LAKE-MARGIN CARBONATE δ18O, δ13C, AND 87SR/86SR, NORTHERN BRIDGER BASIN EOCENE GREEN RIVER FORMATION, WYOMING

Positive covariance of δ¹⁸O and δ¹³C in ancient and modern lacustrine carbonate facies has commonly been interpreted to reflect variable residence time of water in endorheic (underfilled) lake basins. We test this hypothesis by integrating δ¹⁸O and δ¹³C values with ⁸⁷Sr/⁸⁶Sr ratios from Eocene lake-marginal carbonate facies preserved at Little Mesa. These deposits mark the maximum expansion of a long-lived hypersaline lake. Carbonate facies comprise a complex mosaic of grainstone to boundstone (microbialite mound) fabrics, with widely varying proportions of calcite versus dolomite. δ¹³C ranges from -1.77 to 3.37 ‰ VPDB, and δ¹⁸O ranges from -7.20 to -3.10 ‰ VPDB (n=27). δ¹⁸O and δ¹³C are strongly positively correlated (r²=0.85). Higher δ¹⁸O and δ¹³C values generally occur in more calcitic samples, which is surprising in light of the expected greater fractionation of ¹⁸O into dolomite. This result also appears to be inconsistent with previous dolomitization models invoking evaporative concentration of mudflat pore waters.

 ⁸⁷Sr/⁸⁶Sr ratios are the lowest yet measured for the deposits of Eocene Lake Gosiute, ranging from 0.71022 to 0.71140, consistent with previous studies showing that lakewater became less radiogenic as lake levels rose. ⁸⁷Sr/⁸⁶Sr ratios are strongly positively correlated with both δ¹³C (r²=0.83), and δ¹⁸O (r²=0.74). Because meteoric processes do not significantly fractionate ⁸⁷Sr/⁸⁶Sr, this suggests that the observed covariance between δ¹⁸O and δ¹³C may reflect varying relative contributions of water from two geochemically distinct sources, rather than evaporative distillation. Baddouh et al. (2016) proposed that lake highstands reflect increased drainage from thick, Sr-rich marine carbonate strata located west of the basin. In contrast, more radiogenic waters drained from Precambrian-cored uplifts located to the north, south, and east. ⁸⁷Sr/⁸⁶Sr ratios in the Little Mesa carbonates are also weakly correlated with carbonate mineralogy, with lower ratios associated with more dolomitic samples. This could indicate that dolomitization was preferentially associated with elevated Mg/Ca ratios in waters draining from the west. Dolomitization may also have been influenced by the ecology of the mounds themselves, via organic matter mediation, or by burial diagenesis.