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Geology of the Northern Chugach Mountains, Southcentral Alaska

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Geology of the Northern Chugach Mountains, Southcentral Alaska GEOLOGY OF THE NORTHERN CHUGACH MOUNTAINS, SOUTHCENTRAL ALASKA BY L.E.Burns, G.H.Pessel, T.A. Little, T.L. Pavlis, R.J. Newberry, G.R. Winkler, and John Decker Professional Report 94 Published by STATE OF ALASKA DEPARTMENT OF NATURAL RESOURCES DMSION OF GEOLOGICAL & GEOPHYSICAL SURVEYS Alaska Department of NATURAL RESOURCES GEOLOGY OF THE NORTHERN CHUGACH MOUNTAINS, SOUTHCENTRAL ALASKA BY L.E. Burns, G.H. Pessel, TA. Little, T.L. Pavlis, R.J. Newberry, G.R. Winkler, and John Decker Cover: View looking east into valley of a tributary of Coal Creek in the Anchorage C-4 Quadrangle. Photo shows Mesozoic schist (Mzsh) and a very mixed unit of Jurassic diorite-quam diorite (Jdqd) inmdcd by Creraceow rrondhjemite (Kt). A smaU fault-bounded sliver of Jurassic gabbroic rocks (Jgu) is imbedded in the rrondhjemite. A mkture of Jurassic plutonic rocks, including compositionsfrom gabbroic to quom diontic, crops our in the background. Foreground is composed of a rock glacier (Qrg) and glacier (Qg). Photo by G.H. Pessel, 1983. Professional Report 94 Division of Geological & Geophysical Surveys Fairbanks, Alaska 1991 STATE OF ALASKA Walter J. Hickel, Governor DEPARTMENT OF NATURAL RESOURCES Harold C. Heinze, Commissioner DIVISION OF GEOLOGICAL & GEOPHYSICAL SURVEYS Thomas E. Smith, Acting Director and State Geologist DGGS publications may be inspected at the following locations. Address mail orders to the Fairbanks office. Alaska Division of Geological & Geophysical Surveys 794 University Avenue, Suite 200 400 Willoughby Avenue, 3rd floor Fairbanks, Alaska 99709-3645 Juneau, Alaska 99801-1796 U. S. Geological Survey Earth Science Information Center 605 West 4th Avenue, Room G684 4230 University Drive, Room 101 Anchorage, Alaska 99501-2299 Anchorage, Alaska 99508-4664 This publication, released by the Division of Geological & Geophysical Surveys, was produced at a cost of $10 per copy which includes the text printed in Delta Junction, Alaska and the oversize maps which were printed in Washington, D.C.Publica- tion is required by Alaska Statute 41, "to determine the potential of Alaskan land for production of metals, minerals, ...[and] advance knowledge of the geology of Alaska." CONTENTS Page Abstract ............................................................................................................................................................... Introduction ........................................................................................................................................................ Description of the study area ..................................................................................................................... Location, access, and topography ............................. .............................................................................. Previous work ............................................................................................................................................... Current study ................................................................................................................................................ Geologic setting ................................................................................................................................................ Peninsular terrane ...................................................................................................................................... Chugach terrane ....................................................................................................................................... Rocks common to both terranes ........................ .................................................................................. Descriptive geology of the Peninsular terranc ............................................................................................... Paleozoic(?) and Mesozoic metamorphic rocks ...................................................................................... Petrology and lithology ....................................................................................................................... Amphibolite and marble ................................................................................................................ Pelitic schist and quartzite .............................................................................................................. Mixed assemblages of metamorphic and plutonic rock .............................................................. Alteration and metamorphism ............................................................................................................. Age, correlation, and interpretation .................................................................................................... Jurassic system igneous rocks .................................................................................................................... Volcanic rocks of the Talkeetna Formation ....................................................................................... Plutonic rocks ......................................................................................................................................... Cumulate ultramafic rocks ............................................................................................................. Petrology and lithology ............................................................................................................. Mineralogy .................................................................................................................................. Alteration and metamorphism .............................................................................................. Deformation .............................................................................................................................. Gabbroic rocks ................................................................................................................................. Petrology and lithology ........................................................................................................... Mineralogy .................................................................................................................................. Gabbronorite clan ............................................................................................................. Pyroxene-hornblende gabbronorite .................................................................................. Alteration and metamorphism .............................................................................................. Age and correlation ................................................................................................................... Intermediate composition plutonic rocks ..................................................................................... Quartz diorite and tonalite ....................................................................................................... Granodiorite within quartz diorite-tonalilc plutons .............................................................. Small plutons of quartz monzodiorite and granodiorite ....................................................... Age and correlation ................................................................................................................ Interpretation ......................................................................................................................................... Cretaceous sedimentary rocks ................................................................................................................... Matanuska Formation ........................................................................................................................ Petrology and lithology .................................................................................................................. Age and correlation ......................................................................................................................... Interpretation ................................................................................................................................... Descriptive geology of the Chugach terrane .................................................................................................. Cretaceous system ....................................................................................................................................... McHugh Complex ................................................................................................................................. Petrology and lithology ................................................................................................................... Alteration and metamorphism ....................................................................................................... Age and correlation ......................................................................................................................... Interpretation and deformation ..................................................................................................... Valdez Group ........................ .............................................................................................................. Petrology and lithology ..................................................................................................................
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