The Miocene Columbia River Basalt Group within the Troy basin and adjacent Blue Mountains of northeastern Oregon and southeastern Washington consists of Grande Ronde, Wanapum, and Saddle Mountains Basalts erupted during uplift and basin formation. By Wanapum and Saddle Mountains times (15.6 to 6.0 Ma) the Troy Basin was sufficiently developed to begin receiving thick accumulations of lacustrine and alluvial sediments along with a wide variety of relatively thick basalt flows. As subsidence, sedimentation, and volcanism proceeded, stratigraphic relationships became more complex, with some Saddle Mountains flows forming invasive and intracanyon phases and a possible sill, in addition to the subaerial sheet phases mapped by Ross (1978). This chapter reviews and revises the stratigraphy of the Saddle Mountains Basalt in view of recent mapping and additional major- and trace-element analyses. Principal revisions are: the Elephant Mountain Member consists of the Wenaha flow represented by subaerial sheet, intracanyon, and invasive phases; the Buford Member consists of 1 (Buford) or 2 (Buford and Mountain View) flows represented by subaerial sheet, intracanyon, invasive phases, and possibly a sill, interfingered with the Wenaha flow and sedimentary interbeds; the Eden flow consists of subaerial and invasive phases and is elevated to member status within the Saddle Mountains Basalt; the Umatilla Member consists of the Sillusi flow overlain by the Bear Creek flow, the latter being a new Umatilla Member flow. An andesite breccia that may pre-date the cessation of Columbia River Group volcanism in the area is also recognized. Also addressed is the problem of determining if the Wenaha and Buford Member phases repeated within the section represent discrete eruptions of identical lavas or invasive and subaerial phases of single lavas. The Wenaha phases are believed to have resulted from a single subaerial eruption that spread as a sheet flow until encountering thick, wet sediments, which it invaded. Locally the invasive phase extruded as intracanyon lavas into shallow valleys eroded into the invaded sediments. The subaerial sheet, intracanyon, and invasive phases of the Buford Member may also have resulted from 1 eruption (possibly with local sills to the east of the study area) or they may represent 2 discrete lavas.

Abstract This field trip guide covers a two-day trip to examine the characteristics of Columbia River Basalt Group flows and the Yakima fold belt. This field trip focuses on the main physical characteristics of the lavas, compositional variations, and evidence for their emplacement, and on the geometry of the anticlinal ridges and synclinal valleys of the fold belt and deformational features in the basalts.

Abstract Plutonk-relcttcd gold deposits in interior Alaska occur in the apexes of middle Cretaceous (mostly 93-86 Ma) reduced plutons and in spatially associated sedimentary and metamorphic rocks. Hydrothermal micas from these deposits have cooling ages 0 to 2 m.y. younger than those of magmatic micas and amphiboles from hosting intrusions. These deposits have alteration features and trace element, fluid inclusion, and stable and radiogenic isotope signatures indicative of predominantly magmatic metallogenesis. Plutonic-related gold deposits of intetior Alaska share the following charactetistics: 1. The deposits are solely intrusion hosted (Fort Knox, Democrat, Joker-88, Table Mountain, Elephant Mountain), solely schist hosted (Christina, Cleay Hill), or hosted in both intrusions and schist (Ryan Lode, Liberty Bell). Gold occurs in contemporaneous W-Au skarns and replacement deposits. 2. The gold is typically present in and around the tops of intmsions. It occurs in closely spaced anastomosing or planar quartz veins with feldspar-white mica or quartz-sericite halos but is most predominant in brlttlely deformed planar quartz-sericite (± carbonate) shear zones and veins. Tourmaline is common in systems associated with smaller intrusions. Propylitic alteration occurs distally within intrusions. 3. Arsenopyrite and stibnite are the most common sulfides. Bismuthinite, bismuth telluride, and bismuthlend-antimony sulfosalt are common gold associates; a Bi-Au correlation is significant at many deposits. 4. Arsenopyrite geothermometry indicates temperatures or 400° to 480°C for feldspathic alteration and 300° to 350°c for scricitic alteration. Gold-associated fluid inclusion temperatures plot mostly at 300° to 350°c, Aplite, sphalerite, fluid inclusion geobaromehy, and intrusion textures indicate intrusion and mineralization pressures or 0.5 to 1.5 kbars. 5. Plutons related to or hosting gold mineralization may be multiphase. Intrusion compositions range from syenormmxonitc to granite. The most altered and/or mineralized phasc is porphyritic granite, granodiorite, or quartz monzonite. Ilmenite series intrusions with very low primary iron oxide contents are gold associative. Trace element signatures and Pb isotope ratios indicate that the plutons are 1 type. The intrusions are subduction related. Intrusive isotopic ratios indicate contamination by strongly radiogenic crust. 6. Lead and sulfur isotope ratios from sulfides ( 206 Pb/ 204 Pb = 19.0-19.45; 207 Pb/ 204 Pb = 15.58-15.7; δ 34 S = 0 ± 5‰) are similar to those of feldspars from related intrusions. Isotopic ratios are distinct from those measured from Yukon-Tanan terrane volcanogenic massive sulfide deposits and from nonplutonic-related mesotbermal gold deposits. Fluid calculated carbon, oxygen, and hrdrogen isotope ratios (δ 13 C = −9 to -10‰; δ 18 O = 5-10‰; δD = −47 to -100‰) indicate magmalic devolatilization. 7. Fluid inclusion microthermometry of plutons and gold veins indicates a wide range of salinities, the main control being the differentiation index of the host pluton. Most gold-associated inclusions are of moderate salitely (0-10 wt %), though some deposits have as much as 40 wt percent NaCI. Most of the gold-associated veins contnin abundant CO 2 (7-22 mole % avg) and exhibit evidence of H 2 O-CO 2 immiscibility. Geologic, geochronological, geocbemical, and empirical data support a model in which, high CO 2 , goldbearing fluids fractionate from the more differentiated and more potphyritic end members of reduced 1-type intrusions. The relatively low oxidation state associated with gold-favorable intntsions allows ror gold fractionation into magmatic hydrothermal fluids and favors efficient gold transport as a bisulfide complex in a reduced, vapor-rich fluid. Plutonic-related deposits in southwestern Alaska are younger (≈70 Ma), emplaced at lower pressures (0-0.4 kbarn>), and are not within the Yukon-Tanana terrane. CO 2 content is lower than that of interior plutonicrelated deposits, but carbonate alteration is more abundant. Similarilies in (1) tectonic setting and oxidation stale of the plutons, (2) alteration style and minerals, (3) fluid inclusions, and (4) stable isotope ratios suggest a similar origin for plutonic-related gold deposits in interior and southwestern Alaska.

ABSTRACT In portions of Washington, Oregon, and Idaho, the Columbia River Basalt Group (CRBG) hosts a regional aquifer system that is the primary, and in many cases the only, water supply for numerous communities, small water systems, individual homes, industry, and agriculture. In much of the semiarid Columbia Plateau, portions of the CRBG aquifer system have seen significant water-level declines and do not appear to receive significant, if any, natural recharge. Aquifer horizons within the Columbia River basalt generally are associated with intraflow structures at the top (e.g., vesicular flow-top breccias) and bottom (e.g., flow-foot breccias, pillow lava and hyaloclastite complexes) of sheet flows. The interiors of thick sheet flows (in their undisturbed state) have extremely limited permeability and act as aquitards, typically creating a series of stacked, confined aquifers within the Columbia River basalt aquifer system. The dominant groundwater flow follows horizontal to subhorizontal pathways along individual, laterally extensive, interflow zones. Vertical groundwater movement through undisturbed basalt flow interiors is greatly restricted except where basalt flow interiors are disturbed (such as by folds or faults), truncated (such as by flow pinchouts and erosional windows), or where they are cross-connected by wells.

ABSTRACT The Miocene Columbia River Flood-Basalt Province is one of the youngest and perhaps the best studied flood-basalt province on Earth. This field guide describes a three-day field trip through the central, eastern, and western portions of the Columbia Plateau region of this province, visiting field localities that have been key to understanding the geologic and structural history of this province. The guide provides a brief summary of our current understanding of the geologic and tectonic evolution of this flood-basalt province. Recent refinements in Columbia River basalt stratigraphy have confirmed the huge size of many of the Columbia River basalt flows (1000– 5000 km3 in volume) and a wide range of emplacement rates. The emplacement rate estimates range from as low as one to two months to as high as three to four years. Many aspects of Columbia River basalt volcanism appear to be associated with regional-scale deformation (e.g., regional-scale subsidence, folding, and faulting).

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.

The Salton Trough was formed by the late Cenozoic interaction of three major geologic systems: the Gulf of California, Colorado River, and San Andreas fault (Fig. 1). On this field trip we will examine the stratigraphic record of these three systems as exposed in the western Salton Trough. Each day will be spent in one of the three localities ~ Split Mountain, Fish Creek Basin, and Coyote Mountains ~ that provide the best leverage on chronology, facies, and paleogeography of the early Salton Trough. Although the Salton Trough is not a significantly petroliferous basin, we will also examine sedimentary geometries, lateral variability, and reservoir-seal relationships as they might pertain to exploration and production in analogous basin settings.