Site 797

Abstract

1 Tamaki, K., Pisciotto, K., Allan, J., et al., 1990. Proc. ODP, Init. Repts., 127: College Station, TX (Ocean Drilling Program). Principal results: The objectives at Site 797, located in the southern Yamato Basin, were (1) to determine the age and nature of acoustic basement, (2) to measure the direction and magnitude of the present stress field, and (3) to characterize the sedimentation, subsidence, and oceanographic evolution of the area. The results at Site 797 are summarized below and in Figure 1. The principal results at Site 797 are as follows:1. Acoustic basement and an overlying seismic interval at this site comprise basalts and dolerites and interlayered volcaniclastic sandstones, siltstones, and silty claystones. The age of the sediments which overlie the shallowest basalt layer is estimated to be about 19 Ma. Based on paleodepth estimates using microfossils within these sediments and the sedimentologic interpretation of the underlying volcaniclastic sediments, this part of the Yamato Basin probably subsided rapidly during the early Miocene. The uppermost basalts most likely represent submarine lava flows, whereas the bulk of the underlying igneous units are sills and dikes. The upper nine units are high-Al basalt in composition, consistent with generation during the early stages of arc rifting. However, the units below these are mildly alkaline basalts and enriched tholeiites, which are unrelated to the overlying basalts and were probably derived from a different mantle source.2. The in-situ stress field could not be determined, because poor hole conditions precluded packer/hydro fracture tests. A borehole televiewer log over one basalt interval showed fractures but no borehole breakouts for stress directions.3. The lithologic and paleontological data indicate at least four principal stages in the evolution of this part of the Yamato Basin: (a) an early Miocene period of submarine basaltic volcanism and concomitant deposition of gravity-flow and current-reworked volcani- 323 SITE 797clastic sandstones and siltstones on a shelf or slope, probably outboard of a delta; (b) a late early to early middle Miocene phase of diminishing submarine volcanic activity, warm surface waters, and deposition of calcareous and slightly phosphatic hemipelagic claystones at lower middle bathyal depths (1500-2000 m) in a poorly oxygenated basin; (c) a late Miocene to late Pliocene cooling period beginning with an interval of anomalously slow sedimentation (5 m/m.y.) or a hiatus and characterized by dominant but variable diatomaceous sedimentation at lower middle bathyal depths (1500-2000 m); and (d) a latest Pliocene to Holocene stage during which diatomaceous sedimentation diminished volcanic-ash production and terrigenous input increased, subsidence continued, and climate oscillated to produce a sequence of interlayered light-and dark-colored silty clays having variable organic-carbon contents. In contrast with the more northern sites drilled during Leg 127, the sediments at Site 797 contain more carbonate suggesting ...

Beyond Receiver Functions: Green's Function Estimation by Transdimensional Inversion and Its Application to OBS Data

“…(2) The sediment must consist of more than two layers to generate significant negative peaks (phase B) immediately after the phase A1; otherwise, we would need an anomalous low-velocity layer a few kilometers beneath the sediment-basement boundary. Again, we stress that similar two-layered structures have been identified by previous studies (Hirata et al, 1989;Nakahigashi et al, 2013;Tamaki et al, 1990).…”

“…The estimated S wave velocity for the upper sediment layer, 0.126 km/s, is notably low. We attribute it to abundant water in the layer, which is estimated to be~60% from 0 to 300 m beneath the seafloor, according to the drilling survey (Tamaki et al, 1990). We note that the synthetic Green's function explains the amplitude of the direct P arrival well, corroborating the velocity estimate for the upper sediment layer.…”

“…According to these studies, the subsurface structure is laterally homogeneous beneath the central part of the basin where the seafloor topography is relatively flat. Seafloor drilling was conducted in the northwestern edge of the basin, unveiling a~550-m-thick sequence of sedimentary rocks that overlie an igneous basement (Tamaki et al, 1990). Laterally coherent seismic reflectors have been identified within the sediment sequence and at the top of basement (Hirata et al, 1989;Nakahigashi et al, 2013;Tamaki et al, 1990).…”

“…Finally, we forward modeled the estimated R component Green's functions to infer sediment properties. We assumed that the sediment comprises two layers, as implied from reflection profiles (Hirata et al, 1989;Nakahigashi et al, 2013;Tamaki et al, 1990). An acoustic basement half-space was placed beneath the sediment layers.…”

“…The other parameters were fixed at typical values (Table 3). We also required that the total thickness of the sediment layers be similar to that reported by the drilling survey (~550 m, Tamaki et al, 1990).…”