Hydrogeochemistry, ODP Leg 110: an Overview

Abstract

Drilling of the northern Barbados accretionary complex transect during Leg 110 of the Ocean Drilling Program provided a unique opportunity to study the effects of sedimentary thickening, overthrusting, and faulting on the evolution of the chemical composition of interstitial waters. The production of low-chloride fluids farther in the complex has enabled evaluation of the main pathways of fluid expulsion from the complex. These are (1) along faults in the accretionary prism itself and (2) along the decollement and through underlying sandstones. In the latter case, methane anomalies, presumably caused by methane production as a result of thermal decomposition of organic matter farther into the complex, accompany the chloride anomalies. Major cation distributions (Ca 2+ , Mg 2+ , K + , Na +) are primarily affected by alteration of volcanic ash and exchange with formation waters in underlying basement rocks. However, as a result of tectonic activity in the complex, exchange with the overlying ocean is diminished and concentration-decreases in Mg 2+ , S0 4 2_ , and K + become larger in an arcward direction. Major cation distributions rule out the importance of pervasive advection through the sediments, thus confirming that the main pathways for fluid expulsion are along faults, the decollement, and underlying sandstones.

Permeability and fluid flow in natural mudstones

“…Anomalously fresh water and the high levels of thermogenic methane detected in both the d6collement proper and in the incipient d6collement, seaward of the deformation front, were attributed to smectite dewatering and organic maturation, implying lateral transport of 50-70 km along the d6collement, from depths (3.5-5km) where the smectite dehydrationorganic maturation window was attained (Gieskes et al 1990;Brown 1994). Furthermore, high temperatures in some of the thrust faults and the d6collement indicated movement of warm fluids along these fault zones (Fisher & Hounslow 1990).…”

“…The low average vertical permeabilities in the sediment wedges tend to promote extensive lateral fluid migration along stratigraphically controlled tectonic conduits. In the Barbados accretionary complex, geochemical and fluid pressure anomalies point to the role of the d6collement as a barrier to vertical fluid migration in the wedge (Moore et al 1988;Gieskes et al 1990). Here, high thermogenic methane contents are noted in and beneath the d6collement, whereas thrust faults above the d6collement that are conducting fluids have no thermogenic methane signature (Gieskes et al 1990).…”

“…In the Barbados accretionary complex, geochemical and fluid pressure anomalies point to the role of the d6collement as a barrier to vertical fluid migration in the wedge (Moore et al 1988;Gieskes et al 1990). Here, high thermogenic methane contents are noted in and beneath the d6collement, whereas thrust faults above the d6collement that are conducting fluids have no thermogenic methane signature (Gieskes et al 1990). The underthrust sediments beneath the d6collement are highly overpressured but almost undeformed, indicative of the vertical barrier to fluid migration caused by the localization of the dScollement in a 40 m thick smectite-rich clay unit (Moore et al 1988), generally the most impermeable of clay minerals (e.g.…”

Fabric and hydraulic conductivity of sheared clays

“…Hydraulic conductivity was measured using the¯ow pump method (Olsen, 1966). Tests were conducted using local tap water Dewhurst, D. N., Clennell, M. B., Brown, K. M. & Westbrook, G. K. (1996) in the case of kaolinite and silty clay and an arti®cial pore¯uid to simulate the¯uid chemistry of the Barbados clay (Gieskes, Vrolijk & Blanc, 1990). The pore¯uid was back-pressured at 300 kPa (Lowe, Zaccheo & Feldman, 1964).…”

Oxygen and hydrogen isotopic ratios and Cl- concentration of saline water in the Neogene siliceous sediments of Horonobe, Hokkaido, Japan

“…Blanc et al, 1991Gieskes et al, 1989Kastner et al, 1991Cl Kastner et al, 1991Bekin et al, 1994Kastner, 1981 Egeberg and Dickens, 1999…”