IDAHO GEOLOGICAL SURVEY DIGITAL WEB MAP 145 MOSCOW-BOISE-POCATELLO IDAHOGEOLOGY.ORG PHILLIPS

GEOLOGIC MAP OF THE ST. ANTHONY QUADRANGLE, FREMONT AND MADISON COUNTIES, IDAHO Qb Basalt, undivided (Pleistocene)—Basalts encountered in water wells and CORRELATION OF MAP UNITS shown in the cross section. Basalts are generally described in drill logs as black or gray, hard, with horizons of brecciated or fractured material some- William M. Phillips Artificial Unit Alluvial Units Eolian Units Volcanic Units times containing silt, clay, sand, and/or cinders. Many wells encounter more than one flow, as indicated by interbeds of gravel and sand. Total basalt thickness between unconsolidated surficial deposits and Huckle- 2012 Deposits of Teton Dam Flood Basalt Flows Rhyolite Tuff and Flows berry Ridge Tuff (Qyh) ranges from <33 to >90 m (<100 to >300 ft), with m Qtfg Qtfs Qtff HOLOCENE thickness generally increasing to the northwest. Source of individual flows Qa Qas Qes A is not discernible from water well information. Based on regional patterns, 11.7 ka most flows probably originated from the north (Spencer-High Point volca- Qt Qt 11P22 Qblc 2 1 nic rift zone), from the northeast (Ashton-Fall River area), or from the south- late Qes east (Rexburg Bench volcanic rift zone). Qes/Qbmp Qbmp PLEISTOCENE Qgh Qel

Qes/Qbmp Qao 378448 Qbmp Qes/Qbmp 120 ka Qte Rhyolite Welded Tuff Qbc QUATERNARY Mackerts middle Qyh Huckleberry Ridge Tuff (Early Pleistocene)—Welded rhyolitic ash-flow tuff Pond Qes/Qbmp erupted from the Henrys Fork Caldera of the Yellowstone Plateau Qb PLEISTOCENE Qbm (Christiansen, 2001). Not exposed in quadrangle; encountered in water

Qes/Qbmp 781 ka wells along eastern portion of map and also in a geothermal test well near Sugar City (Embree and others, 1978). Exposed in adjacent Newdale and Qbmp Qbsa early Qes 11P20 Qbsp Moody quadrangles. Drill logs typically report the unit as a relatively soft, PLEISTOCENE Qyh white, reddish, or gray rhyolite (the rock is also described as a “tuft,” Qes Qes “shale”, or “sandstone”). Fractures causing loss of circulation of drilling Qes 2.588 Ma fluids are frequently mentioned. Correlated with Unit B of the Huckleberry BP82311-9 Qes/Qbmp Qgh PLIOCENE Ridge Tuff (Christiansen, 2001, p. G58, Fig. 33). Total-fusion and Qte Qbmp incremental-heating ages of sanidine are 2.059 ± 0.004 Ma (Lanphere and 5.0 Ma TERTIARY Qbsp Thv others, 2002). late Qes/Qbmp Qes/Qbmp MIOCENE Thv Heise volcanic rocks (Pliocene-Late Miocene)—Welded rhyolite ash-flow tuffs Qte and pyroclastic deposits erupted from Heise nested calderas between Qes Qa Qa ~6.6-4.5 Ma (Morgan and McIntosh, 2005; Watts and others, 2011). Also Qes Qes includes tuffaceous sedimentary rocks and post-Heise basalts and rhyolites Qas Qte of Pliocene age. Not exposed at surface. Postulated to occur in the subsur- Qes face of the quadrangle based on Sugar City geothermal exploration well Qes Qa (Embree and others, 1978), Newdale area geothermal exploration wells Qa Qa (Phillips, 2010), and Pliocene units present in the adjacent Juniper Buttes INTRODUCTION area (Kuntz, 1979). Qgh Qa Qa Qa This map depicts bedrock and surficial geology of the St. Anthony quad- TERRACE DEPOSITS OF HENRYS FORK Qgh GEOLOGIC HISTORY Qte Qbsa? rangle. Exposure of bedrock is largely limited to basalt lava flows along the Qt1 Alluvium of first terrace above the modern flood plain (Holocene-Late Henrys Fork. One of these lava flows (map unit Qbsa) is responsible for the Pleistocene)—Pebble to cobble gravel and sand similar to unit Qa. Qbsa? Qte scenic waterfalls within the city limits of St. Anthony. Surficial deposits Separated from unit Qa by a scarp about 4-6 m (13-20 ft) high. Best devel- The oldest rocks in the study area are late Miocene to Pliocene (~6.6-4.5 376781 Qa include gravel and sand, which are widely mined for aggregate. The St. oped downstream from St. Anthony falls. Ma) rhyolitic welded tuffs, pyroclastic deposits, and tuffaceous sediments of Qa Anthony area is known as a hub for recreational activities on the Henrys the Heise volcanic field (Morgan and McIntosh, 2005; Watts and others, Qt Qa Qa Fork and within the St. Anthony dune field (located to northwest of the 2 Alluvium of second terrace above the modern flood plain (Holocene-Late 2011). Although not exposed on the surface, these caldera deposits likely map), and for the damage received during the 1976 Teton Dam failure. Pleistocene)—Pebble to cobble gravel and sand similar to unit Qa. Present underlie the quadrangle as shown by geothermal exploration wells near Qel/Qbsp only on the north side of Henrys Fork and downstream from St. Anthony Sugar City (Embree and others, 1978) and near Newdale (Phillips, 2010). In 11P02 Qas falls. Separated from the Egin Bench surface (Qte) and Qt by scarp 1.5-3 m addition, post-Heise basalts and rhyolites of Pliocene-early Pleistocene age Qt TETON DAM DISASTER 1 Qbsa Qa 1 Qbsp (5-10 ft) high. Qbsa are exposed at Juniper Buttes (Kuntz, 1979) and also probably underlie at Qbsa least northern portions of the quadrangle. The Huckleberry Ridge Tuff was Qgh Glacial outwash deposits of the Henrys Fork (Late Pleistocene)—Cobble gravel Qt2 Qt1 m The Teton Dam was constructed on the adjacent Newdale quadrangle. erupted from the Yellowstone Plateau at ~2.06 Ma (Christiansen, 2001). Qa Qa Qbsa Qt1 On June 5, 1976, the Teton Dam failed catastrophically, killing 14 people composed of well-rounded quartzite, rhyolite, and basalt clasts. Bedding is This distinctive rhyolitic welded tuff is encountered in many shallow water and causing $400 million to $1 billion dollars (1976 dollars) in flood thickly planar, separated locally by thin, cross-bedded sand beds. Sand is wells along the east side of the St. Anthony quadrangle, and is exposed in Qa medium grained, composed of subangular obsidian, quartz and feldspar Qbsa Qa damage. The causes of the disaster have been extensively studied (e.g. Seed the adjacent Newdale and Moody quadrangles. Qa and Duncan, 1987) and show that poor understanding of the geologic crystals, and rock fragments of basalt and rhyolite. Sand is typically black Qte 382416 because of high obsidian content. Thickness uncertain because older units Post-Huckleberry volcanic units consist of more than 90 m (300 ft) of basalt Qa Qbsp conditions at the dam site contributed directly to the disaster. Geologic features of the dam site and former reservoir are shown in Embree and cannot be reliably separated in water well logs. Thickness of gravel above lava flows that were erupted from vents outside of the quadrangle. The basalt ranges from about 5 to 23 m (15 to 75 ft). Gravel thickness increases m others (2011) and Embree and Phillips (2011). oldest basalts exposed on the surface have reversed magnetic polarities, 408464 toward southwest to >27 m (>90 ft), and is 36 m (118 ft) in the Sugar City hence are probably older than 781 ka. The youngest lava flows, probably Flood waters from the breached Teton Dam flowed through the southern geothermal exploration well (Embree and others, 1978). Unit is extensively late Pleistocene (<120 ka) in age, were erupted from an unknown vent in portion of the St. Anthony quadrangle. The following description of flood mined for sand and gravel in the quadrangle. These sediments are part of the Spencer-High Point volcanic rift north of the quadrangle (Kuntz, 1979). Qa Qa effects and deposits is taken from Scott (1977). the regional braided-stream outwash plain deposited during the Pinedale These flows have normal polarities and little sedimentary cover save for Qa Qbsa Qgh Qt2 glaciation by meltwaters from the Henrys Fork headwaters in Island Park Holocene sand dunes. Qt1 Qel/Qbsp Peak discharge of the flood was more than 100 times greater than the and along the western edge of the Yellowstone Plateau (Scott, 1982). Qa largest flood for the Henrys Fork at St. Anthony from 1890 to 1970. Sedimentary deposits consist largely of gravel and sand derived from the Qas Undated in the quadrangle. Optically stimulated luminescence ages in Geologic effects of the dam break provide a perspective for comparing correlative deposits of the Snake River in the Idaho Falls area are between Island Park-Yellowstone Plateau-Grand Teton region. During at least two Qt1 Qbsa Qa Qbsa Qa exceptionally large paleofloods produced by failure of natural dams. 25.2 and 12.6 ka (Phillips and others, 2009). Pleistocene glacial periods, the Yellowstone Plateau and Grand Tetons Qt2 Qa Qao contained a large ice sheet and numerous smaller valley glaciers (Licciardi Qa 11P23 Flood waters moving out of the Teton River canyon into the quadrangle Qbm Older alluvium of Teton River (Late Pleistocene-Middle Pleistocene)—Loess, gravel, and Pierce, 2008). Outwash from these glaciers delivered enormous Qbm extended westward 5 km (3 mi) to the towns of Teton and Wilford. The sand, and clay overlying basalt of Moody Creek (shown as Qao/Qbm on volumes of gravel and sand to the Snake River Plain via the Henrys Fork and Qa leading edge of the flood was a 5 m (16 ft) wall of water. Flood waters also the map) adjacent to the Teton River; very poorly exposed in the quad- Teton River. The best documented of the outwash deposits is map unit Qgh, Qbsa moved north along the eastern boundary of the quadrangle, then poured rangle. Water well logs indicate about 5-14 m (16-45 ft) of sediment is which dates from the Pinedale glaciation (~25-14 ka). Egin Bench (map unit Qa westward over loess- and sediment-covered basalts. Locally, loess was present. Soil maps show at least 1.5 m (5 ft) of loess capping the unit. East Qte) is likely an older outwash deposit, dating perhaps from the Bull Lake removed or eroded to basalt or to a buried carbonate-silica duripan. Flood 382056 of Wilford, an unusual triangular geomorphic surface underlain by Qao glaciation (~160-140 ka). Loess was also deposited throughout the region Qt1 waters were shallow over the triangular-shaped geomorphic surface Qgh over Qbm slopes smoothly downstream (westward). A 6 m (20 ft) scarp during Pleistocene glacial periods. The source of the loess was outwash Qtff (Qoa/Qbm) east of Wilford. Deep flow along the sides of the feature Qt1 Qas separates the surface from adjacent Qgh and Qa surfaces. The surface is deposits along the Snake River. Most surface loess deposits in the upper Qtfg Qtfg scoured the scarp, primarily on the north side. Merging of the two streams separated from loess-covered basalt of Chester (Qbc) by an irregular 6-9 m Snake River Plain are Pinedale age (Phillips and others, 2009), although of flood waters occurred in Wilford, where 110 of 154 houses were swept Qao Qa Qas Qel/Qbsp (20-30 ft) scarp. East of Teton, the surface is hummocky. Scarps ranging some extend back to older glaciations (Pierce and others, 2011). During the away, and most remaining houses damaged. from 3 to 9 m (10 to 30 ft) separate Qao from adjacent Qa surfaces, and a Holocene, sand was deposited in the St. Anthony area. The mapped area Qtfg sinuous former channel of the Teton River (presently occupied by an irriga- lies just outside the famous St. Anthony dune field where deposits have a Qa Qgh Deposits of gravel, sand, and fine sand/silt are shown on the map. Many Qtfg tion canal) separate Qao from loess-covered Qbm. Mapped by Scott (1982) complicated Holocene history linked to cycles of climate change (Gaylord deposits are difficult to see today because of agricultural practices and Qa Qgh Qgh as “older alluvium of mainstreams.” Age uncertain. Geomorphic relation- and others, 2000). Qtff repair of roads and other infrastructure. Qgh ships and loess cover suggest unit is younger than Qbc and older than Qgh. Channel changes of the Henrys Fork and Teton River are another feature of Qa Qel/Qbc Qte Alluvium of Egin Bench (Late Pleistocene-Middle Pleistocene)—Sandy pebble the mapped area. The Henrys Fork is contained within an incised flood Qt1 Qtfg SOURCES OF MAP INFORMATION gravel and pebbly sand, 4.6-7.6 m (15-25 ft) thick. Gravel consists of plain for most of its passage through the quadrangle. Near Twin Groves, the subrounded to rounded quartzite, obsidian, welded tuff, and basalt. Sand is river is within about 3 m (10 ft) of the Qgh surface. This permitted the river, Qtff Qtff rich in obsidian, with lesser quartz and feldspar. Generally overlain by 1-3 at some point during the Holocene in an exceptionally high flood, to flow Geologic maps by Kuntz (1979), Mitchell and Bennett (1979), Proska and m (3-10 ft) of sand. Best exposures of the unit are in the adjacent Parker southward and produce a meandering channel that probably linked to the Qa Embree (1978), and Scott (1982) were consulted. Location and descriptions quadrangle along the erosional scarp between units Qte and Qt1 (Kuntz, Teton River. This event is indicated by relic channels and poorly drained of surface deposits and extent of flooding from the 1976 Teton Dam failure 1979). In the St. Anthony quadrangle, Egin Bench appears to lie beneath alluvial deposits and soils between Twin Groves and Wilford. The Teton Qgh Qtfg 20 are taken from Scott (1977) and Thomas and others (1976). Field work was basalt of Mackerts Pond (Qbmp) and Holocene eolian deposits (Qes). The River also has a history of shifting channels. Numerous relic channels are Qtfg conducted in 2011 and 2012. Paleomagnetic and geochemical analyses Egin Bench surface is traceable at least 30 km (19 mi) southwest to north present north of the North Fork. Early topographic maps of the region made were used to correlate basalt flows within the mapped area. Domestic Menan Butte where it disappears beneath basalt of probable late Pleisto- around 1875 show a network of channels along the North Fork, bifurcating Qa Qtfg water well logs were used to gain understanding of subsurface units. The Qgh Qtff Qbc Qtfs cene age (Phillips and Welhan, 2011). Age and origin are uncertain. Coarse downstream (Hayden, 1877). Qas logs were downloaded from the Idaho Department of Water Resources grain size suggests origin as glacial outwash or glacial outburst flooding 387550 Qgh Qel/Qbc (Idaho Department of Water Resources, 2012) and checked for location derived from the Island Park-Yellowstone Plateau area, perhaps during errors, with preference given to logs with GPS locations. Marine Isotope Stage 4 (circa 75-64 ka) or Bull Lake (circa 160-140 ka) STRUCTURE Qt1 Qbc glacial periods. Allison (2001) proposed that massive obsidian-rich gravel and sand in the Market Lake area south of Menan Buttes correlates with Egin Qtff Qbc Bench and was caused by outburst flooding late in the Bull Lake glaciation. Qtfg The eastern Snake River Plain subsided following termination of Heise Qtfg caldera volcanism after about 4.5 Ma. This subsidence was accomplished Qgh Qao/Qbm without recognized faulting. The cross section shows a swell-like structure SYMBOLS EOLIAN DEPOSITS along the contact between Qyh and Qb units. This feature probably reflects Qas Qao/Qbm Qtfg topographic relief on Qyh rather than folding. In the adjacent Newdale and Qtfg Qtfg Qtfg Qbm Contact: dashed where approximately located; dotted where Qes Qgh Sand Dunes (Holocene-Late Pleistocene)—Medium sand, well sorted, Linderman Dam quadrangles (Embree and others, 2011; Embree and Qtff concealed. composed of obsidian and quartz with lesser feldspar and mafic minerals. Qtfg Qtfs Phillips, 2011), relief on Qyh developed in response to subsidence and by Qtfg Consists of active to stabilized dunes draped over hummocky basalt lava Qtff Boundary of area inundated by floodwater from Teton Dam Flood rheomorphic deformation during emplacement. flows. Some deposits are black because of high obsidian content. Depos- Qtfs 327358 of June 1976. Hachures on side of inundated area. ited by southwesterly winds, mostly during the middle to late Holocene, Qtff Qtff Qao/Qbm Asymmetric ripples in deposits of the Teton Dam failure; ripples from sources near present day Market Lake. Deposits are on the edge of the Qtff Qtfg St. Anthony dune field (Gaylord and others, 2000; Kuntz, 1979). Qa Qel/Qbc are 1 to 10 ft thick and 6.5 to 165 ft long. The larger ripples Qa Qtff Qtfg occur on bars in the Teton River canyon and at the canyon Qa Qel Loess (Late Pleistocene-Middle Pleistocene)—Massive, light gray to light Qtff Qtfg mouth; smaller ripples occur in downstream areas. Qbc brownish gray silt, clay, and very fine sand; calcareous except where REFERENCES Qtfg 395898 Area of erosion caused by floodwaters of the Teton Dam failure. leached; carbonate-silica duripan locally present. Thickness is <1.5-3 m Qtfg Qtff Qtff Qtfg Qtff Qtfs (<5-10 ft) over units Qbm, Qbsp, and Qbc (shown as Qel/Qbm, Qel/Qbsp, Qtfg Qa Allison, R.R., 2001, Climatic, volcanic, and tectonic influences on late Pleistocene Qtff Qa Qel/Qbc). Thin loess (not mapped) also covers parts of Qao and Qgh. Qtff Qgh Qtfg Qtff Qa Qtfs 327642 Water well. sedimentation along the Snake River and in Market Lake: Bonneville, Jefferson, Qtff Qtff Qa Derived from deflation of fine-grained sediment from outwash deposits and Madison counties, Idaho: Idaho State University M.S. thesis, 153 p. 11P20 Paleomagnetic sample location (see Table 1). along the Henrys Fork and Snake River by southwesterly winds during Qa Christiansen, R.L., 2001, The Quaternary and Pliocene Yellowstone Plateau Qtfg glaciation of the Snake River headwaters. Several depositional units of loess Qbm volcanic field of Wyoming, Idaho, and Montana: U.S. Geological Survey- Qtff Qgh BP82311-9 Geochemical sample location (see Table 2). separated by buried soils are present in the eastern Snake River Plain (Pierce Qtfg Professional Paper 729-G, 145 p., 3 plates. Qtfg and others, 1982; Scott, 1982). Ages derived from luminescence dating Qtfg Embree, G.F., M.D. Lowell, and D.J. Doherty, 1978, Drilling data from the Qtfg Qa Qa indicate loess deposition occurred between 25-15 ka, 46-36 ka, ~79-68 ka, Qao/Qbm Sugar City Exploration Well, Madison County, Idaho: U.S. Geological Qtff Qtff and ~140-130 ka (Pierce and others, 2011; Phillips and others, 2009). Qtfg Survey Open-File Report 78-1025, 1 plate. Qtfg Embree, G.F., and W.M. Phillips, 2011, Geologic map of the Linderman Dam Qtff Qa quadrangle, Fremont, Madison, and Teton counties, Idaho: Idaho Geologi- Qtff Qtfg VOLCANIC ROCKS cal Survey Digital Web Map 133, scale 1:24,000. Qa Qa Qa DESCRIPTION OF MAP UNITS Basalt Lava Flows Embree, G. F, W. M. Phillips, and J. A. Welhan, 2011, Geologic map of the Qtfg 382721 Newdale quadrangle, Fremont and Madison counties, Idaho: Idaho Qtfg Qtff Qes Basalt of Mackerts Pond (Late Pleistocene)—Dark gray, porphyritic to nonpor- Geological Survey Digital Web Map 122, scale 1:24,000. Qtfg Qbm Qbmp MADE DEPOSITS phyritic flows erupted from unknown vent about 35 km (22 mi) north of the Gaylord, D.R., J.J. Coughlin, A.J. Coleman, M.R. Sweeney, R.H. Rutford, and Qa Qbmp others, 2000, Holocene sand dune activity and paleoclimates from Sand Qa Qtfg Qtfg map in the Spencer-High Point volcanic rift zone. Contains 2 mm olivine Qtff Qtff Qa m Gravel and sand (Holocene)—Artificial fill beneath US Highway 20 where it and plagioclase phenocrysts in a diktytaxitic groundmass of <0.5 mm Creek, St. Anthony dune field, Idaho [abs.]: Geological Society of America Qel/Qbm crosses Qa along the Henrys Fork. plagioclase, olivine, and augite (Kuntz, 1979). Flow lobes of Qbmp appear Abstracts with Programs, v. 32, p. 5. Qtff Qao/Qbm Qgh Qtfg to have advanced over the Egin Bench surface in St. Anthony and adjacent Hayden, F.V., 1877, Ninth annual report of the United States Geological and Qtff Parker quadrangles. Sand dune and sheet deposits cover much of the flows, Geographical Survey of the Territories, embracing Colorado and parts of adjacent Territories, being a report of progress of the exploration for the Qtfg DEPOSITS OF THE TETON DAM FAILURE OF JUNE 5, 1976 especially at flow edges, but are generally too thin to obscure flow Qa Qa (AFTER SCOTT, 1977) morphology. Loess does not appear to be present in significant quantities. year 1875: U.S. Government Printing Office, Washington, DC, 827 p. Qa Qtff Qa Idaho Department of Water Resources, 2012, All Permitted Wells, in GIS Data, Qtff Qtfg Paleomagnetic and geochemical analyses (Tables 1 and 2) show that two Qtfg Qtfg Qtfg Gravel (Holocene)—Mixed pebbly gravel and sand to cobble gravel; clasts flows were sampled. Samples 11P20 and 11P21 are from the same flow, Maps and Spatial Data, Idaho Department of Water Resources. Available Qa Qa Qa rounded to subrounded; derived from the fill of the Teton Dam, gravel along and 11P22 is from another flow. The flows have not been dated. Absence of online at http://www.idwr.idaho.gov/GeographicInfo/GISdata/wells.htm. Qtfg the Teton River, and from gravel in road fills, irrigation canals, and terrace significant loess cover, preservation of primary surface features, and normal Accessed 2/14/2011. Qa Qtfg Qa 357372 scarps. Deposits are as thick as 1 m (3.3 ft) and are locally covered by <10 polarity suggest a late Pleistocene (<120 ka) age. Kuntz, M.A., 1979, Geologic map of the Juniper Buttes area, eastern Snake Qtff Qtfg cm (4 in) of fine silt and sand. Gravel composed of a variety of igneous and River Plain, Idaho: U.S. Geological Survey Miscellaneous Investigations Qa 394875 Qel Qa sedimentary rocks. Surface of deposits locally display asymmetric ripples. Basalt of Chester (Middle Pleistocene)—Medium gray, fine-grained basalt with Series Map I-1115, scale 1:48,000. Qbc sparse phenocrysts of plagioclase. Shown as Qel/Qbc where overlain by Lanphere, M.A., D.E. Champion, R.L. Christiansen, G.A. Izett, and J.D. Obra- Qtff Qbc 327642 Qtfs Sand (Holocene)—Sand and pebbly sand forming sheet-like deposits. Derived loess. Partially fills a north-south-trending paleovalley eroded along the dovich, 2002, Revised ages for tuffs of the Yellowstone Plateau volcanic Qtff in part from scour of road fills, irrigation canals, and terrace scarps. Depos- contact between the Huckleberry Ridge Tuff and unit Qbsp. Vent area field-Assignment of the Huckleberry Ridge Tuff to a new geomagnetic Qtfg its are <1 m (3.3 ft) thick. uncertain but probably north of Falls River in the adjacent Lemon Lake or polarity event: Geological Society of America Bulletin, v. 114, p. 559-568. Qtfg Qtfg Qa Qbm Qtff Ashton quadrangles. Normal magnetic polarity (Embree and Phillips, 2011) Licciardi, J.M., and K.L. Pierce, 2008, Cosmogenic exposure-age chronologies Qtfg Qtfg Qbm BP82311-1 Qtff Fine sand and silt (Holocene)—Fine sand, silt, and clay on flood plains, depres- and trace element concentrations (Table 2) clearly distinguish it from Qbm, of Pinedale and Bull Lake glaciations in greater Yellowstone and the Teton Qa sions, and in fields surrounded by dikes, roads or irrigation canals. Located 40 39 Qtfg Qbsa, and Qbsp. Dated by Ar/ Ar at 256 ± 44 ka in SE¼, sec. 2, T. 7 N., Range, USA: Quaternary Science Reviews, v. 27, p. 814-831. where slack water ponded during flooding. Mapped where thicker than 5 R. 41 E. of the Newdale quadrangle (oral commun., D. Champion, 2010). Mitchell, V.E., and E.H. Bennett, 1979, Geologic map of the Driggs quadrangle, Qtfg Qtfg Qtfg Qao/Qbm cm (2 in). Sediment sources were compacted loess core of the Teton Idaho: Idaho Bureau of Mines and Geology Geologic Map 6, scale 1:250,000. Qa Dam, eolian sand, fine-grained artificial fills, and the fine fraction of Qel Basalt of Moody Creek (Middle Pleistocene-Early Pleistocene)—Medium gray, Qa 327938 Qbm Morgan, L.A., and W.C. McIntosh, 2005, Timing and development of the Heise Qtfg gravelly embankments. fine-grained, locally diktytaxitic basalt with sparse phenocrysts of plagio- Qtff Qbm volcanic field, Snake River Plain, Idaho, western USA: Geological Society Qa clase as much as 5 cm in length and 7-10 percent fine-grained olivine. Qtfg Qel/Qbm Qel/Qbm of America Bulletin, v. 117, no. 3/4, p. 288-306. Qa Qtff Qel/Qbm ALLUVIAL DEPOSITS Shown as Qel/Qbm where overlain by loess. Shown as Qao/Qbm where Pierce, K.L., M.A. Fosberg, W.E. Scott, G.C. Lewis, and S.M. Colman, 1982, overlain by older alluvium. Poorly exposed; best exposures are in adjacent Loess deposits of southeastern Idaho – Age and correlation of the upper two Qa Alluvium of Henrys Fork and Teton River (Holocene)—Along the Henrys Fork, Moody and Newdale quadrangles, and in drill core from the Sugar City generally thin (<3m to 5 m; <10 ft to 16 ft) sandy gravel composed of loess units, in Bill Bonnichsen and R.M. Breckenridge, eds., Cenozoic A' geothermal test well (Embree and others, 1978). Paleomagnetic and Geology of Idaho, p. 717-725. Base Map Field work conducted 2010. quartzite, welded tuff, basalt, and obsidian-rich sand. Grain size of gravel geochemical properties make this flow one of the most distinctive in the ranges from cobble to granules; rare boulders of basalt locally present. Pierce, K. L., D.R. Muhs, M.A. Fosberg, S. A. Mahan, J.G. Rosenbaum, J. M. BLACK LEMON Base digitally scanned from 24,000-scale USGS film MN KNOLL LAKE This geologic map was funded in part by the U.S. Geological region. The Moody Creek basalt has reverse polarity with a steep inclination SCALE 1:24,000 BIG GRASSYRIDGE SE Licciardi, and M. J. Pavich, 2011, A loess-paleosol record of climate and separates, 1948. Survey National Cooperative Geologic Mapping Program, Humic silt deposits present in some sloughs. Deposits are contained within 1 0.5 0 1 of ~70° (Embree and others, 2011) and relatively elevated TiO2, Zr, and Nb glacial history over the past two glacial-interglacial cycles (~150ka), southern GN USGS Award No. G11AC20207. the incised flood plain and form a series of low islands, terraces, and point Shaded elevation from 10 m DEM. MILE (Table 2). Thickness ranges between about 18 to 43 m (60 to 140 ft). Vent Jackson Hole, Wyoming: Quaternary Research v. 76, p. 119-141. 0o 29 16.5o FEET Digital cartography by Loudon R. Stanford and Collette bars. Near St. Anthony, deposits lie directly on basalt exposed during low Topography by planetable methods from aerial location uncertain but probably lies in the Moody quadrangle south of the Phillips, W.M., 2010, Well logs for the Union Oil UNST-7 and UNST-8 1000 0 1000 2000 3000 4000 5000 6000 7000 PARKER NEWDALE photographs taken 1946. ST. ANTHONY Gantenbein at the Idaho Geological Survey’s Digital flows. Unit thickens downstream from St. Anthony. Unit is very poorly map. A 25 m (82 ft) section of three flows, each with well-developed Mapping Lab. drained, frequently flooded and ponded, with saturation at the surface geothermal test wells, Newdale 7.5-minute quadrangle, Idaho: Idaho Projection: Idaho coordinate system, east zone KILOMETER pillows as much as 0.5 m in diameter at the base, is exposed on the north- Geological Survey Staff Report 10-9. 1 0.5 0 1 Reviewed by John D. Kauffman and Mel Kuntz. during May-October. Along the Teton River, unit consists of sandy pebble (Transverse Mercator). 1927 North American UTM Grid and ern wall of the Teton River canyon east of the map (sec. 25, T. 7 N., R. 41 E.; WHITE OWL Phillips, W.M., T.M. Rittenour, and G. Hoffmann, 2009, OSL chronology of late IDAHO MOODY Datum. BUTTE Map version 7-9-2012. gravel of quartzite, welded tuff, limestone, sandstone, basalt, and granitic 1979 Magnetic North Contour interval 10 feet REXBURG Embree and others, 2011). The pillows indicate temporary damming of the Declination at Center of Map Pleistocene glacial outwash and loess deposits near Idaho Falls, Idaho [abs.]: 10,000-foot grid ticks based on Idaho coordinate PDF (Acrobat Reader) map may be viewed online at rocks. Thickness ranges from about 15 m (50 ft) to <3 m (<10 ft). The sand Teton River by the flows. In the Sugar City geothermal well (sec. 4, T. 6 N., system, east zone. idahogeology.org. fraction has lower obsidian content than on the Henrys Fork. The Teton Geological Society of America Abstracts with Programs, v. 41, p. 12. QUADRANGLE LOCATION ADJOINING QUADRANGLES R. 40 E.) near the southwest corner of the map, seven flows totaling 36.5 m Phillips, W.M. and J.A. Welhan, 2011, Geologic map of the Menan Buttes 1000-meter Universal Transverse Mercator grid River bifurcates downstream to form the North Fork and South Fork. The (120 ft) are present. Reverse magnetic polarity suggests that age is between quadrangle, Jefferson and Madison counties, Idaho: Idaho Geological ticks, zone 12. earliest reliable topographic maps of the region (Hayden, 1877) show the 40 39 781 ka and 2.059 Ma (age of unit Qyh). Recent Ar/ Ar dating yielded an Survey Digital Web Map 137, scale 1:24,000. US Highway 20 addded from NAIP 2009 imagery. Table 2. Major oxide and trace element chemistry of samples collected in the St Anthony quadrangle. bifurcation, suggesting that it did not arise from human activities, such as age of 440 ± 50 ka (B. Turrin, written commun., 2012). However, the Prostka, H.J., and G.F. Embree, 1978, Geology and geothermal resources of the canal construction. Fluvial activity along the North Fork is focused within 40 Major elements in weight percent Trace elements in parts per million sample contains excess Ar, plateau steps are disturbed, and the global Rexburg area, eastern Idaho: U.S. Geological Survey Open-File Report Table 1. Paleomagnetic data for the St. Anthony quadrangle. narrow, steep-walled, sinuous channels containing thin sandy gravel geomagnetic polarity timescale shows no reverse periods at 440 ± 50 ka. Sample Map deposits. Abandoned channels are evident from soil, air photo, and 78-1009, Plate 1, scale 1:48,000. Sample Unit Demag number Latitude Longitude Unit name unit SiO TiO Al O FeO* MnO MgO CaO Na O K O P O Sum LOI Ni Cr Sc V Ba Rb Sr Zr Y Nb Ga Cu Zn Pb La Ce Th Nd U Scott, W.E., 1977, Geologic effects of flooding from Teton Dam failure, south- 2 2 2 3 2 2 2 5 topographic evidence. Overbank flooding is indicated by thin (0.6-0.9 m; Qel Basalt of Snake River Plain (Early Pleistocene)—Light gray, fine-grained, number name Latitude Longitude n D I α R κ Polarity Level (mT) Qbsp eastern Idaho: U.S. Geological Survey Open-File Report 77-507, 11 p., 1 95 BP82311-1 43.88228 -111.63789 Basalt of M oody Creek Qbm 45.34 3.84 13.52 15.81 0.24 6.01 9.12 2.68 0.99 1.47 99.01 -0.65 57 121 31 301 746 20 294 524 63 54 23 35 184 9 69 138 3 71 2 24-36 in) stratified soils containing fine sand, silt, and clay adjacent to the diktytaxitic and glomeroporphyritic basalt. Shown as Qel/Qbsp where 10P01 Qbsp 43.9837 -111.6237 8/8 200 -67 2.2 7.989 647 Basalt of Chester 1 Qbsp plate, scale 1:48,000. R 60 BP82311-2 43.94130 -111.61906 Qbc 47.14 1.97 16.44 12.92 0.19 7.80 10.18 2.79 0.32 0.30 100.06 -0.44 105 82 31 276 215 5 268 133 29 11 20 45 113 1 14 32 0 20 2 channels (i.e. Labenzo silt loam; Soil Survey Staff, 2012). The South Fork overlain by loess. Abundant 1 mm plagioclase phenocrysts and moderately 1 Scott, W.E., 1982, Surficial geologic map of the eastern Snake River Plain and 10P02 Qbsa 43.9649 -111.6803 8/8 352 70 1.7 7.993 1034 N 60 BP82311-3 43.97000 -111.61980 Basalt of Snake Plain Qbsp 48.47 1.78 15.27 11.56 0.18 9.43 10.03 2.51 0.46 0.30 99.97 -0.34 183 372 30 244 277 10 255 151 29 14 19 55 105 4 20 37 2 22 1 has a broader floodplain incised slightly along its eastern side. Narrow, abundant ~2 mm olivine phenocrysts. Water well logs indicate at least two 2 adjacent areas, 111º to 115º west, Idaho and Wyoming: U.S. Geological 11P20 Qbmp 43.9899 -111.6765 7/8 50 69 3.9 6.975 244 N 60-80 BP82311-4 44.01744 -111.58165 Basalt of Snake Plain Qbsp 48.45 1.78 15.24 11.40 0.18 9.13 10.22 2.46 0.48 0.30 99.64 -0.40 168 371 31 250 262 9 256 149 29 13 19 51 104 2 16 34 2 24 1 steep-walled, meandering, active and abandoned channels traverse this flows separated by thin clay and gravel deposits. Thickness ranges from Survey Miscellaneous Investigation Series I-1372, 2 plates, scale 1:250,000. 11P21 Qbmp 44.0106 -111.7187 8/8 40 72 1.9 7.992 884 N 60-80 BP82311-6 43.98300 -111.62463 Basalt of Snake Plain Qbsp 49.03 1.87 15.63 11.55 0.18 8.35 10.39 2.52 0.43 0.32 100.27 -0.25 136 317 32 256 273 8 264 158 32 16 18 54 105 2 20 39 3 21 0 floodplain. The floodplain becomes increasingly wider and poorly about 13 to 41 m (44 to 136 ft). Thickness appears to be greatest adjacent Seed, H.B., and J.M. Duncan, 1987, The Failure of Teton Dam: Engineering BP82311-7 43.98361 -111.62460 Basalt of Snake Plain Qbsp 48.26 1.83 15.15 11.31 0.18 8.32 10.11 2.43 0.48 0.43 98.51 -0.51 145 331 30 254 321 11 262 157 31 15 19 50 105 3 18 46 2 22 1 drained downstream. to the Henrys Fork. Not dated in the quadrangle. Reverse magnetic polarity 11P22 Qbmp 43.9997 -111.7451 8/8 26 42 3.8 7.968 217 N 60 Geology, v. 24, p. 173-205. BP82311-9 43.98681 -111.67759 Basalt of Mackerts Pond Qbmp 46.86 2.92 14.99 14.12 0.21 7.65 9.63 2.46 0.46 0.48 99.80 -0.25 109 236 31 290 451 7 331 231 37 21 20 47 137 5 18 53 1 30 1 (Table 1) and geochemistry (Table 2) are distinctive within the quadrangle. 11P23 Qbsa 43.9495 -111.7187 7/8 2 73 3.5 6.980 297 N 60 Qas Alluvium of side streams (Holocene)—Thin (<3 m; <10 ft) deposits of gravelly Soil Survey Staff, 2012, Natural Resources Conservation Service, United States 11P01 43.98372 -111.62374 Basalt of Snake Plain Qbsp 48.69 1.87 15.59 11.30 0.18 8.24 10.46 2.50 0.42 0.31 99.56 -0.32 136 312 32 262 265 8 266 155 30 14 21 49 107 4 16 39 2 23 3 Correlated with basalts erupted from a low shield volcano in the Ashton sand and fine silt contained in several sinuous, narrow abandoned 40 39 Department of Agriculture. Soil Survey Geographic (SSURGO) Database for n = number of cores used / number of cores measured. 11P02 43.96489 -111.68025 Basalt of St. Anthony Qbsa 47.07 2.70 14.76 13.61 0.20 8.02 9.71 2.49 0.60 0.62 99.77 -0.71 130 312 32 275 418 11 313 246 38 23 18 49 144 4 27 59 1 35 3 quadrangle (sec. 14, T. 8 N., R. 42 E.) with an Ar/ Ar age of 909 ± 13 ka D = site mean declination of characteristic remanent magnetization (ChRm). channels cutting the Qgh outwash plain between Twin Groves and Wilford. Fremont County, Idaho. Available online at http://soildatamart.nrcs.usda.gov. 11P20 43.98990 -111.67651 Basalt of Mackerts Pond Qbmp 47.50 2.77 15.26 13.75 0.20 7.79 9.61 2.45 0.41 0.37 100.12 -0.11 120 251 32 276 363 6 327 213 35 18 20 62 130 4 23 40 1 25 1 and paleomagnetic directions of D = 197° and I = -69° (oral commun., D. I = site mean inclination of ChRM. Accessed 2/16/2012. α = confidence limit for the mean direction at the 95% level. The channels reflect flow of the Henrys Fork onto the Qgh outwash plain 95 11P21 44.01057 -111.67326 Basalt of Mackerts Pond Qbmp 46.44 2.78 15.02 13.67 0.20 7.60 10.07 2.36 0.38 0.37 98.88 0.46 114 244 32 276 358 7 323 214 35 18 21 59 129 4 21 42 1 26 1 Champion, 2010). k = precision parameter. that probably connected to the Teton River south of Wilford. Unit is poorly Thomas, C.A., H.A. Ray, and H.F. Matthai, 1976, Teton Dam flood of June Polarity: N = normal; R = reverse. 11P22 43.99966 -111.74509 Basalt of Mackerts Pond Qbmp 47.04 1.26 16.23 10.75 0.18 10.48 11.18 2.05 0.22 0.22 99.62 -0.14 213 466 37 248 195 4 195 113 25 9 16 86 81 3 13 27 0 16 0 drained and subject to seasonal ponding and near-surface saturation. Qbsa Basalt of St. Anthony (Early Pleistocene)—Light gray, dense to diktytaxitic 1976, St. Anthony Quadrangle, Idaho: U.S. Geological Survey Hydrologic Demag Level: alternating field demagnetization used to isolate ChRM. 11P23 43.94946 -111.71872 Basalt of St. Anthony Qbsa 47.00 2.66 15.07 13.42 0.20 7.85 9.69 2.50 0.47 0.56 99.42 -0.42 128 248 31 273 385 8 324 233 38 21 19 50 129 4 28 61 2 35 2 Not all locations on map. basalt. Abundant olivine and plagioclase <0.5 mm. Sparse plagioclase Investigations Atlas HA-0566, 1 plate, scale 1:24,000. All analyses performed in IGS paleomagnetism laboratory. *Total Fe expressed as FeO. phenocrysts to 1 mm. At St. Anthony falls, consists of a single flow 6 m (20 Watts, K.E., I.N. Bindeman, and A.K. Schmitt, 2011, Large-volume rhyolite genesis 1. In Newdale 7.5-minute quadrangle. ft) thick. Base not exposed. Westward from the falls, basalt forms nearly in caldera complexes of the Snake River plain; insights from the Kilgore Tuff of 2. In Lemon Lake 7.5-minute quadrangle. continuous strath along south side of Henrys Fork for ~2.6 km (~1.6 mi). the Heise volcanic field, Idaho, with comparison to Yellowstone and Bruneau- All analyses by XRF performed at Washington State University, GeoAnalytical Lab, Pullman, Washington. Jarbidge rhyolites: Journal of Petrology, v. 52, p. 857-890. Not all locations on map. Paleomagnetic and geochemical properties (Tables 1 and 2) indicate this unit is not correlative with other sampled basalts in the quadrangle. Age A and vent unknown. Reverse polarityA’ suggests age is >781 ka. Teton River City of St. Anthony well Egin Bench Henrys 327642 327938 378448 376781 408464 382416 382056 Limit of Teton Dam Flood 327358 382721 394875 357372 Qbmp Fork Qgh 387550 395898 Qao 5,000 Qao 5,000 Qa Qbm Qbm ACKNOWLEDGMENTS Qte Qgh Qbsa Qb Qb 4,800 TD 156’ TD 144’ 4,800

TD 220’ FEET Qb TD 195’ TD 200’ TD 200’ TD Property owners are thanked for permission to perform geologic mapping TD 200’ TD 233’ Qyh TD 205’ ? TD 240’ TD 223’ Qyh on private property. Fremont County provided land ownership data in GIS FEET TD 300’ format. Dan Moore (BYU-Idaho) and his students assisted with paleomag- 4,600 4,600 netic sample collection. Emily Forsberg (University of Idaho) assisted with Qyh Thv analysis of paleomagnetic samples. Duane Champion (USGS), Glenn Thv Embree (BYU-Idaho), and Mel Kuntz (USGS) generously shared data and Thv 4,400 4,400 provided insights gained from their many years of work in the upper Snake 5x vertical exaggeration. Water wells shown with Idaho Department of Water River Plain. Thanks to John Kaufman and Mel Kuntz for map reviews. Resources WellID number. Water well logs can be found at http://www.idwr.idaho.gov/apps/appswell/RelatedDocs.asp? WellID=xxxxxx where “xxxxxx” is the six-digit WellID.

Published and sold by the Idaho Geological Survey University of Idaho, Moscow, Idaho 83844-3014