U-PB DETRITAL ZIRCON GEOCHRONOLOGY AND PROVENANCE OF THE PALEOGENE-NEOGENE KOOTZNAHOO FORMATION, SOUTHEAST ALASKA: A SEDIMENTARY RECORD OF COAST MOUNTAINS EXHUMATION

Quartzo-feldspathic arenites of the Kootznahoo Formation in southeast Alaska contain detrital zircon U-Pb age distributions dominated by plutonic zircon populations at 190-160, 95-85, and 65-50 Ma. These populations suggest derivation from the adjacent Coast Mountains Batholith complex, and maximum depositional ages suggest that most of the Kootznahoo Formation in the study area was deposited between 60-25 Ma. We use the Kolgomorov-Smirnov statistical test to demonstrate a depositional affinity between the Kootznahoo Formation exposed in Little Pybus Bay on Admiralty Island and the lower portion of the Kootznahoo stratigraphy in the Keku Straits region to the south on Kupreanof Island. An abrupt influx of early Paleogene-aged zircons (65 to 50 Ma) implies that the initial un-roofing of the Coast Plutonic Complex, east of the Coast Shear Zone, is captured within the lower part of the Kootznahoo stratigraphy between 60-50 Ma. Our data also suggests the development of a depositional hiatus or erosional unconformity at about 50 Ma followed by sedimentation between 40 and 25 Ma. The development of this unconformity may be due to uplift and subsidence driven by subduction of an oceanic ridge at approximately 50 Ma. A change in lithofacies from arkosic sandstones and conglomerate with plutonic clasts to dominantly volcaniclastic sandstones and conglomerate with volcanic clasts occurs in the upper most part of the section. This change coincides with a pronounced influx of 27-25 Ma zircons most likely derived from volcanic rocks related to the Admiralty Island Volcanics.