; Stratigraphy of the Keku "Islets and Neighboring 1 Parts of Kuiu and < Kupreanof Islands -3 Southeastern Alaska , By L. J. PATRICK MUFFLER ' CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY BULLETIN 1241-C - Geologic map and preliminary stratigraphic * interpretation of a well-exposed sequence ^of sedimentary and volcanic rocks ranging ^ in age from Late Silurian to Tertiary ^UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1967 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Abstract. _________________________________________________________ Cl Introduction._____________________________________________________ 2 Physical setting_______________________________________________ 2 Previous work.._______________________________________________ 4 Purpose of present study...____________________________________ 5 Fieldwork and acknowledgments.________________________________ 5 Silurian System.__________________________________________________ 6 Bay of Pillars Formation____--_______..__.._._____.____.._..___ 6 Kuiu Limestone_.._-____.__...._.___________ _________________ 11 Red arkose___________________________________________________ 12 Volcanic graywacke and argillite.---________.______________.-__--- 13 Silurian or Devonian System___________________.____________________ 14 Volcanic breccia.___---_____________________-___-______-__-_--- 14 Devonian System..________________________________________________ 15 Gambier Bay Formation _______________________________________ 15 Limestone._ __________________________________________________ 18 Carboniferous Systems._______________.______--__-_-_______-------- 19 Crinoidal limestone._______________________ ___________________ 19 Saginaw Bay Formation.____________.___-_._-___-___._---_---_- 20 Permian System.---______________________________________-_---_--- 22 Halleck Formation________________ ___________________________ 22 Cannery Formation.___________________________________________ 23 Pybus Formation..._______--_____- ___________________________ 25 Triassic System.____.______._.._.___.___.__.____._______.---.-.--- 26 Keku Volcanics_ ______________________________________________ 26 Hyd Group____._.._...._.______._._..._...____.._....____.... 28 Burnt Island Conglomerate.________________________________ 29 Cornwallis Limestone___-_-_______-___-__-----____--------- 34 Hamilton Island Limestone.__________-_____--_____--------- 34 Hound Island Volcanics____________________--______----_--- 35 Jurassic and Cretaceous Systems-----.---.-.------------------------ 44 Seymour Canal Formation.____________________________----_---- 44 Cretaceous System.________________________________________________ 44 Lithic sandstone and mudstone--.___-_--_----------------------- 44 Cretaceous (?) System _________________-_____---__------__---------- 45 Hornblendite..-.___-__._._____--_-_--._---___----------------- 45 Granitic rocks.________________________________-______--------- 45 Tertiary System..___________________________-_____-----_---------- 47 Kootznahoo Formation..__________________________---__-------- 47 Altered dellenite flows____--__-____--_-----------_------------ 48 Gabbro and microgabbro___________________-__----------------- 49 Structure.. __________________________________--__----------_------ 50 References._____________________________________-_--__--_--------- 52 in IV CONTENTS ILLUSTRATIONS Page PLATE 1. Geologic map of Keku Islets and neighboring parts of Kuiu and Kupreanof Islands, Alaska.____________ In pocket FIGURE 1. Index map showing location of the Keku Islets area and of other recently studied nearby areas in southeastern Alaska. _____-------------------- ------------_-- C3 2-5. Photographs showing 2. Interbedded calcareous sandstone and argillite in the Bay of Pillars Formation._____________ 7 3. Thin-bedded argillite and limestone in the Bay of Pillars Formation._._______.._...__..._____ 8 4. Anticline in interbedded argillite and thin-bedded very fine grained calcareous lithic sandstone in the Bay of Pillars Formation..______________ 9 5. Anticline in thick-bedded marble in the Gambler Bay Formation ____________________________ 17 6. Geologic map of the area near Little Hamilton Island____ 31 7. Geologic map of the northwest tip of Hamilton Island-... 32 8. Diagram showing stratigraphic relations of Upper Triassic rocks of the Hyd Group and the Keku Volcanics. _____ 33 9-15. Photographs showing features of the Hound Island Volcanics: 9. Basaltic pillow flQW-----------___-_--_----___ 37 10. Basaltic pillow lava._________________________ 38 11. Basaltic isolated-pillow breccia_-_-_-_____-_____ 39 12. Basaltic broken-pillow breccia _________________ 40 13. Basaltic broken-pillow breccia _________________ 41 14. Aquagene tuff matrix of basaltic broken- pillow breccia_--___________________________ 42 15. Contact between pillow breccia and a limestone inclusion __________________________________ 43 TABLES Page TABLE 1. Age assignments of Upper Triassic fossil collections from the Keku Volcanics_.______________________________________ C28 2. Age assignments of Upper Triassic fossil collections from the Hyd Group___---__-__-_---___-_---__________________ 30 3. Estimated percentages of various rock types in the Hound Island Volcanics _______________________________________ 36 CONTRIBUTIONS TO GENERAL GEOLOGY STRATIGRAPHY OF THE KEKU ISLETS AND NEIGHBORING PARTS OF KUIU AND KUPREANOF ISLANDS, SOUTHEASTERN ALASKA By L. J. PATRICK MUFFLER Bocks ranging in age from Late Silurian to Tertiary are well displayed in shoreline exposures at the north end of Keku Strait between Kuiu and Kupreanof Islands. The section, one of the most nearly complete in southeastern Alaska, is at least 15,000 feet thick, and many units are exceedingly fossiliferous. Upper Silurian rocks crop out extensively on Kuiu Island and include two new formations: the Bay of Pillars Formation, composed of calcareous lithic sandstone, argillite, and limestone, and the Kuiu Limestone, composed primarily of massive limestone that contains sporadic conglomerate and thin-bedded limestone. An unnamed unit of unfossiliferous red arkose is inferred to overlie the Kuiu Limestone and to underlie an unnamed unit of volcanic graywacke and argillite of Late Silurian age. Volcanic breccia, either of Silurian or of Devonian age, crops out in one large fault slice. The Gambier Bay Formation, of Middle (?) Devonian age, is composed of greenstone, greenschist, phyllite, and marble. It crops out on Kupreanof Island but not on Kuiu Island or in the Keku Islets. An unnamed brown fetid limestone of Late Devonian age is exposed at two localities in the Keku Islets, where it is stratigraphically overlain by.an un­ named crinoidal limestone of probable Mississippian age. The Saginaw Bay Formation (new), of Mississippian and Pennsylvanian age, crops out on Kuiu Island and in the Keku Islets. The formation is divided into four informal members: a volcanic member, a black chert member, a chert and limestone member, and a silty limestone member. Permian units comprise the Halleck Formation (new), the Cannery Forma­ tion, and the Pybus Formation (previously described as the Pybus Dolomite on Admiralty Island, 25 miles to the north). The Halleck, which consists of siltstone, sandstone, limestone, conglomerate, and basaltic volcanic rock, is of Early Permian age (probably Leonard) and appears to be restricted to Kuiu Island and the Keku Islets. The Halleck is coeval with the Cannery Forma­ tion, a sequence of volcanic graywacke, volcanic argillite, and chert that crops out on Kupreanof Island and on Admiralty Island but is not found in the Keku Islets or on Kuiu Island. Both formations are overlain by the Pybus Forma­ tion, a prominent cliff-forming unit of white limestone, dolomite, and chert. Cl C2 CONTRIBUTIONS TO GENERAL GEOLOGY The Pybus Formation is overlain unconformably by a sequence of felsic and mafic volcanic rocks, herein named the Keku Volcanics. The unit is restricted to Kuiu Island and is of Late Triassic age, probably early and late Karnian. The Upper Triassic Hyd 'Group, previously described as the Hyd Formation on Admiralty Island, is herein elevated from formational rank and divided into four new formations the Burnt Island Conglomerate, the ornwallis Limestone, the Hamilton Island Limestone, and the Hound Island Volcanics. The Burnt Island Conglomerate is recognized only in the Keku Islets and on Kupreanof Island, where it rests unconformably on either the Cannery Formation or the Pybus Formation. Stratigraphically equivalent 'beds on Kuiu Island are in­ cluded in the uppermost part of the Keku Volcanics and in the lowermost part of the Cornwallis Limestone. The Cornwallis 'Limestone is a medium- to thick- bedded brown-weathering medium-gray oolitic limestone that crops out on Kuiu Island. It is coeval with the Hamilton Island Limestone, a very thin bedded dark-gray to black fetid aphanitic limestone exposed in the Keku Islets and on Kupreanof Island. The Hound Island Volcanics consist of 'basaltic pillow breccia and pillow lava, andesitic volcanic breccia, aquagene tuff, and limestone. Breccia, flow, and aquagene-tuff structures are superbly displayed, and the tuffs locally contain fragments of glass that has not yet 'been devitrified. Forty-seven Upper Triassic (Karnian and Norian) fossil collections from the Keku Islets area make the Hyd Group