Geology and structure of the central Ruby Range, Madison County, Montana JOII:\ M. GARIHA:--J Department ofGeosciences, Indiana University Northwest, Gary, Indiana 46408 Geological Society of America Bulletin. Part II. v, 90, p. 695-788, I ~ figs., 2 tables, April 1979, Doc: no. M90402. basement rocKS immediately encircling the INTRODUCTION range crop out beneath a spotty cover of The study area, encompassing about 260 sedimentar~ and volcanic rocks in the Black- km2 within ~adison county, is situated 19 km tail, Snowcrest, Greenhorn, Gravelly, ~Iadison, due east of Dillon, .Montana. It lies Tobacco Root, and Highland Ranges (Klepper, athwart the middl~ third and narrowest part 1950;'Keenmon, 1950; Berg, 1976; ~adley, of the Ruby Range, and cletaLled mapping pro- 1969~, 1969b; }lillholland, 1976; ~1cThenia, vides data for a geologic history and com- 1960; Burger, 1969; Cordua, 1974; Duncan, plete cross sections, basin to basin, through 1976; and numerous others). the uplifted block. Included are all of the Schist, gneiss, and metaquartzite in. the :Iine Gulch and parts of the Metzel Ranch, vicinity of the Stinking Water (Ruby)" Range Beaverhead Rock Southeast, Belmont Park were first recognized by F. V. Hayden's sur- Ranch, and Red Canyon 7!i-minute quadrangles. vey in 1871 (Hayden, 1872). Later, Hinchell The Ruby Range is one of several non- (1914) correlated these rocks with the Cherry parallel mountain systems in southwestern Creek Group, as defined by Peale (1896) for Montana whose exposed cores are composed of . a thick pile of metamorphosed sedimentary multiply folded, high-grade Precambrian rocks underlyin~ the east flank of the Grav- (pre-Beltian) mctamorphic rocks (Fig. 1). elly Range south.of Enni~. In addition to Similar exposures of complexly deformed these Cherry Creek units, Heinrich (1950a,. 695 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/4_Part_II/695/3444201/i0016-7606-90-4-695.pdf by guest on 26 September 2021 '.. ...., _.. - ~ . -- . ..' . .~ ~tU(b area ~ norMal fault MONTANA o. o 112 00 Figure 1. Index map showing location of the study arga and outline of ranges of southwestern Montana and adj~c~nt "Idaho. 1: Blacktail fault;~: Blacktail Creek; 3: Blacktai~ basin; 4: Beaverhead River basin; 5: Rudy Dam; 6: Ruby basin; 7: Greenhorn thrust; 8: Snowcrest thrust. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/4_Part_II/695/3444201/i0016-7606-90-4-695.pdf by guest on 26 September 2021 97 ". 1950~, 1960) recognized extensive tracts in are scattered over the entire area:or not the Ruby Range underlain by the Dillon "gran uncommonly\are localized along the eastern ite gneiss" and pre--Cherry Creek rocks. range-bounding faults. Gently folded basin This threefold division of units is used deposjts, late Eocene to middle Pliocene in successfully within the Precambrian metamor- age (Becker, 1961; Honroe, 1976), on the phic complex or basement of the Ruby and eastern range flank are faulted against or Blacktail Ranges, but it generally h~s not unconformably overlap the baseme~t crystal- been recognized'or applied elsewhere (Gil- line rocks. etti, 1966). However, the generalized mdp- Prior to any regional mapping, the older ping designations are useful where a number geologic' Li t e r a t ur c covering the Ruby Range of diverse rock types are interlayered meso- tended to describe the diversc' nonmetallic scopically, and they are retained where deposits of economic interest, such as appropriate on the geologic maps (Figs. 2, graphite (Winchell, 1911; Bastin, 1912; 3)·. At variou~ times in the Precambrian, Ford, 1954), talc (Perry, 1948), corundum the :~tamorphic rocks have been i~truded by (Heinrich 1950b) and sillimanite (Heinrich, pegmatite, ultramafic rock, and diabase. 19~Oa), or dealt petrologically with specific Northwest-trending wedges of faulted and rock occurrences, such as pegmatites (Hein- tilt~~. locally folded Paleozoic and younger rich, 1949), ultramafic rocks (Sinkler, rocks (chi e f Ly Tertiary) unc onfo rmab Ly rest 1942), and carbonate rocks (Heinrich, 1947). on the basement units (Fig. 4). The rocks A second generation of studies, including underlie the higher elevations of the north- many.by U.S. Geological Survey workers, has ern one-third of the range (Tysdal, 1976a), involved careful geologic mapping and sam- the resistant Paleozoic carbonates being pl i.ng of small areas--for example, the Carter prominent ledge formers. Hinor small Terti- Creek (James and Wier, 1972; Ross and others, , ary stocks, lava flows, and tuffaceous 1969; Dahl, 1977) and Kelly (James and Wier, deposits (basaltic to rhyolitic in composi- 1960) iron deposits. Hore recently, detailed tion) in contac.t with rhe Precambrian rock regional structural studies within the range . Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/4_Part_II/695/3444201/i0016-7606-90-4-695.pdf by guest on 26 September 2021 o I 2 MILES I" ii' o 2 3 KILOMETERS D 17 N N " 18° PREGAMBRIAN ROCKS ~(1961) Cherry Creek rocks - ee Dillon gneiss - d pre- Cherry Creek rocks - pee diabase - di . PALEOZOIC ROCKS -P T8S TERTIARY SEDIMENTARY ROCKS -T T9S TEF\TIARY VOLCANIC ROCKS - Tv QUATERNARY ~LLUVIUM - Qql Figure 2. Generalized geologic map of central Ruby Range. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/4_Part_II/695/3444201/i0016-7606-90-4-695.pdf by guest on 26 September 2021 For Figure 3 . see page 703 this row. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/4_Part_II/695/3444201/i0016-7606-90-4-695.pdf by guest on 26 September 2021 I~m - IfTli J .' <?A Pu 780CX 5800' '- 5800' o Ikm Imi I II 'I I," -.5000' ,0 I ! Ikm I Figure 4. Selected cross section, central Ruby Range. See Figure 3 for explanation of symbols and Figure 2 for location of cross sections. Figure 4 continues on the following frame. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/4_Part_II/695/3444201/i0016-7606-90-4-695.pdf by guest on 26 September 2021 O Ikm Imi 1 8 I-" -----..!....--.f-----_II 8 6700'.., , ,,~ I " ,' IQol/QC" II , t1u I KT~h , ~ ';;r=> , " 'T~ d/ I I J '\ \ I f 1 I I / ~- - ,--.' r". \ I II - r //1 ~ ~ lf1,~ 1\ d'} I I I C/ /1 ~ I\ H Vst/ l { I I \ ,j 1 I I I 1'/d; 4700' i , t:?; 4700' c' _?-- /' ' ........... / ........ / >-. / '? / - '-?/. '" Oil " /, " /. \. ( .t /"I "',7, '" / 0 " \ . ( '~"""~ I ,,1I I '/' ''.I',,11, 1"- """"/./ '" ' \ ,I \ ,,,. "I 6600' /.I " , ,.....I E1 I /r : I / ,/ v /" Tv 7000' ] if II! 1, / ~ III ;VII,01"'1, I V"'1 j1ld I J //j " 5000' ~ / i I 1/I"[; J " ~ J I, figure 4. (Continued) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/4_Part_II/695/3444201/i0016-7606-90-4-695.pdf by guest on 26 September 2021 have be,en undertaken, directly to the south TOPOGRAPHY AND ROCK EXPOSURE of the present map.ianea (Okuma, 1971; James and others, 1969) and to the north (Tysdal, The Ruby Range is a northeast-trending, 1976b; ~mphasizing·~aramide s~ru~tures in rectangular region approximat~ly ~ by 48 Paleozoi~ rocks; L. Karasevich, in prep). km, bounded by normal faulfs along which the Because of its geographic position, the block was uplifted in Tertiary time. Eleva- central part of the Ruby ~ange is a'key tions across 'the area range from 1,6.80 to ~rea in the interpretation of the range's 2,550 m (5,600 to 8,500 ft), with the Dillon metamorphic and structural history. For gneiss generally at elevations ab0ve 2,040 m example, a ~ignificant Precambrian retrograde (6,800 ft)_ I.ower Paleozoic carbonates are metamorphic episode is, indicated by cordi-' the most resistant rocks ar.d generally are erite-garnet relationships in anthophyllite found Ln.i tho northern part of the study area schist and t ce r t a i.n metapelites (P. Dahl and as t r ec-covor ed cliff-formers above 2,300 m J. Garihan, in prep.). The episode is_~n- (7,500 ft). Local relief across major known in' the southern Ruby Range. Thetsnudy drainages rarelyexcee2.s 300 m (I,OOOft). area is a link between Precambrian structure ,Major streams generally flow perpendicular in allochthonous (?) Cherry Creek roc~~ to to' the northeast axis of the range, and their the ~.outh and the Laramide structural s t yle dra inage lineartty is a result of a northwest o f upthrus t ing to the north (Fi~',. 5). Two trend of faults. The area 'between these major Prec~mbrian fo~d generations and one faults:c~nbe considered as blo~ks whose re- induced in Laramide-time are represented in lief is due to differential vertical movement autochthonous rocks of the central Ruby on/the faults (particularly the Stone Creek Rangev Moreover, in the study area one can c. and Sweetwater 'faults, which are quite visible demonstrate late Cenozoic (Pliocene)' (Hon-, in high-altitude photog~aphs). Well-developed roe, 1976) ~otion on at least one of the fault scarps mqdiMed by erosion form the northwes~-trending normal faults that dissect southwestern boundary of the downdropped Sage the,metamor~hic complex. Creek basin and the larger Sweetwater basin Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/4_Part_II/695/3444201/i0016-7606-90-4-695.pdf by guest on 26 September 2021 Figure 3. Geologic map of central Ruby Range, southwestern Montana. Figure 3 appears on 4 frames; pages 705, 706, 719, and 720. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/4_Part_II/695/3444201/i0016-7606-90-4-695.pdf by guest on 26 September 2021 Explanation of figure 3 Quaternary Qal alluvium Qc colluvium Qh hot-spring limestone Tertiary v v Tv ' volcanic .rock s Tu sedimentary rocks, undivided UNCO~FOR~11TY Cretaceous-Paleocene KTbh Beaverhead formation lj'~CO:--JFOR~IITY Paleozoic .