Rock Springs Uplift

This north-south-trending Laramide uplift divides the Greater Green River Basin (GGRB) equally into an eastern part and a western part. To the east are the Washakie and Great Divide basins, separated by the Wamsutter arch, an eastward extension of the Rock Springs uplift. To the west are the Green River and Bridger basins. The Rock Springs uplift is a large, complex asymmetric anticline, with its western flank steeper than its eastern flank. The anticlinal axis plunges on both the north and south ends of the structure and a westward-directed thrust fault on the western flank occurs at depth below the surface outcrops. This uplift is a miniature version of some Wyoming mountain ranges, except that when it formed during the Late Cretaceous and early Tertiary, it was not uplifted enough to expose the Precambrian core as in the larger mountain ranges. The oldest rocks exposed in the center of the uplift are Upper Cretaceous marine shales and sandstones, with progressively younger sedimentary rocks forming bands of outcrops around the uplift. The uplift is cut by a number of east- and northeast-trending faults that displace the outcrops by varying amounts.

An extensive cover of Eocene rocks once extended across the center of the uplift, but these have been eroded back to form the edges of the surrounding basins. For example, White Mountain west of Rock Springs preserves lacustrine and evaporite rocks of the Green River Formation that once extended across the uplift. On the northeast flank of the uplift are a number of flat-topped buttes and mesas and isolated protrusions of volcanic rocks called the Leucite Hills. These are very young (approximately one million years old) rocks that erupted over the eroded flank of the Rocks Springs uplift. Several of the extrusive rocks are area landmarks, including Boars Tusk north of Rock Springs and Pilot Butte on top of White Mountain.

Travelers on I-80 across southern Wyoming can easily discern the size of the Rock Springs uplift by observing the general dip of the rocks over which they pass. At the town of Green River, the interstate is cut into nearly flat-lying Eocene rocks along the river of the same name. The highway then follows the canyon occupied by Bitter Creek eastward into Rock Springs, where coal-bearing Upper Cretaceous rocks dip steeply westward. Rock Springs is noted for the extensive underground coal mining in, around, and under the town; the mines supplied coal to the Union Pacific Railroad. East of Rock Springs near the airport, the interstate crosses the center of the uplift, where easily eroded, flat-lying marine shales form the feature called Baxter Basin. Even though the axis of the Rock Springs anticline trends through Baxter Basin, the interior of the anticline is a topographic depression caused by differences in resistance to erosion of varying rock types. Farther east on the interstate, the rocks dip eastward, with excellent exposures of thick sandstone beds near Point of Rocks. Eastward beyond Point of Rocks, the highway continues through progressively younger rocks, and near Black Buttes, coal is mined from latest Cretaceous and earliest Tertiary rocks. The Jim Bridger power plant north of the interstate uses this coal and other coal mined north of the plant. Eastward beyond Black Buttes, the rocks continue to dip eastward, but at lesser angles, and eventually pass from the flank of the uplift into flat-lying Tertiary rocks of the Great Divide Basin.

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