Paleoyardangs: Wind-Scoured Topography at Permian Unconformity

David W. Tewes, David B. Loope

At least 28 elongate, parallel ridges exhibiting 5-14 m of positive relief are preserved on the upper surface of the White Rim Sandstone (Permian) near its eastern stratigraphic pinch-out in Canyonlands National Park, southeastern Utah. The ridges show extreme consistency in size, shape, and directional trend. Ridges average 250 m in width, are less than one to possibly several kilometers in length, and trend N20°W ± 5°. Ridge flanks dip as steeply as 30° and are commonly covered with a thin lag of coarse sand. Sand-filled polygonal fissures are abundant on ridges and intervening flats.

The unimodal southeastward dip of the cross-strata within the White Rim and the relationship between eolian bounding surfaces and ridge morphology negate a depositional origin for the ridges. The presence of lag grains within polygonal fissures indicates that the lithification required for fissuring took place prior to deposition of the lag.

We interpret the ridges as paleoyardangs that were shaped from partially lithified eolian sand by the same unidirectional winds that prevailed during deposition. Similar wind-eroded topography has been repeatedly documented from modern hyperarid regions with unimodal winds but has not been previously described from the stratigraphic record. Wind erosion as well as deposition must be considered when investigating ancient eolian sequences.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.