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1
Read before the Association at Atlantic City, New Jersey, April 28, 1960. Manuscript received, June 1, 1960.
2
Associate Professor of Geology, Colorado School of Mines. The Colorado School of Mines Foundation, Inc., gave financial aid to defray field expenses and expenses in the final preparation of material for this paper. The close similarity of Cretaceous depositional environments to present depositional environments along the Gulf and Atlantic Coasts was pointed out to the writer by Chester Cassel during discussions in 1954. Mr. Cassel, in unpublished work, determined the close relationship between present shoreline environments and the Cretaceous environments of the San Juan basin. By use of these environmental studies he was able to demonstrate stratigraphic control for the large gas accumulations in that area. His ideas also apply to the areas described herein. To J. D. Haun, Chester Cassel, and Fred E. Moore, the writer extends his gratitude for reviewing the manuscript.

Abstract

The Cretaceous of the Rocky Mountain region contains sandstones that were deposited in marine, transitional, and nonmarine environments. Spatial dimensions of sandstones deposited in shallow neritic and transitional environments are regular in character and are easily defined. Only this type of sandstone is here considered, and examples illustrating minimum and maximum geographic distribution are treated.

Minimum size sand bodies are well shown by the Fox Hills 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 southeastward to marine shale (Lewis shale). Detailed surface analysis of one barrier bar shows a thickness of 30 feet and a width of 6–7 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 upper part of the Judith River formation of central and eastern Montana exemplifies a transitional and marine sandstone unit having a maximum width. The unit is 140 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 ranges from a wedge edge 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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