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doi:10.2204/iodp.proc.308.103.2006 Site U13191Expedition 308 Scientists2Background and objectivesGeological setting of Brazos-Trinity Basin IVBrazos-Trinity Basin IV is the southernmost of a chain of five north-south-oriented basins on the continental slope of the Gulf of Mexico (Figs. F1, F2) (Satterfield and Behrens, 1990; Winker, 1996). It is located ~250 km south-southeast of Houston, Texas (USA). It forms part of the Brazos-Trinity Fan, a large latest Pleistocene sediment gravity flow depositional system. These basins are a classic area for study of modern deepwater turbidite systems (Morton and Suter, 1996; Winker, 1996; Beaubouef and Friedmann, 2000; Fraticelli, 2003; Anderson and Fillon, 2004). Early work in these basins established the “fill and spill” model whereby deposition in downslope basins would not occur before all upslope basins were filled (Satterfield and Behrens, 1990; Winker, 1996). Subsequent work elaborated on this conceptual model (e.g., Beaubouef and Pirmez, 1999; Friedmann and Beaubouef, 1999). Badalini et al. (2000) and Fraticelli (2003) proposed a contrasting model wherein deposition was contemporaneous in each of these basins. More recently, Beaubouef et al. (2003) used high-resolution three-dimensional (3-D) seismic data over Brazos-Trinity Basin IV and further refined the work of Beaubouef and Friedmann (2000). Most recently, Mallarino et al. (in press) used piston coring at the margins of Brazos-Trinity Basin IV to characterize its paleogeographic evolution. They described six sedimentary units (Fig. F3) and used biostatratigraphic analysis to constrain the age of these sediments; Brazos-Trinity Basin IV formed during the stepwise sea level fall between 115 and 15 ka, ending just prior to the meltwater pulse to the Gulf of Mexico at 14 ka. Seismic surfacesPrior to drilling, we mapped six reflectors in Brazos-Trinity Basin IV from closely spaced, high-resolution, two-dimensional (2-D) seismic reflection data: the seafloor reflector (SF) and Reflectors R10, R20, R30, R40, R50, and R60 (Figs. F4, F5, F6, F7, F8; Table T1). Time-depth conversions were estimated prior to drilling with the following velocity model.
where
Equation 1 results from a compilation of regional check shot (velocity) data (C. Winker, Shell International Exploration and Production Company, pers. comm., 2005). The seismic reflection character of the basin sediments (between Reflector R40 and the seafloor) can be divided into an alternating succession of transparent units and more reflective layered units. We mapped three reflectors (Figs. F5, F6, F7):
Reflector SF caps a high-amplitude but somewhat chaotic package that becomes more layered in the downdip (southern) direction. The reflective layered seismic facies are interpreted as fan deposits, whereas the chaotic and/or transparent deposits are interpreted as debris flows and/or mass transport complexes (Badalini et al, 2000; Winker and Booth, 2000; Beaubouef et al., 2003). Figure F3 illustrates how our mapped surfaces tie to the sediment packages defined in previous work. Location of Site U1319Site U1319 is located in ~1430 m of water on the southern flank of Brazos-Trinity Basin IV (Figs. F4, F5). Two-dimensional seismic data illustrate that Site U1319 holes penetrate the flank of the basin. At this location, basin sediments (seafloor to Reflector R40) are projected to be only 35 m thick (Table T1). Beneath the basin sediments (beneath Reflector R40), parallel reflectors from hemipelagic sediments are present. Site U1319 drilling objectivesThe primary drilling objectives at this site were the following:
To achieve these objectives, Hole U1319A was continuously cored to terminal depth (TD) at 157.5 meters below seafloor (mbsf). Advanced piston coring (APC) was used to 114.6 mbsf, followed by extended core barrel (XCB) coring. One advanced piston corer temperature (APCT) tool measurement was taken at the bottom of Core 308-U1319A-5H at ~42.5 mbsf. Special tool deployments in Hole U1319A included one deployment of the temperature/dual pressure (T2P) probe just above the seafloor and one deployment at 80.5 mbsf. A dedicated second hole (Hole U1319B) was drilled to conduct the LWD/MWD operations to a TD of 180 mbsf. This was done to generate a complete set of logging parameters for correlation with core and wireline logging data from Hole U1319A. |
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