ABSTRACT Lower Mesozoic clastic rocks that unconformably overlie Paleozoic rocks within the Northern Sierra terrane provide clues regarding the evolution of the terrane during a 60 m.y. interval spanning the late Carnian through Bajocian. New detrital-zircon data provide fresh insights into the ages and provenance of these clastic rocks, together with new inferences about the Mesozoic tectonic evolution of the terrane. Previous studies have shown that from the late Carnian to the Sinemurian (~40 m.y.), a 1-km-thick section of subaerial to shallow-marine clastic arc-derived sediment accumulated and shallow-marine carbonate was deposited. At the base of this section, detrital-zircon results suggest the Northern Sierra terrane was located near a source area, possibly the El Paso terrane, containing Permian igneous rocks ranging in age from 270 to 254 Ma. By the earliest Jurassic, the detrital-zircon data suggest the Northern Sierra terrane was located near a source containing latest Triassic–earliest Jurassic igneous rocks spanning 209–186 Ma. The source of this material may have been the Happy Creek volcanic complex of the Black Rock terrane. A deep-marine, anoxic basin developed within the Northern Sierra terrane ca. 187–168 Ma. Approximately 3.5 km of distal turbidites were deposited in this basin. Previously reported geochemical characteristics of these turbidites link the Northern Sierra terrane with arc rock of the Black Rock terrane during this interval, except for a short time in the late Toarcian, when the terrane received an influx of quartz-rich sediment, likely derived from Mesozoic erg deposits now exposed on the Colorado Plateau. Clastic deposition within the Northern Sierra terrane ended in the Bajocian. Eruption of proximal-facies, mafic volcanic rocks and intrusion of hypabyssal rock and 168–163 Ma plutons reflect development of a magmatic arc within the terrane. These igneous rocks represent the first unequivocal evidence that the Northern Sierra terrane was located within a convergent-margin arc during the Triassic and Jurassic. Because detrital-zircon data from Lower Mesozoic strata within the Northern Sierra terrane indicate that it was depositionally linked with differing source areas through time early in the Mesozoic, the terrane may have been mobile along the western margin of Laurentia. There is little evidence from sediment within the Lower Mesozoic section of the terrane that can clearly be tied to the craton or the continental-margin Triassic arc prior to the late Toarcian. The absence of Upper Triassic or Lower Jurassic plutonic rocks within the terrane prior to the mid-Bajocian is also consistent with some form of isolation from the continental-margin arc system. While new detrital-zircon results place the Northern Sierra terrane proximal to the western margin of Laurentia in the late Toarcian, the current location of the terrane likely reflects Early Cretaceous offset along the Mojave–Snow Lake fault.

Abstract The Wallowa terrane is one of five pre-Cenozoic terranes in the Blue Mountains province of Oregon, Idaho, and Washington. The other four terranes are Baker, Grindstone, Olds Ferry, and Izee. The Wallowa terrane includes plutonic, volcanic, and sedimentary rocks that are as old as Middle Permian and as young as late Early Cretaceous. They evolved during six distinct time segments or phases: (1) a Middle Permian to Early Triassic(?) island-arc phase; (2) a second island-arc phase of Middle and Late Triassic age; (3) a Late Triassic and Early Jurassic phase of carbonate platform growth, subsidence, and siliciclastic sediment deposition; (4) an Early Jurassic subaerial volcanic and sedimentary phase; (5) a Late Jurassic sedimentary phase that formed a thin subaerial and thick marine overlap sequence; and (6) a Late Jurassic and Early Cretaceous phase of plutonism. Rocks in the Wallowa terrane are separated into formally named units. The Permian and Triassic Seven Devils Group encompasses the Middle and Late(?) Permian Windy Ridge and Hunsaker Creek Formations and the Middle and Late Triassic Wild Sheep Creek and Doyle Creek Formations. Some Permian and Triassic plutonic rocks, which crystallized beneath the partly contemporaneous volcanic and sedimentary rocks of the Seven Devils Group, represent magma chambers that fed the volcanic rocks. The Permian and Triassic plutonic rocks form the Cougar Creek and Oxbow “basement complexes,” the Triassic Imnaha plutons, and the more isolated Permian and Triassic plutons, such as those in the Sheep Creek to Marks Creek chain and in the southern Seven Devils Mountains near Cuprum, Idaho. The Seven Devils Group, and its associated plutons, are capped by the Martin Bridge Formation, a Late Triassic platform and reef carbonate unit, with associated shelf and upper-slope facies, and overlying and partly contemporaneous siliciclastic, limestone, and calcareous phyllitic rocks of the Late Triassic and Early Jurassic Hurwal Formation. Younger rocks are a subaerial Early Jurassic volcanic and sedimentary rock unit of the informally named Hammer Creek assemblage, and a Late Jurassic overlap sedimentary unit, the Coon Hollow Formation. Late Jurassic and Early Cretaceous plutons intrude the older rocks. Lava flows of the Miocene Columbia River Basalt Group overlie the pre-Cenozoic rocks. Late Pleistocene and Holocene sedimentation left discontinuous deposits throughout the canyon. Most impressive are deposits left by the Bonneville flood. The latest interpretations for the origin of terranes in the Blue Mountains province show that the Wallowa terrane is the only terrane that, during its Permian and Triassic evolution, had an intra-oceanic (not close to a continental landmass) island-arc origin. On this field trip, we travel through the northern segment of the Wallowa terrane in Hells Canyon of the Snake River, where representative rocks and structures of the Wallowa terrane are well exposed. Thick sections of lava flows of the Columbia River Basalt Group cap the older rocks, and reach river levels in two places.

Abstract This field guide covers geology across north-central Idaho from the Snake River in the west across the Bitterroot Mountains to the east to near Missoula, Montana. The regional geology includes a much-modified Mesozoic accretionary boundary along the western side of Idaho across which allochthonous Permian to Cretaceous arc complexes of the Blue Mountains province to the west are juxtaposed against autochthonous Mesoproterozoic and Neoproterozoic North American metasedimentary assemblages intruded by Cretaceous and Paleogene plutons to the east. The accretionary boundary turns sharply near Orofino, Idaho, from north-trending in the south to west-trending, forming the Syringa embayment, then disappears westward under Miocene cover rocks of the Columbia River Basalt Group. The Coolwater culmination east of the Syringa embayment exposes allochthonous rocks well east of an ideal steep suture. North and east of it is the Bitterroot lobe of the Idaho batholith, which intruded Precambrian continental crust in the Cretaceous and Paleocene to form one of the classical North American Cordilleran batholiths. Eocene Challis plutons, products of the Tertiary western U.S. ignimbrite flare-up, intrude those batholith rocks. This guide describes the geology in three separate road logs: (1) The Wallowa terrane of the Blue Mountains province from White Bird, Idaho, west into Hells Canyon and faults that complicate the story; (2) the Mesozoic accretionary boundary from White Bird to the South Fork Clearwater River east of Grangeville and then north to Kooskia, Idaho; and (3) the bend in the accretionary boundary, the Coolwater culmination, and the Bitterroot lobe of the Idaho batholith along Highway 12 east from near Lewiston, Idaho, to Lolo, Montana.

Abstract 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. Day 2 of the field trip is focused on Jurassic–Cretaceous magmatism in the Baker terrane. Late Middle Jurassic to Early Cretaceous igneous rocks in the Blue Mountains Province record three distinct pulses of plutonism that are characterized by distinctive spatial and geochemical signatures. These episodes consist of: (1) late Middle to Late Jurassic small gabbro to quartz diorite plutons (ca. 162–154 Ma; low Sr/Y); (2) Late Jurassic to Early Cretaceous plutons and batholiths (ca. 148 and 137 Ma; includes spatially distinct belts of low and high Sr/Y at 147–145 Ma); and (3) Early Cretaceous small plutons of tonalitic and trondhjemitic composition (ca. 124–111 Ma). Temporal transitions in geochemical characteristics between these suites raise fundamental questions regarding the origins of plutonism in the Baker terrane. In particular, the transition from low Sr/Y (group 1) to high Sr/Y (group 2) magmatism in the Greenhorn subterrane occurred ~ 7 Ma after regional contraction, and may record partial melting of thickened crust as a direct result of Late Jurassic orogenesis.

The Late Triassic (Carnian-Norian) time interval is well represented by strata in the Wallowa and Baker terranes and in the Izee-Suplee area of central Oregon. Strata of the Olds Ferry terrane, by contrast, have received little attention beyond field mapping and reconnaissance-level efforts. This paper describes the sedimentology of Carnian-Norian deposits in the Olds Ferry terrane and describes a model for arc-flanking mixed carbonate-volcaniclastic sedimentation. A distinctive thin-bedded limestone unit in the lower Huntington Formation is underlain and overlain by volcanic and shallow intrusive rocks. Ammonite and bivalve age assignments indicate that the limestone unit spans the late Carnian to early Norian; portions of the section may, more specifically, represent the welleri or dilleri ammonite zones. Four lithofacies—Graded Skeletal Packstone and Mudstone, Tuffaceous Peloidal Grainstone, Skeletal Peloidal Packstone, and Lapilli-Tuff Breccia—record episodic sedimentation influenced by earthquakes, storms, and volcanic eruptions on an oxygen-poor carbonate slope apron. The limestone unit represents an echinoderm- and mollusk-dominated heterozoan faunal assemblage characterized by abundant delivery of unlithified carbonate sediment to deeper water.

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