Description: Ocean Drilling Program holes 744A and 748B represent key sections for calibration of Southern Ocean Eocene-Oligocene biostratigraphic zonations. Sites 744 and 748 were above the carbonate compensation depth throughout this time interval and contain good planktonic foraminiferal, calcareous nannofossil, and diatom biostratigraphic records. In particular, the Southern Ocean diatom biostratigraphic zonation for the Oligocene critically hinges on calibration of these two holes. Previous low-resolution magnetostratigraphic studies at these sites were hampered by limited sampling and technical difficulties, which prompted our high-resolution reinvestigation of the magnetostratigraphy. Magnetic polarity zonations for holes 744A and 748B were constructed after inspection of vector component plots at 1-cm stratigraphic intervals from continuous u-channel measurements. The magnetizations are generally stable and a robust polarity stratigraphy has been obtained for both holes. The increased resolution of our study and identification of persistent secondary overprints, which were not recognised in previous studies, suggests that the previously published interpretations need to be revised. Our magnetostratigraphic interpretations for both holes are constrained by foraminiferal and calcareous nannofossil datums, as well as by Sr isotope ages. We have calibrated four diatom datums, which are synchronous at the two studied sites, to the geomagnetic polarity timescale, including the first occurrence (FO) of Lisitzinia ornata (27.8 Ma), the FO of Rocella vigilans var. B (27.8 Ma), the FO of Cavitatus jouseanus (30.9 Ma) and the FO of Rhizosolenia oligocaenica (33.8 Ma). The synchroneity of these datums suggests that diatom biostratigraphy has considerable potential for Palaeogene biostratigraphic correlation in the Southern Ocean. Although the ages of some datums are obscured by an unconformity in Hole 744A, our age model from Hole 748B suggests age estimates for the last common occurrence of Rocella vigilans var. A (~29.0 Ma), the FO of Rocella vigilans var. A (30.0 Ma) and the FO of Rhizosolenia antarctica (33.2 Ma). It should also be noted that the last occurrence of the calcareous nannofossil Chiasmolithus altus occurs in Chron C8r rather than C8n in our revised magnetostratigraphic interpretation, which indicates that this datum is not diachronous between low and mid latitudes as had previously been suggested. Significant unconformities are documented in both holes, in the middle Oligocene and in the middle late Oligocene, respectively, which probably resulted from periods of enhanced circumpolar deep-water circulation.

Description: The 624.15 m glaciomarine sedimentary succession recovered in the CRP-2/2A drillcore comprises lower Oligocene (〈c. 31 Ma) to lower Miocene (18.5 Ma) strata that are overlain by a thin succession of Pliocene and Pleistocene strata. The age model for the CRP-2/2A drillhole, as presented in this paper, is based on combined microfossil biostratigraphy. 40Ar/39Ar age on volcanic material, 87Sr/86Sr analyses on mollusc shells, and correlation of a magnetic polarity zonation to the magnetic polarity time scale (MPTS). Between 25.92 and 109.05 metres below sea floor (mbsf), several alternative correlations to the MPTS are possible, all of which suggest that sediment accumulation rates averaged ~180 m/m.y. between unconformities, although actual sedimentation rates may have been higher. Between 109.05 and 306.65 mbsf, the age model is straightforward and average sedimentation rates were much higher (〉1000 m/m.y.). Between 306.65 mbsf and the bottom of the drillcore (624.15 mbsf), fewer datums are available to constrain the age model and unique correlation with the MPTS is not possible, although high average sediment accumulation rates are likely. A significant unconformity is identified at 306.65 mbsf, which may represent as much as 5 m.y. of missing time. Additional unconformities at 25.92 and 130.27 mbsf account for c. 16 and 2.5 m.y. of missing time, respectively. The Oligocene-lower Miocene interval documented in the CRP-2/2A drillcore spans about 13 million years, however, it is possible that more time is missing in sequence-bounding unconformities than is represented in the stratigraphic record recovered in the CRP-2/2A drillcore.

Description: An 823 m thick glaciomarine Cenozoic section sitting unconformably on the Lower Devonian Beacon Supergroup was recovered in CRP-3. This paper reviews the chronostratigraphical constraints for the Cenozoic section. Between 3 and 480.27 mbsf 23 unconformity bounded cycles of sediment were recorded. Each unconformity is thought to represent a hiatus of uncertain duration. Four magnetozones have been recognised from the Cenozoic section. The record is complex with several 'tiny wiggles'' recorded throughout. Biostratigraphical or Sr ages, which could be used to link these magnetozones to the magnetic polarity time scale are restricted to the upper 190 m of sediment. Two diatom datums (Cavitatus jouseanus at 48.9 mbsf and Rhizosolenica antarctica at 68.60 mbsf), together with five Sr-isotope dates derived from molluscan fragments taken from between 10.88 and 190.29 mbsf indicate an early Oligocene (c. 31 Ma) age for this interval. The appearance of a new species of the bivalve ?Adamussium at about 325 mbsf, suggests that the Oligocene age can be extended down to this level. This confirms that the dominantly reversed magnetozone (RI), recorded down to about 340 mbsf, is Chron C12r. The ages imply high sedimentation rates and only minimal time gaps at the sequence boundaries. Below 340 mbsf there are no independent datums to guide the correlation of the magnetozones to the magnetic polarity time scale. However, the absence of in situ dinocysts attributable to Transantarctic Flora, if not a result of environmental control, limits the age of the base of the hole to between c. 33.5 and 35 Ma.