ORIGINAL RESEARCH PAPER

Growth patterns of a proximal terrigenous margin offshorethe Guadalfeo River, northern Alboran Sea (SW MediterraneanSea): glacio-eustatic control and disturbing tectonic factors

F. J. Lobo Æ A. Maldonado Æ F. J. Hernandez-Molina ÆL. M. Fernandez-Salas Æ G. Ercilla Æ B. Alonso

Received: 3 March 2008 / Accepted: 27 August 2008 / Published online: 17 September 2008

� Springer Science+Business Media B.V. 2008

Abstract The shelf-upper slope stratigraphy offshore and

around the Guadalfeo River on the northern continental

margin of the Alboran Sea, Western Mediterranean Basin,

has been defined through the interpretation of a grid of

Sparker seismic profiles. We tried to identify evolutionary

trends in shelf growth, as well as to determine the regional/

local factors that may modify the influence of glacio-eu-

static fluctuations. Four major depositional sequences are

identified in the sedimentary record by a detailed seismic

interpretation, which defines three significant intervals of

shelf-upper slope progradation, dominated by deposition of

shelf-margin wedges, which resulted in uniform patterns

of shelf-margin growth in response to significant sea-level

falls. In contrast, the record of transgressive intervals is

more variable, mainly as the result of distinct patterns of

regressive-to-transgressive transitions. Major prograda-

tional wedges are internally composed of seaward-

prograding, landward-thinning wedges, interpreted to

represent shelf-margin deltaic deposits. In contrast, the last

aggradational interval is composed of shelf-prograding

wedges that show distinct characteristics, in terms of

seismic facies, morphology and distribution when com-

pared with previous shelf-margin wedges. These shelf

wedges are thought to represent the particular case of

Regressive Systems or Shelf Margin Systems Tracts, and

their development seems to be controlled by a drastic

change in main depocenter location, which moved from the

upper slope to the shelf during the Pleistocene. The

stacking pattern of seismic units, the shallowness of the

acoustic basement and the migration of the shelf break are

used to infer spatial and temporal changes in tectonic

subsidence-uplift rates, which interact with low-order gla-

cio-eustatic changes. For much of the Pliocene-Quaternary,

uplifted sectors alternated laterally with sectors experi-

encing more subsidence. Subsequently, a significant

change from lateral outgrowth to vertical accretion is

recognised. This stratigraphic change could be related to

the combined influence of increased subsidence rates on

the shelf and the onset of higher-frequency glacio-eustatic

cyclicity after the Mid Pleistocene Revolution that occurred

around 1 Ma.

Keywords Alboran Sea � Pliocene � Quaternary �Proximal margin evolution � Seismic stratigraphy �Sequence stratigraphy

Introduction

The Messinian Salinity Crisis in the Mediterranean Sea

was accompanied by a significant sea-level fall that began

at 5.96 Ma (Gautier et al. 1994; Krijgsman et al. 1999), due

to the combined influence of global eustatic lowering and

tectonic uplift in the Strait of Gibraltar region (Lofi et al.

F. J. Lobo (&) � A. Maldonado

CSIC-Instituto Andaluz de Ciencias de la Tierra, Facultad de

Ciencias, Avenida de Fuentenueva s/n, 18002 Granada, Spain

e-mail: pacolobo@ugr.es

F. J. Hernandez-Molina

Departamento de Geociencias Marinas, Universidad de Vigo,

36200 Vigo, Spain

L. M. Fernandez-Salas

Instituto Espanol de Oceanografıa, Centro Oceanografico de

Malaga, Puerto Pesquero s/n, Apartado 285, 29640 Fuengirola,

Spain

G. Ercilla � B. Alonso

CSIC-Instituto de Ciencias del Mar, Paseo Marıtimo de la

Barceloneta 37-49, 08003 Barcelona, Spain

123

Mar Geophys Res (2008) 29:195–216

DOI 10.1007/s11001-008-9058-5

2003; Duvail et al. 2005). The basin was inundated during

the Early Pliocene, in response to a sudden warming epi-

sode and associated rise in global sea level (Haq et al.

1987). Afterwards, climatic and eustatic sea-level oscilla-

tions were characterised by two distinct evolutionary

trends. Most of the Pliocene was dominated by a general

cooling trend, punctuated by several marked warming and

cooling episodes (Zachos et al. 2001). The onset of sig-

nificant glaciations in the northern hemisphere recorded by

large d18O oscillations and also by a major sea-level fall at

about 3.8 Ma reflects a cooling episode (Jansen and Raymo

1996; Hardenbol et al. 1998; Zachos et al. 2001; Lambeck

et al. 2002). Another major global cooling event and sig-

nificant fall in global sea level is observed at about 2.4 Ma

(Haq et al. 1987).

The Quaternary was a period also characterised by

major climatic and sea-level changes, but in contrast gla-

cial–interglacial variations alternate at shorter time

intervals. The most significant change is known as the Mid-

Pleistocene Revolution or Mid-Pleistocene Transition,

which was initiated as early as 1.5 Ma and lasted until

0.6 Ma (Raymo 1992; Rutherford and D’Hondt 2000).

Before 1.5 Ma, 41 k.y. cycles were dominant in the d18O

isotopic record. From that time onwards, larger amplitude

100 k.y. cycles began to develop until 0.8–0.6 Ma, when

these cycles dominated (Clemens and Tiedemann 1997;

Raymo 1997; Clark et al. 1999). A major sea-level drop

inferred from d18O records and from outcrop data occurred

within the Mid-Pleistocene Revolution at 1.0–0.9 Ma

(Lowrie 1986; Haq et al. 1987; Kitamura and Kawagoe

2006). The 100 k.y. cycles are asymmetric, with long

(*90 k.y.) fluctuating cooling phases and rapid (10 k.y.)

warming terminations (Clark et al. 1999).

The study of the sedimentary record of those significant

environmental variations in proximal margin settings at the

Pliocene-Quaternary time scale can be regarded as rather

limited, in contrast to the relatively well known late Qua-

ternary shelf sedimentary record (Farran and Maldonado

1990; Ercilla et al. 1994; Chiocci et al. 1997; Chiocci 2000;

Hernandez-Molina et al. 2000; Tesson et al. 2000; Trincardi

and Correggiari 2000 amongst others). The Pliocene

and Quaternary shelf-upper slope sediment record of

siliciclastic mid-latitude continental margins is mainly

composed of shelf-margin deltas that can provide significant

information about the sedimentary response to sea-level

change, but to date this remains less well documented.

Only a few recent studies have focused on the large-scale

constructional patterns of proximal margin settings, par-

ticularly in the Mediterranean Sea (Lofi et al. 2003; Pepe

et al. 2003; Duvail et al. 2005). These studies document a

strong dominance of progradational packages, in contrast

to transgressive intervals, which are either not recognised

or tend to be under-represented (Hernandez-Molina et al.

2002). The study of the Pliocene-Quaternary sedimentary

record on terrigenous margins should help to improve low-

order sequence stratigraphic models. In addition, the

climatic and glacio-eustatic changes may interact at those

timescales with tectonically-induced changes in basement

subsidence, rock uplift and tectonic tilting, which all

influence the depositional patterns of sediments on conti-

nental margins and result in complicated sequence

stratigraphic patterns.

The Pliocene and Quaternary evolution of the conti-

nental margins of the southern Iberian Peninsula in the

Alboran Sea has been analysed in order to define the

general stratigraphic patterns. The Alboran Sea is a tec-

tonically active basin and the evolution of the depositional

bodies is largely influenced by the seismic activity, in

addition to climatic and eustatic oscillations (Maldonado

et al. 1992; Woodside and Maldonado 1992). However, a

dominant role for superimposed and hierarchical glacio-

eustatic fluctuations has been proposed for the northern

Alboran Sea shelf (Hernandez-Molina et al. 2002).

In this paper we focus on a specific sector of the central

continental shelf and upper slope of the northern Alboran

Sea, which shows significant physiographic variability, in

order to determine not only the glacio-eustatic control, but

also local tectonic and depositional factors influencing the

stratigraphic architecture. In particular, the recognition of a

dominantly aggradational interval in the most recent sedi-

mentary record, as well as the identification of different

types of regressive–transgressive transitions, may be

indicative of significant modifications of the original gla-

cio-eustatic control, or at least of the influence of distinct

patterns of relative sea-level changes. Another noteworthy

aim is to identify hierarchical patterns controlling deposi-

tional sequence generation and their role in constraining

proximal margin growth patterns. To complete these goals,

we describe the distribution, as well as the stratal and

growth patterns of the Pliocene and Quaternary deposits.

Physiography and surficial morphology

The Alboran Sea is the westernmost basin of the Medi-

terranean Sea. It is partially land-locked by Spain to the

north and Morocco to the south (Fig. 1). The northern

margin of the Alboran Sea comprises a narrow continental

shelf (2–20 km wide) with gradients less than 18 (Fig. 1b).

The minimum width of 2 km occurs offshore Cape Sacratif

(Carter et al. 1972). Maximum widths more than 20 km are

found offshore Malaga and Almerıa (Munoz et al. 2008).

The shelf break is located at variable water depths of 100–

125 m (Alonso and Maldonado 1992; Munoz et al. 2008).

The average gradient of the continental slope is close to 28(Carter et al. 1972).

196 Mar Geophys Res (2008) 29:195–216

123

The study area is located in the central part of the

northern Alboran Sea between La Herradura Beach to the

west and Cape Sacratif to the east, covering the continental

shelf and the upper slope. The coast has a subtropical cli-

mate, but the drainage area is constituted by the central

mountain ranges of the Betic Chains, which reach altitudes

of more than 3,300 m within a distance of 30 km from the

coast and have a snow reservoir for most of the year. The

main sediment supply to this margin sector is derived from

the Guadalfeo River, one of the main regional fluvial

sources with a permanent flow. The sediment contributions

from seasonal streams, such as the Verde River whose river

mouth is at Almunecar, have been much smaller (Fig. 2).

Limited prior studies on this area have reported the main

surficial morphologies and recent sedimentary processes

(Lobo et al. 2006; Fernandez-Salas et al. 2007). The most

significant recent deposits in the study area occur off river

mouths, such as the Guadalfeo and Verde rivers. There,

prodeltaic wedges exhibit shallow offlap breaks and the

frequent occurrence of crenulated sea floor; these mor-

phologies are related to rapid sedimentation processes led

by high-density sediment flows and limited lateral distri-

bution. Areal distribution of prodeltaic wedges changes

according to the size of the feeding river systems (Lobo

et al. 2006). The prodeltaic wedge offshore the Verde River

is limited to the inner shelf. Seaward, several morpholog-

ical highs occur on the middle shelf, establishing the

boundary with a flat outer shelf, which is covered by sev-

eral oblique large-scale bars at 90–100 m water depth.

In contrast, the outer shelf offshore the Guadalfeo River

prodelta shows numerous straight gullies, at water depths

greater than 88 m (Fig. 2).

Other significant morphological features are submarine

canyons, that are found in the eastern part of the study area

composing the Guadalfeo Canyon System (Perez-Belzuz

1999). Two main canyons with distinct morphological

characteristics are found in the study area. Several canyon

heads which connect with the Motril Canyon cut the outer

shelf south of Motril, at water depths as shallow as 70 m.

In contrast, the Carchuna Canyon cuts the entire shelf just

south of Cape Sacratif (Lobo et al. 2006) (Fig. 2).

Evolution of the Alboran Sea and its margins

Stratigraphic evolution

The opening of the Alboran Sea Basin took place from

Early to Late Miocene (Comas et al. 1992; Garcıa-Duenas

et al. 1992), when at least three sequences, mostly com-

posed of hemipelagic sediments and turbidites, were

deposited in the deep basin. Miocene deposits are overlaid

by a strong seismic reflector (M) attributed to the basin

dessication during the Messinian Salinity Crisis (Campillo

et al. 1992; Comas et al. 1992).

The present-day continental margins around the Alboran

Sea were built during the Pliocene and Quaternary. Four

major marginal constructional episodes during that period

Fig. 1 Geographical location

of: (a) the Alboran Sea in the

southwestern Mediterranean

Sea; and (b) the study area in

the northern margin of the

Alboran Sea. Coastline and

shelf break are highlighted in

red and orange colors in b

Mar Geophys Res (2008) 29:195–216 197

123

have been defined either as seismic units or depositional

sequences (Alonso and Maldonado 1992; Campillo et al.

1992; Ercilla et al. 1994; Perez-Belzuz et al. 1997; Perez-

Belzuz 1999; Hernandez-Molina et al. 2002). The main

growth patterns of the Alboran Sea are related to large

fluctuations of sea level, because the interpreted major

depositional sequences are bounded by regional erosive

discontinuities attributed to periods of global cooling and

significant sea-level falls, during the Early Pliocene, Late

Pliocene and mid-Pleistocene times, respectively (Perez-

Belzuz et al. 1997; Perez-Belzuz 1999; Hernandez-Molina

et al. 2002). Although these authors considered a Lower

Pliocene sequence, this should record only part of the Early

Pliocene because a regional hiatus has been identified in

the northern Alboran Sea margin at the base of the Pliocene

(Rodrıguez-Fernandez et al. 1999). These sequences are

also recognised in the basin deposits (Alonso and Maldo-

nado 1992; Campillo et al. 1992; Ercilla et al. 1994).

The Quaternary record of the northern Alboran Sea

margin was studied offshore and west of Malaga (Ercilla

et al. 1992, 1994; Ercilla and Alonso 1996). There, several

Quaternary depositional sequences are mainly composed of

lowstand systems tracts, represented on the shelf and shelf

break by shelf-margin deltas (Ercilla et al. 1992, 1994).

The most recent depositional sequence is dominated by a

regressive wedge (Hernandez-Molina et al. 2002), although

transgressive and highstand systems tracts are also sig-

nificantly preserved within the last Late Quaternary

depositional sequence, being the highstand system tract

mainly represented by stratified prodeltaic wedges (Ercilla

et al. 1992, 1994; Fernandez-Salas et al. 2003). This

depositional sequence is internally structured by higher-

frequency sub-orbital cycles (Hernandez-Molina et al.

1996).

Tectonic evolution

The opening and expansion of the basin occurred from

Early to Late Miocene during several rifting stages. The

basin underwent a significant tectonic change at the end of

the Late Miocene, when a general N–S compressive regime

involving folding and strike-slip faulting modified the

architecture of previous Miocene basins and margins

(Comas et al. 1992; Vazquez 2001; Ballesteros et al. 2008).

General N–S compression was still active during the

Early Pliocene, although a transtensive regime with an

E–W to ESE–WNW direction was active in the central and

western Alboran Sea (Campillo et al. 1992). Compression

was NNW–SSE directed during the Late Pliocene, causing

the development of structural inversions and a change to a

transpressive regime (Campillo et al. 1992; Campos et al.

1992; Ballesteros et al. 2008).

The compressive regime has remained during the Qua-

ternary, causing vertical movements in basins and margins

(Campos et al. 1992). As a consequence, the northern

margin of the Alboran Sea can be regarded as tectonically

Fig. 2 Multibeam bathymetry of the study area and position of seismic network used in this study. Interpreted seismic sections displayed in this

work are highlighted in blue color

198 Mar Geophys Res (2008) 29:195–216

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active during the Quaternary. Tectonic processes have

interacted with sea-level variations (Alonso and Maldonado

1992; Ercilla et al. 1994; Ercilla and Alonso 1996). High-

frequency sequences exhibit high lateral variability under

the direct influence of tectonic tilting, which is related to

the compressional regime (Ercilla et al. 1992). Areas with

high tilting show limited development of sequences and

shelf-margin deltas, because of lack of shelf accommoda-

tion space. Laterally, areas with more subsidence show

better preserved shelf-margin deltas (Ercilla et al. 1994;

Ercilla and Alonso 1996).

A pattern of tectonic lateral variability due to the com-

pressive regime is also observed in the study area (Perez-

Belzuz 1999), where a central subsiding area offshore the

Guadalfeo River is laterally bounded by two uplifted sec-

tors. The zone offshore and west of Verde River has

undergone tectonic tilting for much of Pliocene-Quaternary

time. Finally, folds and strike-slip faulting caused base-

ment uplift in the southeastward part of the study area.

Methodology

Seismic stratigraphy concepts were applied to a regional

grid of seismic profiles collected with different seismic

systems of variable resolution (Fig. 2). A Sparker source

and a 3.5 kHz sub-bottom profiler were used during the G-

83-2 survey in 1990, when about 1,650 km of seismic lines

were collected onboard the research vessel ‘‘Investigador’’

within the framework of the project called ‘‘Mapa Geol-

ogico de la Plataforma Continental Espanola y Zonas

Adyacentes’’ carried out by the Spanish Geological Survey

(Fig. 2). For the purposes of the present study, we mainly

used seismic profiles obtained with the 1,000–4,500 J

Sparker source. The vertical scale of seismic profiles was

1 s, and the acquisition filter was 30–500 Hz. Seismic

profiles were collected obliquely to the coastline with a

primary NNE–SSW orientation and a secondary NW–SE

orientation.

Identified seismic units were classified as progradational

or aggradational units. In some seismic units a higher

frequency architectural level was identified, and these

elements were defined as sub-units. From the seismic

stratigraphy analysis, isopach maps of seismic units were

delineated from La Herradura Beach to the east of Motril

whenever possible. Maps of upper boundaries or fronts of

progradational units were also generated, in order to pro-

vide an estimate of changing paleotopographic conditions.

Both isopach and paleotopographic maps are given in

milliseconds of two-way travel time (TWT).

After the seismic stratigraphy approach, the ensuing

stratigraphy was interpreted by applying sequence stratig-

raphy concepts (Posamentier and Vail 1988). The early

work considered a depositional sequence to be composed

of a lowstand/shelf margin, transgressive and highstand

systems tract, which were defined relative to a curve of

eustatic fluctuations. The lowstand and shelf margin sys-

tems tracts are similar concepts, as they are related to the

sea-level fall through to early rise period. Lowstand sys-

tems tract are linked with type 1 sequence boundaries,

formed during a stage of rapid sea-level fall; in contrast, a

shelf margin systems tract is predicted to form during a

stage of slow sea-level fall (Posamentier and Vail 1988).

We have also included in the sequence stratigraphy

interpretation an additional systems tract to account for

deposits generated during relative sea-level falls, in order

to make a distinction with deposits generated during rela-

tive lowstands, which we include within the lowstand

systems tract. Although there are various terms to describe

that kind of deposits, such as ‘‘forced regressive wedge

systems tract’’ (Hunt and Tucker 1992) or ‘‘falling-stage

systems tract’’ (Plint and Nummedal 2000), we refer

generically to them as ‘‘regressive systems tracts’’ for

coherence with previous work in the region (Hernandez-

Molina et al. 2002). Under this assumption, sequence

boundaries are interpreted to be lowstand erosional sur-

faces truncating clinoforms of underlying regressive

system tracts and generated in response to major sea-level

falls. This style of sequence boundary is well documented

at the Pliocene and Quaternary scale (Trincardi and Corr-

eggiari 2000; Burger et al. 2002; Hernandez-Molina et al.

2002; Browne and Naish 2003).

Stratigraphic architecture and stacking patterns

Six seismic units that can be correlated at regional scale

were recognised overlying the acoustic basement and were

classified as progradational units or aggradational units

according to their seismic configuration, external shape

and/or internal stacking pattern (Figs. 3–8). Three progra-

dational units (PU) have been designated as Units PU C,

PU B and PU A from older to younger. Additionally, three

aggradational units (AU) named Units AU C, AU B and

AU A are intercalated between progradational units.

Careful examination of the internal architecture of those

units has allowed defining seismic sub-units within all

progradational units and within Unit AU A.

Acoustic basement occurrence

In the study area, three distinct morpho-tectonic sectors

were recognised according to the occurrence of the

acoustic basement, which is identified as zones of chaotic

and contorted reflections below well-stratified sediment

packages (Figs. 3, 4, 8). In the western sector offshore the

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Fig. 3 Line drawing interpretation of Sparker profile located in the western part of the study area. The profile depicts the three major periods of

shelf progradation (PUs) interrupted by sediment sheets (AUs). See Fig. 2 for location

Fig. 4 Line drawing interpretation of Sparker profile located offshore the Verde River. Sharp-based progradational deposits merging offshore

into a progradational wedge represents most of the record within Unit AU A. See Fig. 2 for location

200 Mar Geophys Res (2008) 29:195–216

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Fig. 5 Line drawing interpretation of Sparker profile taken offshore the Guadalfeo River, perpendicular to the shelf. The two main phases of

continental margin build-up (progradation and vertical growth) are clearly illustrated in this profile. See Fig. 2 for location

Fig. 6 Line drawing interpretation of Sparker profile offshore the Guadalfeo River, oblique to the shelf. The last phase of shelf aggradation

during development of Unit AU A is well represented. See Fig. 2 for location

Mar Geophys Res (2008) 29:195–216 201

123

Verde River the acoustic basement is located at shallow

depths ranging between 50 and 100 ms TWT below present

sea-level in the inner shelf (Figs. 3, 4, 6), although it may

be locally covered by channelised facies, interpreted as

Miocene fluvial deposits, as previously reported by Perez-

Belzuz (1999). The acoustic basement is not identified in

Sparker seismic profiles in the middle sector offshore the

Guadalfeo River. Finally, a shallow basement high occurs

in the eastern sector to the southeast of the Guadalfeo

River, generating a narrow shelf less than 4 km wide and

an abrupt slope. The basement high also controls the

location of the heads of the Motril Canyon. The acoustic

basement is detected in the upper slope where it develops a

structural high at depths greater than 360 ms (Figs. 8, 9a).

Here, the basement exhibits chaotic facies. The basement

high is bounded seaward by the Gualchos Fault, which

influences the high topographic gradient of this margin

segment (Perez-Belzuz 1999).

Progradational units

Progradational units onlap previous units and/or the acoustic

basement in the inner shelf domain. Internal configurations

of progradational units are dominated by seaward-directed,

parallel-oblique stratified reflections, which downlap the

lower boundaries and are erosionally truncated by the upper

boundaries, particularly in the mid to outer shelf. Upper

terminations tend to evolve to concordance on the upper

slope, although the upper boundaries also may show local

erosional truncation beyond the shelf break (Figs. 3–5).

The upper boundaries of progradational units show well-

defined shelf-to slope profiles (Fig. 9). The depths of the

upper boundaries in inner shelf settings are around or

slightly higher than 100 ms TWT. Seaward, the associated

shelf breaks occur at decreasing depths from older (at about

250 ms) to younger units (\200 ms). Paleotopographic

maps also show that progradational units are incised by a

Fig. 7 Line drawing interpretation of Sparker profile of the shelf southeastward of the Guadalfeo River, where the complex internal architecture

of Unit AU A is well shown. See Fig. 2 for location

202 Mar Geophys Res (2008) 29:195–216

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tributary of the Motril Canyon, in the vicinity of the shelf

break (Fig. 9).

Externally progradational units exhibit well-developed

wedge shapes thoroughout most of the study area.

Landward terminations occur on the inner shelf west of

Salobrena, at variable distances of 2–4 km from the

present-day coastline (Fig. 10). The distribution pattern is

more complex in the eastern part of the study area

Fig. 8 Line drawing interpretation of Sparker profile from the eastern part of the study area, showing the occurrence of acoustic basement close

to the sea bottom, as well as significant erosion of most of the units in the proximity to Carchuna Canyon. See Fig. 2 for location

Fig. 9 Isobaths in milliseconds as two-way travel time (TWTT) of

the upper boundaries of progradational units. From bottom to top: (a)top boundary of Unit PU C; (b) top boundary of Unit PU B; (c) top

boundary of Unit PU A. A profile sketch from the middle part of the

study area highlighting the stratigraphic position of each upper

boundary is included in each map

Mar Geophys Res (2008) 29:195–216 203

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Fig. 10 Isopach maps in milliseconds as two-way travel time (TWTT) of seismic units identified in the study area, with the exception of Unit PU

C, which lower boundary was not resolved in many places due to the presence of multiple; (a) AU C; (b) PU B; (c) AU B; (d) PU A; (e) AU A

204 Mar Geophys Res (2008) 29:195–216

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southeastward of Motril, as the proximal unit termination

is not observed. In addition, progradational units pinch out

distally in several places, such as: (a) Unit PU C thins out

or disappears on the upper slope; (b) Units PU B and A

pinch out in the upper slope, possibly in relation with

canyon erosion processes.

Sediment thickness values are moderate on the shelf

(20–30 ms) but tend to increase in a seaward direc-

tion (Figs. 3–5). As a consequence, thicker sediments

are generally found on the upper slope. Unit PU B is

20–50 ms thick on the upper slope, although maximum

values close to 100 ms are related with the sediment

infilling of the Motril Canyon tributary (Fig. 10b). Two

main Unit PU A depocenters are recognised on the upper

slope: (a) offshore Almunecar, forming a W–E depo-

center which acquires a ENE–WSW orientation to the

east, with maximum thickness exceeding 160 ms; (b)

offshore Motril, where the depocenter seems to open

seaward, because we only recognise proximal distribu-

tion. The maximum thickness of this depocenter is at

least 200 ms. In between the two upper slope depocen-

ters, thickness values generally do not exceed 100 ms

(Fig. 10d).

Internal architecture of progradational units

Progradational units appear to be constituted of a number

of small-scale sub-units or shelf-margin wedges, bounded

by offlap surfaces with toplap to erosional truncations

(Fig. 11). Internal configuration is oblique-parallel and

changes from low-angle on the shelf to higher-angle on

the shelf break and upper slope. Upper slope facies also

show common sediment disruptions. In most of cases, the

shelf margin wedges may be considered as shelf-margin

deltas.

The shelf-margin wedges are generally disposed in a

fore-stepping stacking pattern, leading to the progressive

seaward migration of the shelf break. Each wedge dis-

plays its maximum thickness close to the offlap break,

reducing in thickness landward, where they pinch out on

the mid to outer shelf, and seaward, where distal toe-sets

amalgamate. This internal architecture is best observed in

the western sector of the study area offshore the Verde

River (Fig. 11a).

In other places, however, it is possible to distinguish

within progradational units a number of lenticular-shaped

units intercalated with progradational sub-units. This

Fig. 11 Drawing line interpretations of profiles perpendicular to the

margin highlighting a secondary order architectural level: (a) shelf-

margin wedges forming the progradational units; (b) shelf deposits

are occasionally found within progradational units, mainly in the shelf

offshore the Guadalfeo River; (c) shelf wedges stack vertically within

Unit AU A. See Fig. 2 for location. The different colors highlight the

separate seismic subunits within seismic units

Mar Geophys Res (2008) 29:195–216 205

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pattern is best evidenced in the central sector of the study

area offshore the Guadalfeo River (Fig. 11b), suggesting

that progradational units are composed of minor shelf

wedges and shelf-margin wedges.

Aggradational units

These seismic units rest on underlying progradational units,

and are characterised by the general absence of internal

reflections (Fig. 3). Locally, some internal reflections tend

to be subparallel to unit boundaries, describing onlap rela-

tionships with regard to the lower boundary (Figs. 4, 5).

Unit AU A clearly differs from the general trend, as it shows

an overall aggradational pattern, best observed in the shelf

offshore the Guadalfeo pro-delta (Figs. 5–7). Consequently,

this unit is better described as a complex aggradational unit.

Aggradational units tend to pinch against the acoustic

basement on the inner shelf (Figs. 3, 6, 10a), except Unit AU

A, whose landward termination is not detected. On the shelf,

the thicknesses of Units AU C and B are almost constant, do

not exceed 20 ms, and normally are below 10 ms. As a

consequence, aggradational units exhibit sheet to tabular

external shapes on the shelf. Maximum thickness values are

found in the vicinity of the shelf break (Figs. 10a, c). The

thickness distribution pattern of Unit AU A is more irregular

on the shelf, as two main depocenters were identified

(Fig. 10e): (a) an outer shelf depocenter offshore Almune-

car, showing a W–E orientation for more than 5 km and with

maximum thickness above 50 ms; (b) an inner shelf depo-

center offshore Motril, showing a NW–SE orientation with

maximum thickness around 100 ms.

On the upper slope, thickness distributions show con-

trasting patterns between the three aggradational units:

(a) Unit AU C exhibits an upper slope depocenter

eastward of the Verde River, and mainly offshore

the Guadalfeo River (Fig. 5), with thicknesses above

80 ms and increasing eastward to more than 180 ms

offshore an upper slope acoustic basement high

(Fig. 10a).

(b) Unit AU B pinchs out in the upper slope, where it is

only recognised eastward of the Motril Canyon

tributary. The unit shows internal subparallel reflec-

tions and thicknesses above 50 ms (Fig. 10c).

(c) Unit AU A has a minimum thickness at the shelf

break. On the upper slope, thicknesses increase again,

at least offshore the Guadalfeo prodeltaic area, where

it is 70 ms thick (Fig. 10e).

Internal architecture of aggradational units

As noted before, the internal architecture of Unit AU A is

more complex than the architecture of underlying

aggradational units, which can be considered as simple

units because they do not show internal subdivisions on

the shelf. In detail, the aggradational pattern within Unit

AU A is generated by the vertical stacking of several sub-

units or shelf wedges (Fig. 11c). The vertical stacking

causes the shelf break to remain stationary or even to

migrate landward (Figs. 4, 5, 8). The sub-units tend to be

bounded by planar or undulating surfaces across the entire

shelf; the upper termination of internal reflections is

generally toplap offshore the Guadalfeo River, but ero-

sional truncation is also identified locally. A channelised

upper surface is also found above specific shelf wedges.

Each sub-unit shows parallel-oblique internal configura-

tions of diverse inclinations on the shelf, grading to

subparallel on the upper slope. Shelf sheets may be

observed intercalated between shelf wedges. Individual

wedges show relatively constant and moderate thickness

across the shelf and toward the shelf break, where they

become thinner. Seaward, they continue on the upper

slope as sheet deposits. Their landward termination is

generally located on the inner shelf, or even continuing

further landward.

The geometric characteristics observed offshore the

Guadalfeo River are indicative of deltaic wedges (Fig. 5),

which are considered the dominant shelf depositional type

in the Alboran Sea during the late Quaternary (Ercilla

et al. 1994). In contrast, sharp-based deposits without

aggraded bottomsets found offshore the Verde River

(Figs. 3, 4) probably suggest a lack of accommodation

space and the influence of higher energy depositional

environments, with development of shoreface and/or sand

ridge systems. Dominance of sharp-based deposits has

been linked to strong wave dominance in the Gulf of

Mexico (McKeown et al. 2004). The occurrence of high-

angle clinoforms downlapping on top of low-angle clino-

forms is observed in the outer shelf offshore the Verde

River, and offshore from a sharp-based deposit on the

middle shelf (Fig. 4). This stratigraphic pattern is attrib-

uted to the presence of sharp-based shoreface-delta front

deposits, which are considered to be an indicator of a

forced regression process (Posamentier and Morris 2000).

Moreover, it may indicate development of a long-distance

regression, favored by the low shelf gradients in this area.

Finally, mounded morphologies are also recognised off-

shore the Verde River. Similar geometrical patterns in the

stratigraphic record have been interpreted as sand ridges

(Yang 1989; Knutsen and Larsen 1996).

Stacking patterns of seismic units

The three distinct morpho-tectonic sectors recognised

according to the occurrence of acoustic basement also

control the stacking patterns of seismic units.

206 Mar Geophys Res (2008) 29:195–216

123

In the western sector offshore the Verde River, progra-

dational units show a fore-stepping, prograding pattern.

The shelf break migrated seaward offshore the Verde River

(Figs. 3, 4, 12).

In the middle sector offshore the Guadalfeo River,

progradational units stack vertically and the shelf break has

remained stationary (Figs. 5, 12).

Finally, in the eastern sector to the southeast of

Guadalfeo River, progradational units show limited or no

development, as in the case of Unit PU C, and the shelf

break may be either stationary or even migrate landward

(Figs. 7, 8, 12). The erosion associated with submarine

canyons and sediment instabilities due to the high slopes

associated to the basement high have also influenced this

evolution.

Sequence stratigraphy and chronostratigraphic

framework

Depositional sequences

On the basis of the proposed sequence stratigraphy termi-

nology explained above, we consider that the bulk of the

progradational units can be interpreted as regressive system

tracts (Fig. 13). Geometrical patterns such as prograding

clinoforms, wedge shapes and top erosion have been linked

to regressive deposition, leading to rapid margin progra-

dation in other Mediterranean margins at the Pliocene-

Quaternary scale, such as the southern continental margin

of the Alboran Sea (Gensous et al. 1986), the Gulf of Lions

(Lofi et al. 2003; Duvail et al. 2005) and the southern

Fig. 12 Seaward location of

different paleo-shelf breaks

developed within progradational

units and Unit AU A (present).

Former Units AU C and B did

not contribute to change paleo-

shelf break location

Fig. 13 Drawing line showing the sequence stratigraphy interpretation representative of the study area. This interpretation is based on seismic

line off Guadalfeo River provided in Fig. 5

Mar Geophys Res (2008) 29:195–216 207

123

Tyrrhenian Sea (Pepe et al. 2003). The dominance of

regressive wedges in the Pliocene and Quaternary succes-

sion has also been documented on several Atlantic margins

(Coppier and Mougenot 1982; Hernandez-Molina et al.

2002, 2006; Le Roy et al. 2004).

The identification of internal sub-units downstepping

towards the basin and leading to progressive seaward shelf

break migration and long-distance progradation can be

considered as a reliable indicator of deposition during

falling sea-level conditions (Browne and Naish 2003;

Hubscher and Spieß 2005). In addition, the major pro-

grading packages invariably show erosive surfaces on their

top boundary, related to the major sea-level falls at the

Pliocene-Quaternary scale (Estrada et al. 1997; Hernandez-

Molina et al. 2002). Regressive deposits show similar,

uniform geometric patterns in the interpreted sequences,

providing evidence that continental shelf construction

proceeded following a similar pattern during earlier phases.

It is possible, however, that the proximal part of progra-

dational units may also comprise high stand system tracts,

although poorly preserved due to major erosion concomi-

tant to ensuing sea-level falls.

Aggradational units are represented on the shelf either

by thin sheets or by aggradational packages, and onlap

relationships are observed locally. All these stratigraphic

features are compatible with an origin linked to rising sea

levels. We estimate that transgressive system tracts con-

stitute the core component of aggradational units (Fig. 13),

which would be equivalent to ravinement shoreface

deposits, interpreted as thin, high-energy sheet deposits

lying on an erosional surface (Proust et al. 2001; Hubscher

and Spieß 2005). However, we admit that those units could

have formed, at least partially, during highstand stages.

However, we do not have enough resolution to enable

discrimination between transgressive and highstand

deposits, which usually exhibit offlap and downlap patterns

(Browne and Naish 2003). Only in the case of Unit AU A is

the high stand system tract clearly recognised, as it is

represented by a seaward-thinning wedge with great

thickness in its proximal parts. This sediment wedge

comprises pro-deltaic deposits and laterally continuous

sediment wedges (Lobo et al. 2006). Previous studies in the

Alboran Sea interpret the main development of the proxi-

mal wedge as occurring during the Holocene highstand

(Ercilla et al. 1992, 1994; Ercilla and Alonso 1996).

Upper slope depocenters of aggradational units display

distinct stratigraphic relationships, with lateral shelf facies.

As a result the sequential interpretation is also variable

(Fig. 13). In the case of the upper slope depocenter of Unit

AU C, which is characterised by onlap terminations and a

major component of vertical aggradation, we interpret this

to represent lowstand deposition overlying the sequence

boundary. This upper slope wedge depocenter would be

equivalent to a progradational-aggradational set corre-

sponding to the lowstand systems tract in different shelf-

margin settings (Anderson et al. 1996; Proust et al. 2001).

The sequential interpretation of Unit AU A’s upper slope

depocenter is more controversial, as in most of the zones

shelf and upper slope deposits composing Unit AU A thin

or are eroded near or at the shelf break, making shelf-upper

slope correlation problematic. In some places, however,

upper slope facies seem to constitute the seaward prolon-

gation of shelf facies and should be considered as coeval;

therefore, upper slope facies are also considered to be part

of the transgressive system tract (Fig. 13).

According to this interpretation, the observed shelf and

upper slope sedimentary record may be classified into four

major depositional sequences of ‘‘n’’ order, separated by

regional erosional surfaces, which establish the upper

boundary of regressive system tracts (Fig. 13):

(a) A lower depositional sequence (DS 4) of which we

only observe the upper part, interpreted as a regres-

sive system tract (Unit PU C).

(b) A second depositional sequence (DS 3) characterised

by a lower aggradational interval (Unit AU C) and an

upper progradational interval (Unit PU B). The

aggradational interval is represented by an upper

slope depocenter interpreted as a lowstand systems

tract, and a sheet shelf deposit interpreted as a

transgressive system tract. The upper progradational

interval is considered to be a regressive system tract.

(c) A third depositional sequence (DS 2) characterised by

a lower transgressive system tract (Unit AU B) and an

upper regressive system tract (Unit PU A).

(d) An upper depositional sequence (DS 1) is basically

aggradational and interpreted as a transgressive

system tract-high stand system tract complex (Unit

AU A).

Chronostratigraphic framework: integration with DSDP

and ODP legs and other stratigraphic architectures

A realistic chronostratigraphic framework can be estab-

lished from comparison with previous studies that

correlated a regional grid of medium-resolution seismic

profiles with scientific drillings in the Alboran Sea such as

the Deep Sea Drilling Project (DSDP) Site 121 and the

Ocean Drilling Program (ODP) Site 976 (Comas et al.

1996, 1999), and also by the regional identification of the

acoustic basement in the continental margin, interpreted to

be the M reflector, developed during Miocene sea-floor

subaerial exposure (Perez-Belzuz 1999). In addition,

Hernandez-Molina et al. (2002) provides a general model

of Quaternary stratigraphic stacking patterns in the conti-

nental shelves of the southern Iberian Peninsula.

208 Mar Geophys Res (2008) 29:195–216

123

We found that the interpretation of the upper boundary

of Unit PU A as the Mid-Pleistocene regional discontinuity

is quite reasonable, for the following reasons: (a) the upper

boundary of Unit PU A is a widely recognized erosional

truncation across the shelf; (b) this surface represents the

most significant change of the stratigraphic stacking pattern

observed in the study area, as it separates seaward-pro-

grading packages below from progradational units

vertically stacked on top, i.e., a net change from shelf

progradation to shelf aggradation; (c) the Mid-Pleistocene

Revolution constitutes the most significant climatic change

during the Pliocene-Quaternary, as it marks the start of

100 k.y., high-amplitude glacio-eustatic cycles (Clemens

and Tiedemann 1997; Raymo 1997; Clark et al. 1999).

Taking into account this basic premise, our DS 1 would

have developed after the Mid-Pleistocene Revolution.

The chronostratigraphic framework of the three lower

sequences (DSs 2, 3 and 4) is more problematic. By cor-

relating sequence boundaries with well-known climatic and

eustatic changes during the Pliocene and early Quaternary,

two main hypotheses may be proposed:

(a) The observed shelf stratigraphy comprises the entire

Pliocene and Quaternary record overlying the M-

reflector on the Alboran Sea continental margin

(Perez-Belzuz 1999). Under this interpretation, the

three lower sequences would have developed during

the Early Pliocene, Late Pliocene and Early Quater-

nary (Table 1), led by circa 1.5 m.y. third order

cycles. The top erosional discontinuities would be

equivalent to the Lower Pliocene, Upper Pliocene and

Mid-Pleistocene regional discontinuities that have

been generated during periods of major sea-level falls

and important climatic changes (Hernandez-Molina

et al. 2002) (Table 1).

(b) The base of the Pliocene has not been preserved

according to some authors, because a regional hiatus

has been identified in the northern Alboran Sea

margin (Rodrıguez-Fernandez et al. 1999). In this

case, the lower depositional sequence would be

incomplete. By considering this hypothesis, the upper

boundary of Unit PU C, which is well marked, would

be interpreted as the Upper Pliocene regional discon-

tinuity, related to a relevant sea-level fall (Hernandez-

Molina et al. 2002). In contrast, the upper boundary of

Unit PU B shows less evidence of erosion and would

represent the Pliocene-Quaternary boundary, which is

less recognisable than the Upper Pliocene or the Mid-

Pleistocene regional discontinuities (Table 1).

Discussion

Stratigraphic variability of sequences

Major-scale sequences: regressive–transgressive

transitions

A major contribution of this work is the identification of

geometrical sedimentary patterns associated with trans-

gressive intervals at the Pliocene and Quaternary scale.

Previous interpretations have highlighted the poor repre-

sentation of transgressive and highstand intervals, leading

to pronounced asymmetrical sequences due to the strong

Table 1 Tentative chronologies based on correlation of the regional seismic grid with DSDP 121 and ODP 976 sites and previous chrono-

stratigraphic schemes of Perez-Belzuz (1999) and Hernandez-Molina et al. (2002)

LST: lowstand systems tract; TST: transgressive systems tract; HST: highstand systems tract; RST: regressive systems tract; SB: sequence

boundary; M: Late Messinian discontinuity; LPR: Lower Pliocene regional discontinuity; UPR: Upper Pliocene regional discontinuity; QB:

Pliocene-Quaternary boundary; MPR: Mid-Pleistocene regional discontinuity

Mar Geophys Res (2008) 29:195–216 209

123

dominance of regressive and lowstands deposits (Ercilla

et al. 1994; Chiocci et al. 1997; Hernandez-Molina et al.

2002). In the study area, each transgressive interval shows

a particular signature, which could indicate a relationship

with different transitions between lowering and rising sea-

levels. According to the stratigraphic representation of

transgressive deposits in the study area, it is possible to

observe different types of regressive–transgressive (R-T)

transitions (Fig. 14). There are two asymmetrical transi-

tions between DS 4 and 3, and between DS 3 and 2.

Between DS 4 and 3, the asymmetrical pattern (thick

wedge-shaped regressive system tract and thin transgres-

sive system tract) is interrupted by the presence of an upper

slope aggradational wedge interpreted as a lowstand sys-

tems tract. Between DS 3 and 2, the asymmetrical pattern

of systems tracts is clearer, as a wedge-shaped regressive

system tract is replaced landward by a sheet-shaped

transgressive system tract.

In contrast to earlier schemes, the transition between DS

2 and 1 seems to follow a more symmetrical pattern. This

cycle shows a relatively thick transgressive to highstand

interval represented by Unit AU A overlying the typical

wedge-shaped regressive system tract (Fig. 14c). On the

New Jersey margin, the identification of a thick, trans-

gressive component at the Pliocene-Quaternary scale was

related to long-term eustatic rise (Metzger et al. 2000). In

our study area, such a sea-level evolution following the

previous fall would result in a relatively symmetrical pat-

tern, with both gentle sea-level fall and rise.

We relate the existence of different kinds of regressive–

transgressive transitions to the combined influence of gla-

cio-eustatic changes and additional disruptive factors, such

as tectonic control and/or drastic variations of sediment

supply. The most common asymmetrical pattern would

support the leading control of glacio-eustatic processes.

Most of the shelf sequences at the Pliocene-Quaternary

scale are asymmetrical consistent with the leading control

of asymmetric sea-level changes (Chiocci et al. 1997;

Hernandez-Molina et al. 2002). The distinction between

observed asymmetrical cycles (between DS 4 and 3, and

between DS 3 and 2) refers to the different representation

of predominantly upper slope aggradational facies, inter-

preted as lowstand deposits. This difference may be

attributed either to different shapes of sea-level reversal

(Porebski and Steel 2003), or to drastic changes in sedi-

ment supply. Thus, the higher development of lowstand

facies observed in the transition between DS 4 and 3 could

indicate that the transition was not abrupt but gradual

(Fig. 14a). In contrast, the more strongly asymmetrical

pattern between DS 3 and 2 could indicate an abrupt

transition in the dominant sea-level cycle pattern, when

prolonged sea-level falls were followed by rapid, rather

abrupt rises (Fig. 14b). An alternative hypothesis should

consider a period of increased sediment discharge during

the transition between DS 4 and 3, in contrast to the

transition between DS 3 and 2, when the upper slope

underwent sediment starvation.

Finally, the identification of a more symmetrical tran-

sition between DS 2 and 1 must be considered a significant

anomaly in the otherwise generally symmetric pattern; we

ascribe this symmetric pattern to the disturbing effect

caused by tectonism acting on a regional to local scale, and

more precisely to enhanced subsidence.

Minor-scale sequences: implications for shelf build-up

The significant differences in terms of geometry, regional

distribution and internal configuration between shelf-mar-

gin wedges composing progradational units and shelf

wedges within Unit AU A are considered to reflect dif-

ferent types of generation under a sequence stratigraphy

perspective. Over the long term, the two contrasting

depositional modes lead to distinct patterns of shelf

construction.Fig. 14 Sketches summarizing the different styles of regressive to

trangressive transitions identified in northern Alboran Sea margin

210 Mar Geophys Res (2008) 29:195–216

123

Shelf-margin wedges may be considered as higher-order

(n ? 1) depositional sequences, almost exclusively com-

posed by regressive system tracts (Fig. 15). The lateral

stacking pattern and the limited extent of upper boundaries

would suggest the influence of eustatic cycles of moderate

amplitude, typically, 41 k.y. cycles, which dominated

before the Mid-Pleistocene Revolution (Clark et al. 1999;

Lisiecki and Raymo 2005). This inference agrees with our

observations, since shelf-margin wedges are found within

progradational units. The lack of identification of distal

aggradational facies implies that lowstand systems tracts

probably did not develop or instead that their development

was very limited.

In contrast, shelf wedges typically show two depocen-

ters, on the shelf and on the upper slope (Fig. 15). Shelf

wedges may also be interpreted as regressive system tracts,

but their widespread distribution and the existence of shelf-

wide unconformities are suggestive of the influence of

glacio-eustatic cycles of higher amplitude, such as the

dominant 100 k.y. cyclicity after the Mid-Pleistocene

Revolution. The influence of 100 k.y. cycles on the gen-

eration of shelf depositional cycles has been documented

from the Mediterranean Sea (Chiocci 2000; Trincardi and

Correggiari 2000; Rabineau et al. 2005). Another alterna-

tive would be to consider the shelf wedges as shelf margin

system tracts, which would highlight the different contri-

bution of the two types of wedges to shelf growth. Shelf-

margin wedges (typical regressive system tracts) would

prograde over a pre-existing shelf margin wedge, leading to

progressive seaward shelf break migration and thus causing

shelf lateral accretion (Fig. 15). In contrast, shelf wedges

(particular case of regressive or shelf margin system tracts)

would prograde above pre-existing wedges, but in this case

the shelf break would remain more or less stationary. The

consequence for shelf build-up would be that vertical

growth is favored, but lateral accretion is limited. All those

shelf wedges are overlain by a seaward-thinning inner

wedge, whose development has been ascribed to the

Holocene highstand (Lobo et al. 2006) (Fig. 15).

Finally, shelf wedges included within progradational

units need to have different sequential interpretations, as

they seem to be intercalated between shelf-margin wedges,

interpreted as regressive system tracts. Consequently, shelf

wedges would be deposited between successive sea-level

falls, and must be considered as transgressive and/or

highstand features.

Stacking patterns of depositional sequences: out versus

upbuilding

Spatial variation of stacking patterns: local tectonic

influences

The existence of laterally variable styles of continental

margin construction, as evidenced by different stacking

patterns and styles of shelf-break migration, is particularly

significant during deposition of progradational units. The

stacking pattern of depositional sequences is an indicator of

the balance between subsidence versus uplift and ulti-

mately on the general tectonic evolution of any particular

setting (Chiocci et al. 1997; Ridente and Trincardi 2002).

In addition, patterns of shelf-break migration can provide

information about the style of continental margin growth

(Fulthorpe and Austin 1998). The lateral variability of shelf

growth patterns (Fig. 16) can be attributed to the complex

interplay between low-order eustatic cycles and changing

tectonic regimes experienced in the study area during the

Pliocene-Quaternary. This variability has also been evi-

denced in neighboring areas of the Alboran Sea (Alonso

and Maldonado 1992; Ercilla et al. 1994; Estrada et al.

1997).

We estimate that the differences between morpho-tec-

tonic sectors identified in the study area are strongly

determined by the paleomorphology and lateral tectonic

variability (Figs. 16, 17). The lateral changes took place

over a long period during the Pliocene and Quaternary and

may be related to changes in the relationship between

subsidence and uplift (Fig. 17). In the southeastward part

of the study area folds and strike-slip faulting caused

basement uplift; as a consequence, shelf construction was

limited due to the absence of accommodation space and

Fig. 15 Idealized model of the internal architecture of progradational

units and the youngest Unit AU A: (a) summary of geometry and

internal architecture of a progradational unit; (b) summary of

geometry and internal architecture of Unit AU A

Mar Geophys Res (2008) 29:195–216 211

123

because of sediment instability processes related to can-

yon-headward erosion (Fig. 16). To the west, the rest of the

study area subsided more uniformly, although the dominant

compressive regime also caused an increase in tectonic

tilting towards the west, as documented in other sectors of

the northern Alboran Sea margin (Ercilla et al. 1992).

Thus, the stacking pattern tends to be aggradational off-

shore the Guadalfeo River, whereas the increased tectonic

tilting favored the strongest shelf margin progradation

offshore the Verde River. As a consequence, sediment

supply seems to be subordinated to tectonic behavior off-

shore the Guadalfeo and Verde Rivers, as major shelf break

Fig. 16 Fence diagrams showing the general stratigraphic architec-

ture in the study area. The shelf offshore the Verde River shows the

importance of progradational units that stack laterally. Eastward, the

shelf offshore the Guadalfeo River is affected by an important

subsidence that influences thick progradational and aggradational

units, whereas the occurrence of shallow acoustic basement in the

eastern part of the study inhibit the development of thick sequences,

favoring the erosion of deposits due to headward erosion by

submarine canyons and enhancing the development of Unit AU A

212 Mar Geophys Res (2008) 29:195–216

123

migration occurs offshore a minor river such as the Verde

River; in contrast, a stationary shelf break occurred off-

shore a major river at a regional scale such as the

Guadalfeo River.

This pattern agrees relatively well with onland infor-

mation of vertical displacements from Late Miocene to

present in the coastal zone of the Betic Cordilleras. The

zone around Motril corresponds to a boundary between the

eastern sector, characterised by uplift, and the western

sector between Almunecar and Salobrena (Fig. 2), which

has been dominated by tectonic subsidence (Sanz de

Galdeano and Lopez-Garrido 2000).

Temporal variation of stacking

patterns and shelf growth

The identification in the study area of a recent aggrada-

tional interval represented by Unit AU A can be considered

a significant peculiarity of this margin (Figs. 5–8). This

interval is recognised in each morpho-structural sector

defined above, although with greater development to the

southeast, where the basal boundary is below the seafloor

multiple (Fig. 17). In other words, a seismic unit equivalent

to Unit AU A is not recognised in other areas of the

Alboran Sea margin. This effect would support the influ-

ence of a local factor in generating the observed

architecture.

The existence of dominantly aggradational patterns

since the mid-Pleistocene phase can be attributed to

increased subsidence rates, as reported for example also

in the Gulf of Lions (Lofi et al. 2003). Geologic data

from onland areas support this theory (Armijo et al.

1977), as although tectonic tilting is considered to be

dominant during the Quaternary due to the NNW–SSE

compressive regime, there are specific margin segments

of more than 50 km length that have significantly sub-

sided, such as the continental margin offshore Motril

(Armijo et al. 1977).

The identification of the aggradational pattern in every

morpho-sedimentary sector of the study area indicates

isochronous and uniform subsidence. This inference also

implies a significant change in the tectonic regime within

the study area, and is particularly evident in the eastern

sector, which previously experienced uplift (Fig. 17).

When observed in detail, the aggradational interval

is composed of a number of progradational wedges

stacked vertically and separated by shelf-wide erosion

surfaces. The recognition of these shelf-wide surfaces

also indicates the effect of increased glacio-eustatic

fluctuations, particularly after the Mid-Pleistocene

Revolution. These high-frequency, high-amplitude

oscillations have been reinforced during the Pleistocene,

as reported in several other Mediterranean margins (e.g.,

Pepe et al. 2003; Duvail et al. 2005). As a consequence,

we estimate that the onset of higher amplitude fluctua-

tions combined with enhanced subsidence rates have

been responsible for the generation of the younger

aggradational interval.

Fig. 17 Synthetic seismic stacking patterns and relationship with the

tectonics of the different sectors (western, middle and eastern) in the

study area. Configurations A and B occur offshore Guadalfeo and

Verde rivers, which have undergone continuous subsidence during

most of Pliocene-Quaternary time. The more pronounced prograda-

tional stacking pattern observed offshore the Verde River is related to

increase tectonic tilting towards the west. Configuration C occurs in

the eastern part of the study area, where uplifting of acoustic

basement related to strike-slip faulting has occurred during a

significant Pliocene-Quaternary time interval. A change to increased

subsidence rates in this sector is indicated by enhanced development

of AU A. The tectonic scheme is modified from Perez-Belzuz (1999)

Mar Geophys Res (2008) 29:195–216 213

123

Conclusions

A sector of the northern margin of the Alboran Sea reveals

contrasting stacking patterns of depositional sequences,

which ultimately result from the complex interaction

between low-order glacio-eustatic fluctuations and regional

tectonics. The regional correlation suggests that most of the

observed shelf and upper slope record is Pliocene and

Quaternary in age.

Two levels of depositional sequence were defined.

Four major depositional sequences are represented by the

alternation between progradational and aggradational

units. Progradational units are top bounded by erosive

surfaces linked to the most pronounced Pliocene and

Quaternary climatic changes and marked sea-level falls.

They are mainly interpreted as regressive system tracts,

whose generation leads to uniform, relatively constant

shelf progradation. In contrast, aggradational units are

mainly interpreted as the result of transgressive intervals,

and their representation is variable within depositional

sequences. As a consequence, we identified at least three

types of regressive–transgressive transitions: strongly

asymmetric cycles with either gradual or abrupt transi-

tions, and a relatively symmetric cycle conditioned by the

unusual preservation of the transgressive to highstand

phase.

We also identified higher-order depositional sequences,

mainly within the progradational units but also within the

most recent aggradational unit. Within these higher-order

sequences, regressive deposition was again dominant in

the preserved stratigraphic record, but two contrasting

depositional styles are characterised. Shelf-margin wed-

ges, interpreted as deltaic lobes, stack laterally to

construct progradational units. In contrast, shelf wedges

with different origins (pro-deltaic, shoreface to sand ridge

systems) stacked vertically during the generation of the

most recent aggradational unit. The transition from shelf-

margin wedge to shelf delta development caused a sig-

nificant change of the overall shelf growth patterns, which

changed from mainly lateral accretion to a predominantly

vertical growth with limited lateral construction.

The stacking pattern of major depositional sequences

and the associated shelf break migration reveals spatial

changes for much of the Pliocene and Quaternary. A sub-

siding area offshore the Guadalfeo River was laterally

bounded by uplifted sectors, caused by tectonic tilting, or

by the activity of fault-controlled basement highs. The

most recent period, represented by a relatively thick ag-

gradational unit composed of vertically stacked

progradational wedges, reveals a significant temporal

change of subsidence rates, probably coupled with the

onset of high amplitude, 100 k.y. glacio-eustatic fluctua-

tions after the Mid-Pleistocene Revolution.

Acknowledgements Seismic profiles used to execute this work

were collected in the framework of the project entitled ‘‘Mapa Ge-

ologico de la Plataforma Continental Espanola y Zonas Adyacentes’’

executed by the Spanish Geological Survey. F.J. Lobo received

support from the Ramon y Cajal program, 2004 call. We also

acknowledge the work of two anonymous reviewers and the Editor in

Chief Peter Clift, which provided very detailed and constructive

comments that helped to reshape and improve the initial version. This

paper is a contribution to IGCP project n8 526: Risks, Resources, and

Record of the Past on the Continental Shelf.

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