TUFFACEOUS MICROBIAL MAT MARKER BED, GREEN RIVER FORMATION, WILDHORSE DRAW, WYOMING, CONSTRAINS TIMING OF SILICIFICATION IN SPRING MOUND HORIZON TO ~50 MA

A 5.0 cm-thick tuffaceous, non-stromatolitic microbial mat marker bed in the Green River Formation at Wildhorse Draw, southwest Wyoming, overlies a horizon with ~130 spring mounds deposited on Henry’s Fork Thrust below lacustrine strata of the Laney Member. The horizon of small (~2 m high) travertine spring mounds was deposited in the Wilkins Peak. Some mounds include opaline sinter and euhedral quartz in fracture networks that provide evidence of hydrothermal fluid input. The silica fractures in the mounds occur at the bases of the spring mounds and only below the marker bed, which constrains the timing of silicification in this horizon of spring deposits to the uppermost Wilkins Peak Member at ~50 Ma.

The composition of the marker bed includes lapilli-tuff, sanidine, albite, quartz, calcite cement, euhedral dolomite, pyrite, and iron oxides, with a microcrystalline upper crust of calcite and silica. Three main horizons comprise the bed. (1) At the base is a 0.3 cm-thick microbial mat layer, which is moderately oxidized and well cemented, displays oscillation fractures attributed to loading. (2) The next horizon is a volcanic tuff deposited in shallow water with low turbulence. A distinct dark band in the middle of the bed is attributed to a concentration of iron-oxides and pyrite. (3) The top horizon is interpreted as a microbial mat, and is characterized by a 0.4 cm-thick layer of irregular, meandering shrinkage crack networks and fragmented pieces of the mat. Some of the cracks preserved in the top crust appear to have been initiated by insect mat-feeding traces that created planes of weakness.

The marker bed supports the hypothesis that silica-rich fluids in the spring mound horizon impacted the uppermost Wilkins Peak at Wildhorse Draw. The fluids, which may have been hydrothermal, appear to have continued to influence lake conditions, as evidenced by the non-stromatolitic microbial mat on the tuffaceous marker bed and its cementation by calcite, dolomite, and silica.