ARCHIVED REPORTS_1989_1 - Laserfiche WebLink

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The Corcoran Clay contains approximately 0 to 25 percent fine grained quartz sand.The sand generally occurs as disseminated grains and thin (0.01-to 0.02-inch) sandy ' interbeds within a fine-grained matrix of clays and silts. These thin sandy interbeds being more permeable,may act as a shallow perched water-bearing zone. However, the ' thin sandy interbeds which are subparallel to clay laminations are generally concentrated in the upper half of the unit. The sandy interbeds were not observed within tine unit in ' boring SB-2. Several of the thicker sandy interbeds appear to contain some sedimentary structures including: low angle ripple marks and flame structures. The unit ' contains from a trace to abundant fine muscovite localized along clay partings and sandy interbed/clay contacts. In addition, the Corcoran Clay contains a trace to moderately fine (0.01-inch) oxidized iron concretions and a few thin zones of abundant rootlets. The sediments above the Corcoran Clay include surficial fill emplaced during landfill ' operations and alluvial sediments. Surficial deposits in the area consist of both imported materials and residual soils derived from alluvial fan deposits. The surficial ' soils consist of approximatly 2 to 7 feet of silty sands, clayey sands. and sandy gravels. Soil permeability in the surficial soils varies from very low to moderate. ' Underlying the surficial fill and on top of the Corcoran Clay are alluvial deposits ' consisting of a heterogeneous assemblage of unconsolidated sands and gravels with a few thin lenses of sandy silts and clays. The sediments are composed predominantly of angular to subrounded clasts of quartz,jasperoidal chert, feldspars, and metaimorphics. ' No apparent bedding or sedimentary structures were observed. Because of the similarity between the sediments in the upper Tulare Formation and the Quaternary, the age of these sediments cannot be determined. ' The sediments beneath the Corcoran Clay Member can be divided into an upper and lower zone. The upper zone consists of approximately 200 to 250 feet of a ' heterogeneous mixture of alluvial clayey sands and clayey gravels consisting predominantly of subangular to subrounded clasts of quartz, jasperoidal chert, and brown quartzite.. These coarse-grained, poorly sorted sediments do not form a single ' unit; rather, they are a series of lenticular interbeds. The unit has an overall low permeability due to the high percentage of clay in the matrix. Several laterally ' discontinuous beds of sandy clays and sandy silts are interbedded within the coarse- ' STL 939 4VW n0 As �