A field guide to Newberry Volcano, Oregon

This field trip guide describes a two-day excursion through Mesozoic accreted terranes of the Blue Mountains Province in northeastern Oregon. Day 1 is focused on sedimentary rocks of the Izee Terrane. These deposits are divided into two unconformity-bounded megasequences, MS-1 and MS-2, that record two stages of syntectonic basin formation. MS-1 (Late Triassic to Early Jurassic) accumulated in fault-bounded marine sub-basins on the flank of an inferred growing Baker terrane thrust belt. MS-1 sandstones, derived from the Baker terrane, contain abundant Paleozoic, Late Paleoproterozoic, and late Archean detrital-zircon grains. These observations suggest affinity of the Baker Terrane and MS-1 in the Izee area to portions of the Klamath and Sierra Nevada Terranes that contain similar detrital-zircon age distributions. MS-2 (Early to early-Late Jurassic) accumulated in a large marine basin that received input from low-grade metavolcanic rocks to the east (modern coordinates). Detrital zircons are dominated by Mesozoic, Neoproterozoic, and Mesoproterozoic grains. Two possible interpretations for MS-2 are: (1) the Jurassic Izee Basin was fed directly by the large Mesozoic trans-cratonal sediment-dispersal system, or (2) trans-cratonal sediment was deposited in a Triassic backarc basin in Nevada and was later recycled into the Jurassic Izee Basin during Cordilleran orogenesis.