Abstract Jonah field in the northwestern part of the Greater Green River basin produces gas from the Lance Formation of Late Cretaceous (Maastrichtian) age. The Maastrichtian in the central Rocky Mountain region was a complex period of major transitions associated with the onset of the Laramide orogeny. This period of mountain building and basin development, which ultimately divided the central part of the Rocky Mountain foreland basin into much smaller Laramide basins and uplifts, began during latest Campanian and early Maastrichtian time and continued until near the end of the Eocene. The Maastrichtian section consists largely of continental rocks in the western part of the study area and interbedded continental and nearshore marine rocks in the eastern part. Marine shales deposited in the Maastrichtian seaway are effective seals of regional extent in many parts of the central Rocky Mountain region. Coaly source rocks occur throughout a broad area near the limit of maximum westward marine transgression during the Maastrichtian in this region. Jonah field, however, is located west of the marine shale seals and west of the area with significant Maastrichtian coaly intervals. Coaly intervals in the underlying Campanian Rock Springs Formation at Jonah may be the source of much of the gas, and a shale interval near the top of the Lance Formation may be the top seal for Jonah. An isopach map of Maastrichtian rocks reveals a complex pattern, including thickening trends toward newly developing Laramide uplifts as well as thickening toward areas of active thrusting along the Sevier orogenic belt. Jonah field is located in a deep, northwest-trending trough that developed southwest of the Laramide Wind River Range during the Maastrichtian. The Maastrichtian interval is overlain by lower Paleocene rocks throughout much of the study area, except near later Laramide uplifts. These thickening trends are the result of both variations in rates of subsidence during the Maastrichtian and differential erosion of Maastrichtian rocks prior to deposition of overlying Paleocene rocks. The Maastrichtian section is thin throughout a broad area in the southwestern part of the Greater Green River basin and Uinta basin. The lack of active thrusting and resultant thrust loading along the adjacent part of the Sevier orogenic belt during the Maastrichtian may be responsible for this thinning. Erosion of the Sevier highlands along this inactive portion of the orogenic belt may have instead produced broad uplift in the adjacent basin areas because of unloading. Paleocurrent studies in Maastrichtian rocks indicate that eastward-flowing drainage systems off the Sevier orogenic belt were maintained throughout the central Rocky Mountain region despite highly varying subsidence rates. Major Maastrichtian trunk streams tended to remain in place for extended periods of time, creating east–west-trending belts with thick stacks of fluvial sandstones surrounded by much less sandy intervals. Jonah field is located in a major southeast-flowing drainage system that was confined to the rapidly subsiding trough southwest of the Wind River Range. Stream systems that were flowing off rising Laramide uplifts appear to have been relatively short tributary streams to the major east-flowing trunk streams. These tributary streams were commonly mud choked as Cretaceous marine shale sequences were eroded off Laramide uplifts during the initial stages of uplift.

This paper describes the geology of an area of approximately 430 square miles in northwestern Wyoming and includes the southern margin of the Absaroka Range, the western end of the Owl Creek Mountains, and the northwestern part of the Wind River Basin. Most of the area lies within the southern half of the Kirwin quadrangle and the southeastern part of the Younts Peak quadrangle and extends from 25 to 55 miles southeast of Yellowstone National Park. Except for a few square miles in Hot Springs County, the entire region is in Fremont County, the northern part being in the Washakie National Forest and the southern part in the Shoshone Indian Reservation. RESULTS OF INVESTIGATION A geologic map including 430 square miles was made (Pl. 17). The stratigraphy, structure, lithology, and paleontology of the 15,500 feet of post-pre-Cambrian rocks were studied. Every system except the Silurian is represented. Uppermost Cretaceous and Paleocene rocks are absent. A remarkably complete section of Lower Tertiary rocks is preserved on the crest of the folded and faulted Washakie Range, in the central part of the area. Eight localized pulsations of the Laramide Revolution are believed to have affected the rocks of the region: (1) At the close of Lance time; (2) at the close of the Paleocene; (3) during early Early Eocene time; (4) at the close of early Early Eocene time; (5) at the close of late Early Eocene time; (6) at the close of Middle Eocene time; (7) at the close of Late . . .

Abstract About 420 million bbl of oil has been produced from Salt Creek field, Natrona County, Wyoming. Most production is from the second Frontier sandstone, which is one of many sandstone bodies interbedded with marine shale in the lower part of the Upper Cretaceous in the Rocky Mountain area. The stratigraphic interval between the Mowry Shale and the Niobrara Formation contains the Frontier and equivalent formations. The interval containing this section is more than 1,000 ft thick in central, northeastern, and west-central Wyoming and in southeastern Montana. Another area of thick sediments within this interval is in northwestern Montana and western Alberta. In some areas this section is entirely marine shale; in other areas it contains numerous sandstone bodies. The sand was transported by a series of river systems that formed deltaic complexes or lobate sand concentrations at several places along the margins of the early Late Cretaceous sea. These deltaic deposits are represented by the “D” sandstone of the Denver basin, the Ferron Sandstone Member of the Mancos Shale of Utah, the Cardium and Bad Heart Sandstones of Canada, and the Frontier Formation of Wyoming. The second Frontier sandstone—producing formation at Salt Creek field—is an offshore bar associated with the eastern terminus of one stage of Frontier deposition. The sandstone body is several miles wide, more than 60 mi long, and as much as 100 ft thick. Salt Creek anticline (formed in late Late Cretaceous or early Tertiary time) is in an area of excellent sandstone development where stratigraphic traps developed in which oil accumulated before secondary migration into present structural positions took place. There are other sandstone bodies included in the Frontier depositional complex which contain strati- graphically trapped oil, but these are not draped over an obvious anticline. The Wind River and Big Horn basins, and parts of the Green River and Powder River basins, probably contain more “Salt Creeks”!