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Paleomagnetic results are described from IPOD Holes 400/400A, 401, and 402A in the northeast Bay of Biscay, and Sites 403,404, and 405 at the southwest extremity of the Rockall Plateau. A detailed Pleistocene-late Miocene polarity reversal record has been established for Hole 400A, and comparison with the established Pleistocene and late Neogene polarity time scale allows the determination of precise sediment accumulation rates at this site for the past 10 million years. A fairly complete magnetic polarity record also has been derived for the sequence of Early Cretaceous (Aptian-Albian) "black-shales" in Holes 400A and 402A; this record indicates that the entire section recovered was deposited during the long Early Cretaceous interval of dominantly normal magnetic polarity. A sequence of rapid reversals has been identified in late Albian sediments in both holes, and is correlated with the "mixed polarity zone" of similar age at DSDP Site 263 (Green and Brecher, 1974). No evidence has been found for a prolonged interval of reverse polarity in the uppermost Aptian sediments that might correspond with the late Aptian "Gatan zone" identified by Pechersky and Khramov (1973), and correlated by some authors with marine magnetic anomaly MO. The mean inclination of stable magnetism from the 150 samples of "black shale'' from Hole 402A has been used to derive the paleolatitude of this site in Early Cretaceous times, and the resultant value is in good agreement with the Early Cretaceous pole position for western Europe, derived by van der Voo and French (1974), from a synthesis of marine magnetic anomaly data and paleomagnetic determinations from the circum-Atlantic continents. Discontinuous magnetic polarity records have been derived from the Paleocene to middle Eocene sediments recovered from Hole 400A and Site 401, and normal polarity intervals, corresponding with magnetic anomalies 22 and 23 are tentatively identified at sub-bottom depths of approximately 560 and 585 meters in Hole 400A, and 172 and 183 meters at Site 401. At Sites 403 and 404 on the southwest margin of the Rockall Plateau, a thick sequence of volcanic tuffs, siltstones, and glauconitic muds of late Paleocene-early Eocene age was penetrated. The entire 130-meter section at Site 403 and the lowermost 90 meters at Site 404 is reversely magnetized throughout, and is thought to have been deposited during the long reverse polarity interval which preceded anomaly 24. Biostratigraphic studies of these sediments provides important new control on the age of anomaly 24, which has been identified at a depth of 280 meters at Site 404, near the base of nannofossil Zone NP 12, and within dinocyst Zone II of Costa and Downie (this volume). Since this is the oldest magnetic anomaly identified in the northeast Atlantic, these results provide a precise biostratigraphic determination of the age of opening of the northeast Atlantic. Finally, magnetic susceptibility and intensity values are shown to provide a useful basis for correlation between sites. For example, a sharp decrease in susceptibility and intensity occurs within the NP 14 nannofossil zone, close to the early/middle Eocene boundary at both Hole 400A and Site 401. This is considered to represent an abrupt reduction in the rate of supply of terrigenous material at the end of the early Eocene, due to a change in circulation pattern or sediment source area. Similarly, narrow zones of high susceptibility and intensity at certain depths in the volcanogenic sediments of Sites 403 and 404 appear to represent specific tuffaceous horizons, which can be correlated from one site to the other.

Paleomagnetism of Late Mesozoic to Holocene Sediments from the Bay of Biscay and Rockall Plateau, Drilled on IPOD Leg 48