The Lewiston Structure is located in southeastern Washington and west-central Idaho and is a generally east-west–trending (~075°), asymmetric, noncylindrical anticline in the Columbia River Basalt Group that transfers displacement into the Limekiln fault system to the southeast and the Silcott fault system to the southwest. A serial cross-section analysis and three-dimensional (3-D) construction of this structure show how the fold varies along its trend and shed light on the deformational history of the Lewiston Basin. Construction of the fold’s 3-D form shows that the fold’s wavelength increases and amplitude decreases near its eastern and western boundaries. Balanced cross sections show ~5% shortening across the structure, which is consistent with the Yakima Fold Belt. An angular unconformity below the Grande Ronde Basalt N1 magnetostratigraphic unit, in addition to a variation of N1 unit thickness across the structure, suggests that the fold was forming before N1 time. Analysis of structural data using the Gauss method for heterogeneous fault-slip data indicates north-south (~350°) shortening prior to and after N1 emplacement. The presence of a reverse fault on the southern limb of the Lewiston Structure is controversial. This fault crops out to the east of the field area where Grande Ronde Basalt magnetostratigraphic unit R2 is thrust over Pliocene(?) gravels. However, better control on unit thicknesses and map contacts rules out an exposed reverse fault on the southern limb of the fold west of the Washington-Idaho border, suggesting the fault either dies out or becomes blind.

Sedimentary interbeds preserved between flows of the Columbia River Basalt Group provide a record of the depositional and erosional conditions that characterized the Columbia Plateau between eruptions of basalt. Examination of the sedimentary, stratigraphic, and petrologic character of the Sweetwater Creek interbed from within the Lewiston basin of southeastern Washington and north-central Idaho allows insight into the paleogeographic conditions that existed following eruption of the Priest Rapids Member of the Wanapum Basalt, ca. 14.5 Ma. The Sweetwater Creek interbed is composed of generally unconsolidated and inter-stratified beds of clay, silt, sand (with local thin gravel stringers), and volcanic ash-rich sediment. Three broadly defined sedimentary facies are identified on the basis of lithology and texture. The spatial distribution of these facies, abundance of clay- and silt-rich sediment, and internal sedimentary structures suggest that deposition of the interbed resulted primarily from fluvial and mixed fluvial-lacustrine sedimentation. Fluvial drainages that headed in the ancestral Clearwater Mountains entered the Lewiston basin on the east and exited to the northwest. Basin streams appear to have been primarily of the meandering, mixed-load type. Channel sands deposited by these streams were concentrated east and north of the basin center, and transported extrabasinal sediments are characterized by plutonic and metamorphic sand- and gravel-sized clasts. Fine-grained silt- and clay-rich flood-plain and associated lacustrine deposits extend across the basin, but are thickest near the basin center. The Umatilla basalt flow entered the Lewiston basin during deposition of the Sweetwater Creek interbed and locally invaded fine-grained lacustrine sediments. A later flow, the Wilbur Creek basalt, partially buried the interbed. Complete burial of the Sweetwater Creek interbed sediments followed eruption of the Asotin flow.

ABSTRACT The Ordovician Bronson Hill arc and Silurian–Devonian Central Maine basin are integral tectonic elements of the northern Appalachian Mountains (USA). However, understanding the evolution of, and the relationship between, these two domains has been challenging due to complex field relationships, overprinting associated with multiple phases of Paleozoic orogenesis, and a paucity of geochronologic dates. To constrain the nature of this boundary, and the tectonic evolution of the northern Appalachians, we present U-Pb zircon dates from 24 samples in the context of detailed mapping in northern New Hampshire and western Maine. Collectively, the new geochronology and mapping results constrain the timing of magmatism, sedimentation, metamorphism, and deformation. The Bronson Hill arc formed on Gondwana-derived basement and experienced prolonged magmatic activity before and after a ca. 460 Ma reversal in subduction polarity following its accretion to Laurentia in the Middle Ordovician Taconic orogeny. Local Silurian deformation between ca. 441 and 434 Ma may have been related to the last stages of the Taconic orogeny or the Late Ordovician to early Silurian Salinic orogeny. Silurian Central Maine basin units are dominated by local, arc-derived zircon grains, suggestive of a convergent margin setting. Devonian Central Maine basin units contain progressively larger proportions of older, outboard, and basement-derived zircon, associated with the onset of the collisional Early Devonian Acadian orogeny at ca. 410 Ma. Both the Early Devonian Acadian and Middle Devonian to early Carboniferous Neoacadian orogenies were associated with protracted amphibolite-facies metamorphism and magmatism, the latter potentially compatible with the hypothesized Acadian altiplano orogenic plateau. The final configuration of the Jefferson dome formed during the Carboniferous via normal faulting, possibly related to diapirism and/or ductile thinning and extrusion. We interpret the boundary between the Bronson Hill arc and the Central Maine basin to be a pre-Acadian normal fault on which dip was later reversed by dome-stage tectonism. This implies that the classic mantled gneiss domes of the Bronson Hill anticlinorium formed relatively late, during or after the Neoacadian orogeny, and that this process may have separated the once-contiguous Central Maine and Connecticut Valley basins.

Abstract The late Mesozoic accretionary boundary in west-central Idaho has played a critical role in tectonic models proposed for the northwestern U.S. Cordillera. From west-to-east, major elements include the Permian to Jurassic Wallowa island-arc terrane, a poorly understood transition zone consisting of the Riggins Group assemblage and deformation belt along the west side of the island arc-continent boundary, Late Jurassic to Cretaceous arc-continent boundary, and Precambrian North American margin intruded by the Cretaceous–Paleogene Idaho batholith. We focus on the transition zone in the area between White Bird and Riggins, Idaho, which includes a contractional belt in variously deformed and metamorphosed rocks of island-arc affinity. We propose that the rocks of the entire transition zone, including those originally defined as the Riggins Group, are likely of Wallowa terrane origin and/or related basinal assemblages. Ultramafic rocks in the transition zone are possibly related to a Jurassic or Cretaceous basinal assemblage that includes the Squaw Creek Schist of the Riggins Group. Our recent work addresses the kinematic history of structures in the contractional belt. The belt was reactivated in the Neogene to accommodate mostly brittle normal faulting that strongly influenced preservation of the Miocene Columbia River Basalt Group at this location along the eastern margin of the flood basalt province. This field guide provides a road log for examining the geology between Moscow and New Meadows, Idaho, along U.S. Highway 95.

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.

A Google Earth–based virtual field trip, part of an introductory geology class, has been developed to illustrate the geology of the Presidential Range, New Hampshire. During a class field trip to Mt. Washington, the highest peak in the Northeast, students record GPS locations of exposures and collect information in the form of field notes and digital images from outcrops. Students upload the GPS waypoints into Google Earth and their images into a class PicasaWeb album, and they also make video clips that are uploaded into a class YouTube account. In Google Earth, the students embed and geologically annotate their images and embed their video clips. The final product is a Google Earth .kmz file or what is termed a mashup. The mashup provides a permanent record of the excursion and, if made available on the Internet, allows any user the ability to easily view the geology at any time. Constructing the mashup from the real field trip initiated reflective, independent, student-motivated learning and group work using technology that the students regularly use and enjoy doing. The resulting mashups have been very good, with an appropriate level of geologic content for an introductory course. Grading, which normally is onerous, is actually enjoyable, entertaining, and easy.

ABSTRACT The Mesozoic tectonic margin in west-central Idaho, USA, continues to be a world-class geologic venue, instigating debates and inspiring hypotheses that shape our understanding of tectonics processes and related strain regimes worldwide. This field guide 1 is a snapshot of up-to-date interpretations along with new data that fuels ongoing discussion of the processes that have shaped this corner of the tectonic margin—a field-based road-log tour of the enigmatic structural “elbow,” defined by an ~90° bend (a transition from N-S to E-W orientation) in the Mesozoic arc-continent boundary at the latitude ~46°N. This trip is composed of three transects that showcase the rocks and structural markers documented within this complicated region. These transects are organized as follows: (1) along the N-S segment of the terrane boundary near Riggins, Idaho; (2) a W-E orientation along the South Fork of the Clearwater River to Elk City, Idaho, roughly perpendicular to tectonic grain; and (3) along and across the complex “elbow” bend, where the boundary’s orientation shifts abruptly from N-S to E-W. Bedrock exposures and outcrops along these transects yield many opportunities to see field-based evidence for the complex, protracted tectonic and structural evolution of the arc-continent boundary, while also considering the critical preexisting tectonic framework that exerted influence on the overprinting accretionary event, as well as the late-stage, extensional system that has since reactivated and dismembered the region.

Location The gorge of the Niagara River extends northward along the U.S.-Canada boundary for 7.1 mi (1 1.4 km) from Niagara Falls (Niagara County, New York-Welland County, Ontario), to the Niagara Escarpment at Lewiston, New York-Queenston, Ontario (Fig. 1). Sites, described below, are located on the U.S.G.S. Niagara Falls and Lewiston 7½-minute quadrangles. For purposes of this guide, stops have been selected along Niagara Gorge, from Lewiston southward to Niagara Falls. Alllocations, except for Stop 1, are readily accessible from either the Robert Moses Parkway (New York) or the Niagara Parkway (Ontario), and are well-known tourist areas with well-markedparking lots. For detailed road logs of these and other sites onboth U.S. and Canadian sides of the Niagara Gorge, see Krajewskiand Terasmae (1981) and Friedman and others (1982). The following sections of this guide provide an overview of the two major geologic aspects of Niagara Gorge: (1) Pleistocene-Holocene geomorphology and geology, and (2) Paleozoic stratigraphy. Description of stops and their locations then follows.

Abstract The Channeled Scabland of east-central Washington comprises a complex of anastomosing fluvial channels that were eroded by Pleistocene megaflooding into the basalt bedrock and overlying sediments of the Columbia Plateau and Columbia Basin regions of eastern Washington State, U.S.A. The cataclysmic flooding produced huge coulees (dry river courses), cataracts, streamlined loess hills, rock basins, butte-and-basin scabland, potholes, inner channels, broad gravel deposits, and immense gravel bars. Giant current ripples (fluvial dunes) developed in the coarse gravel bedload. In the 1920s, J Harlen Bretz established the cataclysmic flooding origin for the Channeled Scabland, and Joseph Thomas Pardee subsequently demonstrated that the megaflooding derived from the margins of the Cordilleran Ice Sheet, notably from ice-dammed glacial Lake Missoula, which had formed in western Montana and northern Idaho. More recent research, to be discussed on this field trip, has revealed the complexity of megaflooding and the details of its history. To understand the scabland one has to throw away textbook treatments of river work. —J. Hoover Mackin, as quoted in Bretz et al. (1956, p. 960)