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GEOCHEMISTRY AND PHYSICAL PALEOLIMNOLOGY OF PICEANCE CREEK BASIN OIL SHALES
by
andJohn Ward Smith
Consultant,
Laramie, Wyoming
Kwang K. Lee
Professor, Engineering
University of Wisconsin, Milwaukee
ABSTRACT
The remarkable persistence and uniformity of Green River Formation oil shale demonstrate genesis mecha
nisms which dominated geologic disasters for millions of years. Postulated geochemical conditions capable of
explaining the required stability are summarized and used to explain how 10 present-day observations came
about. These include evaluation of how the lake limited mineral input, how the saline minerals formed, why
the organic matter is uniform, and how the lake formed dolomite as a major component of its sediment. All of
these depend on development and persistence of a stratified lake. The lake that existed during Mahogany zone
time was modeled for "worstcase"
conditions and tested for stability under extreme wind stress. The lake
proved emphatically, persistently stable and capable of providing the environment required for the postulated
geochemical and depositional conditions to exist.
INTRODUCTION
The Eocene Green River Formation, its oil
shales, and its saline minerals form a sedimentary
sequence unmatched in the world's massive sediments.
Nothing like it exists. Much of the Green River
Formation study has concentrated on the detectable
variations in the Green River Formation, but varia
tions are not the Formation's peculiarity. Two
words characterize the oil -shale generatingsystem-
persistence and uniformity. The oil shale deposi
tional system developed and then maintained itself,
doing the same thing continuously for a very long
time. When geologic disasters interrupted this
sedimentation pattern, the chemical conditions
reestablished themselves and went right on making
oil shale. Smith and Robb in 1973 and Smith in 1974
postulated geochemical conditions capable of ex
plaining the persistence and uniformity of Green
River Deposition. These conditions are summarized.
Smith (1974) applied these conditions to explaining
seven requirements made on the depositional geo
chemistry by present day observations. This list,
now numbering 10, is reviewed. The lake stratifica
tion required to make this chemistry work is modeled.
Its stability and ability to reestablish itself is
tested, demonstrating the long-term stability and
persistence of the oil shale depositional conditions.
This paper will concentrate its discussion on
Colorado's Piceance Creek Basin. Here, oil shale
deposition lasted longer, and the geochemical condi
tions matured to a greater degree. However, with
minor modifications the chemical conditions that
formed Colorado's oil shales extrapolate nicely into
the depositional conditions that made Green River
oil shale in Utah and Wyoming.
LAKE UINTA'S POSTULATED CHEMICAL HISTORY
Green River Formation deposition in the region
to become Colorado began in ancient Lake Uinta in
the general area of the early depositional center
outlined in Fig. 1. Normal lacustrine sediments are
found around here lying deep under the oil shale.
The sediments gradually change to dark shale and
finally to oil shale (Trudell et al . , 1970) in
moving up the stratigraphic column. This lake may
have been a rather deep intermontane lake especially
subject to thermal stratification.
Sodium-rich silicates hydrolyzing in the lake
water built up a sodium ion concentration. This
hydrolysis consumed acid, making the lake more
basic. Coupled with C02 produced by organic matter
decomposing on the lake bottom, this sodium accumu
lated in the water as sodium carbonate and bicarbo
nate. Eventually a year came when the lake didn't
undergo normal spring and fall overturn because of
the increasing carbonate concentration. This rein
forced the stratification by permitting accumulation
of more sodium carbonate in the lower layer. The
result was a lake structured like Fig. 2, permanent
ly stratified by a difference in density.
Figure 2 names the lake strata the mixolimnion
is the upper, low-density layer, and the lower,
high-density layer is the monimolimnion. These
layers are separated at a lake horizon labeled the
chemocline. This forms a real barrier. The two
layers circulate independently, divided at the
chemocline. They do not mix. The mixolimnion is
101
R,00'>** Ull
,MJ
-,R95*
1"-^
1 1
B 100 W R 99 *
LEGEND
Early oil jho! dtpoiitional ctnlei
' Mahogany zont and latir drpoiitionol cinttr
****. Approumote outlint of Grim River Formollon
Ka Ton
? 0 2 4
FIGURE 1. Colorado's Piceance Creek Basin with Saline Zone and
Mahogany Zone Depositional Centers
MixolimnionChemocline
Monimolimnion
FIGURE 2. Meromictic (stratified) Lake Unita
102
exposed to the atmosphere, is oxygenated, and sup
ports life. All fresh water entering the lake
(rain, streams, etc.) joins the mixolimnion. As
indicated in Figure 2, an area around the lake's
outer edge consisted only of the mixolimnion, form
ing a shallow normal lake near shore. As will be
indicated during modeling this lake, the water
balance between the layers automatically adjusts to
maintain maximum stability.
Density stratification has important effects on
the chemistry of the lake. The mixolimnion com
pletely covers the monimolimnion, excluding air from
the lower layer. Organic debris falling into the
lower layer insistently consumes available oxygen.
A strongly reducing (anoxic) condition is generated
and continuously maintained in the lower layer by
stability of the lake. Since hydrolysis of sili
cates consumes acid, the monimolimnion became more
basic. Smith (1974) defined the limits of pH and
oxidation-reduction potential in the sediment to
become oil shale and in the water of the lower layer
(monimolimnion, Figure 2) above the sediment. He
indicated that the water above the sediment reached
a pH of 10.5 or higher with a reductive potential
near the equilibrium stability point for decomposi
tion of water to Hp. This limiting value may be
calculated for any pH from the following equation:
Eh (volt) =-0.059 pH
This is the lowest Eh value possible for survival of
aquatic systems. When this water was trapped by
depositing organic rich sediment, its pH dropped
because of evolution of C02 from the organic matter.
Decrease of the pH of the interstial water was
limited to about 8.4 by bicarbonate buffering. The
pH in the sediment probably averaged about 9.
Because of its organic content, the sediment's
reductive potential was probably even lower than
that found in the water of the monimolimnion.
Permanent stratification strongly limited the
materials available to the lake's lower layer. From
effects of stratification, the currents in both
layers of the lake were very small (Lee and Smith,
1976). Consequently, the lake water itself was not
very efficient in transporting solid material very
far from the shores. Only very tiny mineral par
ticles capable of remaining suspended in slow cur
rents would travel very far. The organic matter
supply to the lakes lower layer came as debris from
life in the upper layer. Much of the mineral supply
over all but the edges of the lake arrived as air
borne mineral particles'. Both the amount and the
composition of the mineral and organic materials was
limited and largely continued to be so through the
time oil shale was deposited.
OBSERVATIONS REQUIRING GEOCHEMICAL EXPLANATION
Observations from present day conditions can be
used to test the geochemistry postulated above. Ten
of these observations will be evaluated briefly,
using background data developed and referenced by
Smith (1974) and Smith and Robb (1973).
1. Preservation of Organic Matter
Organic matter recently part of living things
is incredibly reactive chemically, particularly to
oxidative atmospheres. The stratified Uinta lake
moved organic debris from oxidative to reductive
conditions in a relatively short distance. Although
chemical reactions degrading organic matter still
occurred, the reductive environment maximizes or
ganic preservation. Once lithified in oil shale,
the Green River organic matter is like a canned
recent sediment. Loss of carboxyl groups with
increasing depth of burial (Smith, 1963) is the only
alteration apparent after deposition. Only deep
burial such as occurred in the northern Uinta Basin
of Utah creates substantial natural alteration of
the organic matter in the rock.
2. Uniform Hydrogen-Rich Organic Matter
As organic matter dropped into the reducing
environment of the monimolimnion, it encountered a
strongly basic environment. In such a basic reducing
environment many organic structures would be attacked
and digested. Only a particular group of compounds
could survive. If the growth conditions in the
mixolimnion remained relatively stable, The organic
debris entering the bottom layer was always similar,
and this same group always survived. Straight and
branched chains, porphyrins, and pyrroles are exam
ples of surviving compounds. These tend to be
relatively rich in hydrogen. The strongly reducing
(hydrogenating) environment reacted to add hydrogen
to any possible point in the organic matter, en
hancing its hydrogen content and making the deposi
ted material hydrogen-rich. The reducing environ
ment actually intensified in interstitial water of
the sediment. Stability of the depositional condi
tions is required to explain the uniformity of
Colorado's Green River oil shale.
103
3- Continuous Deposition of Organic Matter
Once oil shale deposition started, it would
persist as long as the conditions persisted. In the
northern part of Piceance Creek Basin, continuous
oil shale reaches present-day thicknesses of more
than 2100 feet (640 m). Continuous organic deposi
tion demonstrates persistence of the conditions. At
USBM Colorado Corehole No. 1, the oil shale deposi
tional conditions were overridden by a major clastic
influx from the north, but they reestablished them
selves to deposit Mahogany zone oil shale (Robb et
al . , 1978). After a second clastic pulse killed oil
shale deposition at this site and moved the deposi
tional center of the lake southeast to the second
position shown in Figure 1, an additional 600 feet
(180 m) of continuous oil shale was deposited by the
same lake. Continuous organic matter deposition
through perhaps 6 million years demonstrates remarka
ble persistence which must be part of the deposit's
geochemistry.
4. Organic Concentration Around the Depositional
Centers
Ancient Lake Uinta never produced gigantic
bursts of organic matter. It produced and deposited
relatively little organic matter at any one time.
Only the limitation of mineral matter deposition in
the lake permitted formation of the organic- rich oil
shales. Since the organic matter with lower density
could remain suspended better than tiny mineral
particles, the organic matter would do a bit better
job than the mineral raw material of accumulating
around the circulation centers. The postulated
depositional conditions explain the required limita
tion of mineral influx.
5. Varves and Their Preservation
The tiny and probably annual lamina called
varves accumulated as a pair of layers, one light
and the other dark. The light one is richer in
dolomite, and the dark one is richer in organic
matter and silicate minerals. Varves are too small
to see in Figure 3, a picture of larger laminations
in oil shale, but their existence can be detected
microscopically. Smith and Robb (1973) describe a
mechanism for varve generation. Varves are particu
larly present in the Mahogany zone. If varves are
expressions of annual cycles, the varves in the
shale block shown in Figure 3 at average thicknesses
of 30 x 10 meters indicate it took over 800 years
to generate an inch of finished oil shale.
The tiny varve layers depositing on the sedi
ment surface make several requirements only met by a
permanently stratified lake. The first is that
overturn could not have occurred without destroying
the varves. The second is that circulation immedi
ately above the sediment had to be very slow to
prevent destroying the minute layers. The third is
that bottom-dwelling macro! ife must certainly have
been absent. Even microlife was probably absent
from the sediment because gas bubbles generated by
microbes might break the delicate structure. How
ever, this is not absolute because most of the
possible gases microbes might generate would dis
solve in the basic water. The strongly basic,
strongly reducing water of the monimolimnion would
certainly limit possible bottom-dwelling life forms.
6. Lateral Persistence of Varves and Layers
The layered character of Green River Formation
oil shale is well illustrated in Figure 3. Although
varves can't be visible in this picture, they cer
tainly persisted across this block. Their patterns
could be correlated using microprobe and photo
graphic microdensity scans at several places across
the block's face. Lateral persistence across the
block is an appropriate relative measure in terms of
varve thickness. The laminations visible in Fig
ure 3 represent deposition variations over substan
tially longer times than one year. During the
period represented by the darker lamina, more or
ganic matter in relation to mineral matter was
incorporated in the oil shale. These conditions
were regional, producing the same deposition pattern
over wide areas. The classic demonstration of this
lateral persistence of layers is the photograph of
precisely matching lamina in three cores spanning 65
miles in Colorado and Utah published by Trudell et
al . (1970). This matching is the basis for their
development of a time-strati graphic coordination
system particularly useful for resource evaluation.
The lateral persistence of the varves and layers is
a direct consequence of the stability of the postu
lated geochemistry.
7. Uniform Mineralogy
The primary mineral group in Green River Forma
tion oil shales is monotonously similar laterally,
and it shows only some specific variations strati -
graphically. Robb et al . (1978) plot and evaluate
the distribution of minerals and organic matter
through the Formation in the area of the early
104
depositional center. In the Colorado Formation,
dolomite, quartz, potash feldspar, soda feldspar,
pyrite, and i 11 i te are ubiquitous. Calcite occurs
only sporadically, appearing consistently as a major
mineral component only after the first massive
clastic influx from the north. Analcime, resulting
from alteration of ash falls, occurs spottily in and
above the Mahogany zone but is absent from the
saline zone. Analcime is probably missing from the
saline zone because the ash falls that produced it
were totally dissolved. When the lake grew during
Mahogany zone time, the lower layer became less
basic and would permit the ash falls to survive.
The saline minerals, nahcolite and dawsonite, and
the ill ite distributions are to be discussed later.
Essentially, the Green River oil shales contain
the same minerals. Nothing surrounding the Forma
tion could have supplied these with consistency.
Only chemical processing of incoming mineral matter
could produce such a consistent mineral suite through
millions of years. The primary minerals are auto
genic, forming mostly in the lake sediment. Smith
(1974) and Smith and Robb (1973) provide geochemical
explanations of the mineral genesis. Their mecha
nisms depend on the stratified lake, chemical diges
tion of incoming mineral material, and mineral
formation due primarily to C02 production from
organic matter in the sediment. This C02 lowered
the pH in the sediment to initiate formation of
quartz, albite, and potassium feldspar.
One novelty in the Colorado silicate mineral
suite deserves discussion. Although both minerals
formed in the sediment, albite (NaAlSi-Og) and
potassium feldspar (KAlSi30g), are completely sepa
rate. No potassium appears in the soda feldspar,
and no sodium appears in the potassium feldspar. No
calcium or other substitution appears in either
mineral. The albite has the crystal form corres
ponding to low albite. No mineral name has been
specifically tied to the potassium feldspar because
its X-ray diffraction spacings don't correspond
precisely with any of the ordinaryK- feldspar forms--
microcline, adularia, etc. However, the crystal
spacings in the K- feldspar of Colorado's Green River
Formation indicate the lowest of low temperature
forms--an unnamed crystal. Direct formation of
albite from the effect of C02 on interstitial water
chemistry is straightforward, but why was
K- feldspar formed cleanly and independently in a
sodium-rich solution? Apparently, potassium was
collected on the residual silicate particles which
managed to enter the sediment, a known process in
more normal lakes. Collection must have been
rather efficient because no potassium evaporates
have been detected, and potassium occurs at rather
low levels in the halite and nahcolite. These
potassium-bearing residues either formed the ubiqui
tous illite or were directly altered to potassium
feldspar in the sediment. Illite and potassium
feldspar appear to be inversely related in the
mineral profiles presented by Robb et al . (1978).
Illite is hard to detect and quantify because of its
poor crystal! inity, but its high concentration below
the Blue Marker and its low concentration in the
overlying saline mineral zone are inversely reflec
ted in the K-feldspar quantities. So is the in
creased illite content in the Mahogany zone.
These postulated mechanisms were strongly
supported by mineral relationships developed from
mineral analysis of hundreds of samples representing
the entire stratigraphic sequence of the Formation
(Robb et al . 1978). The consistency of the minerals
in oil shale depends on the stability of stratifica
tion and the consequent geochemical stability of
ancient Lake Uinta.
8. Clay-to-Dolomite Mineral Change
A marked change in mineralogy takes place near
the boundary between the Parachute Creek Member and
the underlying Garden Gulch-Douglas Creek unit of
the Green River Formation. This change progresses
and regresses for perhaps 100 feet, but in the basin
center it is complete about 30 to 50 feet above the
Blue Marker, usually designated as the bottom of the
Parachute Creek Member. Below this point, illite
makes up perhaps two-thirds of the total mineral
material. Above the transition zone, dolomite is
the primary oil shale mineral and illite becomes
only a small component. Both sections are oil
shale, and the organic matter itself is the same in
both sections. This change from illite to dolomite
requires geochemical explanation. Smith (1974)
provided one.
When the stratification came into existence,
the minerals entering the lake were attacked chem
ically. Sodium was released, and some silica was
dissolved. To accomplish this, acid was consumed,
gradually making the water more basic and raising
the pH of the lake's lower layer. The undissolved
105
residue after this extraction was incorporated in
the sediment and altered to clay. When the water's
pH was lower initially, the clay formed was perhaps
kaolin. As the pH of the monimolimnion gradually
increased, more silica was dissolved and the clay
formed became smectite. With continued pH increase,
more silica was dissolved, and the relativelysilica-
poor illite became the stable clay formed. Most of
the mineral matter entering the lake contributed to
formation of illite because aluminum released during
hydrolysis formed a protective hydroxide gel around
the particle. This protective coating transported
the particles to the bottom. Production of this
protective coating continued until the pH of the
water in the lower layer passed 10. At this point,
aluminum hydroxide becomes soluble amphoterically.
The protective coating disappeared, and the lake
water could attack the mineral particles more com
pletely. Only a fraction of the tiny particles
survived to reach the sediment. In sediments deposi
ted after the lake's lower layer reached pH 10,
illite forms a much smaller part of the mineral
fraction. Dolomite becomes the primary mineral
constituent and continues to be so.
9. Calcium-Magnesium Balance
A good deal of heavy weather has been made
about the inadequacy of the amount of magnesium in
the Green River system to explain formation of
dolomite. This arises because magnesium must be in
substantial excess for dolomite to precipitate
directly from solution. The mineral material enter
ing the lake probably had a composition similar to
igneous rocks. In these rocks the Ca content is
slightly larger than the Mg content when expressed
as chemical equivalents. No opportunity existed to
build up sufficient magnesium concentration to
precipitate dolomite directly from the lake water.
It didn't. In Green River oil shale, dolomite is a
matrix mineral formed in the sediment after deposi
tion.
Dolomitization in the sediment completely
alters the requirement for massive magnesium buildup
in the entire lake. In the geochemistry outlined
for carbonate mineral formation, Smith and Robb
(1973) point out that calcium is supplied to the
sediment as calcium carbonate either as calcite or
as aragonite. Calcite formed immediately from
calcium released to the sodium carbonate water of
the lake's lower layer. This release occurred
during chemical attack on the mineral particles
dropping through the monimolimnion. Aragonite
formed in the surface water. Both crystals found
their way into the sediment. Magnesium and iron
released in the lake's high pH lower layer precipi
tated as hydroxides and also joined the sediment.
Iron was all reduced to the ferrous state.
In the sediment CaCO- remained insoluble, but
the magnesium and ferrous iron hydroxides dissolved
as pH decreased. In effect, all of the Mg was
mobile, while none of the calcium was. Dolomitiza
tion proceeded directly in place by the mechanism
shown in the following equation.
CaCO- (solid) + Mg +
C03= *
CaMg(C03)2 (solid)
Lippman (1968) described this reaction for low
temperature formation of dolomite after observing
the development of norsethite, BaMg(C03)2, the
barium analog of dolomite first described in natural
occurrence in the Green River Formation. Ferrous
iron substitutes for some of the magnesium in the
dolomite formed. Smith and Robb (1966) demonstrated
this from enlarged X-ray diffraction spacings,
concluding that in the oil shale samples they stu
died ferrous iron made up about 15 mole percent of
the dolomite's magnesium layer. Because this did
not explain the size of the spacing enlargements
observed, they investigated other possible substitu
tions. A significant correlation between Sr and the
enlargements beyond that due to ferrous iron was
detected. However, this really couldn't explain the
residual enlargement. Only much later did the
proper interpretation developcalcium (and stron
tium) substitution for Mg in the magnesium layer.
This is an expected result of dolomitization of
solid calcium carbonate. Dolomitization of calcium
carbonate in sediment to produce dolomite continu
ously as a major oil shale component requires stable
existence of the stratified lake.
10. Saline Minerals
The Green River Formation is noted for huge
collections of novel minerals, particularly sodium
carbonate minerals. In Wyoming, trona, nahcolite,
wiegscheiderite, shortite, and even dawsonite have
been observed. Utah has nahcolite and shortite.
Colorado's saline mineral suite is truly unique
because it contains most of the world's natural
supply of dawsonite [NaAl (0H)2C023 and nahcolite
106
(NaHC03). Formation of these minerals in huge
quantities over a long time period requires extreme
ly rare natural conditions. Smith (1974) neatly
explained the chemistry behind their natural develop
ment from chemical conditions in stratified Lake
Uinta.
The raw materials for generation of nahcolite
and dawsonite continually accumulated in the lower
layer of the stratified lake. Sodium accumulated
from hydrolysis and solution of minerals entering
the lower layer of the lake. Aluminum accumulated
in solution as the aluminate ion after the lower
layer passed the pH of 10. Carbonate accumulated
from C02 arising from decomposition of the organic
matter. Continuous accumulation of the necessary
materials was enhanced by gradual loss of water from
the lake. Eventually the ion concentrations reached
the point where dawsonite began to form in the
sediment. This process is exactly analagous to a
dawsonite synthesis process developed by Bader and
Esch in 1944 in which dawsonite precipitated directly
upon slow addition of gaseous C02 to a sodium alumi
nate solution containing a large excess of sodium
ion. This formation of dawsonite was shown to be
related to processes which formed the nordstrandite
[Al (OH) ] which always accompanies dawsonite in the
oil shale but has not been found separately in the
oil shale. (Smith and Young, 1975).
Colorado's nahcolite is also a product of this
process. Addition of C02 to a sodium carbonate
solution shifts the balance toward sodium bicarbo
nate as the following equation indicates:
Na2C03+
C02+ H20 ->
2NaHC03
By this process 106 grams of sodium carbonate becomes
168 grams of sodium bicarbonate, and sodium bicarbo
nate is less soluble than sodium carbonate by a
factor of 3 or 4. Saturated sodium bicarbonate
solutions crystallize nahcolite as found in Colorado
oil shale. Nahcolite can only crystallize from
systems in effect maintaining high C02 pressure. In
Colorado, the stratified lake persisted throughout
saline mineral deposition. Otherwise the product
formed would have been trona (NaC03*
NaHC03
2H20) as occurred in Wyoming where the lake went to
dryness and lost its top. Halite interspersed with
nahcolite appeared as a final lake concentration
product in Colorado, and in Wyoming halite is incor
porated in trona. In each case, oil shale deposi
tion reestablished itself. In Colorado, oil shale
reappeares immediately on top of the salt, while in
Wyoming a clastic deposit may appear before oil
shale was again deposited. This is best explained
by persistence of the stratified lake in Colorado
and redevelopment of the stratification in Wyoming
by solution of sodium carbonates as the water came
back.
Continued stability of the stratified lake and
an ability to reestablish itself make this explana
tion of the saline mineral deposition valid.
PALE0LIMN0L0GY MODEL
The previous discussions of chemical mechanisms
operating to form Green River oil shale emphasize
how important the stability of the lake stratifica
tion is. All of the mechanisms depend on the de
velopment, stability and persistence of the strati
fied lake. Assuming this stratification's develop
ment and long-term stability is a fine tool, but at
least part of it can be tested. A model of the
ancient Lake Uinta (Lee and Smith, 1976) was enlisted
to test the lake for stability and persistence.
Trying to project a model back through 50
million years is a procedure that makes direct
observation difficult. As with the chemical postu
lates, the only information available to test against
is present-day observations. Since the lake that
generated Colorado's Green River oil shale has no
modern analogs, no comprehensive current model can
be used. However, lake behaviors somewhat analagous
to that required in evaluating ancient Lake Uinta
have come under study with the growth of computers
capable of handling the vast masses of numbers
required. For example, both wind coupling and
meromixis (stratification) have been investigated.
Study of the physical limnology of ancient Lake
Uinta involves examination of the properties of
stratification, lake morphology and circulation
patterns as they relate to depositional conditions.
The first step was examining the dynamic character
of ancient Lake Uinta. Since the mathematical
simulation technique can be applied to the entire
lake which changed continually in shape and size, a
specific point in time had to be selected. The time
of deposition of the Mahogany zone was chosen, and
the lake circulation during this time was modeled.
Lee and Smith (1976) report the required equations
and wind coupling factors.
107
To model a lake, its dimensions must be known
and described mathematically. The Mahogany zone has
some limits in Colorado which make some of its
boundaries better than guesses. The east boundary
of the Mahogany zone was provided by the Grand
Hogback, a geologic uplift present during deposition
and still present. A west boundary is offered by
the Douglas Creek Arch, active and elevated during
Mahogany zone time. Lake Uinta reached maximum
expansion during Mahogany zone time, and its south
ern boundary was postulated as gradually rising
land. The northern boundary is more difficult
because some positive connection existed into the
Uinta Basin. However, because circulation of the
stratified lake through this connection would be
limited, the north end of Mahogany zone deposition
was postulated as roughly rectangular.
One additional dimension, depth, remains to be
postulated. We chose extremely shallow contours to
impose the worst possible condition on stability of
the stratified lake. The real lake in place was
undoubtedly deeper, but any deeper lake would be
more stable. Lake geography and depth contours
tested for stability are shown in Figure 4. We
chose relatively smooth bottom contours because of
the lateral uniformity in the deposit and because we
have no better information. It must be emphasized
again that depth and bottom contour assumptions are
"worstcase"
choices, not projections of reality.
We now postulate the stratified lake of Figure 2
into the Piceance Creek Basin. The names mixolim
nion and monimolimnion are applied to emphasize the
chemical nature of the stratification. The name
chemocline is applied to the lake level separating
the layers. These names are used to distinguish
these effects from the normal thermal stratification.
A density stratified lake is a stable system
because it requires increasing the potential energy
of the lake to mix the heavier bottom layer with the
lighter upper layer. Thermal stratification can be
readily overturned just from changes of water densi
ty with temperature. This effect is quite small,
however, in relation to chemically induced density
differences. Stability of a stratified lake can be
thought of as the work required to raise the center
of gravity of the stratified lake to the level of
its center of volume. Meromictic stability (S) per
unit area of the lake can be expressed as
where AQ is the surface area of the lake, Azis the
lake's cross-sectional area at a depth z, (Pn~Pz)
is the difference between the density of a complete
ly mixed lake and that at any depth z, g is the
gravity constant, and Z is the maximum depth of them
lake. Also, z is the center of the volume of the
lake calculated as
- I fZmv gy
zAzdz
= _9_
Aooj^ z-zg)Az(ph-pz)d2
where V is the total volume of the lake.
The meromictic stability for the postulated
ancient Lake Uinta shown in Figure 4 was calculated
for static conditions. As can be seen from the
equations, the meromictic stability of the lake at
rest depends on the density difference between the
two layers and the location of the chemocline divid
ing them. Figure 5 gives meromictic stability of
the modeled lake as a function of depth for three
density differences. The maximum meromictic stabili
ty appears at about 5 meters (16 feet) for the
postulated lake. At depths above and below this,
the stability becomes appreciably less. This forces
shifts in the chemocline location and the lake
stabilizes itself near the depth of maximum stabili
ty.
For an ordinary thermally stratified lake with
the upper layer at 15C, the bottom layer density3
would be about 1.0028 gm/cm . At this density it2
requires 350 joules/m to completely mix the lake
with this density difference. During the time of
Mahogany zone deposition, the monimolimnion density3
is postulated at 1.03 gm/cm , a conservative value
for water that sometimes formed nahcolite. Mixing
these two layers requires 3500 joules/m , work not
available from temperature changes. At earlier
times during saline mineral deposition the monimo
limnion density might have been as high as 1.06
3 2gm/cm , requiring 7000 joules/m to mix the lake.
The effect of the density difference on stability is
cumulative because when the water doesn't mix, the
density difference must increase.
The meromictic Lake Uinta is basically a sta
tically stable body of water. The buoyancy of the
water in the upper layer tends to inhibit mixing
across the chemocline. In addition, sodium carbo
nate doesn't diffuse well. Only violent external
forces can alter this stability. Tides, earthquakes,
and seiches move the lake bodily and fail to induce
108
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0)
2
o
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E
u
0)
a
a
100 1000
Meromictic Stability (Joule/m2)
FIGURE 5. Meromictic Stability in the Modeled Lake as a Function of Depth
urbulent mixing. Only wind can realistically be
considered a mixing agent. So we tested wind effects,
Wind stress applied over the free surface of
the lake can generate strong surface currents in the
mixolimnion and produce a small free surface tilt.
This will also generate weaker opposing currents in
the monimolimnion and a larger opposing tilt in the
chemocline. Figure 6 illustrates these tilts. The
velocity difference across the chemocline, when it
becomes large enough, can cause instability which
leads to a violent breakdown of stratification at
and around the chemocline. This instability, known
as Kelvin-Helmholtz type instability, is practically
independent of water viscosity but is dependent on
the velocity difference across the interface. If
this velocity difference is below a critical value,
the buoyancy force on the mixolimnion will suppress
the disturbance. When that velocity difference is
exceeded, the disturbance will grow as waves along
the interface which will become steep fronted and
break into patches of turbulent mixing. If the
sheer force created by the velocity difference is
not sustained, the turbulence will decay as the
mixed layer spreads out.
The stability of a stratified lake under wind
stress can be expressed in terms of the overall
Richardson number, R, which is essentially the ratio
of the buoyancy force to the inertia force. This
can be written as
R =
where H is depth, p2and
p1are the densities in the
monimolimnion and mixolimnion, respectively, g is
110
N
Wind
Surface
Chemocline
Wind
Surface
Chemocline
FIGURE 6. Wind-Generated Surface and
the gravity value, and AU is the velocity differ
ences between the two layers. Using this overall
Richardson number as a stability criterion, Miles
(1961) stated that a sufficient condition for an
inviscid stratified flow to be stable is that the
Richardson number be larger than 0.25. Mortimer
(1974) inferred this same stability judgment in a
lake.
A computer program was developed to compute the
overall Richardson number in the Mahogany zone under
various postulated dynamic conditions. This program,
an extension of the program described by Lee and
Smith (1976) for their initial circulation model for
ancient Lake Uinta, includes wind coupling factors.
Using this program, the overall Richardson number
can be computed at every location in the modeled
lake including the sensitive fringe of the chemo
cline. The program can accommodate a variety of
wind speeds, directions, and durations, as well as
assorted density values for the two layers. The
program plots the base 10 logarithm of the overall
Richardson number to get the wide range of magnitude
Chemocline Tilts in a Meromictic Lake
into manageable sized numbers. The stability cri
terion for the overall Richardson number becomes
-0.6 (log 0.25). Any log value less than 0 is
classed as indicating a potentially unstable area.
Log numbers larger than this indicate more stable
areas.
It is postulated that prevailing winds came
from the northwest for purposes of this report.
Other directions were tested with results not appre
ciably different. Wind speeds of from 20 miles per
hour (8.94 m/s) to 60 miles per hour (26.82 m/s)
from the northwest were tested. The entire strati
fication at all density differences was stable
through winds of 40 miles per hour (17.88 m/s)
blowing for 40 hours. An average wind of 40 miles
per hour lasting through 40 hours is not too likely
to have occurred during even a million years. Winds
60 miles per hour blowing for 40 hours were required
to develop any indication of instability of the
stratification. This average speed for 40 hours is
substantially less likely than 40 miles per hour.
Figures 7, 8, and 9 show plotted contours of loga-
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113
riths of the overall Richardson numbers for density
differences as follows:
Figure 7; monimolimnion 1.03 gm/cm , mixolimnion
31.00 gm/cm .
Figure 8; monimolimnion 1.06 gin/cm , mixolimnion
31.00 gm/cm .
3Figure 9; monimolimnion 1.02 gm/cm , mixolimnion
1.002 gm/cm3.
The shaded areas indicate unstable and poten
tially unstable areas in the lake's area developed
under extreme wind stress. They appear only along
the edges of the chemocline. These are definitely
only a small fraction of the total lake area. The
lake would remain stable under wind stress. The
highest numbers in the figures correspond to the
most stable areas. These seem to concentrate around
the Mahogany zone depositional center.
Computer modeling of stability of stratified
Lake Uinta arranged to test the least stable con
figuration and the most ferocious wind stresses
indicates that the lake would be persistently stable.
This would provide the environment required for all
of the postulated geochemical and depositional
conditions to exist.
LITERATURE CITED
Bader, E. , and U. Esch, 1944. Versuche der
drucksynthese des dawsonits, Zeitschrift fur Elec-
trochemie, V. 50, pp. 266-268.
Lee, Kwang K. , and John Ward Smith, 1976.
Paleolimnology and Oil Shale Genesis in the Green
River Formation, Colorado, Wasserwirtschaft und
Gewinnung Fossil er Energietrager, Symposium of the
International Water Resources Association, Dussel-
dorf, Bundesrepublik Deutschland, Sept. 1976, Paper
15, 17 pp.
Lipprnan, F. ,1968. Synthesis of BaMg(C03)2
(Norsethite) at 20C and the Formation of Dolomite
in Sediments in Recent Developments in Carbonate
Sedimentology in Central Europe, G. Muller and G. M.
Freidman, eds., Springer-Verlag, New York, 255 pp.
Miles, J. W., 1961. On the Stability of Hetero
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pp. 496-508.
Mortimer, C. H. , 1974. Lake Hydrodynamics,
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197.
Robb, W. A., J. W. Smith, and L. G. Trudell,
1978. Mineral and Organic Distribution and Rela
tionships across the Green River Formation's Saline
Depositional Center, Piceance Creek Basin, Colorado,
Laramie Energy Technology Center Rept. Invest. 78/6,
39 pp.
Smith, John Ward, 1963. Stratigraphic Change
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Oil Shales, Colorado, Bull. Am. Assoc. Petrol.
Geol., v. 47, pp. 804-813.
Smith, John Ward, 1974. Geochemistry of Oil
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in Energy Resources of the Piceance Creek Basin,
D. K. Murray, ed., Rocky Mountain Association of
Geologists, Denver, pp. 71-79.
Smith, John Ward, and William A. Robb, 1966.
Ankerite in the Green River Formation's Mahogany
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490.
Smith, John Ward, and William A. Robb, 1973.
Aragonite and the Genesis of Carbonates in Mahogany
Zone Oil Shales of Colorado's Green River Formation,
U.S. BuMines Rept. Invest. 7727, 21 pp.
Smith, John Ward, and Neil B. Young, 1975.
Dawsonite: Its Geochemistry, Thermal Behavior, and
Extraction from Green River Oil Shale, Colo. School
of Mines Quart., v. 70, No. 3, pp. 69-93.
Trudell, L. G., T. N. Beard, and J. W. Smith,
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Correlations in the Piceance Creek Basin, Colorado,
U.S. BuMines Rept. Invest. 7357, 252 pp.
114