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doi:10.2204/iodp.proc.308.106.2006 Site U13221Expedition 308 Scientists2Background and objectivesGeologic setting of Mars-Ursa BasinThe Mars-Ursa salt-withdrawal basin (hereafter referred to as “Ursa Basin”) is located 210 km south-southeast of New Orleans, Louisiana (USA), on the northeastern Gulf of Mexico continental slope in 800–1400 m water depth (Fig. F1). The 500 and 1000 m bathymetric contours shift to the south in this location and record the center of deposition for late Pleistocene sediments from the Mississippi River (Fig. F1). Rapid sedimentation obscured the otherwise hummocky nature of the seafloor in the northern Gulf of Mexico slope (Fig. F1). The study area is bounded to the west by Mars Ridge, a prominent north-south–trending bathymetric high (Fig. F2). Eastward from Mars Ridge, the seafloor slopes down to a zone of slope failures, including one failure described as one of the largest submarine slope failures in the world (Fig. F2) (McAdoo et al., 2000). In the north-central Gulf of Mexico, the late Pleistocene shelf, shelf-margin, and minibasin turbidite deposits that were sourced from the Mississippi River are termed the Eastern Depositional Complex (Winker and Booth, 2000). The shelf and shelf-margin deposits were described by Coleman and Roberts (1988) and McFarlan and LeRoy (1988). The deepwater strata explored during Expedition 308 comprise one component of this system. These strata accumulated outboard of the shelf break during marine isotope Stages (MIS) 2–4 in response to late Wisconsinan continental glaciation (Winker and Booth, 2000; Winker and Shipp, 2002). They overlie the MIS 5 condensed section that contains the extinction events of the planktonic foraminifer Globorotalia flexuosa (70 ka) and the calcareous nannofossil Pontosphaera 1 (~70 ka) (Styzen, 1996; Winker and Booth, 2000). This regional datum has been identified in the Mississippi Fan and in other research and industry drill holes in the Gulf of Mexico (Joyce et al., 1990; Martin et al., 1990). Late Pleistocene strata in Ursa Basin have received attention since the 1990s when overpressured and unconsolidated sands in the shallow section plagued drilling operations (Eaton, 1999; Ostermeier et al., 2000; Winker and Shipp, 2002). This drove the acquisition of high-resolution three-dimensional seismic data and several geotechnical cores to study the geological and geotechnical framework in the region (Fig. F3). Most recently, Sawyer et al. (submitted) presented a comprehensive analysis of the Mars-Ursa latest Pleistocene depositional system. Stratigraphic successionSediments deposited in the last ~70 k.y. in Ursa Basin can be divided into four successive depositional units, from the older to the younger (Fig. F4):
Blue UnitThe Blue Unit is composed of interbedded sand and mud. The base of the Blue Unit is the base of the deepest sheet sand that ties to a weak negative amplitude reflection within the shallow sedimentary section (Fig. F4). The top of the Blue Unit ties to a strong positive amplitude seismic reflection that marks an increase in impedance with depth at the top of sands within the Blue Unit. The top of the Blue Unit is difficult to correlate because of its complex seismic character and because it is eroded in places. The top sand within the Blue Unit is truncated by deformation zones associated with two channel-levee systems in the western part of the study area (Fig. F4). In areas where the top of the Blue Unit is not present, it has been mapped as the base of the deformation zones (Sawyer et al., submitted). Channel-levee systemsThe Ursa Canyon channel-levee system contains a core, deformation zone, and levees (Fig. F4). The core has high-amplitude, chaotic seismic reflections. A deformation zone characterized by rotated slump blocks surrounds the channel core. Both the Blue Unit and levee material are deformed, but not the core. The levees have thin subparallel reflectors and gullwing shapes; they flank each side of the channel and thin laterally away. The Southwest Pass Canyon channel-levee system is younger and lies west of the Ursa Canyon channel-levee system (Fig. F4). It eroded much of the western levee of the Ursa channel and completely buried the Ursa core and eastern levee. It is similar to the Ursa Canyon channel-levee system, although it is significantly larger. Mass transport depositsMultiple mass transport deposits (MTDs) are present (Fig. F4). The bases of the MTDs are marked by a prominent horizontal reflection that separates parallel reflectors below from the nonreflective MTD above. The edges of the MTDs are marked by the abrupt, near-vertical termination of the levee facies (Fig. F4). Mapped surfaces at Site U1322Seismic surfaces that could be correlated across all three drill sites are delineated as S10–S80. Surfaces that are particular to a site are delineated by adding the site name to the reflector name, for example “S50-1322” (Table T1). A predrilling time-depth model was composed based on regional check shot data:
where tbsf = two-way traveltime below seafloor (seconds). Location of Site U1322Site U1322 is the easternmost Expedition 308 site in Ursa Basin (Fig. F3). It is located within Mississippi Canyon Lease Block 855 (Fig. F4). Eight seismic surfaces are mapped (Figs. F4, F5; Table T1). Seismic Reflectors S10, S20, S30, and S80 are regional surfaces that span all three drill sites. Seismic Reflector S30 is a detachment surface that underlies one of the MTDs, seismic Reflector S80 is the base of the Blue Unit, seismic Reflector S40-1322 underlies a reflective package and caps an MTD, and seismic Reflectors S50-1322, S60-1322, and S70-1322 mark detachment surfaces for a series of MTDs. Drilling objectivesThe primary drilling objectives at this site were the following:
To achieve these objectives, a dedicated first hole (Hole U1322A) was drilled to conduct logging-while-drilling/measurement-while-drilling (LWD)/(MWD) operations to a terminal depth (TD) of 238 meters below seafloor (mbsf). Hole U1322B was cored to a TD of 238 mbsf. Advanced piston coring (APC) was used from the top to the bottom of the hole. Special tool deployments included three deployments of the temperature/dual pressure (T2P) probe. Two additional geotechnical holes (Holes U1322C and U1322D) were drilled, and eight T2P and four Davis-Villinger Temperature-Pressure Probe (DVTPP) deployments were performed in these holes. |
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