Geochemistry of Lacustrine Microbialite of the Eocene Green River Formation

M'bark Baddouh, Brian L. Beard, Alan R. Carroll, and Clark M. Johnson
University of Wisconsin-Madison

Microbialite of the Wilkins Peak member of the GRF forms mound-like structures of about 6-12m thick each, separated by carbonate grainstone. They are exposed in the Little Mesa region in the northwest part of the Green River Basin. Previous studies have reported that the mounds are associated with calcified caddisfly cases, encrusted logs, and ostracods. We report C, O, and Sr isotope data for 27 samples from the Little Mesa, Wyoming. The ? 13C values range from -1.77 to 3.37 with an average 1.58 ‰ VPDB (n=27), a typical range for lacustrine rocks, and ?18O values range from -7.20 to -3.10 with an average -4.80 ‰ VPDB (n=27), consistent with moderately high lake water evaporation rates, but not previous interpretations that snowmelt was a major water source. Stable isotope data also show strong positive correlation (R2=0.85), which related to lake morphology, climate and geographic setting. In addition, covariant trend of the stable isotopes confirms hydrological closure of the lake and presence of long-term stability of the water source composition and hydrological characteristics (Talbot, 1990). The 87Sr/86Sr ratios range from 0.71022 to 0.71140 with an average of 0.71102 (n=27). These 87Sr/86Sr ratios represent the lowest yet measured for the deposits of Eocene Lake Gosiute in Wyoming, but are similar to those of the Paleocene-Eocene Flagstaff formation in central Utah (Gierlowski-Kordesch et al., 2008). The Sr isotopes also show strong positive correlation with both ?13C (R2=0.83), and ?18O (R2=0.74). These data shows that water source evolved from less radiogenic to more radiogenic values or probably a mixing between two end-members water sources. In general, these observations are consistent with stratigraphic evidence that GRF microbialite was deposited during a major transgression of Lake Gosiute, and show that lake level rise was caused by increased import of less radiogenic waters.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013