14 ORIGIN OF PRIMARY AND DIAGENETIC CARBONATES, GREEN RIVER FORMATION, COLORADO AND UTAH

compositions of approximately -6 %c. Relative to lake in-
flow, these waters had evolved isotopically as much as 6 %o.
Presumably, the water mass became isotopically even heavi-
er during periods of extreme evaporation that ended in sa-
line-mineral deposition. At the same time evaporite
minerals were forming in the Piceance Basin, lake waters
with estimated !80 compositions of approximately —4 %o
were depositing oil shale in the Uinta Basin. These waters
also were isotopically modified by about 8 %c relative to the
inflow waters. Following the formation of the evapor-
ite-mineral facies in the Piceance Creek Basin, the lakes in
both basins increased in size and productivity. During this
period, precipitating waters in the Piceance Creek Basin are
estimated to have had '80 compositions of about -7 %o,
whereas, in the Uinta Basin, the water mass was slightly
heavier, about -5 %o. The 2 %e '8O enrichment in the Uinta
Basin lake is consistent with lake waters farther from the
source of inflow; these waters were thus affected by greater
evaporation. A similar conclusion was reached by Tuttle and
Goldhaber (1993) on the basis of sulfur isotope geochemis-
try of the formation.

ISOTOPIC TRENDS ON CARBONATE CURVES

Temporal trends on the isotope curves of carbonate
(dashed lines in figs. 4 and 5) document the evolution of the
carbon and oxygen cycles in the Green River lakes. Pertur-
bations superimposed on the trends record alterations in
these geochemical cycles over much shorter time periods.
Variations in the carbon and oxygen budget of Lake Uinta
probably were controlled by changes in climate and tectonic
conditions during the Eocene.

In the Piceance Creek Basin, decreasing 8!3C values of
carbonate (and nahcolite) from the bottom of the argilla-
ceous-shale facies to the top of the evaporite facies parallel a
slight increase in 8!80 of carbonate (fig. 4). In contrast, in-
creasing 5!3C of carbonate corresponds to slight decreasing
8!80 values of carbonate in the oil-shale facies. In the Uinta
Basin, increasing 8!3C of carbonate extends upward through
the oil-shale facies and is accompanied by no associated
long-term changes in 8!80 values (fig. 5).

Superimposed on the 8!3C trends in carbonate is a
rhythmic succession of small- to large-scale (30 to 100 m),
positive and negative excursions (figs. 4 and 5). The 5!80
trends, in contrast, display only minor fluctuations. The
8!3C excursions generally correlate with zones of organ-
ic-matter richness (oil yield) and with carbonate mineralogy
(see Grabowski and Pavear, 1985). On the basis of their da-
ta, positive excursions correspond to zones that have in-
creased oil yield and higher dolomite-to-calcite ratios, and
negative excursions correspond to zones that have decreased
oil yield and lower dolomite-to-calcite ratios. A positive
correlation between these three parameters in the wells on
which the present study is based is indistinct because of the
large sample spacing.

CARBON TRENDS

A combination of biologic processes in the lake waters
and in the sediments produced the evolutionary trends on the
carbon curves of carbonate. The observed !3C-enrichment
trends in the Uinta Basin are what would be expected in an
organic-productive lake closed to freshwater inflow. The in-
crease in 8!3C to heavier values initially was related to in-
tense photosynthetic activity through removal of
13C-depleted organic carbon; however, as productivity in-
creased with time, rates of organic-matter burial became
very high, which promoted bacterial methanogenesis in the
sediments and the formation of carbonate precipitates with
heavier 8!3C values. This same set of processes can explain
the carbonate trends in the oil-shale facies in the Piceance
Creek Basin.

As shown in figure 4, the trends in 8!3C versus 8!80 in
the evaporite facies (Piceance Creek Basin) diverge through
time. Thus, during the long period of evaporite-mineral dep-
osition, the carbon-isotope compositions of the carbon reser-
voir progressively decreased as the oxygen-isotope
compositions of the oxygen reservoir systematically in-
creased, which suggests that carbonate preci
lated to increased lake-water salinity caused by evaporation.
Lazar and Erez (1992) have shown that atmospheric invasion
of !2C-enriched CO, into saline, alkaline water causes a
large negative carbon-isotope fractionation (about —9 %c) be-
tween brine and atmospheric CO2 because of intense photo-
synthetic activity. The same photosynthetic effect has been
reported for a freshwater lake (Herczeg and Fairbanks,
1987). The possibility of !3C-depletion due to this kinetic
isotope fractionation in the Piceance Creek Basin is compat-
ible with the measured 8!3C values and with the !80 enrich-
ment in carbonate; however, it is inconsistent with the small
changes (—29 + 3%c) in the carbon-isotope compositions of
organic matter. In Eocene Lake Uinta, the partial pressure of
dissolved CO) was much greater than that of atmospheric
CO} because of the large bicarbonate reservoir in the water.
Consequently, the organic carbon fixed would have only
been depleted by a few per mil (Calder and Parker, 1973;
Herczeg and Fairbanks, 1987). On the basis of these
geochemical conditions, it is concluded __ that
brine-to-atmospheric CO exchange was not the dominant
process controlling carbonate precipitation in the Piceance
Creek Basin during deposition of the evaporite facies.

A more reasonable explanation of the carbonate
13C-depletion trends in the evaporite facies is that they
reflect the effects of increased respiration. Oxidation of
organic matter is a source of !2C-enriched CO. A high
input of isotopically light CO2 originating from organic
matter to the water mass by means of diffusion would have
shifted the composition of the carbon reservoir in the water
and the resulting carbonate precipitates to lighter values.
The increased effects of respiration can also account for the
8 %o decrease in nahcolite from the bottom to the top of the


ORIGIN OF PRIMARY AND DIAGENETIC CARBONATES, GREEN RIVER FORMATION, COLORADO AND UTAH

8

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CONTENTS

Abstract .
Introduction
Analytical Methods
Results
Carbonate Mineralogy
Stable Isotopes, Carbonate-Mineral Abundance, and Oil Yield
Distribution of Carbon and Oxygen Isotopes
Discussion ....
Primary Versus Diagenetic Carbonate .
Carbon Isotopes of Carbonate
Primary Carbonates ..
Diagenetic Carbonates
3C-depleted (Primary?) Carbonates

Oxygen Isotopes of Carbonate
Isotopic Trends on Carbonate Curves
Carbon Trends

Oxygen Trends
Conclusions
Acknowledgments
References Cited .

FIGURES
Generalized geologic map of east-central Utah and northwest Colorado 2
Generalized stratigraphy of the Eocene Green River Formation, Colorado and Utal 3
Ternary diagram of the range of CaCO3-MgCO3-FeCO3 compositions of carbonate in the Piceance Creek
and Uinta Basins... 4
Chemostratigraphic profiles of stable-isotope ratios, carbonate mineralogy, and organic-matter richness
in the USBM OIA well, Piceance Creek Basin .... see 7
Chemostratigraphic profiles of stable-isotope ratios, carbonate mineralogy, and organic-matter richness
in the USGS Coyote Wash | well, Uinta Basin... 8
8'3C and 8!80 plots showing degree of correlation of 5!3C in calcite and dolomite and 8!80 in
calcite and dolomite.. 9

Relationship of 5!3C versus 5!80 in primary and diagenetic carbonate minerals in the USBM OIA well,

Piceance Creek Basin ... 10
Relationship of 8!3C versus 5!80 in primary and diagenetic carbonate minerals in the USGS Coyote

Wash | well, Uinta Basin...

Isotopic compositions of primary and diagenetic carbonate minerals in Lake Bosumtwi, Ghana 12
Schematic hydrologic model summarizing temporal changes in bicarbonate !3C source, lake level,

and salinity in the Lake Uinta system...... 16

TABLES

Generalized lithofacies, depositional environments, and hydrologic states in the Green River Formation .... 3
Stable isotopes, carbonate mineralogy, and oil yield in the Green River Formation,

Piceance Creek Basin, Colorado 5
Stable isotopes, carbonate mineralogy, and oil yield in the Green River Formation, Uinta Basin, Utah. 6



ACKNOWLEDGMENTS 15

evaporite facies. Positive 5!3C values (as high as +20 %c) in
diagenetic nodules near the base of the facies clearly indicate
generation of methane and associated !3C-enriched CO} in
organic-rich sediments. It has been demonstrated that nah-
colite precipitates when COz pressure in water is at least 10
times that of atmosphere (see fig. 19 in Bradley and Eugster,
1969). Lower values (as low as —4 %c) in bedded nahcolite
at the top of the facies can be attributed to precipitation in
overlying lake waters where photosynthesis/respiration pro-
cesses were active.

The ratio of biogenic CO to organic matter in the
Green River lakes varied substantially over short time inter-
vals as recorded by the succession of broad positive and neg-
ative 8!3C excursions on the carbon trends in carbonate,
particularly in the Piceance Creek Basin. Positive excur-
sions reflect increases in organic-carbon burial and indicate
the influence of bacterial methanogenesis; thus, they gener-
ally do not preserve the original (primary) carbon trend. In
contrast, negative excursions reflect decreases in organ-
ic-carbon burial and represent the effects of photosynthe-
sis-respiration; hence, they record a predominantly primary
carbon trend. Rhythmically alternating carbon excursions
ultimately may provide an historic record of paleoclimate
change modified by local sedimentation effects.

OXYGEN TRENDS

Temporal trends on the 5!80 curves of carbonate record
the inflow-evaporation history of the Green River lakes and
the effects of long water residence time on the composition
of the oxygen reservoir in the water. In the Piceance Creek
Basin, slight increases in 5!80 parallel increased concentra-
tions of nahcolite, implying progressive evaporation and
shallowing of the Piceance Creek Basin lake through time.
The buildup of the heavy isotope of oxygen was small in
comparison to modern saline lakes, which suggests the lake
received minor inflow that mitigated the effects of extreme
evaporation. Long-term decreases in 5!80 of carbonate in
the oil-shale facies are consistent with expansion and deep-
ening of the lake. Near-vertical 5!80 trends in the Uinta Ba-
sin indicate a water body that tended toward long-term
hydrologic stability. A stable water body implies only minor
alterations in the ratio of freshwater inflow to evaporative
loss and, hence, also implies isotopic continuity of the water.

Positive and negative 5!80 excursions on the
oxygen-isotope curves record alterations in hydrologic bal-
ance; positive excursions correspond to increased lake-water
evaporation and negative excursions correspond to increased
lake inflow. In the Piceance Creek Basin, there is a pro-
nounced positive 8!80 excursion at the base of the oil-shale
facies that is not observed in the same stratigraphic position
in the Uinta Basin. This excursion immediately overlies an
interval where nahcolite formerly was widespread. If, in the
Piceance Creek Basin, nahcolite precipitated in carbonate

mudflats during the period of maximum lake regression and,
later, upon lake expansion, was reworked to the center of the
lake, it may have added a component of heavy !80 to the ox-
ygen reservoir in the water that caused the composition of
the precipitated carbonates to shift to heavier values. The re-
turn to light values above this interval suggests that the effect
of this !8O was short lived because of the large reservoir of
180 in the water. The fact that a positive 5!80 excursion is
not observed in the Uinta Basin may reflect the absence of
nahcolite, assuming it was the source of the heavy oxygen.

CONCLUSIONS

Geochemical evidence presented in this study indicates
that lake processes in the Piceance Creek and Uinta Basins
promoted !3C enrichment relative to inorganic carbon (ini-
tially about 0 %oc) in the Green River systems. More enriched
values (greater than ~+4 %o) were caused by bacterial meth-
anogenesis in sediment pore waters during early diagenesis.
Less enriched values (less than ~+4 %c) were produced by
photosynthesis/respiration in lake waters, which resulted in
organic matter being deposited beneath anoxic bottom wa-
ters (fig. 10).

The degree of lake-water stratification strongly influ-
enced the processes (primary versus diagenetic) governing
carbonate precipitation. When lake waters were well strati-
fied, high levels of organic productivity and carbon burial
promoted bacterial methanogenesis and the formation of di-
agenetic carbonates in anoxic sediments. Conversely, when
lake waters were poorly stratified, photosynthesis/respira-
tion processes resulted in primary carbonate precipitation in
the water. Decomposition (respiration) of organic matter
when the lake in the Piceance Creek Basin was poorly strat-
ified reversed the bicarbonate !3C enrichment caused by
photosynthesis.

5!80 values of carbonate indicate that carbonate-pre-
cipitating waters in the Green River lakes were predominant-
ly brackish to slightly saline (relative to seawater) except
during periods of evaporite-mineral deposition when the wa-
ters presumably were brines. Periods of lake-level rise and
fall are recorded as variations in carbonate '80. During
much of Eocene time, the Green River lakes were hydrolog-
ically closed, which resulted in significant increases in salin-
ity due to evaporation and in the isotopic evolution of the
180 in the water.

ACKNOWLEDGMENTS

The work on which this study is based was funded by
the Evolution of Sedimentary Basins Program of the U.S.
Geological Survey. Stable-isotope analysis was performed
on the carbonate assemblage by Augusta Warden in the Pe-
troleum Geology isotope geochemical laboratory (USGS,


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REFERENCES CITED 17

Dean, W.E., and Anders, D.E., 1991, Effects of source, deposi-
tional environment, and diagenesis on characteristics of
organic matter in oil shale from the Green River Formation,
Wyoming, Utah, and Colorado, in Tuttle, M.L., ed.,
Geochemical, Biogeochemical, and Sedimentological Studies
of the Green River Formation, Wyoming, Utah, and Colo-
rado: U.S. Geological Survey Bulletin 1973-F, p. FI-F16.

Eugster, H.P., and Surdam, R.C., 1973, Depositional environment
of the Green River Formation of Wyoming—A preliminary
report: Geological Society of America Bulletin, v. 84, p.
1115-1120.

Friedman, I., and O’Neil, J.R., 1977, Compilation of stable isotope
fractionation factors of geochemical interest: U.S. Geological
Survey Professional Paper 440-KK.

Grabowski, G.J., and Pavear, D.R., 1985, Sedimentology and
petrology of profundal lacustrine sediments, Mahogany zone
of the Green River Formation, Piceance Creek Basin, north-
west Colorado, in Society of Economic Paleontologists and
Mineralogists Core Workshop 6—Deep-Water Carbonates,
Buildups, Turbidites, Debris Flows, and Chalks: Society of
Sedimentary Geology, p. 386-428.

Herczeg, A.L., and Fairbanks R.G., 1987, Anomalous carbon iso-
tope fractionation between atmospheric CO) and dissolved
inorganic carbon induced by intense photosynthesis:
Geochimica et Cosmochimica Acta, v. 51, p. 895-899.

Keith, M.L., and Weber, J.N., 1964, Carbon and oxygen isotopic
composition of selected limestones and fossils: Geochimica et
Cosmochimica Acta, v. 28, p. 1787-1816.

La Rocque, A., 1960, Molluscan faunas of the Flagstaff Formation
of central Utah: Geological Society of American Memoir 78,
100 p.

Lazar, Boaz, and Erez, Jonathan, 1992, Carbon geochemistry of
marine-derived brines—I, !3C depletions due to intense

Published in the Central Region, Denver, Colorado
Manuscript approved for publication May 1, 1996
Edited by Richard W. Scott, Jr.
Graphics prepared by William E. Sowers;

use made of author-drafted material
Photocomposition by Gayle M. Dumonceaux

photosynthesis: Geochimica et Cosmochimica Acta, v. 56, p.
335-345.

Ryder, R.T., Fouch, T.D., and Elison, J.H., 1976, Early Tertiary
sedimentation in western Uinta Basin, Utah: Geological
Society of America Bulletin, v. 87, p. 496-512.

Stanfield, K.E., and Frost, I.C., 1949, Method of assaying oil shale
by a modified Fisher assay retort: U.S. Bureau of Mines
Report of Investigations 4477, 13 p.

Talbot, M.R.. 1990, A review of the palaeohydrological interpre
tation of carbon and oxygen isotopic ratios in primary lacus-
trine carbonates: Chemical Geology, [Isotope Geochemistry
Section], v. 80, p. 261-279.

Talbot, M.R., and Kelts, K., 1986, Primary and diagenetic carbon-
ates in the anoxic sediments of Lake Bosumtwi, Ghana:
Geology, v. 14, p. 912-916.

1990, Paleolimnological signature from carbon and oxy-
gen isotopic ratios in carbonates from organic-rich lacustrine
sediments; in Katz, B.J., and Rosendahl, B.R., eds., Lacus-
trine Exploration—Case Studies and Modern Analogues:
American Association of Petroleum Geologists Studies in
Geology, p. 99-112.

Tuttle, M.L., and Goldhaber, M.B., 1993, Sedimentary sulfur
geochemistry in ancient Lakes Uinta and Gosiute—Implica-
tions for deposition and diagenesis of the Paleogene Green
River Formation: Geochimica et Cosmochimica Acta, v. 57,
p. 3023-3039

Walters, L.J., Jr., Claypool, G.E., and Choquette, P.W., 1972, Reac-
tion rates and 8!80 variation for the carbonate-phosphoric acid
preparation method: Geochimica et Cosmochimica Acta, v. 36,
p. 129-140.

Weber, J.N., 1964, Carbon-oxygen isotopic composition of Flag-
staff carbonate rocks and its bearing on the history of Pale-
ocene-Eocene Lake Flagstaff of central Utah: Geochimica et
Cosmochimica Acta, v. 28, p. 1219-1242.

st U.S. GOVERNMENT PRINTING OFFICE: 1996 - 774-045 / 20053 REGION NO. 8


12. ORIGIN OF PRIMARY AND DIAGENETIC CARBONATES, GREEN RIVER FORMATION, COLORADO AND UTAH

sc
+30
425 L . ome)
420-1
3 ne Diagenetic
415 . trend
ra
.
+10-- go os *
oe
.
+57
i \ 1 ' n f
t t t 0) t t 1.18
“15 -10 5 +5 +10 +15 50
Primary trend -5 +

10+
= Calcite / aragonite
a '8C.enriched calcite
© Dolomite
© Siderite

Figure 9. Isotopic compositions of primary and diagenetic
carbonate minerals in Lake Bosumtwi, Ghana (from Talbot and
Kelts, 1990). Solid lines depict primary and diagenetic
(methanogenic) trends.

calcites and dolomites in the Green River that have slightly
positive 8!3C values (0 to ~+4 %o) were formed mainly by
primary precipitation in the water mass, whereas carbonate
phases that have moderate to high positive 8'3C values
(greater than ~+4 %o) were formed predominantly by diage-
netic processes close to the sediment-water interface. Cal-
cites and dolomites that have negative 5!3C values in the
Piceance Creek Basin are interpreted to have formed by
primary precipitation rather than by diagenesis.

CARBON ISOTOPES OF CARBONATE

The carbon isotopes of carbonates in the Green River
Formation record the composition of the dissolved inorganic
carbon (DIC) reservoir in the lake water and in the sediments
at the time of carbonate precipitation. The carbon reservoir
in the lake was modified by rates of organic productivity,
atmospheric exchange of CO , and bacterially mediated
diagenetic reactions. The degree of basin closure and
lake-water residence time also greatly influenced the DIC
compositions.

PRIMARY CARBONATES

In the Uinta Basin, the carbon-isotope compositions of
calcites and dolomites along the primary trend (see fig. 8)
are interpreted to record the evolution of aqueous bicarbon-
ate as the lake shifted from a hydrologically open, short wa-
ter-residence-time regime to a hydrologically restricted,
long water-residence-time regime. The most negative val-
ues on the trend (0 to ~-3 %c), correspond to the initial
(starting) values for 5!3C of lake-water bicarbonate and are
within the range reported for marine carbonate in
near-shore and alluvial rocks (Keith and Weber, 1964; We-
ber, 1964)—this suggests that ground water containing dis-
solved COp from eroded Paleozoic carbonate rock was the
principal recharge source for the basin. The more positive
values on the carbonate trend (~+1 to ~+4 %c) reflect the in-
fluence of increased basin closure and water residence time
on the composition of the carbon reservoir. Values en-
riched in !3C coupled with generally high organic-carbon
richness of the host shales argues for a DIC reservoir regu-
lated by the increased effects of photosynthesis paralleling
progressive lake-water eutrophication. The effects of pho-
tosynthesis on the carbon-isotope composition were further
enhanced by increased salinity stratification of the water.
CO, exchange between the atmosphere and lake water ap-
pears to have played a less important role in the composi-
tion of the carbon reservoir during basin closure.

DIAGENETIC CARBONATES

In the Piceance Creek and Uinta Basins, there is
substantial evidence suggesting that pore-water bicarbon-
ate, rather than lake-water bicarbonate, governed the
precipitation of !3C-enriched (diagenetic) carbonates in
organic-rich shales. First, the 5!3C values of the carbonates
are moderately to highly positive (~+4 to ~+13 %o; figs. 7
and 8). The rapid reduction of sulfate accompanying a large
carbon flux from organic matter undoubtedly caused
pore-water alkalinities to increase greatly. This process led
to the formation of organic-carbon compounds, methane,
and associated CO via acetate metabolism that, in turn,
promoted the precipitation of diagenetic carbonate minerals
enriched in !3C. Second, the carbon-isotope compositions
of '3C-enriched carbonates are displaced to more positive
values relative to a primary carbonate trend similar to di-
agenetic (methanic) calcites and dolomites in modern Lake
Bosumtwi (compare figs. 7 and 8 with fig. 9). Diagenetic
carbonates in Lake Bosumtwi form in sediment pore waters
where the overlying lake waters have high rates of evapora-
tion and methanogenesis (Talbot and Kelts, 1986). Third,
changes in the carbon-isotope compositions of !3C-en-
tiched calcites and dolomites generally vary independently
relative to changes in the carbon-isotope compositions of
organic matter (figs. 7 and 8). This inverse relationship


4 ORIGIN OF PRIMARY AND DIAGENETIC CARBONATES, GREEN RIVER FORMATION, COLORADO AND UTAH

USBM OIA WELL
Calcite, aragonite
Caco,

Low-Mg calcite—

USGS COYOTE WASH WELL.
Calcite, aragonite

CaCO 3
Ca dolomite ~_ x Low-Mg calcite—
(5 mole % excess CaCO3) oxi) Ca ankerite 50
dolomite ~ XXX ankerite t \
(Mg:Ca =1:1) oO a \
x 2 hx .Caankerite
a ~ 50
Mg siderite ankerite
MgCO 3 Ey
EXPLANATION

LITHOFACIES.

Oil shale o deri

Evaporite x Mg siderite

Argillaceous Oo

shale MgCO3 50 FeCOg
id

EXPLANATION Siderite
LITHOFACIES
Oil shale a
Mixed carbonate- x

clastic

Figure 3. Ternary diagram showing the range of CaCO3-MgCO3-FeCO3 compositions of carbonate in the Piceance Creek

and Uinta Basins.

dolomite to calcite is reported as peak height above back-
ground. The very fine grain size and complexly intergrown
nature of the carbonate minerals prevented an accurate cal-
culation of the weight percent calcite or dolomite. Oil-yield
determinations (in gallons per ton—“ton” throughout this re-
port refers to “short ton” or 2,000 pounds) were made using
high-precision Fischer Assay pyrolysis (Stanfield and Frost,
1949) and are reported for samples containing carbonate.
This technique is a measure of a rock’s ability to generate oil;
thus, it provides a means of estimating organic-matter
content.

RESULTS

CARBONATE MINERALOGY

Finely disseminated Ca-Mg-Fe carbonate minerals are
abundant constituents in the organic-rich and organic-lean
shales examined in the Piceance Creek and Uinta Basins in

this study. X-ray diffraction analysis revealed considerable
compositional variability between individual carbonate
phases in the two wells (fig. 3). Varieties of dolomite rang-
ing from ideal dolomite to Ca-dolomite to Fe-rich dolomite
(ankerite) tend to predominate over calcite in intervals that
have high oil yield. Calcite containing small amounts of Mg
and Fe is most abundant in zones that have low oil yield. In
the upper part of Parachute Creek Member, calcite is ubiqui-
tous. Aragonite, siderite, and Mg-siderite are sparse
throughout the formation.

STABLE ISOTOPES, CARBONATE-MINERAL
ABUNDANCE, AND OIL YIELD

Carbonate isotope ratios (8!3C and 8!80), organic-car-
bon isotope ratios (§'3C), carbonate mineralogy (dolo-
mite-calcite peak-height ratios), and oil yield (gallons per
ton) contents of whole-rock samples from the Piceance
Creek and Uinta Basins are reported in tables 2 and 3 and are
illustrated relative to depth and stratigraphic position in


ORIGIN OF PRIMARY AND DIAGENETIC CARBONATES
IN THE LACUSTRINE GREEN RIVER FORMATION
(EOCENE), COLORADO AND UTAH

By Janet K. Pitman

ABSTRACT

Stable isotope geochemistry was used to gain a better
understanding of the origin of carbonate-rich, kerogen-bear-
ing shales in the Eocene Green River Formation, which was
deposited in an ancient lake complex. Isotopic, mineralog-
ic, and oil-yield data are reported for two basin-center wells,
one in the Piceance Creek Basin, Colorado, and the other in
the Uinta Basin, Utah. The lakes in these basins evolved in-
dependently as separate water bodies, which led to unique
depositional histories and lake-water chemistries. Results
show that Ca-Mg-Fe carbonate minerals, the dominant con-
stituents in oil shales in the formation, are primary or early
diagenetic precipitates whose isotopic compositions were
determined by environmental conditions and lake state at
the time of deposition. Initially, primary carbonate precipi-
tation took place in a hydrologically open lake regime that
had abundant inorganic carbon dissolved from marine
Paleozoic rock. Later, when the basin was hydrologically
closed and organically productive, precipitation of primary
carbonate was promoted by photosynthesis-respiration pro-
cesses; CO2 exchange between the atmosphere and lake wa-
ter had a minor influence. Diagenetic carbonate
precipitated in anoxic, lake-bottom sediments where bacte-
rial methanogenesis was active due to high burial rates of
organic carbon. Relative to seawater, carbonate-precipitat-
ing lake waters were brackish to slightly saline; during peri-
ods of saline-mineral precipitation, they were brines.

Evolutionary trends on carbon- and oxygen-isotope
curves of carbonate provide an historic record of the effects
of basin closure and hydrologic residence time on carbon
and oxygen geochemical cycles. Major carbon trends
record biologic processes that are related to levels of
paleoproductivity and carbon burial. Major oxygen trends
reflect changes in the ratio of lake inflow to evaporation.
Cyclic excursions on the long-term carbon and oxygen
trends record these changes over short time periods.

INTRODUCTION

The Eocene Green River Formation was deposited in a
large lake complex, known as Lake Uinta, which occupied
the Piceance Creek Basin, Colorado, and the Uinta Basin,
Utah (fig. 1). During the early Eocene, the lakes were shal-
low, hydrologically connected water bodies open to fresh-
water input from areas to the south. Later, in the middle
Eocene, the lakes were closed, each with its own depocenter,
which led to distinct depositional histories and lake-water
chemistries. Thick sequences of carbonate- and organic-rich
marlstone, referred to as oil shale, were deposited in the
lakes. In the Piceance Creek Basin, abundant nahcolite (so-
dium bicarbonate) accumulated along with organic-rich
shales in the central part of the lake; nahcolite was deposited
only locally in the Uinta Basin lake. Thinly bedded oil shale
comprises rhythmically alternating, millimeter to submilli-
meter, light- and dark-colored laminations. The light lami-
nations are composed predominantly of carbonate minerals,
whereas the dark laminations contain primarily silicate min-
erals. Considerable controversy remains regarding the ori-
gin of the organic-rich shales and their fine-grained
carbonate minerals, calcite and dolomite.

Two lake-basin models have been proposed for deposi-
tion of the Green River Formation and its carbonate-rich
shales. The stratified lake model proposed by Bradley and
Eugster (1969) defines a deep (greater than 30 m), perma-
nently stratified, saline, alkaline lake that has high organic
productivity in surface waters and organic-matter deposition
in anoxic bottom waters. Bradley and Eugster (1969) infer
that calcite in organic-rich shales was chemically precipitat-
ed in the water column, but they did not determine whether
dolomite was primary, diagenetic, or a replacement mineral.
The playa-lake model, originally advanced by Eugster and
Surdam (1973), suggests that broad playa flats surrounding
a large, ephemeral lake were sites of extreme evaporation.
The model proposed that calcite precipitated in the capillary

1




DISCUSSION 13

suggests that either the isotopic record of organic matter is
more complex, and thus distinct from. that of carbonate, or
the calcite and dolomite formed from a carbon reservoir not
controlled exclusively by photosynthesis—the latter was
more likely the case.

On the basis of the evidence presented above,
13C-enriched calcites and dolomites (5'3C greater than ~+4
%o) in the Green River Formation are early diagenetic
products that formed in a bacterial regime where methano-
genic processes were active and sedimentation of organic
matter and burial rates of carbon were high. That bacterial
methanogenesis had a strong affect on pore-water DIC and,
in turn, the isotopic composition of diagenetic carbonates is
supported by the occurrence of *4S-enriched organically
derived sulfur and disseminated disulfide minerals (+16 to
+52 %c, and +12 to +49 %o, respectively) in organic-rich
shales (Tuttle and Goldhaber, 1993). Tuttle and Goldhaber
(1993) indicate that bacterial sulfate reduction was essen-
tially completed in the lake water, which kept the sediments
undersaturated with respect to sulfate minerals. After sul-
fate was depleted by sulfate-reducing bacteria, methanic
bacteria became dominant. The high iron content in some
carbonate species may have resulted from insufficient
amounts of hydrogen sulfide to precipitate all the available
ferrous iron as pyrite (Cole and Dyni, 1985).

13C. DEPLETED (PRIMARY?) CARBONATES

Isotopically light 8'3C values of calcites and dolomites
in brecciated shales of the evaporite facies (Piceance Creek
Basin; fig. 7) can be interpreted in two ways. The values
could reflect reequilibration (recrystallization) with meteor-
ic water containing soil CO} late in the burial history; how-
ever, the oxygen-isotope compositions do not show lower
values as would be expected if carbonate precipitation were
related to recrystallization. Therefore, dissolution-zone car-
bonates are not an artifact of recrystallization. A more rea-
sonable explanation for the light 5'3C values is that they
reflect photosynthesis/respiration processes related to de-
creased organic-carbon burial rates. Respiration should
have increased the !2C content of CO} in the intermediate
to deep waters of Lake Uinta. Upon reflux of this CO, into
the overlying waters, the composition of surface-water bi-
carbonate and resulting carbonate precipitates would have
shifted to lighter 5!3C values. Increased circulation of CO2
through the water column indicates a water body that tended
toward less stratification.

OXYGEN ISOTOPES OF CARBONATE

The oxygen isotope compositions of carbonate
minerals in the Green River Formation are predominantly a

function of the !80/!6O ratio of the water that precipitated
the carbonates. The distribution of oxygen-isotope compo-
sitions along the primary carbonate trend in the Uinta Basin
(fig. 8) records the isotopic evolution of Lake Uinta as it
shifted from an open, short water-residence-time regime to
a closed, saline, long water-residence-time regime. The
8180 values of carbonate near the negative end of the trend,
including data for shell carbonate from lake-margin envi-
ronments (La Rocque, 1960), reflect the composition of in-
flow water, the freshest waters in the lake. 5!8O values near
the more positive end of the trend record compositions of
the most evolved waters in the lake. These waters occurred
in areas where evaporation was high. Progressive isotopic
evolution of the water body paralleled increased lake-basin
closure and residence time of the water, which resulted in
primary carbonate minerals enriched in !80 relative to the
inflow water. The continuity of the primary trend through
time suggests that major changes in the source of fresh wa-
ter to the lake did not occur.

Like the primary carbonate minerals, diagenetic phas-
es in the Green River also preserve a record of Lake Uinta’s
isotopic evolution. In both basins, 5!80 values of diagenet-
ic carbonates partially to completely overlap the composi-
tional range of oxygen in primary carbonate minerals (see
figs. 7 and 8), which suggests that isotopically modified
pore waters had essentially the same 5!80 composition as
overlying evaporated lake waters. The 8!80 values of di-
agenetic carbonates in the Piceance Creek Basin (fig. 7)
show wide scatter compared to a more limited range of val-
ues for diagenetic carbonates in the Uinta Basin. The more
negative values reflect less saline pore waters and indicate
shallow, fresher water conditions in the lake during the ini-
tial stages of lake-basin closure. The more positive values
reflect more saline pore waters and imply highly evaporated
waters in the overlying closed-basin lake. The linear 8!3C
and 8!80 trend in the oil-shale facies (see facies plots, figs.
7 and 8) is consistent with pore waters that became progres-
sively evolved as evaporation and residence time of the wa-
ter in the lake increased.

The extent of lake-water evaporation and, in turn, iso-
topic evolution of the Green River lakes during the time
they were closed and precipitating carbonate was evaluated
using the carbonate-water fractionation equation of Fried-
man and O'Neil (1977) for calcite. Calculated water com-
positions (in SMOW, standard mean ocean water) are based
on an estimated carbonate precipitation temperature of
10°C and average carbonate 8!80 values specific to various
lithofacies. The freshest water existed when the lakes were
open, short water-residence-time regimes and is estimated
to have had an !80 composition of about -12 %o. Later,
upon closure of the basins, the lake waters evolved to heavi-
er compositions. During periods of oil-shale formation,
which alternated with episodes of evaporite-mineral depo-
sition in the Piceance Creek Basin, carbonate-precipitating
waters were brackish to slightly saline and had !80



ANALYTICAL

METHODS

UINTA BASIN
1

OIL-SHALE
ZONE
UTAH
COLORADO.

| Douglas Creek
arch

Oil yield (gallons/ton)

EOCENE

GREEN RIVER FORMATION (part)

Top of Mahogany oil-shale zone

PICEANCE CREEK BASIN
2

Oil yield (gallons/ton)
40

ZONE

JOIL-SHALE | m

° 80

RG

YRS

R4

Evaporite facies

R38

R2

RA

EXPLANATION

Dolomitic oil shale

Argillaceous oil shale,
mudstone, marlstone

Limestone, siltstone,
argillaceous shale, sandstone

EA Evaporite minerals-nahcolite, halite
(interbedded with dolomitic oil shales)

——Z_ Intertonging contact 1
Z Member boundary (heavier line)

_L Sampled interval

USGS Coyote Wash | well
sec. 22, T.9 S.,R. 22 E.

2  USBM OIA well
sec. 29, T.1S.,R.97 W.

uz Mahogany oil-shale zone

Boundary of rich oil-shale zones

Figure 2. Generalized stratigraphy of the Eocene Green River Formation, Colorado and Utah (W.C. Cashion, written commun., 1994).

Diagram not to scale.

Table 1. Generalized lithofacies, depositional environments, and hydrologic states in the lower to
middle Eocene Green River Formation, Colorado and Utah.

["Open lacustrine” corresponds to lake-center deposition; “Marginal lacustrine” corresponds to lake-margin deposition)

Member Lithofacies Depositional environment! Hydrologic state
Piceance Creek Basin, Colorado
Parachute Creek —_—Oil shale, evaporite Open lacustrine Closed
Garden Gulch Argillaceous shale Open lacustrine Semi-restricted to closed
Douglas Creek” Mixed carbonate-clastic Marginal lacustrine Open
Uinta Basin, Utah
Parachute Creek — Oil shale Open lacustrine Closed

Douglas Creek

Mixed carbonate-clastic Marginal lacustrine Open to semi-restricted

1 After Ryder and others (1976).
? Not cored in U.S. Bureau of Mines O1A well.

the Na-bicarbonate mineral nahcolite was performed using
standard techniques (Global Geochemistry Corporation,
written commun., 1989), and measurements of 8B3C in
organic matter (see Dean and Anders, 1991) were made on
splits of samples for which carbonate isotopes were

analyzed. All isotope results are reported as the per mil (%o)
difference relative to the Peedee belemnite (PDB) standard
using the delta (8) notation and are accurate to +0.2 %o.
Dolomite/calcite ratios were determined by X-ray
diffraction for isotopically analyzed samples. The ratio of




U.S. GEOLOGICAL SURVEY BULLETIN 2157



6 ORIGIN OF PRIMARY AND DIAGENETIC CARBONATES, GREEN RIVER FORMATION, COLORADO AND UTAH

Table 3. Stable isotopes, carbonate mineralogy, and oil yield in the Green River Formation, Uinta Basin, Utah.

[Data from U.S. Geological Survey Coyote Wash I well. Leaders (--) indicate not determined; isotope values in per mil PDB]

Depth Calcite Dolomite Organic Dolomite-calcite Oil yield
(meters) matter peak-height (gallons
83c 880 83¢ l8o ae ratio per ton)
PARACHUTE CREEK MEMBER (closed hydrologic facies)
Oil-shale facies
670 9.31 -0.70 9.55 -0.10 32.45 750.00 35.4
672 1.93 -2.86 9.75 0.89 - ~ 23.3
681 11.13 0.13 11.76 0.30 -28.50! 4.52 35.4
685 12.90 0.89 13.38 0.96 - - 61.4
697 8.80 =1.15 10.45 1.05 - 717 12.2
706 7.16 -1.3 7.52 0.92 31.15 2.31 16.0
716 4.95 2.11 6.40 -1.67 -30.80! 400.00 22
725 4.52 2.62 4.28 ~0.60 27.17 427.50 2.0
734 4.87 1.88 5.11 -1.37 31.07 5.25 16
752 6.99 0.44 761 0.22 -30.99 20.00 12.8
762 6.27 =1.55 6.49 ~0.42 31.53 2.65 3.5
TM 5.44 0.68 5.58 0.43 30.32 300.00 48
789 6.37 ~0.21 6.54 0.00 -28,331 11.80 15.8
807 1.91 ~2.02 241 0.05 28.33 2.35 3.0
815 141 2.22 3.46 0.43 -30.30! 77 13
825 2.88 0.08 3.03 0.09 30.84 570.00 176
843 4.94 -1.98 6.45 1.45 32.02 - 36.7
862 8.13 ~0.67 10.09 0.87 30.45 280.00 14
871 6.23 ~0.05 6.64 0.07 30.71 297.50 13.8
880 5.54 0.12 5.78 0.04 ~30.30! 500.00 149
898 3.75 -2.68 531 0.21 29.97 10.46 1.9
907 1.82 ~3.08 2.24 2.18 30.80! 300.00 3.0
DOUGLAS CREEK MEMBER (open to semi-restricted hydrologic facies)
Mixed carbonate—clastic facies
917 1.08 A419 2.09 =1.97 28.35 300.00 -
926 2.35 ~6.00 0.48 3.73 26.45 - -
935 2.12 ~6.86 0.13 3.29 -26.33 600.00 -
953 0.25 4.86 2.10 1.97 -30.16 0.93 3.2
971 - - - - 28.11 0.50 -
981 1.30 ~6.20 ~0.69 4.60 27.10 0.27 -
990 1.19 -8.15 ~0.59 5.25 -27.99 0.17 ~
1,008 -2.74 9.48 =1.05 5.53 26.13 0.32 -
1,026 4.07 4.63 3.59 ~6.02 29.85 0.50 14.5
1,035 2.04 8.54 0.21 3.85 26.04 0.33 -
1,043 5.44 -2.97 5.92 3.21 -30.50! 500.00 93

1 Value reported by Dean and Anders (1991).

figures 4 and 5. Also shown on table 2 and figure 4 are car-
bon-isotope ratios (5!3C) for the sodium-bicarbonate min-
eral nahcolite.

Carbonate minerals in the Green River generally are
enriched in !3C and depleted in '80 relative to the PDB
standard. On the average, 5!3C and 8!80 values of calcite
and dolomite in the same sample tend to show only small
differences (figs. 6A and 6B). In the Piceance Creek Basin,
the range of 8!3C values for calcite and dolomite is highly
variable, ~1 to +10 %o and —2 to +11 %o, respectively; the
majority of values fall between +4 and +8 %c. Negative

813C values (0 to -2 %c) are from carbonate in brecciated
shales at the top of the evaporite facies. 8!3C values of car-
bonate in the Uinta Basin also show large variations, —2 to
+12 %o calcite and -1 to +13 %c dolomite. Most values are
between +4 and +10 %o.

Compared to 8!3C, the 5!80 values of carbonate tend
to show less variation. In the Piceance Creek Basin, 8!80
ranges from 0 to -10 %o (calcite and dolomite). In the Uin-
ta Basin, 5!80 values of carbonate generally range from +2
to -3 %o (calcite) and +1 to —2 %e (dolomite) except in the
open to semi-restricted hydrologic facies where the 5!80


RESULTS

Table 2. Stable isotopes, carbonate mineralogy, and oil yield in the Green River Formation, Piceance Creek Basin, Colorado.

[Data from U.S. Bureau of Mines O1A well. Leaders (--) indicate not determined: isotope values in per mil PDB]

Depth Calcite Dolomite Naheolite Organic Dolomite-calcite Oil yield
(meters) matter peak-height (gallons
(xo) (5!80) (3c) (5'80) (3¢) (oq) ratio per ton)
PARACHUTE CREEK MEMBER (closed hydrologic facies)
Oil-shale facies
268 7.51 -3.46 9.08 -2.03 - -29.90! 3.27 15.0
272 5.34 5.23 6.01 -3.48 - 28.29 1,513.00 40
285 7.34 2.96 8.60 -1.19 - -31.75 2,025.00 14.0
290 3.83 ~5.00 6.31 3.53 - -30.90! 0.84 70.0
304 7.29 —2.90 8.52 1.84 - -30.68 1.14 22.0
321 6.90 3.30 748 2.27 - -29.80! 0.78 21.0
328 5.72 3.30 5.98 2.36 - -31.20 1.73 20.0
340 5.50 ~2.29 5.33 -1.56 - -30.59 2.15 36.0
358 9.75 0.39 10.36 0.58 - 29.20! 10.23 15.0
378 2.63 4.60 3.09 3.62 - -31.56 1,340.00 8.0
Leached zone
395 -1.20 4.08 2.40 3.18 - -30.70 38.02 21.0
415 0.18 2.86 -0.40 -0.81 - -30.10! 11.08 16.0
429 - - - ~ - -33.21 1,325.00 27.0
Evaporite facies
437 0.90 421 1.49 -3.75 8.46 -31.40! 7.10 17.0
448 2.33 3.48 3.25 -3.73 4,92 32.54 36.54 27.0
468 3.74 3.64 5.69 2.19 - -30.60! 12.77 34.0
474 3.95 -3.36 5.83 2.02 4,76 31.13 14.76 21.0
486 5.55 -3.37 7.10 2.17 31.48 27.83 27.0
504 2.51 ~2.98 4.05 —2.22 5.15 -30.90! 7.53 4.0
510 1.20 3.18 1.27 -2.85 2.68 -31.25 14.50 21.0
521 2.10 -3.61 2.90 2.63 497 -30.10! 462.00 40
S41 3.68 3.78 446 -3.17 - -31.51 10.11 25.0
547 4.08 3.24 6.75 -1.61 - —29.68 1,632.00 30.0
559 4.52 -3.69 6.42 2.43 - -31.10! 156.80 30.0
578 5.95 3.78 6.94 -3.06 - -33.65 16.05 18.0
583 5.19 4.69 4.66 4.42 12.41 -30.53 272.00 11.0
595 4.73 4.17 7.17 2.73 - -31.00! 33.60 37.0
614 1.86 -3.28 3.12 4.75 13.99 -30.99 388.00 28.0
625 2.46 4.55 3.31 -5.49 - -30.73 3.54 20.0
632 447 -5.30 6.48 2.64 11.53 -32.03 - 16.0
651 5.43 4.70 6.11 4.31 9.17 -32.30! 1,714.00 17.0
657 7.05 4.29 777 -3.00 11.08 -30.79 15.33 32.0
669 6.69 4.64 7.62 427 - -32.60 29.35 27.0
677 - - - - 7.99 -31.46 - -
679.5 - _ - - 12.99 -32.90 - -
679.6 467 -5.97 5.02 5.29 11.68 -30.00 - -
680 4.60 5.71 4.34 5.92 11.40 - - -
687 5.50 6.43 4.68 —5.85 19.92 -31.12 534.00 46.0
GARDEN GULCH MEMBER (closed hydrologic facies)
Argillaceous shale facies
705 6.84 2.70 7.60 -2.65 - -30.90! 3.60 41.0
17 3.36 -7.95 3.09 8.74 - —30.84! 484.00 23.0
728 - - - - ~ -29.90 276.00 28.0
743 7.16 -7.78 6.37 —9.87 - ~ 185.00 42.0
755 6.04 -8.15 5.66 8.56 - _ 930.00 12.0
760 3.10 8.29 1.51 -7.53 - - 0.34 9.0
773 10.50 4.99 11.40 4.04 - - 2,704.00 12.0

1 Value reported by Dean and Anders (1991).


2 ORIGIN OF PRIMARY AND DIAGENETIC CARBONATES, GREEN RIVER FORMATION, COLORADO AND UTAH

112° 110° 109° 108°
are S bo WY oa feche ee
‘ Sand Wash
a *:; Basin’.:">
oO : a
aan
2
i
oO
o,
g
or ai
| as
yi]
i HY
i 5
| U
a } f

ler

100 Kilometers

Explanation

~_] Tertiary volcanic and intrusive
rocks

Green River Formation (Tertiary)
Sedimentary rocks

Evaporite deposits

Precambrian intrusive rocks

Precambrian sedimentary rocks

Figure 1.

——— _ Contact
O Fault
“oa = Thrust fault

@ Wells that provided core samples:

1 USGS Coyote Wash 1 well
sec. 22,T.9S.,R.22E.
2 USBM O1A well
sec. 29,T.1S.,R.97W.

Generalized geologic map of east-central Utah and northwest Colorado showing the location

of the Piceance Creek and Uinta Basins and the two study wells.

zone near the surface of the mudflats and later was incorpo-
rated into deeper water sediments where the calcite was al-
tered to dolomite.

This study utilized isotopic, mineralogic, and oil-yield
data to (1) delineate major hydrologic states within the Lake
Uinta system, (2) establish the origin of carbonate mineral
phases (primary versus diagenetic) within these regimes,
and (3) determine the controls on the geochemical cycles of
carbon and oxygen. Two basin-center wells that have con-
tinuous core (U.S. Bureau of Mines OIA well, Piceance
Creek Basin, and U.S. Geological Survey Coyote Wash |
well, Uinta Basin) were analyzed (fig. 2). As part of the
study, successive members of the formation were subdivid-
ed into major lithofacies and assigned to interpreted lake
states and depositional environments (as defined by Ryder
and others, 1976) within the Lake Uinta system (table 1).

ANALYTICAL METHODS

Representative samples of carbonate-bearing shales in
the Green River Formation were sampled for stable isotope
analysis. Because the carbonate fractions on which isotopic
analysis was performed represent mixtures, a timed-dissolu-
tion procedure based on different reaction rates for chemical-
ly distinct carbonate phases was used to obtain carbon and
oxygen isotope ratios (Walters and others, 1972). To pre-
vent contamination by CO> from organic matter during acid
digestion, kerogen was removed from the samples by ashing
prior to reaction. The inability to physically separate dis-
crete carbonate phases in most samples led to attributing
CO gas evolved in the first hour to calcium carbonates (cal-
cite) and CO} gas evolved after several hours to Mg-Fe car-
bonates (dolomite and ankerite). Carbon isotope analysis of


16

ORIGIN OF PRIMARY AND DIAGENETIC CARBONATES, GREEN RIVER FORMATION, COLORADO AND UTAH

Bicarbonate '°C source
Lithofacies Lake state Photosynthesisi hy . Lake-water salinity Lake level
respiration | Methanogenesis |, High | Low High
TS =]
a
| | ———
Oil shale Ye = :
LL om # _
Leached zone oo |
ee
Closed at =
“I 7
Evaporite Vi >
ov
TM~
~
KK |
— <> —
Argillaceous =
< —
shale ca
A
: Bicarbonate '9C source
Lithofacies Lake state Photosynthesis) hi | Lake-water salinity Lake level
; respiration Met lanogenesis| Low High | Low _ High
Oil shale Closed
}
I
¢ |
Carbonat Open/ )/
arbonate- 4
clastic semi ff | — |
restricted a <
| | |
l | _ _ Zz
B
Figure 10. Schematic hydrologic model summarizing temporal changes in bicarbonate !3C source, lake level, and salinity in the

Lake Uinta system; A, Piceance Creek Basin; B, Uinta Basin.

Denver, Colo.); analyses were also performed under con-
tract to Global Geochemistry Corporation (La Habra,
Calif.). The Laramie Energy Research Center kindly pro-
vided core material and Fischer Assay oil-yield analyses for
the study.

REFERENCES CITED

Botz, R.P., Stoffers, E.F., and Tietze, K., 1988, Isotope geochem-
istry of carbonate sediments from Lake Kivu (East-Central
Africa): Chemical Geology, v. 69, p. 299-308.

Bradley, W.H., and Eugster, H.P., 1969, Geochemistry and
paleolimnology of the trona deposits and associated authigenic
minerals of the Green River Formation of Wyoming: U.S.
Geological Survey Professional Paper 496-B, 71 p.

Calder, J.A., and Parker, P.L., 1973, Geochemical implications of
induced changes in !3C fractionation by blue-green algae:
Geochimica et Cosmochimica Acta, v. 37, p. 133-140.

Cole, R.D., and Dyni, J.R., 1985, Origin of dolomite/ankerite in a
low-sulfate lacustrine environment—Parachute Creek
Member of the Green River Formation, Colorado: Society of
Economic Paleontologists and Mineralogists Abstracts and
Programs, v. 2, p. 20.




U.S. DEPARTMENT OF THE INTERIOR
BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY
Gordon P. Eaton, Director

For sale by U.S. Geological Survey, Information Services
Box 25286, Federal Center
Denver, CO 80225

Any use of trade, product, or firm names in this publication is for descriptive purposes only and
does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

Pitman, Janet K.
Origin of primary and diagenetic carbonates in the lacustrine Green River
Formation (Eocene), Colorado and Utah / by Pitman, Janet K.
p. cm. — (U.S. Geological Survey bulletin ; 2157)
Includes bibliographical references.
Supt. of Docs. no. : I 19.3:
1. Rocks, Carbonate—Colorado. 2. Rocks, Carbonate—Utah. 3. Geology,
Stratigraphic—Eocene. 4. Green River Formation. I. Fitle. II. Series.
QE7SB9 no. 2157
(QE471.15.C3]
557.3. s—de20
[552’.58’09788] 96-24533

cIP


RESULTS 9

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2 §13¢
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om
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Figure 6. 5!3C and §!80 plots showing degree of correlation of
8!3C in calcite and dolomite and 8!80 in calcite and dolomite: A,
USBM OIA well, B, USGS Coyote Wash | well. Each point on the
graph represents an isotopic value of carbon versus oxygen for one
sample.

values are more variable (—3 to —9 %o calcite; -2 to -6 %e
dolomite).

8!3C values of nahcolite in the Piceance Creek Basin
display considerable variation and substantial enrichment
ranging from +2 to +20 %c. A comparison between values

of 8'3C in nahcolite and other carbonate minerals in the
same sample show that nahcolite is enriched in !3C by 2 to
10 %o relative to calcite and dolomite.

The 83C values of organic matter in the Piceance
Creek and Uinta Basins remain relatively constant (-26 to
—32 %o) throughout the formation (Dean and Anders, 1991;
this study). The highest values occur in the open to semi-re-
stricted hydrologic facies in the Uinta Basin. The oil yield
of shales generally varies significantly, ranging from about
4 to 70 gallons per ton. The highest oil yields are in the
closed hydrologic facies; the lowest oil yields are in the
open to semi-restricted hydrologic facies.

DISTRIBUTION OF CARBON AND OXYGEN
ISOTOPES

Three populations or types of carbonate are recognized
in the Green River Formation on the basis of the range and
distribution of 8!3C and 5!80 compositions (figs. 7 and 8).
One type characterizes the open to semi-restricted hydro-
logic facies in the Uinta Basin. This type has slight positive
to negative 5!3C values (~+4 to ~-3 %c) and generally neg-
ative 5!80 values (0 to ~—9 %c) that define a linear trend and
a high degree of correlation between 8!3C and 8!80 (R =
0.8; fig. 8). Stratigraphic equivalents were not cored in the
study well in the Piceance Creek Basin; however, because
comparable depositional and hydrologic environments ex-
isted in both basins, the Uinta Basin primary carbonate
trend was superimposed on the O1A isotope plot (solid line
in fig. 7) to facilitate interpretation of the data. The second
type of carbonate is widespread in the closed hydrologic fa-
cies in both basins and displays extreme positive 5!3C val-
ues (~+ 5 to ~+13 %c) and generally negative 5!80 values (0
to ~—5 %e) that are displaced well above the Uinta Basin lin-
car carbonate trend; no correlation exists between 8!3C and
8!80 (figs. 7 and 8). The third type, which occurs only in
the Piceance Creek Basin, has slight negative 8!3C values (0
to —2 %o) and slight negative 5!80 values (0 to —4 %c) that
are displaced below the Uinta Basin linear trend (fig. 7).
Some compositional overlap between the first two carbon-
ate types may occur within individual lithofacies (see facies
plots in figs. 7 and 8).

In the evaporite facies of the Piceance Creek Basin, two
populations of nahcolite are distinguished on the basis of the
range of 8!3C values (table 2). Nodular nahcolite has mod-
erate to extreme positive 5!3C values (+8 to +20 %c) and gen-
erally is restricted to the middle and lower part of the facies.
Bedded nahcolite has slight to moderate positive 8!3C values
(+2 to +8 %o) and occurs in the upper part of the evaporite
facies.

Figure 7 (following page). Relationship of 83C¢ versus 8180 in
primary and diagenetic carbonate minerals in the USBM O1A
well, Piceance Creek Basin. Primary carbonate trend in the Uinta
Basin shown for reference. Isotope data for individual lithofacies
are to right of plot. Note similarity between diagenetic trends in
the Green River Formation and Lake Bosumtwi (see fig. 9).


DISCUSSION

13 : 14
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ri4y 3
a
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a7 .
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-10 8 6 4 2 0 2
Hydrologic
Lithofacies state Calcite Dolomite
Oil shale Closed °
Carbonate-clastic Open/semi- o
restricted 8180
Shell carbonate Open x

Figure 8. Relationship of §!3C versus 8!80 in primary and diag

enetic carbonate minerals in the USGS Coyote Wash | well, Uinta Ba-

sin. Primary carbonate trend shown as solid line drawn through the data points. Note compositional overlap between carbonate types in
each facies; anomalous values were not included in regression calculations. Isotope data for individual lithofacies are to right of plot. Note
similarity between diagenetic trends in the Green River Formation and Lake Bosumtwi (see fig. 9).

DISCUSSION

PRIMARY VERSUS DIAGENETIC
CARBONATE

The degree of correlation between carbon- and
oxygen-isotope compositions distinguishes primary versus
diagenetic calcites and dolomites in modern closed, saline,
alkaline lakes that have high organic productivity (Talbot and
Kelts, 1986; data from Botz and others, 1988, replotted by
Talbot, 1990). One such example is Lake Bosumtwi in

Ghana, Africa (Talbot and Kelts, 1986) (fig. 9). Carbonate
minerals that form as primary precipitates in Lake Bo-
sumtwi tend to show slight positive to negative 8!3C values
(+6 to -2 %o) that have a high degree of correlation with
5!80. Diagenetic carbonate precipitates display variable,
moderate to extreme positive 5!3C values (+7 to +27 %o)
shifted above the primary carbonate trend. In the Green
River Formation, carbonate minerals exhibit an isotopic
distribution pattern comparable to Lake Bosumtwi carbon-
ates (see figs. 7 and 8) despite a lower range in 813C and
8'80 values. On the basis of the Lake Bosumtwi data,


Origin of Primary and Diagenetic Carbonates in the
Lacustrine Green River Formation (Eocene),
Colorado and Utah

By Janet K. Pitman

U.S. GEOLOGICAL SURVEY BULLETIN 2157

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1996




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Hydrologic
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Leached zone Closed +

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ORIGIN OF PRIMARY AND DIAGENETIC CARBONATES, GREEN RIVER FORMATION, COLORADO AND UTAH

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