TECTONIC INTERPRETATION OF OLIGOCENE PALEO-STREAM CHANNEL DEPOSITS AND EROSION SURFACE IN THE RATTLESNAKE HILLS, CENTRAL WYOMING

The Rattlesnake Hills in central Wyoming are the eroded remnants of a northwest plunging Laramide anticline on the north flank of the Sweetwater Arch. Archean crystalline rocks, exposed in the core with Paleozoic and Mesozoic sedimentary strata on the flanks, are unconformably overlain by Tertiary sediments. Approximately 50 discrete intrusive bodies, radiometrically dated as Late Eocene, form 3 groups, a central alkalic cluster with felsic intrusives grouped to the northeast and southwest. The pyroclastic debris in the Middle and Late Eocene Wagon Bed Formation appears to be primarily from the felsic eruptive centers.

After the Laramide orogeny ended in the earliest Eocene, the Sweetwater Arch continued to rise and resulted in the formation of a horst at the end of Early Eocene time. The Oligocene was a time of crustal stability in the Rattlesnake Hills, when a major erosion surface was cut on the central Rattlesnake Hills. Boulder channel deposits of northeast flowing Oligocene streams now form linear ridges resulting from topographic inversion on the north flank of the Rattlesnake Hills. Systematic lithologic changes in the Oligocene stream deposits indicate drainage reversal in the southern half of the Rattlesnake Hills and large scale regional warping and faulting of the Granite Mountains of central Wyoming during the late Cenozoic. Younger (Plio-Pleistocene) stream deposits were formed by streams flowing south out of the Rattlesnake Hills after the Granite Mountains subsided in the Miocene. A postulated original northward gradient on a remnant of the Oligocene erosion surface is used to estimate a minimum of 3200 feet of subsidence of the central part of the Granite Mountains. This evidence supports the existence of a major east-west mountain range in central Wyoming during the middle Tertiary as determined by David Love.