TERTIARY PALEOVALLEY SYSTEM, ALLUVIAL ARCHITECTURE, AND BURIED AQUIFER IMPLICATIONS FOR THE NORTHERN BASIN-AND-RANGE OF SOUTHWEST MONTANA

The Paleogene Renova Fm is the earliest record of sedimentation within the intermontane basins of southwest Montana. Recent interpretations regarding basin origin and internal architecture include those for initially closed extensional basins as well as widespread braidplain-like deposition followed by Neogene tectonic segmentation. However, recent outcrop data yield new information regarding basin evolution, alluvial architecture, and aquifer characterization.

Paleocurrent and compositional data from alluvial fan facies document radiating dispersal away from paleo-uplifts coincident with modern uplifts. In contrast, basin-interior fluvial bodies represent axial, through-going, river systems. Braided-fluvial conglomerate and coarse sandstone make up transmissive bodies reaching 20 m thick and >400 m wide. Also present are smaller scale, coarse- to fine-grained sandstone bodies of an anastomosing facies style. Both facies types are most commonly bound by mudstone. Erosional basin margins mantled by colluvium and paleovalley-marked floors of crystalline basement rock suggest bedrock-channel erosion of intermontane-scale paleovalleys during the Paleocene. The basins parallel the structural and lithologic fabric of the Sevier-Laramide orogen, reflecting strong infrastructural control on paleovalley erosion and subsequent hydrostatic framework. The basin-interior architecture includes upward fining from coarse longitudinally interconnected systems with wide channel belts to labyrinths of anastomosed sandstone bodies. Basin-margin tributary channel bodies with springs and local wells document the presence of aquifers emanating from basin-margin paleovalleys that extend as exhumed valleys into the modern uplifts.

Overall, deposition of the Renova Formation marks a switch from fluvial incision of intermontane-scale paleovalleys to basin back filling with a heterogenous style characterized by interconnected longitudinal fluvial systems within a largely non-transmissive fill of lacustrine and other alluvial facies. Fluvial bodies in the Renova and all overlying formations record an organized paleodrainage network similar to that of the modern Missouri River headwater system. Thermal sites along the paleodrainage system may provide aquifer-thermal energy potential.