DEPARTMENT OF THE INTERIOR FRANKLIN K, LANE, Secretary UNITED STATES GEOLOGICAL SURVEY GEORGE OTIS SMITH, Director Bulletin 631 THE YUKON-KOYUKUK REGION ALASKA, BY HENRY M. EAKIN WASHINGTON GOVERNMENT PRINTING OFFICE 1916 ADDITIONAL COPIES OF THIS PUBLICATION MAY BE PROCURED FROM THE SUPERINTENDENT OF DOCUMENTS GOVERNMENT PRINTING OFFICE WASHINGTON, D. C. AT 20 CENTS PER COPY CONTENTS. Page. Preface, by Alfred H. Brooks_________________________ 7 Introduction ___________________________'.______________ 11 Purpose of report______________________________ 11 Acknowledgments________________'._______________________ 11 Field work__________^_______________________ 12 Previous investigations ____________-_____________ 13 Geography ________________________________'_____ 14 Location and extent ______:__________________ 14 Relief____________________________________ 14 General character ___________________________ 14 ..Mountainous areas ___________ ___ _________ 14 Nonmountainous uplands_______________________ 15 Lowland plains_____________________________ 16 Drainage______________________.____________ 16 Lowland divides_______________'._____________________ 16 Canyons ______________ __ ._-_ ___ ___ 17 Back hand drainage __________________________ 17 Climate______________________________..____ 17 Vegetation __.___^>______ ___ __ ________ 19 Animal life ____________________________________ 21 Population _________________________________.._ . 22 Communication________ __ ___ _______________ 22 Industries______________ _ _______________ 23 Descriptive geology __ __ . ___ 23 General features _________ ______ ___ ________ 23 Metamorphic complex__ ...__. 25 Distribution_____________________ ________ 25 Lithology ________,____________. ____________ 26 Local features________ __ _ _____ 27 Ray-Dall divide ________~___. ____________ 27 Ray-Kanuti divide____.._______. ____________ 27 Yukon section above Melozitna River______________ 27 Gold Hill district_________________________ . 28 Ramparts section___ _ 28 Stratigraphy_________ .. 29 Structure___________ 29 Age and correlation____ _ 30 Greenstones ___________ 31 Distribution________________^______ ._________ 31 Lithology__________ 32 Local features_______ _ 33 Fort Hamlin area____ 33 Kanuti River area_..__ 34 Upper Melozitna River area _ 34 3 4 CONTENTS. Descriptive geology Continued. Greenstones Continued. Local features Continued. page. Lower Melozitna River area___________________ 35 Gold Hill area___...______________________.__ 35 Tanana area ____________________________ 35 Structure__________.._____________________ 35 Age and correlation__:________________________ 36 Older granites _______________________________ 37 Distribution_______"_______________________:_ 37 Lithology±____________________________.______ 38 Structure _____________..__________________ 39 Age and correlation___________________________ 40 Mesozoic sedimentary rocks_______________________ 41 Distribution_______________________________ 41 Stratigraphy _______________________________ 41 Lithology _________________________________ 42 Structure_________________________________ 43 Metamorphism_____________________________ 44 Types_________________________________ 44 Igneous metamorphism__^__________________'_ 44 Age and correlation___________________________ 47 Younger granites ______________________________ 50 Distribution_______________________________ 50 Lithology _________;_________________________ 50 Age__________'_:______________.._________ 50 Tertiary sedimentary rocks_________________________ 51 Distribution____:______________ T__________________ 51 General features__________-_________________ 51 Structure__________________..______________ 52 Age and correlation_____________________________ 53 Quaternary deposits _________________________-___ 53 General features ____________________________ 53 Glacial deposits _______________________ __ 54 Lacustrine silts and beach gravels__________._.;_i_____ 55 General features__________________________ 55 Local features______________i____________ 56 Ray River basin_________________________ 56 Tukon section________________________ 56 Kanuti basin__________________________ 57 Mentanontli-Indian River basin_..____________ 58 Melozitna basin________________________ 58 Tozitna basin _________________ _______ 59 Koyukuk Valley ___________..____________ 59 Alluvial plairi deposits______________________..___ 59 Modern stream gravels______________ ___________ 60 Summary of structural geology_________________________ 61 Geologic history_______________________________________ 63 Introduction ________________ ________ ______ 63 Devonian and earlier time__ _ -__ 63 Later Paleozoic time___ _ 64 Lower Cretaceous time - 64 CONTENTS. 5 Geologic history Continued. , Page. Upper Cretaceous and early Eocene (?) time______________ 65 Tertiary time________:________________________ 65 Quaternary period___________ __________________ 67 Geomorphology__________________________________ 67 Pertinent data___________________________________ 67 Peneplain stage________________________________ 68 Later Tertiary uplift and erosion___ _________________ 69 Quaternary period_____________________________ 69 Physiography_________________________________ 69 Glaciation_________________________________ 71 Inundation_________________________1_________ 73 Later Quaternary erosion_______________________ 74 Other morphologic processes_____________________ 75 Weathering _____________________________ 75 Solifluction_____________________________ 76 Processes _________________-______ 76 Results ______________,_____________ 77 Major and minor features________________ 77 Solifluction slopes__ _____ 77 Altiplanation terraces__________________ ' 78 Mineral resources_______ _____ ________ _ 82 Prospects___________________ _______ _ 82 Indian River gold placers ___________________________ 83 General features____________________ ______ 83 History of development_______ _________ ______ 84 Summary and conclusions___________________________ 84 Index__________________________________ _____ 87 ILLUSTBATIONS. Page. PLATE I. Topographic map, Yukon-Koyukuk region and adjacent terri­ tory____ _________________________ In pocket II. Geologic map, Yukon-Koyukuk region____________ In pocket III. A, Burned and partly fallen timber, Bay River basin; B, Gran­ ite pinnacles near camp 11; C, Close folding in greenstone, Ray-Dall divide__________________________ 12 IV. A, Melozitna Valley, 30 miles from the Yukon; B, Melozitna Canyon, 5 miles from the Yukon; C, Glaciated valley, southwestern headwater of Ray River___________ 16 V. A and B, Augen gneiss illustrating mechanical alteration of granite; C, Granite________________________ 38 VI. A, Outwash plain near head of Tozitna River; B, Erratic bowlder, Ray Mountains______________________ 54 VII. A, Solifluction slope at camp 17; B, Solifluction slope in pro­ file 1 mile west of camp 12; C, Solifluction slopes of mature development, Koyuk River region_______________ 76 VIII. A, Solifluction slopes encroaching upon a granite boss near camp 5; B, Altiplanation terrace developed in quartzite schist near camp 10; C, Altiplanation terraces near Indian River placer camp___._____________________ 77 I.. A, Altiplanation terraces, mountains east of Pocahontas Creek, Indian River district; B, Altiplanation terraces, Eagle dis­ trict ; C, Altiplanation terrace, Hogatza Mountains_____ 78 X. A, Altiplanation feature, 10 miles north of Tanana; B, Frost- heaved mound, Lake Clark-Kuskokwim region; C, Frost- heaved mound, showing fresh rock fragments, 10 miles north of Tanana_________________________ 79 FIGURE 1. Map showing distribution of timber in Yukon-Koyukuk region. 20 2. Diagrammatic vertical cross section of frost-heaved mound illustrating segregation of coarse and fine materials and movements of detritus under frost action 80 6 PREFACE. By ALFRED H. BROOKS. The information presented in this volume is based on exploration of the Yukon-Koyukuk region, which previous to Mr. Eakin's jour­ ney of 1913 was but little known. This journey was undertaken as part of the general plan to carry reconnaissance surveys over the unmapped areas of Alaska as fast as conditions permit. The funds available for this work did not allow the organization of a large party or the detail of an experienced topographic engineer. Therefore it fell to Mr. Eakin not only to survey the geology but also to construct his own base map. He deserves great credit for having, in a single season, mapped so large an area, and his work has yielded many new data bearing on the larger geologic problems of the Yukon basin. The economic results of the investigation are largely negative, for but few mineral deposits occur in the region. The only mining has consisted in the small-scale placer operations in the Indian River district. On the other hand, certain intrusive granites which are shown by Mr. Eakin to be the mineralizing, agents are rather widely distributed, and therefore the region, in spite of the meager results thus far achieved by the prospectors, should be classed as one in which there is hope of finding auriferous deposits. Mr. Eakin, in this volume, presents a valuable analysis of the problems relating to the origin of the drainage system of the middle Yukon region. In this analysis he clearly shows that there have been very extensive changes of drainage during Quaternary time. These changes, Mr. Eakin believes, are due to the advance of glaciers from the mountains which bound the central plateau region. While Mr. Eakin's conclusion is worthy