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doi:10.2204/iodp.proc.308.108.2006 Site U13241Expedition 308 Scientists2Background and objectivesGeological setting of Mars-Ursa BasinThe geological framework of Mars-Ursa Basin is treated in detail in the “Site U1322” chapter. The reader is referred to this chapter regarding detailed background information, and a more extensive compilation of the data available before drilling operations began (Figs. F1, F2, F3, F4 in the “Site U1322” chapter; Table T1 in the “Site U1322” chapter). Overview of seismically mapped surfacesSite U1324 is the westernmost of the sites drilled in Ursa Basin during Expedition 308 (see Figs. F2, F3 in the “Site U1322” chapter), is at the shallowest water depth (1057 meters below sea level [mbsl]), and has the deepest sediment penetration (612 meters below seafloor [mbsf]). It is located within Mississippi Canyon Lease Block 897. Eight seismic surfaces are mapped along the Ursa Basin transect (Fig. F1; see also Fig. F4 and Table T1 in the “Site U1322” chapter). Among those, seismic Reflectors S10, S20, S30, and S80 are regional surfaces that span all three drill sites. Seismic Reflector S10 at 1458 ms two-way traveltime (TWT) (35 mbsf) represents the probable base of the hemipelagic drape sediments. Seismic Reflector S20 at 1548 ms TWT (105 mbsf) separates distal levee muds from the underlying fine-grained clastics of the eastern Southwest Pass Canyon levee. Seismic Reflector S30 (1623 ms TWT; 165 mbsf) is a detachment surface that underlies one of the mass transport deposits (MTDs). Between seismic Reflectors S30 and S60-1324 (2136 ms TWT; 612 mbsf) horizontal parallel reflectors alternate with chaotic zones. The top of the Blue Unit (S80) is delineated by a weak, negative polarity reflector of irregular geometry ~20 ms TWT beneath the terminal depth (TD) of Holes U1324A and U1324B. Local summary of borehole expectationsLatest Pleistocene to Holocene sedimentation at Site U1324 is characterized (from youngest to oldest) by a hemipelagic drape underlain by a packet of muddy sediments belonging to distal levee deposits. Beneath this, the deposits of the eastern levee of Southwest Pass Canyon are underlain by sediments belonging to the western levee of Ursa Canyon. The Ursa Canyon levee deposits cut into the underlying sand-dominated Blue Unit (Fig. F1; see also Fig. F4 in the “Site U1322” chapter). Drilling objectivesThe primary drilling objectives at this site were the following:
To fully achieve these objectives, Hole U1324B was continuously cored to TD at 608 mbsf. Advanced piston coring (APC) was used to 357.90 mbsf, followed by extended core barrel (XCB) coring to 368 mbsf. From there, further APC coring was carried out to 394.50 mbsf, again followed by XCB coring to TD. Special tool deployments included five deployments of the temperature/dual pressure (T2P) probe and seven deployments of the Davis-Villinger Temperature-Pressure Probe (DVTPP). Before coring operations, a dedicated hole (Hole U1324A) was drilled to conduct logging-while-drilling/measurement-while-drilling (LWD/MWD) operations to a TD of 612 mbsf. This was followed by wireline logging and a vertical seismic profile to generate a complete set of logging parameters for correlation with core data and observations from Hole U1324A. After coring in Hole U1324B, a dedicated third hole (Hole U1324C) was drilled to 511.8 mbsf in order to obtain additional temperature and pressure measurements, followed by one spot core at each measurement station. There were five T2P probe and three DVTPP deployments in Hole U1324C. |
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