Stratigraphic Variations and Relationships of the Mineralogy of the Green River Formation, Piceance Basin, Western Colorado
Jeremy Boak, Sheven Poole, Kati Tanavsuu-Milkeviciene, Jufang Feng, and J. Frederick Sarg
Quantitative X-Ray Diffraction was performed on 82 samples spanning the Green River Formation, representing basin margin (outcrop sections in Douglas Pass), and basin center (John Savage 24-1 and Shell 23X-2 core) depositional environments. Bulk mineralogy is integrated into a sequence stratigraphic framework of lake stages, from early brackish to later saline conditions, incorporating variations in organic richness. 28 Douglas Pass samples consist of argillaceous and siliceous mudstone, marlstone, and siltstone, composed of feldspar (ave. 23 wt%), clay (22%), quartz (18%), carbonates (18%), analcime (9%), and organic matter (7%). 39 Savage well samples cover the upper 2/3 of the basin center section, consisting of siliceous mudstone, marlstone and siltstone composed of feldspar (34%), carbonates (23%), quartz (12%), dawsonite (7%), nahcolite (7%), and organic matter (13%). Feldspars may be largely secondary, as buddingtonite ((NH4)AlSi3O8*0.5H2O) comprises a significant fraction. 15 Shell well samples cover the lower 1/2 of the basin center section. The upper 8 samples overlap the lower Savage well section, and show similar mineralogy. The lower 7 Shell samples consist of argillaceous and siliceous mudstone, marlstone and siltstone composed of carbonates (32%), clay (24%), quartz (19%), feldspar (10%), organic matter (9%) and only 3% dawsonite/nahcolite. Overall, basin center samples are clay-poor (6%), and organic matter-rich (12%), whereas, basin margin samples are clay-rich (22%), with more diverse clay types, and less organic matter-rich (7%). Carbonates are dominantly dolomite/ankerite, with only 2% calcite in the upper basin center, 6% in the lower part, and 7% in the margin. 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 (NaAlSi2O3*H2O) is abundant in the margin (max 37%), and sparse in the center. Changes in mineralogy occur at important lake stage boundaries, including a sharp reduction in clay and increase in feldspar content in Stage 2 (transitional lake), and a fall in quartz and dawsonite at the top of Stage 3 (rapidly fluctuating lake) in the basin center. These changes are attributed to reactions controlled by variations in salinity and silica concentration. Halite and nahcolite reflect saturation in the deep lake center. Differences between basin margin and center reflect stratification of the lake through its history.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012