Abstract

Valley anticlines of two types exist in the Needles District of Canyonlands National Park: (1) large, upright anticlines following major drainages, and (2) small, generally asymmetrical anticlines confined to rocks along the floors of smaller tributary valleys. The first type includes the Meander anticline along the Colorado River and anticlines following four deep tributary valleys east of the river. The Meander anticline is about 35 km long and follows the sinuosities of the Colorado River. The tributary anticlines also follow valley sinuosities. Severity of folding decreases upward from the valley floors, but arching is still evident at the level of canyon rims, as much as 600 m above the valley floors. These large valley anticlines appear to be due to upward flow of evaporites of the Paradox Member of the Hermosa Formation (Pennsylvanian). The differential stress driving this flow is a result of the unloading generated by canyon erosion.

Much smaller anticlines occur in the Salt Creek, Butler Wash, and Chesler Canyon drainages 4 to 17 km east of the Meander anticline. These small anticlines are confined to 25 m of thin-bedded rocks straddling the base of the Cutler Formation (Permian) and have been found only where these thin-bedded rocks occur on valley floors. Most have straight limbs and sharp hinges, and all appear to die out downward and laterally so that the rocks of the valley walls and rocks at depth beneath the valley floors are unfolded. The folded rocks show no evidence of plastic behavior of any kind. Although some of the folding could be due purely to elastic rebound following valley erosion, fold amplitudes are best explained by excess horizontal compressive stress.

A close correspondence between structure and landforms generally is interpreted as being due to structural control of landform development through differential erosion. In contrast, fluvial valleys predate folding and have determined the locations and mechanics of formation of anticlines in the Needles District and in a few other localities in the world.

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