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A complex history of Laramide tectonism, erosion, and deposition is recorded in lower Cenozoic rocks of the northwestern Wind River basin and Washakie Range, Wyoming. These rocks include the Indian Meadows Formation (lower Eocene), Wind River Formation (middle to uppermost lower Eocene), and Aycross Formation (middle Eocene), separated by unconformities.
Major structures developed during latest Cretaceous through earliest Eocene time, prior to deposition of preserved Tertiary rocks. During this interval, the Washakie Range arched upward and thrust southwestward over the basin axis and erosion stripped more than 5,000 ft (1,500 m) of upper Mesozoic rocks from the area, forming the basal Tertiary unconformity.
During the Eocene, the Indian Meadows Formation accumulated as alluvial fans spread from deep mountain valleys southward across the basin. Large Paleozoic masses slid southward off the oversteepened range front. Near the end of Indian Meadows deposition, a new fold arched and thrust southward, depressing and downfaulting the older range front and disturbing rocks in the basin.
During the middle early Eocene, the Wind River Formation accumulated as new alluvial fans spread southward from canyons of the rejuvenated range. Erosion had breached the Precambrian core of the range, flooding downstream areas with granitic debris.
During the early middle Eocene, the range collapsed along normal faults. The Aycross Formation accumulated in flood plains in the basin and in broad valleys that developed along normal fault traces in the range. During this time, the Absaroka volcanic field to the north became the dominant local source. By the end of Aycross deposition, the basin and all but the highest parts of the collapsed, deeply eroded range were buried by volcaniclastic debris.
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