Abstract Dating of detrital zircons from well cuttings is a useful technique to constrain stratigraphic ages and structural interpretations in complexly deformed terranes. This technique was applied in the Helena Salient of the Montana Disturbed Belt to determine whether the Norcen, Kimpton Ranch 1-11 and Buckhorn, Federal 2-24 wells penetrated Phanerozoic strata beneith the allochthonous Precambrian rocks carried on the Lombard thrust. Since some Phanerozoic strata have reservoir potential, their presence below the Lombard thrust has important implications for the oil and gas potential of this structural province.

We interpret the Lewis and Clark line of the Montana-Washington Cordillera to represent a rotational shear zone that initiated within the Mesoproterozoic Belt-Purcell basin as northern and southern parts of the basin rotated clockwise about eccentric Euler poles during Early Cretaceous to late Paleocene crustal shortening. The Lewis-Eldorado-Hoadley slab, north of the Lewis and Clark line, rotated about a pole near Helena, Montana, producing the Alberta salient, whereas the Sapphire and Lombard slabs, south of the Lewis and Clark line, rotated about poles located in Idaho, producing the Helena salient. Between the salients, the left-lateral transpressive Lewis and Clark line developed a flower-like structure with steep cleavage and tight folds at deep levels, reverse faults at intermediate levels, and en echelon box-folds at shallow levels. Comparison of older parts of the Lewis and Clark line in the west and younger parts in the east indicate that strain increased with progressive rotation. A topographic ridge was extruded along the Lewis and Clark line and created progressive angular unconformities at the base of early Campanian strata of west-central Montana and influenced sediment dispersion in the foreland basin. Deformation continued along the Lewis and Clark line through Maastrichtian and Paleocene time as the Rocky Mountain fold-thrust belt advanced along the leading edges of the rotating slabs during growth of the Alberta and Helena salients.

Paleomagnetic investigations throughout the overthrust belt in the Wyoming and Helena salients show that buttressing effects of the Rocky Mountain foreland have caused local rotations of thrust sheets along the margins of the salients. Two previous studies of Triassic, Upper Jurassic, and Lower Cretaceous rocks in the Wyoming-Idaho overthrust belt involved rocks from the Prospect, Bear, Darby, and Absaroka thrust sheets. These studies documented as much as 60° counterclockwise rotation in the northern edge, and as much as 30° clockwise rotation in the southeastern edge of the Wyoming salient. It has been suggested that these rotations were buttressing edge effects, since the rotations do not appear to have been transferred back into the interior of the Wyoming salient. New work in the southern portion of the Helena salient in southwestern Montana has documented a similar situation. Here, samples from the Lower Cretaceous Kootenai Formation show a maximum of 54° clockwise rotation in the region of the McCarthy Mountain salient, 23° clockwise rotation within the southwest Montana transverse zone, and 35° clockwise and 30° counterclockwise rotations in the nose of the salient. These rotations vary greatly from site to site, suggesting that the thrust sheets were not deformed in a coherent fashion, but rather, they broke and rotated as individual pieces where the effects of buttressing against the Rocky Mountain foreland were the greatest.

A distinct structural style is present in southwestern Montana where the Cordilleran frontal fold and thrust belt encountered basement surface irregularities in the adjacent cratonic foreland. This structural style is characterized by footwall thrust ramps that have a close spatial and genetic relationship to the basement surface irregularities. These basement irregularities are normal fault blocks, of Proterozoic ancestry, oriented parallel to the direction of tectonic transport on the décollement, and southwest-facing fault blocks, bounded by steeply northeast-dipping reverse faults, oriented obliquely to the direction of tectonic transport, that acted as buttresses to thrust propagation. A major east-trending Late Cretaceous and Paleocene structural boundary is the southwest Montana transverse zone that separates a terrane composed of “thin-skinned” thrusts in the Middle Proterozoic and younger rocks on the north in the Helena salient from basement-involved “thick-skinned” thrusts and reverse faults of the Rocky Mountain foreland on the south. This first-order structural boundary is interpreted to be superimposed on a down-to-the-north normal fault zone that marked the southern margin of the Proterozoic Belt basin. West-to-east shortening in the salient was manifested in right-reverse slip movements on the faults of the transverse zone. The principal fault of the zone is interpreted as a major oblique-to-lateral ramp formed over one or more normal-fault steps. Second-order features, produced when thrust sheets impinged upon abrupt basement surface irregularities, include strongly anastomosing patterns of thrusts, younger-over-older thrust relations, and the localization of small, convex-eastward thrust salients. These features are directly controlled by Late Cretaceous, northwest-trending, left-reverse slip faults of Middle Proterozoic ancestry. Basement-cored anticlines on the hanging-wall sides of these faults deflected frontal thrust sheets into zones of oblique ramps that locally place younger rocks over older rocks. The convex-eastward shape of the McCartney Mountain salient is probably the result of local changes of the direction of west-southwest-east-northeast principal shortening adjacent to two northwest-trending en-echelon foreland blocks along the northeastern side of the salient. The principal sets of foreland structures and thrust belt structures along the front of the thrust belt in southwestern Montana overlap closely in time. Both structures are interpreted to have been produced by the same, dominantly eastward- or east-north-eastward-directed forces during Late Cretaceous and early Paleocene time. Structures in the foreland probably continue for some distance into and beneath the thin-skinned structures of the thrust belt.

Abstract The Sawtooth Range is located in northwest Montana in parts of Pondera, Teton, and Lewis and Clark counties. The range is part of the Rocky Mountain fold and thrust belt, or Disturbed Belt, and lies between Glacier National Park to the north and the Helena salient to the south. In an analogy with the Canadian Rocky Mountains, the Sawtooth Range would represent the Front Ranges of the Rocky Mountain thrust belt (Mudge, 1972; Bally and others, 1966). The Sun River and its tributaries form the major drainage system in the southern part of the Sawtooth Range. Two dams have been constructed on the Sun River creating Gibson Reservoir and Diversion Lake (Fig. 1). The entrance to the Sun River Canyon is approximately 20 mi (32 km) northwest of Augusta, Montana. Turn west on Montana 435 (U.S. Forest Service Road 208) at the intersection of highways U.S. 287 and Montana 434–435 in Augusta. Montana 435 is a year-round gravel road that ends at Gibson Dam (Fig. 1). Spectacular exposures of imbricate thrust faults and related structures are found in the lower part of the canyon downstream from Gibson Dam.