Spatial variability of prodeltaic undulations of the Guadalfeo River prodelta: New clues for their...

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ORIGINAL RESEARCH PAPER1

2 Spatial variability of prodeltaic undulations on the Guadalfeo3 River prodelta: support to the genetic interpretation4 as hyperpycnal flow deposits

5 F. J. Lobo • J. A. Goff • I. Mendes •

6 P. Barcenas • L. M. Fernandez-Salas •

7 W. Martın-Rosales • J. Macıas • V. Dıaz del Rıo

8 Received: 22 April 2014 / Accepted: 8 August 20149 ! Springer Science+Business Media Dordrecht 2014

10 Abstract Two fields of prodeltaic undulations located off

11 the Guadalfeo River were studied by integrating surficial

12 (multibeam bathymetry, backscatter, sediment samples)

13 and sub-surface (seismic profiles, sediment cores) data. Our

14 main motivation was to analyze the along- and across-shelf

15 variability of the seafloor undulations, in order to obtain

16 useful insights into genetic mechanisms. A geostatistical

17 analysis was performed, based on the determination of

18 characteristic parameters and derived relationships. The

19 undulations occur over a concave-upward surface which

20 shows a seaward-decreasing slope. Most of the undulations

21 are symmetrical to asymmetrically-oriented toward the

22 coast. Two main fields are correlated with the present and

23 previous river mouths. The western field, associated with

24 the modern river mouth, is highly symmetrical, with the

25 higher undulations in an axial position and diminishing the

26 width/height relationship both laterally and downslope. In

27 contrast, the eastern field, associated with an historic river

28 mouth, shows lower-amplitude undulations, the width/

29height changes are less pronounced, and the undulations

30are more elongated. The two undulation fields exhibit

31subseafloor reflections that are subparallel to the seafloor,

32with peaks that migrate upslope upward in the stratigraphic

33column and which appear to correlate with coarse-grained

34layers. We support the contention that prodeltaic undula-

35tions off the Guadalfeo River should be regarded as sedi-

36ment waves. Assuming a sediment-wave process, a strong

37normal-to-contour sediment flows with a riverine origin

38(e.g., hyperpycnal flows) may have been active during

39undulation generation. Both morphometric parameters of

40the river basin and estimations of sediment concentration

41during exceptional flood events are in agreement with an

42episodic activity of high freshwater discharges. Most of the

43geomorphic parameters and stratigraphic observations

44indicate a change of sediment supply conditions related to

45the change in river mouth position, attributed to a temporal

46change in the activity of hyperpycnal flows. 47

A1 F. J. Lobo (&)A2 Instituto Andaluz de Ciencias de la Tierra, CSIC-Universidad deA3 Granada, Avenida de las Palmeras no. 4, 18100, Armilla, SpainA4 e-mail: [email protected]

A5 J. A. GoffA6 Institute for Geophysics, Jackson School of Geosciences,A7 University of Texas at Austin, Austin, TX 78758, USA

A8 I. MendesA9 CIMA, Universidade do Algarve, Edifıcio 7, Campus de

A10 Gambelas, 8005-139 Faro, Portugal

A11 P. Barcenas ! J. MacıasA12 Departamento de Analisis Matematico, Facultad de Ciencias,A13 Universidad de Malaga, Campus de Teatinos s/n, 29080 Malaga,A14 Spain

A15 L. M. Fernandez-SalasA16 Instituto Espanol de Oceanografıa-Centro Oceanografico deA17 Cadiz, Muelle de Levante s/n, Apdo. 2609, 11006 Cadiz, Spain

A18 W. Martın-RosalesA19 Departamento de Geodinamica, Facultad de Ciencias,A20 Universidad de Granada, Avenida de Fuentenueva s/n,A21 18002 Granada, Spain

A22 V. Dıaz del RıoA23 Instituto Espanol de Oceanografıa-Centro Oceanografico deA24 Malaga, Puerto Pesquero s/n, 29640 Fuengirola, Spain

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123Journal : Large 11001 Dispatch : 13-8-2014 Pages : 25

Article No. : 9233h LE h TYPESET

MS Code : MARI-D-14-00037 h CP h DISK4 4

Mar Geophys Res

DOI 10.1007/s11001-014-9233-9

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48 Keywords Alboran Sea ! Guadalfeo River prodelta !49 Submarine undulations ! Multibeam bathymetry !50 Geostatistics

51 Introduction

52 Seafloor undulations, or crenulations, are the most signifi-

53 cant morphosedimentary structures built up upon Medi-

54 terranean prodeltaic wedges. These features were initially

55 recognized through the interpretation of high-resolution

56 seismic records and short sediment cores, thus with a

57 limited ability to visualize their spatial extent. The later

58 advancement of multibeam imagery, with complete cov-

59 erage of some shallow-water settings, has revealed that

60 seafloor undulations extend over widespread areas. Such

61 features are conspicuous in many Mediterranean prodeltaic

62 settings (Urgeles et al. 2011), such as: (1) southern

63 (Fernandez-Salas et al. 2007; Barcenas et al. 2009) and

64 north-eastern Iberia (Dıaz and Ercilla 1993; Urgeles et al.

65 2007); (2) the Tyrrhenian Sea (Trincardi and Normark

66 1988; Bellotti et al. 1994; Budillon et al. 2005; Sacchi et al.

67 2005, 2009; Milia et al. 2008); (3) the Adriatic Sea, where

68 they have been extensively reported occurring along mud-

69 dominated coastal wedges, even in areas lacking a direct

70 fluvial source (Correggiari et al. 2001; Cattaneo et al. 2004;

71 Marsset et al. 2004; Berndt et al. 2006; Sultan et al. 2008);

72 and (4) the Hellenic Arc (Lykousis 1991; Hasiotis et al.

73 2006; Lykousis et al. 2009). In addition, equivalent mor-

74 phologies have been reported to occur in deeper water,

75 such as the Gulf of Taranto (Rebesco et al. 2009) and the

76 Caspian Sea (Levchenko and Roslyakov 2010).

77 Undulations imaged in multibeam surveys show dis-

78 tinctive dimensions, with wavelengths up to 300 m, heights

79 up to 5 m and lateral extensions of several kilometers

80 (Cattaneo et al. 2004; Urgeles et al. 2007, 2011). Seismi-

81 cally, they have been depicted as the surficial expression of

82 plane-parallel to discontinuous seismic facies on steep

83 slopes, generally related to basal detachment layers, high

84 and/or rapidly changing accumulation rates, and frequent

85 occurrence of gas within the sediments (Ercilla et al. 1994,

86 1995; Correggiari et al. 2001). Also, they usually show a

87 dominant muddy sediment composition (Trincardi and

88 Normark 1988; Dıaz and Ercilla 1993; Bellotti et al. 1994).

89 Most of the early and some recent interpretations favor a

90 deformational origin, as prodeltaic undulations were

91 regarded as gravity-induced, lowly-displaced instabilities

92 such as syn-sedimentary creep (Dıaz and Ercilla 1993;

93 Bellotti et al. 1994; Correggiari et al. 2001; Budillon et al.

94 2005; Sacchi et al. 2005) or rotated block slumps (Hasiotis

95 et al. 2006; Lykousis et al. 2003, 2009). Other recent

96 interpretations support a composite origin (depositional-

97 deformational), although with a dominant morphogenetic

98role of a sediment wave-forming process (Lee et al. 2002;

99Berndt et al. 2006). For example, undulations in shallow-

100water sectors of the Adriatic Sea are considered to result

101from the growth of sediment waves under the action of

102bottom flows sweeping an irregular sea-floor, due to early

103sediment deformation processes that controlled relief

104growth (Cattaneo et al. 2004; Marsset et al. 2004; Berndt

105et al. 2006; Sultan et al. 2008). In other cases there is no

106strong evidence of the occurrence of gravitational insta-

107bilities or deformation processes, and prodeltaic undula-

108tions are interpreted as the result of: (1) fluvially-derived

109sediment flows (e.g., hyperpycnal flows) normal to bathy-

110metric contours (Lee et al. 2002; Urgeles et al. 2007; Milia

111et al. 2008; Rebesco et al. 2009); or (2) sediment resus-

112pension by internal waves or bottom current activity

113(Trincardi and Normark 1988; Urgeles et al. 2011).

114The collection of extensive multibeam coverage has

115enabled the identification of seafloor undulations on the

116shelves of southern Iberian Peninsula to the east of the

117Strait of Gibraltar similar in dimensions to those reported

118in other Mediterranean settings (Fernandez-Salas et al.

1192007). Initial observations revealed that the most signifi-

120cant undulation field occurs over the Guadalfeo River

121prodeltaic body, a sediment wedge occupying most of the

122shelf in the northern margin of the Alboran Sea. Fernandez-

123Salas et al. (2007) discovered the existence of two main

124zones of undulations, referred to as eastern and western

125fields, where the undulation are most developed and which

126also appeared to correlate with the main river outlets

127(Fig. 1). Taking into account the morphobathymetric

128measurements and additional seismic evidence, it was

129initially hypothesized that the Guadalfeo prodeltaic undu-

130lations were generated by strong sediment flows emanating

131from the river mouths, either present or ancient (Fernan-

132dez-Salas et al. 2007).

133In this paper we present the results of an analysis of

134seafloor undulations in the prodeltaic area of the Guadalfeo

135River (Fig. 1). Our approach will be more comprehensive

136than previous analysis of prodeltaic undulations, most of

137which utilized only a limited number of bathymetric sec-

138tions, and which did not examine along- and across-slope

139morphological variability. We utilize a geostatistical

140characterization that enables estimation of morphological

141parameters by inversion of second-order statistics (Goff

142and Jordan 1988, 1989a, b). This approach allows us to

143examine in detail the variations of small-scale submarine

144topography associated with the Guadalfeo River undulation

145fields, both along- and across-slope. We combine the

146bathymetric data with available surficial (backscatter,

147sediment samples) and sub-surficial information (seismic

148profiles, sediment cores). Our primary objective is to

149constrain possible genetic interpretations, by establishing

150diagnostic criteria linked either to creeps or to sediment

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151 waves, and to ascertain potential submarine geohazards in

152 this area. In particular, we seek to establish the potential

153 role played by hyperpycnal flows in undulation genesis by

154 integrating available information regarding the Guadalfeo

155 River drainage basin.

156 Regional setting

157 Terrestrial setting

158 Climate, hydrology and sediment supply

159 Alternating dry and wet periods have been documented for

160 the last 500 years in the southern Iberian Peninsula

161 (Rodrigo et al. 1999). Drastic climatic conditions increase

162 toward the east, where the catchments show a semi-arid

163 behavior (Liquete et al. 2005). As a consequence, water

164 discharges are low on an annual basis, but they exhibit a

165 marked seasonal variability and the drainage basins have

166 undergone numerous catastrophic floods in response to

167 torrential rains (Brazdil et al. 1999). Both changing cli-

168 matic conditions and variations in water discharge have

169 been linked to fluctuations of the North Atlantic Oscillation

170 (Rodrigo et al. 2000; Liquete et al. 2005). In addition, the

171 change of land use (trees were replaced by vineyards)

172 during the last centuries favored the reduction of the veg-

173 etation cover and soil erosion (Brazdil et al. 1999).

174 The Guadalfeo River is one of the longest (72.5 km) rivers

175 in the northern Alboran Sea margin, with a 1,312 km2 drain-

176 age basin that extends from altitudes higher than 3,000 m in

177 the nearby Sierra Nevada Mountains. Average river slope is

178 2.21", but slope values decrease from the hinterland to the

179 deltaic plain,where theyare\0.5". The upstreamcourse of the

180 river is controlled by E-W trending mountain ranges such as

181 Sierra Nevada, Sierra de Lujar and Sierra de la Contraviesa

182(Fig. 1a). The emerged deltaic area of the fluvial system

183covers 8.6 km2, with coarse-grained sediments ranging from

184medium sands to boulders generating a 6 km long deltaic

185coastline (Jabaloy-Sanchez et al. 2014) (Fig. 1b).

186Average rainfall is *550 mm/year in the drainage basin,

187although with contrasting patterns in the deltaic plain and the

188highlands (Jimenez-Sanchez et al. 2008). In the coastal

189domain, average rainfall is *400 mm/year and the pluvio-

190metric regime is semi-arid. In the mountains, however, aver-

191age rainfall is[1,000 mm/year, frequently as snow (Jimenez-

192Sanchez et al. 2008). However, very high rainfall values have

193been recorded in the Guadalfeo River drainage basin during

194episodic events. For example, values of 400–600 mm/day

195were recorded during a storm event in 1973 in different

196locations of the basin. Those constitute the most intense pre-

197cipitation events per day recorded in the Iberian Peninsula.

198Water discharge is controlled by snow contribution in

199the upper lands, as well as by strong spatial and temporal

200variability of rainfalls. The average water discharge for the

201period 1942–2000 is 0.6 m3/s. Maximum monthly dis-

202charges ([0.8 m3/s) occur from December to May, and the

203rest of the year the discharge decreases substantially

204(Liquete et al. 2005). Two dams constructed in 1986

205(Beznar) and 2003 (Rules) regulate 85 % of the total run-

206off. At Rules, the estimated average water discharge since

207the opening of the dam in 2007 is 2.86 m3/s, although

208values of an order of magnitude higher (23 m3/s) have been

209measured in response to precipitation events higher than

21060 mm/day. The river has a high capacity of hydraulic

211erosion and sediment transport due to the steep basin

212physiography and low vegetation density. Mean sediment

213load is 2.7 kg/s, and the mean sediment yield is 65.1 t/

214km29 year (Liquete et al. 2005).

215Recent coastline changes

216The river mouth and lowermost course of the Guadalfeo

217River have undergone modifications that have influenced

218coastline evolutionary patterns (Rodrıguez-Berzosa and De

219la Pena 1994) (Fig. 2). Two deltaic distributaries (the pres-

220ent-day river channel and a channel located to the east of the

221present-day position) have been active during historic times.

222A general drawing of the coastal domain around the river

223mouth dated in 1932 places the river mouth in the eastern

224channel (Almagro 1932). A geographic map dated in 1940

225depicts the lowermost river course in its present-day position

226(Fig. 2). Therefore, we infer that the eastern channel became

227inactive during the 1930s. This significant change largely

228modified the coastline evolution, as it promoted a significant

229coastal advance of thewestern part of the deltaic system. The

230coastal segment between the historic course and the Motril

231Port also experienced high progradation rates compared to

232accumulation driven by east–west sediment flux distribution

bFig. 1 Geographical setting of the study area, located in the northernmargin of the Alboran Sea, western Mediterranean Basin. a TheGuadalfeo River drainage basin shows an abrupt, steep morphology,dictated by nearby elevations. Two major dams (Beznar and Rulesdams) have regulated the water and sediment production of thedrainage basin in the recent past. Onland shaded relief generated froma 5 m resolution topographic grid extracted from the ‘‘Base Topog-rafica Nacional’’. Bathymetric image generated from a 50 m resolu-tion grid extracted from multibeam bathymetric data from the‘‘Instituto Espanol de Oceanografıa’’ database. Artificial illuminationis from the northwest. Water depth contours are given in meters.b Location of sediment samples, sediment cores and seismic profiles,superimposed on the multibeam bathymetric data covering the shelfdomain. Onland shaded reliefs generated from a 5 m resolutiontopographic grid extracted from the ‘‘Base Topografica Nacional’’.Bathymetric data derived from the 5 m resolution grid generated frommultibeam bathymetric data collected within the ESPACE project.Artificial illumination is from the northwest. Water depth contours aregiven in meters

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233 (Jabaloy-Sanchez et al. 2014) (Fig. 2). The most recent

234 evolution since the 1980s to the present has been dominated

235 by a large decrease of the sediment supply, due to a number

236 of anthropic activities, such as dam construction (Beznar and

237 Rules dams), channel and margin protection, arid extraction

238 and increased agricultural exploitation of the drainage basin

239 (Rodrıguez-Berzosa and De la Pena 1994). As a conse-

240 quence, the Guadalfeo River coastline has been almost sta-

241 tionary during this recent most interval, with slow erosion

242 around the ancient river mouth and accretion around the new

243 one (Jabaloy-Sanchez et al. 2014).

244 Marine setting

245 Hydrodynamics

246 The southern Iberia coast is a non-tidal, low-wave energy

247 setting. The wind regime is characterized by alternating

248 easterlies and westerlies, which account for two dominant

249 wave directions without either being dominant. As a con-

250 sequence, littoral drift shows a high spatial variability.

251 Shallow-water oceanographic circulation is influenced by

252 the incoming Atlantic Jet, which enters the Mediterranean

253 Sea through the Strait of Gibraltar and flows up to

254 150–200 m water depth, with a typical width of 30 km and

255 an eastward advection velocity of around 1 m/s (Perkins

256 et al. 1990; Garcıa-Lafuente et al. 1998). The changing wind

257 regime influences the location of the Atlantic Jet and sec-

258 ondarily the enhancement of shelf circulation, as two

259 main situations have been described: (a) under westerlies

260dominance, the Atlantic Jet is enhanced and flows closer to

261the coast; (b) under easterlies dominance, the Atlantic Jet is

262displaced southward and an enhanced cyclonic circulation

263appears in the shelf area (Macıas et al. 2008). Recent

264hydrodynamic modeling indicates that along-shelf shelf

265currents show alternating directions (westward and east-

266ward) in response to prevailing winds. On the shelf off the

267Guadalfeo River, westward currents ([ 0.1 m/s) are slightly

268more intense than eastward currents (Barcenas et al. 2011).

269Morpho-stratigraphy

270The northern shelf of the Alboran Sea is narrow, with

271widths\30 km. The shelf edge is at *100–125 m water

272depth (Munoz et al. 2008). Its inclination is generally\1",

273although steeper surfaces may occur in coincidence with

274recent deltaic wedges (Ercilla et al. 1994; Fernandez-Salas

275et al. 2003). The main morpho-stratigraphic shelf elements

276comprise late Quaternary shelf-margin deltas and Holocene

277highstand inner prodeltas. Late Quaternary shelf-margin

278deltas constitute the bulk of depositional sequences, where

279they were interpreted as regressive-to-lowstand features

280(Ercilla et al. 1992, 1994; Lobo et al. 2008). The most

281recent sequence also incorporates transgressive and high-

282stand deposits (Hernandez-Molina et al. 1994). Holocene

283highstand deposition is mainly composed of wedge-shaped,

284oblique clinoforms. The most significant of those inner

285shelf wedges are located off the most important river

286mouths, such as the Guadalmedina, Guadalhorce and

287Guadalfeo (Ercilla et al. 1992, 1994, 1995).

Fig. 2 Paleogeographic reconstructions of the deltaic coastline andlowermost river course of the Guadalfeo River deltaic system up to1986, where the coastline remained roughly stationary. Coastline

positions extracted from Rodrıguez-Berzosa and De la Pena (1994);physiographic sketch of 1932 extracted from Almagro (1932)

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288 Distinctive morphological features of shelf prodeltas

289 include foresets and bottomsets with slopes higher than

290 2.5" and 1", respectively, shallow and close to the coast

291 offlap break, lobate shapes and coarse sediment composi-

292 tion (Lobo et al. 2006). Among other small-scale features,

293 sea-floor undulations occur over several prodeltaic bodies;

294 the undulation field located on top of the Guadalfeo River

295 prodelta is the most extensive one. Average height,

296 wavelength and lateral extension values are 0.85, 80 and

297 210 m, respectively (Fernandez-Salas et al. 2007). They

298 appear as two concentric loci emanating off the present and

299 a previous river mouth location. The working hypothesis

300 considers an origin influenced by strong sediment flows

301 normal to bathymetric contours, possibly coupled with

302 subsequent slow sediment deformation processes.

303 Methodology

304 Several types of data were considered for this study: (1)

305 multibeam bathymetric data, which were used to perform a

306 geostatistical analysis; (2) surficial sediment samples and

307 backscatter data; (3) seismic profiles with different reso-

308 lutions; (4) sediment cores and (5) determination of sedi-

309 ment concentration.

310 Multibeam bathymetry

311 Bathymetric data were collected over the northern Alboran

312 Sea shelf with a 300 kHz Simrad EM3000D multibeam

313 echo-sounder (Fig. 1b). Data were acquired using DGPS

314 navigation referred to the WGS-84 ellipsoid. Accurate

315 navigation and real time pitch, roll and heave corrections

316 were provided by the Seatex Seapath 200. The EM3000D

317 multibeam echo-sounder provides 254 beams (1.5" 9 1.5"

318 beam width, 0.9" beam spacing) of depth information from

319 a swath width up to five times the water depth with a

320 maximum ping rate of 25 Hz. Collected bathymetric data

321 provide complete seafloor coverage from \5 m in the

322 shoreface to more than 170 m in submarine canyon axis.

323 Multibeam data were processed with NeptuneTM software

324 and a 5 9 5 m resolution grid was produced. The grid was

325 integrated in an ArcGISTM project, where derived maps

326 such as hillshade and slope maps were generated.

327 Terminology

328 We use the generic term (i.e., without a genetic connota-

329 tion) ‘‘undulations’’ to refer to the conspicuous seafloor

330 morphologies identified over the prodeltaic foresets, in

331 analogy to other studies of equivalent morphologies around

332 the Mediterranean Sea (e.g., Cattaneo et al. 2004; Berndt

333 et al. 2006; Fernandez-Salas et al. 2007; Urgeles et al.

3342007, 2011; Rebesco et al. 2009). Some authors have used

335the term ‘‘crenulation’’ as equivalent to undulation (Corr-

336eggiari et al. 2001; Budillon et al. 2005). Undulations/

337crenulations could indicate the existence either of sediment

338waves, formed by the action of a current flowing across the

339seabed (Trincardi and Normark 1988; Lee et al. 2002;

340Wynn and Stow 2002; Levchenko and Roslyakov 2010), or

341of creep folds, developed as a consequence of soft sediment

342deformation (Lee and Chough 2001; Wynn and Stow 2002;

343Levchenko and Roslyakov 2010).

344Geostatistical analysis

345Geostatistical analysis of undulations is utilized to estimate

346geomorphic metrics that can be used in spatial analysis.

347Our procedure is comprised of three main steps:

348(1) Sampling of bathymetric data. We initially selected a

349number of rectangular samples (1–6 from northwest

350to southeast) along the Guadalfeo River prodeltaic

351area, in order to cover the lateral variability of the

352seafloor undulation fields (Fig. 3). Later, each initial

353sample was subdivided into three smaller subsam-

354ples (A, B and C from shallow to deep water) in

355order to focus on the across-section undulation

356variability (Fig. 3). The samples and subsamples

357were selected from the original grid by using the

358Extract by Mask tool, from the ArcToolbox Spatial

359Analyst Tool. The resulting files were exported to

360ASCII format by using the Raster to ASCII tool,

361from the ArcToolbox Conversion Tools.

362The initial samples 1–6 were chosen to be approximately

363normal to seafloor undulations, and with similar across-

364shelf extensions as much as possible according to wide-

365spread across-shelf occurrence. The sampling location was

366also dictated by the previous study of Fernandez-Salas

367et al. (2007) which found two main undulation fields

368named as the western and eastern fields. Thus, samples 1–3

369are expected to cover the western field, whereas the eastern

370field should be covered by samples 4–6 (Fig. 3). The

371subsamples were generated with the same across-slope

372length, but in general fall within three well-defined

373bathymetric intervals: 30–50, 50–70 and 70–90 m for A, B

374and C subsamples, respectively (Fig. 3).

375(2) Grid adaptation. The ASCII file exported from

376ArcGIS was used as the input information for a

377new Fortran program we named ‘‘gridconv’’, which

378enables the adaptation of the ArcGIS grid in order to

379input it in the inversion program. Gridconv takes as

380input the initial ArcGIS ASCII file, resamples with

381inverse distance weighting interpolation to reorient

382the rows and columns to be oriented along the long

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383 and short axes of the grid, respectively, and detrends

384 the data. Detrending is accomplished on each row

385 individually utilizing the ‘‘trend1d’’ program pro-

386 vided by the GMT graphics package (Wessel and

387 Smith, 1991). The trend1d program fits and removes

388 a best-fit polynomial function. In most of the cases, it

389 was found that a polynomial of order 3 or 4 was

390 appropriate for removing larger-scale trends and

391 variations and isolating the small-scale pattern of

392 prodeltaic undulations.

393 (3) Data inversion. Output grid files from gridconv are

394 used as the input information for grinv, a program

395 that inverts the data to obtain estimates of the von

396 Karman covariance parameters for two-dimensional

397 data. A detailed description of the von Karman

398 covariance function, its defining parameters, and the

399 method for obtaining those parameters through

400 inversion of data, are provided by Goff and Jordan

401 (1988, 1989a, b); a succinct summary is provided

402 here. Initially, selected cross-covariance functions

403 among the detrended rows of the input grid are

404 computed (Fig. 4). These row-oriented cross-covar-

405 iances represent cross-sections of the two-dimen-

406 sional autocovariance. Geostatistical parameters are

407 estimated by weighted, least-squares inversion of the

408 cross-covariance functions, fitting the von Karman

409 covariance model. One standard deviation errors are

410 also estimated. The von Karman covariance model is

411 specified by five parameters: the topographic vari-

412 ance, H2; scale parameters Kn and Ks, which define

413the rate of decay of the autocovariance in the normal

414to strike and strike orientation of the bedforms,

415respectively; the orientation, fs, of the strike direc-

416tion in grid coordinates; and the Hearst number, l,

417which is related to the fractal dimension by the

418relationship D = 3—l for surfaces. These parame-

419ters completely define the von Karman covariance

420model, and provide meaningful information about

421the genetic processes and the development of

422underwater morphological features. In all the cases,

423l was fixed to a value close to 1, taking into account

424the low roughness of the shelf setting, which helped

425to stabilize the inversion.

426Through a trial-and-error approach, the modeled curve is

427first fitted to the estimated cross-covariances, and an eyefit

428model is obtained. The resulting parameters are used as

429initial values for an iteration process that attempts to

430minimize the errors between the cross-covariance estimates

431and model curves. The iteration process is executed until

432we reached convergence, usually after the 3rd or 4th iter-

433ation (Fig. 4). Once the von Karman parameters are

434obtained, we also obtain derivative estimates of the model

435parameters: (1) the root mean square (rms) height

436(H) which is a measure of the overall amplitude of undu-

437lations; (2) the strike azimuth with respect to north (Hs);

438(3) the characteristic width (Ln) which quantifies the

439dominant visual scale in the normal-to-strike direction; (4)

440the characteristic length (Ls) which quantifies the dominant

441visual scale in the strike direction; (5) the wave or vertical

442form index (Ln/H), that provides a measure of the steepness

Fig. 3 Position of samples(1–6, from northwest tosoutheast) and subsamples (A toC from shallow to deep water)used in the geostatisticalanalysis, in order to focus in thealong- and across shelfvariability of prodeltaicundulations. The location of twophysiographic profilesrepresentative of the twoundulation fields is alsoindicated by violet lines. Basemap is multibeam bathymetry,artificially illuminated from thenorthwest. Bathymetriccontours are plotted at 10 mintervals

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443 of the undulations; and (6) the plan-view aspect ratio (Ls/Ln)

444 which characterizes how elongated the undulations are. The

445 characteristic width and length are defined by the width of

446 the covariance in the normal-to-strike and strike directions,

447 respectively, and are quantitatively derived by the relation-

448 ships (Goff and Jordan, 1988).

Ln "2

!!!!!!!!!!!!!!!!!!!!!

2#l$ 0:5%p

Kn

; Ls "2

!!!!!!!!!!!!!!!!!!!!!

2#l$ 0:5%p

Ks

#1%

450450 The fitting of the von Karman covariance model to the row

451 cross-covariance functions is not always perfect. In partic-

452 ular, the cross-covariances tend to be more convex near the

453 origin than the model allows, even at the largest values of l

454 (Fig. 4). A better-fitting functional form might be possible.

455 A Gaussian model, for example, would allow for greater

456 convexity at the origin, but would likely overcompensate;

457 something between a von Karman and a Gaussian functional

458 form might be appropriate. Nevertheless, the von Karman

459model does a good job at characterizing the basic form of

460the cross covariances: height, width, and move-out of the

461peak with increasing row lag (Fig. 4), which defines the

462asymmetry of the bedforms. The principal drawback in fit-

463ting the von Karman function is that the inversion tends to

464overestimate the variance slightly (Fig. 4).

465Determination of morphological asymmetry

466The determination of undulation asymmetry involved the

467following steps after Xu et al. (2008): (1) selection of

468bathymetric profiles, (2) profile detrending, (3) calculation

469of the first derivative, (4) calculation of the ratio H/L

470(height/wavelength) based on the first derivative function,

471and (5) calculation of the asymmetry index for each

472undulation. A positive index indicates asymmetry towards

473the left (landward), whereas a negative index indicates

474asymmetry towards the right (seaward).

Fig. 4 Example of modeledcross-covariance curves, for alarge-scale sample (a; Sample2) and a small-scale subsample(b; Sample 3B). Solid blue linerepresents cross-covariancefunctions computed from thedata, and dashed red linesrepresent best-fit von Karmanmodel

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475 Surficial sediment samples and backscatter data

476 To complement the bathymetric information, seafloor sur-

477 ficial information off the Guadalfeo River deltaic coastline

478 has been included in this study. Several surficial sediment

479 samples (Fig. 1b) extracted from the ESPACE (‘‘Estudio

480 de la Plataforma Continental’’) database provide informa-

481 tion about relative percentages of mud, sand and gravel.

482 Additionally, indirect lithological information is provided

483 by the backscatter distribution, as a positive correlation was

484 found between backscatter intensity and mean grain size.

485 Backscatter intensities were classified as high, medium and

486 low. A detailed description of the relationship between

487 backscatter intensity, average grain size and backscatter

488 classification is given in Barcenas et al. (2011).

489 Sediment cores

490 Cores MS_V12 and MS_V13 were collected from the

491 study area, in the western and eastern fields, respectively

492 (Fig. 1b). The cores were obtained by using a light-

493 weighted vibrocorer, in November 2008, in the framework

494 of the MOSAICO project. Core MS_V12 with 145 cm

495 length and core MS_V13 with 118 cm length were col-

496 lected at 58.5 m and 57 m water depth, respectively. Grain

497 size analyses were performed in intervals of 1-cm thick

498 slices, each 10 cm, or whenever macroscopic changes were

499 observed in the sediment. Relative percentages of gravel,

500 sand, silt and clay were obtained.

501 Seismic data

502 A grid of seismic profiles collected with different seismic

503 systems of variable resolution was also included in this

504 study (Fig. 1b). A 1,000–4,000 J Sparker source and a

505 3.5 kHz sub-bottom profiler were used during the G-83-2

506 survey in 1990, when about 1,650 km of seismic lines were

507 collected onboard the research vessel ‘‘Investigador,’’ part

508 of a project called ‘‘Mapa Geologico de la Plataforma

509 Continental Espanola y Zonas Adyacentes’’ carried out by

510 the Spanish Geological Survey (Fig. 1b). The depth of

511 penetration of Sparker seismic profiles was 1 s two-way

512 travel time (twtt), and the acquisition filter was 30–500 Hz.

513 Seismic profiles were collected obliquely to the coastline

514 with a primary NNE-SSW orientation and a secondary

515 NW-SE orientation. The Sparker profiles were not included

516 in the earlier study of this region by Fernandez-Salas et al.

517 (2007).

518 Determination of sediment concentration

519 To estimate the possibility of generating density flows from

520 the rivers, we applied the procedure of Mulder and Syvitski

521(1995), which is based on the determination of the peak

522sediment concentration during flood conditions (Cflood)

523obtained from several physiographic and hydrological

524parameters, and subsequent comparison with the critical

525sediment concentration (Cc).

526For the estimation of Cc (kg m-3) we used seasonal

527temperature ("C) and salinity (%) data from the nearest

528point to the Guadalfeo River mouth available in the World

529Ocean Atlas 2009 (http://www.nodc.noaa.gov/OC5/WOA09/

530pr_woa09.html) (Antonov et al. 2010; Locarnini et al.

5312010). We calculated the seasonal seawater density

532(1.025–1.027 9 10-3 kg m-3), and then Cc by assuming a

533sediment density of 2,650 kg m-3.

534For the estimation of Cflood (kg m-3) we use the fol-

535lowing equation (Mulder and Syvitski 1995):

Cflood " CQflood

Qav

" #b

; #2%

537537where C " QsavQav

is the average sediment concentration

538(kg m-3), Qav is the average discharge (m3 s-1), Qsav is the

539average sediment load (kg s-1), Qflood is the maximum

540possible discharge (m3 s-1) and b is an empirical rating

541exponent (unitless) that, together with a rating coeffient

542a ((kg s-1)/(m3 s-1)b), defines the relationship between

543sediment concentration (C) and water discharge (Q) for a

544given river (See Eq (2) of Mulder and Syvitski (1995)).

545For the Guadalfeo River, Qav and Qsav are reported in

546Liquete et al. (2005) (Table 1). Qav was calculated from

547water discharge time series (1942–2000), and Qsav from

548several empirical erosion models.

549For the estimation of Qflood we applied a drainage area-

550maximum flood relation (Matthai 1990) that has been

551applied previously for similar calculations (Mulder and

552Syvitski 1995):

logQflood " &0:07 logA2 $ 0:865 logA$ 2:084

for A\106km2; r2 " 0:99;

#3%

554554where A is basin area (km2).Ideally, the rating parameters

555should be calculated empirically from real values of sedi-

556ment concentration and water discharge obtained in

557hydrological surveys (Mulder and Syvitski 1995). Empiri-

558cal data are lacking for the Guadalfeo River. However, the

559rating parameters a and b can also be estimated by several

560formulas that use different river basin properties (Syvitski

561et al. 2000). In this case, we use the formulas that explain

562the higher percentage of variance:

log a# % " 2:93& 1:72 log#Qav% & 0:058Lat; #4%

564564where Lat is latitude ("N), and

b " 0:64& 0:50 log Qav# % $ 0:23 log Qsav# % & 0:32 log#a%:

#5%

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566566 Results

567 Analysis of bathymetric and derived data

568 General physiographic characterization

569 The Guadalfeo River prodelta undulation fields occur off

570 the delta plain, where the coastline is oriented *WNW–

571 ESE. An inflection occurs at the present-day river mouth,

572 where the coastline changes its orientation to*NNW–SSE

573 and a prodeltaic bulge is identified at\20 m water depth.

574 Another inflection changes the coastline to *NE–SW to

575 the east of the historic river mouth; in this zone another

576 prodeltaic bulge is recognized at shallow water, slightly to

577 the west of the coastline inflection and with a well-marked

578 offlap break at about 20 m water depth (Fig. 1b).

579 The shelf is about 4 km wide, with the shelf break

580 approximately coincident with the -110 m isobath. The

581 shelf break is approximately parallel to the main coastline

582 orientation of the deltaic front. The area where the undu-

583 lations are readily identifiable extends*3 km offshore and

584 *5–6 km alongshore (Fig. 1b).

585 The undulations appear to be more developed off the

586 two nearshore prodeltaic bulges, from about 20–90 m

587 water depth; these two areas constitute the western and

588 eastern fields (Fig. 1b). The seaward disappearance of the

589 undulations is accompanied by the inception of a number

590 of straight channels that dissect the shelf break and con-

591 tinue seaward onto the upper slope (Fig. 1b).

592 The gradients of shelf areas surrounding the prodeltaic

593 undulations are generally[2", but with numerous instances

594 of greater local slopes in the undulation area. In the western

595 and eastern fields, most of the undulations generate local

596 slope values between 4" and 6"; values [6" tend to be

597 restricted to the undulations located in\60 m water depth.

598 In cross-shelf sections, the prodeltaic wedge shows a

599 concave-upward shape which becomes less evident in the

600transitional area between both undulation fields. Average

601slopes show a progressive seaward-declining trend (Fig. 5),

602with average values nearly below 2" in the proximal

603foresets (between 30 and 50 m water depths) to average

604values\1" in the distal bottomsets. The undulations occur

605across the prodeltaic wedge independently of the overall

606seaward slope decline (Fig. 5). The undulations show

607either a slight landward-directed asymmetry or symmetric

608character. The landward-directed asymmetry is dominant

609in the central part of the western undulation field; in con-

610trast, the undulations in the central part of the eastern

611undulation field are more symmetric (Fig. 5).

612Along-slope variations of geostatistical parameters

613The along-slope variation of geostatistical parameters is

614investigated using 6 NNE-SSW oriented rectangular boxes.

615A summary of the resulting parameters is provided in

616Table 2 and represented in Fig. 6.

617Sample 2, directly offshore of the present-day course of

618the Guadalfeo River, displays the highest RMS height

619(H) value in the study area (1.12 m) (Fig. 6a); adjacent

620samples 1 and 3 both exhibited lower H values of 0.77 m.

621The next-highest H values (0.87 m) were measured at

622Sample 5, directly offshore of the historic course of the

623river. Adjacent values are likewise reduced, although much

624more at Sample 4 (0.54 m) than at Sample 6 (0.85 m).

625The characteristic width (Ln) exhibits contrasting

626behavior between the western and eastern field (Fig. 6b). In

627the western field, wider undulations (101 m) occur in the

628middle Sample 2, and Ln decreases in adjacent Samples 1

629and 3 (both 91 m). However, the difference in character-

630istic length,*10 m, is smaller than the estimation errors of

631*13–15 m (Table 2). This pattern is inverted in the east-

632ern field, with the middle Sample 5 displaying the lowest

633characteristic lengths (87 m), and adjacent Samples 4 and 6

634displaying the largest values ([ 110 m). These differences

635are better-resolved given the estimation errors of

636*13–19 m.

637The characteristic length distribution (Ls) does not show

638well-defined patterns around the two river outlets (Fig. 6c).

639We only note that the western field shows overall lower Ls

640values than the eastern field, and that the errors are quite

641high for this parameter.

642A clear pattern is recognized in the distribution of strike

643orientation (H), as the undulations are NW–SE oriented in

644sample 1 and become almost E-W oriented in sample 6,

645depicting a radial pattern that is consistent with a lobate

646shape characteristic of prodeltaic settings (Fig. 6d). The

647only exception to this trend is the negligible angle change

648observed between samples 3 and 4.

649The vertical form index (Ln/H) also displays a clear

650pattern with respect to the modern and historic river outlets

Table 1 Morphometric and hydrological parameters of the Guadal-feo River basin

Basin area (km2) 1,312.2

River length (km) 72.5

Basin maximum elevation (m) 3,243

River maximum elevation (m) 2,793

River slope (") 2.21

Delta area (km2) 8.6

Deltaic coastline length (km) 6.0

Mean discharge (m3 s-1) 0.6

Mean sediment load (kg s-1) 2.5

Mean sediment yield (t km2 year-1) 65.1

Extracted from Liquete et al. (2005)

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Fig. 5 Representative physiographic profiles of the study area.a Profile located in the middle part of the western undulation field.b Undulation elevation (with removal of the slope) in the westernfield profile. c Estimate of the asymmetry index in the western fieldprofile. d Profile located in the middle part of the eastern undulation

field. e Undulation elevation (with removal of the slope) in the easternfield profile. (f) Estimate of the asymmetry index in the eastern fieldprofile. Profile locations (a and d) are indicated in Fig. 3. A positiveindex indicates asymmetry toward the left (landward)

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OOF651 (Fig. 6e). Samples 2 and 5, located in the middle parts of

652 the western and eastern undulation fields, show the lowest

653 vertical form indexes, with values\100. Samples adjacent

654 to these exhibit values[100. The pattern is symmetrical in

655 the western field, but asymmetrical in the eastern field, as

656 the vertical form index is considerable higher in sample 4

657 than in sample 6. We also observed that the overall Ln/H

658 values are lower in the western field than in the eastern

659 field.

660 The samples with higher aspect ratios (Ls/Ln) are the

661 outermost 1 and 6, whereas the middle samples 3 and 4

662 show the lowest ratio values (Fig. 6f).

663 Across-slope variations of geostatistical parameters

664 The across-slope variation of geostatistical parameters

665 (Fig. 7, Table 3) was investigated by subdividing the initial

666 sampling rectangles in three subsamples, upper (A), middle

667 (B) and lower (C) (Fig. 3). Strike orientations were com-

668 puted but not presented here, as they do not provide

669 meaningful information.

670 The western field shows a consistent pattern of down-

671 slope variation in RMS height (H): increasing from A to B,

672 and then decreasing from B to C, often by *half (Fig. 7a).

673 The highest values of the study area are found in subs-

674 amples 2A and 2B, directly offshore of the modern river

675 outlet, where H values are 1.23 and 1.39 m, respectively.

676 Subsample 5B, offshore of the historic river outlet, also

677 exhibits H[1 m.

678 Characteristic width (Ln) exhibits more variability in the

679 pattern of across-slope variation than does H (Fig. 7b). In

680 the western field, the highest Ln values are found in middle

681 (B) subsamples, except in sample 1 where subsamples A

682 and B show similar values. In the eastern field there is no

683 consistent pattern, with Ln values decreasing then

684 increasing in Sample 4, increasing monotonically in Sam-

685 ple 5, and increasing then decreasing in Sample 6 (although

686 these changes are not well resolved given the errors).

687 As with H, characteristic length (Ls) values generally

688 show increasing-then-decreasing downslope trends, par-

689 ticularly in the western field offshore of the modern river

690 outlet (Fig. 7c). Samples 4 and 5 in the eastern field are

691 exceptions. In Sample 4, Ls values decrease-then-increase

692from subsamples A to C. In Sample 5, Ls values increase

693monotonically down slope. The higher error estimates for

694Ls (Table 3) render some of these downslope changes in Ls

695values uncertain, however.

696The across-slope variations of the vertical form index

697(Ln/H) do not appear to be consistent, although the highest

698values tend to occur in the most distal subsamples

699(Fig. 7d). Finally, the pattern exhibited by the aspect ratio

700(Ls/Ln) is also scarcely predictable, although the highest

701values tend to occur in the middle subsamples (Fig. 7e).

702Surficial sediments and backscatter distribution

703The available sediment samples (Fig. 8) are more abundant

704in the western field, where sand percentages are higher than

70525 % in almost all the samples, with some higher than

70650 % (Fig. 8). No sediment samples are available in the

707central part of the eastern field, but the samples located in

708the proximal topsets and in the distal bottomsets reveal

709sand percentages even higher than their equivalents in the

710western field. The amount of muds increases laterally from

711the undulation fields. Thus, mud percentages tend to be

712higher than 50 % both to the north-northwest and to the

713south-southeast of the fields (Fig. 8).

714The backscatter data (Fig. 8) exhibit high values over

715the prodeltaic topset ([-20 dB), indicative of high sand

716contents and small amounts of gravels. The eastern field

717shows backscatter intensities ranging from -24 to -26 dB,

718whereas backscatter values in the western field are between

719-27 and -28 dB, suggesting that the eastern field exhibits

720a coarser sediment composition than the western field.

721Seaward, most of the prodeltaic surface is characterized by

722low backscatter intensities (\-24 dB), attributed to fine to

723coarse sands. The rest of the prodeltaic surface, including

724the middle area between both undulation fields, shows

725backscatter intensities lower than -28 dB, correlated in

726many cases with muddy sediments with high amounts of

727sands (Fig. 8).

728Sediment cores

729In both cores, grain size analyses revealed that the coarse

730fraction is composed primarily of sand (gravel\0.2 %) and

Table 2 Summary ofgeostatistical parameters forsamples 1–6, constructednormal to seafloor undulations

Sample H Ln Ls H Ln/H Ls/Ln

1 0.77 ± 0.04 91 ± 14 434 ± 131 -52 ± 2 119 ± 21 4.8 ± 1.3

2 1.12 ± 0.06 101 ± 15 340 ± 84 -62 ± 4 90 ± 16 3.4 ± 0.8

3 0.78 ± 0.04 91 ± 14 264 ± 56 -69 ± 4 119 ± 21 2.9 ± 0.6

4 0.54 ± 0.03 112 ± 19 374 ± 105 -68 ± 4 209 ± 42 3.3 ± 0.9

5 0.87 ± 0.05 87 ± 14 372 ± 103 -71 ± 3 99 ± 19 4.3 ± 1.1

6 0.85 ± 0.06 111 ± 18 498 ± 172 -82 ± 3 130 ± 26 4.5 ± 1.4

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731 the fine fraction is composed mostly of silt (clay\15 %)

732 (Fig. 9). In MS_V12 (western field), two distinct units were

733 identified along the core (Fig. 9a). The lower unit, from the

734core base to *60 cm core depth, showed increasing

735alternations between coarse and fine sediments, which can

736attain fractions higher than 70 % for either. The upper unit,

Fig. 6 Map showing the samples with different geostatisticalparameters, focusing on along-shelf variability. a Root mean square(rms) height (H). b Characteristic width (Ln). c Characteristic length

(Ls). d Strike azimuth with respect to north (Hs). e Vertical formindex (Ln/H). f Plan-view aspect ratio (Ls/Ln)

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737 from *60 cm to the core top, was characterized by rela-

738 tively constant higher percentages of clayey silt, which

739 reached more than 80 % of the total sediment. In core

740 MS_V13 (eastern field), variations between coarse and fine

741 fractions were observed throughout the core. Higher

742abundances of sands were verified near the core base

743(85 %), at 50 and 25 cm core depth, with percentages of 60

744and 73 %, respectively. Percentages of the fine fraction

745higher than 80 % were observed between 95 and 70 cm

746core depth (Fig. 9b).

Fig. 7 Map showing the subsamples with different geostatistical parameters, focusing on across-shelf variability. a Root mean square (rms)height (H). b Characteristic width (Ln). c Characteristic length (Ls). d Vertical form index (Ln/H). e Plan-view aspect ratio (Ls/Ln)

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747 Seismic stratigraphy

748 In the seismic profiles from the eastern (Fig. 10) and

749 western (Fig. 11) fields, the undulations are observed

750 within the Holocene prodeltaic package defined by Lobo

751 et al. (2006), above the Basal Downlap Surface (BDS). In

752the eastern field, the seismic facies observed in Sparker

753profiles (Fig. 10a) above the BDS are sub-parallel, evolv-

754ing seaward to landward-dipping reflections below the

755most prominent undulations. Upward the reflection pattern

756changes from sub-parallel to slightly undulated (Fig. 10a).

757In the 3.5 kHz profile (Fig. 10b), the lower section exhibits

Table 3 Summary ofgeostatistical parameters forsubsamples A, B, C in eachinitial sample 1–6

Subsample H Ln Ls Ln/H Ls/Ln

1A 0.76 ± 0.08 96 ± 20 380 ± 123 126 ± 33 4.0 ± 1.2

1B 0.80 ± 0.10 94 ± 21 422 ± 157 118 ± 35 4.5 ± 1.6

1C 0.36 ± 0.02 60 ± 10 187 ± 38 167 ± 33 3.1 ± 0.6

2A 1.23 ± 0.08 65 ± 11 189 ± 40 53 ± 10 2.9 ± 0.6

2B 1.39 ± 0.24 147 ± 40 496 ± 248 106 ± 39 3.4 ± 1.6

2C 0.54 ± 0.06 103 ± 21 323 ± 107 190 ± 50 3.1 ± 1.0

3A 0.55 ± 0.04 60 ± 10 195 ± 43 109 ± 23 3.2 ± 0.7

3B 0.82 ± 0.08 92 ± 18 248 ± 75 111 ± 27 2.7 ± 0.8

3C 0.56 ± 0.07 58 ± 15 177 ± 73 103 ± 35 3.1 ± 1.2

4A 0.34 ± 0.03 75 ± 13 245 ± 69 219 ± 49 3.3 ± 0.9

4B 0.41 ± 0.03 65 ± 11 211 ± 52 159 ± 33 3.2 ± 0.8

4C 0.38 ± 0.03 92 ± 17 225 ± 61 239 ± 56 2.4 ± 0.6

5A 0.61 ± 0.04 51 ± 7 151 ± 29 84 ± 15 3.0 ± 0.6

5B 1.03 ± 0.10 88 ± 17 285 ± 97 85 ± 22 3.2 ± 1.0

5C 0.47 ± 0.05 100 ± 21 289 ± 102 215 ± 57 2.9 ± 1.0

6A 0.66 ± 0.06 83 ± 14 252 ± 69 127 ± 28 3.0 ± 0.8

6B 0.87 ± 0.10 98 ± 21 440 ± 219 112 ± 32 4.5 ± 2.1

6C 0.33 ± 0.03 79 ± 14 202 ± 51 235 ± 52 2.6 ± 0.6

Fig. 8 Surface sediment samples of the study area, showing mud, sand and gravel percentages, overlaid on a color image of acoustic backscatterintensities (values in dBs). Artificial illumination of the bathymetry from the northwest is also applied

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758 subparallel reflections consistent with Sparker profile

759 observations. The upper section exhibits increased chaotic

760 reflectivity, as well as several strong reflections subparallel

761 to the seafloor undulations in the uppermost 2 m of the

762 section. A 3.5 kHz profile located toward the southeast

763 termination of the eastern undulation field (Fig. 10c)

764 exhibits seismic reflections that are subparallel to a gently

765 undulating seafloor. These undulations occurring on the

766 seafloor and/or reflection horizons are observed in the

767 distal parts of the prodeltaic feature, where seafloor slopes

768 are low (Fig. 10b, c).

769 In the western field, the seismic pattern of the undula-

770 tions in Sparker profiles is similar to that of the eastern

771 field, with sub-parallel continuous reflections over the BDS

772 evolving seaward and upward to an undulatory pattern

773 (Fig. 11a). In 3.5 kHz profiles (Fig. 11b), we observe

774 multiple reflections that are subparallel to the undulations

775 in the uppermost 4.5 m, with peaks that migrate upslope

776upward in the stratigraphic column. We also observed

777increased overall reflectivity near the seafloor (Fig. 11b).

778West of the western field (Fig. 11c, d), the seafloor is

779mainly smooth, with only minor undulations over the distal

780bottomsets. Internally, however, the sub-parallel configu-

781ration above the BDS transitions upward to a somewhat

782discontinuous undulatory pattern (Fig. 11c). In the 3.5 kHz

783profile, this upward transition is represented by an increase

784in reflectivity of the uppermost 2–4 m (Fig. 11d).

785The distribution of the undulation crests is overlain on

786the thickness distribution of the Holocene prodeltaic wedge

787in Fig. 12. The undulations are present in sediment thick-

788nesses ranging from *50 m at the proximal edge of the

789prodelta to *10 m and less at the distal edge. The distri-

790bution of the two undulation fields does not appear to be

791strongly controlled by the thickness distribution of the

792prodeltaic wedge, as the wedge exhibits a rather homoge-

793neous thickness paralleling the nearby coastline (Fig. 12).

Fig. 9 Photography, schematic description and grain size fraction variation of cores a MS_V12 from the western field and b MS_V13, from theeastern field

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794 Calculations of sediment concentrations

795 Sediment concentration, Cc, in the Guadalfeo River ranges

796 between 41.10 kg m-3 in summer and 44.01 kg m-3 in

797 winter. These values fall within the global range of the

798 critical sediment concentration needed to produce a

799 hyperpycnal plume in marine water (35–45 kg m-3)

800 (Mulder and Syvitski 1995). The obtained value (Eq. 5) of

801 the rating exponent b is 0.47. By substituting the different

802 estimated values, the resulting Cflood is 630 kg s-1.

803Discussion

804Sediment waves versus creep folds

805We evaluate the different morphological and stratigraphic

806criteria available in the case of the Guadalfeo undulations,

807in order to propose a likely origin that can be compatible

808with existing evidences in other similar undulation fields.

809We therefore attempt to distinguishing whether the

810observed undulation characteristics are more coherent with

Fig. 10 Seismic stratigraphy of submarine undulations in the easternfield. a Sparker profile located off the ancient river mouth position.b 3.5 kHz profile located off the ancient river mouth position.c 3.5 kHz profile located in the eastern termination of the Guadalfeo

River delta. The base of the Holocene highstand wedge or BasalDownlap Surface (BDS) is indicated by a red line. The position ofseismic examples is indicated in Fig. 1, as well as in inset maps

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811 sediment waves or with creep folds. The correct genetic

812 interpretation of prodeltaic undulations is critical for a

813 proper geohazard evaluation in shallow-water settings, as

814 many anthropogenic structures are placed over those

815 dynamic environments. If the undulations were to be

816 interpreted as slowly deforming creeps, they could poten-

817 tially evolve to more significant mass movement process,

818with a higher potential risk for submarine human infra-

819structures (Urgeles et al. 2007; Sultan et al. 2008).

820Overall morphology

821The undulations in the study area occur over seaward

822decreasing slopes (average slopes tend to be lower than 2")

Fig. 11 Seismic stratigraphy of submarine undulations in the westernfield. a Sparker profile located obliquely in relation to the present-dayriver mouth position. b 3.5 kHz profile located obliquely in relation tothe present-day river mouth position. c Sparker profile located in thewestern termination of the Guadalfeo River delta. d 3.5 kHz profile

located in the western termination of the Guadalfeo River delta. Thebase of the Holocene highstand wedge or Basal Downlap Surface(BDS) is indicated by a red line. The position of seismic examples isindicated in Fig. 1, as well as in inset maps

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823 defined by concave surfaces; they are also common over

824 the distal bottomsets, where slopes are lower than 1"

825 (Fig. 5). The existence of undulations over concave-

826 upward surfaces is considered to be a common feature of

827 sediment wave fields (Lee et al. 2002; Ponce and Carmona

828 2011). In contrast, morphological features resulting from

829 sediment deformation have been reported as more common

830 on slopes higher than 2" (e.g., Wynn and Stow 2002;

831 Rebesco et al. 2009). Although in some settings deforma-

832 tional features such as rotated block slumps have also been

833 reported in relatively low slopes (0.5–2"), in those cases

834 such features are accompanied by other distinctive features,

835 such as headwall scarps or zones of evacuation (Lee et al.

836 2002; Sacchi et al. 2005; Berndt et al. 2006), growth faults

837 and/or gas-charged layers (Hasiotis et al. 2006; Lykousis

838 et al. 2003, 2009). Such observations are not found in the

839 study area.

840 The geomorphological patterns of individual undula-

841 tions in the study area are also more compatible with the

842 sediment wave origin than with the creep fold interpreta-

843 tion. For example, the undulations described off the Gua-

844 dalfeo River outlets display well-defined crests, with a

845 tendency toward landward asymmetry (Figs. 5, 10, 11).

846 The fact that downslope limbs tend to be longer than

847 upslope limbs would indicate differential deposition rates

848 (Lee et al. 2002; Cattaneo et al. 2004). Coast-directed

849 asymmetry has been considered as indicative of sediment

850 waves (Levchenko and Roslyakov 2010). This geomor-

851 phological pattern differs from the pattern observed in

852 creep folds, where the crests tend to be broad and poorly

853 defined and the troughs narrow (e.g., Lee and Chough

8542001; Levchenko and Roslyakov 2010). In plan-view, the

855undulations on the Guadalfeo River prodelta appear as

856approximately linear, although with various degrees of

857sinuosity (Fig. 12), which is consistent with sediment wave

858morphology. In contrast, creep folds tend to exhibit arcuate

859patterns (Wynn and Stow 2002).

860Spatial variability

861The Guadalfeo undulations show morphological charac-

862teristics similar to the undulations documented on the

863Llobregat River prodelta, Catalonia margin (Urgeles et al.

8642007). Both sets occur in water depths of *30–90 m and

865exhibit similar geomorphological parameters (H\1.3 m, Ln

866\200 m, Ls of several hundreds of meters). This general

867comparison suggests a similar depositional environment for

868the Guadalfeo and Llobregat undulations.

869To our knowledge, this study represents the first com-

870prehensive attempt to characterize the lateral variability in

871geomorphological parameters of prodeltaic undulations;

872previous studies have focused on downslope variability

873(Cattaneo et al. 2004; Urgeles et al. 2007). Most of the

874undulation statistical parameters in the study area are

875arranged in fairly organized and consistent patterns cen-

876tered about the present-day and historical river mouths

877(Fig. 6). The existence of distinctive trends in the dimen-

878sions is considered to be a characteristic of sediment waves

879(Lee et al. 2002; Cattaneo et al. 2004). In particular,

880wavelength and height typically decrease downslope (Lee

881et al. 2002; Urgeles et al. 2007). In contrast, instabilities

882features such as creep folds do not show consistent

Fig. 12 Thickness distributionin milliseconds (ms) twtt of theHolocene Guadalfeo Riverprodelta, with superimposedundulation axes

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883 distribution patterns and tend to exhibit a more random

884 scatter of dimensions (Lee and Chough 2001; Wynn and

885 Stow 2002). As an example, the most significant instabil-

886 ities in a fan delta of the Gulf of Corinth occur in a lateral

887 flank, unrelated to the river mouth and the distribution of

888 the depositional body (Hasiotis et al. 2006).

889 We found that H and Ln/H ratio provide the most con-

890 sistent geomorphological patterns with respect to the

891 present-day and historic river mouths (Fig. 6a–e). Along-

892 shelf, the largest H values occur offshore the river mouths,

893 decreasing to either side (Fig. 6a), whereas Ln/H ratio is

894 smallest off the river mouths, and increases to either side.

895 Across-shelf, the highest H values tend to occur at mid-

896 depths (*60 m) in both undulation fields (Fig. 7a). The

897 downslope variation of the Ln/H ratio in the central axis of

898 the undulation fields shows a strong increasing downslope

899 trend (Fig. 7e), indicating that the undulations become

900 flatter and smaller in a downslope direction.

901 Both H and Ln/H distribution patterns give further

902 support to the sediment wave interpretation, as similar

903 patterns have been observed in other sediment wave areas.

904 For example, maximum sediment wave dimensions

905 (H) occur at mid depths in a sediment wave field located on

906 the Fraser River Delta; there, it has been proposed that

907 dune dimensions are most likely controlled by the relative

908 rates of bedload versus suspended load transport, and

909 ultimately by current speed (Carle and Hill 2009). In the

910 case of the vertical form index, the downslope increase of

911 the Ln/H ratio has been related in fjord settings to the

912 generation of sediment waves by sediment fluxes (Bøe

913 et al. 2004). Those distribution patterns are clearly different

914 from the distribution of creep fold dimensions reported in

915 one of the most outstanding studies regarding such defor-

916 mation features (Lee and Chough 2001). In fields of sub-

917 marine creep deposits, the highest reliefs of the undulations

918 tend to occur in the most distal low-slope settings.

919 Characteristic widths, Ln, do not show a consistent

920 downslope pattern, as the cross-slope distributions differ in

921 both fields (Fig. 7b). Systematic variation in undulation

922 width with depth was also not observed in prodeltaic set-

923 tings (Cattaneo et al. 2004; Urgeles et al. 2007) or in deep-

924 water undulation fields (Rebesco et al. 2009).

925 Stratigraphic patterns

926 Undulations in the study area show consistent stratigraphic

927 patterns in both fields (eastern and western) as observed in

928 3.5 kHz profiles, such as landward-migrating crests

929 evolving upward in the stratigraphic section, lateral conti-

930 nuity of internal reflectors from one wave to the other,

931 continuity beyond undulation fields into areas of sub-par-

932 allel reflectors without any significant sediment disruption,

933 and regularity of reflector spacing (Figs. 10, 11). Those

934compelling evidences have been ascribed to sediment wave

935fields (Lee et al. 2002; Wynn and Stow 2002; Urgeles et al.

9362007; Milia et al. 2008; Levchenko and Roslyakov 2010),

937as the development of sediment waves occur in response to

938changing environmental conditions (Lee et al. 2002).

939The shallow internal reflectors occurring in the upper-

940most sediment column could be correlated with the coarse-

941grained peaks identified in the sediment cores. Similar

942sandy or coarse-grained layers have been documented in

943other prodeltaic Holocene wedges around the Mediterra-

944nean Sea, where they are usually interpreted as the result of

945flood-derived rapid deposition (e.g., Budillon et al. 2005;

946Hasiotis et al. 2006; Milia et al. 2008). We propose a

947similar interpretation for the study area. Core MS_V13,

948located in an area influenced by the river discharges of the

949historic course, revealed the sedimentary deposition that

950occurred until the 30 s of the 20th century, when the course

951of the river was deviated to its present position. The lower

952part of core MS_V12 reflected the sedimentation occurred

953after the river course deviation, which alternated between

954periods of increase supply of sand material to the shelf,

955linked with periods of increased runoff, and periods of

956lower river discharges, with the deposition of fine material.

957The upper part of this core represents sediments deposited

958after the stabilization of the river course to its present-day

959location, when only fine material is deposited in this area of

960the shelf, until the present.

961In contrast to those evidences, creep folds show dis-

962placement along failure planes (Lykousis 1991; Corregg-

963iari et al. 2001; Hasiotis et al. 2006; Lykousis et al. 2009)

964and do not display lateral migration (Lee and Chough

9652001; Wynn and Stow 2002).

966Undulations observed in the seismic profiles in the study

967area generally increase in dimensions upward in the

968stratigraphic column (Figs. 10, 11). This pattern has been

969observed in diverse marine settings (both shallow- and

970deep-water), where it has been associated to progressive

971development of sediment waves over long time periods

972(Trincardi and Normark 1988; Lee et al. 2002; Bøe et al.

9732004; Urgeles et al. 2007; Levchenko and Roslyakov

9742010).

975Sedimentological patterns

976The two undulation fields display higher backscatter

977intensities than the surrounding prodeltaic surface (Fig. 8),

978indicative of coarser material (predominantly sandy) that

979we infer is sourced from the two river outlets. This pattern

980also supports the hypothesis that the genesis of the undu-

981lations are related to the transport of sand-sized sediments

982beyond the littoral zone during flooding events, as docu-

983mented in other settings (Mulder et al. 2003; Warrick and

984Milliman 2003). The pattern is not consistent with

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985 expectations under the creep hypothesis for these features,

986 as this would require finer-grained sediments within the

987 undulation field (Lee and Chough 2001).

988 Possible genetic mechanisms for sediment waves

989 Under the assumption that the undulations off the Gua-

990 dalfeo River are sediment waves, their presence should not

991 be considered an indicator for the occurrence of instability

992 processes, and consequently they likely do not pose sig-

993 nificant hazard for offshore development. In this section we

994 discuss possible triggering processes for those prodeltaic

995 undulations in the study area. Two main possibilities can

996 be envisaged (e.g., Cattaneo et al. 2004; Urgeles et al.

997 2011): (1) influence of oceanographic processes; and (2)

998 river discharges with high sediment concentrations (i.e.,

999 hyperpycnal flows).

1000 Oceanographic processes

1001 Elsewhere around the Mediterranean Sea, the formation of

1002 prodeltaic undulations has been attributed to the sediment

1003 transport induced by oceanographic processes like shelf-

1004 depth currents or internal waves (Trincardi and Normark

1005 1988; Cattaneo et al. 2004; Berndt et al. 2006; Urgeles

1006 et al. 2011).

1007 (a) Bottom currents. Under the influence of bottom

1008 currents, sediment waves usually share several of the

1009 following attributes, such as: (1) significant variation

1010 of deepest occurrence or occurrence in relatively

1011 deep shelf waters; (2) dominance of sub-horizontal

1012 upslope limbs; (3) asymmetrically oriented seaward;

1013 (4) no consistent seaward decrease of wave dimen-

1014 sions; (5) high lateral continuity (up to tens of

1015 kilometers); (6) evidences of erosion at different

1016 water depths (Trincardi and Normark 1988; Cattaneo

1017 et al. 2004; Berndt et al. 2006).

1018 The undulations mapped in the Guadalfeo River study area

1019 do not meet these criteria because: (1) they occur in a well-

1020 defined water depth range (30–90 m); (2) they show a

1021 preferential landward-directed asymmetry, with steep

1022 upslope limbs, and the dimensions show a consistent sea-

1023 ward pattern; (3) they show much lower lateral continuity

1024 that undulations generated by laterally continuous shelf

1025 current flows; and (4) available knowledge about shelf

1026 hydrodynamic conditions indicates the prevalence of

1027 along-shelf flows, subparallel to the main undulation ori-

1028 entation in response to alternating winds (Barcenas et al.

1029 2011). Therefore, we conclude that the undulations are not

1030 related to bottom flows running parallel to bathymetric

1031 contours.

1032(b) Internal waves. Recent publications suggest that, in

1033some cases, internal waves may be responsible for

1034the generation of prodeltaic undulations (Puig et al.

10352007; Urgeles et al. 2011). The occurrence of long,

1036linear crests (i.e., kilometric scale) with elongated

1037undulations parallel to the shoreline is regarded as

1038the main morphological criteria to suggest the

1039leading influence of internal waves (Puig et al.

10402007). In addition, such undulations tend to exhibit a

1041preferential seaward-directed asymmetry, decreasing

1042their dimensions in the onshore direction and with

1043L/H relationships between 190 and 230 (Urgeles

1044et al. 2011).

1045The morphology and distribution of the undulations in the

1046Guadalfeo River prodelta is not consistent with these cri-

1047teria: (1) they exhibit low Ls values (hundreds of meters);

1048(2) they form at relatively short distance from the river

1049mouth; (3) the undulations show a concentric distribution

1050around the river outlets; (4) they have preferential land-

1051ward asymmetry; (5) the highest undulations tend to occur

1052at mid-water depths; and (6) L/H relationships are\100.

1053We therefore also conclude that internal waves are not a

1054viable formative mechanism for the Guadalfeo prodelta

1055undulations.

1056High-sediment-concentration river discharges

1057Hyperpycnal flows are another plausible mechanism for

1058sediment wave generation in prodeltaic slopes. These are a

1059particular kind of turbidity current generated by river dis-

1060charge (Normark and Piper 1991), where a negatively

1061buoyant plume with a density higher than the ambient

1062seawater flows along the basin floor. For hyperpycnal flows

1063to occur, critical sediment concentration (Cc) for plung-

1064ing in the marine environment should be between

106535–45 kg m-3 (Mulder and Syvitski 1995). Those values

1066should be considered as conservative estimates, as Cc

1067values are lowered under specific circumstances, such as

1068the occurrence of convective instability of the flow

1069(Cc = 5 kg m-3) (Parsons et al. 2001) or the existence of

1070easily erodible deposits (Mulder et al. 2003). Rivers able to

1071generate annual hyperpycnal flows are designated as

1072‘‘dirty’’, but most rivers are only able to produce hyper-

1073pycnal flows during extreme events, such as major floods.

1074Most of those rivers are small to medium-size, drain

1075mountainous and steep fluvial basins with easily erodible

1076sediments and have low average annual discharges

1077(\380–460 m3 s-1) (Mulder and Syvitski 1995; Imran and

1078Syvitski 2000).

1079The continental margins in the Mediterranean side of the

1080southern Iberian Peninsula are good candidates for the

1081generation of hyperpycnal flows: mountainous and with

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1082 drainage basins that are small and have pronounced slopes.

1083 In addition, the river basins are located in the hottest, driest

1084 and less vegetated regions of the peninsula (Liquete et al.

1085 2005), providing greater sediment availability. Climatic

1086 conditions show contrasting patterns in the drainage basins,

1087 with intense torrential rainfall events occurring episodi-

1088 cally and remaining dry the rest of the year. These factors

1089 lead to significant episodes of sediment discharge. The

1090 Guadalfeo River can be considered as a high mountain

1091 river with a low size drainage basin; the delta plain is

1092 poorly developed and the submarine delta morphology

1093 leads to a marked gradient increase, from\0.5" in the delta

1094 plain to about 2" in the upper foresets. In this kind of river

1095 systems the likelihood of delivering large percentages of

1096 the sediment load directly to the sea is high (Milliman and

1097 Syvitski 1992).

1098 The rating exponent b (Eq. 5) provides an indication of

1099 the possibility of occurrence of events, so a simple clas-

1100 sification of rivers is possible. Rivers with b B 1 are

1101 regarded as moderately dirty, with return periods of

1102 \100 years. The theoretical condition for hyperpycnal

1103 flow generation would be than Cflood[Cc. However, in

1104 reality the only rivers that can be considered to produce

1105 hyperpycnal flows are those with large ([300 kg m-3)

1106 Cflood values (Eq 2), because in this case the condition

1107 would be fulfilled even with b ' 1 (Mulder and Syvitski

1108 1995). Taking into account the estimations, the Guadalfeo

1109River would be classified as a moderately dirty river

1110(b = 0.47) able to produce hyperpycnal flows, as the

1111estimated Cflood = 630 kg m-3.

1112Morphological observations are consistent with the

1113hypothesis that assumes hyperpycnal flow generation in the

1114river basin (Fig. 13). We found that H and Ln/H ratio

1115provide the most distinctive geomorphological patterns.

1116Most of the Ln/H values in the prodeltaic undulations off

1117the Guadalfeo River are *100, and in some cases, such as

1118the axis of undulation fields, even lower (Tables 1, 2).

1119Those low values would be indicative of the possible

1120activity of hyperpycnal flows (Urgeles et al. 2011), as the

1121ratio ranges between 100 and 400 in the Adriatic Sea

1122(Cattaneo et al. 2004) and the Llobregat Prodelta (Urgeles

1123et al. 2007).

1124The observed differences between the two undulations

1125fields in terms of geostatistical parameters (Figs. 6, 7) as

1126well as seismic reflection features and the sedimentary

1127record (Figs. 10,11), suggests important differences in the

1128process of undulation formation between them. We

1129hypothesize, in particular, that the morphological and

1130stratigraphic differences are linked to temporal changes

1131undergone by the hyperpycnal flows associated with the

1132modification of the river outlets.

1133The overall lower H values and higher Ln and Ln/H

1134values in the eastern undulation field (Figs. 6, 7) could be

1135indicative of the inactivity of these sediment features since

Fig. 13 Schematic 3Drepresentation showing thecharacteristic undulationdimensions in the two fields,western and eastern. Arrowsindicate proposed sedimentflows leading to undulationgeneration. In the western field,higher H and lower Ln/H and Ls

values occur in the middle ofthe undulation field, suggestingintense flows that decreaserapidly both laterally anddownslope. In the eastern field,lower H values and higher Ln

and Ln/H values may indicate asedimentary regime of lowerintensity, favoring anasymmetric development of thedepositional features

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1136 the change in river course. The lateral variability patterns

1137 in the geostatistical parameters are also different. The H

1138 values in the eastern field are not as symmetric as for the

1139 western field. From this we infer that the eastern field

1140 sedimentary regime would be of lower energy. In addition,

1141 depositional conditions would have developed an asym-

1142 metrical sediment lobe, as the area with higher undulations

1143 appears to be broader and displaced eastward (Fig. 6). In

1144 addition, seismic data do not show abundant intra-undu-

1145 lation reflections, which would be in agreement with the

1146 field inactivity (Fig. 10b).

1147 Higher H values and lower Ln/H values in the western

1148 undulation field (Fig. 6) could be indicative of the inten-

1149 sification of riverine flows after the river deviation took

1150 place. The shallow subsurface stratigraphy also shows

1151 frequent subparallel reflections that provide evidence of

1152 active shoreward migration (Fig. 11b). The identification

1153 of symmetrical patterns of several parameters (e.g., H, Ln

1154 and Ln/H distributions) around the present river mouth in

1155 the western field may indicate that the undulations in the

1156 western field result from highly focused flows, where

1157 current velocity and capacity for sediment transport

1158 decrease rapidly to either side. Consequently, a temporal

1159 change in the nature of the sediment supply can be inferred,

1160 as the riverine flows became more focused and intense after

1161 the river deviation. The sedimentary texture of the western

1162 undulation field reveals a slightly finer grain size compo-

1163 sition that may indicate that the most recent influence on

1164 sediment supply of dam construction in the drainage basin.

1165 Conclusions

1166 Our analysis provides morphologic and stratigraphic evi-

1167 dence supporting the hypothesis that the prodeltaic undu-

1168 lations can be considered as a type of sediment waves. Key

1169 geostatistical parameters, particularly H and Ln/H, exhibit

1170 fairly consistent distribution patterns, especially in the

1171 along-shelf direction, with respect to historic and present-

1172 day outlets of the Guadalfeo River. This variability can be

1173 used to make predictive assessments in other areas where

1174 the primary wave-forming process is unknown.

1175 Morphometric parameters of the drainage basin and

1176 estimations of sediment concentration during episodic

1177 flood events are compatible with the major influence of

1178 hyperpycnal flows and deposition at short distance from the

1179 coast. The morphological characteristics of the Guadalfeo

1180 River prodelta undulations suggest the imprint of intense

1181 downslope sediment flows emanating from river outlets,

1182 with increased proportions of bedload sediment transport in

1183 relation with other undulation areas. Lateral changes in the

1184 geomorphological variables and in the seismic and sedi-

1185 mentological features are considered to be indicative of the

1186temporal change undergone by hyperpycnal flows associ-

1187ated with the shift in river outlet. Future work in the area

1188should be aimed at validating the estimations of sediment

1189concentration by collecting sediment load data under dif-

1190ferent water discharge conditions.

1191Acknowledgments Multibeam data were collected within the1192framework of the ESPACE (‘‘Estudio Geologico de la Plataforma1193Continental Espanola’’) project, executed by the ‘‘Instituto Espanol de1194Oceanografıa (IEO)’’ and the ‘‘Secretarıa General de Pesca Marıtima1195(SGPM)’’. This work is a contribution to the research projects1196MOSAICO, TESELA, CTM2005-04960/MAR and CGL2011-30302-1197C02-02. This contribution was elaborated during a research stage of1198the first author at the Institute for Geophysics, Jackson School of1199Geosciences, University of Texas at Austin, during May to August12002009. This stage was funded by the ‘‘Jose Castillejo’’ program1201(JC2008-00210), call of the Spanish Ministry for Science and Inno-1202vation to support short stages for young doctors in foreign research1203centers. Isabel Mendes thanks to the Portuguese Foundation for Sci-1204ence and Technology (FCT) for grant SFRH/BPD/72869/2010. Use-1205ful comments and remarks concerning hyperpycnal flow development1206were made by James P.M. Syvitski (University of Colorado at1207Boulder) and Thierry Mulder (Universite de Bordeaux 1). Two1208anonymous reviewers are sincerely thanked because of their con-1209structive suggestions that helped to improve the manuscript. UTIG1210Contribution #2717.

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Spatial variability of prodeltaic undulations of the Guadalfeo River prodelta: New clues for their...

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