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The composition of lower Eocene fluvial sandstones in the Rocky Mountain region was studied to aid in interpretation of the regional geochemical environment of uranium and other mineral deposits in rocks of early Eocene age. The study was based on 216 samples, from 18 basins, collected according to a hierarchial plan that involved the random selection of three pairs of beds in each of 36 sample areas. The geometric means of the constituents (in percent) and of the minor elements (in parts per million) for these 216 samples are as follows (geometric deviation in parentheses) : Si02, 72.9 (1.2); AbO;~, 6.6 (1.9); total iron as Fe20s, 1.2 (2.1); MgO, 0.6 (3.0); CaO, 2.4 (4.3); Na20, 0.5 (4.6); K 20, 1.4 (2.3); H20+, 1.4 (1.6); Ti02, 0.2 (1.9); P 2 0;-,, 0.05 (2.6); MnO, 0.06 (2.1); C02 , 0.7 (12.6); B, 13 (2); Ba, 415 (2); Co, 4 (2); Cr, 13 (3); Cu, 8 (2); Ga, 9 (2); La, 12 (2); Ni, 9 (2) ; Pb, 8 (2) ; Sr, 138 (3) ; V, 24 (2) ; Y, 10 (2); Zr, 113 (2) ; U, 2.4 (2) ; and Th, 4.3 (2). The sampled rocks include feldspar-rich sandstones, which were derived from the erosion of crystalline rocks, and quartzrich sandstones and carbonate-rich sandstones, which were derived from reworked older sedimentary strata. Rocks rich in both carbonate and feldspar are classed as intermediate. Feldspar-rich sandstones are characteristic of a broad area of central Wyoming, much of Colorado, and northern New Mexico. Carbonate-rich sandstones are characteristic of a narrow belt through central Utah with extensions in the adjacent States. Intermediate rocks lie between carbonaterich and feldspar-rich sandstones in western Wyoming and northeastern Utah. Quartz-rich rock was found in one area of the Bighorn Basin of Wyoming. The data from chemical, X-ray diffraction, and petrographic analyses were analyzed statistically. Principal-component analysis of the correlation matrix was used to identify five geochemical groups-a quartz group, a feldspar group, a carbonate group, and two minor-element groups between the feldspar and carbonate groups. Boron, titanium, and zirconium occur in resistate minerals and correlate poorly with everything else except possibly with quartz. Cobalt, chromium, copper, nickel, and vanadium tend to be associated with the intermediate rocks. The enrichment of uranium tends to correlate with feldspar-rich sandstones. Statistical analysis of dominantly red, orange, yellow, or green subsets of the samples shows that the dominantly green samples differ significantly from the other subsets and that the greatest difference is between the green and the red subsets. The subset of 11 green sandstones has a greater concentration of mica, cobalt, chromium, copper, nickel, strontium, vanadium, gallium, total iron, titanium, and the feldspars, especially plagioclase, than the average of all samples. Early diagenetic alteration is evidenced by widespread authigenic clay, carbonate, and zeolite minerals, the etching of quartz, the presumed dissolution of mafic silicate minerals, and the enrichment of minor elements in areas of intermediate rock rather than in areas rich only in feldspar, quartz, or carbonate. In many areas, red sandstone probably represent rocks that were origi~ally greenish gray and that were altered at a later stage. Although rock color can be the result of many processes, the red alteration may have been caused by the introduction of oxygenated meteoric ground. waters and the accompanied leaching of iron and many of the more mo-

Geochemistry of lower Eocene sandstones in the Rocky Mountain region, with a section on direct-reader spectrometric analyses