Boggs S. Principles of Sedimentology and Stratigraphy
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6 Carbonaceous Sedimentary Rocks: Coal, Oil Shale, Bitumens
235
energy than the energy required to produce the oil initially (Hutton, 1995). The
term oil shale is actually a misnomer because relatively little free oil occurs in
ese rocks, although small blebs, pockets, or veins of asphaltic bitumins may be
present. Oil shales are of particular interest because of their potential to generate
oil when refined into fuel at sufficiently high temperatures. At least fifty countries
of the world have reserves of oil shale that have the potential to be exploited as a
fuel resource in the future. More than 80 percent of the organic matter in oil shales
is present in the form of kerogen, which yields oil when heated to a temperare of
about 350"C. The principal constituents of oil shales are shown in Figure 7.24. Or-
ganic constituents commonly do not exceed about 25 percent of the rock. Kerogen
is disseminated organic matter that is insoluble in nonoxidizing acids, bases, or
organic solvents (Durand, 1980; Horsfield, 1997). It consists of masses of almost
completely macerated (disintegrated by biochemical or chemical processes) or-
ganic debris. the basis of petrographic characteristics, this organic matter con-
sists primarily of liptinite macerals (see Table 7.4). Vitrinite and inertinite macerals
are commonly present in only minor amounts (Hutton, 1995). The organic matter
in oil shale is derived from three primary sources: terrestrial plants, lacustrine
(lake) algae, and marine organisms such as algae and dinoflagellates. Organic
matter from terrestrial plants is commonly less important an that from lacus-
ine algae and marine organisms.
Not all so-called oil shales are actually shales. Some are organic-rich silt
stones, limestones, and impure coals. Carbonate-rich oil shales are those in
which the principal nonkerogen constituents are calcite, dolomite, ankerite,
siderite, and various amounts of siliciclastic silt (Duncan, 1976). Silica-rich oil
shales are shales which the main constituents apart from kerogen are fine
graed quartz, feldspar, and clay minerals. They may also contain chert, opal,
and phosphatic nodules. Siliceous oil shales are generally dark brown or black.
Cannel shale is an oil shale that consists predominantly of organic matter that
completely encloses other mineral grains. Cael shales are sometimes classified
as impure cannel coals. Many oil shales are characterized by distinct lamination
caused by alternations of millimeter-thick organic laminae with either siliciclastic
or carbonate laminae.
The amount of oil that can be extracted from oil shales through heating and
retorting ranges between 10 and 150 gallons of oil per ton of rock (Duncan, 1976).
e potential world supply of oil from oil shale is estimated to be 2000 trillion tons
(Russell, 1990, p. 4). On the other hand, many technological problems exist with
respect to mining, extracting, and refining oil shale. Oil shale is not now being
processed commercially because it cannot compete economically wi petroleum.
Oil shales form in environmen where organic matter is abundant and
anaerobic or reducing conditions prevent oxidation and total bacterial decomposi
tion. They are deposited in both lacustrine (lake) and marine environments where
e above condions are met. The three principal environments are large lakes;
shallow seas or continental platforms and continental shelves in areas where water
Oil shale
{ Quartz
Feldspars
Inorganic matrix
Clays (mainly illite and chlorite)
Carbonates (calcite and dolomite)
Pyrite and other minerals
Bitumens (s oluble in CS2)
Kerogens (insoluble in CS
2
containing U, Fe, V, Ni, Mo)
86%
3%
11%
Fire 7.24
Principal constituents in oil
shales. [After Yen, T. , and
G. V. Chilingarian, 1976, In
troduction to oil shales, in
Ye n, T. F., and G. V. Chilin
garian (eds.), Oil shales: De
velopments in petroleum
science 5, Fig. 1.2, p. 3,
reprinted by permission of
Elsevier Science Publishers,
Amsterdam.}
236
Chapter 7 I Other Chemical/Biochemical and Carbonaceous Sedimentary Rocks
Figure 7. 25
Schematic representation of
three kinds of petroleum
traps: A. Anticlinal trap. B.
Fault trap, with an imperme
able fault gouge or mineral
seal along the fault. C. Strati
graphic (pinch-out) trap.
circulation was restricted and reducing or weakly oxidizing conditions existed;
and small lakes, bogs, and lagoons associated with coal-producing swamps. Oi•
shales formed in lakes or swamps may be associated with impure cannel or bog·
head-type coal, tuffs and other volcanic rocks, or even evaporites. Many oil shales
deposited in large lakes are carbonate-rich types and tend to have high oil yields,
apparently owing to enhanced preservation potential of organic material in lake
environments. Oil shales deposited in marine environents are characterisfcally
the silica-rich type and have lower oil yields, ahhough some Te rtiary and Meso
zoic siliceous oil shales have rich oil yields. Oil shales exted mrer wide geograph·
ic
areas and are commonly associated with limones, cherts, sandstones, and
phosphatic deposits (Yen and Chilingarian, 1976).
Petroleum and Natural Bitumins
Petroleum.
Petroleum is not sedimentary rock but a carbon-rich, organic sub
stance that accumulates predominantly in sandstones and carbonate rocks. For
this reason, it is included here with the carbonaceous sedimentary rocks. Petrole
um forms from plant and animal organic matter by a complex maturation process
during burial that involves initial microbial alteration and subsequent thermal al
teration that forms a complex organic substance called kerogen. Kerogen can sub
sequently dergo thermal degradation (cracking) at burial depths exceeding
about 1000 m and temperatures of about 50° to l20°C to form liquid petroleum, a
process called catagenesis (Ht, 1996, p. 60; Tissot and We lte, 1984, p. 69). Liqu�d
petroleum may subsequently be cracked at temperatures ranging from about 150°
to 200°C to form natural gas (e.g., methane).
The source materials for the organic matter that eventually converts to pe
troleum are contaed primarily in organic-rich shales and carbonate rocks. After
petroleum has formed from organic materials in these fine-grained source rocks,
at
substantial burial depths, it migrates out of the source rocks (a process referred
to as primary migration) into coarser grained, porous, and permeable sandstone or
carbonate rocks. It then migrates through water-fill pore spaces in these rocks
(secondary migration) to structurally higher sites, where it eventually acculates
in traps such as anticles (Fig. 7.25). The rocks in wch petroleum accumulates
A
c
D
D
m
B
CONGLOMERATE
SANDSTONE
SILTSTONE
SHALE
LIMESTONE
7. 6 Carbonaceous Sedimentary Rocks: Coal, Oil Shale, Bitumens
237
these traps, called reservoir rocks, are primarily porous sandstones, limestones,
and dolomites.
Petroleum is composed dominantly of carbon (about 85 weight percent) and
hydrogen (about percent), with about 2 percent sulfur, nitrogen, and oxygen
(Hunt, 1996). Despite its simple elemental chemical composition, the molecular
structure of petroleum can be exceedingly complex. The molecules in petroleum
range from the simple methane gas molecule (CH4) with a molecular weight of
16 to larger molecules with molecular weights in the thousands. Several hundred
different hydrocarbons have been recorded in natural crude oils; however, all hy
drocarbons can be grouped into a few basic classes or series having common mol
ecular structural form. These structural forms are complex and are not explained
in detail here, but the main hydrocarbon series are as follows:
1. paraffins (alkanes)-open chain molecules with single covalent bonds
between carbon atoms (Fig. 7.26)
2. napthenes (cycloparaffins)losed ring molecules with single covalent
bonds between carbon atoms (Fig. 7.27)
3. aromatics (arenes)-one or more benzene ring structures with double cova
lent bonds between some carbon atoms (Fig. 7.28)
Most natural gases as well as many liquid petroleums belong to the paraffin
series of hydrocarbons. Most napthene hydrocarbons are liquid petroleums, al
ough two occur as gases at normal temperatures. The aromatics, which are
named for their strong aromatic odor, are liquid petroleums. They commonly
make up only a small percentage of the petroleums in natural crude oils.
Natural Bitumens. These substances are hydrocarbons such as natural asphalts
and mineral waxes that occur in a semisolid or solid state. Most natural bitumens
probably formed from liquid petroleums that were subjected to loss of volatiles,
oxidation, and biologic degradation after seepage to the surface. Others may
never have existed as light oils. Bitumins occur as seepages, surface accumula
tions, impregnations occupying the pore spaces of sandstones or other sedimenta
rock (e.g., the Cretaceous Athabasca tar sands of Canada), and in veins and
A
Butane, C
4
H
10
CH3(CH2bCH3
H H H H
I I I I
H-C-C-C-C H
I I I I
H H H H
A Cyclopentane, C
5
H
10
CH
2
CH2CH
2
CH2
CH2
B Pentane
,
C
5
H
12
CH3(CH2hCH3
H H H H H
I I I I I
H-C-C-C-C-C H
I I I I I
H H H H H
B Cyclohexane, C6H
12
CH2CH2
CH
2
CH
2
CH2CH2
gure 7. 26
Schematic structure of paraffin hydrocarbons having the general
formula
C0
H
2n+
2
where n refers to the number of carbon or hy
drogen atoms. A. Butane. B. Pentane.
gure 7. 27
Schematic structure of naphthene (cycloparaffin) hydrocarbons
having the general formula CnH
2
0
. A. Cyclopentane. B. Cyclo
hexane.
238
Chapter 7 I Other Chemical/Biochemical and Carbonaceous Sedimentary Rocks
Figure 7.28
H
I
H
C
H
"/�/
c
c
II
I
c c
/""
Schematic structure of aromatic hydrocarbons having the general for
mula CnH2n_6. A. Benzene. B. Toluene.
H
C
H
I
H
Figure 7.29
dikes. They are black or dark brown and have a characteristic odor of pitch or
paraffin.
Natural bitumens have roughly the same elemental chemical composition as
liquid petroleum, but the percentage of carbon and hydrogen tends to be some
what lower and the content of sulfur, nitrogen, and oxygen somewhat higher.
They are divided into two main types on the basis of solubility in carbon disulfide
(CS2), an organic solvent (Fig. 7.29): soluble bitumens and pyrobitumens. The sol
uble bitumens are further divided on the basis of ease of fusibility or melting into
three groups: mineral waxes, natural asphalts, and asphaltites.
Mineral waxes are solid, waxy, light-colored substances that consist largely
of paraffinic hydrocarbons of high molecular weight. Most represent the residuum
of high-wax oils exposed at the surface. The most important native mineral wax is
ozocerite, which consists of veinlike deposits of greenish or brown wax. Montan
wax is an extract obtained from some kinds of brown coals or lignites.
Natural asphalts are soft, semisolid bitumins that occur as seeps, surface
pools, or viscous impregnations in sedents (tar sands). They are dark colored,
plastic to fairly hard, easily fusible, d soluble in carbon disulfide. Varietal names
I
I Mine W I I
I
e
Montan wax
Sche
I
NATURAL BITUMENS
I
Soluble in carbon
Insoluble in ca
disulfide (CS2)
disulfide (C
I
I
Bitum I
Pyrobit umens
Fusible
I
I
Natural phalt
l
Athabaaca
inidad Lake
Ta bbylte
Diicult to fuse
I
I
phalt ita
I
GH&onlte
G
rahamite
G
lance pitch
Hydrogen/Carbon Hydrogen/Carbon
ratio > 1 ratio < 1
Te rminology of principal kinds
of naturally occurring solid hy
drocarbons. Based on Rogers,
McAiary, and Bailey, 1974;
Hunt, 1979; Cornelius, 1987;
Meyer and De Witt, 1990 .
lmpsonite
Anthraxolite
Shungite
Further Readlng
239
for asphalts from different areas are shown in Figure 7.29. Asphalts are commonly
associated with active oil seeps.
Asphaltites occ primarily in dikes and veins at cut sediment beds. They are
harder and denser than asphalts and melt at higher temperatures. ey a largely
soluble carbon disulfide. Names applied to varieties of asphaltites that differ
slightly in density, fusibilit and solubility are gilsonite, glance pitch, and grahamite.
robitumins, like asphaltites, occur in dikes and veins but are infusible and
largely insoluble in carbon disulfide. Several varieties of pyrobitumins are recog
nized, which can be placed into two general groups on the basis of hydrogen/ car
bon ratio (Fig. 7.29). Those with H/ C ratios > 1 include elaterite, a soft elastic
substance rather like India rubber, and wurtzlite, also a softer form. More indurat
ed forms are albertite, a black, solid bitumin with a brilliant jetlike luster and con
choidal fracture, and ingramite. The metamorphosed pyrobitumins, impsonite,
anthraxolite, and shungite, are indurated forms that have have H/C ratios <1.
The solid hydrocarbons are of interest to geologists because their presence at
the surface is an indication of petroleum at depth in a region, and because study of
their occurrence may help to solve the problems related to the origin and alter
ation of petroleum. Also, many of the solid hydrocarbons are of commercial value
themselves. (See also Chilingarian and Ye n, 1978; Cornelius, 1987; Meyer, 1987;
and Meyer and De Witt, 1990.)
FURTHER READING
Evaporites
Busson, G., and Schreiber, B. C. (eds.), 1997, Sedimentary deposi
tion in rift and foreland basins in France and Spain: Columbia
University Press, New York, 479 p.
Melvin, J. L (ed.), 11, Evaporites, petroleum and mineral re
sources: Elsevier, Amsterdam, 555 p.
hreiber, B. C. (ed.}, 1988, Evaporites and hydrocarbons: Colum
bia University Press, New Yo rk, 475 p.
Wa rren, J. K., 1989, Evaporite sedimentology: Prentice Hall, En
glewood Cliffs, N.J., 285 p.
Wa rren, J., 1999, Evaporites: Their evolution and economics:
Blackwell Sciences Ltd., Oxford, 438 p.
Siliceous Sedimentary Roc
Heaney, P. J., C. T. Prewitt, and G. Gibbs (eds.), 1994, Silica:
Physical behavior, geochemistry and materials applications:
Mineralogical Society of America Reviews in Mineralogy, v.
29, 606 p.
Hein, J. R. (ed.), 1987, Siliceous sedimentary rock-hosted ores
and petroleum: Van Nostrand Reinhold, New York, 304 p.
Hein, J. R., and J. Obradovic (eds.), 1989, Siliceous deposits of the
Te thys and Pacific regions: Springer-Verlag, New York, 244 p.
lijima, A., J. R. Hein, and R. Siever (eds.), 1983, Siliceous deposits
in the Pacific region: Elsevier, Amsterdam, 472 p.
In-rich Sedimentary Roc
Appel, P. W. U., and G. L. LaBerge, 1987, Precambrian iron
formations: eophrastus, S. A., Athens, Greece, 67 4 p.
Melnik, Y. P. , 1982, Precambrian banded iron-formations: Devel
opments in Precambrian Geology 5: Elsevier, Amsterdam, 310
p. (Translated from the Russian by Dorothy B. Vitaliano).
Petnek, J., and B. Van Houten, 1997, Phanerozoic ooidal iron
stones: Czech Geological Survey Special Papers 7, Czech Ge
ological Survey, Prague, 71 p.
Trendall, A. , and R. C. Morris (eds.), 1983, Iron-formation facts
and problems: Developments in Precambrian Geology 6: El
sevier,
Amsterdam, 558 p.
Van Houten, B., and D. P. Bhattacharyya, 1982, Phanerozoic
oolitic ironstone: Geologic record and facies models: Ann.
Rev. Earth and Planet. Sci., v. 10, p. 441-457.
Young, P., and W. E. G. Taylor (eds.}, 1989, Phanerozoic iron
stones: Geol. Soc. Spec. Pub. 46, The Geological Society, Lon
don, 251 p.
Phosphorites
Baturin, G. N., 1982, Phosphorites on the sea oor: Origin, com
position and distribution: Developments in Sedimentology
33,
Elsevier, Amsterdam,
343 p. (Translated from Russian by
Dorothy B. Vitaliano.)
Bentor, K. (ed.}, 1980, Marine phosphorites-geochemistry, oc
currence, genesis: Soc. of Econ. Paleontologists and Mineralo
gists Special Publication No. 29, Tu lsa, Okla., 249 p.
Burnett, C., and S. R. Riggs (eds.), 1990, Phosphate deposits of
the world: v. 3: Neogene to modem phosphorites: Cambridge
University Press, Cambridge, 464 p.
Cook, P. J., and J. H. Shergold (eds.), 1986, Phosphate deposits of
the world: v. 1: Proterozoic and Cambrian phosphorites: Cam
bridge University Press, Cambridge, 386 p.
Glen, C. R., L Prev-Lucas, and J. Lucas (eds.}, 2000, Marine authi
genesis: From global to microbial: SEPM Spec. Pub. 66, 536 p.
Notholt, A. J. G., and L Jarvis (eds.), 1990, Phosphorite, research
and development: The Geological Society Special Publication
52, Bath, U.K., 326 p.
Notholt, A. ]. G., R. Sheldon, and D. Davidson (eds.), 1989,
Phosphate deposits of the world, v. 2: Phosphate rock re
sources: Cambridge University Press, Cambridge, 566 p.
Coal, Oil Shale, Bitumen
Bustin, R. M., A. R. Cameron, D. A. Grieve, and W. D. Kalkreuth,
1985, Coal petrology, its principles, methods, and applica
tions: Geol. Assoc. Canada Short Course Notes, v. 3, 230 p.
Chilingarian, G. V., and T. Yen, 1978, Bitumins, asphalts and tar
sands: Elsevier, New Yo rk, 331 p.
Cobb, J. C., and C. B. Cecil (eds.), 1993, Mode and ancient coal
forming environments: GSA Special Paper 286, 198 p.
240
Chapter 7 I Other Chemical/Biochemical and Carbonaceous Sedimenta Rocks
Crelling, ). C., and R. R. Dutcher, 1980, Principles and applica
tions of coal petrology: Soc. Econ. Paleontologists and Miner
alogists Short Course Notes No. 8, Tulsa, Okla., 127 p.
Dissel, C. , 1992, Coal-bearing depositional systems: Springer
Ve rlag, Berlin, 721 p.
Hunt, ). M., 1996, Petroleum geochemistry and geology, 2nd ed.:
W. H. Freeman, New York, 743 p.
Meyers, R. A. (ed.), 1982, Coal structure: Academic Press, New
York, 340 p.
Meyer, R. (ed.), 1987, Exploration for heavy crude oil and nat
ural bitumens: AAPG Studies in Geology 25, Amer. Assoc.
Petroleum Geologists, Tulsa, Okla., 731 p.
Russell, L., 1990, Oil shales of the world: Their origin, occur
rence and exploitation: Pergamon Press, Oxford, 736 p.
Snape, C. (ed.), 1995, Composition, geochemistry and conver
sion of oil shales: Kluwer Academic Publishers, Netherlands,
505 p.
Stach, E., M.-Th. Mackowsky, M. Te ichmi.iller, G. H. Ta ylor, D.
Chandra, and R. Te ichmiller, 1982, Handbook of coal petrol
ogy, 3rd ed.: Gebri.ider Bomtraeger, Berlin-Stuttgart, 535 p.
Thomas, L., 1992, Handbook of practical coal geology: john Wiley
& Sons, Chichester, 338 p.
Ti ssot, B. , and D. H. Welte, 1984, Petroleum formation and oc
currence, 2nd ed.: Springer-Ve rlag, Berlin, 699 p.
Van Krevelen, D. W., 1993, Coal: Typology, physics, chemistry,
composition: Elsevier, Amsterdam, 979 p.
Ward, C. R. (ed.), 1984, Coal geology and coal technology: Black
well, Melbourne, 345 p.
Wignall, B., 1994, Black shales: Oxford University Press, New
Yo rk, 127 p.
Yen, , and G. V. Chilingarian (eds.), 1976, Oil shales: Elsevier,
New York, 292 p.
PAR1V
Depositional
Environments
Thompson River north of To k Junction, southeast Alaska.
241
242
T
he characteristic properties of sedimentary rocks are generated through the
combined action of the various physical, chemical, and biological processes
that make up the sedimentary cycle. We athering, erosion, sediment trans
port, deposition, and diagenesis all leave their impress in some way on the final
sedimentary rock product. The sedimentary processes and conditions that collec
tively constitute the depositional environment play the primary role in determin
ing the textures, structures, bedding features, and stratigraphic characteristics of
sedimentary rocks. The close genetic relationship betvveen depositional process
and rock properties provides a potentially powerful tool for interpreting ancient
depositional environments. This linked set of reactions beteen environments and
facies is commonly referred to as process and response (Fig. P4.1). If geologists can
find ways to relate specific rock properties to particular depositional processes and
conditions, they can work backwards to infer the ancient depositional processes
and environmental conditions that created these particular rock properties.
Environmental analysis thus involves identifying response elements or prop
erties that have environmental significance. These properties include sedimentary
structures and textures (which reflect depositional processes such as current flow
and suspension settling of grains), sedimentary facies associations (such as fining
and coarsening-upward successions of facies, which indicate shifts in environ
mental conditions), and fossils (which are useful indicators of salinity, tempera
ture, water depths and water energy, and turbidity of ancient oceans). These
properties can be used to construct facies models for each major depositional en
vironment. A fades model is a general summary of the characteristics of a given
depositional system. Such summary models act as a norm for the purposes of om
parison
and interpretation. ey provid
e a kind of "mental picture" of the proper
ties of rks deposited a given enviroent. Few of us have had enough field
experience, have read enough books and papers, or have good enough memories
Sedimenta Environment
(Process Element)
namic e of e ent
s: wave a cut
; ;
changes: ttonism
: ,
prion, auesis
Biolical b
prip; biologic ng
siment; yn
eleta of tt. environmt
lite
Wat« dtn
y
onal s edlmt eupply)
Clte
Figure P4.1
.
�
S.edimenta Facies
(Response Element)
Geomet of e d
k, prism, , etc.
Prima s8dlll pe
: g and contact relationips;
simentary textures and strtus:
composition
al: - a tra
ical: f content ld
abundances ol
Derived sediment
ty and
t pr laound trsm
Electrical lvit�
Radioactivity
Process-response model illustrating the relationship between sedimentary environments
and sedimentary facies.
to carry around a mental picture of each important depositional environment. For
tunately, we can draw on the experiences of many geologists through their pub
lished data and ideas to construct facies models that will provide the reference
amework we need for interpreting ancient depositional environments.
Sedimentary rocks have been deposited through time in three major deposi
tional settings: continental or terrestrial, i.e., on land; marginal-marine, the bound
between the sea and the land; and marine, the ocean proper. Each of these
first-order depositional settings is divided into several major environments,
which in tum are divided into subenvironments (Table P4.1). The following four
chapters discuss the major environments in the continental, marginal-marine, and
marine settings. Although not labeled specifically as such, facies models of vari
ous kinds are used throughout these chapters to summarize distinguishing char
acteristics of the principal sedimentary facies generated in these major
sedimentary settings.
Primary depositional setting
Continental
Marginal-marine
Marine
*
Fluvial
*
Desert
Lacustrine
*
Glacial
*
Deaic
*
Beach/barrier island
*
Estaurine /lagoonal
Tidal flat
Neritic
Ocean
'Dominant
l
y silicic
l
astic deposition
"Dominantly carbonate deposition
Major environment Subenvironment
{
*
Alluvial fan
*
Braided stream
*
Meandering stream
{
*
Delta plain
*
Delta front
*
Prodelta
{ Continental shelf
**
Organic reef
{ Continental slope
Deep ocean oor
Environments not marked by an asterisk(s) may be sites of si
l
iciclastic, carbonate, evaporite, or mixed sediment deposi
on depending upon conditions.
243
Continental (Terrestrial)
Environments
8.1 INTRODUCTION
W
e tum now to the study of continental or terrestrial depositional sys
tems. Geologists recognize four major kinds of continental environ
ments: uvial (alluvial fans and rivers), desert, lacustrine (lake), and
glacial. Although treated in this book as separate depositional systems, similar
kinds of sediments can be generated in more than one of these environments. For
example, eolian (windblown) sediments can accumulate both in desert environ
ments and in some parts of glacial environments. Lacustrine sediments form in
lakes in any environment, including deserts and glacial settings. Fluvial sedi
ments are deposited mainly in river systems of humid regions, but they are gener
ated also in rivers within desert areas and glacial environments.
Facies deposited in continental environments are dominantly siliciclastic
sediments characterized by general scarcity of fossils and complete absence of ma
rine fossils. Nonsiliciclastic sediments such as freshwater limestones and evapor
ites occur also in continental environments, but they are distinctly subordinate to
siliciclastic deposits. Continental sedimentary rocks are less abundant overall than
are mare and marginal marine sediments, but they nonetheless form an impor
tant part of the geologic record in some areas. Te rtiary fluvial sediments of the
Rocky Mountain-Great Plains region of the United States, Jurassic eolian sand
stones of the Colorado Plateau, Tertiary lacustrine sediments (Green River Forma
tion) of Wyoming and Colorado, and the late Paleozoic glacial deposits of South
Africa and other parts of ancient Gondwanaland are all examples of continental
deposits. Some terrestrial sediments have economic significance. They may con
tain important quantities of natural gas and petroleum, coal, oil shale, and urani
um.
We now examine, in tu, each of the major continental environments.
8.2 FLUVIAL SYSTEMS
Fluvial deposits, also referred to as alluvial deposits, encompass a wide spectrum
of sediments generated by the activities of rivers, streams, and associated gravity
flow processes. Such deposits occur at the present time under a variety of climatic
conditions and in various continental settings rangg from desert areas to humid
and glacial regions. Although alluvial settings can be classified in many ways
245