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Site U13161

Expedition 307 Scientists2

Background and objectives

Site 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:

  1. Gain insight into the history of drift deposits on the downslope flank of Challenger Mound and the off-mound transport of mound-related skeletal and nonskeletal grains. Because of the close proximity of the site to Challenger Mound, the Quaternary deposits at Site U1316 should yield information on development of the mound itself as well as the depositional history of the mound environment, particularly coral-shedding events. Furthermore, they will provide full information on the genesis of seismic Unit P3 and records of high-frequency ice-rafting episodes.
  2. Investigate the character and age of the sigmoid units observed in the upper part of seismic Unit P1 that appears to form the substratum of Challenger Mound. These units, characterized by a high-amplitude sigmoid top reflector, may indicate a possible microreservoir of shallow gas or be the result of a past fluid expulsion event. Therefore, the sediments in these lenticular bodies might provide clues to a past geofluid migration event and past microbial activity leading to mound initiation. Alternatively, the acoustic character of the sigmoid clinoforms might be related to the presence of coarse-grained sediments expected in high-energy drift deposits or a change in lithology in combination with the geometric effect.
  3. Evaluate whether the sigmoid-shaped units contain high concentrations of gas and thus represent a potential hazard for drilling in Challenger Mound. Further drilling of Challenger Mound (Site U1317) was made contingent on investigation of the sediments along its flank at Site U1316.

1 Expedition 307 Scientists, 2006. Site U1316. In Ferdelman, T.G., Kano, A., Williams, T., Henriet, J.-P., and the Expedition 307 Scientists. Proc. IODP, 307: Washington, DC (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/​iodp.proc.307.103.2006

2 Expedition 307 Scientists’ addresses.

Publication: 14 October 2006
MS 307-103