Seismic-induced slump in Early Pleistocene deltaic deposits of the Baza Basin (SE Spain)

Sedimentary Geology 179 (2005) 279 – 294 www.elsevier.com/locate/sedgeo

Seismic-induced slump in Early Pleistocene deltaic deposits of the Baza Basin (SE Spain) L. Gibert a,*, C. Sanz de Galdeano b, P. Alfaro c, G. Scott d, A.C. Lo´pez Garrido b a

Dept. Enginyeria Minera i Recursos Naturals, Universitat Polite`cnica de Catalunya, Farinera 2, 08211 Castellar del Valle`s, Barcelona, Spain b Instituto Andaluz de Ciencias de la Tierra, (CSIC-Univ. Granada), Facultad de Ciencias, Univ. Granada. 18071 Granada Espan˜a, Spain c Dpto. Ciencias de la Tierra y del Medio Ambiente, Universidad de Alicante, Ap. 99, E-03080 Alicante, Spain d Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA, 94709, USA Received 19 December 2003; received in revised form 9 March 2005; accepted 3 June 2005

Abstract Early Pleistocene lacustrine deposits of the Baza Basin in SE Spain show a 2 m thick sheet with deformation structures intercalated within horizontal strata. Along a 180 m exposure we recognise an alternation of compressional (thrusts) and extensional (normal faults and tensional joints) structures. These structures are interpreted as a planar slump. Analysis of the different structures together with the sedimentary facies indicates that this slump moved towards the north in a subaqueous deltaic environment 2.5 m deep with a paleoslope of b 18. An earthquake is presumed to have triggered the mass movement and was followed by other shaking of lesser intensity. These events occurred in rapid succession, during the active deposition of a single delta lobe, which then covered and preserved the structures. The general stratigraphic position of this subaqueous slump matches other nearby liquefaction structures (seismites), indicating a period of active paleoseismicity. This appears to coincide with renewal of tectonic activity in the adjacent Betic Cordillera, which changed the source area of detrital deposits, infilled the NE margin of the Basin, and modified the morphology of the paleo-Lake Baza. Paleomagnetic and biostratigraphic data constrain the age of the structures to the Early Pleistocene, between 1.2 and 1.1 Mya. D 2005 Elsevier B.V. All rights reserved. Keywords: Landslides; Slump; Baza Basin; Pleistocene; Paleoseismicity; Gilbert delta; Seismites

1. Introduction * Corresponding author. E-mail addresses: [email protected] (L. Gibert), [email protected] (C. Sanz de Galdeano), [email protected] (P. Alfaro), [email protected] (G. Scott), [email protected] (A.C. Lo´pez Garrido). 0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2005.06.003

In lacustrine sediments of the Baza Basin (Betic Cordillera, SE Spain), we identify a deformed sheet bounded by horizontal strata. This report

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describes the internal structure of these deformed beds, and identifies the processes that generated the deformation. The studied area (Fig. 1) is in the NE sector of the Baza Basin between the towns of Orce and Huescar, (37845V35W N, 002830V25W W, 969 m a.s.l). Exposed on both sides of the San Clemente Canal are 5 m of horizontal strata except that the middle beds have anomalous tilts and disruptions. Alfaro et al. (1997) described different types of load structures induced by liquefaction and interpreted as seismites in the eastern Baza Basin. Examples of soft sediment deformation (seismites) can be seen at Galera (3 km south), Orce (6 km southeast) and Cullar (20 km south). The San Clemente Canal outcrops are in a similar stratigraphic/topographic position, but with a different style of sediment deformation, showing lateral movement of thin slide-sheets followed by tensional faults and joints.

8 44

4

0

4

2. Geological setting Located in the contact between the Betic Internal and External Zones neotectonic deformations created and isolated the Baza Basin by the Late Miocene (Este´vez and Sanz de Galdeano, 1983; Sanz de Galdeano and Vera, 1992). Infilling then occurred, with at least 700 m of Neogene through mid-Pleistocene continental deposits. In the NE margins of the Basin, the deposits alternate between palustrine–lacustrine and distal alluvial environments reflecting the expansion and contraction of the paleo-Lake Baza (Gibert et al., 1999). In general, the paleo-lake was shallow with an evaporitic area in the centre (~800 km2) surrounded by a palustrine fringe. Falling lake levels would change the extensive marginal areas from palustrine to distal alluvial environments, with an increase in the concentration of salinity (mainly sulphates) in the remaining lake water. Short-term lake level oscillations appear related

8

40 36 32

Hercinian External zones

Murcia

Baza basin

Córdoba

Internal zones Betic Neogene Basins

Sevilla Granada Sierra de la Sagra Sierra de Castril

Almería

Cádiz

Sierra de la Zarza

Huéscar Study area V. Micena

Galera

Mediterranean sea Strait of Gibraltar

Orce

Benamaurel Cullar

Sierra de Orce

BAZA

0

10km

Fig. 1. Geography of the studied area, showing the Betic Ranges and the Baza Basin.

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to changes in the Plio–Pleistocene climate (Gibert et al., 2001), superimposed on longer-term trends in regional and local tectonic processes. Deposits in the Baza Basin can be divided into carbonate-rich (palustrine–lacustrine), clastic (clay to gravel) and evaporitic (mainly gypsum) sediments. Carbonate-rich deposits occur in marginal areas iso-

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lated from significant alluvial input; an example is the palustrine facies of the Orce–Venta Micena area (6 km SE), with numerous quarries for vertebrate fossils. Carbonate-rich deposits also occur in the more central areas of the Basin where subaqueous conditions were more persistent. Clastic deposits vary from conglomeratic sandstones to mudstones depending on the pa-

Fig. 2. Stratigraphy of the area, showing the structures near the top of the sedimentary succession. The correlation panel shows the different deposits in the marginal northern part of the Basin. Note the down dropped block with fossil mammals, site PL. Paleocurrents in this area change from southwest to north when metamorphic grains arrive, and then change back to the southwest, again supplying Mesozoic grains to this part of the basin.

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Fig. 3. Paleogeographic reconstruction of the studied area showing the paleo-Lake Baza during periods of contraction (A), expansion (B), and when the delta front slide-sheets occur (C). Soft sediment deformation structures (seismites) are also common during this time. The fluvial deposits are prograding northwards from the rising metamorphic mountains in the Internal Betic Ranges.

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leogeographical situation. Generally, the most marginal deposits are coarser, becoming finer grained towards the centre of the basin. In some areas, sandstones and gravels are cemented with gypsum, and mudstones contain lenticular gypsum crystals. The studied outcrops appear on both sides of the Canal San Clemente cut, within horizontal Pleistocene strata, and below an extensive erosional surface locally called bLlanos de OrceQ (Fig. 2). The observed structures occur associated to a unique but, widespread clastic deposit occurring within the shallow paleolake as a set of prograding Gilbert deltas. As part of a sediment provenance study, we have determined that the lower stratigraphic layers (distal alluvial sandstones and siltstones with paleochannels) contain clasts of Mesozoic carbonates. Local source areas for these types of Mesozoic grains are the surrounding mountains of the External Betic Zone, 5 km to the NE, and 6 km SE. However, we found the upper stratigraphic layers (including the deformed sandstones and siltstones) contain grains of white mica, hornblende, and metamorphic lithics. The closest source areas for these

Lake

Prodelta muds

metamorphic grains are the mountains of the Internal Betic Zone, along the southern margin of the Basin, 20 km to Sierra de las Estancias, and 35 km to Sierra de Baza. This change to metamorphic provenance occurs synchronous with a change in paleocurrent direction, originally towards the west and southwest and later towards the north-northeast. It appears that the sustained neotectonic uplift of these southern mountains produced a northward expanding paleoslope that transported metamorphic grains across to the opposite side of the Basin in the Early Pleistocene (Fig. 3). These metamorphic centres are young (mid-Tertiary), topographically high, and have an active history of tectonic and erosional denudation (see Johnson et al., 1997).

3. Facies association and depositional environment The San Clemente Canal cuts are a cross-section of the subaerial alluvial environment being flooded by the expanding paleo-Lake Baza. In this area the lacustrine environment began with fine-grained lake bottom

nel

level

283

an Ch

s

bar

s and nt s ls o r f e ta- av Deland gr

Gilbert type lobe Braided gravelly channels Gravelly topsets Gravelly and sandy foresets

Sandy bottomsets

Fig. 4. Sketch of a typical Gilbert delta (modified from Garcı´a-Monde´jar, 1990).

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deposits, but was overwhelmed by sand and gravel from an advancing delta. Different sedimentary facies can be identified and classified as corresponding to a Gilbert-type delta (Fig. 4) (Gilbert, 1885): – Prodelta muds—exposed in the lower part of the sedimentary succession, these clay deposits represent an open lake environment. As the Gilbert delta approaches, the clays become interbedded with fine sand showing wave and traction ripples. – Sandy bottomsets—immediately above the prodelta muds are sand layers with plane and parallel stratification, commonly with wave and traction ripples. This facies thins to form a low-angle (N18) wedge in the N15E direction (towards the

deeper part of the basin). This angle represents the paleoslope of the original depositional surface. – Gravel and sand foresets—layers of coarse sand and gravel are the prograding faces of the Gilbert delta lobes. The foreset layers were deposited on surfaces inclined 12–148 toward N15E. This dominant lobe’s progradation direction represents paleocurrent directions. The maximum height of the foresets (2.3 m) represents the minimum water depth (Fig. 5). To this height we added 0.2 m, a minimum estimate of the channel depth at the river mouth. Thus, the paleo-lake was at least 2.5 m deep. – Infill facies—consisting of clastic layers which infill temporary depressions, the initial deposits were large locally derived blocks, which fell from

Fig. 5. Different facies in a small Gilbert delta present in the studied outcrop (A) with clinoforms 2.3 m high and foreset beds with inclination of 148. Stratigraphic section from the outcrop (B), showing the position of the paleomagnetic samples and the paleocurrent directions measured from the foresets.

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the newly formed walls. The next clastics in the depressions were poorly sorted, irregularly bedded gravels and sands. Then the depressions were filled and overtopped by the continuing deposition of the deltaic foresets. Throughout this facies, the general trend was fining upwards. In summary, the studied structures occur primarily in the distal deltaic facies of subhorizontal (b 18) sandy bottomsets and prodelta muds (Fig. 4). As a consequence of the continued progradation of the deltaic system, these structures were covered with clastic foreset deposits of the delta front. The topset facies of planar delta deposits are not present in these outcrops because of more recent erosion. The progradation of the deltaic lobes was towards the NNE, away from the probable source areas of metamorphic clasts. At similar stratigraphic level to the studied outcrop but 5 km SW, there are more fluvial gravels and sands with metamorphic grains at the distal edge of other small deltas with 1.5 m high gravel foresets. Continuing up the paleoslope, abundant fluvial channels with metamorphic clasts can be found 15 km S coming from the Sierra de las Estancias.

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sand units (Figs. 6 and 7). The entire packet of marl and sand moved over the footwall (undeformed marls) and for each low-angle fault formed a ramp anticline (or fault-bend folds). The thickness of the deformed sheet (1–2 m) limits the amplitude of folds (1–6 m) formed in association with thrusting. Occurring at the back of each imbricate thrust zone are conjugate sets of normal faults (Fig. 8a). Two generations of subparallel faults can be distinguished, the product of two successive tensional movements displacing the slide towards the north. The initial faults are associated with the dominant slide movement and form the walls of the depressions. The second generation of faults has smaller offsets, and only affect the sediments within the depressions. Additionally, there are subvertical tension joints in the sand layers, with an average direction (N75E) subparallel to the normal faults. The ramp anticlines have equivalent hinge lines (N115E) exposed on both sides of the Canal. The perpendicular to the hinge lines (N25E) should represent the direction of movement. This direction is parallel to the paleocurrent (N15E) of the foreset beds (Fig. 8b). The cumulative lateral displacement can be estimated at 10–20 m (Fig. 9).

4. Deformation morphologies 5. Interpretation The San Clemente Canal cut shows undeformed subhorizontal marls and silts overlain by sands and gravels, covered by the present-day soil. Within these deposits and extending laterally for 180 m, is a 2 m thick sheet with various deformation structures (Figs. 6 and 7). The deformed sediments are composed of two different units: (1) a clay-rich (marl) bed, which served as detachment surfaces and (2) sandstone beds, which appear as more competent rock units. The lower cohesive beds, deform as a ductile body (plastic solid or viscous liquid according to their original water content) (Allen, 1982). The overlying beds show zones of compressional (thrusts) and extensional (normal faults and tensional joints) structures, as well as folds related to thrusts. In the compressional zone a stack of three imbrications can be recognised; two of these have three bedding-plane thrusts, the other has only two. Movement on the detachment surfaces produced several low-angle ramps (58 and 208) onto the overlying sand unit. These low-angle faults repeat the

The deformation structures are confined to a thin stratigraphic sequence, without affecting the underlying and overlying strata. These well developed overthrust structures are not indicators of the general tectonic stress in this part of the Baza Basin. Instead, these structures indicate downslope gravitational instabilities. The internal deformations with folds, thrusts, detachment surfaces, normal faults and tension joints are arranged in an architecture of downslope compression at the toe and upslope tension at the head. This is compatible with a detached sheet or slump sliding on a subaqueous slope of less than 18. The main strain in this process is gravitationally induced shear, resulting in folding and thrusting of multilayers (Allen, 1982). Some additional observations argue for the rapid displacement and burial of the slide sheet. The ramp anticline ridge crests lack erosional modification, as would be expected from extensive exposure to trac-

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Fig. 6. Sketch of the San Clemente Canal cuts, with details of 2 areas on the east wall (A, B) and on the west wall (C). See corresponding photos in Fig. 7.

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Fig. 7. Photos of the canal cut showing the overthrust structures. Photos A, 1, and 2 were taken in the east wall; photo 3 was taken on the west wall, of the same structure. A and 3 show a hammer for scale. See corresponding sketches in Fig. 6.

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Fig. 8. Orientation of joint planes and faults (A), and paleocurrent directions measured on the foreset beds (B).

tion currents. For the case of synsedimentary movement, the foresets would have progressive deformation (or tilting) and then incorporation within the growing structure. A significant and prolonged neotectonic uplift of the southern highlands (internal Betic Mountains) is indicated by the arrival of large amounts of metamorphic clasts across and into this northern sector of the paleo-Lake Baza. The rapid progradation of this delta front with a few meters of sediments on top of lacus-

trine muds created the potential for a subaqueous slump. After a local detachment of the front of a newly formed lobe, the continued advance of the delta buried the slide-sheet.

6. Deformation stages The overthrust toes and the headwall depressions indicate a lateral translation event followed by minor

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289

Fig. 9. Reconstruction of the landslide immediately after the initial movement.

adjustments as normal faulting continued in the headwall area. These deformation events apparently occurred in rapid succession, during the active deposition of a single delta lobe. Within the newly formed depressions, the infilling material and its distribution indicates a quick sequence of events, with two types of conglomeratic sands. Initially, there were blocks (up to 80 cm) falling down and unsorted detritus of stratified sand and gravel partially filling the depressions, the detritus from pre-existing bottomset layers and foresets. Then there were new crossbedded gravels and sands that covered the locally derived blocks, at least partially, (Figs. 10 and 11) as the progradation of the delta lobe continued. The following sequence of events can be inferred within paleo-Lake Baza (Fig. 10):

3)

4)

5)

6) 1) In an open water setting, on the northern edge of the Lake, a prograding Gilbert delta approached from the south. Starting at or near the delta front, a detached slide-sheet of sand and clay moved tens of meters toward N25E along a flat, clay-rich surface (estimated paleoslope ~0.68). The movement produced at least three imbricate thrusts along the advancing toe, with associated normal faults and tensional joints along the headwall scarp. This created new relief (~1 m) on the paleo-lake floor consisting of ramp anticline ridges and depression troughs (Fig. 10b). 2) The headwall depressions began to fill with blocks and sand and gravel collapsing from the newly

formed walls. These coarse unsorted deposits were then buried by foresets of gravel and sand from the prograding delta lobe (Figs. 10c and 11). The delta lobe continued to advance, covering the ramp anticline ridges with sand and gravel. These new materials were deposited unconformably across portions of the landslide mass (Fig. 10d). Other small movements occurred on normal faults and fractures within the headwall depressions that do not extend into underlying units. These normal faults have displacements of ~30 cm (Figs. 10e and 12). Deposition continued with crossbedded deltaic and lacustrine strata covering the entire slide area. When paleo-Lake Baza contracted, extensive fluvial deposits returned with westward paleocurrents and Mesozoic carbonate provenance (Fig. 10f). Late in the Pleistocene, the Baza Basin was opened to external drainage (Calvache and Viseras, 1997). This episode of erosion continues into the Holocene, forming the dLlano and BarrancoT landscape of today (Fig. 10g).

7. Trigger mechanism Several mechanisms are capable of producing gravity induced shear and have developed similar subaqueous slump structures. Slumps have been interpreted as the product of overloading (related to rapid sedimentation), over steepening of slopes by deposition, and/or earthquakes (see references in Allen,

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Foresets

Lake paleodeep=2.5m Bottomsets

Paleoslope=0.6º

0

2m

A

Movement

B

C

D

E

F

G

Fig. 10. Stages in the development of the compressional and tensional structures. The prograding delta (A) is shown with the initial slump (B) on the front of a delta lobe, and the advancing delta lobe (C). The structures were partially buried (D), then reactivated to form tensional faults and joints (E). The continued burial under lake sediments (F) is shown, and finally, the present-day situation (G).

1982; Keefer, 1984; Owen, 1987; Van Loon and Brodzikowski, 1987; Moretti, 1996). In the San Clemente Canal case, the mechanism of over steepening can be discarded since this subaqueous landslide was produced on a low slope (0.68). Less easily discarded is the mechanism of overloading, since rapid sedimentation at a site has been shown to increase the

pore-pressure and produce both slumps and synsedimentary deformation (Postma, 1983; Dasgupta, 1998; Moretti et al., 2001). In the present case, overloading appears to be of secondary importance owing to the small thickness (2.3 m), and the lack of deformation in the immediate overlying units that were part of the accumulating load at the advancing delta front.

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Fig. 11. Two views of the same depression formed after the initial translation, on the east side of the canal cut (A), and on the west side (B). This depression has sediment and blocks (N70 cm) fallen from the adjacent walls, followed by an infilling of conglomeratic sand from the advancing delta lobe.

Earthquakes have been proposed as a primary factor in triggering subaerial landslides (Malamud et al., 2002). For subaqueous environments in modern lakes, there are several studies that describe sheet slumps with seismic origins (Siegenthaler et al., 1987; Niemi and Ben-Avraham, 1994; Chapron et al., 1999). Most recently, Schnellmann et al. (2002) described numerous slumps triggered by earthquakes in Lake Lucerne, Switzerland. Field et al. (1982) described movements on a low-angle slope (0.258) in the Klamath River delta (California) induced by an earthquake (1980; magnitude ~7.0). Although much

larger (1  20 km) and in a marine environment, the similarities to the paleo-Lake Baza slide include sheet thickness, lithology (delta sands, over mud), youth of the deposit at the time of sliding, and numerous compression ridges and scarps parallel to the toe. The specific characteristics of the San Clemente Canal sheet slump suggest seismic shaking as a trigger for the initial lateral motion, followed by other shaking events to produce the ancillary faults and fractures. Apparently, the deformation process occurred in a relatively short time. There is ample evidence for tectonic activity in the Baza Basin from the Miocene

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Fig. 12. Small faults (30–40 cm displacement) affecting the infilling materials in the partially filled depressions. These small faults indicate a second, minor tensional event.

to the Present. Modern earthquakes continue to shake neighbouring towns, e.g. a magnitude 4.8 in 1964 at Orce–Galera; 3.1 in 2001 at Baza; and 4.9 in 2005 at Bullas (Andalusian Institute of Geophysics, 2005). Also, several types of load structures in nearby parts of the Basin might be the result of Early Pleistocene seismic liquefaction (Alfaro et al., 1997) (Fig. 13). We have seen examples of seismites at Galera and Orce,

all of which occur in the same stratigraphic position as the slide-sheet described in this paper. There appears to be a regional stratigraphic zone that has been affected by a paleoseismic event(s) producing either brittle or ductile deformation depending on the physical state of the sediments. We propose this episode of seismic activity as the cause for both the load structures (autochthonous seismites), and the sheet slumps

Fig. 13. A soft sediment deformation structure interpreted as a seismite 8 km SW from the outcrop.

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(allochthonous seismite) in different parts of Pleistocene Lake Baza. The origin of these seismic events is speculative, but could be an increase in tectonic activity to the south (uplift in the Betic Ranges), which could also have produced the concomitant changes in paleoflows, sediment provenance and lake morphology. This is consistent with a tectonic pulse defined in the Betic Cordillera for the Early Pleistocene (Sanz de Galdeano and Vera, 1992).

8. Age of the slump The deposits in the studied area are generally described as late Pliocene to early Pleistocene on the basis of paleontological and paleomagnetic data. All upper stratigraphic levels in this sector of the Baza Basin are dominated by reverse polarity, thus assigned to the late Matuyama magnetochron (Oms et al., 2000; Gibert et al., in press), implying an age somewhat older than 1.07 Ma. Ten paleomagnetic samples were collected from the actual Canal exposures, in addition to 8 samples in nearby stratigraphic sections (containing two mammal-bearing horizons) (Fig. 2). Demagnetisation experiments and analysis were made at the Berkeley Geochronology Center, using a 3-axis cryogenic magnetometer. Reverse magnetisation appears as the characteristic remanence, but can rarely be isolated without some normal polarity contribution. However, there is an ambiguity between the ancient normal direction and the modern normal direction, dictating a higher standard for acceptance of suggested Plio/ Pleistocene normal magnetozones. To be classified as normal, a specimen’s demagnetisation directions must not only be dominated by normal polarity, but trends toward reverse directions must be absent. None of our sampled horizons can be assigned to normal polarity. One-third of the specimens are clearly of reverse polarity, while the rest appear to be mixtures of ancient reverse and modern normal vectors. The nearest faunal remains are at Puerto Lobo-1 (3.5 km north) and Loma Quemada (3 km northwest) (Fig. 2). Both sites are placed in the MQ-3 biozone, which corresponds to the middle Biharian and is included within the Allophaiomys nutidens zone (Agustı´, 1985; Agustı´ et al., 1986). These biostratigraphic criteria combined with reverse paleomagnetic results in-

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dicate an age of 1.1–1.2 Ma for the deformed beds (latest parts of the magnetochron C1r.2r). Other European sites with similar fauna and magnetostratigraphy are Solehilach and Mosbach-1; assigned to the Jaramillo normal subchron (1–1.07 Ma), (Agustı´ et al., 1986).

9. Conclusions Within the undeformed, Early Pleistocene, lacustrine deposits of the Baza Basin, a deformed 2 m thick sheet is exposed along the cuts for the Canal San Clemente. Within this sheet and along 180 m of outcrop there appear several folds, thrusts, normal faults and tensional joints. The contemporaneous presence of compressional and tensional deformation in the same stratigraphic unit indicates a low-angle subaqueous sheet slump. This movement generated at least three sheet slumps. Each are characterised by ramp thrusts in the front (toe) and normal faults and tensional joints in the back (head wall). Horizontal translation ranges from 10 and 20 m on a detachment surface of pro-delta muds. These particular slide-sheets were part of a deltaic environment, under 2.5 m of water, on a slope of less than 18 toward the NNE. The probable trigger mechanism was an earthquake (or series of earthquakes) to generate these seismic slumps in the delta facies, as well as liquefaction (seismites) in other areas of the paleo-Lake Baza. Small-scale sedimentary features indicate that the structures were formed and then buried quickly within a deltaic environment. Tensional deformation within the upslope part of the slide-sheet affected only the detached strata, creating troughs that were filled with unsorted blocks, gravel and sand. Smaller tensional deformations then affected these initial fill deposits. Afterwards, the headwall depressions along with the slide-sheets were completely covered by active deltaic foresets. Paleoflow continued unchanged as the delta front advanced past this area towards the NNE. The presence of seismically induced slide-sheets within the deltaic environment, along with a new and distant sediment source implies the relative uplift of the metamorphic southern highlands during the Early Pleistocene (1.2–1 Ma). Prior to this uplift, the Orce– Huescar sector of the Baza Basin was dominated by rivers coming from the Eastern Highlands (now eroded, and depressed by faulting) and by local drainages.

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This change in Early Pleistocene paleogeography presented the Sierra de las Estancias (20 km south) as a new dominate fluvial source, forming an extensive braided fluvial plain and small deltas all the way across to the northern shore of paleo-Lake Baza.

Acknowledgements We thank Ayuntamiento de Orce for a grant to L. G. This research is supported by the research group n8217 from the Junta de Andalucı´a, the Acc. Esp. BTE2001-5230-E and the Earthwatch Institute. We thank J. Menzies and M. Moretti for review and useful comments on this manuscript.

References Agustı´, J., 1985. Bioestratigrafı´a de los depositos Plio–Pleistocenos de la despresion de Guadix–Baza (prov. Granada), Paleoantol. Evol. 18, 13 – 18. Agustı´, J., Moya`-Sola`, S., Pons-Moya`, J., 1986. Venta Micena (Guadix–Baza basin, South-Eastern Spain): its place in the Plio–Pleistocene mammal succession in Europe, Geol. Rom. 25, 33 – 56. Alfaro, P., Moretti, M., Soria, J., 1997. Soft-sediment deformation structures induced by earthquakes (seismites) in Pliocene lacustrine deposits (Guadix–Baza basin, Central Betic Cordillera), Eclogae Geol. Helv. 90, 531 – 540. Allen, J.R.L., 1982. Sedimentary Structures: their Character and Physical Basis, vol. II. Elsevier, New York. 663 pp. Andalusian Institute of Geophysics, 2005. University of Granada (Spain), Website, http://www.ugr.es/~iag/div.html. Calvache, M.L., Viseras, C., 1997. Long-term control mechanisms of stream piracy processes in Southeast Spain, Earth Surf. Process. Landf. 22, 93 – 105. Chapron, E., Beck, C., Pourchet, M., Deconinck, J.F., 1999. 1822, earthquake-triggered homogenite in Lake Le Bourget (northwest Alps), Terra Nova 11, 86 – 92. Dasgupta, P., 1998. Recumbent flame structures in the Lower Gondawana rocks of the Jharia Basin, India — a plausible origin, Sediment. Geol. 119, 253 – 261. Este´vez, A., Sanz de Galdeano, C., 1983. Ne´otectonique du secteur central des Chaˆines Be´tiques (Basins de Guadix–Baza et de Grenade), Rev. Ge´ol. Dyn. Geo´gr. Phys. 24, 23 – 24. Field, M.E., Gardner, J.V., Jennings, A.E., Edwards, B.D., 1982. Earthquake-induced sediment failures on a 0.258 slope, Klamath River delta, California, Geology 10, 542 – 546. Garcı´a-Monde´jar, J., 1990. Sequence Analysis of a Marine Gilbert-Type Delta, La Miel, Albian Lunada Formation Of Northern Spain, Coarse-Grained Deltas, IAS, Spec. Publ., vol. 10, pp. 225 – 269.

Gilbert, G.K., 1885. The topographic features of lake shores, Annu. Rep. U.S. Geol. Surv. 5, 69 – 123. Gibert, Ll., Maestro, E., Gibert, J., Albaladejo, S., 1999. Plio– Pleistocene deposits of the Orce region (SE Spain): geology and age. In: Gibert, J., Ribot, F., Sanchez, F., Gibert, Ll. (Eds.), The Hominids and their Environment in the Middle and Lower Pleistocene of Eurasia. Museo de Prehistoria y Paleontologı´a de Orce, pp. 127 – 144. Gibert, Ll., Ferra`ndez, C., Scott, G., 2001. Plio–Pleistocene Lacustrine Sedimentation in the Baza Basin (SE Spain) and its Relation with Climatic Shifts, European Space Agency (ESA) Spec. Publ., vol. 463, pp. 171 – 175. Gibert, Ll., Scott, G.R., Ferra`ndez-Can˜adell, C. Evaluation of the Olduvai Sub-Chron in the Orce Ravine (SE Spain): implications for Plio–Pleistocene mammal biostratigraphy and the age of Orce archaeological sites, Quat. Sci. Rev., in press. Johnson, C., Harbury, N., Hurford, A.J., 1997. The role of extension in the Miocene denudation of the Nevado–Filabride Complex, Betic Cordillera (SE Spain), Tectonics 16, 189 – 204. Keefer, D.K., 1984. Landslides caused by earthquakes, Geol. Soc. Amer. Bull. 95, 406 – 421. Malamud, B.D., Turcotte, D.L., Guzzetti, F. 2002. Landslides and earthquakes. Landslides: new monitoring techniques and models, AGU Fall meeting San Francisco, vol. 83, 47, 557. Moretti, M. 1996. Le strutture sedimentary deformative. Studio delle modalita di deformazione e dell’origene attraverso esempi fossili e modellizzazione in laboratorio. PhD Universita` degli Studi di Bari, Italy. 232 pp. Moretti, M., Soria, J.M., Alfaro, P., Walsh, N., 2001. Asymmetrical soft-sediment deformation structures triggered by rapid sedimentation in turbiditic deposits (Late Miocene, Guadix Basin, Southern Spain), Facies 44, 283 – 294. Niemi, T.M., Ben-Avraham, Z., 1994. Evidence for Jericho earthquakes from slumped sediments of the Jordan River delta in the Dead Sea, Geology 22, 395 – 398. Oms, O., Pare´s, J.M., Martı´nez-Navarro, Agusti, J., Toro, I., Martı´nez-Fernandez, Turq, A., 2000. PNAS 97 10666–10670. Owen, G., 1987. Deformation processes in unconsolidated sands. In: Jones, M.E., Preston, R.M.F. (Eds.), Deformation of Sediments and Sedimentary Rocks, Geological Society Special Publications, vol. 29, pp. 11 – 24. Postma, G., 1983. Water escape structures in the context of a mass-flow dominated conglomeratic fan-delta (Abrioja Formation, Pliocene, Almeria Basin, SE Spain), Sedimentology 30, 91 – 103. Sanz de Galdeano, C., Vera, J.A., 1992. Stratigraphic record and paleogeographical context of the Neogene basins in the Betic Cordillera, Spain, Basin Res. 4, 21 – 26. Schnellmann, M., Anselmetti, F.S., Giardini, D., McKenzie, J.A., Ward, S.N., 2002. Prehistoric earthquake history revealed by lacustrine slump deposits, Geology 30, 1131 – 1134. Siegenthaler, C., Finger, W., Kelts, K., Wang, S., 1987. Earthquake and seiche deposits in Lake Lucerne, Switzerland, Eclogae Geol. Helv. 80, 241 – 260. Van Loon, A.J., Brodzikowski, K., 1987. Problems and progress in the research on soft-sediment deformations, Sediment. Geol. 50, 167 – 193.