Permian Root Traces From Natural Bridges National Monument

Shawn Duffy
Natural Bridges National Monument, Box 1, Lake Powell, UT 84533


Abstract—Original interpretations of the Cedar Mesa Member of the Cutler Formation in and adjacent to Natural Bridges National Monument in southeastern Utah concluded it to be a near shore marine deposit (Barrs, 1972, 1989). Other studies concluded an aeolian origin (Stanesco & Campbell, 1989). The recent discovery of in situ root traces in the Cedar Mesa Member provide supporting evidence for the latter interpretation.

Introduction

The Cedar Mesa Member of the Cutler Formation in south eastern Utah dominates the canyons in and adjacent to Natural Bridges National Monument. Primarily composed of white sandstone with pink arkosic shale and limestone lenses, the member displays strong cross bedding suggesting an aeolian origin. Because of the presence of fragmentary marine creatures within the sandstone, other interpretations concluded a marine origin (Barrs, 1972, 1989). This latter interpretation does not fully take into account the abundant root casts and traces found throughout the member. Originally, the casts and traces were believed to have been transported to their present position from regional streams, however, the orientation of the roots suggest an insitu origin.

The Roots


Figure 1—Detail of rootsite. Only the larger roots are of Permian age. The smaller roots appear to be calcification of recent roots.

The roots occur as casts, molds, and traces staining the surrounding host rock. Most roots are an average of 2.5 to 5 centimeters thick and roughly 30 centimeters long. Several roots were discovered to be over 3 meters in length and exhibit a branching or radiating pattern which suggest connecting either to each other or to a centralized point like a tree trunk. The sandstone within the Cedar Mesa Member is primarily white, however, most localities in which the roots occur display strong pink and brown mottled patterns suggesting a paleosol (Stanesco & Campbell, 1989).

During the spring of 1997, a site was discovered in Natural Bridges National Monument in which the root casts were over 2 meters in length (Figure 1). The most interesting feature about the site was the fact that the roots did not follow the bedding planes within the sandstone as in most sites, but rather cut across the bedding planes. This pattern suggest that the roots were growing down through the sand before lithification. Along with the larger roots were masses of smaller intertwining roots covering the sandstone surface. It has been determined thus far that these smaller roots are not connected to the larger ones and most likely represent recent calcification of park flora.

Conclusion

Taken as a whole, the random orientation, definite spacing, and the cutting across of bedding planes along with the association of mottled patterns strongly suggest that we are dealing with an insitu origin for the roots. This in turn supports the interpretation that the Cedar Mesa Member of the Cutler Formation represents a series of terrestrial dunes containing islands of vegetation similar to today's coastal sabkhas.

References

Barrs, D.L. 1972. The Colorado Plateau: A Geologic History, University of New Mexico Press, Albuquerque, New Mexico.

———. 1993. Canyonlands Country: Geology of Canyonlands and Arches National Parks, University of Utah Press, Salt Lake City, Utah.

Stanesco, J.D. J. A. Campbell. 1989. Eolian and Noneolian Facies of the Lower Permian Cedar Mesa Sandstone Member of the Cutler Formation, Southeastern Utah, U.S. Geological Survey, Denver Federal Center, MS939, Denver , Colorado, and Red Rocks Community College, Lakewood Colorado, Department of Geology, Fort Lewis College, Durango, Colorado.