ASSESSMENT OF A BRAIDED FLUVIAL SYSTEM IN THE PALEOGENE RENOVA FORMATION AS A PROXY FOR BURIED AQUIFERS IN INTERMONTANE BASINS OF SOUTHWEST MONTANA

Recent outcrop analysis by others has yielded new information regarding Paleogene alluvial architecture in the intermontane basins of southwest Montana. The basinfill is heterogenous and characterized by interconnected, longitudinal, through-going, trunk-fluvial systems within a largely non-transmissive fill of lacustrine and mudstone lithofacies. Limited seismic data verify the presence of coarse-grained fluvial systems in the subsurface that may be similar to exposed coarse fluvial bodies. Thus far, the aquifer potential of the buried systems is undetermined. A large, boulder-cobble, braided-fluvial body in the Renova Formation provides an initial proxy for buried-aquifer assessment.

The dominantly framework-supported conglomerate reaches 20 m thick and extended >400 m in width and >15 km in length prior to modern truncation. In the subsurface, although the paleofluvial systems were interconnected between basins, axial lengths in the various intermontane basins are estimated to range between 20 and 80 km. Internally, the conglomerate is a composite body consisting of laterally and vertically stacked channel, subhorizontal planar, and low-angle bar units. Based upon laboratory measurement of sand-matrix porosity and visual framework:matrix proportions estimated from outcrop photos, porosity of the conglomerate body was assessed to be between 19 and 23%. Given the absence of slug test data from within the study area, a hydraulic conductivity of 3,456 m/day was assumed to calculate transmissivities of 17,280 m²/day and 69,120 m²/day for 5 m- and 20 m-thick bodies, respectively. Complicating factors regarding the evaluation of conductivity and transmissivity include heterogeneity due to clast imbrication with a variable muddy sand to open framework as well as the presence of alternating tabular units of different grain size, sorting, and degree of imbrication. Overall, the character of the surficial body is that of a productive system that bodes well as a proxy for the buried coarse systems.