Kootznahoo Formation

Unit symbol: Tk
Age range Tertiary, lower Miocene to upper Eocene (66 to 15.97 Ma)
Lithology: Sedimentary
Group name: Kootznahoo Formation
Light-greenish-gray, nonmarine, lithofeldspathic sandstone, siltstone, mudstone, and conglomerate that contains coal fragments and lenses in the Zarembo and Kuiu Islands region (Brew and others, 1984) and on Admiralty Island (Lathram and others, 1965) of southeast Alaska. Polymictic conglomerate contains sandstone, argillite, coal, phyllite, limestone, chert, schist, gneiss, granitic rocks, volcanic rocks, and quartz. Includes lignite and subbituminous coal beds up to 1.5 m thick. Outcrop area is wedge-shaped at Kootznahoo Inlet near Angoon, apparently filling an asymmetric graben, and unit is up to 2,000 m thick at south margin of graben. Plant collections from Port Camden on Kuiu Island, on Kupreanof Island, and at Little Pybus Bay on Admiralty Island were assigned an Eocene age by Knowlton in Buddington and Chapin (1929), but were reevaluated as Paleocene by Jack Wolfe in Lathram and others (1965). Plant fossils in Kootznahoo Inlet on Admiralty Island include “Ulmus” pseudobrauni, Octea sp., Dilleniaceae, and Dryophyllum sp., of late Eocene to earliest Oligocene age (Lathram and others, 1965), Carya magnifica, Juglans orientalis, Alnus alaskana, Cercidyphyllum aff. C. crenatum, Vitis atwoodi, Populus sp., and Ficus alaskana of Oligocene age, and Fagus antipofi, Quercus furujelmi, Comptonia naumani, and Populus lindgreni of early Miocene age (Lathram and others, 1965). Age reported to be Paleocene and Eocene in the Zarembo-Kuiu Islands region and Eocene to Miocene on Admiralty Island (Brew and others, 1984)

Source map information

Source map Brew, D.A., Ovenshine, A.T., Karl, S.M., and Hunt, S.J., 1984, Preliminary reconnaissance geologic map of the Petersburg and parts of the Port Alexander and Sumdum 1:250,000 quadrangles, southeastern Alaska: U.S. Geological Survey Open-File Report 84-405, 43 p., 2 sheets, scale 1:250,000.
Symbol Tk
Unit name Kootznahoo Formation (?)
Description Nonmarine arkosic sandstone, sandstone, shale, and conglomerate; medium- to very thick-bedded; locally cross-bedded; dominant rock type is medium- to very coarse-grained lithic feldspathic quartz arenite.
Lithology Sedimentary

Correlated geologic units

Label Tkz
Description Kootznahoo Formation, light greenish gray, medium- to coarse-grained, friable, lithofeldspathic fluvial sandstone and siltstone containing leaf fragments and rare, cm-scale, coal lenses
Geologic age Paleocene to Early-Miocene
Geologic setting Sedimentary, continental
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major
Arkose < Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Carbonaceous Indeterminate, major
Shale < Mudstone < Clastic < Sedimentary Bed Incidental