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A New Middle .Tertiary Volcanic Formation in the Central Alaska Range

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A New Middle .Tertiary Volcanic Formation in the Central Alaska Range THE MOUNT GALEN VOLCANICS--A NEW MIDDLE .TERTIARY VOLCANIC FORMATION IN THE CENTRAL ALASKA RANGE BY JOHN E. DECKER AND WYATT G. GILBERT GEOLOGIC REPORT 59 STATE OF ALASKA Jay S. Hammond, Governor Robert E. LeResche, Commissioner, Dept. of Natural Resources Ross G. Schaff, State Geologist Cover photograph - Mount Galen from ridge 1.0 km north of Eielson visitors center. Photo by W.G. Gilbert. For sale by Alaska Division of Geological and Geophysical Surveys. P.O. Box 80007, College. 99708; 3327 Fairbanks St.. Anchorage. 99601; P.O. Box 7438, Ketchikan. 99901; and Pouch M. Juneau. 99811. CONTENTS Page Abstract ................................................................................................................................................................ Introduction ......................................................................................................................................................... Previous investigations .......................................................................................................................................... General description of Mount Galen volcanics ...................................................................................................... Petrology .............................................................................................................................................................. Hornblende andesite ....................................................................................................................................... Two-pyroxene andesite ................................................................................................................................... Basalt ............................................................................................................................................................. Geochemistry ........................................................................................................................................................ Age and correlation ............................................................................................................................................... Conclusion............................................................................................................................................................ Acknowledgments ................................................................................................................................................. References cited .................................................................................................................................................... ILLUSTRATIONS Page FIGURE 1. Distribution of Mount Galen Volcanics and location of study area ................................. .............. 2 . Geologic map of Mount Galen area. Mount McKinley National Park. Alaska ................................. 3. Looking northwest from ridge 1.0 km north of Eielson visitors center .......................................... 4 . Photograph showing crude stratification of light-colored volcanic breccia overlain by darker flow.......................................................................................................................................... 5 . Volcanic breccia composed of rounded to subangular clasts with a tuffaceous matrix ................... 6 . Photomicrograph of hornblende andesite from Mount Galen Volcanics ......................................... 7 . Photomicrograph of two-pyroxene andesite from the Mount Galen Volcanic sequence ................. 8. Plot of normative color index versus normative plagioclase composition for seven rocks from the Mount Galen Volcanics ....................................................................................................... 9 . Alkali-silica diagram with seven rocks from Mount Galen Volcanics, showing three different alkaline-subalkaline dividing lines.............................................................................................. 10 . AFM diagram with seven rocks from Mount Galen Volcanics ......................................................... 11. A1203 .normative plagioclase diagram after Irvine and Baragar (1971) showing seven volcanic rocks from Mount Galen Volcanics ........................................................................................... Page TABLE 1. Geochemical analyses and CIPW norm calculations (in percent) ..................................................... 2 . Analytical data for K-Ar age determinations.................................................................................. THE MOUNT GALEN VOLCANICS--A NEW MIDDLE TERTIARY VOLCANIC FORMATION IN THE CENTRAL ALASKA RANGE BY John E. ~eckerland Wyatt G. Gilbert2 ABSTRACT PREVIOUS INVESTIGATIONS The Mount Galen Volcanics are named for a series of The earliest known reference to rocks belonging to andesitic and basaltic lava flows, breccia, and tuff of the Mount Galen Volcanics is by Brooks (1911), who late Eocene-early Oligocene age in the west-central noted the presence of volcanic rocks and lignite on his Alaska Range. These rocks, once considered part of the geologic map. In 1919 Capps described the pyroclastic Cantwell Formation, cover at least 25 square kilometers rocks near Stony Creek as sediments containing abun- (km2) and locally reach a thickness of 1,000 meters (m). The formation can be divided petrologically into dant blocks of extrusive rocks interbedded with colum- hornblende andesite, two-pyroxene andesite, and basalt. nar lava, and assigned this occurrence to the Coal- Hornblende andesite with calc-alkaline characteristics Bearing Formation of probable Eocene age. In the is the most common rock type and occurs as tuff, lava Mount Galen area Capps mapped similar rocks as un- flows, flow breccia, and clasts and tuffaceous matrix in differentiated intrusive and extrusive rocks of pre- volcanic breccia. Two-pyroxene andesite most com- Ordovician to Tertiary age. Later, Capps (1927, 1932) monly occurs as lava flows and flow breccia. Basalt described this undifferentiated sequence as dikes, sills, occurs as lava flows and typically contains phenocrysts stocks, and lava flows of various compositions, mainly of titanaugite and plagioclase (+ olivine) and has of Tertiary age. In 1933 Reed mapped volcanic rocks olivine tholeiite characteristics. in the Mount Eielson district that consist dominantly of Ten K-Ar mineral ages from seven rock samples range porphyritic andesite, but include tuff, basalt, dacite, from 32.3 5 1.0 m.y. to 43.2 52.6 m.y. with a mean and mode of 37-38 m.y. obsidian, and breccia as well. These rocks were con- Taken together, the Mount Galen Volcanics and sidered to be of Eocene age on the basis of correlation lower Oligocene plutonic rocks of the Alaska-Aleutian with volcanic rocks associated with the Cantwell For- Range batholith constitute a middle Tertiary magmatic mation. In addition, Reed (1933) mapped porphyritic arc that roughly parallels the southern Alaska con- granodiorite of probable Jurassic age that the authors tinental margin. believe are volcanic rocks of the Mount Galen Volcanics. The change in age of the Cantwell Formation from INTRODUCTION Eocene to Cretaceous (Chaney, 1937) led Capps (1940) to assign the' volcanic 'rocks -in the Mounts Galen and This report names and describes the Mount Galen Eielson areas to the late Mesozoic. In 1961, J.C. Reed, Volcanics, a discontinuous series of andesitic and ba- Jr. included rocks of the Mount Galen Volcanics with saltic volcanic rocks of late Eocene-early Oligocene age the Cantwell Formation. East of the Mount Galen area in the west-central Alaska Range (fig. 1). These rocks Paleocene volcanic rocks once considered part of the are preserved in the western part of a block-faulted Cantwell Formation have recently been renamed the terrane that formed north of the McKinley strand of Teklanika Formation (Gilbert and others, 1976). the Denali fault in late Cenozoic time (Gilbert, 1975). Field studies of the Mount Galen Volcanics east of GENERAL DESCRIPTION OF Stony Creek were begun8byGilbert in June 1974, and MOUNT GALEN VOLCANICS Decker mapped and described the major part of the formation, -which lies northwest of MOOS; Creek and The Mount Galen Volcanics are a discontinuous Creek, during and August series of andesitic and basaltic volcanic breccia, flow extended mapping south to the Thorofare River during breccia lava flows, and tuff that covers more than 1975, and both authors briefly examined the volcanic 25 km5 in a band from the Thorofare River to a point rocks in the Gorge Creek area and west of the Muldrow 4 km east of Stony CreeK (fig. 2see centerfold). These Glacier in 1976. rocks unconformably overlie Triassic(?) metabasalt and '~eology Department, University of Alaska, Fairbanks. AK sedimentary rocks of the Paleocene Cantwell Formation 99701; present address: Geology Department, Stanford Un- iversity. Stanford, CA 94305. and are conformably overlain by Tertiary lignite-bearing 2~~~~,P.O. BOX 80007, College. AK 99708. continental sedimentary rocks (fig. 3). The formation is 2 GEOLOGIC REPORT 59 Figure 1. Distribution of Mount Galen Volcanics and location of study area, overlain unconformably by Quaternary glacial, fluvial, about 1.5 km southwest of VABM Thoro, where there and colluvial deposits. On the northwest the volcanic are rugged outcrops of volcanic breccia which cuts rocks are generally bounded by high-angle, north-side- Triassic(?) metabasalt
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