Tertiary palaeoclimatic history of the southeastern Colorado Plateau
Abstract On the southeastern Colorado Plateau Tertiary erosion surfaces have been preserved below the cover of the Eocene Chuska Sandstone and the late Tertiary Bidahochi Formation. Investigations of the geomorphological attributes of the erosion surfaces and laboratory analyses of the soils and weathering materials as well as the sediments on the surfaces gave substantial information on the climate and the relief-forming processes which were responsible for their formation. In some places the surfaces cut across the underlying Mesozoic sedimentary rocks with a marked angular unconformity, but these truncating reaches are restricted to areas where soft material crops out. Resistant rocks from lithologically controlled cuestas, hogbacks and residual hills below the Tertiary cover. X-ray analysis of the clay minerals shows that the surfaces have not been subject to intensive chemical weathering and soil-forming processes. There is no change in the mineral content from the bedrock to the weathered material. These results apply to the (early) Eocene to Palaeocene Tsaile surface (underlying the Chuska Sandstone) and the Miocene Hopi Buttes surface (underlying the Bidahochi Formation). The results demonstrate that the climatic conditions have not changed decisively during the Cenozoic and were much the same as today. It is concluded that the Colorado Plateau has been dominated by a dry climate since the Laramide orogeny with the possible exception of the early Palaeocene. Thus the Colorado Plateau is out of phase with the general palaeoclimatic situation in western North America in the early Tertiary where at that time warm and moist conditions prevailed. The Colorado Plateau, before its separation from the Basin and Range province, was situated in a topographically lower position in the rain shadow of the southern Rocky Mountains and the Central Arizona Highlands, which at that time formed a barrier stretching from western Arizona to New Mexico.