Short-Lived Marine Transgression Documented through Proximal Tidal Facies at the Southern Limit of An Early Cretaceous (Barremian) Seaway, Kootenai Formation, MT

Exposures of tidal flat, tidal channel, and tidal shoreface facies near the depositonal zero edge of the Sunburst Sandstone Member of the Kootenai Formation document the southern extent of an Early Cretaceous (Barremian) transgression into western Montana. Equivalent subsurface strata extend northward into southern Canada.

The tidal flat facies are predominantly upward-fining tabular successions and vary from sandstone to mudstone-dominated. In general, subtidal facies are sandier than intertidal facies. Scattered channel bodies range from being confined to a single tidal flat unit to those that transect multiple units. The variations in channel size and fill are consistent with channel variability seen in modern intertidal and subtidal settings. The combined tidal flat and channel facies represent the shallowest (most proximal) marine deposits in the basin.

Stacked, upward-coarsening, tidal-shoreface successions occur northward (seaward) of the tidal flat facies. Textural and structural trends in the vertical successions indicate wave influenced subtidal sand flats and bars adjacent to finer grained offshore sand and mud, similar to trends in modern tide-dominated shoreline settings such as along the North Sea and Georgia Bight shoreface.

Overall, the Sunburst represents short-lived transgressive-to-highstand systems development in a N-S-striking basin-scale embayment. The irregular coastline was characterized by estuaries along paleo-valley tracts and tide-dominated shoreface regions. A thin discontinuous conglomerate at the bottom of the Sunburst succession marks the base of the marine system and indicates erosion prior to marine incursion. Minor seaward/basinward offset in vertically stacked tidal flat and shoreface facies indicate an aggradational to slightly progradation stacking pattern, interpreted to reflect deposition in hightand conditions. Non-marine facies in the overlying Kootenai member represent full regression of the seaway.