West-central Montana Research Articles

Non-target effects of an introduced biological control agent on deer mouse ecology.

Release of exotic insects as biological control agents is a common approach to controlling exotic plants. Though controversy has ensued regarding the deleterious direct effects of biological control agents to non-target species, few have examined the indirect effects of a "well-behaved" biological control agent on native fauna. We studied a grassland in west-central Montana infested with spotted knapweed (Centaurea maculosa) to examine the effects of knapweed invasion and two gall flybiological control agents (Urophora affinis and U. quadrifasciata) on the native deer mouse (Peromyscus maniculatus). Stomach-content analysis revealed that Urophora were the primary food item in Peromyscus diets for most of the year and made up 84-86% of the winter diet. Stomach contents indicated that wild-caught mice consumed on average up to 247 Urophora larvae mouse-1 day-1, while feeding trials revealed that deer mice could depredate nearly 5 times as many larvae under laboratory conditions. In feeding trials, deer mice selected knapweed seedheads with greater numbers of galls while avoiding uninfested seedheads. When Urophora larvae were present in knapweed seedheads, deer mice selected microhabitats with moderately high (31-45% cover) and high knapweed infestation (≥46% cover). After Urophora emerged and larvae were unavailable to Peromyscus, mice reversed habitat selection to favor sites dominated by native-prairie with low knapweed infestation (0-15%). Establishment of the biological control agent, Urophora spp., has altered deer mouse diets and habitat selection by effecting changes in foraging strategies. Deer mice and other predators may reduce Urophora populations below a threshold necessary to effectively control spotted knapweed.

Montana Anglers' Inability to Identify Bull Trout and other Salmonids

Abstract The efficacy of fishing regulations relies on anglers knowing and understanding regulations as well as their ability to identify fish correctly. While the legal harvest of both bull trout (Salvelinus confluentus) and westslope cutthroat trout (Oncorhynchus clarki lewisi) has been restricted or eliminated in west central Montana to restore these native populations, anglers unintentionally may be harvesting these trout because of an inability to identify them. We studied the ability of 681 anglers to identify 6 salmonid species commonly found in west central Montana and found that 44% correctly identified bull trout, and 76% correctly identified westslope cutthroat trout. Overall, anglers correctly identified salmonid species 63% of the time but frequently confused related species. Resident and more-experienced anglers identified salmonid species better than nonresident and less-experienced anglers, respectively. Managers must develop educational measures to give anglers identification skills and h...

Distribution, characteristics, and genesis of high fineness gold placers, Ninemile Valley, central-western Montana

The high gold fineness, geographic distribution, and general lack of bedrock lode sources make the Ninemile district of west-central Montana one of the most interesting gold placer localities in Montana. Analysis of the distribution of the gold-bearing gravels and the petrography and geochemistry of the gold grains leads to the conclusions that the gold is detrital, of very high original purity, and derived from lode deposits hosted by Proterozoic-age Belt Supergroup rocks located within the Ninemile Valley. The limited known lode occurrences, type, and metamorphic grade of bedrock in the Ninemile Valley, and gold grain characteristics are compatible with slate hosted-type, gold-bearing quartz veins.The lode deposits, based upon relationships noted in the Ninemile Valley and elsewhere in the region, were probably emplaced during Laramide-age felsic plutonism. The gold, however, may ultimately have been derived from the Prichard Formation, the basal unit of the Belt Supergroup, which underlies the entire area and hosts gold-bearing quartz veins elsewhere in the Belt basin. The stratigraphic interval encompassing the middle Belt carbonate through the lower part of the Missoula Group may also have been the source for some of the vein gold. We speculate that the distribution of placer deposits resulted from dextral offset of lode sources by the northwest-striking Ninemile fault during the Tertiary. This fault may have displaced the northeast half of the formerly circular- to ellipically shaped lode district up to 15 km to the southeast.

Structural and U-Pb geochronological evidence for 1.47 Ga rifting in the Belt basin, western Montana

Structural mapping and U-Pb dates from mafic sills in the Perma culmination of west-central Montana constrain the timing of a major rifting event in the early history of the Belt basin. Magmatic zircon from the Plains and Paradise mafic sills within the Prichard Formation yielded U-Pb ages of 1469 ± 2.5 and 1457 ± 2 Ma, respectively. The older sill invaded unconsolidated, wet sediments at a depth of less than 1.6 km, and its emplacement is coeval with widespread soft-sediment folding, mud diapirism, and bathymetric collapse of the basin between members E and F of the Prichard Formation. The youngest sill intruded near the base of the exposed section after significant accumulation of sediment had occurred in the basin. Correlation of these data with recent geochronologic results from the Moyie sills in British Columbia and Purcell lava in Montana suggests that the rifting and subsidence event was basin-wide, and that at least two thirds of the deposition of the Belt Supergroup occurred between ~1470 and 1440 Ma. These results support a continental rift model for the formation of the Belt basin.

Winter Segregation by the Sexes of White-Tailed Deer

Abstract Winter habitat selection by the sexes of white-tailed deer (Odocoileus virginianus) in west-central Montana was compared. Radio-collared males were concentrated in the eastern 1/3 of the study area, while most females used the western 2/3. The area used by males was at higher elevation, less steep, had less logging, and more trees. Browse plants and twigs were more abundant in the area used by females, but browse use was similar or lower in the area used by males. Our data suggest that males select areas with more dense hiding cover. Where males use areas peripheral to where females winter, dense stands of mature evergreens and adequate hiding cover should be retained adjacent to traditional deer wintering areas. West. J. Appl. For. 8(1):28-33.

Blood Parasites of Blue Grouse (Dendragapus Obscurus) In Western North America

Three hundred thirty-three blue grouse (Dendragapus obscurus) were examined for blood parasites from 11 sites: southern Yukon Territory, southeast coastal Alaska, northern and central interior British Columbia, south coastal British Columbia, northcentral Washington, southcentral Oregon, northwestern California, eastcentral Nevada, northwestern Colorado, and westcentral Montana. Three species of protozoan parasites (Leucocytozoon lovati, Haemoproteus mansoni, Trypanosoma avium) and a splendidofilariid nematode (Microfilaria sp. B) were found in nearly all locations. Prevalence levels were consistently high for L. lovati (92%). The other hematozoa were found less frequently (H. mansoni 29%; T. avium 46%; and microfilaria 29%). The range of these parasites in blue grouse was extended to a more northern (Yukon Territory) and more southern distribution (Nevada than previously reported. Ranges were also extended to blue grouse populations in Alaska, Washington, Oregon and California.

A new Late Cretaceous paleomagnetic pole from the Adel Mountains, west central Montana

North America's apparent polar wander path has been poorly defined between the mid‐Cretaceous and Paleocene reference pole positions. Existing data allowed 13° of apparent polar motion over about 22 m.y. (87–65 Ma) roughly coinciding with the beginning of Laramide deformation (∼80 Ma). We report on a paleomagnetic study of the Adel Mountain Volcanic rocks to refine the North American apparent polar wander path for this interval. The shonkinite rocks of the Adel Mountain Volcanic field are on the eastern edge of the Cretaceous‐Paleocene fold and thrust belt; some of these structures disturb the western edge of the volcanic pile. We obtained two new K‐Ar dates from the Adel rocks. One date, on biotite ( from a shonkinite dike that crosscuts most of the volcanic rocks, is 71.2±2.7 Ma. The other, a whole rock date from a flow deep in the volcanic pile, is 81.1±3.5 Ma. We collected six to nine paleomagnetic samples from each of 34 sites in roadcuts and natural outcrops of flows, dikes, and laccoliths. Positive fold and conglomerate tests, along with alternating field and thermal demagnetization, indicate that our characteristic remanent directions are primary magnetizations acquired before Late Cretaceous to Paleocene thrust belt deformation. Averaging the virtual geomagnetic poles from 26 reliable sites, all of normal polarity, yields a paleopole at 82.2°N, 209.9°E (α95 = 6.80°, k = 18.38). This pole is concordant with the Paleocene reference pole (82.0°N, 170.2°E, α95 = 3.5°, k = 18.6 (Diehl et al., 1983)) and is 11.6° from the Globerman and Irving (1988) mid‐Cretaceous pole at 71°N, 196°E. The youngest information in the Cretaceous stillstand pole is from the Niobrara Formation (Shive and Frerichs, 1974) at about 85–89 Ma. If we take the average age of the Adel Mountain Volcanics to be 76 Ma, then ∼12° of apparent polar motion occurred between 87 Ma and 76 Ma. Thus, rapid apparent polar motion correlates well with the onset of Laramide deformation.

Late Pleistocene Glaciers and the Climate of Western Montana, U.S.A.

Western Montana is ideally situated to have both its climate affected by the presence of Pleistocene ice sheets and those effects be apparent in its distribution of mountain glaciers. Paleoequilibrium line altitudes (paleoELAs) determined through a weighted average of cirque floor elevations, highest lateral moraines, and interpreted glacial extents define a complex pattern which reflects sources of moisture, directions of airflow, and sites of local convergence and divergence of airmasses. The regional trend of paleoELAs is parallel to that of present glacier ELAs but lies about 450 m lower. The parallelism suggests that the regional controls on moisture availability during late Pleistocene time were similar to those at present. Numerical reconstructions of precipitation required to maintain glaciers assuming a uniform regional 10?C summer temperature depression indicate a Pleistocene decrease in precipitation, relative to present, of about 25 cm H20. Decreases were greater (more than 50 cm H20) over the high mountains of southwest Montana and snowfall may have increased (by more than 50 cm H20) in west-central Montana. Glacial Lakes Missoula and Great Falls may have served as significant local moisture sources during periods of maximum glaciation. Windflow patterns were similar to those at present, except that convergence of prevailing westerlies and katabatic winds from the Laurentide and Cordilleran ice sheets apparently generated local forceful uplift and increased precipitation.

Faults of the central part of the Lewis and Clark line and fragmentation of the Late Cretaceous foreland basin in west-central Montana

The Lewis and Clark line is a prominent zone of strike-slip, dip-slip, and oblique-slip faults that extends from near Wallace, Idaho, to east of Helena, Montana. Faults of this zone have been intermittently active from Middle Proterozoic to Holocene time, and because of numerous tectonic overprints, controversy continues about displacement directions and times of displacement along specific faults. Geologic mapping shows evidence that many principal faults of the Lewis and Clark line, such as the St. Marys-Helena Valley, Bald Butte, Ninemile, and Osburn faults, had right separation or slip that ranged between 28 and 11 km, and this displacement probably occurred during Late Cretaceous time. Other faults, such as the Elevation Mountain, Placer Creek, and Ranch Creek faults, have Late Cretaceous right separations that range between 8 and 3.2 km, and the Mount Sentinel fault zone has between 6.5 and 3 km of right separation of probable Late Cretaceous age. Subsidiary structures of the Lewis and Clark line postdate Paleozoic and Lower Cretaceous rocks and predate Late Cretaceous stocks at some places; subsidiary faults and folds that have age constraints have slip directions compatible with right slip along adjacent, principal faults. Sedimentation patterns of Lower and Upper Cretaceous rocks indicate that faults of the Lewis and Clark line fragmented the foredeep region of the foreland basin into separate northern and southern basins in Late Cretaceous time. The Lower and Upper Cretaceous Blackleaf Formation (Albian and lower Cenomanian) was deposited in a continuous foredeep basin that extended across the Lewis and Clark line from north of the Canadian border to southwestern Montana, a distance of about 450 km. North of the Lewis and Clark line, middle and upper Cenomanian rocks are absent, and a thin sequence of uppermost Cenomanian to Campanian rocks was deposited in a marine environment that changed to a strand-line and continental environment in early Campanian time. South of the Lewis and Clark line, middle and upper Cenomanian deposits are also absent, but a thick sequence of Turonian-to-Campanian rocks was deposited in brackish water and strand-line environments, and during later Campanian time, in a continental environment. In the region between the St. Marys-Helena Valley and Bald Butte faults, a barrier may have formed that served as a local sediment source between foredeep regions in the northern and southern foreland basin during the period 91 to 75 Ma. South of the Bald Butte fault, an extensional tectonic regime contributed to a higher sediment-accumulation rate in the foredeep region along the north border of the southern basin (30 cm/1,000 yr), as compared to lower sediment-accumulation rates (6.9 and 7.8 cm/ 1,000 yr) in the foredeep region of the south part of the northern basin.

Late Pleistocene Glacier Dynamics and Paleoclimate Of South-Western Montana and North-Eastern Idaho. U.S.A.

Reliable reconstructions of paleoglaciers using topographic maps and aerial photographs allow calculation of effective basal shear stresses along the longitudinal profiles of these glaciers. Glacial flow theory applied to these shear stresses provides an estimate of the component of mass flux due to internal deformation. Assuming basal slip to be zero at the point where deformation mass flux is a maximum, minimum average accumulation gradients (above the equilibrium-line altitude (ELA)) and ablation gradients (below the ELA) can be calculated and minimum mass flux at the ELA can be estimated using the continuity equation. Average net winter accumulation can also be calculated by dividing the mass flux at the ELA by the accumulation area. Because local climate controls the mass balance of a glacier, and therefore the accumulation and ablation gradients, this model provides information on the climatic setting of these paleoglaciers. This model also allows estimation of basal slip as a factor in point estimates of glacial flow. Application of the continuity model above and below the ELA generates additional estimates of mass flux at discrete points along the glacier. The difference between deformation mass flux and continuity flux at these points yields a first approximation of basal slip, which is highly variable along the glacier. The model was tested on the Big Timber glacier of west-central Montana and applied to several other late Pleistocene glaciers in the northern Rocky Mountains of south-western Montana and north-eastern Idaho. Low ablation gradients (<4.0 mm m-1) suggest a climate during the late Pleistocene comparable to the present-day climate of the Brooks Range in Alaska. Calculated average net winter accumulation for the area is well below modern values, again indicating that the climate was much drier during the full glacial period. Basal sliding accounts for most (>90%) of the glacial flow near the terminus of each glacier but is variable along the rest of the glacier. While the mass-balance values are minima, they are assumed to be reasonable approximations of the actual values unless very high basal slip rates occurred along the entire length of each glacier.

Late Pleistocene Glacier Dynamics and Paleoclimate Of South-Western Montana and North-Eastern Idaho. U.S.A.

Reliable reconstructions of paleoglaciers using topographic maps and aerial photographs allow calculation of effective basal shear stresses along the longitudinal profiles of these glaciers. Glacial flow theory applied to these shear stresses provides an estimate of the component of mass flux due to internal deformation. Assuming basal slip to be zero at the point where deformation mass flux is a maximum, minimum average accumulation gradients (above the equilibrium-line altitude (ELA)) and ablation gradients (below the ELA) can be calculated and minimum mass flux at the ELA can be estimated using the continuity equation. Average net winter accumulation can also be calculated by dividing the mass flux at the ELA by the accumulation area. Because local climate controls the mass balance of a glacier, and therefore the accumulation and ablation gradients, this model provides information on the climatic setting of these paleoglaciers.This model also allows estimation of basal slip as a factor in point estimates of glacial flow. Application of the continuity model above and below the ELA generates additional estimates of mass flux at discrete points along the glacier. The difference between deformation mass flux and continuity flux at these points yields a first approximation of basal slip, which is highly variable along the glacier.The model was tested on the Big Timber glacier of west-central Montana and applied to several other late Pleistocene glaciers in the northern Rocky Mountains of south-western Montana and north-eastern Idaho. Low ablation gradients (<4.0 mm m-1) suggest a climate during the late Pleistocene comparable to the present-day climate of the Brooks Range in Alaska. Calculated average net winter accumulation for the area is well below modern values, again indicating that the climate was much drier during the full glacial period. Basal sliding accounts for most (>90%) of the glacial flow near the terminus of each glacier but is variable along the rest of the glacier. While the mass-balance values are minima, they are assumed to be reasonable approximations of the actual values unless very high basal slip rates occurred along the entire length of each glacier.

Dillon cutoff-Basement-involved tectonic link between the disturbed belt of west-central Montana and the overthrust belt of extreme southwestern Montana

The front of the Cordilleran fold and thrust belt in western Montana follows the disturbed belt in the north, merges with the southwest Montana transverse zone in the west-central part of the region, and in southwestern Montana is marked by a broad zone characterized by complex interaction between thrust belt structures and basement uplifts. The front margin of the thrust belt in Montana reflects mainly thin-skinned tectonic features in the north, an east-trending lateral ramp that curves southwest in the central part into the Dillon cutoff, an oblique-slip, thick-skinned displacement transfer zone that cuts through basement rocks of the Lima recess, and a zone of overlap between thin- and thick-skinned thrusts in extreme southwestern Montana. The transverse ramp and basement-involved thrust faults are controlled by Proterozoic structures.

Dynamics of the Late Pleistocene Big Timber Glacier, Crazy Mountains, Montana, U.S.A.

AbstractThe late Pleistocene Big Timber glacier of west-central Montana was used as the test case for a model which calculates the mass balance of a paleoglacier using glacial flow theory. Application of Glen’s flow law to a detailed reconstruction of the glacier provided an estimate of the component of mass flux due to internal deformation. Assuming basal slip to be zero where mass flux due to deformation was a maximum, the mass flux at the equilibrium-line altitude (ELA) an ablation gradient of 3.0 ± 0.6 mm/m, and an accumulation gradient of 1.0 ± 0.2 mm/m were determined. Application of the continuity model above and below the ELA generated a second estimate of mass flux at discrete points along the glacier. The difference between deformation flux and continuity flux yields a first approximation of slip, which is highly variable along the glacier. Since the mass-balance gradients are climatically controlled, this model provides information on the paleoclimatic setting of the glacier. The low gradients indicate that, during the last glacial maximum, the east side of the central Rocky Mountains experienced a cold, dry environment much like that of modern-day glaciers in the Brooks Range of Alaska.

A Statistical Model of Mineral Wealth in West-Central Montana: ABSTRACT

A Statistical Model of Mineral Wealth in West-Central Montana: ABSTRACT

Character and Regional Significance of Great Falls Tectonic Zone, East-Central Idaho and West-Central Montana

The Great Falls tectonic zone, here named, is a belt of diverse northeast-trending geologic features that can be traced from the Idaho batholith in the Cordilleran miogeocline, across thrust-belt structures and basement rocks of west-central and southwestern Montana, through cratonic rocks of central Montana, and into southwesternmost Saskatchewan, Canada. Geologic mapping in east-central Idaho and west-central Montana has outlined a continuous zone of high-angle faults and shear zones. These structures (1) extend more than 150 km (93 mi) northeastward from near Salmon, Idaho, toward Anaconda, Montana, (2) had recurrent movement from middle Proterozoic to Holocene time, (3) controlled the intrusion and orientation of Late Cretaceous to early Tertiary dike swarms, and (4) ontrolled the uplift and orientation of the Anaconda-Pintlar Range. Recurrent fault movement in this zone and strong structural control over igneous intrusion suggest a fundamental tectonic feature that has influenced the tectonic development of the Idaho-Montana area from at least middle Proterozoic time to the present.

The Age of Glacier Peak Tephra in West-Central Montana

At Sheep Mountain Bog, near Missoula, Montana, a late-glacial tephra, that probably fell in late summer, is preserved as an 8-mm-thick graded bed overlain by another 8 mm of redeposited ash mixed with lake deposits. Sediment surrounding the ash was 14C dated to about 11,200 yr B.P. Electron-microprobe analyses of the volcanic glass and hornblende phenocrysts from this ash layer indicate that they are similar in major-element chemistry to those of Glacier Peak layer G previously considered to be about 12,000 yr old or older.

Great Falls Lineament, Idaho and Montana: ABSTRACT

The name Falls is given to a northeast-trending zone of diverse geologic features that can be traced northeastward from the Idaho batholith in the Cordilleran miogeocline of the United States, across thrust belt structures and basement rocks of west-central and southwestern Montana, through the cratonic rocks of central Montana, and into southwesternmost Saskatchewan, Canada. The zone is well represented in east-central Idaho and west-central Montana where geologic mapping has outlined northeast-trending, high-angle faults and shear zones that: (1) extend more than 150 km (93 mi) from near Salmon, Idaho, northeastward toward Anaconda, Montana; (2) define a nearly continuous zone of faulting that shows recurrent movement from middle Proterozoic to Holocen time; (3) controlled the intrusion and orientation of some Late Cretaceous to early Tertiary batholithic rocks and early Tertiary dike swarms; and (4) controlled the uplift and orientation of the Anaconda-Pintlar Range. Recurrent movement along these faults and their strong structural control over igneous intrusions in this region suggest that northeast-trending faults represent a fundamental tectonic feature of the region. Figure Geologic features that are similar to those mapped in the Salmon-Anaconda region are present to the southwest and the northeast. In central Idaho, these structures include numerous northeast-trending faults and pronounced topographic lineaments that cut across the southern part of the Idaho batholith, and a northeast alignment of Tertiary igneous rocks that cut the Idaho batholith and adjacent rocks. East and southeast of the Anaconda-Pintlar Range, subparallel, high-angle faults and topographic lineaments are present in the Highland, Pioneer, Ruby, and Tobacco Root Mountains. High-angle faults may have in part controlled the orientation of the northeast-elongate Boulder batholith. Northeast-trending structures are not easily traced across the thrust belt of western Montana or across he Lewis and Clark line. In the central Montana plains, northeast of the disturbed belt, however, a broad zone of colinear, northeast-trending structures is present, and includes: parallel, buried basement highs that in part controlled depositional patterns of some Paleozoic and Mesozoic sedimentary rocks; major physiographic features, such as the remarkably straight, 175-km (109-mi) long segment of the Missouri River, and equally long, buried river channels in southwestern Saskatchewan; a northeasterly alignment of highly differentiated igneous rocks and a belt of ultrabasic intrusions and related diatremes; End_Page 1350------------------------------ and a well-defined pattern of northeast-trending gravity and aeromagnetic anomalies underlying this part of central Montana and southwesternmost Saskatchewan. Taken together, all these geologic features define a broad, northeast-trending zone at least 150 to 200 km (93 to 125 mi) wide and more than 1,000 km (620 mi) long. The zone is approximately colinear but not demonstrably continuous with the well-exposed boundary in eastern Saskatchewan and Manitoba between the Archean Superior and the Proterozoic Churchill provinces of the Canadian Shield. This boundary is also characterized by: high-angle faults, shear zones, and topographic lineaments; pronounced linear gravity and magnetic anomalies; igneous intrusions; and fault controlled depositional patterns and mineralization. That the Great Falls lineament is controlled by a similar Precambrian boundary between the Archean Wyoming province of southwestern Montana and early Proterozoic terrane to the north is speculative; however, the geologic features found along the Great Falls lineament share many common characteristics with features present along the Archean-Proterozoic boundary in Canada. End_of_Article - Last_Page 1351------------

Tectonic Framework of Disturbed Belt of West-Central Montana

The Disturbed Belt of west-central Montana consists of a zone of Laramide (Late Cretaceous-early Tertiary) convex-eastward thrusts and folds that include the major Eldorado-Lombard overthrust that has brought Precambrian rocks of the Belt Supergroup eastward over Paleozoic and Mesozoic strata. Most of the Disturbed Belt is east of and structurally below the Eldorado-Lombard overthrust. Deformed rocks range in age from middle Precambrian (Proterozoic or Y) to early Tertiary. This part of the Disturbed Belt forms a salient that extends eastward into the foreland and is bounded on the north by thrusts having a left-lateral component of movement and on the south by folds characterized by right-lateral shift. Deformation is most intense in the western part of the Disturbed Belt and dies out eastward where gentle folds merge with structures of the Rocky Mountain foreland. This salient appears to be underlain by an eastward-yielding decollement fault that merges with tear thrusts on the north and dies out in the subsurface beneath the folds marking the eastern and southern margins of the Disturbed Belt. Deformation resulted from piling up of imbricate thrusts and folds where the major Eldorado-Lombard overthrust sheet of the Cordilleran orogene moved upward and onto the foreland margin. Thus, the strata of the Disturbed Belt have been crumpled and deformed independently of the basement rocks, with crustal shortening above the decollement. Hydrocarbon exploration in this area should be guided by the tectonic features described in subsequent sections of this paper. Thrusts in the western part of the Disturbed Belt have displacements of several miles and may conceal structures beneath the thrusts, but in the eastern part of the area thrusts have smaller displacements and probably do not conceal major structures below. Some anticlines are broken by subsidiary thrusts, resulting in offset axial surfaces at depth. Also, many folds have inclined axial surfaces and the fold axes therefore will migrate with depth.

Overthrust and Disturbed Belt of West-Central Montana--Implications for Hydrocarbon Exploration: ABSTRACT

The Overthrust Belt of west-central Montana consists of a zone of Laramide (Late Cretaceous-early Tertiary) convex-eastward thrusts and folds that include the major Eldorado-Lombard overthrust that has brought Precambrian Belt rocks eastward over Paleozoic and Mesozoic strata. East of and structurally below the Eldorado-Lombard overthrust is the Disturbed Belt with deformed rocks ranging in age from middle Precambrian (Proterozoic or Y) to early Tertiary. The Disturbed Belt here forms a salient that extends eastward into the foreland and is bounded on the north by thrusts having a left-lateral component of movement and on the south by folds characterized by right-lateral shift. Deformation is most intense in the western part of the Disturbed Belt and dies out eastward where gentle folds merge with structures of the Rocky Mountain foreland. This salient appears to be underlain by an eastward-yielding decollement fault that merges with tear thrusts on the north and dies out in the subsurface beneath the folds marking the eastern and southern margins of the Disturbed Belt. Deformation resulted from piling up of imbricate thrusts and folds where the major Eldorado-Lombard overthrust sheet of the Cordilleran orogen moved upward and onto the foreland margin. Thus, the strata of the Disturbed Belt have been crumpled and deformed independently of the basement rocks, with crustal shortening above the decollement. Hydrocarbon exploration in this area should be guided by the tectonic features. Thrusts in the western part of the Disturbed Belt have displacements of several miles and may conceal structures beneath the thrusts, but in the eastern part of the area have smaller displacements and probably do not conceal major structures below. Some anticlines are broken by subsidiary thrusts, resulting in offset axial surfaces at depth. Also, many folds have inclined axial surfaces and the fold aces therefore will migrate with depth. End_of_Article - Last_Page 574------------

Major tectonic elements and tectonic problems along the line of section from northeastern Oregon to west-central Montana: Map summary

Research Article| August 01, 1979 Major tectonic elements and tectonic problems along the line of section from northeastern Oregon to west-central Montana: Map summary DONALD W. HYNDMAN DONALD W. HYNDMAN 1Department of Geology, University of Montana, Missoula, Montana 59812 Search for other works by this author on: GSW Google Scholar Author and Article Information DONALD W. HYNDMAN 1Department of Geology, University of Montana, Missoula, Montana 59812 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1979) 90 (8): 715–718. https://doi.org/10.1130/0016-7606(1979)90<715:MTEATP>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation DONALD W. HYNDMAN; Major tectonic elements and tectonic problems along the line of section from northeastern Oregon to west-central Montana: Map summary. GSA Bulletin 1979;; 90 (8): 715–718. doi: https://doi.org/10.1130/0016-7606(1979)90<715:MTEATP>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract No Abstract Available. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.