Paragenesis of Lacustrine Microbialites in the Upper Wilkins Peak Member, Green River Formation, Wyoming

Microbialites from the upper Wilkins Peak Member were investigated to determine their paragenesis and to help interpret lake chemistry. Two specific microbialite beds were analyzed that are associated with the “layered tuff”. Samples were collected from correlated sections along a NW to SE, 24 km line of section from lake margin towards lake center. XRD, SEM/EDS, and petrographic analysis show differences in diagenesis above and below the layered tuff. Microbialites below the tuff bed contain some primary calcite with diagenetic dolomite and abundant secondary silicification. Later, dolomite replaces both calcite and quartz. Pores are commonly filled with 20 μm euhedral dolomite cements and in some samples with 10 μm crystals of quartz replacing dolomite. The diagenetic sequence for microbialites below the tuff consists of: calcite, secondary dolomite, quartz, pore-filling dolomite, late silicification, late euhedral dolomite and calcite replacing dolomite. Microbialites above the tuff bed have approximately equal amounts of calcite and dolomite with no significant silicification. Pores, in general, are partially filled with euhedral dolomite or occasional euhedral calcite. Ostracods embedded in the microbialites are replaced by dolomite with secondary calcite cement coating the dolomitized shell. However, the associated matrix contains unaltered ostracods (original calcite). The order of diagenesis for microbialites above the tuff is: calcite, secondary dolomite, rare silicification, pore-filling dolomite, and calcite replacing pore-filling dolomite. Observed diagenetic relationships show: a) characteristic differences in diagenesis above and below the layered tuff bed, b) no significant lateral differences in diagenesis within individual microbialite beds along the margin to basin transect and c) a complex diagenetic history similar to the matrix diagenetic history. Differences in mineralogy and paragenesis of stromatolite and matrix in units below and above the layered tuff bed indicate different stages of lake and pore water chemical variations. These stages are: 1) Microbialite formation in a freshwater lake; 2) Dolomitization of microbialites via evaporative pumping and capillary draw; 3) Early post-burial microbialite silicification in the unit below the layered tuff; 4) High degree of microbialite cementation post-silicification on unit below the layered tuff; 5) Post-silicification dolomitization and dedolomitization as a result of the freshening of pore/lake water after tuff burial and the initiation of microbialite growth in the unit above the tuff. Diagenetic changes in the unit above the layered tuff indicate stages 1, 3 and 5