Incised valley-fill system development and stratigraphic analysis of the Lower Cretaceous Kootenai Formation, northwest Montana

Abstract

The Lower Cretaceous Kootenai Formation in northwestern Montana records some of the first deposition of siliciclastic sediment into the Cordilleran foreland basin system. These rocks are also of particular interest due to their viability as reservoirs for hydrocarbons. The delineation of incised valley-fill systems within in this stratigraphic interval is of specific importance to this study as these stratigraphic entities have proven to record significant changes in base level fluctuations as well as preserving productive reservoir facies. For this study a densely spaced collection of well logs, limited core, and analogous outcrop exposures were used to investigate the Cretaceous Kootenai Formation. The specific objectives of this research are three fold: (1) construct a regional stratigraphic framework of the Kootenai Formation and immediately adjacent strata in order to reconcile lithostratigraphic and chronostratigraphic units, (2) utilize the framework to delineate the stratigraphic position and architecture of local-scale incised valley-fill systems, and (3) construct a valley-fill model that incorporates environments of deposition interpreted from the observation of lithofacies in core and analogous outcrop. The stratigraphic analysis revealed a classic non-marine to marginal marine depositional sequence (Mitchum et al., 1977) within the Kootenai Formation. A sequence boundary at the base of the depositional sequence separates highstand marine strata of the Jurassic Ellis Group from lowstand incised valley-fill strata of the basal Sunburst member of the Kootenai Formation. Within the Lower Cretaceous depositional sequence, four distinct depositional environments stack to form the lowstand, transgressive, and highstand systems tract following fluvial incision into underlying Jurassic highstand strata. The first depositional environment is remnant amalgamated fluvial channel sandstone with pebble mud-clast lags. As base level began to rise a transgressive estuarine system developed. A transgressive surface is interpreted where incised valley-fill caps the valley. A lateral shift in depositional environments led to deposition of mud and fine-grained sand interpreted to represent tidal mud flats. Finally, as the rate of base level rise slowed an alluvial plain system developed, which marked the onset of the highstand systems tract.