Middleton G.V. (Ed.) Encyclopedia of Sediments and Sedimentary Rocks
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FABRIC, POROSITY, AND PERMEABILITY
Introduction
Rocks are generally classified by their texture and composition.
Texture includes the size and shape of" the various solid
particles, and their arrangement in space: it therefore also
includes the geometry of the pore spaces included within the
rock (or sediment). There is no generally accepted term to
include alt the types of solid particles, but in this article we
follow Lucia (1995) in using the term partictc to iticlude both
chetiiically precipitated crystals, and clastic ^ra//j,v, which may
themselves be composed of one or more crystals. Unlithified
sediments are generally composed exclusively of grains which
originated either outside Ihe basin of deposition (so are atto-
genic) or within the basin of deposition (so are auio^cnic). but
•A
few seditnents or sedimentary rocks may be composed of
crystals precipitated directly on the sediment surface. The size
and shape of sediment particles is an important part of texture
(see Grain Size and Shape) but it is not the whole story: it is
very important how the particles are arranged in spaee,
because this determines some of the bulk (geophysical)
properties, including the size,, shape, and orientation of the
pore spaces. These in turn delermine two of the most
economically important bulk properties of a sediment:
(I) The percentage volume of pore space, or porosily. and (2)
The ease with whieh fluids can flow through a rock, which is
generally quantified as ils pcnncatiitity.
Fabric
The arrangement of particles in spaee is described as the fabric.
Early workers (for a review, see Martini, 1978) described fabric
in terms of two tnain properties: (1) The orientation ol" the
particles: either the actual orientation ofthe grains themselves
Utiniciisionat orientation), or the orientation of the crystals
eotnposing
the
particles {crysialloi^rapliic orienialion):
and (2)
The packing of the particles. Packing can be described using
theoretical models for regular packings of sphere, or (some-
what tnore realistically) for random packings. For real
particles, the packing density (see Kahn, 1956: or Martini,
197^,
for quantitative definitions) is a measure ofthe porosity,
and the packing proximity describes the number of contacts
between particles. The types of contacts are also very
important: for grains, Taylor (1950) introduced the terms
tangential, long (essentially planar), concavo-convex, and
sutured to describe increasitig close contact between grain in
sandstones, and showed that only tangential and a few long
contacts are present in uncompaeted sands, but concavo-
convex and sutured contacts become more significant as sands
are compacted by deep burial. Later studies have stressed the
importance of using techniques such as cathodoluminescence
(q.v.) to distinguish original clastic grains from chemically
deposited (diagenetic) cements in assessing the nature of grain
contacts. Many apparently concavo-convex and sutured
contacts are not produced by pressure solution
((/,
i',) but by
deposition of cement (see Cements and Cementation). For
more reeent approaches to packing sec Allen (1982, Chapter
4),
Bideau and Dodds (1991) and Rogers eiat. (1994).
Grain orientation
The way that grains become oriented is an interesting part of
the depositional process. It has also been studied as an
indicator of the depositional process
itself,
particularly for
deposits that do not show obvious internal stratifieation. such
as tills, debris flow deposits, and some turbidites (fbr a review
see Allen. 1982, Chapter 5), Most types of deposits show
elongate grains oriented in the direction of the flow, but
transverse and bimodal orientations are also known, particu-
larly for roller-shaped pebbles (see Size and Shape of Gruins).
Flat pebbles in fluvial deposits commonly dip in the up-flow
direction, but dip directions on beaches are more variable (see
Imbrication and Fhm-Orientect
Cla.'its).
Most investigations, of grain orientation have been moti-
vated by the need to obtain paleocurrents from such deposits
(see Patcocurrcnt .itiaty.sis). The statistical techntques required
to establish the fabric type and the preferred orientation (If
any) have been reviewed by Fisher (1993) and Fisher cl at.
(1987):
see Middleton (2000, Chapter 9) fbr references. Particle
orientation may be produced or modified by compaction (q.v.)
266
FABRIC, POROSITY,
AND
PERMEAfJIllTY
or diagenesis
(q.v.).
particularly
in
fine grained, clay rich
sediments
(see
Deposilional Fabric
of
Muilrock.s).
Most sedi-
mentary rocks have anisotropie fabrics,
i.e., the
grains
and
pore spaces show
a
preferred orientation, with
the
result that
bulk propertie.s. especially permeability,
are
also markedly
anisotropie. Permeability
is
almost always much larger
in the
plane
of
stratification than normal
to
that plane. Fabrics
generated
by
physical depositional processes
are
frequently
modified
by
biological processes
(see
Biogenie Siruclures,
and
the symposium organi;^ed
by
Kemp, 1995).
Porosity
Techniques
to
measure porosity
arc
reviewed
by
Monicard
(1980).
It is
generally useful
to
distinguish total porosity from
that part that consists
of
interconnected pores {effcciivcporos-
ity):
e.g.,
pumice
may
have
a
large porosity,
but
fluids cannot
move through
the
pores because they
are not
interconnected.
Geophysical techniques
may
measure
the
total water
(or
hydrogen) content
ofa
rock, some
of
which
may be too
tightly
bound
lo
mineral surfaces
lo
move freely. Porosities
of
uneonsolidated satids
are
generally
35 to 40
percent
but
tnay
be higher (particularly
for
irregularly shaped bioclasts):
fbr
recently deposited tnuds they
are
generally
50 to 80
percent,
with considerable consolidation taking place
in the
first meter
or
two of
burial.
Geologists
are
generally interested
in
classifying pore spaces
according
to
their genesis.
In
sands part ofthe porosity
may
be
deposilional inlergraniilarporosity,
due to the
original deposi-
tional packing
(or
what
is
left
of
it. af^ter compaction): another
part
may be
secondary porosity produced
by
dissolution
of
grains dutung diagenesis. Sotne
may
even
be
iniragranutar.
due
to pore spaces within
the
grains, either present already
at the
time
of
deposition,
or
produced later during diagenesis.
Diagenetic processes tend
to
decrease porosity, especially
during deep burial
(see
Compaction)
but
there
are
exceptions.
For further details,
see
Petrophy.sic.s
of Sand
and
Sanilsume.
Carbonates, even original carbonate sands, tend
not to
retain their original intergranular porosity
fbr
long,
due to
extensive early diagenesis
(see
Neomorphism
and
Recrysudliza-
lion:
Carbonate
Diagenesi.s
and
Microfacies).
The
resull
is
that
extensive classifications
of
pore types have been developed
fbr
carbonate rocks. Choquette
and
Fray (1970) divided pores into
two major categories, according
to
whether their presence
was
determined
by a
preexisting fabric (fabric selective)
or not.
Much secondary porosity
is
fabrie selective, because
it is
determined
by
dissolution
or
recrystallization
or
replacement
of fossil fragments- Large vugs
and
fracture systems, however,
are generally
not
fabric selective,
and cut
indiscrimately across
the original depositional fabrie. Lucia (1995)
has
proposed
a
revised system, based more closely
on
petrophysical rock
properties
(see
below).
It
recognizes
two
major categories
of
pore space:
(i)
interparticte:
and (ii)
yuggy. subdivided into
separate,
and
totiching (i.e.. interconnected vugs).
Permeability
Permeability expresses
the
ease with which fluids flow through
rocks,
Henry Darcy first established
the law (now
called
Da
rey'.s
law) that
the
discharge
of
fluid
per
unit cross-sectional
area ofthe porous medium.
(/, is
proportional
to the
hydraulic
gradient (d/i/d.v).
The
coefficient
of
proportionality
is
called
the livdrautieeonductivity. Hydraulic conductivity is comtnonly
used
by
hydrogeologists (e.g., Freeze
and
Cherry, 1979),
but
has
the
disadvantage that
it
depends
not
only
on the
porous
medium
but
also
on the
properties
of the
fluid.
A
fbrm
of
Darcy's
law
which separates
the
fluid
and
rock properties
is:
q=-k(y/fi){dh/d.x]
(Eq,
1)
Here
•; is the
specific weight
of the
fluid,
and /; is the
fluid
viscosity.
The
coefficient
of
proportionality,
k. in
equation 1
is
called
the
permeability,
or
intrinsic permeability,
and fbr
rocks
saturated with
a
single fluid (e,g,, water
or
air)
it is
determined
only
by the
rock properties.
Its
units, therefore,
are
length
squared. Permeability
is
related
to
some "average size"" ofthe
pore system, though
the
"average cross-section" ofthe pores.
is
not
given
by the
units
of
intrinsic permeability.
For
intergranular porosity, permeability
is
related
to the
size
and
sorting ofthe grains
(see
Peirophv.sic.s
of
Sanit.s
and
Sand.ftonc;
Chapman, 1981; Middleton
and
Wilcock, 1994. Chapter
6).
Unfortunately
the
same
law
does
not
apply (without major
modification)
for
rocks
or
sediments thai contain more than
one fluid,
e.g,, air and
water
in the
vadose zone;
oil or gas and
water
in
petroleum reservoirs.
It is
possible, however,
to
define
a "relative pertncability" which expresses
the
permeability
to a
single fluid,
at a
given fluid content,
as a
proportion
of the
intrinsic permeability:
but
once again, relative penneability
is
not
a
property only
of the
porous medium,
but
also
of the
other fluids present
in the
pores.
Permeability
of
relatively porous, high penneability rocks
is
generally determined
by
fbrcing
gas
through
dry
rock samples,
or
by
pumping tests
on
wells.
The
older unit
of
permeability
was
the
darcy. Very permeable coarse sediments
may
have
a
permeability ofthe order
of
one darcy.
but the
permeability
of
most reservoir rocks
is
best measured
in
mitltdarcies. Many
good reservoirs have permeabilities
of
only tens
of
millidarcies.
The
SI
unit
is
meters squared,
a
very large value
(for
comparison,
a
darcy
is
roughly
one
micrometer squared!),
A
portable "micropermeameter'" that
can be
used
on
outcrops
in
the field
has
been developed (Davis
et at..
1994), though
the
results
do not
always correlate well with measurements made
on cores (Tidwell etal.. 1998). Almost
all
sedimentary rocks
have some permeability,
and
although
it is not
easy
to
determine
the in
situ permeability
of
rocks with very
low
permeability, such
as
shales,
it can be
done
(see
Neuman
and
Neretnieks, 1990). Sueh rocks have measured pet"meabilities
as
low
as
10"^ darcies.
Petrophysics
The term petrophysics
was
introduced
by
Archie (1942,
1952)
as
a
discipline that
is
concerned with relating
the
physical
properties
of
porous t"ocks,
as
measured,
for
example,
by
electrical conductivity,
to the
texture
and
composition
of the
rock. Most petrophysieal studies
are
concerned with
the
detailed geotnetry
of the
pore space, because this determines
not only tlic average bulk properties
of the
rock, such
as
porosity
and
permeability,
and
electrical resistance when
saturated with water,
but
also with
the
way
the
rock transmits
two fluids,
ln the
petroleum industry,
the two
fluids
are
water
(because essentially
all
reservoir roeks contain water)
and oil
or natural
gas. For
introductions
see
Greenkorn (1983)
and
Bideau and^Dodds (1991),
FABRIC, POROSITY, AND PERMEAL3IL1TY 267
To understand two-phase flow, consider the sitnplest case:
movement of two fltiid phases in a straight unifbrm capillary,
with circular cross-section of diameter d. If the two fluids are
water and air. then the water tends to be drawing into the tube,
displacing air. The interface or
mcui.scus.
makes a contaci angle
0 (theta) with the solid surface. The disptacetiwnt pressure. P.
tending to drive the water along the capillary is given by
P ^
-4--co^0/d
(Fq, 2)
where •; (gamma) is the interfacial tension between water and
air(it isabout 70 x 10"'*N/mforwater.and20-40 x 10~"'N/m
fbr crude oil. decreasing with temperature). For water, air and
glass (or most mineral surfaces) the contact angle is about 30 .
fbr oil and mercury it is greater than 90 . What this means is
that water weis ihe mineral surface, but oil and tnercury do
not. Fxperiencc shows that almost all rocks are water-wet
below the vadose zone: even when they are "saturated" with
oil or natural gas, there is always a residual film of water
between the mineral surface and the nonwetting fluid.
The geometry of pore space in sedimentary rocks is com-
plex (see Petrophysics of Sand and Sandstone: Pitman and
Duschatko, 1970; Fnos and Sawatsky. 1981) and differs
significantly from a bundle of capillaries. Nevertheless, the
simple relationship linking displacement pressure with capil-
lary size makes it possible to determine some important
geometrical properties of interconnected pores by monitoring
the displacement pressure needed to force a known volume of
a nonwetting fluid, such as mercury into a dry. air-saturated
sample ofthe rock, up to pressures high enough to (almost)
saturate the roek with mercury, and then monitor the pressure
again as the mercury is allowed to flt)w out, A typical graph
(Figure Fl) shows the displacement pressure (on a logarithmic
scale, pressure increasing downward on the ordinate) plotted
again the percentage of mercury saturation. It is also a way of
displaying the size distribution ofthe pore system. As can be
seen in the figure there is a hysteresis loop: No mercury enters
the pore space until a certain critical pressure is exceeded
(determined by the size ofthe largest pores), and progressively
more mercury enters as mercury is fbreed into smaller pores (or
the interconnecting throats between pores) up to 100 percent
saturation, but when the pressure is released some mercury
retnains in the pores, because the internal pressure is no longer
sufficient to fbrce the mercury through the small throats
connecting the larger pores. This factor limits not only the
rccoyerv of mercury from test samples, but also the recovery
of oil frotn petroleum reservoirs. Note that extremely high
displacement pressures arc required to fbrce fluids such as
petroleum through very stnall pores: this is the t"eason why
rocks of very low permeability, such as shales, will slowly
"leak" water i'rom underlying aquifers, but will serve as
effective seals to fbrm traps for petroleutn.
Displacement pressure on such mercury injection plots
serves as a proxy for pore or throat size (the smallest throats
require the highest displacement pressure). For further
discussion sec
Petrophysics
of Sands and Sandstone. Diagenetic
processes, particularly cotnpaction and cetnentation
(q.v.),
frequently decrease the size ofthe throats (and the coordina-
tion) more than they do the absolute porosity. Burial generally
produces a roughly linear decrease in porosity, but an
exponential decrease in permeability, with depth. In sand-
stones, clay cements (illite and mixed layer clays) are
100
Percent Pore Saturation
Figure Fl Sthem<itic gr,iph showing result ot d inerc:ury injection test,
Atter the displacement pressure reaches
<i
ihresholrl v<3lue, mercury is
driven into the (gas tilled) pore spate: as the pressure increases, more
mercury enters hm.Tller pores, until satur^Tlitin is approt^ched
asymptotically. On decreasing ihe pressure, mercury flow out but
not all if is tan be recovered, Tbe displacemeni pressure can be
considered to be s proxy tor pore (and tbroat} size: tbe largest pores
correspond to the least displacement pressure, and the smallest pores
the most pressure, so tbe "in" t:urve corresponds r{)ughly to tbe size
distribution of tbe pores.
particularly cfTcctive in clogging throats. There .
exceptions, where diagenetic dissolution at depth produce
secondary porosity with l itti tht (
Diagenesi.s).
are.
however,
tic dissolution at depth produces
large interconnecting throats (see
Diagenesi.s).
Mercury injection is not the only technique available to
provide a more detailed measure of pore strueture than bulk
measuretnents of porosity and pertneability or their geophy-
sical proxies (see Geophysical Properties of Sediiiicnt.s:
1994
p
Prensky. 1994),
Gerard V. Middleton
Ttieir Ctniraeter aud
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268 FACIES MODELS
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Cross-references
Cements and Cementation
Compaction (Consolidation) of Sediments
Diagenesis
Petrophysics of Sand and Sandstone
FACIES MODELS
Models are idealized simplifications set up to aid our under-
standing of complex natural phenomena and processes.
They have been extensively used in the interpretation of
sedimentary rock facies.
The concept of facies, that is a body of rock with specified
characteristics, has been used ever since geologists, engineers
and miners recognized that features found in particular rock
units were useful in correlation and in predicting the
occurrence of coal, oil, and mineral ores. The term was
introduced by Gressly (1838) who used it to embrace the sum
total of lithological and paleontological aspects of a strati-
graphic unit. Since that time the term has been the subject of
considerable debate, well summarized by Middleton (1973)
and discussed extensively by Anderton (1985), Walker and
James (1992), Reading (1987, 1996) and Miall (1999),
Ideally a rock facies should be a distinctive body of rock
that formed under certain conditions of sedimentation,
retlecting a particular process, set of conditions, or environ-
ment. It should differ from those bodies of rock above, below
and laterally adjacent. It may be a single bed or group of
similar beds, A facies may be subdivided into subfacies or
grouped into associations of facies (facies associations).
Where sedimentary rocks can be handled at outcrop or in
cores,
a facies can be defined on the basis of color, bedding,
composition, texture, fossils, and sedimentary structures. If the
biological content of the rock is its dominant aspect, then it
should be called a biofacies, an ichnofacies being distinguished
by its suite of trace fossils. If fossils are absent or of little
consequence and emphasis is to be placed on the physical and
chemical characteristics of the rock, then it should be called
lithofacies. Where definition depends on features seen in thin
section, as is often the case with carbonates, the term
microfacies is used,
Facies models developed out of facies classifications that
had been based on observable and measurable features, but, as
time went by and our understanding of process and of
environment increased, the word facies came to be used in
more genetic senses, that is for the product of a proeess by
which the rock was thought to have formed or the environ-
ment in which it was deposited. Although all these senses
overlap, it is necessary to be aware of the sense in which the
word facies or the phrase facies model is being used, in an
objectively defined descriptive sense, or as an interpretation of
the generating process or the environment in which it was
deposited,
Facies classifications
The earliest facies classification in clastic sediments was that of
Mutti and Ricci Lucchi (1972) for deep water facies. It has
continued, with slight modifications, to the present day. Deep
water sediments are divided into 7 classes, i6 groups, and 59
facies based on grain size, internal organization, composition,
bed thickness, and sedimentary structures. In alluvial sedi-
ments, a scheme initiated by Miall (1977) is also widely used
with extensive modifications to suit particular sequences. It
recognizes three major grain size classes, gravel, sand and fines,
and several modes of bedding (e,g,, massive, trough cross-
bedded, planar/tabular cross-bedded, rippled, horizontal
laminated, shallow scours, etc). These facies schemes are
invaluable in giving a rigorous objective record of the rocks
and enabling different geologists to produce consistent results;
and they also force the observer to look at every aspect of the
lithology. However they can become unwieldy and time
consuming. The observer should therefore be prepared to
modify such schemes according to the time available for the
study and the objective of the exercise.
Classification schemes for limestones began, as with clastic
sediments, with ones based on grain size. Limestones and
dolomites were divided into calcilutite/dololutite (grains
<62)im), calcarenite/dolarenite (62|xm to 2 mm) and calcir-
udite/dolorudite (>2mm). From this facies scheme, built on
grain size, there developed the two classical, and still used,
classifications of Folk (1959) and Dunham (1962), These
depended more on texture than on grain size, in particular on
the proportion of grains to matrix, and on the manner in which
the grains were supported. Whilst utterly objective and
descriptive, the divisions reflected, as with any grain size
subdivision of facies, the energy level of the depositional
environment. For example, a wackestone, where the grains are
supported by the surrounding mud and fine-grained carbonate
(micrite) indicate deposition in quiet water; a grainstone
FACIES MODELS
269
formed of detrital grains, supporting each other and lacking
mud indicates agitated water.
Process facies models
Facies models based on the inferred process by which a
sediment is considered to have fonned generally have the suffix
"-ite".
The term evaporite {q.v.) has long been used for
precipitates formed by the evaporation of sea or lake waters in
climates where losses by evaporation exceed additions by
precipitation and influx of less saline waters either fresh or
from the sea. In the 1950s, as it began to be realized that many
"graywackes", that is sandstones with a high matrix content,
had iDeen deposited by turbidity currents it became common to
use the term "graywacke facies" for the deposits ofa turbidity
current. However, not all graywackes were formed by turbidity
currents; nor does every deposit ofa turbidity current have the
high matrix content of a graywacke. The problem was solved
by the introduction of the word turbidite
{q.
v.)
for all rocks
thought to have been deposited by a turbidity current. The term
"graywacke" could then be restricted to a sandstone of a well
defined composition regardless of whether the high matrix
content was the result of deposition from a turbid current or
diagenetic alteration of unstable Fe-Mg minerals as some had
postulated.
The turbidite model, the classic facies model, was first
developed by Kuenen (1950) and then more widely publicized
in the paper by Kuenen and Migliorini (1950), They combined
knowledge of graded beds as seen in outcrops of ancient rocks,
flume experiments, and the increasing knowledge of present
day deep oceanic processes into a genetic turbidite model.
Their pioneering work was further developed by Bouma (1962)
whose field measurements quantified the range and variety of
turbidite faeies, Bouma's study was essentially descriptive.
Harms and Fahnestock (1965) and Walker (1965), on the other
hand, used our expanding understanding of the physics of
sedimentation to explain the Bouma sequence of sedimentary
struetures in terms of tlow regimes.
The turbidite model is only one of an inereasing number of
models for deep water mass-gravity flows. Soon after its arrival
it was realized: (1) that turbidity currents could be of low
density as well as of high density, as Kuenen had thought; (2)
that grain flows (with grain-to-grain interactions), liquefied
flows (where grains settle downward, displacing fluid upward)
and fluidized flows (where fluid moves upward) may each
produce their own suite of sedimentary structures. In addition,
coarser material than sand can be transported into deep water
by sudden falls of rock, by slumping, by sliding, and by slurry-
like debris flows
{q.
v.)
that yield a faeies known as a debrite,
Sinee the processes of transport and deposition ean never be
known with surety, and since post-depositional modifications
can add to or obliterate depositional structures, debate
continues fiercely to this day as to how these deep-water facies
should be categorized. Massive sandstones, for example,
though generally thought to have been formed from high
density turbidity eurrents, are argued by Shanmugam (2000) to
have formed as sandy debris flows and therefore should be
debrites, not turbidites.
Other deep water facies are pelagites, pelagic deposits
consisting of various proportions of calcareous and siliceous
microfossils and clay formed by slow settling in the oceans.
Rocks formed in this way include chalk, chert, Fe-Mn nodules,
phosphates and red days, Hemipelagites are fine-grained
sediments that have not only the same biogenic and clay
material as pelagites but also contain a significant terrigenous
component (25 to 90 percent) derived from an adjacent
land area,
Contourites are somewhat ambiguously termed. They form
from relatively continuous semi-permanent currents in all
depths of oceanic waters, and in lakes. Until the 1960s it was
generally thought that the oceans were rather stagnant bodies
of water where deposition below wave-base (approximately
200 m) was confined to continuous pelagic settling interspersed
with rare eatastrophic slumps and turbidity current flows that
could bring sands and other coarser material into deep water.
As it became possible to measure mass water flows in the ocean
it was realized (e,g,, Heezen et al., 1966) that the oceans
contained an enormous range of semi-permanent currents
which are the deep-water expression of oceanic thermohaline
circulation. Deep-ocean bottom water is formed by the cooling
and sinking of surface water in polar regions that flows toward
more equatorial regions with surface waters, such as the Gulf
Stream, returning waters from the tropies to the poles. The
distribution of land and oeean-bottom topography, and the
Earth's spin, all cause complications in any general pattern. Of
partieular importance is the Coriolis Force, due to the Earth's
rotation, that causes an anticlockwise rotation in the Northern
Hemisphere and thus produces currents such as the Western
Boundary Undercurrent that flows southwestward, paralleling
the contours on the continental slope and rise. Such currents
are known as contour currents and the facies is termed
contourites.
In shallow marine and coastal waters, the two principal
physical processes of sedimentation are tidal currents and
storms. On some confined continental shelves, particularly in
estuaries, tidal currents may be very powerful, giving rise to a
facies known as tidalites, that can record the daily or, more
commonly twice daily (semidiurnal) tidal rise and fall that give
rise to flood and ebb tides. The very strong currents produced
by these tides can give a faeies dominated by tidal currents
with their own distinctive pattern of cross beds, sometimes
even bipolar, formed during the two ebb and flood current
stages and mud drapes deposited during the intervening slack
periods.
Where tidal currents are weak or absent the faeies on
continental shelves are documented by storm deposits {tem-
pestites),
coarse-grained sandy or bioclastic sediments, often
graded, interbedded with finer, quiet water sediments. These
graded, sharp-based sandstones have often been confused with
graded turbidites because the sequence of sedimentary
structures is very similar. They ean be distinguished by the
presence of wave ripples indicating that they formed above the
level of wave base {see Storm Deposits).
Facies relationships, associations, and sequences
The interpretation ofa faeies ean seldom be made in isolation,
A rootlet bed can indicate deposition very close to or just
above water level. But it does not tell us whether it formed in a
backswamp, on an alluvial fan or river levee. Graded
sandstone beds may be formed by a river flood, a storm in
the sea or by the collapse of continental slope sediments into
deeper water. Only the eontext in which we find the graded bed
tells us the environment or the process by which it fonned. We
have to recognize the limitations of individual facies taken in
isolation.
270
FACIES MODELS
Facies have to be interpreted by reference to their neighbors
and are consequently grouped into facies associations that
are thought to be genetically or environmentally related
(Collinson, 1969), The association provides additional evi-
dence whieh makes interpretation of the environment easier
than treating each facies in isolation, particularly in the
elimination of alternative interpretations,
Facies associations are therefore the essential building
blocks of facies analysis and model groups of facies associa-
tions have been constructed for most environments. For
example, Miall (1985) postulates eight architectural elements
in fluvial environments and Mutti and Normark (1991) suggest
there are five primary elements in deep-sea fans, including
large-scale erosional features as one of their elements.
In some successions, the facies within a succession are
interbedded randomly. In others, the facies may lie one above
another in a preferred order with predietable upward and
downward changes. The importance of facies sequences has
long been recognized, at least since Walther's Law of Facies
(quoted in Middleton, 1973) whieh states that "only those
facies and facies areas can be superimposed, without a break,
that can be observed beside each other at the present time".
This concept has been taken to indicate that facies occurring in
a conformable vertical sequence were formed in laterally
adjacent environments and that facies in vertical contact must
be the product of geographieally neighboring environments.
However, Walther's Law only applies to successions without
major breaks, A break in the succession marked either by an
erosive contact, or simply by a hiatus in deposition may
represent the passage of any number of environments whose
products, if they were deposited, were subsequently removed.
Some breaks in the succession are essentially autocyclic, that is
they reflect natural sedimentary processes sueh as the switching
ofa delta, the lateral migration and avulsion ofa river channel
or the build up of a carbonate platform. Other breaks,
generally more laterally extensive, may be the result of
allocychc controls, external to the local environment due to
tectonic movements or changes in climate, sediment supply or
sea level.
Environmental facies models
The ultimate object of facies modeling is to interpret not only
the depositional environment of the sedimentary body being
examined, but the total context within which it formed, the
climate, oceanic circulation patterns, rates of sedimentary
supply, rates and nature of sea-level changes both local and
eustatic, and local, regional and global tectonics. The
stratigraphic age is also important; facies models change as
organisms have evolved, and Precambrian oceans were very
different in their chemistry to those of Mesozoic or Cenozoie
times.
To this end we have not only to describe the facies and
determine the physical, chemical, and biological processes
involved, but to understand the depositional environment.
Environmental facies models were developed initially from
studies of modern environments. Classic studies of these
environments include: (1) Glennie (1970) on desert sedimen-
tary environments: he was sent to study deserts by Shell after
the discovery of huge amounts of gas in Permian desert
deposits in the Netherlands; (2) Oomkens and Terwindt (1960)
on estuaries, especially deposition in tidal channels; (3) Fisk
(1944) and Bernard and Major (1963) on fluvial, especially
meandering river deposits; (4) Fisk et al. (1954) on delta
deposits; (5) Illing (1954) on the carbonate platform of the
Bahamas; and (6) Shearman (1966) on evaporites forming in
sabkhas
{q.v).
Environmental facies models were then further developed by
measurement of outcrops, particularly analysis of sequences
and cycles, research into which had been considerable in
previous decades (see Cyclic Sedimentation), the interpretation
of sedimentary structures in terms of physical processes, and of
composition by analogy with chemical theory and experimen-
tation, and by comparing structures and composition with
eomparable features in modern sediments.
The best known and most influential studies were those of
Allen 1964, 1965, 1970 who published a series of papers that
interpreted fining upward cycles of the alluvial Old Red
Sandstone of England and Wales as a consequence of lateral
migration of point bars in meandering rivers (see Meandering
Channels).
The full cycle consisted of a sharp, channelled base
overlain by an intraformational conglomerate of caliche
fragments that passed gradually upward through parallel-
bedded, cross-bedded, cross-laminated sandstones to siltstones
capped by a calcrete soil bed. This fining upward meandering
river model was used extensively by workers all over the world,
sometimes to the detriment of alternative explanations for such
fining upward cycles. One alternative model, particularly
suitable for sediments deposited in semi-arid climates such as
that of the Old Red Sandstone, is that the cycles are the result
of ephemeral rivers that flow fqr limited periods of tirrie, the
channelling and deposition of sandstones pccurririg during t;he
wet (pluvial) periods and with flows diniinishing ^s, rainfall
j^
reduced. Another explanation for many ofthe smaller cycles is
that they were caused by flash floods of very limited duration
that can give rise to graded beds of some thickness.
Models for a wider spectrum of alluvial deposits were
gradually developed over many years by Schumm (1972) who
separated alluvial systems into streams dominated by bed-
load, by mixed load and by suspended load sediments, by
Miall (1985) who emphasized successions made up of building
blocks termed arehitectural elements and by Orton and
Reading (1993), who showed that models needed to be
developed for a spectrum that went from alluvial fans (^, v,)
through braidplains to the finer grained, low gradient normal
river systems.
Delta models were developed very early on by the
eonvergence of studies of coarsening upward cyclothems,
especially in the Upper Carboniferous sediments of the USA
and Europe, and studies of modern deltas, initially the
Mississippi, The abundance of petroleum reservoirs in such
deltaie deposits led to petroleum finaneed studies of other
deltas such as the Rhone, Rhine, Niger, and Ganges/
Brahmaputra and it was soon realized that delta shape and
sediment accumulation patterns, especially reservoir sand
bodies varied substantially according to whether the dominant
processes of sedimentation were by river diseharge or by
marine reworking by waves, storms or by tidal currents and in
particular the calibre of sediment supplied by the alluvial
system (Orton and Reading, 1993),
Environmental facies models for carbonate sediments were
initiated by Illing (1954) working on the Bahama platform and
by Ginsburg from 1956 onward studying the Florida
shelf.
Their work on modern environments was then extended in the
1970s by Wilson (1975) who combined knowledge of ancient
carbonate facies with the increasing researeh on modern
FACIES MODELS
271
carbonate environments into a model of nine lateral facies
belts that extended from a deep water basinai, euxinic
environment through open, oxygenated marine, slope, organic
build up to open or restrieted platform. Each environment
could be recognized by its facies, especially biotas. This very
general facies model was then supplemented by a range of
lateral facies models for isolated platforms, shelves attached to
the land and earbonate ramps with gently sloping surfaces of
less than 1°, In addition, models had to be created for
platforms that do not exist today sueh as the very extensive
epeiric platforms that were widespread in the late Preeambrian
and Paleozoic, They were some hundreds or thousands of
kilometers across, developing on stable cratonic interiors and
are eharaeterized by distinctive sags or intraplatform basins.
At the platform margins, reefs {q.v.) may develop. Here,
biological sediment production is realized to its maximum
extent. Over geologic time a range of different organisms have
contributed to the building of reefs so the biofacies is
stratigraphically controlled. Corals and algae may predomi-
nate today, but in Cretaceous times rudist bivalves built reefs;
at other times building may have been by bryozoa, sponges or
crinoids.
Beyond the platform margin, a range of different slopes
occur. Some margins, especially below reefs, are steep-sided;
others are more gentle. Some are erosional; others are
accretionary. Windward and leeward margins also differ. At
the base of slope carbonate fans and aprons may form.
The three principal environments of deep water deposition,
basin floor, submarine fan, and slope apron were first
delineated in modern Californian basins by Gorsline and
Emery (1959), However, facies models for submarine fans were
developed primarily from the study of ancient rocks in the
English Pennines (Walker, 1966) and Italian Apennines (Mutti
and Rieci Lucchi, 1972) with the single "all-purpose" fan
model of Walker (1978) widely used by everyone in the next
decade. The reason for models being driven by studies of
ancient roeks, rather than modern sediments was because,
whilst the shape of modern submarine fans could be very
roughly outlined (e,g,, Normark, 1970) it was not possible to
determine the nature ofthe sediments other than in the top few
decimeters. On land, however, whilst submarine fan shapes
and channels were only rarely exposed, vertical sections,
kilometers in length could be measured. By analogy with the
fining upward fluvial model and the coarsening upward delta
model great emphasis was placed on sequences of bed
thickness in submarine fans, thinning-upward sequences being
thought to indicate channels and thickening-upward sequences
to indicate prograding lobes. As DSDP, ODP and petroleum
company cores have been recovered, vertieal sequence models
have now been developed for a much wider range of deep sea
environments. For example. Stow (1985) differentiates 18
vertical sequences for turbidites, contourites, debrites, pela-
gites,
and hemipelagites in the deep seas.
However, in the last 20 years, these earlier simplistic facies
models have been shown to be inadequate. Ironically, our
understanding of deep-sea sediments has been increasingly
based on studies of modern environments rather than ancient
rocks.
This is not only due to the increasing number of cores in
oceanic sediments, but is mainly the result of seismic profiling.
Because seismie profiling is much easier in deep water than in
shallow water or on land, models for deep water sediments are
now derived more than any other facies models from studies of
modern seas. Since the paper of Mitchum et al. (1977) in the
classic memoir edited by Payton (1977), seismic facies, based
on reflection configuration, continuity, amplitude, frequency
and interval velocity, together with the external form of the
unit has become the standard tool for 3-dimensional facies
patterns which are now well established for deep-sea sedi-
ments. The architectural elements of Mutti and Normark
(1991) and the 12 distinctive facies models of Reading and
Richards (1994) are largely based on data obtained from
seismic profiles of base-of-slope depositional systems. The
latter showed that separate models needed to be created,
distinguished by the volume and calibre (grain-size) of the
sediment supply and the nature of the supplying system
whether it is a single point source, a multiple source or a linear
slope apron.
Summary
Facies classifications were initially mainly descriptive, based on
observable, measurable features such as the composition and
texture of sedimentary rocks. As time went by, facies models
were developed based on the inferred process of formation.
Such genetic facies models generally have the suffix "-ite", e,g,,
"turbidite". Simultaneously, it was appreciated that individual
faeies could not be interpreted in isolation but had to be
studied with reference to their neighbors. Such models
emphasized the association of facies, sequences of facies, in
particular those that coarsened or fined upward, and the
presence or absence of breaks in the succession.
The establishment of environmental facies models is the
ultimate objective of facies modeling. Only in this way is it
possible to establish the nature of past climates, oceans, sea
levels and global and local tectonic patterns. Environmental
models are developed by the interaction of studies on modern
and ancient rock sequences. Initially relatively simple, all
embracing models were developed. These have become
increasingly complex as our knowledge of the variability of
nature has increased.
Complex though these models are, they are only simplifica-
tions of reality. In nature there are no models and the majority
of past environments differed in some respect from modern
environments. It must be remembered therefore that each
environment and rock sequence is unique,
Harold G, Reading
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Cross-references
Alluvial Fan
Bioturbation and Trace Fossils
Caliche-Calcrete
Classification of Sediments and Sedimentary Rocks
Cyclic Sedimentation
Debris Flow
Evaporites
Gravity-Driven Mass Flows
Meandering Channel
Reefs
Sabkha, Salt Flat, Salina
Sedimentologists
Submarine Fans and Channels
Tills and Tillites
Turbidites
FAN DELTA
The most commonly adapted deRnition of fan delta is a coastal
prism of sediments derived from an alluvial-fan feeder system
and deposited mainly or entirely subaqueously at the interface
between the active fan and a standing body of water (Nemec
and Steel, 1988), The importance of the feeder system as a
criterion for fan delta recognition roused significant discussion
among students of fan deltas on what is an alluvial fan and
how to recognize it in the fossil record, an issue that is still not
resolved. Most fan delta researchers, however, refer to alluvial
fan systems as steep gradient, often gravelly, cone-shaped
fluvial systems, which can be dominated by either sediment
gravity-flow processes or by shallow streams feeding the delta
front essentially as a line source.
Fan deltas are taken to be sensitive recorders of climate
change and tectonics. The response time of fan deltas is short
compared to river deltas, because their drainage basins are
generally small and the gradient of their feeder system is
relatively steep permitting high rates of sediment transport.
FAN DELTA
Hence, supply pulses generated by elitrtate and tectonics are
likely
to
be preserved
in
the delta body
in
the form
of
bed
thickness variation, grain size and seditnent composition, while
changes in relative sea-level would leave its trace in the form of
stacking patterns of delta components.
Seismic studies of the subaqueous part of modern fan deltas
that started
in
the late seventies (Prior etal.. 1981) added
significantly
to
researchers' insight into
the
sedimentary
processes that governed the morphology
of
the delta slopes.
Due
to
the combination
of
extensive marine and outcrop
studies
of
fan deltas in various tectonic and climate settings,
progress
in
fan-delta research was significant
in
the eighties
and early nineties (Kosters and Steel, 1^84; Netnec and Steel,
1988b; Colella atid Prior, 1990; Dabrio elal., 1991; Chough
and Otton, 1995).
Fan delta concept
Fan delta morphology and architecture
is
the result
of
the
interaction of die alluvial fan feeder system with the receiving
basin. Figure F2 shows how delta morphology is defined by
alluvial processes (lluvial regime) on the one hand and basinai
processes (basinai regime) on the other. The lluvial regime
is
controlled by hinterland characteristics such as drainage area,
geology (e,g,, lithological variation) and topographical relief of
the catchment, which are all subject to continuous change. The
rates al which these changes take place depend on the rate of
variation
in
climate, tectonic
and
sea-level change.
For
instance, periods with inereased run-olT at titnes
of
climate
deterioration,
or
periods
of
tectonic uplifi
of
the hinterland
will lead to an increase in sediment supply to Ihe delta due to
increased erosion rates
in the
hinterland.
But
also other
factors, like vegelalion and not
to
forget civilization, add
to
the total of the fluvial regime. The basinai regime is controlled
by charaeteristics
of
the basin. Geographical setting, shape,
depth and size ofthe basin define, by and large, its dynamics iti
terms
of
available tide and wave energy,
but
also olher
variables tike the density ofthe basin water relative to the river
water plays
a
role in the morphology of the t~an-delta.
The control ofthe iluvial regime on fan delta architecture is
best shown by fan deltas of low energy basins. i,c.. in basins
where wave and tidal inlUience
is
small, F""igure F3 shows
6
prototype deltas based on the type of feeder system and the
Hinterland characteristics
Clrmate. ledonics
subsidence
topography
Recelvirig t3astn
charaderislics
basin depth. Feeder system type-A represents steep gradient
(more than
a few
degrees), often gravelly, cone-shaped
systems, dominated
by
mass-flow processes
and
type-B
represents
a
steep-gradient
(-.0,4')
feeder system that
is
characterized by highly mobile (unstable) bed-load channels
feeding the della fronl essentially as
a
line source (cf, Postma.
in Colella and Prior. 1990). The steepness ofthe fan-delta front
depends tnost
on ihe
initial basin relief (e,g., absence
or
presence ofa shelf) and the fluvial regime at the river outlet. In
settings where the depth ratio (channel depth over basin depth)
is stnall (e.g.. fault scarps) and where bed-load transport
is
high, deltas with steep gradient delta slope (Gilbert-type deltas
called after G,K, Gilbert: see Scdimcniologisis. types 3 to 6, in
Figure F.3) will form. In settings of high depth ratio and with
high-competence streams
of
high suspension-bed-load ratio
(occuring
for
instance during flooding), deltas will
be
characterized
by
shoal water profiles, possibly with mouth
bars in the stream outlets (Hjulstrom-type deltas, types I and 2
in Figure F}).
The role of the basinai regime in creating delta morphology
can be pictured as the relative contribution of wave and tide
processes to river processes. Wave-dominated fan deltas have
been described frequently from the modern and ancient record.
They are often recognized by straightened, wave-worked delta
fronts and gravelly beach facies overlying a flat erosion surfaee
paved with a gravel lag and truncating the delta foreset. Tidal
influence in steep gradient fan deltas is comtnonly assutned to
be minimal.
Feeder
system
Type
A
Type
B
SHALLOW
WATER
DELTAS
DEEP
WATER
DELTAS
HJULSTROM -TYPE
Shoal-
waler profile
Classic GilOen-type
GILBERT-TYPE
Debris cones
Gravitation aily
modilied GilOerl type
Figure F2 Conceptual framework ior tan dflta
and facies.
Figure F3 Murphology and architecture of variuus lypcb ol Ian delta.
Deltas built by ihe high gradieni alluvial streams are generally small
in
size ifew km's radius).
274
I-AN DELTA
WAVE-DOMINATED
FAN
DELTA
wavB
action
alluvial
fan
delta
front
wave-cut platform)
delta
slope
Figure F4 Wave-dominaied fan delta with gravelly beach and storm-
berms prograding onto the wave-cut platform (modified from Dabrio
et
^1.,
1991b).
(A)
Gifbert type Shoal water type
(B)
FAN MAINTAINS
SAME RADIUS
DURING FAULTING
FAN PROGRADES
DURING PERIOD
OF FAULT SLIP
Fu Cu
map view
Fu
CU4FLI
Figure F5 lA) Tet:lonit:ally controlled delta architecture (Gulf of
Corinth,
redrawn from Leeder ef
.i/,,
1988); {B| Stacking of fan deltas in
strike slip settings resLilting in fining and coarsening u|>W£ird cycles
(Hornelen Basin, redrawn from Steel, 1988),
Physical signatures of allocyclic change
Physical signatures of clitnate change in delta successions
occur at various scales and frequencies. Very high frequency
chatiges occur in the form of catastrophic flood deposits. On a
time scale of Mitankoyitch cyctes
(q.v.).
orbital-forced climate
change have been found to be recorded as distinct lurbidite
sequences in the prodelta of deep-water delta systetns, where
turbidite deposition occurred mainly during the wet periods of
the precession cycle {Postma. 2001),
Sea-level changes are preserved in the form of distinct
stratigraphical styles of delta development: Progradation,
aggradation and retrogradation define the large-scale archi-
tecture of a delta and are principally governed by sediment
yield and relative sea-level change (Postma. in Chough and
Orton. 1995: Steel and Marzo. 2001).
Physical signatures of tectonics comprise distinct patterns of
stacking, rotation, supply and erosion of delta systems. The
development of grabens can significantly influenee the supply
to delta systems by closure and opening of sediment transfer
zones,
Gilbert-type deep water delta systetns are likely to
form against the footwall, while shoal-water type deltas
develop on the hanging wall crest. Strike-slip related
subsidence may lead to a stack of rotated and offset
stacked fan bodies, whose architecture is controlled by
the relative rates of fan progradation and fault slip
(Figure F5),
Current investigations and concluding remarks
Current research is foeusing on the impact of alloeyclie
eontrois on the architectural style and distribution. Recently
documented studies of the Eocene Montserrat and Sant
Llorenc del Munt clastic wedges in northern Spain in Marzo
and Steel (2000) illustrate nicely the progress that has been
made in linking fan delta architecture and facies with
tectonically controlled subsidenee and relative sea-level
changes. The itnportanee of the work lies in eareful
3-dimensional high-resolution reconstruction of the fan delta
complexes, which allows detailed estimates of sediment
volume and distribution in relation to relative sea-level
change and climate. However, the values of the allocyelic
variables cannot be determined from the architecture of
deposits alone and need to be verified against other inde-
pendent evidence.
Georce Posttna
Bibliography
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