--> Water Chemistry Evolution of the Green River Formation, Piceance Basin: A Signal Extracted by Mineral Assemblage in Different Lake Stages

AAPG ACE 2018

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Water Chemistry Evolution of the Green River Formation, Piceance Basin: A Signal Extracted by Mineral Assemblage in Different Lake Stages

Abstract

The Green River Formation of Colorado was deposited in a large lake, Lake Uinta, during Eocene time. Throughout Lake Uinta’s existence, the mineral distributions showed temporal and spatial variations across the Piceance Basin, where the basin center was enriched in Buddingtonite ((NH4)AlSi3O8 0.5H2O), and the saline minerals, including Nahcolite (NaHCO3), Dawsonite (NaAl(CO3)(OH)2), and even halite (NaCl); the basin margin was characterized by large amount of analcime (NaAlSi2O6 H2O), and clay minerals.

Major changes in relative mineral proportions and assemblages occur in the basin center at two transition zones, which divide the stratigraphic column into three distinct mineral units. The lower mineral unit is represented by quartz, clay minerals, ferrodolomite, and calcite. The lower transition zone is characterized by decreases in clay mineral and quartz quantities, and increases in dawsonite, feldspars, and buddingtonite. The middle mineral unit is mainly composed of nahcolite, dawsonite, feldspars and ferrodolomite, representing the most saline period of lake stage. The upper transition zone is characterized by a sharp reduction in the occurrence of dawsonite, a second decrease in quartz and buddingtonite, but with another increase in feldspar and clay minerals. The major phases in the upper mineral unit are feldspars and ferrodolomite, with increased contributions by analcime and calcite input.

The mineral assemblages in the basin were mainly controlled by water compositions of the lake, which was especially reflected in the aspects of alkalinity, salinity, silica activity and absorbed CO2 concentration. We tried to depict the chemical evolution of the ancient lake by using Geochemistry’s Workbench to stimulate the mineral stability in the system K+-Na+-H+-SiO2 in the presence of CO2. We infer that the lake water ranged from fresh to brackish in the stability field of illite, quartz and calcite, transitioned from saline to hypersaline condition in the field of dawsonite, nahcolite, feldspar and ferro-dolomite. Halite was primarily formed by evaporative concentration. A relatively quantitative water chemistry condition can be defined during the evolutionary history of the paleolake.