Ocean Drilling Program (ODP) Leg 191 was a cruise devoted primarily to engineering, specifically the emplacement of a borehole seismometer beneath the abyssal plain of the northwest Pacific Ocean and testing of the hard rock reentry system (HRRS; also known as the "hammer drill"). Only one site was drilled for science (Site 1179), and therefore logging, coring, and studies of core samples were secondary goals of the expedition. The ship carried only a small contingent of scientists whose research involved the study of core or logging data, and as a result the number of postcruise science contributions cannot be easily melded into a scientifically focused theme. This chapter describes post-cruise science from Site 1179 as a guide for the reader of this volume. Results from HRRS testing are not described here because the engineering tests resulted in progress reports but no samples were recovered, and, hence, no scientific papers were produced from this aspect of drilling.
Site 1179 was drilled on abyssal seafloor, at a depth of 5564 meters below sea level (mbsl), located on magnetic Anomaly M8, ~240 km northwest of Shatsky Rise in the northwest Pacific Ocean (Fig. F1). A 375-m-thick sedimentary section was cored before penetrating 100 m into the upper oceanic crust. Although a total of five holes (1179A through 1179E) were drilled at the site, coring was done in only two, Holes 1179B and 1179C (Shipboard Scientific Party, 2001a).
The Site 1179 sedimentary section consists of four distinct units (Figs. F2, F3). The uppermost unit is a thick (221.5 m) layer of late Miocene to late Pleistocene age consisting of clay- and radiolarian-bearing diatom ooze. Considering the distance from land and generally low mid-ocean sedimentation rates, this unit is anomalously thick. It appears to be thickest in the waters east of Japan and probably results from high productivity related to the Kuroshio Current and its convergence with the Oyashio Current (Shipboard Scientific Party, 2001a). The diatom ooze sits atop a 24.5-m-thick layer of clay-rich, diatom-bearing radiolarian ooze of late Miocene age, which in turn rests on a 37.5-m-thick layer of barren brown pelagic clay of uncertain age. Within lithostratigraphic Units I and II, 13 discrete vitric ash layers were identified (Shipboard Scientific Party 2001a). The lowermost unit consists of chert layers within a unknown host sediment. The host sediment is unknown because only chert fragments were recovered within the 93.7-m unit above igneous basement. The age of the sediment is known to include the Early Cretaceous because of a few specimens of radiolarian tests that were recovered from chert fragments.
Physical properties of the Miocene and younger sediments are unusual because of the high content of diatom frustules. These low-density silica particles have high porosity and, as a result, the Unit I sediments have unusually high porosity and low density for sediments buried by tens or hundreds of meters of sediment. Nevertheless, these sediments produced a useful magnetic stratigraphy as well as a biostratigraphy based mainly on radiolarian assemblages with a few datums from dinoflagellates and one from an anomalous layer of nannofossils (Shipboard Scientific Party, 2001b). Age data from the different fossil types are in good agreement and indicate slow sedimentation in the Miocene (~2 m/m.y.), which gave way to rapid sedimentation in the last 5 m.y. (40–43 m/m.y.).
Basement coring was reasonably successful, with 43.5% recovery over the 100-m interval cored. Recovered igneous rock mostly consists of aphyric basalts in massive flows, pillows, and breccia. Based on lithologic and textural differences, sediment interbeds, and flow and cooling unit boundaries, the basement section was divided into 48 units. Furthermore, it was also possible to divide the igneous section into three different groups, based on the abundance of the mineral olivine. The uppermost 8 units consist of olivine-poor basalts, whereas the middle 16 units are characterized by olivine-free basalts and the lower 24 units are olivine-rich basalt (Shipboard Scientific Party, 2001b). Alteration of the igneous rocks appears low in hand specimens and thin sections, which indicated zeolite facies metamorphism and temperatures below 30°C.
Logging data from Leg 191 were few because only one site was drilled and there were hole stability problems in Hole 1179D, the single hole logged. Although the first tool string was initially lowered to 300 meters below seafloor (mbsf), the run was reversed for calibration tests and a bridge developed at 260 mbsf, thwarting attempts to log deeper in the hole (see Fig. F2).