Site 931

Abstract

Principal results: Site 931 (proposed Site AF-10) is the most easterly site to be drilled on the Amazon Fan. It is located on a flat terrace on the western levee of the buried Channel 5 system. The holes penetrate a thin acoustic unit interpreted as muds with minor turbidite silt overlying the Channel 5 system, then the thick Channel 5 levee sequence passing down into highamplitude reflection packets (HARPs) interpreted as sandy lobes associated with channel avulsion. Three HARP units overlie a thick acoustically incoherent unit (interpreted as a debris flow) that laps out to the west against the crest of the unusually thick Bottom Levee Complex. The site was selected from a Ewing seismic profile (E9209; 1637UTC on 20 Sept.). Following Site 930, we ran a 12-hr seismic survey across proposed Sites 940, 932,933, and 931.Hole 931A was cored to 53.6 mbsf and recovered 56.99 m. Hole 931B was cored by advanced piston corer (APC) to 76.8 mbsf, then by extended core barrel (XCB) to 421.3 mbsf. Recovery was poor from 142.2 to 219.1 mbsf (15.6%) in alternating sand and mud. Total hole recovery was 296.1 m (70.3%). Temperature measurements were obtained using the ADARA at 67.3 mbsf, and using the WSTP at 103.5 mbsf and 142.2 mbsf, yielding a mean geothermal gradient of 34°/km. There was gas expansion in many cores. Methane was found throughout the hole, with a sharp increase at 10 mbsf, and peaks at about 50 mbsf and 400 mbsf. Only trace amounts of ethane and propane were detected in a few deep cores.Because some pipe had to be raised to retrieve a stuck core barrel, Hole 93IB was conditioned for logging prior to taking the final two cores. On the first logging run, the sonic and natural gamma tools of the Quad combo flooded. They were replaced, and the tool was run to 251 mbsf where a bridge prevented further passage downhole. The hole was logged from 251 to 90 mbsf, the pipe was raised to 72 mbsf, and the interval 150 to 72 mbsf was logged. A similar procedure was used on subsequent log runs with the FMS, geochemical, and GHMT combinations. Hole condi-123 SITE 931 tions were good, except in a few parts of the interval from 140 to 240 mbsf, where sand beds had washed out. Hole 931C was then cored to 32.6 mbsf and recovered 34.43 m (105.6% recovery).Five lithologic units are recognized: Unit I (0-0.57 mbsf) is a Holocene nannofossil-and foraminifer-rich clay that is bioturbated and contains about 50% carbonate.Unit II (0.57-189.63 mbsf) consists of mud with interbedded laminae and beds of silt and very fine sand. The mud contains about 3% carbonate. Subunit IIA (0.57-6.70 mbsf) comprises bioturbated and color-banded mud. Subunit IIB (6.70-27.50 mbsf) consists of mud with <20% silt laminae. These two subunits represent sediment that accumulated since the abandonment of Channel 5. Subunit IIC (27.50-199.20 mbsf), corresponding to the crest of the Channel 5 levee, contains mud with thin to thick beds of silt and fine sand, which increase in frequency and thickness toward the base of the subunit. The thickest sand beds contain...