Insoluble residues of the fine-grained sediments from DSDP Leg 67 Holes

Insoluble residues of Late Cretaceous to Quaternary deep-sea samples from slope, trench, and oceanic plate sites south of Guatemala were examined, specifically for the distribution of clay minerals in the <2-µm fraction and of silt grains in the 20-63-µm fraction. Widespread "oceanic" particles (biogenic opal, rhyolitic glass) and their diagenetic products (smectite, clinoptilolite, heulandite) were distinguished from terrigenous material - illite, kaolinite, chlorite, plagioclase, quartz, and heavy minerals. The main results of this investigation are: (1) At Site 494 on the slope immediately adjacent to the trench, terrigenous supplies testify to a slope position of the whole sequence back to the Late Cretaceous. (2) At Site 495 on the Pacific Cocos Plate, "oceanic" and terrigenous sedimentation are clearly separated. Whereas the pelagic sedimentation prevailed in the early Miocene, terrigenous minerals appeared in the middle Miocene in the clay fraction, and in the early Pliocene in the coarse silt fraction. These terrigenous supplies are interpreted as having been transported by suspension clouds crossing the slope and even the trench. The alternative, however, an eolian transport, cannot be excluded.

Geochemistry at DSDP Leg 70 Holes

A total of 32 holes at five sites near 1°N, 86°W drilled on Deep Sea Drilling Project (DSDP) Leg 70 (November- December 1979) provide unique data on the origin of the hydrothermal mounds on the southern flank of the Galapagos Spreading Center. Hydrothermal sediments, primarily Mn-oxide and nontronite, are restricted to the immediate vicinity of the mounds (< 100 m) and are probably formed by the interaction of upward-percolating hydrothermal solutions with seawater and pelagic sediments above locally permeable zones of ocean crust. Mounds as high as 25 meters form in less than a few hundred thousand years, and geothermal and geochemical gradients indicate that they are actively forming today. The lack of alteration of upper basement rocks directly below the mounds and throughout the Galapagos region indicates that the source of the hydrothermal solutions is deeper in the crust.