Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.

Abstract

New lithostratigraphic and chronostratigraphic, geochronologic, and sedimentary petrologic data illuminate the history of development of the North American Cordilleran foreland basin system and adjacent thrust belt from Middle Jurassic through Eocene time in northwestern Montana. The oldest deposits in the foreland basin system consist of relatively thin, regionally tabular deposits of the marine Ellis Group and fl uvial-estuarine Morrison Formation, which accumulated during Bajocian to Kimmeridgian time. U-Pb ages of detrital zircons and sandstone modal petrographic data indicate that by ca. 170 Ma, miogeoclinal strata were being deformed and eroded in hinterland regions. Sandstones of the Swift and Morrison Formations contain detrital zircons derived from the Intermontane belt. The Jurassic deposits probably accumulated in the distal, back-bulge depozone of an early foreland basin, as suggested by the slow rates of tectonic subsidence and tabular geometry. A regional unconformity separates the Jurassic strata from late Barremian(?) foredeep deposits. This unconformity possibly resulted as a combined effect of forebulge migration, decreased dynamic subsidence, and eustatic sea-level fall. The late Barremian(?)-early Albian Kootenai Formation is the fi rst unit that consistently thickens westward, as would be expected in a foredeep depozone. The subsidence curve at this time begins to show the convex-upward pattern characteristic of foredeeps. By Albian time, the foldand-thrust belt had propagated to the east and incorporated Proterozoic rocks of the Belt Supergroup, as indicated by sandstone compositions, detrital zircon ages in the Blackleaf Formation, and by crosscutting relationships in thrust sheets involving Belt Supergroup rocks in the thrust belt. A major episode of marine inundation and black shale deposition (Marias River Shale) occurred between the Cenomanian and mid-Santonian, and was followed by a regressive succession represented by the Upper Santonian-midCampanian Telegraph Creek, Virgelle, and Two Medicine Formations. Provenance data do not resolve the timing of individual thrust displacements during Cenomanian-early Campanian time. The Upper Campanian Bearpaw Formation represents the last major marine inundation in the foreland basin . By latest Campanian time, a major epi sode of slip on the Lewis thrust system had commenced, as recorded in the foreland by the Willow Creek and St. Mary River Formations in the proximal foredeep depozone.The fi nal stage in the evolution of the Cor dilleran fold-and-thrust belt and foreland basin system is recorded by the Paleocene-early Eocene Fort Union and Wasatch Formations, which were preserved in the distal foreland region. Regional extensional faulting along the fold-and-thrust belt began during the middle Eocene. The results presented here enable the establishment of links between previous geological work in Canada and the better known parts of the Cor dilleran foreland basin in the United States.

A Late Jurassic to Early Cretaceous record of orogenic wedge evolution in the Western Interior basin, USA and Canada

“…Jurassic deposits record marine to nonmarine basin filling, which is typical of foreland basin clastic wedges (Table 1) (Cant and Stockmal, 1989;Fuentes et al, 2011;Miles et al, 2012;Kukulski et al, 2013;Raines et al, 2013). Overall, the thickness of these units is controlled by variations in accommodation and differential erosion of pre-Cretaceous units during incision of the sub-Cretaceous unconformity ( Fig.…”

“…8). These areas include Gibson Reservoir of western Montana (Fuentes et al, 2011), the Grande Cache area of west-central Alberta Quinn et al, 2016), and Cold Lake in east-central Alberta (Blum and Pecha, 2014). The lithostratigraphic nomenclature for each area is presented in Figure 2.…”

“…Gibson Reservoir, Montana Gibson Reservoir , Montana (Fuentes et al, 2011) Cold Lake, Alberta (Blum and Pecha, 2014) Grande Cache, Alberta Quinn et al, 2016) Synthetic age population Upper Jurassic strata of the Monteith Formation in the Grande Cache area have been assigned to the foredeep on the basis of thickness, sandstone provenance, and composition (Price and Mountjoy, 1970;Monger et al, 1982;Cant and Stockmal, 1989;Leckie and Smith;1992, Miles et al, 2012. The similarity between the detrital zircon spectra in the Late Jurassic system and the occurrence of Cordilleran magmatic-arc grains in these deposits do not support the hypothesis that a forebulge significantly influenced sediment dispersal in the basin during the Late Jurassic.…”

“…The Upper Kootenai Member is largely nonmarine at Great Falls and Gibson Reservoir (Table 1) (Fuentes et al, 2011). The Spirit River Formation at Grande Cache consists of marine shoreline units at its base, which transition upwards into nonmarine deposits of the Falher and Notikewin Members (Quinn et al, 2016).…”

“…New data are integrated with previously presented data sets from across the basin in order to consider the history of burial and exhumation of the frontal toe of the orogenic wedge ( Fig. 1) ( Fuentes et al, 2011;Leier and Gehrels, 2011;Raines et al, 2013;Benyon et al, 2014;Blum and Pecha, 2014;Quinn et al, 2016). This study emphasizes the potential impact of wedge-top dynamics on sediment dispersal across foreland basins and the structural evolution of the orogen.…”