Using precruise seismic surveys we estimated that the igneous basement at Site 1153 was buried under ~300 m of sediment. Our drilling strategy was to wash in the drill pipe and begin rotary coring once we contacted basement. Rather than wash down with a center bit, we opted to run a core barrel and retrieve it as required to ensure we recovered the uppermost basement. As a result, we recovered several cores that contained sediment from intervals significantly longer than the length of the core barrel. Because of the uncertainty in actual depth of the intervals sampled and the sediment core disturbance inherent in coring with a rotary bit, all sediment cores were designated as wash cores and given the core type notation of W. Since all of these cores had less than full core barrels, for curation purposes we adopted the same strategy as for hard rock cores, that is, shunting all the recovered material to the tops of sections. Despite the uncertainty and disturbance mentioned above, there is potential for valuable stratigraphic and paleontologic information to be gained from these cores.
Based mainly on composition, we subdivided the sediments recovered from this site into two units. Sedimentary Unit A (0 mbsf to between 233 and 243 mbsf) is a dark brown to very dark grayish brown clay. Some indistinct lighter-hued laminae are sporadically present, but several recovered intervals appear homogeneous. Trace phases in the sediments account for significantly <1% of the recovered core and include, in descending order of abundance, (1) subangular to subrounded brown translucent grains of volcanic glass (2-3 µm in size), (2) angular to prismatic or tabular grains of quartz and plagioclase of similar size, and (3) rare siliceous microfossils. Sedimentary Unit B (top: between 233 and 243 mbsf; bottom: between 243 and 267.6 mbsf) is a calcareous ooze that includes the same igneous trace phases as in sedimentary Unit A in similar abundances. The contact between these two units is sharp and marked by a 2-mm-thick coarse, sandy layer derived from a manganese crust. We cannot determine if the fragmentation of this crust was drilling induced, but there is a thin layer of fine calcareous sand just beneath it, suggesting it was recovered with little disturbance. Below the contact the color of the sediment gradationally changes color to light yellow brown over a short recovered interval. Throughout this gradational color change, the core also exhibits evidence of core disturbance as lighter-hued clay is marbled and mottled through a matrix of dark clay.
Fortunately the contact between the units is reasonably well fixed, considering the nature of our drilling operation. Our strategy was to recover a core barrel whenever the driller noted a significant change in drilling conditions or penetration rate, with the hope of sampling the top of the igneous basement. Core 187-1153A-5W had been retrieved after washing down to 233 mbsf, but drilling conditions changed noticeably at about 243 mbsf, so the core barrel was retrieved. Thus, the wash barrel, which included the transition (Core 187-1153A-6W), only penetrated ~10 m from the bottom of the previous recovered interval. Therefore, the contact between carbonate-free sediment and calcareous ooze must lie between 233 and 243 mbsf, most likely near the bottom of that interval. The total thickness of carbonate sediment above the basaltic basement can be no more than ~30 m.
Considering the water depth of Site 1153 (>5700 m below sea level), we did not expect to see carbonate sediment overlying igneous basement. However, using the estimated age of the basement at this site from paleomagnetic data (~28 Ma) and the theoretical seafloor subsidence algorithm of Sclater et al. (1985), we can estimate that, at the time of formation, the depth below sea level of the igneous rocks recovered from Site 1152 was <4000 m, which is within the carbonate compensation depth range of the modern Indian Ocean.