Middleton G.V. (Ed.) Encyclopedia of Sediments and Sedimentary Rocks
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DEF'OSITIONAL FABRIC OF MUDSTONBS
D
Figure D11 The "self-similar" nature (ii s<>climents. Magnititalion jumps by a factor of 10-20 between successive photos. (A) shale packages
alternate with sandslone-rich units (Blackhawk Fm., Crelaceous. Utah); (R) sandslone-rich units from A consist of alternating sandy ledges
and shatey intervals; (Cl sh.iley intervals trom R show softer shale alternate with centimeter-thick sandstone/si I tstone beds; ID) thin section of
soil interval trom C shows graded siltstone layers and variably bioturbated mudstone. The resistant beds are storm deposits throughout ifrom
HCS betis in B, to distal lempestltes in D).
and they are probably more abundant in mudstones and shales
ihan commonly recognized (Potter etal.. 1980; C'uotno and
Rhoads. 1987). They are indicators of organic activity, and arc
best seen in thin section, where they typically differ frotn the
shale tiiatrix with respect to texture, color, and organic
content. They may range in size from 0.2 mrn to 2 mm and
arc generally of elliptical outline. Compaction tends to flatten
the pellets to varying degrees, and if matrix and pellets are
sitnilar in eotnposition and composed of particles of similar
grain si/e. their ideniifieation can be challenging. The
composition of fecal pellets can give clues on whether they
were produced by benthic or planktonic organistns. Investiga-
tion ol modern environments suggests that potential microhial
colonization o\' tnud surfaces has to be taken into account
when studying ancient mudstones. Although difficult, reeogni-
tion of tnierobial latninae is possible, and can lead to
substantially different interpretations of mudstone environ-
ments (Schieber. 1999a).
Notwithstanding the usefulness of small-scale sedimentary
structures, the study of particle relatiotiships (texture) also
eonstitutes an important avenue of inquiry. This aspect of
shale fabries. studied by electron microscope, is frequently
referred to in the literature as microjdbric (O'Brien and Slatt.
1990).
Microfabric features of mudstones reeord theeombined
impact of processes active during transport, deposition, and
burial at the scale of individual grains and grain aggregations.
The ArgiUaccuus Rock Atlas by O'Brien and Slatl (1990)
contains case examples from a variety of sedimentary
environments (as well as tnany references) and illustrates
tnudstone fabries with SEM photos and phototnierographs.
Experimental work has shown that mierofabrics ean for
example be related to depositional proeesses, such as
206
DFPOSITIONAI. FABRIC
OF
MUDSTONFS
floceulation and single-grain sedimentation (Mattiat.
1969:
O'Brien
and
Slatt. 1990).
The
book Microstructure
of
Fine-GrainedScditncitts (Bennett etal.. 1991) contains
a
series
of papers that employ microfabric analysis
in the
study
of
modern muds.
A
major drawback with
the
application
of
modern tnierofabrics
to the
interpretation
of
depositional
conditions
is the
fact that muds undergo serious compaction
and diagenetic changes
on the way to
becoming rocks.
To
determine from
an
SEM study which fabrics are secondary
and
which features
are
reflective
of
original depositional fabrics
is difficult
and
requires considerable experience.
In
many
instances
the
successful interpretation
of
microfabrics hinges
on
the
availability
of
other information, sueh
as
small-scale
sedimentary features, palcogeographic setting,
and
paleoeeo-
logical information (e.g.. O'Brien cial.. 1994). Nonetheless,
a
comparison
of
modern
and
fossil examples suggests that under
favorable circutnstances. fabrics produced
by
fioeculation.
settling
of
dispersed clays, bioturbation, agglutination
on
microbial mats, deposition
of
biosediment aggregates ("marine
snow"),
compaction,
and
tecal pellets
can
still
be
recognized
and differentiated (Bennett etat., 1991).
Fabric
and
environment
The broader signilieance
of
mudstone fabrics relates
to the
observation that seditnentary rocks
and
sedimentary succes-
sions have
an
inherent fractal quality (Mandelbrot, 1983).
Eor
example, large-scale stratification features
can be
repeated
at
different scales within individual slratigraphic packages,
in
the details
of
bedding,
and
even
in the
tninutiae
of
latnination
(Figure
Dil).
Figure
Dll
illustrates that
at
increasingly
higher levels
of
magnification,
the
basie theme
of
alternating
soft
and
resistant intervals remains unchanged. Likewise,
and
regardless
of
seale. scoured bases, cross-stratification,
and
grading characterize resistant intervals,
and
bioturbation
is
more intense
in
soft intervals. This "self-similarity" from otie
magnification level
to the
next
is at the
heart
of
fractal
geotnetry (Mandelbrot. 1983).
and in our
exatnplc
(Figure
Dll) it
reflects
the
overall conditions under which
the succession accumulated.
In
that context
we may
even
be
tempted
to
speculate whether
the
information that
can be
extracted frotn fabrie elements
of a
thin seetion
or
hand
specimen (Figure
DI
I(D)). will lead
us to
conclusions that
are
comparable
to
those derived from
the
study
of an
entire
outcrop
as
shown
in
Figure Dtl(B). Studies
of
shale fabrics
from rocks ranging
in age
from Proterozoic
to the
Tertiary
have positively shown that shale fabrics
do
reflect
the
conditions
of the
broader sedimentary setting (Schieber.
1986.
1989. 1990:
O'Brien
and
Slatt,
1990;
Schieber etal..
1998).
Comparing what
can be
learned about depositional
eonditions
in a
mudstone-rich sedimentary succession from
paieoecological observations
and the
study
of
interbedded
sandstones, with
the
information derived from
a
detailed study
of shale fabrics, shows that shale fabries
can
provide
a
tnore
concise account
of
depositional conditions than
the
other
two
approaches {Schieber. 1999b).
Summary
Mudstone fabrics
are the
cumulative record
of the
history
of
transport, deposition,
and
burial
of
muddy sediments. Their
final "look" depends upon
the
interplay between
a
whole range
of physical, biological,
and
chemieal processes.
As
such.
successf\il studies
of
mudstone fabrics require
the
integration
of
all
available paleontologic (body
and
trace fossils),
sedimentologic.
and
petrographic observations.
If
done
thoroughly,
the
information derived from
a
sarnple
of
mudstone (fabric study
of
hand specimen
and
thin section)
can accurately portray depositional conditions
and
environ-
tneiits
of the
interval from which
it was
colleclcd.
If
there
is a
choice between studying
an
entire outcrop
of
more resistant
interbedded lithologies (e.g.. sandstone),
and the
careful study
of a mudstone sample from
the
same locality,
the
information
derived from mudstone fabrics
may
well
be
superior.
Juergen Sehieber
Bibliography
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R.H..
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W.R., and
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tonigen Sedimenten, Geolo-
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R.M., 1990.
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Brett.
C.E.. and
Taylor.
W.L.. 1994.
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in
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m
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oJ
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Glossary
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J.. 1989.
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Protcro/oic Newland Formation. Belt basin. Montana. U.S.A.
Sc<ltmenlology.
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Sehieber.
J., 1990.
Significance
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I999ii. Mierobial mats
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ol
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J.,
1999b. Distribution
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mentary Research.
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909-925,
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Zimmerle,
W.. and
Sethi,
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(eds.), 1998. Shales and
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2
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Trcwin,
N.. 1988. Use of the
scanning electron microscope
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fn
Tucker,
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Cross-references
Algal
and
Bacterial Carbonaie Sediments
Bedding
and
Internal Structutes
DESERT SEDIMENTARY ENVIRONMENTS
207
Bkiek Shales
Desiccation StriiLiurcs (Mud Cracks, etc.)
Flocculaiion
Mudrocks
X-ray Radiography
DESERT SEDIMENTARY ENVIRONMENTS
Introduction
Deserts arc defined as areas of sparse vegetation due to aridity
(cf. Glcnnic, 1970: Cooke and Warren. 1973: Cooke and
others. 1993: Thomas. 1997). The United Nations study of
desertification (UNEP. 1992) measured aridity by the ratio of
preeipitation to potential evapotranspiration and defines
hyperarid areas with ratios of less than 0.05 and arid areas
with ratios of 0.05 0.20. These areas cotnprise nearly
20 percent of" the earth's land surface, primarily concentrated
along the high pressure belts of both the subtropical horse
latitudes and the poles. Many large deserts are found along
these belts as well as the midlatitude intracontinental steppes
of Asia and North Atnerica in areas classified as semiat"id
(l)-2() 0.50). Smaller desert areas less dependent on latitude
oeeur in orographic rain shadows and in areas associated with
cold oeeanic upwelling. Fhe distribution of deserts in the
geologic past was strongly influenced by the configuration of
continental land masses (for instance, the Pangean super-
continent), tectonic activity, and the cycles of variation in solar
indux.
The range of sedimentary environments eneountered in
deserts is summarized in Strakhov (1970). Glennie (1970.
1987). Cooke atid Warren (1973(. Hardie and others (1978).
Warren (1989). Fryberger and others (1990). Smoot and
Lowenstein (1991). Cooke and others (1993), Lancaster (1995).
and Thomas (1997). Desert sedimentary environments are
characterized by: (1) poor vegetation, resulting in greater
erosion by wind and water, (2) low rainfall, leading to greater
desiccatioti and evaporation and highly sporadic sedimenta-
tion,
and (3) water input from outside the desert, producing
local facies assoetations. Desert environments are strongly
intlueneed by the interplay of eolian processes, intermitleni
wetting and drying of seditnent, and evaporation of standing
water and groundwater.
The characteristically low vegetation cover of desert
environments allows wind to erode surfaees and transport
large quantities of sediment. Wind erosion may sculpt bedrock
into fantastic fortns or polish ihe surfaces of pebbles and
boulders into eharaeteristie fortns ealled ventifacts. Wind
erosion (deflation) tnay form small catchtnent basins a few lens
of feet in diameter or larger basins that are hundreds of meters
deep and thousands of square kilometers in area. Precipitation
of saline minerals on exposed bedrock enhances its breakdown
and makes it more susceptible to wind erosion.
Soils in deserts reflect low vegetation, intermittent wetting,
and frequent cotnplete desiccation. Many soils are regoliths
with little horizon development. Microphytes (cyanobacteria.
lichen,
moss, ete.) may form a significant cover to the soil
surface, protecting it from erosion or acting as a trap for
windblown
dusl.
Microphyte trapping of windblown dust is
also thought to contribute to the formation of desert varnish
on rock outcrops and boulders. Wetting and drying of desert
soils produce complex desiccation crack patterns and may
build up compressive forces that churn the sediment and force
pebbles to the surfaee as pavements. These eompressive forces
may also produce slickensided shear planes. Vesicular fabrics
are produced by air trapped below wet sediment or rock
pavements. Windblown dust may introduce soluble minerals
that are reworked by rainfall in the
soil,
forming caliche,
silcrete. or evaporite minerals that precipitate as isolated
nodules, erystals. or cemented crusts.
E\aporative concentration of solutes in groundwater and
surface water makes these waters less hospitable to organisms
and may lead to precipitation of saline minerals. Groundwater
evaporation occurs throughout the capillary zone and may be
aided by evapotranspiration by plants. A process called
evaporative pumping has been considered an itnportant source
of hydrostatic head in the eapillary zone, but Macumbcr
(1995) argues that evaporation actually depresses the surface
of regional groundwater discharge. The path of solute
concentration is controlled by the initial groundwater eotnpo-
sition.
whieh reflects the drainage area lithology. and the early
precipitation of
less
soluble phases (Eugster and Hardie, 1978).
Similarly, the initial inflow waters control the eotnposition of
lakes or isolated tnarine embayments. At higher concentra-
tions,
the brine composition is complicated by a variety of
other parameters, such as temperature, biological activity,
mixing of different waters, solute recycling, dehydration, and
reactions with minerals in the sediment. Where heavy saline
brines sink into the water table, producing a lens-shaped
groundwater body, the fresher groundwater may be refracted
upward along the contact to produce artesian springs.
The geologically most itnporlant desert enviroimients are
sumtnarized under the broad headings of eolian, fluvial,
lacustrine, playa, and marine coastal environments. The
discussions of these environments will foeus on the features
unique to desert settings. Additional details about these
environments can be found under other headings.
Eolian environments
Vast deposits of wind-blown sand, called ergs or sand seas,
tnay dotninate the desert landscape. The primary cotnponents
of
these
eolian sand deposits are dunes and sand sheets, Eolian
dunes have a variety of morphologies and sizes that rellect
sedimeni supply and the variability of the sand-transporting
winds. Dune heights tnay reach hundreds of meters and
lengths may extend kilometers. Eolian dunes are primarily
classified by their crest configuration. Dune ridges that are
transverse to witid direction include barchan, crescentic ridges,
and parabolie dunes; ridges parallel to wind direction are
called linear
dunes;
and dunes with eotnplex multiple ridges are
called star dunes. All of these dunes internally produce tabular
cross-bedding from slip-face migration and low-angle tangen-
tial cross-bedding primarily from wind ripple migration.
Eolian deposits dominated by transverse dune ridges have
largely unidirectional eross-bedding parallel to the net wind
direction,
whereas those dominated by linear dunes have
bimodal cross-bedding at right angles to the net wind
direction.
Cross-bedding sets are divided by erosional bound-
ing surfaces that represent the stoss-side erosion of climbing
dunes, or larger scale bounding surfaces that represent the
stoss-side erosion of large compound dunes (alsoealled draa)
or deflation of a dune fleld to the local groundwater table.
Interdune areas may be represented by pebble lags frotn
208 DESERT SEDIMENTARY ENVIRONMENTS
Figure D12 Eulian dtine environments. (A) Crescentic ridge dunes in
forefjround and barchan dunes in background. Note muddy interdune
area in foreground. iBi Deflation flat with small coppice dunes. Note
alluvial fan in background. (C) Linear dunes cut by ephemeral lake
margin.
deflation, thin mudcrackcd mud layers or evaporilcs from
ponding of water, or Mat bedding frotn the migration of wind
ripples or adhesion ripples.
Sand sheets are broad areas of eolian sand that have low
relief.
These form around tbe margins of sand seas or in areas
that are unfavorable for dune formation due to coarse grain
size,
vegetative cover, frequent flooding, eetnentation. or a
high groundwater table. Most sedimentation in eolian sand
sheets is from the migration of wind ripples producing
relatively flat lamination. Zibars are low-relief (Im to lOm)
and long-wavelength (IOO-5OOm) dune forms with rounded
crests and no slip (ace. These produce low-angle inelined
lamination. Coppice dunes (or nebkhas) are mounds of wind-
blown sand that are trapped by vegetation. Eolian sand sheets
that form near evaporite-prodncing areas may have their
surfaces cemented and deformed by effloreseent salt crusts.
Sedimentary layers may be defomied into upturned polygons
or blister like knobs. Wind-blown dust trapped on the
efflorescent salt crust surface produces irregular thin mud
partings lhat accentuate these features.
Large areas of some deserts are dominaled by wind deflation
leaving exposed bedrock eroded into charaeteristie shapes and
lags of wind-blasted cobbles. Detlaiion surfaces covered by
lags of wind-smoothed pebbles and cobbles are atsocalled
gibber plains or regs. Windblown dust (loess) may form thick
deposits in areas marginal to deserts that have some vegetative
eover. and it is often incorporated into the deposits of olher
desert environments. Blowouts may produce small depressions
that host lakes or saline pans. Mud pellets and sand eroded by
wind may accumulate on the downwind edges of these basins,
producing lunettes or parna dunes. Salts eroded from the fiats,
particularly gypsum, may contribute significant portions of
sand to adjacent dune fields.
Fluvial environments
riuvial systems in desert regions can be divided into those
whose drainage extends out of an arid region (allogenic) or
ihose whose drainage area is entirely within the arid
environment (endogenic). Allogenic fluvial systems typically
have fiowing water all the time as perennial streams and rivers.
Endogenic fiuvial systems frequently are dry. fiowing only
during sporadic rainfall events. These fiuvial systems include
ephemeral streams and alluvial fans. Endogenic streams
terminate wilhin the arid region (endoreic). whereas allogenic
streams and rivers may be endoreic or they may fiow through
the desert region (exoreic).
Perennial rivers and streams in deserts may have mean-
dering, braided, or anastomosing channel morphologies
similar to rivers of more humid regions. The size and
distribution of cross-beddiiig from bedforms and bars in a
ehannel deposit and the lateral distribuiion of channel deposits
refieet Ihe channel morphologies similar to rivers in humid
regions. Since perennial rivers in deserts are a rare source of
fresh water, vegeiation tends to cluster around ehannel
margins. Roots may preferentially obscure channel bedding
features rather than those ofthe adjacent fioodptain unlike the
relationship for humid regions.
Ephemeral streams have ehannels lhat are dry e.vcept during
sporadic floods. Most ephemeral streams have small braided
channels thai are dominated by horizontal lamination and
low-angle tabular foresets. Ephemeral streams dominaled by
pebbles and cobbles display gravel bar lenses, pebble clusters,
and shadow fabrics. Larger ephemeral streams include
abundant trough cross-bedding and other features similar to
perennial braided streams. Channel deposits of ephemeral
streams may be capped by mudcracked mud drapes or they
may bave eolian overprints. Mud inlraelasls derived by
reworking of mud drapes or desiccated flood-plain deposits
may be a major component of ephemeral stream deposits.
Eluvial fioodplain deposits in deserts lack the vegetative
cover typical of humid lloodplains. Thick muddy floodplains
are disrupted by complex desiccation cracks and may have
desert soil features sueh as evaporites, caliche, or silcrete.
Windblown sand may fill desiccation cracks. Sandy floodplains
DESERT SEDIMENTARY ENVIRONMENTS
209
Figure D13 Mudcracked dry mudflat, (A) Surface view of complex
mudcracks including partially filled older cracks cut by newer open
tracks. Knife is 3Scm
long.
(Bl Cross-seclion of above showing narrow
irregular cracks (c), ovale vesicles (black), and flattened vesicles (fv).
Scale is
7
mm wide.
tnay be reworked into eolian deposits including small dunes
and sandsheets. Thin mud drapes break up into polygonal
cracks that have upcurled edges that may be buried by
windblown sand or incorporated as intraelasls in subsequent
floods.
Alluvial fans are cones of sedimenl deposiied where stream
drainages deboueh from mountain fronts. Alluvial fans are not
restricted to deserts, but are commonly the dominant fluvial
deposits in mountainous desert areas. Sedimentation on
alluvial fans occurs during infrequent floods producing stream
and/or debris-flow deposits. Streams are typically shallow and
high-gradient with deposits of thin, poorly sorted gravel bars
and planar bedding. Larger stream channels may develop
tabular cross-bedding from bars and low-angle antidune
backsets. Debris flows form levees, lobes, and sheets of poorly
sorted muddy conglomerates with steep tightly packed
boulder-cobble margins. During the long periods between
debris-flow events, the deposits are commonly modified by
rainfall, stream erosion of fines, and wind deflation. Debris-
flow deposits tend to be more sheet-like and thinner al the toes
of alluvial fans. Some stream-dotninaled fans have an apron of
unchaneled sand at the toe called a sandflat. These deposits
consist of sandy ihin beds with thin, mudcracked mud
partings. Broad, low-angle alluvial fans may develop where
perennial rivers enter a desert basin over a mountain front.
Figure D14 EltlorescenI salt crusl on a saline mudflat. (A) Surf^ace view
of (ieolian sand parlially filling depressions wilhin a thick, mud-rich,
efflorescent crust. Stole is J5cm
long.
IB) Cross-section of crust above
showitij;
mud-rich, porous efflorescent crus! ler) with pockets of
aeolion sond (s) that are also visible as patches in poorly sorted mud
with scattered microcryslalline evaporites (ev). View is about iiOcm
high.
Lacustrine environments
Lakes in desert conditions are commonly temporary features
formed by sporadic flooding events. Perennial lakes form at
the terminus of allogenic rivers or in basins with high
grounduater flow. Like lakes in tnore humid settings, desert
lakes are sites for accumulation of fine-grained seditiienl with
coarser sedimenl accutnuiaiing near the margins. Lake
sediments may be massively bioturbated or they may have
flat lamination from settling of different grain sizes or types, or
they may have layering defined by sandy storm layers. The
surface area and depth of the lake eontrois the amount of
wave-sorting that occurs wilhin the lake and the development
of shoreline deposits. Wedges of deltaic sediment form uhere
river or streams intersect the lakes. LInlike most lakes in humid
regions where lake depih is controlled by the spill level, desert
lakes are typically hydrologically closed with Iheir depth con-
trolled by the balance between inflow and evaporation. These
so-called terminal lakes change depth and surface area radi-
cally over short periods of time, even completely desiccating.
210
DESERT SEDIMFNTARY ENVIRONMENTS
SANDFLAT WITH
ZIBARS
DEFLATION PLAIN
ALLOGENIC
RIVER
Figure D15 Distribuiion of sedimentary environments in a sand sea setting. Perennial lake has beacb ridges and a birdfoot delta. Dry mudt'lat
remains from a former lake embaymenl within the dunes.
and they accumulate solutes the longer they remain as standing
water bodies.
Shoreline deposits range from well-developed spits and bars
to thin sheets of rippled sand or imbricated pebbles. Shifting
shorelines often leave the deposits stranded away from the lake
to be modified by wind, flooding, and desiccation. Deltaic
deposits include steep Ibrset "Gilbert-type'" deltas and streatn-
dominaled birdfoot deltas. The frequent changes in lake level
make these delta deposits more sheet-like and may result in
stacks of delta fronts separated by subaerial surfaces. Where
alluvial fans impinge on lake margins, the boulders and
cobbles may be sorted by waves forming tabular sets that dip
lakeward. Algal tufa deposits may coat cobbles or form
mounds and lowers along ihe shorelines, particularly where
spring water mixes with carbonate-saturated lake water.
Desert lakes may accutnulalc enough solutes over time to
supersaturate with saline minerals, such as gypsutii, halile,
trona, or borates. The minerals precipitated primarily depends
upon the eomposition of the inflow water, but they are also
controlled by variables sueh as temperature, depth, mixing,
and organic productivity. Evaporite minerals may precipitate
at the water surface producing cumulate crystal layers as they
settle, or the crystals may precipitate on the lake floor
producing crusts or vertically oriented surface crystals. Dense
saline brines may sink through the underlying sediment
precipitating crystals or altering matrix composition as they
interaet with previously deposited sediment or mix with
interstitial water. Deep saline lakes are otten strongly stratified
with dense brine at the lake bottom inhibiting mixing and
inducing bottom anoxia.
Playa environments
Playas include a variety of geomorphic features such as salars,
Salinas, chotts, inland sebkhas. and pans, among others, that
comprise the "dry lakes'". These deposits occur in depressions
within the desert and are mostly dry with infrequent flooding.
Some playas are superimposed on the remnants of former
desert lakes, whereas others are aggrading. Playas may be dry
muddy surfaces or they may be dominated by saline minerals.
Wind erosion of mud and saline minerals may be very impor-
tant on playa surfaces, particularly where powdery efflorescent
crusts are developed. The major varieties of playa environ-
ments are dry tiiudflats, saline mudflats, and saline pans.
Dry mudflats are muddy surfaces dominated by desiccation
cracks. They form in areas of intermittent surface flooding
where the groundwater table is well below the surface.
Bedding, when present, consists of graded sand-mud layers
representing individual flood deposits. More commonly, the
only sediment accumulation is within mudcracks producing a
massive mud with complex sediment-filled cracks and open
vesicles. Dry mudflat surfaces may develop powdery "puffy
ground" efflorescent crusts where windblown soluble minerals
are dissolved by rainfall or flooding and reprecipitated within
the sediment surface.
Saline mudllats are muddy surfaces that contain abundant
saline minerals precipitated within the sediment and/or on the
surface as efflorescent crusts. These form in areas with shallow
groundwater tables or groundwater discharge near ihe
sediment surface and where there are abundant windblown
soluble minerals available, lntrasedimenl crystals form in
equilibrium with groundwater along its flowpath as it is
evaporated producing large euhedral crystals in the phreatic
zone and smaller crystal clusters or dehydrated forms in the
vadose zone. Windblown dust may contribute to saline mineral
formation in vadose zones when soluble materials are moved
down by rainfall or flooding. This produces vertical profiles of
upward coarsening crystals, opposite of the groundwater
situation. Efflorescent crusts form by the complete evaporation
of porewater at the sedimenl air interface and are eomprised
ofa mixture oCtiny crystals including ihe most soluble phases.
These crusts are rarely preserved as they are dissolved on the
bottom by the undersaturated groundwaier. Hard efflorescent
crusts made of halite trap windblown dust on the crystal
DESERT SEDIMENTARY ENVIRONMENTS
211
surfaces and coarser tiiaterial in surface pits and irregularities
thereby producing a poorly sorted mud with irregular sand
patches. Powdery efflorescent crusts disrupt the upper
sediment surface and form pelleted textures in mud that may
be redeposited into cracks and surface irregitlarities.
Saline pans are salt-encrusted surfaces within saline mud-
flats ihal are intermitlently occupied by shallow saline lakes.
Saline pan lakes form during flooding events and the solutes
are mostly derived by the dissolution of efflorescent crusts.
This causes the saline pans to be dominated by the most
soluble minerals. As the temporary lake waters evaporate,
crystals are preeipitated first as eumulus crystals then a bottom
growth crystal crust. During prolonged periods between
flooding, the erust may be modified by polygonal expansion
cracks wilh formation of efflorescent crusts along their edges.
The sinking brine will precipitate crystals in pore spaces and
within sediments. The nexi flood will dissolve the efflorescent
crust and part of the bottom crust before the cycle of
deposition begins again. This meehanistn may produce thick
monomineralie deposits that are thin bedded.
Marine coastal environments
Where deserts border marine environments, the influence of
seawater and marine seditnenl prodtiees distinctive deposits.
Many of the environmctits found within inland deserts may
also occur in a coastal setting. Gypsutn and halite character-
istically dominate saline soils, saline mudflats, and saline pans
along desert coastlines due to the influence of seawater
chemistry. Eolian sands may contain a significant component
of marine shelly material.
Sabkhas are broad, salt-encrustcd flats that bound a marine
coast. Clastic sedimentation on sabkhas is largely by storm
surge tides, producing graded thin beds or lamination, or by
wind. Thick algal mats commonly cover the sediment surface
in nearshore areas modifyitig or controlling the sediment
layering. Gypsum crystals form within the sediments that are
saturated with seawater. Halite effloreseent crusts cover most
of the inland part of the sabkha. Saline pans precipitating
halite or gypsum crusts are common. Dehydration of gypsum
crystals in distal areas produces anhydrite nodules and layers.
Marine carbonate seditiienl is commonly altered lo dolotnite in
this selling. Refraction of groundwaier by the hypersaline
seawater commonly produces artesian springs or spring-fed
ponds along the coast within which carbonate erusts and
micrilic mud preeipitate.
Isolated etnbayments of seawater in deserts become more
saline by evaporation and less hospitable lo normal marine
organisms. If circulation with normal seawater is strongly
curlailed. these embaymenls may become supersaturated with
gypsutn or halile producing cumulus crystal layers or bollom
crusts similar lo those of saline perennial lakes. Shallow.
intermittently desiccated marine pools are called salterns.
These produce layered evaporites similar lo those of saline
pans.
In the geologic past, large embayments such as the
Mediterranean Sea have becotne hypersaline producing
extensive bedded evaporiies.
Desert environment associations
Sedimeniary deposits of desert environments have been
recognized in rocks throughout the geologic record to the
Precatnbrian. The distribittion of these desert deposits through
lime reflects changes in the Earth's tectonic regime and global
changes in climate. Some desert environments appear to have
left little record in the pre-Ouaternary despite their areal
abundance in the modern world. Polar deserts and high-
latitude saline lakes have not been noted in the geologic record.
Deflation plains have only been recognized within eolian sand
deposits and never as a regional surface in the pre-Quaternary.
The dearth of these deposits in the geologic record probably
reflects the facl thai they do not aggrade making ihem less
likely to be preserved.
Although the combinaliotis of desert environments are as
varied as the potential eombination of climatic and tectonic
histories, there are certain associations that are commonly
repeated. Sand sea deposits are eommonly developed on stable
cratons or broad structural sags where low relief is ideal for
erosion and transport by wind. Perennial lakes associated with
these sand seas are broad and shallow with superimposed dry
playa mudflat deposits common. Foreland basins of thrust
belts in desert regions produce extensive ephemeral and
perennial river plains that terminate into broad basins wilh
perennial lakes and playas. Isolated marine basins may
produee ihiek evaporite deposits ranging from deep water to
suhaerial. Rifl basins produce thick sedimenl accumulations
with alluvial fans, perennial lakes, and playas as the common
associations. Thick sabkha and marine embaymenl deposits
may also be associated with alluvial fans in this setling.
Climatic instability can shiTi a region from humid conditions
to arid and back again repeatedly in the span of thousands of
years.
Desert deposits may be interbedded with non-desert
deposits at the meter scale, or desert fabries may be super-
imposed on non-deserl deposits or vice versa. This is particularly
true for orographic desert situations lhat exist closer to the
equator or at higher latitudes than the high-pressure belts. Sea-
level shifts in response to global clitnate change may result in
marine inundation of coastal desert deposits or isolation of
marine bays to form saline embayments or salterns.
Joseph P. Smoot
Bibliography
Cooke, R.U.. and Wurren. A., 1973. Geomorpholoi-y in De.serts.
Los .Atigclcs: University of Californi;i Press.
Cooke. R.LL. Warren, A.', and Cioiidie. A.S..
199.1.
De.sm Geomor-
phology. London: University College London Press.
Eugsier. H.P., and Hardie, L.A.. !978. Saline lakes. In Lerman, A.
(ed.J. Lakes: Geocheinistrv.
Geoloi;v,
and Pln\m\s. Springer-Vcriag,
pp.
237 293.
Fryberger. S.G.. Kryslinik. L.F,, and Sclienk. C.J.. 1990. Modernand
.Ancienl F.olian Deposits: Petroleum Fxploralion and Production.
Denver: Rocky Mouiuain Section. SEPM.
CJlennie, K.W.. 1970. Desert Sedimentary
F.nvironmenls.
Flsevier.
Glentiie. K.W,, 1987. Desert sedimentary environ men
is.
past and
present a summitry. Seilimeniary
Geology.
50: 135 165.
llardie, L.A..Smoot. J.P., and Eugsier. H.P..
1978.
Saline lakes and iheir
deposits: a sedimentologieal approaeh. In Matter, A., and Tueker.
M.E. (eds.). Modern and .Ancient Lake Sediments. International
Assoeiation of Sedimeniology, Speeial Publication. 2. pp. 7 -41.
Lancaster. N.. 1995. Geomorphology of Desert Dunes. New York:
Rouilcdge.
Macumber. P.G.. IWl, huenielion between Groundwater ami Surface
Svstenis in Sorthcrnl'ieloriii. Melbourne. Department of Conserva-
lion and Environment Victoria.
Rosen, M.R., 1994. The importanee of groundualcr in playas:
A review of playa elassifiealions and llie sedimentology and
hydrology of playas. In Rosen. M.R. (ed.).
I'aleoclimiite
and Basin
212
DFSICCATION STRUCTURES (MUD CRACKS, ETC.)
Fvohition
oJ
Playa Systems. Geological Society ot" .America, Speeial
Paper. 289. pp." 1-18.
Smool. J.P., and Louenslein, T.K,. 1991, Depositioiial environments
of non-marine evaporites. In Melvin, J.L. (ed,).
Evaporile.s.
Petro-
leum and Mineral Resources. Elsevier, pp, 189-347.
Thomas. D.S.G., 1997. Arid Zone Geomorphology. John Wiley and
Sons.
UNEP, 1992.
World
Atlas of Desertifieation. Seven Oaks. UK: Edward
Arnold.
Warren, J.K., 1989. Evaporile Sedimenlolofiy: Its Importance in
Hydroearhon Accumulalions. Prentiec-Hall.
DESICCATION STRUCTURES
(MUD CRACKS, etc.)
Desiccation structures originate as shrinkage cracks formed by
the evaporation of water from the surface of clay-rich
sediment. Previously called mud cracks, they are of subaeriai
origin, and arc caused by the slow drying-out of muddy
sediments which have been exposed to the action of sun and
wind. The volume decrease that results from this loss of fiuid
gives rise lo tensile stresses distributed equally in all directions
within the bedding plane, that are relieved by the formation of
a characteristic pattern of open polygonal cracks on the
surfaee of the sediment.
Arrays of polygonal cracks resulting from this process are
commonly seen on dried-out surfaces in puddles and slurry
pits,
around the margins of lakes, reservoirs and playas. and
on tidal mudflats. Where well-developed, the desiccation
structures form striking prismatie columns a few centimeters
to .several meters across, separated by deep fissures, and
divided internally into seeondary sets of cracks. Following a
change in local climatic conditions and a rise in water level, the
cracked surfaees become submerged, and the cracks and
fissures may be filled with sediment and preserved. Once
the.se sediments have been lithified, and later exposed at the
Earth's surface, the host miidroek is more easily eroded than
the sandy material infilling the cracks, and a cast ofthe sand-
filled cracks is generally preserved on the base ofthe overlying
sandstone bed.
Ancient desiccation cracks preserved in this way are not
only useful as paleoenvironmental indieators. but provide vital
evidenee of way-up in rock sequenees affected by folding and
faulting. The crack patterns have geometrie features in
common with many other types of shrinkage cracks, such as
cooling joints in basaltic dykes and sills, ice-wedge cracks in
frozen ground (see Tills and Tillites). and with microcracks
formed during the drying of thin layers of ceramic glaze,
polymer, corn starch, paint and other materials. This feature
can be used to advantage in experimental and theoretical
studies (Weinberger, 1999, 2001). but their similarity to other
sedimentary structures, and especially interstratal dewatering
structures (Tanner. 1998), and syneresis cracks (sec Syneresis)
means that all sand-lilled craeks have to be examined with
great eare. to determine Iheir origin.
Morphology
The terminology used to describe desiccation structures
was reviewed and illustrated by Allen (1984). following
Lachenbruch (1962). The morphology of intertidal desicealion
cracks was analyzed in a seminal study by Allen (1987), and
that of filled desiccation cracks in vertic paleosols is described
by Paik and Lee (1998).
In plan view, desieeation cracks vary from straight to
curved. They may form random, unconnected, even radial,
patterns (incomplete erarks) but are most commonly seen as
closed shapes bounded by 3. 4, or 5 cracks, with Y- or T-
shaped intersections (complete cracks). Although the linked
hexagonal pattern (nonorihogonaltype) is that most eommonly
portrayed in textbooks for mud cracks, the polygons in
naturally occurring desieeation eraeks are most commonly 4-5
sided, with the majority of individual cracks meeting at right
angles (orihogotial type) (Figure DI6). Polygon size is a
function of the thickness of the desiccated layer, with thick
layers producing the larger polygons.
In cross-section, individual desieeation eraeks form at right
angles to bedding and generally laper downward (V-shaped
profile) from the surface of the cracked layer, ln three
dimensions, the sets of polygonal cracks divide the mud layer
into separate columns or pillars which may be cut by
horizontal cracks whieh connect with the bounding fractures.
At an advanced stage of desicealion, and especially in thinly
bedded mudrocks, bedding laminae at the top of the pillars
become dish-shaped and may beeome distorted to form trieorn
shapes or "mud eurls" (Figure D17).
The infilling of the eraeks with sand and silt grains may
occur in a single event, or be multi-stage as is common in
intertidal situations. Repeated cycles of desiccation and
infilling also oeeur. leading to a complex internal erack
morphology. Vertically stratified crack-inlills have been
reported from vertic paleosols. Alternatively, eraeks formed
by drying-out of the surface of a very wel substrate may
subsequently be intruded by mud or fiuidized sediment from
below, to form polygonal patterns of elastic dykes.
H
tstory
Geologists working in the Appalachian Mountains had
already reeognized the environmental significance of mud
cracks by the mid-19th century; the first use of mud eraeks , or
''suncracks'", as a way-up indicator is not known, but
according to Sehroek (1948) the technique was already
established by 1908. This author provides an excellent account
of the morphology of desicealion craeks, together with a
comprehensive review of earlier work. Fundamental advances
in understanding the mechanics of the desieeation process were
made by investigators studying the formation of elosely related
structures: ice-wedge cracks in frozen ground (Lachenbruch,
1962;
Corte and Higashi, 1964). This phase was followed by
furiher careful documentation of the morphology of mud
craeks. foremost amongst these being the studies by Allen
(1984,
1987). It was also a period in which there was
eonsiderable debate over the criteria that could be used to
distinguish desieeation cracks from syneresis, or subaqueously-
formed, craeks (see Syneresis). In recent years there has been a
revival of interest in these superficially simple desiccation
struetures and further progress has been made in under-
standing their mechanics of formation (see below).
Mode of formation
The desieccation process requires subaerial exposure of a
muddy layer of sediment. As moisture is lost from the upper
DESICCATION STRUCTURES IMUD CRACKS, ETC.i
213
Figure D16 (Orthogonal desicealion cracks in lime-rich murl Irom
Milings pJI in SW England Isee text for description].
Figure D17 Mud curls surmounling o desiccdtion polygon a! the same
locality as Eigure D16.
surfaee of the layer, the traditional model based on field
relationships postulates that shrinkage eraeks initiate at
nueleation points on the surface, lhat in some cases ean be
idenlilied as imperfections on the surface caused by bird's
footprints, raindrops, bivalve shells, vegetation, ete. Aided by
high local stress coEicentrations al their tips, the early eraeks
propagate both laterally and downward, and in most cases link
up to Ibrm elosed networks of polygonal cells. As in the case of
eooling joints in igneous rocks, the pseudo-hexagonal form of
these eells is controlled by the principle of least work: a
detailed mechanical analysis of the proeess was given by
Laehenbruch (1962).
Crack formation takes place in stages, with a network of
primary or first-order eraeks dividing the drying layer into a
series of polygonal shapes, each of which is then subdivided by
second-order eraeks (Figure D16). Once a crack has formed it
represents a principal or "free"* surfaee. and new eraeks
propagating toward it turn to meet with it at right angles, so
explaining the preponderance of orthogonal junctions over
•'ideal" 120 contacts (Laehenbrueh, 1962). The more fine-
grained, smeeite-rieh. and homogeneous the starting material,
the more perfeelly formed and nearly hexagonal the pattern of
cracks which results. A striking feature of some sets of mud
cracks is the development of seeondary and even tertiary sets
of fractures sub-dividing the polygons formed by their
predecessors (Figure DI6).
Current investigations
Recent work has been aimed at analyzing the desieeation
process in detail, both in the field and by studying the
formation of desiccation eraeks in the laboratory using analog
materials such as corn starch water mixtures.
In both natural and experimental situations, the surfaces of
the cracks are commonly deeorated with plumose struetures,
which consist of a series of faint ridges radiating out from a
central axis. Plumose markings enable the aetual nueleation
points for each erack to be idenlilied. as well as showing the
direction of propagation ofthe rupture surface, so enabling a
detailed kinematic picture of the growth of the array to be
assembled (Corte and Higashi, 1%4; Weinberger. 1999, 2001).
In eontradielion to the aeeepted model, new field evidence
shows that, in some cases al least, desiccation cracks nucleate
at the base of the bed and propagate upward (Weinberger,
1999.
2001). In experimental situations the speed of crack
propagation can be monitored using a video camera, and was
found to vary from 100 mm per second at initiation to
<IOmm/min during amplification {Muller. 2001),
Summary
Desiccation structures result from the uniform layer-parallel
shrinkage of homogeneous layers of mud that are rich in
swelling clays. This proeess occurs as a result of subaerial
exposure of recently deposited sediment and gives rise lo
familiar patterns of polygonal evaeks that are analogous to the
cooling joints found in igneous dykes and sills. Individual
eraeks are V-shaped in cross-section and when subsequently
filled with silt or sand, and preserved in the geologieal record,
provide a reliable way-up indicator, A feature diagnostie of
sub-aerial exposure is the formation, in thin, dried-out layers
of mud at the top of the polygonal columns, of concave-
upward plates or "mud eurls".
Desiccation structures are indicative of subaeria! exposure
of muddy sediment whether in an intertidal. lacustrine, or
playa-lake environment. However the simple patterns shown
by these structures should not be eonfused with those shown
by interstratal dewatering structures (see Fluid Escape Struc-
tures) or syneresis cracks (see Syneresis). Polygonal patterns
of sand-lilied cracks seen on bedding planes are not necessarily
proof of subaerial exposure and their misinterpretation can.
and has. led to serious errors in identifying the depositional
environment.
P.W. Geoff Tanner
Bibliography
Allen. J.R.L.. 1984. Sedimentary Siruclures. iheir Character and
Physieal basis. Dt'velopmems in Sedimeniology 30. Amsterdam:
Elsevier.
2t4
DIACiENESIS
Allen.
J.R.L.. !9K7. Dt:sicc7Hion of mud in the tempcriilc iniertidiil
zone:
studies tVom the Severn Esiuary and eastern England. Philo-
sophicalTransaelionsof
the
Royal
Society.
B31S; 127-156.
Corte, A.E.. and Higashi. A.. 1964. Hxperimcnlal researeh on
desiccation eraeks in
soil.
U.S. Amiy Sno». Ice and Pernuifrost
Establishnwnl. 66.
Lachenbruch.
.'\.H..
1962. Mechanics of ihermal contraclion cracks
and ice wedge polygons in permafrost.
Geological Society
of.imer-
ica
Speeial
Paper.
70; I 69.
Muller. G.. 2001. ExperimenUil simulation of joint morphology.
Jour-
nal of Slructmal
Geology.
23; 45 49.
Paik. l.S.. and Lee. Y.I.. 1998. Desieeation eraeks in vertie palaeosols
of the Cretaceous Hasandong Formation. Korea: genesis and
palacoenvlioninental implications. Sedimeniary Geology. 119;
161-179.
Shroek, R.R.. 1948.
.Set/uence
in Layered
Rocks.
New York; McGraw-
Hill.
Tanner. P.W.G.. 1998. Inlerstralal dewatering origin for poiygoniil
patterns of sand-tilled eraeks; a ease study tYom Late Protcro/oic
metasedimenls of Islay. Scotland. Sedimentology. 45: 71-89.
Weinberger. R.. 1999. Inilialion and growth of eraeks during
desiccation of stratified muddy sediments. Journal of Siruciural
Geology. 21: 379 3X6,
Weinberger. R.. 2001. Evolution of polygonal pallerns in siralitied
mud during desiccation: (he role ol" llaw dislribuliori and layer
boundaries.
Geological Society
of
.America
Bullelin. 113:
20-.'ll.
Cross-references
Clastic (Ncpiuniiiii) Dykes and Sills
Fluid Escape Structures
Scpl;irian Concretions
Syncresis
diagenesis. Study
of the
widcspreiid role
of
microbial activity
in diagenetic processes, both physical and chemical,
is an
area
of currently active research.
Conditions
of
diagenesis
Althongh
it
begins
at the
sediment/water
or
sediment/air
interlace, diagenesis lakes place largely
in the
subsurface
and
primarily under conditions
in
which
the
rock pore spaees
are
filled with waler.
No
particular depth
is
implied,
the
scale
ranging,
potentially, from millimeter
to
kilometers. Important
diagenetic fluids include seawater, meteoric water, hypersaline
evaporative fluids, brackish mixed fluids, and pore fluid
of
any
of these types lhat has been altered
in
varying degrees
by
fluid/
rock interactions. Extreme degrees
of
diagenesis ean yield pore
fluids that
are so
substantially modified that their original
composition
(e.g..
marine versus meteoric)
is
undecipherable.
The temperature range
for
diagenesis extends from subzero
(in
sublimalion brines)
to
25()'C
and
perhaps
as
high
as
3OO'^C.
Rocks heated above this maximum temperature range may still
have
an
ideniiCiable sedimentary prololith.
but
tend
to
have
lost
the
fabrics (grain-cement-pore space)
and
pervasive
solid-
phase disequilibrium that characterize sedimentary rocks.
The
pressure regime
of
diagenesis ranges from
the
conditions
al
the Earth's surface
up to and
including eonditions
of
fully
lithostatic pressure (Piiuid
=
Pk.dd)-
Few
subsurface samples
are
available below
5
km.
but
samples obtained from
as
deep
as
7
km
are
still clearly sedimentary
as
long
as
their temperature
has
not
exeeeded 250"C
for
very
long.
DIAGENESIS
Introduction
Diagenesis encompasses
all the
physical
and
chemical
pro-
cesses that affect sediments
and
sedimentary rocks
in ihe
portion
of the
rock cycle that includes
the
period
of
burial
between deposition
and
weathering,
and
also between deposi-
tion and
the
onset
of
metamorphism. Proeesses that induce
the
lithification
of
sediments constitute
a
major aspect
of
diagen-
esis.
Determination
of
historical sequences
of
physical
and
chemical processes
is an
important element
of
diagenetic
studies. Diagenesis
is key to
understanding
the
evolution
of
roek composilion
and
texture with depth, time,
and
tempera-
ture,
and
to
deciphering the mechanisms
by
which elements
are
cycled between
the
atmosphere, ocean,
and
crust.
Diagenetic processes
are
responsible
for the
generation
of
coal,
oil. gas, and
"hydrothermal'"
ore
deposits,
and
exert
a
major control
on
the composition
of
subsurface water and
the
distribuiion
of
porosity
in the
subsurface.
The
effects
of
diagenesis may erect barriers
to
fluid fiow
or
establish selective
passageways
for Ihc
migration
of
fiuids. Thus, there
are
compelling practical aspects
to
diagenetic studies.
Recognition
of the
substantial magnitude
of
chemical
and
physical reorganization that takes place during transformation
o'i sediments into sedimentary rocks
has
been
a
major
conclusion
of
diagenelic studies over
the
past quarter century.
Many processes formerly viewed
in
terms
of a
narrow
affiliation with either weathering, metamorphism,
or
tectonic
deformation
are now
seen
as
integral and essential elements
of
Rock components
in
diagenesis
Recognition
of the
initial versus
the
modified chemical
and
textural states
of
sedimentary rock components
is
essential
to
deciphering diagenetic processes. Rock components that exist
at
the
time
of
deposition
are
known
as
primary, diagenetic
modifications
of
primary components yield features described
as
secondary.
Primary mineral components
in
sediments
and
sedimentary rocks
are of two
types. Primary chemical
precipitates
are
characteristic
of
evaporative environments
and,
by
definition, must precipitate from fiuids concentrated
by solar evaporation (Warren. 1999).
A
larger group
of
primary minerals
are
called detrital
in
recognition
of
their
existenee
as
transportable particles
or
grains, either formed
within
the
depositional environment
(alloehems)
or
derived
from older rocks outside
the
basin
of
deposition
(terrigenous
debris).
Pore space constitutes another important primary
component
of
sediments
and may be
either intergranular.
occurring between
the
detrital grains,
or
primary ititragrauu-
lar pore space, occurring within
the
grain interior. Primary
intragranular pore space
is
observed
in all
types
of
grains,
but
occurs mosl commonly and most extensively within alloehems.
Some roek types are completely authigenic and diagenetic
in
the sense that they contain little
or no
strictly detrital material
and instead
are
constituted almost entirely
of
minerals
precipitated IVom aqueous solution either
at the
depositional
surface
or in the
shallow subsurface. Such "chemical
sediments'" include caliche, chert,
and
nodular anhydrite,
and
halite.
However, most sediments,
at
least initially,
are
constituted primarily
of
detrital grains
and
pore space.
All
of the
primary rock components described above
are
subject
to
profound modifications
by
both chemical
and
physical processes during diagenesis. Primary pore space
can