FLUID INCLUSION MICROTHERMOMETRY FROM HALITE IN THE EOCENE GREEN RIVER FORMATION, PICEANCE CREEK BASIN, COLORADO, USA: EVIDENCE FOR A PERENNIAL STRATIFIED SALINE LAKE

The Eocene Green River Formation Piceance Creek Basin (PCB), Colorado is characterized by thick deposits of oil-shale and the sodium carbonate mineral nahcolite (NaHCO₃) interbedded with halite (NaCl). Depositional sequences of nahcolite and halite can be correlated across the (PCB). Saline units are important paleoclimate indicators because the formation of evaporites is closely linked to regional climate conditions. Primary bottom-growth halite crusts (bottom waters) and air-water interface halite cumulates (surface waters) were identified petrographically. Primary fluid inclusions in halite occur as linear 3-D arrays of single-phase (aqueous) inclusions trapped at the time of deposition. Microthermometry of >1000 fluid inclusions was done on a Linkam THMSG 600 stage; calibrated with pure H₂0 for an accuracy of ± 0.1^◦C and reproducibility of ±0.5^◦C. Weighted average homogenization temperatures of fluid inclusions in halite crystals precipitated from bottom waters are constant throughout the saline sequence, 21^◦C. However, cumulate halite fluid inclusions show an increase in the weighted average temperature from the Lower (23.6^◦C) to 26.5^◦C in the Middle and a decrease in the Upper to 23.1^◦C evaporite units of the PCB. These data suggest that average surface water temperatures fluctuated through time but that average bottom water temperatures remained constant. This further supports the hypothesis of a stratified perennial lake.