CONSTRUCTION OF A SPRING-LAKE SYSTEM IN THE JURASSIC NAVAJO SANDSTONE, UTAH—EVIDENCE FROM STABLE ISOTOPES OF OXYGEN AND CARBON

A carbonate spring-lake system in the Lower Jurassic Navajo Sandstone near Moab, Utah, provides information on the chemistry of the spring waters, the influence of climate on the chemical evolution of the waters after emergence, and the relationship between carbonate facies and the chemistry of the waters. The overall lithology of the Navajo is sandstone, exhibiting both large- and small-scale crossbed sets typical of sand dunes, indicating an arid climate. The locality studied consists of four interdune carbonate deposits interbedded with sandstone. The upper two interdune deposits have tufa mounds interpreted to have been formed by freshwater, ambient-temperature artesian springs. The uppermost interdune deposit and a tufa mound associated with it were the targets of this study because the stratigraphy is well exposed.

In the tufa mound carbonate, δ¹⁸O_VSMOW is ‑9.4‰ to -4.7‰ ( x̄ = -7.0‰) and δ¹³C_VPDB -5.2‰ to -2.3‰ (x̄ = -2.4‰); in the lacustrine carbonate beds, δ¹⁸O_VSMOW is -9.3‰ to -5.2‰ (x̄ = -7.5‰) and δ¹³C_VPDB -1.4‰ to -4.0‰ (x̄ = -3.4‰). As would be expected in the Navajo desert environment, the lacustrine carbonate is more enriched in ¹⁸O than the tufa mound carbonate as a result of evaporation of the emergent waters as they flowed from the spring into the adjacent lake. The lacustrine carbonate is also more enriched in ¹³C than the tufa mound carbonate. We attribute this enrichment to a combination of evaporative effects, mixing with atmospheric CO₂, and biological activity during and after emergence of the spring waters.

Biological activity can be assessed through an analysis of the carbonate facies in the spring mound. The tufa mound is primarily thrombolite. The lacustrine deposits are predominantly peloidal or laminated. The δ¹³C values for the peloidal facies are intermediate between those of the tufa and those of the laminated facies. Previous workers have argued that peloidal facies may represent slightly higher-energy environments, suggesting that microbial activity could be disrupted; this would explain the intermediate δ¹³C signatures.