Leg 174B Scientific Report


Site 1074
Sixty-four meters of sediment and 0.58 m of basalt were recovered at the single hole drilled at Site 1074. A total of eight cores were recovered; Cores 1074-1H through 7H contain sediments, and Core 8X is basalt. Two lithologic units were defined: Unit I contains nannofossil ooze with varying amounts of foraminifers, clay, radiolarians and sand, and nannofossil clay, and overlies Unit II, a unit of aphyric basalt (Fig. 8). The sedimentary Unit I was divided into two subunits based on the magnetic susceptibility record, clay content, and the presence or absence of graded sand layers. Unit IA includes the upper 62 m of sediments; Unit IB, a red clay, occupies the lower 2 m. The clay content gradually increases, and the occurrence of sand (either foraminifer ooze or lithic fragments) decreases with depth in the hole. The magnetic susceptibility increases in intervals with increasing clay content, bioturbation, and sand layers. Foraminifer oozes are characterized by very low susceptibility values. The magnetic susceptibility increases sharply at the Subunit IA/IB contact and remains high in Subunit IB. Density and sonic velocity show normal gradients in the upper 10-20 m, below which high values correlate with sand layers.

The composition of interstitial waters at Site 1074 generally shows only minor variations as the result of diagenetic alteration. There is little evidence of microbial decomposition of organic matter, suggesting low organic matter content in the sediments. Potassium and H4SiO4 increase at the base of Subunit IA, indicating a source for these constituents in the clay-rich sediments (Subunit IB) at the base of the sedimentary section. There appears to be little diffusive exchange between sedimentary and basement pore fluids, perhaps as a result of the presumably low permeability basal clay. Downhole temperatures measured on Cores 3H through 6H are consistent with purely conductive heat transfer. Thus, there is no evidence for fluids vertically advecting through the sediment column at Site 1074. This observation supports the model proposed by Langseth et al. (1984, 1992) that fluid circulation is confined to the basement beneath the sediment pond and that heat transfer through the sediments is predominantly conductive.

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