Abstract

The Frontier Formation (Late Cretaceous) of western Wyoming contains a range of wave- and tide-dominated deltaic facies. Two examples are investigated in detail: one in which the tide-dominated facies occur in a protected bay, and the second in which the tide-influence deltas occur on the open shelf. The first example is from the Oyster Ridge Sandstone Member of the Frontier Formation, exposed along the north–south oriented Hogsback Thrust. The facies becomes progressively more tide-dominated to the north progressing from shoreface parasequences in the south, to mixed-influence deltas, to south-prograding ebb-tide-dominated deltas, to flood-tidal bars in the far north. The tide-dominated delta and flood-tidal bars have broadly similar parasequences: Basal facies are bioturbated mudstone to muddy sandstone that grade upward into cross-bedded and current-rippled sandstones with common mud drapes. All of these facies are interpreted to have been deposited in a north–south oriented bay formed in a foredeep basin, bounded on the west by the thrust front, and on the east by a broad, low uplift, and open to the ocean to the south. Tidal facies predominated in the bay, where deposition was not influenced by waves and where tides may have been enhanced due to tidal amplification.

The second example comes from the subsurface First Frontier Member of the Frontier Formation, also in western Wyoming. Approximately 700 closely spaced wells, including several with cores, were used to describe and map this system, which is composed of a series of northeastward-prograding delta lobes. The facies stacking pattern observed in core is similar to the flood-tidal bars described from the outcrop. Maps of net sandstone show distinct linear sandstone bodies oriented northeast–southwest that are interpreted as tidal sandstone ridges similar to features in modern tide-dominated deltas. Significantly, regional well-log correlations do not show any evidence of a barrier that would have protected the system from wave influence. These deltas appear to have been tide-influence on the open shelf.

This and other studies of ancient tidal deltas reveal some common elements. Tidal deltas are characterized by coarsening-upward parasequences with abundant current ripples, large-scale cross beds, and mud drapes. Prodelta faces may be either well laminated or bioturbated, and in the former case may preserve evidence of tidal cyclicity. Sandy delta-front facies commonly have steeply dipping clinoforms and may form shore-normal elongate tidal bar systems. Delta-plain facies are rarely preserved, but where present are composed mostly of tidal-flat and tidal-channel deposits.

The tide-dominated delta parasequences and mid-estuary tidal bars can have very similar stacking patterns, so that single cores or well logs alone are not sufficient to distinguish the setting or sandstone geometry. In this study the stacking patterns of the Oyster Ridge Member tidal bars and tidal deltas appear very similar to First Frontier Member tidal deltas, and all have bioturbated prodelta facies, which contrasts sharply with tidal deltas from central Wyoming and elsewhere. Thus stacking patterns alone are not predictive of depositional setting (shelf vs. estuary) or parasequence geometry (delta vs. tidal bar).