Abstract

Crater Lake, in the collapse caldera of a Pleistocene composite volcano in the Cascade Range of Oregon, has a maximum depth of 1932 feet. Immature and generally unsorted volcanic sediments have been deposited on the steep slope and nearly flat floor of the lake. In locations protected from terrigenous influx, the sublittoral bottom and summits of volcanic cones have accumulations of diatomaceous and moss oozes. Poor sorting of patchy deposits of medium sand on the slope and shore suggests deposition by sublacustrine and subaerial mass movements. The thickest deposits, layers of mud and sand, are found on the flat floors of the basins. Organic content, poor sorting, and silt-size median diameters suggest that surface sediments of the basin floor, mud layers of the basin floor, and sediments of the intracaldera volcanic structures result from continuous sedimentation of suspended materials. These suspended particles are introduced into the water by winds picking up material from within and outside the caldera, and by streams, sheet wash, and mass movements down the caldera wall. Good sorting, median diameters of fine sand size, and the mineral composition of basin sand layers indicate that mass movements from the slopes occasionally generate turbidity currents that carry sand across the flat basins to the lake center. Different sources and varying mineral content of sand layers in the same core, and lack of correlation of sand layers from core to core indicate the limited lateral extent of turbidity currents. The thinner, finer-grained, better sorted, and fewer number of sand layers toward the center of the lake suggest that turbidity currents lose competence progressively away from their source. Crater Lake turbidites are most similar to those of the offshore ocean basins of Southern California.

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