Crazy Mountains Research Articles

Patterns of faunal turnover and diversity in the Wyoming-Montana Paleogene in relation to regional and global events

The Bighorn Basin of Wyoming and its northerly extension, the Crazy Mountains Basin of Montana, contain a continuous record of the middle Paleocene through early Eocene mammalian biota. This interval, spanning some eight million years, is subdivided into 18 faunal zones. Durations of the zones, ranging from 0.3 to 0.75 million years, are extrapolated from intrabasinal depositional rates and correlated with the early Tertiary paleomagnetic reversal record. This stratigraphic resolution allows us to document in detail regional diversity and turnover.In the Bighorn and Crazy Mountains basins, standing generic richness (estimated number of genera at the midpoint of an interval) decreased slightly between the Torrejonian and Tiffanian (middle and late Paleocene), increased from the latest Tiffanian and Clarkforkian (latest Paleocene) to the early Wasatchian (early Eocene), and declined during the later Wasatchian.Changes in diversity reflect first and last appearances (including originations, extinctions, and dispersal). Because first and last appearance rates are in part dependent on diversity and the duration of each interval, we plot generic turnover as rate quotients, ratios of observed to expected first or last appearances. Expected turnover rates are calculated from multiple regression of turnover on duration and generic richness. Turnover rates are evaluated in relation to the quality of the fossil record for each interval. Rate quotients show that the slight decline in generic richness in the Bighorn and Crazy Mountains basins between the Torrejonian and Tiffanian resulted from a higher than expected disappearance rate at the end of the Torrejonian. Lower than expected disappearance rates and higher than expected appearance rates in the early Clarkforkian and, particularly, the earliest Wasatchian contributed to the increase in richness during this period. The middle through late Wasatchian decline in generic richness resulted from surprisingly low appearance and high disappearance rates.There was little change in taxonomic composition of faunas between the middle and late Paleocene; faunas from this interval were dominated by archaic groups (e.g., “condylarths”, multituberculates, plesiadapiforms). In contrast, faunal composition changed markedly during the latest Paleocene and the early Wasatchian, with the introduction of major modern groups (e.g., rodents, artiodactyls, perissodactyls, primates) and the decline in diversity of the archaic forms that dominated earlier Paleocene faunas. There is strong evidence that this faunal change was influenced by global climatic factors: climatic warming, beginning in the latest Paleocene, opened new routes for intercontinental dispersal, and had a major effect on worldwide patterns of faunal composition and diversity, including those of the Bighorn and Crazy Mountains basins.

Taphonomy of Paleogene and Neogene vertebrate assemblages

The Paleogene of Wyoming and Montana and the Neogene Siwaliks of Pakistan contain deposits representing a wide variety of terrestrial environments. Although fossils are preserved in all of these environments, fossil vertebrates are abundant in only certain facies. These principal preservational environments vary within and, particularly, between formations in each region.The Bighorn and Crazy Mountain basins of Wyoming and Montana contain abundant remains of Paleocene and Eocene vertebrates. Over 1800 localities in this region have been established in the Paleocene Fort Union and Paleocene to Eocene Willwood formations. The distribution of vertebrate remains changes dramatically upsection in this sequence. The Fort Union consists of thick fluvial sandstones and minor swampy floodplain mudstones in the lower part, and more widely separated channel sandstones interbedded with better drained floodplain deposits that include paleosol horizons in the upper part. These changes reflect a local increase in aggradation rate. Vertebrates are preserved almost exclusively in channel mud-clast conglomerates in the lower part of the formation, but appear in a wider variety of environments (channels, splays, swamps, and paleosols) in the upper part. The Willwood Formation continues the changes evident in the upper Fort Union. With increased aggradation rates, floodplain deposits became thicker and more well drained. Except for rare occurrences in other environments (channels, a variety of calcareous environments, and oxbows), fossils are recovered primarily from paleosol horizons developed in fine grained floodplain deposits.The Fort Union channel deposits contain assemblages that are often highly biased samples of faunal composition with large aquatic taxa well represented and small terrestrial forms unevenly represented. The upper Fort Union and Willwood floodplain paleosols contain more homogeneous (and therefore comparable) assemblages. These paleosols differ systematically, however, in terms of soil maturity and vertebrate composition, and they are commonly biased against large or aquatic taxa.The distribution of Siwalik vertebrate localities among sedimentary environments is known for four formations spanning early middle to late Miocene: Kamlial, Chinji, Nagri, and Dhok Pathan. The principal deposits in which fossils occur are: major stream channel complexes, secondary (floodplain) channels, crevasse splays, and floodplains. The Kamlial and Nagri formations, dominated by coarse lithologies (>50% sandstone), are notably less productive of fossil localities than the Chinji and Dhok Pathan formations, dominated by fine grained lithologies (>50% mudstone). In the Kamlial and Nagri formations, major channel complexes form the most prevalent environment, and more localities are associated with these channels than with any other environment. In the Chinji and Dhok Pathan formations, the most prevalent environment is floodplain, but more localities are associated with the secondary channels on these floodplains than with any other environment. In the Chinji Formation, most localities developed in secondary channels occur in fining-upward fill sequences, whereas in the Dhok Pathan Formation, most sites in these channels occur in their lag deposits. In these more productive formations, the abundance of certain mammalian taxa is correlated with specific depositional environments. Change upsection in the abundance of these taxa could result largely from change in facies productivity.The taphonomy of the two areas is similar in that channel lag deposits are an important source of fossil vertebrates. The major difference is that the Rocky Mountain Paleogene contains fewer, but highly productive, environments that changed significantly through time, including the Eocene emergence of floodplain paleosols as the primary environment of vertebrate preservation.

Geochemistry of igneous rocks from the Crazy Mountains, Montana, and tectonic models for the Montana Alkalic Province

Tectonic models have postulated that subduction of a flat‐lying oceanic plate was intimately linked with mid‐Eocene magmatism in the northwestern United States and have assumed that this linkage is expressed in the geochemical characteristics of igneous rocks. Geochemical data from the Crazy Mountains in south central Montana can be interpreted as indicating the presence of a subduction‐related chemical component. Because radiogenic isotopic data for these rocks restrict the major chemical events in their mantle source to the Late Cretaceous‐Early Tertiary (50–100 Ma) and the mid‐Proterozoic (1.6–1.8 Ga), subduction during one or both of these periods is the most plausible explanation for the origin of the arc‐like geochemical features. Assessment of geologic and tectonic constraints, however, indicates that Late Cretaceous‐Early Tertiary subduction cannot explain the regional distribution, the contemporaneity, and the compositional range of the Eocene igneous rocks. The Eocene magmatic event does not show interpretable age progressions, as most subduction‐related models suggest. There is considerable geographic overlap in the chemical characteristics of the Eocene igneous rocks, so that chemical zoning cannot be demonstrated south of the U.S.‐Canada border. The zone of igneous activity was diffuse and discontinuous, unlike most subduction‐related magmatic arcs. The presence of thick lithospheric mantle beneath the Archean Wyoming craton presents difficulties for models that involve subduction of a flat‐lying slab. The lithospheric mantle probably controlled the depth of subduction and probably forced the slab to depths greater than those of plausible source regions for subalkalic, mantle‐derived basalts. Flat slab subduction models cannot account for the thermal inputs required for extensive magmatism. Subduction‐related models fail to explain the CO2‐rich character of alkalic magmatism in central Montana. The arc‐like geochemical patterns in the Crazy Mountains samples could result either from subduction unrelated to Late Cretaceous‐Early Tertiary events, or from mantle geochemical processes that are unrelated to subduction. If geochemical processes unrelated to subduction are involved, trace element discrimination diagrams are misleading and are inadequate criteria for interpreting the tectonic environment. The evidence for mid‐Proterozoic chemical modification of the mantle beneath the Wyoming craton suggests that the arc‐like character could be inherited from subduction events between 1.6 and 1.9 Ga, but a specific mid‐Proterozoic subduction geometry cannot be identified. Alternative tectonic models for Eocene magmatism can be built around plausible heating or decompression mechanisms for regional magma generation. The advection of a thermal anomaly by mantle upwelling, or regional uplift related to the thickening‐flexure‐rebound cycle in the Cordillera to the west could have triggered the regional Eocene magmatic event.

Eocene potassic magmatism in the Highwood Mountains, Montana: Petrology, geochemistry, and tectonic implications

Potassic volcanics and intrusives of Eocene age (52±1 Ma) in the Highwood Mountains of north‐central Montana provide evidence for interaction of asthenospheric magmas with Archean mantle lithosphere of the Wyoming Craton. Diverse rock types were produced by shallow level degassing, fractional crystallization and magma mixing within two separate magma series whose parental liquids were latite and olivine minette. Halogen systematics of apatite microphenocrysts and other evidence have established that shallow degassing of the phlogopite‐diopside‐phyric minettes to yield heteromorphic leucite‐salite‐phyric mafic phonolites and shonkinites was an important process. Geochemical and mineralogical data suggest that fractional crystallization of olivine, clinopyroxene, mica and leucite, accompanied by widespread magma mixing, produced a spectrum of more evolved magmas such as leucite phonolites, malignites, alkali syenites and trachytes along with mica clinopyroxenite cumulates. Sr, Nd and Pb isotopic compositions and trace element data for the primitive olivine minette magmas are explicable by a multistage model involving three source components. One component is ancient Ba‐LREE‐enriched, U‐Th‐HFSE‐depleted subcontinental lithospheric mantle, which has been recognized in other alkalic rocks of the region, including those from the Crazy Mountains and Smoky Butte. Glimmerite‐veined harzburgite and phlogopite dunite xenoliths (one with ‐εNd of −33 at 52 Ma) found in the most primitive Highwood olivine minette are probably samples of this material. The other two components are asthenospheric mantle with isotopic composition near that of bulk earth (and dominant in Montana alnöitic diatremes) and a young subduction‐related component (probably Eocene, but possibly as old as late Cretaceous), which is required to explain the Rb/Sr‐87Sr/86Sr systematics of the Highwood rocks. A consistent model for the petrogenesis of the Highwood parental mafic magmas involves partial melting of asthenospheric mantle wedge triggered by infiltration of melts released from the metasomatized carapace above a low‐angle subducted slab of Farallon Plate lithosphere, followed by assimilative interaction of these melts with ancient, phlogopite‐rich, metasome veins upon ascent through the Wyoming Craton mantle keel.

Paleomagnetic and isotopic dating of thrust-belt deformation along the eastern edge of the Helena salient, northern Crazy Mountains Basin, Montana

Thrust-belt deformation along the eastern edge of the Disturbed Belt in the Helena salient of Montana has not been well dated owing to a lack of syn- or postorogenic strata. In situ paleomagnetic data from alkaline intrusions exposed in the easternmost folds of the salient in the northern Crazy Mountains Basin, previously described as pre-, syn-, or posttectonic with respect to deformation, are well grouped (Dec. = 343°, Inc. = 61°, α95 = 5.0°, k = 46, n = 16 sites); a negative fold test is significant at minimally 95% confidence. Intrusion and magnetization acquisition thus postdate fold and thrust deformation. K-Ar age determinations of selected intrusions range from 52 to 48 Ma. Paleomagnetic and isotopic age data, combined with stratigraphic information, indicate that folding in the northern Crazy Mountains Basin is middle to late Paleocene in age. This age is in agreement with suggested dates for deformation in the northern Montana Disturbed Belt but is recognizably older than the youngest episodes of frontal deformation in the Utah-Idaho-Wyoming salient. Data from this study and existing structural and stratigraphic information demonstrate that deformation in the overthrust belt and foreland provinces of southwest Montana overlapped temporally and spatially, ranged from Late Cretaceous to earliest Eocene in age, and progressed from west to east through time.

Magnetic Polarity Stratigraphy and Biostratigraphy of Middle-Late Paleocene Continental Deposits of South-Central Montana

Exposures of the Fort Union Formation on the divide between Hunt Creek and Cub Creek in the northern Clark's Fork Basin, Carbon County, Montana, were selected for magnetostratigraphic study of the transition between the Torrejonian and Tiffanian Land-Mammal Ages. Paleomagnetic samples were collected from 25 sites within a 160 m-thick section of the Fort Union Formation at that location. Rock-magnetic analyses indicate that alternating-field demagnetization to peak fields in the 10 to 40 mT interval successfully removed secondary components of natural remanent magnetism (NRM). Characteristic NRM directions define three polarity zones, a 50 m-thick normal polarity zone bracketed by two reversed polarity zones. The Cub Creek local faunule CC-2 () occurs within the upper portion of the normal polarity zone. Cub Creek local faunules CC-1, CC-3, and Eagle Quarry (all occur in the upper reversed polarity zone. These data, along with faunal and magnetostratigraphic data from the San Juan Basin, New Mexico, and the southern Clark's Fork Basin, Wyoming, allow the transition between the Torrejonian and Tiffanian Land-Mammal Ages to be correlated with the later portion of chron 27. Paleomagnetic and paleontologic data from isolated quarries in the southern Clark's Fork Basin allow Mantua Quarry (Pui) to be correlated with chron 29r, while Rock Bench Quarry correlates with the later portion of chron 27r. Data from the Crazy Mountain Basin in Montana indicate that Silberling Quarry () correlates with chron 27r, while Douglass Quarry (TiO, Scarritt Quarry (), and Locality 13 () correlate with chron 26r.

Regional Middle Proterozoic enrichment of the subcontinental mantle source of igneous rocks from central Montana

Research Article| January 01, 1987 Regional Middle Proterozoic enrichment of the subcontinental mantle source of igneous rocks from central Montana Francis Ö. Dudás; Francis Ö. Dudás 1Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802 Search for other works by this author on: GSW Google Scholar Richard W. Carlson; Richard W. Carlson 2Department of Terrestrial Magnetism, Carnegie Institute of Washington, 5241 Broad Branch Road, N.W. Washington, D.C. 20015 Search for other works by this author on: GSW Google Scholar David H. Eggler David H. Eggler 3Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802 Search for other works by this author on: GSW Google Scholar Author and Article Information Francis Ö. Dudás 1Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802 Richard W. Carlson 2Department of Terrestrial Magnetism, Carnegie Institute of Washington, 5241 Broad Branch Road, N.W. Washington, D.C. 20015 David H. Eggler 3Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1987) 15 (1): 22–25. https://doi.org/10.1130/0091-7613(1987)15<22:RMPEOT>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 Francis Ö. Dudás, Richard W. Carlson, David H. Eggler; Regional Middle Proterozoic enrichment of the subcontinental mantle source of igneous rocks from central Montana. Geology 1987;; 15 (1): 22–25. doi: https://doi.org/10.1130/0091-7613(1987)15<22:RMPEOT>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 SocietyGeology Search Advanced Search Abstract Trace-element and Pb, Nd, and Sr isotope analyses demonstrate that the Tertiary subalkalic and alkalic igneous rocks of the Crazy Mountains, Montana, derive from separate mantle sources. The alkalic source region is heterogeneous and strongly enriched in most incompatible elements. Although it has Sr isotopic compositions near bulk earth values, it has low 206Pb/204Pb (16.5–17.4) and ϵNd (−2.6 to −153). The source of the subalkalic rocks is less enriched but is similar isotopically, except that ϵNd ranges between −13.4 and −17.1, Model ages based on Nd and Pb isotopic data show that these mantle source regions have a multistage differentiation history that probably commenced shortly after stabilization of the Archean crust at 3.0 Ga. The isotopic character of both alkalic and subalkalic sources was significantly influenced by a mantle-enrichment event at 1.5–1.8 Ga.Comparison of the Crazy Mountains with other Cretaceous to Holocene igneous centers in Montana and Wyoming shows that the Middle Proterozoic enrichment event is recorded from the Bearpaw Mountains to the Leucite Hills. This regionally extensive ancient subcontinental mantle is complementary to mid-ocean ridge and ocean island basalt sources and is distinct from the sources of many other alkalic rocks. 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.

A Reliable Celloidin Technique for Dental Cementum Analysis: A Comment

havior and evolution. Univ. Chicago Press, Chicago, Ill. 383pp. HANSON, W. 0. 1950. The mountain goat in South Dakota. Ph.D. Thesis, Univ. Michigan, Ann Arbor. 92pp. HARMON, W. 1944. Notes on mountain goats in the Black Hills. J. Mammal. 25:149-151. HIBBS, L. D. 1967. Food habits of the mountain goat in Colorado. J. Mammal. 48:242-248. HOLECHEK, J. L., M. VAVRA, AND R. D. PIEPER. 1982. Botanical composition determination of range herbivore diets: a review. J. Range Manage. 35:309-315. JOHNSON, B. K., R. D. SCHULTZ, AND J. A. BAILEY. 1978. Summer forages of mountain goats in the Sawatch Range, Colorado. J. Wildl. Manage. 42: 636-639. KLEIN, D. R. 1953. A reconnaissance study of the mountain goat in Alaska. M.S. Thesis, Univ. Alaska, Fairbanks. 121pp. PECK, S. V. 1972. The ecology of the Rocky Mountain goat in the Spanish Peaks area of southwestern Montana. M.S. Thesis, Mont. State Univ., Bozeman. 54pp. SAUNDERS, J. K. 1955. Food habits and range use of the Rocky Mountain goat in the Crazy Mountains, Montana. J. Wildl. Manage. 19:429-437. SMITH, B. L. 1976. Ecology of Rocky Mountain goat in the Bitterroot Mountains, Montana. M.S. Thesis, Univ. Montana, Missoula. 203pp. SPARKS, D. R., AND J. C. MALECHEK. 1968. Estimating dry weight in diets using microscope technique. J. Range Manage. 21:264-265. TODD, J. W., AND R. M. HANSEN. 1973. Plant fragments in the feces of bighorns as indicators of food habits. J. Wildl. Manage. 37:363-366. VAVRA, M., AND J. L. HOLECHEK. 1980. Factors influencing microhistological analysis of herbivore diets. J. Range Manage. 33:371-374. -, R. W. RICE, AND R. M. HANSEN. 1978. A comparison of esophageal fistula and fecal material to determine steer diets. J. Range Manage. 31:11-13.

Laramide Interactions of Structural Elements in Southwestern Montana: ABSTRACT

The present geologic framework of southwestern Montana is the result of the interactions of several structural units during the Laramide orogeny. The Lewis and Clark lineament is the oldest of these, having been in existence since the Precambrian when some of its elements bounded the Belt embayment. It is composed of five or six parallel features with differing geologic histories. During the Laramide orogeny, horizontal compression initiated deep-seated wrench faulting along the Lewis and Clark lineament. This faulting allowed the transition from the continued Sevier End_Page 1348------------------------------ style of thin-skinned overthrust deformation, which occurred north of the lineament, to the foreland basement-involved thrust deformation, which occurred to the south. The differential motion of the basement was absorbed by small amounts of left-lateral transform motion. This lineament may also have created a weak zone in the crust into which the Boulder batholith intruded during the early stages of the Laramide. The voluminous volcanic material associated with the batholith created a supracrustal load which downwarped the adjacent lithosphere. If the batholith itself slid eastward, as advocated by Hyndman in 1979, the load was enhanced. Decoupling of the lithosphere along the southernmost elements of the Lewis and Clark lineament localized and accentuated the load-induced subsidence, creating the Crazy Mountains basin and localizing the accumulation of the thick volcaniclastic sediments of the Livingston Group. End_of_Article - Last_Page 1349------------

Crazy Mountains Area: Geology, Exploration History, and Production Potential of Natural Gas Province in South-Central Montana: ABSTRACT

Crazy Mountains Area: Geology, Exploration History, and Production Potential of Natural Gas Province in South-Central Montana: ABSTRACT

Rocky Mountain Cretaceous Source-Rock Types, Maturity, and Relation to Exploration Future of Crazy Mountains Basin: ABSTRACT

Rocky Mountain Cretaceous Source-Rock Types, Maturity, and Relation to Exploration Future of Crazy Mountains Basin: ABSTRACT

Structural Geology of Crazy Mountains Basin: ABSTRACT

The Crazy Mountains basin is a physiographic depression, as well as a structural basin in south-central Montana. The basin is bounded on the north by the Castle Mountains, Shawmut anticlinal trend, and the Huntley-Lake basin fault zone; on the west by the Bridger and Big Belt Mountains, on the south by the Nye-Bowler lineament and the Beartooth Range; and on the east by the Pryor-Big Horn uplift. Separation of the Crazy Mountains basin from the Big Horn basin to the south is arbitrarily placed along the Nye-Bowler lineament. The Crazy Mountains are a passive intrusive complex occupying the deeper part of the basin. This setting is analogous to the Raton basin of the Colorado and its associated Spanish Peaks intrusives. The structural pattern of the Crazy Mountains basin is somewhat enigmatic, being most varied and diverse. Structures within the basin resulted from the superposition of Laramide compression on an earlier fragmented Precambrian crystalline basement. The resulting structures are folds related to thrusting; drape folds over faults with dip-slip movement; and en echelon folds with gash fractures related both to left-lateral and right-lateral transcurrent fault movement. End_of_Article - Last_Page 956------------

Resume of Depositional and Structural History of Western Montana

The western part of Montana is not a depositional basin in the sense used in this symposium, but its depositional and structural history is related to events of nearby areas. The area discussed lies partly in the shelf region and partly in the marginal area of the Cordillera geosyncline. The decipherable part of its history begins with late Precambrian (Belt) sedimentation, during which the fundamental structural framework of western Montana evolved. Thick Belt strata are present in the western extremities and in an eastward-projecting embayment. Subsequent depositional patterns and present structural configuration are closely related to distribution of that thick sedimentary wedge. Coarse arkosic conglomerate was deposited along the southern fault-controlled margin of th Belt embayment. Cambrian through Mississippian formations and parts of the Cretaceous section are typically thicker in east-west zones, essentially coincident with the old Belt embayment, than they are north or south of the embayment. Along the southwestern Montana and Idaho border a positive arch existed against which Cambrian through Devonian formations thin or disappear. This positive element became strongly negative during Mississippian and later depositional intervals as geosynclinal subsidence encroached on the cratonic margin. Important facies changes take place in stratigraphic units across the northeast-trending Greenhorn fault in the Greenhorn-Snowcrest Range. These changes, which occur in short distances, suggest faulting or strong flexure along this zone during post-Ordovician to pre-Late Devonian and during Mississippian times. Pennsylvanian, Permian, and Triassic thicknesses also seem to be mildly influenced by relatively negative movements in this area. Other northeast-striking thickness trends in several stratigraphic units are apparent in the Sweetgrass arch area, where they seem to coincide with known present-day subsurface faults. Northeast-striking structural trends apparently also control the thickness of Upper Cretaceous and Paleocene strata in the Crazy Mountains basin. In general, Triassic, Permian, Pennsylvanian, and Mississippian formations successively underlie Jurassic beds from south to north, a relationship that has been explained by southward tilt and beveling during pre-Jurassic erosion. Irregularities in the truncational pattern and general thinning of each formation beneath the next younger unit indicate that much of the northward pinch-out is related to depositional thinning on which southward tilt was superimposed. During deposition of the marine Jurassic several large islands remained above for part or all of that interval. Late in Jurassic time the western seaway along which earlier seas had transgressed the region was destroyed by increasing tectonism in the area west of Montana, and a flood of debris was carried eastward to form the non-marine Morrison Formation. The basal conglomerate of the Kootenai Formation (Lower Cretaceous) indicates a particularly strong uplift in areas that could not have been far west of Montana. When the seas returned to this region, they came from the north and south and spread westward, inundating western Montana. In the eastern part of the area, Cretaceous and Paleocene rocks are generally separable into rock and time-rock units; however, on the west the corresponding sequence is almost entirely non-marine, sparsely fossiliferous, and exceedingly diverse in lithologic character. Four major westward advances of the sea punctuated Cretaceous deposition in an increasingly unstable tectonic setting. Locally, volcanic debris is very abundant in the Colorado Group, and strong increase in thickness westward attests to further encroachment of geosynclinal downwarping onto the cratonic margin. Laramide orogeny began in the Montana area coincident with deposition of the Eagle-Claggett and correlative units. Local areas of strong uplift, erosion, and volcanism, and the large influx of andesitic volcanic debris in these stratigraphic units, are evidence of the initial stages of orogeny. Accumulation of very thick volcanic sequences in at least two separate fields during Judith River time attests to increasing intensity of orogenic processes. Strong deformation and erosion, followed by deposition of coarse erosional products, and volcanism in the southwestern and central parts of the area, intrusion of granitic plutons in the west-central part of the area, and thick accumulation of coarse gravels in the Crazy Mountains basin, all during Lancian and Paleocene time, coincide with the culmination of orogenic activity. Some intense folding and thrusting post-date those events just mentioned, but it is reasonably certain that Laramide compressional deformation had ceased before middle Eocene time in western Montana. End_Page 1801------------------------------

Resume of Depositional and Structural History of Western Montana: ABSTRACT

The western part of Montana is not a depositional basin in the sense of this symposium, but its depositional and structural history are related to events of nearby areas. The decipherable part of this history begins with Late Precambrian (Belt) sedimentation during which the fundamental structural framework of western Montana evolved. Thick Belt strata are present in the western extremities of the state and in an eastward-projecting embayment. Subsequent depositional patterns and present structural configuration are intimately related to distribution of that thick sedimentary wedge. Course arkose conglomerates were deposited along the southern fault-controlled margin of the Belt embayment. Cambrian through Mississippian formations and parts of the Cretaceous section are t pically thicker in east-west zones essentially coincident with the old Belt embayment than they are to the north or south of the embayment. A positive arch existed along the southwestern Montana and Idaho border against which Cambrian through Devonian formations thin and/or disappear. This positive element became strongly negative during Mississippian and later depositional intervals as geosynclinal subsidence encroached on the cratonic margin. Abrupt changes in stratigraphic units across the northeast trending Greenhorn fault in the Greenhorn-Snowcrest Range suggest faulting or strong flexure along this zone during post-Ordovician to pre-Late Devonian and during Mississippian time. Pennsylvanian, Permian, and Triassic thicknesses also seem to be mildly influenced by relatively negative movements in this area. Other northeast thickness trends in several stratigraphic units are apparent in the Sweetgrass arch area, where they seem to coincide with known present-day subsurface faults. Northeast structural trends apparently also control the thickness of Upper Cretaceous and Paleocene strata in the Crazy Mountains basin. In general, successively older (Triassic, Permian, Pennsylvanian, Mississippian) formations underlie Jurassic beds from south to north, a relationship that has been explained as a result of southward tilt and beveling by pre-Jurassic erosion. Irregularities in the truncational pattern and general thinning of each formation beneath the next younger unit indicate that much of the northward pinchout is related to depositional thinning on which southward tilt was superimposed. During deposition of the marine Jurassic several large islands remained above the sea for part of all of that interval. Late in Jurassic time the western seaway along which earlier seas had transgressed the region was destroyed by increasing tectonism in the area west of Montana, and a flood of debris was carried eastward to form the nonmarine Morrison Formation. The basal conglomerate of the Kootenai Formation (Lower Cretaceous) marks a particularly strong uplift in areas that could not have been far west of Montana. When the seas returned to this region, they came from the northeast and east. In the eastern part of the area, Cretaceous, and Paleocene rocks are generally separable into rock and time-rock units; however, to the west the corresponding sequence is almost entirely nonmarine, sparsely fossiliferous, exceedingly diverse in lithologic character, and subdivision is very difficult. Four major westward advances of the sea punctuate Cretaceous deposition in an increasingly unstable tectonic setting. Locally, volcanic debris is very abundant in the Colorado Group and rapid increase in thickness westward attests to further encroachment of geosynclinal downwarping onto the cratonic margin. Laramide orogeny began in the Montana area coincident with deposition of the Eagle-Claggett and correlative units. Local areas of strong uplift, erosion, and volcanism, and the strong influx of andesitic volcanic debris in these stratigraphic units is evidence of the initial stages of orogeny. Accumulation of very thick volcanic sequences in at least two separate fields during Judith River time attests to increasing intensity of orogenic processes. Strong deformation and erosion followed by deposition of coarse erosional products and volcanism in the southwestern and central parts of the area, intrusion of granitic plutons in the west central part of the area, and thick accumulation of coarse gravels in the Crazy Mountains basin, all during Lancian and Paleocene time, coincide with th culmination End_Page 1877------------------------------ of orogenic activity. Some intense folding and thrusting post-dates those events just mentioned but is reasonably certain that Laramide compressional deformation had ceased before middle Eocene time in western Montana. End_of_Article - Last_Page 1878------------

Structural Geology of the Beartooth Mountains, Montana and Wyoming

The Beartooth Mountains—an 80 by 40 mile elevated crystal block of Precambrian crystalline rocks—are in the Middle Rocky Mountains east of the great north-south belt of overthrusts. The Beartooth Mountains are bounded on the east by the Bighorn Basin (average structural relief, 15,000 feet); on the northeast and north by the Nye-Bowler lineament (along which movement has been dominantly left-lateral); and on the north by the Crazy Mountain syncline (average structural relief of at least 10,000 feet). The southwest side (Yellowstone-Absaroka Mountains) is mostly covered by thick Tertiary volcanic rocks. Detailed mapping, supplemented by results of earlier workers, provides an integrated geologic picture of the tectonic development of the block. Much of the tectonic history is recorded in the structures developed in the Paleozoic and Mesozoic sedimentary rocks exposed around the edge of the mountains. The sedimentary rocks played a relatively passive role in the dynamic rise of the block during Laramide time, as they draped, folded, broke, and slid along the edge of the block. A thrust plane(s) dipping west-southwest is the major structural feature between the Clarks Fork of the Yellowstone and Nye, Montana. Near Red Lodge numerous tear faults displace the upper plate of the thrust as much as 10,000 feet. From Nye to Livingston, Montana, the major bounding structure is a north-northeast-dipping thrust plane(s) terminating in several left-lateral tear faults. Near Gardiner a high-angle thrust plane dipping northeast bounds part of the southwest Beartooth. Several structural mechanisms apparently operated during deformation: (1) Uplifting, tilting, and depressing of large crustal blocks. These commonly form ramps from the Beartooth massif to adjacent basins; during uplift smaller blocks were rotated in the opposite sense. (2) Vertical raising of the northwest and southeast corners. Here, the general lack of horizontal displacement relative to the adjacent basins limits the total horizontal displacement of the block. (3) Thrust faulting. Most of the frontal thrusts show moderate dips. Imbrication is developed particularly at the unconfined northeast corner. The major thrust steepens with structural depth to the south. The horizontal thrust displacement is not directly measurable at most places but is limited by the lack of horizontal movement at the two fixed corners of the block. (4) Tear faulting. Several major tear faults (displacements up to 10,000 feet) bound large keystone-shaped blocks near Red Lodge. (5) Lateral shearing. During the late stage of uplift, horizontal movement toward the unconfined Red Lodge corner created major lateral shears, associated with imbrication, along the north and east sides, with folding along vertical axes and consequent shortening of the sedimentary section. Tectonic development is attributed to (1) horizontal compressive forces, responsible for early crustal block movements and subcrustal plastic shifting, and (2) vertical forces of fluid pressures from the continued transfer of subcrustal material which probably helped raise the block. Uplift ceased when one or both of these forces terminated.

A New Multituberculate from the Paleocene Torrejon Fauna of New Mexico

In 1948, I found in middle Paleocene rocks (Torrejon fauna, Nacimiento formation) exposed at the head of Kimbetoh Arroyo, San Juan County, New Mexico, a lower jaw of a small multituberculate species hitherto unknown. Although representing a species new to the Torrejon fauna, this specimen is similar to specimens of Ectypodus? sinclairi which are rather common in beds of roughly comparable age exposed in the Crazy Mountains area of Montana (Lebo formation), and there obtained by quarrying. Our specimen, No. 7908, Museum of Natural History, University of Kansas, seems worthy of record because: (1) it represents an addition to the Torrejon fauna, and (2) it may eventually be of value in attempts at precise correlation of the Torrejon fauna with the Gidley and Silberling quarries (Simpson, 1937: 31) of the Crazy Mountain field of Montana.

Food Habits and Range Use of the Rocky Mountain Goat in the Crazy Mountains, Montana

Food Habits and Range Use of the Rocky Mountain Goat in the Crazy Mountains, Montana

A Two-Year Study of the Rocky Mountain Goat in the Crazy Mountains, Montana

A Two-Year Study of the Rocky Mountain Goat in the Crazy Mountains, Montana

GEOLOGY OF THE BRIDGER RANGE, MONTANA

Research Article| November 01, 1955 GEOLOGY OF THE BRIDGER RANGE, MONTANA WILLIAM J McMANNIS WILLIAM J McMANNIS MONTANA SCHOOL OF MINES, DEPARTMENT OF GEOLOGY, BUTTE, MONTANA Search for other works by this author on: GSW Google Scholar Author and Article Information WILLIAM J McMANNIS MONTANA SCHOOL OF MINES, DEPARTMENT OF GEOLOGY, BUTTE, MONTANA Publisher: Geological Society of America Received: 08 Jun 1953 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1955, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1955) 66 (11): 1385–1430. https://doi.org/10.1130/0016-7606(1955)66[1385:GOTBRM]2.0.CO;2 Article history Received: 08 Jun 1953 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation WILLIAM J McMANNIS; GEOLOGY OF THE BRIDGER RANGE, MONTANA. GSA Bulletin 1955;; 66 (11): 1385–1430. doi: https://doi.org/10.1130/0016-7606(1955)66[1385:GOTBRM]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 In and near the Bridger Range area north and east of Bozeman, Montana, in Gallatin County, about 27,000 feet of sedimentary rocks, from Beltian to Recent, is exposed. Strata of known Ordovician, Silurian, Permian, and Triassic age are not present. Middle Cambrian Flathead sandstone is underlain by coarse Beltian arkoses in the northern part of the area and by Archean (?) metamorphics in the southern part. The depositional edge of the Belt arkoses has been fault-controlled. The Sappington formation (Berry, 1943) is separated from the underlying Three Forks formation by a minor disconformity. The Big Snowy group is recognized in the Bridger Range for the first time. The problem of the Livingston formation is reviewed, and it is apparent that the andesitic phases of these Montanan, Lancian, and Paleocene sediments can be used to interpret the tectonic history of the Crazy Mountain basin and adjacent mountain areas.Analysis of the structural features of the Bridger Range and adjacent areas indicates two phases of folding and faulting. One developed generally north-trending structures in mid-Paleocene time, and the other developed north-northwest trending structures probably in latest Paleocene or early Eocene.In Oligocene time isostatic arching produced normal faults on the west side of the Bridger Range. The range was lifted relative to the Gallatin Valley, and the east-flowing drainage was impeded so that sediments accumulated in the basins. These movements and deposition have continued sporadically to the present. 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.

Central Montana Tectonics: ABSTRACT

The major tectonic elements of Central Montana include: (1) the Little Belt-Big Snowy-Porcupine anticlinorium, (2) the northern High Plains region with its igneous intrusions, (3) the Sweetgrass arch, and (4) a southern area that consists of the Crazy Mountain syncline, Lake Basin fault zone, and the northern end of the Bighorn-Pryor uplift. The economic and academic significance of these features and their important subdivisions are reviewed. The literature is reviewed in an effort to present a picture of the geological development of these tectonic elements. In order to do this, outstanding contributions including those by Erdmann, by Sloss, and by other geologists have been slightly modified by use of new information available at this time. Revised isopach maps and generalized lithofacies maps are presented for central Montana for the Madison-Colorado shale interval. The effect or significance of certain features or conditions is discussed. These include: (1) the generally positive Sweetgrass arch, (2) the generally negative Beltan (Montana) trough-Big Snowy axis, (3) shallow and deep-seated igneous intrusives of the High Plains, (4) en echelon faults in central Montana, and (5) thrust faulting east of the main Rocky Mountain front. End_of_Article - Last_Page 2615------------