The sediments covering igneous basement along the Leg 168 transect are a mixture of hemipelagic mud (closest to basement, particularly above local basement highs) and turbidites (Underwood and Hoke, Chap. 5; Cavin et al., Chap. 6; both this volume). Sediments within the shallowest part of the section vary in texture depending on proximity to distributary channels. Correlation between sediment physical properties and seismic profiles allows extrapolation of interpretations laterally away from ODP boreholes (Sun, Chap. 3, this volume).
Fluid seepage through shallow sediments was evident from pore water profiles at Sites 1030 and 1031 (Shipboard Scientific Party, 1997). Sediments at these sites were found to have anomalous seismic properties (Zühlsdorff et al., 1999) when compared to sediments from equivalent depths at adjacent sites, but subsequent laboratory testing revealed variations in primary lithology that could account for differences in chemical and physical properties (Inoue, 1999; Giambalvo et al., 2000) and little direct evidence for sediment alteration caused by fluid seepage, except immediately above basement (Su et al., Chap. 4, this volume; Bautier et al., in press). Pore waters at Sites 1030 and 1031 appear to be maintained at a state close to equilibrium with their host sediment (Monnin et al., Chap. 8, this volume). Alteration within ridge flank sediments depends strongly on the reactivity of organic carbon (Rudniki et al., in press).
Low-temperature alteration within upper basement is ubiquitous along the Leg 168 transect, with extent and intensity increasing along with age and temperature from west to east (Hunter et al., 1998; Hunter et al., 1999; Marescotti, et al., Chap. 10; Porter et al., Chap. 12; both this volume). Secondary clays in veins, vesicles, and replacing olivine and glass account for several percent to 10%-20% of the recovered rock by volume and represent the earliest alteration products. A later alteration stage is characterized by calcium carbonate (calcite and aragonite) in veins and vesicles. A general progression from hydrologically open, oxidizing alteration to hydrologically isolated, reducing alteration is evident in the penetrative alteration and vein mineralogy (Marescotti et al., Chap. 10; Porter et al., Chap. 12; both this volume; Hunter et al., 1999). The minor and trace element chemistry of low-temperature secondary minerals such as calcite and aragonite provide information about hydrothermal fluid chemistry and the budgets for a number of elements (Yatabe et al., Chap. 11, this volume). Alteration is consistent with a thermal history in which the rocks have never seen temperatures greater than those at present (Marescotti et al., Chap. 10, this volume).