EPHEMERAL LAKES OF THE LOWER CRETACEOUS CEDAR MOUNTAIN FORMATION, WOODSIDE ANTICLINE, UTAH

Details on the depositional environments of the basal Cedar Mountain Formation (CMF) in the San Rafael Swell have thus far been elusive. While it has been established that the CMF records terrestrial deposition dominated by fluvial processes, subfacies can range through a spread of interfluvial environments. Explanation of the details is critical for understanding the history of the Sevier Orogeny and the resulting changes to the landscape. Our research of the CMF throughout the San Rafael Swell has recorded a broad range of environments from the deposition of the Buckhorn Conglomerate through the Ruby Ranch Member. Herein we focus on the intervening interval containing rare limestone found in the Woodside Anticline area. The limestone is primarily massive micrite with ostracodes and rare charophyte gyrogonites in beds 0.5 to 2 meters thick. The limestone occurs in a sequence that begins with conglomerate grading up to low-angle, cross-bedded sandstone. Similar limestone can be found southeast toward Arches and west toward Castle Dale. Beds of oncoids in this interval are found only at the Woodside Anticline. The oncoids coat clasts of chert, sandstone, conglomerate, eroded dinosaur bones, and wood. In some horizons, the oncoids are in turn coated by ~30 cm thick calcrete that we interpret to represent ‘beachrock' precipitated along a shoreline. The thinness of the oncoid and limestone horizons, coupled with the limited lateral extent of 10s of meters suggest deposition in ephemeral lakes. Carbon and oxygen isotope ratios of oncoids and associated carbonate define a diagenetic trend from presumably less altered material with high ratios and to more altered materials with lower ratios. Using data from less altered material and assuming a range of reasonable (lake) temperatures, the oxygen isotope ratio of lake water associated with oncoid growth is estimated to have been ~ -9 per mil. Using data from more altered material and assuming a range of reasonable ground temperatures, the oxygen isotope ratio of precipitation-derived ground water is estimated to have been ~ -11 per mil. The inferred oxygen isotope difference between lake water and local ground water is consistent with the isotopic enrichment of lake water via preferential loss of ¹⁶O during evaporation from these small water bodies.