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