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doi:10.2204/iodp.proc.341.106.2014

Core-log-seismic integration

Three seismic profiles cross Site U1420: GOA2505 (Fig. F30) and GOA2502 (Fig. F31), acquired in 2004 aboard the R/V Maurice Ewing, and STEEP09 (Fig. F2), acquired in 2008 aboard the R/V Marcus Langseth. In preparation for core-log-seismic integration, we interpreted key seismic horizons that mark a change in acoustic facies or a reflector truncation surface. Horizons H1, H2, and H3 were previously interpreted by Worthington et al. (2008, 2010). Here, we name subhorizons using the Worthington et al. (2008, 2010) naming convention. Additional internal packages are broken out, defined by either a high-amplitude, continuous reflector or a minor change in seismic character.

For preliminary correlation between Site U1420 lithostratigraphic and logging units with features observed in seismic data, we converted lithostratigraphic and logging unit boundaries from depth in meters CSF-A/WMSF to TWT using the average sonic velocity of each unit. Average P-wave velocity was derived from core physical properties measurements using data from the PWC within lithostratigraphic Unit I and the downhole sonic logs at depths between ~94 and 282 m WMSF (see “Physical properties” and “Downhole logging”). Below ~282 m WMSF, we used values calculated from a linear trendline of the downhole sonic log. We weighted the trendline with the lower velocity PWC-determined values measured deeper in the hole. Based on correlations between the sonic logs and PWC values at previous sites, PWC velocities appear to be reduced within intervals of diamict. Because most of the recovered lithology was diamict, we favor the higher velocity trend recorded in the sonic log data. Detailed correlations in this part of the drilled interval require postcruise core-log-seismic analyses.

Each of the seismic profiles exhibits a distinct change in stratal architecture across the regional unconformity marked by Horizon H1 (Fig. F1). At Site U1420, the seismic packages above Horizon H1 are acoustically semitransparent and semichaotic (Figs. F30, F31). Three subpackages are present, bounded by Subhorizons H1A and H1B. Truncations above and below each of these subhorizons indicate that these are erosional surfaces, likely related to glacial dynamics. According to the TWT-depth conversion using both the PWC and extrapolated sonic log values, Subhorizon H1A likely corresponds to the boundary between lithostratigraphic Units I and II (Fig. F11). Lithologically, Unit I is characterized by muddy clast-rich diamict intervals interbedded with clast-poor diamict.

In lithostratigraphic Unit II, core recovery was <10% and the recovered material consisted primarily of washed pebbles and drilled clasts of varying lithologies (see “Lithostratigraphy”). Assuming that the drilled length of the clasts is representative of the average maximum length, some of the clasts in Unit II are boulder grain size. Using the sonic log for TWT-depth conversions, logging data (from ~95 to 282 m WMSF) can be used to characterize a portion of lithostratigraphic Unit II that starts between Subhorizons H1A and H1B and continues across the H1 unconformity and deeper than Subhorizon H2A (Fig. F11). Logging Subunit 1B, defined by an abrupt decrease in both resistivity and NGR (see “Downhole logging”), may coincide with Subhorizon H1B and a transition from seismic transparent facies to stronger coherent reflectors above Horizon H1 (Fig. F11). Logging Subunit 1D, defined by a section of high velocity (average = ~2300 m/s) and high resistivity (see “Downhole logging”), appears to tie with Horizon H1. Logging Subunit 1E coincides with the uppermost aggradational packages that are truncated by Horizon H1 and includes Subhorizon H2A.

Deeper than logging Subunit 1E (>282 m CSF-A/WMSF; Fig. F11), we observe increasing disparity between P-wave velocities measured by the PWC and those extrapolated from the downhole sonic log. This velocity discrepancy creates potential errors in TWT calculation that are cumulative with depth in borehole. For example, the possible TWT at the base of the drilled interval has a range of up to ~200 ms. Figure F11 includes both sets of correlations, but postcruise analysis will be essential for further interpretations. Shipboard results indicate that the lithostratigraphic Unit II/III boundary lies somewhere within the package of bright, continuous reflectors deeper than Subhorizon H2B. Core recovery increased at the top of lithostratigraphic Unit III, and the sediment within this unit consists primarily of clast-poor and clast-rich muddy diamict with occasional intervals of mud with or without clasts (see “Lithostratigraphy”). Based on the traveltime range estimated from our depth-TWT relationships, two intervals of low core recovery (at ~500–540 and ~705–750 m CSF-A) within lithostratigraphic Unit III may well be associated with distinct seismic features deeper in the section than Subhorizon H2B.

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