Late Pleistocene and Holocene sedimentary facies on the SW Galicia Bank (Atlantic NW Iberian Peninsula)

Marine Geology 249 (2008) 46 – 63 www.elsevier.com/locate/margeo

Late Pleistocene and Holocene sedimentary facies on the SW Galicia Bank (Atlantic NW Iberian Peninsula) B. Alonso a,⁎, G. Ercilla a , D. Casas a , F. Estrada a , M. Farrán a , M. García a , D. Rey b , B. Rubio b .a

Instituto de Ciencias del Mar-CSIC, Paseo Marítimo de la Barceloneta, 37-49, 08003 Barcelona, Spain b Universidad de Vigo, Facultad de Ciencias del Mar, 36310 Vigo, Spain Accepted 20 September 2007

Abstract Five main Pleistocene–Holocene lithofacies are defined in three different sedimentary environments (fault scarp, sedimentary lobe and inter-lobe channel) of the SW Galicia Bank: (1) turbidites (biogenous and terrigenous), (2) hemipelagites, (3) pelagites, (4) debrites, and (5) Heinrich sediments. In the sedimentary lobe and inter-lobe channel, the stratigraphic record consists mainly of turbidites interbedded with debrites and Heinrich sediments and hemipelagites that are covered by hemipelagites or pelagites. In the fault scarp, the stratigraphy comprises hemipelagites and turbidites covered by pelagites. Frequency of turbidite events has varied between 1/1.2 ka and 1/3.1 ka, since 31.3 ka BP. At least four turbidite events (1 to 4) between 9.1 and 31.3 ka BP, have been correlated between the different sedimentary environments. The downslope of turbidity flows prevailed until 9.1 ka; after which, vertical settling and slow lateral advection have controlled sedimentation. The source area of turbidites and debrites is the fault scarp. Erosion of the slope near-surface pelagites/ hemipelagites and ancient outcropping deposits could explain the presence of biogenous turbidites and terrigenous turbidites respectively. The rhythmic development of turbidites interrupted by hemipelagites could represent the manifestation of different pulses of sedimentary instability induced by a combination of oversteepening (up to 29°) and occurrence of earthquakes. © 2007 Elsevier B.V. All rights reserved. Keywords: Late Quaternary; turbidite; sedimentary processes; sedimentary instability; stratigraphy; Galicia Bank

1. Introduction The Galicia Bank (Fig. 1A) is a structural high in the western Galicia continental margin (Atlantic NW Iberian Peninsula). From the 1970 s to the 1990 s this

⁎ Corresponding author. E-mail address: [email protected] (B. Alonso). 0025-3227/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2007.09.012

area was the subject of numerous geophysical and geological studies aimed at determining its geodynamic evolution (Montadert et al., 1974; Laughton et al., 1975; Dupeuble et al., 1976; Mauffret et al., 1978; Groupe Galice, 1979; Boillot and Winterer, 1988; Murillas et al., 1990). Most studies of the sedimentary evolution of the Galicia Margin and surrounding areas (Iberian and Biscay Abyssal Plains, Portugal and Galicia continental margins) have concentrated on the Upper Cretaceous and Cenozoic, using sedimentology, biostratigraphy and

B. Alonso et al. / Marine Geology 249 (2008) 46–63

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Fig. 1. (A) Map of the NE Atlantic Iberian continental margin showing the study area in the SW of the Galicia Bank (white rectangle). The location of Sites 637, 638, 639, 640 and 641 of ODP-Leg 103, Site 398 of DSDP-Leg 47B and Sites 897, 898, 899, 900, 901 of ODP-Leg 149 is also displayed; (B) 3-D multibeam bathymetry of the study area with the location of cores and sedimentary environments. Location of the Prestige (stern and bow) is also shown. Bathymetric contours are in metres; and (C) Location of the cores in topographic profiles perpendicular and along-strike to the study area (I′–I, II′–II, III′–III and IV′–IV). Legend: Smt, seamount.

magnetometry from ODP-Leg 103 Sites 637 to 641 on the Deep Galicia Margin of the Galicia Bank (Boillot et al., 1987; Comas and Maldonado, 1988), DSDP-Leg 47B Site 398 on the southern margin of the Vigo Seamount (Maldonado, 1979), and ODP-Leg 149 Sites 897 to 901 on the Iberian Abyssal Plain (Alonso et al., 1996; Milkert et al., 1996) (Fig. 1A). These Upper Cretaceous and Cenozoic deposits include turbidites, pelagites/hemipelagites, contourites and debrites. Turbidites and pelagites dominate Plio-Pleistocene sequences, whereas contourites and debrites occurred mostly during the Late Miocene. However, sedimentary studies of the Late Pleistocene and Holocene history of the Galicia Bank Region do not seem to have attracted significant attention just since the Prestige naufrage. This paper intends to fill this gap and presents a detailed study of the Late Pleistocene to Holocene sedimentary history in different sedimentary environments of the SW flank of the Galicia Bank based on the analysis of sediment cores (Fig. 1B). This study was included as part

of the detailed geological studies conducted in the area in which the Prestige oil tanker wreck is located (Grupo Prestige, 2004). The main objectives are to define the lithological facies and stratigraphy that characterize the sedimentary environments as well as the near-surface sedimentary history. The interest of this study lies in the fact that it provides knowledge about recent sedimentation on the Galicia Bank Region and also, due its geostructural framework, it will contribute to knowledge of the type of processes and factors (local/regional) governing Late Pleistocene and Holocene sedimentation associated to a deepwater structural high, isolated from hinterland sources (Fig. 1A). 2. Geological setting The study area is located on the SW flank of the Galicia Bank in the Galicia continental margin (NW Iberian Peninsula) (Fig. 1A). The Galicia continental margin resulted from rifting and final break-up between

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B. Alonso et al. / Marine Geology 249 (2008) 46–63

Table 1 Coordinates, water depth, core length and sedimentary environments of the nine gravity cores used in this study Core number

Latitude (N)

Longitude Depth Core Sedimentary (W) (m) length environment (cm)

TG1 TG2 TG3 TG4bis TG6bis TG8 TG9 TG10bis TG11

42°10.8′ 42°12.6′ 42°12.6′ 42°10.9′ 42°12.6′ 42°10.9′ 42°10.5′ 42°10.5′ 41°10.3′

12°03.2′ 12.02.7′ 12°01.9′ 12°01.4′ 12°06.4′ 12°04.1′ 12°03.5′ 12°04.3′ 12.03.7′

3766 3424 3104 3363 4078 3822 3811 4171 3833

293 62 34 66 259 283 275 277 263

Inter-lobe channel Fault scarp Fault scarp Fault scarp Inter-lobe channel Sedimentary lobe Inter-lobe channel Inter-lobe channel Sedimentary lobe

the North American and Iberian Plates during the Early Cretaceous time (Montadert et al., 1974; Boillot et al., 1979a). The overall tectonic style of the Early Cretaceous main rifting event is characterised by a series of tilted blocks bounded by listric faults (Olivet et al., 1984). Tectonic movements observed on the Galicia Bank can be correlated with the kinematics of the North Atlantic, and these movements can be divided into three different stages: pre-rift, syn-rift, and post-rift (Mauffret

and Montadert, 1987; Murillas et al., 1990). The topography was created by faulting, with the crest being subaereally eroded while the half-graben was not exposed (Boillot et al., 1979b; Mauffret and Montadert, 1987). From the structural point of view, the Galicia Bank, together with the Vigo and Porto seamounts, forms a NNW–SSE trending alignment of elevated highs (Monatschal and Bernoulli, 1999) (Fig. 1A). The Galicia Bank is a structural high and is bounded to the north and the west by the Biscay and Iberian Abyssal Plains respectively and represents an isolated region from hinterland source. The region of the Galicia Bank is defined by several morpho-stuctural provinces by Vázquez et al. (2008-this volume), and the study area is located at the half-graben province from 3324 to 4171 m water depth. The top of this bank is located at b 700 m and the surrounding region displays a water depth range of about 5000 m on the northern and western sides and 3000 m on the eastern and southern sides (Ercilla et al., 2006). The Galicia Bank Region displays an irregular and complex morphology (Llave et al., 2008-this volume; Vázquez et al., 2008-this volume). In particular, the study area shows relatively high slope gradients (up to 29°) (Ercilla et al., 2006; Llave et al., 2008-this volume) (Fig. 1).

Table 2 Sedimentological data (textural data, carbonate content, mean grain size, sorting and compositional sand fraction) that characterised to the Late Pleistocene to Holocene sediments in the SW Galicia Bank Lithofacies

Sand (%)

Silt (%)

Clay (%)

CaC03 (%)

Mean grain size (phi)

Sorting (phi)

Biogenic components (%)

Broken planktonic foraminifer (%)

Terrigenous components (%)

Quartz (%)

Turbidites TB TB + Q TT TAB

2–14 0.1–10 2–15 50–84

17–50 24–37 21–37 15–24

43–79 62–74 46–76 0–48

35–60 17–35 17–25 47–78

8.5–9.3 8.5–9.3 7.4–8.6 3.2–5.9

2.0–2.3 2.0–2.5 2.1–2. 8 0.5–1.2

70–90 57–90 23–45 99–100

30–88 67–80 20–30 0–13

10–33 10–50 55–77 0–1

0–10 0–28 20–57 0

Hemipelagites HA HB H/T

11–19 2–16 10–22

16–23 26–32 18–26

59–61 51–68 52–70

69–73 23–57 35–60

7.9–8.2 7.8–8.7 7.4–8.5

2.4–2.9 1.9–2.6 2.4–2.8

95–100 79–100 43–70

0–10 2–25 27–41

0–5 0–21 30–57

0–2 8–14 1–12

Pelagites Pm Psm

6–16 23–64

24–26 14–26

59–61 23–51

56–73 67–74

7.9–8.4 5.6–7.3

2.2–2.7 2.9–3.2

100 100

0 0

0 0

0 0

Debrites D

55–84

8–19

0–31

79–85

2.4–5. 3

0.8–3.6

80–90

0

0

0

2–13

25–40

46–65

15–26

7.4–9.0

1.8–2.4

51–71

5–22

28–83

20–67

Heinrich layers H1, H2, H3

TB, carbonate-rich biogenous turbidite muds; TB + Q, carbonate-poor biogenous turbidite muds; TT, terrigenous turbidite muds; TAB, biogenous turbidite sands; HA, carbonate-rich hemipelagite muds; HB, carbonate-poor hemipelagite muds; H/T hemiturbidites carbonate-poor biogenousterrigenous hemipelagites muds; Pm, pelagite muds; Pms, pelagite sandy muds; D, debrites; and HL, Heinrich layers.

B. Alonso et al. / Marine Geology 249 (2008) 46–63

Here, the main topographic features are a fault scarp in the eastern area, sedimentary wedges and lobes and inter-lobe channels in the central area, and a Main Channel in the western area (Llave et al., 2008-this volume) (Fig. 1). These morpho-sedimentary features are formed by masswasting deposits eroded from the scarp and deposited at its base (Ercilla et al., 2006; Llave et al., 2008-this volume). The sedimentary lobes have elongate positive relief of variable length (4300–9500 m long) and width (1300–4000 m) covering areas up to 30 km2 reaching heights of 65 m to 90 m. They are separated by inter-lobe channels that show linear to slightly sinuous erosive relieves. These valleys display a “V”-shaped cross-section in the proximal zones and a “U”-shape cross-section in the distal parts. Their pathways are slightly sinuous, and all together they form a system that drains the SW flank of the Galicia Bank. The head of these channels are located on the steepest lower zone of the fault scarp. The Main Channel has a NNE–SSW orientation with a “V”-shaped asymmetric cross profile (Llave et al., 2008-this volume). This channel coming down from the Galicia Bank. It goes after passing the study area toward the Iberian Abyssal Plain. This channel represents a collector of eroded sediment from the scarp fault toward the abyssal plain (Ercilla et al., 2008-this volume; Hernández-Molina et al., 2008-this volume).

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3. Materials and methods 3.1. Location The study area covers about 200 km2 and was defined to include the Prestige wreck site and the surrounding area. Nine gravity cores were collected around the stern and bow, divided into four transects, three E–W (I–I′, II–II′, and IV–IV′) and one N–S (III–III′) (Fig. 1B and C). Water depth varies from 3104 to 4171 m and core lengths are from 34 to 293 cm (Table 1). Table 1 summarises their depth, location, recovered lengths, and sedimentary environment. 3.2. Methods Continuous, non-destructive high-resolution measurements of the whole-round cores were obtained with the GEOTEC Multi-Sensor Core Logger (MSCL). The measured parameters include wet-bulk density (by Gamma Ray Attenuation), magnetic susceptibility and P-wave velocity (Weber et al., 1997) at 1 cm intervals. Representative lithological samples were taken from the split cores at 5–10 cm intervals and in some selected sections at 1 cm intervals for analysis of grain size, sand composition, and carbonate content.

Table 3 Thickness lithofacies (in cm and %) in the fault escarp, sedimentary lobe and inter-lobe channel sedimentary environments Location Core

TB Length (cm)

Fault scarp Nothern area TG2 62 cm TG3 34 cm Southern area TG4bis 66 cm

TB + Q (%)

(cm)

TT

TAB

(%) (cm) (%)

HA + HB

(cm) (%) (cm) (%)

0 0

0 0

0 0

0 0

0 0

0 0

0 0

0

0

0

0

0

0

44

29.8 11.3 0

0

0

0

29.8 11.3

14.2

0

0

8.5

3

19.8

7

0

0

0

0

0

5.2

2

207

0 0 0

0 0 0

0 0 26.9

Sedimentary lobe Flank TG11 263 cm 110.6 42 Crest TG8 283 cm 220.7 78

5

Inter-lobe channel Northern area TG6bis 259 cm 13 5 0 0 Southern area TG1 293 cm 192 65.5 0 0 TG9 275 cm 89.4 32.5 107.3 39 TG10bis 277 cm 148 53.4 33.2 12

10.6 3.6 30.5 11.1 11 4

0 0

32 24

HB (cm)

66.7 0

(%)

HT

P

D

(cm) (%)

(cm) (%)

(cm) (%) (cm) (%)

HL

52 70.6

0 0

0 0

0 0

0 0

30 10

48 29.4

0 0

0 0

0 0

0 0

0

0

0

0

0

22

33.3

0

0

0

0

0

0

0

0

0

0

29.8 11.3

0

0

0

0

0

0

0

19.8

7

80

0

0

33.6 13

0

0

0

0

0 0 9.7

63.1 24.11

11.1 3.8 0 0 40.2 14.5

40.1 13.7 30 11.1 0 0

0 0 0

0 0 0

28.7 9.8 10.5 0 0 17.3 0 0 17.7

3.6 6.3 6.4

TB, carbonate-rich biogenous turbidite muds; TB + Q, carbonate-poor biogenous turbidite muds; TT, terrigenous turbidite muds; TAB, biogenous turbidite sands; HA + HB, carbonate-rich hemipelagite muds and carbonate-poor hemipelagite muds of the core top; HB, carbonate-poor hemipelagite muds between turbidites units; H/T, hemiturbidites, P, pelagites; D, debrites; and HL, Heinrich Layers.

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Fig. 2. Selected core photograph (TG10bis) showing the lithofacies types of the SW of the Galicia Bank. Note that the turbidite lithofacies are dominant. Examples of the sand fraction components of the turbidite lithofacies are also shown. Legend: P, pelagite muds; HA, carbonate-rich biogenous hemipelagite muds; HB, carbonate-poor biogenous hemipelagites muds; H/T, carbonate-poor biogenous–terrrigenous hemipelagite muds; D, debrites; TB, carbonate-rich biogenous turbidite muds; TT, terrigenous turbidite muds, TB + Q, carbonate-poor biogenous turbidite muds, TAB, biogenous turbidites sands; and HL, Heinrich layers (H1, H2, H3). Numbers 1 to 3 correspond to location of samples at core.

Sedimentological logs were made of all cores. Special attention was paid to colour, grain size, sand composition, bed thickness variations, primary physical sedimentary

structures and physical properties. Textural analysis was performed using settling-tube techniques for the coarsegrained fraction (b 4 phi) and the Sedigraph-X ray

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technique for fine fraction (N 4 phi) (Micromeritics 5100) (Giró and Maldonado, 1985). The standard deviation adopted for the classification of sediments refers to sorting classes (Friedman and Sanders, 1978; Swan et al., 1979; Alonso and Maldonado, 1990; Alonso et al., 1999). They are the following: 0.50 phi to 0.80 phi for well sorted sediments; 0.80 phi to 1.40 phi for moderately sorted sediments; 1.40 phi to 2.00 phi for poorly sorted sediments; and 2.00 phi to 2.60 phi for very poorly sorted sediments. The sand fraction composition was studied using a binocular microscope counting about 300 grains per sample. The following components were identified and counted: light minerals (quartz, mica, feldspar, and others), heavy minerals, rock fragments, neoformation minerals (pyrite), bioclasts (planktonic foraminifera, benthic foraminifera, pteropods and others that include undifferentiated remains). Core chronology was based on the relative positions of the lithofacies in all nine cores examined and physical properties. Age control is based on 14 C dating. The chronology of sediments refers to the calibrated BP ages which have been determined by Rey et al. (2008-this volume). 4. Lithofacies Five main lithofacies are identified based on texture, colour, carbonate content, sand composition, planktonic foraminifera preservation, and physical properties: (1) turbidites, (2) hemipelagites, (3) pelagites, (4) debrites, and (5) Heinrich sediments. Each type of lithofacies is interpreted in terms of a specific sedimentary process. The main sedimentological data (textural and compositional sand fraction) of these lithofacies are illustrated in the Table 2. Brief descriptions of each of the lithofacies follow. 4.1. Turbidites Four subtypes of turbidite lithofacies were identified: (TB) carbonate-rich biogenous muds, (TB + Q) carbonate-poor biogenous muds, (TT) terrigenous muds, and (TAB) carbonate-rich biogenous sands. The dominant turbidites are carbonate-rich biogenous (TB, up to 74% in thickness) and carbonate-poor biogenous turbidites muds (TB + Q, up to 35% in thickness) (Table 3, Fig. 2). The turbidite muds belong to the E turbidite division of Piper (1978) and correspond to fine-grained turbidites, which are described and well documented in the literature (Piper, 1978; Stow and Shanmugam, 1980; Stow, 1985). We identified graded basal units (e.g. mean 8.0 to 9.0 phi) passing up to ungraded muds (mean about 8.6 phi), which are characterised by moderate to poor

51

sorting (2.0–2.8 phi). These fine-grained muds display intervals of laminated colour bands, finely laminated silts and black lenses of silt and homogenous structureless sediment (Figs. 2 and 3). The subtype TB is composed of dark brown (greyish olive-10Y4/2, pale olive-10Y6/2; light olive brown5Y5/6) muds (mean 8.5 to 9.3 phi) with a high carbonate content (35–60%) (Table 2). These turbidite muds have low sand content (2–14%) which is composed mainly of poorly preserved planktonic foraminifera (up to 88%) (Table 2, Fig. 4). The average compressional-wave velocity is 1438 m/s and the average density is 1.54 g/ cm3. The magnetic susceptibility values are relatively low (6.23 × 10− 5 SI average) with minimums of 1.92 × 10− 5 SI (Figs. 5 and 6). The subtype TB + Q consists of moderate olive brown (5Y4/4) and light olive grey (5Y5/2) muds (mean 8.5– 9.3 phi) with a lower carbonate content (17–35%) than the type TB (Table 2, Fig. 4). These turbidite muds have low sand content (0.1–10%) and their composition consists predominantly of poorly preserved planktonic foraminifera (67–80%) (Table 2); in addition, there is a high presence of some terrigenous components, such as quartz which reaches 28% and other light minerals (mica, feldspar) (15%). The average compressionalwave velocity is 1429 m/s and the average density is 1.48 g/cm3. The average magnetic susceptibility is low (5.81 SI), with a peak at 15.28 × 10− 5 SI at the base of some units (Figs. 5 and 6). The subtype TT consists of muds (mean 7.4–8.6 phi) that are moderate olive brown (5Y4/4) and light olive grey (5Y5/2) in colour, with a low carbonate content (17–25%) (Table 2, Figs. 2 and 4). These turbidite muds have low sand content (2–15%) and their composition is formed predominantly by quartz (20–57%) (Fig. 4), other light minerals (mica, feldspar) and other terrigenous constituents (rock fragments and iron oxide); the biogenic grains (b 45 %) include poorly preserved planktonic foraminifera (20–30%) (Table 2, Fig. 4). The average compressional-wave velocity is 1,392 m/s and the average density is 1.54 g/cm3. The average magnetic susceptibility is relatively high (11 × 10− 5 SI) (Fig. 6). The subtype TAB comprises silty sands (mean 3.2– 3.9 phi) and muddy sands (mean 3.5–5.9 phi) (Table 2), both pale yellow brown (10YR6/2) in colour (Fig. 3). The carbonate content is high (47–78%) (Table 2). The mean grain size shows two types of texture: coarse sands (b3.5 phi) and finer sands (N3.5 phi). These sands are moderately well sorted (0.50 to 0.80 phi) and moderately sorted sediments (0.8 to 1.20 phi) (Table 2). The sand fraction is composed predominantly of well preserved planktonic foraminifera (Table 2). These coarse-

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B. Alonso et al. / Marine Geology 249 (2008) 46–63

Fig. 3. Core photographs showing the sedimentary features (structures and colours) of the lithofacies in the SW Galicia Bank. For legend, see Fig. 2.

grained turbidite display intervals of colour banded, finely laminated with moderately yellowish brown in colour (Fig. 3). The average compressional-wave velocity is 1435 m/s and the average density is 1.56 g/cm3. The average magnetic susceptibility is low (3.75 × 10− 5 SI) (Fig. 7). We note that these calcareous biogenic turbidites show similitudes to “pelagic turbidites” described by Kelt and Arthur (1981) from the sedimentological and physiographic setting point of views. They are generally inferred to be the result of injection of low-concentration, sluggish turbidity currents from spreading flanks (Kelt and Arthur, 1981). 4.2. Hemipelagites Hemipelagites are defined as “deep-sea sediments containing a small amount of terrigenous material as well as remains of pelagic organisms” (Bates and Jackson, 1987). The hemipelagic sediments include structureless, homogenous and highly bioturbated muds (mean 7.4 to 8.7 phi) (Table 2, Fig. 3). They consist of biogenic muds (planktonic foraminifera) with a presence (b57%) of

terrigenous components (quartz, mica, light minerals). They are poorly and very poorly sorted sediments (1.9 to 2.9 phi). Three subtypes of hemipelagites were identified: (HA) carbonate-rich biogenous muds, (HB) carbonate-poor biogenous muds and (H/T) carbonatepoor biogenous-terrigenous muds (Table 2). Subtype HA occurs at the top of most of the cores (Figs. 5 and 6). It consists of light-brown colour (10 YR7/4) mud with high carbonate content (69–73%) and intact planktonic foraminifera (Table 2, Fig. 2). Subtype HB occurs below subtype HA and between turbidite units (Figs. 2, 5 and 6). It comprises dark brown (5Y6/4 and 10YR6/6) and light-grey colours (5Y5/6, 5Y6/4) mud (Fig. 2) with low carbonate content (23–57%), and intact planktonic foraminifera (Table 2). Subtype H/T has a local presence, occurring only at the top of two cores (Figs. 4 and 6). It consists of light-grey colours (5Y5/6) mud with low carbonate content (b60 %) (Table 2, Fig. 4). Mixed terrigenous–biogenous components highly fragmented planktonic foraminifera (27–41%) and a decrease in the clay/silt ratio are observed in these units (Fig. 4). These characteristics allow these beds to be interpreted as hemiturbidites (Stow and Wetzel, 1990).

B. Alonso et al. / Marine Geology 249 (2008) 46–63

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Fig. 4. Analytical data from a selected core (TG 9): sand, silt and clay (%); mean grain size (phi); carbonate content (%); sand composition (%) (biogenous and terrigenous components); intact planktonic foraminifer (Plank. Foram.) tests (%); broken planktonic foraminifers tests (Plank. Foram) (%) in the sand fraction; and quartz content (%) of the turbidites, hemiturbidites and Heinrich layers. For legend, see Fig. 2.

For subtype HA, the average compressional-wave velocity is 1457 m/s, the average density is 1.50 g/cm3, and the average magnetic susceptibility is 4.59 × 10− 5 SI (Figs. 5 and 6). For subtype HB, the average compressional-wave velocity is 1475 m/s, the average density is 1.62 g/cm3, and the average magnetic susceptibility is the lowest (1.64 × 10− 5 SI) (Figs. 5 and 6). For H/T the average compressional-wave velocity is 1426 m/s, the average density is 1.59 g/cm3, and the average magnetic susceptibility is 2.02 × 10− 5 SI (Fig. 6). 4.3. Pelagites Pelagites are defined as “deep-sea sediments without terrigenous material” (Bates and Jackson, 1987). These sediments consist of light-coloured moderately yellowish brown-10YR5/4 (Fig. 3); very pale orange (10 YR 8/ 2) calcareous biogenic muds and sandy muds with intact planktonic foraminifera (Table 2). The carbonate

content is high (56–73%) and texturally are very poorly sorted sediments (2.2–2.6 phi) (Table 2). The mean grain size shows two types of textures: muds (N7.9 phi) and sandy muds (b7.3 phi) (Table 2). The average compressional-wave velocity is 1372 m/s, the average density is 1.81 g/cm3 , and the average magnetic susceptibility is 9.1 × 10− 5 SI (Fig. 7). 4.4. Debrites Debrites consist of pteropods gravel size (millimetres in size) within a matrix-supported bioclasts of also pteropods. The matrix is composed of sands (mean 2.4–3.5 phi), muddy sands (mean 5.6–5.6 phi) and sandy muds (mean 7.1–7.4 phi) (Table 2) that are light olive grey (5Y5/2) in colour (Fig. 8). The carbonate content is high (up to 85%) (Table 2). The sand fraction is composed predominantly of fragmented pteropods shells (80–90%) (Fig. 8). This sediment is poorly stratified, with an upward coarsening

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B. Alonso et al. / Marine Geology 249 (2008) 46–63

Fig. 5. Lithofacies logs and physical properties of cores in the sedimentary lobe environment. The location of the cores is shown in Fig. 1B, and C. Vp, compressional-wave velocity (m/s); D, density (g/cm3); and MS, magnetic susceptibility (x10–5 SI). For legend, see Fig. 2.

grain size trend of the matrix, and a major presence of millimetre-sized pteropod fragments (Fig. 8). The average compressional-wave velocity is 1476 m/s, the average density is 1.48 g/cm 3 , and the average magnetic susceptibility is relatively high (8.69 × 10− 5 SI) (Fig. 6). 4.5. Heinrich sediments Three Heinrich Layers (H1, H2, H3) have been recognised and characterised base on their magnetic properties assemblages by Rey et al. (2008-this volume). These layers were found within specific turbidite mud units and between hemipelagites. They display low carbonate content (17–26%) (Table 2). These levels

contain in most beds abundant terrigenous components (up to 83%) in the sand fraction, mainly transparent quartz (up to 67%). They display an increase magnetic susceptibility and density in most beds (Figs. 5 and 6). The average compressional-wave velocity is 1370 m/s, the average density is 1.6 g/cm3, and the average magnetic susceptibility is 20.75 × 10− 5 SI (Figs. 5 and 6). 5. Chronostratigraphy 5.1. Sedimentary environments Here we refer to the sedimentary stratigraphy of the three modern sedimentary environments previously

B. Alonso et al. / Marine Geology 249 (2008) 46–63

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Fig. 6. Lithofacies logs and physical properties of cores in the inter-lobe channel environment. The location of the cores is shown in Fig. 1B and C. Vp, compressional-wave velocity (m/s); D, density (g/cm3); and MS, magnetic susceptibility (x10–5 SI). For legend, see Fig. 2.

defined by morpho-seismic study by Ercilla et al. (2006) and Llave et al. (2008-this volume) of the SW Galicia Bank. They are: (a) a fault scarp, (b) sedimentary lobes, and (c) inter-lobe channels (Figs. 5 to 7). The different vertical arrangements of the five types of lithofacies (turbidites, pelagites, hemipelagites, debrites and Heinrich Layers) within these sedimentary environments are described in detail. The age of the Heinrich Layer H3 top has been dated at 31.3 ka by Rey et al., while the ages of the other Heinrich Layers H1 (16.6 ka) and H2 (24.7 ka) are taken from Thouveny et al. (2000) (Rey et al., 2008this volume). (a) The stratigraphy of the fault scarp environment (2790 and 3600 m water depth) displays differences from the northern to southern areas (Fig. 7). In the northern area, it is represented by carbonate-rich hemipelagites muds (HA, average

core thickness 61%) that change to pelagites (average core thickness 39%) toward the top (Table 3, Fig. 7). In the southern area, the stratigraphy is defined by biogenous turbidite sands (TAB, core thickness 66.7%) that are covered by pelagites (P, core thickness 33.3%) (Table 3, Fig. 7). The boundary between these lithofacies is diffuse. Three units of pelagites and one unit of carbonate-rich hemipelagites muds are dated respectively at 22.4 ka, 22.2 ka, 5.9 ka and 25.9 ka (Table 4, Fig. 9). (b) The stratigraphy of the sedimentary lobe environment (3500 to 4400 m water depth) shows differences in thickness and lithofacies between the flank and crest (Fig. 5). The flank displays highly variable lithofacies with carbonate-rich biogenous turbidite muds (TB, core thickness 42%), which alternate with carbonate-poor

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Fig. 7. Lithofacies logs and physical properties of cores in the fault scarp environment. The location of the cores is shown in Fig. 1B and C. Vp, compressional-wave velocity (m/s); D, density (g/cm3); and MS, magnetic susceptibility (x10–5 SI). For legend, see Fig. 2.

biogenous turbidite muds (TB + Q, core thickness 11%), carbonate-poor hemipelagite muds (HB, core thickness 24%) and Heinrich Layers (HL, core thickness 11%), (Table 3). Toward the top, the lithofacies change to hemipelagite muds (HA + HB)

(core thickness 11%). Three units of hemipelagic muds yielded calibrated ages of 10.2 ka, 19.5 ka and 19.7 ka (Table 4, Fig. 9). In contrast, the crest of the sedimentary lobe is stratigraphically much more homogeneous, being characterised by abundant carbonate-rich biogenous turbidite muds (TB, core thickness 74%), alternating with a few thin units (b5 cm) of carbonate-rich biogenous turbidite sands (TAB, average core thickness 3%), carbonate-poor biogenous turbidite muds (TB + Q, core thickness 5%) and Heinrich Layers (HL, core thickness 11%) (Table 4, Fig. 5). Hemipelagites prevail at the top of Table 4 Calibrated BP ages (14C dated) of the lithofacies and sediment rates

Fig. 8. (A) Photographs showing an example of a debrite unit composed of pteropods sand size within a matrix-supported bioclast of also pteropods overlying turbidite muds. Note the negative grading and the erosive basal boundary. (B) Photograph showing broken pteropod tests.

Sedimentary environment

Core number

Level (cm)

Lithofacies type

Age (ka)

Fault scarp

TG2

Fault scarp Fault scarp Sedimentary lobe (crest) Sedimentary lobe (flank)

TG3 TG4bis TG8

29 53 4 11 33

P HA P P HB

22.4⁎ 25.9⁎ 22.2⁎ 5.9 13

Inter-lobe channel

TG1

Inter-lobe channel Inter-lobe channel

TG9

29 94 140 30 43 104 120 158 24 30 29 96

HB HB HB H/T H/T D HB D H/T H/T HA HB

10.2 19.5 19.7 8.6 11.1 20.2 17.6 18.6 7.2 9.1 10.7 16.9

TG11

TG10bis

⁎Refers to datings below the precision limit of the 14C method (N21.7 ka) (for details see Rey et al., 2008-this volume).

B. Alonso et al. / Marine Geology 249 (2008) 46–63 Fig. 9. Correlation of turbidite events in the SW of the Galicia Bank. The summarised lithological logs and the core location are also shown. Numbers 1 to 4 refer to the turbidite events from younger to older from 9.1 ka to 31.3 ka respectively. For legend, see Fig. 2. 57

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the section (HA + HB, core thickness 7%) and hemipelagites bases have been dated at 13 ka and 31.3 ka (Table 4, Fig. 9). In both sectors, flank and crest, Heinrich Layers (H1, H2, H3) are present. The inter-lobe channel environment (3600 to N 4600 m water depth) has different stratigraphies in the northern to southern areas (Fig. 6). In the north, it consists mainly of carbonate-poor hemipelagite muds (HB, core thickness 80%) alternating with a few thin (b3 cm) carbonate-rich biogenous turbidite mud units (TB, core thickness 5%) (Fig. 6); toward the top this association of lithofacies changes to pelagites (average core thickness 13%) interrupted by two thin (b 3 cm) units of carbonate-poor hemipelagite muds (HB, core thickness 2%) (Fig. 6). By contrast, the southern area stratigraphy is more heterogeneous (Fig. 6). Here, there are alternations of carbonaterich biogenous turbidite muds (TB, average core thickness 50%), carbonate-poor biogenous turbidite muds (TB + Q, average core thickness 17%), terrigenous turbidite muds (TT, average core thickness 6%), debrites (D, average core thickness 3%), carbonate-poor hemipelagite muds (HB, average core thickness 3%) and Heinrich Layers (HL, average core thickness 7%) (Table 3, Fig. 6). Toward the top these alternating lithofacies, mixed terrigenous– biogenous hemipelagites (H/T, average core thickness 8%) and hemipelagites (HA + HB, average core thickness 3%) prevail (Table 3). The debrites have been dated around 18.6 ka and 20.2 ka, which are similar to Portuguese continental margin pteropods muds (17.8 and 24.6 ka) (Bass et al., 1997). Dating of hemiturbidites yielded calibrated ages of 8.6 ka and 7.2 ka. The ages of hemipelagite mud units are: 17.6 ka, 16.9 ka, 10.7 ka and 9.1 ka (Table 4, Fig. 9). 5.2. Correlation: turbidite distribution The correlation of turbidites through basins has been described many times and various criteria have been used to perform it (Weaver et al., 1986; Weaver and Rothwell, 1987; Davies et al., 1997). However, in this work, it was not possible to correlate all the identified turbidites. Nevertheless, we were able to correlate four individual turbidite events, named 4 to 1 from older to younger, and dated between 31.3 ka and 9.1 ka (Fig. 9). These events have been defined in the lobe and inter-lobe channel environments. They were correlated according to several criteria, including texture, mean grain size, carbonate content, composition of sand fraction, relative stratigraphic position, thickness, age of the underlying hemipelagites and vertical distribution pattern of the physical properties.

Event 4 comprises mainly carbonate-rich, biogenous turbidite muds (TB) and carbonate-poor biogenous turbidite muds (TB + Q) that overlies the Heinrich Layer H3 (31 ka) and underlying the Heinrich Layer H2 (24.7 ka); this event has been defined in those cores at relatively shallow sites (Fig. 9). Both types of sediment belong to the same event, because we consider that carbonate-poor biogenous turbidite muds may have evolved downslope to rich-carbonate biogenous turbidite muds. The explanation offered is the turbidity flow charged mostly with biogenous and some of terrigenous (quartz), moves depositing the quartz particles at a shallowest position whereas the tail of skeletal particles of extremely low effective densities incorporate in the upper part of the carbonate-poor biogenous turbidite muds; downslope, once lost most of the terrigenous, the turbidity flow would deposit the biogenous ones forming the carbonaterich biogenous muds. Event 4 is an important unit taking into account its maximum thickness (up to 110 cm) and extent (Fig. 9). This event affects two sedimentary environments: the inter-lobe channel and the sedimentary lobe (Fig. 9). Vertical mean grain size variation fairly graded muds at the turbidite base (mean 8.3 to 8.8 phi) and ungraded muds at the top (mean 8.9 phi). The lateral distribution of the mean grain size from core to core shows no variation across the different sedimentary environments. Based on Heinrich Layer H2 chronostratigraphy and the results of calibrated ages at base of this event, an age between 31.3 ka and 24.7 ka is assigned for the emplacement of the turbidite event 4 (Fig. 9). Event 3 is defined by terrigenous turbidite muds (mean 7.4 to 8.4 phi) and affects the inter-lobe channel environment (Fig. 9). This turbidite event forms a thin (average thickness 8 cm) fining-upward succession that overlies the Heinrich Layer H1 (16.6 ka). The mean grain size from core to core does not vary. 14C dating of the overlying carbonate-poor hemipelagite muds and the Heinrich Layer H1 chronostratigraphy suggest this event occurred at some time between 11.1 ka and 16.6 ka (Fig. 9). Event 2 consists of carbonate-poor, biogenous turbidite muds and affects inter-lobe channel and sedimentary lobe environments. This pattern of distribution is quite similar to those observed for Event 4 (Fig. 9). Event 2 always underlies the carbonate-poor hemipelagite muds and overlies the Heinrich Layer H1 (16.6 ka). This turbidite event forms a thin unit (average thickness 20 cm) displaying graded muds (mean 7.6 to 8.3 phi). Mean grain size does not vary across the different sedimentary environments. This event occurred between 13 ka and 16.6 ka (Fig. 9). Event 1 is characterised by carbonate-rich biogenous turbidite muds and appears along the inter-lobe channel

B. Alonso et al. / Marine Geology 249 (2008) 46–63

environment. It represents a thin unit (average thickness 20 cm). The vertical distribution of the mean grain size shows graded muds (mean 8.5 to 9.3 phi). The spatial distribution of mean grain size does not vary along the inter-lobe channel environment. This event always underlies the carbonate-poor hemipelagite muds and mixed terrigenous–biogenous hemipelagite muds. The stratigraphic control suggests that this event occurred before 9.1 ka (Fig. 9).

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terrigenous turbidites. We thus interpreted three periods of high terrigenous input: after and before of Heinrich Layer H1 (16.6 ka) and before of Heinrich Layer H3 (31 ka) (Fig. 9). Most of the turbidites are fine-grained sediments. They occur in all sedimentary environments except on the fault scarp environment (southern sector) where coarse-grained turbidites (biogenous turbidite sands) have been deposited. These, which are also called pelagic turbidites (Kelt and Arthur, 1981) represent sediments redeposited in the same source area.

6. Discussion and conclusions 6.1. Sediment source

6.2. Sediment transport and deposition during Late Pleistocene–Holocene

The literature indicates that deep water turbidites are mostly introduced into the marine environment from fluvial and glacial discharges, coastal erosion and eolian transport (Stow, 1985). However, these sources do not seem to have played a relevant role in the study area due to the Galicia Bank represents an isolated region from hinterland sources. Here, the source area of turbidites and debrites is represented by the fault scarp environment (Ercilla et al., 2006; Hernández-Molina et al., 2008-this volume; Llave et al., 2008-this volume). Ancient deposits outcrop in this faulted scarp and its exhumation has triggered mass-movements on the fault scarp, building the major depositional lobes and interlobes channels. Hernández-Molina et al. (2008-this volume) based on the analysis of acoustic facies also suggest that depositional lobes are make up mud/debris flow deposits and inter-lobe channels are formed by turbidity flows. Mass-movements could be triggered by (a) the tectonism responsible for the formation of the fault scarp and (b) the steep slopes (up to 29°). These facts would give an unstable scarp, favouring a depositional/erosive downslope transport that contributes to the development of a rill and gully topography and the accumulation of biogenous turbidite sands on the fault scarp, and of carbonate-rich biogenous turbidite muds, carbonate-poor biogenous turbidite muds and terrigenous muds on the depositional lobes and inter-lobe channels. The source area affects the nature and texture of the turbidites. With respect to the nature of the turbidites, changes in the sand components are observed establishing 2 types: biogenous and terrigenous. The nature of turbidites indicates that sediment supplied by the fault scarp comes from the erosion of the near-surface pelagites/hemipelagites which is supported by the major hiatus at the top of TG3 (the fault scarp), where sediment at 4 cm core depth is dated at 22.2 ka (Fig. 7). In addition, the ancient outcropping deposits (Middle Eocene to Valanginian in age) contribute to the presence of

The last few decades have been times of major reorientation in the sedimentologist's view of deep-sea sediment transport and deposition, and the mechanisms that operate on continental margins and the deep-sea ocean (Evans et al., 1998; Stow and Myall, 2000; Wynn et al., 2002). The sedimentary processes have been divided into three categories taking into account the direction of movement: predominately downslope, alongslope, and vertical flux (Evans et al., 1998). On the SW flank of the Galicia Bank, our sedimentological study indicates that downslope processes predominate mainly in the form of turbidity flows, although the presence of debris flows are also detected. These flows would result from slope failures in the fault scarp. Less common are vertical settling (synonymous to pelagic settling) and slow lateral advection (hemipelagic settling). With respect to the turbidity flows, according to the results of the ODP Site 637 (Fig. 1 A) (Comas and Maldonado, 1988), they began in the upper Pliocene and continued into the Pleistocene. In addition, the Pleistocene and Holocene stratigraphy of this study confirms that turbidite sedimentation predominated during the Late Pleistocene on the SW flank of the Galicia Bank. Here, the 14C dating suggests that the oldest dated turbidite event has occurred about 31.3 ka (Fig. 9). Assuming that each unit corresponds to a single event, we observe that the number of turbidite events varies between different sedimentary environments as well as through the geographic areas (northern and southern). The maximum number (at least 9) was registered in the inter-lobe channel of the southern area. This suggests that here the channelised flows were more frequent and only some of them (at least 3) affected the inter-lobe channel of the northern area (Fig. 9). The relative low presence of turbidite events in the northern area could be related to the predominance of short distance turbidity currents that do not reach the distal most positions of the inter-lobe channels. In fact, the

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analysed core in the northern sector is located in a distal most position. Nevertheless, a relative minor occurrence of turbidity currents with respect to the southern area could be also considered as another explanation. Furthermore, those turbidity currents that travelled short distances are responsible for the formation of biogenic turbidite sands (or “pelagic turbidites”) in the southern area of the fault scarp (Fig. 9). Their low erosive capability could explain the redeposition of planktonic sediment with good preservation of the shells. The erosive mass-movements processes that occur on the fault scarp would favour the redeposition of carbonate-rich biogenous turbidite sands (TAB) accumulated only at areas close to the scarp (b3363 m water depth). The occurrence of these mass-movements has been also evidenced by seismic facies and stratigraphical analyses (Ercilla et al., 2008-this volume; Hernández-Molina et al., 2008-this volume). The great variability of the turbidite events displayed by the cores in spite of the short distance between them (60–100 m), and the differences in thickness (8 to 110 cm) and extension of the four correlated events could be the result of interplay between the uplift movements of the fault scarp, rill and gully topography, gradients, occurrence of channelised and unchannelised flows, and lateral adding of sediment from erosion of the adjacent depositional lobe walls. The combination of these elements in a small size area (b200 km2) favoured a complex turbidite deposition that contrasts with similar studies in larger areas (hundreds of kilometres), where turbidity flows have travelled long distances before their deposition, forming turbidites with a spatial evolution of grain size, e.g. on the Madeira, Agadir, Iberian, and Horeshoe Abyssal Plains (Weaver et al., 1986; Lebreiro et al., 1998, Wynn et al., 2002). Finally, we can tentatively estimate the frequency of emplacement of turbidites based on the number of units and chronological framework. This estimation has been calculated for those cores located on the southern area. In general, the frequency ranges from about 1 turbidite event every 1 ka to 3 ka. The general distribution of this frequency is similar for each sedimentary environment. Thus, the inter-lobe channel environment displays frequencies of about 1/1.2 ka to 1/3.1 ka and the sedimentary lobe crest frequencies of about 1/1.2 ka. The rhythmic development of the turbidites interrupted by hemipelagites could represent the manifestation of different pulses of sedimentary instability. These pulses may be induced by a combination of oversteepening and occurrence of earthquakes at the Galicia Bank (Engdahl & Villaseñor, 2002; Díaz et al., 2008-this volume). With respect to seismicity, the results from the OBS experiment

at the Galicia Bank Region detected seismic events with magnitudes varying between 2.5 and 3 (Ercilla et al., 2006; Díaz et al., 2008-this volume). All the data together allow the authors to state that although the Galicia continental margin displays a moderate level in the global catalogue of seismicity, it can be considered to have a low to moderate level of local seismicity (5 N Mag N 1.5). With respect to the debris flows, this process is responsible for the deposition of two layers of debrites mostly composed of pteropod fragments in the interlobe channel environment (Fig. 6). In order to explain the genesis of these deposits, it is important to consider two general aspects: (a) the pteropod sediments are restricted to only 2.4 % in the deep ocean of the Atlantic due to rapid dissolution of aragonite; and (b) similar pteropod-rich layers have been identified throughout the region from Portugal to Senegal (Diester-Hass and Van Der Spoel, 1978; Sarnthein et al., 1982; Bass et al., 1997; Kalberer et al., 1993) and also on topographically high areas in the Atlantic, Pacific and Indican Oceans (Berger, 1978). The formation of these layers not yet fully understood. In this sense, several pre- and post-depositional processes which are yet fully understood may lead to the formation of these sporadic occurrences of pteropodsrich layers. Detailed sedimentological studies by Sarntheim et al. (1982) and Melker et al. (1992) in Atlantic Ocean reveal that theses layers are correlated with warming pulses combined with reduced coastal upwelling in the surface waters. In our study area we have to take into account that these layers appear only in one core (TG1), being absent in the rest of the cores, event in the nearest ones, and that the pteropods shells are very poor preserved. Because of that we can tentatively consider an increased in pteropods sedimentation due to a mass mortality deposition on shallower areas of the Galicia Bank Region (b 700 m water depth), and subsequent redeposition by mass-movements to form debrites far away at 3766 water depth, on the interlobe channel environment (TG1 in Fig. 6). Hemipelagic settling of marine microskeletons and terrigenous particles, and pelagic settling with absence of terrigenous particle input, are responsible for the deposition of hemipelagites and pelagites respectively. Likewise, hemiturbiditic flows resulted in the deposition of hemiturbidites defined in those core sites close to the fault scarp (TG1 and TG9) (Fig. 7). These have been defined previously as a fine, muddy sediment with partially turbiditic and partly hemipelagic characteristics. We therefore infer that such beds were deposited close to the source area from an essentially stationary suspension cloud that is formed directly from, but

B. Alonso et al. / Marine Geology 249 (2008) 46–63

beyond and above, the dying stages of a lowconcentration turbidity current (Stow and Wetzel, 1990). The deposition of pelagites and hemipelagites corresponding to the core tops occurred mostly during the Holocene (7.2 ka). While deposition of pelagites and hemipelagites in the sedimentary lobe and inter-lobe channel environments occurred during late glacial period (13 ka). In contrast, in the fault scarp, the deposition of the pelagite and hemipalagite lithofacies is older (25.9 ka) in the northern area (Fig. 9). The Heinrich Layers were formed during distinct periods of instability of the Laurentian Ice Sheet leading debriscarrying icebergs into the North Atlantic during the Late Pleistocene (Heinrich, 1988; Broecker et al., 1992). Finally, the change of sediment transport mechanism from Late Pleistocene to Holocene (from predominantly downslope to pelagic/hemipelagic settling) could be related to the lack of sedimentary instability pulses during this period; in this situation, sedimentation is mainly controlled by vertical settling. This interpretation is also supported by other geological works done in this area. For example, the seismic facies study by Hernández-Molina et al. (2008-this volume) and the stratigraphy study by Ercilla et al. (2008-this volume) reveal that mass-gravitational processes should be more active during fault scarp reactivation periods, through the relief rejuvenation, new exposed deposits, and earthquake activity. In addition, taking into account that (a) the sediment source is a fault scarp, (b) the sedimentary lobes and inter-lobe channels result from exhumation of this scarp, (c) the Galicia Bank is an isolated seamount in the continental rise, and (d) the hinterland sources are at a great distance and it is unlikely to consider sea-level fluctuations as a controlling factor in the sedimentary evolution of the SW Galicia Bank. This would be so, only when the tectonic pulse is dominant and obliterates any signal of sea-level fluctuations. Nevertheless, taking into account that the Galicia Bank is b 700 m, this control could be possible. In this sense, Rey et al. (2008-this volume) have recognised the oceanographic changes occurred during the deposition of our recent most pelagites and hemiplagites based on magnetochemical proxies. Also, paleontological studies on hemipelagites and pelagites would be of great interest in order to paleoclimatic approach. Acknowledgements The authors wish to thank the Commander Officer and crew of the BIO Hespérides for their help in collecting the data and the UTM-CSIC technicians for

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their assistance during the cruise. This work was supported by the Comisión de Coordinación Científica (MEC) Special Action, CICYT (MEC) ERGAP project (Ref. VEM 2003-20093-CO3) titled Identificación de riesgos geoambientales potenciales y su valoración en la zona de hundimiento del buque Prestige (Identification of Potential Geoenvironmental Risks in the Sinking Zone of the Prestige, and their Assessment), and CICYT (MEC) SAGAS project (Ref. CTM2005-08071-C0302/MAR-C03) titled The Gibraltar arc system: active geodynamic processes in the south Iberian margins of the Spanish. Likewise, we would like to thank E. Gonthier and L. Carter for corrections and beneficial comments on the manuscript. References Alonso, B., Maldonado, A., 1990. Late Quaternary sedimentation patterns of the Ebro turbidite systems (northwestern Mediterranean): two styles of deep-sea deposition. In: Nelson, C.H., Maldonado, A. (Eds.), The Ebro Continental Margin, Northwestern Mediterranean Sea. Mar. Geol., vol. 95 (3/4), pp. 353–378. Alonso, B., Ercilla, G., Martínez-Ruiz, F., Baraza, J., Galimont, A., 1999. Plio-Pleistocene sedimentary facies at Site 976, ODP Leg. 161: depositional history in the Northwestern Alboran Sea. In: Zahn, R., Comas, M.C., Klaus, A. (Eds.), Sci. Results Ocean Drilling Program vol. 161, 57–68. Alonso, B., Comas, M.C., Ercilla, G., Palanques, A., 1996. Data report: textural and mineral composition of Cenozoic sedimentary facies off the western Iberian Peninsula, Sites 897, 898, 899 and 900. In: Whitmarsh, R.B., Sawyer, D.S., Klaus, A., Masson, D.G. (Eds.), Proc. ODP Sc. Results, 149, College Station, TX 149, pp. 741–754. Bass, J.H., Mienert, J., Abrantes, F., Prins, M.A., 1997. Late Quaternary sedimentation on the Portuguese continental margin: climate-related processes and products. Palaeogeogr. Palaeoclimatol. Palaeocol. 130, 1–23. Bates, R.L., Jackson, J.A. (Eds.), 1987. Glossary of Geology, 3rd ed. Am. Geol. Inst., Alexandria, V.A.. 788 pp. Berger, W.H., 1978. Deep-sea carbonate pteropod distribution and the aragonite compensation depth. Deep-Sea Res. 25, 447–452. Boillot, G., Winterer, E.L., Meyer, A.W., Shipboard Scientific Party, 1987. I., Proc., Init. Repts. (Pt.A). ODP 103, 663. Boillot, G., Auxietre, J.L., Durand, J.P., Dupeuble, P.A., Mauffret, A., 1979. In: Talwai, M., Hay, W., Ryan, W.B.F. (Eds.), Deep Sea Drilling Results in the Atlantic Ocean: Continental Margins and Paleoenvironment, Maurice Ewing Series 3. Am. Geophys. Union, Washington, pp. 138–153. Boillot, G., Dupeuble, P.A., Malod, J.A., 1979. Subduction and tectonics on the continental margin off northern Spain. Mar. Geol. 32, 53–70. Boillot, G., Winterer, E.L., 1988. Drilling on the Galicia margin: retrospect and prospect. In: Boillot, G., Winterer, E.L., et al. (Eds.), Proc. ODP Sc. Results, 103, College Station, TX 149, pp. 809–828. Broecker, W., Bond, G., KLaus, M., Clark, E., McManus, J., 1992. Origin of the northern Atlantic's Heinrich events. Clim. Dynamics 6, 265–273. Comas, M.C., Maldonado, A., 1988. Late Cenozoic sedimentary facies and processes in the Iberian Abyssal Plain, Site 637, ODP Leg 103. In: Boillot, G., Winterer, E.L., et al. (Eds.), Proc. ODP, Sc. Results, 103, College Station, TX, pp. 635–655.

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