basin research (2001) 13, 199–216 chronostratigraphic …forth/publications/garces01.pdf ·...

Chronostratigraphic framework and evolution of theFortuna basin (Eastern Betics) since the Late MioceneM. GarceÂs,* W. Krijgsman² and J. Agusti³

*Institute of Earth Sciences Jaume Almera, CSIC, SoleÂ i SabarõÂs

s/n, 08028-Barcelona, Spain

²Paleomagnetic Laboratory Fort Hoofddijk, Budapestlaan 17,

3584 CD Utrecht, The Netherlands

³Institut de Paleontologia Miquel Crusafont, Escola Industrial 23,

08201 Sabadell, Spain

ABSTRACT

A Tortonian to Pliocene magnetostratigraphy of the Fortuna basin supports a new

chronostratigraphic framework, which is signi®cant for the palaeogeographical and geodynamic

evolution of the Eastern Betics in SE Spain.

The Neogene Fortuna basin is an elongated trough which formed over a left-lateral strike-slip zone

in the Eastern Betics in the context of the convergence between the African and Iberian plates.

Coeval with other basins in the Alicante±Cartagena area (Eastern Betics), rapid initial

subsidence in the Fortuna basin started in the Tortonian as a result of WNW±ESE stretching.

This led to transgression and deposition of marine sediments over extensive areas in open

connection with the neighbouring basins. Since the late Tortonian, N±S to NW±SE

compression led to inversion of older extensional structures. The transpressional tectonics along

the NE±SW-trending Alhama de Murcia Fault is related to the rising of a structural high

which isolated the Fortuna basin from the open Mediterranean basin. The progression of basin

con®nement is indicated by the development of restricted marine environments and deposition

of evaporites (7.8±7.6 Ma). The new basin con®guration favoured rapid sediment accumulation

and marine regression. The basin subsided rapidly during the Messinian, leading to the

accumulation of thick continental sequences. During the Pliocene, left-lateral shear along the

Alhama de Murcia Fault caused synsedimentary folding, vertical axis block rotations and uplift

of both the basin and its margins. The overall sedimentary evolution of the Fortuna basin can

be regarded as a developing pull-apart basin controlled by NE±SW strike-slip faults. This

resembles the evolution that has taken place in some areas of the Eastern Alboran basin since

the late Tortonian.

INTRODUCTION

The Neogene tectonic evolution of the Betic, Rif and

Alboran region, at the western end of the Alpine±

Mediterranean collisional system, continues to puzzle

geoscientists (Sanz de Galdeano, 1990; Maldonado &

Comas, 1992; Lonergan & White, 1997; Comas et al.,1999; TorneÂ et al. 2000). Controversy surrounds the

origin of the Alboran basin, the geodynamic evolution of

the Betic and Rif cordilleras, and the timing of closure and

palaeogeography of the late Messinian marine passages

across the Gibraltar Arc. To achieve a complete picture of

the tectonic and sedimentary processes acting in the area

since the early Miocene, onshore and offshore strati-

graphic records must be integrated. Moreover, correlation

between the intramontane basins of the Betic Cordillera is

crucial in understanding the vertical crustal movements

that controlled the post-collisional evolution of the overall

region. A high-resolution chronostratigraphic frame is

therefore required before onshore and offshore basin

stratigraphy can be combined with the results of deeper

crustal geophysical studies to produce satisfying forward

models of the basin-forming mechanisms.

The predominance of restricted marine and continental

facies, with poor biostratigraphic resolution, found in the

Neogene intramontane basins provides a weak chronos-

tratigraphic framework (Sanz de Galdeano & Vera, 1992).

As a result, interbasinal correlations have been made

without a well constrained time frame. Recent studies

(GarceÂs et al., 1998; Krijgsman et al., 2000) have

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challenged earlier correlations of the stratigraphic record

of the Fortuna and Lorca basins. In particular, the

interpretation of the evaporitic and diatomitic formations

in these basins as related to the Messinian Salinity Crisis

(Santisteban, 1981; MuÈller & HsuÈ, 1987) was shown to be

clearly untenable. In this paper, we present fuller and

more extended magnetostratigraphic and biostratigraphic

dataset that support these results. A new Tortonian to

Pliocene chronology places the stratigraphic record of the

Fortuna basin in a more consistent palaeogeographical

and geodynamic context, with wider implications for the

evolution of the eastern Betics.

GEODYNAMIC ANDPALAEOGEOGRAPHICAL SETTING

The Betic±Rif mountain belt constitutes the western edge

of the Mediterranean Alpine thrust-belt, which formed

during the Cenozoic in response to the convergence

between the African and European plates. The palaeo-

geographical reconstruction of the belt is complicated by

large-scale tectonic transport oblique to the belt axis,

coupled with very important extension in the central parts

of the chain (Andrieux et al., 1971; Durand Delga &

FontboteÂ, 1980; Sanz de Galdeano, 1990). Lower to

middle Miocene collision led to thrusting and westward-

directed migration of the Alboran domain units (Internal

zones) over the relative autochthonous units of the Iberian

and African continental margins (External zones) (Fig. 1).

One striking feature of the Betic and Rif Cordilleras is the

substantial subsidence accommodated in the Internal

Zones, leading to the generation of the Alboran basin

since the early Miocene. Subsidence of the Alboran basin

re¯ects important crustal thinning, largely accomplished

by means of subcrustal detachments and low-angle

extensional faults as identi®ed along the uplifted ¯anks

of the Alboran basin (GarcõÂa-DuenÄas et al., 1992). A

number of models that involve the removal of subcrustal

continental lithosphere, such as convective removal (Platt

& Vissers, 1989) or delamination (Docherty & Banda,

1995; Platt et al., 1998), have been invoked as a possible

extension driving mechanism. Other authors (Doglioni

et al., 1997; Lonergan & White, 1997) suggest alternative

scenarios in which the Alboran basin, together with

the South Balearic basin, is regarded as a consequence

of the back-arc extension related to a rapidly retreating

subduction of the old Tethyan oceanic crust.

Following continental collision and the emplacement of

the thrust sheet units, the oblique convergence between

Africa and Iberia during the late Miocene caused

deformation along NE±SW and NW±SE strike-slip

faults and the generation of intramontane basins (Sanz

de Galdeano & Vera, 1992). During the Tortonian, the

Alboran basin occupied a wider area than its present-day

Fig. 1. Sketch showing the principal tectonic elements of the Gibraltar arc and Alboran basin region. WAB: West Alboran

basin; EAB: East Alboran basin; SBB: South Balearic basin; AR: Alboran Ridge.

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con®guration, extending towards the north and south to

encompass the Betic and Rif regions. The intramontane

Betic basins are therefore regarded as an uplifted ¯ank of

the Alboran basin (Comas et al., 1992). Among these, the

Fortuna basin is an elongated (15±20 km wide and 60 km

long) trough, sealing the contact between the Internal

and the External zones in the Eastern Betics (Figs 1

and 2). Two major NE±SW-trending shear zones, the

Crevillente Fault and the Alhama de Murcia Fault, limit

its north-western and south-eastern margins, respectively

(Montenat et al., 1990a). The main terrigenous sediment

sources were the External Betics massifs, mainly consist-

ing of Mesozoic and Cenozoic carbonates. Detrital input

from the Internal Betics was less signi®cant and mostly

restricted to the alluvial sequences north of Murcia

(Fig. 2), where an emerged domain known as the SeguraMassif (Montenat, 1973) was present since the Tortonian.

Some present-day relicts of an Internal Betics palaeo-

margin to the east of the Alhama de Murcia Fault are

the Orihuela and Callosa massifs (Fig. 1). The prolonga-

tion of this uplifted block to the SW is detected below

the Quaternary sediments of the Guadalentin corridor

(Fig. 3) by means of a gravimetric anomaly (Gauyau et al.,1977). On the SW edge of the basin, the Sierra EspunÄa

mountain range was an uplifted block of the Internal

Zones between the Fortuna and Lorca basins. Proximal

basin-margin sediments on the sides of the Sierra

EspunÄa indicate the presence of this emergent domain

since the Tortonian (Montenat et al., 1990b; Lonergan &

Schreiber, 1993). On the NE edge of the Fortuna basin,

the Crevillente strait (Montenat, 1973) allowed a marine

connection with the Mediterranean basin in the

Tortonian. The uplift of the eastern margin of the

Fortuna basin, resulting from the transpressional left-

lateral strike-slip tectonics along the Alhama de Murcia

Fault, played an important role in the closure of marine

gateways and in the subsequent isolation of the Fortuna

basin from the Mediterranean Sea, as discussed below.

THE FORTUNA BASIN SEDIMENTARYRECORD

The sedimentary in®ll of the Fortuna basin can be simply

grouped into three basinwide units: (1) a lower trans-

gressive marine unit; (2) a regressive marine to transi-

tional evaporitic unit; (3) a very thick continental alluvial

and lacustrine unit.

Lower Marine unit

An approximately 500-m-thick lower marine succession is

represented by turbidites and pelagic marls, ®rst referred

to as Fortuna marls by Montenat (1973), and later

informally termed the Los BanÄos Fm (MuÈller & HsuÈ,

1987). These marls grade laterally into deltaic facies as

well as reef complexes towards the western and southern

margins of the basin (Santisteban, 1981; Lonergan &

Schreiber, 1993). Pelagic facies yielded lower to upper

Tortonian planktonic assemblages (Montenat, 1973).

Other studies have suggested a Messinian age for the

upper part of the Fortuna marls (Lukowski et al., 1988),

but solid biostratigraphic evidence for this has not been

demonstrated. Magnetostratigraphic data from the Lorca

and Fortuna basins (Krijgsman et al., 2000), however,

con®rmed a late Tortonian age for this unit, as had been

suggested by Montenat (1973).

Evaporitic unit

Overlying the Fortuna marls, an almost 200-m-thick

regressive sequence, with gypsiferous marls, diatomites,

massive gypsum and terrigenous deposits, is referred to as

the RõÂo Chicamo Formation (MuÈller & HsuÈ, 1987). In the

Chicamo section, this unit is grouped into two members: a

lower gypsum unit (Tale Gypsum) and an overlying

cyclic diatomitic±evaporitic sequence (Chicamo diato-

mites). These two members have been lithostratigraphi-

cally traced to the SW edge of the Fortuna basin, in the

Librilla area, where a similar evaporitic sequence has been

described (OrtõÂ et al., 1993; PlayaÁ et al., 1999). Because of

the extreme restriction of the depositional environment,

no reliable marine biostratigraphic markers have been

reported in this unit. Nevertheless, the evaporites of the

Fortuna basin have generally been correlated with the late

Messinian Mediterranean Salinity Crisis (MuÈller &

HsuÈ, 1987; Lukowski et al., 1988; DinareÁs-Turell et al.,1999). Yet, GarceÂs et al. (1998) provided contradictory

data suggesting an older age, and recently has it been

demonstrated that they correspond to a local Tortonian

phase of basin restriction, affecting both the Fortuna and

the Lorca basins (Krijgsman et al., 2000).

Continental unit

The Rio Chicamo Fm. grades upwards into a very thick

continental succession. At the Chicamo section only the

lower part (150 m) of this sequence is exposed, where it

consists of a thinly bedded alternation of evaporites, clays

and sandstones informally known as the Rambla Salada

Fm. (MuÈller & HsuÈ, 1987). More complete exposures

of this upper continental unit occur around Molina de

Segura and in the Librilla area, where it reaches a thick-

ness of about 1000 m. The continental deposits mainly

consist of lacustrine marls and limestones alternating

with alluvial red clays and conglomerates, arranged in an

overall coarsening-upward sequence.

The lower levels of this continental unit have yielded

fossil rodents of middle Turolian age (MN12 Mammal

Neogene Unit, late Tortonian to early Messinian) at the

classical site of Casa del Acero (Mein & AgustõÂ, 1990),

7 km to the south of Fortuna, as well as in some other new

sites in the sections of this study (Figs 4 and 6). The

overlying alluvial±lacustrine sequences are rich in fossil

mammals of late Turolian (MN13, Messinian) to early

Ruscinian (MN14, early Pliocene) age (AgustõÂ et al., 1985).

The Neogene Fortuna basin, Betic Cordillera

# 2001 Blackwell Science Ltd, Basin Research, 13, 199±216 201

Fig. 2. Geological map of the Fortuna basin and neighbouring areas of the eastern Betics (south-east Spain). m: transgressive marine beds recording the Pliocene ¯ooding in the Librilla

area. Redrawn and modi®ed from Montenat et al. (1990a).

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Fig. 3. Cross-section of the southern sector of the Fortuna basin and the Guadalentin corridor.

The

Neogene

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STUDIED SECTIONS

Earlier magnetostratigraphic studies in the Fortuna basin

focused on the late Turolian (Messinian) continental units

of the Librilla and Molina de Segura areas (GarceÂs et al.,1998) and, more recently, on the evaporitic sequences

of the Chicamo section (DinareÁs-Turell et al., 1999;

Krijgsman et al., 2000) (Fig. 2). In this study, we have

extended the magnetostratigraphic and biostratigraphic

analysis to the complete outcropping sequences in the

Librilla and Molina de Segura areas, where tectonic

deformation has favoured the exposure of thick contin-

uous sequences spanning from the Tortonian marine

marls up to the Pliocene continental units.

The Librilla area

In the area to the north of Librilla, at the southern edge of

the Fortuna basin, the upper Fortuna marls, the Rio

Chicamo Fm and the overlying continental units were

sampled in the Barranco de la Salada and the SifoÂn de

Librilla sections (Figs 2 and 3).

The Barranco de la Salada section

The Barranco de la Salada (BS) section represents a

500-m-thick regressive sequence, consisting of marine

marls and sandstones (uppermost Fortuna marls),

evaporites and terrigenous deltaic deposits towards the

top (Fig. 4). The presence of desiccation cracks, raindrop

impressions and bird footprints indicates that the entire

succession was deposited in a shallow water environment

with periods of subaerial exposure. The lower marls

alternate with thinly bedded sandstones and, rarely, with

conglomerates containing debris from Tortonian basin

fringing reefs. The evaporitic sequence starts with a thick

laminated gypsum unit (Librilla gypsum). A second gypsi-

ferous and marly unit followed by laminated diato-

mitic clays has been lithostratigraphically correlated

with the evaporitic±diatomitic cycles of the Chicamo

section (OrtõÂ et al., 1993), at the NE edge of the basin

(Fig. 2). The upper part of the BS section consists of

terrigenous fan delta sediments corresponding with the

marine±continental transition in the basin. Conglomer-

ates contain clasts of Cretaceous and Palaeocene lime-

stones, suggesting a western source in the External Betics

and in the exhumed North-Betic Foreland basin to the

Fig. 4. Magnetostratigraphy of the Barranco de la Salada section and correlation with the Chicamo section (see location in

Fig. 2). Closed circles indicate reliable palaeomagnetic directions; open circles represent low intensity samples with uncertain

palaeomagnetic directions. Lithostratigraphic units: 1: Lower Marine Unit (Fortuna marls); 2: Evaporitic units; 3: Upper

Continental Unit (see Fig. 2 for location). LG: Librilla Gypsum; TG: Tale Gypsum.

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north of the Sierra EspunÄa (Fig. 2). Palustrine ¯oodplain

deposits at the top of the section have yielded fossil

rodents of middle Turolian age (MN12), similar to the

Casa del Acero faunal assemblage (Mein & AgustõÂ, 1990).

The SifoÂn de Librilla section

The base of the SifoÂn de Librilla (SL) section is

about 2 km to the north of Librilla (Fig. 2). An earlier

palaeomagnetic study focused on the very rich fossilifer-

ous lower part of the section (GarceÂs et al., 1998). In the

present paper we extended the sampled section to the

complete 800-m-thick outcropping sequence. The SL

section is grouped into four members (Fig. 5), from

bottom to top:

1 An alluvial±palustrine member (400 m) consisting ofcyclic alluvial red beds and palustrine organic-rich greymarls and minor lacustrine limestones. Fossil mammalsindicate an MN13 (late Turolian=Messinian) age.2 A prograding conglomeratic member (30 m) thatrepresents a marked basin-wide spread of the alluvialwedges fed from the western margin (External Betics).3 A shallow marine member (30 m), consisting of greysandstones and marls with bivalves, echinoderms andbenthonic foraminifera indicative of shallow (<30 m)water depths and an early Pliocene age. This probablyrepresents the basal Pliocene marine transgression thatfollowed the re-establishment of marine conditions inthe Mediterranean after the Messinian Salinity Crisis.This is the ®rst report of the presence of marinePliocene sediments in the Fortuna basin.4 An upper alluvial member (350 m), consisting ofred silts and channellized conglomerates. Clast com-position again indicates a sediment supply from theExternal Betics. Incised channels and scours indicateNE±SW palaeocurrents, parallel to the long axis ofthe basin and to the axis of a syndepositional synclineaffecting the sediments of this member. Fossil mammallocalities yield early Ruscinian (MN14, early Pliocene)faunas.

The Molina de Segura area

In the Molina de Segura area the two stratigraphically

overlapping Chorrico (CH) and Salinas de Molina (SM)

sections merge into a 900-m-thick composite section

spanning most of the upper continental unit (Fig. 6). The

SM section is about 2 km to the north-east of Molina

de Segura (Fig. 2), and can be approximately anchored

to the top of the CH section on the basis of the bio-

stratigraphic and lithostratigraphic content (Fig. 6).

The Molina de Segura succession was deposited in a

continental environment. The sediments are organized in

an overall coarsening-upward trend re¯ecting the pro-

gradation of an alluvial conglomeratic fan delta system

derived from a western source and advancing over a pre-

existing evaporitic lacustrine environment. The fan delta

deposits inter®nger over the upper part of the section

with red alluvial deposits fed from the eastern margin

(Orihuela±Callosa massifs, Internal Zones).

Fossil mammal localities in the CH section have

yielded a series of middle to late Turolian (MN12 to

MN13) faunas, similar to those described in the Librilla

area. The SM section yields an MN13 age in its lower

part, while Ruscinian (MN14) faunas are found in its

uppermost beds (AgustõÂ et al., 1985).

Fig. 5. Magnetostratigraphy of the SifoÂn de Librilla (SL)

section (see Fig. 2 for location). Closed circles indicate

reliable primary palaeomagnetic directions; open circles

represent secondary palaeomagnetic directions (remagnetized

samples). Lithostratigraphic members: 3.1: alluvial±palustrine;

3.2: prograding alluvial; 3.3: shallow marine; 3.4: upper

alluvial (see text for explanation). e: erosional surface. The

same legend of Fig. 4.



PALAEOMAGNETIC ANALYSIS

All the studied sections were sampled at approximately

1±2 m stratigraphic intervals with a portable drill and

orientated in the ®eld with a magnetic compass.

Palaeomagnetic analysis in the laboratory included

standard stepwise (<50 uC steps) thermal demagnetiza-

tion and measurement of the natural remanent magne-

tization decay by means of a three-axis SQUID

magnetometer. The majority of the samples displayed a

two-component magnetization (Fig. 7a±c), consisting of

a subrecent normal polarity overprint and a dual polarity

characteristic remanent magnetization. Successful isola-

tion of the characteristic magnetization was possible

after moderate thermal demagnetization (250±350 uC).

Stepwise demagnetization was continued until complete

removal of the characteristic magnetization. We employed

least squares analysis for the estimation of the direction

of the characteristic remanent magnetization.

An exception to the typical two-component natural

remanent magnetization was observed in a number of sites

sharing the same organic-rich grey clay lithology, in the

lower part of the SL section. In those speci®c sites, the

addition of a prefolding reversed magnetization produces

Fig. 6. Magnetostratigraphy of the Molina de Segura composite section (see Fig. 2 for location). See legend to Fig. 4.

M. GarceÂs et al.


a triple-component magnetization (Fig. 7d). This has

been explained as a remagnetization event linked to the

fall in the sea level at the end of the Messinian, which

favoured the ¯ow of meteoric ground waters to deeper

levels across the sedimentary basin in®ll (GarceÂs et al.,

1998; GarceÂs & Krijgsman, 2000). Oxidation of meta-

stable iron sulphides to magnetite would have produced a

new chemical remanent magnetization parallel to the

reversed geomagnetic ®eld that occurred at that time.

Palaeomagnetic directions show varying degrees of

rotation depending on the location of the sites studied

(Table 1). The mean palaeomagnetic directions from the

CH and SM sections are rotated anticlockwise 50u and

35u, respectively (Fig. 8). This indicates that the overall

Fig. 7. Representative Zijdervelt demagnetization diagrams and natural remanent magnetization decay plots.

Table 1. Mean palaeomagnetic directions and Fisher statistics (Fisher, 1953) of the studied sections. k: precision parameter; a95:

95% con®dence limit.

Site Polarity N

Bedding coordinates

dec inc k a95

Librilla area

Bco de la Salada Normal 115 353 47 11 4

Reverse 19 177 x13 3 25

All samples 134 353 43 7 5

SifoÂn de Librilla Normal 86 354 43 20 4

Reverse 75 185 x45 11 5

All samples 161 359 44 12 3

Molina de Segura area

Chorrico Normal 69 317 30 12 5

Reverse 52 120 x22 11 6

All samples 121 309 27 10 4

Salinas de Molina Normal 05 334 45 6 33

Reverse 47 148 x37 8 8

All samples 52 329 38 8 7



Fig. 8. Structural sketch of the Fortuna basin and equal area projection of palaeomagnetic directions of the studied sections. Note signi®cant anticlockwise rotations in the Molina de Segura

area (Chorrico and Salinas de Molina sections). See legend to Fig. 2.

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Molina de Segura area was signi®cantly affected by

sinistral block rotations associated with the left-lateral

shear along the Alhama de Murcia fault. Vertical axis

rotations as a result of the strike-slip deformation,

however, did not occur homogeneously along the

Alhama de Murcia shear zone. Sites in the Librilla area

do not record any signi®cant rotation despite being close

to the shear zone. It is noticeable that block rotations in

the Molina de Segura area appear to be associated with a

bend of the Alhama de Murcia Fault (Fig. 2), where the

sinistral shear displacement probably resulted in local

E±W extension. It is suggested that the extensional

component oblique to the direction of strike-slip trans-

port played a role in favouring vertical axis rotations

in such a transpressive context.

MAGNETOCHRONOLOGY OF THE BASININFILL

In each section we have inferred a local magnetic strati-

graphy by calculating the virtual geomagnetic pole

latitude from each single palaeomagnetic direction

(Figs 4±6). Palaeomagnetic directions from the CH and

SM sections were unrotated 50u and 35u, respectively,

before VGP calculation in order to align them with

the axial geomagnetic mean dipole. In addition to the

magnetostratigraphic results of this study we incorpo-

rated the magnetostratigraphy of the Chicamo section in

the NE edge of the basin (Fig. 4) (Krijgsman et al., 2000).

Basinal correlations

Magnetostratigraphic correlation between the Librilla and

the Molina de Segura areas is determined on the basis of

the coincident reversal patterns and the biostratigraphic

records in the SL and CH sections (Fig. 9). Particularly

signi®cant is the faunal datum provided by the entry

of the fossil rodent Paraethomys, an African genus of

Asian origin that entered the Iberian peninsula at 6.2 Ma

(chron C3An.1n) through the Gibraltar passageway

(GarceÂs et al., 1998). In addition to its chronostratigraphic

signi®cance, this bioevent represents further evidence

for the progression of the land-locking in the Gibraltar

area during the pre-evaporitic late Messinian.

Magnetostratigraphic results from the lower marine

units cannot be independently tied to the upper con-

tinental sections because of insuf®cient stratigraphic

overlap. Their integration into a single magnetostrati-

graphic framework requires the incorporation of the

geomagnetic polarity time scale to the correlation chart

(Fig. 9).

The BS section cannot be stratigraphically tied to its

neighbouring SL section because of a fault contact

(Fig. 2). However, on the basis of the mammal biostrati-

graphic content and the basinal lithostratigraphic frame-

work we know that BS is older than the SL section. A

middle Turolian (MN12) age is determined for the

evaporitic units in all the studied areas (Fig. 9).

Furthermore, there is good agreement between the

magnetic stratigraphy of the BS and Chicamo sections

(Fig. 4), as well as with the previously suggested

lithostratigraphic correlation of the evaporitic units

between the Librilla and the Abanilla areas (OrtõÂ et al.,1993).

Correlation with the time scale

In this paper we take as numerical ages of geomagnetic

reversals those provided by the astronomically calibrated

geomagnetic polarity time scale (Hilgen et al., 1995;

Krijgsman et al., 1999). Earlier results (GarceÂs et al.,1998) con®rmed a Messinian age for the late Turolian

continental units of the Fortuna basin in agreement with

the marine±continental relationships described in the

locality of La Alberca (Fig. 2) (Mein et al., 1973), on the

north-western slopes of the Carrascoy massifs, and in the

Crevillente area (Fig. 2) (Bruijn et al., 1975). The long

and distinctive record of magnetic reversals in the SL

section results in a perfect match with the expected

magnetic polarity sequence of the Miocene±Pliocene

time transition (Fig. 9). The resulting correlation with the

time scale is further supported by the identi®cation of

transgressive marine sediments corresponding to the basal

Pliocene within chron C3r as detected in the SL section,

in the Librilla area (Fig. 2).

The magnetostratigraphy of the marine and evaporitic

units shows a characteristic pattern that perfectly cor-

relates with the late Tortonian chrons C4n.2n, C4n.1n

and C3Br.2n. Correlation with older ages can be ruled

out since MN12 terrestrial faunas are necessarily

younger than 8 Ma (Krijgsman et al., 1996; Opdyke

et al., 1997).

Our correlation challenges the conclusions of an earlier

magnetostratigraphic study (DinareÁs-Turell et al., 1999;

PlayaÁ et al., 1999) which suggested a Messinian age for the

Rio Chicamo Fm. Their proposed correlation is appar-

ently supported by radiometric ages of volcanic rocks

inter®ngered with the sediments of the transitional

evaporitic units (Bellon et al., 1983). However, it

should be noted that whole-rock K/Ar dates are prone

to yield younger ages as a result of rock alteration and/or

Ar loss from the glass fraction. Moreover, their suggested

correlation is in con¯ict with the integrated mammal

biostratigraphic, lithostratigraphic and magnetostrati-

graphic frame as shown in this study (Figs 9 and 10).

Chronology of the basin in®ll

This new chronology indicates that marine sedimentation

in the Fortuna basin ®nished in the late Tortonian at

about 7.6 Ma, after a period of progressive basin

restriction and deposition of evaporitic and diatomitic

formations (7.8±7.6 Ma) (Fig. 9). The development of

diatomitic facies was largely isochronous, not only at basin



Fig. 9. Magnetostratigraphic correlations between the studied sections and absolute ages based on the astronomically calibrated

Polarity Time Scale (Hilgen et al., 1995; Krijgsman et al., 1999).

M. GarceÂs et al.


scale, but also between the Fortuna and Lorca basins

(Krijgsman et al., 2000), suggesting the existence of

a marine connection between the two basins at this

stage. In contrast, their associated gypsiferous units

show a time-transgressive character: the youngest marine

evaporitic deposits of the Fortuna basin are about

200 kyr older than the main gypsiferous member of

the La Serrata Fm in Lorca (Krijgsman et al., 2000).

This facies distribution agrees with the overall marine

regression towards the south-west in a palaeogeographical

scenario which is regarded as a restricted elongated gulf

with a connection to the Mediterranean in the south

(Montenat, 1973).

After deposition of the evaporitic units, continental

environments developed all over the basin since the late

Tortonian. Ongoing subsidence led to the accumulation

of thick alluvial sequences throughout the Messinian.

Only at the base of the Pliocene a rapid and brief marine

transgression affected the south and eastern areas of the

basin. These results depict an evolution of the Fortuna

basin which challenges earlier studies based on erroneous

age interpretations (Santisteban, 1981; MuÈller & HsuÈ,

1987; Lukowski et al., 1988; PlayaÁ et al., 2000).

BASIN EVOLUTION

Based on the new chronostratigraphic framework for the

Fortuna basin we have carried out a geohistory analysis of

the basin in®ll (Fig. 11). High-resolution magnetochro-

nology, however, has only been available since the late

Tortonian as provided by the present study. For older

units we rely on previous regional stratigraphic studies

which constrain the onset of marine sedimentation to

the early Tortonian (Montenat et al., 1990a). Another

Fig. 10. Plot of stratigraphic thickness with absolute time. TSC: Tortonian Salinity Crisis; MSC: Messinian Salinity Crisis.

Fig. 11. Geohistory analysis of the Fortuna basin

stratigraphic composite sequence. Age of onset of rifting as

well as palaeobathymetry of the older pelagic sediments is not

precisely constrained. Therefore, the slope of the ®rst

segment of the subsidence curve has a signi®cant uncertainty

(see text for explanation). The two curves in the subsidence

graphs refer to the maximum and minimum bounds of

palaeowater depth.



limitation for the geohistory analysis is the lack of precise

palaeobathymetric estimates for the lower marine unit,

which makes estimates of tectonic subsidence for the

earliest rifting stages inaccurate. The lower members

of the Fortuna marls contain a rich pelagic planktonic

assemblage which is characteristic of a deep basin

environment. In contrast, micropalaeontological studies

in the upper part of the Fortuna marls point to a relatively

shallow basin environment (Sierro et al., 1992). This

indicates that maximum palaeodepths were reached in

the early stages of the basin in®ll. For younger units

palaeobathymetry is well constrained as being very

shallow to emergent. Palaeoaltitudes for the continental

units are assumed to be relatively low since the basin

maintained feasible connection with the marine environ-

ments towards the south. The fact that the basal Pliocene

marine ¯ooding was recorded in the Fortuna basin

suggests that sedimentation in the basin was close to

sea level.

Tectonic subsidence was calculated by means of back-

stripping techniques, adopting a local isostasy model

under the assumption of no ¯exural lithospheric response.

Such simpli®cation can be assumed given the prime

control of the crustal-scale bounding faults (de LarouzieÁre

et al., 1988) in the evolution of the region, and the very

low lithospheric strength as a result of older stretching

events and the elevated thermal gradient of the region

(Cloetingh et al., 1992; TorneÂ et al., 2000).

The geohistory analysis indicates that the Fortuna

basin accommodated 1 km of tectonic subsidence from

the Tortonian to the early Pliocene. This amount of

subsidence is comparable with other basins such as the

Sorbas and Atalaya basins in the same area (Cloetingh

et al., 1992). A signi®cant feature of the Fortuna basin is

that half of the tectonic subsidence (500 m) occurred

during the Messinian, clearly post-dating the main period

of extension that affected many of the eastern Betics

Neogene basins (Cloetingh et al., 1992).

During the early stages of basin evolution, subsidence

largely exceeded sedimentation, causing deepening of the

basin ¯oor and the accumulation of pelagic sediments

(Fortuna Marls). The Librilla area accumulated nearly

1000 m of marine marls. The Fortuna basin was in open

connection to the north (Crevillente strait) and south

(Guadalentin corridor) with neighbouring troughs of the

eastern Betics, where Tortonian marine sedimentation

shows a large thickness variability (Montenat et al.,1990a), revealing a complicated horst and graben struc-

ture and prime tectonic control by vertical faults on the

distribution of sediment depocentres.

After this initial open marine evolution, the onset

of evaporitic sedimentation at about 7.8±7.9 Ma marks

a drastic change in basin con®guration. Restriction of

marine water circulation and the initiation of hypersaline

conditions in the Fortuna basin must have been related to

the presence of an eastern ridge, controlled by the tectonic

deformation along the Alhama de Murcia Fault. Several

lines of evidence challenge sea level lowering as a prime

controlling factor on basin restriction as previously sug-

gested (Santisteban & Taberner, 1983; MuÈller & HsuÈ,

1987). First, a coeval regression is not observed in

neighbouring basins. On the contrary, the late Tortonian

is generally characterized by transgressive marine deposits

in the eastern Betics (Montenat et al., 1990a). Second, the

shallowing-upward sedimentary sequence in the Fortuna

basin does not provide evidence of a sea-level lowering.

No basin-scale seaward progradation of marginal clastic

wedges is observed. No low sedimentation rates are

recorded as a result of a decreasing accommodation

space in the basin. On the contrary, restriction and

evaporite precipitation in the Fortuna basin is coeval

with increasing sedimentation rates (up to 1.0 m kyrx1)

(Fig. 10). This suggests a large accommodation space

which does not easily match a scenario of lowering base

level. The observed sedimentation pattern is more con-

sistent with a scenario of increasing accommodation

space as a result of the tectonic uplift of the basin margins.

In this context, the shallowing sequence in the Fortuna

basin represents a rapid sediment ®lling favoured by the

rising of an eastern margin and the transformation of the

Fortuna basin into an effective sediment trap. A tectonic

control on the presence of an eastern ridge is favoured

by the existence of stratigraphic angular unconformities

related to a late Tortonian intensi®cation of tectonic

deformation along the NE±SW-trending shear zones in

the Eastern Betics domain (Montenat & Ott d'Estevou,

1990).

Sediment input outpacing subsidence led to rapid basin

®lling and marine regression. Once the basin reached an

over®lled stage, sedimentation decreased to average rates

of 30±40 cm kyrx1 (Fig. 10), matching the generation of

new accommodation space due to the on-going subsidence

(Fig. 11). Complete isolation of the Fortuna basin from

the Mediterranean margin probably did not occur, and

sediment excess was probably transported towards the

marine platforms to the south-east. In contrast to other

eastern Betic basins, Messinian sedimentation was

characterized by high subsidence and sedimentation

rates (Fig. 11).

The scenario during the Messinian was that of a poorly

drained, con®ned, continental basin, with the develop-

ment of shallow lacustrine and palustrine environments in

the distal and marginal areas of the prevailing alluvial fan

systems. Connection to the marine environments was still

feasible towards the south-eastern margin, as can be seen

on the NW slopes of the Carrascoy massif: Messinian

marine sediments inter®nger with late Turolian con-

tinental units in La Alberca (Mein et al., 1973). The

recognition of a basal Pliocene marine transgression on

top of a thick Messinian shallow-marine to continental

sequence in the Librilla area indicates that continuous

tectonic subsidence occurred throughout the Messinian

(Fig. 10).

The Mediterranean sea-level ¯uctuations in¯uenced

the sedimentation in the Fortuna basin. The sea level

drop accompanying the Messinian Salinity Crisis at

M. GarceÂs et al.


5.55 Ma (Krijgsman et al., 1999) led to a marked decrease

in the basin accommodation space, fast basinward advance

of the alluvial fringing wedges, sediment bypassing

towards outer deeper troughs, and local erosion (Figs 9

and 10). The incision of the drainage network caused

shallow lakes and ponded areas to dry out. A basin outlet

was probably located in the area near Librilla, where a

sediment hiatus of 0.5 Myr indicates that this was an

area exposed to sediment bypassing and erosion at the

end of the Messinian. The late Messinian environmental

changes apparently did not in¯uence the sedimenta-

tion in the Molina de Segura area. There, sedimentation

is apparently continuous across the Miocene±Pliocene

transition (Fig. 10), indicating that erosion at that time

was probably restricted to the areas of valley incision near

the basin outlet.

Linked to the Messinian Salinity Crisis, a remagnetiza-

tion event is detected in the Librilla area. It is expected

that the late Messinian sea level drop caused a deepening

in phreatic levels near the coast. This probably resulted in

an increased ¯owing of oxidizing groundwater through

the buried sediments, a feasible mechanism causing

remagnetization as discussed above (see Palaeomagnetic

analysis). The thickness of the remagnetized stratigraphic

interval in the Librilla section suggests a deepening of the

ground-water table of at least 300 m.

The re-establishment of normal marine conditions in

the Mediterranean is recorded in the Fortuna basin by

means of a brief and areally restricted basal Pliocene

transgression. This marine in¯ux was probably con®ned

to a narrow depressed area following the axis of a growing

syncline parallel to the NW±SE-striking Alhama de

Murcia Fault (Fig. 2).

The last stage of basin evolution was determined by a

phase of transpressional left-lateral shear along the

Alhama de Murcia Fault that caused the basin uplift in

the Pliocene. The contractional nature and age of this

event is recorded by the syndepositional folding of the

Pliocene sediments in the Librilla area. In addition, its

left-lateral strike-slip component is indicated by the

anticlockwise rotations of the Messinian to Pliocene

sediments in the Molina de Segura area (Fig. 8).

THE FORTUNA BASIN IN THE CONTEXTOF THE EASTERN BETICS AND THEALBORAN BASIN EVOLUTION

The overall regressive sequence from open marine to

restricted evaporitic±diatomitic and continental deposits

in the Fortuna basin is also recognized in the Lorca basin.

Magnetostratigraphic correlation between the two basins

(Krijgsman et al., 2000) con®rms a parallel sedimentary

evolution, a result which is to be expected since both

basins were tectonically controlled by common major

structures. One striking feature of their sedimentary

record is the `Tortonian Salinity Crisis' (TSC)

(Krijgsman et al., 2000), followed by continentalization

at 7.6 Ma as a result of the regional tectonic uplift of

their eastern margins (Internal Zones) during the late

Tortonian. The installation of continental conditions in

the Lorca and Fortuna basins appears to be roughly coeval

with the uplift and marine regression in the Guadix±Baza

and Granada intramontane basins in the Central and

Western Internal Betics (Soria et al., 1998). However,

signi®cant differences between these basins prevents a

single interpretation of the late Tortonian marine

regression in the Betic zone. In the Guadix±Baza basin,

marine sediments are overlain by a not very thick (about

300 m) late Tortonian to Pleistocene continental succes-

sion, which progressively onlap the relatively passive

fault-bounded basin margins. In the Granada basin, more

active Pliocene±Pleistocene extensional faulting has taken

place, associated with the recent uplift of the Sierra

Nevada mountain range (FernaÂndez et al., 1996). This

led to a marked asymmetric basin in®ll, with migrating

depocentres towards the north, and the accumulation of

variable thickness of lacustrine to alluvial sediments of up

to 1 km (FernaÂndez et al., 1996). The marine regression

in both the Guadix±Baza and the Granada basins resulted

from a regional uplift of the central Betics from late

Tortonian to Present (Soria et al., 1998). This has been

interpreted as a ¯exural rift shoulder uplift, where the

Betic domain represents the uplifted ¯ank of a subsiding

Alboran Sea basin (van der Beek & Cloetingh, 1992).

Alternatively, lithospheric mantle thinning beneath the

topographic highs of the central and eastern Betics has

also been suggested to account for the observed recent

uplift of the chain (TorneÂ et al., 2000).

In the eastern Betics, rift shoulder uplift is not a

plausible mechanism since earlier stretching and the high

thermal gradient would have prevented the lithosphere

from supporting ¯exural stresses, resulting in a local

isostatic compensation (Cloetingh et al., 1992). The size

and geometry of the Miocene sedimentary troughs may

also account for the weakness of the lithosphere in the

eastern Betics. The Fortuna basin has accommodated a

thick (up to 1 km) continental succession in a relatively

narrow trough. In contrast to the intramontane basins of

the central Betics, inter®ngering with marine Pliocene

sediments shows that the rapid sedimentation in the

Fortuna basin was associated with a marked subsidence of

the basin ¯oor. Instead of a long-wavelength regional

uplift, contrasting vertical and lateral motions between

blocks in the eastern Betics agree with a strike-slip

structural setting, with coalescent uplifting blocks and

pull-apart basins. Under a scenario of strike-slip tectonics

only subtle changes of the stress ®eld (Montenat & Ott

d'Estevou, 1990) are required to produce block uplift.

The tectonic activity along the Alhama de Murcia Fault

evolved into a more transpressional component during the

Pliocene as the direction of the local compression shifted

from N±S to NNW±SSE (Montenat & Ott d'Estevou,

1990). This caused the recent uplift of the Fortuna

basin and its eastern margin (Carrascoy massifs) (Sanz de

Galdeano et al., 1998). Early Pliocene syndepositional



folding in the Librilla area and anticlockwise vertical axis

block rotations in the Molina de Segura area (Fig. 9)

provide evidence for this last stage of basin evolution.

The tectonic and sedimentary evolution of the Fortuna

basin has been appropriately associated with NE±SW

left-lateral strike slip deformation in a context of a late

Tortonian to present N±S to NW±SE compression in the

eastern Betics (Montenat & Ott d'Estevou, 1990;

Montenat et al., 1990a), which resulted from the oblique

NW±SE convergence between the African and Eurasian

plates (Dewey et al., 1989; Srivastava et al., 1990). The

results of the present study are in agreement with this

scenario. On the other hand, we found no evidence

for a general NW±SE shortening and piggyback basin

evolution, as has been suggested (Poisson & Lukowski,

1990), a model which is not substantiated by a reliable

basin chronostratigraphy nor by basin-scale balanced

cross-sections.

As already envisaged by de LarouzieÁre et al. (1988),

such a scenario for the eastern Betics resembles the one

described for the southern and eastern Alboran basins

where NE±SW strike-slip faulting has controlled dis-

tribution of sedimentary troughs since the late Tortonian

(Comas et al., 1992; Alvarez-MarroÂn, 1999). It should

be pointed out that strike-slip tectonics is a feasible

mechanism to explain substantial extension in an overall

convergent setting. This fact should be taken into account

when applying subsidence analysis in order to detect

patterns of migrating depocentres in the Alboran basin

and to constrain models of basin formation. In particular,

limited observations of younger subsidence in the Eastern

Alboran basin might not be a suf®cient evidence for a

south-eastward migration of delaminating continental

lithosphere as proposed by other authors (Docherty &

Banda, 1995; Tandon et al., 1998). The Fortuna basin is

located beyond the limits of the thinned lithosphere of the

Alboran basin, but underwent suf®cient stretching and

subsidence to accumulate 2000 m of sediments in about

5 Myr. Similar structures have been described in the

Eastern Alboran basin, where subsidence has been

associated with a process of tectonic escape within an

area bounded by conjugate strike-slip faults (Alvarez-

MarroÂn, 1999). Therefore, in order to substantiate an

eastward migration of the Alboran basin it is necessary to

show that this extension is not con®ned to pull-apart

structures.

CONCLUSIONS

After an early stage of extension and marine transgression

which affected extensive areas in the eastern Betics, a late

Tortonian marine restriction is detected in the Fortuna

and Lorca basins. A rising eastern ridge prevented the

exchange of marine waters and favoured hypersaline

conditions and precipitation of evaporites. This ridge also

acted as an ef®cient sediment trap, causing increasing

sediment accumulation and marine regression in the

Fortuna and Lorca basins. The shallowing succession

does not represent the initiation of uplift of the basin

itself. On the contrary, continuous subsidence favoured

the accumulation of thick sequences of continental

sediments during the Messinian followed by a brief and

areally restricted marine transgression at the base of the

Pliocene. This evolutionary pattern contrasts with the

general trend in other neighbouring basins to the east

where subsidence was primarily accommodated in the

Tortonian during the main rifting stage which affected

the overall region. It also differs from the evolution of the

intramontane basins of the central Betics, which were

uplifted together with the chain since the late Tortonian.

The Messinian subsidence recorded in the Fortuna basin

as well as the tectonic inversion and uplift from the

Pliocene to the present is consistent with an evolving pull-

apart basin controlled by the strike-slip activity of the

Alhama de Murcia and Crevillente Faults. This is in

agreement with the late Miocene scenario depicted for

both the Alboran and the Betic regions where folding,

strike-slip tectonics and inversion of previous normal

structures developed in a regime of overall convergence

between the African and Eurasian plates.

ACKNOWLEDGMENTS

This research was supported by PB96-0815 MEC project.

Conxita Taberner and Mary Russell are acknowledged

for their assistance in the ®eld. Thanks are also due to

Bert van der Zwaan for analysing the benthic foram-

inifera association at the Pliocene marine beds in the SL

section. Constructive reviews by L. Lonergan, C. Sanz de

Galdeano and P. Haughton improved the manuscript.

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Received 30 August 2000; revision accepted 5 February 2001

M. GarceÂs et al.


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