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Paleomagnetic studies resolve stratigraphic and correlational problems of several ignimbrites exposed along the margins of the eastern Snake River Plain (ESRP) in eastern Idaho and western Wyoming. Each ignimbrite exhibits a consistent magnetic remanence direction from its base to top. All units studied are high-silica, densely welded, metaluminous ignimbrites, the radiometric ages of which range from 7 Ma to 4 Ma. This study investigates whether ignimbrites of the Heise volcanic field—the tuffs of Blacktail Creek, Blue Creek, Elkhorn Spring, and Kilgore—correlate with the lithologically and chronologically similar Walcott and Conant Creek tuffs, two ignimbrite sheets marginal to the Heise field. Correlation of the tuff of Elkhorn Spring with the tuff of Blue Creek appears to be erroneous. Rather, paleomagnetic and chemical data indicate that the tuff of Blue Creek is equivalent to the Walcott Tuff. Results also suggest that the Conant Creek Tuff may actually consist of two separate ignimbrites: (1) an older unit that is tentatively correlated with the tuff of Elkhorn Spring and (2) a younger unit that is correlative with the tuff of Kilgore. The older unit appears to be restricted to the Heise cliffs and to the west side of the Teton Range. The tuff of Blacktail Creek remains a separate older unit. The Blue Creek and Kilgore calderas are now recognized as sources for the Walcott Tuff and the younger Conant Creek Tuff. The source of the older Conant Creek Tuff remains unidentified. Minimal areal distribution estimates are 15,000 km 2 for the Walcott Tuff and 18,000 km 2 for the tuff of Kilgore.

Abstract Neogene drainage development in southeastern Idaho has been influenced by drainage capture, Basin and Range faulting, volcanism, and the Late Pleistocene Lake Bonneville overflow and Bonneville Flood. In Marsh Valley, the Middle to Late Pleistocene sedimentary sequence is dominated by alternating lacustrine/paludal and alluvial sediments, which have yielded new 40Ar/39Ar, amino acid racemization, and luminescence age estimates. The pattern of sedimentation through time indicates poor drainage integration of southern Marsh Valley through most of the last ca. 640 ka and suggests slow basin subsidence along Quaternary faults mapped on the basin edges. Marsh Valley initially incised into that valley fill sequence ca. 19 ka, shortly before the Bonneville Flood. Marsh Creek is a markedly underfit stream occupying a meandering, broad valley carved into the valley fill sequence. These geomorphic and sedimentologic patterns suggest non-catastrophic Lake Bonneville overflow before and after the Bonneville Flood. In Portneuf Valley, ca. 8.5–7.4 Ma basin fill and a bedrock pediment are perched 800 m above the modern valley floor. Major incision of basin fill and bedrock by the ancestral Portneuf drainage system occurred prior to the Middle to Late Pleistocene, when two cut-fill events resulted in accumulation of alluvial fan deposits extending ~10–60 m above the modern valley floor and basalt extending ~10 m below to 20 m above the modern valley floor. Final incision by Lake Bonneville overflow is evident but relatively minor in comparison to the cumulative downcutting. Overall, incision is attributed to isostatic subsidence of the eastern Snake River Plain, which served as base level for the Portneuf drainage system after passage of the Yellowstone hot spot in late Miocene time.

Quaternary rocks northeast of American Falls Reservoir, Idaho, include more than 140 m of poorly indurated sedimentary rocks and intercalated basalt flows. Previous stratigraphic studies in the region include reconnaissance and detailed mapping and topical studies based mostly on surface exposures. The present study extends the Quaternary stratigraphy into the shallow subsurface by using lithologic data from drillers’ logs of about 240 water wells. Pre-Quaternary rocks of the area include Miocene bimodal volcanic and volcani-clastic rocks and Pliocene basalt of Buckskin Basin. The surface developed on the pre-Quaternary rocks slopes gently to the west and northwest except locally, as at Ferry Butte, where doming is inferred to be the result of shallow emplacement of a rhyolite intrusive body. Quaternary rocks of the study area consist of coarse-grained alluvium deposited by the Snake River, thin distal edges of basalt flows from sources to the northwest, fine-grained alluvial fan deposits derived from highlands to the southeast, two fine-grained fluvial and lacustrine deposits (Raft Formation and American Falls Lake Beds), and deposits of the Bonneville Flood. The Raft Formation, present only in the subsurface in the area, is correlated with the Raft in exposures along the Snake River southwest of American Falls and at the northern end of the Raft River basin on the basis of stratigraphic position and virtually identical thickness and lithology. The minimum age of the Raft Formation in the study area is constrained by an overlying basalt flow dated at 0.2 Ma. Driller’s logs and surface exposures show that the American Falls Lake Beds are at a higher elevation and are coarser grained northwest of the Snake River than they are southeast of the river. These differences are attributed to slightly different depositional environments caused by the geometry of the lake basin. Dating of a basalt flow that overlies the American Falls Lake Beds north of Springfield will provide valuable information on the history of American Falls Lake.

Models of basaltic volcanism of the eastern Snake River Plain (ESRP), Idaho, provide estimates of crustal basalt magma-driving pressures, magma-supply rates, dike geometry, mass-eruption rates, and durations of eruptions. Magma-driving pressures are 0.1 to 0.2 kb and about 1.2 kb for basalt magma rising from magma reservoirs at 40 and 60 km, respectively. Magma-driving pressures are 0.2 to 0.3 kb and 1.2 kb for basalt and trachyandesite magmas of the Craters of the Moon lava field that rise from a reservoir at 40 km. The lack of vents in several tracts of the ESRP is believed to be related to thick fill of low-density rocks in buried calderas that creates a density barrier that impedes or prevents eruption of basalt magma. Magma injection by dikes is influenced by regional tectonic extensional stress orientations; eruptive fissures and volcanic rift zones of the ESRP typically are oriented northwest-southeast, perpendicular to minimum regional stresses. Calculated model mass-eruption rates are 9 × 10 3 kg/sec m for typical ESRP basalt eruptions. Calculated durations of eruptions on the ESRP range from about 25 to 30 hours for the 0.005 km 3 Kings Bowl lava field to about 2 months for the 6 km 3 Wapi lava field. An inferred magma-plumbing system includes a magma source region in the mantle at depths of 60 km, a magma reservoir at or near the Moho, magma-filled cracks that transfer magma upward from the reservoir, an upper-level reservoir beneath the Craters of the Moon lava field, and dikes that lead from the reservoir to surface vents. The relatively unfractionated character of ESRP basalts, long repose intervals between eruptions on most volcanic rift zones, and relatively small volumes of lava fields suggest episodic local melting, little or no accumulation of magma in reservoirs, and rise of magma to the surface fairly directly from the source depth.

More than 95% of the eastern Snake River Plain (ESRP) is covered by basaltic lava flows erupted in the Brunhes Normal-Polarity Chron; thus they are younger than 730 ka. About 13% of the area of the ESRP is covered by lava fields of latest Pleistocene and Holocene age <15 ka. More than 90% of the basalt volume of the ESRP is included in coalesced shield and lava-cone volcanoes made up dominantly of tube- and surface-fed pahoehoe flows. Deposits of fissure-type, tephra-cone, and hydrovolcanic eruptions constitute a minor part of the basalt volume of the ESRP. Eight latest Pleistocene and Holocene lava fields serve as models of volcanic processes that characterize the basaltic volcanism of the ESRP. The North Robbers, South Robbers, and Kings Bowl lava fields formed in short-duration (a few days), low-volume (each <0.1 km 3 ), fissure-controlled eruptions. The Hells Half Acre, Cerro Grande, Wapi, and Shoshone lava fields formed in long-duration (several months), high-volume (1 to 6 km 3 ), lava cone- and shield-forming eruptions. Each of these seven lava fields represents monogenetic eruptions that were neither preceded nor followed by eruptions at the same or nearby vents. The Craters of the Moon lava field is polygenetic; about 60 flows were erupted from closely spaced vents over a period of 15,000 years. Most of the basaltic volcanism of the ESRP is localized in volcanic rift zones, which are long, narrow belts of volcanic landforms and structures. Most volcanic rift zones are collinear continuations of basin-and-range-type, range-front faults bordering mountains that adjoin the ESRP. It is not clear whether the faults extend into the ESRP in bedrock beneath the basaltic lava flows. The great bulk of basaltic flows in the ESRP are olivine basalts of tholeiitic and alkaline affinities. The olivine basalts are remarkably similar in chemical, mineralogical, and textural characteristics. They were derived by partial melting of the lithospheric mantle at 45 to 60 km, and they have been little affected by fractionation or contamination. Evolved magmas having SiO 2 contents as high as 65% occur locally in and near the ESRP. The chemical and mineralogical variability of the evolved rocks is due to crystal fractionation in the crust and to contamination by crustal minerals and partial melts of crustal rocks. The trace-element compositions of the olivine basalts and the most primitive evolved basalts do not overlap, suggesting that the evolved rocks were derived from parent magmas that are fundamentally different from the parent magmas of the olivine basalts. The distribution and character of volcanic rift zones in the ESRP are partly controlled by underlying Neogene rhyolite calderas. Areas that lack basalt vents and have only poorly developed volcanic rift zones overlie calderas or parts of calderas filled by thick, low-density sediments and rocks, which served as density barriers to the buoyant rise of basaltic magma. Volcanic rift zones are locations of concentrated extensional strain; they define regional stress patterns in the ESRP.

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.