Tide-Dominated Facies Complex at Southern Terminus of Sunburst Sea, Cretaceous Kootenai Formation, Great Falls, Montana

Schwartz, Robert K. ¹, Susan M. Vuke² (1) Allegheny College, Meadville, PA (2) Montana Bureau of Mines and Geology, Butte, MT

The Sunburst Member of the Lower Cretaceous Kootenai Formation in the Great Falls area represents the southern terminus of the earliest marine transgression into the Cretaceous foreland of western Montana. Ripple bundles, mud drapes, reactivation surfaces, rhythmically stacked two-part and neap-spring bundles of mud-sand laminations, flaser bedding, and flat-crested wave ripples document tidal dominance. Brackish to marine conditions are indicated by low ichnofossil diversity and extremely rare ammonoid fossils. A 7-14 m thick, upward-fining sandstone lithofacies that contains channel- shaped erosion surfaces and stacked subhorizontal to low-angle inclined beds of large-scale trough and tabular cross-stratification represents sand-wave shoal development atop a tidal ravinement surface. The sand-wave lithofacies is overlain by less than 34 m of stacked, NE-SW trending, sandstone- and heterolithic-filled channels. Sedimentary structures within the sinuous channels indicate bank accretion, slumping, rhythmic tractive-slack sedimentation, bipolar flow, and minor wave influence. Toward the eastern and southern basin margin, stacked, tabular, upward-coarsening tidal bar successions and mudstone-dominated lithofacies merge laterally into thin tidal flat and tidal creek deposits. Tidal flat and non-marine coastal plain facies regionally cap the entire assemblage. Although the Sunburst overlies the NW-SE-striking Sweetgrass Arch, fluvial paleocurrent and isopach data for the total Kootenai indicate a northward paleoslope and possible paleovalley control of Sunburst deposition along a local, elongate, NE-SW zone that transects the South Arch. Overall, the Sunburst represents southward transgressive-to-highstand systems development in a N-S-striking basin-scale embayment. The irregular coast included tide-dominated shorezone regions and estuaries along paleovalley tracts.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana