“ALGAL HEADS” AND TRAVERTINE ON THE SOUTHERN LAKE MARGIN OF THE EOCENE GREEN RIVER FORMATION, WYOMING AND COLORADO

Hundreds of ~2 m-high stromatolitic travertine mounds, known as “algal heads”, characterize multiple horizons on the southern lake margin of the Eocene Green River Formation during the expansion of the oil shale-rich Laney Member (~50 Ma). Two localities on thrust faults were investigated north of the Uinta Uplift at: (1) Sparks Ranch Thrust (SRT), Sand Wash Basin, CO; and (2) Henry’s Fork Thrust (HFT), Bridger Basin, WY. Clastic mudstones and fine-grained quartz sandstones surrounding the horizon at SRT are consistent with subaerial growth of the mounds prior to deposition of lacustrine shales. Fine-grained quartz sandstones of alluvial fan toes, through which spring fluids contributing to mound growth may have flowed, underlie the mound horizons at HFT. Larger mounds (~20 m high) found within the overlying lacustrine Laney Member are aligned along the HFT, and preserve fracture systems with calcite-dominated travertine.

External morphologies of the mounds indicate domal stromatolitic build-up and draping by carbonate and siliceous bands, some of which are interpreted to have formed via vents opening at the tops of the mounds as hydrothermal “cones”. Cylindrical openings in the mounds are comparable to fumeroles and vents. Massive porous carbonate and fenestral laminae typically form the cores of the mounds, and are surrounded by bands of sinter-like flat laminae, stromatolitic laminae, and/or shrubby dendrites. Internal features include aragonite needle splays, calcite, euhedral and saddle dolomite, and microscale and macroscale porosity filled with opal, chert, chalcedony, and blocky and euhedral megaquartz.

The broadly linear distribution of the mounds is associated with active thrust faults and the transition from subaerial to lacustrine facies. Variability in the textures and geometry suggests that abiotic precipitation from spring fluids, in addition to stromatolite growth, contributed to the development of the travertine “algal heads”. Aragonite needles and dolomite indicate that waters may have been high in CO₂ and Mg²⁺ and/or that CO₂ degassed rapidly for precipitation of aragonite. The presence of syndepositional silica precipitates, along with aragonite, may also signify high temperatures of the fluids.