Paper No. 27
Presentation Time: 3:30 PM


MUSTAIN, Monica, Illinois State University, Normal, IL 61761, MALONE, David, Geography-Geology, Illinois State Univ, Campus Box 4400, Normal, IL 61761-4400 and CRADDOCK, John P., Geology Department, Macalester College, 1600 Grand Avenue, St. Paul, MN 55105,

The Eocene volcanic rocks at Hominy Peak represent a southwestern outlier of the Absaroka Volcanic Supergroup. As part of this study, we used U-Pb dating methods to determine the zircon age spectrum on an ash fall tuff (#32), a polymict agglomerate matrix (#85) and a quartzite cobble conglomerate matrix (#77) from the Hominy Peak Formation. The quartzite cobble conglomerate has a zircon spectrum that is 41% Archean, 30% Proterozoic, 20% Eocene, and 8% Mesozoic. Age of the 10th youngest grain is 50.44 +0.96 -0.68 Ma. The polymict agglomerate had a surprisingly diverse zircon suite that includes 48% Proterozoic, 25% Eocene, 13% Archean, 4% Paleozoic, and 3% Mesozoic ages. The age of the 10th youngest grain for this unit is 49.25 +0.91 -1.20 Ma. The ash fall tuff spectrum consisted of 63% Eocene zircon, with a variety of Paleozoic and Precambrian zircons also present. The weighted mean age of this unit is 50.3 +/- 1 Ma. The zircon provenance of the Hominy Peak Formation is diverse. These rocks contain a surprisingly high proportion of non-Eocene zircons in what has been interpreted to be a primary volcanic unit. During Hominy Peak time, the source areas included the uplifted Laramide Teton Range to the south that produced the Archean ages, the active Absaroka volcanic rocks to the north that yielded the Eocene ages, and the underlying Pinyon and Harbelle Formations that yielded that Proterozoic and and Mesozoic Zircons. The age of 50.3 +/- Ma would make these rocks correlative to other Bridgerian rocks to the east, which would include the Wapiti Formation.