shelf of the Alboran Sea : the effects of hydrodynamic forcing and supply of sediment by rivers

the eastern shelf of the northern Alboran Sea margin. This study highlights the high spatial variability of sedimentary processes in a sediments with backscatter distributions extracted from multibeam echo sounder data; in addition, a wind-forced tidal hydrody! " # ! " ! $ ! % ! " ter intensities appear to be controlled at large by a major physiographic feature (the Carchuna Canyon) which separates a western # ! " & ! ' ' + 6 7 ' '! ISSN (print): 1698-6180. ISSN (online): 1886-7995 www.ucm.es /info/estratig/journal.htm Journal of Iberian Geology 37 (2) 2011: 195-214 doi: 10.5209/rev_JIGE.2011.v37.n2.8 196 = $ > $ 7 (% $ , ? BC D F # H" /


Introduction
Since the inception of multibeam echo sounders 0%(6 DV D SULPDU\ SURFHGXUH IRU VHDÀRRU PDSSLQJ and exploration of shallow environments, the analysis of backscatter strengths has been a valuable tool for obtain-LQJ PHDQLQJIXO LQIRUPDWLRQ DERXW VHDÀRRU VHGLPHQW GLVtribution, composition and geological processes (Glynn et al., 2008;Medialdea et al., 2008).By extension, it enables the determination of benthic habitats, the monitoring of natural and/or anthropogenic changes and the interpretation of dynamic characteristics of water masses (e.g., Kostylev et al., 2001;Glynn et al., 2008).
Backscatter intensity is modulated by the interaction of several parameters (Jackson et al., 1986;Gadner et al. 1991) such as surface and volume scattering, the angle of LQFLGHQFH RI WKH LQFRPLQJ DFRXVWLF ZDYH WR WKH VHDÀRRU and the acoustic impedance contrast of the sub bottom sediment interfaces.The effect of the referred parameters on the backscatter data in order to obtain quantitative in-IRUPDWLRQ DERXW WKH VHDÀRRU PDWHULDOV KDV EHHQ VWXGLHG E\ a number of authors (Jackson et al., 1986;Gadner et al., 1991;Mitchell, 1991Mitchell, , 1993;;1995).Within those factors, VXUIDFH VFDWWHULQJ LV SDUWLFXODUO\ VLJQL¿FDQW IRU UHWULHYLQJ JHRORJLFDO DQG HQYLURQPHQWDO LQIRUPDWLRQ IURP VHDÀRRU data, as it is mainly dependant on surface roughness, which may be produced by a number of sedimentological, physiographic and biological properties (Collier and Brown, 2005).Sediment-related factors would include grain size, density, composition and sediment structures (Collier and Brown, 2005;De Falco et al., 2010).Physi-RJUDSKLF IDFWRUV GHSHQG RQ VHDÀRRU URXJKQHVV JHQHUDWHG by rock outcrops or small-scale bedforms (Glynn et al., 2008).Biological factors comprise bioturbation and the presence of benthic organisms (De Falco et al., 2010).
Shallow shelves represent one of the environments where backscatter variability can be more directly linked to grain size, due to their low bathymetric changes.As thus, most of the approaches acknowledge that seabed properties can be extracted from digital backscatter/side scan sonar data, while accompanied by ground-truth data tion has been found to be particularly robust in the case of well-sorted sediments, as poorly-sorted sediments show more variable backscatter responses (Goff et al., 2000;Collier and Brown, 2005).Disturbing effects on the expected grain size-backscatter correlation may be caused by a number of physiographic and biological factors.For example, the presence of small-scale bedforms and bioturbation may increase surface roughness (Glynn et al.DV D FRQVHTXHQFH ¿QHJUDLQHG DUHDV PD\ be characterized with high backscatter (Urgeles et al., 2002).The presence of benthic organism may also be VXFK DV VXU¿FLDO VHGLPHQW VDPSOHV .RVW\OHY et al., 2001;Lathrop et al., 2006;Glynn et al., 2008;Brown et al., 2011).In

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The study area is located in the northern margin of the Alboran Sea, western Mediterranean Sea, comprising the shelf stretch between Torre del Mar and Balerma towns (Fig. 1).The Alboran Basin is located along the Eurasia-Africa plate boundary, and it is surrounded by the Betic-Rif Arc.The Alboran Sea was structured during the the Oligocene-late Miocene through a number of rifting stages (Comas et al., 1992).Since the Tortonian, however, the basin has been subjected to convergence between Eurasia and Africa.Most of the previous structures have been reactivated as strike-slip faults (Campillo et al., 1992;Estrada et al., 1997).
The shelf of the northern Alboran Sea margin is narrow (several kilometers wide), although it may reach locally more than 20 km in width off Málaga and Almería, due to the sediment supply of major rivers (Muñoz et al., 2008).The shelf break is located at a mean water depth of 110 P 9i]TXH] 7KH PRVW VLJQL¿FDQW LQQHU VKHOI PRUphologies include prodeltaic bodies in front of the main ÀXYLDO LQSXWV DQG LQIUDOLWWRUDO SURJUDGLQJ ZHGJHV ,3:V ODWHUDOO\ IURP WKH PDLQ ÀXYLDO HQWULHV :LGH VHJPHQWV RI the outer shelf are covered either by sand ridges or by erosional and/or tectonic morphologies such as abrasion surfaces, submarine terraces and escarpments (Hernández-Molina et al., 1994, 1996;Vázquez, 2005;Lobo et al. 7KH PRVW VLJQL¿FDQW HURVLYH IHDWXUH LQ WKH study area is the Carchuna Canyon head, which is located at less than 500 m from the coastline off Cape Sacratif and cuts the entire shelf with a main N-S trend, showing several distributaries (Lobo et al., 2006).

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Tides are determined by the entrance of Atlantic waters across the Strait of Gibraltar with a main semidiurnal periodicity (Parrilla and Kinder, 1987).Dominant wind directions alternate from the west and from the east, generating a low-to-moderate wave climate (Puertos-del-Estado, 2007).
The location of the Alboran Sea to the east of the Strait RI *LEUDOWDU JUHDWO\ LQÀXHQFHV LWV RFHDQRJUDSK\ DV VXUface current patterns are controlled by the entrance of $WODQWLF ZDWHUV GHVLJQHG HLWKHU WKH $WODQWLF ,QÀRZ $, or Surface Atlantic Water (SAW) through the strait with an estimated speed of 1 m s -1 .The AI is mixed with vari- able amounts of water masses of Mediterranean origin generating the Atlantic Jet (AJ) that feeds two anticyclonic gyres, the quasi-permanent Western Alboran Gyre (WAG) and the more elusive Eastern Alboran Gyre (EAG) (Tintoré et al., 1988;Perkins et al., 1990).The AJ originates a strong thermohaline front between cold and dense Mediterranean waters to the left and Atlantic waters to the right of the jet (Vargas-Yáñez and Sabatés, 2007).

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In deltaic coastlines of the northern Alboran Sea margin most sediments are supplied via relatively short, mountainous rivers and streams.In the study area, the rivers DQG VWUHDPV FDQ EH FODVVL¿HG DFFRUGLQJ WR VHYHUDO SDUDPeters, such as basinal areas, river lengths and slopes, water discharges and sediment loads, in the following types (Fig. 1): (a) major rivers (Guadalfeo and Adra); (b) minor rivers (Güí, Torrox, Chillar, Jate, Seco and Verde), which are mostly located in the western part of the study area; (c) discontinuous streams (Gualchos, Haza de Trigo, Albuñol DQG +XDUHD ZKLFK GR QRW KDYH D SHUPDQHQW ÀRZ DQG DUH preferentially located to the east of the Carchuna Canyon (Table 1).Overall, mean discharges decrease from west to east, but in contrast mean sediment loads and yields tend to increase to the east, indicative of an increasing torrential character (Liquete et al., 2005).This supply pattern is conditioned by: (a) the abrupt coastal physiography, as the Betic Mountains occur at short distance from the coast, and in several places show recent tectonic activity OHDGLQJ WR VWHHSHQLQJ RI SK\VLRJUDSKLF SUR¿OHV DQG ULYHU incision (Carvajal and Sanz de Galdeano, 2008); (b) the Mediterranean climate, with increasing aridity toward the east.Due to the torrential character, most of the rivers are very effective in transporting sediments from the drainage basins toward the shelf and eventually into deeper water (Liquete et al., 2005).
Sedimentation is siliciclastic in the western and central parts of the northern shelf of the Alboran Sea.Siliciclastic sediments are mainly derived from suspended load deposition, developing muddy prodeltaic facies ZLWK FRDUVHQLQJ¿QLQJ XSZDUG VHTXHQFHV DQG YDULDEOH amounts of sands (Ercilla et al., 1994).The sediments are WUDQVSRUWHG ODWHUDOO\ HLWKHU E\ OLWWRUDO GULIW RU E\ $, LQÀXence.Postglacial relict sediments composed of reworked sands and gravels carpet the outer shelf (Ercilla et al., 1994;Hernández-Molina et al., 1994).Spillover facies mainly composed by sands tend to occur in the proximity of the shelf break; these facies are related to reworking and resuspension by storm currents and by upper slope gravitational processes (Ercilla et al. 1994).

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Backscatter data were collected by using a 300 kHz Simrad EM3000D MBES during several geological surveys (ESPACE01 and ESPACE02).The EM3000D echosounder generates 254 beams (1.5°×1.5°wide, 0.9° spacing) that includes bathymetric information in a ten times water depth wide band, with a maximum ping rate of 25 Hz.Backscatter data were processed with the Neptune™ VRIWZDUH DW D [ P JULG 7KH JULG ZDV ¿OWHUHG E\ DSSO\-LQJ DQ $UF*,6 VRIWZDUH VWDWLVWLFDO ¿OWHU WKDW FDOFXODWHG the mean backscatter value each 180 m, assuming (1) this value as the horizontal MBES resolution and (2) the application of a perpendicular angle to the direction of the boat during data acquisition.The backscatter intensity has been represented by a colour scale, ranging from blue (high intensity) to red (low intensity).
%DFNVFDWWHU FODVVL¿FDWLRQ ZDV SHUIRUPHG WKURXJK WKH establishment of intervals of intensity values by using the ArcGIS™ software.Those intervals were generated E\ XVLQJ WKH -HQNV 1DWXUDO %UHDNV &ODVVL¿FDWLRQ RU 2S-WLPL]DWLRQ V\VWHP ZKLFK LV D GDWD FODVVL¿FDWLRQ PHWKRG designed to optimize the arrangement of a set of values into natural classes, as the method reduces the variance within classes and maximizes the variance between classes (Jenks, 1967;McMaster, 1997).In our case, the acoustic data are not completely quantitative, due to the lack of correction of the different controlling parameters (Jackson et al., 1986;Gadner et al. 1991).This study is mainly based on the data correlation obtained with different techniques instead of detailed backscatter data analysis.

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To reproduce shelf hydrodynamic conditions, a number RI QXPHULFDO H[SHULPHQWV ZHUH SHUIRUPHG E\ XVLQJ D ¿nite volume shallow-water hydrodynamical model (Castro el at., 2006).The numerical model was run with initial calm conditions by imposing variable in time and constant in space winds at the sea surface and tidal forcing at the open sea boundaries.Sea surface elevation and the WZR KRUL]RQWDO FRPSRQHQWV RI WKH PHDQ ÀRZ YHORFLW\ were obtained.
Wind forcing derives from observational data of the WANA station located off the Adra River (Puertos del Estado, 2007) (Fig. 2).The WANA data set is formed by time series of wind and wave parameters generated from numerical modeling.In order to run the model, two periods with contrasting wind conditions were selected: a 61-day period of prevailing easterly winds starting at 14 th February 2003 and a 60-day period of prevailing westerly winds starting at 1 st October 2003 (Fig. 3).Within  2007) implementados en el modelo, considerando dos periodos con vientos dominantes de direcciones opuestas (a la izquierda, periodo con vientos dominantes de levante; a la derecha periodo con vientos dominantes de poniente).
each period, we focus on hydrodynamic conditions related to high-energy storm events (wind velocities higher than 6 m s -1 ).To produce a smooth transition from daily observed data, model imposed winds were continuously interpolated in time.
At the open sea boundaries of the computational domain, boundary conditions that simulate tidal elevation are imposed as: the water depth at the steady-state solution at this point, A n (x B , y B ) the amplitudes, Z n the frequencies, and a n the phases of the four tidal components considered (M2, S2, O1 and K1).The total phase a n is decomposed into a n =g n + V n (t 0 ), with g n the Greenwich phase and V n (t 0 ) the astronomical argument.The latter is used to initiate the forcing at a given date.Amplitudes, frequencies and Greenwich phases were extracted from FES2004 (Lyard et al., 2006).Astronomical phases were computed in order to initiate the tidal forcing at the starting dates of each of the two simulations (02/14/2003 and 01/10/2003) to produce more realistic and coherent results.where d is the median grain diameter, z is the height above the sea bed (in this study = 1 m) and u z is the current velocity at a height of z meters above the sea bed, GH¿QHG E\ 6RXOVE\ DV for 0 ] K, where u is the depth-average current speed obtained by the wind-forced tidal hydrodynamic model and h is the water depth.

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Seven sediment types have been found in the study area: sandy gravel, muddy sandy gravel, gravely sand, sand, gravely muddy sand, muddy sand and sandy mud (Fig. 4).The most common sediment type is muddy sand (Fig. 4), with sand contents > 70% over most of the study area (Fig. 5b).In shallow waters (< 25 m), the sediments are mainly composed by sandy gravels and sands (Fig. 4).From 25 m water depth to the shelf break (> 100 m) all the granulometric classes are present, although muddy sandy gravels and sandy gravels show very limited occurrence (Fig. 4).As a general rule, the sediments of the study area show seaward decreasing gravel contents and seaward increasing mud contents (Fig. 5).As there are VLJQL¿FDQW DORQJVKHOI FKDQJHV RI VXU¿FLDO VHGLPHQWV WKH study area was subdivided in two sectors (western and eastern) limited by the Carchuna Canyon (Fig. 4).

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The dominance of muddy sands is particularly evident in this sector, where a seaward gradation of sediment sizes is usually found (Fig. 4a).Muddy sands show sand percentages generally higher than 50% and mud percentages are below 40% (Figs.5a, 5b).
7KHUH DUH VKHOI ]RQHV RI ¿QHU RU FRDUVHU JUDLQ VL]HV of limited extend.A patch of sandy mud with mud percentages higher than 50% occurs between the Verde and Guadalfeo rivers (Fig. 4a).The zones with coarser grain sizes occur: (a) off the Torrox River, where gravelly sands extend across the shelf with sand contents higher than 80% and gravel contents below 15%; (b) off the Chillar River, where sands form a cross-shelf strip extending from the river mouth to the shelf break; (c) over the infralittoral between the Chillar and Verde rivers and between the Motril Port and the Carchuna Canyon (Figs. 4a, 5), where sandy gravels show maximum gravel percentages higher than 40%.

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The shelf is characterized by higher grain sizes to the east of the Carchuna Canyon, as gravelly sands and sands occur over widespread zones (Fig. 4b), although muddy sands are also widely distributed.Mud contents are considerably lower than in the western sector, showing in many zones percentages lower than 15% (Fig. 5c).
Gravelly sands with sand contents higher than 70% and gravel contents below 25% cover extensive mid to outer shelf zones between the Carchuna Canyon and the Haza de Trigo Stream (Figs. 4b,5a,5b).
Muddy sands cover inner shelf zones to the east of Calahonda town, extending over the entire shelf with mud percentages up to 50 % between the Haza de Trigo and Albuñol streams.Muddy sands are restricted to mid-shelf water depths off the Huarea Stream, being substituted to the east by a small patch of sandy muds with mud percentages higher than 50% (Figs.4b, 5c).
Most of the shelf to the east of the Huarea Stream is covered by coarser sediments, such as sands (with sand contents higher than 80%), gravelly sands (with sand contents higher than 70% and gravel contents below 25%) and small isolated patches of sandy gravels (with gravel contents higher than 40%) located over the topsets of the Adra River prodelta and in the eastern termination of the study area (Figs. 4b,5).Finer sediments occur off the Adra River where a muddy sand patch covers the prodeltaic protuberance (Fig. 4b).
The correlation between backscatter intensity and mean grain size is depicted in a scatter plot (Fig. 6).The arithmetic mean was used to calculate the mean grain size of WKH VXU¿FLDO VHGLPHQWV where AM represents the arithmetic mean of the grain size, n the granulometric range number, CM the expres- where Ls and Li are the upper and lower limits of grain size of interval respectively, and f i the relative frequency of the class i.
A positive correlation (R 2 =0.63) between backscatter intensity and mean grain size is found (Fig. 7).This correlation is given by the function BS = 9.73 Ln(AM) -21.43.
Based on the sediment size-backscatter correlation, we have characterized the distribution patterns of backscatter intensities (high, medium and low) and their associated sediment sizes (Table 2).higher than 75 m.However, some distal areas may show low backscatter strengths, such as the stretches between the Chillar and Jate rivers, between the Seco River and the Carchuna Canyon and between the Haza de Trigo and Huarea streams.

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In accordance with the grain size distribution, we also depict the backscatter distribution in both sectors (western and eastern) (Fig. 6).

Western sector
The western sector is mainly covered by low-to-medi-XP UHÀHFWLYLW\ YDOXHV DOWKRXJK LQ VKDOORZ ZDWHU m) high and medium backscatter zones alternate laterally )LJ D D 7KH PRVW VLJQL¿FDQW VWUHWFK RI KLJK EDFNscatter in shallow water occurs between the Motril Port and the Carchuna Canyon.
In deeper water (> 25 m) a low backscatter patch occurs on the inner shelf in the western termination of the study area.To the east, a clear reduction of backscatter values is observed.The shelf between the Güi and Chillar rivers mostly exhibits medium backscatter values, with a major high backscatter patch off the Torrox River.The stretch between the Chillar and Seco rivers mostly displays medium backscatter values, with two elongated FRDVWSDUDOOHO ORZ UHÀHFWLYLW\ SDWFKHV VHSDUDWHG E\ D QDU-URZ PHGLXPUHÀHFWLYLW\ EDQG H[WHQGLQJ IURP WKH &KLOODU to Jate rivers.The shelf sector between the Seco River and the Carchuna Canyon is covered by an extensive low UHÀHFWLYLW\ SDWFK

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The across-shelf backscatter distribution shows that shallow waters (< 25 m) are covered by high and medium backscatter strengths (Fig. 6).Mid-shelf water depths between 25-75 m mostly exhibit irregular patches of medium and low backscatter.Some exceptions to this general trend are: (a) the stretch between the Verde River DQG WKH &DUFKXQD &DQ\RQ ZKHUH ORZ UHÀHFWLYLW\ YDOXHV are observed in 25-75 m water depths and (b) the eastern part of the study area shows high to medium backscatter values in 25-75 m water depths (Fig. 6).Medium to high backscatter values are usually found at water depths

Current patterns and bed shear stress values
Simulation results depict the distribution maps of depth-averaged current velocities matched with current direction vectors and of bed shear stress for two endmember situations (easterlies and westerlies dominance).Although wind speeds are similar for both main wind directions, both depth-averaged current velocity and bed shear-stress values are higher under easterlies dominance.Across-shelf, the maximum current velocity and bed shear-stress values occur up to 25 m water depth; basinward, the shelf break establishes a net boundary ZLWQHVVLQJ D VLJQL¿FDQW FXUUHQW YHORFLW\ GHFUHDVH )LJV 8, 9).Along-shelf, higher values of both variables occur in the eastern sector.

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The most common depth-averaged current velocities on the western sector are lower than 0.1 m s -1 .Maximum depth-averaged currents velocities (> 0.2 m s -1 ) usually occurs at water depths less than 25 m.On average, shelf current velocities are higher under easterlies dominance.
Maximum current velocities during both easterlies and westerlies dominance occur between Torrox and Chillar rivers, with velocities higher than 0.2 m s -1 in the nearshore (Fig. 8-a1).Under easterlies dominance, shelf sectors between the Seco and Verde rivers and the Guadalfeo River and the Carchuna Canyon are also affected by relatively high current velocity values (above 0.1 m s -1 ) (Fig. 8-a1).During easterlies/westerlies dominance, the minimum velocity values can be observed in the westernmost part of the study area (< 0.03 m s -1 ) and in the shelf sector between the Verde and Guadalfeo rivers (Fig. 8-a1, a2).
The shelf between the Haza de Trigo Stream and the shelf widening to the west of Adra shows the less energetic current conditions in the eastern sector.There, maximum current velocities (above 0.2 m s -1 ) occur at water depths less than 25 m.The rest of the shelf exhibits much lower current velocities (around 0.1 m s -1 ), with alternating directions according to wind dominance (Fig. 8-b1, b2).
The easternmost shelf shows the most energetic current conditions in the study area, as most of the shelf is affected by current velocities around 0.2 m s -1 during easterlies and westerlies dominance b2).During easterlies dominance, velocity values near 0.2 m s -1 can be found up to the shelf break (Fig. 8-b1).During westerlies dominance, current velocities are slightly less intense, as  , 2008).There, sediment mobilization is considered to be mainly led by wave energy, resulting in across-shelf grain-size gradations.In the study area, a laterally continuous coarse-grained (sandy gravels and sands) and high backscatter infralitttoral environment DERYH P ZDWHU GHSWK LV ERXQGHG VHDZDUG E\ DQ RIÀDS break which is characteristic of shallow-water wedges in the northern margin of the Alboran Sea, such as prodeltas and IPWs (Hernández-Molina et al., 2000;Fernández-Salas, 2008).The proximal coarse-grained granulometry DQG KLJK EDFNVFDWWHU UHVSRQVH ZRXOG UHÀHFW WKH OHDGLQJ LQÀXHQFH RI VWRUPZHDWKHU K\GURG\QDPLF FRQGLWLRQV on the infralittoral, both during easterlies and westerlies dominance, with maximum current velocities (> 0.2 m s -1 ) and the highest values of bed shear stress (> 0.2 N m -2 ).This bed shear-stress value represents the threshold for motion of grains with diameters higher than 0.05 mm (Soulsby, 1997).Direct current measurements made off Marbella (located about 80 km to the west of the western boundary of the study area) indicated variable velocities lower than 0.3 m s -1 (Janopaul and Frisch, 1984).Those values are therefore consistent with our estimations in the study area (i.e., maximum current velocities around 0.2 m s -1 ).
In the study area, along-shelf sediment composi-WLRQ DQG EDFNVFDWWHU JUDGLHQWV DUH PRUH VLJQL¿FDQW 7KH DORQJVKHOI JUDGDWLRQ VHHPV WR EH GLFWDWHG DW ¿UVW E\ WKH Carchuna Canyon that dissects completely the shelf, SUHYHQWLQJ IURP VLJQL¿FDQW ODWHUDO VHGLPHQW WUDQVSRUW In addition, local sediment supplies combined with bidirectional currents resulting from the alternance of easterlies and westerlies also leave an imprint.Indeed, the study area receives the contribution of a number of small PRXQWDLQRXV ULYHUV ZKRVH PRVW VLJQL¿FDQW VHGLPHQWDWLRQ events occur during wet storm conditions.
The shelf located to the west of the Carchuna Canyon is dominated by the terrigenous sediment contributions of the major regional river (the Guadalfeo River), as well as the added contributions of several small rivers (Güí, maximum values near 0.2 m s -1 mainly occur on the inner shelf, whereas the outer shelf shows current velocity values below 0.15 m s -1 (Fig. 8-b2).
The distribution of bed shear stress in the eastern sector documents more energetic conditions affecting deeper areas during easterlies dominance (Fig. 9).The maximum values (>0.2 N m -2 ) occur at water depths less than 25 m.Along-shelf, the highest bed shear-stress values are found between the Carchuna Canyon and the Gualchos Stream and to the east of the Huarea Stream in the easternmost part of the study area (Fig. 9-a1, b1).
A positive correlation (R 2 EHWZHHQ VXU¿FLDO JUDLQ sizes and backscatter intensities was found in the study area, which is in agreement with the observations reported in other sandy-dominated shallow-water environments (Davis et al., 1996;Goff et al., 2000;Kostylev et al., 2001;Collier and Brown, 2005;Ferrini and Flood, 2006;Lathrop et al., 2006;De Falco et al., 2010).Thus, levels of backscatter (high, medium and low) broadly correspond with sandy granulometric classes, with variable amounts of other textures, such as gravels and muds.As a consequence, the lateral changes in backscatter mainly UHÀHFW WKH FKDQJHV LQ FRPSRVLWLRQ IURP JUDYHO WR PXG LQ D VDQG\ VHDÀRRU HQYLURQPHQW $FURVV DQG DORQJVKHOI VHGLPHQW and backscatter changes 3UHYLRXV DFFRXQWV RI WKH VXU¿FLDO VHGLPHQW GLVWULEXWLRQ of the northern shelf of the Alboran Sea are simple, as they consider muddy prodeltaic deposits and poorly-sorted sandy relict facies (Ercilla et al., 1994) (LWWUHLP et al., 2002).
Erosional sedimentary responses are more extensive DQG VLJQL¿FDQW DW WKH ORFDWLRQV ZKHUH VKHOI FXUUHQWV UHDFK the highest velocities, such as (1) the wide easternmost shelf of the study area (affected both by eastward and westward currents), (2) the shelf between the Carchuna &DQ\RQ DQG WKH +D]D GH 7ULJR 6WUHDP PDLQO\ LQÀXenced by westward currents, and (3) the shelf between WKH 7RUUR[ DQG &KLOODU ULYHUV PDLQO\ LQÀXHQFHG E\ HDVWward currents.In general, those high-energy shelf environments broadly occur off wide protuberances, where FXUUHQWVHDÀRRU LQWHUDFWLRQ LV HQKDQFHG b) Mixed-energy depositional environments.Those environments occur in sandy shelf sectors developing elongated along-shelf medium backscatter distributions that suggest linear sediment sources.In the California Shelf, similar elongated depocenters show higher lateral conti-QXLW\ UHYHDOLQJ D KLJKHU HI¿FLHQF\ RI DORQJVKHOI WUDQVport processes, as the majority of the along-shelf sediment transport is related with strong storm events (Ogston and Sternberg, 1999;Edwards, 2002) and coupling with ÀRRGLQJ HYHQWV 6RPPHU¿HOG DQG 1LWWURXHU ,Q WKH study area, however, those environments would be subjected to a less effective oceanographic dispersal system due to weak oceanographic conditions (with maximum velocity values below 0.15 m s -1 and bed shear-stress values below 0.2 N m -2 ) combined with moderate to low ÀXYLDO VXSSOLHV DV WKH\ WHQG WR RFFXU RII VPDOO ULYHUV EHtween the Chillar and Verde rivers) in the western sector and off ephemeral streams (between the Gualchos Stream DQG WKH $GUD 5LYHU LQ WKH HDVWHUQ VHFWRU 7KRVH ÀXYLDO systems exhibit drainage basins of tens of km 2 and river lengths between 8 and 25 km, with low sediment supplies (mean sediment load is estimated at 0.4 kg s -1 for the case of the Verde River) (Liquete et al., 2005).
The Adra River also develops a medium backscatter ¿QJHUSULQW LQ VSLWH RI WKH KLJKHU VL]H RI WKH FDWFKPHQW Torrox, Chillar, Jate, Seco and Verde rivers).As a con-VHTXHQFH VXU¿FLDO VKHOI VHGLPHQWV DUH GRPLQDWHG E\ muddy sands that generate medium to low backscatter intensities.The relationship between lower backscatter YDOXHV ZLWK ¿QHU VHGLPHQWV LV FRQVLVWHQW ZLWK WKH UHVXOWV of many studies (e.g., Goff et al., 2000Goff et al., , 2004;;Collier and Brown, 2005;Ferrini and Flood, 2006).In contrast, the shelf located to the east of the Carchuna Canyon is clearly dominated by the hydrodynamic regime, as shelf currents and bed shear-stress values are stronger than in the western sector, and terrigenous supplies are less important.The more intense oceanographic regime is related to the slight dominance of easterly-driven currents, leaving the western sector more protected to the dominant activity of ZHVWZDUG FXUUHQWV ,Q DGGLWLRQ WKH H[LVWHQFH RI D VLJQL¿cant shelf widening in the eastern part of the study area could also favour the interaction of shelf currents with the VHDÀRRU 2Q WKLV ZLGH VKHOI WKH FRQWULEXWLRQ RI WKH VHFond regional river (the Adra River) in terms of discharge would be moderate, as their sediment left over would be restricted to the inner shelf.The rest of the eastern shelf would only receive the contributions of ephemeral, sea-VRQDO VWUHDPV ZLWK GLVFRQWLQXRXV ÀRZV *XDOFKRV +D]D de Trigo, Albuñol and Huarea).

6PDOOVFDOH VHGLPHQW YDULDELOLW\ and backscatter zonation
$W D ¿QHU VFDOH WKH PDWFK IRXQG EHWZHHQ WKH VXU¿FLDO sediment composition and the backscatter intensity enables us to distinguish a high lateral variability of sea-ÀRRU VHGLPHQWDWLRQ DQG GLIIHUHQW GHSRVLWLRQDOHURVLRQDO shelf regimes with a distinctive backscatter response (high, medium and low).These relationships have been evidenced in estuarine settings (Nitsche et al., 2004), but most notably along sectors of the California Shelf, such as the Monterey Bay (Edwards, 2002 Bárcenas et al., 2009).Amongst all the deltaic systems in the study area, the Guadalfeo River represents the major regional source (basin area of more than 1,300 km 2 , river length of 72.5 km and mean sediment load of 2.7 km s -1 according to Liquete et al. (2005)).Here, relatively high current velocity and bed shear-stress values less than 0.1 N m -2 (particularly during easterlies dominance) are not able to ZLQQRZ WKH VHGLPHQWV DQG WR UHPRELOL]H WKH ¿QHU JUDLQ size fractions.Another factor to consider is the sheltering effect provided by physiographic boundaries such as the Carchuna Canyon and the landward coastal promontory, which would subdue the activity of oceanographic and storm-related processes, as evidenced in Monterey Bay (Edwards, 2002).

Conclusions
The distribution of surface sediments shows a good correlation with the backscatter response over a wide segment of the northern shelf of the Alboran Sea.High, intermediate and low backscatter responses broadly correspond with sandy sediments with variable amounts of accessory grain sizes (from gravels to muds in consonance with decreasing backscatter intensities).
7KH VXU¿FLDO VHGLPHQW GLVWULEXWLRQ RI WKH VKHOI XQGHU FRQVLGHUDWLRQ GLIIHUV IURP RWKHU VXU¿FLDO VHGLPHQW JUDGD-WLRQ SDWWHUQV DV DORQJVKHOI FKDQJHV DUH PRUH VLJQL¿FDQW that across-shelf.Off from the coast, the most consistent pattern is provided by a high backscatter infralittoral band of gravelly sands generated by storm conditions, with maximum current velocities (>0.2 m s -1 ) and the highest bed shear-stress (> 0.2 N m -2 ) occurring during both easterlies and westerlies dominance.Seaward, across-shelf FKDQJHV LQ VXU¿FLDO VHGLPHQWEDFNVFDWWHU SDWWHUQV DUH OHVV conspicuous.A lateral segmentation is evident, primarily conditioned by the Carchuna Canyon head dissecting the shelf but also by the interaction between an active, bi-GLUHFWLRQDO ÀRZ UHJLPH DQG ORFDO ÀXYLDO VXSSOLHV IURP mountainous, small rivers.The shelf sector to the west RI WKH FDQ\RQ LV GRPLQDWHG E\ WKH FRQWULEXWLRQ RI ÀXYLDO systems; in contrast, the shelf sector to the east of the can-\RQ LV PDLQO\ LQÀXHQFHG E\ WKH K\GURG\QDPLF UHJLPH At a small scale, the backscatter response was indicative of variable depositional/erosional shelf regimes.7KUHH PDLQ HQYLURQPHQWV ZHUH GH¿QHG VWRUPGRPLQDW-HG PL[HG DQG ÀXYLDOO\ GRPLQDWHG 0D[LPXP FXUUHQW velocity and bed shear-stress values show a very high correlation with gravelly sands and high backscatter intensities, thus strongly controlling the location of storm-GRPLQDWHG HQYLURQPHQWV 0L[HG DQG ÀXYLDOO\GRPLQDWHG HQYLURQPHQWV RFFXU LQ UHVSRQVH WR WKH FRPSHWLQJ LQÀX-(i.e., the drainage basin extends for 750 km 2 , and the river OHQJWK LV NP DQG PRUH VLJQL¿FDQW VHGLPHQW FRQWULbution (the mean sediment load is 4.8 kg s -1 ).In addition, this river is particularly outstanding by its extremely high sediment yields (more than 200 t km-2 yr-1 ), attributed to an increase in regional erosion and torrential character to the east (Liquete et al., 2005).The fact that the Adra River generates a sediment-backscatter response very similar to that generated by much smaller rivers would LPSO\ WKDW WKH ÀXYLDO VXSSO\ LV REOLWHUDWHG LQ WKLV UHODWLYHly wide shelf segment by hydrodynamic forcing, as shelf bed shear stress attains its maximum values at this location.Those high current velocities could be conductive to the generation of gravity-driven, downslope transport RI UHODWLYHO\ ¿QH VHGLPHQWV DV GRFXPHQWHG LQ QXPHURXV deltaic settings, like the shelf off the Eel River, northern California (Wright et al., 2001).This downslope trans-SRUW RI VHGLPHQW PD\ FRQVWLWXWH D VLJQL¿FDQW PHFKDQLVP of across-shelf transport during storms (Wright and Friedrichs, 2006).c) Fluvial-dominated depositional environments.Low backscatter intensities are mainly restricted to the west of the Carchuna Canyon, in relation with the dominance RI PXGG\ VDQGV 7KH SUHYDOHQFH RI WKRVH UHODWLYHO\ ¿QH grained facies across the shelf would indicate the main LQÀXHQFH RI ULYHUGHULYHG VHGLPHQW SOXPHV DV GRFXmented in several settings of the California margin, such as Monterey Bay (Edwards, 2002;Eittreim et al., 2002) and the Eel Shelf (Goff et al., 1999), and off the Waiapu River, New Zealand (Wadman and McNinch, 2008).Those shelves receive greater-than-normal sediment con-WULEXWLRQV EHFDXVH ÀXYLDO GHOLYHU\ LV PDGH WKURXJK VWHHS mountainous rivers (Storlazzi and Reid, 2010).
In comparison with the California Shelf, in the study DUHD ÀXYLDOO\GHULYHG GHSRVLWV WHQG VKRZ D FRDUVHU FRPposition with substantial amounts of sands.Although the physiographic settings are similar (i.e., small and steep drainage basins), a extreme torrential character for most of the rivers draining into the Alboran Sea would increase the availability of easily erodible materials, favouring the RFFXUUHQFH RI K\SHUS\FQDO ÀRZV /LTXHWH et al., 2005;Urgeles et al., 2011).

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LJ 3HUFHQWDJH GLVWULEXWLRQ PDSV RI JUDYHO D VDQG E DQG PXG F RI VXU¿FLDO VHGLPHQWV LQ WKH VWXG\ DUHD )OXYLDO V\VWHPV DUH LQGLcated by numbers 1-12.See location in Fig. 1. )LJ &DUWRJUDItDV GHO FRQWHQLGR HQ JUDYD D DUHQD E \ IDQJR F GH ORV VHGLPHQWRV VXSHU¿FLDOHV HQ OD ]RQD GH HVWXGLR /RV VLVWHPDV ÀXYLDOHV VH LQGLFDQ FRQ ORV Q~PHURV GHO 9HU ORFDOL]DFLyQ HQ OD )LJ sive high backscatter patch occurs to the east of the Adra River, covering the inner shelf up to 50 m water depth.In deeper mid-to-outer shelf water depths, zones with high backscatter occurs off the Gualchos Stream, between 50 and 75 m water depth, and in the easternmost part of the study area at water depths higher than 70 m.In the transitional sector between the Gualchos Stream and the Adra River, there are two main low backscatter bands trending W-E and surrounded by medium backscatter values; a shallower, relatively continuous but irregular band occurs at 50-75 m water depth, and a more discontinuous band occurs at around 100 m water depth.