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Operational strategy

The region between the paleoshoreline and the paleo-inner to middle shelf is the most sensitive region for studying past sea level variations and was therefore targeted by Expedition 313 to obtain estimates of eustatic amplitudes. Reliability of these estimates depends on the accuracy of two separate measurements: (1) paleowater depths determined by lithologic facies and benthic foraminifer associations and (2) basin response to the spatial and temporal changes in sedimentation. Biofacies associations are generally clearest and paleodepth resolution optimal in nearshore to inner neritic environments (<30 m paleodepth), but unfortunately these associations are difficult to date. Consequently, biologic indicators of paleowater depth are best determined in nearshore to middle neritic facies; they become less precise in facies deeper than middle neritic (>100 m; see examples in Miller and Snyder, 1997). Work onshore New Jersey has shown that the best results can be obtained by targeting sequences deposited between 0 and 60 m paleodepth (Kominz and Pekar, 2001). Basin response is a sum of dewatering/compaction of the underlying sediment, thermal subsidence of the lithosphere, and regional bending of the crust due to the load of deposited sediment. Several of these parameters are insufficiently constrained to provide reliable estimates for sea level calculations if applied to just one location; a transect of sites along which smoothly changing values can be assumed provides the best opportunity for calculating these effects on depositional history. In view of these constraints, and following guidelines developed by a JOIDES Sea Level workshop (Watkins and Mountain, 1990) and the JOIDES Sea Level Working Group (JOIDES SL-WG, 1992), the ideal drilling strategy is summarized in Figure F6.

Holes M0027A–M0029A target upper Oligocene to middle Miocene seismically imaged prograding clinoforms that were deposited in inner–middle neritic paleodepths (based on coeval onshore strata deposited in nearshore/prodelta settings). We obtained excellent seismic profiles of these clinoforms at locations that are most likely to record the full amplitude of sea level change: immediately landward of and near the toes of the clinoforms (i.e., across the clinoform inflection point). Modern water depths in Holes M0027A–M0029A are around 34–36 m (Fig. F7), a fortunate "crossover" depth between being too far landward for detailed control on sequence geometry (i.e., thorough seismic control on land is not possible) and too far seaward for affordable commercial drill rigs. Holes M0027A–M0029A were optimally located to sample several clinoform packages across a 22 km transect.