Elsevier

Marine Geology

Volumes 291–294, 1 January 2012, Pages 63-72
Marine Geology

Geochemical and mineralogical characterization of surficial sediments from the Northern Rias: Implications for sediment provenance and impact of the source rocks

https://doi.org/10.1016/j.margeo.2011.11.003Get rights and content

Abstract

A multidisciplinary study of the elemental geochemistry and mineralogical characteristics of the marine surficial sediment in the Northern Rias (NW Iberian Peninsula) has been carried out. The linkages between the marine sediment composition and their potential sources were examined.

The influence of the river-borne sediments is only detected in the innermost part of the three Rias. Regional variations of the mineral assemblages are governed by the source-rock composition of the different geological complexes and the relative source-rock contribution controlled by the continental hydrology. Mineralogical composition of the Ortigueira Ria and adjacent shelf surficial sediments are mainly made up of mafic and ultramafic rocks of the Cape Ortegal complex indicated by the high content of Mg, Mn and chrysotile and riebeckite minerals. In areas nearby Ortegal complex the imprint of heavy minerals present in the surrounding rocks has also been recorded. Barqueiro and Viveiro Rias bed-sediments are influenced by granitic and metamorphic rocks from the Ollo de Sapo complex as revealed by the high contribution of muscovite and quartz.

Mining activities in the continental domain left strong imprints on marine surficial sediments. Pyrite content is high in the innermost areas of the Ortigueira Ria since this mineral is exploited in the Mera River basin, whereas high muscovite percentages characterize the Viveiro Ria owing to the abundance of granitic rocks and its exploitation in the Landro River basin. Quartz content is high nearby Cape Estaca de Bares, induced by the presence of an important excavation of this material.

Highlights

► Geochemistry and mineralogy of the marine sediment in the Northern Rias is presented. ► Spatial variations are controlled by source rocks cropping out in the hinterland. ► Geological setting and fluvial loads determine the geochemical and mineralogical pattern.

Section snippets

Background and objectives

The chemical composition of marine surficial sediments is determined by the composition of river-derived material, that is dependant of the catchment petrology (Cho et al., 1999) as well as the biogenic contribution (Lackschewitz et al., 1994). Variations in minerals, lithogenic components, organic material and trace elements abundance, therefore, are a tool for deciphering possible sediment sources and for discriminating physico-chemical processes affecting the geological record.

The

Geography and geology of the studied area

The north coast of Galicia shows a rough morphology, with cliffs up to 100 m high made up of igneous, plutonic, mafic, ultramafic and metamorphic rocks. Eastward of 8°W (Cape Ortegal), and along the 43.6°N are located the Northern Galician Rias Ortigueira, Barqueiro, and Viveiro (locally named Northern Rias or Rias Altas; Torre-Enciso, 1958). These Rias are coastal inlets with an external open area dominated by marine processes and a partially enclosed estuarine shallow area. Different

Material and methods

Seventy one samples were recovered on July 10–12, 2007 in the Ortigueira, Barqueiro and Viveiro Rias and on May 21, 2008, fifteen sediment samples were collected from the continental shelf (Fig. 1). Sampling surveys were conducted onboard the R/V Mytilus and R/V Lura for stations located at depths greater than 10 m, using a 30-liter Van Veen grab sampler. Sediment sampling from shallower stations was carried out onboard small boats using a 5-liter Van Veen grab sampler. The uppermost sediment

Sediment composition

The Northern Rias sediments were mainly composed of sandy fractions, only the sedimentary material of the innermost parts was muddy (Fig. 2). Sediments from Ortigueira Ria contained a wide range of CaCO3 (0.4–35 wt.%) being even higher offshore in the continental shelf where values reached 90 wt.% in the Station C12. C/N molar ratios (Fig. 3) were higher landwards and closer to the Mera, Landoi and Baleo River mouths reaching 20.4.

As found in the Ortigueira Ria CaCO3 content in Barqueiro

Discussion

Seabed sediments of the Northern Rias and shelf are predominantly composed by sand (Fig. 2), due to the high energetic hydrodynamic conditions and the NW swell (Lorenzo et al., 2007b). The origin of the sediment is related to fluvial input, being one of the most relevant sources of fine detrital material in the innermost part of the Rias. Another possible source of supplies is related to the eroded material from exposed coastal cliffs located from Cape Ortegal area and adjacent beaches. Organic

Concluding remarks

Mineral and geochemical composition of the Northern Rias and shelf sediments were used to reconstruct the pathways of modern sediment inputs from the continental domain to Rias and inner shelf. Surficial sediments in these Rias integrate several sources (marine/terrigenous), petrological, lithological characteristics of the river basins draining the area and the human activities on land. The influence of the river-borne sediments in only detected in the innermost part of the three rias, whereas

Acknowledgments

The authors sincerely thank the crew and staff of INTERESANTE cruise on board the R.V. Lura and Mytilus for their kind cooperation during the sampling. This paper was supported by the Spanish Ministry of Science and Technology through project ‘INTERESANTE’ Ref. CTM2007-62546-C03-01/MAR. We thank two anonymous reviewers for comments that contributed to greatly improve the manuscript.

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