Abstract

The Parachute Creek Member of the Green River Formation in the southern Piceance Creek basin displays four major lacustrine and marginal-lacustrine facies: deltaic-interdeltaic mudflat, carbonate mudflat, proximal open lacustrine, and distal open lacustrine. These facies are strongly gradational across the basin and their definition is not clearly evident from study of the physical characteristics of each facies. Additional evidence from whole-rock mineralogy (defined by X-ray diffraction) and sulfur isotopes, however, help to clarify each facies.

The deltaic-interdeltaic mudflat facies contains channel-form cross-stratified sandstone intercalated with pyritic mudrock and marlstone. Detrital quartz, K-feldspar, Na-plagioclase, and rock fragments characterize the sandstone. The mudrock and marlstone contain both calcite and dolomite plus detrital silicate minerals; δ34S values of pyrite from this facies range from 2 to 26 per mill and average 12 per mill. The carbonate mudflat facies is dominated by dolomitic algal stromatolite, oolite, pisolite, and interclastic carbonate rock, and interbedded green to gray pyritic marlstone. The marlstone contains both calcite and dolomite and considerable authigenic analcime; δ34S values range from 5 to 40 per mill and average 16 per mill.

Proximal open-lacustrine deposition produced well-stratified marlstone and lean oil shale, and rare algal stromatolite. Detrital silicate minerals become less abundant and are volumetrically replaced by dolomite, Fe-dolomite, excess-Ca ankerite, analcime, and rare dawsonite. The δ34S values of pyrite from this facies range from 22 to 54 per mill and average about 30 per mill. The distal open-lacustrine facies is represented by richer grades of oil shale (greater than 15 gal/ton) and saline minerals (nahcolite and halite). The oil shale contains abundant Ca-Fe-Mg-carbonate phases, authigenic quartz, K-feldspar, Na-plagioclase, and dawsonite. Analcime is rare. The δ34S values of pyrite and marcasite range from 18 to 66 per mill and average 35 per mill.

Overall, the Parachute Creek Member shows that detrital silicate minerals and calcite are concentrated in the marginal-lacustrine facies but authigenic silicate minerals, complex Mg-Ca-Fe-carbonate minerals, and saline minerals are concentrated in the open-lacustrine facies. Sulfur-isotope data show that iron sulfide minerals become progressively enriched in 34S toward the open-lacustrine environment of deposition.

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