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

Facundo Fuentes, Peter G. DeCelles, Kurt N. Constenius and George E. Gehrels
Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.
Geological Society of America Bulletin (March 2011) 123 (3-4): 507-533


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 fluvial-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 first 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 fold-and-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-mid-Campanian 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 episode 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 final stage in the evolution of the Cordilleran 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 Cordilleran foreland basin in the United States.

ISSN: 0016-7606
EISSN: 1943-2674
Serial Title: Geological Society of America Bulletin
Serial Volume: 123
Serial Issue: 3-4
Title: Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A.
Affiliation: University of Arizona, Department of Geosciences, Tucson, AZ, United States
Pages: 507-533
Published: 201103
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 155
Accession Number: 2011-022474
Categories: StratigraphySedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With GSA Data Repository Item 2011002
Illustration Description: illus. incl. sects., strat. cols., 2 tables, geol. sketch maps
N44°30'00" - N49°00'00", W116°01'60" - W104°01'60"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201113
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