Crazy Mountains Basin Research Articles

Magnetostratigraphy of Upper Cretaceous (Lancian) to Middle Paleocene (Tiffanian) strata in the northeastern Crazy Mountains Basin, Montana, U.S.A.

Magnetostratigraphy of Upper Cretaceous (Lancian) to Middle Paleocene (Tiffanian) strata in the northeastern Crazy Mountains Basin, Montana, U.S.A.

Late Paleocene high Laramide ranges in northeast Wyoming: Oxygen isotope study of ancient river water

The distribution and initial timing of the establishment of high surface elevations in the Rocky Mountains during the Early Cenozoic remain controversial despite the importance of these data in testing tectonic models for this region. We track the timing and distribution of high elevation in the Rocky Mountains during the Late Cretaceous–Early Eocene by examining annual and seasonal δ 18O values of the ancient river water, which are calculated from the δ 18O values of well-preserved freshwater bivalve shells. In the Powder River basin of the eastern Laramide province, the δ 18O values of the ancient river water vary between − 23.0‰ and − 8.0‰ SMOW in both seasonal and annual records in the Late Paleocene–Early Eocene. The large variation suggests that the ancient rivers were fed yearly or seasonally by snowmelt from highlands of 4.5 ± 1.3 km. This can be explained by the existence of the Bighorn Mountains and Black Hills with a drainage pattern similar to the present in northeast Wyoming. The δ 18O values of ancient river water along the front of the Sevier thrust belt generally follow a trend from lower values in north, − 14.2 ± 1.4‰ in the Early Paleocene Crazy Mountains basin, to higher values in south, − 11.1 ± 0.8‰ in the Late Paleocene Bighorn basin, and − 7.1 ± 1.6‰ in the Early Eocene Washakie basin. The variations within each basin are relatively small. These rivers most likely rise in the Sevier thrust belt, and may reflect highland elevation of 1–2 km. The δ 18O values in the Alberta foreland and Williston basin are very low (− 20.5‰) in the Late Cretaceous, indicating the rivers were fed by snowmelt from the Canadian Rocky Mountains of 4.3 ± 1.0 km high. The attainment of high elevation in the eastern Laramide province prior to the western province could be explained by southwestward progression of back-thrusts soled into an earlier east-directed master detachment, which may be formed by the westward rollback of subducted shallow slab.

40Ar/39Ar dates from alkaline intrusions in the northern Crazy Mountains, Montana: Implications for the timing and duration of alkaline magmatism in the central Montana alkalic province

Intrusive rocks in the Crazy Mountains, Montana, consist of numerous stocks, dike swarms, laccoliths and/or sills of strongly alkaline to subalkaline affinity that have intruded and metamorphosed Tertiary sedimentary strata of the Crazy Mountains Basin. The subalkaline rocks form stocks, sills, and radiating dikes and are located primarily in the southern Crazy Mountains (e.g., Big Timber stock, Loco Mountain stock). With the exception of the Ibex Mountain sill (?), the alkaline rocks are restricted to the northern Crazy Mountains. New 40 Ar/ 39 Ar dates are reported for alkaline rocks of the northern Crazy Mountains, with results ranging between 50.61 Ma and 50.03 Ma. Five dates from the strongly alkaline nepheline and mafic nepheline syenites of the Ibex Mountain sill (?), Robinson anticline intrusive complex, and Comb Creek stock (?) and dike swarm give tightly clustered dates suggesting that they were emplaced during a restricted time interval at ∼50.1 Ma. The dates from the alkaline rocks of the northern Crazy Mountains are slightly older than those previously reported from the subalkaline Big Timber stock in the southern Crazy Mountains (i.e., 49.3–49.2 Ma, biotite 40 Ar/ 39 Ar) (du Bray and Harlan, 1996). However, the limited span of dates (i.e., 50.6–49.2 Ma) and the geographic proximity between the alkaline and subalkaline rocks indicate that the magmas represented by these different geochemical groups were closely associated in both time and space. Furthermore, all the igneous rocks in the Crazy Mountains were emplaced in a narrow time interval of 1–2 m.y. On a regional scale, the 51–49-Ma age span from the Crazy Mountains is similar to that of most of the igneous centers of the central Montana alkalic province and is coeval with the peak of widespread volcanism in the Absaroka-Gallatin volcanic field immediately to the south of the Crazy Mountains Basin.

New specimens of Elphidotarsius russelli (Mammalia, ?Primates, Carpolestidae) and a revision of Plesiadapoid relationships

ABSTRACT Significant new material of the plesiadapiform Elphidotarsius russelli is described from Montana and Alberta. Previously the most poorly documented carpolestid, this species is now known from numerous isolated teeth and many jaw fragments. New material from the Crazy Mountains Basin, Montana substantially extends its known geographic range and shows that E. russelli is the most derived species of Elphidotarsius, with many features that foreshadow characteristics of Carpodaptes; consequently, emending the diagnosis of Elphidotarsius has been necessary. A phylogenetic analysis based on 73 characters scored for all species of Elphidotarsius, Pronothodectes, and Saxonella, as well as Purgatorius unio, Purgatorius janisae, Pandemonium dis, Chronolestes simul, and Carpodaptes hazelae, indicates that Elphidotarsius is a paraphyletic stem taxon at the base of Carpolestidae. Numerous crossing synapomorphies among these taxa show that a single anagenetic lineage can no longer represent the evolution of early carpolestids. This analysis also clarifies basal plesiadapoid relationships. Carpolestidae is the sister taxon to Plesiadapidae, and Saxonellidae is sister group to the carpolestid/plesiadapid clade. Contrary to previous hypotheses, Chronolestes and Pandemonium are found to lie outside of a clade including the other members of Carpolestidae, Saxonellidae, and Plesiadapidae, implying that they are basal plesiadapoids that diverged before the evolution of the common ancestor to the three main plesiadapoid families.

Abstract: Thermal Maturity of Cretaceous and Tertiary strata, Crazy Mountains Basin, South-Central Montana 

Abstract: Thermal Maturity of Cretaceous and Tertiary strata, Crazy Mountains Basin, South-Central Montana 

A new species of Purgatorius (Mammalia; Primatomorpha) from the Lower Paleocene Bear Formation, Crazy Mountains Basin, south-central Montana

Purgatorius titusi, represented by 51 teeth and jaw fragments, is described from the lower Paleocene Bear Formation of the Crazy Mountains Basin in south-central Montana. Several synapomorphies, including a weak postprotocingulum and mesial inflection of molar trigonids, are indicative of the taxon's “plesiadapiform” affinities. Other features, such as wider stylar shelf and a less squared lingual outline on upper molars, are more primitive than those of Purgatorius unio, heretofore the earliest occurring species of well-represented primatomorphs.

Mammalian generic diversity and turnover in the late Paleocene and early Eocene of the Bighorn and Crazy Mountains Basins, Wyoming and Montana (USA)

Abstract Patterns of mammalian generic turnover, richness, and faunal composition were investigated for faunas from 17 biostratigraphic zones in middle Paleocene through early Eocene deposits of the Bighorn and Clarks Fork basins of northern Wyoming and the Crazy Mountains Basin of south-central Montana. Significance of turnover was evaluated (1) by comparison of observed turnover to expected turnover (calculated from the multiple regression of turnover on zone duration and generic richness), and (2) by comparison of observed turnover to a bootstrapped turnover distribution. Patterns of turnover and richness also were assessed in light of relative sampling quality of each faunal zone. The analysis identified four intervals of significant faunal change: the Torrejonian Tiffanian transition, the late Tiffanian, the earliest Wasatchian, and the middle-to-late Wasatchian. The first interval was characterized by a high number of last occurrences in the latest Torrejonian, resulting in a decrease in standing generic richness in the earliest Tiffanian, but no major changes in ordinal composition. During the next interval of significant turnover, the late Tiffanian, higher-than-expected first occurrences resulted in an increase in standing richness and a change in faunal composition, most probably reflecting the immigration of taxa from outside North America. The third, and most dramatic, interval of significant generic turnover took place in the earliest Wasatchian and was distinguished by a high number of first occurrences, but relatively few last occurrences. This led to a marked increase in generic richness, a pattern similar to that for the early Wasatchian of North America as a whole. The major change in faunal composition, as in the late Tiffanian, was largely composed of immigrants from other continents. The pattern of faunal change during the early Wasatchian of the Bighorn Basin, along with evidence for global warming at the Paleocene-Eocene boundary, supports previous interpretations associating this episode in mammalian evolution with the opening of high latitude intercontinental dispersal routes. During the fourth interval of interest, the middle to late Wasatchian, the Bighorn Basin fossil record shows a drop in generic richness. This differs from the overall North American pattern, and may be, in part, an artifact of still inadequate sampling for the latest part of the stratigraphic sequence in the Bighorn Basin.

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.

Biochronology of uppermost Cretaceous and Lower Tertiary nonmarine Mollusca of the northern Great Plains, U.S.A. and Canada

The biochronologic use of nonmarine mollusks commenced in the northern Great Plains with the pioneering studies of F.B. Meek and F.V. Hayden in the 1850s and ‘60s. Their research focused on the chronostratigraphic organization of nonmarine and marine strata and included the naming of the Fort Union Group, which represents the Paleocene throughout the northern plains. Current research has focused on ascertaining the biostratigraphic distribution of nonmarine taxa throughout the post-Fox Hills Formation sections in the northern Great Plains, specifically in the Williston (WB: North Dakota, Montana, Saskatchewan), Powder River (PRB: Montana, Wyoming), and Crazy Mountains (CMB: Montana) Basins. The strata represented include the uppermost Cretaceous Lance (PRB) and Hell Creek (WB, PRB, CMB) Formations; Paleocene Fort Union Group, including (in ascending order) the Ludlow (WB)—Tullock (WB, PRB)—Bear (CMB), Slope (WB)—Lebo (PRB, CMB), Bullion Creek (WB)-Tongue River (WB, PRB)-Melville (CMB), and Sentinel Butte (WB)—lower “Wasatch” (PRB) Formations, and the Bear Den Member of the Golden Valley (WB) Formation; and the Lower Eocene Camels Butte Member of the Golden Valley Formation (WB) and main body of the “Wasatch” (PRB) Formation. About 2000 nonmarine molluscan localities have been collected from these strata and correlated with all of the approximately 200 relevant mammalian local faunas. The geologic section represented by each study area has a segment of the section that is biostratigraphically and biochronologically relatively well controlled by either molluscan or mammalian occurrences or both. These segments overlap sufficiently well to provide a nearly continuous composite section for the uppermost Cretaceous, Paleocene, and Lower Eocene. In the Montana portion of the Williston Basin, the uppermost Cretaceous (Lancian) taxon ranges of nonmarine mollusks and mammals are stratigraphically well controlled. Less abundant or less well-controlled occurrences elsewhere support the view that a diverse, molluscan assemblage, largely composed of highly sculptured unionid bivalves, is extinguished, relatively suddenly, near but before the end of the Cretaceous. The lower and lower middle Paleocene (Puercan and Torrejonian) section, as evidenced in isolated fossil occurrences, is a time of a substantially reduced number of low-diversity molluscan faunules consisting of a small mesogastropod-dominated snail fauna, a few taxa of which cross the Cretaceous-Tertiary boundary, and a small, unornamented (except for the umbonal region) fauna of unionids. The upper middle Paleocene (Torrejonian) and lower upper Paleocene (Tiffanian) contain a record indicating a resurgence in the molluscan fauna, particularly in the upland Paleocene terrains preserved in the upper Lebo and lower Melville Formations of the Crazy Mountains Basin. A relatively large number of localities contain a diversifying mesogastropod-dominated snail fauna, an increasing number of terrestrial taxa, and an increasingly diversified, but unsculptured, unionid fauna. The middle upper Paleocene (Tiffanian 3–4) faunal record, representing the “classic” Fort Union molluscan fauna of Meek and Hayden, was comparatively diverse, abundant, and dominated by freshwater snail taxa. Although known from substantially fewer localities, this interval also contains an increase in diversity and novelty of aquatic and terrestrial pulmonates. Unionids are locally abundant, but remain relatively undiverse. The uppermost Paleocene (presumed Clarkforkian) record in the Powder River Basin documents a continued increase in pulmonate taxa and a sustained, but compositionally different, diversity of mesogastropods. The unionid fauna is relatively unchanged. The Paleocene-Eocene (Wasatchian) boundary, based on mammalian, palynomorph, and plant criteria, is clearly shown by a widespread change in the composition of the molluscan assemblage. The overall faunal diversity is somewhat reduced, with a noted reduction in the diversity of certain freshwater mesogastropods, a few unionid taxa, and a continued increase in the representation of pulmonate taxa. In summary, uppermost Cretaceous and lower Tertiary strata in the northern Great Plains contain a sequence of nonmarine molluscan faunas that are sufficiently diverse and changing so as to permit the construction of a substantially more resolved biochronology than previously considered possible.

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.

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------------

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------------