TRANSECT ACROSS AN ESTUARY FACIES COMPLEX IN THE EARLY CRETACEOUS FORELAND BASIN OF WESTERN MONTANA: SUNBURST MEMBER OF THE KOOTENAI FORMATION
Near the basin center, a 10-15 km wide, <1-28 m thick, estuary kaolinite-mudstone/sandstone complex disconformably overlies the basal Kootenai lithic-fluvial sandstone (Cutbank Member) and remnants of reddish Kootenai paleosols. This is overlain by a 5 to 15 m-thick, quartzose, upward-fining, estuary mouth (linear-bar) facies with a concavo-convex erosional (tidal ravinement) base. The estuary mouth-bar facies is superposed by a > 5 km-wide x <40 m thick tract of regressive estuary channel facies which, in turn, is overlain by regionally widespread nonmarine coastal plain facies. Laterally, toward the eastern basin margin, the estuary channel facies is absent and coastal plain facies directly overlie the estuary mouth-bar facies. Here, an incised upper-Kootenai paleovalley with coarse fluvial fill fully truncates the Sunburst member. Laterally, the mouth-bar facies pinches out where it is flanked by an ~ 3.5 km-wide tract of stacked upward coarsening tidal shoreface units within which an incised late-Sunburst paleovalley with estuarine mudstone fill fully truncates the shoreface succession. The tidal shoreface lithosome then undergoes a lateral transition into gray estuarine mudstone containing scattered upward coarsening tidal bar(?) units followed laterally by thin tidal flat units encased in reddish paleosols. Overall, the transect demonstrates change from high to low energy tide-dominated settings across an estuary and compounding preservational effects due to subsequent base fluctuation and paleovalley development.