Sedimentary response to orogenic exhumation in the northern Rocky Mountain Basin and Range province, Flint Creek basin, west-central Montana

Abstract

Middle Eocene through Upper Miocene sedimentary and volcanic rocks of the Flint Creek basin in western Montana accumulated during a period of significant paleoclimatic change and extension across the northern Rocky Mountain Basin and Range province. Gravity modelling, borehole data, and geologic mapping from the Flint Creek basin indicate that subsidence was focused along an extensionally reactivated Sevier thrust fault, which accommodated up to 800 m of basin fill while relaying stress between the dextral transtensional Lewis and Clark lineament to the north and the Anaconda core complex to the south. Northwesterly paleocurrent indicators, foliated metamorphic lithics, 64 Ma ( 40 Ar/ 39 Ar) muscovite grains, and 76 Ma (U-Pb) zircons in a ca. 27 Ma arkosic sandstone are consistent with Oligocene exhumation and erosion of the Anaconda core complex. The core complex and volcanic and magmatic rocks in its hangingwall created an important drainage divide during the Paleogene shedding detritus to the NNW and ESE. Following a major period of Early Miocene tectonism and erosion, regional drainage networks were reorganized such that paleoflow in the Flint Creek basin flowed east into an internally drained saline lake system. Renewed tectonism during Middle to Late Miocene time reestablished a west-directed drainage that is recorded by fluvial strata within a Late Miocene paleovalley. These tectonic reorganizations and associated drainage divide explain observed discrepancies in provenance studies across the province. Regional correlation of unconformities and lithofacies mapping in the Flint Creek basin suggest that localized tectonism and relative base level fluctuations controlled lithostratigraphic architecture. Résumé : Les roches sédimentaires et volcaniques (Éocène moyen au Miocène supérieur) du bassin de Flint Creek dans l'Ouest du Montana se sont accumulées au cours d'une période de changements paléoclimatiques importants et d'extension significative à travers la province Basin and Range des montagnes Rocheuses septentrionales. La modélisation de la gravité, les données obtenues des trous de forage et la cartographie géologique du bassin Flint Creek, indiquent que la subsidence était surtout concentrée le long de la faille de chevauchement Sevier, fortement réactivée; cette subsidence a permis une accumulation atteignant 800 m de remplissage de bassin tout en transférant les contraintes entre le linéament de transtension dextre Lewis et Clark vers le nord et le complexe de noyau Anaconda vers le sud. Des indicateurs de paléocourants vers le nord-ouest, des unités lithologiques métamorphiques foliées, des grains de muscovite âgés de 64 Ma ( 40 Ar/ 39 Ar) et des zircons âgés de 76 Ma (U-Pb) dans un grès arkosique d'environ 27 Ma concordent avec l'exhumation et l'érosion du complexe à noyau Anaconda durant l'Oligocène. Le complexe à noyau et les roches volcaniques et magmatiques dans l'éponte supérieure ont créé une importante ligne de partage des eaux au cours du le Paléogène, dispersant des débris vers le nordoues...

Paleogene postcompressional intermontane basin evolution along the frontal Cordilleran fold-and-thrust belt of southwestern Montana

“…Moreover, the magnitude of extensional deformation decreased eastward across the eastern Idaho and southwestern Montana region, becoming progressively more obscure beyond the eastern limit of the study area. Thus, the effect of Eocene extension upon relief and accommodation may have been signifi cantly less within our study area than suggested for westward locations (e.g., Portner et al, 2011). Therefore, we submit that during the latest Cretaceous and early Paleogene, fl uvial incision was a primary mechanism for forming an interconnected network of large-scale paleovalleys that served as primary depocenters (basins) throughout Cenozoic time.…”

Determining the source of placer gold in the Anaconda metamorphic core complex supradetachment basin using detrital zircon U-Pb geochronology, western Montana, USA

“…The northern footwall of the AMCC, proximal to the Pioneer District placers, is composed of folded Mesoproterozoic-Mesozoic metasedimentary and sedimentary rocks that have been intruded by several granite and granodiorite plutons (Fig. 2;Emmons and Calkins, 1913;Grice, 2006;Portner et al, 2011). The dominant plutons include the 69-60 Ma Royal stock and the 65 Ma two-mica (muscovite and biotite) Mount Powell batholith (Fig.…”

“…Many of the strata now exposed in the hanging wall of the AMCC are interpreted to have been deposited in an Eocene-Oligocene supradetachment basin (the "Flint Creek Basin") that developed contemporaneous with slip along the Anaconda detachment (Janecke et al, 2005;Stroup et al, 2008). The late Oligocene-early Miocene Cabbage Patch Formation and mid-Miocene Squaw Gulch conglomerate beds were deposited atop the hanging wall of the Anaconda detachment and have a combined thickness of ~800 m, constituting most of the Flint Creek Basin fill (Fields et al, 1985;Loen, 1986;Stroup et al, 2008;Portner et al, 2011). It has been hypothesized that the Cabbage Patch Formation and Squaw Gulch beds were the original depositional units of the Pioneer District placer deposits prior to subsequent glacial transport and redeposition as fluvial placers in the Pleistocene (Loen, 1986).…”

“…Several studies interested in the sedimentation histories of extensional basins in western Montana conducted provenance analysis of the Paleogene-Neogene rocks exposed in the Flint Creek Basin (e.g., Stroup et al, 2008;Portner et al, 2011). These studies were focused specifically on Eocene-Miocene basin development and did not consider the history of gold placer deposition, but they provide valuable DZ and sedimentological data for the Cabbage Patch (Stroup et al, 2008), and paleocurrent analysis of Oligocene two-mica sands of the Flint Creek Basin indicate northwest-directed paleoflows (Portner et al, 2011). Paleoflow indicators in the late Miocene Barnes Creek gravel beds also record sediment derivation from the south-southwest, suggesting that sediment dispersal patterns in the Flint Creek Basin remained broadly consistent during the Cenozoic (Portner et al, 2011).…”

“…These studies were focused specifically on Eocene-Miocene basin development and did not consider the history of gold placer deposition, but they provide valuable DZ and sedimentological data for the Cabbage Patch (Stroup et al, 2008), and paleocurrent analysis of Oligocene two-mica sands of the Flint Creek Basin indicate northwest-directed paleoflows (Portner et al, 2011). Paleoflow indicators in the late Miocene Barnes Creek gravel beds also record sediment derivation from the south-southwest, suggesting that sediment dispersal patterns in the Flint Creek Basin remained broadly consistent during the Cenozoic (Portner et al, 2011). This interpretation is also supported by the orientations of Pleistocene lateral moraines emanating from the northern Flint Creek Range, which are oriented predominantly northeast-southwest (e.g., Loen, 1986).…”