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Regional Stratigraphic Setting of the Maastrichtian Rocks in the Central Rocky Mountain Region
U.S. Geological Survey, Denver, Colorado, U.S.A.
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.
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