IDAHO GEOLOGICAL SURVEY MOSCOW-BOISE-POCATELLO DIGITAL WEB MAP 137 PHILLIPS AND WELHAN WWW.IDAHOGEOLOGY.ORG GEOLOGIC MAP OF THE MENAN BUTTES QUADRANGLE, JEFFERSON and MADISON COUNTIES, IDAHO C OR R ELAT I ON OF M A P U N I T S Alluvial Units Artificial Unit Mass Movement Units Eolian Units Units m Volcanic Units Qls Qa Qes Qas HOLOCENE Qc 11.43 ka* LATE Qt William M. Phillips and John A. Welhan Qto PLEISTOCENE Qblg ? Qg Menan Buttes Volcanic Complex 2011 QUATERNARY Qtcb Qtca Qtu Qtn Qtns Qtna Qts Qtss Qtsa Qtc 126 ka* ? MIDDLE Qto Qa PLEISTOCENE Qblg I NT R OD U CT I ON Qa Older terrace alluvium of Egin terrace (late? to middle? Pleistocene)— Medium sand and pebbly sand forming extensive fill terrace. Sand largely obsidian but mixed with quartz and rhyolitic lithics near surface. Pebbles consist of subrounded to rounded quartzite, obsidian, rhyolitic tuff, and basalt (Kuntz, 1979). Generally lies beneath basalt of Little Grassy Butte (Qblg). However, about 3 m (10 ft) of coarse fluvial sand correlated with Egin terrace overlies Qblg in a now-closed landfill in sec. 26, T. 6 N., R. 38 E. (G. Embree in Ferdock, 1987, p. 45). Also, reported to be both under and on top of Qblg in the Juniper Buttes area to the northeast (Kuntz , 1979). The surface northeast of North Menan Butte in sec. 35, T. 6 N., R. 38 E. mapped as Qes may be a partially dissected portion of Egin Bench covered by Holocene eolian sand and colluvium. Age of unit uncertain. May partially record high discharge glacial outburst flooding in the headwaters of the Henrys Fork during Bull Lake glaciation at ~140 ka (W. Scott quoted in Allison, 2001, p. 18). Water wells indicate minimum thickness of 37 m (120 ft) in map; thickness is 13-30 m (43-98 ft) in Juniper Buttes area (Kuntz, 1979). Qto Qto Qa This map depicts bedrock and surficial geological units in the Menan Butte quadrangle. The area sits on the edge of the eastern Snake River Plain, a major crustal downwarp associated with the Yellowstone hotspot. Late Miocene–Pliocene rhyolitic volcanic rocks of the Heise Volcanic Field were erupted in this portion of the Snake River Plain between 6.62–4.45 Ma as the hotspot passed beneath the region (Morgan and McIntosh, 2005). At 2.06 Ma, when the hotspot was located near its present position, the Huckleberry Ridge Tuff was erupted from the Henrys Fork Caldera (Christiansen, 2001). In the Menan Buttes quadrangle, both the Heise rhyolites and the Huckleberry Ridge Tuff are covered by Snake River alluvium and basaltic lava flows. The map lies at the junction of the two major tributaries of the Snake River. On the north, the Henrys Fork drains highlands largely underlain by rhyolitic caldera deposits of the Island Park-Yellowstone area. These rocks tend to break down relatively quickly to form sandy alluvium rich in obsidian. To the east, the South Fork originates in the Grand Teton area underlain by Precambrian, Paleozoic, and Mezosoic rocks. These rocks produce the diverse clast lithologies found in the South Fork. During the course of at least two glaciations at ~140 ka and 25-13 ka (Licciardi and Pierce, 2008), the Snake River transported enormous quantities of gravel onto the Snake River Plain (Scott, 1982). In the map, the two streams filled a subsiding basin with hundreds of feet of alluvium. When basaltic magma erupted into this basin, steam explosions occurred that mixed quenched magma with gravel and sand to form the tuff cones of Menan Buttes. These structures are unusual for the Snake River Plain and are among the largest tuff cones in the world (Ferdock, 1987). Following the formation of the tuff cones, a voluminous lava flow erupted from Little Grassy Butte about 24 km (15 mi) northwest of the map. This flow impinged onto the floodplain of the Henrys Fork, causing the stream to move eastward. During the Holocene, sand carried by northeast-directed winds was trapped in the craters of Menan Buttes, and formed small dunes and sand sheets on the irregular topography of the Little Grassy Butte lavas and on Egin Bench. The Henrys Fork and South Fork (including small splays such as Texas Slough and Bannock Jim Spring Slough) reworked alluvium and deposited new sediments. On June 5, 1976, much of the area flooded when the Teton Dam failed catastrophically (Thomas and others; 1976). This flood was about 100 times larger than any historic Snake River flood; hence it provides perspective on the effects of exceptionally large prehistoric events (Scott, 1977). Qa Qa Qa Qblg Qa Qa Qa Qa Qblg Qa Qa Qa Qa Qa Qto Qblg Qls Qa Qa Qa The map is based upon compilation and consultation of master thesis studies (Ferdock, 1987; Allison, 2001; Creighton, 1982), county soil surveys (Noe, 1981; Jorgensen, 1979), regional geologic mapping (Scott, 1982), domestic water well logs (available from Idaho Department of Water Resources at http://www.idwr.idaho.gov/apps/appswell/searchWC.asp), and field work conducted in 2007. Qa Qa Qto Colluvium (Holocene-late Pleistocene)—Massive, semi-indurated, brown to tan, sand, cobbles, and pebbles in a clayey matrix. Composed of sideromelane sand and rounded tuff fragments. Thickness 0.5-2.5 m (1.6 8.6 ft). Best developed on flanks of the Menan Buttes. Includes alluvial fans composed of bedded silt and sand on northeast sides of North and South Menan Butte. Qls Landslide (Holocene-late Pleistocene)—Rotational slump of older terrace alluvium of Egin (unit Qto) onto the floodplain of Henrys Fork. Scarp has been modified by road construction. Qa ARTIFICIAL UNIT ALLUVIAL UNITS m Qa Alluvium of active channels and floodplain of the Snake River and main tributary streams (Holocene)—Sand, gravel, and sandy silt. On Henrys Fork and South Teton River, sand consists of quartz, black obsidian and rhyolitic lithic grains while gravel clasts consist of rhyolite, basalt, and lesser quartzite, sandstone, and granitic cobbles. Thickness <10 m (33 ft). On South Fork of Snake River, dominated by hard, well-rounded quartzite cobbles with lesser sandstone, basalt and limestone. Forms small islands and bar-tops exposed at low water levels. Subject to flooding and high water tables during spring and early summer. Parent material for poorly drained, channeled Haplaquolls soils (Noe, 1981; Jorgensen, 1979). Qa Qt Qa m Qa Qes Qa Qa Qtns Qa Qa Qas Alluvium of side streams (Holocene)—Gravel, sand, and sandy silt. Forms islands, and bar-tops and beaches exposed at low water levels; also consists of deposits in numerous relic channels. Thickness <10 m (33 ft). Side streams of the South Fork of the Snake River are dominated by quartzite cobbles and lesser sandstone, metamorphic and granitic rocks, while side streams of Henrys Fork contain obsidian sands and rhyolite cobbles as well as lesser quartzite, sandstone, and granitic rocks. Subject to flooding and high water tables during spring and early summer. Qt Terrace alluvium of the Snake River and tributary streams (late Pleistocene)— Sand and gravel similar in clast composition to unit Qa; forms fill terraces separated by 1.5-3 m (5-10 ft) scarps from flood plain and active channels of the Henrys Fork and South Fork of Snake River. Terrace riser height generally increases to the north along the Henrys Fork. Terrace surfaces have gentle northwest slope indicating source of most terrace alluvium is South Fork of Snake River. Thickness uncertain because unit cannot be distinguished from older or younger alluvial units in water well logs. Minimum thickness about 10 m (33 ft). Unit interpreted to have been deposited during period of waning discharge and stream incision during termination of Pinedale glaciation at ~13-14 ka. Parent material for the Blackfoot, Labenzo, Heiseton, and Harston soils (Noe, 1981; Jorgensen, 1979). Locally poorly drained with water levels <1.5 m (<5 ft) from surface, as indicated by soils with aquic textures. Qa Qtn 32 Qc 16 2 Qtn 14 Qa 13 20 13 Qtn Qg 11 Qtns Qtn Qtn Qc 34 31 Qtns 2 Qc Qtns Qtn Qt Qas Qa Qas Qt Qas Qa Qtna Qtns Qtn Qes Qas Qas Qa Qt 3 Qtn Qtns 21 Qtn Qa 13 6 Qtna Qtns Qa Qtns Qas Qa 15 32 Qtn Qtna Qc Qas 4 35 Qc Qtns Qas Qas Qa Qtn Qas Qt Qa Qa Qtn 23 Qt Qa Qa Qa 13 Qtns 15 Qblg Qtna Qa Qa Qtns 5 Qtn Qas Qa 12 Qtn 20 Qa 12 Qtna Qtn Qtn Qtn Qtn Qtns Qtn Qtn 4 5 10 Qtu Qtn Qtn Qtu Qtu Qtc Qa 15 Qtn Qtns Qc Qa Qts Qtn Qtca 8 Qc 31 Qa 17 11 Qa Qts C 22 Qes Qtsa Qa Qa Qt Qtss Qc 11 33 Qa Qa Qa Qt Qt Qa Qblg Qas AC KN OWLEDGMENTS We thank the landowners in the area for access to their property. K. Othberg (IGS) and C. Kersey (University of Idaho) assisted with paleomagnetic sample collection and analysis. G. Embree (BYU-Idaho) led several helpful field trips to the Menan Buttes Volcanic Complex. 5,600 A' South Menan Butte Qa Qts Qtc Qts Qtsa sand and gravel 4,600 basalt 4,400 sand and gravel ? ? basalt ? clay gravel and sand 4,400 gravel and sand ? basalt ? basalt of vent (projected) 4,200 basalt of vent (projected) basalt of vent clay ? 4,200 ? 4,000 4x vertical exageration Qas Qa O ER ET NE AN RK PL MA LA RK KE PA G UR XB N ES MILE DE PA ER RK S FEET IS RI W IDAHO Contour interval 10 feet RI E E KILOMETER Y 1 7000 VI 0 6000 LL 5000 GB 4000 RE 1 3000 RI 0.5 2000 ME BU NA TT 0 1000 QUADRANGLE LOCATION ADJOINING QUADRANGLES B’ 6,000 6,000 5,800 5,400 5,400 clay FEET Strike and dip of tuff. 4,200 Strike and dip of overturned tuff. basalt basalt basalt basalt basalt of vent C C'' 5,400 C' 5,200 slumped vent deposits 5,000 D l α95 Polarity Demag level (mT) 7 334.6 67.9 2.6 540.9 N 20 8 322.5 64.9 4.0 189.1 N 40 FEET n 43.81417 -112.00854 5,000 Qtc 4,000 4,800 clay gravel and sand Qtc sand and gravel 4,600 4,600 gravel and sand basalt n = number of oriented cores. D = site mean declination of characteristic remnant magnetism. I = site mean inclination of characteristic remnant magnetism. α95 = confidence limit for the mean direction at the 95% level. k = precision parameter. N = normal polarity. 4,400 clay basalt basalt gravel and sand basalt sand and gravel 4,200 4,400 4,200 basalt of vent 4,000 Published and sold by the Idaho Geological Survey University of Idaho, Moscow, Idaho 83844-3014 4x vertical exageration Modified from Ferdock (1987). 4,000 FEET Latitude Longitude 43.82689 -112.04051 5,200 profile at maximum extent Qtca Qtc Unit 5,400 Central Menan Butte Extent of 1976 Teton Dam flood (Thomas, Ray, and Harenberg, 1976). Qblg Modified from Ferdock (1987). 4x vertical exaggeration. Crater rim. Qblg basalt sand and gravel 4,000 4,600 4,400 sand and gravel sand and gravel Landslide block: slump blocks of tuff (Qtn and Qts) on flanks of North and South Menan Buttes. 07P021 4,000 basalt basalt basalt clay sand and gravel clay sand and gravel sand and gravel Horizontal tuff. 07P020 Qblg Qtn sand and gravel gravel and sand gravel and sand 4,600 4,400 Site number* Qes Qtn Table 1. Paleomagnetic data for basalt of Little Grassy Butte. Samples are from the Deer Parks quadrangle. k 5,200 5,000 Qtna Qg Normal fault: ball and bar on downthrown side. 12 slump folds 4,800 Contact: dashed where approximately located. 3 Qtna Qes Qtn 5,000 4,200 4,000 FEET SY MBOL S 5,800 5,600 Qtn Qa Noe, H.R., 1981, Soil survey of Madison County area, Idaho: U.S. Department of Agriculture, Soil Conservation Service, 128 p., 29 map plates, scale 1:24,000. Phillips, W.M., T.M. Rittenour, and Glenn Hoffmann, 2009, OSL chronology of late Pleistocene glacial outwash and loess deposits near Idaho Falls, Idaho: Geological Society of America Abstracts with Programs, v. 41, p. 12. Rittenour, Tammy, and H.R. Pearce, 2009, Drought and dune activity in the Idaho Falls dune field, Snake River Plain, southeastern Idaho: Geological Society of America Abstracts with Programs, v. 41, no. 7, p. 619. Scott, W.E., 1982, Surficial geologic map of the eastern Snake River Plain and adjacent areas, 111º to 115º W., Idaho and Wyoming: U.S. Geological Survey Miscellaneous Investigation Series Map I-1372, scale 1:250,000. Scott, W.E., 1977, Geologic effects of flooding from Teton Dam failure, southeastern Idaho: U.S. Geological Survey Open-File Report 77-507, 11 p., 1 plate, scale 1:48,000. Thomas, C.A., H.A. Ray, and W.A. Harenberg, 1976, Teton dam flood of June 1976, Menan Buttes quadrangle, Idaho: U.S. Geological Survey Hydrologic Investigation HA-570. profile at maximum extent North Menan Butte 5,600 5,200 RE F E RE NC ES Modified from Ferdock (1987). B Field work conducted 2007. This geologic map was funded in part by the U.S. Geological Survey National Cooperative Geologic Mapping Program, USGS Award No. 07HQAG0070. Digital cartography by Collette Gantenbein,Theresa A. Taylor, Loudon R. Stanford, and Jane S. Freed at the Idaho Geological Survey’s Digital Mapping Lab. Reviewed by J.D. Kauffman, Idaho Geological Survey. Map version 10-11-2011. PDF (Acrobat Reader) map may be viewed online at www.idahogeology.org. 5,000 4,000 gravel and sand clay 4,000 0.5 1 5,200 Qes Central Menan Butte Qa Qa 0 ? 5,400 slumped vent deposits Qtn Qtn ? SCALE 1:24,000 Allison, R.R., 2001, Climatic, volcanic, and tectonic infuences on late Pleistocene sedimentation along the Snake River and in Market Lake: Bonneville, Jefferson, and Madiison counties, Idaho: Idaho State University M.S. thesis, 153 p. Christiansen, R.L., 2001, The Quaternary and Pliocene Yellowstone Plateau volcanic field of Wyoming, Idaho, and Montana: U.S. Geological Survey Professional Paper 729-G, p. G1-G145. Creighton, D.N., 1982, The geology of the Menan Complex, a group of phreastmagmatic constructs in the eastern Snake River Plain, Idaho: The State University of New York, University at Buffalo M.S. thesis, 76 p. Ferdock, G.C., 1987, Geology of the Menan Volcanic Complex and related volcanic features, northeastern Snake River Plain, Idaho: Idaho State University M.S. thesis, 171 p., geologic map and cross-sections, scale 1:12,000. Forman, S.L., and J. Pierson, 2003, Formation of linear and parabolic dunes on the eastern Snake River Plain, Idaho in the nineteeth century: Geomorphology, v. 56, no. 1-2, p. 189-200. Gaylord, D.R., J.J. Coughlin, A.J. Coleman, M.R. Sweeney, and R.H. Rutford, 2000, Holocene sand dune activity and paleoclimates from Sand Creek, St. Anthony dune field, Idaho: Geological Soceity of America Abstracts with Programs, v. 32, no. 5, p. 10. Jorgensen, Wendell, 1979, Soil survey of Jefferson County, Idaho: U.S. Department of Agriculture, Soil Conservation Service, 219 p., 66 map plates, scale 1:20,000. Kuntz, M.A., 1979, Geologic map of the Juniper Buttes area, eastern Snake River Plain, Idaho: U.S. Geological Survey Miscellaneous Investigations Map I-1115, scale 1:48,000. Licciardi, J.M., and K.L. Pierce, 2008, Cosmogenic exposure-age chronologies of Pinedale and Bull Lake glaciations in greater Yellowstone and the Teton Range, USA: Quaternary Science Reviews, v. 27, p. 814-831. Morgan, L.A., and W.C. McIntosh, 2005, Timing and development of the Heise volcanic field, Snake River Plain, Idaho, western USA: Geological Society of America Bulletin, v. 117, no. 3/4, p. 288-306. Tuff of Menan Buttes, undivided (late Pleistocene)—Indurated, gray-green to brown, poorly sorted, massive to thin bedded, palagonitic, lapilli to fine tuff. Cannot be reliably correlated with eruptive source. Qtu Qt Qt LE 10,000-food grid ticks based on Idaho coordinate system, east zone. 1000-meter Universal Transverse Mercator grid ticks, zone 11. Qtn 4,600 Qa Qa 1000 Qblg sand and gravel Qa Qas 1 UTM Grid and 1979 Magnetic North Declination at Center of Map Agglutinate spatter of Center Menan Butte (late Pleistocene)—Poorly exposed, broad, circular mound, 250 m (830 ft) in diameter, 11 m (36 ft) high. Composed of oxidized, welded, scoriaceous spatter bombs and pillow lava fragments. Interpreted to be remnants of late-stage spatter ramparts and pillows formed when magma erupted into small lake occupying Center Menan Butte. Qtna Qa Qa Qa MN 17o Qtca basalt of vent 5,000 Qg Qa Qa Qg Qa Qa GN 0o 39 Explosion breccia of Center Menan Butte (late Pleistocene)— Unconsolidated lapilli and block-sized, angular clasts of dense black, and red/black scoriaceous basalt fragments, 1-6 cm thick. Shown as stipple pattern where found only within colluvium. Interpreted to be remnants of lava lake destroyed by explosion. A'' Qes Qtna A'' Base map scanned from USGS film positive, 1979. Shaded elevation from 10 m DEM. Topography from aerial photographs by Kelsh plotter and by plane-table surveys 1951. Aerial photographs taken 1950. Revisions from aerial photographs taken 1976 and other source data. Map edited 1979. Not field checked. Projection: Idaho coordinate system, east zone (Transverse Mercator). 1927 North American Datum. Qtcb slumped vent deposits 4,000 Qa Qa Qa Qc Tuff of Center Menan Butte (late Pleistocene)—Indurated gray-green to tan, lapilli to fine, palagonitic tuff. Bedding is weakly developed, thin to medium, and planar. The tuff grades from coarse lapilli tuff on interior of the cone to medium tuff on the outer flanks. Tuffs on the interior of the cone are composed of 35 percent fresh black, rounded, scoriaceous to dense lapilli; 10 percent angular to rounded, sand to boulder-sized accidental lithics of quartzite and dense basalt, and 55 percent tan to dark gray-green ash matrix. Accretionary lapilli and vesicles within the tuff are rare. Scattered on the surface of the tuff beds are large accidental lithics of broken cobbles of red, green, and white quartzite, and broken boulders of vesicular basalt. North Menan Butte Qtn Qa Qa Qc Qts 12 Qa Qa Qa Qa Qts Qc Qa Qa Qa 37 25 Qa 50 Qa Qts 9 Qtsa Qc 23 5 Qc 17 12 Qc 8 Qa Qa Qc 22 Qc 22 11 25 Qa Qtc Menan Buttes, Undivided Tuff of North Menan Butte (late Pleistocene)—Indurated, gray-green to brown, poorly sorted, massive to thin bedded, palgonitic, lapilli tuff to fine tuff. Average grain sizes of tuff decrease from medium to coarse ash on vent interior, to fine ash in distal deposits. Accidental lithics composed of basalt are rare; quartzite and sandstone also decrease in grain size from proximal to distal deposits. The concentration of accidental lithics on North Menan Butte is the lowest in the Menan Volcanic Complex. Thin sections of tuff average 56.5 percent angular sideromelane, 27.6 percent pore space, 8.5 percent tachylite, 2.6 percent phenocrysts (1.1 percent plagioclase and 1.5 percent olivine), 4.3 percent palagonite, and 0.5 percent accidental material. Along the crater rim and more rarely along slopes, cobble- to boulder-sized accidental basalt clasts are concentrated by erosional processes that remove surrounding tuff. Many of the larger accidental clasts display ventifacting. Where the tuff is well exposed and undisturbed by redeposition, reverse and normally graded beds are present. Most beds are planar, with rare mantle and cross-bedding structures, and lobate beds of unarmored vesicular lapilla interbedded with fine tuff. Armored and accre- Qa Qts Center Menan Butte is a partially eroded and poorly exposed tuff cone covering an area of about 1.8 km2 (1.1 mi2) and rising about 42 m (135 ft) above the surrounding landscape. Tuffs from both North and South Menan Buttes partially bury the cone. It has the largest rim crater diameter of any of the Menan Butte Complex structures as well as a unique, late stage agglutinate ring and explosion breccia inferred to mark the position of lava erupted into a small lake. North Menan Butte Qtn Altered tuff of South Menan Butte (late Pleistocene)—Massive, orange to brown palagonitic tuff, similar to unit Qtna. Restricted to exposures in the southwestern inner crater but inferred to be more common within the butte at depth. Center Menan Butte The age of Menan Volcanic Complex is not precisely known. It probably falls between 140 and 10 ka. The complex was erupted into water-saturated sediments of late Pleistocene age formed from the outwash of glaciers in the headwaters of the Henrys Fork and South Fork drainages. These glacial deposits date to the Bull Lake and Pinedale glaciations at ~140 ka and ~22-14 ka (Licciardi and Pierce, 2008), indicating an age for the complex of less than 140 ka. This is supported by water well logs showing that ash deposits from the Menan Buttes Complex appear to lie upon alluvial deposits of Egin Bench (unit Qto). North of the map, the Egin Bench deposits contain black obsidian gravels (Kuntz, 1979) thought to have formed from glacial outburst flooding along the Henrys Fork during the Bull Lake glaciation (W. Scott quoted in Allison, 2001, p. 18). The Menan Butte deposits lie beneath basalt lava flows erupted from Little Grassy Butte (unit Qblg), believed to be ~10-20 ka. 5,400 3 14 6 Qtn Ash tuff of South Menan Butte (late Pleistocene)—Black, thinly laminated to thinly bedded, cross- to planar-bedded, moderately sorted, fine to medium sideromelane ash. Similar to unit Qtna but less common than on North Menan Butte. 5,200 Qc 22 33 Qtss FEET B 14 Tuff of South Menan Butte (late Pleistocene)—Indurated, gray-green to brown, poorly sorted, massive to thin bedded, palgonitic, lapilli tuff to fine tuff. Similar to tuff of North Menan Butte except for accidental lithic concentration. Tuff of South Menan Butte contains more than 1.5 times the accidental lithics by volume as tuff of North Menan Butte. Accidental clasts are most common on the rim and interior of the crater and tend to be concentrated in thin beds with large lapilli of juvenvile vesicular basalt. The basalt lapilli have nearly the same petrographic composition as those of North Menan Butte. The accidental clasts range from cobble to sand size for quartzite clasts, and boulder size (as large as 1.5 m) for vesicular basalts. Most of the accidentals show signs of breakage. The quartzite-to-basalt ratio is about 4:1. Minor amounts of granite, gneiss, and rhyolitic tuff are also present. Qtsa The Menan Volcanic Complex consists of phreatomagmatic tuff cones produced by the injection of basaltic magma into alluvial sediments and basalts of the Snake River Plain aquifer. Magma quenching and steam explosions produced tuffs composed of basaltic glass (sidermelane and trachylite), hydrothermally altered glass (palagonite), and phenocrysts of plagioclase and olivine. Fragments of the sediments and basalts underlying the cones (accidental lithics) together with bombs of quenched basalt and clumps of pebble-sized tephra forming small balls (accretionary lapilli) were also incorporated into the tuffs. Slumping and faulting of the cones occurred both during and shortly after eruptions along with hydrothermal alteration of the tuff. The volcanic edifices define a north-northwesttrending lineament that probably mirrors basalt dike orientations. Prevailing winds during the eruptions caused the cones to be elongated to the northeast. Mapping and unit descriptions of the Menan Volcanic Complex are taken from the detailed study of Ferdock (1987). Qa 4 13 Qa Qa Qc Qc Qc Basalt of Little Grassy Butte (late Pleistocene)—Gray to dark-gray, porphyritic to nonporphyritic, tube-fed pahoehoe lava flows erupted from Little Grassy Butte, about 24 km (15 mi) northwest of Menan Butte quadrangle (Kuntz, 1979). Consists of rare phenocrysts of olivine (1 mm) and plagioclase (3 mm) in a diktytaxitic groundmass of plagioclase, olivine, and augite crystals (<0.5 mm). Pressure ridges and tumuli as much as 9 m (30 ft) in height with well-preserved pahoehoe flow surfaces are surrounded by local accumulations of eolian sediment. Well-drained loam soils have formed on these deposits (Mathon-Modkin-Bondranch complexes; Noe, 1981). Water well logs and exposures at edges of unit indicate flow thicknesses of 2-10 m (635 ft). Paleomagnetic measurements show the unit to have normal polarity (Table 1). Unit is undated; possibly 10-20 ka (Kuntz, 1987, quoted in Ferdock, 1987, p. 45). The basalt overlies ash deposits of Menan Butte Complex and both underlies and overlies alluvium of Egin Bench (unit Qto; Kuntz, 1979). 5,600 Qas Qa Qc Qc 21 18 South Menan Butte Qts A Qc Qc Qc 18 Altered tuff of North Menan Butte (late Pleistocene)—Massive, brittle, relatively featureless, orange to brown palagonitic tuff. Found on the southwestern rim and locally within the crater; interpreted to form large portion of the cone at depth (see cross section A-A’). In contrast to the craggy, alveolar weathering of the unaltered tuff, the altered tuff weathers to smooth, exfoliating slopes punctuated by occasional accidental basalt clasts. Composed of about 50 percent orange-yellow palagonite, and medium ash-sized brown sileromelane and black tachylyte coated with palagonite. Relative to unaltered ash, pore space has been reduced to about 12 percent. Contacts between unaltered and altered ash are usually sharp. The contacts cross bedding and locally show control by fractures. Qa 8 7 A' Qts Qas Qa Qtc Qts Qc Qtc Qtn Qc Qtcb C' Qtc Qtc Qt Qa Qa Qtc Qtc Qas C'' Qtn 4 Qtn Qtc Qtc FEET Qa Qtna Menan Volcanic Complex Alluvium of Snake River outwash (late Pleistocene)—Gravel and sand composed dominantly of very hard pink, purple and gray quartzite with lesser rhyolite, basalt, sandstone, gneiss, and granitic rocks. Poorly exposed in map. Exposures in nearby gravel pits indicate unit is thickly planar- to cross-bedded, separated locally by thin, cross-bedded sand layers. Gravel is mostly pebble- to cobble-sized, clast-supported, locally normally graded and imbricated. Gravel framework is filled by fine to medium sand composed of subangular black obsidian, quartzite, quartz and feldspar crystals, muscovite, and fragments of basalt and rhyolite. Sand beds are locally black because of high obsidian content. Water well logs suggest minimum thickness of ~50 m (164 ft). Thickness uncertain because possible older units cannot be reliably separated in water well logs. Unit is part of the regional braided-stream outwash plain deposited during the Pinedale glaciation by meltwaters from the Snake River headwaters (Scott, 1982). OSL ages between 25.2 ka and 12.6 ka (Phillips and others, 2009) are consistent with cosmogenic surface exposure ages of Pinedale-age moraines in the Yellowstone headwaters (Licciardi and Pierce, 2008). Qg Qa Qa Qblg Artificial fill (Holocene)—Landfill (garbage dump). m Qa Ash tuff of North Menan Butte (late Pleistocene)—Black, thinly laminated to thinly bedded, cross- to planar-bedded, moderately sorted, fine to medium sideromelane ash. Ash has the appearance and consistency of sand. Locally contains small channels, stoss and lee structures, rip-up clasts, armored lapilli, and sag structures. Also occurs as thin interbeds within the main tuff unit and at distances well away from the vent. Total thickness varies with position relative to the northeastward dispersal direction of tephra from the cone. At least 50 m (160 ft) thick on the northeast flank. Composed of 59.2 percent silderomelane, 34.9 percent open spaces, 2.7 percent black tacylyte, 2.9 percent olivine and plagioclase phenocrysts, 0.4 percent accidental lithics, and trace palagonite. Interpreted to represent dry surge eruptions. North of Menan Butte in an abandoned quarry at SE1/4, NW1/4, sec. 34, T. 6 N., R. 38 E., about 1 m (3 ft) of planar-bedded black tuff is exposed beneath basalt of Little Grassy Butte (unit Qblg). The tuff is reddened for a thickness of about 30 cm by baking from the basalt. A similar contact can be viewed at SW1/4, SW1/4, sec. 26, T. 6 E., R. 38 E. The contact between Qblg and about 1 m (3 ft) of the black ash is also present in the Lower Teton Observation Well #1 (IDWR Permit number 818955) drilled by the U.S. Bureau of Reclamation in SW1/4, NE1/4, sec. 25, T. 6 N., R. 38 E. VOLCANIC ROCKS D ES CR I P T IO N O F M AP U NI T S Qa B' Dunes and sand sheets (Holocene)—Loose, tan to brown, medium sand. Composed of rounded quartz, obsidian, sideromelane, and basaltic tuff grains. Forms active small dunes and sand sheets on the crater floors of the Menan Buttes where it is also bedded with pebbly colluvium. At least 6 m (20 ft) thick in crater of South Menan Butte. Northeast of North Menan Butte in sec. 35, T. 6 N., R. 38 E., silty-sand covers the low-relief surface lying topographically above the active floodplain of Henrys Fork. Parent material for the Mathon sandy loam soil (Noe, 1981). Undated; assigned a Holocene age based upon regional studies of sand dune activity (Gaylord and others, 2000; Forman and Pierson, 2003, Rittenour and Pearce, 2009). Qc Qa Qblg A Qtns MASS MOVEMENT UNITS SOURCE OF DATA Qblg tionary lapilli as large as 2 cm are commonly found about midway down outer flanks, particularly on the northwest and southeast flanks. Vesicular lapilli of juvenile basalt are also present. Thin sections of the lapilli average 67.4 percent black opaque glass groundmass, 28.7 percent vesicles, 3.9 percent phenocrysts (2.5 percent plagioclase and 1.4 percent olivine), 0.9 percent palagonite, and 0.2 percent accidental lithics. EOLIAN UNITS Qes Qa Qa 781 ka* *Stage boundaries of the Pleistocene from Gradstein, F.M., J.G. Ogg, A.G. Smith, Wouter Bleeker, and L.J. Lourens, 2004, A new geologic time scale, with special reference to Precambrian and Neogene: Episodes v. 27 no. 2, p. 83-100. Qblg
