PROVENANCE STUDY OF THE GOOSE ROCK CONGLOMERATE, CENTRAL OR: CHARACTERIZING THE MARGIN OF THE CRETACEOUS OCHOCO BASIN

Marine Cretaceous rocks of the Ochoco Basin onlap terranes of the Blue Mountains Province and crop out in the Mitchell Inlier in central Oregon. Isolated Cretaceous deposits at Goose Rock, Antone Ranch, and Bernard Ranch crop out east and south of the Mitchell Inlier and may represent marginal Ochoco Basin deposits. Conglomerate clast compositions, sandstone petrography, and detrital zircon age dating of these Cretaceous deposits will help assess proposed links between the fluvial Goose Rock Conglomerate, shallow marine deposits at Antone Ranch, and deeper marine deposits within the Mitchell Inlier. The Goose Rock Conglomerate formed in a high-energy fluvial system, and is characterized by abundant cobble conglomerate, discontinuous sand lenses, and an absence of mudstone. Conglomerate clast compositions are primarily volcanic (trachyte and rhyolite) and sedimentary (chert) with clast sizes from 2 to 30 centimeters. Medium- to coarse-grained, quartz-rich sandstone lenses are trough cross-bedded and include pebble stringers. Paleocurrent measurements of trough cross-beds indicate primarily southwest-directed paleoflow, which suggest source regions to the northeast. Depositional age of the Goose Rock Conglomerate is poorly known due to a lack of fossils, but is considered Cretaceous because of similarities to rocks within the Mitchell Inlier. Detrital zircon age dating of Goose Rock sandstone will provide maximum depositional age and permit age signature comparisons between the Goose Rock and Mitchell Inlier. This more complete characterization of the Ochoco Basin sedimentary record can then be used to evaluate proposed connections between the Ochoco basin and Hornbrook basin currently located in southwestern Oregon.