The Idaho-Wyoming segment of the Cordilleran thrust belt is characterized by west-dipping folded thrusts that place older strata over younger, by thrust plates that have lateral continuity and distinctive stratigraphic sequences, and by a gently west-dipping uninvolved basement beneath the thrust plates. Northwestward across the Snake River Plain. frontal thrusts and thrust plates of the Idaho-Montana segment of the Cordilleran belt exhibit the first two characteristics, but differ in that basement rocks locally are involved in the thrusts, indicating that these Idaho-Montana thrust plates overrode a previously faulted foreland. Distribution of basement rocks indicates that the faulted foreland consisted of west-northwest- and east-northeast-trending faults of probable Proterozoic ancestry in the area of the shelf west of the Montana craton, and northeast-trending, northwest-dipping, basement-rooted Cretaceous thrust faults of the southwestern Montana craton to the east. The hanging wall of the Cordilleran Cabin thrust contains Archean(?) rocks in a fragment of the Cabin block, a regional Proterozoic basement uplift cut by the thrust as it propagated northeastward. Hanging walls of structurally lower Cordilleran thrusts contain segments of northeast-trending Cretaceous foreland thrusts and fold structures, such as the Snowcrest Range thrust system and the Little Water syncline. Renewed movement on foreland thrusts subsequently locally folded Cordilleran thrusts. Available paleontological data and radiometric age determinations indicate that major movements on both foreland and Cordilleran thrusts took place in Late Cretaceous time in the Beaverhead Mountains and vicinity. Major Cordilleran thrust plates in the Beaverhead Mountains are, from west to east: the Hawley Creek, Fritz Creek, Cabin, Medicine Lodge, Four Eyes Canyon, and Tendoy. A west-to-east deformational sequence is assumed for all of the plates except part of the Cabin. Diagrammatic cross sections of the southern Beaverhead Mountains suggest that locally the Cabin may have overridden the Medicine Lodge, and is out of sequence. The redefined Cabin thrust plate is thick and more than 200 km in length. It has been thinned secondarily by several younger-over-older Cenozoic normal faults, some of which were mapped previously as thrusts. Archean(?) through Triassic rocks make up the plate in the central and southern Beaverhead Mountains, and Proterozoic Yellow-jacket Formation and Lemhi Group rocks make up the plate in the northern Beaverhead Mountains. There, Proterozoic rocks are thrust over Belt Supergroup strata of the Grasshopper thrust plate and are part of a structural culmination in the position of the Salmon River Arch. A large lateral ramp in the hanging wall of the Cabin thrust marks the northern margin of the 75-km-long transported segment of the Proterozoic Cabin block. Structural and stratigraphic throw diminish to the south near Bannack Pass, but increase north of the ramp near latitude 45°, where the Cabin thrust cuts down section with respect to the hanging wall to include several thousand meters of Proterozoic Yellowjacket Formation. The northern margin of the Cabin block may compose the northern margin of Archean basement beneath the thrust belt in south-central Idaho. Northeastward translation of the Cabin and Medicine Lodge thrust plates is about 40 km in the southern Beaverhead Mountains; thus, Archean(?) crystalline basement rocks of the block originally were at least as far southwest as the present Lemhi Range. These old crystalline rocks constitute a western projection of the southwestern Montana reentrant.

A recent 1:24,000 scale mapping project within the northern Beaverhead Mountains along the Idaho-Montana border has resulted in a reinterpretation of both the Mesoproterozoic stratigraphy and the regional structural framework. A 15-km-thick stratigraphic section of the Mesoproterozoic Lemhi subbasin was initially deformed by northeast-southwest shortening into giant northwest-striking, northeast-verging folds, probably during Cretaceous Sevier orogenesis. These initial folds were then dissected by a system of subparallel and anastomosing, oblique-slip reverse, thrust, and normal faults that generally strike northwest, but that exhibit east-west–oriented lineations, suggesting components of strike-slip displacement. Contractional faulting appears to have been followed by Eocene to Miocene extensional faulting, with many normal faults following the preexisting fabrics. Extension opened Tertiary basins along some of these faults, including the Salmon Basin along the southwestern side of the Beaverhead Range. Subparallel faults in the surrounding region appear to have a similar complex history, and all appear to be part of a major northwest-striking Cretaceous fold-and-thrust belt that was later dissected by Tertiary extension. Although the faults of the Beaverhead Mountains are significant and long-lived, they are not terrane-bounding structures separating the Belt and Lemhi sedimentary sequences. Instead, Lemhi strata extend across the range and northward to Missoula, where they grade into correlative Missoula Group strata.

Abstract During Late Cretaceous (late Albian-Cenomanian) through Paleocene and probably early Eocene time, southwestern Montana and adjacent Idaho were the depocenter for thick accumulations of syntectonic quartzite conglomerate, limestone conglomerate, and minor amounts of sandstone collectively known as the Beaverhead Formation. In this study, thedepositional and deformational history of the Beaverhead is documented in detail in an attempt to understand the tectonic and topographic development of southwestern Montana and east-central Idaho. Clast imbrication and composition measurements suggest that two fundamentally different source areas existed for theBeaverhead. The limestone conglomerate clasts were derived primarily from Mississippian and Triassic carbonate rocks exposed by uplift of the Blacktail-Snowcrest basement arch and the Ancestral Beaverhead Range along the present-day Montana-Idaho border. These deposits, which derived their detritus from an area less than 25 km away, probably represent coalesced alluvial fans. Although the quartzite conglomerate beds were deposited simultaneously with the limestone clasts, the quartzite was derived from a more distant source. The vast quantity of Belt quartzite clasts that initially reachedwhat is now southwestern Montana in late Albian time apparently originated from the large area of Belt Group exposure on the northeast side of the Idaho batholith approximately 80 km to the west and northwest of the study area. Gradual expansion of this source area by active uplift continuously provided steep slopes needed for the transport of gravel by braided stream systems to the adjacent subsiding alluvial plain. Recycling of the Belt clasts owing to Late Cretaceous uplift in the present eastern Snake River plain carried them as far east as western Wyoming, where they became incorporated in the Harebell and Pinyon conglomerates of western Wyoming. The Beaverhead Formation exhibits two distinct structural patterns: (1) an earlier cratonic pattern composed of northeast-trending, gently plunging, open folds and associated high-angle faults, and (2) a northwest-trending “;geosynclina” pattern characterized by major upthrusts such as the Tendoy, Cabin, and Fritz Creek faults, and large, gendy plunging, open folds. The older structural pattern seems to be related to the tectonism of the Late Cretaceous toearly Paleocene that created the Blacktail-Snowcrest basement arch. The younger, more pervasive northwest trend may have a causal connection with the large uplift in the region of the Idaho batholith. It is plausible that gravitative energy created by this uplift was available not only for eastward fluvial transport of Belt clasts, but also for northeastward downslope tectonic transport. According to this hypothesis, large sheets of bedded rock slid eastward along detachment surfaces within the Belt, and as these surfaces were folded and locked, new ones developed to the east. This eastward migrating tectonic front reached the Lima, Montana, region soon after major Beaverhead sedimentation had ceased during middle to late Paleocene or early Eocene time. At this time, the Beaverhead rocks, which in pan had inherited the earlier northeast trends, were cut by large upthrusts, folded into broad anticlinal structures, and overridden by low-angle sheets of Carboniferous limestone.

Laramide-style deformation of the Rocky Mountain foreland began in the Lima region of southwest Montana in Coniacian to Santonian (Late Cretaceous) time with the growth of the Blacktail-Snowcrest uplift. The Lima Conglomerate of the Beaverhead Group locally onlaps its deformed source terrane, the Laramide-style (thick-skin) Snowcrest-Greenhorn thrust-fault system of the foreland, along the southeastern margin of this uplift. Associated sandstones as old as Coniacian to Santonian, also derived from this uplift, are here reinstated into the Beaverhead Group. Northeast of Lima, the Snowcrest thrust transported Archean gneiss, marble, and schist southeastward over deformed Phanerozoic rocks. These Phanerozoic rocks are locally overturned and intensely fractured, and they exhibit many cross-faults. The Archean rocks exhibit locally intense cataclasis, microfaults, and pressure solution at grain boundaries. The first incursion of Sevier-style (thin-skin) thrusting into the Lima region followed Campanian erosion of the Blacktail-Snowcrest uplift, locally to Archean basement. This thrusting shed thick quartzite-roundstone conglomerates eastward. These are placed in a new informal stratigraphic unit, the Little Sheep Creek conglomerate unit, which appears to conformably overlie the fining-upward sequence at the top of the Lima Conglomerate, palynologically dated as mid-Campanian. Quartzite clasts in the Little Sheep Creek conglomerate unit were probably recycled from proximal fans adjacent to deeply eroded hinterland thrust sheets to the west. However, this unit contains large slide blocks of Mississippian limestone from the front of the closer Four Eyes Canyon sheet, the lower bounding thrust of which reached the land surface, probably in late Campanian time. The Little Sheep Creek conglomerate unit is overridden by the Tendoy thrust. Consequently, the Tendoy is younger than the Four Eyes thrust to the west. Complex structural imbrication of upper Paleozoic and Triassic through Lower Cretaceous rocks of the Tendoy thrust sheet occurs in the Lima Peaks area, above the inferred southwestern extension of the older Snowcrest-Greenhorn thrust system. This imbricate stack, transported east-northeast on the Tendoy thrust, subsequently was folded about a N70°E axis, together with the Tendoy thrust and Lima Conglomerate of its footwall, by possible later reactivation of Snowcrest-Greenhorn thrust system. Structural relationships within the Little Water Canyon and McKnight Canyon areas northwest of Lima indicate that northeast-trending structures of probable foreland origin developed in these areas prior to emplacement of the Tendoy thrust sheet, but subsequent to emplacement of the Four Eyes Canyon thrust sheet. Therefore, these northeast-trending structures are younger than the Snowcrest-Greenhorn thrust system east of Lima and may be Maastrichtian, based on structural involvement of Beaverhead rocks palynologically dated as late Campanian to early Maastrichtian in the northern part of the McKnight Canyon area. Cessation of thrusting in the Lima region is still poorly dated. The youngest Beaverhead conglomerates, those derived in part from the Tendoy thrust sheet, underlie middle to upper Eocene basin beds northeast of Dell.

The central Beaverhead Range forms the Continental Divide along the Idaho-Montana border north of the Snake River Plain. This general region is located at, or near, the eastern edge of the former Rocky Mountain miogeosyncline. Pre-Laramide sediments consist of Belt and lower Paleozoic quartzitic sandstones and upper Paleozoic carbonates, separated by Upper Devonian to Lower Mississippian dolomite, shale, and shaly limestone. Mesozoic sediments were eroded in post-Laramide time. Sedimentary processes were influenced by faulting during Late Pre-cambrian, Late or post-Ordovician, and Middle to Late Devonian time, and by moderate folding (Skull Canyon Disturbance) between Late Pre-cambrian and Middle Ordovician time. The Beaverhead Pluton, a leucocratic granite-syenite body, was emplaced during Late Ordovician or Early Silurian time at shallow depth, as indicated by miarolitic cavities, aplite, granophyre, and the hyper-solvus (perthitic) nature of most of the pluton. The Laramide structural framework can be divided into an infra-level and a supra-level, which behaved disharmonically with respect to each other, the detachment horizon being the mid-Paleozoic shaly rocks. Early Laramide compressive stress, oriented NE.–SW., caused major arching, superposed folding, and faulting in both levels. Sub-vertical faults, and thrusts growing steeper with depth, later became avenues of mafic intrusion. The pluton was raised into a faulted dome and carried north-eastward, creating plan-curvature of earlier folds and faults. In the supra-level, gravitational gliding and cascading of Carboniferous carbonates took place down the flanks of the early arches and radially away from the pluton dome. Continued compression caused reactivation of earlier faults. Post-orogenic NE.–SW. extension resulted in mid-Tertiary to Recent normal range front faulting and in north-south, left-lateral wrenching, as interpreted from regional oroclinal bends and from local curvature of fold axes. Folding of the Tertiary sediments in the down-dropped blocks could have resulted from intermittent regional NE.–SW. compressive spasms, or from local accommodation of the beds to a narrowing space between converging faults. Crustal downwarping in the Snake River region caused a southeast plunge of all tectonic axes and an extensional stress in that direction which was resolved along northeast-trending normal faults and, in nearby areas, along north-trending, right-lateral wrench faults.

Near Bannack, Montana, the Cordilleran thrust belt overlaps a Laramide Rocky Mountain foreland structure. The Precambrian-cored Armstead anticline formed along a steep, basement-rooted fault. Erosion breached the anticline, and Upper Cretaceous Beaverhead Group rocks, including distinctive conglomerates and tuffs, overlapped the truncated structure with angular unconformity. The Ermont thrust and associated structures then advanced into the area, disrupted the preexisting foreland structure, and tightly folded the Beaverhead Group. The interaction between thrust structures and the Rocky Mountain foreland structure resulted in younger-over-older thrust faults and thrusts that cut down-section in the direction of transport.