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

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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

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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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109

0)

2

o

"uO

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

geneous Shear Flow, Jour, of Fluid Mechanics, V. 10,

pp. 496-508.

Mortimer, C. H. , 1974. Lake Hydrodynamics,

Mitt. International Verein. Limnology, v. 29, pp. 124-

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

in Organic Composition Demonstrated by Oil Specific

Gravity-Depth Correlation in Tertiary Green River

Oil Shales, Colorado, Bull. Am. Assoc. Petrol.

Geol., v. 47, pp. 804-813.

Smith, John Ward, 1974. Geochemistry of Oil

Shale Genesis in the Piceance Creek Basin, Colorado,

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

Zone, Jour. Sedimentary Petrology, V. 36, pp. 436-

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,

1970. Green River Formation Lithology and Oil Shale

Correlations in the Piceance Creek Basin, Colorado,

U.S. BuMines Rept. Invest. 7357, 252 pp.

114