Sequence Stratigraphy in Mixed Lake Systems, Organic Richness and Climate - Green River Formation, Lake Uinta, Part III, Mineralogy and Geochemistry
Boak, Jeremy; Poole, Sheven; Sarg, Rick; Tanavsuu-Milkeviciene, Kati
Quantitative X-Ray Diffraction performed on 82 samples spanning the Green River Fm. and representing basin margin (outcrop sections in Douglas Pass), and basin center (John Savage 24-1 and Shell 23X-2 core) depositional environments reveals mineralogic relationships to the sequence stratigraphic framework of lake stages, from early fresh-brackish/mesosaline to later hypersaline conditions. Most basin center samples are mudstone and marlstone composed of quartz, feldspar, and Fe-dolomite with varying amounts of clay minerals and calcite. Basin margin samples vary from mudstone and marlstone to siltstone and fine-grained sandstone composed of quartz, feldspar, clay minerals, dolomite and calcite. Buddingtonite, nahcolite (NaHCO3), dawsonite (NaAl (CO3) (OH2)), and halite (NaCl) are common in the basin center, but sparse to absent in the margin, whereas analcime (NaAlSi2O6*H2O) is abundant in the margin, and sparse in the center. Halite and nahcolite reflect saturation in the deep lake center. Differences between basin margin and center reflect chemical stratifica¬tion of the lake through its history. Mineralogic and geochemical features suggest that the deeper part of the basin was dysoxic to anoxic and mesosaline from very early in its history.
Transitional changes in mineralogy especially in the basin center are related to important lake stage boundaries. These include: 1) sharp reduction in clay and increase in feldspar and dawsonite content in the lower part of Stage 2 (transitional lake), and 2) a fall in quartz and dawsonite and increase in feldspar in the middle of Stage 3 (highly fluctuating lake) in the basin center. Three mineralogic units are defined in the basin center, based on the two transitions, and appear to have counterparts in the basin margin. These changes are attributed to shifts in diagenetic mineral stability controlled by variations in salinity as well as silica and CO2 activity in lake and interstitial waters. The first transition reflects the change from mesosaline to hypersaline conditions as indicated by the appearance of first dawsonite, then nahcolite. The second transition appears to reflect an increase in silica activity, leading to diagenetic feldspar formation in place of dawsonite, even as nahcolite continued to form in the lake and interstitial water. The silicate assemblage resembles more nearly the mineralogy of the basin margin, possibly reflecting some reduction in salinity before and during Stage 4 (rising lake).
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013