ESTUARINE SEDIMENTATION AT THE SOUTHERN TERMINUS OF A PRE-ALBIAN SEAWAY IN THE CRETACEOUS FORELAND BASIN OF WESTERN MONTANA - LOWER KOOTENAI FORMATION

The Lower Cretaceous Sunburst member of the Kootenai Formation near Great Falls, Montana, marks the southern marine terminus of a quartzose transgressive-to-highstand system tract within the alluvial-dominated foreland of western Montana. Tidal facies define a lobate, northward opening, estuarine complex that developed along and south of the eastern flank of the Sweetgrass Arch, at the southern end of the pre-Kootenai Whitlash paleovalley tract. Northward opening of the estuary is consistent with a northward paleoslope as indicated by fluvial systems in underlying and overlying Kootenai members. Facies types, bearing diagnostic tidal structures and trace fossil assemblages, include estuary mudstone, estuary-mouth linear-bar, and estuary-channel facies that typify a central basin area. Tide-dominated shoreface facies lie adjacent to the linear-bar facies, and thin tidal-flat and channel sandstones, locally interbedded with paleosol units, typically mark basin margins near the zero edge of deposition. The stratigraphic succession documents: (1) maximum pre-Sunburst erosion along the central basin (pre-existing valley) axis, (2) estuary flooding and entrapment of non-marine-derived kaolinite mud as well as tidal flat/bar development, (3) a high-energy shoal zone in the estuary mouth consisting of linear sand bars and intervening channels, (4) a flanking tidal shoreface setting, and (5) overall superposition by estuary-channel and tidal-flat facies. Headward encroachment of the linear bar system during transgression is evidenced by a tidal ravinement surface over the estuarine mud-rich facies. The anomalous quartzose composition of Sunburst sandstone compared to otherwise lithic-rich sandstones in the Kootenai resulted from a cratonic (Canadian) source and net headward (southward) transport by tidal currents, consistent with Sunburst paleocurrent data and modern analogues. With regression, the estuary system was abandoned and capped by estuary-channel and tidal-flat facies followed by non-marine coastal plain facies of the overlying Kootenai member.