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Abstract

One of the most difficult challenges of sequence stratigraphy is the establishment of synchrony between events observed in widely separated depositional basins. Problems arise primarily because the chronostratigraphic control in most passive margins is not adequate to constrain the ages of sequence boundaries to better than plus or minus a few million years. This resolution is often insufficient for the correlation of third-order sequences. Furthermore, unless a common mechanism affecting eustasy is assumed, such as variations in the volume of ice on the planet, there is no a priori reason to expect that sequences of similar age in widely separated basins are indeed synchronous.

The stable oxygen isotope composition (δ18O) of marine carbonates is an independent proxy for ice volume (sea level) which has been under utilized in sequence stratigraphic analyses. This is somewhat surprising given the number of studies that have established a good relationship between foraminifera δ18O and ice volume in Pliocene to Pleistocene units. This paper builds on the work of Miller et al. (1987, 1991) and Abreu and Savini (1994) in identifying major Oligocene to middle Miocene isotope events and correlating them to sequence stratigraphic events. Identification of isotope events is based on δ18O data from DSDP sites 522, 529, 563, and 608, and ODP Site 747, drilled in abyssal water depths in the Atlantic and Indian oceans. These isotope records were used by Miller et al. (1991) to define Oligocene and Miocene oxygen isotope zones. In addition to the DSDP/ODP sites, we present oxygen and carbon isotope data from Petrobrás Well A drilled in bathyal water depths in the campos Basin on the Brazilian passive continental margin. Detailed biostratigraphy indicates that this well contains a fairly complete Oligocene to middle Miocene record.

Ages of common isotope events in DSDP and ODP sites and Well A correspond remarkably well with the ages of Oligocene to middle Miocene sequence boundaries identified by Hardenbol et al. (this volume) and Vakarcs et al. (this volume) and correlated to the new time scale of Berggren et al. (1995). Because of the good correlation between the isotope and sequence stratigraphic records, we reconfirm that ice-volume change is the common mechanism driving both oxygen isotope and sea-level fluctuations from Oligocene to present time. We propose four previously unidentified early Oligocene to middle Miocene heavy oxygen isotope events that correlate with sequence boundaries identified in the Pannonian Basin (Vakarcs et al., this volume) and presented in the new cycle chart of Hardenbol et al. (this volume). Additionally, we suggest new chronostratigraphic positions for most of the heavy oxygen isotope zonal boundaries observed previously by Miller et al. (1991). We also present the chronostratigraphic positions for minimum ice-volume events (maximum flooding surfaces) determined from the isotopic record.

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