Blackleaf Formation Research Articles

Osteology and phylogenetic relationships of the mid-Cretaceous neornithischian dinosaur Oryctodromeus cubicularis Varricchio, 2007

ABSTRACT The vertebrate assemblages of the Albian to Cenomanian Wayan Formation of southeastern Idaho and southwestern Montana’s coeval Vaughn Member of the Blackleaf Formation are dominated by the small, burrowing orodromine dinosaur Oryctodromeus cubicularis. Here, we describe in detail the osteology of Oryctodromeus based on new specimens from Idaho and Montana that add substantially to the preliminary description of the types from Montana, and provide a suite of additional diagnostic characters for the taxon: ilium with elongate preacetabular process; elongate cervical vertebra centra with an anteroposterior length 1.6 times the dorsoventral height; elongate dorsal vertebra centra with an anteroposterior length 1.4 times the dorsoventral height; more than 55 elongate caudal vertebrae enveloped in hypaxial and epaxial ossified tendons; and a femoral head on an elongate neck—similar to that of Koreanosaurus—projecting from the greater trochanter at about 35°. The tail, comprising two-thirds of the animal’s roughly 3 meters length, and associated tendon sheaths in the axial column indicate greater flexibility than previously supposed for ossified tendons or, alternatively, suggest that the Oryctodromeus burrows had separate, or multiple entrances and exits. The elongated and angled femoral head likely facilitated digging via a braced splayed-leg posture. Our phylogenetic analysis incorporates new characteristics and supports the monophyly of Orodrominae, a clade of neornithischian dinosaurs from the middle to Late Cretaceous of Asia and western North America that utilized burrowing.

Timing of the Transition From Sevier‐ to Laramide‐Style Tectonism in Southwestern Montana Based on the Provenance of the Frontier Formation, North American Cordillera

AbstractThe Upper Cretaceous Frontier Formation in southwestern Montana is coeval with the transition from Sevier‐ to Laramide‐style tectonism in the Idaho‐Montana sector of the North American Cordillera. To better constrain the timing of initial exhumation above the Laramide‐style Blacktail‐Snowcrest arch, we use biostratigraphic data, sandstone petrography, and detrital zircon (DZ) geochronology to determine the provenance and depositional age of the Frontier Formation. Near Lima Peaks, erosion of Lower Cretaceous strata from the frontal Sevier‐style Tendoy thrust sheet provided sediment to the foreland basin. In the Western Centennial Mountains, sediment sources included those same sources as well as Neoproterozoic and Cambrian strata and Mesoproterozoic plutons in the Belt basin. In contrast, near the Gravelly Range, sediment eroded from Pennsylvanian‐Upper Cretaceous strata atop the Blacktail‐Snowcrest basement‐cored uplift, documenting unroofing to Pennsylvanian‐Permian stratigraphic levels by 87–85 Ma. Palynology corroborates recycling into the Frontier Formation, including from the underlying Blackleaf Formation. The ubiquitous presence of 100–85 Ma DZ ages coupled with different interpreted source regions suggests that an ash‐fall source contributed young zircon grains to the Frontier Formation. The timing of exhumation above the Blacktail‐Snowcrest arch provided by these new data presented herein suggest that Laramide‐style tectonism in Idaho‐Montana may be unrelated to shallow‐angle subduction within a narrow corridor as envisioned by current models. Instead, upper plate controls, such as the locations of inherited faults and basement highs, the distribution and thickness of pre‐orogenic sedimentary cover, and the availability of detachment surfaces, may be responsible for Laramide‐style tectonism during early Late Cretaceous time.

Foreland basin response to middle Cretaceous thrust belt evolution, southwestern Montana, USA

Abstract The middle Cretaceous Blackleaf Formation records the first major transgressionregression of the Western Interior Seaway into the southwestern Montana retroforeland basin. Although Blackleaf sedimentology is well documented, sediment provenance and potential linkages with regional tectonics are not. Recent characterization of hinterland tectonics, fold-thrust belt detrital zircon signatures, and advances in high-n detrital zircon analysis allow for significant provenance refinement. We present new detrital zircon ages (n = 5468) from ten samples from the upper Blackleaf Formation (Intervals C and D) in southwestern Montana. Based on maximum depositional ages, sedimentation spanned from 106 to 92 Ma. Jurassic and Cretaceous grains were primarily derived from the older portion of the Cordilleran magmatic arc in western Idaho. Triassic and older grains were recycled from older central Idaho sedimentary strata inboard of the arc. Three depositional stages are identified based on statistical modeling of detrital age distributions. Stage 1 (106–104 Ma) records sourcing from lower Paleozoic strata in central Idaho. Stage 2 (105–101 Ma) records initial unroofing of upper Paleozoic–Triassic strata via propagation of the fold-thrust belt into eastern Idaho, accommodating shortening of Mississippian and younger rocks above the Lemhi Arch. Stage 3 (102–100 Ma) records continued unroofing in central Idaho down to Cambrian stratigraphic levels and distal mixing of sources in the eastern part of the basin. Exhumation in the fold-thrust belt beginning at ca. 105 Ma is coincident with marginwide fault slip-rate increases. We infer that increased sedimentation rates and lowmagnitude flexural loading from shallow thrusting in eastern Idaho drove clastic wedge progradation across the basin.

Nevadadromeus schmitti (gen. et sp. nov.), a New Basal Neornithischian with Affinities to the Thescelosaurinae, from the Upper Cretaceous (Cenomanian) Willow Tank Formation of Southern Nevada

In 2008, and subsequent collecting trips, the remains of a partial basal neornithischian were recovered from the Cenomanian Willow Tank Formation of southern Nevada. Bones identified include the proximal femora, a series of vertebrae missing neural arches, several pedal phalanges, fragments of ossified tendons, and some as yet unidentified elements. Size and shape of the femora are consistent with other known basal neornithischians of both orodromine- and thescelosaurine-grade. The round femoral head exhibits a convex anterior side but a concave posterior surface. The neck beneath the head projects proximodorsally at an obtuse angle (∼100°) from the femoral shaft. The greater trochanter sits slightly posterior to and offset from the neck of the femoral head. Anterior and lateral to the greater trochanter is a pointed lesser trochanter. The three-sided lesser trochanter bows slightly posterior toward the greater trochanter. A prominent and deep notch separates the lesser from the greater trochanter and is characteristic of thescelosaurine-grade ornithischians. This deep intertrochanteric notch is absent in the femora of orodromines. A raised but taphonomically truncated base on the posterior femoral diaphysis likely represents the remnants of a pendant fourth trochanter. The vertebrae of the Nevada basal neornithischian are similar to both thescelosaurine- and orodromine-grade morphology. The laterally biconcave vertebrae are asymmetrical in having a boss on the posteroventral end of the centrum. The centra are nearly twice as long as they are tall with the oval articular surfaces, taller than they are wide. Due to the very fragmentary nature of this specimen, parsimonious phylogenetic analysis yields statistically insignificant results. Nevertheless, a few taxonomically important characters, particularly those of the femur, support the hypothesis that this is a thescelosaurine, and a new genus and species, herein referred to as Nevadadromeus schmitti gen. et sp. nov. This would represent the earliest occurrence of thescelosaurines in the fossil record of North America as all other thescelosaurines from the continent date to the Maastrichtian. The geographic position of the Willow Tank Formation depocenter, very proximal to the Sevier highlands of the time, likely experienced some biogeographic insularity from other areas represented by contemporaneous units of western North America, e.g., Mussentuchit Member of the Cedar Mountain Formation of Utah, the Wayan Formation of Idaho, and the Blackleaf Formation of Montana.

Middle Albian provenance, sediment dispersal and foreland basin dynamics in southwestern Montana, North American Cordillera

AbstractThe Lower Cretaceous Blackleaf Formation in southwestern Montana records sedimentation in the Idaho‐Montana retroforeland basin of the North American Cordillera. Regional‐scale sedimentology suggests that during Albian time southwestern Montana was partially flooded by an early marine incursion of the Western Interior Seaway during deposition of the Blackleaf Formation. We use sandstone petrography, large‐n (n = 600) U‐Pb detrital zircon geochronology and mixture modelling to determine the provenance of these strata. Our analysis suggests three distinct provenance groups: Group 1 sandstones occur in the eastern region of the study area, are quartz‐rich and have zircon age‐probability peaks of ca. 110, 160, 420–450, 1050 and 1160 Ma; these sandstones match with a primarily Appalachian provenance. Group 2 sandstones occur in the western region of study area, are lithic‐rich and have peaks of ca. 110, 160, 1780, 1840, 1920, 2080 and 2700 Ma; the primary source for these sandstones was exhumed lower‐middle Palaeozoic strata from the Idaho sector of the Sevier belt. Group 3 sandstones occur in the western region of the study area, are lithic‐rich and have prominent peaks of ca. 115, 170, 430, 600, 1085, 1170, 1670 and 1790 Ma; the primary source for these sandstones was exhumed Triassic‐upper Palaeozoic strata from the Idaho sector of the Sevier belt. Our provenance data record a sharp change that coincides with the western shoreline of the seaway, and we infer that it may indicate the position of an irregular, submarine forebulge depozone influenced by dynamic subsidence during a period of reduced thrusting in the adjacent fold‐thrust belt. Albian‐aged sediments in southwestern Montana were delivered by rivers with headwaters in the Sevier belt as well as transcontinental river systems with headwaters in eastern North America. In southwestern Montana, west‐flowing transcontinental fluvial systems were flooded by the Western Interior Seaway as it encroached from the north.

Taphonomy of and new burrows from Oryctodromeus cubicularis, a burrowing neornithischian dinosaur, from the mid-Cretaceous (Albian-Cenomanian) of Idaho and Montana, U.S.A.

Vertebrate assemblages from the mid-Cretaceous (Albian-Cenomanian) Wayan Formation of Idaho and Vaughn Member of the Blackleaf Formation of Montana are dominated by the small burrowing dinosaur, Oryctodromeus cubicularis. Specimens can be assigned to three distinct taphofacies dependent on degrees of skeletal articulation and associated lithofacies. Taphofacies A comprises mostly to fully articulated specimens in bioturbated sandstones. Taphofacies B comprises partially articulated to associated collections of skeletal elements in pedogenic mudstones, siltstones, and fine-grained sandstones. Taphofacies C comprises isolated skeletal elements within the debris-flow deposit of the Robison Bonebed. Except for one associated nodosaur skeleton, Oryctodromeus is the only vertebrate represented by significant associations of skeletal elements or portions of articulated skeletal elements within the Wayan Formation/Vaughn Member units, and occurrences represent a paleocommunity dominated by a small cursorial and fossorial vertebrate. The co-occurrence of multiple individuals, representing presumed juvenile and adult growth stages, reinforce previous suggestions of parental care in this species. In addition, newly reported Oryctodromeus burrows compare in overall morphology and size with the burrow containing the holotype of Oryctodromeus. These data suggest the possibility that evidence for additional fossil taxa having had a communal and fossorial lifestyle may be discerned through further focused taphonomic investigations.

Photogrammetry of the Oryctodromeus cubicularis type locality burrow and the utility of preexisting, standard field photographs for three dimensional digital reconstruction

ABSTRACT Oryctodromeus cubicularis is a small-bodied ornithischian dinosaur from the mid-Cretaceous Blackleaf and Wayan Formations of Montana and Idaho, respectively, and is the only documented dinosaur with evidence of a burrowing ecology. The type locality, from the Blackleaf Formation of southwestern Montana, consisted of an infilled burrow structure containing the disarticulated skeletons of one adult and two juveniles and represents a rare association of body fossils and trace fossils of a single dinosaur taxon. While these fossils and their paleobiological implications were unprecedented, collection nevertheless necessitated removal of the burrow structure from its geological context and its destruction during body fossil preparation. Here we present the first photogrammetric models of the type locality burrow in its original geological context and three dimensional morphology, constructed from photos taken at three stages in excavation. Because the terminal extent of the burrow was collected prior to the burrow’s complete exposure, the structure was never documented in its entirety. A composite model provides the only holistic documentation of the entire burrow morphology as originally preserved. These models supplement the original burrow description, serve as comparative tools for identifying and diagnosing future discoveries, and demonstrate the utility of pre-existing photographs for later photogrammetric reconstruction.

Paleontological and chronostratigraphic correlations of the mid-Cretaceous Wayan-Vaughn depositional system of southwestern Montana and southeastern Idaho

ABSTRACT The Mussentuchit Member of the Cedar Mountain Formation of eastern Utah represents the best source of data on Cenomanian vertebrate assemblages from North America. However, increasing data has recently been forthcoming from the late Albian to Cenomanian Wayan Formation of Idaho and Vaughn Member of the Blackleaf Formation of Montana, which are both at least partially coeval with the Mussentuchit Member. While the paleontological assemblages of the Wayan Formation and Vaughn Member are fragmentary, numerous vertebrate forms are represented, with the small burrowing neornithischian Oryctodromeus cubicularis dominating these assemblages. The differences between the Wayan Formation and Vaughn Member assemblages, the Mussentuchit and few other mid-Cretaceous assemblages are likely a result of some combination of preservational biases, paleogeography, and paleoenvironmental differences. The chronostratigraphy, fossil content, bracketing facies, and ages of the Wayan Formation and Vaughn Member suggest these sediments represent the same depositional system prior to disruption by subsequent tectonic and volcanic events. This assemblage from the Wayan Formation and Vaughn Member is here termed the Wayan-Vaughn Assemblage (WVA). Continued work in the WVA, particularly with microvertebrate materials, may reveal additional shared taxa between the WVA and other coeval units.

Taphonomy and taxonomy of a vertebrate microsite in the mid-Cretaceous (Albian–Cenomanian) Blackleaf Formation, southwest Montana

The vertebrate fauna of the Cretaceous Blackleaf Formation of southwest Montana remains largely undocumented. A microsite (BL1) discovered in the Flood Member in the Lima Peaks area, Montana, consists of a green siltstone and yields taxa previously unreported from the formation, including several dinosaurs: a hypsilophodont, dromaeosaurid, tyrannosauroid, hadrosaurid and an ankylosaurian. Non-dinosaurian taxa include goniopholidid and Bernissartia crocodilians; Glyptops, cf. chelydrid and other turtles and at least two neopterygiian fish. This diversity corresponds well with the fluvial–deltaic–estuarine environment interpreted for the uppermost unit of the Flood Member. Taphonomic data and sedimentologic relationships suggest that this assemblage represents a floodplain depression accumulation. Comparisons with contemporaneous faunas from around the Western Interior of the USA suggest a remarkably consistent faunal makeup, at least at the family level, existed across western North America in the mid-Cretaceous.

EXPERIMENTAL MODELING OF A POSSIBLE ORYCTODROMEUS CUBICULARIS (DINOSAURIA) BURROW

Abstract The hypsilophodont Oryctodromeus cubicularis from the Cretaceous Blackleaf Formation of Montana represents the first dinosaur interpreted to be from a burrow fill. The skeletal remains occurred within an incompletely preserved chamber at the end of an S-shaped tunnel. Unlike many fossil vertebrates associated with burrows, the Oryctodromeus remains were disarticulated and elevated within the graded fill. To test whether this skeletal arrangement reflected in situ burial from within or transport into the burrow, we constructed a half-scale burrow model using PVC pipes and conducted a series of sediment infilling experiments with appropriately scaled, disarticulated rabbit skeletons. Experimental trials varied incoming flow velocities, sediment supply, dry or water-filled chamber conditions, and initial placement and weight of skeletal elements. Twelve of the 13 trials resulted in complete rearrangement of the skeletons, six trials left bones elevated above the base of the chamber floor, and six re...

Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.

New lithostratigraphic and chronostratigraphic, geochronologic, and sedimentary petrologic data illuminate the history of development of the North American Cordilleran foreland basin system and adjacent thrust belt from Middle Jurassic through Eocene time in northwestern Montana. The oldest deposits in the foreland basin system consist of relatively thin, regionally tabular deposits of the marine Ellis Group and fluvial-estuarine Morrison Formation, which accumulated during Bajocian to Kimmeridgian time. U-Pb ages of detrital zircons and sandstone modal petrographic data indicate that by ca. 170 Ma, miogeoclinal strata were being deformed and eroded in hinterland regions. Sandstones of the Swift and Morrison Formations contain detrital zircons derived from the Intermontane belt. The Jurassic deposits probably accumulated in the distal, back-bulge depozone of an early foreland basin, as suggested by the slow rates of tectonic subsidence and tabular geometry. A regional unconformity separates the Jurassic strata from late Barremian(?) foredeep deposits. This unconformity possibly resulted as a combined effect of forebulge migration, decreased dynamic subsidence, and eustatic sea-level fall. The late Barremian(?)-early Albian Kootenai Formation is the first unit that consistently thickens westward, as would be expected in a foredeep depozone. The subsidence curve at this time begins to show the convex-upward pattern characteristic of foredeeps. By Albian time, the fold-and-thrust belt had propagated to the east and incorporated Proterozoic rocks of the Belt Supergroup, as indicated by sandstone compositions, detrital zircon ages in the Blackleaf Formation, and by crosscutting relationships in thrust sheets involving Belt Supergroup rocks in the thrust belt. A major episode of marine inundation and black shale deposition (Marias River Shale) occurred between the Cenomanian and mid-Santonian, and was followed by a regressive succession represented by the Upper Santonian–mid-Campanian Telegraph Creek, Virgelle, and Two Medicine Formations. Provenance data do not resolve the timing of individual thrust displacements during Cenomanian–early Campanian time. The Upper Campanian Bearpaw Formation represents the last major marine inundation in the foreland basin. By latest Campanian time, a major episode of slip on the Lewis thrust system had commenced, as recorded in the foreland by the Willow Creek and St. Mary River Formations in the proximal foredeep depozone. The final stage in the evolution of the Cordilleran fold-and-thrust belt and foreland basin system is recorded by the Paleocene–early Eocene Fort Union and Wasatch Formations, which were preserved in the distal foreland region. Regional extensional faulting along the fold-and-thrust belt began during the middle Eocene. The results presented here enable the establishment of links between previous geological work in Canada and the better known parts of the Cordilleran foreland basin in the United States.

First trace and body fossil evidence of a burrowing, denning dinosaur

A fossil discovery in the mid-Cretaceous Blackleaf Formation of southwest Montana, USA, has yielded the first trace and body fossil evidence of burrowing behaviour in a dinosaur. Skeletal remains of an adult and two juveniles of Oryctodromeus cubicularis gen. et sp. nov. a new species of hypsilophodont-grade dinosaur, were found in the expanded distal chamber of a sediment-filled burrow. Correspondence between burrow and adult dimensions supports Oryctodromeus as the burrow maker. Additionally, Oryctodromeus exhibits features of the snout, shoulder girdle and pelvis consistent with digging habits while retaining cursorial hindlimb proportions. Association of adult and young within a terminal chamber provides definitive evidence of extensive parental care in the Dinosauria. As with modern vertebrate cursors that dig, burrowing in Oryctodromeus may have been an important adaptation for the rearing of young. Burrowing also represents a mechanism by which small dinosaurs may have exploited the extreme environments of polar latitudes, deserts and high mountain areas. The ability among dinosaurs to find or make shelter may contradict some scenarios of the Cretaceous-Paleogene impact event. Burrowing habits expand the known range of nonavian dinosaur behaviours and suggest that the cursorial ancestry of dinosaurs did not fully preclude the evolution of different functional regimes, such as fossoriality.

Influence of the Ancestral Sweetgrass Arch on Sedimentation of the Lower Cretaceous Bootlegger Member, North-Central Montana

ABSTRACT In north-central Montana the Bootlegger Member of the upper Lower Cretaceous Blackleaf Formation is well exposed near Great Falls and at one locality in the Sweetgrass Hills. In both areas, Bootlegger strata comprise four stratigraphic successions. In the Great Falls study area each succession consists of an upward-coarsening (-shoaling) package of interbedded very fine sandstone and shale grading upward into fine to medium sandstone, abruptly overlain by a transgressive erosion surface and capped in many places by a conglomeratic transgressive lag deposit. The stacking of these successions is attributed to episodic changes of relative sea level, probably related to episodic reactivation of the ancestral Sweetgrass Arch. During episodes of arch uplift accommodation space was reduced a d caused the Bootlegger shoreline to prograde eastward. Subsequently, because of arch subsidence and/or the ongoing basinwide eustatic rise, the shoreline migrated westward. Erosion associated with transgressive shoreface retreat was most intense near the crest of the arch. As a result, in the western study area, which lies above the arch crest, successions are thin and typically consist only of inner-shelf strata (Lithofacies 1); strata deposited in shallower environments were removed mainly by later transgressive erosion. By contrast, in the eastern area, farther from the arch crest, the upward-shoaling part of each succession is more completely preserved. At the end of Bootlegger time, however, movement of the arch either ceased or simply could no longer keep pace with the eustatic ri e. Consequently, the shoreline continued to transgress westward, and only fossiliferous silt and mud accumulated in a distal shelf environment. This marked a eustatic drowning and termination of rhythmic Bootlegger sedimentation.

Correlation Chart of Lower and Lower-Upper Cretaceous Blackleaf Formation, Eastern Pioneer Mountains, Southwestern Montana, to Drummond, Central-Western Montana

Correlation Chart of Lower and Lower-Upper Cretaceous Blackleaf Formation, Eastern Pioneer Mountains, Southwestern Montana, to Drummond, Central-Western Montana

Quasi-Planar-Laminated Sandstone Beds of the Lower Cretaceous Bootlegger Member, North-Central Montana: Evidence of Combined-Flow Sedimentation

ABSTRACT Lower-shoreface to shallow-shelf strata of the Bootlegger Member of the Lower Cretaceous Blackleaf Formation are characterized by interbedded sandstone and mudstone. Sandstone beds are characterized by a peculiar planar lamination showing a subtle although perceptible undulation; spacing-to-height ratios of the undulation are generally 100 or more. Typically the undulation shows no evidence of lateral accretion but only vertical aggradation, and as a result most beds consist of a single laminaset. Aspects of quasi-planar-laminated beds indicate single-event storm sedimentation, and paleocurrent data indicate offshore sediment transport. By its sedimentary characteristics and its similarity with a bed configuration generated in un experimental wave duct, quasi-planar lamination is produced by high-energy combined flows. This style of stratification should be common in the shallow-marine stratigraphic record, and its recognition should aid in interpreting high-energy, combined-flow depositional events.

The Upper Bow Island (Blackleaf) Formation of Southwestern Alberta: Geological Aspects and Exploration Approaches

The upper parts of the Bow Island Formation (Albian) of southwestern Alberta are significant gas reservoirs. The main westernmost reservoir zone is part of a complex package of interbedded lenticular sandstones, mudstones, and localized chert pebble conglomerates. The depositional setting for these sediments comprised a wave-dominated shoreline with conglomerates found proximal to drowned river mouths. The coarse nature of the upper Bow Island is related to tectonic movements associated with Crowsnest (Vaughn) volcanism. Conglomerates form the most impressive Bow Island reservoirs because of their thickness (up to 25 m) and petrophysical properties (17% porosity, 24 d permeability). Diagenesis dominantly comprises compaction features within grain-supported conglomerates. Increasing quartz content is related to decreasing grain size and is associated with porosity occlusion by quartz overgrowths. Bow Island reservoirs in southwestern Alberta are cool (under 50C) and significantly underpressured (0.2 psi). The high permeabilities and low pressures at depths of 1,000 to 1,500 m suggest the potential for formation damage is high, and many wells in the region were targeted for deeper, high-pressure zones. In spite of the low pressures, however, many Bow Island wells are capable of excellent gas deliveries with individual well recoveries of up to 10 bcf. All significant Bowmore » Island porosity in the deepest, undisturbed parts of southwestern Alberta is gas saturated with updip aquifers flanking the gas. Seismic definition of the thickest Bow Island targets is feasible but has been hampered, in part, by difficult surface conditions and a prior emphasis on deeper targets.« less

Correlation of Lower and Upper Cretaceous Blackleaf Formation, Lima Peaks Area to Eastern Pioneer Mountains, Southwestern Montana Foreland Basin: ABSTRACT

Correlation of Lower and Upper Cretaceous Blackleaf Formation, Lima Peaks Area to Eastern Pioneer Mountains, Southwestern Montana Foreland Basin: ABSTRACT

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.

Cretaceous Fluvio-Lacustrine Facies of Upper Kootenai Formation and Flood Member of Blackleaf Formation, Southwestern Montana: ABSTRACT

The Flood Member of the Albian Blackleaf Formation is exposed in the eastern foothills of the Pioneer Mountains, southwestern Montana. Two localities were studied along a 10-mi north-south outcrop belt: Frying Pan Gulch (south) and Apex (north). Strata are 625 ft thick and composed of mixed siliciclastics and carbonates; they are grouped into coarse and fine-grained facies (CGF and FGF) sequences. The Flood Member is underlain conformably by the upper carbonate member of the Kootenai Formation.

Sandstone Diagenesis and Source Rock Analysis of Cretaceous Blackleaf and Frontier Formations in Parts of Beaverhead, Madison, and Gallatin Counties, Montana: ABSTRACT

The mid-Cretaceous Blackleaf and Frontier Formations and equivalent strata in parts of Beaverhead, Madison, and Gallatin Counties, Montana, are composed of a marine and nonmarine sequence of clastic and carbonate rocks deposited along the western margin of the Western Interior seaway. At present seven dry holes have penetrated this sequence in southern Beaverhead County. Sandstones in the formations are diagenetically diverse and exhibit calcite, silica, sericite, zeolite, and clay mineral alteration. Alteration is more pervasive in the upper Blackleaf and lower Frontier Formations because of compositional heterogeneity and the presence of volcanic detritus. Lower Blackleaf alteration is less complex and is mainly due to quartz overgrowths and calcite cements. Porosity in lower Blackleaf quartz arenites in eastern Beaverhead, Madison, and Gallatin Counties has been reduced significantly by quartz overgrowths. Lower Blackleaf selitharenites in Beaverhead County contain calcite pore-filling cement but exhibit the best reservoir rock potential of both formations. Vitrinite reflectance ranges from a maximum of 3.62% for shales in the lower Blackleaf Formation in the Pioneer Mountains in northern Beaverhead County to a minimum of 0.43% for the lower Frontier Formation in the Lima Peaks area to the south. Maximum reflectance values are related to the proximity of Cretaceousmore » and Tertiary intrusive bodies. Reflectance values for all other samples within the study area lie just below or at the base of the oil generation window. Total organic carbon (TOC) values for both formations are higher in the Gallatin Range (maximum value equals 8.1%), but Rock-Eval pyrolysis data indicate generally poor source rock potential. Black shales of the lower Blackleaf and lower Frontier Formations provide the best source rock for generation of hydrocarbons.« less