Middleton G.V. (Ed.) Encyclopedia of Sediments and Sedimentary Rocks
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Cross-references
Cements and Cementation
Climatic Control of Sedimentation
Diagenesis
Mutation, Organic
Porewaters in Sediments
Provenance
Seawater
Tectonic Controls of Sedimentation
K
KAOLIN GROUP MINERALS
Kaolin is a tiame given to a group of phyllosilicate mitierals
whose layers have a
1
;
1
structure with a composition of
AI:Si:Os(0H)4. The
1
:1 designation indicates the individual
layers are composed of sheets of comer sharing tetrahedra in a
distorted hexagonal arrangement and a sheet of edge sliaring
octahedra. The tetrahedra are occupied by silicon atoms
and the octahedra are occupied by aluminum atoms in a
dioctahedral manner. The join between the two sheets is
effected by sharing of what would be the apical oxygen atoms
of the tetrahedra with atoms of the octahedra. This creates an
asymmetrical layer since one of the surfaces of Ihe layer
consists of basal oxygen atoms of the tetrahedral sheet while
the opposite surface is populated by hydroxyl groups. The
stacking of the layers and the nature of the material, if any,
between the layers generate the indi\iduai minerals in the
group.
The bonding between the layers is due to hydrogen bonding
involving hydrogen atoms of one layer and the basal oxygen
atoms of the adjacent layer (Hendricks. 19.39). To this should
be added the interfacial interactions of the layer surfaces. The
geometric requirement necessary to create a nuinber of
different stable stackings is to enable the hydroxyls (the
hydrogen bond donors) of one surface to be close to the basal
oxygen atotns (the hydrogen bond acceptors) of the adjacent
layer. Newnham (I96t) analyzed this and incorporated other
factors such as the superposition of eatlons of the two layers
that would be a destabilizing contribution. He determined that
there were theoretically 36 possible stackings of the layers
taken two at a time. When considering
1-layer
structures, two
of the stackings were judged to be most stable. These
correspond to a right and left-handed kaolinite structure.
For two layer structures only 2 of the 36 were likely because
the other stackings were destabilized by other factors sueh
as tetrahedral rotation and tilting and the distortions of
the individual layers. These two correspond to dickite and
nacrite.
Thus,
of the large number of possible stackings of the
1
: I
kaolin layers, only three are observed. These are kaolinite,
dickite and nacrite. There is normally a fourth mineral added
to the kaolin group; halloysite, a hydrated (interlayer water)
version. Of these four minerals, kaolinite is by far the most
common and, along with haltoysite. the only mineral that is
connnercially exploited.
Structures
Kaolin minerals, in commoti with many clay minerals, occur
only as small particles so that traditional single crystal
diffraction studies are not possible. There are oecasions when
one tinds what appear to be macroscopic crystals, but these are
always cither not single crystals or they are highly disordered
and not suitable.
Kaolinite derives its name from a locality in China where it
was tirst mined (Grim. 1968). Early attempts to understand the
structure of these tninerals were based on X-ray powder
diffraction experiments. This is difficult because of the low
symmetry, Hendricks (1936) observed the characteristic 7.lA
basal spacing. Knowing the thickness of the layer structure
allowed Pauling (1930) to outline the architecture of the
kaolinite structure. There were a number of attempts to refine
the structure suggested by Pauling. Initially, the structure was
thought to be monoclinic (Gruner. 1932: Hendricks. 1936), but
later Brindley and Robinson (1945) demonstrated that the unit
cell is really triclinic and this allowed them to determine the
positions of the non-hydrogen atoms. The structure is based
on a simple shift of —«/3 where a is the unit eell axis. The
vacant octahedral site is the same in each of the layers.
The hydrogen atoms clearly are extremely important in
the interlayer bonding of these minerals so there has been great
interest in locating their positions so as to understand the
interlayer hydrogen bonding. Various attempts were made
to infer the hydroxyl orientations using infrared absorption
spectroscopy (Farmer. 1964). There were great problems in
these efforts. Later, potential energy calculations were used to
locate the OH orientations that corresponded to the minimum
energy (Giese and Datta, 1973: Giese. 1982).
KAOLIN GROUP MINERALS
399
Figure K1 Clinogriiphic view ot the kiiolinite structure showing two
1 :
1
layers. The hydrogen .ltoms are shown as small circles tind the
hydrogen bonding is indicaled by the longer solid lines to oxygen
atoms in Ihe ddjatent layer (drawing produced with the program
ATOMS, by Shape Software, www.shapesotlware.cnm).
The kaolinite structure has been refined several times since
then with increasing accuracy aided by improvements in the
refmement of struetures based largely on powder diffraction
data (see Ciiese, 1988., for a discussion). Oddly enough,
relincments still seem to be in progress even though there
seems little jtistilication for such work.
Dickite is named after the mineralogist (Dick. 1888: see
Newnham and Brindley. 1956). The major features of the
dickite structure were determined by Newnham and Brindley
(1956) and there are a number of modern refinements. The
stacking is ---a/3 as for kaolinite. The dickite structure differs
from that of kaolinite in that the vacant octahedral site
alternates its position in adjacent layers giving a two-layer
structure. It is possible to find small but useful single crystals
of dickite and these have eased the search for the hydrogen
atoms.
Nacrite was named for its nacreous appearance. The
structure is different from those of kaolinite and dickite
(Blount etal.. 1969). The stacking between layers follows the
S.9A a axis and there is a rotation of the 1:1 layer by 180''
between successive layers. The unit cells and space groups ean
be found In recent refinements of the structures: viz. Bish and
von Dreele (1989) for kaolinite. Joswig and Drits (1986) for
dickite and Zheng and Bailey (1994) for nacrite.
Defect structure
It was noticed early on that there were wide variations in the
X-ray diffraction patterns of kaolinite samples from different
loealities. These varied from patterns with narrow Bragg
reflections to those in v\hich many of the typical kaolinite
diflVaction bands were diffuse and there was typically a high
background between peaks. This was correctly interpreted as
some sort of stacking disorder. The peaks most affected had
indices with k 7^3. This suggested that the disorder was due to
random shifts of
+/J/3
(Brindley and Robinson. 1945). It was
also pointed out that rotations of + 2nB would result in a shift
in the \'aeant octahedral site from layer to layer while
maintaining the interlayer hydrogen bonding (Murray, 1954).
The variations in the diffraction patterns were of some use in
detertnining the physical properties of kaolinites, so there were
several attempts to fortntilate an empirical estitnate of the
degree of disorder. The most widely used index was that of
Hincktey (1963). It should be noted that Hinckley used the
term "crystallinity"" in the title of the paper placed in quotes,
presumably to indicate that he was not using the term in strict
sense of the word. Unfortunately, subsequent usage has
emphasized the normal meaning of crystallinity. Typical
values of the Hinckley inde.\ range frotn roughly 0.2 to 1.7
with the larger values corresponding to a low level ofstructural
defects. Recent work has shown that for values of the index
greater than 0.43. the samples are mi.xtures of two kinds of
kaolinite. one having few defeets and the other having a high
concentration of defects (Plancon and Giese. 1988. Bookin
eui!,. 1989. Plancon
f-ra/..
1989).
Chemistry
The kaolin minerals in general arc reasonably pure with small
amounts of iron and titanium. The chemistry is somewhat
clouded because kaolinite is now known to occur with
interstratified smectite. The admixed smectite introduces alkali
and alkaline earth elements, increases the surface area and
accounts for the small amount of water often found in
samples. Pure kaolinites have small surface areas (approxi-
mately <20m'/g and very modest cation and anion exchange
capacities. Typical chemical analyzes are shown below
(Murray and Keller, 1993).
Component
SiO.
FcOi
TiO.
MfiO
CiiO
Na^O
K:6
LOI (water)
(icorgia
kaolinite
45.30
38.38
0.30
1.44
0.25
0.05
0.27
0.04
13,^7
England
kaolinite
46,77
37,79
0,36
(J,02
0,24
0,13
0,115
1,49
12,97
Theoretical
46,55
39,50
—
—
—
—
Common mineriil impurilifs in khiilinite depoiiits are anatase. smectite, and
ilmcnilc.
Recent measurements of the enthalpy of formation of the
kaolin minerals has allowed more accurate determinations of
the standard free energy of formation (de Ligney and
Navrotsky. 1999). The values show that kaolinite is the stable
mineral while the other forms are metastable. Further, the
variation in the standard free energy of formation does not
vary measurably as a function of the defect structure.
Origin and occurrence
There are two types of kaolin deposit: primary and seeondary.
The primary kaolins are derived from feldspar and muscovite
in granite or rhyolite under conditions of high rainfall,
moderate to high tetnperatures above the water table. The
large amounts of water are necessary to remove the potassium
400
KFROCFN
and prevent the formation of illite rather than kaolin. A higher
temperature regime can result in hydrothertnal kaolins. If the
kaolin remains in place, the deposit is termed residual. On the
other hand if the kaolin is transported by running water and
deposited elsewhere, one has a secondary deposit. The
formation of kaolinite is favored by low pH and low
concentrations of alkali ions as shown in Faure (1998),
Hydrothermal deposits are known in China, Japan.
and Mexico. These deposits are exploited primarily lor
ceramic clay. Residual deposits are found in Australia.
Czechoslovakia. Germany and Indonesia, These are used in
paper filling and ceramic nianufacturitig. Deposits which were
formed from a combination of hydrothermal and chemical
weathering are found in Cornwall. England and New Zealand.
The Cornwall kaolinite is particularly well-ordered and pure,
and is extensively used in paper coating. There are large
deposits of sedimentary kaolin in Australia, Brazil. Germany.
Spain, and the United States, principally in Georgia and South
Carolina (Murray and Keller. 1993). There are many
occurrences of kaolinite "books" in sandstones where they
have formed under diagenetic conditions.
Rossman F. Giese, Jr.
Bibliography
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structure of kiiolinitc. Clays and Clay Minerals. 37: 289 2^)b,
Blount. A,M,. Thrcadgold,
1,'M..
and Bailey. S,W,. 1969, Refinement
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185-194,
Bookin. A,S.. Drits. V.A.. Pkincon. A,, and Tchoubar. C, 1989.
Stacking faulLs in kaolin-group minerals in ihe light of real
siructure features.
Clays
and Clay Minerals. 37: 297-307,
Brindley, G,W,. and Robinson, K,. 1945, Structure of kaoliniie.
Nature. 156; 6(il-663,
Farmer. V,C,. 1964, inlrared absorption of hydro,xyl groups in
kaolinite, .SaVmc. 145: 1189 I hK),
Faure. G,. 1998,
Pritieipies
and Applications of Geoehenti,stry. 2nd edn,
Preniice Hall.
Giese, R.F,, 1982, Tbcorctical studies of the kaolin minerals:
electrostatic calculations, Btdletin Mineralogy. 105: 417-424.
Giese. R.F,, 1982, Kaolin minerals: strnctiLres and stabilities, Cliaptci'
3,
In Bailey. S,W. (ed.). Hydrous Phvllosih'caies. Reviews in
Mineralogy."
19:
29 66.
Giese. R,F.. 1988, Kaolin minerals: structures and stabilities. In Bailey.
S,W, (ed,).
HydrousPhyllo.silieates
{exclusiveofntieas). Mineralogical
Society of America, pp, 29-66,
Giese. R,F.. and Datta, P,. 1973. Hydroxyl orientations in kaoliniic.
dickite. and nacrite. American Mineralogist. 58: 471 479,
Grim. R.E,. 1968, Clay Mineralogy, McGraw-Hill.
Gruner. J,W,, 1932. The crystal structure of kaolinite, Zeiischrift fur
Krislallographie, 83: 75-88,
Hendricks. S.B.. 1936, Concerning the crystal structure of kaoliuite.
AliO-,
2SiO2 2H2O and the composition of anauxitc, Zeit.schrift
fur Kristallographie. 95: 247 252,
Hendricks. S,B,. 1939. The crystal siructure of nacriie
ANOi
•
2SiO2
•
2H2O and the polymorphism of tbe kaolin minerals.
Zeitsehriflfiir Kristallographie. 100: 5(W 518,
Hmckley. D.N,. 1963, Variiibility in "crystailinity" values among the
kaolid deposits of the coastal plain of Georgia and Sotith Carolina.
ClaysandClay Minerals. 11: 229-235.
Joswig, W,. and Drits. V,A,. I9K6, The orientation of the hydroxyl
groups in dickite by X*ray diffraction, Neues Jahrhmhfur Minera-
logieMh,.. HI: 19 22,
de Ligny. D.. and Navrotsky. A., 1999, Energetics of kaolin
polymorphs, Atnericait Mineralogist. 84: 506 516,
Murray. H,H,. 1954, Strnclural variations of some kaoiinites in
relation to dehydrated hiilloysite. Ameriean Mineralogist. 39:
97 108,
Murray. H.H.. and Keller. W.D,, I99.V Kaolins, kaolins and kaolins.
In Murray. H,H,. Bundy. W,M,. and Harvey. CC (eds.). Kaolin
Genesis
and Vlilization, Tiie Clay Minerals Society, pp,
I
-24,
Newnham. R,E,. 1961. A rehnemeni ol" the dickite structure and some
remarks on polymorphism in kaolin minerals, Mineraloi;iea!
Magazine. 32: 683 704,
Newnham, R,E,. and Brindley. G.W,. 1956. The crystal structure of
dickite, Ada Crystallographica. 9: 759 764,
Pauling. L.. 1930. The structure of chlorites. Pnxeedin^s of the National
.Academyof ScteneesV.S.A.. 16: 578 582.
Plancon. A,, and Giese. R.F,. 1988, The hinekley index for kaolinites.
Clay Minerals. 2y. 249 260,
Plancon. A,. Giese. R.F,, Snyder. R,. Drits. V,A,. and Bookin, A,S..
19S9,
Slacking faults in the kaolin-group minerals delecl strtictiires
of kaolinite. Claysand Clay Minerals. 37: 203-210.
Zheng. H.. and Bailey, S.W.. 1994. Retinement of the nacrite structure.
Clays and Clay Minerals. 42: 46 52,
Cross-references
Cliiy Minerals
Diagenesis
Weathering, Soils and P;ileosols
KEROGEN
Kerogen is the solid, high moiecular-wcight fraction of
sedimentary organic matter (OM) that is insoluble in organic
solvents (e.g.. chloroform) as opposed to the soluble fraction
called bitumen. Kerogen is a macroniolecule of condensed
cyclic nuclei linked by hcteroatomic bonds or aliphatic chains.
The importance of differentiating the soluble and insoluble
fractions ol'OM in standard organie solvents is based on the
origin and formation of petroleum in sediments. The soluble
fraction contains '"free" hydrocarbons (HC) formed during
geological processes, whereas the insoluble fraetion has large
proportions that are liable to form HCs (Durand, 1980). The
insoluble nature is linked to condensation of OM and thtis
represents polycondensed or polymerized states of OM in
contrast to the biogenic polymers of the living matter.
Introduction
Kerogen has been defined in several studies, which arc
summarized in Durand (19S()). Durand (1980) proposed to
call kerogen sensu strieto the fraetion of sedimentary OM that
is insoluble in standard organic solvents. The distribution of
kerogen in the earth's crust is estimated to be IO"'t, with most
of the kerogen occurring in the fine silt or clay fraction
deposited on continent shelf and slope (Figure K2), Kerogen
makes up the largest part of the total organic matter and can
represent more than 95 percent of the weight of recent OM.
Kerogen content is generally estimated on the basis of organic
carbon content of the sediment, which yields often an
underestimated value that has to be corrected by a factor of
1.5 or 1.6 (br slightly evolved sediments. The proportion of
kerogen progressively changes during burial and the formation
of soluble and/or volatile products, such as HCs, whieh makes
it often difficult to estimate the average proportion of kerogen
of the total sedimentary OM (Durand. 1980),
KEROGfN
401
Natural
generation
(A) 2,0
Shale oil
5x1011t
In oil
shales:
Asphalts
Oil
4x10"t
Figure K2 Comparison of kerogen abundance in (he earth's crust and
iht' ullimale resources of
I'ossil
fuels. Modified after Durand 11980),
Historical development
Scientists studying the oil-shales of the Lothiiins of Scotland
end of the 19th century named the oil-yielding, \va,\y part of
OM "kerogen" (from Greek keros=uax) or "'pyrobitumen"
(Bailey. 1927. p.158). Kerogen and kerogen maturation
received much attention throughout the 20th century, because
ol"
its importance for fossil fuels. Numerous methods identify-
ing kerogen type and kerogen maturity have been de\eloped
and are summarized in Barnes etal. {1990), Bustiii elal. (1983),
Bustmc/i//. (1990). Durand (1980). Hnnt (1979). Peters (1986).
Taylor elal. (1998). and Tissot and Welte (1984).
Characterization of kerogen
Accumulated organic matter in sedimentary deposits provides
the potential source for kerogen and henee HC formation
during diagenesis. The kerogen concentration, composition,
and state of maturation are important parameters for the
formation of oil and gas. The OM in sedimentary deposits
derives from either living or dead particulate OM or dissolved
OM. Amount and type of kerogen will determine the amount
of oil or gas that has been or will be generated from OM in
sedimentary deposits,
Kerogen type is commonly determined either by optical
(petrography) or chemical methods, Petrographic analyzes
allow morphological classification of OM of various deposi-
tional environments, sueh as marine or lacustrine deposits, and
the level of maturation, while geochemical analysis allow the
classification based on chemical properties. In coal geology,
three main groups of
maceral.s
(kerogen constituents of organic
matter) are distinguished; liptinite, huminite/vitrinite, and
inertinite (Figure K3), During progressive diagenesis. these
maceral groups beeome enriched in carbon but eaeh compo-
nent follows a predetermined and distinct coalilication path
(figure K3). Vitrinite refleetance (Ro percent) is widely used as
a parameter of the digenetic level of kerogen (see Maiaralion.
Organic).
In petroleum geoehemistry. four kerogen types (I to IV) are
commonly differentiated (Figure K3), All kerogen types are
determined soIeK on H-. 0-. and C-content. Type I. consisting
I 1.0H
0.5-
I highly oil prone
, II oil prone
777///////,, gas
?, H26',
P''°"®
\ biogenic CH4-
METAGENESIS
I
0,1
Atomic 0/C
0,3
(B)
2,0 -
O
i
o
E
o
1.0
I
Waxes, cuticles
II
Cellulose
I I I I I I I I I [
0 0.2 0,4 0,8 1.0
Atomic 0/C
Figure K3 (A) Kerugen evoluiion illustrLifed on a v,in Krevelen
diagram.
Several evolution stages otcur during progressive diagenesis,
which have difterent principal HC production depending on original
type Iii kerogen. During progressive diagenesis, kerogens become
enrJLhed in carbon. Note (hat typt.' IV organit matter does not have an
evolution
path.
Modified after Tisbot and Welte (1978). (Bl van
Krevelen diagram ol the Hiagenctic pathway ol' ctial macerals (kerogen
types determined un mnrphologyi. During progressive diagenesis, the
three maceral groups (li|)tinile, inertinite, huminite) become enriched
in carbon. The diagram shows the changes in terms of H/C and O/C
ratios.
Modified after Buslin et ,il. (1983).
of mainly algal and amorphous kerogen that is highly likely to
generate light oil; Type II, mixed terrestrial and marine
material (algae, spores, cuticles) that can generate wa,\y oil and
some gas; Type III, woody terrestrial source material that
typically generates gas; and Type IV. alsoealled residual or
refractory kerogen. composed of resedimented. oxidized OM.
402
KEROGEN
that has no potetitial for oil and gas generation (Tissot and
Welte. 1984).
minor amounts of methane are formed from the residual
kerogen during the last stage of diagenesis.
Diagenesis of kerogen
Upon deposition and burial of sedimentary deposits. OM (e.g..
biogenic polymers, sueh as proteins, carbohydrates) is
transformed (biochemical degradation) and polycondensed
struetures (geopolymers) are formed whieh are the precursors
of kerogen. The initial stage of diagenesis. called eogenesis
(Barnes elal.. 1990), ineludes proeesses of bacterial oxidation,
sulfate reduction and fermentation. The most important HC
formed during this stage is CH4 (Figures K3. K4). Oxygen
elimination of kerogen takes place and results in the formation
of CO:. HiO. and some heteroatomic (N. S, O)-petroleum
compounds of high-moleeular weight (e.g.. asphaltenes and
resins) are liberated. Most carboxyl groups are removed
(Barnes elal.. 1990; Bustin
i-t
al., 1990). Type III kerogen
may produce some gas (Figure K3).
Increasing burial and progressive diagenesis {increase in
temperature and pressure) of the sediments results in the
production of liquid petroleum from kerogen. During this
stage of thermal maturation, ealled catagenesis, both liquid oil
and condensate, including methane, are formed (wet gas
phase) (Figure K4). HC molecules, sueh as aliphatic chains.
are produced from kerogen and eracking results in the
production of light HCs, Because of the chemical structures
present, immature kerogens that are hydrogen-rich will
generate crude oil and hydrogen-poor kerogens will yield only
gas.
As burial depth and temperature increase, the residual
kerogen becomes depleled in H (labile compounds) and has an
atomic H/C ratio of about 0.5 by the end of the catagenesis
{Tissot and Welte. 1984).
Further inerease in pressure and temperature leads to the
stage of metagenesis. During this stage, kerogen is structurally
reorganized and a higher degree of ordering is obtained. Only
0
CH4 (biogenic)
Geochemical fossils
EOGENESIS
CATAGENESIS
Gas
o
onlyCH.
.METAGENESIS og
Hydrocarbons generated
0.0
0.5
o
GC
1.35
2.0
4-5
Figure K4 HyHrot.irbon gL'neration changes in sediments during tiioir
evolution of progressive diagenesis, Vilrinite reflectance (R,, %)
corresponds to the degree of organic matter maturalion and is
dependent on type of kerngen, buri.1l history and geolhermal gradient.
Modified after Bustin e(
.1/,
(1990).
Determining level of kerogen maturity
With increasing levels of organie diagenesis. the color of
kerogen in transmitted light changes progressively and several
methods have utilized such characteristics for kerogen
classification, for example the ihcrmal alteration iiule.x. a
numeric seale used to classify the color changes of palyno-
morphs. Because of the widespread abundance of palyno-
morphs in sedimentary deposits, optical determination of the
level of kerogen maturity is seldom a problem. Another optieal
method is fluorescence microscopy, which determines the
irradiation level of liptinite components (algae, spores, resin.
cutin).
In coals, similar changes are recorded by analyzing the
yitriniterefleclaiice (R,,), The vitrinite reflectance is commonly
used to classify coal rank and OM maturity level (see Maliira-
liofi. Organic).
Classification of kerogen by chemical means is limited to the
pre and early oil generation maturities because generation and
expulsion ol' hydrogen-rich oil and gas alters the hydrogen
content of the original kerogen (Figure K.3). Numerous
methods are used for determining the geoehemistry of kerogen.
The most important methods are; pyrolysis, ultimate analysis,
infrared spectroscopy, electron paramagnetic resonance, car-
bon isotope studies, proximate analysis, caloric value, etc..
{Bustin etal.. 1990; Garcette-Lcpecq etal.. 2000). Pyrolysis or
progressive heating of the sample up to 550 C, utilizing Rock-
Eval" instruments (see Peters. 1986). measures three types of
gases:
(I) the HCs residual in the sample {S]); (2) HCs
generated during thermal cracking between 300'C and 55OX
(S2,
Tinax)- ii'id (3) CO: (S3)- This allows determination of the
level of organic diagenesis and type of HCs the kerogen will
yield. This method is widely used despite the fact that some
surface samples tend to be prone to alterations and thus
erroneous T,,,a,.i (see Copard etal.. 2002),
L'ltimaleanalysis determines elemental coneentrations of C
H. and O and H/C and O/C ratios are plotted against eaeh
other in a van Krevelen diagram, van Krevelen diagrams
allow characterization of kerogen type and maturity level
(Figure K3).
Infrared spectroscopy quantifies the main functional groups.
During progressive diagenesis of OM. carboxyl. carbonyl. and
saturated HCs decrease and aromatic C-H groups are formed
and removed (Bustin eial.. 1990), Kerogen type, the precursor
of the functional groups as well as diagenetie stage can be
determined.
Summary
Kerogen is the fraction of OM in sedimentary deposits that is
insoluble in standard organic solvents. It is a macromoleeule of
condensed cyclic nuclei linked by heteroatomic bonds or
aliphatic chains. Different types of kerogen (1 to IV) occur
depending on the eomposition of the organic matter, most of
whieh generate oil and gas upon diagenesis. Burial of the
sediments, including increasing temperature and pressure,
results in the rearrangement of kerogen. During eogenesis,
elimination of heteroatomic bonds and functional groups
occurs and CO2. H2O and some heavy N, S, O compounds are
released, while CH4 is produced. During catagenesis, HC
chains and cycles are eliminated and crude oil and gas are
KEROGBN
403
formed. During metagenesis, aromatic sheets of kerogen are
rearranged and form clusters while some gas is generated.
Progressive changes of HCs occur with progressive diagenesis.
Whilst biomolecules are dominant during eogenesis. medium
to low molecular weight HCs become predominant during
catagenesis and CH4 is practically the remainder during
metagenesis,
Raphael A.J, Wiist and R. Marc Bustin
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Cross-references
DiagenesiN
Maluralion. Organit:
LACUSTRINE SEDIMENTATION
The lake is a system, that is, a hounded entity possessing a set
of unique, interconnected elements, especially of process and
response, that can be recognized as such regardless of how they
are dealt with by scicntitii: enquiry (Strahler, 1980). Further,
the lake is an open system; processes beyond the boundaries
relate to those within the system to produce mass and energy
exchange across the boimdaries. For a lake in equilibrium with
its environment, the components of these exchanges lead, over
the long term, to a zero-sum result; what goes in must eome
out. While that is generally true of energy and of water,
sediment flux is normally not balanced because input and
storage mueh exeeed loss from the system.
Sedimentary processes and deposits in lakes arc determined
by the physical, chemieal and biological environment of the
lake and of its watershed and airshed. The proeesses and
produets of this environment are controlled in complex
interactions among climate, hydrology, biology, geomorphol-
ogy. geology, and, inereasingly, human actions. Figure Ll sets
out a conceptual framework that assoeiates these factors and
indicates qualitatively the way in which the sedimentology of
the lake is controlled.
Limnologic processes affecting sedimentation
The processes in a lake are driven by the energy of the
laeustrine system. The most important input of energy is
through solar heating as part of the radiation balance which
reaches an average of about 2(X)W/iir in summer in mid
latitudes. The major ways in whieh heat energy is lost from the
lake are by sensible heat transfer and evaporation. Solar
heating is partieularly effective because the mean albedo
(refleetivity) of the water surface is less than IU percent, one of
the lowest of any natural surface. Solar radiation penetrates
water up to several tens of meters before being absorbed and
heating the upper part of the lake, thus redueing the density of
the water and leading to a tripartite strueture of the water
column: an upper epilimnion separated from a colder, higher
density hypolininion by a /one ot" thermal change, the
metalitnnion. In mid-latitude lakes the isolation of these zones
breaks down twice eaeh year in spring and autumn as water
in the epilimnion warms or eools, respectively, through the
temperature of the hypolimnion. This semi-annual overturn
and mixing of the lake water, referred to as (limi,\is. is
important in mass (including sediment) and energy transfer
throughout the lake. In high latitudes and tropieal to
subtropical regions heating or cooling to the near-bottom
temperature oecurs only in summer or winter, respectively,
giving rise to one period of overturn, or
ni<)n<>nii.\i,s.
In lakes
less than about 2m to 10m deep depending on feteli (Gorham
and Boyee, 1989) wind-generated Ibrees frequently overcome
thermal stabilization of the water column, leading to repeated
or near continuous overturn of the lake water referred to as
polytui.xis.
Mass exehange of water (the uater balance) is itnportaiit to
sedimentary proeesses beeause normally a signitieant amount
of sediment is delivered to lakes by rivers. The annual pattern
of preeipitation is espeeially important, leading to tlushes of
water and its sediment load in seasonal events, for example
related to snowmelt in the mid to high latitudes or monsoons
in the sub-tropics, and to irregular, eatastrophie. major storm
events. Where evaporation trom the lake exceeds precipitation
and ruEioff into the lake, outflow does not oecur and dissolved
salts accumulate in the lake water. This commonly leads to a
persistent sii ucture based on lower density (iVesher) water near
the surface, and high-density, in some lakes hypersaline, water
at depth which does not permit overturn and is referred lo as
mcromi.xis. In extremely arid environments the entire lake may
be hypersaline. Salts may also be present in lakes lifted from
the sea by tectonic processes, A review of the water and energy
balances of lakes is provided by Lerman etal. (1995),
The components of the water balanee determine the rcsi-
dcncc litnc of water in the lake which is an important factor
inllueneing sedimentary proeesses. This is the time that water
spends in the lake on average between when it enters the lake
as runotr or precipitation and when it leaves the lake as
outflow or through evaporation. It is a I'unetion of the relative
volume of the lake with respeet to the volume of input, and ean