Even though many dredge hauls and rock cores from over Galicia Bank (sensu latu) and its flanks have collected continental metamorphic and plutonic igneous rocks (Capdevila and Mougenot, 1988; Mougenot et al., 1986), only at Sites 900 and 1067 along the drilling transect have in situ continental basement rocks been cored. Such rocks are expected on several grounds, one being that a new basement relief chart (Fig. F3) clearly shows continuity between the roughly north-south ridges of, and adjacent to, Vasco da Gama Seamount and the three buried ridges on which Sites 1069, 900, 1067, 1068, and 1065 are located. Site 901 may also lie on a fourth similar ridge.
The cores from Sites 901, 1065, and 1069 all have features indicative of underlying continental crust. Sites 901 and 1065 are 21 km apart on two different tilted fault blocks, each flanked by a normal fault on its western (seaward) side. At both sites, Upper Jurassic (Tithonian) sediments contain clasts of metasediments, which indicate exposure and erosion of nearby Hercynian basement rocks (similar lithologies have been dredged from Vasco da Gama Seamount) (Capdevila and Mougenot, 1988; Mougenot et al., 1986) (Fig. F2). Clasts of shallow-water carbonates are similar to Tithonian limestones recovered from the Galicia Bank margin during Leg 103 (Boillot, Winterer, Meyer, et al., 1987). Tithonian sedimentation in a restricted basin, which was only briefly and intermittently connected to the open ocean, is indicated by the paucity of calcareous nannofossils, the general lack of bioturbation (indicating, together with common nannofossil coccospheres, anaerobic conditions), and the relatively high amounts of organic carbon (Concheryo and Wise, Chap. 7, this volume). Benthic foraminifer assemblages from Site 901 indicate a similar environment (Collins et al., 1996). Sedimentary features at both sites suggest deposition below wavebase by turbidites, with clays transported by low-density currents and sandstones by higher density flows, which carried clasts of metasediments and possibly coeval shallow-water carbonate grains from nearby basement highs. Thus, evidence suggests that during the Late Jurassic, deposition at Sites 901 and 1065 occurred at depths of only a few hundred meters. Assuming Airy isostasy and in the complete absence of any evidence of excessive volcanism or thermal uplift from plume activity, this indicates the sites were underlain by only slightly thinned continental crust at that time.
Site 1069 is located farther west over the southern extension of the ridge that lies immediately west of Vasco da Gama Seamount and dies out southward (Fig. F3). Several tens of pieces of hard low-grade metasediment were recovered from the lowermost 86 m. Micas in this sediment have been 40Ar/39Ar dated at 348 Ma (Manatschal et al., in press). At least three pieces appear to be rounded pebbles. Despite the very hard lithologies recovered, each core was drilled quickly. This suggests a very friable matrix, a conclusion supported by the clay mineral-rich nature of a few small samples that were retained in the cores. Above this equivocal sequence, a thin bed of reworked shallow-water limestone clasts is capped by 10 cm of dark clay with a thin band of latest Tithonian nannofossil ooze, analogous to that encountered at Sites 901 and 1065 (Concheryo and Wise, Chap. 7, this volume). The clay is overlain by lowermost Cretaceous (upper Berriasian-lower Valanginian) slumped nannofossil chalk of an outer shelf-slope, open-marine facies. The chalk is separated by a 50-m.y. hiatus from an overlying deep-water uppermost Cretaceous (uppermost Campanian) turbidite/hemipelagite sequence. Hence, here too, the evidence suggests the site originally lay in water depths of at most a few hundred meters, which again indicates the underlying crust is almost certainly continental, before it subsided during the Cretaceous.
The mafic basement cores from Sites 1067 and 900 are also considered to represent lower continental crust. The evidence for this is described in the next section.