RE-INTERPRETATION OF KOOTENAI FORMATION IN WESTERN MONTANA ON THE BASIS OF NEW TIDAL EVIDENCE FOR EARLY CRETACEOUS (NEOCOMIAN) TRANSGRESSION

Tidal facies in the Barremian Sunburst Member of the Kootenai Formation near Great Falls, MT, mark the southern terminus of a previously undocumented marine highstand within the fluvial-dominated foreland of western Montana. The tidal facies, previously accepted as fluvial and lacustrine, provide a basis for re-interpreting the underlying Neocomian facies succession. Depending upon location within the tidal basin, two architectural styles of facies change occur above a basal lowstand unconformity and lowermost braided lithic-fluvial unit. A relatively thick reddish mudstone- and paleosol-rich succession of non-marine facies (20-40 m) occurs along the marine margin in between the basal braided unit and capping tidal bar and tidal flat facies. Within the succession, a unit of alluvial plain deposits containing isolated, fine-grained, fluvial sandstone overlies the basal braided unit and is succeeded by localized ephemeral carbonate-lake deposits and increased paleosols. Next, lithic-rich fluvial channel and hyperconcentrated flood deposits mark a return to higher-energy alluvial plain conditions. This is followed by gray rootlet-bearing mudstone and bioturbated, clayey, channelized quartz-rich sandstone and heterolithic bank-accretion deposits prior to occurrence of the tidal bar and flat facies. In some other areas, tidal flat facies directly overlie red coastal plain mudstone. In contrast, in the “central basin” area, a 7-14 m sand-wave facies and underlying tidal ravinement surface reflect high-energy shoal (bar-and-channel) development in a more open-basin setting. Here, the succession above the basal fluvial system is thin (<8 m) to non-existent and gray kaolinite-mudstone dominated. Tidal bedding occurs locally within the mudstone. Underlying lacustrine carbonate, where present, is thicker and wider spread than along the basin margin. The mudstone unit reflects estuary entrapment of fines following an earlier phase of larger carbonate-lake development, both in a paleo-low region. Overall, the lower Kootenai facies succession reflects two transgressive pulses during the earliest Cretaceous, one causing base-level increase and lake development and the other resulting in Sunburst tidal deposition.