The Late Cretaceous Frontier Fm (up to 200 m thick) exposed in the Wind River basin consists of superposed decametre-scale depositional cycles (storm-dominated shelf and shoreface to tidal-channel distributary systems). Detailed lateral mapping and measurements of the cycles over a 20 km strike distance illustrates the presence of two distinct types of intraformational erosional surfaces. The first is a sequence boundary recognised as a sharp surface having decimetre-scale scouring and superposing shoreface (hummocky and swaley stratified) and tidal channel (trough cross-bedded, clay drapes, tidal bundles) sandstones on mid-outer shelf mudstones. This surface cuts downsection and, importantly, it locally emplaces the shoreface-tidal channel sand-stone packet on similar sandstones of the underlying cycle yielding the appearance of one continuous and genetically related shoaling cycle. The second type of erosional surface is associated with tidal channel sandstones. These sandstones are several 10's of metres thick, broadly incise interfluve mudstones, but are laterally discontinuous over several to many hundreds of metres. This erosional surface is part of the autocyclic processes of channel migration and thus not necessarily indicative of a sequence boundary. This underscores the need to document lateral facies and stratal geometries for correlations rather than trying to match the number of sandstone packets present at any given locality or in any given well log.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah