DETRITAL ZIRCONS FROM LATE MIOCENE CHALK HILLS FORMATION AND EQUIVALENTS, LAKE IDAHO, SOUTHWEST IDAHO AND SOUTHEAST OREGON

Detrital zircons from late Miocene strata of the Vale area along the Oregon-Idaho border support previous studies that suggest no connection to provenance areas in eastern Idaho containing recycled Grenvillean-age grains. The through-going Snake River did not develop until the Pliocene. Our provenance studies of the Sucker Creek, Chalk Hills correlative, and Glenns Ferry formations suggest local sources north, south, and west of Lake Idaho. The Sucker Creek Fm. (15 Ma) from the Devils Gate area north of Succor Creek State Park, OR, contains Late Cretaceous Atlanta Lobe (70 to 80 Ma), Eocene Challis Volcanic (46 Ma), and late Miocene Yellowstone-Snake River Plain volcanic zircons. Similar detrital-zircon age populations are found in Chalk Hills Fm. cuttings from 2030 to 2400 ft in the Champlin Well southwest of Boise, ID, provided by Dr. Spencer Wood, and from 5830 to 5910 ft in the Ore-Ida well north of Ontario, OR. The Chalk Hills correlative south of Vale, OR (<8.1, >6.4 Ma) contains 112 Ma grains and 40-43 Ma Eocene zircons, both with permissive sources in the Pine Forest Range in northern Nevada. It also contains dark-colored chert pebbles for which a source in the King Lear Fm. in northern Nevada (Martin et al., 2010) seems likely. The Chalk Hills Fm. correlative from Vines Hill, west of Vale, contains 128 to 132 Ma zircons with bladed and euhedral shapes. These ages are not present in the Idaho batholith and fall between the two Cretaceous magmatic age-peaks of the Cordilleran arc. A western source in Oregon seems more likely than a source in Idaho. Finally, the Glenns Ferry Fm. on Malheur Butte north of Vale, contains mainly Neogene volcanic grains with ages as young as 3.0 Ma. Snake River Plain-Yellowstone latest Miocene and Pliocene volcanic zircon grains, that must have been transported westward by aeolian processes, are present in all samples and are progressively younger in stratigraphically higher samples.