GEOLOGICAL SURVEY BULLETIN 907 FRONTISPIECE WINTEK VIEW OF MOUNT McKINLEY FROM THE NORTHWEST. UNITED STATES DEPARTMENT OF THE INTERIOR Harold L. Ickes, Secretary GEOLOGICAL SURVEY W. C. Mendenhall, Director Bulletin 907 GEOLOGY OF THE ALASKA RAILROAD REGION BY STEPHEN R. CAPPS UNITED STATES GOVERNMENT PRINTING OFFICE WASHINGTON : 1940 For sale by the Superintendent of Documents. Washington, D. C. ..... Price $1.25 (paper) ft 3 -vu> CONTENTS Page Abstract. _._._.-._.._.___._______-_---.--_--__------_--..-.-.-__._ 1 Introduction.._.._..._..__-__-__-------l---_._-...--.-.---_-__.-_. 1 History., of, exploration. _..__._.._._-----..--_.___-____.-_-____.-__.. 4 Present report__....--..-__._-.--------.----...----_----_----.-._ 21 Geography.. .-...._-.___-_--.-------_------_-__--.-_-----___.._ 23- Geographic provinces._______----_--_----_-_._---.---_.--...._. 23' Chugach-Kenai Mountains._----_---_.__--_--_---.---...-.. 24 Talkeetna Mountains._-_-------_---__--._.____._-..._-._.. 25 Cook Inlet-Susitna lowland.-----_----__--_---.-_---_-,-_-__ 26 Copper-Susitna lowland._------------..___-..._.--_.-.-.-._ 27 Alaska Range.______.___------__----_-_-__--_-_-__________ 27 Tanana-Kuskokwim lowland.._-..__.__._.__._._____.__.____ 29 Yukon-Tanana'upland. __--_------------__--------------.-- 30 Climate. _-_-__-------_---_-_-----------_-----------._-_-_-_--_ 31 Vegetation... _._._.__._-:___-.._.---.___.--._----__-_-__--._-__ 33 Animal life.___-_---_-_--_----_-----------_-------__.-__.---._ 35 Population..................----.-.----I.-.-..--.-.-.......... 38 Routes of travel._-_-_-.-_-_.--.------------__-------._--_..-._ 40 The Alaska Railroad.__.____.___-_-______.______-__.___.._...__ 44 Agriculture and stock raising.,.-.-._.__._____..._________.__._._ 46 Mount McKinley National Park._........_.._..._............_. 48 Geology _...._-.__..__-_-___-_...---.-----.-_.----.-._-.-......... 51 Principal features'.' .--_----_--_--------------------_---_.-____ 51 Kenai-Chugach Mountains____---------_-------_---__--_-_..... 53 General succession_________.___.-_________.___-___...__._. 53 Undifferentiated Mesozoic slates and graywackes____..__.._.._ 56 Character.___________________________________________ 57 Structure and thickness.__.-._-.-.-___-_-.-...._......_ 58 Age and correlation.___.__________._____.._..__._..___. 59 M.esp,?j,oicigr.eernstones_ ___-_.-----_----__-----_-__-..--_-_.. 59 Greenstone tuff...-_-__--_------------_------__-'-__----__._ 60 Granitic intrusives-__---------_-------_-_-----_------.-.--_ 61 Tertiary rocks._.._..._._.-----_-.-._-._-.--_--...-._.... 61 Quaternary deposits.____--------_-_--_-_-----__-.....,_.._ 62 Structure of the Kenai-Chugach Mountains __--.--...-..._.. 63 Geologic history of the Kenai-Chugach Mountains.---_.-..-.-_ 64 Talkeetna region-_-----_---..------___---_-------_--.-_..._.__ 65 Mica schist.._'____-._._.___.---_________.__-__._.___.._... 66 Sedimentary rocks of questionable Paleozoic age....-___-...... 67 Mesozoi'C, rocks. _!__..___ ---_---------.--_----_--.-_--.-.__ 69 Undiff erentiated Mesozoic sedimentary rocks._.._._..... 69 Jurassic rocks..-----------------_---__------_-________ 72 Lower Jurassic volcanic rocks-____._.-__.-_...__.._. 72 Middle Jurassic rocks........._._._...._..__...__.._ 74 Upper Jurassic rocks.-_-_--__-_--.--.---.__.__-_-_. 75 'S-Z6SO^ ra lv CONTENTS Geology Continued. Talkeetna region Continued. Mesozoic rocks Continued. Cretaceous rocks_-___i__.-___-_._....__._..__...._._._ 76 Lower Cretaceous rocks.__----_____-______.____.._. 76 Upper Cretaceous rocks.____-.---________________.. 77 Mesozoic intrusive rocks____--__-_____._-______-___-__ 78 Tertiary rocks_____________________________________________ 80 Undifferentiated Tertiary rocks__---.__---_______-___.- 81 Chickaloon formation__________-_______________________ 84 Tertiary igneous rocks_______________________________ 85 Lavas and tuffs.____-__.-----_--_-_--_----__--.___ 85 Intrusive rocks________--------__-_-_-_--__-.______ 86 Quaternary deposits_______-__-____-_-----__--____---._____ 87 Geologic history of the Talkeetna region._____________________ 87 Alaska Range._______________ _________________________________ 93 Pre-Cambrian rocks_________.____._________ ______________ 95 Birch Creek schist.____________________________________ 95 Undifferentiated Paleozoic rocks____________________________. 97 Middle Devonian limestone.________________________________ 103 Totatlanika schist.__________________^__. i ____________._.__ 104 Carboniferous (Permian?) rocks___--_--_-__---_---_-___._.__ 107 Meso/oic rocks-___-_-_______________.___.__________________ 108 Undifferentiated Mesozoic rocks.-.._____________________ 108 Triassic rocks...._____________________________________ 110 Jurassic or Cretaceous rocks--__------_---------_-__.--_ 111 Cretaceous rocks.__-----__--_----_--_--__---_-____.___ 112 Cantwell formation._______________________________ 112 Tertiary rocks---_-__-_-----_-_-_--_--_-----_-.--__________ 118 Tertiary coal-bearing formation___._...____-_______-_____ 118 . Nenana gravel._________-__-_-__-_----_-__--_-__-__-__ 123 Igneous rocks. ___-____-____-__--_-__-__-_--____..-----.--__ 128 Quaternary deposits._______________________________________ 130 Summary of geologic history of the Alaska Range______--_--_- 130 Yukon-Tanana region._____---____-_--_-_--______-__-_---_--_-- 133 Pre-Cambrian rocks_-_-_----_-----_----_----------_------- 134 Birch Creek schist._________-____----______-.____-_-___ 134 Undifferentiated Paleozoic rocks earlier than Middle Ordovician. 136 Ordovician volcanic rocks______--_--_-_---------_--_------- 137 Silurian rocks.____________________________________________ 138 Tolovana limestone._.____.______________-_-_____-__..- 138 Undifferentiated Paleozoic rocks later than middle Silurian..... 139 Mississippian rocks.___________-______-_____--_-_--------_- 141 Rampart group._____________________ _-____--__------ 141 Cretaceous rocks._____-_--_---_---_------_---------------- 142 Tertiary rocks---.-.---.--.-----.-------------------------- 143 Igneous rocks.-_-_----------------.----------------------- 144 Upper Devonian basic greenstone intrusives._____-__-_-__- 145 Mesozoic granitic rocks..------------------------------ 145 Tertiary granitic rocks.______________-----_-_---------- 146 Geologic history of the Yukon-Tanana region.______------ -_ 147 CONTENTS V Geology Continued. Pleistocene and Recent geology of the Alaska Railroad region ________ 150 Preglacial conditions.-________-___-___-_-____-__.-----____- 150 Glacial epoch___--__-___------------------_--_---_-------- 151 Extent of glaciation__._______..______-_--------_--_-_- 152 Evidences of more than one glacial advance.______________ 155 Effects of glaciation upon land forms___________________ 155 Retreat of the ice.._-_______-__--_-___'__--____'-___-_------_ 160 Present glaciers.___-_-__-.----____---_--_--_---_-------___- 161 Recent deposits.__ __ _______- _____ _ - _ 162 Terrace gravel. --------------------------------------- 162 The "muck" problem....______________________________ 164 Present stream gravel-______-____-____._____-_.._-___-__ 165 Soilflows___.-_. _ _ _ _ _ _ 167 - Ecdriomic geology.____---------_------_-----_-_-------_-__.------- 170 General features.---___--_-_-_--__---_-_---_---___-_-_-_______- 170 Kenai Peninsula...._-_----_.----_------_--_-_-----.--..__----- 172 Gold lodes-....------------------------------------------- 172 Gold placers._______--_.._-_---_-_-_-_ - _ -_- -- 173 Copper lodes----------.------------.---------------------- 174 Talkeetna region._____________________________________________ 175 Gold lodes.....__._....__....._...__.._.._._.__.._...._... 175 Gold placers____ _ _ _ ____- _- 176 Copper lodes---------------------------------------------- 178 Antimony lodes.__-__-__-_-----_---._-----___-_-_-___--__- 179 Silver lodes. --.-------------------_-----------_-__-_--- 179 Alaska Range._____--_______-_____-_____--_________._-_-_____- 180 Yentna district__________________________________________ 180 Gold lodes ......_._......-_...:...._....-..._._.._.. 180 Gold placers-...... ....-____ ._..-.. _........_.... 180 Chulitna region._________________________________________ 182 Lode deposits___-_------_--_--__-_-_-._-__.-_-.__-___- 182 Valdez Creek district._____________________________________ 184 Gold placers__-_-----------------------__-_-_--__-____ 184 Gold lodes.___._... .. ...... _......._..__. 184. Bonnifield region-__________________________________________ 184 Gold placers.__--__-_---_-_-_____-_________._______.._ 184 Goldlodes.... _-_ . .. _.'..._ ......I. 185 Kantishna region.___-____-__--_-_______-_.___-____l_______- 186 Gold placers____________________________________________ 186 Lode mines --..-___-_--_-_----_-_ -_-_-----___-_-. 187 Fairbanks district__________--__----__-___--.-___-____________ 188 Gold placers---------------------------------------------- 189 Gold lodes--..-- _________________________________________ 190 Antimony lodes.._-_______-___'__.__.___.__________________ 191 Tungsten lodes.__-----------------------------------_--___ 191 Rampart district._------_------------_-_-----___--_-_-_--___-_ 191 Hot Springs district __---_---_------__---_---_--_-_-__ __ 192 Tolovana district-__----_--__-_--------__--_--__--__-._________ 192 Coal resources of the railroad region _______________-__-______ 193 Index... __ _ _ -. _ _ ____.__ ___ _____ 197 ILLUSTRATIONS Paga FRONTISPIECE. Winter view of Mount McKinley from the northwest. PLATE 1. Geologic map of the Alaska Railroad region, Seward to Mata- nuska coal