Morphology, petrography, age and origin of the Fogo Seamounts, eastern Canada

The Fogo Seamounts are located approximately 500 km offshore from Newfoundland to the southwest of the Grand Banks of Newfoundland. This complex seamount chain is Early Cretaceous in age and is partially buried under later continental slope deposits. The seamounts are developed along the northeastern transform margin of the Jurassic central Atlantic Ocean. The Narwhal F-99 well was drilled in 1986 on the continental slope into one buried seamount. In this study, we have compiled unpublished data on the bathymetry, seismic-reflection character, and distribution of the Fogo Seamounts and interpret new petrographic, geochemical and geochronological data from a dredge sample from seamount G in the central part of the seamount chain and from the Narwhal F-99 well. Petrographically, the seamount samples consist of vitrophyric basalt, with clinopyroxene at Narwhal and kaersutite in the dredge sample. Chemically, the samples are olivine basalt with a low Mg number and low concentration of transition metals. Trace element and REE abundances are similar to those of other early Cretaceous volcanic rocks on the southeast Canadian margin, except that LILE are more enriched in basalts on the continental shelf. Sm/Nd isotopes suggest mantle derivation, with ,Nd ranges from 2.3 to 4.7. Overall, the Narwhal basalts are more tholeiitic whereas the basalts at seamount G are more alkalic, showing OIB characteristics, and their range of compositions is well matched by basalts from Hawaii. The dredge sample at seamount G gave a 40Ar/39Ar age of 130.3 ± 1.3 Ma on kaersutite (top Hauterivian on the Gradstein et al. 2004 time scale). A K/Ar age from the Narwhal F-99 well of 127 ± 6 Ma is inconsistent with biostratigraphic zonation, which shows that sedimentary rocks overlying basalt in the well are at least as old as early Berriasian (ca. 145 Ma). Seismic reflection profiles show that a series of coastal transgressions can be recognised above the basalt, with final submergence probably in the Berriasian. Distribution of the seamounts, based on bathymetry and in the case of buried seamounts, magnetic and seismic data, shows that there is no clear linear trend, but rather a broad zone within which seamounts have formed. New bathymetric data shows that flat tops to seamounts, resulting from coastal erosion prior to subsidence and platform carbonate deposition, show no systematic pattern. The broad extent of the seamounts on the margin probably results from the influence of a series of upper crustal faults along the transform margin.