DATA ACQUISITION AND PROCESSING

Seismic Data

The SCREECH survey acquired >3000 km of MCS, magnetic, gravity, and multibeam bathymetric data and 1000 km of wide-angle seismic reflection/refraction data off Newfoundland (Fig. F1). This was a two-ship program with MCS, magnetic, gravity and bathymetric data acquired aboard the Maurice Ewing (Cruise 00-07) and wide-angle seismic reflection/refraction data acquired by ocean-bottom seismometers/hydrophones deployed and retrieved from the Oceanus (Cruise 359-2). Coincident MCS reflection data and wide-angle seismic reflection/refraction data were collected along three primary transects (Fig. F1). MCS data were also collected on lines parallel and perpendicular to all transects, including the locations of both Leg 210 sites indicated by white stars in Figure F1. MCS data were recorded on the 6-km, 480-channel streamer of the Maurice Ewing; these data have a sampling interval of 4 ms, a shot-spacing of 50–62.5 m, a fold of 45–60, a recording length of ~16 s, and a common midpoint (CMP) spacing of 6.25 m. The tuned, 8540-in3, 20-gun array of the Maurice Ewing was the seismic source for both wide-angle and MCS seismic data. SCREECH Line 2, the central of the three primary transects, crosses Leg 210 Sites 1276 and 1277 (Figs. F1, F2), and the MCS data from this line are used for comparison with synthetic seismograms created from laboratory measurements. The profiles presented here were created by prestack time migration; a full description of processing is given by Shillington et al. (2004).

Core Data

Lithology

At Site 1276, 936.9 m of core were recovered between 800.0 and 1736.9 mbsf (Shipboard Scientific Party, 2004). Cored sediments range in age from early Oligocene to earliest Albian–latest Aptian. The sedimentary section was subdivided into five units based on variations in lithology and breaks in age as defined by biostratigraphy (Fig. F3). A brief description of each unit is given below; a complete description of the lithology and biostratigraphy can be found in Shipboard Scientific Party (2004).

  1. Lithologic Unit 1 (753–864.7 mbsf [includes wash Core 210-1276A-1W from 753 to 800 mbsf], middle Eocene–lower Oligocene): Unit 1 is hemipelagic, burrowed mudstone and claystone (~85%) with minor amounts of disorganized muddy sandstone and sandy mudstone deposits (~8%) and grainstone beds (~7%) emplaced by turbidity currents and debris flows. Soft-sediment deformation features are common.
  2. Lithologic Unit 2 (864.7–929.3 mbsf, upper Paleocene–middle Eocene): This unit is dominated by beds that range from grainstone and calcareous sandstone (~40%) to marlstone (~40%), interlayered with lesser amounts of mudrock (~20%). Grainstone and sandstone contain significant amounts of CaCO3 (typically between 30 and 60 wt%), and there is a high proportion of graded beds deposited from turbidity currents.
  3. Lithologic Unit 3 (929.3–1028.0 mbsf, lowermost Campanian–upper Paleocene): Unit 3 consists primarily of mudstone and claystone (~80%) deposited by gravity flows in dominantly graded beds. It also contains subordinate grainstone and calcareous sandstone, marlstone, calcareous siltstone, and sandy mudstone. Grainstone is much less abundant than in Unit 2.
  4. Lithologic Unit 4 (1028.0–1067.2 mbsf, Turonian–upper Santonian): Unit 4 is mostly siliciclastic muddy sandstone, sandstone, mudstone, sandy mudstone, and siltstone. These rocks have reddish hues and are highly bioturbated.
  5. Lithologic Unit 5 (1067.2–1736.9 mbsf, uppermost Aptian/lowermost Albian–Turonian): Overall, Unit 5 is dominated by mudrock (60%–90%) deposited by gravity flows. This thick unit is divided into three subunits. Subunit 5A (1067.2–1129.8 mbsf) is characterized by gravity-flow deposits interspersed with minor amounts of hemipelagic sediments. Subunit 5B (1129.8–1502.1 mbsf) is dominantly hemipelagic, variably calcareous, and carbon-rich mudrock with minor turbidites. Subunit 5C (1502.1 mbsf to the total depth of the hole) contains poorly organized gravity-flow deposits that include sandy debris and graded beds. Diabase sills occur at 1612.7–1623.0 mbsf (Subunit 5C1) and 1719.2 to >1736.9 mbsf (Subunit 5C2) within these sediments. The upper sill is ~10 m thick. The thickness of the lower sill is unknown because drilling terminated before reaching its base, but it is at least 18 m thick. Whole-rock Ar40/Ar39 radiometric dating yielded ages of 105.95 ± 1.78 and 104.7 ± 1.7 Ma (average = 105.3 Ma) for samples from the upper sill and 99.7 ± 1.8 Ma and 95.9 ± 2.0 Ma (average = 97.8 Ma) for samples from the lower sill (Hart and Blusztajn, 2006).

Approximately 96 m of sediment lie between the two sills, including a ~17-m-thick interval from 1693 to 1710 mbsf that has very low velocities (~1.7 km/s) and densities (2.05 g/cm3). The explanation for the undercompaction of these sediments is still uncertain, but it is possible that the sills sealed off this interval and thus prevented normal compaction (Karner and Shillington, 2005; Shipboard Scientific Party, 2004).

Physical Property Measurements

As part of the standard shipboard analysis of all recovered cores, laboratory measurements of physical properties (e.g., density, compressional [P]-wave velocity, thermal conductivity, natural gamma radiation, etc.) were made on whole cores and/or selected samples. Of particular value to the present study are velocity and density (Fig. F3). Horizontal (x) and vertical (z) velocities were measured on representative sediment and rock samples every ~2 m throughout the recovered section (Shipboard Scientific Party, 2004). Cubes of rock ~8 cm3 in size were cut from the working half of the core, and P-wave velocity was measured in three directions using the P-wave velocity sensor 3 modified Hamilton Frame velocimeter. An acoustic signal of 500 kHz is transmitted and received between two transducers, passing though the sample, whose thickness is measured by a digital caliper. We chose to use the measurements of vertical velocity to create synthetic seismograms because they most closely approximate the path of seismic waves recorded in the seismic reflection data. This choice has implications for the time-depth relationship established below 800 m. The difference between vertical and horizontal velocity increased downhole from ~4%–5% at 800 mbsf to ~10% in the deepest sediments (Shipboard Scientific Party, 2004). Horizontal velocities are often faster in sediments because of grain orientation and cementation along near-horizontal bedding planes. The longest offset arrivals in the MCS reflection data will have a significant contribution from horizontal velocities. Therefore, the use of vertical velocities indicates that the depth-time relationship established below 800 mbsf from laboratory measurements will represent the slow end-member.

Two types of density measurements were taken on each core: (1) gamma ray attenuation (GRA) bulk density and (2) moisture and density (MAD). GRA data are evenly spaced measurements of density obtained over the full core before splitting. Although this procedure provides continuous, finely spaced measurements (~2.5 cm), the consolidated sediments and rocks retrieved using rotary core barrel drilling typically fracture when they are recovered, leading to breaks in the core, reduced core volume, and significant artificial variations in the density data. GRA density measurements are also too low because recovered cores do not completely fill the core liner (Shipboard Scientific Party, 2004). These artificial variations in density would cause significant noise when computing synthetic seismograms, and thus the GRA densities were not used. The MAD technique determined wet and dry bulk density, grain density, and porosity on discrete samples taken from every section of each core. Because the samples analyzed were small and hand-selected, they were not as compromised by the fractures that degrade the GRA density measurements, although they are affected by decompaction. Also, because of noncontinuous sampling and imperfect (85%) core recovery, they do not sample the details of density and velocity changes that would be measured by downhole logging. Densities were obtained from the same samples measured for velocity, so any sampling biases will be present in both data sets. Nonetheless, because these data contain fewer artificial variations because of core breaks or core diameter compared to GRA data, they are more suitable for our purposes.

Processing of Physical Property Data

The use of physical property data instead of traditional sonic and density logs requires several additional considerations to ensure that velocity and density measurements are used in a way that best represents the overall core properties:

  1. First, velocities measured on samples that were described as concretions by sedimentologists were removed. This included eight measurements in the hole with vertical component velocities ranging from 2.126 to 4.625 km/s. Concretions are small and anomalous, and thus they are unlikely to contribute to the bulk seismic properties of the subsurface at the frequencies of MCS data considered here (i.e., ~10–100 Hz). If these data were included in the velocity and density functions used to create synthetic seismograms, they could significantly affect the seismograms, which are sensitive to small velocity perturbations. Because these measurements comprise only 8 of 533 data points, their exclusion does not impact the overall depth-time relationship established below 800 mbsf using the remaining data.
  2. Another important consideration when using laboratory velocities is the difference in velocity expected at in situ pressures versus atmospheric pressures where the velocities are measured. When cores are brought to the surface, they undergo decompaction and fracturing because of the change in confining pressure and drilling disturbance; these effects lower the bulk modulus of cored sections, which leads to a decrease in P-wave velocity (Carlson et al., 1986). As with our use of vertical velocities, the decompaction effects will shift our velocity model toward the slow end-member. This effect is considered when we link synthetic seismograms to reflection profiles.
  3. Finally, the physical property measurements were made on hand-selected samples spaced at ~2 m, and thus they might contain biases toward anomalously high- or low-velocity material (Carlson et al., 1986) (for comparison, the sampling interval of the downhole wireline Sonic Digital Tool used for acquiring in situ measurements during logging is 6 inches [0.1524 m]). The danger of bias in laboratory measurements certainly arises in turbidites, where a variety of lithologies occur over a small range in depth. We examined this issue by measuring horizontal velocity through turbidites at 2-cm intervals on the uncut archive half of several cores (Shipboard Scientific Party, 2004). Only horizontal velocities were measured because the measurement of vertical velocities would have required cutting the cores at 2-cm intervals. These measurements show that velocities change downward through the turbidite sequences, from lower velocities (~2.0–2.5 km/s) in finer grained mudrocks at the top of the turbidites to higher velocities in the fine-grained sandstones (~4.5–5.0 km/s). Velocities decrease within the coarse-grained bases of the turbidites (~3.0–4.5 km/s).

These results show that care must be taken when interpolating physical property measurements between locations. For example, a measurement on a sample from a thin interval of a particular lithology could be extrapolated to exist over 2 m, or more in the case of gaps between cores. This could affect the reflection characteristics of a synthetic seismogram significantly, particularly if the measured velocity is much higher or lower than the average velocity trend. It may at least partially account for discrepancies between MCS data and synthetic seismograms created at sea during Leg 210. Those synthetic seismograms contained several bright reflections that could not be easily related to reflections in the MCS data (Shipboard Scientific Party, 2004). This was particularly true of synthetic reflections arising from within Unit 5, which is largely mudrock and corresponds to an interval of low-amplitude, discontinuous reflections in the MCS data (Shillington et al., 2004; Shipboard Scientific Party, 2004).

We tried three different approaches to processing physical property data in order to explore the effects of sampling bias on synthetic seismograms (Fig. F4):

  1. All measured velocities and densities were included except for those from concretions. No modifications were made to the apparent vertical extent of each velocity or density measurement.
  2. High-velocity (>3 km/s) measurements from thin (<1 m thick) intervals shallower than the upper sill were removed from Unit 5 (1067.2–1612.7 mbsf), where high-velocity materials were comparatively unrepresentative.
  3. The thicknesses associated with physical property measurements on samples taken from high-velocity (>3 km/s) beds throughout the cored section were adjusted to match the thicknesses of beds with corresponding lithologies from which they were sampled (Shipboard Scientific Party, 2004).

The processing steps described above screen the data points in different ways to account for the presence of potentially anomalous samples and to test the effects of sample bias.

The final step was to create an "earth model" consisting of a series of layers, and it was ultimately used for the calculation of synthetic seismograms. These layers were interpreted by hand within the Nucleus software package to group together similar velocities and densities; 252 layers were included in this analysis. Given that there are 533 input data points, there is an average of <2 data points per layer. For each layer, P- and S-wave velocity, density, and P- and S-wave attenuation (QP and QS, respectively) must be given. The density and P-wave velocity assigned represent average values calculated between the interpreted top and base of the layer based on input data points. The result of this averaging step is that layers in the final earth model are assigned velocities that are neither as high nor as low as velocities in the original velocity function (Fig. F5). Both the input data points and the average velocities assigned to the layers in the earth model are shown in Figure F6. S-wave velocity is estimated from P-wave velocity. We use a QP of 1000 and a QS of 5000 for water, 200 and 100 for sediments, and 400 and 100 for sills (Fuchs and Muller, 1971; Minshull and Singh, 1993).

We generated synthetic seismograms for earth models derived from each of the three scenarios described above (Fig. F6) to examine the robustness of the reflections generated and to test their sensitivity to potential sampling biases in laboratory measurements. All three cases are discussed in "Results and Discussion."

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