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Cobbles in Eocene alluvial-plain deposits of California indicate that major changes have occurred in these first-cycle sediments. Migrating pore water has caused diagenetic alteration in the upper parts and authigenic iron sulfide growth in the lower parts of these braided-river gravels. Analysis of thin sections and X-ray diffraction patterns reveals that mica, amphibole, and feldspar in cobbles of various compositions are replaced predominantly by kaolinite. Original minerals found in fresh cores of some cobbles are replaced completely in altered rims by hematite and limonite. Sand samples from the upper parts of the gravels contain abundant quartz with kaolinite pseudomorphs of biotite and feldspar and hematite pseudomorphs of iron sulfide. The heavy-mineral suite cons sts of chemically stable minerals including zircon, ilmenite, tourmaline, and the alteration products anatase and leucoxene. Sand in the basal Eocene gravels contains abundant detrital iron-bearing silicates and authigenic sulfides. Cobbles in the lower unit show little evidence of alteration. Apparently, decomposition of trapped organic matter and pyritic slate clasts reduced the pH and Eh of the pore water percolating through the gravels, causing the hydrolysis of silicate minerals. Some constituent ions migrated in solution away from reaction sites, and iron was precipitated as sulfides as it reached saturation. At some later time, lowering of the water table permitted oxidation of the sulfides in the upper parts of the gravels. This interpretation suggests that the subtropical alluvi l-plain environment is ideal for elimination of chemically unstable minerals and rock fragments from sediment being transported through the river system.
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