Boggs S. Principles of Sedimentology and Stratigraphy
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266
Chapter 8 I Continental e rrestrial) Environments
Figure 8.20
Estimates of the minimum and maximum areas of de
position of the Navajo Sandstone and its lateral
equiv
alents. [After Marzolf, j. E., 1988, Controls on late
Paleozoic and early Mesozoic deposition of the west
ern United States: Sedimentary Geology, v. 56, Fig. 6,
p. 179, reproduced by permission.]
APPROXIMATE ORIGINAL DEPOSITIONAL LIMITS
PRESENT LIMITS OF NUGGET AND NAVAJO SANDSTONES
Figure 8.21
Navajo Sandstone (Jurassic) in Zion National Park, Utah, showing large sets of cross strata
generated by migration from left to right of ancient eolian sand dunes. Note the promi
nent interdune bounding surfaces (see Fig. 8.19) between cross-bed sets.
bedding, which are reported from modern dune sands that have been wetted, are
also common.
As
described in preceding paragraphs, sediment deposited during migra
tion of dunes across a desert may be preserved in part, commonly the lower part
of the foresets, owing to tlse in water table, basin subsidence, or both. Succeeding,
8.3 E
o
lian Desert Systems
267
successive migrations across a subsiding basin result in vertical stacking of eolian
facies separated by terdune bounding surfaces and super surfaces. At the mar-
gins of dtme fields, the boundary between eolian and other (e.g., fluvial or ma-
rine) environments may shift back and forth, generating a vertical succession of
facies in which eolian and noneolian sediments are interbedded. For example, the
schematic illustration in Figure 8.15 shows an eolian succession in Column A and
an interbedded fluvial-eolian succession in Column B.
The Navajd Sandstone provides a real example of this principle, as shown
by intertonguing eolian deposits of the Navajo and fluvial deposits of the
Kayenta Formation in northeastern Arizona (Fig. 8.22). Three fluvial to eolian
drying-upward cycles are illustrated, each representing the advance of the
Navajo erg across the Kayenta alluvial plain, probably in response to an increas
ingly arid climate. Return to wetter conditions terminated the advance of the
erg, allowing fluvial deposits of the Kayenta to, in turn, advance over an ero
sional Navajo surface. Thus, in this succession, cross-bedded Navajo eolian
dune deposits and flooded interdune deposits are interbedded vertically with
floodplain and other fluvial deposits of the Kayenta Formation.
m c GENETIC UN ITS
Increasing
grain size
AEOLIAN DUNE
FLOOD ED
INTERDUNE
AEOLIAN DUNE
FLOODPIN
EPHEMERAL
FLUVIAL
AEOU DUNE
��
_
AEOLIAN DUNE &
DRY INTERDUNE
FLDPIN
EPHEMEL
FLUVIAL
FLOODPIN
AMALMATED
INTERDUNE
AEOLIAN DUNE
FLOODPLAIN
Figure 8.22
Representative intertonguing Jurassic eolian (Navajo) and fluvial (Kayenta) facies
in northeastern Arizona. To tal thickness of the column is about 100 m. Note three
major drying-up cycles, indicaing advance and retreat of the Navajo erg. [After
Herries, R. D., 1993, Contfasting styles of fluvial-eolian interaction at a downwind
erg margin: Jurassic Kayenta-Navajo transition, northeastern Arizona, in North, C.
P., and D. ). Prosser (eds.), Characterization of fluvial and aeolian reservoirs, Geo
logical Society London Special Publication No. 73, Fig. 17, p. 21 0, reproduced by
permission.]
268
Chapter 8 I Continental e rrestrial) Environments
Other Ancient Desert Deposits
Ancient sandstones interpreted to be windblown deposits have been described
from sedimentary successions as old as the Precambrian from many parts of the
world. One of the most extensive and intensely studied eolian records is from the
late Paleozoic and Mesozoic of the western interior of the United States (Blakey,
Peterson, and Kocurek, 1988). addition to the Navajo Sandstone, described
above, eolian deposits are widespread from Montana to Arizona and include
Pennsylvanian (e.g., Weber and Tensleep), Permian (e.g., Cedar Mesa and Coconi
no), Triassic (e.g., Jelm and Wingate), and Jurassic (Entrada) formations. This im
pressive eolian system consists of thick, extensive assemblages that represent
deposition from different types of dunes and eolian complexes and interaction
among eolian, fluvial, marine, and lacustrine environments.
Examples from other continents include the Permian Rotliegendes of nor
weste Europe, the Jurassic-Cretaceous Botucatu Formaon of the Parana Basin of
Brazil, the Permian Lower Bter Sandstone of Great Britain, the Permo-Triassic
Hopeman Sandstone of Scotland, the Permian Corrie Sandstone of Scotland, and
the Proterozoic (Precambrian) of India and northwestern Africa. The Rotliegendes
has been particularly well studied (e.g., Glennie, 1986). It accumulated in a series
of graben (fault) basins as interbedded eolian, uvial, lacustrine (lake), and
sabkha (evaporite) deposits, again illustrating the complex interaction of eolian
and noneolian systems. Other examples of ancient eolian deposits can be found
"Further Reading-Eolian Systems" at the end of this chapter.
8.4 LACUSTRINE SYSTEMS
Lakes cover about 1-2 percent of Earth's surface. Because the world's continents
are presently in a higher state of emergence than was typical of much of Phanero
zoic time, lake sedimentation is more prevalent today than it was during much of
the geologic past. In fact, ancient lake sediments appear to be of only minor im
portance volumetrically in the overall stratigraphic record, although they have
been reported in stratigraphic successions ranging in age from Precambrian to
Holocene. Although not abundant in the geologic record, lake sediments are
nonetheless important. Lake chemistry is sensitive to climatic conditions, making
lake sediments useful indicators of past climates. For example, several studies
have
shown that ancient episodes of wet and dry climates can be deciphered on
the basis of lake sediment chemistry and mineralogy. Also, some lake deposits
contain economically significant quantities of oil shales, evaporite minerals, coal,
uranium, or iron. Many lake sediments also contain abundant fine organic matt
that may act after burial as a source material for petroleum (Katz, 1990).
Origin and Size of lakes
The basins, or depressions, in which lakes form can be created by a variety of
mechanisms, including tectonic movements such as faulting and rifting; glacial
processes such as ice scouring, ice damming, and moraine damming; landslides or
other mass movements; volcanic activity such as lava damming or crater explo
sion and collapse; deflation by wind scour or damming by windblown sand; and
fluvial activity such as the formation of oxbow lakes and levee lakes. Many exist
ing lakes appear to have originated directly or indirectly by glacial processes (Pi
card and High, 1981) and thus may not be typical of ancient lakes, which formed
predominantly by tectonic processes. the oer hand, we know that some large
modern lakes also formed by tectonic processes (e.g., Lake Tanganyika in the East
African rift system, Lake Baikal in the Baikel rift system in Siberia) and volcanic
processes (e.g., Crater Lake, Oregon). Of the twenty-five largest lakes by surface
8.4 custrine Systems
269
area today, ten are of glacial origin, seven occupy cratonic depressions, and four
are in rift valleys (Smith, 1990).
Modern lakes range in areal dimensions from a few tens of square meters to
tens of thousands of square kilometers. The largest modem lake is the saline, in
land Caspian Sea with a surface area of 436,000 km2 (Van der Leeden, 1975). Other
large Jakes with surface areas ranging between 50,000 and 100,000 km2 include
Lake Superior, Lake Huron, and Lake Michigan in North America; Lake Victoria,
located between Uganda and Kenya in east-central Africa; and Lake Aral east of
the Caspian Sea. Water depths of modern lakes range from a few meters in small
ponds to more than 1700 min the world's deepest lake, Lake Baikal, Siberia. Water
depth and surface area are not necessarily related; us, some of the largest Jakes
have very shallow depths and vice versa. For example, Lake Victoria has a surface
area of 68,000 km2 but a maximum depth of only 79 m, whereas Crater Lake, Ore
gon, with a surface area of about 52 km2 has a maximum depth of about 580 m.
Preserved lacustrine sediments show that ancient lakes also ranged in size
from small ponds to large bodies of water exceeding 100,000 km2. Three of the
largest ancient lakes recognized are the Late Tr iassic Popo Agie Lake of Wyoming
and Utah, which had a minimum areal extent, based on the preserved sediment
record, of 130,000 km2 (Picard and High, 1981); the Jurassic T'oo'dichi' Lake of the
eastern Colorado Plateau, with an area of 150,000 km2 (Turner and Fishman,
1991); and the Eocene Green River Basin, with an area of about 100,000 km2 (Eug
ster and Hardie, 1978). According to Bohacs, Caroll, and Neal (2003), ancient lake
sata in the Cretaceous svstem of the South Atlantic and eastern China and the
Permian system of wester� China extend up to 300,000 km2 Reported thickness of
pserved ancient lake sediments ranges from less an 20 m to as much as 9000 m
(e.g., Pliocene Ridge Basin Group, California; Link and Osborne, 1978). Lake size
and character is a complex function of four main variables: basin-floor dep, sill
height, water supply, and sediment supply (Bohacs, Carroll, and Neal, 2003).
Lake Seings and incipal Kinds of Lakes
Modem lakes occur in a variety of environmental settings, including glaciated in
land plains and mountain valleys, nonglaciated inland plains and mountain re
gions, deserts, and coastal plains. They exist under a spectrum of climatic
conditions ranging from very hot to very cold and from highly arid to very humid.
Most lakes are filled with fresh water, but others, such as the Caspian Sea and
many lakes in arid regions (e.g., Great Salt Lake, Utah) are highly saline. Many
lakes are associated with other types of depositional systems, notably glacial, flu
vial, eolian, and deltaic systems. The depositional processes that occur in lakes are
influenced both by climatic conditions and by a variety of physical, chemical, and
biological factors that include the chemistries of their waters and fluctuations in
their shorelines and siliciclasc sediment supply. Some attributes of lacustrine de
positional environment are similar to those of marine environments; however, im
portant diffrences exist in terms of such factors as basin size, water chemistry,
physical processes (e.g., no tides in lakes), and biologic processes (e.g., Gierlowski
Kordesch and Kelts, 1994b).
Open lakes are those that have an outflow of water and a relatively stable
(fixed) shoreline and in which inflow and precipitation are approximately bal
anced by outflow and evaporation. Siliciclastic sedimentation commonly pre
dominates in open lakes; however, chemical sedimentation can occur in open
lakes that have a low supply of clastic sediment. Closed lakes do not have a
major outflow and have uctuating shorelines; inow is commonly exceeded by
evaporation and infiltration. These conditions lead to concentration of ions in
lake water and a predominance of chemical sedimentation, although siliciclastic
sediments may accumulate also.
270
Chapter 8 I Continental (Terrestrial) Environments
Factors Controlling Lake Sedimentation
The kinds of sediments deposited in lakes are the result of a complicated balance
among physical, chemical, and biological processes (Fig. 8.23). Climatic factors af
fect lake sedimentation in numerous ways. For example, the global distribution of
lakes reflects global climate patterns. Water level in lakes is maintained by the bal
ance between evaporation and precipitation. Climate can determine whether a
lake is filled to overflowing (open) or acts as an internal drainage basin (closed).
The kind of chemical sedimentation lakes strongly reflects climatic conditions.
For example, chemical sedimentation in lakes of arid regions is dominated by pre
cipitation of gypsum, halite, and various other salts, but in humid climates chem
ical sedimentation is dominated by carbonate deposition. Sediment input to lakes
is uenced by the vegetation cover in the drainage area of the lakes and is great
est in arid regions with low vegetation cover. In cold climates, seasonal drops
temperature lead to freezing of lakes, causing decrease in sediment input and ces
sation of wave activity, allowing deposition of fine-grained suspended sediment
during these quiet-water conditions. Climate and the physiography of lake set
tings also determine the local weather conditions over lakes. Severe, localized
storms with high winds can cause considerable shore erosion, coupled with sedi
ment transport and deposition, during short periods of time.
Physical Processes
Physical processes that interact in lakes to bring about sediment transport and de
position include wind, river inflow, and atmospheric heating. Wind processes are
of major importance because winds create waves and currents. River inflow may
generate plwnes of fine sediment that extend in surface wa ters far out into a lake
(Fig. 8.23), or it may generate density underflows, or turbidity cunents, at carry
sediment along the bottom toward the basin center. River inflow can also create
currents that flow along the margins of lakes. Other currents may be generated by
flow-through of water along the lake bottom toward a point of lake discharge. At
mospheric heating, which is a function of climate, is responsible for deRsity differ
ences in lake water. These differences can cause stratification of water on the one
hand (heating of surface water) or, under some conditions, generation of density
Water balance
(controlled by
precipitation,
inflow, evaporation)
� up
Thermocline
(controlled by
climate, season)
Lake depth
(controlled by
tectonics,
water balance)
Figure 8.23
Solar radiation
(controls water
mixing, stratification,
overturn, biologic
productivity
Waves
(controlled by
wind intensity,
lake size [fetch],
depth)
River inflow
(affects sediment
supply, density
flows, water balan)
I
pH and ion concentration
(ntrolled by precipitation, water
inflow, evaporation, ra tes of chemical
sedimentation)
Sedimentary processes in lakes involve a balance between clastic input by rivers, oshore
and longshore redistribution of clastics by waves and wave-produced currents, transport
of fine clastics as turbid interflows, downslope movement of fine and coarse clastics by
turbidity currents, and in situ production of biological and chemical sediment. The term
thermocline refers to the boundary between warm, low-density suace water and colder,
higher density deeper water. [Diagram based on numerous sources.]
8.4 Lacustrine Systems
271
currents (by cooling of surface water) that produce mixing and lake overtu.
Also, temperature variations may cause alternate freezing and melting of lake sur-
face waters, thereby affecting sediment transport within the lake.
Thus, a variety of sediment transport and depositional mechanisms operate
in lakes. Deposition of siliciclastic sediment in the calmer, deeper portion of lakes
can take place by settling of fine particles that were suspended in the water col
umn owing to wave and current activity, or deposition may occur from turbidity
currents generated where sediment-laden streams discharge into lakes. Sedimen
tation can occur also along the shallow shoreline of lakes from wind-generated
traction currents or river-inflow currents deflected along the lake margin. One
type of lake sedimentation process that appears to be particularly characteristic of
cold-climate lakes is the formation of vaes, which are very thin, alternating
light- and dark-colored sediment layers. Thicker, light-colored, coarse-graed
laminae accumulate suspension settling of fine sediment during summer condi
ons. Thinner, finer grained, organic-rich, dark laminae form by slow suspension
settling during winter months when lakes are frozen.
Chemical Processes
Deposition of chemically formed sediment is particularly common in closed lakes.
e chemistry of lake waters varies from lake to lake but is dominated by calcium,
magnesium, sodium, potassium, carbonate, sulfate, and chloride ions. us, the
most common chemical sediments in the lakes of humid regions are carbonates, al
though phosphates, sulfides, cherts, and iron and manganese oxides are present in
some lakes. arid regions, where rates of evaporation are high, chemical lake sedi
ments are dominated by carbonates, sulfates, and chlorides. The evaporite deposits
of lakes include many common marine evaporite minerals such as gypsum, anhy
drite, halite, and sylvite, but they also include several minerals such as trona, borax,
epsomite, and bloedite that are not common in marine evaporites (Chapter 7). e
pH of lake waters commonly falls between 6 and 9; however, it can range om less
than 2 (highly acidic) in some volcanic lakes to as much as 12 (highly alkaline) in
some closed desert lakes. Although chemical sedimentation pcesses are most im
portant in closed lakes, they may predominate also in some open lakes where the
clastic sediment supply is low.
Biological Processes
Organisms play an important role in lake sedimentation by extracting chemical
elements from lake water to build shells and the subsequent deposition of these
shells, extraction of C02 during photosynthesis (thereby aiding precipitation of
CaC03), contributing plant remains to form plant deposits, and bioturbation of
sediments. Many kinds of organisms live in lakes and contribute their skeletal
and nonskeletal remains to lake sediments. Siliceous diatoms are particularly
widespread and noteworthy. Diatoms carry out photosynthesis and are the only
important type of lake organism that produces siliceous tests. Their remains form
important diatomite deposits in many Pleistocene lakes. Pelecypods, gastropods,
calcareous algae, and ostracods also abound in many lakes and are important
contributors of calcium carbonate sediments. Blue-green algae (cyanobacteria)
carry on photosynthesis and also trap fine sediment to form stromatolites. Many
dierent types of higher plants live in lakes. Under the reducing conditions and
high sedimentation rates that exist some lakes, the remains of higher plants
may be partially preserved to eventually form peat and coal. Considering the
small size of many lakes and their generally lower alkalinity and buffering capac
i, compared to those of the open ocean, the assimilation of C02 by plants during
photosynthesis is a much more important factor in controlling the pH of lakes
an that of the ocean. Thus, increase in pH caused by photosynthetic removal of
272
Chapter 8 I Continenta
l e rrestrial) Environments
C02 likely exerts a dominant control in facilitating carbonate sedimentation in
lakes. Finally, organisms such as pelecypods, freshwater shrimp, and worms may
burrow and rework lake sediments, destroying laminations and other primary
sedimentary structures.
Characteristics of Lacustrine Deposits
Bohacs
et al. (2000) suggests that lakes can be divided into three types on
the basis
of fill characteristics: overfi, balance-ll, and underfilled. Overfilled lake bas
have persistently open hydrology, freshwater lake chemistry, progradational
shoreline architecture, and commonly interbedded fluvial deposits. ey occ
when rate of supply of sediment + water consistently exceeds accomodation
space (the space available in which sediments can accumulate). Balance-fill
basins have intermittently open hydrology, fluctuating lake water chemistry, ther
mal and chemical stratification, mixed progradational and aggradational architec
ture, and varied interbedding of clastic and carbonate strata. is lake-basin type
occurs when rates of sediment + water supply and accomodation are roughly
balance. Underfilled lake basins have persistently closed hydrology, characterisc
chemical stratification, high solute content of lake waters, extensive desiccation
(drying) features, highly contrasting lithologies, common association with evap
ote deposits, and domantly aggradational shoreline architecture. This basin
type occurs when rates of accomodation consistently outstrip available water and
sediment supply, resulting in closed basins with ephemeral lakes interspersed
with playas or brine pools or both.
The sediment of most hydrologically open lakes are dominated by siliciclas
tic deposits, derived mainly from rivers-but possibly including windblown, ice
rafted, and volcanic detritus. Much of this sediment is deposited along the shores
of lakes, particularly near river mouths. Gravelly sediment may be present in the
toes of alluvial fans or fan deltas that extend to the lake edge or into the lake. Sand,
likewise, accumulates mainly along the lakeshore in deltas, beaches, spits, or bar
riers. Sand may also carried by turbidity currents into the middle of the lake
(Fig. 8.23), however, deeper parts of the lake are characterized particularly by the
presence of fine silt and clay. Some muddy sediment is transported into deeper
water by surface overflows. In density-stratified lakes, muddy sediment may also
be carried as a turbidity terflow above cold, denser lake water. Coarser particles
in such interows settle fairly quickly and accumulate as silt layers. Finer particles
settle more slowly to form clay layers. Thus, the siliciclastic deposits of open lakes
may
consist of deltaic sands and muds (and possibly alluvial-fan gravels), tur
bidite sands and silts, and homogeneous to laminated muds.
In open lakes where the clastic sediment supply is low, chemical and bio
chemical processes predominate, resulting in deposition of largely chemical sedi
ments. Primary inorganic carbonate precipitation (caused by loss of C02
through
plant photosynthesis and/ or increase in water temperature or mixing of water
masses) and production of shells (by calcium carbonate- or silica-secreting organ
isms) account for most of the sedimentation. The principal types of invertebrate
remains in lacustrine sediments include bivalves, ostracods, gastropods, diatoms,
and charophytes and other algae. Chemical lake deposits consist maly of car
bonate sands and muds (less commonly siliceous diatom deposits). Stromatolites
produced by blue-green algae (cyanobacteria) are common also in some lake de
posits. Various amounts of noncarbonate organic matter and some siliciclastic sed
iment may be present. Plant life is commonly abundant in shallow water around
lake margins, and plant deposits may become important during the late stages of
lake filling. Carbonate sediments may interfinger along the lake margin with sili
ciclastic deltaic or alluvial deposits. Typical facies in an open lake with low sili
clastic sediment input are illustrated in Figure 8.24.
Shoreline
sand
gure 8.24
Bioturbated carbonate
{charophytic) sands,
silts, and muds
Interbedded
turbidites and
oanic-rich,
limy mudstones
River
input
8.4 Lacustrine Systems
273
Deltaic Alluvial fan
sand
conglomerate
Sediment types in open lake characterized by low siliciclastic sediment input include both
chemical/biochemical and siliciclastic sediment. By contrast, the deposits of open lakes
with high clastic input consist predominantly of siliciclastic sediment. [Redrawn from Eug
ster, H. P., and K. Kelts, 1983, Lacustrine chemical sediments, in Goudie, ]. j., and K. Pye
(eds.), Chemical sediments and geomorphology: Academic Press, New York, Fig. 12.2,
p. 333, reproduced by permission.]
Hydrologically closed lakes occur in regions of interior drainage where lake
levels may experience considerable fluctuation owing to seasonal flooding. Allu
vial fans are commonly present around the borders of such lakes, and the sandy
aprons (sandflats) of such fans may extend into the lake. During high water, the
edges of these sandflats can be reworked by wave action, resulting in redeposition
of wave-rippled sandy sediment along the lake edge. Most sedimentation in
dosed lakes takes place by chemical/biochemical processes in waters made saline
by
high rates of evaporation. Two kinds of closed lakes are recognized. Perennial
basins receive inflow from at least one perennial stream. They commonly do not
dry up completely from year to year, although some may dry up occasionally.
Most perennial lakes are saline, but some are dilute. The deposits of perennial
la kes include carbonate muds, silts, and sands, commonly with intergrowths of
evaporite minerals, and may include stromatolites (Fig. 8.25). Bedded evaporites
may be present in the central part of the lake. Ephemeral salt-pan basins are fed
by ephemeral runoff, springs, and groundwater and are generally dry through
part of each year. Ephemeral salt-pan deposits may also contain carbonate sedi
ments, including spring travertine or tufa, but bedded salt deposits are much
more important. Saline deposits interfinger with siliciclastic sandflat deposits
arod e margin of the salt pan.
Bioturbated
carbonate muds,
silts, and sands
Bedded salt and
organic-rich
laminated
carbonate muds
Carbonate muds,
silts, and sands
with desiccation
cracks and
growth of salts
Fringing
alluvial
fan
gure 8.25
Depositional subenvironments
and sediment types in a hydro
logically closed, perennial saline
lake basin. [Redrawn from Eug
ster, H. P., and K. Kel, 1983,
Lacustrine chemical sediments,
in Goudie, ]. J., and K. Pye
(eds.), Chemical sediments and
geomorphology: Academic
Press, New York, Fig. 12.8 and
12.9, p. 351, reproduced by
permission.)
274 Chapter 8/ Continental (Te rrestrial) Environments
Numerous kinds of sedimentary structures occur in lake sediments, in
cluding laminated bedding, varves, stromatolites, cross-bedding, ripple marks,
parting lineations, graded bedding, groove casts, load casts, soft-sediment de
formation structures, burrows and worm trails, raindrop and possible ice-crystal
impressions, mudcracks, and vertebrate footprints. Varves are one of the mo
diagnostic characterisc of lake sediments, but light and dark laminae resem
bling varves have also been reported in nonlacustrine sediments (e.g., some lam
inated marine deposits). Another distinguishing characterisc of lake sediments
is that individual lake beds tend to be thin and laterally continuous compared to
associated fluvial deposits (although total lake sediments can be very thick).
Otherwise, no uniquely diagnostic structures occur in lake sediments. Many
sedimentary structures of lacustrine deposits are similar to those of shallow ma
rine sediments.
Owing to high sedimentation rates in lakes and the fact that they are essen
tially closed systems with respect to sediment transport, all lakes are ephemeral
features. Lake basins eventually fill with sediment, and most are converted into
fluvial plains as they are overrun by fluvial systems. Therefore, lake filling is
commonly
regarded as a regressive process. That is, coarser, nearshore sediments
are believed to gradually encroach on ner lake basin sediments and to be cov
ered in turn with fluvial sediments. This postulated process of filling theoretical
ly generates shallowing and coarsening upward successions of lake facies.
Although the ultimate filling of lakes and their encroachment by prograding
fluvial or other coarser grained deposits may generate a gross coarsening-up
ward pattern of facies, ideal coarsening-upward successions of lake sediments
probably rarely occur, except perhaps in some very small lakes (Picard and
High, 1981).
Ancient Lake Deposits
Lake sediments are preserved in a variety of tectonic settings, including exten
sional rift systems, strike-slip basins, foreland basins, and cratonic basins. Ancit
lake sediments are known from many parts of the world in sedimentary succes
sions ranging in age from Precambrian to Holocene (e.g., Gierlowski-Kordes
and Kelts, l994a).
Some of the better-known lacustrine deposits in North America include the
Pliocene Glenn Ferry Formation of the Snake River Pla; the Eocene Green River
Formation of Utah, Colorado, and Wyoming, own for its oil-shale deposits;
much of the Jurassic Morrison Formation of the Colorado Plateau, renowned for
its dinosaur remains; parts of the Triassic Chugwater Group of Wyoming; Tr iassic
Supergroup rift basins of eastern North America; e Devonian Escuminac Forma
tion of souern Quebec; and the Carboniferous Strathloe Formation of Nova
Scotia. Some well-known lake deposits from other parts of the world include the
Cenozoic rift-basin deposits of East Africa; the Cretaceous rift-basin deposits of
Brazil (which are source-rocks for much of Brazil's oil; Abrahao and Wa rme, 1990);
the clastics, evaporites, and carbonates of the Tr iassic Keuper Marl of south Wales;
e Permo-Triassic Beaufort strata of the eastern Kao Basin, Natal, South Africa;
parts of the Lower Permian Rotliegend deposits of southwest and easte Ger
many; and the middle Devonian sediments from the Old Red Sandstone of the Or
cadian Basin of northeast Scotland, Many of these lake deposits throughout the
world include thick successions of organic-rich shales that are important source
rocks for petroleum (Katz, 1990). A recent monograph edited by Gierlowski
Kordesch and Kelts (2000), entitled "Lake Basins through Space and Time,"
summarizes the characteristics of 60 additional ancient lakes ranging in age fm
Carboniferous to Quaternary.
8.4 custrine Systems
275
Because
lakes vary widely in size and hydrologic characteristics (e.g.,
open vs. closed), and lake sediments are correspondingly diverse, it is not pos
sible to select a single example that illustrates a "typical" ancient lake. The
Green River Formation (Eocene), which extends over more than 100,000 km
2
in
Wyoming, Colorado, and Utah, provides an example of a large, well-studied,
lacustrine deposit at formed under fluctuating hydrologic and climatic condi
ons. The formation exceeds 2 km in thickness and has vast reserves of oil shale
and trona, a sodium carbonate. It was deposited in two Eocene lakes, Lake Uinta
and Lake Goshiute. Lake Uinta, which extended over the Uinta Basin of Utah and
e Peance Basin of Colorado (e.g., Ryder et al., 1976), was a perennial, moder
ately deep lake in which oil shales and carbonates were deposited. Lake Goshiute,
located the Green River Basin of Wyoming, was a shallow, ephemeral playa
that appears to record changes from pluvial (wet) climatic conditions during
its early history to arid and then back to pluvial through time.
The principal deposits of the Green River Formation in the Green River
Basin (Lake Goshiute) are shown in Figure 8.26, after Roehler (1992). During its
early history (early Eocene) when the Luman To ngue and Tipton Shale Member
we deposited, Lake Goshiute was a freshwater lake enriched in calcium car
bonate and fine-size organic matter. These conditions favored deposition of oil
shales and dolomitic mudstones, together with mudstone, sandstone, tuff, and
limestone. [As discussed in Chapter 7, oil shales are dark-colored shales that con
tain significant quantities of kerogen, which can be converted into oil by heating.)
During deposition of the Wilkins Peak Member in early to middle Eocene time,
arid conditions prevailed and the lake became hypersaline. Beds of evaporites
(trona and halite) as much as 10 m thick were deposited along with dolomitic
mudstones, oil shales, sandstone, and algal limestone. Evaporite deposits grade
laterally to mudflat, sandflat, and alluvial-fan deposits. Wet conditions returned
middle Eocene time, bringing about a change from evaporitic deposition to de
position of freshwater oil shales, mudstones, siltstones, and algal limestones that
make up the Laney Member. The lacustrine deposits of the Green River Forma
tion as a whole interfinger laterally with fluvial deposits of other, equivalent-age,
formations (Wasatch Formation, Battle Springs Formation, Bridger Formation).
Flgure 8.26
Lacustrine deposits of the
Green River Formation in the
Green River Basin, Wyoming.
[Redrawn from Roehler, H.
W., 1992, Correlation, com
position, areal distribution,
and thickness of Eocene
stratigraphic units, greater
Green River Basin, Wyoming,
Utah, and Colorado: U.S. Ge
ological Survey Professional
paper 1506-E, Fig. 1, p. E2,
reproduced by permission.]