Abstract

The Medicine Bow Mountains, Sierra Madre, and intervening Saratoga Valley have been interpreted as structural features formed chiefly during the Laramide orogeny, but this concept needs revision. In this article, it is argued that Laramide regional contraction created a single Precambrian-cored range which was subsequently disrupted by Neogene normal faulting forming the present-day Saratoga Valley, Sierra Madre, and Medicine Bow Mountains. The originally broad Laramide range was bordered on the east by a thrust fault system directed toward the Laramie Basin and on the west by downwarped Paleozoic and Mesozoic strata which descend into the Washakie Basin. Erosion of the Laramide range during uplift exposed Precambrian basement rocks whose clastic debris were deposited in adjacent Paleocene basins. Following an episode of late Paleocene to early Eocene erosion, airborne pyroclastic ash from remote sources progressively filled the broad Laramide basins during the middle Eocene, Oligocene, and early Miocene. Ensuing Neogene epeirogenic uplift was accompanied by Miocene deposition, as the west flank of the Medicine Bow Mountains subsided along a normal fault thus creating the northeast-facing front of the Sierra Madre and associated half graben of Saratoga Valley. During this extensional event the North Platte River established its northward course from the floor of North Park Basin in Colorado through Saratoga Valley. The valley is interpreted as a northern manifestation of the Rio Grande rift which disrupted a broad Neogene uplift extending from New Mexico into southern Wyoming.

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