COLORATION AND DIAGENETIC HISTORY OF JURASSIC NAVAJO SANDSTONE AT COYOTE BUTTES, VERMILION CLIFFS NATIONAL MONUMENT, ARIZONA

The Coyote Buttes, in Jurassic Navajo Sandstone straddling the Arizona and Utah border at the northern margin of the Vermilion Cliffs National Monument, is renowned for its stunning diagenetic coloration expressed within delicately sculpted, cyclic eolian cross-strata. The wide range of red, orange, pink, and purple hues is largely due to iron oxide grain coatings and cement that is predominantly hematite. Yellow to brown coloration is indicative of goethite. White (bleached) color occurs where some of the iron-oxide coatings have been removed. Coloration is constrained by both sedimentary and tectonic structures at microscopic to outcrop to regional scales.

Sandstone coloration is categorized into four main, large-scale (10's m thick) diagenetic facies: 1) a primary, basal red facies; 2) a red and white banded transition facies; 3) a bleached upper facies; and 4) a secondary red facies associated with the re-introduction of iron-rich fluids along a fault trace. Both the red and white banded and bleached facies are commonly overprinted with two cross-cutting, Liesegang-related zones (m's thick): one of numerous, narrow, cm-scale, multi-colored Liesegang bands, and the second containing multiple orange chemical reaction fronts including iron oxide micro-concretions.

Stratigraphic relationships of reaction fronts show that large-scale diagenetic facies are due primarily to advective fluid flow and iron mobilization within the host rock. Small (cm-scale) and large (10's-m scale) bleaching patterns in the outcrop clearly indicate the upward migration and accumulation of a chemically reducing and bleaching fluid. Smaller-scale, Liesegang-type reaction fronts are due to diffusive mass transfer causing the chemical precipitation of iron oxide mineralogies.

The location draws thousands of wilderness enthusiasts and photographers each year, primarily to a unique geomorphic feature – “The Wave” – where impressive colors accent cross-strata resembling a cresting ocean wave. This study contributes to a better understanding of the intense diagenetic coloration and fluid flow history that distinguishes the Coyote Buttes as an exceptional geologic resource.