Marine sedimentation which took place on the Wyoming Cratonic Shelf during upper Mississippian, Pennsylvanian, and lower Permian Periods left its record as the transgressive Sacajawea shale, the "stillstand" Amsden carbonate rock, and the regressive Tensleep sandstone.

Primary source of sand-size detritus occurring in these three formations was the Precambrian Canadian Shield. Immediate source of 80 per cent of the sand was well rounded orthoquartzite and/or subgraywacke sandstone cropping out north and east of northern Wyoming. The remaining 20 per cent sand may have come directly from the Shield area. Extensively dissolved and eroded argillaceous Madison limestone probably served as a main source of clay for the overlying Sacajawea Formation. Solution of Madison carbonate rock also made calcium and magnesium ions abundantly available for reprecipitation in Amsden limestone and dolomite.

A change from highly kaolinitically weathered feldspar gains in the Sacajawea Formation to vacuolized feldspar in the Tensleep, plus a strong decrease in intensity of weathering, suggests a climatic shift from warm-humid conditions during the transgressive phase of the cycle to more arid conditions during regression. A mixture of strongly altered feldspar grains with moderately altered grains is characteristic of the Darwin Member (sandstone) of the Sacajawea Formation. In contrast, Tensleep feldspar is more uniformly weathered. This suggests moderate topographic relief in the adjacent source terrane during Sacajawea accumulation (greater erosive activity), but very subdued relief during deposition of Tensleep sediment (minor erosive activity).

Sacajawea strata were deposited during a period of regional transgression in a variety of continental and nearshore marine environments which may have included fluvial, tidal flat, lagoonal, estuarine, and neritic sites. The sandstones of the Darwin Member are river deposits, and as such form poor horizon markers for stratigraphic correlation. Amsden carbonate rock is a neritic and littoral deposit, and marks the period of greatest inundation during the transgressive-regressive cycle. Tensleep sandstone was deposited in neritic and littoral environments during a north-to-south withdrawal of the sea.

Cross-bedded Tensleep strata display bimodal size distribution characterized by a minor coarse mode. They may have been developed by high-energy bottom currents during readjustment of the foreshore profile of equilibrium following large storms. Average direction of cross-bed dip is southwest, suggesting a bathymetric slope in that direction. There is no evidence that final accumulation took place under eolian conditions. Thinly and evenly bedded strata separating the cross-laminated units display a unimodal or positively skewed-bimodel grain-size distribution, and were probably deposited by longshore and offshore currents.

The broad pattern of quartz cement distribution in the Tensleep sandstone suggests ultimate derivation from a northeastern land source. Carbonate cement was precipitated directly as microcrystalline dolomite "mud" along with the accumulation of terrigenous quartz grains, or as microcrystalline calcite which altered to dolomite at or near the sediment-water interface before lithification. The presence of anhydrite, particularly in the western Bighorn basin, indicates restriction of marine circulation over the shallow, cratonic shelf. Restriction may have been related to reef growth, or simply a result of width of shelf and aridity of climate during Tensleep deposition.