Paleomagnetic Results from DSDP Site 398

Abstract

Paleomagnetic investigations have been conducted on over 300 sediment samples from sub-bottom depths of between 125 and 1740 meters, and ranging in age from late Pleistocene to Early Cretaceous. Although NRM intensities show a considerable total variation, the section can be divided into clearly defined intervals within which the range of intensities is rather narrow. The transitions from one intensity interval to another generally coincide with lithological changes, particularly color changes, and the most marked intensity change corresponds almost exactly with a hiatus between Cenomanian and younger sediments. Most samples have been subjected to progressive a.f. demagnetization and the majority reached good stable end-points at fields of between 100 and 300 Oe. There is some evidence that the stable remanence is carried by both magnetite and hematite, and that the magnetite and hematite magnetizations are in essentially the same direction. The deviation of the hole from vertical below 1000 meters introduced an uncertainty into the remanence inclinations in unbedded samples. However, by assuming the hole deviation azimuth to be constant with depth and using it as a fixed reference, it was possible using clearly bedded samples to demonstrate that their remanence declinations were very similar, and to determine an average remanence declination with respect to the hole deviation azimuth. It was then possible to apply an appropriate correction to the inclination of unbedded samples. Down to the upper Santonian, approximately equal numbers of normal and reversed stable magnetizations are present; below this, virtually all stable magnetizations are normal. The sampling density does not allow the recognition of a detailed polarity sequence, but the results do not support the existence of an appreciable reversed interval within the Aptian corresponding to marine magnetic anomaly MO, and they suggest that the long Cretaceous normal interval extends back at least as far as the middle Barremian. Stable inclinations show some scatter but, when averaged over à dozen or so samples, are consistently shallower than would be expected from continental reconstructions for the periods of time represented, and it is tentatively suggested that this may be due to sediment compaction.

Magnetostratigraphy of Pliocene Sediments From the Stirone River (Po Valley)

“…We therefore conclude that characteristic primary directions have undergone a -10" inclination shallowing following deposition in these soft Pliocene clays. Such shallowing has been observed in sediments of similar lithology and have been the subject of recent work (Blow & Hamilton 1978;Morgan 1979;Laj et al 1982). The mechanism for this shallowing is unclear in the case of the Stirone section.…”

The ocean-continent boundary off the western continental margin of Iberia-I. Crustal structure at 40°30'N

“…This was correlated with a downhole Aptian/Albian unconformity at 1400m depth (Sibuet et al 1979). The magnetic stratigraphy of the cores down to 1737m depth strongly suggests that polarity interval MO is absent, thereby appearing to confirm the acoustic and biostratigraphic correlation, although sampling was not entirely continuous (Morgan 1979).…”

Application of an Anisotropy‐Based Correction to Relative Paleointensity Estimates of Experimentally Deposited Sediments

“…While inclination shallowing has been described in some deep-sea sediment cores (e.g., Arason & Levi, 1990;Celaya & Clement, 1988;Gordon, 1990;Jarrard & Cockerham, 1975;Kent & Spariosu, 1982;Morgan, 1979), we note that this study may not be directly applicable to natural deep-sea sediments which are used to obtained RPI records, and have been shown to record inclinations close to those expected by the Earth's dipole. Particularly important is that magnetic inclinations obtained from sediment cores are difficult to correct for inclination shallowing because of the limited sample size for each depth interval, which is too small for a significant anisotropy study.…”