Magnetic polarity dating of tectonic events at passive continental margins

Abstract

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.