Geology of the Iniskin -Tuxedni Region, Alaska GEOLOGICAL SURVEY PROFESSIONAL PAPER 512 Geology of the Iniskin -Tuxedni Region, Alaska By ROBERT L. DETTERMAN and JOHN K. frARTSOCK GEOLOGICAL SURVEY PROFESSIONAL PAPER 512 A study of the highly fossiliferous Jurassic rocks on the west side of Cook Inlet and an account of the petroleum exploration on Iniskin Peninsula UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1966 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director Library of Congress Catalog No. GS66-220 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Page Abstract--_-_-___-_-______________________---_---_-_ 1 Descriptive geology of the bedded rocks Con. Introduction, ______________________________________ 2 Tertiary System.____________________--_------_- 55 Location and area.'_____________________________ 2 Kenai Formation.___________--_----_--_---- 55 Early geologic investigations,____________________ 2 Tertiary lava flows_____-____-__--__------_ 56 Recent investigations__________________________ 3 Quaternary System._____________--__--__------- 57 Methods of work___________________________ 3 Iliamna lava flows.______--__--__--__-----_- 57 Acknowledgments. __________________________________ 3 Surficial deposits....______-_-___------------ 57 Geography.-_ ______________________________________ 4 General relations.___-_____-------__---- 57 Relief and drainage.____________________________ 4 Residual deposits.__________-___--__---- 59 Iniskin Peninsula.__________________________ 4 Glacial deposits.-.._________---_-------_ 59 Chinitna Bay to Tuxedni Bay______-_________ 4 Modern glaciers.______-__-__------_ 61 Accessibility. __________________________________ 5 Colluvial deposits_______________---_-- 61 Wildlife.. _________________________ 5 Alluvial deposits___________---__----__ 62 Vegetation.____________________________________ 5 Littoral deposits._________--_--_----_--- 63 Climate. ___..-__._____________________________ 6 Igneous rocks.___________-___---------------_-_-_-- 63 Descriptive geology of the bedded rocks.______________ 6 Plutonic rocks____________--_-_-_-_-----_----_-- 64 Regional setting._______________________________ 6 Quartz diorite. ----------------------------- 65 General relations.______________________________ 7 Quartz monzonite _ ----------------------- 65 Stratigraphic units._-_-_-____________._.____ 7 Dikes and sills____-_----_------------------- 66 Stratigraphic relations.______________________ 9 Structural geology._____-_____------------_--------- 66 Triassic System_______________________________ 9 Regional setting.____-___-_------_----------- 66 Metamorphic rocks undifferentiated.._________ 9 Folds____.__________________________ _ 67 Jurassic System._______________________________ 12 Fritz Creek anticline-________-___--_--_----_ 67 Lower Jurassic rocks._______________________ 12 Tonnie syncline _-_-_--_---___-_---_---_- 68 Talkeetna Formation__________________ 12 Faults.._ ____________-_--_--_----------------_- 68 Marsh Creek Breccia Member________ 13 Bruin Bay fault-________-_____--_----_----- 68 Portage Creek Agglomerate Member. _ 14 Cross faults. ------------------------------- 68 Horn Mountain Tuff Member. _______ 18 Hinge faults.. .____--_--__-_-_---_------- 68 Middle Jurassic Rocks_____________________ 20 Normal faults.________-__-_-__-_--_-------- 69 Tuxedni Group.________________________ 20 Joints.____________-_------------_------------- 69 Red Glacier Formation._____________ 21 Age of folding and faulting ___________________ 69 Gaikema Sandstone.________________ 25 Geologic history. ----------------------------------- 69 Fitz Creek Siltstone_______________ 29 Mineral resources. ---------------------------------- 71 Cynthia Falls Sandstone.____________ 31 Petroleum. ------------------------------------ 72 Twist Creek Siltstone.______________ 34 Oil and gas seeps._______--___------_------- 72 Middle(?) and Upper Jurassic Rocks_________ 35 Early exploration and development-__________ 72 Tuxedni Group.________________________ 35 Recent exploration and development__________ 72 Bowser Formation._________________ 35 Iniskin Bay Association well 1 _________ 72 Upper Jurassic Rocks.______________________ 40 Beal well 1_________________________ 73 Chinitna Formation.____________________ 40 Antonio Zappa well l_______---_-------- 74 Tonnie Siltstone Member.___________ 40 Petroleum possibilities ____________________ 74 Paveloff Siltstone Member.__________ 42 Miscellaneous mineralization- -------------------- 75 Naknek Formation____________________ 47 Chisik Conglomerate Member________ 48 Magnetite. -------------------------------- 75 Lower Sandstone Member.__________ 49 Copper. ----------------------------------- 75 Snug Harbor Siltstone Member_____ 51 References cited-_______---___--------_---_---_----_ 75 Pomeroy Arkose Member.___________ 53 Index.________-___-_---------_--------_--_-------- 77 m IV CONTENTS ILLUSTRATIONS [Plates are in pocket] Page PLATE 1. Geologic map and structure sections of the Iniskin- FIGURE 1. Index map of Iniskin-Tuxedni region__ _ 2 Tuxedni region. 2. Depositional relationships of Jurassic Strat­ 2. Tectonic map of part of Alaska Peninsula and igraphic units________________________ 23 Cook Inlet region. 3. Vertical stereoscopic view of Iniskin Penin­ 3. Map showing location of fossil collections, sam­ sula__ ____________________________ 26 ples examined in thin section, mineral occur­ rences, oil well sites, and oil and gas seeps. 4. Oblique view of Iliamna Volcano.________ 58 4. Stratigraphic sections of Upper(?) Triassic and 5. Vertical view of Tuxedni Glacier showing Lower Jurassic. deposits.____________________________ 60 5. Stratigraphic sections of Middle and Upper Juras­ 6. Aerial view of landslide dam at Hickerson sic and Tertiary. 6. Maps and sections showing interpretation of Lake______________________________ 62 structure and stratigraphy of Fitz Creek anti­ 7. Vertical view of master joint sets southwest cline. end of Fitz Creek anticline.____________ 70 TABLES Page Page TABLE 1. Stratigraphic units in the Iniskin-Tuxedni TABLES 10-12. Geographic and Stratigraphic distribution region, Alaska______________________ of fossils in the 2. Comparison of past and present nomencla­ 10. Tonnie Siltstone Member of the ture of the bedded Jurassic rocks in the Chinitna Formation._____________ 44 Iniskin-Tuxedni region, Alaska. ________ 11. Paveloff Siltstone Member of the 3. Correlation chart for the Jurassic of the Chinitna Formation._____________ 48 Iniskin-Tuxedni region, Alaska.________ 16 12. Naknek Formation.________________ 51 4-9. Geographic and Stratigraphic distribution 13. Succession of igneous rocks exposed in the of fossils in the Iniskin-Tuxedni region.___-________-- 64 4. Red Glacier Formation^_________ 24 14. Quantitative mineralogical content of 5. Gaikema Sandstone..--____.____ 28 6. Fitz Creek Siltstone. ____________ 32 selected rocks from Aleutian Range 7. Cynthia Falls Sandstone.________ 34 batholith.. __________________---__-- 65 8. Twist Creek Siltstone___________ 36 15. Analyses of crude oil and natural gas from 9. Bowser Formation._____________ 38 Iniskin Bay Association well 1 ______ 73 GEOLOGY OF THE INISKIN-TUXEDNI REGION, ALASKA By ROBERT L. DETTEEMAN and JOHN K. HAETSOCK ABSTRACT region. Breaks in the faunal record are marked by corresponding The Iniskin-Tuxedni region, on the west side of Cook Inlet, local unconformities in the stratigraphic sequence. Alaska, is an area of about 710 square miles between Iniskin The Chinitna Formation of Late Jurassic (Callovian) age has Bay and Tuxedni Bay. Systematic geologic work was begun a maximum thickness of 2,680 feet in this region. The forma­ along the coast in 1903, 1904, and 1909 by the U.S. Geological tion is predominantly a massive arenaceous siltstone, but it Survey. Additional work was done in 1920 and 1921. More can be divided into two members separated by an interval of intensive exploration was started in 1944 and continued in sandstone. The Tonnie Siltstone Member, at the base, is 1946, 1948-51, and 1957-58 as part of the Geological Survey mainly a red-brown-weathering siltstone containing small lime­ program of investigations of the natural resources of Alaska. stone concretions; it is overlain by the Paveloff Siltstone Member, Iliamna Volcano (10,016 ft) dominates this mountainous which weathers gray and has large ellipsoidal limestone con­ region; the average relief is about 2,500-3,500 feet. Ten glaciers cretions. Ammonites and pelecypods found in the formation radiate from Iliamna Volcano and cover about 15 percent of indicate a correlation with middle Callovian faunal zones of the area. Tuxedni Glacier, the largest, is 16 miles long and northwest Europe. extends to the tidal flats at Tuxedni Bay. The region has a The Naknek Formation of Late Jurassic (Oxfordian and cool maritime climate with abundant rainfall in summer and Kimmeridgian) age is as much as 5,200 feet thick and is divided snow in winter. Dense brush and grass cover the lowlands and into four members: Chisik Conglomerate, lower sandstone, mountains to about 2,000 feet; spruce and cottonwood grow to Snug Harbor Siltstone, and Pomeroy Arkose Members. A about 800 feet. local unconformity is present at the base of the formation. The oldest rocks exposed are metalimestone, metachert, The Kenai Formation of Oligocene(?) and Miocene age overlies quartzite, argillite, and metabasalt of Late(?)