Ocean Drilling Program, United States

We conducted a combined paleomagnetic and biostratigraphic study on the Cenozoic sedimentary sequences of the Iberia Abyssal Plain west of Portugal, focusing on Ocean Drilling Program Sites 897, 898, and 900. Magnetic directions at these sites were obtained by both shipboard pass-through cryogenic magnetometer measurements and onshore progressive alternating-field and thermal demagnetization experiments of more than 1000 discrete samples. Sedimentary sections of late Neogene age from Holes 897C, 898A, and 900A have recorded a pattern of magnetic polarity reversals that correlates well with the known magnetic polarity time scale for the past 5 Ma, and allows the determination of accurate sediment-accumulation rates. The polarity patterns from the Pliocene-Pleistocene turbidite sequence show that a reliable magnetostratigraphy can be established from early Pliocene to Holocene, including the Gilbert/Gauss boundary (3.58 Ma) through the Matuyama/Brunhes boundary (0.78 Ma). Below the middle Miocene angular unconformity that separates the Pliocene-Pleistocene turbidites from the Miocene and earlier strata, a reliable magnetostratigraphy could not be established because of the extremely weak magnetization of the sediments. This sudden decrease in magnetic signal corresponds in general to a large decrease in iron content and an increase in sulfate concentration, suggesting a magnetic mineral dissolution. Downhole magnetic susceptibility and intensity values provide a useful basis for correlation of this change between sites, and present a magnetic boundary among the Leg 149 sites. This magnetic boundary may be caused by the termination in supply of the terrigenous material at the end of the early Miocene and reflects the occurrence of the tectonic folding event at this time in much of western Iberia. A short sedimentary hiatus in the early Pleistocene, or a decrease in accumulation rate, was recognized at all three sites. This hiatus may represent an oceanographic event that affected a significant part of the Iberia Abyssal Plain region, such as the Arctic glaciation in Pleistocene.