Late Cretaceous Stratigraphy, Deformation, and Intrusion in the Madison Range of Southwestern Montana

Late Cretaceous stratigraphy, deformation, and intrusion in the Madison Range of southwestern Montana

R. G. TYSDAL \ R. F. MARVIN > U.S. Geological Survey, Federal Center, Denver, Colorado 80225 ED DEWITT j

ABSTRACT

Dating of orogenic rock units in the central part of the Madison Range shows that Lara- mide deformation was virtually completed by the end of the Cretaceous. Early Campanian K-Ar dates of about 79 m.y. were obtained from welded tuffs in the basal part of the Liv- ingston Formation, a volcanic and volcani- clastic assemblage that is conformable with underlying Cretaceous clastic rocks and with the overlying Sphinx Conglomerate. No dat- able materials were obtained from the Sphinx, but both it and the Livingston were deformed by the Hilgard fault system, a series of thrust faults and associated folds which extend along the western side of the southern two-thirds of the range. This north-trending fault system represents the culmination of Laramide shortening within the range. K-Ar and 40Ar/39Ar dating of hornblende from dacitic laccolithic rocks that intruded the Hil- gard fault system in the central part of the range indicates an approximate date of 68-69 m.y. B.P. for emplacement of the igneous suite. The dacite postdates movement along faults of the Hilgard fault system, and postdates the synorogenic Sphinx Conglomerate.

INTRODUCTION

The Madison Range trends north from the northwest corner of Yellowstone National Park (Fig. 1) and is in the foreland of the Rocky Mountains of southwestern Montana. Our study area encompasses the part of the range that is south to southeast of the Shell Canyon-Lone Mountain area and east to northeast of Lone Mountain (Fig. 2); data of the Fan Mountain area and northward are generalized from Swan- Figure 1. Index map, showing location of Madison Range and major structural features.

Geological Society of America Bulletin, v. 97, p. 859-868, 8 figs., 4 tables, July 1986.

859

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MADISON:

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Qu, Ku, AND DACITE UNDIVIDED

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45° 1 5' GALLATIN

SCARFACE PLATE RANGE

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< Qu Unconsolidated deposits (Quaternary! > Qu Dacite intrusive rocks {Upper Cretaceous) Ks Sphinx Conglomerate (Upper Cretaceous} m-< Kl Livingston Formation (Upper Cretaceous) El Everts!?) Formation and Virgelle Sandstone (Upper o• • Cretaceous) .•jj' : Ku Cretaceous rocks older than Virgelle Sandstone rov (Upper and Lower Cretaceous) a Sedimentary rocks, undivided (Mesozoic and Paleozoicl Ü Metamorphic rocks, undivided (Archean) Contact Fault. D, downthrown side; U, upthrown side 45° Normal fault of range front system; bar and ball 00' on downthrown side «a Ku -à A. Thrust fault. Teeth on upper plate i Anticline •4* Overturned anticline Syncline "V 4 MILES Bedding £ 67 inclined KILOMETERS -d- overturned 1 • Sample locality

Figure 2. Gen eralized geologic map of the central part of the Madison Range. Data are from Tysdal and Simons (1985); Tysdal (in press); unpublished data of Tysdal; and, west and north of Fan Mountain, from Swanson (1950).

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son (1950). The Madison Range has a basement dal and Simons (1985) to the Late Cretaceous Flathead Sandstone. One pebble of bull quartz, of Archean rocks metamorphosed about Everts(?) Formation. probably a Precambrian metamorphic rock, was 2,750 m.y. ago (James and Hedge, 1980) and an The middle member of the Livingston found, but no other metamorphic clasts were overlying Phanerozoic sedimentary sequence Formation is chiefly a vent-facies assemblage of seen. as much as 4,700 m thick. Paleozoic rocks reddish-purple dacite to andesite flow-breccia. No strata overlie the Sphinx Conglomerate, (900-1,200 m thick) are chiefly carbonate strata Less abundant rocks include trachyte and tra- but the contact with the underlying Livingston but also include a few units of clay shale and chyandesite flows and flow-breccia, basalt, and Formation is conformable almost everywhere, quartz sandstone. In contrast, Mesozoic rocks minor welded tuff. The member is estimated to and the compositional change from one forma- (2,500-3,500 m thick) are almost entirely clastic be 300 to 400 m thick at the northwest end tion to the other is transitional. Clasts of Cre- strata. The uppermost two Mesozoic units, the of the belt, but it thins rapidly toward the south- taceous and Jurassic rocks found in the basal Cretaceous Livingston Formation and the east end. beds of the Sphinx also are present in the upper Sphinx Conglomerate, are critical to the chro- The upper member is an alluvial-facies se- part of the Livingston. Along the southwest side nology of Laramide orogenic events in the quence as much as 200 m thick. Its lower two- of Sphinx Mountain and The Helmet (Fig. 2), range, and summary descriptions of them are thirds is conglomerate, composed of well- the Sphinx Conglomerate rests discordantly on presented in this report. The synorogenic Sphinx rounded volcanic clasts in a matrix of volcani- the middle member of the Livingston, and this is of particular interest because it previously was clastic sandstone. The upper third is chiefly sand- area of discordance apparently is what led pre- considered to be an Eocene deposit that indi- stone, which contains a few lenses of conglom- vious workers (Peale, 1896; Beck, 1960; Hall, cated orogenic activity in Tertiary time. In this erate, and thin units of red and green mudstone. 1961; and Hadley, 1980) to conclude that the paper, we show that (1) the Sphinx was de- Volcanic clasts predominate in the conglomer- contact was unconformable. Beck (1960) stated formed by thrust faults of the Hilgard fault sys- ate, but carbonate clasts from Mesozoic forma- that it appeared conformable near the eastern tem, which represents the culmination of Lara- tions occur in increasing amounts upward in the part of Sphinx Mountain. It would not be un- mide shortening within the range; and (2) the sequence. usual, however, for a terrigenous deposit such as Hilgard system was later intruded by laccolithic the Sphinx Conglomerate to lie unconformably rocks that yielded Late Cretaceous radiometric SPHINX CONGLOMERATE on Livingston rocks locally and conformably on ages. The time span of deformation in the range them elsewhere. The area of discordant contact also could be structural, a possibility that we is bracketed between these radiometric ages and The Sphinx Conglomerate of the Madison discuss below, in the section on tectonics. by radiometric and paleontologic ages from the Range is preserved only at Sphinx Mountain Livingston rocks, which underlie the Sphinx and The Helmet (Fig. 2), where it is beautifully Conglomerate. Age data are presented after dis- exposed in cliffs and on steep slopes. The forma- INTRUSIVE ROCKS cussion of structural relationships, which aid in tion is a reddish-orange sequence, about 600 m interpretation of the ages. thick, of interlayered conglomerate and sand- A series of tabular igneous intrusives are pres- stone. The sandstone layers, as well as the sand- ent at Lone and Fan Mountains (Fig. 2); these LIVINGSTON FORMATION stone matrix of the conglomerate layers, are were termed "Christmas tree laccoliths" by fine- to coarse-grained, pebbly, poorly sorted Swanson (1950). The intrusives exhibit a variety The Livingston Formation of the Madison rock composed of calcite-cemented quartz, of cross-sectional shapes, including those of lac- Range was mapped originally by Peale (1896), chert, and fragments of limestone. Hematite is a coliths, sills, and dikes, but for ease of reference, who correlated it with the Livingston of the type minor cement; it coats many of the sand grains we call the bodies "laccolithic intrusives," re- area near the town of Livingston in west-central and clasts of the conglomerate and gives the gardless of shape. Montana. Hadley (1980) believed that the unit formation its color. The sandstone layers are typ- The intrusive rocks are chiefly porphyritic da- was an erosional remnant of the lower member ically about a metre or two thick. Conglomerate cite, although andesite is present locally. Whole- of the Elkhorn Mountains Volcanics of western layers are commonly a metre to a few metres rock chemical analyses of four laccolith samples Montana, which crop out about 75 km to the thick, constitute about 75% of the formation, from widely separated localities are shown in northwest. Tysdal and others (in press) de- and consist of rounded pebbles, cobbles, and Table 1, and rock classification is from the scribed the unit in some detail and reassigned it boulders as much as 75 cm in diameter (Peale, method of De la Roche and others (1980). to the Livingston Formation. 1896; Beck, 1960). Phenocrysts are plagioclase (25% to 35%), which The Livingston of the Madison Range con- Clasts in the Sphinx Conglomerate were typically is zoned; and hornblende (20% to tains three informal members (Beck, 1960; Had- eroded from Mesozoic and Paleozoic forma- 25%), which is pleochroic from dark green to ley, 1980; Tysdal and others, in press). The low- tions. From the base to the top of the Sphinx, the yellowish brown. Quartz (<5%) and opaques er member is a 60- to 250-m-thick heterogen- clasts record a sequence that is the inverse of the (<2%) are minor constituents. Biotite is un- eous alluvial-facies assemblage of volcaniclas- normal stratigraphic order of the formations rep- common. X-ray study shows that the ground- tic sandstone composed of basaltic debris, and resented. Cretaceous, Jurassic, Permian, and mass (30% to 40%) is chiefly plagioclase and less abundant conglomeratic lahar deposits, oli- Pennsylvanian formations are represented in the potassium feldspar with minor quartz and vine basalt, and welded tuff. The basaltic sand- lower beds of the Sphinx Conglomerate. Lime- hornblende. stone is the thickest and most extensive unit. At stone clasts of the Mississippian Madison Group The main center of intrusion is at Lone the southeast end of the outcrop belt (Fig. 2), it dominate in the middle part (about three-fifths) Mountain; Fan Mountain is only a subsidiary contains interbeds of mudstone, bentonite, tuff, of the Sphinx but are present in the lower and center. The positive gravity anomaly (Fig. 3) and thin discontinuous layers of coal. Lenses of upper parts as well and are the chief component associated with Lone Mountain is of higher am- conglomerate, some as much as several metres of the formation. Pebbles and cobbles of the plitude than that at Fan Mountain, and intru- thick, are common in the northwest end of the Devonian Jefferson Formation become com- sives are thicker and more abundant. At Lone outcrop belt. The basaltic sandstone is conform- mon in upper beds of the Sphinx and are suc- Mountain itself, which is located at the north able with the underlying unit of sandstone, ceeded upward by abundant clasts of Cambrian end of its associated gravity anomaly, Creta- siltstone, and minor mudstone assigned by Tys- Pilgrim Limestone, Meagher Limestone, and ceous sedimentary strata form thin, laterally

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disrupted layers within the intrusive rocks TABLE 1. WHOLE-ROCK CHEMISTRY AND LOCATION DATA FOR CRETACEOUS IGNEOUS ROCKS FROM CENTRAL PART OF MADISON RANGE (Fig. 4), whereas in the south and southwest

areas of the gravity anomaly, intrusives form Sample no. Location Rock name Geollgic sills between the Cretaceous strata. (lai. N, long. W) (classification occuirence scheme of De la Roche Dikes of porphyritic dacite, similar to that of and others, 1980) the laccolithic intrusives, crop out along the west 82MTzl765 45<'12'34"N Dacite porphyry Lacoilith side of the Madison Range in a several- Hl°3r46"W kilometre-wide zone that extends north from 81MTzl264 45°16'00"N Jo— Sphinx Mountain lo the northern limit of sedi- 111°27'17"W

mentary rocks (Fig. 2). Dikes range from 1 m to 82MTzl766 45°17'11"N Andesite porphyry several metres thick and commonly are widely 111®26'24"W spaced; at a few locations, however, the dikes 82MSÌ827I 45°14'23"N Dacite porphyry are closely spaced, almost sheeted. All dikes are 1II°31'00"W vertical, or nearly so, and trend northward, ex- 82MTzl903 45°12'53"N 11I°33'45"W cept in the southern part of the zone where they 82MTzI784 4S°I4'02"N Rhyodacite trend northwestward. lll°33'36"W

STRUCTURAL GEOLOGY

Broad, regional tilting or uplift in the Madison Range area occurred prior to deposition of the Livingston and is reflected mainly in the occur- rence of nonmarine strata of the Everts(?) For- mation between thi; Livingston and the marine Virgelle Sandstone. No significant folding or faulting occurred prior to deposition of the Liv- ingston, and no evidence was found for angular discordance with the underlying Everts(?) strata; the two units are interlayered locally (Hadley, 1980; Tysdal and others, in press). Intense tec- 45° 20' tonism apparently started in the later stages of deposition of the upper member of the Living- ston. Clasts of Lower Cretaceous and Jurassic rocks occur in conglomerate lenses of the unit and reflect erosion well down into the Mesozoic section.

Northwest-Trending Faults

Two major noithwest-trending faults, the Buck Creek and Spanish Peaks faults, cut through the central part of the range and are flanked by genetically related folds (Fig. 2). The 15' Buck Creek fault, the more southerly of the two faults, is nearly vertical at present levels of expo- sure. It has a maximum displacement of about 1,800 m northeast of Sphinx Mountain near the western edge of the range, where it meets the northeast-trending Bear Creek fault. The Buck Creek fault is flanked on the northwest by the Buck Creek anticline and on the southwest by the Sphinx Mountain syncline, in which is preserved the thickest section of strata within the Figure 3. Bouguer gravity map of the Lone Mountain area. Contours in milligals. range. The Spanish Peaks fault spans the entire Dots represent sites of measurement (contoured from data of Hassemer and Kauf- width of the range, dips about 60° to the north- mann, 1984). east, and exhibits a minimum vertical displacement of about 4,900 m. The Lower Basin syncline flanks the fault on its southwest side. son Range, Reid (1957) and Wooden and evidence of a Precambrian ancestry for the Buck The Buck Creek and Spanish Peaks faults are others (1978) demonstrated that several north- Creek fault is available because no Precambrian two of a prominent series of northwest-trending west-trending faults were first formed during the rocks are exposed along or near it. The Spanish fractures of the foreland in southwestern Mon- Precambrian, and that many of the faults were Peaks fault, however, is believed by Schmidt tana. In ranges of the foreland west of the Madi- reactivated during the Laramide orogeny. No and Garihan (1983) to be a segment of a fault

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trending faults had ceased. We demonstrate this TABLE I. (Continued) relationship (1) in the south where the north-

Chemical analysis (wl %) west-trending Sphinx Mountain syncline is deformed by the Hilgard system and (2) in the Si02 AI2O3 FeTOj* MgO CaO Na20 K20 Ti02 P205 MnO LOI north where the northwest-trending Lower Basin syncline south of the Spanish Peaks fault is 63.1 15.9 4.73 2.64 4.25 4.35 2.47 .54 .18 .06 1.14 overlapped by the north-trending Andesite anti-

62 15.5 4.95 3.34 4.33 4.24 2.55 .53 .18 .10 1.43 cline of the Hilgard system.

59.9 15.2 5.61 3.88 5.23 4.04 2.34 .63 .21 .08 1.81 Relative Ages of North- and Northwest- Trending Structures 63.2 16.4 4.56 2.52 3.96 4.32 2.39 .49 .18 .06 1.02 The Sphinx Mountain syncline probably al- Not analyzed ready existed before thrusting along the Hilgard

66.2 15.6 3.16 1.94 3.21 4.18 2.62 .42 .15 .03 1.18 fault system, or at least thrusting continued after folding had ceased along the northwest-trending axis of the fold. The syncline is asymmetric for •Total iron reported as FeTOj. its entire length, with its steeply dipping north limb adjacent to the Buck Creek fault (Fig. 2). The south limb dips gently northward, except near the west end, adjacent to the Beaver Creek thrust plate of the Hilgard fault system. The effect of thrusting along the Beaver Creek fault becomes increasingly evident westward from about the eastern end of Sphinx Mountain. Fig- ure 2 shows the steepening dip of the Livingston as its strata are traced from east to west along the south side of Sphinx Mountain. The steepening is attributed to deformation caused by emplacement of the Beaver Creek thrust plate. The western end of the Sphinx Mountain syncline also was deformed by the Scarface thrust plate, which overlaps the Beaver Creek thrust plate. Deformation caused by the Scarface plate is evident along the southwest side of Sphinx Mountain, where beds of the Livingston Formation change trend from westerly to northerly (Fig. 2). From east to west toward the Scar- Figure 4. East side of Lone Mountain, a laccolithic intrusive center. Outlines enclose some face plate, the Livingston and underlying of the disrupted strata of the Upper Cretaceous Frontier Formation. Everts(?) strata become progressively steeper, then vertical, and finally overturned. Beds of the Sphinx Conglomerate also swing to a northerly system that, to the west, includes the Bismark basement rocks are thrust over unmetamor- strike along the southwest flank of Sphinx fault, which Reid (1957) showed to have a Pre- phosed Paleozoic and Mesozoic strata; (2) the Mountain and The Helmet (Fig. 2). At The cambrian ancestry. thrust is a zone of faults; (3) the dip of the faults Helmet, the Scarface plate deformed the west- is variable, ranging from as low as 20° to nearly ernmost extent of the conglomerate into north- North-Trending Faults: The Hilgard vertical; (4) overturned strata of Paleozoic and trending upturned beds, as pointed out by Beck Fault System Mesozoic age occur within the zone; and (5) (1960). strata beneath the zone of thrust faults may, or In the area southwest of Sphinx Mountain The Hilgard fault system trends northward may not, be strongly deformed. and near The Helmet, the Sphinx Conglomerate along the western side of the Madison Range Rocks west of the Hilgard fault system repre- overlies the middle member of the Livingston from Hebgen Lake to the Spanish Peaks fault sent the toe of a thrust plate which moved Formation; the actual contact is not exposed, (Fig. 1). It extends for more than 80 km, and in eastward during regional shortening (Tysdal, in but in many places, the two formations can be its northernmost part includes a segment known press). The western part of the sheet was severed traced to within a few metres of each other. The as the "Jack Creek fault" (Swanson, 1950). by the Madison Range frontal faults, a series of contact may be unconformable, a common oc- The Hilgard fault system is a mountain flank en echelon Cenozoic normal faults along which currence for terrigenous units in a tectonically thrust (Tysdal and Simons, 1985; Tysdal, in the main part of the thrust sheet was down- active area. The contrasting style of structural press). Its features are similar to those described dropped to the west and now lies beneath the deformation exhibited by the Livingston and by many authors (for example, Berg, 1962, Madison Valley, concealed by Cenozoic de- Sphinx strata, however, suggests that the contact 1981; Gries, 1983) for mountain flank thrusts in posits. may be a fault. The incompetent Livingston Wyoming and Colorado. Similar features in- Movement along the Hilgard fault system strata were deformed into folds that have steeply clude the following: (1) crystalline Precambrian continued after movement along the northwest- dipping limbs (Fig. 2), whereas the competent

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Sphinx strata dip moderately northeast. The Liv- ingston beds may have been squeezed beneath the western edge of the Sphinx Conglomerate during thrusting of the Scarface plate. The Spanish Peaks fault is older than structures of the Hilgard fault system, and we demonstrate this relationship along a segment of the Spanish Peaks fault in the Madison Range. In both the Madison and Gallatin Ranges, the Spanish Peaks fault is flanked on the south by the Lower Basin syncline (Figs. 1, 2) (Swanson, 1950; Hall, 1961; Garihan and others, 1983). The axis of the syncline is consistently parallel to the fault, although the axial area is exposed only intermittently. Thi; block of basement rocks north of the Spanish Peaks fault was uplifted a minimum of 4,900 m and had a southwestward component of movement, as indicated by the northeasterly dip of the fault. The Lower Basin syncline resulted from the uplift, and the work of Garihan and others (1983) indicates that the Figure 5. Fault and associated fold intruded by undeformed dacite sill, which is about 30 m sedimentary rocks were deformed uniformly thick. Photograph taken 1 km northwest of sample locality 4 (Fig. 2), looking northwest. along the upturned limb of the fold. Upper Cretaceous rock units, from youngest to oldest, are as follows: Kda = dacite, Near the central part of the Spanish Peaks Kev = Everts(?) Formation; Kv = Virgelle Sandstone; Ktc = Telegraph Creek Formation. fault within the Madison Range, axes of the An- desite and Middlefork anticlines trend perpen- dicular to the northwesterly trend of the Lower acted as the northern limit of a tear zone during was deposited during the Campanian Stage of Basin syncline (Fig. 2). The north-trending anti- thrusting along the Hilgard system. Metamor- the Late Cretaceous. This age assignment is clines cut across the projected axis of the Lower phic rocks northeast of the Spanish Peaks fault based on (1) potassium-argon dates (79.8 ± 2.9 Basin syncline and show no deformation that had already been uplifted and locked into place m.y. and 76.8 ± 2.5 m.y.) of biotite from welded could be related to subsequent formation of the against Phanerozoic rocks on the southwest. A tuffs in the lower member of the Livingston and syncline. Therefore:, we conclude that the anti- zone of tear faulting developed along the junc- (2) palynological determinations from the lower clines postdate formation of the syncline and the tion of the rigid and moving rocks. member of the Livingston. genetically related Spanish Peaks fault. In the area of intersection of the two structur- TIMING OF DEFORMATION Postdeformation Relationships and Ages al trends, the northerly trending anticlinal folds end abruptly at several faults that we interpreted The timing of deformation in the study area is Laccolithic Rocks. These rocks in the vicin- as tear faults (Fig. 2). The most southerly of the bracketed by radiometric and paleontologic ages ity of Lone and Fan Mountains intruded sedi- tear faults trends northwest, dips southwest at a from the Livingston Formation, which predates mentary strata that earlier were deformed into moderate angle, anil truncates the Andesite anti- deformation, and radiometric dates from the in- northeast-trending structures during movement cline. Tear faults farther north generally dip trusive rocks, which postdate the deformation. along the Hilgard fault system. Nortieast- moderately southwest, but steepen as they curve No dates were obtained from the synorogenic striking strata are interrupted near Lone Moun- westward and became parallel to the Spanish Sphinx Conglomerate. tain, which forms the center of a dome that is Peaks fault. Slivers of right-side-up strata be- encircled by a syncline at its base (Fig. 2). Fan tween the faults appear to have an easterly di- Predeformation Relationships and Ages Mountain also is a dome, and reconnaissance rected movement. These tear faults are inclined mapping shows that a syncline exists around at opposite to those described by Garihan and oth- The Livingston Formation is the youngest least part of it. Both domes owe their origin to ers (1983) near Dudley Creek (Fig. 2) at the unit that predates the major structures of the intrusion of the igneous material. In every out- southeasternmost extent of the Spanish Peaks Madison Range. The formation is older than the crop that we examined where structural and fault within the Madison Range. Faults confin- northwest-trending Sphinx Mountain syncline intrusive relationships are unambiguous, the ing the Dudley Creek slivers are inclined to the that it occupies and is older than the north- intrusive rocks clearly postdate thrust-related northeast, and rock of the slivers is overturned to trending Hilgard fault system that deformed the deformation. the southwest. According to Garihan and others syncline. Dates from the Livingston provide a The relationship of an intrusive to fault and (1983, p. 306), the Dudley Creek faults are early maximum age for the two sets of structures. fold structures is illustrated in Figure 5. The fault formed splays of ths master Spanish Peaks fault The age of the Livingston in the Madison shown is nearly vertical; has a throw of about and were severed on their rearward (northern) Range originally was considered Cretaceous 150 m, as determined by relative displacement side (and thus isolated) as reverse-fault move- (Peale, 1896), but subsequently the formation of the Virgelle Sandstone; and is interpreted as a ment continued along the master fault. was assigned to the Paleocene (Hall, 1961). splay of a thrust fault. Left of the fault, the dacite Our interpretation of the relative ages of Tysdal and others (in press) believe that the en- intrusive occurs beneath the Virgelle, but on the movement indicate) that the Spanish Peaks fault tire Livingston sequence of the Madison Range right, it occurs above the Virgelle. The intrusive

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forms a continuous outcrop and is not broken by the fault. The dacite apparently intruded strata that previously were deformed into a fold during movement along the fault. Another example of an intrusive/fault relationship is shown in Figure 6. The fault is interpreted as a minor back-thrust formed during development of the Hilgard fault system. Figure 6A shows a bedding-plane fault that abruptly steepens and cuts upsection. Figure 6B, a close- up of part of the top of the mountain, shows an undeformed dacite intrusive that cuts both the fault and folded strata of the Everts(?) Forma- tion and the Virgelle Sandstone. Becraft and others (1970, p. 7) reported that a laccolithic sill near the Spanish Peaks fault "appears to be cut by a smaller fault." We do not regard this possible faulting of a sill to be significant, for the following reasons. (1) Major movement along the Spanish Peaks fault took place prior to major movement along the Hil- gard fault system, as we described above. We have shown that the laccolithic intrusives of the study area are younger than the northwest- trending faults and younger than the Hilgard fault sytem. (2) Minor, post-intrusive adjustment along a major fault, such as the Spanish Peaks fault, is not surprising and does not invalidate other observations or our general conclusions. According to Swanson (1950), laccolithic rocks within thrust slices of the Hilgard fault system along the west side of the range were intruded before faulting occurred. His interpretation was reiterated by Becraft and others (1970). Reconnaissance traverses by T. H. Kiilsgaard (1984, oral commun.) during the study of Be- craft and others (1970) questioned Swanson's (1950) conclusion about intrusive relationships, and traverses by one of us (Tysdal) failed to find deformed dacitic intrusives along the west side of the range at least as far north as the area of Shell Canyon (Fig. 2). If deformed dacitic intrusives do exist along the western margin of the Figure 6. A. Minor back-thrust fault intruded by dacite sill-like body that is estimated to be range, as indicated by Swanson (1950), then it is 75 m thick. B. Close-up photograph of part of Figure 6A. Photograph taken 1 km north of possible that some thrusting could have occurred sample locality 4 (Fig. 2), looking east. Upper Cretaceous rock units, from youngest to oldest, after intrusion, in addition to the post-thrusting are as follows: Kda = dacite, Kev = Everts(?) Formation; Kv = Virgelle Sandstone; Ktc = intrusion that we found in our study area. At the Telegraph Creek Formation. range margin, however, one must be careful to distinguish deformation caused by thrusting from that caused by later basin-and-range that the Everts(?) Formation, Livingston Forma- Sphinx Conglomerate is absent from the north- faulting. tion, and the Sphinx Conglomerate all had been erly area. This change in trend probably was deposited before the formation of the Sphinx caused by thrusting along the Hilgard fault Relationship of Laccolithic Rocks to Mountain syncline and that subsequently these system. Sphinx Conglomerate rocks and the west end of the syncline were The change in trend occurs immediately deformed during thrusting along the Hilgard southwest of the southwest corner of an uplifted Laccolithic rocks did not intrude preserved fault system. Northwest of Sphinx Mountain block delimited by the Buck Creek and Bear outcrops of the Livingston Formation or the (Fig. 2), the northwest end of the outcrop belt of Creek faults (Fig. 2). Uplift of the block is kine- Sphinx Conglomerate. Nevertheless, we can the Livingston changes from a westerly trend to matically related to the Sphinx Mountain show that the laccolithic rocks postdate defor- a northwesterly trend, and the underlying Cre- syncline (as described above) and occurred be- mation of the two sequences. We showed above taceous strata also reflect this change. The fore thrusting along the Hilgard fault system.

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The change in trend of the Livingston and underlying strata was not caused by uplift of the block, however, but rather by thrusting along the Hilgard system. This is shown immediately west of the Bear Creek fault where the folds and thrust faults are east-facing compressional structures; fault inclinations and axial planes of folds dip west and indicate eastward-directed movement (Tysdal, in press). Thrust faults of the Hilgard system also are juxtaposed upon thu west flank of the northerly trending Livingston and underlying Cretaceous rocks. Laccolithic rock intruded folded northerly Figure 7. Line drawing from sequence of photographs, showing relationship of intrusive trending Everts(?) strata immediately north of rocks (Kda) to sedimentary strata of the Everts(?) Formation (Kev). Drawing is of 300-m-l.iigh, the northernmost Livingston rocks, near sample west-facing cliff-wall about 1 km southwest of sample locality 1. Dashed lines represent locality 1 (Figs. 2, 7). South of locality 1, these bedding. beds underlie Livingston strata that display dips coincident with those of the Everts(?) Forma- tion. The laccolithic rocks are part of the same group of intrusives that we previously showed TABLE 2. K-Ar DATES AND ANALYTICAL DATA FOR SAMPLES FROM CENTRAL PART OF MADISON RANGE (Figs. 5, 6) are younger than structures of the Sample Field Mineral K,0 «Ar- «Ar- Age Hilgard fault system. In addition, a dacite dike at 10 no. no. analyzed <«) lxlO total^Ar (m.y.) this location cuts at a high angle across the mod- moles/g) m :2a

erately inclined bed;; of the Everts(?) Formation, 1 82MTzl765 Hornblende 0.67 avg 0.7153 75 72.7 • 4.6 but no offset of ar.y part of the intrusive has 2 81 MTzl264 do 0.744 0.7948 67 72.7 • .6 3 82MTzl766 do 0.77 avg 0.7718 85 68.3 • .1.2 taken place (Fig. 7). None of these intrusive 5 81 MTzl903 do 0.601 1.035 86 116 • .1.0 rocks appears to have been folded or faulted; they were intruded after folding of the Everts(?) 'Radiogenic argon.

TABLE 3. 40Ar/3,Ar HORNBLENDE ANALYTICAL DATA FOR LACCOLITHIC ROCKS OF MADISON RANGE

37 39 36 39 39 3, Temp. °C «Ar/^Ar Ar/ Ar" Ar/ Ar Ark "^A^ Ark Apparent Apparent (xlO 2) (percent K/Ca(xlO') age Measured Measured Measured of total) {'X) (xlO 14 moles) (moles/mole) (m.y.)

Sample 2: 81MTzl264 hornblerde J = 0.007161; sample wt = 0.30.15 g 850 12.91 5.234 2.121 4.9 51.7 9.93 84.29 • 1000 10.95 1.131 2.003 3.8 46.7 4.59 64.91 • 1050 15.34 5.089 3.391 1.4 37.2 1.02 72.28 . 1100 15.55 7.747 3.544 1.1 36.5 0.67 71.78 . 1159 13.31 7.440 2.730 1.9 43.7 0.70 73.57 . 1200 11.04 6.895 1.870 3.3 54.7 0.75 76.37 . .42 1250 8.04 7.254 1.078 6.8 67.4 0.71 68.62 • .36 1300 7.48 7.410 0.884 10.5 72.7 0.70 68.86 . .35 1325 7.56 7.315 0.898 11.3 72.4 0.71 69.24 . .40 1375 7.79 7.254 0.897 9.6 73.2 0.71 72.07 . .37 1425 8.02 7.118 0.888 12.6 74.1 0.73 75.10 . .40 1475 8.21 7.062 0.983 10.7 71.3 0.73 73.97 ! .43 Fuse 8.52 7.261 0.793 22.1 79.) 0.71 84.97 : .48 Total gas 75.11

Sample 3: 82MTzl766 hornblende J = 0.007375; sample wt = 0.44:10 g

850 11.18 4.803 1.967 48.3 4.2 70.39 • .38 1000 10.63 2.754 1.835 50.9 3.3 70.60 • .40 1100 7.97 7.149 0.994 70.1 10.2 0.72 72.78 ; .36 1200 7.04 7.248 0.716 77.9 9.4 0.71 71.41 i .35 1250 7.05 7.317 0.768 75.8 5.3 0.71 69.65 - .36 1300 6.61 7.176 0.634 80.1 7.5 0.72 68.99 i .37 1350 6.46 6.969 0.638 79.1 8.6 0.74 66.68 I .37 1400 7.34 6.968 0.868 72.4 7.8 0.74 69.20 i .37 Fuse 13.50 7.254 3.021 38.0 2.2 0.71 66.99 • .41 Total gas 69.90

•Measured ratio corrected for r decay Note: analyses by Ed DcWiu. The tsotopic composition of Ar was measured in ihe Branch of l^iopc Geology. K-Ar Laboratory, at the U.S. Geological Survey in Reston. Virginia, using a Model MM-1200B mass spectrometer made by VG Isotopes. Inc. (use of trade nanes and trademarks in this publication is for descriptive purposes only and does not constitute an endorsement by the U.S. Geological Survey). Samples were irradiated in the USGS Central Thimble facility TRIGA reactor (GSTR) at the Federal C enter in Denver, Colorado (Dalrymple ;md others. >981). The neutron flux monitor used in this study was MMhb-l hornblende. SI9.4 m.y. in age (Alexander and others, 1978), and an estimated \'X error in the calculated J values has been assigned. Corrections for irradiation-prodt.ced, interfering isotopes of Ar were made by using ihe values suggested for the Central Thimble facility of the GSTR (Dalrymple and others, 1981). Values corrected for -^Ar were determined by using a decay constant of 8.25 * 10 disintegrations j»er hour for ^ Ar. Apparent K:Ca ratios were calculated by using the equation given by Reck and others (1977). Constants used in the age calculations were those recommended by Steiger and Jager (1977). Concentrations of 39Ar were ca culated by using the measured sensitivity of the mass spectrometer and thus have an estimated precision of only about 10'*. Age plateaus have been assigned by using the criteria of Fleck and others (197-). Error estimates for the apparent ages of individual temperature steps are 1 (67V confidence level) and have been assigned by using the equations suggested by Dalrymple and others (1981).

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of the Everts(?) Formation after it was thrust o ~— into its present position. The beds display a net- work of criss-crossing fractures filled with hair- W thin seams of white quartz, and locally they display fractures filled with quartz veins as much as 2 cm thick. In contrast, no fractures, breccia, : .01 vein quartz, or other evidence of deformation .1 80 exist in the immediately adjacent dikes that intruded the sandstone. [A About 2 km to the north, a similar relationship exists on a west-trending ridge made up of SAMPLE 2 SAMPLE 3 several thrust slices of the Everts(?) Formation. Tmin 69 m.y. Tmin 66.7 m.y. From east to west, over a distance of ~2 km Tmax 85 m.y. tr. Tmax 72.8 m.y. < 20 along the ridge crest, the dip of Everts(?) beds Q_ gradually increases from about 10° west to 30° a. west. Across this same span, though, abundant 100 « 0 39Ark RELEASED (%) 39Ark RELEASED (%) 100 north-trending dikes retain nearly vertical dips; the dikes of this ridge have the same orientation Figure 8. 40Ar/39Ar spectra for samples 2 and 3. Analytical data are shown in Table 3. as do most dikes along the west side of the range. These data suggest that the dikes of the ridge were intruded after folding and thrust Formation. Because the laccolithic rocks are approximate emplacement age of 68-69 m.y. faulting of the Everts(?) beds. younger than the Hilgard system, they also are can be inferred from the analytical data. We Beck (1960) reported that the northwest- younger than the Sphinx Conglomerate. interpret the 40Ar/39Ar and K-Ar data to indi- trending dike near the eastern edge of Sphinx cate that the intrusive rocks were emplaced Mountain was offset by three bedding-plane Age about 68 to 69 m.y. ago, and that the discordant faults. We were unable to confirm this observa- spectra most likely resulted from incorporation tion. We point out that the dike, which is ex- Radiometric dating of hornblende from the of small amounts of excess argon (from intruded posed intermittently for at least 3 km, has a laccolithic intrusives yielded K-Ar dates of 72.7 crustal rocks) into the hornblende during straight linear trace across both limbs of the ± 4.6 m.y., 72.7 ± 1.6 m.y., and 68.3 ± 4.2 m.y. cooling. Sphinx Mountain syncline. It trends at angles of (samples 1, 2, and 3, respectively, of Fig. 2 and 30° to 80° to beds that dip 25° to 55° toward Table 2). Samples 2 and 3 also were dated by Dikes the fold axis. The dike postdates the syncline; the 40Ar/39Ar age spectrum method (Table 3), otherwise the trace would not be straight. We do and they both exhibit discordant spectra (Fig. 8). The dacite dikes of the Madison Range are not consider the possible dike offset to be signifi- The age spectrum of sample 2 shows the greater younger than structures of the Hilgard fault sys- cant because the south limb of the syncline dips discordance, apparent ages ranging between 69 tem. The greatest concentration of dikes is 25° to 55° northward near the southern end of and 85 m.y., with no age plateau. The 85 m.y. within the strata deformed and displaced by the dike, and bedding so steeply inclined likely date is discounted because it is greater than the folds and thrust segments of the Hilgard fault reflects deformation caused by emplacement of 79-80 m.y. date determined by K-Ar dating of system. Over most of the length of the dike zone, the Beaver Creek thrust plate of the Hilgard fault biotite in welded tuff in the Livingston Forma- however, the dikes are vertical and strike about system. If so, the dike must also postdate tion (Tysdal and others, in press), which was north (northwest in the southernmost part of the thrusting. deformed by the Hilgard fault system before in- dike zone). No dike is deformed, is inclined at Age of the dacite dikes relative to the dacite trusion of the igneous rocks. The 75.1 m.y. total- low to moderate angles, or has a strike much laccolithic rocks is uncertain. No clear intrusive 40 39 gas Ar/ Ar date of sample 2 is nearly the different from north; such features would suggest relationships between dikes and laccolithic rocks same as the 72.7 m.y. date determined by the intrusion prior to deformation within the Hil- were observed, but the two are believed to be K-Ar technique when the 1.6 m.y. uncertainty gard fault system. about the same age because they are similar pet- of the K-Ar date is taken into account. Neither Several dikes cut across folded rocks of the rographically and chemically (Tables 1 and 4). the 75.1 nor the 72.7 m.y. date, however, rep- Livingston Formation, and one of them also ex- Radiometric dating of hornblende from one of resents the age of emplacement of the igneous tends across folded beds of the Sphinx Con- the dikes (sample 5) proved inconclusive. The rocks. If the low and high temperature step ages glomerate. These dikes are vertical, strike at calculated K-Ar date of 116 m.y. (Table 2) is of sample 2 can be attributed to excess argon, moderate to high angles relative to bedding, greater than (1) the minimum age of about 84 admittedly a tenuous explanation, then the ages range to more than a kilometre long, and are m.y. determined by palynology (Tysdal and of the intermediate temperature steps that straight. These combined features preclude em- others, in press) for the host Everts(?) Forma- yielded dates of 69 ± 1 m.y. may be considered placement of the dikes prior to folding of the tion, (2) the 77-80 m.y. K-Ar dates for tuffs of representative of the probable "time" of two units. the Livingston Formation, and (3) the age of the emplacement. Relationships described in the preceding par- entire sequence of Upper Cretaceous strata The age spectrum of sample 3 (Fig. 8) is less agraph are further illustrated by field observa- through which the dike material probably discordant than that of sample 2. Dates for the tions made near sample locality 5 (Fig. 2). moved. The 116 m.y. date is attributed to an high-temperature steps range from 67 to 71 m.y. There, closely spaced, vertical dacite dikes in- excess of radiogenic argon, a situation similar to No plateau is exhibited, however, and only an truded steeply dipping to vertical sandstone beds that for the dates obtained from the laccolithic

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TABLE 4. KEVEX X-RAY ANALYSES FOR SELECTED TRACE ELEMENTS OF SILLS AND DIKES Fleck, R. J., Sutter, J. F., and Elliot, D. H., 1977, Interpretation of discordant 40Ar/39Ar age spectra of Mesozoic tholeiites from Antarciica: Geo- chimica et Cosmochimica Acta, v. 41, no. I, p. 15-32. Sample Field Intrusive 6 5 4 3 3 3 Garihan, J. M., Schmidt, C. J., Young, S. W., and Williams, M. A., 1983, no. no. form Rb Sr Y Zr Nb Mo Geology and recurrent movement history of the Bismarl:-Spanish Peaks-Gardiner fault system, in Lowell, J. D., and Gries, Rojbie, eds., 1 82MTzl 765 Laccolith 47 1043 7 128 7 Rocky. Mountain foreland basins and uplifts: Denver, Colorado, Rocky 2 8IMTzl.!64 —do- 47 972 9 129 4 Mountain Association of Geologists, p. 295-314. 3 82MTzl766 —do- 41 1083 7 135 6 Gries, Robbie, 1983, Oil and gas prospecting beneath the Precanbrian of 4 82MSÍ8Ü71 —do- 40 1013 5 138 5 foreland thrust plates in the Rocky Mountains: American Association of 5 82MTzl!J03 Dike 44 787 8 133 8 Petroleum Geologists Bulletin, v. 67, p. 1-26. 6 82MTzl'84 49 873 — 125 5 Hadley, J. B., 1980, Geology of the Varney and Cameron quadrangles, Madi- son County, Montana: U.S. Geological Survey Bulletin 1459,108 p. Note: Detection limit for each element is shown above elemental abbreviation; • \ not detected. Values in parts per million. Hall, W. B., 1961, Geology of pan of the upper Gallatin Valley of sou .hwestern Montana [Ph.D. dissert.]: Laramie, Wyoming, University of Wyoming, 239 p. Hassemer, J. H., and Kaufmann, H. E., 1984, Principal facts for gravity stations in and near the Madison and Gallatin Divide roadless areas, viontana: U.S. Geological Survey Open-File Report 84-847, 31 p. intrusives. Although we were aware that the north of the Madison Range (Fig. 1), Robinson James, H. L., and Hedge, C. E., 1980, Age of the basement rocks of southwest Montana: Geological Society of America Bulletin, v. 91, p. 11-15. fresh-appearing hornblende phenocrysts might (1963, p. 60-64) described a reddish limestone- Klepper, M. R„ Weeks, R. A., and Ruppel, E. T., 1957, Geology of the give a spurious age, the possibility of directly cobble conglomerate that he assigned to the southern Elkhorn Mountains, Jefferson and Broadwater bounties, Montana: U.S. Geological Survey Professional Paper 292,82 p. determining a minimum age for the Sphinx post-Laramide basinal deposits of the Bozeman Nichols, D. J., Perry, W. J., Jr., and Haley, J. C., 1985, Reinterpretation of the palynology and age of Laramide syntectonic deposits, southwestern Conglomerate was worth the effect. (The dike Group. This conglomerate gradationally under- Montana, and revision of the Beaverhead Group: Geology v. 13, that cuts the Sphinx Conglomerate is weathered, lies strata that are conformable beneath dated p. 149-153. Peale, A. C., 1896, Description of the Three Forks Sheet, Monu na: U.S. 40 39 and no attempt was- made to date it.) Ar/ Ar Eocene rocks; hence, an Eocene age was as- Geological Survey Geologic Atlas Folio 24, 5 p. Reid, R. R., 1957, Bedrock geology of the north end of the Tobacco Root age spectrum dating of the hornblende would signed to the conglomerate. Although Robinson Mountains, Madison County, Montana: Montana Bureau of Mines and (1963) used the name "Sphinx" for this con- Geology Memoir 36, 25 p. likely show a highly discordant age spectrum Robinson, G. D,, 1963, Geology of the Three Forks quadrangle, Montina: U.S. indicative of excess Âr. glomerate, our work shows that the Cretaceous Geological Survey Professional Paper 370,143 p. Robinson, G. D., Klepper, M. R., and Obradovich, J. D., 1968, Ovt rlapping Sphinx Conglomerate of the Madison Range plutonism, volcanism, and tectonism in the Boulder batholit i region, (the type area) is a syntectonic Laramide unit Montana, in Coats, R. R., Hay, R. L., and Anderson, C. A., eds, Studies DISCUSSION in volcanology: Geological Society of America Memoir 116, and could not be conformable with post- p. 557-576. Schmidt, C. J., and Garihan, J. M., 1983, Laramide tectonic development of Radiometric dating of volcanic and intrusive thrusting basinal deposits. Therefore, we reject the Rocky Mountain foreland of southwestern Montana, in Lowell, designation of the Sphinx as the basal unit of the J. D., and Gries, Robbie, eds., Rocky Mountain foreland basins and rocks that respectively predate and postdate de- uplifts: Denver, Colorado, Rocky Mountain Association of Geologists, formation bracket i;he time of Laramide defor- Bozeman Group. p. 271-294. Smedes, H. W., 1966, Geology and igneous petrology of the northern Elkhorn mation within the Madison Range. The dates, Mountains, Jefferson and Broadwater Counties, Montana: U.S. Geolog- REFERENCES CITED ical Survey Professional Paper 510, 116 p. along with the structural relationships described Steiger, R. H., and Jager, E., 1977, Subcommission on geochronology: Conven- Alexander, E. C., Jr., Michelson, G. M., and Lanphere, M. A., 1978, MMhb-1: tion on the use of decay constants in geo- and cosmochronoloj y: Earth in the report, also show that the syntectonic A new Âr/Âr dating standard, in Zartman, R. E., ed., Short papers and Planetary Science Letters, v. 36, p. 359-362. of the Fourth International Conference on Geochronology, Cosmo- Swanson, R. W., 1950, Geology of a part of the Virginia City and Eldridge Sphinx Conglomerate of the range is of Cam- chronology, and Isotope Geology: U.S. Geological Survey Open-File Re- quadrangles, Montana: Spokane, Washington, U.S. Geologica. Survey port 78-701, p. 6-8. panian and/or Maastrichtian age, older than Open-File Report, 12 p. Beck, F. M., 1960, Geology of the Sphinx Mountain area, Madison and Gal- Tysdal, R. G., in press. Thrust faults and back-thrusts in the Madison Range of previously believed. The age of the Sphinx thus latin Counties, Montana, in Campau, D. E., and Anisgard, H. W., eds., the southwestern Montana foreland: American Association c f Petro- West Yellowstone—Earthquake area: Montana Geological Society, leum Geologists Bulletin. is in agreement with the time of formation of the 11th Annual Field Conference Guidebook, p. 129-134. Tysdal, R. G., and Simons, F. S,, 1985, Geologic map of the Madison roadless Becraft, G. E., Kiilsgaard, T. H., and Van Noy, R. M., 1970, Mineral resources other major syntectonic conglomerate of south- area, Gallatin and Madison Counties, Montana: U.S. Geological Survey of the Jack Creek basin: U.S. Geological Survey Bulletin 1319-B, 24 p. Miscellaneous Series Map MF-I605-B, scale i:96,000. western Montana, the Beaverhead Group. Paly- Berg, R. R., 1962, Mountain flank thrusting in Rocky Mountain foreland, Tysdal, R. G., Nichols, D. J., and Winkler, G. R., in press. The Livingston Wyoming and Colorado: American Association of Petroleum Geolo- Group in the Madison Range of southwestern Montana: U.S. (Jeologi- nological dating by Nichols and others (1985) gist Bulletin, v. 49, p. 2019-2032. cal Survey Bulletin 1665. 1981, Review of thrusting in the Wyoming foreland: Wyoming Univer- showed the Beaverhead to contain conglomer- Wooden, J. L., Vitaliano, C. J., Koehler, S. W„ and Ragland, P. C, 1978, The sity Contributions to Geology, v. 19, no. 2, p. 93-104. Late Precambrian mafic dikes of the southern Tobacco Root Moun- ates of Campanian and Maastrichtian age. Dalrymple, G. B., Alexander, E. C., Jr., Lanphere, M. A., and Kraker, G. P., tains, Montana: Geochemistry, Rb-Sr geochronology, and relationship 1981, Irradiation of samples for Âr/Âr dating using the Geological to Belt tectonics: Canadian Journal of Earth Sciences, v. 25, Survey TRIGA reactor U.S. Geological Survey Professional Paper Not all reddish limestone-cobble conglomer- p. 467-479. 1176, 55 p. ates in southwestern Montana are of the same De la Roche, H., Leterrier, J., Grandclaude, P., and Marchal, M.. 1980, A age or structural and stratigraphic setting, how- classification of volcanic and plutonic rocks using R,R2-diagram and MANUSCRIPT RECEIVED BY THE SOCIETY JULY 22, 1985 major element analyses—Its relationship with current nomenclature: REVISED MANUSCRIPT RECEIVED JANUARY 3,1986 ever. In the Three Forks Basin, about 25-40 mi Chemical Geology, v. 29, p, 183-210. MANUSCRIPT ACCEPTED JANUARY 13,1986

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