Paragenesis of Lacustrine Stromatolites of the Upper Wilkins Peak Member (Lake Gosiute), Green River Formation, Wyoming

Stromatolites from the upper Wilkins Peak Member were investigated to determine their paragenesis and the consequences of paragenesis toward interpretations about lake chemistry. Two specific stromatolite 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 margin towards basin center. XRD, SEM/EDS, and petrographic analysis show differences in diagenesis above and below the layered tuff.

Stromatolites below the tuff bed contain some primary calcite with diagenetic dolomite and abundant secondary silicification. 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 stromatolites below the tuff is: calcite, secondary dolomite, quartz, pore-filling dolomite, late silicification, late euhedral dolomite and calcite replacing dolomite. Stromatolites 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 stromatolites 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 stromatolites 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 stromatolite beds along the margin to basin transect and c) a complex diagenetic history similar to matrix lithologies.

In conclusion, significant diagenesis has altered the stromatolite beds. Primary calcite remains suggesting initial freshwater lake chemistry. Differences in the mineralogic composition of ostracods within the stromatolites verses the matrix suggest significant syndepositional diagenesis. The stromatolite paragenesis is complex making accurate interpretations about lake chemistry difficult; however the differences in paragenesis (in particular, silica diagenesis) above and below the tuff bed suggest that diagenesis was syndepositional or early post burial.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California