TRANSECT ACROSS AN ESTUARY FACIES COMPLEX IN THE EARLY CRETACEOUS FORELAND BASIN OF WESTERN MONTANA: SUNBURST MEMBER OF THE KOOTENAI FORMATION

The lower Kootenai Formation in the subsurface of northern Montana and the correlative lower Mannville Group in the subsurface of southern Alberta serve as major oil and gas reservoirs. Exposures of the marine-influenced Sunburst member limited to the Great Falls area provide an unusual opportunity to observe tidal facies in outcrop that are recognized in the subsurface to the north. Overall, the exposed Sunburst facies define a lobate estuarine complex that lies at the southern end of the pre-Kootenai Whitlash paleovalley. High-resolution panoramic photography and facies analysis were conducted along the Missouri River gorge in order to demonstrate stratigraphic relationships and facies change along a transect oriented at a high angle to the paleobasin and paleovalley axes.

Near the basin center, a 10-15 km wide, <1-28 m thick, estuary kaolinite-mudstone/sandstone complex disconformably overlies the basal Kootenai lithic-fluvial sandstone (Cutbank Member) and remnants of reddish Kootenai paleosols. This is overlain by a 5 to 15 m-thick, quartzose, upward-fining, estuary mouth (linear-bar) facies with a concavo-convex erosional (tidal ravinement) base. The estuary mouth-bar facies is superposed by a > 5 km-wide x <40 m thick tract of regressive estuary channel facies which, in turn, is overlain by regionally widespread nonmarine coastal plain facies. Laterally, toward the eastern basin margin, the estuary channel facies is absent and coastal plain facies directly overlie the estuary mouth-bar facies. Here, an incised upper-Kootenai paleovalley with coarse fluvial fill fully truncates the Sunburst member. Laterally, the mouth-bar facies pinches out where it is flanked by an ~ 3.5 km-wide tract of stacked upward coarsening tidal shoreface units within which an incised late-Sunburst paleovalley with estuarine mudstone fill fully truncates the shoreface succession. The tidal shoreface lithosome then undergoes a lateral transition into gray estuarine mudstone containing scattered upward coarsening tidal bar(?) units followed laterally by thin tidal flat units encased in reddish paleosols. Overall, the transect demonstrates change from high to low energy tide-dominated settings across an estuary and compounding preservational effects due to subsequent base fluctuation and paleovalley development.

Handouts

1 Takatch, Statza, Schwartz_Transect Estuary Sunburst_2014.pdf (33.6 MB)

2 Takatch, Statza, Schwartz_Transect Estuary Sunburst_2014.pdf (38.6 MB)

3 Takatch, Statza, Schwartz_Gigapans.pdf (49.4 MB)

Session No. 140

T238. Sedimentary Geology: The Now and the Next Generations of Scientists (Posters)

Monday, 20 October 2014: 9:00 AM-6:30 PM

Exhibition Hall C (Vancouver Convention Centre-West)

Geological Society of America Abstracts with Programs. Vol. 46, No. 6, p.353

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