DETRITAL ZIRCON FISSION TRACK AGES OF THE PALEOGENE KOOTZNAHOO FORMATION, SOUTHEAST ALASKA

Arkosic and lithic sandstones of the Paleogene Kootznahoo Formation in the Keku Straits region on Kupreanof Island and the Little Pybus Bay section on Admiralty Island in southeast Alaska record the exhumation of distinct sources adjacent to the Admiralty trough. Fission Track (ZFT) and U/Pb dating of detrital zircon have been used to understand sediment provenance, basin evolution, and exhumation of flanking terranes. ZFT ages from the base of the section have component populations at ~55 and 67 Ma. ZFT age from the middle of the section have component populations at 51 and 39 Ma. ZFT ages from the top of the stratigraphic section have three populations at 35, 48 and 69 Ma. We entertain the idea that sediment of the Kootznahoo Formation record mixing of material from the Coast Plutonic Complex to the east and the Chugach terrane to the west. The ZFT cooling ages from 51-55 Ma likely represent rapid exhumation of the Coast Mountains, which flanks the Admiralty trough to the east. Upwards in the stratigraphy, the pattern is more complicated because both younger and older cooling ages emerge. The young population may represent the uplift and exhumation of the Chugach terrane along the western basin margin. Garnet-bearing schist clasts in strata at the top of the section may have been eroded from the Baranof Schist of the Chugach terrane. The 35-39 Ma ZFT population may show exhumation of the Chugach terrane, which is known to have cooled dramatically in this interval. Exhumation likely caused denudation of Chugach cover strata and produced the 69 Ma population. It is unclear if the 48 Ma population is associated with continued exhumation of the Coast Mountains or erosion of the Chugach. U/Pb ages through the section are consistent with a CPC source, though these data cannot rule out exhumation of the Chugach because it most of these strata are also inferred to have been derived from Coast Mountain Complex. ZFT ages from samples taken adjacent to three dikes show a clear thermal resetting of three samples at 23 Ma, which constrains the minimum depositional age of the Kootznahoo Formation.