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Site survey data and stratigraphic interpretation

Data acquisition

The primary data used for selection of sites are low-fold seismic reflection profiles collected during Cruise TN037 of the R/V Thomas G. Thompson in 1994. This data set was also used for Leg 198 and is described in some detail in the Leg 198 Initial Reports (Klaus and Sager, 2002), including plots of most ship tracks and seismic lines. Cruise TN037 also collected swath bathymetry, gravity, and magnetic data. Bathymetry and magnetic anomaly analyses are described in Sager et al. (1999) and Nakanishi et al. (1999), respectively.

Seismic data were collected with a six-channel Teledyne streamer with 25 m active sections. On any given seismic line, one of two alternate seismic sources was used: (1) a single generator-injector air gun (45/105 in3) or (2) a four–air gun array (80, 108, 150, and 200 in3). The generator-injector air gun was mainly used for seismic lines shot between the main bathymetric edifices. It was towed at ~7 nmi/h and achieved two-fold coverage. The air gun array was used primarily on the mountain tops to penetrate the thicker sediment cover in those locations. The array was typically towed at ~5 nmi/h and it provided three-fold coverage. Data were digitized at a 1 ms rate and recorded in Society of Exploration Geophysicists (file format "Y") format. Final shot spacing was 25–37 m. The data were processed through to migration using Sioseis and ProMax software (Klaus and Sager, 2002). Included in the processing train were band-pass filtering (30–150 Hz to 0.25 s two-way traveltime [TWT] below seafloor, 20–150 Hz from 0.25–1.0 s TWT, and 6–70 Hz below 1.0 s TWT), deconvolution, normal moveout correction, stacking, and finite difference migration.

Seismic stratigraphy

Interpretation of the seismic layering is relatively straightforward because of the drilling previously done on Shatsky Rise. A review of the stratigraphy cored during Leg 198 as well as previous cruises is contained in the Leg 198 Initial Reports (Shipboard Scientific Party, 2002a). In general the summits of the high edifices of Shatsky Rise contain thick sections (up to ~1.2 km thickness) of pelagic sediments. Over the flanks, this sedimentary section is usually much thinner or absent.

The sediment pile is mainly Cretaceous chalk and limestone covered by Cenozoic ooze. Sliter and Brown (1993) divided the section into five units and this was found to be consistent with Leg 198 drilling results (Shipboard Scientific Party, 2002a). Units I and II consist of foraminifer nannofossil ooze with minor clay. Unit I is Neogene in age (often Miocene to Holocene), whereas Unit II is of Paleogene age. Both have similar seismic character, with parallel-continuous layers, and are often separated by a seismic horizon of modest strength. Units III–V consist of Cretaceous chalk and occasional limestone layers with minor clay and abundant layers and nodules of chert and porcellanite. Indeed, the soft chalk layers interspersed with hard chert and porcellanite have frustrated efforts to recover cores from the top of Shatsky Rise since the beginning of scientific ocean drilling. The three Cretaceous units are often divided by two prominent seismic horizons, dubbed R1 and R2 by Sliter and Brown (1993). In addition, the Leg 198 scientific party used R0 for the horizon nearest the top of the Cretaceous section. The uppermost and lowermost Cretaceous layers show depositional character that is most uniform; however, the middle unit is more sculpted and often shows evidence of erosion and onlap with instances of slumping. The uppermost Cretaceous layer (Unit III) is Turonian to Maastrichtian in age and lies between R1 and R0. The mid-Cretaceous layer (Unit IV) is Aptian to Cenomanian in age and is bounded by R2 and R1. At the bottom of the pile is Unit V, which is Berriasian to Barremian in age and resides between igneous basement and R2 on seismic profiles.

The Cruise TN037 seismic data occasionally show some character in the portion of the record interpreted as igneous basement. Usually, seismic basement is a strong, irregular reflector below which few coherent reflections are seen. In some places on the Ori Massif, dipping reflectors in were noted in acoustic basement. These are likely to be dipping lava flows. On the southwest flank of Tamu Massif, igneous basement was a surprise. All along the seismic line over that flank of Tamu Massif, seismic basement has an odd, layered appearance for about 0.1 s TWT below the interpreted top of the igneous section. Drilling at Site 1213 during Leg 198 cored 46 m into acoustic basement with this signature, recovering three flow units interpreted as sills (Shipboard Scientific Party, 2002b). At most sites on Shatsky Rise, acoustic basement is strong, has no consistent internal reflectors, and is interpreted as the top of the lava pile.