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doi:10.2204/iodp.proc.307.103.2006 Site U13161Expedition 307 Scientists2Background and objectivesSite U1316 (proposed Site PORC-4A; 51°22.56′N, 11°43.81′W; 965 m water depth) is located in the downslope sediment deposits ~700 m southwest of Challenger Mound (Fig. F1). Challenger Mound is part of the Belgica mound province of outcropping and buried carbonate mounds on the eastern slope of Porcupine Seabight on the southwest continental margin of Ireland. Surface sediments at Site U1316 consist of rippled sands littered with dropstones (Foubert et al., 2005) and attest to the northward-flowing bottom currents that have kept the western flank of Challenger Mound free of sediments (Van Rooij et al., 2003). Shallow piston coring at Site U1316 (Marion Dufresne Core MD01-2450) recovered 12 m of silty clay with common to abundant nannofossils. Fine to medium sandy intercalations and a few centimeter-scale silty layers are present between the silty clays. Based on the seismic distribution and the sedimentological parameters, this interval is interpreted as drift sediments. The presence of a turbidite layer is evidence of a complex sedimentological environment with combined bottom and downslope sedimentological processes on the southwest flank of Challenger Mound. Glacial–interglacial variations have been identified in the sedimentological record (Foubert et al., 2005). Interglacial deposits are characterized by coarser grain fractions, indicating a more intensive current regime than in glacial periods. It is suggested that the inflow of Mediterranean Outflow Water in the basin during interglacial periods enhanced the current regime. Ice-rafted events in the glacial sediments record sediment delivery from both the Laurentide Ice Sheet and British-Irish Ice Sheet (Foubert et al., 2005). Three main seismostratigraphic units can be identified in the Belgica mound area, separated by two regional discontinuities (Van Rooij et al., 2003) (see Fig. F3 in the “Expedition 307 summary” chapter). The lowermost Unit P1 is characterized by gentle, basinward-dipping continuous parallel strata with moderate to locally high amplitude reflectors. A clinoform pattern formed by a number of superimposed sigmoid reflectors is encountered in the upper strata of Unit P1 below and adjacent to Challenger Mound (Fig. F2). These clinoforms are frequently characterized by a high-amplitude top sigmoid reflector. This seismic facies is interpreted as migrating drift bodies (Van Rooij et al., 2003; De Mol et al., 2005). The upper boundary of Unit P1 is an erosional unconformity which strongly incises the underlying strata. Unit P2 is characterized by a nearly transparent acoustic facies on top of the erosional unconformity bounding Units P1 and P2. Only a few sets of continuous relatively high amplitude reflectors are observed inside Unit P2. The uppermost seismic Unit P3, characterized by slightly upslope migrating wavy parallel reflectors, represents Quaternary drift deposits partly enclosing the mounds. The reflectors of Unit P3 onlap the mound, demonstrating that the mounds were already present before the deposition of the most recent drift. Scouring and moat features around the mounds suggest that the mounds had obtained a significant dimension and affected the intensity of the currents and the deposition of the enclosing sediments. Only seismic Units P1 and P3 are encountered at Site U1316 adjacent to Challenger Mound and are separated by a regional erosional unconformity interpreted as the mound base (Fig. F2). The principal objectives at Site U1316 were as follows:
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