plio-pleistocene boundary-guadix-baza-orce-spain

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0277-3791/$ - se

doi:10.1016/j.qu

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Quaternary Science Reviews 25 (2006) 507–525

Evaluation of the Olduvai subchron in the Orce ravine (SE Spain).Implications for Plio-Pleistocene mammal biostratigraphy and the

age of Orce archeological sites

L. Giberta,�, G. Scottb, C. Ferrandez-Canadellc

aDepartment Enginyeria Minera i Recursos Naturals, Universitat Politecnica de Catalunya, Farinera 2, 08211 Castellar del Valles, Barcelona, SpainbBerkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA

cDepartament d’Estratigrafia, Paleontologia i Geociencies Marines, Facultat de Geologia, Universitat de Barcelona, Martı Franques s/n,

E-08028 Barcelona, Spain

Received 9 June 2003; accepted 4 March 2005

Abstract

The Barranco de Orce (BO) section in the Baza basin (SE Spain) exposes several fossiliferous layers (O-1 to O-7) with

Plio–Pleistocene micro- and macromammals. Biostratigraphic and magnetostratigraphic data from this and other sections in the

basin have been extensively used to calibrate the Plio–Pleistocene chronology based on mammal biozonations. Because of its

stratigraphic and geographic proximity, the BO section has also been used to date the paleontological and archeological sites of

Barranco Leon, Fuentenueva-3 and Venta Micena. This study shows that the BO section crosses a mega-landslide that produces

partial repetitions of the sedimentary sequence. The seven fossiliferous layers are actually the repetition of only two (O-6 and O-7)

which are found in situ in the upper part of the ravine. New paleomagnetic results demonstrate the presence of Reverse

magnetization throughout this section, contradicting the Normal event previously reported and assigned to the ‘Olduvai’ subchron

(C2n). Published and new magnetostratigraphic data show that all archeological and paleontological sites in the Orce area are

within a Reverse magnetochron, presumably C1r.2r (late Matuyama). The use of BO in the magnetobiostratigraphical calibration of

the Pliocene/Pleistocene boundary for western Europe is not advised.

r 2005 Elsevier Ltd. All rights reserved.

1. Introduction

The Guadix-Baza basin, in the Betic ranges (SESpain), exposes a thick sequence of Plio–Pleistocenecontinental deposits, rich in fossil mammal sites. Arecent paper by Alba et al. (2001) has shown, usingPliocene and Pleistocene data from this and otherbasins, that the Neogene mammalian fossil record ofthe Iberian Peninsula is very complete, more than 75%at the specific level, and 90% at the generic one. Becauseof its continuous sedimentary record from the earlyPliocene to the Middle Pleistocene, together with the

e front matter r 2005 Elsevier Ltd. All rights reserved.

ascirev.2005.03.006

ing author.

resses: [email protected] (L. Gibert), gscott@

tt), [email protected] (C. Ferrandez-Canadell).

abundance of rich fossil sites, the Guadix-Baza basinhas been proposed as a parastratotype area for thePliocene–Pleistocene boundary in continental sediments(Aguirre, 1997b).

After a few preliminary studies in the 1960s and1970s, the associations of both micro- and macrofaunafrom the Baza basin were studied in the last twodecades. These studies have produced an extensiveliterature on micromammal systematics, biostratigra-phy, and Neogene faunal replacements (e.g. Agustı,1984, 1986a,b, 1990, 1991, 1998; Agustı et al., 1986,1987a–c, 1989, 1993, 2001; Alberdi et al., 1989; Agustıand Moya-Sola, 1991, 1992, 1998; Marchetti and Sala,2001). Micromammal associations have also been usedto calibrate magnetostratigraphic studies developed inthis region (Oms et al., 1994, 1996, 1999, 2000a,b; Agustı

ARTICLE IN PRESSL. Gibert et al. / Quaternary Science Reviews 25 (2006) 507–525508

et al., 1997; Garces et al., 1997; Oms, 1998). Asignificant effort was made to establish micromammalbiostratigraphic criteria to recognize the Plio–Pleisto-cene boundary (e.g. documenting the first occurrence ofthe Pleistocene marker Allophaiomys pliocaenicus), andto integrate the biostratigraphical data into the strati-graphic and magnetostratigrahic framework (Agustı,1984, 1986a,b, 1991, 2001; Agustı et al., 1986, 1987a,b,c,2001; Agustı and Moya-Sola, 1991, 1992). Part of thedata used in these magnetobiostratigraphical paperscomes from the Barranco de Orce (BO) section.

Three of the calibrated boundary sections forNeogene mammals of western Europe proposed byAgustı et al. (2001) are in the Guadix-Baza basin. TheBO section was specifically designated by these authorsas the continental boundary section for the Tertiary/Quaternary mammalian faunas for western Europe(Table 1 in Agustı et al., 2001). Additionally, biostrati-graphic and magnetostratigraphic data from the BOsection have also been used to discuss paleoclimaticevents in paleoenvironmental studies of the lacustrinesequence (e.g. Agustı and Julia, 1990; Anadon et al.,1994). However, these efforts to extend the biostrati-graphic and magnetostratigraphic data from the Bar-ranco the Orce sites (O-1 to O-7) into neighboringsections, and to use the data in the calibration ofthe micromammal chronobiostratigraphy for the Plio–Pleistocene boundary have been frustrated by numerouschanges in taxonomic assignments, biostratigraphicalzonations and magnetostratigraphical interpretations.

In this paper, we present a significant revision of thestratigraphy of the BO section, as corrected bystructural analysis of a large landslide present in thisarea. We include new paleomagnetic data that changethe previous magnetostratigraphic interpretation of theBO section and its correlation different sites in this partof the Baza basin. Also, we present a summary of theextensive micromammal literature from the BO sectionand argue that the low number of specimens togetherwith the unrecognized repetition of the BO fossiliferouslayers has led to misinterpretations of faunal associa-tions and, therefore, biozones.

2. Geological setting

Neotectonic thrusting and uplift created and isolatedthe Guadix-Baza Basin from the sea in the late Miocene(Estevez and Sanz de Galdeano, 1983; Sanz deGaldeano and Vera, 1992). This large basin consists oftwo sub-basins, Guadix in the SW and Baza in the NE,separated by the Jabalcon mountain. In the Guadixbasin the deposits are basically alluvial with minorinfluence of Palustrine (Viseras, 1991; Vera et al., 1994).The Baza basin was mainly infilled with evaporiticlacustrine sediments in the central area and palustrine

deposits in the margins, having less influence of alluvialdeposits. The studied area is located in the NE sector ofthe Baza basin near the town of Orce (Fig. 1). This areaexposes near 100m of late Pliocene and Quaternaryalluvial and palustrine deposits. The sediments of Orcearea (Gibert, L. et al., 1998, 1999b) include thearcheological sites of Venta Micena, Barranco Leon-5and Fuentenueva-3, which have yielded human remainsand lithic artefacts, together with a rich mammalassociation (Campillo, 1989, 2002; Gibert, J. et al.,1994, 1998, 1999a,b, 2001; Borja et al., 1997; Tobias,1998; Roe, 1995). The studied BO section follows theOrce ravine, with its head located in the Jurassic Sierrade Orce (Subbetic zone of the Betic range) and cutting50m of Neogene sedimentary succession.

3. Structure of the Orce ravine

A preliminary study about the disrupted structure ofthe Orce ravine and its biostratigraphical interpretationswas provided in 1995 (Gibert, L. et al., 1995, 1999a). In1997, when the first paleomagnetic results of this sectionwere published, the authors wrote: ‘‘Definitive samplingwas carried out throughout the section, but disruptionscaused by a large number of faults led us to consideronly those outcrops free from tectonic displacements.Only the topographically uppermost part of the Orcesection was therefore studied, where the O-6 and O-7localities are located’’ (Agustı et al., 1997).

The Orce ravine crosses a lobe morphology 1.5 kmlong and 0.5 km wide, which develops northwards intothe main E–W Velez valley (Figs. 2 and 3). Except forthe uppermost part, the materials of the entire ravineshow different types of structures that reveal movementof the strata due to a mega-landslide. The main listricfault, which separates the in situ southern deposits fromthe slump lobe, can be followed for more than 1.5 kmbetween the adjacent Fuentecica and Leon ravines(Fig. 2B). The whole lobe is crossed by auxiliary faultsseparating different structural blocks. Antithetic faultsare more common in the proximal (southern) areaswhere the movement initially stopped. Many verticalfaults appear to change laterally into subhorizontalfaults (Fig. 4). Antiform structures are producedbetween the auxiliary antithetic and synthetic faults.

Different detachment surfaces can be identified in thiscomplex. The most important is associated with themain fault, developed on a basal layer consisting ofplastic red siltstone about 10m thick (Fig. 4A). Thesealluvial siltstones are overlain by rigid lacustrine lime-stones with intercalations of dark, uncemented, finedetrital material that now form the slide blocks. The lessrigid layers between the limestones are the zones ofnumerous secondary, low angle detachment surfacesusually parallel to the stratification (Fig. 4B–D). These

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Fig. 1. Location of the Orce ravine in the Baza basin in south-east Spain.

L. Gibert et al. / Quaternary Science Reviews 25 (2006) 507–525 509

surfaces are found at the bottom of the slide blocks,which appear undeformed though they can showinternal displacement along other minor plastic layers.The basal layers of the blocks normally correspond to agranular media, where a dark fine matrix includesheterometric fragments of adjacent lacustrine strata thatwere included into the granular media during thedisplacement (Fig. 5A,B). Such granular layers shouldbe considered as fault breccias, and not as conglomer-ates. The fabric and structure in these basal layersindicate a fluidization of the material during the slump(Fig. 5C) (see Anders et al. (2000) for similar examples).These multiple fluidized basal layers played an impor-tant role in the rapid movement of the landslide.

4. Origin of the landslide

Distribution of landslides is largely controlled bybedrock geology (Kawabata and Mokudai, 2002) andearthquakes are the main factor in triggering landslides(Malamud et al., 2002). One major earthquake cangenerate many contemporaneous landslides, as occurredin Taiwan in 1999 (Liao et al., 2002) and Alaska in 2002.

Tectonic activity in the Baza basin continues from theMiocene to the present, producing earthquakes in theneighboring towns, for example: 4.8 on the Richter scalein Orce-Galera in 1964, and another of 3.1 in Baza in2001 (Andalusian Institute of Geophysics, 2002). Thegeological context of the Orce ravine landslide suggeststhat it originated as a single, gravity-driven event,probably induced by a nearby earthquake (Fig. 6).Many other landslides which affect the same Pleistocenedeposits in the Canada de Velez and Rıo Orce valleyscould be contemporaneous to the BO landslide.

5. The age of the landslide

In the Late Pleistocene the Baza basin becameexorreic (Vera et al., 1994), and began an episode oflarge-scale erosion and down cutting that continuestoday. A narrow E–W valley (Velez valley) wasdeveloped in the region of the modern BO. Thelandscape development of rigid, resistant cappingcarbonatic units, underlain by layers of lutites andclays, combined with the seismic activity in the regionfacilitates the occurrence of landslides. The dimensions

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Fig. 2. (A) View of the slide lobe from the other side of the Velez valley. (B) Aerial view of the studied area showing the lobular morphology of the

slumped surface. Altitude curves are separated by 10m. The rectangle shows the amplified area on Fig. 3.

L. Gibert et al. / Quaternary Science Reviews 25 (2006) 507–525510

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Fig. 3. Detailed aerial view of the Orce ravine showing the location of the paleomagnetic samples (Agustı et al., 1997, this paper), the paleontological

sites (O-1 to O-7), the faults, and the measured bedding. The Figure also shows the location of the cross section in Fig. 4.


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Fig. 4. (A) Main detachment surface over the red alluvial clay of Venta Micena cycle. (B–D) Three examples of antithetic faults, one of them (D)

showing subhorizontal movement.


and the geometries of these landslides indicate that theywere produced after the valley was well developed tonear its current depth. Small ravines, such as BO nowerode the BO lobe, produce small alluvial fans at theirmouths. These ravines appear to be pre-existing or atleast partially developed before the landslide occurred.The present morphology of the Velez valley, with a largemeander around the toe of the BO landslide, indicatesthat the position of the modern streambed has beensubstantially modified. Taken together, these featuresindicate that the landslide occurred relatively recentlyduring the latest Pleistocene–Holocene. If we use theactivity classification for landslides in southern Spain(Mather et al., 2002) the BO landslide can be classifiedbetween dormant-old and dormant-mature (inactive)presenting the following features (Figs. 2 and 3):

�
The cause of the movement is still identifiable but notlikely to re-occur. � The main scarp is dissected and vegetated.
�
Vague lateral margins without lateral drainage. � The internal morphology is smooth with undulating
topography and a normal stream pattern.

These types of landslides have an estimated age ofaround 10,000 years (Mather et al., 2002).

6. Paleomagnetic analysis

In this paper we suggest a different magnetostrati-graphy, therefore age, for the BO section. Our initialreason for recollecting the upper part of the publishedmagnetostratigraphy (Agustı et al., 1997) was to refinethe sequence of two short polarity events immediatelyunderlying the apparent Olduvai subchron; short eventsnot reported elsewhere in the world. These novel events,if substantiated, would have been a valuable correlationtool for our project of detailed lithostratigraphiccorrelation and development of the Baza basin.

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Fig. 5. (A) Granular media on a basal layer including large (412 cm)

fragments of different beds. (B) Internal detachment surfaces through

dark clay beds within rigid lacustrine limestones. Note the small

intraclasts included in a dark clay matrix and small normal fault with

about 10 cm of displacement. This layer corresponds to O-2. C: Flame

structure showing fluidification inside the minor detachment layers.


Nine samples were collected in 1998, includingsamples at both the O-6 and O-7 paleontological sites.Blocks (about 10 cm on a side) were collected frombeds that showed the least oxidation/weathering or

iron-hydroxide staining. Part of each block was sawninto three to seven specimen cubes (about 8 cm3),sanded, and cleaned with compressed air. Measurementswere made with a three-axis cyrogenic magnetometer,and demagnetization made in a non-inductive furnace(+/�3nT), all enclosed within a room-sized magneto-static shield.

Arguing for a change in the polarity assignmentrequires some explanation. This argument is made moredifficult since the rocks in question are, in general, poorpaleomagnetic recorders. An additional difficulty is thatthe section is in flat-lying young rocks, which produces acoincidence between Plio–Pleistocene Normal directionsand modern Normal directions. These factors combineto confuse a simple interpretation of the upper BOmagnetostratigraphy. In addition, we will argue thatprevious polarity interpretations based on the sign of themean virtual geomagnetic pole (VGP) can be improvedand superceded.

A paleomagnetic measurement can be viewed as thecomposite resultant vector or vector sum of theremanent components carried by the specimen. Labora-tory demagnetization is an attempt to selectively removesome of these components, especially those morerecently acquired. An example is thermal demagnetiza-tion to 1301, which exceeds the Curie point of goethite(Ozdemir and Dunlop, 1996). Fig. 7 shows thesubtracted vector directions (difference between the251 and 1301 vectors). The mean of these directions iscoincident, as expected, with the ambient (or Holocene)field direction. Ideally, further demagnetization wouldremove other remanent components, isolating andrevealing the characteristic (primary) remanence. Suchideal behavior is not the case in this study and isuncommon in many magnetostratigraphic studies (seeTurner, 2001). We were unable to isolate a simple singlecomponent magnetization above 1301, with either aNormal or a Reverse direction. What we did find weremulticomponent magnetization directions that wereconsistent mixtures of Normal and Reverse directions.

Multicomponent magnetizations are combinations oftwo or more remanent vectors. Each vector is a productof an event that affected iron-bearing minerals and/orthe grain size of iron minerals. Common events areDRM (depositional/early diagenetic remanence), CRM(chemical precipitation/alteration of iron-bearing miner-als), TRM (thermal heating and cooling) and VRM(viscous magnetization acquired through long exposureto a magnetic field). The three expected events in thisstudy are DRM, VRM and a weathering CRM(CwRM). There will be two expected directions fromthis combination of magnetizations, which will dependon the polarity during deposition. This DRM can be ineither a Normal or a Reverse direction. The VRM andthe weathering CwRM are directly related with time ofsurface exposition and will be directed toward the

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Fig. 6. Reconstruction of the landslide showing the identified faults and structures, the location of the paleontological sites and the different alluvial

(dark grey) and lacustrine (light grey) units.


modern Normal direction. In the case of the Normaldirected DRM, in combination with any amount ofVRM+CwRM (Normal), the resultant will be in aNormal direction. In the case of the Reverse directedDRM in combination with any amount of NormalVRM/CwRM, the resultant will be in a Reversedirection or in an intermediate direction as largerproportions of Normal vectors are combined. It is thislater case of multi-component intermediate directionsthat we observe in specimens from the in situ BOsection.

Specimens were thermally demagnetized at six tem-peratures between 1301 and 3201. In general, at thehighest temperatures the specimen directions became

erratic (or the susceptibility increased), limiting thevalue of increasingly higher temperature demagnetiza-tion (see Holm and Verosub, 1988). Two samples (ninespecimens) were inconsistent in direction and magnitudeduring all phases of demagnetization. Seven samples (29specimens) consistently moved away from the Normaldirection upon demagnetization (Fig. 8). The initialsubtracted vector (251 to 1301) was directed toward aNormal direction (Fig. 7), as expected for the removal ofthe remanence carried by goethite and VRM (Ozdemirand Dunlop, 1996; Dunlop and Ozdemir, 2000). Ingeneral, subtracted vectors from temperature stepsbetween 1301 and 3201 were subequal mixtures ofNormal and Reverse directions (Fig. 9). Therefore, no

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Fig. 7. Vector directions related to Goethite and young viscous

magnetizations (VRM), based on subtracted vectors between 251 and

1301. Site means and alpha 95 circles of confidence for Barranco de

Orce and Barranco Leon. A is the modern, long-term average dipole

field; P is the present magnetic field.

Fig. 8. Orthogonal demagnetization diagram for a specimen from the

paleontological site ORCE-6. Removal of a Normal directed

magnetization is shown.

(A)

(B)

Fig. 9. Ratio of Normal to Reverse components based on thermal

demagnetization. All vectors are deconvolved into two components

based on N and R model directions (101 from 000/57, and 201 from

180/�57, in the plane of the data). Starting with the components from

the highest temperature vector, the components are added from

sequentially lower temperature subtracted vectors. Heavy line at 1.0

indicates equal loss of N and R during demagnetization. An example is

shown of a typical Reverse specimen with a Normal overprint which

was successfully removed (sample from a lower stratigraphic position)

(A) Specimens from paleontological site Orce-7 (sample OR.04) and a

site 1.0m above. (B) Specimens from paleontological site Orce-6

(sample OR.01) and a site 0.3m above.


specimen revealed a single component direction ofReverse or Normal polarity. The variable mixing ratiosfound within (and between) sampled horizons reflect thevariation in preservation of the Reverse magnetizationand the relative amount of alteration or remagnetizationin the modern Normal field.

The presence of a Reverse directed magnetization insamples throughout the BO section indicates depositionduring a Reverse magnetochron. Most samples fromthis and the previous study also indicate the presence, invariable amounts, of a Normal directed magnetizationwhich we would argue is of modern origin. The previousstudy assigned a Normal polarity to any sample with amixing ratio where the Normal exceeded the Reverse inmagnitude. The use of this VGP method would assign aReverse Polarity if 51% of the remanence is Reverse and

assign Normal polarity if 49% of the remanence isReverse. This method of polarity assignment appearsarbitrary, but more importantly avoids an evaluation orexplanation of the presence of the Reverse remanentmagnetization in designated Normal samples. Anotherargument against using VGPs for magnetostratigraphyis that the term is misleading in two ways. Firstly, aVGP is intended as a determination of the geomagneticfield direction. Mixtures of magnetic vectors fromdifferent ages will produce an artefact, not a feature ofthe ancient magnetic field. Secondly, the variationbetween specimens in the same sampled horizon is theresult of variations in the relative amounts of the ancientand modern vectors and is not a feature indicatingsecular variation of the ancient geomagnetic field. As asubstitute for the VGP method, we suggest plotting theangular departure from the expected Normal fielddirection: D (Hoffman, 1984) toward the antipodalReverse direction (see Figs. 10 and 11).

We conclude from the presence of mixed polaritydirections that the DRM or early diagenetic remanence

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Fig. 10. Stratigraphic position, paleomagnetic direction, and polarity for the in situ part of Barranco de Orce. Solid diamond symbols are data from

Agustı et al. (1997). Circles with arrow are data from this report: demagnetized specimen directions at the maximum angular distance ðDÞ from the

axial dipole field (Hoffman, 1984), circles with point are samples with no coherent magnetic direction.


is of Reversed polarity (Fig. 10). The in situ fossil-bearing sites O-6 and O-7 are of Reversed polarity.Combined with the previous work, it is concluded thatall of the in situ BO sediments were deposited during theMatuyama Reversed Chron. The presence of theubiquitous Normal polarity overprint derives from acombination of vectors whose origin are modern, arisingfrom viscous magnetic realignment and near surfacealteration of magnetic minerals. It should be noted thatno samples were collected higher in the section owing tothe pervasive iron-hydroxide staining and more subduedtopography. Apparently these features also limited thesampling of Agustı et al. (1997). We suggest that this

uppermost part of the section is an older, pre-landslidesurface with a longer weathering history. The sectionthat was sampled appears to be a younger surface thatrepresents the slightly eroded headwall of the landslidescarp.

Two unpublished paleomagnetic studies were madein the displaced strata of the lower part of Orceravine (Semah, 1985; Oms, 1998), in which both Reverseand Normal sequences were described. Other studies,mainly biostratigraphic ones, also used samples from thelower part of section (e.g. Agustı, 1984, 1986a,b, 1991;Agustı et al., 1986, 1987a,b; Agustı and Moya-Sola,1992).

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Fig. 11. Stratigraphic position, paleomagnetic direction, and polarity for the Barranco Leon quarry site. Solid diamond symbols are data from Oms

et al. (2000b). Circles with arrow from this report: demagnetized specimen directions at the maximum angular distance ðDÞ from axial dipole field

direction (Hoffman, 1984).


6.1. Barranco Leon

The same collection technique as in BO was used atthe west wall of nearby Barranco Leon quarry. Twelvesamples were collected, also in 1998. Samples from thelower 3m (six samples, 21 specimen cubes, 10 cm3 each)are reported here as an extension of the sectionpublished by Oms et al. (2000b). A slightly moredetailed demagnetization procedure was attempted thanfor the correlative BO samples. Alternating field (AF)demagnetization was performed on all specimens atpeak fields of 4 and 10mT. This followed from the workof Dunlop and Ozdemir (2000) showing that AFdemagnetization could remove part of a VRM. Allspecimens were also subjected to eight thermal demag-

netization steps between 801 and 3101. However, multi-component magnetizations similar to those of BO werefound. Five of the six samples clearly have a Reversepolarity component (Fig. 10). These results extend theReversed polarity section (Oms et al., 2000b) downwardby 3m. Thus, all the fossil and lithic artefact-bearingstrata at Barranco Leon (Gibert, J. et al., 1992, 1998,2001; Roe, 1995) were deposited during the MatuyamaReverse Chron.

6.2. Magnetic susceptibility

For specimens from both sections, the lowfield magnetic susceptibility was measured after eachthermal demagnetization step, primarily to monitor


mineralogical changes that might occur during heating.One sample from each section showed an increase insusceptibility between 280 and 320 1C. At the BOsection, our lowest sample OR.01 (which is the O-6paleontological site) increased from 0.95 to 2.20�10�5SI. At the Barranco Leon section, sample BL.05increased from 0.73 to 1.15� 10�5SI. A probablemineralogical change that explains these increases insusceptibility involves non-magnetic sulfide minerals(e.g. pyrite) altering to ferric oxide/hydroxide minerals.The original rock color is olive gray (5Y 4/1) forOR.01 and light olive gray (5Y 6/1) for BL.05 (Munsellcolor numerical designations). After heating, thecolor of both samples became light brownish gray(5YR 6/1) with streaks of moderate reddish orange(10R 6/6).

Samples with extremely low susceptibility also corre-lated with poor paleomagnetic performance. From BO,the two samples with the lowest susceptibility (0.002,�0.095 SI) were inconsistent in direction and magnitudethus no polarity determination was made. Likewise inBarranco Leon, the only sample without a polaritydetermination had the lowest magnetic susceptibility(0.35 SI).

7. Discussion

7.1. Magnetochron assignment

An assignment of early Matuyama age (C2r,1r) wasmade for the lower 9.5m of the in situ BO section(Agustı et al., 1997), thus a simple expansion of thismagnetozone would be justified. Further reinforcementfor an early Matuyama assignment has been added byAgustı et al. (2001), who designated the BO section asthe boundary section for the MN17/MmQ1 boundaryfor western Europe. In direct conflict with this, Omset al. (2000b) assigned a late Matuyama age (C1r.2r) tothe adjacent Barranco Leon section. This conflict issurprising since these two sites are only 500m apart(Fig. 2), can be correlated and considered contempora-neous on geologic grounds (Fig. 12). These conflictingstudies from adjacent sections did not attempt to resolvethe choice in age between mid-late Pliocene or EarlyPleistocene. While this report modifies and extends themagnetostratigraphy of both sections, these paleomag-netic data can support either age assignment, but notdifferent ages for the sections. To help clarify thissituation, our current research is attempting to find anauthentic Normal magnetozone higher or lower in thesedimentary sequence. The ambiguity of having themodern Normal direction coincide in coincidence withPlio–Pleistocene Normal direction, means that paleo-magnetic field tests must accompany future claims offinding the Olduvai or Jaramillo magnetochrons. The

fold test and the facies test are both possible in this partof the Baza basin. Waiting for new paleomagneticresults, the mammalian fossil record (see below)supports a late, post-Olduvai, Matuyama age for theBO and the archeological sites.

7.2. Micromammal chronobiostratigraphy

Mammal fossils are normally found at isolated sites,usually in fluviatile and lacustrine deposits, or in cavedeposits. These occurrences have greatly influenced themammalian biostratigraphy, in the sense that mammalzones are not true biostratigraphic units but punctualassociations of faunas or local faunas defined inreference localities. These local associations are chron-ologically ordered by the evolutionary stages of taxaor by faunal replacements (‘‘faunal events’’) (e.g.Fahlbusch, 1976; Azzaroli, 1983; Azzaroli et al., 1988).A further difficulty is the correlation between micro- andmacromammal faunal events, because these are notalways found together.

During the last few decades, effort has been made tolink these faunal events and reference localities withabsolute dating, and with chronostratigraphic stagesdefined in marine sequences (see Lindsay and Tedford(1990) for a historical review). The continuous Plio–Pleistocene sedimentary record in the Guadix-Bazabasin, with its abundant fossil mammalian record, offersa possibility to recognize faunal replacements, date theirabsolute age, establish a stratigraphical range of faunalassociations and correlate to reference localities inEurope as well as to standard stages defined in marinedeposits. One main goal is to calibrate mammalianevents to the Plio–Pleistocene boundary. The Plio–Pleistocene boundary (base of the Pleistocene Series)was formally defined by a Global Standard StratotypeSection and Point (GSSP) at the base of a claystoneunit conformably overlying the sapropelic bed ‘‘e’’ inthe Vrica section in Calabria (southern Italy) by theSubcommission on Quaternary Stratigraphy of theINQUA Commission on Stratigraphy at the 27thSession in Moscow, 1984 (Aguirre and Pasini, 1985;Bassett, 1985; Pasini and Colalongo, 1997). Theboundary is defined biostratigraphically by LADs andFADs (Last, First Appearance Data) of calcareousnanoplankton and planktonic foraminiferal taxa (Pasiniand Colalongo, 1997), but its correlation potentialincludes astrocyclostratigraphy, magnetostratigraphy,and marine oxygen isotope stratigraphy (Rio et al.,2000). The Vrica GSSP lies just below the top of theOlduvai Subchron (C2n Subchron of Cande and Kent,1995) (Zijderveld et al., 1991; Pasini and Colalongo,1997). Thus the importance of identifying this subchronin the Baza basin, with mammalian fossil associationswould establish a correlative mammalian zones to thedefined Plio–Pleistocene boundary.

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Fig.12.Correlationofthesectionswithpaleomagnetic

data,together

withFuentecica

section.SectionsofBarrranco

LeonandFuentenueva-3

contain

archeologicalsites.Paleomagnetic

results

showReverse

polarity

andare

consistentwiththestratigraphicdata.Thecorrelationshowstheoccurrence

of

All

ophaio

mys

pli

oca

enic

us.Observethelack

ofexposure

inthelower

Barranco

deOrce.



The BO section has been extensively used inbiostratigraphy, both to characterize micromammalassociations and biozones and to calibrate their relativeand absolute chronology using paleomagnetic data(Agustı, 1984, 1986a,b, 1991, 1998; Agustı et al., 1986,1987a–c, 1997, 2001; Agustı and Moya-Sola, 1991, 1992,1998; Oms et al., 1994, 1996, 1999, 2000a,b). Data fromthese papers have then been used by other authors todiscuss the chronology of Plio–Pleistocene mammalianIberian and European local faunas as well as the timingof human dispersal into Europe (e.g. Sese and Sevilla,1996; Aguirre, 1997a,b; Aguirre et al., 1997; Martınezet al., 1997; Gibert, J. et al., 1998, 1999a,b).

On micromammalian biostratigraphic grounds, theearliest Pleistocene is recognized in Europe by the firstoccurrence (FAD) of the arvicolid Allophaiomys plio-

caenicus (Chaline, 1977, 1997). There has not been aspecific relationship established to radiometric dating.Instead, the earliest Pleistocene has been discussed usingdata from the BO section, which has yielded only a fewspecimens of A. pliocaenicus in O-3, 4 and 7.

Based on different biostratigraphical and magnetos-tratigraphical interpretations (e.g. Olduvai subchronbeing located at either O-2 or at O-7), the publishedfirst occurrence of A. pliocaenicus in the Baza basin hasbeen interpreted either as prior to (Agustı et al., 1997) orposterior to (Agustı, 1984; Agustı et al., 1987a) thePlio–Pleistocene boundary. Successive magnetostrati-graphic interpretations of the BO section were accom-panied by different interpretations of the faunal content,identifying up to three different faunal assemblages inBO 1–7 sites (Agustı et al., 1987a,b,c), with threepopulations of A. pliocaenicus: a basal one in O-1 and O-2, a middle one with ‘‘primitive’’ forms in O-4 and O-7,and an upper one with ‘‘evolved’’ forms in O-3.Different interpretations were made of the populationsof A. pliocaenicus from O-7, either as ‘‘very similar’’ (e.g.Agustı et al., 1987a, 1997) or as ‘‘primitive’’ (e.g. Omset al., 2000a) with respect to the population from theVenta Micena site (Fig. 13). The form of A. pliocaenicus

from O-4 was considered much more primitive than thatfrom O-7 and close to the population of the Beftia-2locality (Agustı et al., 1987a–c, p. 79). However, in alatter paper (Agustı and Moya-Sola, 1998) it was thepopulation from O-7 the one considered primitive andclose to that from Beftia. Based on these biostratigra-phical and magnetostratigraphical interpretations, theBO section has been suggested as a boundary section todefine MN 17 to MQ1 (late Villanyian/early Biharianmammal ages of Europe) based on the FAD ofMimomys ostramosensis (Table 1 in Agustı et al.(2001); also see the discussion of the significance of thisspecies at site O-2 in Agustı (1986b) and Agustı et al.(1986)). However, in previous paper (Oms et al., 2000a)these authors state that the presence of this species in theGuadix-Baza basin ‘‘can no longer be accepted’’.

As an overview, biostratigraphic studies are built on asequence of observations and interpretations. The firststep deals with taxonomy and systematics, which areopen to subjective interpretations and need statisticallyrepresentative populations. Micromammals, andespecially arvicolid rodents, show a clear gradualisticevolution (e.g. Chaline and Laurin, 1986; Chaline et al.,1993), and are subject to different taxonomic interpreta-tions (e.g. species assigned to either Allophaiomys,

Arvicola or Euphaiomys by different authors, Sese andSevilla, 1996). Another step is the recognition of faunalassociations and ranges of taxa. This requires detailedstratigraphic studies, but also depend on the previoussystematic and phylogenetic interpretations. The nextstep is the definition of biozones and their correlationwith other biozonations to obtain a relative chronologytaking into account possible biogeographic variations,dispersals and endemisms. Finally, these defined bio-zones can be calibrated with absolute datings toestablish an absolute chronology.

Biostratigraphic studies in the BO section showdeficiencies in all aspects: systematics, stratigraphy,biozonation and absolute datings. Published systematicsare based on scarce specimens, usually less than five,which do not allow the recognition of populationvariability and thus lack statistically precise speciescharacterization and assignment. This feature can beseen in the above-mentioned differing interpretations ofa population of A. pliocaenicus as either ‘‘primitive’’ or‘‘evolved’’ with respect to the population from VentaMicena. The low number of specimens together with theunrecognized repetition of the BO fossiliferous layers bylandsliding led to misinterpretations of faunal associa-tions and therefore, biozones. For example, M. ostramos-

ensis was identified in the ‘‘lower’’ strata (O-1 to O-3) andA. pliocaenicus in the ‘‘upper’’ strata (O-4 to O-7),resulting in two biozones in the BO section (Agustı,1984). Based on this latter species, Agustı (1986a, 1991)and Agustı et al. (1986, 1987b) proposed an initialbiozone in the Quaternary (MnQ-1, biozone of M.

ostramosensis) previous to the inmigration of A. pliocae-

nicus, which posteriorly was located in the late Pliocene(Agustı and Moya-Sola, 1992). Accounting for land-sliding, the sites in the BO ravine correspond torepetitions of only two layers, originally separated byless than 3m, thus the faunal association for all sevensites should be similar. Actually, M. ostramosensis and A.

pliocaenicus are found together in the type locality of M.

ostramosensis (Chaline, 1997, 1986). Recently, it has beenstated that M. ostramosensis no longer occurs in theGuadix-Baza basin (Oms et al., 2000a), although the M.

ostramosensis biozone (Mm Q1) was maintained andredefined as the Mimomys oswaldoreigi zone. Summingup all the successive biostratigraphical interpretations(Agustı, 1984 to Agustı et al., 2001), the faunalassociation from the BO section has been assigned to a

ARTICLE IN PRESS

Fig. 13. Successive biochronostratigraphical interpretations proposed for the paleontological sites of the Orce section. Note the shifts of the

Barranco de Orce sites within the successive scales. BL: Barranco Leon; FN: Fuentenueva; O: Barranco de Orce; VM: Venta Micena.


total range of four biozones, Mn17 to MmQ3 (Fig. 14).We suggest that this range might be a reflection ofbiodiversity at this location.

According to the present study, the BO sites O-1 to O-5 are displaced segments of O-6 and O-7, which are closetogether (o3m) in a continuous sequence. Additionally,sections at BO, Venta Micena (Scott and Gibert, 1999)and Barranco Leon can all be placed within a singlereversed polarity interval. Lacking a polarity boundary,the BO section cannot be used to define an absolutechronology to the micromammal biozones. Neither can

the BO section be directly useful in stating thespecific relationship of such biozones to the Plio–Pleistocene boundary. If the populations of A. pliocae-

nicus from these sites are actually the first occurrence ofthis species in the Baza basin, then it does not coincidewith the end of the Olduvai Normal subchron. Furtherstudies will be needed to identify the Olduvai subchronin this part of the Baza basin and to confirm the firstoccurrence of A. pliocaenicus and its stratigraphic andchronologic relationship with the Plio–Pleistoceneboundary.

ARTICLE IN PRESS

Fig. 14. Synthesis of the biozonations (Agustı et al., 1986–2001) and

the differing biostratigraphical interpretation of the micromammal

faunal associations from the BO section ‘‘sites’’. The ‘‘sites’’ are

actually repetitions of two beds separated less than 3m. Summing up

all successive biostratigraphical interpretations, the faunal association

from the BO section has been assigned to a total range of four

biozones, Mn 17 to MmQ3.


7.3. The age of Orce archeological sites

The polarity determination for the Orce ravine hassignificance since it is located near archeological sites:Barranco Leon-5, Venta Micena and Fuentenueva-3,which have yielded lithic artefacts with evidence ofanthropic action as well as fragmentary human remains(Borja et al., 1997; Gibert, J. et al., 1998, 2001; Tobias,1998). These archeological sites are found in the samesubhorizontal palustrine sequence as exposed in theOrce ravine. Because of the proximity of these sites,especially BL-5 (Figs. 1, 2), and their similar strati-graphic position (Fig. 12), the previous paleomagneticdata from the BO section (Agustı et al., 1997) had beenused to date the archeological sites. The previouspaleomagnetic interpretation of O-7 and higher strataas normal polarity (Agustı et al., 1997) led to anerroneous dating of BL-5, VM, and FN-3, as beinglocated in the Olduvai subchron (Gibert, J. et al., 1998,1999a,b). The incorporation of the new paleomagneticresults from the BO section indicates that the correlativearcheological sites are located within a period of Reversepolarity. This new interpretation of the BO section isconfirmed by the Reversal polarity paleomagneticresults from the adjacent site BL-5 (Fig. 12) (Omset al., 2000b, this paper).

Precise dating of the archeological sites cannot bemade from the available paleomagnetic data, neithercan it be made from a comparison with other Europeansites with similar faunal assemblages owing to the lackof radiometric dates. The most significant fossil speciesin the BO and BL sites is the arvicolid Allophaiomys

pliocaenicus (Agustı et al., 1987b), which is considered a

marker for the Early Pleistocene in Europe/Eurasia(Chaline, 1987, 1997; Aguirre et al., 1997). According toGabunia et al. (2000a), among the late Villafranchianmammalian faunas of western Europe, the faunalassociation from Venta-Micena is the most similar tothat from Dmanisi, which has been dated in 1.7Ma(Gabunia et al., 2000b, 2001). The absence of A.

pliocaenicus in Dmanisi indicates a very latest Plioceneor earliest Pleistocene age for that site (Gabunia et al.,2001). Owing to the similarity in the faunal association,together with the presence of A. pliocaenicus, the age ofVenta Micena and the Orce ravine are slightly youngerthan Dmanisi and thus corresponds to the post-Olduvaipart (C1r.2r) of the Matuyama chron.

8. Conclusions

The Barranco de Orce (BO) section crosses a land-slide. Sites O-1, O-2, O-3, O-4 and O-5 are located in slidblocks and are repetitions of two fossiliferous layers:O-6 and O-7, which remain in their original strati-graphic position at the top of the section. There is noclear evidence for the presence of the Olduvai Normalsubchron in the BO section. Previously identifiedNormal polarities are apparently missinterpretationsof weathering and viscous remagnetization resultingfrom modern processes. The paleontological sitesfrom the Orce ravine were deposited during theMatuyama Reverse time period. The neighboringsection at Barranco Leon, including all fossiliferousand lithic artefact bearing strata, is a lateral equivalentsection, also of Reverse polarity. Considering thatthe fauna of the Dmanisi site (Georgia) is no olderthan 1.7my, and its faunal association is slightlyolder than that from Venta Micena, the first occurrenceof Allophaiomys pliocaenicus in the Baza basin mustoccur during a Reverse period of time in the post-Olduvai, late Matuyama subchron. The presenceof human bones and lithic artefacts in similar strati-graphic horizons (Venta Micena, Fuentenueva-3, Bar-ranco Leon) points to this conclusion. The use of BO inthe magnetobiostratigraphical calibration of the Plio-cene/Pleistocene boundary for western Europe is notadvised.

Acknowledgments

We wish to thank Dr. C. Swisher for his encourage-ment of this detailed magnetostratigraphy samplingprogram and Dr. Josep Gibert for his discussion anduseful comments on the geology and biostratigraphy ofthe Orce region. This study was funded in part by TheEarthwatch Institute.

ARTICLE IN PRESSL. Gibert et al. / Quaternary Science Reviews 25 (2006) 507–525 523

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