Middle Valley

Bent Hill Area


Rock magnetic parameters reflect the varying lithologic units in Hole 856H by abrupt changes in the magnetomineralogy and the respective concentration of magnetic minerals. A transitional shift of magnetic signal indicates a downhole change from relatively high concentration of hematite to low concentration of magnetite, associated with an increase of paramagnetic minerals such as pyrite or hexagonal pyrrhotite. Thus, in general the magnetic signal seems to reflect a strong overprint caused by the hydrothermal alteration of the sediment.

Downhole measurements

The Triple Combo tool, composed of natural gamma ray (HNGS), resistivity (DIT), porosity (APS), sonic velocity (SDT), and density (HLDS) probes, was run in Hole 858H from 495 mbsf to the seafloor. The caliper confirmed that the hole was in a very good condition and the Formation Microscanner (FMS) was run to 491 mbsf. Good hole conditions and significant variability in the composition and physical properties of the different types of formations allowed the clearly identification of very distinct logging units and subunits.

Logging unit 1 (84.5-100 mbsf)

Massive sulfide is characterized by the highest densities (> 4 g/cm3) and the lowest resistivity recorded in Hole 856H and corresponds to the massive pyrrhotite sampled at the base of the massive sulfide mound.

Logging unit 2 (100-210 mbsf)

This unit corresponds to the sulfide stringer zone. All the logs display very strong fluctuation related to variations in the intensity of the hydrothermal alteration and sulfide veining. From 100 to 145 mbsf, the resistivity reaches extremely high values but tends to decrease with depth, as do the density and the velocity, whereas the porosity reaches minimum values. This subunit corresponds to the most intensely sulfide-veined sediment identified in core samples between 105 and 155 mbsf. Between 145 and 187 mbsf, the resistivity and density curves remain relatively low and the porosity fluctuates considerably. This interval corresponds to sediment with some veins and sulfide impregnation. Between 187 and 210 mbsf, resistivity, density and sonic velocity are similar to that of the upper part of the stockwork. Low gamma-ray counts reflect the partly turbiditic nature of the sediment. This interval corresponds to the sulfide-banded sediments observed in the core samples. Over the entire unit FMS images give a very good picture of the veins network and of its change in density with depth.

Logging unit 3 (210-292 mbsf)

Below 210 mbsf the sediments are only slightly altered, and the logs become more stable and more representative of changes in structure and lithology through the succession of interbedded mudstones, siltstones, and sandstones. Logging Unit 3 is characterized in the FMS images by a high level of fracturing, potentially related to faulting. Two intervals (221-239 and 250-270 mbsf) have low resistivity and high porosity. The high gamma-ray counts and low density indicate dominant clayey sediments in this interval. The tops of these two intervals are marked by a contact surface dipping 50 degrees to the west. Between these two intervals the FMS maps a succession of fractures dipping the same direction at about 50-70 degrees. The sharp decrease in potassium above 292 mbsf can be interpreted as the result of fault controlled hydrothermal circulation and alteration.

Logging Unit 4 (292-432 mbsf)

This unit is characterized by a progressive reduction in gamma-ray counts and an increase in resistivity, density, and sonic velocity. From 292 to 323 mbsf, resistivity is constant and FMS images show a succession of very fine beds dipping gently to the south (2 -10°). Some fractures dipping east are possibly conjugate to the main fault. At 323 mbsf, an increase in density, resistivity, and velocity, and a decrease in gamma-ray counts are interpreted to indicate a higher proportion of sand. The beds are thicker but display similar dips and azimuths.

Logging unit 5 (432 mbsf to bottom)

The occurrence of sills correlates with a strong increase in density, sonic velocity, and resistivity, and a drop in porosity and gamma-ray counts. Four sills are identified in the logs, their thicknesses varying between 2 and 6 m. FMS images suggest that some of these units may be pillow lava flows. The deepest part of the FMS record images pillow basalt between 471 mbsf and the bottom of the hole.

Temperature measurements indicate that the heat transfer is predominantly conductive at Site 1035, however, slight non-linearity of the curves for Holes 1035A and 1035E suggests that the regime is not purely conductive.

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