Abstract

A geochemical and petrographic study of lower Eocene fluvial sandstones in the Rocky Mountain region was made to determine their average composition and geochemical history. From 19 basins of deposition, 228 samples were collected according to a random sampling plan. The 228 samples range from highly arkosic sandstones derived from the Pre-cambrian core terrain of the central Rocky Mountains to calcareous quartz sandstones derived from Paleozoic and Mesozoic thrust sheets in western Wyoming and Utah. The average major element composition is (in percent) SiO₂ 73.4, A1₂O₃ 8.0, Fe₂O₃ 1.1, FeO 0.42, MgO 1.0, CaO 5.5, Na₂O 1.1, K₂O 1.9, H₂ 2.1, TiO₂ 0.27, P₂O₅ 0.08, MnO 0.07, CO₂ 4.5. Minor elements detected include (in ppm) Ba 580, Co 5.5, Cr 22, Cu 11, Ga 11, Ni 12, Sr 390, V 32, Zr 140, Th 7.4, U 2.4. These values are relatively high for A1₂O₃, K₂O, CO₂, Ga, Sr, V, Ni, Pb, Co, and U, but low for Fe₂O₃, MgO, Zr, B, and La compared with several estimates of the average composition of sandstones.

Nearly all samples show distinct chemical and mineralogic changes resulting from diagenetic and, locally, hydrothermal alteration. Authigenic minerals include carbonates, zeolites, montmorillonite, kaolinite, mixed-layer clays, and locally epidote and amphibole. Many samples show corrosion of quartz and silicate minerals. Hornblende was probably present in the original arkosic sediments but has since been dissolved. Alkaline carbonate solutions capable of transporting uranium were probably responsible for the alteration observed in many basins. A possible source for such waters in some basins is the connate waters expelled from the intertonguing Green River Formation.