A PRESERVED LATE CRETACEOUS CRYPTOBIOTIC SOIL IN GRAND STAIRCASE-ESCALANTE NATIONAL MONUMENT: PALEOCLIMATE IMPLICATIONS

Modern cryptobiotic soils develop under semiarid to arid conditions and are characterized by a diverse community of microorganisms. Outcrop of the Late Cretaceous capping sandstone member of the Wahweap Formation in Grand Staircase-Escalante National Monument, Utah, contains what is best interpreted as a preserved cryptobiotic soil. This paper describes this rarely reported soil type from the rock record and discusses the paleoclimatic implications.

The capping sandstone of the Wahweap is separated from the overlying Kaiparowits Formation by a change in lithology from quartz arenite to lithic arenite, and a change in paleocurrent direction from southeast to northeast. The cryptobiotic soil is developed about two meters below this contact and covers an area about 20 meters by 5 meters. The soil features, in profile varies from 10 to 15 cm thick. The surface morphology is pedicillated with mm- to cm-scale irregular prismatic columnals, and is strikingly similar to that of modern microbiotic soils. Small deep tracks preserved within the underlying layer demonstrate that the soil was easily penetrated. In thin section, the quartz arenite is poorly sorted, displaying a range in grain size from silt to medium-grain sand in contrast to the well sorted sandstones elsewhere in the capping sandstone. Porosity is highly variable with high porosity zones attributable to bioturbation. No direct evidence is preserved of the former microbial community, except for a mottled color pattern in the thin section sans magnification. In close association with the cryptobiotic soil are medium-scale low-angle eolian cross-beds. The cryptobiotic soil was possibly preserved by eolian burial. This combined evidence strongly supports a semi-arid to arid climate in the uppermost capping sandstone member of the Wahweap Formation.