U.S. DEPARTMENT OF THE INTERIOR GEOLOGIC INVESTIGATIONS SERIES I–2774 U.S. GEOLOGICAL SURVEY Version 1.0

111°52'30" R.1W. D' 111°45'00" 46°52'30" 45 ၤf 46°52'30" CORRELATION OF MAP UNITS DESCRIPTION OF MAP UNITS GEOLOGIC SETTING ၤpp ၤf ၤf Yg ၤm Meagher Limestone (Middle Cambrian)—Limestone, medium gray, medium Khd ၤw IN [Terminology for carbonate rocks from Folk (1962)] ၤm 32 ၤf MOUNTA ၤf dark gray, and medium brownish gray, weathers medium light gray; Ml The Nelson quadrangle is near the north end of the Big Belt Mountains in west- TH 25 23 R Qa common grayish-orange, moderate-brown, and pale-grayish-red silty mottles, central Montana, about 30 km northeast of the capital city of Helena (fig. 1). The Big US Qa Holocene Alluvium (Holocene)—Unconsolidated, stratified to unstratified sand, gravel, silt, 26 T Ql particularly in basal 30 m and in upper 50 m. Finely crystalline limestone 22 S 33 clay, and some boulders, deposited primarily by streams; locally includes Belt Mountains rise as a north-northwest-trending range between the Smith River 25 MDt 65 ၤm R QUATERNARY Mm 26 22 O Qog some colluvium in talus cones and alluvial fan deposits on valley sides, and with channels and beds of medium to coarsely crystalline bioosparite and valley on the east and the Helena valley on the west. The Missouri River flows through 26 O 25 Yg Yg Khd M 20 Pleistocene gravel on low terraces in stream valleys. Thickness as much as 15 m biopelsparite limestone. Discontinuous wavy laminae and beds 0.2–4 cm a narrow canyon and valley incised through the northwest edge of the mountains.

28 35 QTg 50 Dj 20 thick, with discontinuous silty limestone partings; weathers with knobby, Northwest of the Nelson quadrangle, the canyon of the Missouri River was named the Ml Ql Landslide deposits (Holocene and Pleistocene)—Angular rock fragments, F A U locally chicken-wire texture, especially in the lowest quarter and uppermost S Gates of the Rocky Mountains by Lewis and Clark in 1805. LT Pliocene blocks, and soil that have moved downslope as a single unit. Some large O Mm Djb Dm 15 part; forms steep slopes or low cliff. Contains abundant small trilobite The Big Belt Mountains strike across two prominent west-northwest-trending 23 22 U blocks bounded by arcuate faults. Superimposed generations of landslide P Yg fragments, some pelmatozoan(?) plates, and sparse chitinous brachiopod elements: the north end of the Helena valley on the west, and the south margin of the ၤpp HOGBACK Unconformity TERTIARY deposits shown locally by common internal map contacts. Thickness as ၤm 23 MOUNT fragments. Thickness about 110 m, but as much as 177 m, likely as a result Dry Range and Little Belt Mountains on the east (fig. 1). The north edge of Helena 27 Mm 32 28 THRUST F much as 95 m 35 Khd of deformation valley marks a strand of a major tectonic zone, the Lewis and Clark tectonic zone, a 40 AIN Ts Miocene(?) Qog Old gravel (Holocene and Pleistocene)—Gravel, sand, and some silt, generally ၤf AUL ၤw Wolsey Shale (Middle Cambrian)—Silty mudstone and siltstone, light olive to zone of recurrent major strike-slip and vertical fault movement through geologic time 48 47 unconsolidated, stratified and unstratified; deposited by streams; clasts 26 18 T light grayish green, locally olive black; weathers light olive gray, dusky yellow (Billingsley and Locke, 1939; Hobbs and others, 1965; Harrison and Reynolds, 1976; Dm Yg 58 Unconformity rounded to subrounded; at altitudes 15–75 m above recent stream channels. 24 26 22 45 gray, and grayish red. Thin beds of silty sandstone, pale grayish red, are Reynolds, 1977, 1979; Reynolds and Kleinkopf, 1977). Similarly, the south flank of 28 Khd 35 Thickness 1–5 m interbedded in olive green siltstone in basal 5 m. Interval of medium-gray and the Little Belt Mountains and Dry Range is a zone of recurrent faulting that occurred 25 Khd Upper Cretaceous QTg Older gravel (Pleistocene and Pliocene)—Gravel, sand, and some silt, from Middle Proterozoic to at least middle Miocene time. The zone is likely contiguous MDt ၤw medium-brownish-gray limestone, 3 m thick, is present about 60 m above 33 unconsolidated to weakly consolidated; stratified; deposited dominantly by 47 65 base; weathers grayish red, brownish gray, and medium light gray; limestone in the basement with the Lewis and Clark tectonic zone. Thus, the Nelson quadrangle Dj Intrusive contacts CRETACEOUS 25 28 80 streams, some deposited as colluvium; clasts rounded to subrounded, is medium to coarsely crystalline sandy biosparite, with sparse to abundant occupies a geologically significant position at the junction between the Big Belt 25 63 Djb 45 subangular and angular adjacent to Soup Creek fault. Locally contains sandy pellets of glauconite; fossils are trilobite, brachiopod, and pelmatozoan(?) Mountains and major cross-trending tectonic elements. Mm 47 Kk Lower Cretaceous tuffaceous sediment reworked from underlying Tertiary sedimentary deposits. fragments. Limestone laminae, 0.2–3 cm thick, are scattered throughout the Rocks of the Nelson quadrangle are predominantly sedimentary strata that range in MDt 16 45 Dj Ml Thickness as much as 25 m siltstone. In top 30 m is light-gray to light-greenish-gray limestone; weathers age from Middle Proterozoic (about 1,400 million years) to Recent. Pre-Tertiary (pre- ၤw 20 10 Dj Mm 33 Unconformity 18 27 25 27 Khd 15 Ts Sandstone, conglomerate, and tuff (Miocene?)—Lithic and feldspathic lithic pale yellowish brown to grayish orange; trilobite biosparite; some greenish- 66 million years ago) rocks are mostly marine in origin (refer to Description of Map ၤpp 20 28 52 Units). However, some strata, such as the Middle Proterozoic Spokane Formation, Djb 73 Jme JURASSIC sandstone, interbedded conglomerate that has well-rounded clasts as much as gray siltstone chips on parting surfaces. Siltstone in lower third contains 73 Ml parts of the Middle Cambrian Flathead Sandstone, and some Upper Devonian, Upper 35 15 Djb 17 cm across, and interbedded vitric and feldspathic vitric tuff. Locally dips scattered Cruziana and annelid(?) trace fossils. Strata split shaly and platy; 22 30 65 Mississippian Big Snowy, Pennsylvanian Amsden, Jurassic Morrison, and Lower Dm MDt 60 Unconformity east into Soup Creek fault and is commonly masked by unit of older gravel limestone is platy and flaggy. Numerous hornblende diorite sills intrude the Khd Ml MDt 30 ၤf (Qog). Thickness as much as 75 m Wolsey in the upper plate of the Moors Mountain thrust, but only one sill Cretaceous Kootenai strata, originated in nonmarine continental environments.

45 Dj 70 75 30 PERMIAN

38 Pၧpq C Ml Khd Hornblende diorite (Upper Cretaceous)—Sills and irregular intrusive bodies of was identified in the unit in underlying plates. Apparent thickness varies as a Tertiary and Quaternary deposits accumulated on alluvial plains and as fan, talus, and R Mm Dj PENNSYLVANIAN landslide deposits on steep mountain slopes. Beds of air-fall and weakly indurated ash- E Qa hornblende diorite; dark olive gray to olive black; weather medium to light result of local intrusion of mafic sills and local strong internal deformation; 50 E 78 60 20 ၧMab K flow volcanic tuff of possible Miocene(?) age (24–26 million years) are present in the Dm MDt 16 approximately 65 m thick in south-central part of quadrangle to 245 m thick Djb Khd Qa Ml olive brown; fine to coarsely crystalline, equicrystalline. Unit is biotiferous MDt 67 63 in northeast part Tertiary strata. Yg 48 Unconformity Upper Mississippian and locally contains abundant epidote. Large intrusive bodies have altered Ql Igneous intrusive rocks of Late Cretaceous age [about 68–73 million years ago] 28 48 61 A' host carbonate and shale units to hornfels. Intrusive body on the east side of ၤf Flathead Sandstone (Middle Cambrian)—Sandstone and pebbly sandstone Dj ၤpp 38 MISSISSIPPIAN intrude Middle Proterozoic through Devonian rocks. On the upper plate of the Moors 50 ၤm 53 MDt Mm Stump Gulch contains thin intrusive pods of quartz diorite; rock unit is friable that contain very thin siltstone beds at top: pinkish gray, yellowish gray, and 28 Dm ၤm Khd 16 67 Mountain thrust fault in Big Log and Left Fork Gulches (secs. 3 and 4, T. 12 N., R. 2 Dm 20 Ml ၤpp Dj on weathered outcrop. Sills 1–30 m thick very pale red; weathers pale reddish brown, yellowish gray, and light 35 Ql Ml Lower Mississippian W., and sec. 34, T. 13 N., R. 2 W.), small plutons intrude the Cambrian and Devonian MDt Khd 32 ၤm Kk brownish gray; siltstone beds at top are greenish gray and light olive gray, Dj Qa Dm 20 Kootenai Formation (Lower Cretaceous)—Siltstone and silty mudstone that strata (geologic section A–A’) along and adjacent to strike-slip faults that displace strata 21 ၤw weather greenish gray. Sandstone is fine to coarse grained; contains locally 20 MDt contains beds of lithic sandstone and lenticular beds of limestone. Siltstone within the upper plate. Thin sills extend laterally north and south from the plutons into MDt 36 ၤpp ၤw ၤpp 43 38 and mudstone are pale grayish red to grayish red, locally olive gray, split abundant pebbles as large as 2 cm across. Grains and pebbles are quartz, Khd 52 Dj Ql Ml quartzite, red and gray siliceous siltite and chert, and minor feldspar; grains the same host rocks, but are most abundant in shaly and very thin bedded units such as 22 ၤf Dj Unconformity platy to shaly; sandstone is pale grayish red and yellowish brown; limestone, Dj 28 generally well rounded but modified by pressure solution and quartz the Wolsey Shale (geologic sections A–A’, B–B’). Structurally low thrust plates contain Dm ၤm MDt Qa 32 20 pale greenish-gray to light gray. Sandstone units as thick as 18 m are fine to 32 30 42 MDt few sills (sec. 31, T. 13 N., R. 1 W.; sec. 36, T. 13 N., R. 2 W.; and sec. 1, T. 12 N., 25 ၤw 42 Khd Djb Upper Devonian overgrowths; weakly to firmly indurated; common to abundant glauconite 43 Dj DEVONIAN dominantly medium and coarse grained, split platy; abundant rounded black 37 33 Dj Khd 50 50 50 80 grains in uppermost 12 m; cross-laminae to very thin beds; herringbone R. 2 W.; geologic section A–A’). ၤw ၤm 33 MDt chert grains among quartz, feldspar, and light-colored lithic grains produce a 30 Dm cross-bedding near base and in top 15 m; thin sets of trough cross-laminae Several notable stratigraphic and structural-stratigraphic changes are present in the Khd 37 ၤm Dm Qa 64 Unconformity salt-and-pepper appearance. Limestone beds are laminated and very thin common; ripple laminae in upper part; splits flaggy and slabby, locally platy; Nelson quadrangle: (1) North across the quadrangle, the Middle Proterozoic Spokane Djb 57 bedded, aphanocrystalline and very finely crystalline, silty and sandy, and Qa ၤw 63 80 Yg 26 forms resistant ledges and ridge. Thickness 80–140 m Formation thins rapidly beneath the Middle Cambrian Flathead Sandstone from about ၤf Dm Middle Devonian locally contain small gastropods and possible algal filaments. Exposed partial 20 82 32 Dj 865 m at the south edge of the quadrangle to a wedge edge north of Beaver Creek in ၤf ၤf ၤpp thickness approximately 180 m Ys Spokane Formation (Belt Supergroup, Middle Proterozoic)—Siltite and ၤpp U ၤpp Dm sec. 3, T. 12 N., R. 2 W. North and northeast of that area, the underlying upper part D ၤm Unconformity 57 ၤm Jme Morrison Formation and Ellis Group, undivided (Jurassic)—Morrison argillite, pale red and moderate reddish brown, weathers moderate reddish ၤm Yg 78 63 of the Greyson Formation is also truncated from its top downward beneath the T.13N. 38 Ml ၤw 45 T.13.N brown, grayish red, and dusky red; laminated and very thin bedded, cross- 10 ၤw Upper Cambrian Formation: siltstone, minor mudstone, and thin sandstone are greenish gray Flathead Sandstone. The thinning results from Late Proterozoic (post 825 million ၤf Khd ၤpp laminated, locally ripple cross-laminated; dominantly silt and clay on partings Yg 80 ၤw to grayish red; sandstone, grayish orange; discontinuous silty and clayey years ago) to pre-Middle Cambrian (pre-Flathead, estimated 540 million years ago) 46°50'00" Khd Qa Khd 46°50'00" surfaces; locally common ripup clasts and fluid escape structures; scattered Khd 20 ၤm limestone beds contain sand grains, charatophytes, and possible algal uplift and erosion across a west-trending arch, probably areally coincident with the Mm ၤf 68 CAMBRIAN lenses of fine-grained quartz sandstone as thick as 6 cm; beds of sandy T.12N. 43 60 ၤpp T.12.N filaments. Thickness approximately 65 m. Disconformity at base. Ellis general west-northwest trend of the Lewis and Clark tectonic zone (fig. 1; Reynolds, ၤf ၤw Middle Cambrian limestone and rare stromatolitic limestone 20–35 cm thick interbedded with 50 Khd 22 48 Group, undivided, includes the Swift Formation at the top and Sawtooth 1977; Reynolds and Kleinkopf, 1977). Such thinning is not evident in the Spokane 37 Dm 67 ၤw greenish-gray siltite in basal 45 m. Formation is truncated north across the 54 Formation at the base. Swift Formation: sandstone, siltstone, and Formation of the structurally higher and more westerly Eldorado thrust plate (fig. 2; 25 42 54 Qa B' ၤf discontinuous very thin basal conglomerate; light olive gray to pale brownish Beaver Creek area in the upper plate of the Moors Mountain thrust plate Robinson and others, 1969; Robinson and McCallum, 1991). (2) The Middle MDt Yg 25 10 and absent in lower thrust plates as a result of Upper Proterozoic-pre-Middle ၤw 26 Khd F ၤm Unconformity gray, medium and medium dark gray; splits platy and flaggy, locally shaly. Cambrian Flathead Sandstone and Wolsey Shale thicken from Soup Creek, near the ၤf A 18 48 Cambrian erosion. Thickness 0-865 m Dj U 45 Sawtooth Formation: calcareous sandstone, sandy limestone, and lenticular southeast corner of the quadrangle, northward, independent of the thickening by

L Djb T 70 UPPER PLATE OF LOWER PLATE OF calcareous pebble conglomerate containing oyster fossils at base; yellowish Greyson Formation (Belt Supergroup, Middle Proterozoic)—Formation 20 45 30 17 50 intrusion of the Upper Cretaceous sills. (3) The Permian Phosphoria Formation is 28 ၤpp 88 ၤf MOORS MOUNTAIN MOORS MOUNTAIN FAULT AND Khd 30 gray to grayish orange, weather same color; crosslaminated and laminated; truncated downward by Upper Proterozoic-pre-Middle Cambrian erosion mostly removed by erosion from this quadrangle, from the Upper Holter Lake THRUST FAULT UPPER PLATE OF 20 45 45 splits flaggy. Ellis Group is approximately 75 m thick from south to north on upper plate of Moors Mountain fault; only middle quadrangle adjacent on the west (Robinson and others, 1969), and from quadrangles Khd ၤw HOGBACK MOUNTAIN 33 Qa 36 60 45 part of the formation exposed beneath Middle Cambrian rocks in the lower 22 Ys THRUST FAULT Pၧpq Phosphoria and Quadrant Formations, undivided (Permian and to the east. (4) In secs. 7, 8, T. 12 N., R. 1 W., and sec. 12, T. 12 N., R. 2 W., the 23 Dm 45 Ys plate of the Moors Mountain fault and upper plate of the Hogback Mountain Moors Mountain fault cuts abruptly upward through, and structurally truncates, the ၤm 38 53 Pennsylvanian)—Phosphoria Formation (Permian): chert, light brownish gray ၤw Khd 47 ၤf 80 fault. Thickness about 1,070–2,000 m A 24 40 Dj and medium gray, locally contains microfossils; medium-brownish-gray Middle Proterozoic Newland Formation and the overlying lower part of the Greyson 23 55 Yg Yg Belt Supergroup MIDDLE PROTEROZOIC Qog Khd dolomitic limestone containing chert nodules. Generally removed by pre- Yg Upper plate of Moors Mountain thrust fault—Olive gray, light greenish Formation. From the south-central part of the Hogback Mountain quadrangle 55 35 85 48 Dm ၤpp Khd 30 62 (Reynolds, 2003), adjacent on the east to the Nelson quadrangle, the Moors Mountain 15 Qa Yg Yn Sawtooth Formation erosion; only discontinuous remnants less than 2 m gray, and medium olive to medium dark gray; weathers greenish gray, pale 33 57 76 Dj Khd 37 ၤw ၤpp thick preserved along Jme—Phpq boundary within quadrangle. Quadrant olive, dusky yellow, and pale yellowish brown; even parallel laminae, fault cuts north and west up through nearly 2,800 m of the Newland Formation and 10 Qa ၤm 1,100 m of Greyson Formation. This structurally truncated edge of the Newland Djb 3 Formation (Pennsylvanian): sandstone and sandy dolomitic limestone, very discontinuous wavy laminae, and cross-laminae; contains very thin beds of Dm 27 35 30 ၤf Formation in the Nelson Quadrangle is the most northerly and westerly known Ml 50 Qog light gray and light yellowish gray; weathers yellowish gray and pale grayish silty fine-grained quartzite, and in lower and upper middle parts, thin 34 80 50 55 exposure of the Newland in the overthrust belt of Montana. The position of this Khd 38 orange; common quartz cement; very thin and thin laminae and cross concretionary lenses of dolomitic and calcareous siltite; splits shaly and platy Yg Ql 112°30′ 112°00′ 111°30′ 111°00′ 111°52'30" 111°45' 24 44 Yg East 46°52'30" laminae; splits platy and flaggy; forms ledges and local low cliffs. Thickness Yg structural truncation coincides with the stratigraphically higher, but older, truncation of 38 Lower plate of Moors Mountain fault and upper plate of Hogback 32 40 60 37 55 R S the Spokane Formation and the trend of the Lewis and Clark tectonic zone (fig. 1). 35 58 FAULT 50 C O Front m 75–105 m Mountain fault—Medium-gray and medium-dark-gray to dark-gray siltite MDt o er Dj Yg 55 Qog 30 n C v ADEL i 48 i t The geologic structure of the Nelson quadrangle is complex. Three principal ti K R h ၧMab Amsden Formation and Big Snowy Group, undivided (Pennsylvanian and and argillitic siltite, scattered lenses of brownish-gray very fine grained T n Y i q S 25 e r MOUNTAINS structural plates, each deformed internally, are exposed at the surface in the 35 QTg 36 U n u R t Wolf o Upper Mississippian)—Amsden Formation (Pennsylvanian): sandstone and quartzite; weathers olive gray, dark greenish gray, and olive black; even 33 54 80 H a s Khd U 55 55 T l Creek s quadrangle (fig. 2). The structurally lowest plate, here called the Avalanche Butte plate Mm 70 i LITTLE interbedded silty mudstone: grayish red and light reddish brown; interbedded parallel and discontinuous wavy parallel laminae; very thin sets of graded 28 D R q Yn 47°00′ M disturbed belt (fig. 2), is exposed east from Nelson in the canyon of Beaver Creek and on adjacent 25 36 Khd 62 17 i Geologic mapping by 14 Qa Khd Qa q 72 v BELT pinkish-gray limestone and sandstone at top; limestone-pebble conglomerate laminae; cut by thin quartz veins; local intense small- and intermediate-scale e Mitchell W. Reynolds Khd B r mountain ridges. This lowest plate is a complex sequence of recumbent anticlines and 33 50 38 q I MOUNTAINS folding associated with thrusting; splits shaly and platy. In secs. 1 and 12, T. 65 Helena G at base; rests unconformably on underlying rocks of the Big Snowy Group. ၤf Ys 75 15 synclines that trend northwest near the east edge of the quadrangle and are deformed 20 MOO Thickness 78–100 m. Big Snowy Group (Upper Mississippian): in upper 12 N., R. 2 W.; secs 6 and 7, T. 12 N., R. 1 W., and SE1/4SE1/4 sec. 20, ၤw 25 48 RS q 45 20 Valley Lewis and to west-northwest trends in the east-central part of the quadrangle. The folds are 38 40 85 part, mudstone and silty mudstone that contains thin beds of sandy limestone T. 13 N., R. 1 W. U IN DRY QTg ၤm Qa D A 60 75 generally overfolded northeast and are locally folded, in turn, about east-trending axes 12 20 50 OUNT HELENAfault RANGE and dolostone; medium light gray to medium dark gray and dark olive gray; Djb 42 M 60 Yn Newland Formation (Belt Supergroup, Middle Proterozoic)—Calcareous and (Checkerboard Creek area, south of Beaver Creek; geologic sections A–A’, B–B’, and 30 45 53 B Dry 45 York E Range fault zone limestone beds are yellowish gray, weather pale grayish orange; limestone dolomitic silty argillite and argillitic siltite that contain limestone and rare 25 48 L D–D’). A series of folded thrust faults in the upper part of the Avalanche Butte plate zoneClark tectonicT zoneR Smith beds contain crinoid columnals and brachiopod fragments. In middle part, 70 V I S arkosic sandstone intervals in uppermost part. Silty argillite and argillitic D ALLEY Dm 38 35 V M contains folded fault duplexes. The folded thrust faults repeat carbonate strata of the i M E v silty mudstone and very thin limestone, dolostone, and sparse dolomitic 16 MDt 45 TOWNSEND R IT siltite are medium gray and medium dark gray, weather pale grayish orange i Mississippian Mission Canyon Limestone and Lodgepole Limestone. Devonian strata O ၤpp d H 30 20 sandstone beds; mudstone is pale greenish gray to olive gray; limestone, e U Helena Geologic mapping by and grayish orange; very thin even parallel laminated producing a varvelike V R are folded recumbently in the core of one duplex (N1/2 sec. 32, NE1/4 sec. 31, and 40 N A 30 QTg 45 iv William H. Hayes dolostone, and sandstone are light brownish gray to olive gray; laminated 40 T L appearance on weathered surface; local very thin low-angle inclined laminae. Khd M L e SE1/4 sec. 30, T. 13 N., R. 1 W; geologic sections A–A’ and D–D’). About 20 km A r with modifications and O E and very thin bedded; split shaly and flaggy. In lower part, siltstone, sandy Ml 46°30′ I Y Limestone is medium light gray and medium gray, weathers light and Khd N 25 Qa 35 Yg U additions by east-southeast of the Nelson quadrangle (fig. 2), Middle Precambrian strata are S V N 50 40 ALLEY Mitchell W. Reynolds siltstone, and limestone, with limestone and dolostone breccia at base; medium light gray; aphanocrystalline and very finely crystalline, locally silty; 45 T exposed at the surface up-plunge along strike in the core of the Avalanche Butte Dj 27 Yg A ELKHORN I yellowish gray; pale grayish red and grayish red calcareous siltstone sparry calcite ribbons along some curved surfaces; scattered to common Djb Ys N 46°45' structural plate. Yg U MOUNTAINS S MDt 23 interbeds; subangular pebbles and fragments of underlying Mississippian coarse calcite fills fracture cleavage; beds locally highly contorted 0–800 m 51 D Figure 3. Index to geologic mapping in The intermediate structural plate in the Nelson quadrangle is bounded below by the ၤf Townsend rocks in basal part; sandy limestone beds are locally stromatolitic. Group is from Moors Mountain fault. Intervals of limestone beds 0.5–5 m thick. 28 25 the Nelson quadrangle. All mapping overturned Hogback Mountain thrust fault (E1/2 SE1/4 sec. 20, T. 13 N., R. 1 W.; 27 30 Qa C' 165–185 m thick Arkosic sandstone is grayish orange pink, weathers very pale orange and 35 ၤw 20 30 from ground traverses at 200–750 m geologic section D–D’). Both in the Nelson quadrangle and continuously for about 19 30 43 0 10 20 30 40 KILOMETERS Mm light brown; fine to medium grained, local coarse grains and granules; clasts Khd spacing; locations in field determined Mission Canyon Limestone (Upper and Lower Mississippian)—Limestone, km to the southeast (fig. 2), recumbently folded siltite and argillite strata of the Greyson 27 D 15 QTg of quartz, microcline feldspar, polycrystalline quartz, and rare laminated 28 0 10 20 MILES by compass resection and altimetry or medium light to medium dark gray and light olive gray; weathers light gray, Formation rest along the folded thrust fault on the sequence of fault duplexes of the Mm 30 U 25 15 U 42 very light gray, and white; locally dolomitic; scattered nodules of brownish- limestone; beds range from structureless to low-angle cross laminated; local D Figure 1. Generalized physiographic map of west-central Montana, showing by inspection from local distinctive Avalanche Butte structural plate. From near Sawtooth Mountain on the north (NE1/4 30 25 gray chert; generally recrystallized and fractured; coarsely crystalline; internal slump structures; beds 0.04–0.3 m thick. Lower exposed part of sec. 36, T. 13 N., R. 2 W.; geologic section A–A’) to the Moors Mountain fault Qa Dm 35 location of the Nelson quadrangle (purple) in relation to regional tectonic topographic character on the base 30 pressure solution surfaces common; columns and pods of collapse breccia formation splits shaly and platy and forms steep slopes; upper part splits southeast of Nelson, recumbently folded Greyson strata and overlying Paleozoic rocks 30 ၤpp ၤm 36 features. map, scale 1:24,000. 23 25 Qa present in upper third; thin bedded with local thin sets of cross-laminae; shaly; limestone splits platy to slabby, and forms ledges on steep slopes and Djb form the lowest exposed part of the upper plate of the Hogback Mountain thrust fault Ys splits thick or massive; limestone pellets and fragments, crinoid, brachiopod, ridge crest. Formation structurally truncated from top downward by Moors 18 30 30 (fig. 2). Although it now occupies an intermediate structural position, the Hogback 20 Ts and coral fossils and fossil fragments and ooliths visible locally; thin bedded, Mountain fault. Thickness 0–620 m 20 20 30 Mountain and associated faults likely formed before the Avalanche Butte fault 22 Khd QTg 40 MDt 30 112°00′ 111°45′ 111°30′ local current cross-bedding; forms cliffs or rugged slopes. Thickness duplexes. However, the Hogback Mountain and associated faults are older than, and 46°47'30" Qa 15 46°47'30" 30 47°00′ 290–530 m; variable as a result of deformation and dissolution are clearly truncated by, the structurally higher Moors Mountain thrust fault (SE1/4 25 ၤw Ml Lodgepole Limestone (Lower Mississippian)—Limestone and interbedded silty sec. 20, T. 13 N., R. 1 W., and N1/2 sec. 8, T. 12 N., R. 1 W.; fig. 2; geologic 25 Dj H limestone and highly calcareous siltstone; medium light to medium dark gray, sections A–A’, B–B’, and D–D’). ၤf D o 30 Yg 48 l te 25 U r light brownish gray to yellowish gray; weathers light gray, very light gray, The Moors Mountain fault forms the sole of the highest structural plate. Along the Dm Contact—Dashed where approximately located, short dashed where inferred, 40 QTg MONTANA 45 25 La and white, and moderate orange pink to pale yellowish orange in silty fault, Middle Proterozoic strata of the Newland and Greyson Formations are thrust ke dotted where concealed Ml QTg Area of Qa 25 M figure 2 intervals; laminated and very thin bedded; fine- to medium-crystalline U over all lower structural plates. The stratigraphic succession on the Moors Mountain O U 60 35 O Fault—Dashed where approximately located; short dashed where inferred; 70 30 R d biosparite, oosparite, and intrasparite limestone; splits platy, shaly, and D plate is generally tilted uniformly southwest along the western half of the Nelson 20 S a ၤm E Djb o dotted where concealed. U and D show upthrown and downthrown sides of L M flaggy, with intervals of thick beds near center that split slabby; contains quadrangle. In lower Beaver Creek and Big Log Gulch, east-southeast- and east- 35 R 30 D OU 30 O fault; arrows show relative direction of movement on oblique-slip fault. On 20 NT abundant fossils including brachiopods, crinoid fragments, bryozoa, corals, trending high-angle faults, which have dominant strike-slip movement, offset the e 80 Ys R AIN U d A TH oblique-slip fault in cross section B–B’, T shows relative movement toward Mm 40 R a rare trilobite fragments, and trace fossils, including unidentified burrows, uniformly dipping Paleozoic strata and some Middle Proterozoic strata but do not D U S ST c O s observer, A away from observer 35 30 ၤf O F U a scattered Scalerotuba in lower part and rare Zoophycus in upper half. displace the basal thrust fault. In the southwest corner of the quadrangle, Mississippian U A 25 30 ၤpp U C 25 P LT Thickness 215–285 m; variable as a result of deformation and dissolution Thrust fault—Dashed where approximately located; short dashed where and younger rocks are faulted over folded Mississippian and older rocks on an M – s Yg

ၧMab 30 25 I Moors g inferred; dotted where concealed. Sawteeth on upper plate. In cross unnamed thrust fault system (geologic section C–C’). 35 S MDt 30 S n Three Forks Formation (Mississippian and Devonian)—At top of formation i O Mountain r 35 40 sections, arrows show relative direction of displacement; A, movement away Rocks of the Moors Mountain plate rest on a major lateral structural ramp, formed U p includes Cottonwood Canyon Member of Lodgepole Limestone: 40 40 R S 30 MDt I from observer; T, movement towards observer; T with arrow indicates by the structural high of the deformed Hogback Mountain and Avalanche Butte

B carbonaceous mudstone brownish black to grayish black; weathers pale

r C H u oblique movement towards observer structural plates (geologic sections A–A’ through D–D’). The lateral ramp seems to 40 35 R yellowish brown and very pale orange. Three Forks Formation: silty O RS h 20 30 ၤf E G O lp trend and plunge west and has structural relief of at least 3,000 m. Based on the 15 E B O M dolostone, calcareous siltstone, limestone, and mudstone; medium olive gray Overturned thrust fault—Base of sawteeth on upper plate of Hogback A M u Dm K S 24 T C T geometry of the folded lower plate, erosional reentrants in the edge of the thrust plate, ၤw H K N ite and olive gray; weather pale grayish orange, grayish orange, and pale olive; Mountain thrust fault in SE1/4 sec. 20, T. 13 N., R. 1 W. Approximately 45 45 R Wh 40 U Lingshire 35 QTg Dj S ST U and extent of a klippen of the Moors Mountain fault east of the Nelson quadrangle, the T THRU contains intervals of calcareous mudstone, medium light gray, and located. In cross sections, arrows show relative direction of displacement Hogback 20

40 S horizontal displacement on the fault is at least 45 km (fig. 2; Reynolds, unpublished Mountain carbonaceous silty mudstone, olive black to brownish gray. In lower part, Djb O M FA Upper O U Anticline—Showing trace of axial surface and direction of plunge of axis. 35 30 U U 20 LT mapping, 1976–78). West and northwest of the Nelson quadrangle in the Upper 10 Holter N P T limestone and calcareous siltstone are medium brownish gray; weather 20 Nelson A Dashed where approximately located; short dashed where inferred; dotted IN 15 Lake Holter Lake and Beartooth Mountain quadrangles, respectively, rocks of the Middle 35 30 yellowish gray, pale orange pink, and light brown. Local thin lenses of RS T where concealed O H 35 O R 35 M U Proterozoic Greyson and Spokane Formations on the Eldorado fault are part of the 20 QTg solution-collapse limestone breccia at base. Limestone beds in upper part 15 20 30 30 S 15 M T Asymmetric anticline—Showing trace of axial surface and direction of plunge Ts F O structurally highest plate in the area. The Eldorado plate rests on strongly deformed 15 A Qa U F contain common brachiopods and crinoid columnals, some oncolites, and Ml N AU ၧၧpq 25 U R C L of axis. Short arrow indicates steeper limb. Dashed where approximately R TA T Mesozoic and Paleozoic rocks in the upper part of the Moors Mountain plate (fig. 2). 25 L I I trace fossils, including Scalerotuba and unidentified smooth-walled small 15 15 35 V E N T E located; short dashed where inferred; dotted where concealed E The youngest principal structure in the Nelson quadrangle is the Soup Creek fault K U burrows. High total organic content within brownish- and grayish-black 15 R T 40 Ts H 30 RU mudstone units. Splits shaly and flaggy; commonly covered by debris from Overturned anticline—Showing trace of axial surface, direction of dip of limbs, (geologic sections A–A’ through D–D’). Along the fault, pre-Miocene(?) strata of the 15 60 ST ၧၧpq ၧMab QTg NELSON FAULT overlying formation. Thickness variable as a result of deformation, 38–67 m and direction of plunge of axis. Dashed where approximately located; short Moors Mountain plate are faulted down on the west. Overlying Miocene(?) beds are Mm U Hauser ၤf 40 QUADRANGLE Avalanche U tilted as much as 8° east into the fault. The coincidence of the trace of the Soup Creek 15 25 D 46°45′ Lake dashed where inferred; dotted where concealed 25 8 Ts Butte Djb Jefferson Formation (Devonian)—Dolostone, light and medium to medium- 20 25 45 35 Qog Dj fault with a reentrant in the Moors Mountain fault suggests that the Soup Creek fault Jme 20 FA Inverted anticline—Beds in both limbs of fold are overturned and dip toward HEL ULT F dark brownish gray, locally medium gray; weathers medium brownish gray, 25 EN A 35 ၤm A flattens at depth to merge with the Moors Mountain fault. The age of the faulting is 30 ၤpp Ts FA VA U pale yellowish brown, and yellowish gray; fine to medium crystalline; strong the trace of the axial surface shown. Dashed where approximately located ULT LL L 35 E T Miocene(?) and Pliocene, and the fault is interpreted as a basin-range fault associated ZO Y petroliferous fetid odor on fresh surface; local wavy crosslaminae, but Syncline—Showing trace of axial surface and direction of plunge of axis. Dashed NE with extension in and along the transform fault zone of the Lewis and Clark tectonic 20 Yg generally structureless; thin beds of pelletoid limestone; rare very thin beds of ၧၧpq 30 40 where approximately located; short dashed where inferred; dotted where zone during middle and late Cenozoic time (Reynolds, 1977, 1979; Reynolds and 45 Dm Khd Ts dolomitic sandstone; contains lenses of brachiopod and trilobite(?) fragments 25 15 30 Ml Qa 0 1 2 3 4 5 MILES concealed Kleinkopf, 1977). 15 30 ၤw Ts and very thin intervals of stromatoporoid algae. Splits flaggy and blocky. Mm Qa 35 Asymmetric syncline—Showing trace of axial surface and direction of plunge of 10 QTg 0 1 2 3 4 5 KILOMETERS Birdbear Member (Djb) mapped at top of formation in western part of map 25 35 30 axis. Short arrow indicates steeper limb. Dashed where approximately 20 20 30 25 35 area: limestone, medium and medium-dark gray; weathers very light gray located; short dashed where inferred; dotted where concealed 20 QTg Ml 45 30 REFERENCES CITED MDt and white; forms white ledges or low cliff at top of formation. Formation 20 30 Ys 26 Ts 28 Yg [Thrust sheets listed in descending succession of increasing age] Overturned syncline—Showing trace of axial surface, direction of dip of limbs, 20 thickness variable as a result of deformation and dissolution, 165–190 m; 20 30 Billingsley, P.R., and Locke, Augustus, 1939, Structure of ore deposits in the Kk 25 32 Eldorado thrust sheet and direction of plunge of axis. Dashed where approximately located; short 15 Oligocene basalt flows Birdbear Member 20–28 m thick continental framework: New York, American Institute of Mining and Metallurgical 25 dashed where inferred; dotted where concealed Jme 40 Phanerozoic rocks Dm Engineers, 51 p. Jme 30 30 Maywood Formation (Upper and Middle Devonian)—Dolostone, light gray, 35 30 Authochthon, Phanerozoic rocks at surface Inverted syncline—Beds in both limbs are overturned and dip away from the U QTg Ts 25 20 Middle Proterozoic rocks yellowish gray and light brownish gray; weathers very pale orange and pale Folk, R. L., 1962, Spectral subdivision of limestone types, in Ham, W. E., ed., ၧၧpq 35 D Dj ၤf trace of the axial surface shown. Dashed where approximately located 10 Qa 28 QTg grayish orange; very thin intervals of dolomitic siltstone and sandy dolostone; Classification of carbonate rocks—A symposium: American Association of 20 50 25 38 20 35 35 laminated, wavy laminated, and thin bedded; a few laminae contain halite Strike and dip of bedding—In areas of complex structure, ball on end of strike 25 ၧMab Qa Petroleum Geologists, Memoir 1, p. 62–84. Ml 35 Ys Frontal fold and fault belt—Upper Mississippian and younger crystal casts; splits platy and flaggy. At base includes discontinuous unit as line shows top of beds determined from sedimentary features Ts 20 ၤm 18 Harrison, J.E., and Reynolds, M.W., 1976, Western U.S. continental margin—A Djb 30 Ts Yg rocks; includes Adel Mountains Volcanics (Upper Cretaceous) Edge of major thrust sheet—Sawteeth on upper plate thick as 6 m of thin beds and laminae of silty dolostone that contain siltstone 55 Inclined Kk Mm ၤpp 75 stable platform dominated by vertical tectonics in the late Precambrian [abs.]: Jme 20 35 Dm 25 33 partings and very thin limestone and dolostone beds; grayish red and C 5 20 40 35 25 Edge of major thrust sheet, overturned—Base of Moors Mountain thrust sheet 40 Overturned Geological Society of America Abstracts with Programs, v. 8, no. 6, p. 905. 20 Ts greenish gray; weather yellowish gray and grayish red; lenticular beds contain 5 70 35 18 sawteeth on upper plate 30 20 MDt 40 30 30 QTg 32 Phanerozoic rocks 60 Overturned more than 180° Hobbs, S.W., Griggs, A.B., Wallace, R.E., and Campbell, A.B., 1965, Geology of the 46°45'00" 55 20 35 40 46°45'00" Edge of decollement surface at base of frontal pebbles reworked from underlying Upper Cambrian beds and probable fish(?) 46°47'30" R.1W. 111°45'00" Coeur d’Alene District, Shoshone County, Idaho: U.S. Geological Survey D Middle Proterozoic rocks; includes Upper Proterozoic sills fault and fold belt bone fragments. As mapped in steep exposures, locally includes some beds Vertical of Late Cambrian age. Total thickness 16–23 m 15 Professional Paper 478, 139 p. U Major high-angle reverse fault—U on upthrown block; Inclined and crenulated beds—Showing approximate strike and dip Hogback Mountain and related thrust sheets Reynolds, M.W., 1977, Character and significance of deformation at the east end of 18 1/ ၤpp Pilgrim Formation and Park Shale, undivided (Upper and Middle Base from U.S. Geological Survey, 1962 2° SCALE 1:24 000 Geology mapped by Mitchell W. Reynolds some faults have strike-slip component of movement Strike and dip of cleavage—Combined with symbol for strike and dip of Phanerozoic rocks Cambrian)—Pilgrim Formation (Upper Cambrian) forms upper part: pebbly the Lewis and Clark line, Montana [abs.]: Geological Society of America Abstracts Polyconic projection; longitude of central meridian 111°48'45" 1 1/ 2 0 1 MILE in July 1976, August 1977, and August 1988 and Major high-angle normal fault—Bar and ball on bedding at coincident site of measurement William H. Hays, assisted by William D. Quinlivan in 1959 limestone and silty limestone, light gray to light olive gray, weathers light with Programs, v. 9, no. 6, p. 758–759. Latitude of projection origin 46°45'00" Middle Proterozoic rocks downthrown block 62 1000 0 1000 2000 3000 4000 5000 6000 7000 FEET Inclined North American Datum of 1927; 10,000-foot grid based on Digital database by D. Paco VanSistine gray, pale grayish orange, and grayish orange; interbedded silty claystone Reynolds, M.W., 1979, Character and extent of basin-range faulting, western Montana Stratigraphic boundary at termination of thrust sheets Vertical Montana coordinate system, central zone; 1,000-meter grid ticks, zone 12 TRUE NORTH Digital cartography and layout design by Gayle M. Dumonceaux Avalanche Butte and related thrust sheets and claystone, light olive gray, olive gray, weather light olive, dusky yellow and east-central Idaho, in Newman, G. W., and Goode, H. D., eds., RMAG and 1 .5 0 1 KILOMETER or between Phanerozoic and pre-Phanerozoic rocks MAGNETIC NORTH Color design of geologic map by Mitchell W. Reynolds Phanerozoic rocks within a thrust sheet green and grayish green; pebbly limestone and limestone beds 0.3–2 m Minor vein—Showing strike UGA 1979 Basin and Range Symposium: Denver, Colorado, Rocky Mountain Preparation of database files for map layout by Nancy Shock 1962 MAGNETIC CONTOUR INTERVAL 40 FEET thick; silty claystone and claystone intervals 0.1–5 m thick. Limestone is Minor vein—Showing strike and vertical dip Association of Geologists, p. 185–193. NORTH DECLINATION Figures 1 and 2 drafted by Norma Maes Middle Proterozoic rocks biopelsparite, containing common to abundant trilobite fragments, Reynolds, M.W., 2003, Geologic map of the Hogback Mountain quadrangle, Lewis NATIONAL GEODETIC VERTICAL DATUM OF 1929 Edited by F. Craig Brunstein q pelmatozoan fragments, small chitinous brachiopods, ooliths and pellets, and Quartz—Mineral in veins, or mineral extracted from shafts, adits, and prospects and Clark and Meagher Counties, Montana: U.S. Geological Survey Geologic Manuscript approved for publication September 17, 2002 Figure 2. Tectonic map of the northern Big Belt Mountains, Montana, showing thrust sheets, tectonic units, and the distribution of Middle Proterozoic glauconite pellets in sparry calcite matrix. Flat pebbles of laminated Minor anticline—Showing bearing and plunge Investigations Series I–2773, scale 1:24,000. rocks, Phanerozoic rocks, and Oligocene basalt flows in relation to the Nelson quadrangle. aphanocrystalline to silty fossiliferous limestone, 1–8 cm in largest Minor crenulated beds—showing form Reynolds, M.W., and Kleinkopf, M.D., 1977, The Lewis and Clark line, Montana- dimension, are subrounded to rounded and are generally supported at high 3 Lineation—Showing bearing and plunge Idaho—A major intraplate tectonic boundary [abs.]: Geological Society of America angles to bedding in silty, biopelsparite matrix. Claystone and silty claystone Abstracts with Programs, v. 9, no. 7, p. 1140–1141. are very thinly laminated; split fissile and platy. Park Shale (Middle Inclined adit Robinson, G.D., and McCallum, M.E., 1991, Preliminary geologic map of the Cambrian) forms lower part: silty claystone and claystone, sparse very thin Vertical mine shaft—Abandoned A Beartooth Mountain quadrangle, Lewis and Clark County, Montana: U.S. channels filled with pebbly biosparite limestone; light olive gray and light Prospect Geological Survey Open-File Report 91–113, scale 1:24,000. olive; weathers olive gray and grayish green; fissile and platy. Total thickness Ts Outcrop of unit Ts too small to show at map scale—Includes some float of mapped unit varies as a result of strong deformation in claystone and silty Robinson, G.D., McCallum, M.E., and Hays, W.H., 1969, Geologic map of the Upper indicative of formation underlying slope covered by unit QTg claystone intervals and lenticularity of pebbly limestone beds; generally Holter Lake quadrangle, Lewis and Clark County, Montana: U.S. Geological 145–190 m thick Spring Survey Geologic Quadrangle Map GQ–840, scale 1:24,000 T BEND IN SECTION D' – A A' C AUL C' D' D

LT Section FAU – FEET RUST FEET T TH METERS METERS IN D BEND IN SECTION

A Section L NT 7000 MOU 6000

U S 2100 1800

MOOR

A

F

K MDt E Dj MDt Dj E

ၤpp QTg SOUP CREEK F ork-Nelson Road 6000 R 1800 5000 Gulch Dm 1500 Y

C Hunters Ql Mm Djb QTg

P ၤpp ၤw Mm MDt

U ၧMab Ts Yg

O Stump Gulch Ml Ml MDt Big Log Gulch S Mm ၤm Dm 5000 Khd 1500 4000 1200 ၤf Qa Qa ၤf Ml Qa Kk ၧMab Mm Dj MDt Dj Dm Dj ၤw Ml ၤpp Yg Jme Pၧpq Dj ၤf Ys ၤm ၤm 4000 Ml Djb ၤw 1200 3000 900 ၤpp ၧMab Mm Yg MDt ၤpp Khd Khd ၤm Yg ၤf Yn ၤm ၤw 3000 Djb Yg 900 2000 600 LT U Dj FA Mm ၤf LT U T A ST ၤw A U F R ၤf ST 2000 TH ၤw 600 1000 U 300 HR IN T TA Yg N N AI U ၤf T O N OU M Yg M RS Yg 300 S Yg Sea level 1000 O Sea level R O Yn OO M M Traces of beds shown locally to demonstrate disharmonic deformation in formations Sea level 0 -1000 -300

B T B' BEND IN SECTION

AUL D D' FEET FEET 6000 METERS METERS 8000 2400 LT 1800 FAU T T A' D' B' RUS C'

H – – – T – TAIN A N A B D C eek U Section 5000 Section Section Section O Sheep Gulch SOUP CREEK F M ၤf S ၤw ၤpp 7000 T M OR Gulch 1500 O 2100 MO ၤm OR S ၤf ၤpp ၤw AUL T M Pipeline Checkerboard O Khd Qa ၤw T U Checkerboard Gulch A NTA 4000 MDt Bridge Cr IN Yg ၤm A TH ၤf 6000 RU 1800 1200 eek ST Djb FAU ၧMab ၤw ၤf LT eek Mm Mm Ml T H T Sweats Gulch T O 3000 Dj ၤpp UL Yg A Ys A G ၤm F 5000 SOUP CREEK F A 1500 ST 900 QTg Beaver Cr B A RU Soup Cr T Yg Qa Dm Dj Ml H C Dm ၤf N T Qa AI Qa Qa A T K T A T Yg UN Yn M Yg 2000 Ys O Yg Ts A ၤw ၤpp S M Yg 4000 O 1200 Khd OR 600 MDt U O ၤm N M Yg LT U ၤf TA Yg A I F N T ၤw T 1000 S H Ys U R 300 3000 R U 900 H ၤf S T T F Yg AU Yg IN LT TA Yg N Sea level U Yg Sea level 2000 O 600 M S R O Traces of beds shown locally to demonstrate Traces of beds shown locally to demonstrate O -1000 Yn M disharmonic deformation in formations -300 1000 disharmonic deformation in formations 300

GEOLOGIC MAP OF THE NELSON QUADRANGLE, LEWIS AND CLARK COUNTY, MONTANA Any use of trade names in this publication is for descriptive purposes only and does not imply By endorsement by the U.S. Geological Survey For sale by U.S. Geological Survey Information Services Mitchell W. Reynolds and William H. Hays Box 25286, Federal Center, Denver, CO 80225 ARC/INFO coverages and a PDF for this map are available at Printed on recycled paper 2003 http://pubs.usgs.gov/imap/2003/i-2774