Diagenesis Related to the Stratigraphic Sequences in Tensleep Sandstones, Wyoming
University of Wyoming, Laramie, WY
Most important early diagenetic events in the Upper Tensleep Sandstone include mechanical compaction and cementation by dolomite and anhydrite (or gypsum). As the cementation, compaction has dramatically reduced both porosity and permeability in sandstones rich in detrital dolomite grains. Distribution of the detrital dolomite grains and cements are closely related to stratigraphic sequences in these repeated eolian-shallow marine deposits. Each eolian-shallow marine sequence is composed of a lowstand systems tract, a transgressive systems tract, and a highstand systems tract. During the lowstand of sea level, ground water table tends to gradually drop because the eolian sand accumulation moves landward with the shoreline regression. Due to reduction of oceanic water influence, the detrital dolomite grains will become less and less in the eolian sandstones and the dolomite and anhydrite cements precipitated from the seawater-dominated interstitial fluids will also decease. During the transgression period, the ground water table rapidly rises, and the oceanic water intensively floods and penetrates the eolian sands. Under evaporative conditions, formation of the dolomite grains and precipitation of the dolomite and anhydrite cements will potentially increase. The highstand systems tract carbonates were either deposited in the evaporative environments or modified by the hypersaline fluids, causing the original calcite carbonate minerals were dolomitized. Because the carbonates were subaerially exposed during the lowstand of sea level, dissolution and dedolomitization features are very common along bedding surfaces. Combined the stratigraphic sequence and diagenetic modification, the trend of porosity and permeability distribution in the Tensleep deposits can be predicted.