Quantitative mineralogy and distributions of minerals of the Green River Formation, Piceance Creek Basin, western Colorado

Abstract

Over thirty-five minerals have been identified in the Green River formation of the Piceance Creek Basin, using quantitative X-Ray Diffraction (XRD) bulk analysis. Fifteen major phases occur at over 5 weight per cent average in 117 samples spanning the thickness of the formation. Integrated data from cores, outcrops, and the USGS represent two lacustrine depositional environments: basin margin shallow water and basin center deep water. Results are presented in a stratigraphic framework of six lake stages. Mineral distributions support a permanently stratified saline lake model characterized by a chemocline that separates a less saline upper water layer from a more saline, alkaline, and reducing lower water layer. Lateral and vertical variations in salinity, alkalinity, redox conditions, silica and CO2 activities, and depths of the water column and chemocline are proposed to have permitted localized differences in precipitation, preservation, alteration, and formation of authigenic phases across the meromictic lake. Original detrital mineral components contributed to the basin included mainly quartz, clay minerals and feldspars. Reversal of weathering reactions during periods of extreme water chemistry converted clay minerals to authigenic feldspars and other authigenic phases. Distinct assemblages of authigenic minerals differ between the basin center and basin margin locations. Basin center samples are enriched in buddingtonite ((NH4)AlSi3O8[middle dot]0.5H2O), and the saline minerals nahcolite (NaHCO3), dawsonite (NaAl(CO3)(OH2)), and halite (NaCl). Only trace quantities of saline minerals occur in the basin margin samples, instead, high quantities of the zeolite, analcime (NaAlSi2O6[middle dot]H2O), are found. Basin center samples are clay-poor (10 weight per cent ave), and organic matter-rich (12 weight per cent ave). Basin margin samples are clay rich (24 weight per cent ave) and relatively organic matter poor (7 weight per cent ave). Carbonate minerals represent a large proportion of GRF rocks, and are especially iron rich. Calcite and aragonite form only 12 weight per cent ave of total carbonates in the basin center. 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 richest in 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 and represents the most saline period of lake history. The upper transition zone is characterized by a marked reduction in the occurrence of dawsonite, another increase in feldspar, a second decrease in quartz, decrease in buddingtonite, and increase in clay minerals. The major phases in the upper mineral unit are feldspars and ferrodolomite, with increased contributions by analcime and calcite. Quantitative and qualitative relationships between minerals are suggestive of reactions responsible for authigenic mineral formation and differences in distributions due to local depositional conditions. Occurrences of nahcolite and analcime are mutually exclusive, pointing to distinct conditions of formation controlled by alkalinity, salinity and silica activity. Quartz and dawsonite exhibit a positive quantitative relationship in saline lake stages, resulting from a decrease in silica activity, and increase in salinity and alkalinity. Buddingtonite and ferrodolomite both show qualitative correlations with indicators of redox conditions. Sample rock types in the basin margin include fine sandstone, siltstone, mudstone and marlstone. Rock types represented by samples in the basin center include siltstone, mudstone, marlstone, and saline subaqueous evaporite. The majority of siltstone and sandstone samples are arkosic, and most mudstone samples are feldspathic in both basin locations. A rock type classification based on grain size and relative quantities of 4 main mineral groups (saline minerals, Ca-Mg-Fe carbonates, framework silicates, and sheet silicates) reveals that dawsonite and buddingtonite quantities are not similarly distributed in the same rock types as other minerals of the same group. Sample photos, quantitative XRD and inorganic and organic chemistry results in spreadsheet form, raw XRD patterns, additional analyses such as rock types and cation exchange capacity, and enlarged figures are available in supplemental files.