Sediment of fine silt grade (10μm median diameter) has been deposited in a flume on beds of various small slopes. The deviation of the remanence due to the slope (the "bedding error" β) has been measured and its variation with the angle of slope α has been ascertained for a geomagnetic field inclination of 80°. The ratio β/α is compared with that observed by Rees (1964) in natural sediments, and with that predicted from a simple theoretical model. The observed and predicted results are in satisfactory agreement for both natural and artificial sediments.
Palaeomagnetic studies of continental margin sediments drilled during IPOD Legs 47a (NW African margin), 47b (NW Portugal margin) and 48 (NE Bay of Biscay and SW Rockall margins) have provided magnetic polarity records for the late Neogene, early Palaeogene and Cretaceous periods. The general pattern of geomagnetic field polarity reversals during late Neogene times is now well established, and the absolute age of major polarity transitions during the past 5 Ma or so has been determined from palaeomagnetic studies of radiometrically dated igneous rocks (see, for example, McDougall et al. 1977). Consequently, a comparison of the late Neogene polarity sequences identified in the sediments of NE Biscay and the NW African margins, with this ‘standard’ polarity reversal time scale, allows the precise determination of sediment accumulation rates, and the times at which significant changes in these rates occurred. These comparisons have led to the recognition of a short but significant hiatus in early Pleistocene times in both areas. Such information has importance in evaluating the recent geological evolution of these margins. For earlier geological periods the correlation between the magnetic polarity time scale and the geological time scale is less well established but studies of the type described here can still provide important information on the timing of certain geological events recorded in the magnetic anomaly patterns of the oceanic lithosphere. For example, in the early Palaeogene sediments cored at Sites 403-405, off the SW Rockall continental margin, a sequence of magnetic reversals has been identified, which shows a good correlation with marine magnetic anomalies 22-24. Since anomaly 24 is the oldest recognizable anomaly in the Atlantic, and lies immediately adjacent to the continental margins, the biostratigraphic age of this anomaly, determined from the nannofossil and dinocyst zonal determinations at Site 404, provides important information on the date of initial rifting of Rockall (together with the rest of NW Europe) from Greenland. The Site 404 results indicate that this important tectonic event occurred in early Eocene times, at about 52 Ma B.P., rather than at 60 Ma B.P. as was originally proposed by Heirtzler et al .(1968). A further example of the potential value of this type of study is provided by the Cretaceous sediments cored at Sites 397, 398, 400A and 402A. A long section of predominantly normal polarity sediments at the latter three sites appears to correlate with the long Cretaceous interval of dominantly normal polarity identified in marine magnetic anomaly patterns. The combination of palaeomagnetic and biostratigraphic studies allows useful constraints to be placed on the maximum duration of this interval, and on the age of short reversals within and below it. This information has direct relevance to the interpretation of Mesozoic marine magnetic anomaly patterns in terms of the history of seafloor spreading and evolution of continental margins during the early stages of opening of the South Atlantic in Cretaceous times.
A magnetic polarity stratigraphy is established for the sediments of Site 397, which is located on the northwest African continental margin off Cape Bojador at the continental rise/slope juncture.The magnetic stratigraphy can be correlated with established geomagnetic polarity time-scales by some use of concurrent biostratigraphy control.The combination of magnetic stratigraphy with a good biostratigraphy allows for a more precise dating of important paleoenvironmental events in the development of this part of the northwest African continental margin in Neogene and Quaternary times.Sediment accumulation rates can be inferred from the detailed magnetic stratigraphy.Below the major hiatus encountered at this site, the Early Cretaceous sediments show predominantly normal magnetic polarity.Correlation to an established polarity time scale indicates probably an uppermost early Hauterivian age.
At Site 516, Rio Grande Rise, southwestern Atlantic, a continuous depositional sequence is present across the Cretaceous/Tertiary boundary.This paper reviews the principal results of magnetostratigraphic, micropaleontologic, sedimentologic, and geochemical studies made across the boundary.
Rare vitric volcanogenic ash but more abundant clay minerals considered volcanogenic in origin are associated with an expanded and essentially complete K-T boundary sequence from Ocean Drilling Project (ODP) Hole 690 C on Maud Rise in the Weddell Sea off East Antarctica. Results at this writing are preliminary and are still based to some extent on shipboard descriptions. Further shore-based studies are in progress. It would appear, however, that the presence of volcanic ash and altered ash in the Danian section beginning at the biostratigraphically and paleomagnetically determined K-T boundary on Maud Rise can be cited as evidence of significant volcanic activity within the South Atlantic-Indian Ocean sector of the Southern Ocean coincident with the time of biotic crises at the end of the Maestrichtian. This is a postulated time of tectonic and volcanic activity within this Southern Hemisphere region, including possible initiation of the Reunion hot spot and a peak in explosive volcanism on Walvis Ridge (1) among other events. A causal relationship with the biotic crisis is possible and volcanism should be given serious consideration as a testable working hypothesis to explain these extinctions.
A Late Cretaceous magnetostratigraphy for the Rio Grande Rise, southwestern Atlantic Ocean, was determined from a paleomagnetic study of a thick sequence of pelagic carbonates recovered during drilling at Site 516.The observed Santonian through Maestrichtian magnetozones were satisfactorily correlated to those already established from the classic section of the Umbrian Apennines at Gubbio.There was limited evidence from Site 516 of a possible geomagnetic fluctuation immediately after the Cretaceous/Tertiary boundary event.
Measurements of the detrital remanent magnetization (DRM) of redeposited deep-sea sediment of the silty clay grade are described. Variations in the magnitude of an observed remanence inclination error are related to conditions of sediment accumulation, contrasted here as grain-by-grain settling from a dilute dispersion or by settling from a concentrated slurry. For these artificial redepositions post-depositional compaction is shown to be a major factor in shallowing the observed inclination from the ambient field inclination. The term compactive DRM is tentatively assigned to describe such behaviour.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
