A succession of separate tectonic events affected Mesoproterozoic Belt Supergroup strata of NW Montana, just as in the better-displayed Coeur d'Alene Mining District of Idaho. A series of these established a tectonic zone historically known as the Lewis and Clark line, here re-identified as the Lewis and Clark tectonic zone, an apparent product of periodic reactivation of fundamental basement structures and physical constraint of a growth fault on developing folds. Six events identify a partial tectonic history of the west-central Belt Basin. The oldest produced growth faults concentrated along at least two structural lineaments, one of which, the Jocko line, substantially controlled the distribution of subsequent structures; the other, the north-trending Noxon line, is implicated in creation of metal-enriched source rock for Coeur d'Alene veins and provides a marker for right-lateral faulting within the Lewis and Clark tectonic zone. Subsequent deformation produced (1) west-northwest–trending folds, mostly confined to the Lewis and Clark tectonic zone and terminating northward against the Jocko line as the likely result of their having been compressed against this pre–Belt Basin structure; (2) north-trending regional folds, which extend southward from Canada and cross the ultimate Lewis and Clark tectonic zone; (3) foliated shear zones in the Lewis and Clark tectonic zone and associated Coeur d'Alene veins and reverse faults; (4) right-lateral, transcurrent faults, so identified by offsets of the Noxon line, north-trending regional folds, and the Montana overthrust belt and its associated foredeep basin; and, last, (5) Lewis and Clark tectonic zone normal faults and associated kink folds, which extensively reached gigantic, “megakink” proportions. These megakinks locally disrupted all prior structures, greatly confusing local structure; these need to be “unkinked,” so that structures resulting from prior tectonism may be fully recognized and correctly interpreted. Many faults of the Lewis and Clark tectonic zone trend southeasterly in its easterly part, tracking pre–Belt Basin structures separate from those associated with the Jocko line.

Abstract The Helena and Wallace formations, currently of the “middle Belt carbonate”, were deposited in the block–fault Belt basin, within the Proterozoic Columbia continent, which filled from about 1480 to 1400 Ma. Dolomitic argillite–capped cycles of the Helena Formation were thought to represent a marine carbonate shelf deposit along the eastern margin of the Belt basin. Siliciclastic and calcitic rocks of the Wallace Formation were considered to be the western facies of the middle Belt carbonate, deposited in deeper water. This study shows that the Helena–type cycles form a unit across most of the Belt basin that is disconformably overlain by Wallace–type rocks. The Helena and Wallace formations are here revised to reflect the stacked stratigraphic relations. Both are inferred to be deposits of broad, shallow lakes. The Helena and Wallace are assigned to the resurrected and revised Piegan Group. The revised Helena Formation is characterized by cycles one to 10 m thick. The lower half-cycles are composed of light gray, thin, graded, siliciclastic layers 0.3 to 10 cm thick. Some continue upward and become mixed with tan-weathering dolomite in the upper half-cycles. In other cycles siliciclastic graded layers thin and fine upward but remain siliciclastic. The Helena Formation can be divided into lower, middle, and upper informal members. The lower and upper members have centimeter-scale bedded cycles, but the middle membercontains cycles with dark-gray, decimeter-scale hummocky cross-stratified arenite beds. The Grinnell Glacier section of Glacier National Park is selected as the revised Helena reference section. It is 500 m thick and contains 363 thin-bedded, dolomite-capped cycles, averaging 1.4 m thick. The Helena thins to 100 m on eastern thrust plates of the Front Range, thickens to 800 m north of Plains, Montana, but thins to 250 m in the Coeur d'Alene Mining District. Based principally on scattered halite casts, the crosscutting of the siliciclastic lithofacies by the dolomitic cycle caps, and the absence of significant scour at the cycle bases, the Helena Formation is interpreted to have been deposited in an underfilled, periodically hypersaline, broad, shallow lake. Its flat lake floor was everywhere above storm wave base. Stacking patterns of small-scale cycles indicate the Helena represents a large-scale expanding and contracting lake sequence. The revised Wallace Formation is characterized across northwestern Montana by gray-weathering siliciclastic upward-fining andthinning cycles, mostly 2 to 5 m thick. It can be subdivided into the following six members: (1) oolitic member, (2) molartooth member, (3) Baicalia member, (4) pinch-and-swell member, (5) microcouplet member, and (6) the full-cycle member. Across northern Idaho the Wallace members and cyclic patterns merge into a continuous unit of medium-gray arenite lenses in dark-gray argillite. Thinly laminated black argillite and dolomite units previously assigned to the upper Wallace in Idaho along with dolomitic argillite beds of the upper part of the Helena in Montana are here assigned to the lower Missoula Group. The Clark Fork section of northern Idaho is designated the Wallace reference section. It is 400 m thick and has 47 cycles. The Wallace Formation thickens eastward to more than 1,000 m in the Mission Range and thins to 300 m in Glacier National Park. Siliciclastic cycles of the Wallace Formation are similar to those of the Helena Formation. Cycle boundaries lack evidence of significant exposure and erosion, but only the lower and upper Wallace cycles have dolomite caps. For these reasons the Wallace Formation is interpreted to represent an underfilled and balanced-fill lake deposit that expanded and contracted, forming a genetic sequence. Widespread hummocky arenaceous beds indicate that the Wallace lake expanded westward, but its floor was flat and everywhere above storm wave base.

The Mesoproterozoic Belt-Purcell Basin of the United States–Canadian Rocky Mountains formed in a complex intracontinental-rift system. The basin contained three main fault blocks: a northern half-graben, a central horst, and a southern graben. Each had distinct internal stratigraphy and mineralization that influenced Phanerozoic sedimentation; the northern half-graben and horst formed a platform with a condensed section, whereas the southern graben formed the subsiding Central Montana trough. They formed major crustal blocks that rotated clockwise during Cordilleran thrust displacement, with transpressional shear zones deforming their edges. The northern half-graben was deepest and filled with a structurally strong prism of quartz-rich sedimentary rocks and thick mafic sills that tapered toward the northeast from >15-km-thick near the basin-bounding fault. This strong, dense prism was driven into the foreland basin as a readymade, critically tapered tectonic wedge and was inverted into the Purcell anticlinorium. Erosion did not breech the Belt-Purcell Supergroup in this prism during thrusting. The southern graben was thinner, weaker, lacked mafic sills, and was engorged with sheets of granite during thrusting. It was internally deformed to achieve critical taper and shed thick deposits of syntectonic Belt-Purcell–clast conglomerate into the foreland basin. A palinspastic map of the basin combined with a detailed paleocontinental map that juxtaposes the northeastern corner of the Siberian craton against western North America indicates that the basin formed at the complicated junction of three continental-scale rift zones.