Approximately 2,500 feet of Upper Cretaceous post-Cody non-marine sediments in the southwestern part of the Big Horn basin interfinger toward the north and east

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with marine shale. In the eastern part of the basin four formations are readily distinguishable; they are, in ascending order, the Eagle Sandstone, Claggett Shale, Judith River Formation, and Bearpaw Shale. In the western part of the basin the equivalent stratigraphic section includes the Mesaverde (Gebo Formation of Hewitt), and the Meeteetse Formation. Non-marine deposits of the Lance Formation, including locally the Fox Hills Sandstone, overlie the Meeteetse-Bearpaw section.

Major transgressions are evidenced by the Claggett and Bearpaw shales and regressions by the nearshore and non-marine sediments of the Eagle, Judith River, and Lance formations. Minor transgressions and regressions produced a complex interfingering of sand and shale that complicates formational boundaries.

Isopachous maps, utilizing all available well information as well as fourteen partial surface sections, were compiled to show the distribution of each of the formations. The stratigraphic interval containing the Eagle Sandstone thickens from 200 feet near the Wyoming-Montana line to 800 feet along the south margin of the basin, but the sandstone content decreases southward as the over-all interval increases. Eagle sands are particularly well developed in the area between Worland and Coon Creek. The Claggett Shale is recognizable throughout the east half of the basin and reaches a maximum thickness of 275 feet along the eastern margin. The Judith River and Mesaverde formations thin northward and eastward throughout the basin while the overlying Bearpaw Shale displays a reciprocal relat onship ranging in thickness from zero in the southwestern part to 1,000 feet in the north end. In the south part of the basin the Judith River is divisable into three mappable units: upper sandstones, middle continental deposits, and lower sandstones.

Marine and transitional beach environments, that have produced sandstone reservoirs and petroleum accumulations in several other basins in the Rocky Mountains are well represented in the Eagle, Judith River, and Bearpaw of the Big Horn basin. Numerous littoral and neritic sandstones provide a variety of trapping conditions in conjunction with both old and present-day structural features. Embryonic development of Laramide structures during late Cretaceous resulted in (1) thinning of sediments over some of the major structural features and (2) the accumulation of thicker and more porous sandstones along the flanks and over the crests. Examples of depositional thinning of the Claggett Shale may be observed in the vicinity of Five Mile, Worland, Slick Creek, and Sand Creek fields. Local t inning of the Judith River interval occurs at Worland and Slick Creek fields. Similar Laramide structural growth patterns have been mapped in association with oil and gas production in the Patrick Draw-Table Rock area of the Great Divide basin in southern Wyoming.

In approximately 60 per cent of the basin the top of the Judith River and Mesaverde is at a depth of 8,000 feet or less. There is no petroleum production from Upper Cretaceous rocks of the Big Horn basin at the present time. Significant shows have been found at (1) Neiber anticline where a well blew out and burned from gas in the Eagle, (2) Golden Eagle field where three cable-tool wells produced gas from the lower Mesaverde during the early stages of the field's development (1918-1923), (3) Golden Dome and Dry Creek fields in Montana where gas was produced from a zone in the lower Mesaverde, and (4) at the Berwin area northwest of Badger basin where oil was recovered on drill-stem tests of the lower Mesaverde. Smaller shows of oil and gas have been encountered in several other wells. /P>

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