DEPARTMENT OF THE INTERIOR FRANKLIN K. LANE, Secretary UNITED STATES GEOLOGICAL SURVEY GEORGE OTIS SMITH, Director Bulletin 668 THE NELCHINA-SUSITNA REGION ALASKA BY THEODORE CHAPIN WASHINGTON GOVERNMENT PRINTING OFFICE 1918 CONTENTS. Preface, by Alfred H. Brooks............................................... 7 Introduction.............................................................. 9 Location and area..................................................... 9 Early explorations and previous work................................... 10 Present investigation.................................................. 11 Geography............................................................... 12 Surface features...................................................... 12 Drainage.............................................................. 14 Climate............................................................... 16 Vegetation........................................................... 17 Game................................................................. 19 Routes of travel..........................;............................ 19 Population........................................................... 20 Descriptive geology........................................................ 20 General features....................................................... 20 Carboniferous or pre-Carboniferous rocks ............................... 22 Klutina group.................................................... 22 Distribution and character.................................... 22 Structure.................................................... 22 Age and correlation........................................... 22 Greenstone, limestone, and schist.................................. 23 Distribution and character.................................... 23 General features........................................... 23 Greenstone................................................ 23 Limestone................................................ 24 Schist.................................................... 24 Intrusive rocks............................:............... 24 Age and correlation........................................... 24 Triassic (?) rocks..................................................... 26 Triassic (?) lavas and tuffs.......................................... 26 Distribution and character.................................... 26 Age and correlation.........................................'.. 27 Upper Triassic (?) rocks..................... .*. ..................... 27 Distribution and character.................................... 27 Structure and thickness....................................... 28 Age and correlation........................................... 28 Jurassic rocks......................................................... 29 Lower Jurassic rocks............................................... 29 Distribution and character. .................................... 29 Structure and thickness ....................................... 29 Age and correlation........................................... 30 Middle Jurassic rocks.............................................. 32 Tuxedni sandstone............................................ 32 Distribution and-character................................ 32 Age and correlation....................................... 32 3 4 CONTENTS. Deocriptive geology Continued. Jurassic rocks Continued. Middle Jurassic rocks Continued. Page. Chinitna formation............................................ 33 Distribution and character............... i................ 33 Structure................................................ 34 Age and correlation....................................... 34 Upper Jurassic rocks.............................................. 35 Conglomerate. .................................................. 35 Distribution and character................................ 35 Structure and thickness................................... 36 Age and correlation....................................... 36 Naknek formation....................... '. ..................... 36 Distribution and character................................ 36 Structure................................................ 37 Age and correlation.................................. i.... 37 Cretaceous rocks....................................................... 38 Tertiary rocks......................................................... 38 Conglomerate...................................................... 38 Acidic lavas...................................................... 38 Distribution and character.................................... 38 Age and correlation........................................... 39 Basaltic lavas and tuffs............................................ 40 Distribution and character.................................... 40 Age and correlation........................................... 40 Quaternary deposits................................................... 40 Intrusive rocks........................................................ 42 Quartz diorite..................................................... 42 Distribution and character.................................... 42 Age and correlation........................................... 42 Gneiss........................................................... 43 Distribution and character.................................... 43 Age and correlation........................................... 44 Granite.......................................................... 44 Distribution and character..................................... 44 Age and correlation.................. '. ...................:.... 44 Dike rocks....................................................... 45 Geologic history............................................................ 45 Paleozoic time........................................................ 45 Mesozoic time......................................................... 46 Tertiary time............ ?. ............................................ 47 Quaternary time........................................................ 47 ' Glaciation......................................................... 47 Ketreat of the ice.......................... 1........................ 47 Geomorphology............................................................ 48 Land forms............................................................ 48 Lowlands........................................................... 48 Mountains........................................................ 49 Drainage.............................................................. 50 Tertiary development.................................................. 51 Eocene peneplanation.............................................. 51 Eocene diastrophism............................................... 51 Post-Eocene erosion................................................ 52 Geomorphology Continued. Page. Glaciation....................................... .................. 54 Glacial erosion.............................. .................. 54 Sources of the ice and direction of movement. ................... 55 Epochs of glaciation................................................ 56 Thickness of the ice............................................... 57 Postglacial events...................................................... 58 Deposition......................................................... 58 Uplift and erosion................................................. 58 Mineral resources.......................................................... 59 General features....................................................... 59 Source of the gold...................................................... 60 Gold placers........................................................... 60 Crooked Creek............... f ......... Main stream...................... Albert Creek..........,............ Other tributaries of Crooked Creek. Caribou Creek.......................... Little Nelchina Kiver................. Tyone Creek.......................... Yacko Creek...................... Fourth of July Creek............... Daisy Creek....................... Oshetna River......................... Index ILLUSTRATIONS. Page. PLATE I. Topographic reconnaissance map of Nplchina-Susitna region, Alaska....................................................In pocket. II. Geologic reconnaissance map" of Nelchina-Susitna region, Alaska.In pocket. III. J., Lake-dotted moraine, Copper-Susitna basin; B, Tazlina River at mouth of Moose Creek.......................................... 12 IV. Mountains between Susitna and Nenana rivers.................... 14 V. A, Susitna River near the mouth of Maclaren River; S, Contorted gneiss on Tsusena Creek........................................ 16 VI. A, Jointed andesite, Oshetna River; B, Interbedded tuff and andes- ite, Oshetna River............................................. 28 VII. A, Tertiary lavas near the head of Tsusena Creek; B, Volcanic rocks at the head of Alfred Creek.................................... 38 VIII. Sand and silt bluffs along Tazlina River near mouth of Tolsona Creek. 40 I.. A, Dissected peneplain of Talkeetna Mountains and valley of Kosina Creek; B, Talkeetna 'peneplain and Talkeetna Mountains......... 48 X. A, Albert Creek; B, Groundsluicing on Oleen and Getchel claims on Albert Creek............................................... 60 FIGURE 1. Index map of part of southern Alaska, showing area represented on Plates I and II...............................................