Cuaderno Lab. Xeolóxico de Laxe Coruña. 1987. Vol. 12, pp. 35-45 Estructuras de unidades sedimentarias recientes en la plataforma continental de Galicia (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, Spain) 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 Galician Massif in front ofthe «Rías NITTROUNER, 1984). Bajas. (Muros, Arousa, Pontevedra and The analysis of the seismic reflections Vigo). 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 Iberian Peninsula 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).