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doi:10.2204/iodp.proc.341.107.2014 Site U14211J.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 objectivesA primary objective of Expedition 341 was to examine the impact of changing Neogene sedimentation rates on actively deforming orogenic structures. Whereas Site U1420 targeted strata above an inactive thrust fault, Site U1421 was positioned to sample correlative strata on the limb of an actively deforming structure, where more deeply buried seismic sequences on the shelf are observed slightly closer to the seafloor. Site U1421 is located downslope of the Bering Trough above the youngest two thrusts of the Pamplona Zone where they cut obliquely across the slope (Fig. F1B). The slope sediments trapped behind these folds are seismically reflective, and sequences within them are mappable onto the shelf. These sequences appear to aggrade through Horizon H1 roughly parallel with the modern slope surface and include thinned distal extents of shelf sedimentary sequences, as well as slope sequences that are correlative with sequences truncated by unconformity Horizon H1 (Fig. F2). Shallower than Horizon H1, the slope sequence is more uniform in thickness and includes higher amplitude reflections that may represent the formation of a glacial trough mouth fan associated with the arrival of glacier termini at the shelf edge during glacial maxima, which is suggested to have initiated during the mid-Pleistocene transition (Fig. F2) (Berger et al., 2008). At the southeastern end of the STEEP09 seismic profile (Fig. F1A), two active faults (BT1 and BT2) are present at the continental slope, exhibiting less burial than the structures on the shelf. Scarps ~750 and ~300 m high associated with the active slope structures are visible on high-resolution bathymetry of the continental slope (Worthington et al., 2008). Site U1421 is located just landward of Fault BT2, which may have initiated after the Pliocene–Pleistocene transition (Worthington et al., 2010), given the lack of growth strata observed below Horizon H3 (Fig. F1). The presence of two distinct sedimentary packages on Fault BT2 is indicative of either a decrease in slope sedimentation or an increase in deformation rate across Fault BT2. Between Horizons H1 and H2, the angle of the observed growth strata becomes less pronounced, indicating a gradual decrease in fault growth rate during the early–Middle Pleistocene (Worthington et al., 2010) or an increase in accumulation rate. Shallower than Horizon H1, sediments are truncated by the anticline and are very slightly tilted toward the shelf, indicating either minimal deformation on Fault BT2 since Horizon H1 or high accumulation rates. The target depth at this site was designed to penetrate Reflector H2 (expected to lie at ~1 km), which is mapped from the shelf where it marks the latest growth strata associated with a now inactive thrust fault (Figs. F1A, F2). Determining the age of this reflector (hypothesized to be younger than the Pliocene/Pleistocene boundary) will allow us to infer the timing of when loading by increasing sediment accumulation forced accommodation of collisional stresses to be shifted elsewhere in the orogen (Worthington et al., 2010). Crossing the slope equivalent of the angular unconformity Horizon H1 will occur while drilling to the depth of Horizon H2, allowing for a second opportunity beyond Site U1420 to constrain the timing of its formation. Potential lithofacies are alternating diamict (ice-rafted debris and debris flow deposits), turbidites, and hemipelagic mud (Fig. F3). This site is expected to provide a proximal provenance record of sediment supply from the Bering Glacier, which can be used to locate the temporal and spatial loci of glacial erosion in the St. Elias orogen. |
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