Pre-Diagenetic Sedimentary Fractionation of Andesitic Detritus in a Semi-Arid Climate: An Example from the Eocene Datil Group, New Mexico
Published:January 01, 1991
Steven M. Cather, Robert L. Folk, 1991. "Pre-Diagenetic Sedimentary Fractionation of Andesitic Detritus in a Semi-Arid Climate: An Example from the Eocene Datil Group, New Mexico", Sedimentation in Volcanic Settings, Richard V. Fisher, Gary A. Smith
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The lower Datil Group (40–37 Ma) records the early phase of intermediate-composition volcanism in the northern Mogollon-Datil field of New Mexico. Eruptive products consist almost entirely of high-K andesite and dacite that range in SiO2 content from about 58 to 67 weight percent. Lower Datil conglomerate clasts and correlative(?) lavas are characterized by phenocrystic plagioclase (An20–60), amphibole, and titanomagnetite (±biotite, pyroxene) within a groundmass containing alkali feldspar, silica minerals, and plagioclase. Volcaniclastic rocks of the lower Datil Group were deposited by a spectrum of fluvial, debris-flow and lacustrine processes in two semi-arid intermontane basins and differ mineralogically from their volcanic progenitors in the following ways: 1) nonwelded ash is not represented in conglomerate; 2) phenocryst phases are preferentially concentrated in sandstone; 3) groundmass constituents and devitrification products are over-represented in mudstone and detrital matrix in sandstone; 4) opaque reaction rims on free amphibole and biotite grains are poorly developed or not present; 5) amphibole abundance is slightly diminished relative to plagioclase and biotite in sandstone; and 6) potassic riras on plagioclase grains may have been removed by abrasion. Mineralogic fractionation of lower Datil Group volcaniclastic sediments occurred primarily by impact shattering and abrasion during transportation. As a result, provenance determinations for volcaniclastic rocks may be influenced by grain-size effects.
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Sedimentation in Volcanic Settings
We have gained considerable experience with volcaniclastic materials over the past 30 years, but the field has undergone considerable growth in the decade following the 1980 eruption of Mount St. Helens. This eruption resulted in an accelerated research in explosive volcanic products and spurred a renewed interest in volcaniclastic materials as they relate to plate tectonic boundaries and explosive volcanism in general. Since the early 1970s a loosely defined field called â∈œsedimentary tectonicsâ∈ has emerged. A large part of the field of sedimentation and tectonics includes studies of volcaniclastic sedimentation, largely because of the direct association of tectonism, volcanism and sedimentation. This book attempts to illuminate the field and to present its salient features to sedimentologists not generally versed in volcaniclastic particles, deposits or facies.