ABSTRACT

The mineralogy and petrography of forty cores of marine bottom sediments from the Peru-Chile Trench area were studied by X-ray and microscopic techniques. Samples from each core were collected at intervals of 10 cm, or more frequently where visible lithologic variations occur. The -1.3 µ fractions of many of these samples were also studied and the clay minerals identified by means of treatment with ethylene glycol and by heating to 200° and to 450° C.

The most common minerals in the cores are quartz, plagioclase, calcite, and the clay minerals kaolinite, chlorite, illite, and a mixed-layer clay predominantly montmorillonite. Halite and gypsum are important in many cores as results of dehydration of the cores. Volcanic glass, in various stages of devitrification, is extremely common in nearly all the cores; some samples are sufficiently abundant in glass to be called ash beds.

Among the less common minerals are aragonite, dolomite, rhodochrosite, potassic feldspar, glauconite, biotite, phillipsite, pyroxene, and amphibole. Chemically precipitated dolomite is found in one core at a water depth of 4600 meters. Besides dolomite, minerals which are at least partly authigenic are quartz, calcite, rhodochrosite, phillipsite, glauconite, and kaolinite. The observed mineral associations are given in a table.

The gross lithology of the cores includes sand, silt, mud, black mud, and calcareous and siliceous ooze. While large lithological variations variations exist between adjacent cores, within each core the lithology tends to be fairly uniform over the entire length.

The methods of identification of the 7-14 A clay minerals are in an unsatisfactory state. The Bradley criteria are applicable to fine-grained, poorly crystalline chlorite, of relatively lower thermal stability than the well-crystallized chlorite to which the Brindley criteria apply. The very identification of a chlorite in a mineral mixture would thus be predicated on assumptions regarding the detailed nature of the material. The two sets of criteria, however, are not altogether contradictory; by a suitable combination it is possible in many cases not only to identify the chlorite but also to estimate its crystallinity. Most of the chlorites from the Peru-Chile Trench area satisfy the Bradley criteria.

Samples from the seaward side of the Peru-Chile Trench are uniformly fine mid. Samples from within the Trench are largely fine mud but include thin beds of silty to sandy material. Samples from the landward side of the Trench are heterogeneous in size as well as in mineralogical and chemical compositions. Although on the whole the near-shore samples tend to be most coarse-grained, there is no simple correlation between grain size and distance from the coast, nor between grain size and bottom topography. Sediment transport appears to be due largely to bottom currents, the existence of which is independently established by the discovery of graded bedding and cross-bedding in a number of the cores.