A model of lacustrine sedimentation for the Early Pleistocene deposits of Guadix-Baza basin (southeast Spain)

Quaternary International 243 (2011) 3e15

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A model of lacustrine sedimentation for the Early Pleistocene deposits of Guadix-Baza basin (southeast Spain) José Manuel García-Aguilar*, Paul Palmqvist Departamento de Ecología y Geología (Área de Paleontología), Facultad de Ciencias, Campus Universitario de Teatinos, 29071-Málaga, Spain

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 19 February 2011

The Early Pleistocene deposits of Guadix-Baza basin represent a depositional unit with distinctive features in the tectosedimentary history of this intramontane basin, which sedimentary infillings range in age between the uppermost Miocene and 45 ka. This Pleistocene unit has an average thickness of 10 m and is composed of carbonate lacustrine facies arranged in an upward-shallowing sequence. The paleontological richness of this unit is evidenced by its special sedimentological and paleoecological features. Diverse numerical analytic data related with the evolution of the sedimentary sequences and the establishment of the sedimentary and paleoecological scenario are presented. Ó 2011 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction The region of Guadix-Baza (Granada, SE Spain) is a w100 km long, NEeSW directed intramontane basin situated in the contact between the Internal and External Zones of the Betic Chains (Fig. 1). This basin preserves the thickest and most continuous record of continental Plio-Pleistocene sediments of the Iberian Peninsula, showing two depocenters (Guadix and Baza subbasins) separated by a threshold linked to substrate outcrops (Jabalcón mountain, 1494 m). There is a well-known structural control of this basin, which results from the action of N50e70E and N150e170E fractures as well as from the presence of a tectonic accident, the Negratín shearing fault, which is part of a major system that crosses the Betic Chains from one extreme to the other following a NEeSW direction. The continental sediments of this basin are dated between the Turolian (Late Miocene) and the Late Pleistocene (Fig. 2), which cluster in two main sedimentary packages (García-Aguilar and Martín, 2000) with 300 m total thickness dated as Turolian and Plio-Pleistocene, respectively. The Plio-Pleistocene sedimentary infillings comprise five tectosedimentary units (García-Aguilar, 1997) limited by sedimentary discordances linked to hiatuses. The oldest of these is a Early Pliocene lacustrine-carbonate unit and the rest of sediments are composed of diverse alluvial and carbonateemarlyeevaporitic lacustrine units related laterally and vertically by a model of stratigraphic architecture which is complex in detail.

* Corresponding author. E-mail addresses: [email protected] (J.M. García-Aguilar), [email protected] (P. Palmqvist). 1040-6182/$ e see front matter Ó 2011 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2011.02.008

The Guadix-Baza basin was endorheic (i.e., characterized by interior drainage) until very recent times (43 ka), in which the capture of its waters by the Guadiana Menor, a tributary of the Guadalquivir River, led to strong erosive processes resulting in the predominance of badland landscapes with w200 mm of annual precipitation inside the basin (Viseras, 1991; Fernández et al., 1996; Azañón et al., 2006). However, it is worth noting that an age in excess of 180 ka has been recently assigned for this endorheiceexorheic transition (García-Tortosa et al., 2008). Subrecent (w5 ka) travertine deposits up to 10 m thick associated to thermal upwellings aligned with the major fracture systems of the basin are found locally (García-Aguilar, 1997). 2. Objectives The main objective of this article is to develop a model of lacustrine sedimentation for the Early Pleistocene deposits of the Guadix-Baza basin using the sedimentological and paleontological data available, which will allow establishing their patterns of lateral and vertical evolution. The second objective is to use this model in the sedimentary reconstruction of the main paleontological localities of the Orce area, particularly Venta Micena. 3. Architecture of the continental infillings The Guadix-Baza basin is exceptional from a stratigraphic point of view, given the quality of exposure of the outcropping areas and their lateral and vertical continuity, the density of paleontological sites (e.g., Cuevas et al.,1984; Ruiz-Bustos et al.,1984; Martín-Suárez, 1988; Soria and Ruiz-Bustos, 1991, 1992; Sesé, 1994; Palmqvist et al.,

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Fig. 1. Geographic situation and geologic structure of Guadix-Baza basin in the Betic Chains.

1996, 2003, 2008a, 2008b; Arribas and Palmqvist, 1998, 1999; Arribas et al., 2001; Palmqvist and Arribas, 2001; Viseras et al., 2006), the availability of paleomagnetic and geochronologic data (e.g., Garcés, 1993; Oms et al., 2000; Azañón et al., 2006) and the existence of an important documental database of geological studies developed during the last four decades (e.g., Anadón et al., 1995; Peña, 1985; Soria, 1986, 1993; Viseras, 1991; Sanz de Galdeano and Vera, 1992; García-Aguilar, 1997; García-Aguilar and Martín, 2000; Sanz de Galdeano and Peláez, 2007; García-Tortosa et al., 2008). All this information allowed selecting the areas where 90 stratigraphic series were sampled in detail (García-Aguilar, 1997), which resulted in a panel of stratigraphic architecture for the continental sedimentary infillings of the basin (Fig. 2). Five tectosedimentary

units (Megías, 1982) can be differentiated among these infillings, which are separated by sedimentary hiatuses. The first unit is of Late Turolian age (biozones MN12 and MN13). It has 170 m of maximum thickness and an average sedimentation rate of 11 cm/ka. This unit includes two formations, one alluvial and the other lacustrine, related by lateral and vertical changes of facies. The alluvial sediments are conglomerates and sandstones of subbetic origin (External Zones), formed by deltaic and alluvial fans. The lacustrine sediments are composed of pink marls interspersed with sands and microconglomerates. There is a discontinuity at the top of this unit associated to a compressive tectonic phase and a hiatus estimated by biostratigraphy in 600 ka. The unit is tilt 15
to the south and most outcrops concentrate in the northern part of the basin.

Fig. 2. General geologic map of Guadix-Baza basin, showing the marine and continental Miocene substrate, the Gorafe-Huélago, Guadix and Baza (Plio-Pleistocene) formations and the glacis. A specific model for the stratigraphic architecture of the basin is included.

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The second unit is also represented by several marginal outcrops and corresponds to carbonate lacustrine sediments of Alfambrianlowermost Villanian age (biozones MN14, MN15 and base of MN16), with 75 m of maximum thickness and an average sedimentation rate of 4 cm/ka. The lower part of this unit is composed of marly sediments and the upper of alternating marls and (marly) limestones. There is a stratigraphic discontinuity at the top of this unit, which is associated to a sedimentary hiatus estimated by paleomagnetic data and biostratigraphy in 400 ka. The third unit is the most complex from the point of view of the lateral and vertical evolution of facies. This unit has a VillanianEarly Pleistocene age (upper part of biozone MN16 and biozones MN17 and MP 18) and covers a large extent of the inner parts of the basin. There is an alluvial system within this unit that spreads out on the Guadix subbasin and is laterally connected with a lacustrine system mainly represented in the Baza subbasin. The maximum perceptible thickness of these sediments is about 250 m and their average sedimentation rate has been estimated in 15 cm/ka. This unit shows an initial retractive phase followed by an expansive one, which represents a major development of the alluvial facies. The fourth unit is found exclusively in the Baza subbasin and corresponds to 10 m thick lacustrine deposits basically composed of alternating marls and marly limestones of Middle Pleistocene age (lowermost part of biozone MP19). In the Guadix subbasin this temporal unit corresponds to a sedimentary hiatus, with a duration estimated by biostratigraphy in w200 ka. Finally, the fifth unit has Middle PleistoceneeLate Pleistocene age (biozones MP19 and MP20) and is 15 m thick. There is a predominance of coarse detritic sediments of alluvial nature, with reduced swampy and lacustrine areas. At the top of this unit there is a glacis surface developed after the end of the sedimentation in the basin, with an age estimated by radiometric data in 43 ka B.P (Azañón et al., 2006). 4. Paleogeographic evolution Several phases took place during the continental history of the Guadix-Baza basin (García-Aguilar and Martín, 2000). The first one (Late Turolian) corresponds to a lacustrine system of tectonic origin that was inherited from the previous marine basin, with a mountainous northern front very active from an erosive point of view. After its filling, this basin disappeared following a compressive tectonic phase and, as a result, there is a sedimentary hiatus estimated in 600 ka. Later, during the Early Alfambrian, a new lacustrine basin of smaller extent was generated. This basin was aligned in an NEeSW direction and had little influence from the external reliefs, which were heavily worn by the erosion. The outcrops of these lacustrine deposits are placed in the marginal zones of the basin and, although nowadays disconnected, show similar lithostratigraphic features. The origin of the Alfambrian lacustrine basin was linked to the regional tectonic accident N50e70E (García-Aguilar, 1997). Later, during the Late Villanian, the basin took a configuration more similar to the modern one, as a result of extensive tectonic activity associated to a w400 ka hiatus. This induced the individualization of the two depocenters (i.e., Guadix and Baza subbasins). The tectonically active, erosive border corresponded during this stage to the southern margin (Sierra Nevada and Sierra de Baza). At the bottom of this margin, the alluvial fans evolved to more winding fluvial systems and, finally, toward a wide and very shallow endorheic lake of evaporitic nature (i.e., a playa-lake), which spread over the majority of the Baza subbasin and constituted the base level of this entire sedimentary complex. This sedimentary model changed during the Early Pleistocene to a carbonate lacustrine system fed by a drainage net unstable in time and space together with sectors of alluvial plain, which shaped

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a paleoecological setting similar to that of modern wet savannas. This landscape was represented during the Middle Pleistocene by wide and shallow lacustrine systems fed by a poorly developed drainage net. In the MiddleeLate Pleistocene, the configuration of the basin was similar to the modern one, with wide and energetic “braid-plain” alluvial systems originated in Sierra Nevada and Sierra de Baza, which were especially active during the Gunz, Mindel and Riss glacial stages. Only during specific moments this scenery showed small swampy and lacustrine systems, especially on the northern and eastern sectors of the basin. The whole tectosedimentary dynamics of the basin was linked to a number of relative movements of cortical blocks of substrate, with a rate of displacement that fluctuates between 0.2 and 2.2 cm/year during the time interval comprised between the Turolian and the Middle Pleistocene, depending on the stage and the place of the basin considered (Fig. 3). This extensive dynamics resulted in the separation of the older sedimentary units toward the perimeter of the basin, where they outcrop now, in contrast to the most recent sediments, which are found in the central zones. The seismic profiles of this region demonstrate that the older sedimentary complexes are not found under the inner parts of the basin, revealing the lateral expansion of the most modern units and an asymmetric geometry for the base of the basin (García-Aguilar, 1997). 5. Lacustrine sedimentation during the Early Pleistocene The Early Pleistocene (base of biozone MP 18) is characterized in the Guadix-Baza basin by a predominance of carbonate sedimentation (García-Aguilar, 1997), with temporal limits comprised between 1.55 and 1.25 Ma (range of variation of w300 ka). The cartographic distribution of the sediments of this age concentrates in Huélago, Gorafe, Cuevas del Campo, Zújar and, above all, in the triangle GaleraeOrceeVenta Micena, with an average thickness of w10 m and a mean sedimentation rate of 3 cm/ka. The stratigraphic profiles for this unit (Fig. 4) show in detail very diverse lithologies and sequences, especially if we take into account their low relative thickness. The study of these profiles shows that the unit is mainly composed of carbonate facies (marls, limestones, dolomitic limestones and marly limestones) together with residual and minoritary detritic facies (clays, silts, sands and microconglomerates). At an interpretative level, some considerations on this unit can be drawn. First, the position of the outcrops within the basin, which are distributed along several sectors, stands out and implies their wide spatial development. This feature indicates indirectly a paleogeography in agreement with the modern geometric model. From a sedimentologic point of view, the observed facies evidence a carbonate lacustrine system with three major depositional environments: 1) marginal areas connected with external drainage systems, which resulted in facies composed of microconglomerates, sands and lutites; 2) highstand lacustrine areas, which generated marly and calcilutitic facies; and 3) lowstand lacustrine areas, characterized by the deposit of marly limestones, dolomitic limestones and calcarenites (i.e., tractive facies associated to coastal zones). 5.1. Analysis of facies and sequences X-ray diffraction mineralogical analyses of the total sample and of the decarbonated <2 m fraction in oriented aggregate have been performed in the marly facies (Table 1) as well as microtextural studies of limestones (Table 2) and statistical analyses of the distribution of facies and sequences (Tables 3 and 4) for the whole unit (García-Aguilar, 1997). Although these sedimentary sequences show differences in detail, five basic modalities can be

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Fig. 3. Model of tectosedimentary evolution for the Guadix-Baza basin between the Turolian and present time (modified from García-Aguilar and Martín, 2000).

distinguished among them. These sequence types usually have two lithologies, with marls representing their lower term in all cases: marlsemarly limestones (type 1 sequence), marlselimestones (type 2 sequence), marlsedolomitic limestones (type 3 sequence), marlsecalcarenites (type 4 sequence), and marlsesandstones (type 5 sequence). In addition, there are other sequences poorly represented in this sedimentary unit, composed of vertical combinations of all these facies. Below there is a more in-depth description of the sequences: Type 1 sequence: it is composed of light colored isotropic marls, although locally there may be chromatic zonations with diffuse bioturbation and signs of pedogenesis. Above these facies there is a bank of compacted marly limestones with inner structures (nodulization, brechification and bioturbation). It is also frequent the presence of intraclasts. The mean thickness relationship between both facies (i.e., marls and marly limestones) is close to 1:1. Type 2 sequence: it is similar to the previous one, but the level of marly limestones is substituted here by a tabular bank composed of light colored, compact limestones with diverse inner structures and fossil contents. The thickness relationship between both levels (i.e., marls and limestones) is highly variable.

Type 3 sequence: it is also similar to the type 1 sequence, but in this case the marly limestone level is replaced by dark colored, massive dolomitic limestones in tabular strata. This sequence is mainly represented in the Orce-Venta Micena sector. Type 4 sequence: it is found in some marginal sectors such as Cuevas del Campo and Huélago. This sequence consists of a level of white isotropic marls that evolved toward edaphogenic facies and, finally, to a 40 cm thick bank of compact calcarenites. The thickness relationship between marls and calcarenites is 3:1. Type 5 sequence: it represents the transition from a level of light colored isotropic marls and calcilutites to a stratum of sandstones with an erosive bottom and parallel bedding. The thickness relationship between both levels fluctuates between 1:1 and 5:1 in favor of marls.

5.2. Cyclicity Given the moderate thickness recorded in the sedimentary strata and the low repetition rate of the sequential patterns, it is difficult to perform analyses on sedimentary cyclicity. However, cycles with frequencies included within the range of Milankovitch (38e39 ka and 102e107 ka, roughly corresponding to inclination and eccentricity, respectively) have been tentatively established for

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Fig. 4. Stratigraphic profiles for the Early Pleistocene sediments and their location in the basin.

the Pleistocene deposits using Fourier transforms (García-Aguilar, 1997). 5.3. Sedimentary conditions: sediment composition and genetic data Marls with isotropic texture sum up 33% of the thickness of the sedimentary unit (Table 4). The mineralogical analyses of these facies show values of average composition for the whole sample of Table 1 Mineralogical analyses of the samples studied. The values are shown as percentages over the total of each sample (t: traces). Key for minerals in total fraction: C, calcite; D, dolomite; Q, quartz; A, phylosilicates. Key for minerals in fraction <2m: Fd, feldespates; S, smectites; I, illite; K, caolinite; Cl, chlorite; Pl, palygorskite; Se, sepiolite. Sample

C

D

Q

A

Fd

S

I

Venta Micena-4 Venta Micena-3 Venta Micena-2 Venta Micena-1 Orce West-2 Orce West-1 Orce North

e 56 79 e e 49

46 e e 42 47 e

38 16 19 15 22 25

16 28 2 43 31 26

t t e t e t

6 4 10 8 10 55 15

34 38 52 58 50 37 40

Mean

31

15

44

22

22

25

t

K

Cl

Pl 53 50

3

7 4 5 4 5 8 7

36

3

6

27

4 8 4

Se

33 22 31

5

31% calcite, 25% clay minerals, 22% dolomite, 22% quartz, and traces of feldespates. Concerning the source area of the lacustrine system, the presence of micas in most levels of sandstones and conglomerates suggests a water input from the Internal Zones. However, some tractive levels in the Huélago and Orce sectors indicate drainage from the External Zones. Several levels generated by redepositions and intraclasts from this unit have also been found. The marls would have an inherited origin, at least in part (Arribas et al., 1988), because the drainage net would provide calcite to the lacustrine environment in clastic particles and also in ionic form after dissolution (CO32 and Ca2þ). In any case, if we consider the amount of marly limestones, which account for 31% of the total thickness, the biogenic formation of calcite must have been important. After calcite, clay minerals account for the greatest percentage of the composition of all samples. The specific analysis of these minerals (fraction <2 m) shows a composition of 44% illite, 27% palygorskite, 15% smectites, 6% chlorite, 5% sepiolite, and 3% kaolinite. Of these minerals, palygorskite, sepiolite and an undetermined fraction of smectites may be considered as neoformation minerals, while the others were clearly inherited (Sebastián-Pardo, 1979). All of these neoformation minerals are linked to alkaline hydrochemical conditions with scarce detrital input and high silica and magnesium contents, particularly in the case of sepiolite. In any

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Table 2 Analitic data on microfacies of carbonate rocks (thin section). Key for microfacies: MI, micrite with intraclasts; M, micrite; MF, micrite with fossils; G, micrite matrix with grumelar texture; A, general alteration; P, gypsum pseudomorphs. Percentage of holes in the sample (%H). Key for shape of holes (SH): A, circular; B, elongated; C, bifurcated; D, ring-shaped; E, latticed; F, uneven. Key for cementum (C): S, sparitic; MS, microsparitic. Allochemicals (Alc.): G, gastropods; B, bivalves; O-ostracods; I, intraclasts; A, algae. Classification of samples (Clas.) according to Dunham types (in Corrales et al., 1977): Mu, mudstone; Wa, wackstone. Sample, location and abbreviation

Microfacies

%H

SH

%D

C

Alc.

Clas.

Barranco del Canto 12 (Huélago-East). BC Barranco del Canto 11 (Huélago- East). BC Barranco del Canto-10 (Huélago- East). BC Barranco del Canto-8 (Huélago- East). BC Barranco del Canto-7 (Huélago- East). BC Arroyo de Gorafe-16 (Gorafe-North). AG Arroyo de Gorafe-15 (Gorafe- North). AG Arroyo de Gorafe-14 (Gorafe- North). AG Arroyo de Gorafe-13 (Gorafe- North). AG Cuevas del Campo-2. CC Cuevas del Campo-1. CC

MI, G, A M M, G. A MI, G, A MI, G, A MF, G MF, G M MF, G, P MI, G, A MI, G, A

27 18 18 8 20 5 16 23 10 22 22

C, F B, D, F D, F B, C A, B, C A, C C, E, F A, C, F B, C, F B, D A, B, C

3 3 2 10 2 2 2 1 5 1 3

S S S e e MS S, MS S, MS S S MS

I I I B, I I G, I, A G e B, O, I, A I I

Wa Wa Mu Wa Wa Mu Mu Mu Wa Mu Mu

case, the presence of sepiolite in the sedimentary unit is limited to a sample from the Venta Micena sector, which shows a value of 33%. Based on the presence of this mineral and also on the finding of palygorskite, this area would define a "central" zone of the lacustrine system. Quartz sums up 22% of the average composition of samples and is another element of inherited origin, like feldespates. Dolomite, which also represents 22% of total, can be considered as a primary mineral that evidences a high concentration of magnesium, although its origin could be also associated to diagenetic processes (Sebastián-Pardo, 1979). Muller et al. (1972) proposed that dolomite precipitates in lacustrine environments if Mg2þ/Ca2þ > 7, which would imply high magnesium input. Magnesium could provide from the dissolution of rocks rich in this element or even could be contributed by hydrotermal vents. In addition to this autochtonous fraction, we must consider the contribution of detrital dolomite for those cases in which this mineral is included in the lutitic or sandstone facies. The mineral origin of calcite (31% of total) deserves special consideration. Although the genesis of inorganic calcite is an option that must be considered (Kelts and Hsu, 1978), in most cases this mineral has an organic origin in lacustrine environments, either as bioclast or derived from biochemical processes in a broad sense (Freytet, 1973, 1975; Plaziat and Freytet, 1978; Murphy and Wilkinson, 1980; Buczynski and Chafetz, 1991; Koban and Schweigert, 1993; Kempe and Kazmierczak, 1993). Finally, microfacies data were calculated on samples of limestones, calcareous marls and dolomites (Fig. 5), which together represent 31% of the thickness of the sedimentary unit and almost 40% of the levels. Micrite with intraclasts represent 46% of samples, followed by micrite and micrite with fossils (27% in both cases). These analyses reveal a depositional environment characterized by a high sedimentary variability in time and space, and also by the presence of specific periods with high biological productivity in the lacustrine system. The diversity of facies and sequences, as well as by their low lateral continuity in most cases, evidence the variability of this

Table 3 Statistical inform on microfacies for the Early Pleistocene of Guadix-Baza basin according with Table 2 (*: values expressed independently over the total number of samples). 123456-

Microfacies: MI, 46%; M, 27%; MF, 27% Mean percentage of holes: 17% Shape of holes *: A, 36%; B, 54%; C, 73%; D, 27%; E, 9%; F, 54% Mean percentage of detritics: 3% Types of allochemicals*: G, 18%; B, 18%; S, 9%; I, 82%; A, 18% Classification: Mu, 55%; Wa, 45%.

sedimentary unit, which results in the alternation of fluvial, swampy and biogenic lacustrine stages. The periods of high biological productivity are reflected in the deposits of marly limestones and dolomites, together with specific marly levels, which sometimes include true lumaquelas with foraminifers, ostracods, gastropods and vertebrates (Anadón et al., 1986). Another important aspect concerns the hydrodynamics of these lacustrine systems, with episodes of high relative energy reflected in the presence of calcarenites (7%), the high percentage of microfacies with intraclasts (86%) and the abundant tractive levels with centimetric-sized intraclasts and bioclasts that may be associated to lake shore areas. These coastline areas would have been affected by the action of waves and/or currents with some energy. The higher levels of microbial activity during certain periods would result in the production of carbonate sediments in shallow, well-lit waters with high nutrient contents. Thus, the production of micritic sediment would be linked to bacterial activity, as evidenced by the continued presence of grumelar structures in the thin sections analyzed (Arenas, 1993). Holes represent on average 17% of the laminar surface of the samples, with forms that reveal root traces in the sediment (elongated, bi- and trifurcated hollows).

5.4. Sedimentary sequences: genetic models The type 1 sequence corresponds to isotropic marls with occasional presence of bioturbation and dispersed microcrystalline gypsum. Over these marls there is a bank of altered, white marly limestones with the bottom frequently ondulated. This succession has been described in several basins of the Iberian Peninsula (Arribas and Bustillo, 1985; Arribas, 1986; Arribas et al., 1988; Arenas, 1993) as a shallowing process from highstand periods (marls) to lowstand ones (marly limestones). The structural elements and microtextures of the upper level include nodulization, pedogenesis, brecciation, marbling, root bioturbation, rhizoconcretions, microkarstification and generalized disturbance of the sediment, which evidences a lowering of the water sheet. Such structures may be interpreted as resulting from pedogenetic swampy sedimentation and subsequent early diagenetic modification. The low proportion of allochemicals indicates a high organic growth and a low influence of tractive processes. Several authors (e.g., Freytet, 1973, 1975; Plaziat and Freytet, 1978; Freytet and Plaziat, 1982) have analyzed the genetic conditions associated to these swampy-pedogenetic zones, which include periods of sediment emersion and microbial activity. The length of the period of subaerial exposure determines the appearance of different structures. For example, a short period of desiccation results in root-marks and pedogenesis, while a long one produces

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Table 4 Geostatistical parameters of the facies and main sedimentary sequences defined in the Early Pleistocene of Guadix-Baza basin. Facies/sequences

Relative width (%)

Number of relative levels (%)

Mean width of each level or sequence (in m)

Distribution

Conglomerates Sands Lutites Marls Calcarenites Marly limestones Limestones Dolomites Type 1 sequence Type 2 sequence Type 3 sequence Type 4 sequence Type 5 sequence

3 11 6 33 8 9 22 8 13 40 17 13 17

4 3 8 47 7 8 19 4 6 48 24 11 11

1.2 0.3 1.3 1.4 0.8 0.9 0.8 0.5 1.0 2.5 2.8 1.6 1.2

Huélago and Venta Micena sectors Entire Unit Guadix basin Entire Unit Entire Unit Entire Unit Entire Unit Orce-Venta Micena sector Entire Unit. Dispersed Entire Unit Orce-Venta Micena sector Entire Unit. Dispersed Entire Unit. Dispersed

Fig. 5. Microfacies of the Early Pleistocene deposits of Guadix-Baza basin. The location of the samples according to the nomenclature used is at the left of each image and the type of microfacies is at the right. Microfacies nomenclature after Folk (1962; in Corrales et al., 1977): MI, micrite with intraclasts; M, micrite; MF, micrite with fossils.

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Fig. 6. Non-dimensional sedimentary model with the different depositional environments defined for the Early Pleistocene deposits of Guadix-Baza basin. The black arrows mark the evolution through time of these environments according to each of the five sedimentary sequences defined (numbers from 1 to 5). Key for facies: A, conglomerates; B, sandstones; C, lutites; D, marls; E, calcarenites; F, marly limestones; G, limestones; H, dolomites.

nodulization structures and recrystallization. Microkarstification and marbling account for even longer intervals of subaerial exposure. The microtextures associated with desiccation processes are common in all these cases, including root bioturbation, “bird’s-eyes” resulting from gas escapes, halos and channels around micritic nodules, shrinkage and clotted textures, ooid-cracks, grumelar elements and vug shapes. All these elements can be filled with microsparite cement. Similar microtextures in marls-marly limestones shallowing sequences are described by Arribas (1986), Arribas et al. (1988) and Arenas (1993). The transition from highstand to lowstand lacustrine periods does not necessarily imply a shift from central areas to marginal ones. In this case, it is more reasonable a desiccation or a lowering of the water table in a large area of the lacustrine system, which would be driven by the climatic cycles, resulting in the evolution towards a swampy regime. The type 2 sequence is a variation of the previous one, in which the marly limestones are replaced by limestones. This sequence has been described as characteristic of carbonate lacustrine environments under a shallowing trend (Arribas, 1986; Arribas et al., 1988; Arenas, 1993). However, in spite of its similarity with the previous sequence, it has some differences that allow modifying the sedimentological scenario proposed before. For example, the limestone stratum shows new textural elements of organic nature and lacks any sign of nodulization, fissured textures or zoned crusts, which are typical of swampy environments. In contrast, microtextural analyses indicate the predominance of biomicritic grumelar and intraclastic facies with oncolytes, characeans, gastropods, ostracods, plates of algal stromatolites, and colonies of cyanobacteria (Rivularea). With the only exception of the intraclastic facies, linked to tractive processes, all these lithologies demonstrate the existence of very low energy conditions in a permanent water sheet with a depth range comprised between 4 and 10 m (Murphy and Wilkinson, 1980). Koban and Schweigert (1993) describe several microtextures associated to micritic facies, which in many cases are observed here in the limestone levels. Specifically, these authors describe fabrics of microbial origin such as laminar stromatolite crusts, peloid layers (clotted fabrics) and gas bubbles resulting from O2 release during microbial photosynthesis. In addition, Kempe and Kazmierczak (1993) show similar textures in a modern lake and highlight the precipitation of CaCO3 associated with the enzymatic activity of cyanobacteria and the development of laminated or clotted microtextures. In other cases, these limestones show at their top signs of nodulization typical of edaphogenic environments linked to periods of emersion, as well as textures induced by eodiagenetic processes (marbling, gypsum pseudomorphs and black boulders). The appearance of the type 3 sequence is restricted to the OrceVenta Micena sector. From a lithologic point of view, this sequence is a variant of the former, in which the limestone level is substituted by dolomites or dolomitic limestones. The bottom marls show the typical

facies of a highstand level, while the dolomitic beds have a characteristic dark grey tone and preserve ostracods and gastropods. Sebastián-Pardo (1979) has proposed a hydrothermal primary origin for the dolomite of the Guadix-Baza basin. These data suggest that the level of dolomitic limestones has a primary nature, evidencing a fall of the lake level, increased evaporation and magnesium input from hydrothermal upwellings and/or detritic flows. The type 4 sequence is composed of isotropic marls above which there is a 80 cm thick level of calcarenites with variable grain size (0.5e2 mm) and intraclastic or bioclastic composition. This deposit generated in a coastal lake scenario subjected to waves and currents that produced particle size selection and transport. This hypothesis is confirmed by the rounded shape of boulders and by the presence of tractive structures such as bottom marks and crosslaminations. For this reason, this sequence indicates a shallowing trend from a highstand level (marls) to a lowstand one represented by coastal facies subject to the action of waves and currents. The low statistical representation of this sequence (11%) indicates its sporadic nature within this unit. Finally, the type 5 sequence consists of the superposition of a level of sandstones with a very mature texture and a variety of inner sedimentary structures, including cross-lamination developed on isotropic marls or bioturbation and sporadic lamination. This sequence has been interpreted by Arribas et al. (1988) and Arenas (1993) as evidencing mixed conditions of alluvial plain and lacustrine system. In our case, it indicates the expansion of distal alluvial flows from the western sector over a shallow lake with marly sedimentation. These alluvial materials would entry in the lacustrine system through small deltas, which is evidenced by the internal structures observed in some sandstone levels, as well as through wide areas of the lake shore that promoted an intense ecological activity of vertebrates. Figure 6 shows the depositional environments which correspond to each of these sedimentary sequences defined for the Early Pleistocene deposits of Guadix-Baza basin. 6. Modeling the sedimentary scenario and the limnogeological parameters 6.1. Geometry, drainage and feeding flow Cartographic data and stratigraphic analysis allow us to deduce with reliability the geometric model of the Early Pleistocene lacustrine system of Guadix-Baza. These data evidence a scarce diastrophic activity, which resulted in a change in the Plio-Pleistocene record of this basin toward the retraction of the large alluvial systems from the south-southwest and the expansion of both the alluvial plains developed under an edaphogenic regime and the carbonate lacustrine systems. If these data are taken into account, the surface distribution of the Early Pleistocene deposits would

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have been slightly smaller than the Late Pliocene ones in the Guadix subbasin, but much larger in the Baza subbasin, where they theoretically occupied most of its length. Subsequent erosion of the Pleistocene deposits in the central sectors of the Baza subbasin would have resulted in the emergence of the Late Pliocene sediments (marly-evaporitic unit). From these assumptions, a model for the paleogeographic distribution of the whole unit that helps to deduce several geometric parameters is proposed in Fig. 7. For example, the lacustrine system during the period of greatest extension would reach a length of 110 km (Huélago-Venta Micena), with a total perimeter of 300 km and average widths of 6 and 17 km in the Guadix and Baza subbasins, respectively. The total surface estimated for this lacustrine system would be around 875 km2, mostly aligned in N50e70E direction and concordant with the major regional faults. Morphologically, the western part of the basin (Gorafe and Huélago sectors) had a higher length/width ratio, while the eastern part was more equidimensional (Baza subbasin). Both areas would receive detritic materials contributed from low energy alluvial systems originated in the Internal Zones (Sierra Nevada and Sierra de Baza) and, to a lesser extent, also in the External Zones. In this way, it is possible to infer a paleogeography dominated by external reliefs of limited erosive potential and a predominance of shallow sedimentation areas with sporadic subaerial exposures. 6.2. Hydrodynamics and paleoclimatology The scenario described above evidences the hydrodynamic control of the lake by inner factors (currents and waves), with a dominant field of action around the island and coastal areas, which favored the formation and transport of intraclasts and bioclasts. Concerning the paleoclimatic parameters, biostratigraphic data on the abundance of rodent faunas indicate relative conditions of high humidity alternating with periods of desiccation linked to more arid climates (Ruiz Bustos, 1994). These periods would have induced the subaerial exposure of a wide extension of the lacustrine system, which is reflected in the appearance of desiccation structures in the limestones and marls of the unit. Finally, paleontological data point to a mean temperature of around 15
C during this period, but within the cyclic oscillations that could raise this value (Ruiz-Bustos, 1994). 6.3. Hydrochemistry Given the predominance of carbonate sediments in this unit (85% of total thickness), it is possible to define a water ionic content

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composed in its majority of CO32, Ca2þ, Mg2þ and, to a lesser extent, SiO2. This configuration would have favored the formation of sediments through biological production of calcite as well as decantation processes, dissolution-precipitation and mineral neoformation (palygorskite, sepiolite and dolomite). The ionic contents would come from the dissolution of rocks by meteoric waters in the source areas (calcium and magnesium carbonates and metapelites) as well as from hydrothermal inputs rich in Mg2þ and SiO2 from the northeastern edge of the Baza subbasin. In addition, the presence of organic remains in the unit evidences the availability of high nutrient contents and dissolved O2. All these conditions experienced significant changes in both time and space. An example of these variations would be the period of formation of sepiolite in the Venta Micena sector associated with high ionic contents of Mg2þ and SiO2 in the environment, with alkaline pH conditions and a long residence time of the waters, as evidenced by the isotopic data (see below). During the episodes of detritic sedimentation all these parameters would change to conditions of lower ionic contents, increased water renewal and a dystrophic nature, shifting to a fluvio-lacustrine model. 7. Paleoecology The Early Pleistocene locality of Venta Micena provides a good example on how paleoecology can be linked to the sedimentary sequences defined in the Guadix-Baza basin by analyzing the relationship between a number of paleoenvironmental parameters, the type of facies and the taphonomic attributes of the large mammals assemblage. As explained below, the shallowing periods related to the upper limestone beds correspond to conditions of lower salinity and higher temperature of the lake waters, which resulted in an increased hydrological dynamics that is compatible with the model of sedimentary evolution proposed for these sequences. The trace-element and stable-isotope chemistry of lake waters is a sensitive monitor of climate in arid and semi-arid regions (Hu et al., 1998). Fig. 8 shows the abundance of several trace-elements (Mn, Fe, Na, and Mg) and the proportions of carbon-, oxygen- and strontiumisotopes in a number of bulk-rock samples taken along the stratigraphic section of Venta Micena, which lithology is composed of alternate levels of micrite limestones, calcilutites, lutites, silts, and marls. The main excavation quarry of this locality is located within the upper part of the section, in an 80e120 cm thick limestone stratum undisturbed by tectonic activity and composed of homogeneous and porous micrite sediments (98e99% CaCO3) that evidence a lacustrine stage (Arribas and Palmqvist, 1998). The lower

Fig. 7. Proposal of a bidimensional paleogeographic configuration for the sedimentary complex of Early Pleistocene age of Guadix-Baza basin. Key: 1, main direction of detritic inputs; 2, region of lacustrine-swampy sedimentation, 3, region of alluvial sedimentation (Guadix Formation); 4, contact with the substrate of the External Zones; 5, contact with the substrate of the Internal Zones and pre-Pleistocene units. Geographic references: G, village of Gorafe; J, Jabalcón mountain (1494 m); O, village of Orce.

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Fig. 8. Stratigraphic profile of Venta Micena showing the abundance of a number of trace-elements and stable-isotopes that allow to define a shallowing sequence for this Early Pleistocene locality (data from Palmqvist et al., 2008a). VM-2 marks the position of the main excavation quarry of Venta Micena within this sequence.

half of this stratum has carbonate nodules (5e20 cm thick), mud banks and fossil shells of eurythermal freshwater mollusks. During this stage, micrite was precipitated in a shallow (<10 m) and well-oxygenated water sheet not subject to eutrophic conditions, as indicated by the absence of pyrite and carbonate facies rich in organic matter. Above this level there is a 4e15 mm thick calcrete paleosoil, which records a major retreat of the Pleistocene lake and represents a swampy biotope with wide emerged zones (w4 km width) and shallow ponds (<1 m depth, 2e20 m diameter). The upper half of the stratum is composed of micrite sediments showing root marks and mud cracks at the bottom, which record the rise of the lake level. This level preserves a huge assemblage of large mammals with a high density of fossil bones (>50 skeletal elements/m2). The macrovertebrate assemblage of Venta Micena has been dated by biostratigraphy to the Early Pleistocene (Agustí et al., 1987; Martínez-Navarro, 1991; Arribas and Palmqvist, 1999; Duval, 2008), with an age estimated in w1.5 Ma. Analysis of weathering stages indicates a very short period of exposure before burial (less than one year for most bone specimens). The surface of the skeletal remains is not abraded and the longitudinal axes of long bones show no preferred orientation, which indicates that they were not transported by fluvial processes prior to deposition. Furthermore, the ratio of isolated teeth to vertebrae is close to the value expected in the absence of hydrodynamic sorting (1:1) and the frequencies of bones grouped according to their potential for dispersal by water (i.e., Voorhies’s groups) are similar to those in the mammalian skeleton (Arribas and Palmqvist, 1998; Palmqvist and Arribas, 2001). Analysis of mortality patterns deduced for ungulate species from juvenile/ adult proportions has shown that most skeletal remains were scavenged by the giant hyena Pachycrocuta brevirostris from carcasses of animals hunted selectively by hypercarnivores (Palmqvist et al., 1996). Taphonomic analysis revealed that the hyenas transported ungulate carcasses and body parts to the vicinity of their maternity dens, where they fractured selectively the major limb bones for accessing their marrow contents (Palmqvist and Arribas, 2001). Geochemical data reveal several key relationships on the chemistry and temperature of the lake waters. For example, iron and manganese concentrations decrease from level VM-1 to level VM-2 (see Fig. 8), which indicates a shift from a restricted, stratified water table with decreased oxygen levels and stagnation to a more open, well-oxygenated water column resulting from increased water supply and circulation. Sodium concentrations, which may be interpreted as representative of lake salinity levels, decrease systematically from the base of the section to the middle of the section, where

they reach their lowest values, and then suddenly rise to high concentrations below the paleosoil. These changes suggest that the middle part of the section witnessed an increase in the water level, and thus a reduction of salinity, which is supported also by iron and manganese concentrations, although the salinity level dramatically increased just prior to paleosoil development, evidencing the lowering of the water table. The proportion of oxygen-isotopes (18O/16O) relates inversely to water temperature, given the preferential evaporation of the lighter H216O molecules and their storage as glacial ice (Koch, 1998). Magnesium concentrations correlate also negatively with the paleotemperature of lake waters (Chivas et al., 1986). Fig. 8 shows that there is a general trend of decrease in d18O and Mg values through the Venta Micena section, which indicates an overall warming from level VM-1 to level VM-2. Carbon-isotopes (13C/12C) are an archive of more difficult interpretation, however, but it is worth noting that d13C and d18O values show parallel trends in the section: d13C ratios take the maximum value in the marly level at the base of the section, with minor fluctuations in the silty, lutitic and calcilutitic levels, and then show a slight decrease in the upper micrite limestones. Atmospheric CO2 has a higher d13C value (6.5&) than C3 and C4 plants (26& and 12&, respectively), contributing to soil CO2 near the surface. However, the CO2 >30 cm deep in soils is largely supplied by plant decay and root respiration, which both generate CO2 isotopically similar to organic matter. For this reason, the d13C of soil carbonate is strongly correlated to that of the overlying vegetation (Koch, 1998; Palmqvist et al., 2008a). Diffusion of CO2 from the soil to the atmosphere leads to a d13C enrichment of þ4.5& for CO2 at depth in a soil relative to soil organic matter. In addition, temperature-dependent fractionation associated with precipitation of calcite sum to a d13C increase of þ10.5&. As a consequence, modern carbonates forming below 30 cm depth have d13C values 15& higher on average than those of organic matter: 11& for soils with C3 overlying flora and þ3& for those in which organic matter is supplied by C4 plants (Koch, 1998). d13C ratios measured in the Venta Micena section (range: 7.4& to 4.1&; Fig. 8) lie between both values, suggesting a mixed vegetation of C3 and C4 plants. However, d13C values for bone collagen of grazing ungulates (see below) show the absence of C4 grasses in their diet, which implies that other factors may also have been involved in determining bulk-rock d13C ratios: for example, under higher pressures of atmospheric CO2, more of the CO2 at depth in soils is derived from the atmosphere, increasing the difference in d13C values between soil carbonate and organic matter (Koch, 1998).

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The proportion of strontium-isotopes (87Sr/86Sr) is useful for deriving the paleosalinity record of ancient environments if independent constraints on the system’s hydrologic parameters (i.e., evaporation, precipitation and fluvial flux) are available (Flecker et al., 2002). 87Sr/86Sr ratios of river waters are similar to those of terrestrial plants and monitor the soluble strontium in soils, derived from weathering and precipitation (Hoppe et al., 1999). For this reason, 87Sr/86Sr ratios record differences in atmospheric input as well as in bedrock age and composition (Price et al., 1985; Miller et al., 1993). Concerning the stratigraphy of Venta Micena (Fig. 8), bulk-rock 87Sr/86Sr ratios are relatively uniform in the lower part of the section, with the only exception of a slight decrease in level VM-1. This reflects deposition under conditions of hydrological stability. The upper carbonate samples, however, show a significant decrease in 87Sr/86Sr proportions, which reflects the increase in river and/or groundwater input that translated in the rising of the lake’s table in the lacustrine levels. Fig. 9 shows a plot with the average ratios of carbon- and nitrogen-isotopes of bone collagen in 17 species of large mammals from Venta Micena (data from Palmqvist et al., 2003, 2008a, 2008b). The range of d13C values for ungulates (27& to 20&) indicates that all these herbivore species consumed C3 plants, which implies that C4 grasses were absent from southern Spain during Early Pleistocene times. There are, however, significant variations in the relative abundance of carbon-isotopes among ungulates, with perissodactyls Equus altidens and Stephanorhinus hundsheimensis showing the lowest d13C values and bovids Bison sp., Soergelia minor and Hemitragus albus the highest ones. Given the hypsodonty values of these species, such difference does not indicate a different feeding behavior for both groups. Rather, it reflects a lower isotope-enrichment factor for monogastric herbivores compared to ruminants, which relates to physiological differences in the digestive system of hindgut and foregut fermenters

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(Palmqvist et al., 2003). Cervids Praemegaceros cf. verticornis and Metacervocerus rhenanus show lower d13C values than bovids, which suggest a browsing diet in closed canopy areas in contrast to bovids, which were grazers or mixed-feeder in open habitat. Megaherbivores Mammuthus meridionalis and Hippopotamus antiquus show high d13C proportions, similar to those of bovids, which result from their long ingesta passage rates (Palmqvist et al., 2008a). Carnivores Homotherium latidens, Megantereon whitei, Panthera cf. gombaszoegensis, P. brevirostris, Lycaon lycaonoides, Canis mosbachensis and Ursus etruscus show higher d15N values than ungulates except in the case of hippo and Praeovibos sp. The isotopic fractionation between carnivores and herbivores agrees with the enrichment value expected from increasing one trophic level, which indicates that the collagen extracted from the fossils did not undergo diagenetic alteration (Palmqvist et al., 2003, 2008b). The high d15N values obtained for the hippo, which are even more elevated than those of carnivores, suggest that it fed predominantly on aquatic plants in a lake with moderate salinity levels, instead of consuming terrestrial grasses as do living hippos. This is corroborated by the huge body mass (w3200 kg) and shortened metapodials of this extinct species, which resulted in severe limitations for terrestrial locomotion. The elevated d15N value of muskoxen may be indicative of increased recycling of nitrogen from body protein during winter due to a poor quality diet based on lichens in the surrounding mountainous areas. Perissodactyls and bovids show more positive d15N ratios than cervids, which confirms that the latter preferably fed in closed habitat, where their low d15N values would result from soil acidity. Finally, the range of d13C and d15N values for carnivores reflects resource partitioning among sympatric predators and helps in deciphering predatoreprey relationships in Venta Micena, which provides a proxy to reconstructing the trophic web of this ancient community (for details and discussion, see Palmqvist et al., 2008b).

Fig. 9. Bivariate plot showing the relative abundances of carbon- and nitrogen-isotopes in samples of bone collagen from the large mammal species of Venta Micena (data from Palmqvist et al., 2008a). The ranges of d13C and d15N values allow interpreting the habitat adaptations, dietary preferences and ecophysiology of these Early Pleistocene species. Carnivores: dotted symbols, herbivores: open symbols. Horizontal and vertical lines represent one standard deviation above and below the mean for those species in which at least three isotopic measurements were available.

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8. Tectonic activity Most aspects related to the tectonic and diastrophic activity of this sedimentary unit have been discussed before. It is only necessary to emphasize here the decrease in the average sedimentation rate (w3 cm/ka) compared to the Pliocene units, which reflects an attenuated tectonic activity in a stage of relative stability after the significant expansion of the basin during the Late Villanian. These features are complemented by the lower erosive potential of the outer reliefs, as evidenced by the scarcity of alluvial facies in the unit. In this sense, a similar paleogeographic scenery to the modern geometry of the basin can be proposed, dominated by highly variable depositional environments depending on the climatic factors. The only elements that evidence some tectonic activity are the virtual presence of hydrothermal fluids in the Galera-Venta Micena sector, probably associated with the dynamics of the N50e70E major fracturing system present in the basin. 9. Conclusions The w10 m thick Early Pleistocene sedimentary unit of GuadixBaza basin is composed of 2e5 shallowing upward lacustrine sequences that reflect the transition from highstand (marls) to lowstand conditions (limestones s.l, dolomites or sandstones). Sedimentological analyses have revealed a depositional environment characterized by a high sedimentary variability in time and space, and also by the presence of periods with high biological productivity in the lacustrine system. The diversity of facies and sequences, as well as their low degree of lateral continuity in most cases, evidence the variability of this sedimentary unit, which results in the alternation of fluvial, swampy and biogenic lacustrine stages. The periods of high biological productivity are represented by the deposits of marly limestones and dolomites, together with specific marly levels. Trace-elements and stable-isotopes analyses show the changes in the chemistry and temperature of the lake waters associated to these sequences. Specifically, the limestone beds correspond to conditions of low salinity, high temperature and well-oxygenated waters resulting from increased water supply and circulation. In contrast, the marly levels were deposited in a more stratified water table with decreased oxygen levels and stagnation. These depositional environments took place into a tectonic scenario marked by w0,5-1 cm/yr extensional movements, subsidence and hydrothermal inputs. Isotopic analyses of bone collagen samples from the large mammals identified in the Venta Micena assemblage contribute inferences on the feeding preferences and environmental adaptations of these species, providing interesting proxies for reconstructing the trophic web and habitat of this Early Pleistocene paleocommunity. Acknowledgements Funding and analytical facilities for biogeochemical analyses were provided by the Universities of Granada and Málaga, project CGL2008-04896. We gratefully acknowledge J. Soria for his insightful comments and helpful criticism of the original manuscript. References Agustí, J., Arbiol, S., Martín-Suárez, E., 1987. Roedores y lagomorfos (Mammalia) del Pleistoceno inferior de Venta Micena (Guadix - Baza, Granada). Paleontologia i Evolució, Special Memoir 1, 95e107. Anadón, P., De Deckker, P., Juliá, R., 1986. The Pleistocene lake deposits of the NE of Baza basin (Spain): salinity variation and ostracod succession. Hydrobiologia 143, 199e208.

Anadón, P., Utrilla, R., Juliá, R., 1995. Palaeoenvironemental reconstruction of a pleistocene lacustrine sequence from faunal assemblages and ostracode shell geochemistry. Baza basin, SE Spain. Palaeogeography, Palaeoclimatology, Palaeoecology 111, 191e205. Arenas, C., 1993. Sedimentología y paleogeografía del terciario del margen pirenaico y sector central de la Cuenca del Ebro (zona aragonesa occidental). Unpub. PhD Thesis dissertation, University of Zaragoza. Arribas, A., Palmqvist, P., 1998. Taphonomy and paleoecology of an assemblage of large mammals: hyaenid activity in the lower Pleistocene site at Venta Micena (Orce, Guadix-Baza Basin, Granada, Spain). Geobios 31, 3e47. Arribas, A., Palmqvist, P., 1999. On the ecological connection between sabre-tooths and hominids: faunal dispersal events in the Lower Pleistocene and a review of the evidence for the first human arrival in Europe. Journal of Archaeological Science 26, 571e585. 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Modelling the effect of evaporation on the salinity-87Sr/86Sr relationship in modern and ancient marginal-marine systems: the Mediterranean Messinian Salinity Crisis. Earth and Planetary Science Letters 203, 221e233. Freytet, P., 1973. Petrography and paleo-environment of continental carbonate deposits with particular reference to the upper cretaceous and lower Eocene of Languedoc. Sedimentary Geology 10, 25e60. Freytet, P., 1975. Le Danien (Dano-Montien) des petites Pyrénées et du Plantaurel: Etude pétrogràfique et palèogèographique des facies lacustres. Géologie Méditerrannée II-4, 159e178. Freytet, P., Plaziat, J., 1982. Continental carbonate sedimentation and pedogenesis. Schweizerbart-Verlagbusch. Contributions to Sedimentology 11, 1e216. Garcés, M., 1993. Magnetoestratigrafía de los sedimentos lacustres pliocenos de la sección de Galera (Cuenca de Guadix-Baza, Cordilleras Béticas). Unpub. Ms Thesis dissertation, University of Granada. García-Aguilar, J.M., 1997. 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