Preliminary Sedimentologic Analysis of Santonian and Campanian Strata Near Billings, Montana

Jörn Hauer¹, Marc S. Hendrix¹, and Octavian Catuneanu^2
1Department of Geosciences, University of Montana, Missoula, MT
²Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada

Reconnaissance sedimentology has been applied to Santonian-Campanian Eagle Formation outcrops near Billings, Montana along an 85 km long west-to-east transect oriented down depositional dip. Westernmost exposures mapped as Eagle contain at least two distinct regressive units. The lowermost Eagle consists of ~9 m of tabular-bedded rippled sandstone with cm-scale mud partings, overlain by ~5.5 m of well-sorted trough cross-stratified medium sandstone interpreted as upper shoreface deposits. A flooding surface at the top of Eagle shoreface facies is overlain by ~1 m of thoroughly bioturbated sandstone and ~6 m of offshore mud and sand. These offshore facies pass upward into a channelized sandstone complex lacking bioturbation and interpreted to be fluvially-influenced. Eagle exposures stratigraphically down-dip in the Billings area also suggest the preservation of at least two regressive cycles with dm-scale beds of bioturbated, glauconitic muddy sandstone thickening upward into a ~10 m section of well-sorted massive sandstone. A flooding surface capping the lower regressive unit is overlain by swaley cross-stratified sandstone passing upward into tidally-influenced trough cross-stratified sandstone. East of Billings, the Eagle contains several distinct parasequences topped by prominent sandstones. The lowermost sandstone nearest Billings is planar-stratified and interpreted as upper shoreface facies. Overlying sandstones are glauconitic, bioturbated, and interpreted as offshore deposits. A thin transgressive lag is locally present at the top of the uppermost sandstone. In the easternmost part of the study area this surface lacks the pebble lag, is marked by well-preserved trace fossils, and is sharply overlain by offshore silty mud of the Campanian Claggett Formation.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas