The ca. 1.460 Ga Revett Formation is a gray and purple quartzite lithosome in northwestern Montana, and it interfingers eastward into red argillite of the Grinnell Formation in Glacier National Park. The Revett Formation was analyzed in northwestern Montana by identifying sedimentary structures in stratigraphic sections and by interpreting flow processes of the structures using the standard flow regime model (e.g., Simons et al., 1965). The sedimentary structures and thicknesses of the event beds were then organized into eight sediment types (lithofacies) that were grouped into three sediment complexes: the playa complex, the antidune complex, and the sheet sand complex. The arrangements of the sediment types and complexes within the stratigraphic framework of the lower informal Revett member indicated the configurations of the depositional environments in space, and the vertical configurations of the sediment types revealed the depositional history of the lower Revett member. The lower Revett member lithosome interfingers eastward into the red argillite of the Grinnell Formation lithosome, and has eight through-going descriptive, stacked, lithic units, called lithostromes. Lithostromes 2, 4, 6, and 8 (from the bottom up) are composed of the sheet sand complex and extend into playa complexes of the Grinnell Formation. They were deposited by sandy sheetfloods that flowed at grade and terminated as the water sank into the sand substrate. Between lithostromes 2, 4, 6, and 8 are lithostromes marked by playa lakes of the playa complex that spread from the east across western Montana during humid periods. They were overlain by sheetfloods of the antidune complex that built eastward over the playa complex as the playa lakes retreated with increasing aridity. The antidune complex was overlain by the sheet sand complex of a vast sand plain deposited by sheetfloods from the southwest that flowed at grade level across western Montana during arid periods. The sheetflood deposits of the Revett Formation were mostly deposited by the upper-flow regime element of the established fluvial facies model.

The Chamberlain Formation, one of the lower members of the early Mesoproterozoic Belt Supergroup, has previously yielded low-diversity assemblages of microfossils but the reported fossils were of limited utility for inferring paleoenvironmental conditions. Here, we describe substantially more diverse microfossil assemblages from drill core of the Chamberlain Formation obtained from the Black Butte mine locality near White Sulphur Springs, Montana. The Chamberlain Formation biota contains abundant Valeria , Leiosphaeridia , Synsphaeridium , and Lineaforma , with lesser amounts of Satka , Symplassosphaeridium , and Coniunctiophycus. The assemblages partially overlap with, but are distinct from, microfossils recently reported from the Greyson Formation, another unit from the Helena embayment of the Belt Supergroup. Since the overlapping taxa exhibit similar states of preservation but dissimilar relative abundances, we interpret the assemblages as reflective of distinct paleoenvironmental conditions of the sampled sections of the Chamberlain and Greyson Formations. The Chamberlain Formation assemblages are most comparable to microfossil groupings reported from the Bylot Supergroup of Canada and the Roper Group of Australia from sediments from very shallow-water (supratidal to lower shoreface) marine environments. This comparison corroborates previous hypotheses on the basis of sedimentological data that the lower Chamberlain Formation sediments were formed in a lagoonal or mud-flat environment. By contrast, the Greyson Formation assemblages are most comparable to microfossil groupings associated with sediments from shallow-shelf marine environments. The fidelity of comparisons among the 1.2 Ga Bylot Supergroup, 1.49 Ga Roper Group, and 1.45 Ga Belt Supergroup assemblages indicates that the groups of microorganisms that produced these assemblages, and their associations with the paleoenvironments that they inhabited, may have been characteristic of the littoral marine biosphere throughout much of the Mesoproterozoic.

Proterozoic mafic magmatic rocks exposed along the western side of North America, or western Laurentia, from Kimberley, British Columbia, through to northwestern Wyoming have been previously divided into two large igneous provinces: the ca. 1460 Ma Moyie-Purcell and the ca. 780 Ma Gunbarrel large igneous provinces. New geochemical analysis from this study demonstrates that there are additional intraplate mafic magmatic rocks present. Distinguishable by variable normalized rare earth element patterns combined with differing slopes on a binary Ti versus V plot, there are 17 identifiable geochemical signatures in the 307 whole-rock and trace-element analyses from this study. Only seven of these signatures can be linked to the ca. 1460 Ma Moyie-Purcell large igneous province, and one signature to the 780 Ma Gunbarrel large igneous province. This study has identified two groups of intrusions with distinct geochemical signatures previously linked with the ca. 1460 Ma Moyie-Purcell large igneous province but now recognized to be separate events, a single unique geochemical signature with a U-Pb age correlative with the Moyie-Purcell large igneous province and seven other heretofore unidentified signatures interpreted to belong to additional undated events.

Abstract The Lewis and Clark line (LCL) is a major transverse structure that crosses the North American Cordillera from northeastern Washington to central Montana. It initiated as a rift structure within the Mesoproterozoic Belt basin and reactivated several times during the Phanerozoic. This field trip examines the internal structure of the LCL along a transect in central-western Montana. Structural plunge permits examination of a 25-km-thick crustal section of a flower structure that formed along the LCL during Late Cretaceous-late Paleocene sinistral transpression. We will observe changes in structural style from the deepest parts of the Belt Supergroup upward to the syntectonic depositional surface.

The Priest River, Clearwater, Bitterroot, and Anaconda metamorphic core complexes of the northern Rocky Mountains were exhumed in Eocene time by crustal extension, which was linked via dextral displacement on the Lewis and Clark fault zone. Detailed geochronology and thermochronology (U-Pb, 40 Ar/ 39 Ar, and fission-track) from the Bitterroot complex indicates that extension started at 53 ± 1 Ma and continued until after 40 Ma. New U-Pb zircon and 40 Ar/ 39 Ar data from the Anaconda complex and published geochronology from the Priest River complex indicate a similar timing for the onset of major extension and exhumation. 40 Ar/ 39 Ar data from the Clearwater complex, which formed within a relay between strike-slip splays of the Lewis and Clark fault zone, are consistent with exhumation during the same time span. The Lewis and Clark fault zone separates ENE-directed extension in the Priest River complex from ESE-directed extension in the Bitterroot and Anaconda complexes. Large-scale extension was transferred eastward on the south side of this fault zone, where stretching lineations in core complex mylonites are oriented ∼104°–110° and coincide with the general trend of the transcurrent faults. Extension and exhumation of middle crustal rocks along the Lewis and Clark fault zone was concentrated in areas that also experienced voluminous Eocene midcrustal magmatism. Extension was probably initiated by a change in plate boundary conditions combined with the rapid influx of heat from the asthenosphere as a slab window opened beneath the western Cordillera, which led to collapse of the Cordilleran orogenic wedge and widespread early Eocene magmatism.