Showing papers in "American Journal of Science in 1975"

K-Ar dating, Quaternary and Neogene volcanic rocks of the Snake River Plain, Idaho

Abstract: Ninety-eight K-Ar dates for lavas of the Snake River Plain (SRP) and vicinity provide calibration for the stratigraphic framework discussed by Malde and Powers (1962). The Idavada Volcanics, predominantly silicic volcanic rocks, are 9 to 13 m.y. old in the western SRP. Rocks of similar type and comparable stratigraphic position are 8 to 10 m.y. old in the central SRP and as young as 4 to 5 m.y. old in the eastern SRP. This time-transgressive silicic volcanic assemblage merges in the east with the silicic volcanic units erupted during the last 2 m.y. from Island Park, Yellowstone, and related volcanic centers. The Idaho Group, consisting of intercalated basalt flows and sedimentary deposits in the western half of the SRP, ranges in age from about 0.6 to 11.5 m.y. (71 refs.)

Application of the silica geothermometer in low temperature hydrothermal areas in Iceland

Abstract: Temperatures measured in drillholes indicate that the amount of dissolved SiO/sub 2/ in thermal waters at depth in the low temperature hydrothermal areas in Iceland is governed by the solubility of chalcedony at below approximately 110/sup 0/. At above approximately 180/sup 0/, the solubility of quartz governs the amount of dissolved SiO/sub 2/ in the water. The factors that determine with which of these SiO/sub 2/ minerals the dissolved SiO/sub 2/ may reach equilibrium in this temperature range were not shown. Equilibrium with chalcedony appears in some instances to be established at 20/sup 0/, but most commonly cold ground water is SiO/sub 2/ supersaturated with respect to chalcedony solubility. Provided SiO/sub 2/ is the only varying component in the system, SiO/sub 2/ temperatures are minimum temperatures and represent temperatures of last equilibrium with quartz or chalcedony. If mixing with cold ground water takes place and an originally high pH of the thermal water is lowered, the estimated SiO/sub 2/ temperatures are likely to be high compared with the temperature of the mixture. There is a clear relation between the intensity of low temperature hydrothermal activity in Iceland and the age of the rocks in which this activity is located.

Silurian marine red beds

Abstract: Derivation from oxidized source material, followed by rapid burial in quiet marine environments before reduction could take place; Europe and eastern North America