DYNAMICS OF TERTIARY VOLCANISM IN DENALI NATIONAL PARK, ALASKA

Two major Tertiary age volcanic episodes are recorded in the rocks of Denali National Park (DNP). The first includes late Paleocene-early Eocene volcanic rocks of the Cantwell Formation and the second includes the late Eocene-early Oligocene Mt. Galen volcanic rocks. The Cantwell volcanic rocks have a preserved thickness of 2,750 m and include alternating intervals (100’s of m) of basalt-andesite and rhyolite lavas with thinner intervals (10’s of m) of fluvial volcaniclastic and pyroclastic deposits. The pyroclastic rocks include pumice and lithic tuff-breccia, welded tuff, and cross-stratified vitric tuff formed as pyroclastic flows and surges. These rocks form the colorful aretes in the Polychrome Pass area and cap Double, Igloo, and Cathedral Mountains along the park road. Beneath the volcanic rocks are up to 120 m of volcanic conglomerate that records the initial influence of Cantwell volcanism on drainage systems within the Cantwell basin. Within this conglomerate is a 90 m thick volcanic mega-breccia (clasts up to 10 meters) that formed as a debris avalanche deposit during catastrophic slope failure of a volcanic landform. Lateral facies changes from proximal to distal pyroclastic deposits, together with northeast-directed paleoflow in the volcanic conglomerate, indicates that Cantwell eruptive centers were located in the southwestern part of DNP. The Cantwell volcanics are the same age and are geochemically similar to the McKinley sequence granitic plutons, located to the southwest, which may represent the uplifted remnant of Cantwell eruptive centers. The Cantwell volcanics were erupted during the final phases of terrane accretion and at the end of a 72-56 Ma episode of arc magmatism in south central Alaska. The Mount Galen volcanics consist of over 250 m of basalt, andesite, dacite, and rhyolite lavas and dacite and rhyolite tuff and tuff-breccia deposited by pyroclastic flows. The Mount Galen volcanics display a geochemical pattern that is typical of arc rocks and were erupted along the northern end of the 45-30 Ma Alaska-Aleutian subduction-related magmatic belt. Collectively, the Cantwell and Mt. Galen volcanics provide a record of the dynamic eruption processes of two separate magmatic events that shaped the paleogeography and present landscape of DNP.