Project Synopsis
Collaborators:
Dr. Ken Ridgway, Purdue University – lower Cantwell sedimentology, regional geology
Dr. Sun-Lin Chung, National Taiwan University – isotope geochemistry
Dr. Paul Layer, University of Alaska, Fairbanks – Ar/Ar geochronology
A rich volcanic and tectonic history is preserved in the Early Eocene (60-55 Ma) volcanic rocks of the Cantwell Formation in Denali National Park. The Cantwell volcanics are about 3000 meters thick and overlie about 4,000 meters of Late Cretaceous sedimentary rocks of the lower Cantwell Formation. New 40Ar/39Ar ages for the Cantwell volcanics range from 59.8+/0.2 to 55.5+/-0.2 Ma and reveal a 10-20 million year hiatus across an angular unconformity between the lower and upper Cantwell Formation; this unconformity separates pre-volcanic and post-volcanic episodes of deformation. Pre-volcanic deformation of the Lower Cantwell Formation involved north-south shortening that can be attributed to the final collision and suturing of the Wrangellia composite terrane to southern Alaska. Post-volcanic deformation of the entire Cantwell Formation is consistent with right-lateral simple shear along the Denali fault.
Cantwell volcanism began with basaltic lava flows, scoria cones, and minor felsic pyroclastic eruptions. The presence of thin coal seams, abundant plant and tree fossils, and hydromagmatic pyroclastic surge deposits within the lower part of the Cantwell volcanics reveal that the Cantwell basin was a wet and swampy environment at the onset of volcanic activity. High energy rivers and catastrophic debris avalanches cascaded into the basin along the steep leading edge of the new volcanic center. As the volcanic sources grew, thick intervals of basalt-andesite lavas, alternating with rhyolite lavas and pyroclastic deposits, prograded into the Cantwell basin. On the basis of sedimentary and volcanic facies mapping, stratigraphy, and paleoflow data the Cantwell eruption center is interpreted to have been a polygenetic stratovolcano that was located in the vicinity of the Denali fault along the southwest side of Denali National Park.
New geochemical data, including Nd-Sr-Pb isotopes, show that the Cantwell volcanics were derived from an enriched mantle source and that they are similar in composition to the McKinley sequence plutons. The combination of geochemical, radiometric age, stratigraphic, structural data of this study support a new hypothesis that the McKinley sequence plutons represent the uplifted plutonic remnant of the Cantwell eruptive center which had shed volcanic rocks northward and were then offset from the Cantwell volcanics by right-lateral slip along the Denali fault. While this correlation is not a precise piercing point it does provide constraint that, at most, there have been a few tens of kilometers, not hundreds of kilometers, of post-Early Eocene offset along the McKinley strand of the Denali Fault system.