Site 792

Abstract

1 Taylor, B., Fujioka, K., et al., 1990. Proc. ODP, Init. Repts., 126: College Station, TX (Ocean Drilling Program). Hole 792A was spudded for advanced piston corer (APC) operations at 2130Z on 17 May 1989. We cored 95 m at a recovery rate of 76% before an overpull resulted in a severed piston rod, thus ending operations at the hole. Hole 792B re-cored the mud line, washed to 50 m below sea floor (mbsf), and continued with the APC to 69.2 mbsf; at this point we switched to the extended core barrel (XCB) and cored to 146.4 mbsf, with overall recovery of 43%. Holes 792B-D all ended operations in the interval from 137 to 146 mbsf, owing to separation of the XCB cutting shoe in each case. We spudded Hole 792E with a rotary core barrel (RCB) at 0207Z on 19 May, washed to 135.6 mbsf, and then cored to 885.9 mbsf; recovery was 52% in the sediments above 804 mbsf, and 16% in the underlying basement. As we pulled the pipe from the hole to prepare for logging it became stuck, the bit resting at 286 mbsf. After attempts to save the bottom-hole assembly (BHA) proved fruitless, we ran a full suite of standard geophysical and geochemical logs above 878 mbsf. The first use of formation microscanner (FMS) logging in the Ocean Drilling Program (ODP) was successfully completed, and a vertical seismic profile (VSP) was also collected. Finally, we severed the pipe with explosives at 134 mbsf.We defined six lithostratigraphic units at Site 792:1. Unit I (0-183.7 mbsf): upper Pliocene to Holocene nannofossil-rich, vitric silty clay and clayey silt, interbedded with vitric silts and sands and minor pumiceous and scoriaceous gravels.2. Unit II (183.7-357.4 mbsf): middle and upper Miocene sandy mudstone, muddy sandstone, and silty claystone (all with nannofossil-rich intervals), and vitric sand-and siltstone.3. Unit III (357.4-429.3 mbsf): lower Miocene to upper Oligocene intensely bioturbated, nannofossil-rich claystone and nannofossil chalk, and rare crystal-vitric silt-and sandstone.4. Unit IV (429.3-783.4 mbsf): upper Oligocene vitric sandstone and sandy conglomerate with claystone intraclasts, with minor silty claystone and claystone that contain nannofossil-rich intervals.5. Unit V (783.4-804 mbsf): altered volcanic sandstone with claystone intraclasts.6. Unit VI (804-885.9 mbsf): porphyritic andesite with minor basaltic andesite and dacite.Units I-IV were dated by means of nannofossils, foraminifers, and paleomagnetics, with the addition of radiolarians for Units I-II. Four lacunae occur, at 1-2.2, 3.5-6, 8-9, and 13-19 Ma. The second and fourth lacunae correspond to the basal boundaries of the first two units. Average sedimentation rates in the intervals between these lacunae are (in order of increasing age) 90 and 122 m/m.y. (Unit I), and 43 and 23 m./m.y. (Unit II), respectively. Sedimentation rates for Unit III are 14 and 4 m/m.y., respectively, for intervals above and below a normal fault that cuts out strata from 23 to 24 Ma. Sedimentation rates for Unit IV (27-29 Ma) were about 300 m/m.y. and greater prior to 28 Ma, but ...