Mesozoic and Cenozoic Sequence Stratigraphy of European Basins, SEPM Special Publication No. 60 Copyright  1998, SEPM (Society for Sedimentary Geology), ISBN 1-56576-043-3 GLACIOEUSTATIC FLUCTUATIONS: THE MECHANISM LINKING STABLE ISOTOPE EVENTS AND SEQUENCE STRATIGRAPHY FROM THE EARLY OLIGOCENE TO MIDDLE MIOCENE VITOR S. ABREU Petrobra ´s Research Center (CENPES), Cidade Universita ´ria, Rio de Janeiro, RJ, 21949-900 Brazil AND GEOFFREY A. HADDAD Department of Geology and Geophysics, Rice University, Houston, Texas 77005-1892 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 (d 18 O) 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 d 18 O 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 d 18 O 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 Petrobra ´s 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 uniden- tified 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. 1 Current address: UNOCAL Corporation, Sugarland, Texas 77478. INTRODUCTION Vail et al. (1977) defined a depositional sequence as a strati- graphic unit composed of a relatively conformable succession of genetically related strata bounded at the top and base by unconformities or their correlative conformities defined as se- quence boundaries. Vail (1987) later called the depositional se- quence a third-order sequence (duration of 0.5 to 3 my) within a hierarchy of first-order (duration 50 my) to sixth-order (du- ration 0.01 to 0.03 my) cyclicity. Variation of glacioeustasy was proposed as the driving mechanism for third-order depositional sequences throughout Phanerozoic time (Vail, 1987). Assuming a common mechanism for the development of depositional se- quences permitted the correlation of third-order sequences in widely separated depositional basins and led to the age assign- ments shown for third-order sequence boundaries and maxi- mum flooding surfaces in the cycle chart of Haq et al. (1988). Vail’s assumption of a glacioeustatic mechanism for third- order sequences has met considerable skepticism. At the time when third-order sequences were proposed, most geologists be- lieved that the Cretaceous and Cenozoic times were essentially ice free until middle Miocene time. However, a revolution in our knowledge of polar glaciations took place during the 1970s when long pelagic sediment cores with unprecedented strati- graphic continuity were recovered and studied. Using stable oxygen isotope measurements of foraminifers, Emiliani (1955) and Shackleton and Opdyke (1973) demonstrated that there have been many more Pleistocene glaciations than the four identified in continental records from North America and Europe. Stable isotopic measurements of longer deep-sea records re- covered by the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP) have yielded paleoclimate in- formation that has revised our ideas about Tertiary glaciations. Shackleton and Kennett (1975) and Savin et al. (1975) sug- gested that the 18 O enrichment in foraminifera at approximately 14 Ma indicates the initiation of the Antarctic ice sheet. How- ever, many investigators contend that significant ice develop- ment had already occurred by this time (e.g., Matthews and Poore, 1980; Miller et al., 1987, 1991; Prentice and Matthews, 1988; Denton et al., 1991; Woodruff and Savin, 1991; Barron et al., 1991). Continued isotopic research, complemented by sedimentologic and seismic studies of the Antarctic margin and Southern Ocean, now suggests the presence of ice sheets on East Antarctica at least since early Oligocene and probably as early as late middle Eocene time (Poore and Matthews, 1984; Prentice and Matthews, 1988; Hambrey et al., 1989; Miller et al., 1991; Barron et al., 1991; Ehrmann and Mackensen, 1992; Denton et al., 1991; Wright and Miller, 1992). Although the importance of glaciations as a driving mechanism for third- order eustatic fluctuations is questionable during much of the Phanerozoic time, there is little doubt that glaciations have been important during the “ice house world” (Fischer, 1984) extend- ing from at least the early Oligocene to Recent time. Stable isotopes can provide a measure of sea-level (ice vol- ume) change during the early Oligocene to middle Miocene time that is independent of sequence stratigraphic analysis. In- creases in ice volume inferred from stable isotope records can