CARBONATE SPRING MOUNDS AND INTERDUNE LAKES IN THE NAVAJO SANDSTONE (JURASSIC, WESTERN US)—RESULTS OF STABLE ISOTOPE ANALYSES

Numerous carbonate spring mounds in the Navajo Sandstone have been previously reported, and we are working to explain the paleohydrology of the system that led to their formation. Here we report the results of stable isotope analyses of the carbonate mounds and their adjacent lake-carbonate beds. Individual spring mounds have different but overlapping isotopic signatures. Carbonate beds that flank the mounds are interpreted as interdune lakes fed either by the spring water, directly by groundwater, or both, but influx of surface water is not ruled out. Both mound and lake carbonates are almost entirely calcite; dolomite is present in significant amounts (40%) in only one sample. Overall, δ18O values are highly variable, ranging from -20.6‰ to +13.4‰. δ13C values are less variable, ranging from -6.7‰ to -1.3‰. These ranges are marked by outliers; 85% of the data are between 0‰ and -14‰ δ18O and -1.3‰ and -4.5‰ δ13C. On average, both the δ13C and δ18O isotopic signatures of the lake beds are more positive (δ13C -2.6‰, δ18O -5.9‰) than that of the mounds (δ13C -2.8‰, δ18O -6.9‰), possibly reflecting more biotic activity and higher evaporation rates in the lakes compared with the mounds. A general geographic trend is observed, from more negative values in both δ13C and δ18O in the east to more positive values in the west. Our hypothesis is that the springs were fed by groundwater originating in the Uncompahgre uplift east of the current pinch-out of the Navajo Sandstone. The general trend in stable isotope values could reflect evolution of that groundwater as it flowed from east to west.