ESTUARINE MEMBER OF THE LOWER CRETACEOUS KOOTENAI FORMATION, MISSOURI RIVER GORGE, GREAT FALLS, MT

The Third (Sunburst) Member (Kk3) of the Kootenai Formation is a quartz-rich unit that is bound by thick lithic-rich non-marine strata and restricted to the foredeep sector of the Cordilleran foreland in northwestern Montana. Exposures along the Missouri River gorge indicate tidal deposition during an early southward transgression of the Boreal Seaway into the subsiding basin. At this location, the Kk3 erosionally overlies basal Kootenai strata and lies atop the paleo-Sweetgrass forbulge. A transgressive 7-m thick sand wave sheet, correlative with 10-14 m thick sequences to the south, fines upward from medium to fine sand. The facies contains stacked tabular sets of large-scale planar and trough cross-stratification, rare reactivation surfaces and wave ripples. Paleocurrents were locally unimodal but of opposite polarity (SW or NE) in nearby outcrops due to time-velocity asymmetry of laterally separated flow. A regressive 34-m thick channel facies erosionally overlies the sand wave facies. Upward-fining channel bodies (<1-4 m thick) of fine sand and interbedded fine sand and mudstone include gutter casts, rip-up clasts, large-scale trough cross-stratification, abundant ripple bedding, vertically accreted bundles of parallel lamination, wave ripples, slumped and folded bedding, locally abundant trace fossils and up to 100% bioturbation fabric. A lateral change to tidal bar and tidal flat deposits occurs toward the basin margin (east). The tidal bar facies is an 11-m thick tabular body containing upward coarsening sequences (2-3 m thick) of mudstone, fine-grained rhythmically parallel laminated sand, mud-draped bi-directional ripple sets and large-scale trough cross-stratified medium sand. The tidal flat facies, containing interbedded bioturbated mudstone and fine quartzose sandstone, overlies the bar sequence and is transitional with overlying coastal plain deposits. Overall, the absence of body fossils and low trace fossil diversity (including rare Ophiomorpha, Diplocraterion, Planolites, Limulicubichnus, and lingulid burrows) in the Kk3 indicate a brackish setting.