A REASSESSMENT OF THE TIMING OF SEDIMENTARY INJECTITES IN KODACHROME BASIN STATE PARK AND VICINITY, SOUTHWEST UTAH

Large-scale (5-20m diameter) sub-vertical cylindrical injectites of Kodachrome Basin in southwest Utah are a complex of unique sand-hosted intrusions cutting through the Jurassic Carmel and Entrada Formations, and are among the largest of similar occurrences throughout the Colorado Plateau. Initial investigations suggested a Pliocene-Pleistocene age of intrusion based on mixing with an overlying conglomerate of the same age. Recent studies suggest that the pipes formed in Jurassic time, but this model has yet to be adequately evaluated.

To evaluate the time-synchronous mixing model of injectites and the overlying conglomerate, field and lab analyses were conducted on clast composition, matrix elemental composition, and the nature of the basal contact of the conglomerate with underlying injectites. To determine if the injectites are Jurassic in age, cross-cutting relationships of smaller sand dikes, (“thin dikes”) were traced in the field to determine the youngest formation they cut.

Point count comparisons of clasts in injectites with the conglomerate show that the two are compositionally distinct. Clast sizes in injectites are statistically smaller than those in the overlying conglomerate, also showing distinctiveness. X-ray fluorescence (XRF) elemental composition analyses show that the matrix in injectites was likely eroded and re-deposited in the overlying conglomerate. Most importantly, many injectites were clearly eroded to a flat plane after lithification, and later buried by the younger conglomerate, eliminating any possibility of temporal synchroneity. A Jurassic age is suggested by “thin dikes” cutting the Gunsight Butte Member of the Entrada, but no younger units.