Wireline Log-Based Stratigraphy along a Shelf-Slope Transect in the Canterbury Basin, New Zealand
DESCRIPTION: IODP Expedition 317 drilled four sites in the Canterbury Basin to investigate the relative importance of sea level verses local tectonics and sedimentary processes in controlling margin sediment deposition and preservation. Downhole logging data from these sites were reprocessed and used to provide critical ties between the borehole data and the regional seismic stratigraphy, and characterize key seismic sequence boundaries.
OUTCOMES: The first step was to reprocess the velocity data using the Slowness/Time Coherence processing algorithm of Kimball and Marzetta (Geophysics, 1984). This led to a significant improvement in the Vp data, particularly for Holes U1351B and U1352B. Vp logs recorded during the expedition were generally slower than the velocity model between ~250 and 1200 mbsf, indicating the seismic sequence boundaries were likely to be encountered at shallower depths than those predicted by the model. At shallower sub-seafloor depths (< 250 mbsf), Vp recorded at slope Site U1352 was also slower than the model, whereas velocities at shelf Sites U1351, U1353, and U1354 were faster than the model. Using Vp and density logs to construct detailed synthetic seismograms, a time-depth relationship was determined for the logged interval at each of the four drilled sites. Due to unstable borehole conditions, velocity logs were recorded only in the upper several hundred meters of each hole; these data allow us to construct a time-depth relationship for the shallow sediments and to characterize a number of the regional seismic sequence boundaries. Preliminary synthetic seismograms were constructed during the expedition at three of the four sites; post-cruise work incorporated the reprocessed velocity data from all four sites and more detailed analysis, including an iterative stretching/squeezing process to achieve the best fit between seismic reflection data nearest to each borehole and the synthetic seismograms. Across the drilled transect of sites, seismic sequence boundaries U12 through U10 are well-resolved in the synthetics seismograms and correspond to distinct features in velocity, density, and gamma ray logs. These boundaries are located at abrupt transitions from high gamma ray, lower density, and low velocity below to low gamma ray, higher density, and very high velocity above. Although core recovery through these features is low at several sites, the logs indicate that these boundaries correspond to changes from muddy sediments below to sand-, shell-, and gravel-dominated sediments above. Strong similarities between logs recorded at the three shelf sites (U1351, U1353, and U1354) reflect stratigraphic continuity along the continental shelf. The construction of synthetic seismograms for Expedition 317 sites allow for detailed correlation of logs and cores and will permit improved understanding of the seismic stratigraphy in the Canterbury Basin.