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doi:10.2204/iodp.proc.341.103.2014 Site U14171J.M. Jaeger, S.P.S. Gulick, L.J. LeVay, H. Asahi, H. Bahlburg, C.L. Belanger, G.B.B. Berbel, L.B. Childress, E.A. Cowan, L. Drab, M. Forwick, A. Fukumura, S. Ge, S.M. Gupta, A. Kioka, S. Konno, C.E. März, K.M. Matsuzaki, E.L. McClymont, A.C. Mix, C.M. Moy, J. Müller, A. Nakamura, T. Ojima, K.D. Ridgway, F. Rodrigues Ribeiro, O.E. Romero, A.L. Slagle, J.S. Stoner, G. St-Onge, I. Suto, M.H. Walczak, and L.L. Worthington2Background and objectivesSite U1417 is located in the distal Surveyor Fan in the Gulf of Alaska (Fig. F1). The area was originally drilled at Site 178 (~1.5 km from Site U1417) during Leg 18 of the Deep Sea Drilling Project (DSDP) with ~40% recovery, most of which is Late Pleistocene in age (Fig. F2). Site U1417 is ~60 km from the Surveyor Channel, which delivers sediment to this site via overbank processes (Fig. F3). Drilling targets were three major regional seismic boundaries and associated seismic sequences mapped by Reece et al. (2011) using reprocessed U.S. Geological Survey and 2004 high-resolution and 2008 crustal-scale seismic reflection data to correlate stratigraphic changes and fan morphology through time (Fig. F4). Sequences I and II exhibit layered, laterally semicontinuous reflectors consistent with distal submarine fan (turbiditic) deposition. Sequence III is thinly layered and contains reflectors that are laterally continuous, flatter, and smoother than those in the other sequences. Stratal relationships at the sequence boundaries are highly variable and greatly influenced by basement topography. Sequence II onlaps Sequence I in the areas where Sequence I exhibits topography, but it is conformable in other locations. Sequence III downlaps Sequence II in parts of the distal fan where both sequences pinch out farther from the Surveyor Channel sediment source. At Site U1417, Sequences I–III appear to be conformable (Fig. F3). Site U1417 lies at the intersection of two crustal-scale seismic profiles acquired during the 2011 R/V Marcus Langseth U.S. Extended Continental Shelf project for the U.N. Law of the Sea (Fig. F5; Walton et al., submitted). The shallow primary target for Site U1417 is the Sequence II/III boundary from Reece et al. (2011), which has been mapped onto the 2011 profiles. This sequence boundary, which is mappable throughout the Surveyor Fan (Fig. F6), is proposed to represent an increase in sediment supply to the site as a result of an intensification of glaciation at the mid-Pleistocene transition (MPT) and subsequent delivery to the site via the Surveyor Channel, ice-rafting, and hemipelagic/pelagic deposition. The deeper primary target is the Sequence I/II boundary (Reece et al., 2011), which was also mapped onto the 2011 data and which is proposed to correspond to the onset of tidewater glaciation and to the start of Surveyor Fan deposition in the late Miocene. Additional divisions of these sequences for the purpose of core-log-seismic integration are discussed in “Core-log-seismic integration” and displayed in Figure F7. This site was chosen to provide a sedimentary record of Neogene glacial and tectonic processes occurring in the adjacent orogen (Fig. F8). All of Sequences II and III are estimated to correspond with this time period, which would include both the Miocene tectonic uplift of the St. Elias orogeny and the late Miocene to Pleistocene preglacial to glacial conditions, including the initiation of the northwestern Cordilleran ice sheet (NCIS), the intensification of Northern Hemisphere glaciation, and the potential intensification of glacial extent following the MPT. These sequences should also contain a provenance record reflecting the locus of sediment created during the exhumation and uplift of the St. Elias Mountains. By drilling into Sequence I, Site U1417 allows examination of deposition prior to the onset of the sedimentation associated with establishment of the Surveyor Channel. The channel is proposed to have formed in response to increased sediment supply from the adjacent coastal mountains, the timing of which may correspond with the initiation of the NCIS (Reece et al., 2011). Sequence I includes several seismic facies (Fig. F7) that may yield clues into the changes in depositional setting from the late Oligocene (approximate age of basement) through the Miocene, as this site tectonically migrated northwest from somewhere near modern day British Columbia toward southern Alaska. These sediments should yield insight into the initial uplift of the St. Elias orogen, which provided the enhanced elevation needed for later glaciation. The Sequence I/II boundary occurs at ~330 m within the drilled interval at Site 178, within a section of fine-grained sand to silty turbidites and interbedded diatomaceous ooze and mud with increasing diamict upsection (Kulm, von Huene, et al., 1973; Reece et al., 2011). The Sequence I/II boundary was dated to be ~5 Ma by Reece et al. (2011), near the beginning of Glacial Interval A of Lagoe et al. (1993), based on 40Ar/39Ar dating of ash layers (Hogan et al., 1978) (Fig. F2). At ~130 m at Site 178, the Sequence II/III boundary lies within an interval of changing fan lithology. The section from 96 to 141 m contains abundant diamict interbedded with silty clay and diatom-rich intervals, whereas the section from 141 to 280 m contains less diamict, much more silty clay, and fewer diatoms (Kulm, von Huene, et al., 1973; Reece et al., 2011). The Sequence II/III boundary was tentatively dated to ~1 Ma based on correlation with the upper Jaramillo (1r.1n) Subchronozone magnetic polarity reversal identified at Site 178 (Reece et al., 2011) (Fig. F2), making it coincident with the onset of Glacial Interval C of Berger et al. (2008a). Both sequence boundaries are synchronous with doubling of terrigenous sediment flux observed at Ocean Drilling Program (ODP) Site 887 at ~6 and ~1 Ma (Rea and Snoeckx, 1995), but no regional sequence boundary projected into Site 178 appears to correlate with the onset of Glacial Interval B (Reece et al., 2011) based on the 40Ar/39Ar ages of Hogan et al. (1978) (Fig. F2). Key lithologies of interest as observed in Site 178 cores include tephras and other volcaniclastic detritus, variable amounts of ice-rafted debris, biosiliceous-rich intervals, turbidites, and general changes in depositional processes from prefan sedimentation to modern overbank deposition sourced from the Surveyor Channel, ~60 km east of the site. Important contributions to hypothesis testing at this site will come from biostratigraphic and paleomagnetic age models, physical properties changes, and evolution in sedimentary facies in an increasingly glacially influenced setting. |
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