Preliminary Sedimentologic and Stratigraphic Analysis of Santonian and Campanian Strata near Billings, Montana
Sedimentologic analysis has been applied to fluvial through shallow marine clastic strata of the Santonian-Campanian Eagle Formation near Billings, Montana. Outcrops were investigated along an 85 km long west-to-east transect oriented down depositional dip and a 45 km long north-to-south transect oriented parallel to depositional strike. The stratigraphically lowest exposures of the Eagle Formation occur in the east Billings area and consist of cliff-forming sandstone characterized by cm-scale tabular alternations of mud and fine-grained sand containing ripples and burrows. Individual beds wedge out and dip slightly to the west and are truncated to the east, producing a convex-up sand body. This sand body is flanked to the west by a series of offlapping sandstone wedges that each are ~10 m thick and extend laterally for ~2 km. The wedges are composed of fine-grained bioturbated sandstone and are massive or contain cm-scale mudstone interbeds. An erosional surface truncates the tops of the sand wedges, and ~9 m of massive, bioturbated, glauconitic, fine-grained sandstone rests on the erosional surface and overlaps the truncated wedges. This sandstone is sharply overlain by two coarsening upward regressive units separated by a paleosol and a rooted coal layer containing a bentonite. Trough cross-stratification in both units indicates southward directed paleoflow. In the western part of the study area, the lower unit comprises ~9 m of tabular-bedded, rippled, upper shoreface sand with cm-scale mud partings, overlain by ~5.5 m of channelized trough cross-stratified sand interpreted to be fluvial. The upper unit is ~18 m thick and contains a vertical succession consisting of bioturbated glauconitic muddy sandstone, trough cross-stratified sandstone with abundant burrows, and trough cross-stratified sandstone lacking burrows. In the eastern part of the study area, coarsening-upward units inferred to be stratigraphically equivalent are thinner and change down depositional slope from hummocky cross-stratified sandstone overlain by trough cross-stratified sandstone to shales overlain by trough cross-stratified sandstone. A transgressive pebble lag locally marks the top of the uppermost sandstone which is overlain by offshore silty mud of the Campanian Claggett Formation. Parallel to depositional strike, the two coarsening upward units at the top of the Eagle Formation continue, but with variations in coal thickness.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009