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doi:10.2204/iodp.proc.308.107.2006 Site U13231Expedition 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. This also applies to the figures and tables contained in the chapter (see Figs. F1, F2, F3, F4 in the “Site U1322” chapter; Table T1 in the “Site U1322” chapter). Overview of seismically mapped surfacesSite U1323 is the middle location of the three sites drilled along an east–west transect in Ursa Basin (see Figs. F2, F3 in the “Site U1322” chapter). Site U1323 is located in 1262 m water depth and was originally scheduled for coring, wireline logging, and logging-while-drilling/measurement-while-drilling (LWD/MWD) operations to a terminal depth (TD) of 358 meters below seafloor (mbsf), 20 m above the top of the Blue Unit (Fig. F4 in the “Site U1322” chapter). The site is located within Mississippi Canyon Lease Block 898. 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 1731 ms two-way traveltime (TWT), or 33 mbsf, is probably the base of the hemipelagic drape sediments. Seismic Reflector S20 (1812 ms TWT; 97 mbsf) separates distal levee muds from the underlying fine-grained clastics of the eastern Southwest Pass Canyon levee. The upper and lower parts of this packet of strata seem seismically well defined, but the middle part exhibits disturbance and disruption, possibly due to slumping. Seismic Reflector S30 (1938 ms TWT; 197 mbsf) is a detachment surface that underlies one of the mass transport deposits (MTDs). Seismic Reflector S30 can be easily traced from Site U1324 farther to the west (Fig. F4 in the “Site U1322” chapter). In the immediate vicinity of Site U1323 seismic Reflector S30 forms the base of a series of MTDs (Fig. F1). Between seismic Reflectors S30 and S50-1323 (2068 ms TWT; 305 mbsf), the reflection character is complex. The top of the Blue Unit (seismic Reflector S80) is delineated by a weak negative polarity reflector of irregular geometry that can be traced regionally (Fig. F1; see also Fig. F4 in the “Site U1322” chapter). Local summary of borehole expectationsLatest Pleistocene to Holocene sedimentation at Site U1323 is characterized (from youngest to oldest) by a hemipelagic drape, underlain by a packet of muddy sediments of distal levee nature, in part containing MTDs. Below this horizon, the heavily disturbed deposits of the eastern levee of Southwest Pass Canyon are in turn underlain by a body of sediments belonging to the eastern levee of Ursa Canyon, close to the core of the canyon fill. The Ursa Canyon levee deposits locally 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 achieve these objectives, a dedicated LWD/MWD hole (Hole U1324A) was to be drilled to a projected TD of 358 mbsf. Drilling was halted at 247 mbsf and the hole abandoned after >1.5 m sand intervals were penetrated repeatedly and because of a sudden increase in annular pressure while drilling (APWD) on the order of 150 psi at 207 mbsf. For a detailed description, see “Operations.” Despite this, a complete set of LWD/MWD data was collected to TD of 247 mbsf for correlation with core, wireline logging, and LWD/MWD data from Sites U1322 and U1324. |
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