ABSTRACT

Core and grab samples from 40 sites in Lake Tahoe and from six major tributaries were analyzed for mineral and biogenic components and for texture. Clay and silt fractions were analyzed by X-ray diffraction. Sand and gravel were stained to differentiate feldspars, and mineral composition was visually estimated. In addition. the relative abundance of diatoms and pollen was estimated.

The areal distribution of volcanic constituents of sand and gravel fractions reflects volcanic sources in the north and northwest parts of the basin. Volcanic areas contribute montmorillonite to clay fractions whereas vermiculite and chloritic intergrades are characteristic weathering products of granitic sources.

Two distinct types of sediment are present. Pollen-rich diatomaceous ooze (organic ooze) is characterized by the following: (a) abundant diatoms and pollen; (b) chloritic intergrades in the clay fraction; (c) all samples from flat-lying, well stratified beds. The other sediment type (non-organic) is typified by: (a) diatoms and pollen rare or absent; (b) vermiculite/mica/montmorillonite clay fraction; (c) not present in "flat-lying" beds; (d) texturally more varied than organic ooze.

Non-organic samples represent still-exposed depositional products of the Tioga (?) glaciation, reflecting relatively rapid erosion and slumping into deeper parts of the basin. The principal source of non-organic material was the west side where volcanic rocks constitute about half of the area. In contrast, organic ooze samples result from relatively passive post-glacial fluvial erosion. The relative abundance of biogenic components in organic ooze reflects low depositional rates and the clay fraction, rich in chloritic intergrades, points to the dominance of granitic source rocks in the present basin-wide source.