The Messinian–early Pliocene stratigraphic record in the southern Bajo Segura Basin (Betic Cordillera, Spain): Implications for the Mediterranean salinity crisis

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Sedimentary Geology 20

The Messinian–early Pliocene stratigraphic record in the southern BajoSegura Basin (Betic Cordillera, Spain): Implications for the

Mediterranean salinity crisis

Jesús M. Soria a,⁎, Jesús E. Caracuel a, Hugo Corbí a, Jaume Dinarès-Turell b, Carlos Lancis a,José E. Tent-Manclús a, César Viseras c, Alfonso Yébenes a

a Departamento de Ciencias de la Tierra y del Medio Ambiente, Universidad de Alicante, Apdo. Correos 99, 03080 Alicante, Spainb Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605, 00143 Roma, Italy

c Departamento de Estratigrafía y Paleontología, Universidad de Granada, Campus de Fuente Nueva s/n, 18071 Granada, Spain

Received 8 November 2006; received in revised form 9 December 2007; accepted 21 December 2007

Abstract

The analysis of the Messinian and Pliocene stratigraphy of the southern Bajo Segura Basin (Betic Cordillera, Spain) has revealed threehighstand sedimentary phases (Messinian I, Messinian II, and Pliocene) bounded by two lowstand erosional surfaces (intra-Messinian and end-Messinian unconformities). The Messinian I highstand phase is characterized by the progradation of coastal and shallow marine sandstones (LaVirgen Fm) over slope and pelagic-basin marls (Torremendo Fm). After this first phase, a fall in sea level brought about the intra-Messinianunconformity, a subaerial erosional surface with local accumulations of karstic breccias and caliche-like carbonate crusts. The Messinian IIhighstand phase is represented by sandy beaches and muddy lagoons (Garruchal Fm) correlative with shallow marine evaporites (San Miguel Fm);this second phase records the intra-Messinian reflooding of the basin, which characterizes the salinity crisis in the marginal basins of theMediterranean. A new sea-level fall resulted in the end-Messinian unconformity, of which the most significant feature is the presence of a broadpalaeovalley, c. 200 m deep, which, along its course, completely eroded the deposits of the Messinian II phase and part of the deposits of theMessinian I phase. The Pliocene highstand phase begins with coastal and shallow marine conglomerates and sandstones (La Pedrera Fm) whichfill the deep part of the above-mentioned palaeovalley. These bottom deposits evolved gradually upwards towards pelagic marls (Hurchillo Fm),over which shallow marine and coastal sandstones prograded (Rojales Fm). This third phase records the flooding of the basin at the beginning ofthe Pliocene, when the salinity crisis ended in the marginal basins of the Mediterranean.

The combination of calcareous nannoplankton biostratigraphy and magnetostratigraphy has confirmed that both the end of the sedimentation ofthe Messinian I phase, as well as the two lowstand erosional surfaces (intra- and end-Messinian unconformities) and also the onset of the Pliocenephase occurred in the chron C3r (c. 5.9–5.2 Ma). Under the assumption of the classical model of a desiccated deep basin, either of the twoaforementioned erosional surfaces, or even both, could be correlative with the evaporites deposited in the abyssal parts of the Mediterranean.© 2008 Elsevier B.V. All rights reserved.

Keywords: Stratigraphy; Magnetobiostratigraphy; Messinian; Pliocene; Salinity crisis; Mediterranean region

1. Introduction

Ever since the term “Messinian salinity crisis” was coined bySelli (1960) and, particularly, after the discovery of evaporitesin the subsurface of the abyssal plains of the Mediterranean

⁎ Corresponding author.E-mail address: [email protected] (J.M. Soria).

0037-0738/$ - see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.sedgeo.2007.12.006

during the Leg 13 of the DSDP (Friedman, 1973; Hsü et al.,1973a; Nesteroff, 1973), giving rise to the hypothesis of thedeep desiccated basin (Hsü et al., 1973b), the salinity crisis hasbeen widely considered to be one of the most dramatic episodesof oceanic change in the last 20 million years (Krijgsman et al.,1999). The great interest sparked in the scientific community bythis event is due possibly to the attraction of visualizing theMediterranean as a desert at more than 1500 m below global sea

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268 J.M. Soria et al. / Sedimentary Geology 203 (2008) 267–288

level (Hsü, 1983), a scenario which has no precedents in thehistory of the Earth and which has no comparable cases atpresent. The importance of the salinity crisis of the centre of theMediterranean, especially assuming the hypothesis of the deepdesiccated basin, is rooted in the following aspects: 1) it affectsan area of more than 2.5 million km2 (Hsü et al., 1977); 2) itgave rise to the accumulation of more than 1 million km3 ofevaporites (Ryan 1976); 3) it resulted from a sea-level fall of2000 m (Hsü et al., 1973b); 4) it reduced global oceanic salinityby c. 6% (Hsü et al., 1977); and 5) it produced a subaerialerosive surface with deep incised valleys (Ryan and Cita, 1978;Clauzon, 1982). The Messinian salinity crisis was not exclusiveto the centre of the Mediterranean, but rather has been registeredalso in the peripheral basins or marginal sectors, currently atmore than 2000 m of elevation with respect to the central orabyssal basins. In fact, manifestations of the crisis have beendocumented from Cyprus in the eastern Mediterranean to theBetic and Rif Cordilleras (Gibraltar region) in the westernMediterranean. In the marginal sectors, the crisis is expressedby two fundamental events. One is the precipitation ofevaporites, which accumulated in marine basins periodicallyisolated from the main body of Mediterranean water; and theother is the formation of a subaerial erosive surface (Messinianunconformity) related to the sea-level fall that originated thecentral evaporites. In addition, in these basins, other sedimen-tary records are linked to the crisis. One is the called Lago Mareepisode of the end of the Messinian, which occurred after theevaporitic phase of the centre of the Mediterranean as aconsequence of a general inundation; this Lago Mare episode ischaracterized by the development of hypohaline or brackishwater environments, though cyclic sea-level oscillations whichcaused stages of gypsum precipitation (Upper Evaporites). Theother is the complete reflooding of the Mediterranean at thebeginning of the Pliocene (Pliocene transgression), which is theevent that definitively marked the end of the salinity crisis.

The present study centres on a marginal basin of the westernMediterranean: the Bajo Segura Basin. The southern part of this

Fig. 1. (A) Location of the Betic Cordillera in the westernMediterranean. (B) GeologiSegura Basin (after Montenat, 1990).

basin presents one of the most complete Messinian and Pliocenerecords of the Mediterranean margins, both from a temporalstandpoint as well as considering the variety of depositionalenvironments. The aim of this work is to show the sedimentaryfeatures and the chronology of the pre-evaporitic, evaporitic,and post-evaporitic stages, emphasizing the palaeogeographicmeaning of the events that mark the limits between these stages.In addition, the record of the southern sector of the basin iscorrelated with other sectors of the same basin, where previousworks have illustrated the manifestations of the salinity crisis. Insynthesis, the Bajo Segura Basin is presented as a singularscenario to explain how and when the salinity crisis took placein the marginal basins of the western Mediterranean.

2. Geological setting and general stratigraphic architecture

The Bajo Segura Basin, located in the south-eastern IberianPeninsula, belongs to the group of Neogene–Quaternary basinsof the Betic Cordillera. This cordillera, together with the Rif inNorth Africa, forms the western end of the Alpine chainssurrounding the western Mediterranean (Fig. 1A). The BeticCordillera is formed by two main geological domains: theInternal Zones to the south, and the External Zones to the north.The Internal Zones constitute an alloctonous lithosphericfragment (Alborán Block) dominated by metamorphic rocksPalaeozoic and Triassic in age. The External Zones, chieflycomprised of Mesozoic sedimentary rocks, represent the oldpalaeomargin located to the south of Iberia (South-IberianPalaeomargin). The Bajo Segura Basin, like the other Neogene–Quaternary basins, was palaeogeographically defined at thebeginning of the Upper Miocene, after the orogenic movementsthat caused the collision of the Alborán Block (Internal Zones)against the South-Iberian Palaeomargin (External Zones).

The Bajo Segura Basin is the largest of all the basins of theeastern Betic Cordillera and overlies the contact between theInternal and External Zones (Fig. 1B). The sedimentary fillingof the basin, which temporally spanned from the Tortonian to

cal map of the eastern end of the Betic Cordillera showing the position of the Bajo

269J.M. Soria et al. / Sedimentary Geology 203 (2008) 267–288

the Quaternary, presents a wide variety of facies fromcontinental to deep marine (Montenat et al., 1990; Soria et al.,2001). The best and most extensive outcrops are found in thesouthern part of the basin (Fig. 2A), from which it has beenpossible to establish the general stratigraphic architecture, asreflected in Fig. 2B. This latter figure shows that the basinbegan to fill at the beginning of the Tortonian, with atransgressive unit (Garres Fm), which overlies the metamorphicrocks of the basement (Internal Zones); this formation beginswith a lower member composed of deltaic conglomerates andends with an upper member dominated by marine marls,indicating a deepening megasequence. The second unit(Paredon Fm) lies in sharp contact with the former and iscomposed of sandy turbidites. The third, fourth, and fifth of theunits (Atalaya Fm, Columbares Fm, and Pujalvarez Fm,respectively) present as a common feature the progradation ofshallow marine sandstones over open marine marls. These threelatter units end the Tortonian s. str. sedimentation.

The sedimentation during the Messinian and Pliocene, thefocus of the present study, is registered by three units boundedby unconformities. Following the order established in theforegoing section, the sixth unit (latest Tortonian to Messinianp.p. in age) is bounded at its bottom by an erosive surfacethat truncates the Columbares Fm (Fig. 3A) as well as thePujalvarez Fm. The internal organization of this unit consists ofa lower assemblage of open marine marls, the Torremendo Fm,over which an upper assemblage dominated by shallow marinesandstones, La Virgen Fm, progrades (Fig. 3B and C). Theseventh unit of the basin, Messinian in age, consists of lagoonalclays and associated coastal sandstones (Garruchal Fm) inproximal parts, which laterally changes facies with shallowmarine evaporites (San Miguel Fm) in basin centre position.The lower limit of this unit, intra-Messinian unconformity, iswell recognized in the outcrops of the Garruchal sectorseparating La Virgen Fm, underneath, from the Garruchal Fmabove (Fig. 3D). Finally, the eighth unit, early Pliocene in age,is separated from the previous one by an erosional surface, end-Messinian unconformity, which is characterized by the devel-opment of an incised palaeovalley located in the La Pedrerasector (Fig. 2B). This unit is made up of five formations. Thelowest (La Pedrera Fm) filled the aforementioned palaeovalleyand is characterized by the presence of coastal and shallowmarine conglomerates. The four upper formations are organizedaccording to a progradational pattern, which begins with marinemarls at the base (Hurchillo Fm), having coastal and shallowmarine sandstones in an intermediate position (Rojales Fm), andfluvial clays and conglomerates in the upper part (MontesinosFm and Segura Fm).

Tectonically, the structure of the southern Bajo Segura Basinis characterized by the presence of a large anticlinal fold – theTorremendo anticline – with NNE–SSW axial orientation.Other smaller folds, of similar axis, are found to the north ofTorremendo anticline, at the localities of Hurchillo, Benejúzar,and Guardamar (Fig. 2A). This fold affects the eightstratigraphic units mentioned above, the age of which spansthe Tortonian to the early Pliocene. In both limbs of the fold, theMessinian and early Pliocene units (units VI, VII, and VIII)

present monoclinal structure, i.e. with the same strike and dip,which indicate that the deformation by folding and the completeinversion of the basin occurred after the early Pliocene.

3. Messinian–early Pliocene stratigraphyand biostratigraphy

To reveal the manifestations of the Messinian salinity crisisin the southern sector of the Bajo Segura Basin, we haveselected five stratigraphic sections that illustrate both thesedimentary features as well as the unconformities that typifythe Messinian–early Pliocene time interval. Three sections arestudied from outcrops (Garruchal, La Pedrera and San Miguel)and two sections resulting from exploratory oil wells (SM1 andLM). Below, we describe these five sections, for which thegeographical location and stratigraphic positions are indicatedin Fig. 2.

3.1. Garruchal section

This section shows the most complete stratigraphic recordand is the best exposed of the entire Bajo Segura Basin (Fig. 4).The general stratigraphic features of this section wereestablished by Montenat et al. (1990), from which we maintain(with certain changes) the denomination of the lithostratigraphicunits represented. Our description begins with the top of theColumbares Fm (upper member), comprised by conglomeratesof heterometric clasts derived from the Internal Zones. As themost significant feature, shallow marine bivalve fossils (ostreidsand pectinids) of great size are recognized. This formation hasbeen dated as Tortonian (Montenat et al., 1990) on the basis ofthe determination of planktonic foraminifers within the under-lying marine marls (lower member), which show a gradualvertical evolution towards the above-mentioned conglomerates.The upper limit of this formation is a sharp surface underlyingbasal deep marine deposits of the Torremendo Fm. With thissudden deepening event, the sedimentation of the latestTortonian–Messinian begins in the Bajo Segura Basin.

3.1.1. Torremendo FormationThis formation is represented by a succession of 450 m thick

dominated by deep marine marls rich in planktonic organisms(foraminifers and calcareous nannoplankton) and with occa-sional accumulations of bivalves of very thin shells. The lower170 m has a rhythmic alternation of marls and turbiditicsandstones that exhibit Ta, Tb and Ta-b Bouma sequences. Inthe basal part of the formation, which has been called Geacycles (Fig. 3A), the turbidites are organized in thickeningupward sequences (TU) of 10 to 15 m thick, resulting from thestacking of minor-order cycles (also TU), of 2 m mean thickness(Fig. 5A). The Gea cycles are interpreted as small deep-sea fansor prograding turbiditic lobes that accumulated in the deepestpart of the basin during the sedimentation of the TorremendoFm. The rest of the upper part of this formation is dominated bymarls with frequent and irregularly distributed intercalations ofstructureless fine-grained turbiditic sandstones, without appre-ciable Bouma sequences (Fig. 5B), the strata morphology of

ig. 2. (A) Geological map of the southern Bajo Segura Basin (simplified from Montenat, 1990). (B) Stratigraphic architecture of the basin, indicating the position of


F
the study sections.

Fig. 3. Panoramic views of the Garruchal section and neighbouring sectors, where the most complete stratigraphic record from the late Tortonian to the early Pliocene isrepresented. (A) Lower part of the section, showing the basal turbidites recognized in the Torremendo Fm (Gea cycles, within which is the Tortonian/Messinianboundary) and the overlying homogeneous marls from the Messinian. (B,C) Middle part of the section, where eastward progradation of the La Virgen Fm over theTorremendo Fm is recognized. (D) Upper part of the section, where the top of the La Virgen Fm and the entire Garruchal Fm are exposed, both late Messinian in age,separated by the intra-Messinian unconformity. To the right of the image the position of the end-Messinian unconformity is indicated; this unconformity is overlain byearly Pliocene marine units – Hurchillo Fm and Rojales Fm-.


Fig. 4. Stratigraphy and magnetostratigraphy of the Garruchal section. Declination and inclination represent the tilt-corrected ChRM directions. Black-filled circlesrepresent reliable (class A) directions used for the magnetostratigraphy while open circles denote unreliable (class B) directions. Crosses mark the position of discardedunsuitable samples of class C.


Fig. 5. Garruchal section. (A) One of the Gea cycles (basal turbidites of the Torremendo Fm) which display a thickening upward sequence -TU- of decametric scale,formed by the stacking of TU sequences of metric scale. (B) Homogeneous marls of the Torremendo Fm, with regularly spaced layers of turbiditic sandstones -T-.(C) Detail of the morphology of the turbiditic layers (lobes and compensation beds) within the marls of the Torremendo Fm. (D) Thickening upward sequence -TU- inthe La Virgen Fm; the lower sandstone strata show hummocky and swaley cross-stratification -HCS-, while the upper ones are characterized by internal undulatingsurfaces. (E, F) Breccias and caliche-like carbonate crusts over the La Virgen Fm, marking the position of the intra-Messinian unconformity. (G) Outcrop of theGarruchal Fm, where lagoon marls dominate, with thin levels of beach sandstones; at the far of the image the end-Messinian unconformity as well as the Hurchillo andRojales Fms are exposed. (H) Detail of the beach sandstones with wave ripple cross-lamination within the Garruchal Fm. (I) The two marine units of the early Pliocene,which appear in the upper part of the Garruchal section: at the base marine marls rich in planktonics -Hurchillo Fm- and at the top shallow marine sandstones -RojalesFm-. (J) The most characteristic feature of the end-Messinian unconformity: an erosional surface bounding the Garruchal and Rojales Fms.



which reveals lobes and compensation beds in the spacesbetween the lobes (Fig. 5C). An additional feature, althoughoccasional, of this formation is the presence of slumps andintervals of diatomites with fish remains. This uppermost andthickest part of the Torremendo Fm is interpreted as a submarineslope in which the dominant sedimentary process is pelagicsedimentation and in which avalanches of sand were emplacedby grain-flow mechanisms.

The age of the Torremendo Fm has been established as lateTortonian–Messinian by Montenat et al. (1990) on the basis ofplanktonic foraminifers. According to these authors, the lowerpart of the formation contains the species Globorotaliahumerosa and Globorotalia dutertrei, without Globorotaliamediterranea and Globorotalia conomiozea, indicating a lateTortonian in age. The middle and upper parts of the TorremendoFm are characterized by the presence of G. mediterranea andG. conomiozea, which are the biostratigraphic markers of theMessinian. In the stratigraphic section of Venta de la Virgen,near and equivalent to the section of Garruchal, Krijgsman et al.(2006) have made a high-resolution chronological study bycombining biostratigraphic, magnetostratigraphic, and isotopicdatings. These authors have specified that the Torremendo Fmspans from 8.1 to 6.4 Ma, situating the Tortonian/Messinianlimit in the lower half of the formation. The results of Montenatet al. (1990) and Krijgsman et al. (2006) agreed that most of theTorremendo Fm is Messinian in age, an aspect confirmed in ourstudy by calcareous nannoplankton. Thus, the pelagic marls ofthis formation, above the Gea cycles, contained an associationof nannoplankton, where, among other species, Amaurolithusamplificus and Reticulofenestra rotaria have been identified,these being characteristic of the Messinian (Berggren et al.,1995). The presence of Reticulofenestra pseudoumbilicusN7 μm in the middle of the formation is another indicator ofthe Messinian, on the basis that this long-range species appearsabove the Tortonian (from 7.1 Ma; see Krijgsman et al., 2000,for references). The precise chronological position of theTortonian/Messinian limit in the Garruchal succession, whichwe have situated within the Gea cycles, will be justified below,in the Magnetostratigraphy section.

3.1.2. La Virgen FormationThe Torremendo Fm underwent gradual vertical evolution

towards the La Virgen Fm. In the transition zone between thetwo, the upper marls of the Torremendo Fm increased thecontent in the terrigenous sandy fraction, where thin sandstonebeds appeared with traces of Thalassinoides. Above thistransition zone, the La Virgen Fm is 50 m thick and composedof an alternation of sandstones and marls. In detail, the stackingpattern of the sandstone strata defines three thickening-upsequences -TU- (Fig. 5D). The lower sandstone beds that makeup these TU sequences, 50 cm thick, show a basal interval ofrip-up mud clasts and an upper interval with hummocky andswaley cross-stratification (HCS). The TU sequences end withbeds of 7 m in thickness, structureless or with frequent HCS-like internal undulating surfaces. The sandstones are composedmainly of carbonate lithoclasts and calcite cements, a composi-tion accounting for the name of La Virgen “limestones”

proposed by Montenat et al. (1990) for this formation. Itspalaeontological record is very scant, composed only of bivalvefossils and scattered plant remains. The marls interstratifiedwith the sandstones are of a composition similar to that of theTorremendo Fm, with abundant benthonic and planktonicforams. The Virgen Fm is interpreted as a storm-dominatedshelf, where episodes of fair-weather muddy accumulation(marly beds) alternate with the deposition of tempestites(sandstone beds) originated from the erosion of sandy vegetatedcoasts. The regular presence of HCS indicates that thesetempestites are deposited below the main wave level, by meansstorm-ebb surges modelled by high-regime oscillatory flows(see similar models in Walker, 1979; Cheel, 1991; Brenchleyet al., 1993).

3.1.3. Garruchal FormationThe top of the La Virgen Fm is an irregular surface termed as

intra-Messinian unconformity. This surface erodes the uppersandstones of the La Virgen Fm and contains pockets filled bybreccias derived from this formation (Fig. 5E). The brecciaspresent clasts with a grain size varying between 2 and 20 cm,and they are interstratified with caliche-like carbonate crusts(Fig. 5F). These features indicate a subaerial exposure phase ina subarid climate during the formation of the intra-Messinianunconformity. Above this unconformity lies the Garruchal Fm(Figs. 3D and 5G), represented by a succession 100 m thick anddominated by grey and red marls, in which the onlymacrofossils recognized are ostreids (hence the term Oystermarl Formation proposed by Montenat et al., 1990). Themicropalaeontological content of these marls is very poor. Mostof the formation contains no microfossils, except for Chara sp.Certain levels show an association composed of smooth-shelledostracods (Cyprideis), benthic foraminifers (Ammonia beccarii,Elphidium granosum, Elphidium macellum, Haynesina germa-nica and Quinqueloculina laevigata) and small planktonicforaminifers, mostly reworked. Within these marls appearregularly spaced sandstone beds 20 to 50 cm thick, having aninternal structure of wave ripple cross-lamination (Fig. 5H) andforeshore lamination. This formation is interpreted as a muddycoastal lagoon with bands of sandy beaches. The presence ofChara sp. points to predominantly hypohaline water, thoughthe foraminifer association mentioned above would indicateepisodic marine incursions in the lagoon. In the section studied,the Garruchal Fm does not present fossils adequate for dating;its attribution to the Messinian by Montenat et al. (1990) can beupheld on the basis of its correlation with other units of similarfacies and equivalent stratigraphic position that have beendescribed in the northern sector of the Bajo Segura Basin(Caracuel et al., 2004; Soria et al., 2005). Specifically, in thelocalities of Crevillente and Elche, the lagoon and palustrinemarls that overlie the intra-Messinian unconformity includelevels rich in rodent fossils dated as the Upper Turolian orMessinian – biozone MN 13 of Mein (1990) – (Alfaro et al.,1995; Martín-Suárez and Freudenthal, 1998). From thestandpoint of other arguments, the Messinian Age for thisformation is supported both by its position underlying thedeposits of the early Pliocene (Hurchillo Fm, which will be


described below) as for its correspondence with an extensivezone of reverse magnetic polarity identified as the chron C3r(Fig. 12).

3.1.4. Hurchillo Formation and Rojales FormationThe Garruchal Fm is truncated by an erosional surface

designated as the end-Messinian unconformity, overlain by theHuchillo and the Rojales Fms (Figs. 3D and 5G). In the lowerstratigraphic position, the Hurchillo Fm fills erosive incisions ofthe discontinuity. This formation is represented by 5 m of marlsrich in planktonic and benthic foraminifers, with a characteristicassociation of small pectinids, echinoderms and spongespicules. The planktonic foraminifers content is dominated by

Fig. 6. Stratigraphy and magnetostrati

the genera Globigerinoides, Globigerinella and Orbulina; thebenthic foraminifers association is characterized by the generaTextularia, Fursenkoina, Nonion, Heterolepa, Globobulimina,Cancris and Cibicidoides. Both over the Hurchillo Fm as wellas directly over the end-Messinian unconformity appears theRojales Fm (Fig. 5I and J), which is composed of more than20 m of coarse-grained sandstones bearing bioclasts of red algae(rhodoliths) and pectinids. Both formations are marine andindicate a transgression after the erosive phase that characterizesthe end-Messinian unconformity. The thinness of the HurchilloFm and the shallow marine clastic character of the Rojales Fmin the Garruchal section hamper a detailed biostratigraphicstudy of the two. We assigned the first to the early Pliocene,

graphy of the La Pedrera section.



given that sediments that can be stratigraphically correlated atother points of the basin reveal an association of planktonicforaminifers belonging to the Globorotalia margaritae biozone(Montenat et al., 1990; Soria et al., 1996). The assignment of theRojales Fm to the early Pliocene is based on the presence ofrodent fossils dating the MN 14 biozone in coastal deposits ofthe upper part of this formation, which crop out near the studysection (site of Barranco de Fayona; Alfaro et al., 1995).

3.2. La Pedrera section

This section begins in the upper part of the Torremendo Fm(Fig. 6), while the lower part is submerged under the water ofthe La Pedrera dam (the main water reservoir of Alicanteprovince). Previous data related to the stratigraphy of thissection have been provided by Garcin (1987), Montenat et al.(1990), Michalzik et al. (1993), and Martínez del Olmo andSerrano-Oñate (2000).

3.2.1. Torremendo FormationThis formation is represented by a succession 200 m thick

dominated by marine marls rich in planktonic organisms(foraminifers and calcareous nannoplankton) and scatteredremains of small thin-shelled bivalves. The marls contain thinlevels of turbiditic sandstones, mostly structureless andoccasionally with Ta-b Bouma sequences. These turbidites aregrouped in intervals of 5 and 10 m thick, which are deformed byslumps and by slump scars (Fig. 7A). Also, in the central third ofthe succession, intercalations of diatomites and sapropeliticmarls are recognized, these alternating cyclically with homo-geneous marls (Fig. 7B). This formation is interpreted in thesame depositional context suggested in the foregoing Garruchalsection, as a submarine slope dominated by pelagic sedimenta-tion in which sandy avalanches are carried by turbiditic flows s.l.The alternation of sapropel and homogeneous marls is acharacteristic feature of the Torremendo Fm, which is docu-mented in sediments of the same age in other Mediterraneanbasins, such as the Sorbas basin in SE Spain, Caltanissetta inSicily, and Gavdos in Greece (Krijgsman et al., 1999; Nijenhuis,1999; Schenau et al., 1999; Sierro et al., 1999; Krijgsman et al.,2001). According to Krijgsman et al. (2001), this alternation ispredominantly related to precession-controlled dry–wet oscilla-tion in a circum-Mediterranean climate. In agreement with theauthors mentioned above, the sapropels would correspond towet-climate stages (minimum precession, maximum insolation),during which the stratification of the water column (by riverrunoff) would provoke an anoxic bottom appropriate for thepreservation of organic matter. On the contrary, the homo-geneous marls reflect dry climatic stages (maximum precession,minimum insolation), when the increased salinity and surface-

Fig. 7. La Pedrera section. (A) Turbiditic sandstones -T- with slumps, diatomites -d- an(B) Sapropel levels -s- intercalated in homogeneous marls and diatomites in the interend-Messinian unconformity modelled at the top of the Torremendo Fm. This unconbasal interval of conglomerates -cg- followed by alternating sandstones and marinMessinian) have been completely eroded. (D, E) Two details both of the strong erosclasts that form the basal conglomerate of the La Pedrera Fm. (F, G) Sandstone layersin some cases wave ripple cross-lamination at the top of the layers. (H, I) Sandston

water density (by evaporation) trigger a process of mixture withdeep waters, leading to an oxygenation and oxidation (decom-position) of the organic matter.

The age of this formation, from the lower levels to the top,has been determined by the occurrence of R. pseudoumbilicusand A. amplificus, species of calcareous nannoplankton thatdate the Messinian.

3.2.2. La Pedrera FormationThe Torremendo Fm is truncated by an erosional surface

(end-Messinian unconformity), overlain by the La Pedrera Fm(Fig. 7C). This surface marks an extensive palaeovalley nearly200 m deep, which completely eroded the La Virgen andGarruchal Fms (see stratigraphic position in Fig. 2A). The LaPedrera Fm begins with a polygenic, heterometric conglomeratethat fills sharply erosive incisions (palaeogullies), where blocksvarying in size from a few cm to 1 m in diameter can berecognized (Fig. 7D and E). These blocks are not onlyfragments of carbonate rocks derived from the basement thatcrops out north of the Bajo Segura Basin (External Zones), butalso marl and sandstone clasts derived from stratigraphicallyunderlying formations (Torremendo Fm and La Virgen Fm).Above this basal conglomerate, in continuity, appears asuccession of 50 m thick dominated by an alternation ofmarls and sandstones (Fig. 7C and E), within which at least twoconglomerate intervals are recognized, these being similar to thebasal conglomerate. The marls contain a high content in finesand and silt, in addition to calcareous nannoplankton,planktonic foraminifers, and scarce sponge spicules andostracods. The calcareous nannoplankton association is domi-nated by the genera Ceratolithus, Amaurolithus, Discoasterand Sphenaster; the planktonic foraminifers content is char-acterized by the genera Globorotalia, Neogloboquadrina,Globigerina, Turborotalita and Globigerinella. The sands arewell stratified in beds of 10 to 50 cm thick, exhibiting climbingripple cross-lamination (Fig. 7F) occasionally with wave ripplesat the top (Fig. 7G), and hummocky and swaley cross-stratification (Fig. 7H and I). The La Pedrera Fm is interpretedas shallow marine (even coastal) sedimentary fill of theaforementioned palaeovalley, where processes of high-energyfluvial discharge (conglomerates) and high sandy supply(climbing ripples) interfere with oscillatory flows of low andhigh regime (wave ripples and HCS). The age of this formationhas been established from calcareous nannoplankton; thepresence of Ceratolithus acutus throughout the formationsupports a dating of the Zanclean (early Pliocene).

3.2.3. Hurchillo FormationThis formation, succeeding the previous one in gradual

transition, is represented by 140 m of homogeneous marls with

d homogeneous marl -m- as the most significant features of the Torremendo Fm.mediate part of the Torremendo Fm. (C) Erosional surface that characterizes theformity is overlain by the La Pedrera Fm (early Pliocene), which begins with ae marls -s/m-. Note that both the La Virgen Fm and the Garruchal Fm (lateive character of the end-Messinian unconformity as well as the large size of theof the La Pedrera Fm, which exhibit mainly climbing ripple cross-lamination ande layers with hummocky and swaley cross-stratification.


an abundant and varied association of planktonic foraminifersand calcareous nannoplankton, in addition to sponge spicules,indicating pelagic marine depositional conditions. The plank-tonic foraminifers content is represented by the genera Globi-gerinoides, Globigerina, Orbulina, Neogloboquadrina andGlobigerinella; The calcareous nannoplankton association isdominated by the genera Reticulofenestra, Discoaster, Sphe-nolithus, Amaurolithus, Sphenaster and Scyphosphaera. In theupper part thin levels of sand are recognized, these marking thevertical transition towards the shelf and coastal sandstones ofthe Rojales Fm (Fig. 2B), registering a shallowing upwardsuccession identical to the one recognized in the northern sectorof the Bajo Segura Basin (Soria et al., 2005). Its age has beenestablished as early Pliocene on the base of planktonicforaminifers (Montenat et al., 1990). According to theseauthors, the lower part of the formation contains speciesbelonging to the biozone of G. margaritae, while in the upperpart the species Globorotalia puncticulata and Globorotaliacrassaformis have been identified.

3.3. San Miguel section

Near the village San Miguel de Salinas a good outcrop isavailable to study the upper part of the SanMiguel Fm as well as

Fig. 8. Stratigraphy of the San Miguel section (f

its vertical transition towards the Garruchal Fm (Fig. 8).Previous data on the stratigraphy of this section have beenprovided by Montenat et al. (1990) and Michalzik (1996).

3.3.1. San Miguel FormationThe San Miguel Formation is represented by five selenitic-

gypsum intervals of upwardly decreasing thickness, from morethan 5 m to less than 1 m. The thickest show irregular, sharpsurfaces in their interior, in addition to cavities filled with finelylaminated marls (Fig. 9A), which contain planktonic organisms,benthic foraminifers, and siliceous sponge spicules. Thesecavities present a mean value of 2 m wide and 4 m high, inaddition to near vertical symmetry. The thinnest upper gypsumintervals are interbedded with the same type of laminated marls,with plane top and cone-like morphologies at the base (Fig. 9B),similar to the supercones described by Dronkert (1976). This typeof gypsum represents episodes of high concentrations in arestricted marine environment with calm and stratified waters,which enabled the good development and the preservation of themillimetric lamination in the inter-evaporitic intervals. Theirregular internal surfaces and, especially, the aforementionedcavities can be interpreted as features of subaerial exposure andkarstification of the evaporitic layers. It bears mentioning thatcurrent examples of karst developed in gypsum (Sorbas area;

rom outcrops) and the SM1 and LM wells.

Fig. 9. San Miguel section. (A) Upper part of the San Miguel Fm, composed of at least three layers of selenitic gypsum -g1, g2 and g3-, in which karstic cavities filledwith laminated marls -lm- are recognized. (B) Transition zone between the SanMiguel Fm and the Garruchal Fm, characterized by an alternation of gypsum layers -g3,g4 and g5- and intervals of laminated marls with sandstone intercalations interpreted as tempestites -t-. (C) Upper part of the Garruchal Fm. (D, E) Detail of thelaminated marls of the Garruchal Fm, which alternate with very fine-grained sandstones modelled by wave ripples, resulting in flaser bedding -fb- and linsen bedding-lb- geometries. (F, G) Medium-grained sandstone beds -tempestites- of the Garruchal Fm, which exhibit a lower interval of planar lamination -pl- and an upper intervalwith hummock -h- and swale -sw- morphologies.


Calaforra and Pulido-Bosch, 2003) exhibit cavities quite similarboth in size and morphology to those described here. As analternative to this explanation, dissolution cavities might developin gypsum by changes in brine saturation with no subaerialexposure. The occurrence of sponge spicules and planktonicfossils in the cavities can be interpreted as the result of more openmarine episodeswith normal-marinewater salinities after gypsumdeposition, causing first the dissolution of the gypsum beds andthen the deposition of fine-grained sediments.

3.3.2. Garruchal FormationThe succession of laminated marls interbedded with gypsum

continues upwards with the same features described above. Thebase of the Garruchal Fm is marked by a transition zone inwhich gypsum beds coexist with sandstones (Fig. 8). Above,this formation is characterized by a predominance of laminatedmarls, which include regularly spaced beds of sandstones(Fig. 9C). These sandstones present two types of facies: one ofvery fine grain size, forming discontinuous layers of centimetric


thickness that show flaser and linsen structures modelled bywave ripples (Fig. 9D and E); the other, of medium grain size,arranged in beds of 10 to 70 cm thick display mainly hummockyand swaley cross-stratification (HCS). Only in a few cases dothese sandstones present a basal interval of planar laminationand another upper interval of HCS (Fig. 9F and G). In general,this formation is interpreted as the proximal part of the restrictedbasin where the laminated marls and evaporites of the SanMiguel Fm accumulated. The frequent entry of fine sandysediment, persistently modelled by low-regime oscillatory flow,indicates a position near the coastline, in a shallow marine orcoastal environment (shoreface). The sedimentary structures ofthe medium-grained sandstone beds are identified as tempes-tites, formed by storm-ebb surges modelled occasionally byunidirectional flows (planar-lamination interval) and frequentlyby high-regime oscillatory flows (HCS).

3.4. SM1 and LM wells

The outcrops available in the Bajo Segura Basin are notadequate for precise recognition of the complete San MiguelFm, especially its boundary with the underlying unit. To solve

Fig. 10. A–I. Tilt-corrected orthogonal plots of demagnetization data from representatsections in stratigraphic order (see the meter level at the lower left corner of each dplane, respectively. The fitted ChRM component is shown. The intensity of the init

this lack of information two oil exploratory wells (SM1 andLM) have been used, in which the San Miguel Fm is completelydrilled (Fig. 8). Both show that the evaporites overlie asuccession of marine marls assigned to the Torremendo Fm(Montenat et al., 1990). The boundary between the twoformations is defined by a sharp, non-transitional surface,both in the lithological record as well as in the neutron logwireline. This boundary, where the La Virgen Fm is absent(possibly eroded), coincides with the intra-Messinian uncon-formity recognized in the stratigraphic sections obtained fromthe outcrops. Above this unconformity, the San Miguel Fm isrepresented by a succession of 100 m thick composed ofgypsum beds alternating with marls. In the LM well, 15 gypsumbeds were recognized by the lithological record and neutron logwireline. In the SM1 well, where the only lithological record isavailable, 9 gypsum beds were identified. This difference can beexplained by the more precise well-logging technique applied atthe LM well. For the upper part of both wells, we assumed thelithological features observed in the San Miguel section. TheSM1 well reveals that the evaporites are covered by 30 m ofsandy marls that can be assigned to the Garruchal Fm based onthe correlation with the San Miguel section (Fig. 8). Finally, in

ive class A specimens from the Venta del Garrucha (VG) and the La Pedrera (PD)iagram). Solid (open) circles represent projections onto the horizontal (vertical)ial NRM and the demagnetization treatment for some steps are indicated.

Fig. 11. Equal area projections of the ChRM directions before (in-situ) and afterbedding correction for the Garrucha (A) and La Pedrera (B) sections. The 95%confidence ellipse for the normal and reverse mean directions is indicated.Statistical information is given (N, number of samples; Dec., declination; Inc.,inclination: k Fisher's precision parameter; α95, radius of the 95% confidencecone).


both wells, the lithological record ends with marine marls(Hurchillo Fm; according to Montenat et al., 1990) which coverby means of a sharp surface the two previous formations. Thisbrusque stratigraphic change fits well with the features of theend-Messinian unconformity.

4. Magnetostratigraphy

To detail the chronology both of the stratigraphic units aswell as the above-mentioned unconformities, we performed apalaeomagnetic study of the Garruchal and La Pedrera sections;the magnetic polarity of which is expressed in Figs. 4, 6 and 12.

4.1. Palaeomagnetic sampling and results

Sampling for magnetostratigraphy was conducted through-out the 600 m long Garruchal section and the lower 250 m of theLa Pedrera section (Figs. 4 and 6). A total of 147 and 56sampling levels were obtained along the Garruchal and the LaPedrera sections respectively, comprising 2 or 3 oriented hand-samples per site level. Block samples were cut in the laboratoryinto standard regular specimens for palaeomagnetic measure-ments. Natural remanent magnetization (NRM) and remanencefollowing demagnetization steps were measured on a 2GEnterprises DC SQUID high-resolution pass-through cryogenicmagnetometer (manufacturer noise level of 10−12 Am2)operated in a shielded room at the Istituto Nazionale diGeofisica e Vulcanologia (INGV) in Rome, Italy. A Pyrox ovenin the shielded room was used for thermal demagnetizations andalternating field (AF) demagnetization was performed withthree orthogonal coils installed inline with the cryogenicmagnetometer. Palaeomagnetic analyses were conducted on201 VG (Venta del Garruchal) and 56 PD (La Pedrera)specimens. A combined thermal and alternating field (AF)demagnetization protocol was applied to all specimens. Theprotocol consisted in thermal demagnetization at 150 °Cfollowed by progressive stepwise (AF) demagnetization. AFdemagnetization up to 100 mT included 14 steps with intervalsof 4–5 mT, 10 mT and 20 mT. Demagnetization data wereplotted on standard orthogonal plots. NRM components werecalculated using principal component analysis and standardFisher statistics was applied to compute mean directions.

The intensity of the NRM for the Garruchal section samplesranged from 0.02 mA/m to 0.4 mA/m for most of the sectionreaching 1–3 mA/m in some levels of the Garruchal Fm. A fewsamples (25%) provided only scattered non-interpretabledemagnetization trajectories (class C) and are thus unsuitablefor polarity interpretation. The majority of the samples unblocka reverse or a normal component usually above 15–25 mT afterremoval of a viscous component at 150 °C and low AFmagnetic fields, indicating that the magnetic carrier is largely amagnetite-type mineral (class A) (Fig. 10A–F). We considerthis dual polarity component that trends toward the origin of theorthogonal demagnetization diagrams as the characteristicremanent magnetization (ChRM) that constitutes the primarycomponent. Samples of intermediate demagnetization quality(7%) (class B), are retained unreliable and not used for

magnetostratigraphy. The reliable ChRM components delineatea succession of at least 8 polarity zones (Fig. 4). The presence ofmany class B samples in the lower 50 m of the studied sectionand the fact that only a few class A samples define eachdetermined polarity zone along this part render this interval theless reliable throughout the section. Above this lower interval anormal magnetozone is clearly retrieved from about 56 m to90 m. It follows a long reverse magnetozone up to about 270 m,only punctuated by a few apparent normal samples (we havedepicted those as grey bars in the polarity zonation of Fig. 4).Then a normal magnetozone extends up to about 350 m.Between this level and up to about 420 m, several class A bothnormal and reverse samples are present. We have interpretedthis interval as completely reverse (Fig. 4) because we infer thatthe normal samples may represent remagnetizations given thatthey are intercalated with reverse samples and do not define anyconsistent polarity succession. This is in agreement with thedifficulty to find suitable fresh outcrops in the field along thisinterval. Above 430 m, a normal magnetozone follows up toabout 472 m, and finally a final thick reverse magnetozoneextends up to the top of the section.

The intensity of the NRM for the La Pedrera section samplesranges from 0.1 mA/m to 0.8 mA/m. Almost all samples have


provided reliable class A demagnetization data where a dualpolarity ChRM component that decays toward the origin of thedemagnetization diagram can be isolated above 17–20 mTindicating also a magnetite like carrier (Figs. 6 and 10G–I). Atleast 4 polarity zones can be unambiguously determined alongthe upper 2/3 of the section (Fig. 6). The presence of a complexsuccession of normal and reverse samples in the lower part ofthe succession together with the presence of various slumpedintervals prevents us to propose a reasonable magnetostrati-graphy for this interval.

The mean directions before and after tilt corrections fromboth sections are shown in Fig. 11 along with some statisticalparameters. The significant bedding dip along the Garruchalsection that varies from about 50° at the bottom of the section toabout 25° at the top of the section allows evaluating theconsistency of data before and after bedding correction. Theshallow mean directions for both the normal and reversecomponents before bedding correction and a moderate steepinclination after bedding direction more compatible with aMiocene direction for the site latitude, are indications of theprimary nature of the ChRM component. Moreover, thestatistical parameters improve somewhat after bedding correc-tion. The moderate northerly bedding dip around 30° for the LaPedrera section does not unambiguously provide evidence tosupport the primary nature of the ChRM component by

Fig. 12. Correlation between the magnetostratigraphy of the Garruchal and the La Pcalcareous nannofossils events in Berggren et al. (1995).

evaluating the inclination before and after bedding correction.However, an improvement of the statistical parameters after tiltcorrection is obvious.

4.2. Correlation with the geomagnetic polarity timescale(GPTS)

To fit the magnetozones differentiated in the Garruchal andLa Pedrera stratigraphic sections (see Figs. 4 and 6) with theGPTS of Cande and Kent (1995), we apply the scale ofnannofossil events established by Berggren et al. (1995). TheFAD and LAD of three species – R. rotaria, A. amplificus andC. acutus – served to identify the chrons (Fig. 12). R. rotariawas recognized in the middle part of the Torremendo Fm(Garruchal section) in a large reverse zone that fits well withchron C3Ar. The species A. amplificus was found to be presentin a normal zone within the Torremendo Fm (lower part of LaPedrera section), which corresponds to chron C3An.2n; also,this species was found in the upper part of the Torremendo Fm(Garruchal section) in a reverse zone, for which the onlypossible assignment is chron C3An.1r. Finally, C. Acutus, aspecies that marks the beginning of the Pliocene (Zancleanstage), was recognized throughout the La Pedrera Fm incoincidence with a reverse zone corresponding to the upper partof chron C3r. In the Garruchal section, from chron C3Ar on, the

edrera sections with the standard GPTS of Cande and Kent (1995) (CK95) and

Fig. 13. Main palaeogeographic changes that occurred during the Messinian and early Pliocene in the southern Bajo Segura Basin.


rest of the magnetozones were extended downwards followingthe order of the GPTS, resulting that the Tortonian/Messinianboundary would be situated in the Gea cycles (lower part of theTorremendo Fm), in coincidence with a small reverse zone thatfits well with chron C3Br.1r. This determination of the

Tortonian/Messinian boundary in the lower part of theTorremendo Fm coincides with the isotopic and bio-magnetos-tratigraphic data presented by Krijgsman et al. (2006) for theVenta de La Virgen section, situated a few kilometers west ofthe Garruchal section.


5. Events and palaeogeographic evolution

The two unconformities recognized in the study arearepresent events marking the main palaeogeographic changesthat occurred during the Messinian and early Pliocene in amarginal basin of the western Mediterranean (Fig. 13). The firstsedimentary and palaeogeographic stage (Messinian I high-stand) corresponds to the deposit of the La Virgen Fm (coast andshelf deposits) and to the Torremendo Fm (slope and pelagic-basin deposits); both formations were deposited when most ofthe Bajo Segura Basin was occupied by the sea, and therefore sowas the rest of the Mediterranean Basin. An initial sea-level falloriginated a lowstand erosional surface (the intra-Messinianunconformity), for which the only sedimentary record consistsof karstic breccias and caliche-like carbonate crusts, features ofsubaerial exposure. After this erosional surface, the secondsedimentary stage of the basin (Messinian II highstand) tookplace, characterized by an intra-Messinian reflooding andrecorded by the deposition of the Garruchal Fm (beach andlagoon deposits) in the basin margin and by the San Miguel Fm(shallow marine evaporites) in the basin centre. After theMessinian II highstand, a new sea-level fall caused the end-Messinian unconformity, a lowstand erosional surface asso-ciated with the La Pedrera palaeovalley. This is a by-pass fluvialsystem that crosses the entire Bajo Segura Basin from the reliefof the northern margin (origin of the clasts of the conglomeratesforming its fill) to the Mediterranean Sea. According to the

Fig. 14. The stratigraphic record made in the southern sector of the Bajo Segura Basinand the northern sector of the basin.

desiccated deep basin model (Hsü et al., 1973a; Hsü, 1983) thispalaeovalley represents one of the fluvial systems (e.g. Rhôneand Nyle, among others of smaller size) excavated when thedrastic drop in sea level increased the gradients of theMediterranean rivers. A rise in sea level during the earlyPliocene encouraged the marine ingression over the entire end-Messinian surface. In an initial phase, the La Pedrerapalaeovalley filled, forming a deepening or transgressivesequence that began with the interplay between fluvial inputsand shallow marine processes (La Pedrera Fm), and whichended with pelagic sedimentation (Hurchillo Fm). In asubsequent phase, the sea occupied the entire basin (Pliocenehighstand), when coastal and shelf systems (Rojales Fm)prograded over the pelagic sediments.

6. Basinwide-scale correlation

To evaluate the basinwide significance of the Messinianevents in the southern Bajo Segura Basin, we shall establish acorrelation with two other well-documented sectors of the basin(Fig. 14). For the northern sector, we assume the stratigraphicmodel recently proposed by Soria et al. (2005); for the Santa PolaCape sector, we use the data of Esteban (1979, 1996), Montenatet al. (1990), Calvet et al. (1996) and Feldmann and McKenzie(1997), in combination with our own field observations.

In the northern Bajo Segura Basin the Messinian recordindicates two main allostratigraphic units (MI and MII)

(this work) compared to that established in previous works for Santa Pola Cape


separated by the intra-Messinian unconformity. The MI Unitconstitutes a highstand systems tract formed by continentalsystems (alluvial fans) in the westernmost band of the basin andby shallow marine systems (shelf) in the rest of the basin. Theupper boundary of the MI Unit corresponds to the intra-Messinian unconformity, a lowstand erosional surface withpalaeovalleys of 30 m deep generated by a sea-level fall. TheMII Unit is comprised of continental systems throughout mostof the basin and coastal systems in the easternmost part and nearthe current position of the Mediterranean, all of these beingdeposited during a new sea-level highstand after a refloodingthat followed the preceding lowstand. The upper boundary ofUnit MII corresponds to the end-Messinian unconformity,which is another lowstand erosional surface with palaeovalleyssimilar to those of the intra-Messinian unconformity. The end-Messinian unconformity is overlain by the Pliocene Unit (P),which consists of four depositional systems superimposed incontinuity. The first two (coast and open marine) form atransgressive assemblage that fills the palaeovalley carved overthe end-Messinian unconformity. The latter two (shallowmarine to coastal in the lower part, and fluvial in the upperpart) form a regressive prograding assemblage deposited in ahighstand context, when the sea level reached its height in theearly Pliocene. In accord with this stratigraphic scheme, theintra- and end-Messinian unconformities present the samescenario (lowstand sea level) in the northern and southernsectors of the Bajo Segura Basin. The same occurred in the MI,MII and P units documented in the northern sector, for whichthe context in terms of sea level coincides with the threehighstand phases (Messinian I, Messinian II and Pliocene)documented in the southern sector.

In the Santa Pola Cape, the Messinian and Pliocene recordshave been divided into three stratigraphic units separated byunconformities. The lower unit is composed exclusively ofcoral reefs (Porites sp.) with green algae (Halimeda), bivalves,gastropods and other shallow marine benthic organisms. Thislower unit has been considered by Montenat et al. (1990) to be alateral equivalent to the La Virgen Fm, which contains coralreefs at several points both in the northern as well as southernsectors of the Bajo Segura Basin (Montenat et al., 1990;Reinhold, 1995; Calvet et al., 1996). Assuming this correlation,we equate this lower unit with the MI Unit of Soria et al. (2005),deposited during the Messinian I highstand phase proposed inthe present work. The top of this unit is carved by a steppederosional surface which can be correlated with the intra-Messinian unconformity recognized in the northern and south-ern sectors of the basin.

Over this unconformity lies a unit dominated by stromato-lites and trombolites, the lower strata of which are arranged inonlap over the basal erosional surface. This unit has beenclassically assigned to the late Messinian (Montenat et al.,1990), and, in agreement with Feldmann and McKenzie (1997),represents a marine flooding after a sea-level fall. These datasuggest that the stromatolitic unit of the Messinian can becorrelated with the MII Unit of Soria et al. (2005) andconsequently equivalent in time with the Messinian II highstandphase defined in the present study. At the top of this unit an

erosional surface that coincides with the end-Messinianunconformity was recognized. The first deposits that coverthis unconformity are planktonic-rich marine marls from thelower Pliocene (Hurchillo Fm), indicating a sudden marineflooding and the establishment of the Pliocene highstand-phaseconditions documented in the present work for the southernsector of the Bajo Segura Basin.

7. Discussion: implications for the Mediterraneansalinity crisis

The Mediterranean salinity crisis during the Messinian is acomplex, multi-phased event (Rouchy and Caruso, 2006) forwhich one of the principal manifestations is the accumulation ofevaporites both in the central parts (abyssal plains) as well as inthe marginal sectors (peripheral basins) of the Mediterranean.We assume, in agreement with other researchers (e.g. Martínand Braga, 1994; Clauzon et al., 1996; Martínez del Olmo,1996; Riding et al., 1998, 1999, 2000; Roveri et al., 2001), thatthe central and marginal Messinian evaporites of the Mediter-ranean are diachronic, whenever the difference between the twois a minimum of 2000 m. In addition, the presence of gypsum,and especially of halite, in the centre of the Mediterranean(Friedman, 1973; Hsü et al., 1973b; Nesteroff, 1973) forces usto assume a lowstand sea-level context (desiccated deep basin;Hsü et al., 1973a; Hsü, 1983) and, consequently, a subaerialerosional surface throughout the circum-Mediterranean area(Ryan and Cita, 1978).

Our stratigraphic model for the Bajo Segura Basin, a typicalmarginal basin of the Mediterranean, shows that the evaporitesand the correlative deposits of the MII highstand phase, arebounded at the base and top by subaerial erosional surfaces(intra- and end-Messinian unconformities). The key questionconsists of ascertaining which of these two erosive surfacesrepresents the stage of evaporitic precipitation (possibledesiccation) of the centre of the Mediterranean. In this regard,considering data of other authors outside the study area, weexamine two alternatives.

First, based on the two-step model presented by Clauzonet al. (1996) for the correlation between the marginal basins (i.e.Sicily and Sorbas) and the central basins, supports that themajor erosion and accumulation of abyssal evaporites occurredafter the deposition of the marginal evaporites. Synthetically,the model of Clauzon et al. (1996) presents the followingprogression of events related to the Messinian Salinity Crisis. 1)an initial modest sea-level fall (late Messinian; 5.75 Ma), whichcaused the precipitation of the evaporites in perched marginalbasins; 2) a subsequent and more important sea-level fall (end ofthe Messinian; 5.5–5.45 Ma), estimated at around 1500 m,which brought about an erosive surface and incised valleys,contemporaneous with the precipitation of evaporites on theabyssal plains, and 3) a rise in the sea level and reflooding of themarginal basins at the beginning of the Pliocene (5.32 Ma). Thismodel does not explain the existence of a lowstand erosionalsurface (intra-Messinian unconformity) at the bottom of theevaporites and their correlative deposits that form the Unit MIIof the Bajo Segura Basin. However, the model of Clauzon et al.


(1996) is in fact compatible with the existence of a lowstanderosional surface (end-Messinian unconformity) at the top ofthe Unit MII (which contains evaporites) and which ischaracterized by the presence of deep valleys (i.e. La Pedrerapalaeovalley) carved more than 200 m in the underlying units(MI and MII). In addition, this model satisfies a rise in the sealevel at the beginning of the Pliocene, when the Bajo Segurawas completely and definitively reflooded by marine waters, forwhich the sedimentary record is the Unit P. In short, theapplication of the ideas of Clauzon et al. (1996) to the BajoSegura Basin would imply, on the one hand, that our end-Messinian unconformity represents the salinity crisis in thecentre of the Mediterranean, and on the other, that the definitivereflooding and end of the crisis took place in the early Pliocene,coinciding with the Pliocene highstand phase documented in theBajo Segura Basin.

The second alternative, proposed by Riding et al. (1998,1999) in their study of the Sorbas Basin and by Braga et al.(2006) in other Almería basins, consider that the greatesterosion occurred immediately prior to the precipitation of themarginal evaporites, according to the presence of an erosionalsurface (intra-Messinian erosion, according to these authors)that separates the pre-evaporitic marine deposits (Abadmember) from the evaporites (Yesares Gypsum member). Themodel of Messinian events presented by Braga et al. (2006,Fig. 15; modified after Riding et al., 1998), based on theAlmería basins (Sorbas, Níjar and Vera, in SE Spain), consistsof the following phases: 1) pre-evaporitic sedimentation ofnormal salinity, represented by Messinian fringing reefs andcorrelative marine basinal deposits (Abad member); 2) deeperosion (up to 240 m) of the pre-evaporitic sediments caused bya greater sea-level fall at ca. 5.9 Ma, when the evaporitesprecipitated in the centre of the Mediterranean; 3) sea-level riseand marine reflooding at ca. 5.5 Ma, when deposition ofevaporites (Yesares member) in barred basins took place;4) continued sea-level rise (ca. 5.5 to 5.3 Ma) and restoration ofthe open marine conditions in the Almería basins, for which thesedimentary records are the post-evaporitic, normal marine,Messinian deposits (Sorbas member and Terminal Complex).Over these latter marine deposits, in the Sorbas Basin, theMessinian sequence ends with fluvial and lacustrine sediments(Zorreras member). In addition to these four Messinianepisodes, Braga et al. (2006) indicate that the Lower Pliocenemarine sediments unconformably overlie the Messiniansequence, and that the unconformity below the Lower Pliocenerocks is a major erosional surface, as has been documented byother authors (Montenat et al., 1990; Fortuin et al., 1995;Aguirre, 1998; Aguirre and Sánchez-Almazo, 2004). In relationto this unconformity, Braga et al. (2006) present a series of datathat point to its tectonic origin being caused by regional uplift.

This model, based on the Almería basins, is largelyapplicable to the Bajo Segura Basin. The pre-evaporitic phaseof sedimentation is represented in our case by the MI Unit. Thefall in sea level, and the subsequent deep erosion, that took placeafter the pre-evaporitic phase coincides with the first subaerialerosion (intra-Messinian unconformity) recognized in the BajoSegura Basin. The following sea-level rise, when the marine

reflooding takes place in the Almería Basins (which is recordedboth by the precipitation of evaporites as well as by thesedimentation of the Sorbas member and Terminal Complex), isrepresented in the Bajo Segura Basin by the MII Unit. Theerosive surface that crowns the Messinian sequence and overwhich the Lower Pliocene sediments rest, coincides with ourend-Messinian unconformity. Alternatively to the tectonicorigin (regional uplift) presented by Braga et al. (2006) forthe Messinian/Pliocene unconformity, in our study it is viewedas having as its cause a greater fall in sea level. In favour of ourproposal, the structural data indicate that the Messinian andPliocene strata show equal strike and dip, without angulardiscordance that enables us to infer a tectonic event between thetwo. A similar structural arrangement can be recognized outsidethe study area, as occurs both in the northern Bajo Segura Basin(Caracuel et al., 2004; Soria et al., 2005) as well as in the VeraBasin (Fortuin et al., 1995). In these two basins, the Messinianand Pliocene materials form a monoclinal structure, though theM-P limit coincides with an erosional surface, the genesis ofwhich has been associated with a fall in the sea level.

This second model implies that the reflooding of theMediterranean is recorded by the marginal evaporites duringthe Messinian, before the Pliocene reflooding considered in theclassical works on the Messinian salinity crisis (e.g. Hsü et al.,1977). This model can be applied to the Bajo Segura Basin,where the intra-Messinian unconformity has been documentedas an erosive surface separating the pre-evaporitic deposits (LaVirgen Fm and Torremendo Fm; Messinian I phase) from thesyn-evaporitic and evaporitic deposits (Garruchal Fm and SanMiguel Fm, respectively; Messinian II phase). Also, the BajoSegura Basin could have withstood the Messinian reflooding,which would be registered by the Messinian II highstand phase.

8. Conclusions

In the Bajo Segura Basin, we find the most significantsedimentary events related to the Messinian salinity crisis in themarginal basins of the Mediterranean.

From the analysis of the stratigraphic record, three highstandsea-level sedimentary phases have been deduced: Messinian I(pre-evaporitic or pre-crisis), Messinian II (evaporitic), andPliocene (post-evaporitic or post-crisis). These phases areinterrupted by two falls in sea level: intra-Messinian (betweenthe Messinian I and Messinian II phases) and end-Messinian(between the Messinian II and Pliocene phases), which arerepresented by unconformities that exhibit features of subaerialerosional surfaces.

The chronology of the events, established by a combinationof calcareous nannoplankton biostratigraphy and magnetostra-tigraphy, indicates that during chron C3r (c. 5.9–5.2 Ma) tookplace from the end of the Messinian I phase (pre-crisis) to thebeginning of the Pliocene phase (post-crisis).

In relation to the salinity crisis in the centre of theMediterranean,which is manifested in the marginal basins by subaerial erosionalsurfaces, either of the two unconformities (intra- and end-Messinian) recognized in theBajo SeguraBasin could be correlatedwith the lowstand evaporites precipitated in the abyssal plains.


The stratigraphic scheme presented for the Bajo SeguraBasin during the Messinian salinity crisis shows an intra-Messinian reflooding, in agreement with the model proposed byRiding et al. (1998, 1999), and of another Pliocene reflooding,in accord with the model of Clauzon et al. (1996).

Acknowledgements

The authors wish to thank Dr. Juan C. Braga and ananonymous referee for their valuable suggestions and criticalcomments. Financial aid was provided by Research ProjectsBTE2003-05047, CGL2005-06224/BTE, CGL2007-65832/BTE MEC, and GV04B-629 (Generalitat Valenciana) and the“Paleoenvironmental Changes”Group (UA). We are indebted toDavid Nesbitt for the English version of the paper.

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The Messinian–early Pliocene stratigraphic record in the southern Bajo Segura Basin (Betic Cordillera, Spain): Implications for the Mediterranean salinity crisis