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.

Paleomagnetism of the Joban Seamount Chain, Northwestern Pacific.

“…seamounts (BLACK et al, 1964). However this possibility may not be the case for Mizunagidori seamount because this seamount lies along a chain of seamounts that are clearly volcanic in origin.…”

Introduction, Objectives, and Principal Results: Ocean Drilling Program Leg 103, West Galicia Margin

“…Basement rock and overlying pre-rift sediments have been sampled only by dredges (Black et al, 1964;Dupeuble et al, 1976;Boillot et al, 1979;Mougenot et al, 1985), which recovered (1) shallow-water platform carbonates of Late Jurassic to earliest Cretaceous age, locally interbedded or capped by micrite with calpionellids (Dupeuble et al, this volume), (2) unfossiliferous red sandstone, lithologically similar to Triassic continental sediments in the Portugal Basin, and (3) metamorphic or granitic rocks, similar to the crystalline basement rocks cropping out onshore on Galicia or Portugal.…”

The Continental Margin off Galicia and Portugal: Acoustical Stratigraphy, Dredge Stratigraphy, and Structural Evolution

“…When the IPOD project in the Galicia area was initiated, published regional information was scarce (Black et al, 1964;Funnel and Smith, 1968;Montadert et al, 1974). The three French institutions interested in the program (Centre Océanologique de Bretagne-CNEXO, the University of Paris "Groupe d'Etude de la Marge Continentale," and Institut Français du Pétrole) joined the "Galicia Group."…”

“…Numerous samples from Vigo, Porto, and Galicia seamounts and surrounding relief allow us to construct a regional stratigraphic sequence (Black et al, 1964;Funnel and Smith, 1968;Dupeuble et al, 1976Dupeuble et al, , 1977 unpublished data of the Hespérides cruise, 1976) and to identify (to a certain extent) Formations 1 to 4 (observed on reflection seismics) and acoustic basement ( Figure 11).…”