Calibration of Late Neogene Calcereous Plankton Datum Planes with the Paleomagnetic Record of Site 397 and Correlation with Moroccan and Mediterranean Sections

The late Neogene succession continuously cored at Hole 397 has been investigated in detail from Cores 69 to 19 to calibrate datum planes based on calcareous nannofossils and planktonic foraminifers with the sequence of magnetic reversals detected in the cores. Nine datum planes based on calcareous nannofossils, 7 based on planktonic foraminifers, and 15 magnetic reversals were detected from Section 57-3 to Core 19. The interval spans 5 m.y. , and encompasses paleomagnetic Epoch 7 (partim) to the Matuyama Epoch (partim). Correlations are presented for the Bou Regreg section of northwestern Morocco; the Melilla section of northern Morocco; Rio Mazzapiedi section of northern Italy (Tortonian stratotype); Cape Rossello, Sicily (Zanclean stratotype); and DSDP Site 132 (Tyrrhenian Basin). The Tortonian/Messinian, Messinian/Zanclean, and Zanclean/ Piacenzian boundaries are biostratigraphically recognized and located with accuracy. INTRODUCTION Purpose This report has multiple purposes. First multidisciplinary research on the geodynamic significance of the late Miocene salinity crisis in the Mediterranean has provided an immense body of information and experience, which we wished to utilize to better interpret the sediments cored off Cape Bojador. In particular, the foraminiferal and nannofossil contents of a number of carefully measured stratigraphic successions have been investigated from Italy (d'Onofrio et al., 1975, Rio et al., 1976) and from northern Africa (Bossio et al., 1976; Barbieri et al., 1976; Arias et al., 1977). Second, we considered the Cape Bojador succession an important deep-sea reference section for the extra-Mediterranean record during the time interval spanning the "salinity crisis" (see Cita in Smith, 1977). In our opinion, the biostratigraphy founded on calcareous nannofossils has the greatest resolution for the stratigraphic interval under discussion; we have nine datum levels available which are usable as zonal boundaries. In addition, we could utilize 15 magnetic polarity boundaries which can be independently calibrated to the nannofossil datums at other locations. Methods Approximately 200 samples are investigated micropaleontologically: one from each section from Core 19 (late Pliocene) to Core 69 (late Miocene). The material received was split in two groups; alternating samples were studied by R. M. at Pisa, and by I. R. and D. R. at Parma. The purpose of this subdivision was to obtain a better, more reliable, double checked stratigraphically controlled interval. Also, with two working groups, we could take advantage of their slightly different previous experience in regional stratigraphy. One smear slide was obtained from each sample, from which observations were made on the optical polarizing microscope. A general range chart constructed from all the 200 samples is not included here, but it is at DSDP Headquarters at La Jolla and at the disposal of all interested scientists. Approximately 100 samples were investigated in a semiquantitative way, slightly modified after the procedures of Hay and Beaudry (1973). The results of this semiquantitative evaluation of the fossil occurrences are tabulated in Figure 1, where the criteria used for this evaluation are explained in detail. RESULTS From bottom to top, the interval investigated is here subdivided in two major parts: the first encompasses Core 69 to Section 57-4, the second from Section 57-1 to Core 19.