Cuaderno Lab. Xeolóxico de Laxe Coruña. 1987. Vol. 12, pp. 35-45

Estructuras de unidades sedimentarias recientes en la plataforma continental de (NW de España)

Structure of recent sedimentary units in the Galician continental shelf

REY,]. and DIAZ DEL RIO, V.

Los 'perfiles·· de sísmica de reflexión de alta resolución en la plataforma continental gallega nos permiten conocer la estratigrafía de los sedimentos holocenos. De techo a muro se distinguen: - Un nivel superior de limos holocenos marinos con interestratificaciones de arena. - Un reflector basal que aflora hacia ~·ra zona Inedia y externa de la plataforma continental y formado por varias superficies erosionales debidas a lo reducido de los aportes sedimentarios en ese punto. - Un basamento acústico de naturaleza ígnea y metamórfica con una fuerte respuesta sísmica. La plataforma se caracteriza por el desarrollo de dos cuerpos de arena limosa que se extiende hacia mar abierto desde la zona interior de plataforma hacia el sur y una superficie de progradaci6n en la zona externa de la plataforma. El mapa de isopacas demuestra que los sedimentos marinos holocenos forman ... generalmente un lecho contínuo que alcanza su máximo espesor delante de las rías.

Palabras clave: Galicia, plataforma continental, holoceno, cuaternario, estratigrafía, sondeos sísmicos.

High-resolution seismic reflection profiles of Galician Continental Shelf show the shallow seismic stratigraphy of Holocene sediments. Three acoustic units can be distinguished from top bottom: 36 Rey & Díaz del Río

- a continuous upper layer of Holocene marine muds with interstratifications of sand. - a basal reflector which outcrops towards the middle and outer continental shelf, forming several erosional surfaces due to reduced sediment supply. - an acoustic basement, of igneous and metarnorphic nature; with strong seismic response. The shelf is characterized by the development of two bodies of sandy mud that extends offshore from the inner shelf toward the south and a progradation surface on the outer shelf. The isopac map shows that the Holocene marine sediments generally form a continuous layer that reachs a maximum thickness in front of the «rias».

Key words: Galice, continental shelf, holocene, quaternary, stratigraphy, sismic profiles.

REY,).; DIAZ DEL RIO, V. (Instituto Español de Oceanografía, Laboratorio Oceanográfico, Apartado 285, Fuengirola, 29640 Málaga, )

INTRODUCTION by the tectonic revival of Hercynian discon­ tinuities (SCHEU, 1913). The northwestern continental shelf of . The regional bathymetry (Fig. 1) and Spain is currently the object ofa multidisci­ the structure of the continental shelf are plinary oceanographic study in a cooperati­ well known due to the numerous surveys ca­ ve investigation Program carried out by rried out on the continental border as well Spanish and North-American investigators. as in the «Rias. (KOLDI]K, 1968; ARPS The Program includes studies in the and KLUYVER, 1969; DE ]ONG and fields of biology, physics, chemistry and POORTMAN, 1970; PANNEKOEK, 1970; geology. LAMBOY and DUPEUBLE, 1971; LAM­ The objective of this paper is to present BOY and DUPEUBLE, 1975; LAMBOY, the preliminary results of seismic-strati­ 1976; AUXIETRE and DUNAND, 1978; graphic analysis ofthe uppermost sedimen­ MOUGENOTand VANNEY, 1980; VAN­ tary layers on the continental shelfthat bor­ NEY and MOUGENOT, 1981; DIAZ and der the in front ofthe «Rías NITTROUNER, 1984). Bajas. (Muros, Arousa, Pontevedra and The analysis of the seismic reflections ). Information·also is provided by the profiles has provided a spatial view of the correlation of high-resolution seismic reflec­ upper layer of the Holocene sediments as tion profiles with sediment samples collec­ well as the variation in thickness and the ted during the study. The so-called «Rias. 'character of the seismic response. The track are systems of river valleys initiated by the lines include transects in the shelf as well as drowning ofthe continental shelf. The con­ one transeet in each ofthe three Rias; Arou­ cept of«Rias Altas., «Rias Bajas. and Estua­ sa, Pontevedra and Vigo. In additión, three ries., from a geomorphic point of view, .sonograph profiles were made using a side considers climatic effect combined with sea sean sonar system in the Rías (Fig. 1). The level changes and tectonic variations caused analysis of the sonographs shows that the Estructuras de unidades sedimentarias recientes 37

Q 10,

40'

... :.- ..;~··:PORTUGAL .,.:-:.. 40' :.

Fig. 1. Map showing generalized bathymeuy ofthe shelfin front ofcRias Bajas~ and location ofthe seismic profiles and samples collected. The thicker lines show the location ofilie profiles depicted in Fig. 2, 3 and 5. 38 Rey & Díaz del Río bottom is accoustically soft in the inner part ding Galicia and Northern Portugal is an of the Rias, whereas in the mouth sand­ Atla?tic passive type (HEEZEN, 1974). The patches and rocky outcrops have been de­ contInental shelf and break are slightly va­ tected. This event is in part due to the exis­ riable in depth. Bathymetry shows that the tence of a regionally continuous layer of shelf in front of «Ria de Muros» is 35 Km Holocene sediments composed of sandy wide and towards south progressively na­ mud with several interstratified sandy la­ rrows to 25 Km in width in front of«Ria de yers. Pontevedra». From this point, the shelf wi­ dens to 35 Km off the mouth of Miño Ri­ ver. METHODS The shape of the shelf is generally flat a?d slopes gently seaward. Small irregulari­ The surveyed area comprises a part of tles are present due to the existence ofrocky the continental shelf between parallels outcrops and coarse-sandy benches. The in­ 42 0 42' N and 41 0 53' N and includes the ner shelf is characterized by proximal sedi­ Rias of Muros, Arousa, Pontevedra and Vi­ ments produced by coastal erosion and lit­ go. The present study consists mainly of toral drift, but in the mouth of the «rias» sub-bottom profiling (900 Km) from the profluvial sediments occur as submarine inner and outer shelf, to a water depth of «beaches». From the Hydrographic Charts it 200 m (Fig. 1). is possible to infer numerous bathymetric The seismic profiles and samples were irregularities related to a rocky belt which collected aboard the O/V «Cornide de Saa­ borders the entire littoral. In the mouth of vedra» during the oceanographic cruise the «rias» the sedimentary units cover these «Breogan-485» on April 1985 and rocky outcrops. The middle and outer shelf «Breogan-586» on May 1986. For recording are characterized by little to no relief. To seismic profiles an ORE 3.5 kHz high reso­ t~e sout?, there is a consolidated outcrop­ lution profiler and a 100 kHz KLEIN side plng unlt related to subaerial'erosion foHo­ scan sonar (150 m slant range scale) were wing a faH in sea level (VANNEY and used in the survey. Seismic records were MOUGENOT, 1981). complemented with gross-lithologic data from gravity cores (2 m long), Shipeck grab This area is an example of a continental samples, drags and box-cores collected in shelfmolded by an erosion surface ofOligo­ the study area. cene age, which was later modified during Although sedimentological analysis of the Quaternary periodo The origin and the samples are still underway, visual exa­ orientation of the main morphological fea­ mination has helped to interpret the sedi­ tures are structurally controlled (AUXIE­ mentological facies recorded in seismic re- . TRE and DUNAND, 1978). At the Shelf­ flection profiles as well as sonograph profi­ break, the sedimentary units are represen­ les. ted by distal progradational units (MOU­ The navigation systems consisted of ra­ GENOT and VANNEY, 1980). The source dar, satellite an Loran C. of sediment forming the depositional units of the shelf are basically the igneous and metamorphic rocks which constitute the ad­ REGIONAL FRAMEWORK AND MOR­ jacent emergent region. Littoral transport is PHOLOGY to the south and produces sandy shoals down-drift from rocky outcrops, islands and The continental margin in the river mouths (FRAGA and MOURIÑO, Northwestern surroun- 1982). Estructuras de unidades sedimentarias recientes 39

RESULTS rent sedimentary facies: proximal inner shelf, and distal middle and outer shelf. Seismic stratigraphy The Holocene unit is composed of de­ posits with a low acoustic ,impedance, and From the analysis of the 3,5 KHz can be interpreted as fine-grained sediment seismic-reflection records, three main geo­ (mud to fine sand). The internal reflectors logical units are identified: (1) a very reflec­ of the unit have a greater acoustic impedan­ tive acoustic basement; (2) a coarse sand ce, and are interpreted as interstratified la­ unit corresponding to a Holocene basal le­ yers of coarse sediments (gravels and sands) vel; and (3) Holocene deposits. with lateral changes in seismic character. The acoustic basement composed of ig­ Locally, near the basement outcrops in the neous and metamorphic rocks is very reflec­ inner shelf, the seismic response increases in tive and clearly differentiated from the acoustic impedance. In consequence; the other two geological units. Seaward, the general translucent character is progressively continuity of the reflector is progressively less due to the presence of sand and gravel less distinct due to the increase in the thick­ mixed with bioclastic sediments. ness of the sedimentary layers, as well as in the limitations in penetration of the sei~mic equipment. The acoustic basement out­ Holocene unit isopachs crops in the inner shelf, forming a rocky belt near the coast line. Figura 4 shows the thickness of the Ho­ In the middle shelf, two morphological locene deposits. The isolines are in metres, provinces are defined in relation to the out­ based on a sound propagation velocity assu­ cropping basement; (1) the northern sector, med in 1.8 Km/s. Although the Holocene where the existence of outcrops produces deposits are widley distributed in the study scarps and other discontinuities in the relief area, local discontinuities occur (MOUGE­ of the shelf; and (2) the southern sector, NOT and VANNEY, 1980). which is unaffected by the outcrops (Fig. 2). A regionally continuous reflector is charac­ The thickness ofthe Holocene unit, the­ terized by layering on the acoustic base­ refore, is variable over the shelf with maxi­ mento This reflector has an angular discor­ mum thickness of 15 m northward the dance with the uppermost unit and because mouth of Miño River and minimum thick­ ofits high reflectivity and continuity, is easy ness of 2 m in the outer shelf. Elsewhere, to detect in the seismic records (Fig. 3). The thicknesses are 6 m in front of«Ria de Pon­ Arousa~ deepest point of the reflector is reached in tevedra»; 5 m in front of «Ría de Muros~. the middle shelf, having a slightly convex and 8 m in front of «Ria de geometry. It outrops on the outer shelf and From the isopach map, it is possible to produces an irregular surface, where it can infer two axes ofmaximum thickness orien­ be defined as a hardground (Fig. 2). tated in a N -S direction subparallel to the The texture of the reflector in the out­ coastline, and separated by a non­ crop area is composed of gravels and coarse deposition surface. The first sedimentary sands, corresponding to an erosional surface body is located northward of the shelf, rela­ that originated in a transgressive interesta­ ted to contibutions of «Ría de Muros~. The dial periodo An overlying stratified unit is second one with larger thickness is related acoustically translucent composed of para­ to Arousa, Pontevedra and Vigo «Rias~. Ma­ Hel internal reflectors. This unit belongs to ximum thicknesses are associated with pro­ the Holocene, and is characterized by pro­ ximity to the mouths of the «Rias~ where fluvial accumulation surfaces in two diffe- the outcrops trap (Fig. 5). ~ o :::o ~ ~ g ~ N ~ ~ :::o 5'

Fig. 2. 3.5 kHz seismic record showing the outcropping of the Holocene basal surface. (1) Translucent unit; (2) Holocene basallevel; (3) non deposition surface. See Fig. 1 for location. ~ § e ªo.­ o ~ o.::;:3 o.-~ ~ el) no.-

ns· :;:3 E ~. o~ o·n g ~ Fig. 3. 3.5 kHz seismic profile, showing the discordance between the transparent unit and basal level. (1) Accumulation surface; (2) Holocene unit with ~ non-continuous internal reflectors. See Fig. 1 for location. ~ 42 Rey & Díaz del Río

20'

111 1 ~2 :. [J...o::. 3

Fig. 4. Map showing the isopach of the Holocene deposits, based on 3.5 kHz seismic profiles. (1) Rocky outcrops; (2) Erosional and non-depositional surfaces; (3) Shelf break deposits (contour lines are in metres). r,p

2n 8 ~ o.. o c: oo.: o..~ ~ oti) o.. 3' o o E ~. Pontevedra~. Fig. 5. Seismic profile (A) and side sean sonar (B) records showing a granitic outcrop in the mouth ofthe cRia de Differences in the acoustic response ~ n of both sides of the outcrop are detected. See Fig. 1 for location. o' g o VI

~ \jo.) 44 Rey & Díaz del Río

CONCLUSIONS shelf. Later on, during the Holocene trans­ gressive period, a finer depositional unit The Galician Continental Shelf in front was deposited because of the confluence of of the «Rias Bajas» is characterized by the three main contributions: fluvial proximal, development of a Holocene sedimentary fluvial distal and local currents. unit, deposited on an erosive base level. Ackowledgements We thank our coIlea­ Three main so1.Írces have controIled the gues of the Spanish Oceanographic Institu­ variations in the amount of shelf deposits: te, particularly those from the Coastal Cen­ (1) fluvial sediments from the contribution tres of«A Coruña» and «Vigo» for their help of the rivers of the «Rias». These deposits in coIlecting data. We also thank the offi­ are mixed with the organic sediments pro­ cers and crew of the O/V «Cornide de Saa­ duced inside the «Rias» due to the existence vedra» for their help during the cruise. Spe­ of a high productivity level (LOPEZ JA­ cial thanks are due to Dr. Eduardo López MAR, 1978; TENORE et a¡~, 1984), (2) dis­ Jamar for cooperating in sedimentological tal deposits defined as the suspended, finer analysis. sediments removed from the «Rias» to the Fouding for this study was provided by shelf, (3) local currents, producing a sedi- ~ the Spanish-North American Cooperative ment transport in a southward direction Investigation Program N CA 83/023 coor­ (FRAGA and MOURIÑO, 1982). dinated by Dr. Nicolás González. The reflectivity and the geometrical cha­ We extend our thanks to personnel of racter of the seismic reflectors in the Holo­ the Chesapeake Biological Laboratory; Dr: cene unit suggest that the depositional pro­ Kenneth R. Tenore, Principal Investigator cesses have been repetitive during the Holo­ ofthe Program; Dr. Donald L. Rice and Dr. cene periodo The unit is not affected by Thomas Bianchi for participating in cruise; faulting. and to R. Aviles for typing the manuscript. The recent geological history of the con­ Very special thanks are due to Dr. V. J. tinental shelf can be reconstructed on basis Henry for making a critical view and sugges­ of results obtained from the survey. FoIl­ tions. wing the upper Pleistocene, a transgressive erosiona1 surface was formed composed of gravels and littoral sands. The accumulatlon Recibido, 13-//-88 of the deposits was thicker in the middle Admitido, 10-V-88 Estructuras de unidades sedimentarias recientes 45

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