Abstract

Ocean Drilling Program Leg 191 had two main goals: (1) to drill and case a borehole at a site in the northwest Pacific Ocean between Japan and Shatsky Rise and install therein a seismic observatory and (2) to test the drilling and casing emplacement capabilities of the hard rock reentry system (HRRS or "hammer drill") on a basaltic outcrop atop Shatsky Rise. There were also numerous ancillary scientific goals to be addressed using cores and logs obtained from Leg 191 sites. The seismic observatory was successfully installed at Site 1179 and left ready for activation by a future remotely operated vehicle cruise. The hammer drill tests were less successful owing to a streak of bad luck. Early in the leg, 4 days were lost when the JOIDES Resolution had to leave Site 1179 because of a typhoon. A medical emergency cost another 3 days and forced the ship to leave the Shatsky Rise area and return to Japan. In addition, a broken part on the drawworks made it impossible to return to Shatsky Rise for the HRRS test. In an effort to salvage the HRRS program, the hammer drill was tested on a seamount near Guam (Sites 1180 and 1181), but the lithology was unsuitable (soft volcanic ash) and another typhoon forced evacuation of the area. Finally, an abbreviated HRRS test was accomplished at a site atop a basaltic volcano in the Mariana Trough (Site 1182).

Despite the operational difficulties, an excellent set of cores was obtained from Site 1179, which is located on lithosphere of Anomaly M8 age (129 Ma). A 377-m-thick sedimentary column was cored in addition to 98 m of basaltic basement (total depth = 475 meters below seafloor). The sedimentary column can be divided into four lithologic units. Unit I consists of 223.5 m of clay- and radiolarian-bearing diatom ooze of late Miocene to late Pleistocene age. Ash beds are common in this unit, recording volcanic activity from the western Pacific island arcs. Unit II is a clay-rich, diatom-bearing radiolarian ooze of late Miocene age with a thickness of 22.5 m. Unit III contains barren, brown pelagic clay in a 37.5-m-thick layer. Unit IV yielded poor recovery with only chert and porcellanite fragments from an unknown sedimentary matrix within 93.7 m above basement. The upper sedimentary section produced a well-defined magnetic reversal pattern, which shows that sedimentation was low (1.5 m/m.y.) during the mid-Miocene and increased 300-fold (to 40-43 m/m.y.) in the Pliocene and Pleistocene. Biostratigraphy in Units I and II was based mainly on siliceous microfossils and palynomorphs because calcareous microfossils were rare to absent. Sedimentation rates derived from biostratigraphy are in good agreement with those calculated from magnetostratigraphy. The brown pelagic clay of Unit III is barren, and few fossils were recovered from Unit IV; however, radiolarians observed in porcellanite samples indicate an Early Cretaceous age. The physical properties of the upper sedimentary section are unusual because porosities are extremely high (often >80%) and bulk densities actually decrease downhole for the first 150 m. These characteristics probably result from an increasing downward abundance of diatom tests, which have low grain densities and contain large amounts of pore space. The 98-m igneous section consists of aphyric ocean ridge basalts divided into 48 units based on lithologic differences and cooling boundaries. The section consists of massive flows and pillows with small amounts of interunit sediments and volcanic breccia. The basalts are unusually fresh for Early Cretaceous igneous rock, and alteration is restricted to low-grade zeolite facies at temperatures less than ~10°-30°C.