Petrology of the Keetley Volcanics in Summit and Wasatch Counties, North-Central Utah
1994-03
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Petrology of the Keetley Volcanics in Summit and Wasatch Counties, North-Central Utah
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1994-03
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The Keetley Volcanics rest subhorizontally in a structural saddle between the Uinta and Wasatch Mountains ca. 35 km east of Salt Lake City. The Keetley Volcanics consist essentially of Oligocene to Late Eocene volcanic breccias, sandstones, lava flows and porphyritic intrusives. Volcanic breccias are volumetrically the most important, being as thick as 500 m (total) in the vicinity of Keetley. Most of the breccias and intrusives are andesitic. Chemical compositions vary from trachyandesite to silica-poor rhyodacite. The relatively uniform compositions suggest a trachyandesitic source that differentiated by fractional crystallization in a relatively shallow chamber. Large porphyry intrusions are also present west of the Keetley Volcanics. Apparently, the Keetley Volcanics represent a part of magmatic events in the Uinta-Oquirrh mineral belt that started in Late Eocene and continued into earliest Miocene. The Keetley Volcanics may belong to a larger group of mid-Tertiary early to syn-extensional volcanic rocks that were formed before the gradual development of the San Andreas - Basin and Range transform-extensional system. The primary structures and petrography of the Keetley Volcanics were studied in order to define the mechanics and environments of deposition for volcanic breccias and sandstones that can be formed by several processes. An extensive litterature review of volcaniclastic deposits is included. The studied breccia clasts are almost entirely plagioclase-amphibole porphyritic andesites which commonly contain accessory biotite, clinopyroxene and orthopyroxene phenocrysts. Other accessory minerals are apatite, opaques, sanidine and quartz. Staining for K-bearing minerals indicates that the samples commonly contain sanidine microlites. Other volcanic breccia clasts have a dacitic or basaltic-andesitic mineralogy. Volcanic breccias and sandstones with sand- and gravel-sized dacite clasts indicate a dacitic source for at least some of the distinctively polymictic mass flow deposits. Basaltic-andesitic scoriaceous breccias are associated with basaltic-andesitic lava flows. The breccias of the Keetley Volcanics commonly include gravel-sized grayish-white clasts that are strongly altered to clay. Microscopic studies indicate that such clasts commonly are pumice. However, some samples have a scoriaceous rather than a pumiceous texture. Megascopic observation also indicate that some of the clasts are fragments of a pumiceous tuff The textures of observed mafic enclaves could not be attributed to magma mixing; together with the common resorbed phenocrysts and rare xenocrysts, they suggest magma/country rock interactions and/or complex crystallization. Slight variations in mineralogy could result from the fact that the breccias were derived from several source rocks or from a single heterogeneous source such as a compositionally zoned magma chamber. Based on mineralogy only, the intrusive rocks cutting the volcanic breccias could have been the sources for the thick breccias. Because of the narrow mineralogical variation, different andesite lithologies were recognized with difficulty. Porphyritic rocks have been broken into sand-sized clasts that show any combination of one or more phenocrystminerals. Difficulties are also partly due to oxidation, alteration and devitrification. Oxidation has had a stronger influence on the petrography in the basaltic-andesitic clasts than in andesites. Samples of altered or devitrified clasts that have a distinctively different megascopic appearance may appear much less distinctive under a microscope. Different clast lithologies can be defined, based on variations in mineralogy and texture. The results suggest a polymictic and thus, a probable reworked origin for the most of the studied deposits. The studied outcrops and roadcuts at Jordanelle are located in the Heber quadrangle. The roadcuts are along newly built highway 189. The deposits consist of polymictic to monomictic volcanic breccias and sandstones. Beds commonly have channeled bases and are massive. Inverse or inverse to normal grading is common. Volcanic sandstones and pebbly sandstones are massive or stratified. Some of the stratified lithologies show low-angle cross-bedding. Some of the pebbly sandstones and sandstones are inversely, inversely to normally or normally graded. Volumetrically, sandstones form a minor portion. A few basaltic-andesitic scoriaceous breccia sequences associated with the basaltic andesitic lava flows are present in the upper portion of the sequence which is more than 500 m thick. For many depositional units, a reworked (debris flow or hyperconcentrated flow) origin is probable. However, for some of the depositional units the other alternative, a primary pyroclastic depositional mode, cannot be completely discarded. The studied outcrops at Indian Hollow are located at the Kamas Quadrangle. The total thickness of the studied sequence is over 100 m. The lower half consists of andesitic breccias. Thick breccias and conglomerates are laterally persistent but commonly are cut into minor volcanic sandstones and pebbly sandstones, forming gentle channels-fills. Breccia and conglomerate beds are massive or, uncommonly, crudely stratified. Inverse or inverse to normal grading is common. The texture varies from clast-supported to matrix-supported. Most of the breccias and conglomerates are polymictic and contain pieces of columnar-jointed andesites. In a few matrix-supported beds, the clast composition appears monomictic and the sandy matrix is reddish due to oxidation. The beds also contain in situ cooling-jointed blocks. The volcanic sandstones are massive and may show inverse or normal grading. The studied stratified breccia deposits at Indian Hollow are interpreted to have been deposited by lobate- or tongue-shaped debris flows or minor pyroclastic flows. The upper half of the sequence is poorly exposed; it seems to consist of basaltic-andesitic lava flows and sconaceous breccias. A sequence of Keetley Volcanics approximately 150 m thick is well exposed at Silver Creek Canyon, 5.5 to 6.5 kilometers southwest of Wanship along Interstate Highway 80 in the Wanship Quadrangle. Polymictic and clearly matrix-supported conglomerates make up an overwhelming part of the sequence. The thicknesses of matrix-supported conglomerates vary from a few meters to several tens of meters. Clast-supported conglomerate beds several meters thick form a minor proportion. The conglomerates commonly contain rounded pebbles and cobbles of older sedimentary rocks. The studied deposits at Silver Creek Canyon can be almost unequivocally attributed to debris flows or hyperconcentrated flows. Although the Keetley Volcanics rest on the top of volcaniclastic lake deposits (the Peoa Tuff), the studied stratified andesitic to basaltic-andesitic sequences do not support a caldera collapse origin for the Keetley Volcanics. Volcanic breccias and sandstones of the different study areas of this project are similar to debris flow, hyperconcentrated flow and pyroclastic flow deposits that are found at some distance from the source on slopes of modem stratovolcanoes, with the source vent or vents located kilometers to few tens of kilometers from the sites of deposition. The various porphyritic plutons in the Wasatch Mountains 5 to 20 km west of the study area indicate the probable location of the magmatic center and may represent the unroofed remnants of an ancient stratovolcano or an intermediate-silicic multivent center. Some of the material may have been derived from local explosive to gravitational collapses of spines and domes related to shallow intrusions cutting the breccia. Basaltic-andesite flows and scoria breccias are at least partly derived from local parasitic vents. The position of basaltic-andesite lava flows at stratigraphically high levels may, however, indicate compositional zoning in the magma chamber.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Jussi Einari Leveinen in partial fulfillment of the requirements for the degree of Master of Science, March 1994.
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