Image: Tidwell Member of the Morrison Formation over Wanakah Formation (Jurassic; roadcut near Artists Point, Colorado National Monument, Colorado, USA) 1 (23644153939)

Description: Colorado National Monument is a colorful and scenic park just southwest of the town of Grand Junction in western Colorado, USA. It's located in the northeastern part of the Colorado Plateau Physiographic Province, a large section of continental crust that was significant uplifted during the Cenozoic. The rocks in the park are gently folded into a large monocline that has been erosionally dissected into a series of dramatic canyons. The brown-colored ledges near the top of the roadcut are the basal sandstone beds of the Tidwell Member of the Morrison Formation (Upper Jurassic). The Morrison is widely distributed in many western American states and consists of fluvial (river/floodplain) and lacustrine (lake) deposits. Dinosaur bones and dinosaur tracks are moderately common in Morrison Formation sediments (exceptionally rich localities include Como Bluff in Wyoming, Dinosaur National Monument in Utah, Dinosaur Ridge in Colorado, and the Cleveland-Lloyd Quarry in Utah). The grayish- and reddish-colored rocks in the middle of the photo are mudshales of the Middle Jurassic Wanakah Formation (formerly known as the Summerville Formation). Some weathering-resistant siltstone and sandstone interbeds are also present. The Wanakah Formation is mostly nonmarine, but some marginal marine beds have been identified. Relatively few fossils have been found in the Wanakah Formation of western Colorado and eastern Utah, but include bivalves, ostracods, fish, and trace fossils. Stratigraphy: Morrison Formation, Kimmeridgian Stage, middle Upper Jurassic, ~150 to 156 Ma Locality: roadcut along Rim Rock Road next to Artists Point overlook, Colorado National Monument, southwest of the town of Grand Junction, west-central Mesa County, far-western Colorado, USA
Title: Tidwell Member of the Morrison Formation over Wanakah Formation (Jurassic; roadcut near Artists Point, Colorado National Monument, Colorado, USA) 1 (23644153939)
Credit: Tidwell Member of the Morrison Formation over Wanakah Formation (Jurassic; roadcut near Artists Point, Colorado National Monument, Colorado, USA) 1
Author: James St. John
Usage Terms: Creative Commons Attribution 2.0
License: CC BY 2.0
License Link: https://creativecommons.org/licenses/by/2.0
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