Geologic Map of the Northern Part of the Leadore Quadrangle, Lemhi County, Idaho
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IDAHO GEOLOGICAL SURVEY TECHNICAL REPORT 20-03 BOISE-MOSCOW IDAHOGEOLOGY.ORG PARKER AND PEARSON BABY JOE GULCH THRUST DESCRIPTION OF MAP UNITS Mmg McGowan Creek Formation (Mississippian)—Black to dark-gray and light-gray GEOLOGIC MAP OF THE NORTHERN PART OF THE LEADORE QUADRANGLE, LEMHI COUNTY, IDAHO siltstone and claystone and gray- to tan-weathering silty lime mudstone and lime mudstone. Bedding ranges from mm-scale laminations to tabular, decime- Cambrian plutonic rock and Ordovician to Devonian sedimentary strata. We CORRELATION OF MAP UNITS on the Wentworth scale (Lane, 1947). Carbonate rocks are described using ter-scale beds. Recessive, slope-forming unit, distinguished by abundant chippy Lund’s (2018) in the southeastern part of the map area. The Baby Joe Gulch Stuart D. Parker and David M. Pearson Mass-Movement metric units, with imperial units listed in parentheses. Multiple lithologies within cement. Bottom and top are mostly siliciclastic, while middle contains more thrust is planar for most of its trace, from Jakes Canyon to Italian Gulch. Though Department of Geosciences, Idaho State University Deposits Alluvial Deposits a rock unit are listed in order of decreasing abundance. Intrusive rocks are carbonate, including a lavender-weathering lime wackestone bed of a few poorly exposed, our mapping constrains a dip to the south-southwest at 30-40˚, meters thickness. Sharp bottom contact. Gradational upper contact is placed with parallel foliation developed within the pluton. In and west of Thompson 2020 Holocene quartz (Q), alkali feldspar (A), and plagioclase (P) on a ternary diagram (Streckeis- Gulch, the Baby Joe Gulch thrust clearly cut the Thompson Gulch thrust and associated mylonite. These rocks in the immediate footwall of the Baby Joe Qls Qal QUATERNARY en, 1976). Volcanic rock compositions were not studied here in detail. Composi- Huh (1967), “Milligen(?) and Three Forks Formations” of Ruppel (1968), “Three tional names for small exposures of Cenozoic dikes are from Ruppel (1968). Forks” of Lucchitta (1966), and “Trident member of the Three Forks Formation”, Gulch thrust exhibit a foliation that dips more shallowly to the west-southwest Pleistocene CENOZOIC ? “Lower Sappington”, and “McGowan Creek Formation” of Grader and others than constrained for the Baby Joe Gulch thrust, which suggests the presence of Intrusive (2016). May correlate with transgressive systems tract above the “Trident a footwall ramp. East of Italian Gulch, the trace of the Baby Joe Gulch thrust Rocks ALLUVIAL DEPOSITS curves to the northeast and then appears to be truncated by the Railroad A measured at a full section in the head of Jakes Canyon, ~0.75 km (0.5 mi) north Canyon thrust. In several places, the Beaverhead pluton rocks in the hanging 113° 22' 30" A 20' BB 17' 30" 113° 15' Td TERTIARY o Qal Alluvium (Holocene)—Sub-rounded to angular, unconsolidated cobble-gravel and wall of the Baby Joe Gulch thrust are structurally overlain by calcareous mylon- o o of the map area. o o 36 o o o o o 44° 45' o 44° 45' o 25 25 ite. Sense of slip is unclear. Top-to-NW sense of slip was interpreted in the imme- 7 o Mylonitic Rocks Sedimentary Rocks 28 o Msp Mmc? o Msp o 20 20 40 Mr Thickness varies considerably, but is usually from 1-3 m (3-10 ft) in mountain diate footwall on the western side of Jakes Canyon. Top-to-NE sense of slip was o # Mb o Dj Dark- to light-gray dolomitic mudstone, fossilif- Ú ¹ ¹ o Southern Northern Ú¹ Td o o interpreted in the immediate footwall on the eastern side of Jakes Canyon. o canyons. Fill of Beaverhead Valley is likely substantially thicker. o ¹ Mmg? ¹ 38 o Qls ¹ 27 Ú o Qal 45o erous wackestone, and minor tan-weathering quartz arenite. Tabular bedding Mmc 32 16 1 Facies Facies 21 Ü o Ü Ü o Ü 30 20 ¹ o È o 6 45 33 25 ¹ ¹ ¹ ranges in thickness from ~0.5 m to 3 m, occasionally with calcite vugs at the top. ¹ È Dj? 32 m o ¹ o o ¹ o C o o ? ? ¹ o 40 o sc sc 14 42 ¹ Recessive weathering slope former, with isolated resistant beds. Fetid odor on È 24 È 25 m Ü19 o 30 ¹ MASS-MOVEMENT DEPOSITS RAILROAD CANYON THRUST 25 o o # o 56 20 o o sc 15 oo o o o o Ü25 sc PENNSYLVANIAN freshly broken surfaces. Flat to wavy (algal?) laminations on mm-scale are o 30 58 o o # sc o o 22 o o 20 ¹ o 24 20 ¹ o ¹ o o ¹ 38 30 o Mb Qls Landslide deposits (Holocene to Pleistocene?)—Poorly-sorted, unconsolidated common. Rare rip-up clasts, minor scour pits, and ooids. Distinctive, fossiliferous 76 o o 24 o o 29 o 50 o o o10 Qls the southeastern part of the map area. This thrust dips moderately to the SE, and Mr 30 sediments resulting from downhill movement. Mapped on the basis of sections exposed on the Baby Joe Gulch thrust hanging wall. Exposures outside o 36 ¹ o o ¹ Mr Mb o 13 Qls ¹ Mr hummocky topography and the presence of a head scarp. Typically associated of the Baby Joe Gulch thrust hanging wall consist of a thinner section of almost ¹ o o o sc ¹ 80 ¹ o 39 16 # ¹ 74 21 o with the shaley Railroad Canyon Formation. o15 o ¹ ¹ o ¹ ¹ ¹ o ¹ Ü o o Ü17 ¹ o ¹ # Ú 15 Ú 18 o ¹ # o stretching lineations in the mylonite are ~ E-W oriented, parallel to stretching 18 24 77 o the Baby Joe Gulch thrust hanging wall contain a lower unit of unfossiliferous 23 o Msp 15 9 Ü ¹ # o Ü o 17 o ¹ 15 È 31 È 38 # 28 o 45 58 m MISSISSIPPIAN lineations in the mylonite beneath the Thompson Gulch thrust. The footwall Ü o 33 50 Ü m 23 o ÚÜ È 34 o o Ü o o È ¹ ? o o ¹ o 23 30 o m b CENOZOIC INTRUSIVE ROCKS Ü 81 ¹ ¹ Ü ¹ 34 o mylonite lies above the Beaverhead pluton, visible in rare outcrops in Italian o 14 m ¹ ¹ 28 Ú 6 dolomitic wackestone containing bivalves, brachiopods, crinoids, 25 o o ¹ m ? sc Mmc o ¹ 20 o Yqgh o ¹ 40 ¹ PALEOZOIC ¹ ¹ ¹ o m ¹ o 28 o o 38 o o ¹ ¹ Gulch, Baby Joe Gulch, and Jakes Canyon. The relationship between mylonites ¹ 20 ¹ 5 ¹ 31 ¹ (possibly including ) and distinct beds of large ¹ o stromatoporoidea actinostroma ¹ 35 Qal ¹ 18 ¹ 15 9 Ü ¹ Ü Ü 26 ¹ È Ü 25 22 9 25 of the Railroad Canyon thrust, the Thompson Gulch thrust, and the mylonites in Dj? Msp 21 o (cm-scale width) cladopora overlain by smaller (mm-scale width) amphipora. o ¹ È 31 27 ¹ o m o ? o o Mmg Td Diabase (Eocene?)— ¹ ? 16 ¹ o o o Ü ¹ 30 ¹ ¹ 32 ¹ 12 m 26 Ü o ¹ o o o ¹ 18 ¹ o Ü Mb o o Everywhere observed, the bottom contact is sharp and marked by a tan-weath- the hanging wall of the Radio Tower thrust is unclear. The initially low-angle, Ü 20 o ¹ ܹ o 24 porphyritic. Anhedral crystals 2-3 mm in diameter, of altered plagioclase 10 o 38 41 4 16 35 34 34¹ 7 5 ¹ o o Ü 14 Mmg? o 30 ¹ Ü 58 Ü ¹ Ü ¹ Ü ¹ ering quartz arenite. The basal sandstone contains well-sorted, well-rounded ¹ 34 # Ü ¹ 40 ¹ ? r 14 o ? ¹ ¹ 21 38 feldspar. Subhedral to euhedral crystals 1-2 mm long of hornblende. Poorly ¹ 37 ¹ Dj o Ü 40 6 32 r Dj o ¹ ¹ Ü13 Ü Ú11 10 DEVONIAN o ¹ o attitudes from the mylonite. It also appears to be cut by the higher-angle, 55 ¹ Ü ¹ m 22o 25 26 4 ¹ ¹ 70 42 ¹22 sc ¹ exposed. Typically in dikes 1-3 m thick (3-10 ft), less commonly 15-30 m thick # C’ ¹ ¹ Mmc? ¹ ¹ 39 ¹ 20 33 ¹ ¹ 8 ¹ Ú Ü12 Ü 50 ¹ Ü Ü ¹ Ü 4 unconformity basement-involved Baby Joe Gulch and Italian Gulch thrusts. A well-exposed o Ü Msp Mr ¹¹ 21 20 m 20 Ü Ú Ú o ¹ Ü 12 o (50-100 ft). Ü Mb 13 32 40 ¹ 12 Ü Ü ¹ o # 1 o Ü 10 Dj 50 Ü ¹ Ü 20 ¹ Ü ¹ ÚÜ o Ú 23 33 ¹ WNW-ESE trending hanging wall syncline is observable on the eastern side of Ü 12 38 # ¹¹10 unconformable. Upper contact is not observed on the Baby Joe Gulch thrust Ü 12 ¹ ¹ Ú 18SP14 Ü12Ú 4 7 o Ü ¹ o Ü SOs 13 o ¹ ¹ o Ü 47 25 11 47 Ü o 44 ¹ o SILURIAN o Ü ¹ ? ? o o o ¹ Railroad Canyon, just below an unnamed thrust that carries Kinnikinic Quartzite È ¹ o o plate, but is sharp when observed elsewhere. Section exposed below the Baby Ú È o87 24 o Ü È 7 o o Ü 30 Ü SOs o ¹ o 48 17 ¹ 18SP15 Ü 39 È 38 ¹ 28 o ¹ 2 m ¹ 21 o o 50 70 o ¹ o 44 15 s in its hanging wall. ¹ ¹ s o STRONGLY TECTONIZED PALEOZOIC STRATA Joe Gulch thrust is ~50 m (165 ft) thick and is likely Famennian in age (Scholten Ü 8 5 4 ¹ sc ¹ o Ü ¹ o ¹ 87 ¹ ¹ Ü 19 Ü44 Ü Ok 1 ¹ ܹ 40 o 25 ¹ 28 Ú o Ü8 and Hait, 1962; Grader and others, 2016); ~150 m (490 ft) are exposed above the Ü o ܹ Ü Ü o 30 20 Ok m Ü 75 Ü 20 ¹ o Ü20 o 40 ¹ È # È 18 35 os o Ú 41 o Ü o unconformity 1 ¹ s 60 7 Ü Ü Baby Joe Gulch thrust and is likely Frasnian to Famennian in age. o o 29 27 8Ü 15 ¹ 62 RADIO TOWER THRUST 55 o ¹ o ¹ 20 35 o o o o 24 o ORDOVICIAN È o 30 ¹ 19 # È b Gray to tan, massive brecciated Mr ¹ 13 o o 30 o 83 o o Thrust # 14 30 ¹ o 16 o# o 8 Intrusive ¹ 62 ¹ Ü o o 32 sc Ü Ü Ü Ü o Ü o 36 58 Mr 36 ¹ 15 33 15 o ¹ 60 ¹ Rocks ¹ o 21 Metasedimentary Msp o 34 Ü ¹ 40 Ú m Ü 52 19SP40 Ü o o 16 Gulch southwestern part of the map area.