2004 yebenes oncolitos sed geol

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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).


(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).



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).



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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