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www.elsevier.com/locate/sedgeo
Sedimentary Geology 169 (2004) 121–128
Short communication
Early Pliocene transgressive coastal lags (Bajo Segura Basin, Spain):
a marker of the flooding after the Messinian salinity crisis
Jesus E. Caracuel*, Jesus M. Soria, Alfonso Yebenes
Departamento de Ciencias de la Tierra y del Medio Ambiente, Universidad de Alicante,
Apdo. Correos 99, 03080 San Vicente del Raspeig, Alicante, Spain
Received 3 May 2004
Abstract
The Messinian salinity crisis is recorded on the northern margin of the Bajo Segura Basin by lacustrine and fluvial deposits
coeval to the evaporites that precipitated in the basin’s central areas. These syn-evaporitic Messinian deposits are bounded at the
top by an erosional surface caused by a fall in base level (end-Messinian unconformity) on which an early Pliocene sequence is
located. The latter begins with a coastal lag, consisting of oncoliths and carbonate clasts intensely bored and encrusted by
lithobionts, that records the installation of beach environments at the beginning of the Pliocene transgression. A succession of
pelagic marls rich in planktonic and benthic foraminifers lies on this basal lag and indicates the complete marine flooding of the
basin and the definitive conclusion of the salinity crisis.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Messinian salinity crisis; Pliocene transgression; Borings; Bajo Segura Basin; Mediterranean
1. Introduction Cyprus (Orszag-Sperber et al., 2000) and in Morocco
‘‘Leg 42A drilling confirmed the Leg 13 drilling
results that the first Pliocene sediments above the
Messinian are deep and open marine hemipelagic
sediments. The Messinian salinity crisis was ended
by the Pliocene flooding.’’ (Hsu et al., 1977, p. 402).
In most of the Mediterranean marginal basins, the first
transgressive Pliocene sediments, marking the defin-
itive postcrisis reflooding, are recorded as presenting
clearly marine facies. This is the case in Sicily (Butler
et al., 1995), the Apennines (Roveri et al., 2001),
0037-0738/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.sedgeo.2004.05.006
* Corresponding author. Tel.: +34-96590-3400x3337; fax: +34-
96590-3552.
E-mail address: [email protected] (J.E. Caracuel).
(Rouchy et al., 2003). In the southern Spain basins,
these transgressive deposits are marls rich in plank-
tonic organisms with pelagic affinity, as described in
the Malaga (Guerra and Serrano, 2003), Vera (Ott
d’Estevou et al., 1990) and in the Nıjar basins (Van de
Poel, 1992). In other cases, the transgressive deposits
of the earliest Pliocene present shallow marine shelf
facies, as in the Fortuna Basin (Garces et al., 1998). It
is not usual to find records of coastal sedimentation
occurring in the early Pliocene when reflooding began,
forming the base of the Pliocene transgressive se-
quence. In the Melilla Basin in North Africa, evidence
has been found that the Pliocene transgression began
with an intense boring activity on hard substrata
(Rouchy et al., 2003). In the Nıjar–Carboneras Basin
J.E. Caracuel et al. / Sedimentary Geology 169 (2004) 121–128122
(Majada del Curica sector), the basal Pliocene deposits
are clasts bored together with oysters and serpulids
(Montenat et al., 1990). Finally, in the Bajo Segura
Basin, boring has been described in the unconformity
separating the Messinian from the Pliocene (Montenat
et al., 1990). These are all evidence that the Pliocene
transgression began with coastal environments.
The aim of this study is to describe a singular
manifestation of coastal sedimentation in the first
transgressive deposits of the Pliocene (lags of onco-
liths and carbonate clasts, bored and encrusted by
lithobionts). We also describe their relation with both
the evaporitic phase characterising the Messinian
salinity crisis and with the marine sedimentation
marking the complete Pliocene reflooding of the
Mediterranean.
2. General features of the Bajo Segura Basin
The Bajo Segura Basin is a Mediterranean mar-
ginal basin located in the eastern end of the Betic
Cordillera (Fig. 1, upper). Its sedimentary filling
age ranges from the Tortonian to the Quaternary
(Montenat et al., 1990). According to these authors,
the most complete stratigraphic record corresponds to
the late Miocene and Pliocene, where five major
stratigraphic units bounded by regional unconform-
ities can be identified: Tortonian I (TI), Tortonian II
(TII), latemost Tortonian–Messinian (M), Pliocene I
(PI) and Pliocene II (PII). Recent studies (Soria et al.,
2002, 2003) have justified the existence of an intra-
Messinian unconformity dividing the M Unit into two
new units: MI and MII. In addition, the end-Messi-
nian unconformity, with incised valley fillings spo-
radically associated (Unit MIII), has been recognised.
On the basis of previous data (Montenat et al., 1990;
Calvet et al., 1996; Martınez del Olmo and Serrano
Onate, 2000; Soria et al., 2001, 2002, 2003) and
other new data provided by the present study, the
stratigraphic model of the Bajo Segura Basin has
been brought up to date (Fig. 1, lower). According to
this model, Unit MI corresponds to preevaporitic
sedimentation; Unit MII records the intra-Messinian
reflooding, when the evaporites, characteristic of the
salinity crisis in the marginal basins of the Mediter-
ranean, precipitated. Unit MIII records the fluvial
sedimentation syngenetic to the end-Messinian un-
conformity, while Unit P (consisting of Units PI and
PII of Montenat et al., 1990) represents the marine
reflooding of the basin.
3. Stratigraphic and biostratigraphic data of the
study area
This study centres on the Crevillente–Elche sec-
tor, located on the northern margin of the Bajo
Segura Basin, where the boundary between the MII
(Messinian) and P (early Pliocene) units is clearly
exposed (Fig. 2). The MII Unit here is made up of
two laterally equivalent depositional systems: MIa,
consisting of lacustrine limestones and marls, and
MIb, fluvial lutites and gravels. Both systems contain
Upper Turolian or Messinian (biozone MN13; Mein,
1990) rodent fossils (Alfaro et al., 1995; Martın-
Suarez and Freudenthal, 1998). The MII Unit is
crowned by an erosive surface generated by a base-
level fall at the end of the Messinian (end-Messinian
unconformity). The Pliocene deposits begin with a
coastal depositional system (P0) consisting of bored/
encrusted oncoliths and carbonate clasts, forming the
base of a stratigraphically continuous sequence that
continues with three other systems: pelagic marls (P1),
shallow marine and coastal sands (P2) and alluvial
clays and gravels (P3). The planktonic foraminifers
determinations in the P1 system by Montenat et al.
(1990) suggest an early Pliocene age, corresponding to
the Globorotalia puncticulata biozone. Lancis (1998),
studying the calcareous nannoplankton, also obtains
the same age (NN13). The P3 system has not been
dated in the study area, but in other sectors of the basin,
its age was determined as early Pliocene (base of the
MN15 biozone; Mein, 1990) using rodent fossils
(Soria et al., 1996).
4. Depositional system P0—lags of bored/encrusted
oncoliths and carbonate clasts
Four stratigraphic sections, Crevillente A, B and C
and Elche (Fig. 2), were chosen for the detailed study
of the P0 coastal system as the main aim of this study.
The end-Messinian unconformity is defined by an
erosional surface with a palaeovalley morphology
which erodes over 30 m of Unit MII. Examined in
Fig. 1. (A) General sketch of the Western Mediterranean, with the indication of position of the Betic Cordillera. (B) Location of the Bajo Segura
Basin in the eastern end of the Betic Cordillera (simplified from Montenat and Ott d’Estevou, 1990). (C) Stratigraphic organization of the Bajo
Segura Basin (Late Tortonian to early Pliocene). The main lithostratigraphic units are indicated as defined by Montenat et al. (1990).
J.E. Caracuel et al. / Sedimentary Geology 169 (2004) 121–128 123
detail, this surface shows palaeoreliefs with sharp
vertical incisions measuring tens of centimetres.
In the Crevillente A and B sections, the end-
Messinian unconformity separates lacustrine marls
(system MIIa) from lags of bored/encrusted oncoliths
(system P0) (Fig. 3). These lags are absent in the
Crevillente C section where they are replaced by a
hard, ferruginised and bored substrate corresponding
to lacustrine limestones (system MIIa). The layer with
bored/encrusted oncoliths in the Crevillente A and B
sections has a maximum thickness of 7 m and consists
of spherical and subspherical ferruginised elements
ranging from 1 to 30 cm in diameter. The concentric
laminated structure is clearly visible in the elements
smaller than 10 cm, whereas those of large diameter
have external laminae eccentric to the internal ones.
Successive growth phases are occasionally observed
with variation in the eccentricity of the external
laminae. Because they are so small and bad-preserved,
it is hard to recognise the oncolith nuclei, although
some instances have a tubular morphology interpreted
as plant fragment casts.
Regardless of their size, almost all the oncoliths are
bored and encrusted only on their external parts, with
larger sizes of lithobionts coinciding on the larger
oncoliths, which also have a greater variety of colo-
nisers. These lithobionts are found all over the surfaces
of the oncoliths, although the larger ones are usually
colonised more on the upper part. The most common
endolithic borings recorded are due to sponges (Cliona
sp.) and several boring bivalves (Irus sp./Petricola
sp.), as well as large (up to 12 cm) Lihophaga sp. on
the larger oncoliths. A large variety of epilithic lith-
obionts have also been recognised, such as Cyrripeda
Fig. 2. (Upper) Simplified geological map of the northern sector of the Bajo Segura Basin with situation of the studied sections. (Lower)
Stratigraphic relation of the P0 depositional system (coastal lags of bored clasts) as regards the underlying MIIa (lacustrine–palustrine) and
MIIb (fluvial) systems and the overlying P1 system (pelagic basin).
J.E. Caracuel et al. / Sedimentary Geology 169 (2004) 121–128124
(including both complete Balanidae and their basal
plates), Vermetidae, Serpulidae, Ostreidae (colonised
on their inner side by serpulids), Polichaeta (Polydora)
and calcareous algae (Lithophyllum sp.), mainly inside
the borings of Lithophaga. From an ichnologic point
of view, the assemblage described corresponds to the
coastal ichnofacies of Trypanites (Pemberton et al.
1992), which is characteristic of hard biotic/abiotic
substrata, in this case, the only hard elements available
for colonisation by the lithobionts are the oncoliths.
Regarding the origin of the oncoliths, we interpret that
they come from the underlying lacustrine system
(MIIa). There is abundant literature showing that one
of the most common environments for oncolith for-
mation are the littoral zones of lakes (e.g., Dixit, 1984;
Magny and Richard, 1987; Magny et al., 2003) such as
the depositional environment of the MIIa system.
In the Elche section, the end-Messinian unconfor-
mity separates fluvial clays and gravels (system MIIb)
from lags of bored carbonate clasts (system P0, Fig. 3).
These clasts form a 30- to 50-cm-thick layer in which
the elements are well-rounded fragments of Mesozoic
limestones with a maximum size of 10 cm. Their
surfaces have a ferruginous covering and abundant
Fig. 3. Panoramic views of the Crevillente A, and Elche outcrops and detailed photographs of the lags of bored oncoliths and carbonate clasts of
the P0 depositional system (see stratigraphic location in Fig. 2). Note how, on the one hand, the P0 system overlies the erosion surface defining the
end-Messinian unconformity and, on the other hand, how this system rapidly evolves upwards into the P1 system (pelagic marine marls).
J.E. Caracuel et al. / Sedimentary Geology 169 (2004) 121–128 125
J.E. Caracuel et al. / Sedimentary Geology 169 (2004) 121–128126
borings by bivalves (Lithophaga sp.) and sponges
(Cliona sp.), as well as encrusted oysters, cirripeds
and serpulids. As in the Crevillente A and B assem-
blage, this one corresponds to a coastal ichnofacies of
Trypanites developed on a hard, abiotic substrate,
forming a pavement of unassociated elements. The
carbonate clasts thus constitute a set of benthic islands
colonised by coastal lithobionts.
The origin of the Mesozoic carbonate clasts in the
Elche profile is interpreted from the channels of
fluvial gravels recognised in the underlying MIIb
system which coincide in lithology, size and morphol-
ogy with the bored clasts of the P0 system.
In all the sections analysed, sporadic fragments of
bivalves (pectinids and oysters), echinoderms and
gastropods, as well as benthic foraminifers, can be
recognised in association with the bored oncoliths or
carbonatic clasts. These lags of bored oncoliths and
clasts, making up the P0 system, grade upwards
rapidly to marine marls of system P1 (early Pliocene).
In the transition zone between both systems, there are
thin levels of sand with wave ripples and small-scale
hummocky cross-stratification (Crevillente C section).
The P1 system marls are characterised by the presence
of planktonic and benthic foraminifers, as well as
concretions of limonite and highly ferruginised small
bivalves and echinoderms. This vertical transition of
facies reflects a progressive but rapid evolution from
coastal (P0) to marine (P1) environments.
5. Conclusions: evolutionary model
We now summarise the main stages in the sedimen-
tary evolution of the northern margin of the Bajo
Segura Basin, with reference to the significance of
the lags of bored oncoliths and carbonate clast, as
regards the two main events of the Messinian salinity
crisis: the evaporitic sedimentation and the Pliocene
reflooding.
5.1. Syn-evaporitic continental sedimentation
Messinian Unit II (MII) represents lacustrine and
fluvial sedimentation in the Crevillente–Elche sector,
coeval to the precipitation of the evaporites in the
central parts of the Bajo Segura Basin (see Fig. 2).
The genesis of the oncoliths under study is related to
the oscillatory flow of shallow water in the littoral
zones of the lacustrine areas (system MIIa). This flow
kept the oncoliths separated from the substratum,
allowing the development of spherical morphologies
with concentric laminae (similar conclusions were
presented by Dixit, 1984 at Lake Manyara in East
Africa). The marls and limestones that provide the
lithological character of most of the system MIIa were
deposited in the other parts of the lake and neighbour-
ing swampy areas. Lags of gravels, dominated by the
carbonatic clasts studied in this paper, accumulated in
the fluvial channels, which, together with the flood-
plain lutites, form the system MIIb (Fig. 4A).
5.2. End-Messinian unconformity
The upper limit of Unit MII is defined by an
erosional surface (end-Messinian unconformity) on
which the first Pliocene deposits lie. This erosional
phase is related to a fast fall in base level, resulting in
the elimination of most of the oncolithic layers formed
during deposit of Unit MII, but leaving some near the
surface, together with layers of lacustrine limestones
(MIIa) and fluvial gravel channels (MIIb) (Fig. 4B).
5.3. Start of Pliocene transgression
The genesis of system P0, a trangressive coastal
lag, is related to a sea-level rise and the subsequent
occupation of the area studied by beach (shoreface)
environments. The wave effect on the bottom caused
winnowing of the fine sediment (lacustrine marls and
fluvial clays), encouraging both the concentration of
coarse-grained clasts (lags of oncolith and carbonate
clasts) and the exposure of hard substrata on the
bottom (lacustrine limestone). Communities of coastal
organisms colonised the lags and hard strata, while the
oscillatory flow and wave breaking processes rolled
the oncoliths and carbonate clasts. The absence of
traces of organic activity on the soft coastal substra-
tum can be explained by the predominance of erosive
wave processes at the beginning of the Pliocene
transgression (Fig. 4C).
5.4. Complete marine flooding
The rapid rise in sea level during the early Pliocene
caused an increase in bathymetry and, therefore,
Fig. 4. (A–D) Evolutionary model explaining the genesis of the bored oncoliths and carbonate clasts (P0: trangressive coastal lags). Relation of
both the Messinian syn-evaporitic sedimentation (MII Unit, see Fig. 2) and the complete marine flooding of the early Pliocene (P1: pelagic
marls).
J.E. Caracuel et al. / Sedimentary Geology 169 (2004) 121–128 127
J.E. Caracuel et al. / Sedimentary Geology 169 (2004) 121–128128
caused the change from beach (system P0) to pelagic
marine (system P1) environments. At this time, both
the sector examined here and the rest of the Bajo
Segura Basin were completely flooded by the sea, as
occurred in the entire Mediterranean area (Fig. 4D).
Acknowledgements
Financial aid was provided by Research Project
BTE 2003-05047 MCYT. We are indebted to Dr. Ian
McCandless for the English version of the paper.
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