GEOLOGIC SETTING OF A PERMIAN FOSSIL PLANT AND VERTEBRATE SITE, SOUTHEASTERN UTAH

The Indian Creek paleontological site contains abundant Permian vertebrate and plant material that was deposited in shallow standing water. The site is unusual because overlying and underlying rocks reflect deposition in a dominantly semi-arid to arid environment. The Indian Creek site occurs in the transition zone between the Cutler Formation undifferentiated and the Cedar Mesa Sandstone. The primarily fluvial Cutler Formation undifferentiated consists of feldspar-rich sedimentary rocks derived from the Uncompahgre Uplift segment of the Ancestral Rocky Mountains, to the east of the Indian Creek site. The dominantly eolian Cedar Mesa Sandstone includes rounded to well-rounded, well-sorted, quartz-rich sedimentary rocks derived from a source to the west of Indian Creek. Vertebrate fossils are typically absent from the Cedar Mesa, although tree root casts are commonly present beneath deposits left by floods that periodically inundated the Cedar Mesa dune field. The absence of soil horizons or organic material (other than tree root casts) in the Cedar Mesa near Indian Creek, coupled with the occurrence of gypsum casts and high-angle cross-stratification suggest deposition occurred under arid to semi-arid climate conditions. The Indian Creek site is therefore unusual because both plant material and vertebrate fossils occur within sediment deposited in standing water. Deposition of the fossil-bearing layers occurred within an arroyo complex that was cut into eolian sand sheet and interdune deposits. The arroyo complex was initially cut and filled by a northwest-flowing stream carrying debris from the Uncompahgre Uplift. This first arroyo was modified by subsequent erosion and deposition by a northward-flowing stream. Several large coniferous trees were deposited approximately perpendicular to the thalweg of the second stream, suggesting the trees were carried downstream during a flood event. A layer of freshwater limestone containing abundant disseminated plant material and vertebrate bone fragments and teeth is present above the flood deposit. The freshwater limestone was deposited in a shallow lake that extended beyond the edges of the channel. The youngest channel at the site was carved by a westward-flowing fluvial system that partly removed and then covered the fossil-bearing layers with sediment.