the only arvicolid species present in this level, is a small sized, hypsodont species which still retains its primitive, mimomyan features in the young specimens. It strongly resembles, both in size and morphology, Mimomys cf. reidi from the late Villanyian or early Bilharian locality
Calibration of the Late Pliocene-Early Pleistocene Transition in Southeastern Spain
b d
e
f
h
g
FKi. 2. Lower first molars of the mom significant rodant species from the Orce and Galera sections. DoJofaysadrovcri from Galera 1C: (a) occhsal view; (II)labial view. bfhomys medawnsis fmm Galem 1G; (c) occlusal view; (d) labial view. Kidmgia g&i fbm Galera 2: (e) ~~~lusal view. Mimmys cf. reidi 6rom Galera 1I-k(f) owlusal view; (g) labial view. Allophaiomys pliocaenicus fmm Orce 7: (II) occlusal view. Graphic scale: 1 mm.
of Schambach (Koenigswald, 1977). However, it seems that the molars of specimens ftom this fissure infilhng are more hypsodont than those of Galera 1H. Another difference is that Schambach aheady includes some teeth of a rootless arvicolid assigned to? Allophaiomys sp. by Koenigswald (1977). The palaeomagnetism of the Galera section has been extensively reported in Garc6s et al. (in press), and a summary of the results is presented here. These results show that the lower part of the Galera section has three normal and reversed polarity zones. These are followed in the middle and upper Galera section by a major reversed polarity period, and, near the top, by a 10 m thick normal polarity zone (Fig. 3). As a control for the Galera section, we used the marinecontinental correlation established in the locality of Asta Regia in the Jerez basin (Aguirm et al., 1995). In this locality, late Ruscinian (MN 15) levels with a microvertebrate association similar to that of Galera 1C overlie marine marls with planktonic fomminifera belonging to the PL lc Zone of Berggren et al. (1985). In terms of this constraint the lower part of the Galera section (in which the level of Galera lc is included) fits well within part of the Gauss (2An) chron. The large reversed interval
containing the late Villanyian faunas of Galera 1G and Galera 2 is correlated with the lower part of the Matuyama (2r) chron (Fig. 4). The uppermost normal polarity zone in the Galera section is therefore correlated GALERASECTION
OftCE SECTION
LE&ND
l!!r Oral
.‘.
FIG. 3. Stratigraphic and palaemmgnetic sequences of the Galera and
Onx sections.
J. Agustf et al. DECLINATION
INCLINATION
VGP LAT.
POL.
li
LEGEND lithology reddish mudstones limestones dark mudstones
2
paieontological site polarity reversed I.-l normal
FIG. 4. L.ithological log of the Orce section with declination, inclination. VGP latitudes and polarity
with the Olduvai (2n) sub&on. Galera 1H is placed immediately below this interval, at the top of the lower part of the Matuyama (2r) chron (Fig. 4).
The Orce section In the Orce section, situated in the surroundings of Orce town, only the middle and upper members of the Baza Formation are present. In this section the two fossiliferous levels thus far recognized fall within the Upper Calcareous Member of Vera et al. (op. cit.; see Agusti et al., 1987a). The level of Orce 6 yielded scarce remains of large mammals assigned to Equus stenonis spp. and to a large Bovini (cf. Leptobos sp., Agusti et al., 1987a; Marfn, 1987). A palynological analysis was also carried out within this horizon, but the results were not statistically meaningful. However, the pollen association, composed of Abietaceae, Pinus sp., cf. Cuthuyu and Poaceae (Parra and Esteban, 1984) presents some interesting features, such as the persistence of subtropical elements including cf. Cuthuyu, also present in the Piacenzian of Rio Maior in Portugal (Din& 1984). The other fossiliferous level in the series, Orce 7, contains a much more significant faunal association composed of Allophuiomys pliocaenicus, Apodemus mystacinus, Apodemus sylvuticus and Castillomys crusufonti spp. Allophuiomys pliocaenicus from Orce 7 is a large sized form, very similar to the samples from Venta Micena (Agusti et al.,
1987b) and Pirro Not-d 1 (De Giuli and Torre, 1984). and is also very close to Allophuiomys ruffoi from Cava Sud. The anteroconid cap is simple and a very small BSA-4 (sensu Meulen, 1973) is often present giving a ‘nivaloid’ aspect to the teeth. The enamel is of the undifferentiated type or presents a slightly Micro&s type of differentiation (Fig. 2). In the same basin, truly microtoid or pitymyoid morphotypes appear in levels younger than Orce 7, as in Loma Quemada (Agusti, 1986) and Hu&car 2 (Maze et al., 1985). As in the case of the Galera section, a palaeomagnetic analysis has been carried out in the Orce section. The fust magnetostratigraphic reconnaissance in the Orce area was, in fact, carried out by F. Semah in order to determine whether any stable palaeomagnetic behaviour could be observed. No data were published as a result of this work, but an internal report revealed that no clear result could be obtained because most of the samples displayed intensities that were close to the background noise of the magnetometer used. This tirst attempt was restricted to the lower part of the Orce section, in which the Orce 2 and Grce 3 palaeomological sites are located (Agustf et al., 1987a). In 1992 further sampling was carried out in the Orce section to check whether stable demagnetization could be achieved using a cryogenic magnetometer. Cryogenic magnetometers are the most suitable devices for measuring rocks with weak magnetic intensity, such as most of those found in the Orce section. The preliminary laboratory analyses provided good results, encouraging further work.
Calibration of the Late Pliocene-Early Pleistocene Transition in Southeastern Spain
Definitive sampling was carried out throughout the section, but disruptions caused by a large number of faults led us to consider only those outcrops free from tectonic displacements. Only the topographically uppermost part of the Orce section was therefore studied, where the Orce 6 and Orce 7 localities are located (Fig. 4). Sampling involved the recovery of carefully oriented blocks from hard rocks (limestones), while soft material (mudstones) were cored in situ, using a portable manual pushing device (Lerbekmo, 1990). In the mudstones it was necessary to dig large holes, around 1 m wide, in or&r to reach fresh rock, as regolith was well developed. In the whole section 14 sites were sampled over 17 m of sediments, resulting in a mean sampling interval of 1.2 m (2.8 m in the widest case). From each block a minimum of three cores was obtained using an electrically powered drilling machine. Samples obtained with the pushing device required impregnation with sodium silicate to harden the soft materials. The results from Orce and similar sections in the same basin (Oms et al., 1994) indicated that thermal demagnetization would prove more effective, compared to the alternating field method, in isolating the primary magnetic component. In the end, two samples per site were demagnetized thermally and another was treated using the alternating field method. When this latter method was unsuccessful, further thermal demagnetization was carried out. The bulk of the samples were demagnetized in wide steps, but using smaller intervals where unblocking temperature became critical, which depended on lithology. With alternating field demagnetization, samples were heated to 120°C before starting the programme, in order to remove any possible contribution to remanent magnetism induced by high coercivity minerals (such as iron hydroxides) that would otherwise have remained. up, w
97
Natural Remanent Magnetization and initial susceptibility appeared to be more or less closely related to different kinds of lithology. Limestones displayed a natural remanent magnetism varying from 1.6~10-~ Am-’ to 51.8~10-~ Am“, with a mean value around 11.67x 10e5 Am-‘. Reddish mudstones ranged from 7.6x lo-’ Am-’ to 20.7x low5 Am-‘, with median values around 12.25 x lOA Am-‘. Dark mudstones varied from 8.6x lo-’ Am-’ to 42.6~10~~ Am-‘, having an average of 20.30x 10e5 Am-‘. The initial bulk susceptibility record does not correlate fully with the Natural Remanent Magnetization. Limestones ranged from 10.9 x 10B6 SI to 26.6 x 10d6 SI, with a mean value around 5.23~10~~ SI. Reddish mudstones varied from 99.95~10~~ to 142.4~10~~ SI, with a mean of 117.7~10-~ SI. Dark mudstones ranged from 112.0~10-~ SI to 137.3~10~~ SI, with a mean value of 124.3 x 10-6. In general, a relatively stable demagnetization behaviour was observed. The orthogonal demagnetization plots displayed no more than two components. Both normal and reversed polarities could be determined from the sampled materials. In normal polarity samples, only one component appears, as present-day remagnetization cannot be differentiated from the Neogene palaeopole for Iberia. In reverse polarity samples (see Fig. 5) two components could be determined. With thermal demagnetization one low temperature component and another associated with high temperature can be observed. The maximum unblocking temperature of each mineral phase belongs either to low (Ti) or high (Th) temperature components; Th is always below 350°C for most limestones and below 750°C for reddish mudstones, dark mudstones and some limestones (see Fig. 5). Once the T,, is reached, susceptibility increases sharply and magnetiTHERMAL
DEMAGNETIZATION
NRY INTENSllW
3.54XlF
Ah
FIG. 5. StandardZijderveld plot of a hermally demagnetizd limestone with n~ersed polarity. Demagnetization behaviour is simple, with two components. Note secondary viscous component.
J. Agustt et
98 ALTENNAllNQ FIELD DEMAcINETlZATtDN
N
6smT
\ 12 ml
NRMINTewsTTy: %0x10-Nm
%
PIG. 6. Demagnetization orthogonal plot of a limestone with normal polarity. It belongs to a sample d~magneti?.ed using the altemating field pro&we. It is important to note that specimens from the same Core provide. a very similar plot when demagnchd thermally.
zation becomes unstable, with demagnetization plots becoming randomized. Alternating field &magnetization turned out to be useless in the reddish and dark mudstones and could only be applied successfully to some limestones (see Fig. 6).
ii
Chrons
al.
Here, also, two components could be resolved when samples had reversed polarity, with a maximum unblocking low field (I$) always below 20 mT, while Fh is always less than 65 mT; this component could, however, never be fully removed. A generally smooth decrease in the remanent magnetization intensity was observed (Figs 5 and 6), suggesting a wide range of stability in the magnetic minerals. Some samples have a preponderance of minerals which demagnetize below 12O”C, as is evident in some limestones where this step involves a 77% drop in the remnant intensity, which can be attributed to the demagnetization of iron h&oxides. other minerals which demagnetized at higher temperatures could include sulfides and/or low unblocking temperature magnetite. Hematite is unequivocally present in both reddish and dark mudstones. Components of high temperature or high field are considered as characteristic components (or characteristic remanent magnetization), but no normal field stability test could be applied (such as the fold test, the reversal test, etc.). The virtual geomagnetic pole was calculated from the characteristic remanent component of all of the specimens treated from every sampling point, resulting in the latitudes shown in Fig. 5. The entire section is basically reversed in the lower part, having a normal interval towards the top. The remaining upper part of the section has a fully normal polarity record. On the basis of the results from the Galera section, the normal event at the top of the Grce section can have two
~okrity
In 1
1r.lb Ir lr.2 r
2
ig$g$) 3
! ! ! !
I. Gabmc
!
4 PIG. 7. Correlation of the late Pliocene-ear ly Pleistocene Mammal units with the Geomagnetic scale in terms of the results obtained in the sections of Galera, Chce and Chtcs de Baza.
Calibration of the Late Pliocene-Early Pleistocene Transition in Southeastern Spain
possible positions. A first possibility is that it corresponds to the Olduvai (2n) subchron. A second possibility is that the reverse to normal transition in the Orce section could correspond to the Matuyama-Brunhes boundary. However, Allophaiomys pliocaenicus from Orce 7 is a rather primitive form, with undifferentiated or slightly Microtusderived tooth enamel. In contrast, the upper levels of the Atapuerca section, placed at the Matuyama-Brunhes boundary (Par& and Pkrez-Gonzaez, 1995), contain a much more modem rodent association, with fully Microtus morphotypes, including Terricola gregaloides, Terricola arvalidens and Zberomys brecciensis. Moreover, in the same Guadix-Baza basin, the Cortes de Baza section, correlated with the upper Matuyama (Oms et al., 1994, Fig. 7) has yielded, from its upper levels, teeth with typically derived Micro&s enamel. Therefore, the normal interval in the Orce section, with its primitive Allophaiomys pliocaenicus, can only be correlated with normal episode 2n (Olduvai, Figs 3 and 7). This correlation conforms with the field relationships established between the Galera and Orce sections.
CONCLUDING REMARK!3 The results obtained in the Galera and Orce sections in the Baza sector of the basin show the following mammal units to be represented: MN 15 (Galera lC), MN 17 (Galera 2, Galera lG, Galera 1H) and the Allophaiomys pliocaenicus Zone (Orce 7). Allophaiomys pliocaenicus has been used traditionally as a biostratigraphical marker for the early Pleistocene in terrestrial Quatemary biochronology. Palaeomagnetic analyses of the two sections allows a calibration of the boundaries between these mammal units as follows: MN WMN 16 (Ruscinian/ViUmyian boundary): The level of Galera 1C is placed in the lower part of the 2An chron (2An.3n). On the other hand, truly Villanyan faunas with Equus are already recorded within the upper part of chron 2An (chron 2An.ln, Alberdi et al., 1982; Biquand et al., 1981). The boundary between the MN 15 and the MN 16 units can therefore be placed within the 2An chron, between the 2An.3n and the 2An.ln subchrons. MN 16/MN 17: The rodent association of Galera 1G is comparable with that found in other localities belonging to MN 17 with Mimomys tornensis (Osztramos 3; Deutsch Alterburg 3, 10, 2Cl and 4B; Almenara 1). The MN 16/ MN 17 boundary can therefore be placed in the lower part of the lower Matuyama (sub&on 2r.2r.). MN 17lAllophaiomys pliocaenicus Zone: The locality of Galera lH, in which Allophaiomys pliocaenicus is absent, is placed at the top of the lower Matuyama chron (2r.lr). In contrast, the locality of Orce 7, with Allophaiomys pliocaenicus, is placed at the base of the Olduvai subchron 2n. The entry of Allophaiomys pliocaenicus into the Guadix-Baza basin therefore almost coincides with the lower Matuyama/Olduvai boundary (2r.lr/2n boundary), and the dispersal of this arvicolid
99
species must be placed slightly below the presently accepted Pliocene-Pleistocene boundary, ACKNOWLEDGEMENTS This paper has been supported under project DGICYT-PB94-1265 (Spanish Ministry of Education and Science).
Aguirre, J., Castillo, C., Ferriz, F.J., Agusti, J. and Oms, 0. (1995). Marine-continental rnagnetobiostratigraphic correlation of the Dolomys subzone (middle late-Ruscinian): re-examination of the lateRuscinian lower boundary age. Palaeogeography, Palaeoclimatology, Pulaeoecology, 117, 139-152. Agusti, J. (1986). Synthkse biostratigraphique du Plio-Pleistodne de Guadix-Baza (province de Granada, sud-est de l’Espagne). Geobios, 19,4 505-510. Agustf, J., MoyB-Sol& S., Mattin-Stiz, E. and Marin, M. (1987a). Faunas de mamifems en el Pleistcceno inferior de la region de Orce (Granada, Espaila). Paleontologia i Evolucid, Memoria Especial, 1, 73-86. Agusti, J., Arbiol, S. and Martin-Sutkez, E. (1987b). Roedores y lagomorfos (Mammalia) de1 Pleistocene inferior de Venta Micena (depresion de Guadix-Baza, Granada). Puleonrologia i Evolucid, Memoria Especial, 1,95-107. Agusti, J., Galobatt, A. and Martin-Sukez, E. (1993). Kislangia qusii sp. nov., a new arvicolid (Rodentia) from the Late Pliocene of Spain. Scripta Geologica, 103, 119-134. Alberdi, M.T., Arias, C., Bigazzi, G., Bonadorma, F.P., Leone, G., L&z, N., Michaux, J., Morales, J., Robles, F. and Soria, Y.D. (1982). Nuevo yacimiento de moluscos y vertebrados de1 Villafranquiense de la cuenca de1 Jdcar (Albacete, Espaflia). ‘Le Villafranchien mkdircrranicn ‘, pp. 255-27 1. Lille. Anad&, P., Julia R., De Deckker, P., Rosso, J.-C. and Soulie-Marsche, I. (1987). ContribuciQ a la paleolimnologia de1 Pleistocene inferior de la cuenca de Baza (sector Orce-Venta Micena). Paleonrologia i Evolwzid, Memoria Especial, 1, 35-72. Berggren, W.A., Kent, D.V., Flynn, J.J. and Van Couvering, J.A. (1985). Cenozoic geochronology. Geological Society of America Bulletin, 96, 1407-1418. Biquand, D., Cassignol, C., Chanbaudet, A. and Couthures, J. (1981). Nouvelles dotmks chronostratigraphiques concernant les d&&s lacustres de Vialette (Haute-Loire). Bulletin Associarion Frawaise Etudes du Quaremaire. 2, 83-87. Castillo, C. and Agustf, J. (in press). Early Pliocene rodents (Mammalia) from Asta Regia (Jerez basin, South-western Spain). Proc. Ken. Akad. Werensch., Ser. B. Utrecht. Diniz, F. (1984). Apports de la palynologie B la conaissance du Plio&ne portugais. Rio maior: un bassin de r6ference pour l’histoire de la flore, de la vegetation et du climat de la fae atlantique de 1’Europe meridionale. Ph.D. Thesis, Univ. SC. et Tech. Languedoc, 230 pp. Montpellier. Esteban-Aenlle, J. and L&z-Martinez, N. (1987). Les Arvicolidts (Rodentia, Mammalia) du Villanyen ¢ de Casablanca 1 (Castell6n. Espagne). Geobios, 20, 5 591623. Fejfar, O., Mein, P. and Moissenef E. (1990). Early arvicolids from the Ruscinian (Early Pliocene) of the Temel basin, Spain. Int. Symp. Evol. Phyl. Biosfr. Arvicolids, 133-164. Garcts, M., Agusd, J. and Parks, J.M. (in press). Late Pliocene Continental Chronology from the Guadix-Baza basin (Betics, Spain). Earth and Planetary Science Letters. Koenigswald, W.V. (1977). Mimomys cf. reidi aus der villefranchis&en Spaltenfl8lung Schambach bei Treuchtlingen. Min. Bayer. S&at. Pal#ont, 17, 197-212. Lerbekmo, J.F. (1990). A tool for obtaining oriented samples of weakly to moderately indurated sedimentary rocks for palaeomagnetic measurements. Sedimentary Geology, 66, 295-299. Matfn, J. (1987). Equus srenonis gmnatensis en el Pleistocene inferior de Venta Micena (Gmnada, Espaiia). Paleonrologia i Evoluci6, Memoria Especial, 1, 255-282. Maze, A.V., Se&, C., Ruiz Bustos, A. and Pea J.A. (1985). Geologia y paleontologia de 10s yacimientos plio-pleistocenos de H&car (depresibn de Guadix-Baza Granada). Estudios Geoldgicos, 41, 467-493.
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Mein, P. (1975). Proposition de Biozontaion du Ntogbne M&litemu&n a partir des Mammikes. Prep. on Activity ofthe R.Ch4.N.S. working groups (1971~1975), pp. 18-81. Bratislava. Meulen, A. van der (1973). Middle Pleistocene smaller Mammals from the Monte Peglia (Orvieto, Italy) with special reference to the phylogeny of Microru.s (Arvicolidae, Rodentia). Quotemaria, 17,144 PP. Michaux, J. (1971). Arvicolinae (Rodentia) du Plioc&ne terminal et du Quatemaire ancien de France et d’Bspagne. Palaeovertcbruta, 4, 5 137-214. Oms, 0.. Garcts, M., Pan%, J.M., Agusti, J., Anad& P. and JuliB, R. (1994). Magnetostratigraphic characterization of a thick Lower Pleistocene lacustie sequence from the Baza Basin (Betic Chain, Southern Spain). Physics of theEarth and Planetary Interiors, 85.
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QuaternaryInternational,Vol. 40, pp. 93-100,1997. Q 1997 INQUA/ Elsevier Science Ltd All rightsreserved.Printedin GreatBritain. 1040-6182/97 $32.00
CALIBRATION OF THE LATE PLIOCENE-EARLY PLEISTOCENE TRANSITION IN THE CONTINENTAL BEDS OF THE GUADIX-BAZA BASIN (SOUTHEASTERN SPAIN) Jorge Agusti,* Oriol Oms,t Miguel Gamest
and Josep M. Par&t
*Institute of Palaeontology ‘A4 Crusafont’, 08201 Sabadell, Spain tlnstitute of Earth Sciences ‘J. Almera’, CSIC, 08028 Barcelona, Spain
In this paper a survey of the correlationbetween magnetic polarity and the mammahan succession in the Plio-Pleistocene sequence of the Guadix-Baxa basin is presented.In this basin a complete continental sequence from the early Pliocene to the Middle Pleistocene is present, allowing the correlationof mammalian bioxones with the geomagnetic polarity scale. Two sections of the Baxa sector of the basin have been analyzed in orderto calibrate the Pliocene-Pleistocene transition:the Galera (middle Pliocene to early Pleistocene) and the Grce (late Pliocene to early Pleistocene). It is concluded that the boundarybetween the MN 15 (late Ruscinian) and the MN 16 (early Villanyian) is placed between the 2An. 3n chron and the 2An. In sub&ton. The late Villanyian (MN 17) extends up to the upperpartof chron 2r. Jr. The entry of the Pleistocene markerAllophuiomys pliocaenicus can be placed within subchron2n (Olduvai). Q 1997 INQUA/ Elsevier Science Ltd. All rights reserved.
INTRODUCTION
the middle member consists of sediments deposited on an alluvial plain. On the basis of its faunal content and geochemical composition (Anadon et al., 1987), the Upper Member was deposited in a slightly saline lake. Within the Baza Formation the Galera and Orce sections were chosen for a detailed calibration of the PlioPleistocene transition in the basin.
Most problems of Plio-Pleistocene continental biostratigraphy arise from the fact that mammalian biochronology has been based mainly on isolated localities which lack a good stratigraphic context. This applies particularly to terms such as ‘Villafranchian’ or ‘Villanyian’, commonly used by mammalian palaeontologists. In an attempt to resolve these problems, a joint programme of the Institute of Pa&ontology ‘M. Crusafont’ (Sabadell) and the Institute of Earth Sciences ‘J. Almera’ (CSICBarcelona) has been established in recent years. This programme has involved intensive palaeontological and palaeomagnetic sampling of long continental sections which contain a number of fossiliferous levels, instead of attempting correlations with well-known classical localities lacking a consistent stratigraphical context. The Guadix-Baza basin in southern Spain was chosen for study, since this depression provides large continuous outcrops of horizontal sediments ranging in age from early Pliocene to late Pleistocene. The basin is an intramontane depression located at the transition between the Internal and External Zones of the Alpine Betic Chain, and covers an area of 3000 km2 (Fig. 1). Its infilling occurs at about 1000 m a.s.l., and two thick lithostratigraphic units displaying different facies associations have been distinguished (Vera et al., 1985): a marginal unit, constituting the Guadix Formation alluvial sediments, with a large proportion of conglomerate, and a distal unit represented mainly by the lacustrine Baza Formation. Within the Baza Formation three lithostratigraphic units have been distinguished (Vera et al., 1985): a Lower Calcareous Member, a Middle Red Detrital Member and an Upper Silty Calcareous Member. The lower and upper members were characterized as lacustrine deposits, while
METHODOLOGY In order to achieve these goals, intensive palaeontological and palaeomagnetic sampling were carried out in the Galera and Orce sections. A standard sample of 5070 kg was taken from each potentially fossiliferous level and a mean of 1000 kg was sieved from each level which gave positive results. In the palaeomagnetic analysis, two measurements were made initially on each sample: natural remanent magnetism (NRM) and initial susceptibility. A GM-400 triaxial cryogenic magnetometer (Cryogenic Consultants Ltd.) was used to measure natural remanent magnetization as well as further remanent magnetization during progressive demagnetization. Bulk susceptibility was analyzed with a KLY-2 bridge (Geofyzika). Demagnetization was carried out using both the thermal method and alternating fields. Thermal cleaning was performed with a TSD-1 thermal specimen demagnetizer, while alternating field demagnetization was carried out on a GSD-5 tumbling-specimen demagnetizer (both from the Schonstedt Instrument Co.). During tbermal cleaning the KLY2 bridge was used to monitor any new mineral formation due to heating. The demagnetization strategy involved the characterization of the demagnetization behaviour for each lithology from a pilot suite. Where possible, sampling was focus& on fine grained lithologies and 93
94
J. Agusti et al.
FIG. 1. Geological map of the Guadix-Baza basin and location of the studied sections (modified from Anadh et al., 1987).
weakly weathered sediments. Some of the sediments could not be sampled because they were totally unconsolidated and highly weathered. In the Grce section, and similarly in the Galera section, the pilot suite of thermally cleaned specimens was treated using very small temperature increments, with 12 steps from 0 to 550°C. The alternating field pilot demagnetization suite involved up to 16 steps of 5 or 10 mT, from Natural Remanent Magnetization to 100 mT. A minimum of three specimens per site was demagnetized using both thermal and alternating field methods.
RESULTS
The Galera section The Galera section is a hundred meter thick sequence located in the eastern part of the basin where carbonates, evaporites and marls are dominant. Towards the top, this section is dominated by thinly bedded sands, marls and gypsum, culminating in conglomerates. In this section four fossiliferous levels have provided abundant micromammal faunas which allow correlation with the general mammalian sequence of the Neogene-Quatemary of Europe (Mein, 1975). The lowermost, Galera lC, occurs in the Lower Carbonatic Member of Vera et al. (1985). This level includes the following rodent fauna: Dolomys adroveri, Mimomys occitanus, Stephanomys donnezani, Apodemus dominans, Raghapodemus, sp., Castillomys crusafonti, Occitanomys brailloni, Eliomys intennedius and Pliopetaurista bressana. This association indicates a late Ruscinian age (MN 15 mammal unit of Mein, 1975). The most characteristic arvicolid species, Dolomys adroveri, has been found in several localities of this age in the Iberian peninsula, such as Grrios 3, Grrios 7 and Poblado Ib&ico in the Teruel basin (Fejfar et al., 1990), Laina (prov. Soria, NE Spain) and Asta Regia 3, in the Jerez basin (Aguirre et al., 1995; Castillo and Agusti, in
press). In terms of their morphology and degree of hypsodonty, the specimens from Galera 1C are very similar to those from Grrios 3 (with a moderately undulated linea sinuosa, see Fig. 2). The specimens of Stephanomys donnezani and Castillomys crusafonti from Galera 1C fall within the range of variability of the sample from the fissure infilling of Laina. The remaining faunal levels of the Galera section occur within the Upper Calcareous Member of Vera et al. (1985). Galera 1G has provided a small mammal association including the following rodent species: Mimomys medasensis, Mimomys cf. tomensis, Apoaknus dominans and CastiZZomyscrusafonti spp. This association indicates a late Villanyan age, similar to that of the Spanish locality of Islas Medas (Michaux, 1971). Some meters above this locality, Galera 2 has produced an abundant rodent fauna including Kislangia gusii, Mimomys medasensis, Occitanomys sp., Stephanomys cf. thaleri, Castillomys crusafonti spp. and Eliomys intermedius. This assemblage is closely comparable to those found in other late Villanyian localities in Spain, such as Valdeganga 34 and Ahnenara 1. Kishutgia gusii. from Galera 2 (Fig. 2) displays features very similar to the same species from the fissure infilling of Almenara 1 (Agusti et al., 1993). The few specimens recovered of Mimomys mea!asensis(Fig. 2) also fall within the range of variability of the population from Almenara 1 (EstebanAenlle and Lopez-Martinez, 1987). The uppermost fossiliferous level in the Galera section, Galera lH, contains a poorly diversified fauna in which most of the murids from previous levels, such as Stephanomys, are absent and only Castillomys crusafonti spp. and Apodemus dominans persist. Mimomys cf. reidi, the only arvicolid species present in this level, is a small sized, hypsodont species which still retains its primitive, mimomyan features in the young specimens. It strongly resembles, both in size and morphology, Mimomys cf. reidi from the late Villanyian or early Bilharian locality
Calibration of the Late Pliocene-Early Pleistocene Transition in Southeastern Spain
b d
e
f
h
g
FKi. 2. Lower first molars of the mom significant rodant species from the Orce and Galera sections. DoJofaysadrovcri from Galera 1C: (a) occhsal view; (II)labial view. bfhomys medawnsis fmm Galem 1G; (c) occlusal view; (d) labial view. Kidmgia g&i fbm Galera 2: (e) ~~~lusal view. Mimmys cf. reidi 6rom Galera 1I-k(f) owlusal view; (g) labial view. Allophaiomys pliocaenicus fmm Orce 7: (II) occlusal view. Graphic scale: 1 mm.
of Schambach (Koenigswald, 1977). However, it seems that the molars of specimens ftom this fissure infilhng are more hypsodont than those of Galera 1H. Another difference is that Schambach aheady includes some teeth of a rootless arvicolid assigned to? Allophaiomys sp. by Koenigswald (1977). The palaeomagnetism of the Galera section has been extensively reported in Garc6s et al. (in press), and a summary of the results is presented here. These results show that the lower part of the Galera section has three normal and reversed polarity zones. These are followed in the middle and upper Galera section by a major reversed polarity period, and, near the top, by a 10 m thick normal polarity zone (Fig. 3). As a control for the Galera section, we used the marinecontinental correlation established in the locality of Asta Regia in the Jerez basin (Aguirm et al., 1995). In this locality, late Ruscinian (MN 15) levels with a microvertebrate association similar to that of Galera 1C overlie marine marls with planktonic fomminifera belonging to the PL lc Zone of Berggren et al. (1985). In terms of this constraint the lower part of the Galera section (in which the level of Galera lc is included) fits well within part of the Gauss (2An) chron. The large reversed interval
containing the late Villanyian faunas of Galera 1G and Galera 2 is correlated with the lower part of the Matuyama (2r) chron (Fig. 4). The uppermost normal polarity zone in the Galera section is therefore correlated GALERASECTION
OftCE SECTION
LE&ND
l!!r Oral
.‘.
FIG. 3. Stratigraphic and palaemmgnetic sequences of the Galera and
Onx sections.
J. Agustf et al. DECLINATION
INCLINATION
VGP LAT.
POL.
li
LEGEND lithology reddish mudstones limestones dark mudstones
2
paieontological site polarity reversed I.-l normal
FIG. 4. L.ithological log of the Orce section with declination, inclination. VGP latitudes and polarity
with the Olduvai (2n) sub&on. Galera 1H is placed immediately below this interval, at the top of the lower part of the Matuyama (2r) chron (Fig. 4).
The Orce section In the Orce section, situated in the surroundings of Orce town, only the middle and upper members of the Baza Formation are present. In this section the two fossiliferous levels thus far recognized fall within the Upper Calcareous Member of Vera et al. (op. cit.; see Agusti et al., 1987a). The level of Orce 6 yielded scarce remains of large mammals assigned to Equus stenonis spp. and to a large Bovini (cf. Leptobos sp., Agusti et al., 1987a; Marfn, 1987). A palynological analysis was also carried out within this horizon, but the results were not statistically meaningful. However, the pollen association, composed of Abietaceae, Pinus sp., cf. Cuthuyu and Poaceae (Parra and Esteban, 1984) presents some interesting features, such as the persistence of subtropical elements including cf. Cuthuyu, also present in the Piacenzian of Rio Maior in Portugal (Din& 1984). The other fossiliferous level in the series, Orce 7, contains a much more significant faunal association composed of Allophuiomys pliocaenicus, Apodemus mystacinus, Apodemus sylvuticus and Castillomys crusufonti spp. Allophuiomys pliocaenicus from Orce 7 is a large sized form, very similar to the samples from Venta Micena (Agusti et al.,
1987b) and Pirro Not-d 1 (De Giuli and Torre, 1984). and is also very close to Allophuiomys ruffoi from Cava Sud. The anteroconid cap is simple and a very small BSA-4 (sensu Meulen, 1973) is often present giving a ‘nivaloid’ aspect to the teeth. The enamel is of the undifferentiated type or presents a slightly Micro&s type of differentiation (Fig. 2). In the same basin, truly microtoid or pitymyoid morphotypes appear in levels younger than Orce 7, as in Loma Quemada (Agusti, 1986) and Hu&car 2 (Maze et al., 1985). As in the case of the Galera section, a palaeomagnetic analysis has been carried out in the Orce section. The fust magnetostratigraphic reconnaissance in the Orce area was, in fact, carried out by F. Semah in order to determine whether any stable palaeomagnetic behaviour could be observed. No data were published as a result of this work, but an internal report revealed that no clear result could be obtained because most of the samples displayed intensities that were close to the background noise of the magnetometer used. This tirst attempt was restricted to the lower part of the Orce section, in which the Orce 2 and Grce 3 palaeomological sites are located (Agustf et al., 1987a). In 1992 further sampling was carried out in the Orce section to check whether stable demagnetization could be achieved using a cryogenic magnetometer. Cryogenic magnetometers are the most suitable devices for measuring rocks with weak magnetic intensity, such as most of those found in the Orce section. The preliminary laboratory analyses provided good results, encouraging further work.
Calibration of the Late Pliocene-Early Pleistocene Transition in Southeastern Spain
Definitive sampling was carried out throughout the section, but disruptions caused by a large number of faults led us to consider only those outcrops free from tectonic displacements. Only the topographically uppermost part of the Orce section was therefore studied, where the Orce 6 and Orce 7 localities are located (Fig. 4). Sampling involved the recovery of carefully oriented blocks from hard rocks (limestones), while soft material (mudstones) were cored in situ, using a portable manual pushing device (Lerbekmo, 1990). In the mudstones it was necessary to dig large holes, around 1 m wide, in or&r to reach fresh rock, as regolith was well developed. In the whole section 14 sites were sampled over 17 m of sediments, resulting in a mean sampling interval of 1.2 m (2.8 m in the widest case). From each block a minimum of three cores was obtained using an electrically powered drilling machine. Samples obtained with the pushing device required impregnation with sodium silicate to harden the soft materials. The results from Orce and similar sections in the same basin (Oms et al., 1994) indicated that thermal demagnetization would prove more effective, compared to the alternating field method, in isolating the primary magnetic component. In the end, two samples per site were demagnetized thermally and another was treated using the alternating field method. When this latter method was unsuccessful, further thermal demagnetization was carried out. The bulk of the samples were demagnetized in wide steps, but using smaller intervals where unblocking temperature became critical, which depended on lithology. With alternating field demagnetization, samples were heated to 120°C before starting the programme, in order to remove any possible contribution to remanent magnetism induced by high coercivity minerals (such as iron hydroxides) that would otherwise have remained. up, w
97
Natural Remanent Magnetization and initial susceptibility appeared to be more or less closely related to different kinds of lithology. Limestones displayed a natural remanent magnetism varying from 1.6~10-~ Am-’ to 51.8~10-~ Am“, with a mean value around 11.67x 10e5 Am-‘. Reddish mudstones ranged from 7.6x lo-’ Am-’ to 20.7x low5 Am-‘, with median values around 12.25 x lOA Am-‘. Dark mudstones varied from 8.6x lo-’ Am-’ to 42.6~10~~ Am-‘, having an average of 20.30x 10e5 Am-‘. The initial bulk susceptibility record does not correlate fully with the Natural Remanent Magnetization. Limestones ranged from 10.9 x 10B6 SI to 26.6 x 10d6 SI, with a mean value around 5.23~10~~ SI. Reddish mudstones varied from 99.95~10~~ to 142.4~10~~ SI, with a mean of 117.7~10-~ SI. Dark mudstones ranged from 112.0~10-~ SI to 137.3~10~~ SI, with a mean value of 124.3 x 10-6. In general, a relatively stable demagnetization behaviour was observed. The orthogonal demagnetization plots displayed no more than two components. Both normal and reversed polarities could be determined from the sampled materials. In normal polarity samples, only one component appears, as present-day remagnetization cannot be differentiated from the Neogene palaeopole for Iberia. In reverse polarity samples (see Fig. 5) two components could be determined. With thermal demagnetization one low temperature component and another associated with high temperature can be observed. The maximum unblocking temperature of each mineral phase belongs either to low (Ti) or high (Th) temperature components; Th is always below 350°C for most limestones and below 750°C for reddish mudstones, dark mudstones and some limestones (see Fig. 5). Once the T,, is reached, susceptibility increases sharply and magnetiTHERMAL
DEMAGNETIZATION
NRY INTENSllW
3.54XlF
Ah
FIG. 5. StandardZijderveld plot of a hermally demagnetizd limestone with n~ersed polarity. Demagnetization behaviour is simple, with two components. Note secondary viscous component.
J. Agustt et
98 ALTENNAllNQ FIELD DEMAcINETlZATtDN
N
6smT
\ 12 ml
NRMINTewsTTy: %0x10-Nm
%
PIG. 6. Demagnetization orthogonal plot of a limestone with normal polarity. It belongs to a sample d~magneti?.ed using the altemating field pro&we. It is important to note that specimens from the same Core provide. a very similar plot when demagnchd thermally.
zation becomes unstable, with demagnetization plots becoming randomized. Alternating field &magnetization turned out to be useless in the reddish and dark mudstones and could only be applied successfully to some limestones (see Fig. 6).
ii
Chrons
al.
Here, also, two components could be resolved when samples had reversed polarity, with a maximum unblocking low field (I$) always below 20 mT, while Fh is always less than 65 mT; this component could, however, never be fully removed. A generally smooth decrease in the remanent magnetization intensity was observed (Figs 5 and 6), suggesting a wide range of stability in the magnetic minerals. Some samples have a preponderance of minerals which demagnetize below 12O”C, as is evident in some limestones where this step involves a 77% drop in the remnant intensity, which can be attributed to the demagnetization of iron h&oxides. other minerals which demagnetized at higher temperatures could include sulfides and/or low unblocking temperature magnetite. Hematite is unequivocally present in both reddish and dark mudstones. Components of high temperature or high field are considered as characteristic components (or characteristic remanent magnetization), but no normal field stability test could be applied (such as the fold test, the reversal test, etc.). The virtual geomagnetic pole was calculated from the characteristic remanent component of all of the specimens treated from every sampling point, resulting in the latitudes shown in Fig. 5. The entire section is basically reversed in the lower part, having a normal interval towards the top. The remaining upper part of the section has a fully normal polarity record. On the basis of the results from the Galera section, the normal event at the top of the Grce section can have two
~okrity
In 1
1r.lb Ir lr.2 r
2
ig$g$) 3
! ! ! !
I. Gabmc
!
4 PIG. 7. Correlation of the late Pliocene-ear ly Pleistocene Mammal units with the Geomagnetic scale in terms of the results obtained in the sections of Galera, Chce and Chtcs de Baza.
Calibration of the Late Pliocene-Early Pleistocene Transition in Southeastern Spain
possible positions. A first possibility is that it corresponds to the Olduvai (2n) subchron. A second possibility is that the reverse to normal transition in the Orce section could correspond to the Matuyama-Brunhes boundary. However, Allophaiomys pliocaenicus from Orce 7 is a rather primitive form, with undifferentiated or slightly Microtusderived tooth enamel. In contrast, the upper levels of the Atapuerca section, placed at the Matuyama-Brunhes boundary (Par& and Pkrez-Gonzaez, 1995), contain a much more modem rodent association, with fully Microtus morphotypes, including Terricola gregaloides, Terricola arvalidens and Zberomys brecciensis. Moreover, in the same Guadix-Baza basin, the Cortes de Baza section, correlated with the upper Matuyama (Oms et al., 1994, Fig. 7) has yielded, from its upper levels, teeth with typically derived Micro&s enamel. Therefore, the normal interval in the Orce section, with its primitive Allophaiomys pliocaenicus, can only be correlated with normal episode 2n (Olduvai, Figs 3 and 7). This correlation conforms with the field relationships established between the Galera and Orce sections.
CONCLUDING REMARK!3 The results obtained in the Galera and Orce sections in the Baza sector of the basin show the following mammal units to be represented: MN 15 (Galera lC), MN 17 (Galera 2, Galera lG, Galera 1H) and the Allophaiomys pliocaenicus Zone (Orce 7). Allophaiomys pliocaenicus has been used traditionally as a biostratigraphical marker for the early Pleistocene in terrestrial Quatemary biochronology. Palaeomagnetic analyses of the two sections allows a calibration of the boundaries between these mammal units as follows: MN WMN 16 (Ruscinian/ViUmyian boundary): The level of Galera 1C is placed in the lower part of the 2An chron (2An.3n). On the other hand, truly Villanyan faunas with Equus are already recorded within the upper part of chron 2An (chron 2An.ln, Alberdi et al., 1982; Biquand et al., 1981). The boundary between the MN 15 and the MN 16 units can therefore be placed within the 2An chron, between the 2An.3n and the 2An.ln subchrons. MN 16/MN 17: The rodent association of Galera 1G is comparable with that found in other localities belonging to MN 17 with Mimomys tornensis (Osztramos 3; Deutsch Alterburg 3, 10, 2Cl and 4B; Almenara 1). The MN 16/ MN 17 boundary can therefore be placed in the lower part of the lower Matuyama (sub&on 2r.2r.). MN 17lAllophaiomys pliocaenicus Zone: The locality of Galera lH, in which Allophaiomys pliocaenicus is absent, is placed at the top of the lower Matuyama chron (2r.lr). In contrast, the locality of Orce 7, with Allophaiomys pliocaenicus, is placed at the base of the Olduvai subchron 2n. The entry of Allophaiomys pliocaenicus into the Guadix-Baza basin therefore almost coincides with the lower Matuyama/Olduvai boundary (2r.lr/2n boundary), and the dispersal of this arvicolid
99
species must be placed slightly below the presently accepted Pliocene-Pleistocene boundary, ACKNOWLEDGEMENTS This paper has been supported under project DGICYT-PB94-1265 (Spanish Ministry of Education and Science).
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