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.

Approximately 14,500 yr ago, Pleistocene Lake Bonneville discharged 4,750 km 3 of water over the divide between the closed Bonneville basin and the watershed of the Snake River. The resulting flood, released near Red Rock Pass, Idaho, followed the present courses of Marsh Creek, the Portneuf River, and the Snake and Columbia Rivers before reaching the Pacific Ocean. For 1,100 km between Red Rock Pass and Lewiston, Idaho, the Bonneville Flood left a spectacular array of flood features that have allowed for geologic reconstruction and quantitative evaluation of many aspects of the flood hydrology, hydraulics, erosion, and sediment transport. Geologic evidence of maximum flood stages in conjunction with step-backwater flow modeling has furnished estimates of peak discharge and local hydraulic flow conditions for 10 separate reaches along the flood route. Peak discharge was approximately 1.0 million m 3 ·sec −1 at the Lake Bonneville outlet near Red Rock Pass. Downstream, near Lewiston, Idaho, the maximum discharge attenuated to 0.57 to 0.62 million m 3 ·sec −1 . In part, attenuation of the peak discharge was probably the result of flow storage in the wide alluvial valleys of the western Snake River Plain. Diverse geologic and geomorphic environments along the flood route resulted in large spatial variations in the channel geometry as the flood moved downstream. Consequently, the local hydraulic conditions (flow depth, velocity, boundary shear stress) of the Bonneville Flood varied substantially within and between the study reaches. The rate of energy expenditure was also highly varied; local calculated stream power magnitudes ranged from less than 10 watts·m −2 in ponded reaches to more than 100,000 watts·m −2 in major constrictions. More than 50% of the total energy loss at peak discharge was expended over a distance that encompassed less than 6% of the flood route. These spatial variations in local hydraulic conditions were profoundly important in controlling the distribution of flood processes and the resulting flood features. The deposition of tractively transported cobbles and boulders (measured clast diameters ranged from less than 10 cm to more than 10 m) were deposited in reaches of decreasing flow competence within quantitatively definable limits. The hydraulic conditions at areas of erosion were more difficult to precisely evaluate; however, erosion of basalt bedrock was primarily in reaches that had stream power magnitudes that exceeded 20,000 watts·m −2 . Cavitation was probably an important erosional process in areas of most intense flow conditions.

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 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.

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)

Abstract The western Snake River Plain (SRP) is a southeast-northwest–trending complex graben bounded on the SW by the Owyhee Front. This graben, which merges with the central SRP at its southeast end near Bruneau Canyon, is a subsidiary tectonic feature that resulted from southwest-northeast extension as the main SRP–Yellowstone hotspot trend evolved. Silicic volcanism during the late Miocene along the Owyhee Front and in the central SRP resulted in large welded-tuff and rhyolite lava flows being erupted; these are well exposed southwest of the western Snake River Plain (WSRP) and in the western Mount Bennett Hills, northeast of where the western and central SRP merge. As the WSRP graben developed, it held a large lake, into which some of the rhyolite units flowed. At various stops described in this field guide, the characteristics of rhyolite lavas versus rheomorphically deformed welded tuffs, and of subaerially deposited versus subaqueously deposited rhyolite units, are displayed. During Pliocene and Pleistocene time, basaltic volcanism partially filled the WSRP graben and developed a basalt plateau across much of the central SRP. At various stops described in this field guide, the characteristics of subaerial versus subaqueous basalt flows, and of phreatomagmatic vent complexes, are displayed. Altogether, the stops described provide a guide to the wide variety of rhyolitic and basaltic volcanism phenomena in the western SRP, Owhyee Front, western Mount Bennett Hills, and Bruneau Canyon areas.

Abstract This field guide covers the Precambrian geology of the western portion of the Clearwater complex and surrounding area in north-central Idaho in the vicinity of Marble Creek within the St. Joe National Forest. The regional geology of the Marble Creek area includes Precambrian basement orthogneisses, possible basement metasupracrustal rocks, and overlying metamorphosed Belt Supergroup strata. These rocks are exhumed within the western portion of the Cretaceous-Eocene Clearwater metamorphic core complex. This guide focuses on the western part of the Clearwater complex in the vicinity of Marble Creek. Outcrops of Paleoproterozoic basement and overlying Mesoproterozoic metasedimentary units provide a better understanding of the Precambrian magmatic and metamorphic history of this region. The road log in this guide describes the regional geology in a south to north transect from Clarkia, Idaho, to the confluence of Marble Creek with the St. Joe River.

Abstract Prepared for the 2016 GSA Rocky Mountain Section Meeting, this well-illustrated volume offers guides to the lavas of the Columbia River basalts, megaflood landscapes of the Channeled Scablands, Mesozoic accreted terranes, metamorphic Precambrian Belt and pre-Belt rocks, and other features of this tectonically active region.

Abstract This one-day field trip of geologic and historical significance goes from Washtucna, Washington, through Palouse Falls State Park, Lyons Ferry State Park, and Starbuck, and ends in the Tucannon River valley. At Palouse Falls, it is readily apparent why Native Americans crafted stories about the origins of this spectacular area and why geologic debates regarding the role of Pleistocene glacial Lake Missoula floods during the formation of this natural wonderland have been centered here. This field trip focuses on structural geology and the Palouse Falls fracture zone, Columbia River Basalt Group stratigraphy at the falls, and subsequent erosion by glacial outburst floods. Discussion of the falls will include human history and the formation of Palouse Falls State Park. The main stop at Palouse Falls will explore the stratigraphy of the Columbia River Basalt Group, Vantage Member, loess islands, fracture zones, and human history dating back at least 12,000 yr. Driving south through Lyons Ferry State Park and the Tucannon Valley, we will discuss topics ranging from the Palouse Indians to sheep herding and from clastic dikes to terracettes.

Abstract The Middle Miocene Columbia River Basalt Group (CRBG) is the youngest and smallest continental flood basalt province on Earth, covering over 210,000 km 2 of Oregon, Washington, and Idaho and having a volume of 210,000 km 3 . A well-established regional stratigraphic framework built upon seven formations, and using physical and compositional characteristics of the flows, has allowed the areal extent and volume of the individual flows and groups of flows to be calculated and correlated with their respective dikes and vents. CRBG flows can be subdivided into either compound flows or sheet flows, and are marked by a set of well-defined physical features that originated during their emplacement and solidification. This field trip focuses on the Lewiston Basin, in southeastern Washington, western Idaho, and northeastern Oregon, which contains the Chief Joseph dike swarm, where classic features of both flows and dikes can be easily observed, as well as tectonic features typical of those found elsewhere in the flood basalt province.

Abstract The Moscow-Pullman basin, located on the eastern margin of the Columbia River flood basalt province, consists of a subsurface mosaic of interlayered Miocene sediments and lava flows of the Imnaha, Grande Ronde, Wanapum, and Saddle Mountains Basalts of the Columbia River Basalt Group. This sequence is ~1800 ft (550 m) thick in the east around Moscow, Idaho, and exceeds 2300 ft (700 m) in the west at Pullman, Washington. Most flows entered from the west into a topographic low, partially surrounded by steep mountainous terrain. These flows caused a rapid rise in base level and deposition of immature sediments. This field guide focuses on the upper Grande Ronde Basalt, Wanapum Basalt, and sediments of the Latah Formation. Late Grande Ronde flows terminated midway into the basin to begin the formation of a topographic high that now separates a thick sediment wedge of the Vantage Member to the east of the high from a thin layer to the west. Disrupted by lava flows, streams were pushed from a west-flowing direction to a north-northwest orientation and drained the basin through a gap between steptoes toward Palouse, Washington. Emplacement of the Roza flow of the Wanapum Basalt against the western side of the topographic high was instrumental in this process, plugging west-flowing drainages and increasing deposition of Vantage sediments east of the high. The overlying basalt of Lolo covered both the Roza flow and Vantage sediments, blocking all drainages, and was in turn covered by sediments interlayered with local Saddle Mountains Basalt flows. Reestablishment of west-flowing drainages has been slow. The uppermost Grande Ronde, the Vantage, and the Wanapum contain what is known as the upper aquifer. The water supply is controlled, in part, by thickness, composition, and distribution of the Vantage sediments. A buried channel of the Vantage likely connects the upper aquifer to Palouse, Washington, outside the basin. This field guide locates outcrops; relates them to stratigraphic well data; outlines paleogeographic basin evolution from late Grande Ronde to the present time; and notes structures, basin margin differences, and features that influence upper aquifer water supply.

Abstract The Cougar Gulch area near Coeur d’Alene, Idaho, is a newly recognized Paleoproterozoic to Archean basement occurrence located in the southern Priest River complex. Here, a structural culmination exposes deeper levels of the core complex infrastructure, similar to where Archean basement is exposed in the northern portion of the complex near Priest River, Idaho. At Cougar Gulch, the basement rocks are composed of a variety of granitic orthogneisses and amphibolite, which are unconformably overlain by a graphite-bearing orthoquartzite. The orthoquartzite is in turn overlain by the Hauser Lake Gneiss. The similarity of structure, metamorphic fabrics, and kinematics here and in the northern portions of the complex is consistent with the Cougar Gulch area being the southern continuation of the Spokane dome mylonite zone. Neoarchean amphibolites (2.65 Ga) have been identified as part of the basement sequence. These amphibolites had a basaltic protolith and can be distinguished geochemically from amphibolites found within the overlying Hauser Lake Gneiss (Mesoproterozoic, Lower Belt Group equivalent), which are metamorphosed Moyie sills. The Archean amphibolites have steeper REE (rare earth element) slopes and consistently higher REE values. Protoliths of the Paleoproterozoic orthogneisses (1.87–1.86 Ga) are calc-alkaline, “I-type” monzogranites and granodiorites, which exhibit subduction-related geochemical characteristics such as high LILE:HFSE (large ion lithophile element: high field strength element) concentrations, along with characteristic depletions in Nb, Ta, P, Ti, and Eu. A second distinctive geochemical unit of orthogneiss, the Kidd Creek tonalite, exhibits TTG (tonalite-trondhjemite-granodiorite) geochemical characteristics. The Kidd Creek tonalite has Sr/Y and La/Yb ratios, along with Y and HREE (heavy rare earth element) concentrations (no Eu anomalies) similar to Precambrian TTG compositions formed in subduction settings. Detrital zircon data from the orthoquartzite unit, along with characteristic graphite and its consistent stratigraphic level support correlation to the pre-Belt Gold Cup Quartzite in the northern part of the complex.

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.

The Mesozoic Peninsular Ranges batholith, part of a long-lived Cordilleran subduction orogen, is located at a critical juncture at the southwest corner of cratonal North America. The batholith is divided into northern and southern segments that differ in their evolution. In this paper, we focus on the more poorly understood southern Peninsular Ranges batholith, south of the Agua Blanca fault at ~31.5°N latitude, and we compare its evolution with the better-known northern Peninsular Ranges batholith. Adding our new insights to previous work, our present understanding of the geologic history of the Peninsular Ranges consists of the following: (1) stronger connections between the Paleozoic passive-margin rocks in the eastern Peninsular Ranges batholith and similar assemblages in Sonora, Mexico, to the east and the Sierra Nevada batholith to the north that were originally proposed by earlier workers; (2) continuity of the Triassic–Jurassic accretionary prism and forearc basin assemblage from the northern Peninsular Ranges batholith through the southern Peninsular Ranges batholith; (3) possible synchronous subduction of an ocean ridge or ridge transform along the Peninsular Ranges batholith in late Middle Jurassic time; (4) continuity of the Early Cretaceous Santiago Peak continental arc from the northern Peninsular Ranges batholith along the entire margin, including the southern Peninsular Ranges batholith; (5) development of the Alisitos oceanic arc in Jurassic and possibly Triassic time, much earlier than originally thought; and (6) removal of part of the Santiago Peak assemblage in the southern Peninsular Ranges batholith during collision of the Alisitos terrane in latest Early Cretaceous time.