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The Cretaceous of the Rocky Mountain region contains sandstones that were deposited in marine, transitional, and non-marine environments. Geometric patterns of sandstones deposited in shallow neritic and transitional environments are regular in character and are easily defined. Only these sandstones are here considered and units illustrating their minimum and maximum geometric aspects are treated.
Minimum-size sand bodies are well shown by the Fox Hill sandstone where it is exposed on the northeast flank of the Rock Springs uplift, Wyoming. This formation consists of a series of barrier-bar sandstones that change northwestward to lagoonal shales (Lance formation) and change southeastward to marine shale (Lewis shale). Detailed surface analysis of one barrier bar shows a thickness ranging from 30 to 50 feet and a width of 5 miles from the lagoonal shale and sandstone facies to the marine shale and siltstone facies. Each bar is believed to have extended along much of the western margin of the Cretaceous seaway.
The Judith River formation of central and eastern Montana exemplifies a transitional and marine sandstone unit have a maximum width. The unit is 50-75 miles wide and was deposited between lagoonal shale facies to the west and marine shale facies (Pierre shale) to the east. Thickness of the unit varies from 50 to 100 feet.
The geometric pattern of most of the sand bodies that accumulated along the Cretaceous shoreline is similar in character to the above examples and ranges in size between these extremes.
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