Esacanaba Trough

Escanaba Trough Reference Site

Inorganic Geochemistry

The relatively rapid sedimentation rate in the Escanaba Trough helps to preserve the chemical signatures in pore waters, and diffusive processes are likely to influence signals only above about 35 m. This permits interpretation of the pore water concentration-depth profiles in terms of sedimentary provenance and diagenetic features. At 360 mbsf the concentration profile of several species (Na, K, Ca, Cl, B, H4SiO4, NH4, and Na/Cl ratio) are interpreted as showing a sudden change in the principal source of sediment fill in the Escanaba Trough from the Columbia River above to the Klamath Mountains below. These two areas of sediment provenance have rather different mineral assemblages; sediments from the Columbia River are principally volcanic and pyroxene rich, while sediments from the Klamath Mountains are amphibole rich. Diagenetic reaction in these sediments provide a source of Na, Ca, Li, and B to pore waters and a sink for K, alkalinity, and ammonia. A decrease in Mg and K concentration with depth is interpreted as being due to clay mineral formation.

Organic Geochemistry

The organic matter in the sediments of this reference site is generally immature from the top to approximately 450 mbsf. There are a few horizons with more mature bitumen which is interpreted to be derived from recycled older sedimentary detritus carried in by turbidites, however, the low extract yields throughout the hole indicate that there are no petroleum zones. Below 450 mbsf the organic matter is thermally altered, but low concentrations of bitumens reflect in situ maturation without migration/expulsion.

Physical Properties

Porosity decreases from 70% near the seafloor to 40% at the base of the sedimentary section and 10% in the basalt and associated altered sediments below 500 mbsf. The MST logs proved particularly useful in easing identification and classification of sedimentary units. Fluctuations in magnetic susceptibility correspond to boundaries of turbidites or hemipelagic units identified based on grain size distributions.

Downhole Measurements

Hole 1037B was logged from 100 to 535 mbsf using the Triple Combo tool, composed of temperature (TLT), resistivity (DIT), density-caliper (HLDS), porosity (APS), and natural gamma ray (HNGS) probes. A second run was composed of acceleration (HRA), sonic (SDT), and resistivity (DIT) probes. The gamma-ray and resistivity logs correlate well with Pleistocene lithostratigraphic sequences identified from core recovery and also show distinctive fining upward sequences within turbiditic sections. The density, velocity, and porosity logs were affected by irregular borehole conditions as the caliper from the HLDS showed that, for the most part, the hole diameter is over 16 inches. Adara temperatures recorded maximum formation temperatures of 15°C at 85 mbsf which extrapolate to 84°C at 500 mbsf.

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