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Reservoir Architectures of Incised Valley Fill Deposits in the Upper Cretaceous Eagle Formation, South-Central Montana

Auchter, Neal C.*1; Staub, James R.1; Hofmann, Michael H.1; Hendrix, Marc S.1
(1) Department of Geosciences, The University of Montana, Missoula, MT.

The Santonian-Campanian Eagle Formation in south-central Montana is composed of a series of exceptionally well preserved regressive-transgressive cycles that were deposited on the western margin of the Cretaceous Interior Seaway. Our study focuses on the genesis and architecture of a regional correlative basal incision surface and the subsequent fill history of the informal middle member of the Eagle Formation representing the most basinward advance of the Eagle depositional system.

This maximum basinward advance is marked by a pronounced incision surface that erosionally cuts into the underlying laterally extensive marine sands of the informal middle member of the Eagle Formation. The basal incision surface has an undulating profile ranging from less than 3 m to over 20 m of incision and is laterally mappable for over 30 km. Based on its profile variability and vast regional extent we interpret this surface as the basal incision of one or more incised valley(s) formed as a function of limited or decreasing accommodation.

To constrain the lateral continuity of the basal incision surface and to characterize the incision fill deposits, 40 measured sections were obtained in conjunction with geophysical subsurface surveys and high resolution photomosaics. Valley fill architectures associated with areas of significant incision and valley thalwegs are characterized by complexly stacked multistory channels adjacent to single story channels, both reflecting changes in accommodation through time.

Channel fill facies overlying the erosional surface are coarser grained fluvial sands consistent with the basinward shift in facies during time of deep incision. Other channel fill facies includes inclined heterolithic strata, rhythmically bedded mud drapes, doublets, and mud filled abandoned channels, all reflecting the influence of changing fluvial and marginal marine depositional processes. Areas of little to no incision lack any of the above facies changes and are marked by nearly conformable successions of marine strata above and below the erosional surface and are interpreted as valley interfluves. A final increase in accommodation is marked by a thin, laterally extensive coal and a transgressive bioturbated marine sand capping the valley fill deposits.


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California