Carbonate-basaltic sediments from Cobb Seamount, Northeast Pacific: Zonation, bioerosion and petrology

Abstract

Six dives were made with “Pisces IV” at depths of −30 to −700 m, and sediments collected by scoop. Bathymetrically zoned bioclastic carbonates rim the seamount. They reflect the zonation of the living fauna but are complicated by relic Mytilus gravels from the Wisconsin low-stand. Successive faunal zones contain prominent echinoderms: echinoid-bivalve-bryozoan (−30 to −90 m); crinoid-brachiopod (−90 to −180 m); coralbrachiopod-ophiuroid (−180 to −500 m); hydrocoral-crinoid-gorgonian-ophiuroid (−500 to −700 m). Sediments down to −360 m average over 80% CaCO 3. Within the photic zone (down to −185 m) they are dominated by bivalves and bryozoans; below this, by benthic and planktonic foraminifera. Bioerosion occurs within the photic zone by algal boring and echinoid/limpet grazing, but at lower rates than during the low-stand. Wave base is −150 m. A conspicuous belt of ahermatypic reefs occurs on pillowed dykes at −310 to −360 m. Gorgonian coral bases perform a rapid secondary encrusting role, e.g. lining eunicid polychaete tubes associated with the coral Lophelia. Basaltic granules, hyalotuff of basaltic composition, feldspar crystal fragments and hyaloclastite grains dilute the carbonate, particularly in deeper water. Manganese nodules occur in the sample at −385 m. These nodules are small and could have formed in 20,000 yrs. High Mn:Fe ratios indicate their formation in an environment of high productivity. A poorly lithified hyalotuff beach, believed to have originated intertidally, is now seen at −310 m. Well-rounded basaltic granules occur down to −537 m and make up the bulk of the sediment at this depth.