Canyonlands National Park in southeastern Utah (fig. 1) has some of the most spectacular and colorful canyon scenery in the country. One of the most recognizable units exposed along the canyon wall in the park is the White Rim Sandstone Member of the Permian Cutler Formation, which forms a conspicuous white ledge surrounded by dominantly reddish-brown sandstone and shale (fig. 2). The abundant, large-to medium-scale, unidirectional, tabular planar crossbedding in the White Rim prompted early workers in the region to conclude that, for the most part, it represents ancient dunes deposited in an eolian environment (Baker and Reeside, 1929; McKnight, 1940; Baker, 1946; Kunkel, 1958; Heylmun, 1958; and Hallgarth, 1967a).
The first detailed study of the White Rim Sandstone Member was done by Baars and Seager (1970) and was concentrated in the Elaterite Basin area (fig. 1). They concluded the White Rim is almost entirely shallow marine and only the easternmost part, near the base of Shafer Trail (fig. 3), is eolian. This interpretation was based on a detailed analysis of sedimentary structures and on the presence of a problematic (alga?) fossil found in Elaterite Basin. The large-scale, unidirectional cross stratification in the White Rim was interpreted to represent numerous, elongate. offshore bar deposits formed by longshore currents moving from the northwest. Also, according to Baars and Seager (1970), these bar deposits invariably overlie horizontally bedded material in the area between the Green and Colorado Rivers. They interpreted the horizontal bedding to represent a regressive sequence, and the offshore bars a transgressive sequence.
In order to resolve this difference in interpretation, a detailed study of the White Rim was conducted in the area between the Green and Colorado Rivers in Canyonlands National Park (fig. 3). Stratigraphic relationships, sedimentary structures, and petrographic characteristics of the White Rim were examined. Results from this study indicate the White Rim was deposited in an eolian system and contains dune, interdune, and sabkha deposits. Also, the White Rim has a close marine association and represents a coastal dune field (Steele-Mallory, 1981a).
Canyonlands National Park is located in the north-central portion of the Colorado Plateau and is part of the Canyonlands section of the Plateau (Fenneman, 1928). The Canyonlands section is essentially a large plateau surface that has been epeirogenically upwarped and broadly folded (Hunt, 1956). Entrenchment of the Green and Colorado Rivers and their tributaries on this surface in the last 5 million years (Trumble, 1980) has formed a series of deep canyons. The present study was done in the Island in the Sky district of the park, which includes the 60-km-wide erosional basin of the Green and Colorado Rivers (fig. 3).
Almost continuous sedimentary deposition in the Canyonlands region throughout much of the Paleozoic, Mesozoic and Cenozoic Eras resulted in a thick accumulation of sediments, which exhibit diverse compositions. Late Tertiary to Holocene erosion has completely removed the entire Cenozoic and part of the Mesozoic record. Presently, relatively undeformed rocks of Pennsylvanian to Jurassic age are exposed in the park (fig. 4). Baars and Molenaar (1971), Lohman (1974), and Fassett and Wengerd (1975) have discussed the geologic history of Canyonlands.
The White Rim Sandstone Member is excellently exposed in canyon walls of the Green and Colorado Rivers (fig. 2). The White Rim trail, a primitive four-wheel drive road (fig. 3), gives limited access to outcrops. Twelve complete and three partial stratigraphic sections were measured, described, and sampled. Stratigraphic sections were spaced in order to attain maximum coverage of the White Rim throughout the study area (fig. 3), but working access to full sections of the White Rim is limited because it forms steep, unscalable cliffs. Complete descriptions of the measured sections are given in Steele-Mallory (1981b and 1982).
In the course of measuring and describing stratigraphic sections, emphasis was placed on the examination and description of primary sedimentary structures, including their size, orientation, and morphology. Mean cross-bed dip directions were calculated for each measured section by the vector summation method first used by Reiche (1938). Equal-area rose diagrams of all crossbed dip directions were made. These were used to classify dune types and to help reconstruct the depositional history of the White Rim. Local and regional geologic relationships of the White Rim to adjacent stratigraphic sequences also were examined to help reconstruct the White Rim's depositional history.
At each measured section, the White Rim and the overlying and underlying units were sampled for thin sections. A detailed petrographic examination was made of 107 thin sections to identify textures, mineralogy, and diagenetic features. Complete descriptions of the thin sections are given in Steele-Mallory (1981b and 1982).
Last Updated: 09-Nov-2009