| Summary: | Integrated magnetostratigraphic and biostratigraphic studies of DSDP Sites 563 and 558 (western North Atlantic) show that, except for a short (~2 Ma) early Miocene hiatus, deposition was continuous from time of seafloor formation (Site 563, between Anomalies 12 and 13; Site 558, between Anomalies 13 and 15) through the Miocene at both sites. Several biostratigraphic datum levels, which have been correlated firmly with magnetic polarity chrons elsewhere, allow correlation of the magnetostratigraphy with the geomagnetic polarity time scale. Based upon that calibration, sediment accumulation rates were low to moderate (< 10 m/Ma) in the Oligocene-early Miocene and higher (> 10 m/ Ma) in the middle-late Miocene after a short hiatus (Chron C5E is missing at both sites). The established magnetochronology is used to make direct magnetobiostratigraphic correlations. For the Oligocenelower Miocene samples, these correlations agree well with previously published first-order correlations. However, our correlations differ from the earlier indirect middle to upper Miocene correlations. In particular, indirect methods were previously used to correlate magnetostratigraphic Chron 11 ( = Zone NN9) with marine Anomaly 5A (Chron C5AN). However, Zone NN9 ( = Chron 11) and associated Zone N16 occur in a long normal interval at Sites 563 and 558 that best correlates with marine magnetic Anomaly 5. This reassignment (Chron 11 = Anomaly 5 = Chron C5N) requires an approximately 1.5 2 Ma upward shift in nannofossil zonal boundaries NN7/NN8 through NN10/NN11. The stratotype lower upper Miocene (Tortonian) is biostratigraphically linked with Zones NN9-NN11. Because Zone NN9 occurs in magnetic Anomaly 5 correlative (Chron C5N), the middle-upper Miocene boundary (basal Tortonian) is probably near the base of Chron C5N, with an estimated magnetochronologic age of 10.4 Ma.
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