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In recent years magnetic polarity stratigraphy has emerged as an accurate tool for absolute correlation between rapidly and continuously deposited sedimentary sequences (e.g., Opdyke, 1972). Furthermore, under favorable circumstances, comparison of such polarity sequences with the established paleomagnetic reversal time scale can allow the assignment of absolute ages to the sedimentary sequence. Attempts have been made to apply this method of dating to DSDP cores with varying degrees of success (e.g., Ryan, 1973; Allis et al., 1974). In principle the technique is limited by the following factors: 1) Assuming an axial geocentric dipole geomagnetic field model, a magnetic polarity reversal should, theoretically, be represented by a change in declination of 180°, and a change in sign of inclination. Unfortunately, DSDP cores are not azimuthally oriented, and in any case the rotary technique used for coring is likely to lead to relative rotation of different parts of the sediment core, so that even relative azimuthal orientation is not necessarily preserved within a single core barrel. Consequently, measurements of magnetic declination are of very limited use in attempting to define the paleomagnetic stratigraphy. 2) Since magnetic inclination is a function of latitude, it is important that the site is situated at a sufficiently high latitude to allow accurate determinations of inclination to be made. 3) The precision of paleomagnetic determinations will generally depend on the intensity of remnant magnetization, which in turn is related to the sedimentary lithology. In general, clastic sediments are more strongly magnetized than biogenic pelagic deposits. 4) The validity of the magnetic stratigraphy depends on a clear demonstration that the magnetization is stable and of primary origin. 5) The existence of hiatuses in the sedimentary record will generally lead to ambiguities in correlation of the observed magnetostratigraphy with the established polarity time scale. 6) It is important that samples should be taken at sufficiently close intervals to allow the detection of all major magnetic epochs and events. A suitable sampling interval would be in the range of 10 to 10 yr. An attempt has been made to determine a preliminary paleomagnetic stratigraphy for a portion of the continuously cored Eocene sediments at Site 366 (Sierra Leone Rise) and Miocene/Oligocene sediments at Site 368 (Cape Verde Rise). Particular problems inherent in this study are: 1) Site 366 is located at a relatively low magnetic latitude (5°41'N) and is unlikely to have undergone significant latitudinal movement during the Cenozoic. The corresponding low paleomagnetic inclination (approximately 11°) places severe limits on the reliability of polarity changes determined from magnetic inclination alone; 2) At both sites the sedimentary sequences are comprised predominantly, of pelagic oozes, with very weak intensities of magnetization.

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A Preliminary Paleomagnetic Stratigraphy for Lower Eocene Sediments at Site 366 (Sierra Leone Rise) and Miocene and Oliogocene Sediments at Site 368 (Cape Verde Rise), Northwest African Continental Margin