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Preliminary scientific assessment and conclusions

The two overarching goals of Expedition 313 were to (1) recover a complete and measurable record of Oligocene–Miocene eustatic variation and (2) evaluate models of sedimentation on siliciclastic continental shelves during a time of known eustatic oscillations. Achieving both goals will require considerable shore-based analysis and integration among disciplines, and it is too early to gauge how successful we will be. Nonetheless, there are several positive indications, among which are the following.

Target section recovered

  • Despite penetrating the Miocene/Oligocene boundary in only Hole M0027A, the key target interval of middle to early Miocene was cored in all three holes with 80% recovery.

  • Core quality is in general very good to excellent; poorly lithified sand was a drilling and coring challenge in the post–middle Miocene section above roughly 200 mbsf at all holes, as well as in several older intervals (sometimes several tens of meters thick) in Holes M0027A and M0028A.

  • The value of a drilling transect strategy was demonstrated by the many thin (10–20 m) topset units drilled in one or both of Holes M0027A and M0028A that could be matched by age, facies, log, and/or seismic correlation to the toeset strata seaward of clinoform rollovers.

Good geochronologic control

  • An additional purpose of drilling a transect of holes was to improve the chance of more nearly continuous accumulation in a basinward, deeper setting than is found in the more landward, shallower holes; initial geochronology shows this has worked exactly as planned.

  • Mollusk fragments and benthic foraminifers have thus far provided >100 87Sr/86Sr age dates.

  • Calcareous nannofossils are often abundant and provide ages largely consistent with Sr isotope ages.

  • Dinocysts and pollen prepared prior to the OSP are common in many intervals, and additional studies will be beneficial; the presence of distinctive pollen assemblages restricted in time are present in all three holes, potentially providing narrow and unexpected correlation markers.

  • Shore-based preparation of planktonic foraminifers showed they are common in some intervals, and post-OSP analysis will provide additional age control.

  • Although additional demagnetization of low-susceptibility sediments is required to measure remanent inclination values, certain fine-grained intervals have shown that magnetochronology will assist in our final age control.

Reliable paleobathymetric indicators

  • Benthic foraminifer assemblages are found in fine-grained intervals in each hole, providing an indication of water depths (relative sea levels) at the time of deposition.

  • Thanks to excellent core quality, primary sedimentary structures (e.g., low-angle cross-bedding indicating a shoreface environment, storm beds preserved as fining-upward sharp-based sands, etc.) provide an independent measure of paleobathymetry.

  • Vertical facies successions across surfaces and within sequences yield trends in paleodepths that provide broad-scale indicators of depositional environments and suggest a range of sediment transport mechanisms.

Extensive physical measurements

  • Gamma ray response was logged through the pipe for 98% of the 2065 m of drilled hole, vertical seismic profiles were acquired in 83%, and a combination of five logging tools deployed in open hole covered roughly 46% of the entire drilled section; these data provide properties of the sediment and pore fluids even in the ~20% of the cored intervals not recovered.

  • The MSCL provided continuous measurements of core properties and core-wireline log correlation to confirm accurate ties between these data.

  • Excellent quality seismic reflection images provide stratal geometry that, with accurately depth-registered physical characteristics from logs and MCSL data, will lead to confident seismic-core correlation.

These results suggest we will be able to date the lower to middle Miocene section with accuracy sufficient to compare breaks in the record with times of sea level lowering predicted by the ages of δ18O glacio-eustatic proxy. We are confident we will be able to match these breaks to seismic sequence boundaries that have been traced to drill holes dated on the slope and to the shoreline where extrapolation to sediments in the modern coastal plain have also been dated in drill holes. We already know we have recovered sediment (mostly in our most basinward Hole M0029A) not found previously in the coastal plain, presumably because of nondeposition/erosion during times of sea level lowstand at those updip locations. Dating and matching sediments to the seismic record is only part of the objective; incorporating estimates of paleobathymetry, compaction history, and basin subsidence will be needed to backstrip this transect and arrive at estimates for the magnitude of eustatic (global) sea level change. This task lies ahead, but for now we know we have datable, nearly continuously deposited sediment of the right age to compare to a sea level proxy. Furthermore, we will have the ability to build on (1) excellent core quality and continuity, (2) firm core-log-seismic integration, and (3) a transect across several clinoforms in developing a model of siliciclastic successions during a time of known large eustatic variations.