Abstract An ~90-m-thick interval of mixed siliciclastic-carbonate strata, including a cap dolostone unit, overlies diamictite of the upper Scout Mountain Member of the Pocatello Formation in the Fort Hall Mine area south of Portneuf Narrows, southeastern Idaho, and is ≤ ca. 665 Ma. Six facies comprise this interval: silty sandstone (reworked diamictite matrix), laminated dolomite, dolomite and sandstone, sandstone, dolomite-chip breccia, and argillite and limestone. Sedimentary structures and bedding geometries of facies indicate paleoenvironments ranging from below storm wave base to upper shoreface. The edgewise, mounded, and parallel-bedded dolomite-chip breccia indicates slope failure and reworking of the lower shoreface during large storms. Facies relationships allow generalized division of these strata into three units. The lowermost unit, Unit A, comprises intimately interbedded laminated dolomite (“cap dolostone”), dolomite-chip breccia, and sandstone facies and is 17 m thick. Unit A apparently grades upward into Unit B, a 45-m-thick interval of the sandstone facies. Unit C, 28 m thick, rests sharply on Unit B, and comprises a basal laminated dolomite facies and the limestone and argillite facies. Units A and B may indicate a regressive wave-dominated coast that was influenced by large storms (highstand systems tract). Unit C indicates near storm wave base deposition and an overall deepening, as shown by dark argillite beds of the overlying upper member of the Pocatello Formation (transgressive systems tract). δ13C and δ18O values from dolomite and limestone samples of Units A and C are similar to values from local, regional, and transglobal cap carbonate intervals. δ13C values range from −1.9 to −5.6‰ and δ18O values range from −10.2 to −17.4‰, with no systematic correlation between C- and O-isotope values. δ13C values are consistent with previously reported values from the Pocatello Formation and are similar to values from the alleged Marinoan Noonday Formation in Death Valley, California, and the Marinoan Maieberg Formation in Namibia. Collective data from the cap dolostone and associated strata of the Pocatello Formation suggest protracted mixed siliciclastic-carbonate deposition on a stormdominated shelf at ca. 665 Ma.

Abstract Geologic, geomorphic, and geophysical analyses of landforms, sediments, and geologic structures document the complex history of pluvial Lake Bonneville in northern Cache Valley, NE Great Basin, and shows that the outlet of Lake Bonneville shifted ~20 km south after the Bonneville flood. The Riverdale normal fault offsets Bonneville deposits, but not younger Provo deposits ~25 km southeast of Zenda, Idaho. Rapid changes in water level may have induced slip on the Riverdale fault shortly before, during, or after the Bonneville flood. Although other processes may have played a role, seismicity might have been the main cause of the Bonneville flood. The outlet of Lake Bonneville shifted south from Zenda first 11, then another 12 km, during the Provo occupation. The subsequent Holocene establishment of the drainage divide at Red Rock Pass, south of Zenda, resulted from an alluvial fan damming the north-sloping valley. Weak Neogene sediments formed sills for the three overflowing stages of the lake, including the pre-flood highstand. Field trip stops on flood-modified landslide deposits overlook two outflow channels, examine and discuss the conglomerate-bearing sedimentary deposits that formed the dam of Lake Bonne ville, sapping-related landforms, and the Holocene alluvial fan that produced the modern drainage divide at Red Rock Pass. The flood scoured ~25 km of Cache and Marsh Valleys, initiated modest-sized landslides, and cut a channel north of a new sill near Swan Lake. Lake Bonneville dropped ~100 m and stablilized south of this sill at the main, higher ~4775 ± 10 ft (1456 ± 3 m) Provo shoreline. Later Lake Bonneville briefly stabilized at a lower ~4745 ± 10 ft (1447 ± 3 m) Provo sill, near Clifton, Idaho, 12 km farther south. An abandoned meandering riverbed in Round Valley, Idaho, shows major flow of the large Bonneville River northward from the Clifton sill. Field trip stops at both sills and overlooking the meander belt examine some of the field evidence for these shorelines and sills. The Bear River, which enters Cache Valley at the mouth of Oneida Narrows, 17 km ENE of the Clifton sill, was the main source of water in Lake Bonneville. It produced 3 sets of deltas in Cache Valley—a major delta during the Bonneville highstand, a larger composite delta during occupation of two Provo shorelines, and at least one smaller delta during recession from the Provo shoreline. The Bonneville delta and most of the Provo delta of the Bear River were subaqueous in Cache Valley, based on their topsets being lower than the coeval shorelines. The Bonneville delta is deeply dissected by closely spaced gullies that formed immediately after the Bonneville flood. The delta morphologies change sequentially from river-dominated to wave-dominated, then back to river-dominated. These unique shapes and the brief, intense erosion of the Bonneville delta record temporal changes in wave energy, erosion, vegetation, and/or storminess, at the end of the Pleistocene. Stops on a delta near Weston, Idaho, reveal many of the distinguishing features of the much larger deltas of the Bear River in a smaller, more concentrated form. We will see and discuss the ubiquitous gully erosion in Bonneville landforms, the nearly undissected Provo delta, the subaqueous topset of the Provo delta, and the wave-cut and wave-built benches and notches at the upper and lower Provo shorelines.

Abstract Neoproterozoic strata in southeastern Idaho and Utah include the <766 Ma Uinta Mountain Group and Big Cottonwood Formation (Fm.) deposited in an east-trending rift basin and, to the west, the lower part of a westward-thickening rift to passive-margin succession that initiated c. 720 Ma. The latter contains a lower diamictite and volcanic succession, with a complex stratigraphic interval of Cryogenian marine glacial deposits (Pocatello and Mineral Fork formations and correlatives). This is overlain by a mostly terrigenous succession of <667 Ma strata assigned to the upper member of the Pocatello Fm. and Brigham Group in southeastern Idaho, to the Kelley Canyon Fm. and Brigham Group in northern and western Utah, and to the McCoy Creek Group and Prospect Mountain Quartzite in adjacent Nevada. Although the Brigham Group and correlative deposits contain no direct evidence for glaciation, widely developed, though stratigraphically restricted, incised valleys, with erosional relief from a few metres to as much as 160 m, are inferred to represent subsequent times of Cryogenian glacially lowered sea level. Overall interpretations of the stratigraphy and sedimentology of these rocks have changed little in the past 10–15 years. The most important recent advances relate to U–Pb geochronology. In strata that lie unconformably below demonstrable glacial deposits, the lower Uinta Mountain Group (formerly thought to be c. 900 Ma) contains populations of detrital zircons as young as 766±5 Ma. Cryogenian magmatism north of the Snake River Plain in central Idaho is recognized near House Mountain, east of Boise at c. 725±5 Ma, in the Pioneer Mountains Core Complex at about 695 Ma, and in central and east-central Idaho at 685–650 Ma. Clasts interpreted to be from the rift-related Bannock Volcanic Member of the Pocatello Fm. are dated at 717±4 Ma and 701±4 Ma. The overlying diamictite-bearing Scout Mountain Member contains a mafic lapilli tuff near the base (686±4 Ma) and a reworked fallout tuff near the top (667±5 Ma). Strongly negative C-isotope data have been obtained from some of the carbonate rocks, although the latter constitute only a small fraction of the succession. Palaeomagnetic data are available only for the Uinta Mountain Group, and suggest an equatorial palaeolatitude.

Abstract This one-day field trip examines two Neoproterozoic sections in the Portneuf Narrows area, SE of Pocatello, Idaho. Rocks to be examined on the first traverse belong to the Scout Mountain Member, Pocatello Formation, and include <710 Ma glacial diamictites, dolomite cap, 667 Ma tuff, and upper caplike carbonate. The second traverse, in Blackrock Canyon, exposes the cyclic Blackrock Canyon Limestone, with upward-shallowing siliciclastic to carbonate cycles and microbial mounds.

The Putnam thrust has long been recognized as an important Mesozoic structure in the northern Portneuf Range, southeastern Idaho. At most localities, the thrust places Ordovician rocks above Permian and Pennsylvanian rocks, although near its southeastern extent, it ramps laterally downsection to the southeast. At its southeasternmost exposures, Cambrian rocks are juxtaposed above Mississippian rocks. New work indicates that the hanging wall of the Putnam thrust contains three imbricate thrust slices or subplates, which are, from structurally lowest to highest (and generally from north to south), the Lone Pine subplate, the Narrows subplate, and the Bear Canyon-Toponce subplate. The steeply south-dipping, east-trending Narrows thrust overlies the Lone Pine subplate, underlies the Narrows subplate, and is a lateral ramp that merges eastward into the Putnam thrust. Where exposed, the Narrows thrust places Late Proterozoic quartzite of the Brigham Group over Ordovician and Cambrian rocks. The Bear Canyon thrust overlies the Narrows subplate and underlies the Bear Canyon-Toponce subplate, dips eastward along the west side of the Portneuf Range, and places lower Brigham Group quartzite above Cambrian limestone and Cambrian and Late Proterozoic upper Brigham Group quartzite and argillite. At its northern extent, the Bear Canyon thrust curves to the east, where it merges with the Putnam thrust. On the east side of the range, the intensely folded Toponce thrust places upper Brigham Group quartzite above Ordovician rocks; the Toponce is believed to be an eastward extension of the Bear Canyon thrust. East-dipping rocks within the Lone Pine subplate were not strongly deformed during Cretaceous thrusting, in contrast to rocks within the Narrows subplate, where east-vergent recumbent folds, cleavage directions that fan about northerly strikes, and tectonic thickening and thinning of beds indicate intense, thrust-parallel shear. The deformation and thrust geometry within the Narrows subplate suggest that the Narrows subplate actually consists of several horses within a foreland-dipping duplex. Late Miocene and younger basin deposits occur in north-trending valleys adjacent t o the northern Portneuf Range and, to the west, the Bannock and Pocatello ranges. At most places, the Neogene deposits dip to the east by as much as 35°, indicating that late Miocene and younger extension and down-to-the-east rotation occurred along mostly west-dipping listric faults that are inferred to merge on at least one regional detachment. Although range-bounding faults account for a large component of extension and rotation, an additional large component was contributed by numerous, relatively small-displacement normal faults within mountain ranges.

Three segments of the Grand Valley fault (from south to north the Star Valley, Grand Valley, and Swan Valley segments) in southeastern Idaho and western Wyoming were identified by combining evidence from (1) the location and extent of fault scarps on late Quaternary alluvial fans and an early Quaternary basalt flow, (2) the height of the footwall escarpment, (3) the stratigraphy of deposits on the hanging wall adjacent to the fault, and (4) the depth of dissection of these hanging-wall deposits. Multiple latest Quaternary (≤15 ka) surface ruptures are displayed on the southernmost Star Valley segment by fault scarps of different heights preserved on alluvial fans with ages estimated between 11 and 15 ka. Assuming that these scarps were formed by two to four surface ruptures, the inferred average return period for large-magnitude paleoearth-quakes on this segment of the fault is 2,500 to 7,500 years. A latest Quaternary displacement rate of 0.6 mm/yr to 1.1 mm/yr is also inferred from the fault scarps. The lack of fault scarps on surfaces of similar age or older elsewhere along the Grand Valley fault readily distinguishes this segment from the other two. Height of the footwall escarpment, linearity of the range front, and shallow incision by the Salt River into deposits on the hanging wall suggest that a relatively high displacement rate persisted on the Star Valley segment throughout the Quaternary (≤2Ma). The lack of fault scarps on undated surfaces of colluvial, alluvial, and eolian deposits along the trace of the Grand Valley fault or on a fluvial terrace with an estimated age of 15 to 30 ka that crosses the fault suggests that no latest Quaternary surface ruptures have occurred on the central, Grand Valley segment. Marked erosion of the footwall escarpment and deep incision by the Snake River and its tributaries into lower Quaternary and older deposits on the hanging wall contrast with the youthful morphology along the Star Valley segment. Lack of tectonic tilt of sediments that could be as old as 2 Ma suggests that no major Quaternary displacement has occurred on this segment. An average displacement rate on the northernmost Swan Valley segment of only 0.019 mm/yr is estimated from an inferred fault scarp on a basalt flow isotopically dated at 1.5 Ma. No other fault scarps were observed along this segment of the fault, but the only deposits that directly overlie the fault are loess deposits that could be as young as 11 to 30 ka. Assuming single-event surface ruptures of 2 to 6 m, the average return period for large-magnitude paleoearthquakes during the Quaternary is ≥100,000 years. Tectonic tilt of late Tertiary and Quaternary volcanic units in Swan Valley indicates that the displacement rate on the Swan Valley segment has decreased since 2 Ma from a maximum rate of about 1.8 mm/yr between 2 and 4.3 Ma. This change in displacement rate on the Swan Valley segment of the Grand Valley fault, along with the timing of high displacement rates on other late Cenozoic normal faults in the region, suggests that displacement on faults immediately south of the eastern Snake River Plain is probably related to the locations of calderas on the eastern Snake River Plain. These data suggest that large-magnitude paleoearthquakes were common on the Swan Valley segment of the Grand Valley fault between about 2 and 4.3 Ma but have been infrequent since about 2 Ma. The differences in displacement rates combined with inferred differences in the average return periods between large-magnitude paleoearthquakes during intervals of the Quaternary and the contrast in the age of the most-recent surface displacements for the various fault segments are potentially important considerations in assessing the seismic hazard posed by the Grand Valley fault.

Abstract The Menan Buttes are located at the Madison County-Jefferson County border in southeast Idaho, 56 mi (90 km) southwest of Yellowstone National Park and 9.8 mi (15.7 km) west-southwest of Rexburg, Idaho (Fig. 1). From Rexburg, take Idaho 33 west and turn south at an improved light-duty road that is 2.6 mi (4.3 km) past the bridge that crosses the Henry’s Fork (Fig. 1). North Menan Butte lies 1.9 mi (3 km) south of Idaho 33. From Idaho Falls, take U.S. 191/20 northeast to the town of Lorenzo; turn left at Idaho 80 and go past the town of Annis and a double bend in Idaho 80 where the road passes up and over two small volcanic cone remnants (Fig. 1). After passing the remnants, turn north at the second paved road that extends northward. North Menan Butte lies 3.9 mi (6.2 km) north of Idaho 80 on this road. As shown on Figure 1, paved light-duty and unimproved dirt roads encircle North Menan Butte and allow easy access for two-wheel drive vehicles. North Menan Butte is owned by the U.S. Bureau of Land Management and is open to the public, whereas South Menan Butte is privately owned, and permission is required from David South to gain entry. Since the two buttes are similar, it is advisable to study the one with public access rather than asking for access to the privately owned butte.

Abstract Portneuf Narrows is located on the Inkom, Idaho, 1:24,000 Topographic Quadrangle. The area is ideal for a one-day field trip because it is accessible from 1–15 (Exit 63-Portneuf Area) and is only 3 mi (4.8 km) from Pocatello where tourist services and the Idaho State University Library are available. Stops 1 and 3 are on paved roads. Stop 2 is on a dirt road that can be driven by passenger cars in dry weather. The entire area is on public land. Stop 1 is an overview of Portneuf Narrows (Fig. 1A); it is reached by heading southeast from Pocatello on I–15 and taking exit 63S. Turn west (right) onto the frontage road (South Fifth Avenue), proceed 0.7 mi (1.1 km), cross under the freeway, and stop on the east side of the frontage road. If approaching from the south (Salt Lake City), take exit 63N, turn right at the frontage road, and proceed 0.2 mi (0.3 km) to Stop 1 on the left side of the road. The outcrops south of Portneuf Narrows (Stop 2) are reached by heading south on Fort Hall Mine Road from exit 63. Cross the Union Pacific Railroad and the Portneuf River and proceed 0.2 mi (0.3 km) to Portneuf Road. Cross it, and continue straight for 0.6 mi (l km). There, bear left onto dirt road (avoiding road to sanitary landfill) and park at locked gate in Fort Hall Mine Canyon (Stop 2, Fig. 1A). The most complete stratigraphic sections can been seen in the base of the second canyon

Abstract The area described herein is in southeastern Idaho, in Franklin and Bannock counties. It is readily reached via I–15 and various paved state highways from any direction. Each stop is easily accessible by two-wheel drive vehicles when roads are dry. However, the dirt roads leading to stops 1 to 6 can be extremely slick when soaked. Permission from property owners is required if you desire to wander beyond each stop. The tour is keyed to the following 7½-minute quadrangles: Pocatello North, Inkom, Arimo, McCammon, Bonneville Peak, Downey West, Downey East, Oxford, and Swan Lake. The latter two are particularly important for locating features south of the overflow site. The Preston and Pocatello 2-degree quadrangles are also suggested. The trip will take about six hours. It begins in northern Cache Valley at the hamlet of Oxford, proceeds northward through Red Rock Pass and Marsh Valley, and ends in the Pocatello area (Fig. 1). Food and lodging can be found in Preston (dry) and Pocatello.