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Site U13211

Expedition 308 Scientists2

Background and objectives

Geological setting of Brazos-Trinity Basin IV

The geological framework of Brazos-Trinity Basin IV is described in the “Site U1319” chapter. The reader is referred to this chapter regarding necessary background information, along with an extensive compilation of presite data in figures and tables (Figs. F1, F2, F3, F4, F5, F6, F7, F8 in the “Site U1319” chapter; Table T1 in the “Site U1319” chapter).

Overview of seismically mapped surfaces

Site U1321 is located in 1458 m water depth near the center of Brazos-Trinity Basin IV (see Fig. F4 in the “Site U1319” chapter). Two-dimensional (2-D) seismic data show that the drilling location is on the southern margin of a bowl-shaped basin (Fig. F4 in the “Site U1319” chapter). Site U1321 is located on seismic Line 3020 between Sites U1319 and U1320 (Fig. F4 in the “Site U1319” chapter). Prior to drilling we mapped six major reflectors (shown in Fig. F1) in the vicinity of Site U1321: from shallowest to deepest, the seafloor reflector (SF) and seismic Reflectors R10, R20, R30, R40, R50, and R60 (see also Table T1 in the “Site U1319” chapter). Below seismic Reflector R40, sediment packages have approximately equal thickness along seismic Line 3020 (Fig. F5 in the “Site U1319” chapter). We infer that these sediments were deposited before any local topography developed as a result of salt movement or other tectonic factors. At Site U1321, seismic Reflector R60 is at ~2160 ms two-way traveltime (TWT) (174.5 meters below seafloor [mbsf]) and separates a well-stratified package above from an area of short and more chaotically oriented reflections below (Fig. F1; see also Figs. F5, F6, F7 in the “Site U1319” chapter; Table T1 in the “Site U1319” chapter). Seismic Reflector R50 at 2119 ms TWT (139.9 mbsf) is a prominent reflector at the center of this package. Seismic Reflector R40 at 2054 ms TWT (86.2 mbsf) defines the top of the constant-thickness units. Above this reflector, seismic Reflectors R30 (2036 ms TWT; 71.5 mbsf), R20 (2024 ms TWT; 61.8 mbsf), and R10 (1973 ms TWT; 21.2 mbsf) cap three wedge-shaped packages in this marginal position of Brazos-Trinity Basin IV.

Local summary of borehole expectations

A few meters of Holocene hemipelagic mud drape was expected to overlie uppermost Pleistocene muddy and sandy deepwater fan turbidites to a depth of ~86 mbsf, roughly equivalent to the position of seismic Reflector R40 (Fig. F1). The wedge-shaped geometries of the reflective and seismically transparent packages lead to an interpretation that Site U1321 is the location of rapid thinning of the deepwater fan units cored at Site U1320. Between seismic Reflectors R40 and R60, the seismic signature extending from the southern flank of the basin indicates interbedded hemipelagic and turbidite muds of prefan age.

Drilling objectives

We drilled Hole U1321A as a dedicated logging-while-drilling/​measurement-while-drilling (LWD/MWD) hole. The principal objective was to facilitate correlation of lithostratigraphic units and individual sand layers across the south-southwest margin of Brazos-Trinity Basin IV and document the lateral change in petrophysical facies, especially of the deep-sea fan units above seismic Reflector R40. The results of LWD/MWD are documented in the following sections of this chapter.

1 Expedition 308 Scientists, 2006. Site U1321. 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.308.105.2006

2 Expedition 308 Scientists’ addresses.

Publication: 8 July 2006
MS 308-105