Geologic Map of the Nelson Quadrangle, Lewis And
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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 ef 46°52'30" CORRELATION OF MAP UNITS DESCRIPTION OF MAP UNITS GEOLOGIC SETTING epp ef ef Yg em Meagher Limestone (Middle Cambrian)—Limestone, medium gray, medium Khd ew IN [Terminology for carbonate rocks from Folk (1962)] em 32 ef MOUNTA ef 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 em 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 epp HOGBACK Unconformity TERTIARY deposits shown locally by common internal map contacts. Thickness as em 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 ef AUL ew 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 ew 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- ew 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 epp 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 ef (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 Phpq 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 hMab 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 ef Flathead Sandstone (Middle Cambrian)—Sandstone and pebbly sandstone Dj epp 38 MISSISSIPPIAN intrude Middle Proterozoic through Devonian rocks. On the upper plate of the Moors 50 em 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 em 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 epp 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 em 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 ew 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 epp ew epp 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 ef 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’).