The ‘Tortonian salinity crisis’ of the eastern Betics (Spain) W. Krijgsman a; *, M. Garce ¤s b , J. Agust| ¤ c , I. Ra/ d , C. Taberner b , W.J. Zachariasse e a Paleomagnetic Laboratory Fort Hoofddijk, Budapestlaan 17, 3584 CD Utrecht, The Netherlands b Institute of Earth Sciences ‘Jaume Almera’ (CSIC), Sole ¤ I Sabaris s/n, 08028 Barcelona, Spain c Institut de Paleontologia M. Crusafont, Escola Industrial 23, 08201 Sabadell, Spain d Dipartimento di Scienze della Terra, Universita ' ‘G. D’Annunzio’, Via dei Vestini 31, Chieti Scalo, Italy e Department of Geology, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands Received 24 March 2000; received in revised form 11 July 2000; accepted 13 July 2000 Abstract The late Miocene depositional history of the Lorca and Fortuna basins, both occupying an internal position in the eastern Betics of Spain, is marked by a regressive sequence from open marine marls, via diatomites and evaporites, to continental sediments. Based on facies similarities, these evaporites have often been correlated to the well-known Mediterranean evaporites of the Messinian salinity crisis, although this correlation was never substantiated by reliable chronological data. In this paper, we present an integrated stratigraphy of this regressive sequence which shows that the evaporites of the Lorca and Fortuna basins are entirely of late Tortonian age and as such have no relation with the Messinian salinity crisis. The main phase of basin restriction, resulting in deposition of diatomites and evaporites, took place at 7.8 Ma, while the last marine deposits (massive evaporites of the Lorca basin) are dated at 7.6 Ma. Consequently, this ‘Tortonian salinity crisis’ of the eastern Betics had a duration of approximately 200 kyr, while continental deposition prevailed throughout the entire Messinian as also revealed by the fossil mammal record. The ‘Tortonian salinity crisis’ of the eastern Betics is obviously related to a local phase of basin restriction caused by uplift of the metamorphic complexes at the basin margins, probably in concert with strike-slip activity along SW^NE trending fault systems. The development of a submarine sill is of crucial importance for the increase in salinity because it allows marine waters to continuously enter the basin at the surface while it restricts or prevents the outflow of dense saline waters at depth. Furthermore, we show that evaporite and diatomite cyclicity in these restricted basins is predominantly related to precession controlled circum-Mediterranean climate changes and that glacio-eustatic sea level changes only play a minor role. It is remarkable that the lithological sequence of the Tortonian salinity crisis mimics in many aspects that of the Messinian salinity crisis. This suggests that the diatomaceous facies is an essential part of the lithological sequence associated with basin restriction. ß 2000 Elsevier Science B.V. All rights reserved. Keywords: stratigraphy; evaporites; Miocene; Mediterranean region; Spain 1. Introduction The geodynamic evolution of the Miocene ba- sins that straddle the European^African collision zone was largely controlled by vertical motions 0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII:S0012-821X(00)00224-7 * Corresponding author. Tel.: +31-30-253-5246; Fax: +31-30-253-1677; E-mail: [email protected]Earth and Planetary Science Letters 181 (2000) 497^511 www.elsevier.com/locate/epsl
Transcript
The `Tortonian salinity crisis' of the eastern Betics (Spain)
W. Krijgsman a;*, M. Garces b, J. Agust| c, I. Ra¤ d, C. Taberner b,W.J. Zachariasse e
a Paleomagnetic Laboratory Fort Hoofddijk, Budapestlaan 17, 3584 CD Utrecht, The Netherlandsb Institute of Earth Sciences `Jaume Almera' (CSIC), Sole I Sabaris s/n, 08028 Barcelona, Spain
e Department of Geology, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands
Received 24 March 2000; received in revised form 11 July 2000; accepted 13 July 2000
Abstract
The late Miocene depositional history of the Lorca and Fortuna basins, both occupying an internal position in theeastern Betics of Spain, is marked by a regressive sequence from open marine marls, via diatomites and evaporites, tocontinental sediments. Based on facies similarities, these evaporites have often been correlated to the well-knownMediterranean evaporites of the Messinian salinity crisis, although this correlation was never substantiated by reliablechronological data. In this paper, we present an integrated stratigraphy of this regressive sequence which shows that theevaporites of the Lorca and Fortuna basins are entirely of late Tortonian age and as such have no relation with theMessinian salinity crisis. The main phase of basin restriction, resulting in deposition of diatomites and evaporites, tookplace at 7.8 Ma, while the last marine deposits (massive evaporites of the Lorca basin) are dated at 7.6 Ma.Consequently, this `Tortonian salinity crisis' of the eastern Betics had a duration of approximately 200 kyr, whilecontinental deposition prevailed throughout the entire Messinian as also revealed by the fossil mammal record. The`Tortonian salinity crisis' of the eastern Betics is obviously related to a local phase of basin restriction caused by upliftof the metamorphic complexes at the basin margins, probably in concert with strike-slip activity along SW^NE trendingfault systems. The development of a submarine sill is of crucial importance for the increase in salinity because it allowsmarine waters to continuously enter the basin at the surface while it restricts or prevents the outflow of dense salinewaters at depth. Furthermore, we show that evaporite and diatomite cyclicity in these restricted basins is predominantlyrelated to precession controlled circum-Mediterranean climate changes and that glacio-eustatic sea level changes onlyplay a minor role. It is remarkable that the lithological sequence of the Tortonian salinity crisis mimics in many aspectsthat of the Messinian salinity crisis. This suggests that the diatomaceous facies is an essential part of the lithologicalsequence associated with basin restriction. ß 2000 Elsevier Science B.V. All rights reserved.
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that transformed the area into a mosaic of rapidlysubsiding and emerging blocks. The semi-enclosedcon¢guration of the Mediterranean and the dete-rioration of water exchange with the Indian andAtlantic Ocean repeatedly favoured deposition ofevaporites in strongly restricted basins. Prime ex-amples are the middle to late Miocene evaporitesof the Red Sea basin, the middle Miocene (Bade-nian) salt deposits of the Carpathian foredeep,and the Messinian evaporites of the Mediterra-nean. The formation of these evaporites was heav-ily discussed over the last century, but criticalfactors ^ such as a reliable chronology, basinand gateway con¢guration, in£uence of tectonicsand climate ^ are, in many cases, still poorly con-strained.
The Mediterranean Messinian salinity crisis isone of the most impressive examples of evaporiteformation in Neogene history [1]. It was suggestedto be the result of a complex combination of tec-tonic and glacio-eustatic processes in the Gibral-tar area which progressively restricted and ¢nallyisolated the entire Mediterranean Sea from theopen ocean [2,3]. Typical Mediterranean Messi-nian sequences consist of a succession of openmarine marls, diatomite-rich sediments, thickevaporites (carbonates, gypsum and halite), and¢nally continental deposits. Astronomical calibra-tion only recently provided an accurate and high-resolution time frame for the Messinian andshowed that the onset of the Messinian salinitycrisis was a perfectly synchronous event in allstudied basins of the western and eastern Medi-terranean [4,5]. Therefore, evaporite depositionduring the Messinian salinity crisis must have tak-en place independent of local paleogeographicand paleoenvironmental conditions.
The synchroneity of the Messinian evaporites ischallenged by Rouchy et al. [6] and Dinares-Tur-rell et al. [7] who proposed that evaporite deposi-tion in the Lorca and Fortuna basins (E Spain)started earlier than on Sicily and in the nearbySorbas basin. Time constraints on the Lorca andFortuna sequences, however, are extremely poor.In fact, it has not yet been proven that the evap-orites of the Lorca and Fortuna basin are of Mes-sinian age. To establish a reliable and accuratechronology for the salinity crisis of the Lorca
and Fortuna basins, we decided to re-examinethe sedimentary sequences by subjecting themost continuous sections to a detailed and high-resolution integrated stratigraphic study. Thesesequences form an excellent opportunity to com-pare the processes of evaporite deposition in rel-atively small areas, like the Lorca and Fortunabasins, with those with a large and complexarea, like the Mediterranean. The results will pro-vide useful information on the general processesof evaporite formation in the circum-Mediterra-nean area and on the mechanisms of basin evolu-tion in the eastern Betics during the late Miocene.
2. Geological setting and sections
2.1. Lorca basin: the Serrata section
The quadrangularly shaped Lorca basin devel-oped on the substratum of the Internal Units ofthe Betic Cordillera and is bound to the southeastand northwest by two major NE^SW orientedsinistral strike-slip faults (the Alhama de Murciaand the North Betic Fault, Fig. 1). The south-western and northeastern limits were formed byNW^SE and N^S oriented normal faults whichclassify it as a `pull-apart basin' or `rhombgraben'[8]. The Lorca basin obtained its present fault-bound con¢guration during early Tortoniantimes, a period characterised by sedimentationof conglomerates, siliciclastics and carbonates[9]. A paleogeographic di¡erentiation occurredduring the middle-Tortonian resulting in sedimen-tation of coarse grained siliciclastics in the NWand a thick series of open-marine marls (HondoFm. [9]), diatomites and evaporites (Serrata Fm.[9]) in an elongated depocentre parallel to the SEmargin [6,8]. The marine sequences are well-ex-posed along a SE^NW trending ridge (La Serrata)which yields several undisturbed continuous sec-tions encompassing the entire stratigraphic se-quence from marine marls to evaporites. Theseevaporites are in turn overlain by a sequence ofcontinental reddish silts.
For our integrated stratigraphic study, we se-lected the Serrata section, 170 m thick and located5 km to the north of the city of Lorca (Fig. 1). We
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have sampled the lower part of the section alongthe transect of Section 3 of [6], the central andupper part in the Minas Volcan gypsum quarry(Section 4 of [6]) where the entire diatomite suc-cession, including the transition to massive gyp-sum, is clearly exposed. The base of the Serratasection is formed by sediments of the Hondo Fm.,consisting of cyclic alternations of grey homoge-neous marls and white Opal CT-rich dolomiticlayers (Fig. 2). This sedimentary cyclicity is sim-ilar to that of the `Lower Abad' marls of the
Sorbas basin, which was proven to be related toastronomical precession [10,11]. Four sedimentarycycles could be recognised up to the ¢rst promi-nent diatomite bed, which marks the transition tothe Serrata Fm. This latter formation mainly con-sists of alternations of marine diatomites and siltymarls, with interbedded sandstones and ¢negrained limestones or dolostones [9]. The SerrataFm. contains seven intercalations of precursorevaporitic layers (Fig. 2), which laterally gradeinto seven gypsum beds at the basin margin (Cor-
Fig. 1. Geological sketch map of the Neogene basins of SE Spain (modi¢ed after [15]). LS, Lorca Serrata section; SL, Sifon deLibrilla section; CH, Chorrico section; CO, Chicamo section; SMS, San Miguel de Salinas. Major faults: NBF, North BeticFault; CF, Crevillente Fault; AMF, Alhama de Murcia Fault. OCM, Orihuela-Callosa Massif; CM, Carrascoy Massif.
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tijada del Pozuelo section), indicating that theLorca basin was already periodically restrictedand re£ooded by marine waters [6]. The top ofthe Serrata Fm. consists of silty marls with inter-calated sandstone beds, capped by a massive gyp-sum unit of approximately 50 m thick that ismainly composed of laminated, detrital and nod-ular gypsum [6,9,12^14].
2.2. Fortuna basin: the Chicamo and Chorricosections
The Fortuna basin lies on the contact between
the external and internal Betics and is also boundby two major NE^SW shear zones (the Alhamade Murcia and the Crevillente Fault, Fig. 1). Thebasin developed since the early Tortonian and itscon¢guration was largely controlled by sets ofboth (sinistral) strike-slip and normal faults underan overall compressional regime resulting fromAfrica^Europe plate collision [15^17]. The sedi-mentary in¢ll of the Fortuna basin shows a re-gressive sequence from marine marls (Los Ban¬osFm. [18]) to diatomites and evaporites (Rio Chi-camo Fm. [18]) and continental deposits (RamblaSalada Fm. [18]).
Fig. 2. Lithology, magnetostratigraphy and planktonic foraminiferal biostratigraphy of the Serrata section of the Lorca basin.The Hondo/Serrata transition marks the onset of more restricted conditions in the basin. The diatomaceous/opal-rich beds are la-belled a^e. The roman numbering is after [6] and corresponds to precursor evaporitic layers which laterally grade into gypsumbeds at the basin margin. The uppermost massif gypsum unit is overlain by continental deposits of the laminated pelite membersensu [9]. In the magnetostratigraphy column solid dots denote reliable directions, crosses denote unreliable results.
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The Chicamo section, 320 m thick and situatedalong the Rio Chicamo river, 4 km to the south ofthe village of Abanilla, comprises the uppermostpart of the Los Ban¬os Fm., the entire Rio Chica-mo Fm. and the lowermost part of the RamblaSalada Fm. [7,19,20]. The basal part is perfectlyexposed along the river ^ as well as in a neigh-bouring gully complex ^ and consists of marine,bluish coloured, homogeneous marls of the LosBan¬os Fm. (Fig. 3). Occasionally, very thin turbi-dites are intercalated. Upward, these marls show asudden transition to a well laminated, 15 m thick,gypsum unit, informally named Tale Gypsum[18]. Above the Tale Gypsum, an alternation ofsix sedimentary cycles, consisting of diatomiticmarls and evaporites, characterises the restricted
environment of the Fortuna basin during sedi-mentation of the Rio Chicamo Fm. The upper-most evaporite cycle is overlain by a conglomerateunit with the informal name `Wichman bed' [19].The sediments of the upper part (Rambla SaladaFm.) are all of continental origin and consist ofmarls, clays, sands and occasionally gypsum. Thispart is well-exposed in two relatively fresh out-crops along the main road from Abanilla to San-tomera.
The Chorrico section, 750 m thick, is situated inthe Molina de Segura suburbs, 6 km to the northof Murcia. It is the most suitable (longest andcontinuous) section to study the post-evaporiticcontinental Rambla Salada Fm. We had alreadysampled the upper 350 m of the section for a
Fig. 3. Lithology, magnetostratigraphy and fossil mammal sites of the Chicamo and Chorrico sections of the Fortuna basin. TheLos Ban¬os/Rio Chicamo transition marks the onset of more restricted conditions in the basin. The Rio Chicamo Fm. reveals adistinct sedimentary cyclicity of evaporites/diatomites and laminated marls. The Rio Chicamo/Rambla Salada transition marksthe onset of continental sedimentation. Mice denote fossil mammal sites: grey mouse indicates MN12 site, white mice MN13 andblack mice MN13 with the additional presence of Paraethomys (see also [21] and legend to Fig. 2).
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magnetostratigraphic study that focused on theMessinian evolution of the eastern Betics andthe dating of late Miocene mammal events inSpain [21]. For our present study, we extendedthe Chorrico section downward by an additional300 m (Fig. 3) until we reached the suburbs ofMolina de Segura which are built on the evaporiteunits of the Fortuna basin. The downward exten-sion of the section comprised predominantly con-tinental marls, silts and sandstones.
3. Biostratigraphy
3.1. Planktonic foraminifera
Planktonic foraminiferal biostratigraphy isbased on the stratigraphic distribution of fourmarker species plus the coiling ratio of N. acos-taensis and is performed according to standardmethods. The best datable part of the Serratasection is the lower part corresponding with theHondo Fm. The joint presence of Sphaeroidinel-lopsis seminulina and Globorotalia menardii 4 ^including conical forms being indistinguishablefrom Globorotalia miotumida ^ in the basal partof this section (Fig. 2) allows a correlation withthe Metochia (Crete) and Gibliscemi (Sicily) sec-tions from which a similar association has beenreported [22] with an age of 7.892 Ma [23]. Pre-vious reports of Globorotalia conomiozea from thesame level [6] are basically correct, but the asso-ciated faunal elements seem to indicate that thesekeeled and conical globorotaliids belong to anearly (late Tortonian) in£ux of the G. miotumidagroup into the Mediterranean, which precedes theFRO of this group at the base of the Messinianby about 650 kyr.
Supporting biostratigraphic evidence for thecorrectness of this correlation is provided by thepresence of Globoquadrina altispira in several sam-ples from the Hondo Fm. (Fig. 2). This species isrelatively frequent immediately above and belowthe late Tortonian in£ux of the G. miotumidagroup in Crete and Sicily and (almost) absent inyounger Miocene deposits. Also the scattered oc-currences of Globorotaloides falconarae1 in theHondo Fm. and the lower part of the overlying
Serrata Fm. suggests a late Tortonian age for thebase of the Serrata section because the LO of thisspecies in the central and eastern Mediterranean isdated at 7.443^7.456 Ma [22,23]. The isolated oc-currence of S. seminulina at the base of theSerrata section, however, is at variance with Creteand Sicily, where the interval above and below thelate Tortonian in£ux of the G. miotumida group ischaracterised by scattered occurrences of thisspecies. The dominant dextral coiling of Neoglo-boquadrina acostaensis 18 m above the base isalso at odd with Crete and Sicily where sinistralcoiling dominates between the early in£ux of theG. miotumida group and the LO of G. falconarae[22].
Representatives of Bolboformis are relativelycommon, particularly in the lower part of theSerrata Fm. and belong for the larger part toBolboformis intermedia. This again is an indica-tion that the Serrata section is of late Tortonianage, since in Crete and Sicily Bolboformis almostexclusively occurs in the Tortonian with B. inter-media being restricted to the late Tortonian [22].The absence of Bolboformis, G. menardii 4 andG. falconarae in the upper part of the SerrataFm. may be related to increasingly stressed sur-face water conditions, which near the top of thesection resulted in the £ourishing of low-diversefaunas dominated by Globigerina bulloides and/orGlobigerina quinqueloba.
The presence of dominant dextral N. acostaen-sis in several samples from the Serrata Fm. (Fig.2) is puzzling because no dominant dextralN. acostaensis has been observed in time-equiva-lent sediments from Crete, Sicily, and Morocco[22,25]. One explanation is that these dextral pop-ulations re£ect special (but unspeci¢ed) local sur-face water conditions. An alternative explanationis that they have been derived by erosion fromlower Tortonian marine sediments, which in thecentral and eastern Mediterranean are character-ised by dextral coiling of N. acostaensis [22,26].Four samples from the Serrata Fm. contain re-worked Cretaceous and Eocene planktonic fora-
1 Note that the label Catapsydrax parvulus in [22] has beenreplaced by Globorotaloides falconarae (see discussion in [24]).
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minifera, one of which with dextral N. acostaensis.Another two samples containing dextral N. acos-taensis include a few large-sized G. falconarae andone specimen of Globoquadrina dehiscens. Boththese taxa may have been reworked from olderTortonian sediments. All other samples from theSerrata Fm. lack clear evidence for reworking.The only sample from the Hondo Fm. containingreworked Cretaceous and Eocene species is justthe one containing dominant dextral N. acostaen-sis.
Reworking does thus occur, but whether or notthe deviating coiling pattern in N. acostaensisshould be entirely attributed to the process of re-deposition remains unresolved. The problem ofreworking also plays in the nearby Fortuna basin.The marls of the Los Ban¬os Fm. in the Chicamosection contain £uctuating numbers of poorly pre-served planktonic foraminifera. The neogloboqua-drinids compare best with N. acostaensis from thelower Tortonian on Sicily because of their domi-nant dextral coiling and the presence of specimensresembling small-sized Neogloboquadrina atlantica(see [26]). This interpretation is supported by thepresence of specimens resembling Globorotaliapartimlabiata and of G. dehiscens (in one sample).The lack of clear in-situ microfaunas suggests ad-verse environmental conditions and occasionallymassive reworking during the deposition of thesemarls. Clear evidence of substantial reworking inthe marls of the Los Ban¬os Fm recommends cau-tion with regard to the biostratigraphic interpre-tation of the time-equivalent Hondo Fm. The fewsamples from the Rio Chicamo Fm. (being time-equivalent with the Serrata Fm. in the Lorca ba-sin) seems to contain an in-situ planktonic fora-miniferal fauna dominated by G. quinqueloba and/or G. bulloides and a few sinistral N. acostaensis.The absence of dextral N. acostaensis might pro-vide a hint that the dextral N. acostaensis in theLorca basin has been reworked.
The planktonic foraminiferal data presentedabove leaves little doubt that reworking is a seri-ous problem in the Lorca and Fortuna basinswhich prevents any straightforward biostrati-graphic interpretation. If we weigh all the similar-ities and inconsistencies between the Spanish dataand those from Crete, Sicily, and Morocco, then
the scale is tipped to a late Tortonian age for theSerrata section in the Lorca basin.
3.2. Calcareous nannofossils
Biostratigraphic information obtained by thestudy of calcareous nannofossils in samples fromthe Serrata section also show a strong in£uence ofreworking. Moreover, the peculiar environmentalconditions which characterised the sedimentationin the Lorca basin a¡ected the composition of thenannofossil assemblages as well. This resulted inassemblages characterised by the dominance ofreworked (Cretaceous, Paleogene and lower Mio-cene) forms over the autochtonous ones in mostof the studied samples. Eleven out of 87 sampleswere completely barren of any nannofossil. Re-working is particularly strong in the intervalfrom 78 to 125 m, where assemblages completelybarren of in situ nannofossils were observed inmost of the samples. Samples with a consistentpresence of autochtonous nannofossils are thosecorresponding to the lower part of the section(Hondo Fm.) and sparse samples from the SerrataFm, but these assemblages have mainly long-ranging species and lack of biostratigraphicmarkers. Di¡erently to what reported by Rouchyet al. [6], we did not ¢nd any evidence of Messi-nian nannofossil markers. The nannofossil speciesbelonging to the genus Amaurolithus, typically as-sociated with the G. miotumida group at the Tor-tonian/Messinian transition in the Mediterranean,are completely missing. Moreover, from the samelevel where earlier G. conomiozea has been re-ported [6], we did not ¢nd any evidence of thepresence of Reticulofenestra rotaria, a species con-sidered by several authors (e.g. [27,28]) a goodmarker for the Messinian in the Mediterranean(it has a scattered distribution from C3Br.2r toC3Ar- [28]). We focused on other lines of evidencein an attempt to provide a biostratigraphic `mean-ing' to the poor data obtained. The absence ofReticulofenestra pseudoumbilicus (s 7 Wm) in allthe observed in-situ assemblages helped to tenta-tively interpret what has been observed. Thislong-range species, which appeared in the middleMiocene (at about 14.5 Ma) and became extinctin the Pliocene (at 3.7 Ma), temporarily disap-
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peared from the stratigraphic record in the inter-val from 8.8 to 7.1 Ma [29], de¢ning a R. pseu-doumbilicus paracme [30]. This late Miocene inter-val lacking R. pseudoumbilicus was observed invarious ocean basins (see [29], for references).Some diachrony was evidenced for the re-entranceof this species, which seems to occur at the top ofthe interval corresponding to C3Ar. This indicatesthat R. pseudoumbilicus should have occurred(due to its re-entrance) in the Serrata section, toconform to a Messinian age as inferred byRouchy et al. [6], but this is not the case. Onthe contrary, the nannofossil assemblages ob-served both in Hondo and Serrata Fms have thepeculiar `atypical character' of the upper Torto-nian nannofossil assemblages in the Mediterra-nean. They lack of the biostratigraphic markersknown in the oceanic sediments (e.g. discosteridsof the D. berggrenii^quinqueramus group) andshow the e¡ects of the environmental conditions(restricted environment and increased reworkingof older sediments). Particularly, in the samplesfrom Serrata Fm low-diversity assemblages wereobserved, with some of the few autochtonousspecies showing unusual abundances. Bloom ofCoccolithus pelagicus and peaks in abundance ofhelicoliths were recorded, generally in correspond-ence with the opal-rich beds, indicating the envi-ronmental control on the nannofossil distribu-tions.
3.3. Mammals
In the Fortuna basin, middle Turolian (MN 12)fossil mammal assemblages have been found inthe locality of Casa del Acero, located approxi-mately 70^80 m above the marine evaporites [31^33], at the base of the Choricco section, and in theBarranco de la Parra section near Librilla. Bio-zone MN12 is de¢nitely older than 6.8 Ma be-cause younger levels in the Fortuna basin revealMN13 fauna [21]. A late Turolian age (MN13) isgiven to the Librilla, Chorrico and Salinas (nearMolina de Segura) sections [21].
4. Magnetostratigraphy
Paleomagnetic samples have been thermally de-magnetised according to standard procedures.The mean NRM intensity in the Serrata sectionis around 0.1 mA/m. Thermal demagnetisation ofthe NRM reveals the presence of an initial com-ponent of normal polarity consistent with apresent-day ¢eld direction. A characteristic dualpolarity component of magnetisation was ob-tained in 66% of the specimens in the temperaturerange between 200 and 390³C (Fig. 4). Furtherheating above 400³C only revealed viscous ran-dom directions, caused by the oxidation of pyriteinto magnetite. In 33% of the specimens the char-acteristic component could not be isolated withcon¢dence. Declinations and inclinations werecalculated for each characteristic component sta-ble endpoint direction after correction for beddingtilt. Magnetostratigraphic results of the Serratasection reveal three polarity zones. The lowerpart (0^95 m) consists of normal polarities2, asmall reversed zone is found between 95 and 120m, and the top part (120^175 m) is of normalpolarity again (Fig. 2).
NRM intensities in the Chicamo section rangefrom 0.04 to 10 mA/m, although most of the sam-ples show values around 0.1 mA/m. After removalof a weathering induced overprint (T6 240³C) acharacteristic dual polarity component (Fig. 4) isremoved between temperatures of 240 and 400 or680³C (depending on lithology) indicating thatiron oxides, such as hematite and magnetite, arecarriers of the ChRM. Removal of the secondaryoverprint often results in a very weak ChRMcomponent. We generally refrained from inter-preting the Zijderveld diagrams when the ChRMintensity at 240³C is less than 0.02 mA/m. The
2 Rouchy et al. [6] found two single levels with reversed polar-ity in the lower part of their Serrata section corresponding to asandy turbidite in diatomaceous bed (d) and a sulphur-richcarbonate level in bed (III). These suspect lithologies werenot sampled by us, but the marls directly above and below(6 10 cm) show normal polarities. We consider the correlationof these two reversed levels to chrons C3Ar and C3An.1r [6]erroneous and think that these reversed directions are prob-ably diagenetic artefacts.
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only exceptions are made when the ChRM isclearly of reversed polarity. In most cases, theChRM directions can be reliably determined ifintensities are higher than 0.02 mA/m. In general,our results from the Chicamo section are ingood agreement with the earlier paleomagneticdata [7], although upward extension of our sec-tion revealed an additional (N/R) magnetic rever-sal.
Thermal demagnetisation behaviour of samplesfrom the downward extension of the Chorricosection is similar to the results of our previousstudy [21]. A ChRM is isolated above 300³Cand shows both normal and reversed polarities(Fig. 4). The overall mean direction (reverse sam-ples rotated to antipodal) yields a very substantialanticlockwise rotation of approximately 50³ [21],which can be linked to the prevalent left-lateralshear of the associated NE^SW trending wrenchfault [34].
5. Correlation to the astronomical polarity timescale
The magnetic polarity column of the Serratasection reveals three magnetozones with a re-versed interval that is at least four times shorterthan the normal interval below, and two timesshorter than the normal interval above (Fig. 2).Magnetostratigraphically, this characteristic pat-tern can be perfectly correlated to the late Torto-nian part of the late Miocene polarity time scale[23], while correlations to the Messinian are lesslikely. Clearly, the best pattern ¢t is obtained bythe correlation to chron C4n.2n, C4n.1r andC4n.1n (Fig. 5). This magnetostratigraphic corre-lation is in agreement with biostratigraphic inter-pretations indicative of a late Tortonian age forthe Serrata section.
The most striking feature of the Chicamo po-larity sequence, with three normal and three re-versed magnetozones, is that the lowermost twonormal magnetozones are much longer than thereversed magnetozones. This pattern perfectly
Fig. 4. Zijderveld diagrams for samples from the Serrata (LS) section of the Lorca basin and the Chicamo (CO) and Chorrico(CH) sections of the Fortuna basin. Filled symbols denote the projection of the vector end-points on the horizontal plane; opensymbols denote projections on the vertical plane; values represent temperatures in ³C; stratigraphic levels are in the lower left-hand corner.
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Fig. 5. Magnetostratigraphic correlation of the Serrata, Chicamo and Chorrico sections to the astronomical time scale of [23]modi¢ed for the Messinian part by [4]. We de¢ne the term `salinity crisis' as the interval of evaporite deposition [4]. The onset ofthe `Tortonian salinity crisis' (TSC) of the eastern Betics took place at an age of 7.8 Ma and had a duration of approximately200 kyr. Note that this Tortonian salinity crisis occurred almost 1.8 Myr earlier than the Messinian salinity crisis (MSC) of theMediterranean.
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correlates with the late Tortonian polarity se-quence of chrons C4n.2n, C4n.1n and C3Br.2n(Fig. 5). Another good pattern ¢t is obtained inthe early Tortonian (10.4^9.5 Ma) part of theGPTS, but this correlation is regarded unlikelyfrom a biostratigraphic point of view. Correla-tions to the Messinian [7] again clearly result inunrealistic changes in sedimentation rate and/orhiatuses and are in disagreement with the biostra-tigraphic data.
The upper part of the Chorrico section was al-ready correlated to the GPTS [21] with the twonormal zones corresponding to C3An.1n andC3An.2n, respectively (Fig. 5). The downward ex-tension of the Chorrico section reveals an addi-tional ¢ve magnetozones, making a total polaritysequence of ten magnetozones (¢ve normal and¢ve reversed). The downward extension of theChorrico section perfectly correlates to the ex-pected polarity sequence characteristic for the in-terval straddling the latest Tortonian/earliest Mes-sinian and implies that the additional normalzones correspond to C3Bn, C3Br.1n andC3Br.2n (Fig. 5).
6. The `Tortonian salinity crisis' of the easternBetics
The ¢rst open-marine conditions in the Lorca(Hondo Fm) and Fortuna basin (Los Ban¬os Fm)are recorded after some ill-de¢ned period in theearly-middle Tortonian [9]. An important changein basin con¢guration took place in the late Tor-tonian, accompanied by uplift of the basin mar-gins [6,9]. This uplift was most likely related tomajor wrench fault activity along the SW^NEstriking Alhama de Murcia fault system, and re-sulted in an increase of detrital input and rework-ing of earlier deposited sediments. The basinalresponse to this tectonic period was an increasein salinity and a change in facies from marls todiatomites and evaporites. Our new chronologyshows that this salinity crisis began in the upperpart of chron C4n.2n at an approximate age of7.80 þ 0.05 Ma, both in the Lorca and Fortunabasin. The transition to continental deposits,marked in Lorca by the deposition of a very thick
evaporite unit [14], took place at approximately7.6 Ma (chron C4n.1n). Continuous subsidenceof the basin centres accommodated a thick seriesof late Miocene continental deposits (Rambla Sal-ada Fm.), which comprise a time interval of morethan 2.3 Myr.
Evaporite deposition in the eastern Betics thusresulted from a local tectonic phase in the lateTortonian that caused basin restriction by upliftof several structural blocks at the basin margins.Consequently, no relation exists with the Mediter-ranean Messinian salinity crisis. In fact, the Lorcaand Fortuna basins experienced their own phaseof restriction and desiccation, 1.8 Myr earlier thanthe Mediterranean, and thus could better be re-ferred to as the `Tortonian salinity crisis' of theeastern Betics. This Tortonian salinity crisis isnow accurately dated at 7.80 þ 0.05 Ma and hada duration of approximately 200 kyr.
7. Discussion
In the present-day situation of the Mediterra-nean, the loss of water by evaporation is morethan double the gain by precipitation and runo¡.This net loss of fresh water in the Mediterraneanis compensated by the in£ow of saline surfacewaters from the Atlantic and the Black Sea. Toconserve salinity in the Mediterranean, there mustbe export of excess salt. This export is maintainedby out£ow at depth of dense and saline waterthrough the Strait of Gibraltar. Restricting thisout£ow would increase the salinity in the Medi-terranean which would ultimately result in thedeposition of evaporites.
Tectonic uplift in the Gibraltar area during theMessinian [4,35,36] may have restricted the out-£ow of excess salt, while the in£ow of Atlanticsurface waters continued, which in the end re-sulted in the formation of the Messinian salinitycrisis evaporites. During the Tortonian salinitycrisis of the eastern Betics, out£ow has probablybeen restricted by tectonic uplift of the SW^NEstriking metamorphic complexes (Orihuela-Cal-losa massif and Carrascoy massif) of the InternalBetics (Fig. 1), which separated the Lorca andFortuna basins from the Mediterranean. Uplift
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of these sill complexes probably originated in con-cert with strike-slip activity along the SW^NEtrending fault zones like the Alhama de MurciaFault, as a consequence of Africa^Europe colli-sion. These strike-slip faults can be traced
throughout the Alboran basin and Morocco andlargely determined the tectonic evolution of theGibraltar arc. [35,37,38]. Cyclically bedded evap-orites and diatomites were deposited in a re-stricted basin to the northwest, while open marine
Fig. 6. Tentative astronomical calibration of the Serrata section to the insolation curve of Laskar [44] using the paleomagnetic re-versals as age constraints. Astronomical polarity time scale (APTS) is after [23]. Numbered arrows denote the bioevents recog-nised in the Serrata section: 1, level with conical forms of the G. miotumida group, being indistinguishable from G. conomiozea ;and 2, ¢rst level of dominant dextral coiling N. acostaensis. The calibration of the Gibliscemi section from Sicily [23] is shownon the right-hand side for comparison with the Mediterranean record. Downward calibration of the sedimentary cycles of theSerrata section reveals that the level with the G. miotumida group perfectly correlates to one of the two in£uxes in Gibliscemi.Upward calibration of cycles VII and VIII is less straightforward because this interval is marked by low amplitude variationsand precession/obliquity interference patterns in insolation, related to the V400 kyr eccentricity minimum around 7.6^7.7 Ma.Regarding the increased thickness between the diatomite layers, it is possible that the diatomite VII and VIII correspond to therelatively highest peaks in the insolation record. The uppermost part of the section (between cycle VIII and the massif gypsum)is the most problematic because the sedimentary expression of the climate £uctuations is completely lacking. Assuming a rela-tively constant sedimentation rate, the transition to the main evaporites closely coincides with the amplitude increase in insolationfollowing the V400 kyr eccentricity minimum dated around 7.6^7.7 Ma. The normal polarities at the base of the evaporitesclearly demonstrate that the age of the transition to evaporites is older than 7.532 Ma. Note, furthermore, that the two diatomitelevels (white) of Gibliscemi predate the onset of the Hondo/Serrata transition by approximately 80 kyr.
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Mediterranean sequences prevailed to the south-east, including the well-known Messinian evapor-ites of San Miguel de Salinas (Fig. 1).
Quite a few studies have obviously misinter-preted the diatomite and evaporite deposits ofthe Lorca and Fortuna basin as equivalents ofthe Messinian salinity crisis because of facies sim-ilarities. Indeed, both the Messinian and Torto-nian salinity crises are materialised by a regressivesequence and a signi¢cant increase in biosiliceousdeposits (diatomites). This suggests that the dia-tomaceous facies is an essential element in theprocess of basin restriction.
Astrochronology has revealed that evaporitecyclicity during the Messinian salinity crisis isdominantly driven by dry-wet climate oscillationsin the precession frequency band of orbital forc-ing [4,39]. Evaporite deposition occurred duringprecession maxima (insolation minima), duringrelatively dry periods when evaporation exceededprecipitation. During precession minima (insola-tion maxima) and relatively wet periods, highfreshwater runo¡ resulted in deposition of lami-nated marls. Obliquity controlled glacio-eustaticsea level changes may only have added a minorcontribution to the formation of evaporites.Evaporite cyclicity in the Tortonian salinity crisisis best demonstrated in the Chicamo section ofthe Fortuna basin where a regular alternation ofsix evaporite-marl cycles are present immediatelybelow the Wichman-conglomerate. A similar cy-clicity is exempli¢ed by seven gypsum cyles in themarginal facies of the Serrata Fm. (Cortijada delPozuelo section) of the Lorca basin. These gyp-sum layers can easily be traced to the Serratasection where they have been replaced by sul-phur-bearing carbonates [6]. The magnetostrati-graphic results from the Chicamo and Serrata sec-tions indicate that approximately three to fourevaporite cycles are present in the reversed inter-val correlative to C4n.1r. Precessional forcing forthe observed evaporite cyclicity, implying a dura-tion of approximately 60^80 kyr for the reversedinterval, is in good agreement with the astronom-ical duration of chron C4n.1r is 53 kyr [23].Clearly, obliquity and eccentricity can be ruledout because they would result in unrealistic longpolarity intervals.
Precessional forcing for the evaporite cyclicitycan, furthermore, be used to make a tentativecorrelation of the Serrata section to the astronom-ical curves. The paleomagnetic reversals of C4n.1rare located between cycles III and V and betweenVI and VII. Unfortunately, cyclicity is not veryclear in this interval, but the reversed intervalcomprises approximately three or four sedimenta-ry cycles. Downward correlation of the diatomitebeds to subsequent peaks in insolation minima(Fig. 6) reveals that the in£ux of the G. miotumidagroup corresponds to one of the two in£uxes inthe Gibliscemi section of Sicily [22,23].
The evaporite deposits in the Fortuna basinhave long been regarded as evidence for the exis-tence of a Messinian marine gateway between theMediterranean and Atlantic [18]. This so-called`Iberian Portal' or `Betic Corridor' is thought tohave persisted until the late Messinian and, assuch, to have provided the oceanic waters re-quired for deposition of the marine evaporites ofthe Messinian salinity crisis [36]. Our data fromthe Lorca and Fortuna basins, however, clearlyindicate that marine conditions already disap-peared during the late Tortonian, 1.8 Myr beforethe onset of the Messinian salinity crisis. This is inagreement with the pioneering work of Montenat[17] who dated the pre-evaporitic marls as lateTortonian and the evaporites as latest Torto-nian^earliest Messinian because they were foundin direct superposition. A late Tortonian phase ofbasin restriction is also reported from other Span-ish basins that were part of this hypothetical Mes-sinian gateway. Sedimentological studies from theGuadix-Baza and Granada basins, located in amore central position in the Betics, show a changefrom marine to continental sediments in the lateTortonian [40^43]. Hence, our data provide fur-ther evidence that the Betic Corridor which con-nected the Mediterranean with the Atlantic duringthe Tortonian, became emergent well before theMessinian.
8. Conclusions
A detailed and integrated magnetostratigraphic,biostratigraphic, and cyclostratigraphic study of
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the sedimentary sequences of the Lorca and For-tuna basins revealed that the entire marine succes-sion was deposited during Tortonian times. Animportant tectonic event which strongly restrictedthese basins from the open marine waters of theMediterranean occurred at 7.80 þ 0.05 Ma. Thisphase resulted in the deposition of cyclic alterna-tions of diatomites and evaporites, which stronglyfavour a precession-induced climatic control. The¢nal isolation of the basins resulting in a transi-tion from marine to continental deposits is datedat 7.6 Ma. The isolation and desiccation of theLorca and Fortuna basins from the Mediterra-nean was related to a local tectonic phase andhas no relation to the isolation and desiccationprocesses of the Mediterranean that occurred 1.8Myr later. Consequently, the evaporites of theLorca and Fortuna basins do not correlate tothe Messinian salinity crisis of the Mediterranean(5.96^5.33 Ma), but to a Tortonian salinity crisisof the eastern Betics (7.8^7.6 Ma). This late Tor-tonian phase of basin reorganisation is, further-more, in good agreement with the geodynamicevolution of the other basins that formed theBetic Corridor in Tortonian times and providesmore evidence that this marine gateway wasclosed during the Messinian.
Acknowledgements
We thank Marie Russell and Jean-MarieRouchy for their help in the ¢eld. Henk Meijerand Piet-Jan Verplak assisted with the paleomag-netic measurements. Gerrit van 't Veld and GeertIttman processed the micropaleontological sam-ples. Bill Ryan, Josep Pares, Cor Langereis andFrits Hilgen are thanked for their constructive re-views. This work was conducted under the pro-gramme of the Vening Meinesz Research Schoolof Geodynamics (VMSG) and CICYT projectPB96-0815.[RV]
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