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doi:10.2204/iodp.proc.341.105.2014

Core-log-seismic integration

Seismic profiles and seismic units

Two seismic lines cross Site U1419: GOA3101 (Fig. F37) and GOA3102 (Fig. F38), both acquired in 2004 aboard the R/V Maurice Ewing as a part of a site survey cruise for Expedition 341. In preparation for core-seismic integration, we divided the two profiles into seismic units defined by either changes in acoustic facies or truncation surfaces. Some of these seismic units are characterized by distinct internal packages, which are either separated by a high-amplitude, continuous reflector or distinguished by a minor change in seismic character.

At a gross scale, seismic character across both profiles changes from laterally continuous subhorizontal reflectors (seismic Units A and B) to generally more chaotic facies with intermittent semicontinuous units deeper than Unit B (Units C–G) at ~997 ms TWT at Site U1419. On seismic Profile 3101, an erosional surface is present at the bottom of Unit D and the underlying units, E and F, are truncated between common depth points 300 and 500 (Fig. F37). On seismic Line 3102, the bottom boundary of Unit E forms an erosional surface that truncates Unit F in the common depth point range ~650–700. Strata in Unit F are also truncated by the underlying Unit G, which forms a lobate mound and provides an onlap surface for Unit F (Fig. F38). This mound thins to the northeast, but it is unclear whether it tapers off completely. On Profile GOA3101, units deeper than Unit G (Units H–K) appear conformable; however, Profile 3102 reveals a more complicated geometric relationship in which the underlying units are subhorizontal in the northeast–southwest direction, whereas the overlying unit boundaries and the seafloor show a draped geometry deepening both to the northeast and to the southwest.

To correlate lithostratigraphic units and changes in core physical properties to features observed in the seismic data, we first converted CCSF-B to TWT using the average P-wave velocity of each unit. Average P-wave velocity was derived from physical properties measurements at Site U1419 (see “Physical properties”). For the upper ~25 m CCSF-B, we used velocities from both the PWL and PWC. For the interval between ~25 and 95 m CCSF-B, we used the average of the few PWL and PWC measurements to interpolate seismic velocities. From ~100 m CCSF-B to the base of the hole, we used average PWC velocities within each unit.

Seismic Unit A is defined by two high-amplitude, continuous reflectors between the seafloor and ~937 ms TWT. Lithologically, this seismic unit correlates with a gray diatom ooze interval that comprises the uppermost ~6 m CCSF-B of the core (see “Lithostratigraphy”). Physical properties measured in this interval reveal relatively low MS, P-wave velocity, and density (Fig. F39). Seismic Unit B (~937 to ~997 ms TWT) features conformable, subhorizontal, continuous reflectors. An ~18 ms thick section of negative polarity, low-amplitude, seismically transparent material is located at the top and bottom of this seismic unit. In the core, this interval consists of gray mud with lonestones. Some of the MS peaks in this interval may be associated with occasional sand layers (Fig. F39), whereas other peaks may be associated with individual clasts. The presence of diatomaceous material within this interval correlates with lows in MS at ~16 and ~60 m CCSF-B.

Seismic Unit C (~997–1045 ms TWT) is defined by high-amplitude, semichaotic reflectors with some truncations. According to our depth to traveltime conversion, the lower portion of Unit C correlates to an interval of low MS and low density (Fig. F39). Lithologically, this interval consists of mud with lonestones, but an increase in lonestone abundance relative to lithologies corresponds with Unit B (see “Lithostratigraphy;” Fig. F9). The boundary between seismic Units C and D correlates to the boundary between lithostratigraphic Units I and II. In the lithostratigraphy, this boundary is defined by a change from mud with lonestones (Unit I) to muddy diamict interbedded with laminated mud and thin coarse sand beds (Unit II). Seismic Unit D (~1045–1095 ms TWT) is composed of two packages of high-amplitude, semicontinuous reflectors. The lower portion of this unit correlates with an interval of high density and velocity between ~113 and ~120 m CCSF-B as measured on the core (Fig. F39). Lithologically, this interval consists of muddy diamict that is interbedded with laminated mud and thin coarse sand beds (part of lithostratigraphic Unit II).

Because of poor core recovery, correlations below ~120 m CCSF-B are difficult. Given the average velocities of the recovered material, it appears that we were able to drill through seismic Units E and F (Figs. F37, F38); detailed correlation with the lithostratigraphy will require postcruise analysis. Seismic Unit E (~1095–1105 ms TWT) is a low-amplitude interval with negative upper reflector that is seismically transparent. Seismic Unit F is a high-amplitude, semichaotic facies that is truncated along its upper boundary by seismic Unit E. The lower part of the core was primarily composed of muddy clast-poor diamict, with one small interval of diatomaceous material observed.

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