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 wt% 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 CO 2 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 ((NH 4 )AlSi 3 O 8 *0.5H 2 O), and the saline minerals nahcolite (NaHCO 3 ), dawsonite (NaAl(CO 3 )(OH 2 )), and halite (NaCl).Only trace quantities of saline minerals occur in the basin margin samples, instead, high quantities of the zeolite, analcime (NaAlSi 2 O 6 *H 2 O), are found.Basin center samples are claypoor (10 wt% ave), and organic matter-rich (12 wt% ave).Basin margin samples are clay-rich (24 wt% ave) and relatively organic matter-poor (7 wt% ave).Carbonate minerals represent a large proportion of GRF rocks, and are especially iron rich.Calcite and aragonite form only 12 wt% 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 xiii LIST OF ABBREVIATIONS Carb .... Divalent Carbonate Minerals CEC Cation Exchange Capacity (analysis for clay minerals) CO-1 Colorado Core Hole No. 1Section used for comparison data Corg .Organic Carbon (as opposed to carbon in carbonate minerals) COSTAR..Center for Oil Shale Technology and Research CR-2 .... Oil Shale core used for comparison from Dean, Pitman and Harrach, 1981 DP Douglas Pass Outcrop Sections EDS .. Energy Dispersive X-Ray Spectroscopy (SEM-EDS) Fsp Feldspar Ft .. Feet GRF .Green River Formation ICP-MS Inductively Coupled Plasma -Mass Spectrometer IR .Infrared Spectroscopy I/S .Illite / Smectite Clay Minerals
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