Paper No. 3
Presentation Time: 2:10 PM


AUCHTER, Neal C.1, STAUB, James R.2, HOFMANN, Michael H.3 and HENDRIX, Marc S.2, (1)Department of Geosciences, Virginia Tech, Blacksburg, VA 24060, (2)Department of Geosciences, The University of Montana, Missoula, MT 59812, (3)Department of Geosciences, University of Montana, Missoula, MT 59812,

The Santonian-Campanian Eagle Formation in south-central Montana is composed of a series of well preserved regressive-transgressive cycles deposited on the western margin of the Cretaceous Interior Seaway. This study focuses on the genesis and architecture of diachronous and composite erosional surfaces associated with a compound incised valley in the informal middle member of the Eagle Formation.

Two surfaces are of regional significance and were traced continuously for up to 15 km laterally. These were interpreted as a sequence boundary (SB) defined by fluvial channel erosion during the most basinward shift in facies, and a transgressive surface of erosion (TSE) defined by tidal-fluvial channel erosion during a landward shift in facies.

The stratigraphic framework and valley architecture built by these regional surfaces together with the stacking patterns of facies, and facies associations were used to establish a multiphase depositional history of valley evolution: 1) fluvial incision in response to falling relative sea level concomitant with fluvial deposition and sediment storage on abandoned terraces within the valley, 2) rising relative sea level concurrent with flood-dominated mud-rich central valley deposition, 3) tidally influenced fluvial deposition, extensive valley widening, and development of coal over the valley fill deposits during later transgression, and 4) rapid inundation and drowning of the valley system.

The resulting complex valley architecture is bound at its base by a diachronous composite incision surface and sequence boundary which formed in the valley center by fluvial incision during the falling stage (SB), and by tidal-fluvial channel incision near the valley margins during the transgressive stage (TSE = SB). The time-transgressive multiphase nature of valley formation and infilling produced a fill profile with a mud-rich valley center flanked by stacked, sand-rich fluvial and tidal-fluvial deposits toward the valley margins.