SYNDEPOSITIONAL DOMING AND DEVELOPMENT OF A LARGE HYDROTHERMAL SPRING DEPOSIT IN THE GREEN RIVER FORMATION, WILDHORSE DRAW, WYOMING

A large ~50 m-high siliceous travertine spring mound within the Wilkins Peak Member of the Green River Formation (GRF) at Wildhorse Draw, southwestern Wyoming, appears to be the nucleus for multiple horizons of spring mounds within the upper Wilkins Peak and lower Laney members. Growth of the large mound was contemporaneous with movement on Henry’s Fork Thrust (HFT) in the southern Bridger Basin. Strata adjacent to the mound deviate from the regional stratal orientation and dip off the mound itself, suggesting that the mound underwent doming and deformation during its growth. An extraformational conglomerate with Paleozoic carbonates and sandstone clasts provides evidence of alluvial fan deposition adjacent to and within the spring deposit.

Macroscale features of the mound indicate multiple stages of subaerial growth, evidenced by draping of carbonate and siliceous bands throughout the mound and desiccation cracks in adjacent strata. Adjacent deposits indicate interaction with alluvial fan channels and spring pools around the mound. Internally the mound is dominated by porous calcareous tufa, massive finely crystalline dolomite, calcareous and siliceous sinter banding, and calcite and siliceous fracture fill. Opal, iron oxides, reworked sinter and travertine fragments, and plant, microbial mat and ostracode remains are also present.

Abundant silica, euhedral dolomite, and sinter and travertine textures indicate that fluids may have had high temperatures. The draping of spring deposits over deformed lake beds adjacent to the mound, inclusions of travertine hash within adjacent sediments, and plant stem fragments and root traces within the mound indicate that the spring mound was subaerial throughout its growth within the Wilkins Peak Member. The spring fluids that led to the deposition of the mound appear to have also discharged into the lower Laney Member following deposition of the mound, as evidenced by paludal deposits with chert horizons and travertine-containing beds intercalated with lacustrine shales.