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Geologic Map of the Menan Buttes Quadrangle, Jefferson and Madison Counties, Idaho

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Geologic Map of the Menan Buttes Quadrangle, Jefferson and Madison Counties, Idaho IDAHO GEOLOGICAL SURVEY DIGITAL WEB MAP 137 MOSCOW-BOISE-POCATELLO WWW.IDAHOGEOLOGY.ORG PHILLIPS AND WELHAN CORRELATION OF MAP UNITS GEOLOGIC MAP OF THE MENAN BUTTES QUADRANGLE, Artificial Alluvial Units Eolian Units Mass Movement Volcanic Unit Units Units Units m Qls HOLOCENE JEFFERSON and MADISON COUNTIES, IDAHO Qa Qas Qes Qc 11.43 ka* LATE Qt Qto Qblg PLEISTOCENE William M. Phillips and John A. Welhan Qg ? Menan Buttes Volcanic Complex QUATERNARY 2011 Qtcb Qtca QtnQQtns tna QtsQQtss tsa Qtu Qtc 126 ka* ? Qto MIDDLE Qa PLEISTOCENE 781 ka* *Stage boundaries of the Pleistocene from Gradstein, F.M., J.G. Ogg, A.G. Smith, Wouter Bleeker, and L.J. Lourens, Qblg 2004, A new geologic time scale, with special reference to Precambrian and Neogene: Episodes v. 27 no. 2, p. 83-100. Qto Older terrace alluvium of Egin terrace (late? to middle? Pleistocene)— tionary lapilli as large as 2 cm are commonly found about midway down Qblg Qa INTRODUCTION Medium sand and pebbly sand forming extensive fill terrace. Sand largely outer flanks, particularly on the northwest and southeast flanks. Vesicular Qto obsidian but mixed with quartz and rhyolitic lithics near surface. Pebbles lapilli of juvenile basalt are also present. Thin sections of the lapilli average Qa This map depicts bedrock and surficial geological units in the Menan Butte consist of subrounded to rounded quartzite, obsidian, rhyolitic tuff, and 67.4 percent black opaque glass groundmass, 28.7 percent vesicles, 3.9 Qa quadrangle. The area sits on the edge of the eastern Snake River Plain, a basalt (Kuntz, 1979). Generally lies beneath basalt of Little Grassy Butte percent phenocrysts (2.5 percent plagioclase and 1.4 percent olivine), 0.9 major crustal downwarp associated with the Yellowstone hotspot. Late (Qblg). However, about 3 m (10 ft) of coarse fluvial sand correlated with percent palagonite, and 0.2 percent accidental lithics. Miocene–Pliocene rhyolitic volcanic rocks of the Heise Volcanic Field were Egin terrace overlies Qblg in a now-closed landfill in sec. 26, T. 6 N., R. 38 Qa Qa erupted in this portion of the Snake River Plain between 6.62–4.45 Ma as E. (G. Embree in Ferdock, 1987, p. 45). Also, reported to be both under and Qtns Ash tuff of North Menan Butte (late Pleistocene)—Black, thinly laminated Qblg on top of Qblg in the Juniper Buttes area to the northeast (Kuntz , 1979). The to thinly bedded, cross- to planar-bedded, moderately sorted, fine to Qa the hotspot passed beneath the region (Morgan and McIntosh, 2005). At 2.06 Ma, when the hotspot was located near its present position, the Huck- surface northeast of North Menan Butte in sec. 35, T. 6 N., R. 38 E. mapped medium sideromelane ash. Ash has the appearance and consistency of leberry Ridge Tuff was erupted from the Henrys Fork Caldera (Christiansen, as Qes may be a partially dissected portion of Egin Bench covered by Holo- sand. Locally contains small channels, stoss and lee structures, rip-up 2001). In the Menan Buttes quadrangle, both the Heise rhyolites and the cene eolian sand and colluvium. Age of unit uncertain. May partially clasts, armored lapilli, and sag structures. Also occurs as thin interbeds Huckleberry Ridge Tuff are covered by Snake River alluvium and basaltic record high discharge glacial outburst flooding in the headwaters of the within the main tuff unit and at distances well away from the vent. Total Qa lava flows. The map lies at the junction of the two major tributaries of the Henrys Fork during Bull Lake glaciation at ~140 ka (W. Scott quoted in thickness varies with position relative to the northeastward dispersal direc- tion of tephra from the cone. At least 50 m (160 ft) thick on the northeast Qa Snake River. On the north, the Henrys Fork drains highlands largely under- Allison, 2001, p. 18). Water wells indicate minimum thickness of 37 m (120 ft) lain by rhyolitic caldera deposits of the Island Park-Yellowstone area. These in map; thickness is 13-30 m (43-98 ft) in Juniper Buttes area (Kuntz, 1979). flank. Composed of 59.2 percent silderomelane, 34.9 percent open spaces, rocks tend to break down relatively quickly to form sandy alluvium rich in 2.7 percent black tacylyte, 2.9 percent olivine and plagioclase phenocrysts, obsidian. To the east, the South Fork originates in the Grand Teton area 0.4 percent accidental lithics, and trace palagonite. Interpreted to represent underlain by Precambrian, Paleozoic, and Mezosoic rocks. These rocks EOLIAN UNITS 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 Qa produce the diverse clast lithologies found in the South Fork. During the Qes Dunes and sand sheets (Holocene)—Loose, tan to brown, medium sand. black tuff is exposed beneath basalt of Little Grassy Butte (unit Qblg). The Qblg 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 Composed of rounded quartz, obsidian, sideromelane, and basaltic tuff tuff is reddened for a thickness of about 30 cm by baking from the basalt. A Qa onto the Snake River Plain (Scott, 1982). In the map, the two streams filled grains. Forms active small dunes and sand sheets on the crater floors of the similar contact can be viewed at SW1/4, SW1/4, sec. 26, T. 6 E., R. 38 E. a subsiding basin with hundreds of feet of alluvium. When basaltic magma Menan Buttes where it is also bedded with pebbly colluvium. At least 6 m The contact between Qblg and about 1 m (3 ft) of the black ash is also erupted into this basin, steam explosions occurred that mixed quenched (20 ft) thick in crater of South Menan Butte. Northeast of North Menan Butte present in the Lower Teton Observation Well #1 (IDWR Permit number in sec. 35, T. 6 N., R. 38 E., silty-sand covers the low-relief surface lying 818955) drilled by the U.S. Bureau of Reclamation in SW1/4, NE1/4, sec. Qa 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 topographically above the active floodplain of Henrys Fork. Parent material 25, T. 6 N., R. 38 E. tuff cones in the world (Ferdock, 1987). Following the formation of the tuff for the Mathon sandy loam soil (Noe, 1981). Undated; assigned a Holocene age based upon regional studies of sand dune activity (Gaylord and others, Qtna Altered tuff of North Menan Butte (late Pleistocene)—Massive, brittle, cones, a voluminous lava flow erupted from Little Grassy Butte about 24 km relatively featureless, orange to brown palagonitic tuff. Found on the south- Qa 2000; Forman and Pierson, 2003, Rittenour and Pearce, 2009). (15 mi) northwest of the map. This flow impinged onto the floodplain of the western rim and locally within the crater; interpreted to form large portion Henrys Fork, causing the stream to move eastward. During the Holocene, of the cone at depth (see cross section A-A’). In contrast to the craggy, alveo- sand carried by northeast-directed winds was trapped in the craters of MASS MOVEMENT UNITS lar weathering of the unaltered tuff, the altered tuff weathers to smooth, Qa Menan Buttes, and formed small dunes and sand sheets on the irregular exfoliating slopes punctuated by occasional accidental basalt clasts. Qa topography of the Little Grassy Butte lavas and on Egin Bench. The Henrys Qc Colluvium (Holocene-late Pleistocene)—Massive, semi-indurated, brown to Composed of about 50 percent orange-yellow palagonite, and medium Qto Fork and South Fork (including small splays such as Texas Slough and tan, sand, cobbles, and pebbles in a clayey matrix. Composed of ash-sized brown sileromelane and black tachylyte coated with palagonite. Qblg Bannock Jim Spring Slough) reworked alluvium and deposited new sideromelane sand and rounded tuff fragments. Thickness 0.5-2.5 m (1.6 - Relative to unaltered ash, pore space has been reduced to about 12 sediments. On June 5, 1976, much of the area flooded when the Teton Dam 8.6 ft). Best developed on flanks of the Menan Buttes. Includes alluvial percent. Contacts between unaltered and altered ash are usually sharp. The Qls failed catastrophically (Thomas and others; 1976). This flood was about 100 Qa fans composed of bedded silt and sand on northeast sides of North and contacts cross bedding and locally show control by fractures. times larger than any historic Snake River flood; hence it provides perspec- South Menan Butte. tive on the effects of exceptionally large prehistoric events (Scott, 1977). Qa South Menan Butte Qls Landslide (Holocene-late Pleistocene)—Rotational slump of older terrace Qts Qa alluvium of Egin (unit Qto) onto the floodplain of Henrys Fork. Scarp has Tuff of South Menan Butte (late Pleistocene)—Indurated, gray-green to SOURCE OF DATA been modified by road construction. brown, poorly sorted, massive to thin bedded, palgonitic, lapilli tuff to fine tuff. Similar to tuff of North Menan Butte except for accidental lithic The map is based upon compilation and consultation of master thesis concentration. Tuff of South Menan Butte contains more than 1.5 times the Qa Qa studies (Ferdock, 1987; Allison, 2001; Creighton, 1982), county soil surveys VOLCANIC ROCKS accidental lithics by volume as tuff of North Menan Butte. Accidental clasts Qblg (Noe, 1981; Jorgensen, 1979), regional geologic mapping (Scott, 1982), are most common on the rim and interior of the crater and tend to be domestic water well logs (available from Idaho Department of Water Qblg Basalt of Little Grassy Butte (late Pleistocene)—Gray to dark-gray, porphyritic concentrated in thin beds with large lapilli of juvenvile vesicular basalt.
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