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doi:10.2204/iodp.proc.308.104.2006

Site U1320¹

Expedition 308 Scientists²

Background and objectives

Geological setting of Brazos-Trinity Basin IV

The geological framework of Brazos-Trinity Basin IV is described in “Background and objectives” in the “Site U1319” chapter, and the reader is referred to this chapter regarding necessary background information, along with an extensive compilation of precruise data in figures and tables (Figs. F1, F2, F3, F4, F5, F6, F7, in the “Site U1319” chapter; Table T1 in the “Site U1319” chapter).

Overview of seismically mapped surfaces

Site U1320 is located in 1470 m water depth near the center of Brazos-Trinity Basin IV (Fig. F4 in the “Site U1319” chapter). Two-dimensional (2-D) seismic data show that the drilling location is a short distance away from the southern boundary of a bowl-shaped basin (Fig. F5 in the “Site U1319” chapter). Here, the stratigraphic section was expected to be most expanded and most complete. Prior to drilling we mapped six major seismic reflectors (shown in Fig. F1) in Brazos-Trinity Basin IV: from shallowest to deepest, the seafloor reflector (SF) and seismic Reflectors R10, R20, R30, R40, R50, and R60 (see Table T1 in the “Site U1319” chapter). Below seismic Reflector R40, sediment packages are of approximately equal thickness along the seismic section, and we infer that these sediments were deposited before any local topography developed as a result of salt movement or other tectonic factors. Seismic Reflector R60 at ~2310 ms two-way traveltime (TWT) (284.3 meters below seafloor [mbsf]) (Table T1 in the “Site U1319” chapter) separates a well-stratified package above from an area of short and more chaotically oriented reflections below. Seismic Reflector R50 at 2254 ms TWT (229 mbsf) marks a prominent seismic signal at the center of this package. Seismic Reflector R40 at 2189 ms TWT (179 mbsf) defines the top of the constant-thickness beds. Above this reflector, seismic Reflectors R30 (2141 ms TWT; 138 mbsf), R20 (2070 ms TWT; 79.6 mbsf), and R10 (2008 ms TWT; 29.9 mbsf) cap three layered reflective packages in this central position of Brazos-Trinity Basin IV. With the exception of the lower package, which is seismically stratified throughout at least south-southwest, west-northwest, and east-southeast of Site U1320 (Figs. F5, F6 in the “Site U1319” chapter), the lower parts are acoustically transparent and the upper parts show a well-defined, even, stratification.

Local summary of borehole expectations

Beneath a Holocene hemipelagic mud drape a few meters thick, the expected uppermost Pleistocene-age lithologies were muddy and sandy deepwater fan turbidites to a depth of ~180 mbsf, roughly equivalent to the position of seismic Reflector R40 (Fig. F1). The depositional environment of these sediments was anticipated to be that of a deepwater fan, according to the interpretations of Badalini et al. (2000) or Beaubouef and Friedmann (2000) (see Fig. F3 in the “Site U1319” chapter). The fan deposits are underlain by interbedded hemipelagic and turbidite muds. Downsection from seismic Reflector R40 to Reflector R60, the seismic signature extending from the southern flank of the basin was taken to represent the interbedded hemipelagic and turbidite muds sedimented before development of the fan. Seismic Reflector R60 was the final target for drilling at Site U1320. Beneath this depth, rocks of the earlier basin fill are probably slumped, producing a pattern of short, chaotically oriented reflections in seismic sections. Viewed on the scale of the whole Brazos-Trinity Basin IV, the packages bounded by seismic Reflectors R40 and R60 thicken somewhat to the north, perhaps reflecting proximity to the sediment source. Badalini et al. (2000) refer to these strata as hemipelagic deposits. Winker and Booth (2000) refer to the material between seismic Reflectors R40 and R60 as hemipelagic deposits and the other material as “chaotic.”

Drilling objectives

The specific drilling objectives at Site U1320 were the following:

• Establish a complete section for the thick Holocene–latest Pleistocene late basin fill at the center of Brazos-Trinity Basin IV to determine the change in rock and fluid properties in this normally pressured location.
• Determine the age of the stratigraphic section and thus contribute to the derivation of an integrated age model for Brazos-Trinity Basin IV.
• Determine the lithology and facies evolution of the center of this basin, to allow lateral correlation with the drilled and/or logged sections at Sites U1319 and U1321.
• Sample the subsurface biosphere in this reference location.
• Conduct an extensive wireline logging program on the cored hole and a logging-while-drilling (LWD)/​measurement-while-drilling (MWD) program on a dedicated LWD/MWD hole.

To achieve these objectives, Hole U1320A was continuously cored to terminal depth (TD) at 299.6 mbsf and then wireline logged with the triple combination (triple combo) and Formation MicroScanner (FMS)-sonic tool strings and vertical seismic profiling using the single-axis Well Seismic Tool (WST). Special tool deployments in Hole U1320A during the coring program included two deployments of the temperature/dual pressure (T2P) probe at 126.2 and 213.0 mbsf and two deployments of the Davis-Villinger Temperature-Pressure Probe (DVTPP) at 203.4 and 289.9 mbsf. A dedicated second hole (Hole U1320B) was then drilled to conduct LWD/MWD operations to a TD of 320 mbsf. This was done to generate a complete set of logging parameters for correlation with core and wireline logging data from Hole U1320A.

¹Expedition 308 Scientists, 2006. Site U1320. In Flemings, P.B., Behrmann, J.H., John, C.M., and the Expedition 308 Scientists, Proc. IODP, 308: College Station TX (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/​iodp.proc.308.104.2006

²Expedition 308 Scientists’ addresses.

Publication: 8 July 2006
MS 308-104

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