Three non-magnetic seamounts off the Iberian coast

Abstract

The seamounts of the deep oceans are strongly magnetized and are made up of basic volcanic rocks with or without a capping of limestone. Some seamounts of the continental margins differ from these by being weakly magnetized. Three such seamounts are described. Magnetic surveys over them strongly suggest that they do not have cores of basaltic rocks. There is good evidence that pellet limestones are exposed in the flanks of one of them (Vigo Seamount). These rocks resemble pellet limestones of Jurassic age which occur in Portugal. The results of dredging and photography on the top of Vigo Seamount and on Galicia Bank suggest that detrital limestones, porcellanous limestones, and soft foraminiferal limestones, of Cretaceous to Eocene age, are exposed there. The facies of these calcareous rocks resembles that of the Mesozoic limestones of Portugal and southern France. The remainder of the dredged rocks are believed to be ice-rafted erratics from farther north in Europe or America. The results of seismic refraction work on Galicia Bank are consistent with a structure in which 0.4 km of uncemented limestones overlie 4.2 km of hard limestones that rest on a crystalline basement. The mode of origin tentatively suggested for the seamounts involves the formation of limestones during Mesozoic time on a gradually subsiding platform of continental rocks. As there is no substantial gravity anomaly, subsidence must have been accompanied by density changes or by removal of the lower part of the continental crust near the present position of the continental margin.