Illustrations of gravity models of the southeast limb of the Blacktail-Snowcrest uplift, southwest Montana

Kulik, D.M.

doi:10.3133/ofr82823

Cited by 6 publications

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“…38) can be explained by postulating the presence of older structures in the foreland that were truncated by the west-to-east directed thrust faults of the Cordilleran fold-thrust belt. Though Dahlstrom (1970, p. 382) Recent geological and geophysical studies (Perry and others, 1981;Perry and others, 1983;Kulik and Perry, 1982) have indicated the presence of a major subsurface east-northeast-trending, lowangle thrust fault zone northeast of this study area, the sub-Snowcrest thrust, that juxtaposes Archean basement crystalline rocks from the northwest over thick Cretaceous sedimentary rocks on the southeast. Aligned gravity highs identified by Kulik (in Skipp, Lucchitta, and Kulik, 1983), that characterize the sub-Snowcrest fault zone, extend southwestward into the study area ( Fig.…”

Section: Several Large Normal Faults Between Railroad Canyon and Petementioning

confidence: 74%

Contraction and extension faults in the southern Beaverhead Mountains, Idaho and Montana

Skipp¹

1985

Open-File Report

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Geologic mapping of 340 square miles (884 square kilometers) in the southern Beaverhead Mountains demonstrates that the area is a segment of the Mesozoic to early Tertiary Cordilleran thrust belt, and a northward continuation of the Idaho-Wyoming thrust salient. Five thrust plates are bounded by major west-dipping, low-angle folded thrusts that juxtapose older strata over younger, and are characterized by east-verging concentric folds, frontal ramp anticlines, and transverse tear faults. The five thrust plates, each with a distinctive stratigraphic sequence, are, in descending structural order: Hawley Creek, Fritz Creek, Cabin, Medicine Lodge, and Tendoy. The Cabin thrust plate locally includes Precambrian (Archean(?)) basement crystalline rocks along its leading edge. These crystalline rocks probably were part of an older Precambrian block uplift cut by the eastward directed Cabin thrust. Extension faults with youngest movements ranging in age from early Eocene to Holocene offset the five thrust plates. The Divide Creek extension fault zone, along which middle Paleozoic stratigraphic section is deleted, probably is of early Eocene age. Miocene to Holocene basin-range faults offset older structures, and loci of the younger faults have shifted westward through time. Thrusts and thrust plates identified in the study area also make up a large part of the central and northern Beaverhead Mountains. The Cabin thrust plate is the most extensive of these. IX The redefined Cabin thrust is more than 125 miles (200 kilometers) long, has two major lateral ramps, has Archean(?) crystalline rocks along a 48-mile (75-kilometer) segment of its leading edge in central parts of the mountains, and Middle Proterozoic Yellowjacket Formation and overlying Lemhi Group rocks in northern parts. The Cabin plate includes Archean(?) through Lower Triassic rocks in the central Beaverheads. In the northern Beaverheads, rocks of the Yellowjacket Formation and the Lemhi Group have been thrust over Belt Supergroup rocks creating a structural culmination of Proterozoic rocks in east-central Idaho. The foreland that was overridden by the easterly transported Cabin and Medicine Lodge thrust plates was deformed by northwest to southeast directed reverse faults that probably were formed and partly eroded before the thrust plates arrived in Late Cretaceous (Maestrichtian(?)) time.

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Section: Several Large Normal Faults Between Railroad Canyon and Petementioning

confidence: 74%

Contraction and extension faults in the southern Beaverhead Mountains, Idaho and Montana

Skipp¹

1985

Open-File Report

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“…The Sage Creek normal fault forms the southeastern margin of this basin. Gravity modeling (Kulik, 1982;Kulik and Perry, this volume) suggests that this Tertiary fault is listric and merges at depth with the subSnowcrest thrust, dipping somewhat more steeply northwest than the latter, as indicated by Perry and others (1983a, Fig . 12).…”

Section: Sage Creek Normal Fault the Sagementioning

confidence: 85%

“…6, 8B), much more deformed and overturned than those to the north, suggests proximity to a major subsurface thrust, the subjacent sub-Snowcrest thrust of previous reports by and others (1981,1983a). This is inferred from gravity data and gravity modeling to be a major northwest-dipping buried foreland thrust beneath the Snowcrest structural terrane, on which the deformed Paleozoic and lower Mesozoic rocks of the Snowcrest terrane-the hanging wall-moved southeastward over Cretaceous footwall rocks (Kulik, 1982;Kulik and Perry, 1982 and this volume;Kulik, 1982,1983). Subsurface geological data are insufficient to draw well-constrained cross sections across the Snowcrest structural terrane or SnowcrestGreenhorn thrust system.…”

Section: Cretaceous Rocksmentioning

confidence: 97%

Interactions of Rocky Mountain foreland and Cordilleran thrust belt in Lima region, southwest Montana

Perry¹,

Haley²,

Nichols³

et al. 1988

Geological Society of America Memoirs

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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 BeaverheadGroup 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, paiynologically 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.

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“…1). Distribution of geologic map units, differences in thicknesses of upper Paleozoic rock units, ages and stratigraphic relations of Cretaceous synorogenic deposits, the alignment of gravity highs, and the distribution of aeromagnetic contours indicate that the foreland in southwestern Montana was faulted before the Cordilleran thrusts moved across it (Kulik and Perry, 1982;Kulik and others, 1983;Perry and others, 1981;Perry and others, 1983;Ryder and Scholten, 1973;Nichols and others, 1985;. Positions of these foreland faults appear to have had a Precambrian ancestry (Schmidt and O'Neill, 1983;Schmidt and Garihan, 1983).…”

Section: B Skippmentioning

confidence: 96%

Cordilleran thrust belt and faulted foreland in the Beaverhead Mountains, Idaho and Montana

Skipp

1988

Geological Society of America Memoirs

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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 westdipping 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 inthe area of the shelf west of the Montana craton, and northeast-trending, northwestdipping, 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 Yellowjacket 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 237 on June 5, ...

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Illustrations of gravity models of the southeast limb of the Blacktail-Snowcrest uplift, southwest Montana

Abstract: Also included is a map showing the location of gravity stations used in the models.

Cited by 6 publications

References 0 publications

Contraction and extension faults in the southern Beaverhead Mountains, Idaho and Montana

Contraction and extension faults in the southern Beaverhead Mountains, Idaho and Montana

Interactions of Rocky Mountain foreland and Cordilleran thrust belt in Lima region, southwest Montana

Cordilleran thrust belt and faulted foreland in the Beaverhead Mountains, Idaho and Montana

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