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doi:10.2204/iodp.proc.301.106.2005

Geologic context

Operational and scientific objectives for Site U1301 are described in the "Expedition 301 summary" chapter, CORK configuration and experimental goals are discussed by Fisher et al. (this volume), and regional survey data are presented by Zühlsdorff et al. (this volume). Selected aspects of these discussions are presented in this section.

Site U1301 is located ~100 km east of the Endeavour Segment of the Juan de Fuca Ridge, over relatively flat seafloor, above a buried basement high. The basement age has been determined from seafloor magnetic anomalies to be 3.5 Ma. Site U1301 comprises four holes located 0.9–2.0 km south of holes drilled at Site 1026 during Leg 168. A line connecting the two sites is oriented along the trend of basement structural strike; the sites are located along the peak of the same buried basement high. Sites 1026 and U1301 are located between two basement outcrops, Mama Bare outcrop to the north and Baby Bare outcrop to the south, which are the seafloor exposures of partially buried volcanic edifices built upon a ridge-flank abyssal hill (Fig. F1). ODP Site 1027 is located 2.2 km to the east.

Site U1301 is located on the western edge of a major Pleistocene distributary channel for turbidites flowing from the north. Sediments recovered from Sites U1301 and 1026 are dominated by this sedimentary sequence, including some very coarse grained layers that indicate high velocities during transport. The sediment/basement contact is generally clear in seismic lines across Second Ridge, although this boundary often has the rough (diffuse) character that is typical of the uppermost extrusive basalt lavas (Figs. F2, F3). In some places, the basement reflectors are horizontal and clear and appear to be highly layered (e.g., near common depth points [CDPs] 550 and 570 of Line GeoB00-466). A sill was previously cored and recovered in nearby ODP Hole 1027C, but it is not clear if there could be intrusive rocks above true basement at Sites U1301 and 1026. No intrusive rocks have been recovered from these sites.

The blocky nature of upper basement is readily apparent on the expanded segments of the seismic lines (Fig. F3), particularly on Line GeoB00-468, which shows an abrupt offset between uppermost basement near CDP 400. In addition, there are weak events below the eastern side of the highest basement block (where Holes U1301A, U1301B, U1301C, and U1301D were drilled) that may be reflectors from out of the plane of the seismic lines, illustrating characteristics of local basement relief in the along-strike direction.

The thermal state of the crust around Site U1301 has been documented through numerous heat flow surveys, drilling, and postdrilling experiments, although there has not been a heat flow profile run directly across this drill site. Uppermost basement is roughly isothermal in this area, with a temperature of 62°–64°C, making heat flow higher above the buried basement high (where sediment is thinnest) than over the surrounding area. Isothermality of upper basement is caused by rapid fluid convection within basement. Uppermost basement is also known to be overpressured in this area, but the sediment at Site U1301 is too thick to allow upward fluid seepage at thermally or chemically significant rates. The chemistry of uppermost basement fluids at Sites 1026 and 1027 is very similar to that of fluids from seeps on the top of Baby Bare outcrop, suggesting that the shallow basement aquifer in this region is well mixed at a lateral scale of kilometers.

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