Detrital modes of sand and sandstone derived from andesitic rocks as a paleoclimatic indicator

Abstract

The effect of climate on the composition of volcanic sand is tested by determining detrital modes of modern stream sands derived from high-relief, andesitic rocks in areas of contrasting humid and semi-arid climates. The three humid sample areas, Mt. Pelee, Mt. Fuego, and Mt. Rainier, consist of active volcanoes and associated Neogene rocks, whereas the three semi-arid sample areas are underlain by Oligocene and Miocene (southwestern Colorado), Eocene (southwestern New Mexico), and Late Cretaceous (southwestern Montana) volcanic rocks. Point-counts of the medium sand-sized fraction of 66 samples (35 semi-arid, 31 humid) indicate that the ratio of volcanic rock fragments to plagioclase plus volcanic rock fragments (RFP index) and the ratio of volcanic rock fragments to accessory minerals (primarily monocrystalline pyroxene, amphibole, or biotite) plus volcanic rock fragments (RFA index) are statistically lower in humid regions, reflecting a greater degree of chemical breakdown of the parent rock and gravel- and sand-sized rock fragments and release of monocrystalline grains. The RFP index is applied as a paleoclimatic indicator to Late Cretaceous and Paleocene foreland-basin sandstones of the southern Foothills of Alberta, Canada. Interpretations of changes through time in paleoclimate based on the RFP index correlate closely with independent interpretations of paleoclimate based on the distribution of coal and caliche.