Geomorphic and Sedimentary Plio–Pleistocene Evolution of the Nerja Area (Northern Alboran Basin, Spain)

Geomorphology 60 (2004) 89–105 www.elsevier.com/locate/geomorph

Geomorphic and sedimentary Plio–Pleistocene evolution of the Nerja area (northern Alboran basin, Spain)

A. Guerra-Mercha´n*, F. Serrano*, D. Ramallo

Departamento de Ecologı´a y Geologı´a, Facultad de Ciencias, Universidad de Ma´laga, 29071 Ma´laga, Spain Received 3 February 2002; received in revised form 1 April 2003; accepted 17 July 2003

Abstract

The Nerja area (S Spain), on the northern edge of the Alboran basin, underwent palaeogeographic, sedimentary and geomorphic changes during the Pliocene and Pleistocene, that were controlled by tectonics and by eustatic–climatic variations. The existence of NW–SE fault lines and the particular lithology of the area played a fundamental role in differentiating two sectors of divergent geomorphic evolution: the NE sector, which forms part of the Sierra Almijara, and the SW sector, in the area of Nerja. The NE sector contains outcrops of Alpujarride marbles and is characterised as a continental domain that has been permanently emerged and subjected to uplifting at intervals, under which the relief becomes progressively steeper. Today, the SW sector is constituted of Alpujarride schists and Plio–Quaternary sediments. This sector was partially affected by the marine transgression of the early Pliocene, during which time alluvial fans developed; distally, these are connected to littoral and shallow marine environments. During the early Zanclian, a tectonic event separated the Pliocene sediments into two lithostratigraphic units that rest unconformably but without any significant sedimentary interruption. As a consequence of this event, some fluvial courses changed direction. Towards the end of the early Zanclian, falling sea levels led to the emergence of the Nerja region. During the late Pliocene, marine sediments were restricted to the innermost areas of the Alboran Sea, but continental deposits did not develop, probably due to the warm, humid climate that did not favour clastic sediment supply. Another tectonic event in the Pliocene tilted the sediments of the Zanclean II unit in the western sector. During the cold period in the final Pliocene and in the basal Pleistocene, a pediment developed over the Pliocene units and the Alpujarride basement. Three phases of sedimentation of alluvial fans took place during the Pleistocene, becoming progressively more limited towards the SW. The deposition phases are separated by stages of erosion and trenching of the fluvial network, mainly occurring during periods of low sea levels. Another tectonic pulse has been identified, in the mid-Pleistocene, between the first and the second generation of fans, which caused further changes in the direction of some fluvial courses. D 2003 Elsevier B.V. All rights reserved.

Keywords: Geomorphic evolution; Pliocene; Pleistocene; Alboran basin; Nerja; Malaga; Betic Cordillera

1. Introduction

The Betic Cordillera and the Rif constitute the * Corresponding authors. A. Guerra-Mercha´n is to be contacted westernmost extent of the Mediterranean Alpine at Fax: +34-52-132000. mountain ranges. Its tectogenesis is related to a geo- E-mail address: [email protected] (A. Guerra-Mercha´n). dynamic framework of compression (approximately

N–S) between the African and European plates occur- the Alboran block (Andrieux et al., 1971), which today ring from the late Cretaceous to the Miocene (Tappon- forms part of the Betic-Rif Internal Zones, approached ier 1977; Olivet et al., 1982; Olivet, 1996). The last and collided with the continental margins of the paroxysmal tectogenetic phase took place during the Iberian block (Betic External Zones) and of the Afri- early Miocene (Burdigalian), due to a process in which can plate (Rif External Zones) (Durand-Delga and

Fig. 1. Location of the Nerja region within the Betic Cordillera (A) and geological map of the study area (B). A. Guerra-Mercha´n et al. / Geomorphology 60 (2004) 89–105 91

Fontbote´, 1980; Martıń-Algarra, 1987; Sanz de Gal- that is very narrow (5–10 km), which has a steep deano, 1990a, 1997, among others). Crustal thinning gradient (0.5–0.7j) and a shelf-break located at a water in a trans-extensional context led to the formation of depth of 110–120 m (Ercilla et al., 1992; Hernańdez the Alboran Sea (Platt and Vissers, 1989; Comas et al., Molina et al., 2002). The stratigraphic record of the 1992; Sanz de Galdeano, 1997). continental shelf comprises sediments from the Mio- After the paroxysmal tectogenetic phase of the cene to the present day, but contains important gaps Burdigalian, the Betic Cordillera gradually acquired (Comas et al., 1992; Campillo et al., 1992). its present configuration. After the middle Miocene, This study examines the influence of tectonics and distensive and transtensive tectonics configured moun- eustacy on the sedimentary and geomorphic evolution tain reliefs and postorogenic sedimentary basins. The of a sector of this northern edge of the Alboran basin, latter were found in subsidence-affected areas that from the Pliocene onwards, which is when the first were filled by the accumulation of sediments (Mon- postorogenic sediments were deposited in this area. tenat, 1990; Serrano, 1979; Rodrı´guez-Fernańdez, This sector is located at the eastern part of the 1982; Sanz de Galdeano and Vera, 1992; Rodrı´guez- province of Malaga, close to the town of Nerja Fernańdez and Sanz de Galdeano, 1992; Guerra-Mer- (36j45VN, 3j52VW); it extends from the southernmost chań, 1993). During the initial stages (middle and late foothills of Sierra de Almijara to the coast, where the Miocene), these basins, located in both the Internal and towns of Nerja and Maro are situated (Fig. 1B). the External Zones (Fig. 1A), were interconnected by marine waters. Subsequently, although later (the late Miocene or the Pliocene), the basins acquired a con- 2. Stratigraphic record tinental regime (Sanz de Galdeano and Vera, 1992). Tectonic activity and changes in sea levels modified Sierra de Almijara is formed of metamorphic rocks the detrital source areas subjected to erosion, and also of the Alpujarride complex, belonging to the Betic the deposition areas, modelling the landscape to create Internal Zone. The lithologic sequence of this complex the contemporary configuration. Data on these pro- is constituted of three well-differentiated lithostrati- cesses have been provided by Harvey (1984, 1987), graphic units: at the base is a thick Palaeozoic forma- Silva et al. (1992, 1993), Calvache and Viseras (1997), tion of dark graphitic schists with intercalations of Rodrı´guez Vidal et al. (1998), Harvey et al. (1999), quartzites; above this rests a formation of lighter- Mather (2000) and Harvey (2002a). coloured phyllites with a greater abundance of quartz- At present, the Alboran Sea is the great postoro- ite intercalations, dating from the Permian and the genic axial basin of the Betic-Rif orogen. On its early Triassic; above this there is a transition member northern border (Spain), the emerged margin includes comprised of alternating schists, calcoschists and mar- mountainous relief of folded rocks and downfaulted bles, which leads to a thick middle–late Triassic basins filled by postorogenic deposits. The former are formation of dolomitic marbles, passing at the top to found mainly in the domain of the Internal Zones of the limestone marbles with intercalations of calcoschists Betic Cordillera, with the exception of the westernmost (Avidad and Garcıá-Duenãs, 1981; Elorza and Garcıá- reliefs, which form part of the Campo de Gibraltar units Duenãs, 1980; Sanz de Galdeano, 1990b; Andreo et (Fig. 1A). To a large extent, these mountain reliefs are al., 1993). In the area studied, the Alpujarride complex delimited by faults, many of which present a vertical is composed of two thrust nappes: the Almijara nappe slip of several hundred metres, in some locations at the bottom and the Guajares nappe above it, the two reaching 1000 m (Sanz de Galdeano and Lo´pez Gar- presenting a similar lithologic sequence. The Triassic rido, 1991; Rodrı´guez-Fernańdez and Martıń-Penela, marbles of the Almijara nappe outcrop in the NE sector 1993; Martıńez-Dıáz, 2000). These faults favoured the of the area studied, while outcrops in the SW sector uplift of mountain ranges, while the downfaulted comprise mainly Palaeozoic schists of the two nappes blocks were transformed into sedimentary basins, (Fig. 1B). which during the marine stages of the late Miocene The study area contains no outcrops of late Miocene and/or the Pliocene comprised bays within the Alboran age, the closest such being located to the W, in the basin. Today, the submerged part is made up of a shelf Malaga basin (Serrano 1979; Sanz de Galdeano and 92 A. Guerra-Merchań et al. / Geomorphology 60 (2004) 89–105

Lo´pez Garrido, 1991), and to the E at Campo de Dalıás ravine, the lower unit is constituted entirely of marble to the S of Sierra de Ga´dor (Rodrı´guez-Fernańdez and pebbles, while in the upper unit there is a predomi- Martıń-Penela, 1993; Rodrı´guez Vidal et al., 1998).In nance of schist and quartzite pebbles, despite the addition, sediments of this age have been recognised in proximity of the marble outcrops. drillings carried out near the mouth of the Ve´lez River Quaternary deposits rest unconformably on both at Torre del Mar (Carrasco et al., 1978) and on the Pliocene units and on the Alpujarride basement. Three continental shelf of the Alboran Sea, off the coasts of Pleistocene stratigraphic units named Pleistocene I, II Malaga and Almeria provinces (Ryan et al., 1973; and III have been distinguished (Figs. 1B and 2). Jurado and Comas, 1992). In the SE sector of Sierra de These units are constituted of conglomerate and brec- Almijara, Rodrı´guez Vidal and Caćeres (1993) have cia of marble pebbles, which are cemented to a described an erosion surface that they attribute to the considerable degree. Debris flow deposits tend to late Tortonian, by correlation with the erosion surfaces predominate towards proximal zones, while distally studied by Lhenaff (1981) in other sectors of the Betic sheet and channel deposits are also common. These Cordillera. units characterise three systems of alluvial fans, which The Pliocene deposits rest unconformably on the are trenched one by one (Guerra-Merchańetal., Alpujarride metamorphic basement. Their morpholo- 2000b). The alluvial fans present maximum thick- gy is elongated and wedge-shaped and they fill ancient nesses (50–80 m) in the proximal areas and minimum valleys formed during the late Miocene. It is possible values (10–20 m) in the distal zones. to distinguish two stratigraphic units, separated by The Pleistocene I and III units present fining- weak angular unconformity, clearly observable at upward sequences, while the Pleistocene II unit char- Burriana beach (Nerja) and at Torre de Maro acterizes a fining-coarsening-upward sequence in the (Guerra-Merchań and Serrano, 1993; Guerra-Merchań fan distal zones (Fig. 2). The three units comprise et al., 1999). Both units correspond to alluvial fans mainly marble pebbles, which is in accordance with formed by breccia and conglomerates, which distally the location of the apexes of the alluvial fans close to pass to rich-Molluscan sands and microconglomerates the outcrops of Alpujarride marbles (Fig. 1B). Never- from littoral and shallow marine environments. The theless, it should be noted that in the outcrop of the lower unit (80–100 m thick) represents a transgressive Pleistocene I unit to the W of Maro, there is a sequence with marine facies superposed on alluvial predominance of schist and quartzite pebbles, despite facies, while in the upper unit (50–60 m thick) alluvial the proximity of the marbles, a circumstance resem- facies prograde over marine facies (Fig. 2). These two bling that found in the upper Pliocene unit. units as a whole, therefore, represent a deepening– The alluvial fans of the Pleistocene I unit extend shallowing sequence. It has not been possible to throughout the SW sector, with the exception of the determine their age on the basis of the fauna observed Rıó de la Miel sector. Lacustrine deposits intercalated in them, but equivalent transgressive–regressive sed- in this unit are found on the Frigiliana road (Km. 2) imentary cycles from the early Pliocene have been and near the Nerja Cave. The single palaeomagnetic described in the Almayate basin, W of Nerja (Aguirre, datum available from the upper part of the Torre de 2000) and in the Malaga basin (Guerra-Merchań et al., Maro fan (Jorda´ Pardo, 1992) indicates normal polar- 2000a). ity. In the Rıó de la Miel sector, Mayoral and Rodrı´- Concerning the nature of the pebbles, certain differ- guez-Vidal (1990) described a marine calcareous crust ences can be observed between the two Pliocene units. with bioerosive activity at 64–74 m a.s.l., which Lario In the western sector (Fig. 1B), the bottom of the lower et al. (1993) have linked to a marine episode of the unit is formed mainly of schist and quartzite pebbles, early Pleistocene. This crust covers part of the alluvial while upwards, marble pebbles become predominant. deposits of the Zanclean II unit and, given its altitude, In the east, the greater proximity of the marble out- it seems to be related to the Pleistocene I unit. crops means that in the lower unit the pebbles corre- The alluvial fans of the Pleistocene II unit devel- spond mainly to this lithology, while the upper unit is oped only to the W of Maro. To the E the outcrops comprised exclusively of marble pebbles, with the assigned to this unit present an elongated morphology exception of the outcrops west of Maro. In the Maro and characterize the infilling of fluvial valleys. From N A. Guerra-Merchań et al. / Geomorphology 60 (2004) 89–105 93

Fig. 2. Synthetic lithological column and geological sections showing the stratigraphic relations of the Plio–Quaternary deposits and the general characteristics of the relief of the Nerja area. To locate the sections, see Fig. 1B. to S, the trenching of the Pleistocene I unit by The only alluvial fan belonging to the Pleistocene Pleistocene II unit ranges from 10 to 50 m. Towards III unit is located on the Chı´llar River, immediately W distal zones, the Pleistocene II unit rests directly on the of Nerja. Alluvial deposits of the same unit are Pliocene or on the Alpujarride basement. Dura´n et al. exposed in the mouth of the Maro ravine, but for (1993) dated a calcite intercalation in the upper part of reasons of scale these cannot be shown in Fig. 1B. The this unit at approximately 120 ka (late Pleistocene). trenching of the Pleistocene II unit by Pleistocene III The palaeomagnetic data reported by Jorda´ Pardo unit ranges from 10 to 30 m. (1992) on 14 samples from fans located W of Maro Travertine deposits form a broad platform to the SE reveal normal polarity in 11 cases. One sample of Maro (Fig. 1B). The travertines rest over Alpujarr- obtained from the bottom of this unit presents inverse ide schists and present a wedge-shaped morphology, polarity, while the results from two other samples were with a maximum thickness of 30 m in the distal part. indeterminate. Jorda´ Pardo (1988) distinguished two types of facies: 94 A. Guerra-Mercha´n et al. / Geomorphology 60 (2004) 89–105

(a) cascade facies, with a banded internal structure, the marbles from the schists (Fig. 1B). These faults that form the framework of the travertine; (b) present dextral-strike slip (sometimes with a normal vegetable-structure facies, which appear in the mar- component) and normal slip. Sanz de Galdeano ginal zones. The Maro travertine has been dated at (1993) estimated that the principal strike slip attained 46 F 7ka(Dura´n, 1996), corresponding to the late a displacement of tens of kilometres and that these Pleistocene. movements were prior to the most important normal The last, Holocene-age, unit includes fluvial ter- slips. In addition, lesser NE–SW and E–W fault race deposits, piedmont, slope debris, travertines and systems have been observed, with mainly normal slip. recent and contemporary river and beach sediments. Diaclases are also frequent, especially in the marbles, among which the principal directions are NNW–SSE, NNE–SSW and E–W (Sanz de Galdeano, 1993). 3. Principal tectonic features In the study area, there is no evidence that Plio- cene sediments have been affected by the strike-slip The most characteristic tectonic feature of the faults. Therefore, taking into account that these faults study area is the NW–SE fault system that separates postdate the last main folding episode (Burdigalian),

Fig. 3. Topographic map of the study area showing the relief of the two sectors differentiated and the change in direction of the water courses in relation to the principal faults. A. Guerra-Mercha´n et al. / Geomorphology 60 (2004) 89–105 95 the lateral displacements associated with this type of of caves and cavities, resulting from the high dissolu- fault must have occurred during the middle–late tion rates of the dolomitic marbles and the severe Miocene. degree of fracturing. The best example of this is the Vertical movements took place during the Pliocene case of the Nerja Cave (Jorda´ Pardo, 1992; Carrasco, and the Pleistocene, fundamentally due to normal 1993). On the other hand, the considerable folding of faults that caused tilting and angular unconformities. the marbles has impeded the development of surface The two Pliocene units present an angular relationship, karstic forms. such that the strata of the lower unit are tilted 20–30j The SW sector contains the outcrops of Alpujarride towards the S, while those of the upper unit are complex schists and Plio–Quaternary deposits (Fig. inclined 10–15j towards the SW in the outcrops lo- 1B). The schists and the Pliocene deposits produce an cated W of Maro. In contrast, the outcrops E of Maro undulating relief of 10–30j slopes; these are related to are horizontal. the interfluves of first-, second- and third-order fluvial Another unconformity has been recognised be- segments. The valleys are less deep (100–200 m tween the Zanclean II unit and the Pleistocene I unit. incision) and the drainage network is denser than in To the W of Maro it is evident as an angular uncon- the NE sector (Fig. 3). Both the Alpujarride rocks and formity (10–15j), while to the E (at Torre de Maro) it the Pliocene sediments are affected by an erosion is present as a paraconformity, as the sediments of the surface over which Pleistocene sedimentation took two units are horizontal. The Pleistocene I unit, more- place (Fig. 2). over, is locally tilted (Fig. 1B) and affected by small The alluvial fans deposited during the Pleistocene normal faults and diaclases, oriented mainly NW–SE (Figs. 1B and 4) are strongly cemented. The fan and NE–SW. surfaces slopes (3–8j) and are stepped, so that the The last noteworthy feature related to tectonics is more recent ones are lower. The topographically high- the abrupt change in direction of some river courses. In est and best-represented surface (Pleistocene I unit) is the study sector, the main river courses tend to be found between Nerja and Frigiliana (geological cross- oriented S or SSW, but when the rivers pass from the section I in Figs. 2 and 4). The distribution of the NE sector (marble outcrops) to SW sector (schist outcrops and palaeocurrents (Fig. 1B) reveals that this outcrops), the direction changes to SE, approximating surface was created by the coalescence of two alluvial that of the main fault lines (Fig. 3). fans corresponding to the Higuero´n and Chı´llar Rivers. To the E of Nerja, this surface is very eroded and only small, isolated outcrops can be recognised (W and NE 4. Geomorphic features of Maro and at Torre de Maro). The fan surfaces in an intermediate position (Pleis- The NW–