AN UPPER CRETACEOUS COAL IN THE EAGLE FORMATION OF SOUTH-CENTRAL MONTANA AS AN INDICATOR FOR RAPID TRANSGRESSION

A thin coal bed (~0.25 m thick) in laterally continuous cliff-face exposures in the Santonian-Campanian Eagle Formation of south-central Montana was examined with respect to its possible sequence stratigraphic significance. This coal bed caps a ~20 m thick succession of shallow marine to coastal plain deposits. At its base, this succession comprises 7 m of lower/middle shoreface silty shale interstratified with hummocky cross-stratified sandstones. This facies is transitionally or sharply overlain by 6 m of upper shoreface trough cross-stratified sandstone. These units are interpreted as a highstand systems tract and a falling-stage systems tract, respectively. At locations more proximal to the paleo-shoreline, the intervening contact is interpreted as the basal surface of forced regression and, further offshore, the regressive surface of marine erosion. The shoreface sands are overlain by 6 m of tidal to paralic trough cross-stratified sandstone, locally channelized and incised up to 5 m into underlying shoreface sands. The channel sandstone is topped by a 0.80 m thick rooted paleosol, in turn capped by the coal. The base of the channelized sandstone is interpreted as a sequence boundary; the top of the paleosol as a maximum regressive surface; and the intervening strata as a lowstand systems tract. The coal marks the beginning of the transgressive systems tract and passes upward into a 2 m thick glauconitic bioturbated muddy sandstone, interpreted to contain the maximum flooding surface. This surface marks the beginning of the following highstand systems tract which coarsens upward from the glauconitic bioturbated muddy sand into a 4 m thick burrowed trough cross stratified sandstone. The coal layer thickens up depositional dip and contains a bentonite layer. The ichnology in the sands overlying the coal indicate a transgressive event that rapidly drowned a swamp with minimum dimensions of 15 km down depositional dip and 40 km parallel to depositional strike.