Bottom sediments of the Ría de Arosa (Galicia, NW Spain)

The subject of this study is the largest of the ‘classical’ rias in W. Galicia, the Ría de Arosa (length 25 km, depths of over 65 m). The bottom of the ria displays a complex topography. The wide variety of the surrounding rock types results in the supply of heavy minerals and clay minerals of a variegated composition. The different types of sediment occurring in the bay are described. The results of various laboratory analyses of some 400 grab samples form the basis of the discussions, but punch-core and boring samples were also studied. The present sedimentary processes are inferred from data obtained from textural and petrographical analyses in combination with the oceanographical and biological characteristics of the ria. Two types of bioclasts are distinguished: lithothamnion rudite and shell debris. Accumulations of Lithothamnion (two species) are restricted to shallow areas, where they sometimes constitute the entire sediment. The innermost part of the ria lacks calcareous algae; the depth-limit of growth shifts downward on approaching the Atlantic Ocean. Shell debris is found throughout the ria, the amount increasing in the direction of the ocean. Accumulations of shell debris are also encountered in shallow areas. The moderate sorting and the absence of a dominant species in the heavy-mineral association of the samples taken in the bed of the largest river flowing into the ria (the Ulla), indicate that these sediments are not in equilibrium with their present environment. This also accounts for the pebbly sands of the Umia River, the second most important confluent to the ria. The rivers supply sandy particles to the ria only occasionally. Coarse-grained sediments, mainly restricted to the coastal zones, derive from the local bedrock. The distributional boundaries of the eight heavy-mineral associations distinguished in the Ría de Arosa reflect local petrological differences on the mainland, thus indicating the insignificance of long-shore currents. One metamorphic association is exceptional, because the adjoining bedrock is an intrusive granite. The origin of this — presumably relict — sediment found along part of the eastern coast remains a problem. Immaturity of the sandy sediments is confirmed by roundness and quartz/feldspar ratios. Muds and sandy muds are the predominating types of bottom sediments in the Ría de Arosa. Black liquid muds, smelling strongly of hydrogen sulphide, are encountered in the inner part of the ria. Greenish-grey firm inodoriferous muds are found in the central and outer regions. These physical differences are caused by the decrease in both the oxygen content and the velocity of the bottom current flowing into the ria. This water circulation pattern also governs the distribution of the clay minerals. The muds are only partially supplied by the rivers, which carry kaolinite, illite, gibbsite, and other minerals. Marine clay minerals, also consisting mainly of kaolinite and illite but characterized by the additional presence of montmorillonite, are brought into the ria by bottom currents.