Detrital zircon fission track ages of the Paleogene Kootznahoo formation, Kupreanof and Admiralty Islands, southeast Alaska

• Main Author: Ancuta, Leonard
• Format: Text
• Language: English
• Published: Union | Digital Works 2010
• Subjects: terrane; cpw; exhumation; ma; section; Earth Sciences; Geology; Tectonics and Structure; Admiralty Island; Western Basin; Yakutat; Alaska
• Online Access: https://digitalworks.union.edu/theses/1095; https://digitalworks.union.edu/cgi/viewcontent.cgi?article=2235&context=theses

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 39 and 51 Ma. Strata from the top of the stratigraphic section have three populations at 35, 48 and 69 Ma. It is possible that sediment of the Kootznahoo Formation records mixing of material from the Coast Mountains Batholith Complex (CMBC) to the east and the Chugach-Prince William (CPW) 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 CPW terrane along the western basin margin. Abundant garnet-bearing schist clasts in strata at the top of the section may have been eroded from the Baranof Schist of the CPW terrane. The 35-39 Ma ZFT population may show exhumation of the CPW terrane, which is known to have cooled dramatically in this interval. Exhumation likely caused denudation of CPW cover strata or unreset stata and produced the 69 Ma population. It is unclear if the 48 Ma population at the top of the section is associated with continued exhumation of the Coast Mountains or erosion of the CPW terrane. Component probability density plots of the three stratagraphically highest samples appear to be coeval with probability density plots from the Bainbridge area on the Prince William Sound terrane, which strengthens the suggestion that the CPW terrane was a source of detritus. U/Pb ages through the section are consistent with a CMBC source, though these data cannot rule out exhumation of the CPW terrane because most of these strata are also inferred to have been derived from the Coast Mountain Batholith 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 and represent the timing of intrusions that fed the overlying Admiralty Island Volcanics. A model with thermally reset samples and the upper Kootznahoo Formation detrital samples provides distinct similarities with cover strata units of the Yakutat terrane. This finding suggests there may be a distinct link between the CPW and the Yakutat terrane cover strata.