Middleton G.V. (Ed.) Encyclopedia of Sediments and Sedimentary Rocks
Подождите немного. Документ загружается.
TEXTLIKES 715
Examination of micrographs in these papers does not
equivocally support a glacial transporlive environment.
Periglacial environments and possible relative
dating of deposits
SEM stirface textures have been used to help unravel the
complexities oi periglacial environments but the difficulty of
distinguishing between "weathered" and "glacial" environ-
ments applies here too. If left long enough, usually in an alkaline
environment, even quartz grains weather. This ean be related lo
the formation of diagenetic features on grain surfaces. Il is
possible that grains in a near-surface environment (e,g..
moraine ridges) ean produee difTerences of weathering textures
which allow relative ages to be ascribed to the sampled features.
Eolian processes
It is generally believed that eolian grains are distinctive and
have characteristic surface textures (Figure S6I), Various
stages in the rounding process have been identified and it is
probable that silt-sized particles can be produced as rounding
took place. This lalter characteristic has also been used to
assist in the interpretation and characterization of loess
deposits. Silt, especially as loess (Figure S(i2). cotild be useful
in the identification of terrigenous sediments in. for example,
marine deposits. Early investigators suggested that features
known as "upttirned plates" were not only characteristic but
indicative of the velocity of the transporting wind. This has not
however, been followed up as the rotational velocity of
spinning erains appears to be the sienificant factor involved
(Whaliey ('?(//., 1982).
Figure S61 (A) Present day eolian grain from Btihrain dune showing the distinctive rounded outline and smoothed edges and corners while
Id.
Figure SdOl the faces also show impact features. Picture widlh = 250fim, iBl Detail showing a typici! eolian texture wiih micro-scale
impact features. Picture width = 45 |ini.
Figure S62 lA) A s.ind-sized f^rain friim a loessic deposit (EasI Kenl, Enj^landi wiih rounded oulline but some surface [iretipilation which
smoothes out some ol the surface detail. Picture width =
,32O(jm,
(Bl A more general view of some ol the same loess deposit
showing a variety of angular grain outlines in the fine sand fraction as well as silt-sized particles. This is a fracture
surface of a small |)iece of the loess. Picture width - 200 nm.
716
SURFACE TEXTURES
Figure S63
(Al
Present day grus material (Ben Ecdi, Scotland, derived from gneiss) showing both fracture characteristics as well as some
modification
of
the edges and corners which might be due
lo
eolian activity. This
is
a gooci illustration
of
how difficult
it
can
be lo
discern
the
mode
of
origin
of
a grain/deposit unless something
is
known about
ihe
environmental history before interpretation. Picture widlh
,
220j.im,
(Bt A detail
of
Figure S6,^(AI showing the conclioidal fracture and parallel arcuate lines which are typical
of
brittle fracture. The edges and corner
however
do
show that there has been some modification
of
ihe textures and that they
are
akin
to
eolian textures (Figure S60), Picture
width
= 5
Beach processes
There are distinctive surface textures found on beach
sediments with small impact versus resulting from the highly
turbulent and energetic processes of breaking waves on
beaches. Fractured grains, with the fractures cutting aeross
the rounded grain profile and texture, are common and
distinctive.
Volcano-clastic material
Tephra particles identified by SEM are distinctive (Marshall.
1987) and have been found frequently in glacial deposits and
peats and have been used, together with chemical composition.
to mark horizons.
Summary
With relatively inexpensive SEMs and simplicity of use, taking
micrographs of quartz (and heavy minerals, etc.) for surface
texture analysis is a useful adjunct to conventional sedimento-
logieai techniques. How and how intensively it is used, depends
upon the ultimate objective. Only rarely will the teehnique
provide unequivocal evidence ofa transport environment but
it is certainly useful in identifying problem eases and assisting
with interpretations. It seems clear that there is still room for
experimentation to define mechanisms more closely and
perhaps to elucidate the energetics of sedimentological
transport processes and temporal lactors affecting diagenesis.
Bibliography
W. Brian Whalley
anci Dudas.
M.J,, 1987, The
scanning electron
i" selected soils trom Alberta,
Black. J.M.W,.
and
Dudas.
M.J,, 1987
microscopic tnorphology
of
quartz
in si
Canadi(tn Jinirnal of Soil Seienee. 67:
965
Bull.
P,A..
Whiillcy,
W,B,, and
Magee.
A.W.. 1986. An
annotated
bibliography
of
environmental reconstruction
by SEM.
British
Geomorphofogieal Researeh
Group.
Technical
Bulletin.
35,
Norwich:
Cico Books,
Btill. P,A,.
and
Culver.
S,J..
1979,
An
applicLition
of
scanning eleclron
microscopy
to the
study
ol"
ancient sedimentary rocks from
the
Saioniii Scarp. Sierra Leone. Piilaeogeographv, Palaeoehmaiology
andPalaeoeeoloi;y.
26: 159 172.
Ctilver. J,.l,. Bull, P',A,, Campbell.
S..
Shakesby.
R.A,. and
Whalley.
W,B,.
1983,
En\ironmeniai discrimination based
on
quartz grain
stirface texttnes:
a
statistical investiaatioii, Sedinwntology.
30:
129-136,
Giilkiglier.
J,J. Jr. 1987.
Fractography
of
sand grains broken
by
tmiaxial compression.
In
Marshall,
.I,R,
(ed,), Clastie Partieles.
Van Nostnmd Reinhold.
pp, 189 228.
Higgs.
R..
1979, Quariz-grain stntace features
of
Mesozoic-Ccnozoic
sands from
the
Labrador
and
Western Greenland conlinental
margins. Journal of Sedinieniary Petrology. 49: 599-610,
Krinsley, D,H,.
and
Doornkamp. J.C.. 1973, Atlasof Quartz SandSur-
faceTe.xtures. CumbridgL' t,.niversity Press.
Krinsley. D.H,.
and
Marsliall,
J,R,.
1987, Sand grain textur.)! analysis:
an assesstnent,
hi
Marshall,
J,R,
(ed.). Clastie Partieles.
Van
Noslrand Rcinliold.
pp,
2-1.5.
Mahaney.
W,C,. 1995.
Glacial crushing, wealhering
and
diagenetic
histories
of
qtiariz grains interred trom scanning electron micro-
scopy,
[n
Menzies.
J.
(ed.). Modern Glaeial Environments. Butter-
woith-Heinemann.
pp.
487-506,
Marshall.
J,R,
(ed,).
1987,
Clastic Particles.
V:m
Nostrand
Reinliold,
Orford.
J,D.. and
Whalley.
W.B,. 1991.
QuantiUUive grain form
analysis.
In
Syvitski. J,P.M. (ed,). Prineiples.
.Methods
and Applica-
tions
of
Particle .Si:e .4nalysis, Cambridge University Press,
pp.
88-108,
Orr. n.D..
and
Folk.
R,L,.
1983, New scents
on
the chattermark trail:
weathering enhances obscure microfnicutres. Journal
of
Sedimen-
tarv Petrology, 53:
121 129.
Smart.
P.. and
Tovey.
N.K,. 1981,
Eleetron Mieroseopy
of
Soils
and
Sediments: Examples. Oxtord: Clarendon Press,
Smart.
P.. and
Tovey.
N.K.. 1982.
Eleetron Microscopy
of
Soils
and
Sedimenis.Techniijues. Oxford: Clarendon Press,
Stieglitz.
R,D,.
I9fi9, Surface textures
of
quartz
and
heavy mineral
grains from fresh-water environments:
an
application
of
scanning
SWASH
AND
BACKWASH, SWASH MARKS 717
electron microscopy, Geologieal Society
of
America Bulletin,
80:
2091-2094,
Whalley, W,B,, 1972,
The
description
of
sedimentary particles
and the
concept
of
form, Journalof Sedimentary Petrology,
42:
961-965,
Whalley,
W,B,
(ed,), 1978, Scanning
Electron
Microscopy
in
the Study of
Sediments. Norwich:
Geo
Abstracts,
Whalley,
W,B,, 1996,
Scanning electron microscopy.
In
Menzies,
J,
(ed,).
Past
GlaeiatEnvironments. Butterworth-Heinemann,
pp, 357-
375,
Whalley, W,B,, Smith,
B,J,, and
Marshall,
J,R,,
1982, Origin
of
desert
loess from some experimental observations. Nature, 300: 433-435,
Cross-references
Attrition (Abrasion), Fluvial
Attrition (Abrasion), Marine
Cathodoluminescence
Desert Sedimentary Environments
Diagenesis
Eolian Transport
and
Deposition
Features Indicating Impact
and
Shock Metamorphism
Glacial Sediments: Processes, Environments
and
Facies
Grain Size
and
Shape
Peat
Provenance
Sediment Transport
by
LJnidirectional Water Flows
Swash
and
Backwash, Swash Marks
Tills
and
Tillites
Weathering, Soils,
and
Paleosols
SWASH AND BACKWASH, SWASH MARKS
-The terms swash
and
backwash collectively refer
to the
oscillatory motion
of the
shoreline
due to the
continuous
arrival
of
waves. They also describe
the
associated thin lens
of
water behind
the
moving shoreline that periodically covers
and
uncovers
the
beach face. Some researchers
use the
term swash
to describe the complete cycle
of
shoreline oscillation (i,e,, both
landward
and
seaward motion), whereas others
use the
term
swash
to
describe
the
landward motion
of the
shoreline
and
use
the
term backwash
to
describe
its
seaward motion. Other
terms synonymous with swash
and
backwash
are
wave-runup
and -rundown, respectively.
Most
of
the early work relating
to
swash
was
carried
out by
applied mathematicians with
an
interest
in
nearshore oceano-
graphy. Benchmark papers from this discipline
are
those
by
Carrier
and
Greenspan (1958)
for the
case
of
non-breaking
waves,
and
Shen
and
Meyer (1963)
for the
case
of
breaking
waves. Coastal engineers principally interested
in
wave over-
topping
of
structures have also contributed extensively
to our
knowledge, with
the
paper
by
Battjes (1971) providing
the
foundation
for
much
of
the engineering literature. Prior
to the
mid 1970s comprehensive field measurements
of
swash
and
backwash
are
absent from
the
literature. Since that time
the
number
of
publications
has
increased rapidly. Currently
swash-backwash hydrodynamics
and
swasli-zone morpho-
dynamics
are
major research topics
in the
fields
of
coastal-
oceanography
and
coastal-geomorphology/sedimentology
(see
Komar,
1998 and
Hughes
and
Turner,
1999 for
recent
reviews).
Process description
The nature
of
swash
and
backwash varies between
two end-
member beach types: steeply-sloped reflective beaches
and
gently-sloped dissipative beaches.
The
nomenclature relates
to
the predominance
of
reflection
or
dissipation
of
energy
in the
wind-
and
swell-wave bands
of the
nearshore wave spectrum.
On steeply-sloped beaches
the
swash-energy spectrum
(calculated from measured time series
of
shoreline position
or elevation) typically displays
a
narrow peak somewhere
between 0,05
Hz and
0,2 Hz, This frequency
of
shoreline
motion
is
related
to the
continuous arrival
of
wind-
and
swell-
waves
at the
beach face, either
as
surging
or
plunging breakers.
There
is
often
an
additional peak
in the
swash spectrum
at a
frequency that
is
half that
of the
incident wind-
or
swell-wave
frequency,
and
arises
due to the
reflective nature
of
steeply-
sloped beaches.
The
interference between incident
and
reflected
waves produces
a
standing wave field
in the
nearshore zone,
which
can
influence
the
swash-backwash process. Possible
standing wave types
are
leaky modes whose crests
are
parallel
to
the
shoreline
and
edge wave modes whose crests
are
normal
to
the
shoreline.
The
latter results
in a
rhythmic variation
of
the maximum swash limit along
the
beach.
On gently-sloped beaches
a
significant amount
of
the incident
wave energy
is
dissipated
via bed
friction
and
turbulent wave-
breaking across
a
broad surf zone. Consequently
the
shoreline
motion associated with
the
arrival
of
each wind-
or
swell-wave
is often negligible,
and the
swash
is
said
to be
saturated
in
this
frequency band.
In
contrast with steep beaches,
the
swash-
energy spectrum typically displays
a
broad-banded peak
somewhere between 0,05
Hz and
0,005 Hz (termed
the
infra-
gravity frequency band)
and a
logarithmic decrease
in
energy
toward
the
wind-
and
swell-wave frequency band.
The
infragravity shoreline motion dominant
on
dissipative beaches
is associated with long waves that
are
initially bound
to
incident wave groups,
but
subsequently released
at the
seaward
margin
of the
surf zone. These long waves have such
a low
steepness that they
are
reflected
by
even
the
gentlest
of
beach
slopes.
So
despite
the
strong dissipation
of
high frequency
waves
on
gently-sloped beaches, standing waves still develop
in
the nearshore
due to the
interaction
of
incident
and
reflected
long waves. Again
the
standing wave types
can be
either leaky
modes
or
edge wave modes.
Several aspects
of
swash-backwash behavior have been
quantified using
the
Irribaren number,
c,,,.
\0,5
where
P is the
beach slope,
H and L are the
wave height
and
length,
and the
subscript
o
denotes deepwater measurements,
<^n
is
effectively
the
ratio
of the
beach steepness
to
wave
steepness. Swash-backwash driven
by the
arrival
of
wind-
and
swell-waves
on
reflective beaches
is
associated with values
of
io>\,
and
swash-backwash driven
by
long waves
on dis-
sipative beaches
is
associated with values
of
^o
<^
1, In
general
both swash frequency
and
swash height increase with
increasing values
of {„, The
statistical distribution
of
swash
heights
on
both reflective
and
dissipative beaches typically
follows
a
Rayleigh distribution.
The dynamic geometry
and
internal flow field
of
the swash-
backwash lens
is
reasonably well established
for
wind-
and
swell-waves,
but
less well established
for
long waves. When
a
wind-
or
swell-wave arrives
at the
beach face
it
causes
a
nearly
718
SYNERESIS
instantaneous acceleration of the shoreline to its maximum
velocity, termed the initial swash velocity. As a result of this
impulse the shoreline climbs the beach with a velocity that
decreases toward zero at a roughly constant rate. This
deceleration is due largely to gravity opposing the motion.
Throughout the swash the lens of water travelling behind the
shoreline progressively shallows with both time and distance
up the beach. When the shoreline velocity reaches zero the
shoreline is at its maximum landward displacement, terrried
the swash limit, and the swash part of the cycle is complete.
The shoreline now recedes back down the beach with a velocity
that increases at a roughly constant rate due to gravity
assisting the motion. The maximum shoreline velocity during
the backwash occurs near the completion of the cycle. Both
shoreline and internal flow velocities are therefore largest on
the lower beach face. Toward the end of the backwash a
hydraulic jump may occur on the lower beach face.
The internal fiow field within the swash-backwash lens
differs from the shoreline motion. At any position on the
beach the landward-directed fiow during the swash decreases
to zero and then reverses direction before the swash limit is
reached. As a consequence the duration of landward-directed
fiow is shorter than seaward-directed flow. This asymmetry in
flow duration is most pronounced on the lower beach face and
decreases toward the swash limit. In order to sustain a
seaward-sloping beach face with this type of fiow asymmetry,
the relationship between the sediment transport flux and the
flow field cannot be straightforward.
Swash marks and swash-related internal structures
The most widely recognized surface form in the swash zone is
the upper-stage plane bed, which is consistent with the near-
critical to super-critical flow regime that is characteristic of
both swash and backwash. Other bed features observed
include parting lineations, rolling-grain ripples, rhomboid
ripples, antidunes, and swash marks.
Swash marks are typically low (<5mm high), narrow
(<20mm wide) ridges of sand that mark the landward swash
limit. In plan-form they are convex toward the landward
direction, widely spaced on gentle slopes and more closely
spaced on steep slopes. Two explanations have been proposed.
The first is favored on fine-sand beaches and involves sediment
flotation. When the bed jecjjment is not completely wetted
(e,g,, landward of the groundwater effluent zone) low density
or platy grains (e,g,, mica flakes and shell detritus) can be
supported by surface tension on the narrow wedge of water at
the leading edge of the swash lens. At the swash limit, this thin
wedge of water seeps into the beach and deppsib the floating
grains in the form ofa swash mark (Epiery and pale, 1951),
The second explanation is favored for swash marks on steep,
medium-coarse-sand beaches. The reverse grading observed
within these swash marks (sediment fines with depth and also
in the seaward direction) suggests that they are the result of a
narrow, highly concentrated grain flow driven by the leading
edge of the swash (Sallenger, 1981),
Because of the subtle relief of bed features in the swash zone
the preservation potential of related internal stratification is
low. The most likely internal structures to be preserved are
those relating to beach-slope changes resulting from: (I) storm
erosion and post-storm recovery; (2) profile cut-and-fill due to
tidal translation of the swash zone; or (3) migration of beach
cusps.
Beach deposits are characterized by seaward dipping.
continuous planar to curved parallel laminae, often with
alternating fine and coarse grained laminae. Coarsening
upward sequences of beach laminae occur in relation to beach
cusps.
Reverse grading within individual beach laminae is
ubiquitous, and results from dispersive pressure within the
highly-concentrated bed-load transport layer that is character-
istic of swash and backwash fiows (see Grain flow; Physics of
Sediment Transport: the contributions of R.A. Bagnold, for
dispersive pressure). There is currently no proposed methodol-
ogy for quantifying paleohydraulic conditions from ancient
swash-backwash deposits,
Michael G. Hughes
Bibliography
Battjes, J,A,, 1971, Runup distributions of waves breaking on slopes.
Journal of
Waterways,
Harbours, and Coastal Engineering Division,
Ameriean Societyof
Civil
Engineers, 97: 91-114,
Carrier, G,F,, and Greenspan, H,P,, 1958, Water waves of finite
amplitude on a sloping beach. Journal of Fluid Mechanics, 4: 97-
109,
Emery, K,O,, and Gale, J,F,, 1951, Swash and swash mark. Transac-
tions, American Geophysical Union, 32: 31-36,
Hughes, M,G,, and Turner, I,L,, 1999, The beach face. In Short, A,D,
(ed,).
Handbook of Beach and Shoreface Morphodynamics. John
Wiley and Sons, pp. 119-144.
Komar, P,D,, 1998, Beach ProcessesandSedimentation. Prentice-Hall,
Sallenger, A,H,Jr, 1981, Swash mark and grain flow. Journal of Sedi-
mentary
Petrology,
51: 261-264,
Shen, M,C,, and Meyer, R,E,, 1963, Climb ofa bore on a beach—
Part 3, Run-Up, Journatof Fluid Mechanics, 16: 113-125,
Cross-references
Coastal Sedimentary Facies
Grading, Graded Bedding
Grain Fiow
Planar and Parallel Lamination
Physics of Sediment Transport: The Contributions of R,A, Bagnold
Surface Forms
SYNERESIS
Syneresis [aka synaeresis] is the process first identified in
colloidal solutions whereby spontaneous contraction of a gel
results in the expulsion of liquid. It is an intensively researched
and industrially important transformation that takes place
during the maturation of substances as diverse as polymers,
cement paste and foams. An everyday example is provided by
the gel formed by the acidification of milk that, during
subsequent syneresis, expels fluid (whey) as the curd contracts
to <30 percent of its original volume. In earth science it has
been proposed that an analogous process takes place when
recently-deposited clay-rich sediment comes into contact with
a saline solution. As a result of syneresis, water is expelled
from the sediment, and this causes the formation of shrinkage
cracks on the sediment surface. In some instances these cracks
may later be filled with sand or silt, and preserved on the base
of the overlying bed, as in the case of desiccation cracks.
SYNERESIS
719
There are many published descriptions from the geological
record of sand-filled cracks which have been interpreted as
syneresis structures. These structures, which are also referred
to as "subaqueous shrinkage cracks", are inferred to have
formed at, or near, the sediment-water interface. However, in
all of these cases there is a lack of unambiguous evidence to
confirm their mode of origin, and in recent years a number of
workers have questioned the applicability of the syneresis
mechanism in such situations. Likewise, no examples have
been reported of syneresis cracks forming at the present day on
the sediment-water interface in marine or brackish water
environments, although these structures have been reported
from sediments in more saline situations such as salt pans.
In recent years it has been proposed that, in suitable
mudroeks having a high content of swelling clays (smectites),
syneresis may take place at various depths below the sediment-
water interface, during burial of these sediments. On a small
scale this may result in the formation of interstratal dewatering
structures, seen as complexes of sand-filled veins which
commonly form polygonal patterns in plan view (see Fluid
Escape Structures), or on a larger scale they may be seen as
km-scale stratabound sets of randomly oriented normal faults.
History
Jiingst (1934) was responsible for introducing the syneresis
concept into the realm of geological processes. White (1961)
later carried out experimental work on clays and proposed that
syneresis cracks may be found in sediments, in particular those
containing swelling clays (smectites). Application of the
syneresis mechanism to sedimentation gained further credence
following the experiments of Burst (1965) on the action of
strong electrolytes on clays sedimented in fresh water, and
syneresis cracks were reported from many different geological
settings around the world during the 1965-1985 period (e.g,,
Picard, 1966), The problem of correctly identifying syneresis
cracks (Plummer and Gostin, 1981), which have proved
difficult to distinguish from trace fossils, especially in
Neoproterozoic rocks, was exemplified by the dispute over
the origin of quartz-rich sand- and silt-filled cracks from
Middle Devonian lake sediments of NE Scotland, Originally
interpreted as syneresis cracks by Donovan and Foster (1967),
they were reinterpreted by Astin and Rogers (1991) as being
formed during subaerial desiccation, with many of the cracks
having nucleated upon gypsum crystals. This disagreement
heralded an increasing uncertainty over the reality of the
traditional syneresis model as applied to mudroeks at the
sediment surface (Pratt, 1998; Tanner, 1998), with the impli-
cation that it should be abandoned. At the same time, the
importance of seismic shock as a catalyst for promoting a
syneresis-like loss of fiuid from mudroeks after they had been
buried, to form either interconnected sets of sand-filled,
interstratal dewatering structures (Pratt, 1998; Tanner, 1998),
or basin-scale sets of normal faults (Cartwright and Dewhurst,
1998;
Dewhurst etal., 1999), was recognized.
Mode of formation
Syneresis occurs when a subaqueous layer of sediment rich in
fiocculated
clays,
especially smectites (swelling clays with a
high water content), is brought into contact with a saline
solution and spontaneously contracts in volume due to
interparticle attraction. This causes the expulsion of water
which had previously occupied the spaces between the
randomly organized clay particles, and the resulting isotropic
contraction of mud-rich layers in the plane of the bedding at,
or close to, the water-sediment interface leads to the
formation of shrinkage cracks. These cracks have the potential
to be filled with grains of sand and silt, and preserved in the
geological record, in much the same way as desiccation or mud
cracks (see Desiccation Structures).
Following the experiments reported by Burst (1965), the
common mode of occurrence of syneresis cracks was thought
to be as isolated, lenticular, filled-craeks. The cracks are small
in size and restricted to a single mud layer {cf. interstratal
dewatering cracks). The problem which has inhibited further
work on the syneresis mechanism in sediments, is that it has
proved impossible to erect criteria to distinguish between fossil
sand-filled cracks formed by the passive contraction of a
muddy layer due to sudden loss of part of its pore fiuid
(syneresis), and those resulting from the forced evaporation of
the pore fiuid (desiccation). The two processes have many
features in common: as the tensile stress fields in the muddy
layers in each case will be similar, both have the potential to
form linked patterns of polygonal cracks. The precise
morphologies of the resultant cracks, and of the patterns they
make, will be controlled by features such as bed thickness,
rheology, and anisotropy and, with one exception, will not be
diagnostic of the geological process or setting. The exception is
that, as desiccation results in a greater water loss than
syneresis, special features such as mud curls may develop
and confirm that sub-aerial exposure of the wet mudrock was
responsible for their formation. No such positive feature for
the recognition of syneresis cracks has been identified, and the
first geological examples of such structures, figured by White
(1961,
his plates 3 and 4), could just as readily be interpreted as
desiccation or interstratal shrinkage structures.
The interstratal mode of "syneresis" is thought to be caused
by earthquake-induced ground motion which triggers off a
combination of syneresis
(s.s.),
liquifaetion, and water-escape
processes relatively close to the sediment surface (Pratt, 1998),
It differs from that thought by Cartwright and Dewhurst
(1998) and Dewhurst etal. (1999) to be responsible for the
development of a km-scale system of polygonal normal faults,
in that the latter is inferred to occur by syneresis during
compaction, without the mediation of earthquake shaking.
The driving force in this case is the three-dimensional
contraction of colloidal smectitie gels through interparticle
forces in a saline medium, on a major scale.
Summary
tt has been proposed that syneresis cracks form in smectite-
bearing muds at two different levels in a sedimentary basin: at
the sediment-water interface, and in rocks that have already
been buried. The processes that occur at each level are also
different. That taking place at the sediment surface in a marine
environment is true syneresis, due to the spontaneous loss of
fiuid from a gel in contact with a saline solution; that taking
place at deptii apparently requires the intervention of seismic
activity to act as a triggering mechanism, could also involve
compaction-driven expulsion of fiuid, and results in sets of
interstratal, filled fractures. The nomenclature used to describe
the small-scale interstratal cracks formed in this way is a
problem. They are not true syneresis cracks and should be
referred to by a descriptive, non-genetic, name such as
720
SYNERESIS
"interstratal dewatering structures". To name km-scale
fractures as syneresis structures is also misleading: syneresis
may be involved in the production of these structures but it is
probably not the only process involved,
"True" syneresis cracks formed at the sediment-water
interface, if they exist in sedimentary systems apart from
highly saline lakes, are impossible to distinguish from other
shrinkage cracks, especially desiccation cracks, and there is a
strong case for ceasing to use the term in such situations. As its
use to describe the other types of fracture or crack is debatable,
the term is best abandoned,
P.W, Geoff Tanner
Bibliography
Astin, T,M,,
and
Rogers, D,A,, 1991, "Subaqueous shrinkage cracks"
in
the
Devonian
of
Scotland reinterpreted. Journal of Sedimentary
Petrology, 6V. 850-859,
Burst,
J,F,, 1965,
Subaqueously formed shrinkage cracks
in
clay.
Journal of Sedimentary
Petrology,
35;
348-353,
Cartwright, J,A,,
and
Dewhurst, D,N,, 1998, Layer-bound compaction
faults
in
fine-grained sediments.
Geological
Society of America Bul-
letin,
110: 1242-1257,
Dewhurst,
D,N,,
Cartwright,
J,A,, and
Lonergan,
L,, 1999, The
development
of
polygonal fault systems
by
syneresis
of
colloidal
sediments. Marine and
Petroleum
Geology,
t6:
793-810,
Donovan, R,N,,
and
Foster,
R,J,,
1972, Subaqueous shrinkage cracks
from
the
Caithness Flagstone Series (Middle Devonian)
of
north-
east Scotland, Journal of Sedimentary Petrology, 42: 309-317,
Jiingst,
H,, 1934, Zur
geologischen bedeutung
der
synarese, (Geolo-
gical significance
of
syneresis), Geologische Rundschau,
25:
321-325,
[in
German]
Picard,
M,D,,
1966, Oriented, linear-shrinkage cracks
in
Green River
Formation (Eocene), Raven Ridge area, Uinta Basin, Utah, Jour-
nalof Sedimentary Petrology,
36:
1050-1057,
Plummer, P,S,,
and
Gostin, V,A,, 1981, Shrinkage cracks: desiccation
or synaeresis? Journalof Sedimentary
Petrology,
51: 1147-1156,
Pratt, R,B,, 1998, Syneresis cracks: subaqueous shrinkage
in
argillac-
eous sediments caused
by
earthquake-induced dewatering. Sedi-
mentary
Geology,
117:
1-10,
Tanner, P,W,G,,
1998,
Interstratal dewatering origin
for
polygonal
patterns
of
sand-filled cracks:
a
case study from Late Proterozoic
metasediments
of
Islay, Scotland, Sedimentotogy, 45: 71-89,
White,
W,A,, 1961,
Colloid phenomena
in the
sedimentation
of
argillaceous rocks. Journal of Sedimentary
Petrology,
31: 560-570,
Cross-references
Colloidal Properties
of
Sediments
Desiccation Structures
Flocculation
Fluid Escape Structures
TALC
Talc is a 2:1 phyllosilicate with an ideal composition of
Mg3Si40io(OH)2 , The 2:1 layer structure comprises two
tetrahedral sheets of silica tetrahedra bound to one octahedral
sheet of edge-linked oetahedra. The octahedral sheet in talc is
magnesium-rich, trioctahedral, and referred to as brucite-like.
The 2:1 talc layer is electrostatically neutral and is bound to
other layers by van der Waals forces, A small amount of ionic
attraction between the layers may be present and result, in
part, from cation substitutions (Giese, 1975), Although
most talcs have compositions close to the ideal, substantial
amounts of Fe may replace Mg in octahedral sites, minor
amounts of Al may replace both Si and Mg in tetrahedral
and octahedral sites, respectively, and small amounts of Ni,
Mn, Cr, and Ti may substitute into the structure. Traces
of Ca, Na, and K may occupy the interlayer site to satisfy
any layer charge imbalance. The dioetahedral analog of
talc is pyrophyllite, an aluminum-rich, dioetahedral 2: 1
phyllosilicate.
Talc is trielinic (1-layer), although many earlier references
report that it takes a 2-layer monoclinic form (Bailey, 1980).
The mineral is green to white to silvery white, has a specific
gravity of 2.58-2,83, and a hardness of 1, making it the softest
mineral on the Moh's scale. It has a greasy feel, a pearly luster
and is sectile. It often occurs in massive or foliated aggregates,
but rare crystals are tabular and show perfect cleavage
{001}.
Massive talc is referred to as soapstone or steatite. The mineral
forms in high-magnesian rocks, such as ultramafics, basic
igneous rocks and siliceous dolomites that have been subjected
to low grade metamorphism, contact metamorphism, hydro-
thermal alteration, or weathering (Evans and Guggenheim,
1988),
Talc is associated with serpentinization and the
transformation of magnesian-rich silicates such as olivine,
enstatite, tremolite, and chlorite. Talc is used in the production
of ceramics, paint, rubber insecticides, cosmetics, paper, and
plastics. Soapstone blocks are used for ornamental materials
and carvings.
Talc is not a major component of sediments and sedimen-
tary rocks, but its occurrence is more common and widespread
than previously thought. In modern ocean hydrothermal
environments talc forms as a direct precipitate when hot
solutions emanating from fissures and vents mix with cold,
Mg-rich bottom waters. The main hydrothermal vent field
in the Tjornes Fracture Zone (deptli of 400 m) north of
Iceland consists of about 20 large mounds, chimneys, and
spires of anhydrite and talc (Hannington etal., 2001), Sediment
coring through talc- and anhydrite-rich sedimentary layers
that occur up to
7
m below the mounds revealed pore fiuids
with temperatures as high as 250°C, Talc also forms as an
alteration product of basalts through which these hot
fiuids flow. Shau and Peacor (1992) studied trioctahedral
minerals in DSDP drillhole 504B and found a sequence of
hydrothermal alteration products in the basalt as follows:
saponite (to a depth of 624
m);
mixed-layer corrensite-
chlorite + chlorite + corrensite (from 624 m to 965 m); chlor-
ite-)-talc-t-mixed-layer talc-saponite (from 725 m to 1076 m).
Mineral occurrences were not continuous as they were related
to rock permeability and temperature variations. Seafioor
hydrothermal activity has also resulted in the formation of talc
from the alteration of epiclastic sediments and pumaceous
tuffs in the Okinawa Trough (Marumo and Hattori, 1999) and
from transformation of detrital clays in the Dead Sea Rift
(Sandier et al., 2001). In both these examples mineral
assemblages changed with depth because mixing proportions
of hydrothermal fluids and seawater resulted in different
reaction temperatures. Talc has been observed in sediments
and in metalliferous deposits accumulating in the Red Sea, on
the Mid-Atlantic Ridge, in the Gulf of California and along
the East Pacific Rise, Temperatures of the hot, brine solutions
that give rise to the formation of talc on the seafioor are
estimated to vary between 200°C and 400°C, Minerals
associated with talc formation in these hydrothermal environ-
ments include sulfides of copper, iron, and zinc, magnesium-
rich clays (e,g., smectite, chlorite, corrensite), anhydrite,
sepiolite, and zeolites.
In some older sedimentary rocks talc has been reported to
occur as a replacement product, Nesbitt and Prochasaka
(1998) suggested that dolomite, magnesite, and talc miner-
alization in the Middle Cambrian carbonate rocks of
the southern Canadian Rocky Mountains occurred as warm.
722
TALC
Mg-enriched brines (originating from seawater) circulated
through the rock units in the Late Devonian or Early
Mississippian, Noack etal. (1989) reported that oolitic talc in
the Upper Proterozoic rocks of the Congo formed from the
transformation of original stevensite (or sepiolite) during
diagenesis. Contact metamorphism resulted in the reaction of
an assemblage of saponite + quartz + dolomite to produce
grain-coating flakes of talc at temperatures estimated to be
between 130 and 180°C in the arkosic rocks of the Triassie
Sherwood Sandstone Group in Northern Ireland (McKinley
et al., 2001), Talc along with chlorite formed in and near
fractures in Cambrian dolostones in the vicinity of Winter-
boro,
Alabama from hydrothermal solutions and metasomatic
processes (Blount and Vassiliou, 1980),
Talc forms authigenically in evaporitic sedimentary envir-
onments and during burial diagenesis in evaporitic sediments
when fluids and pore waters become highly alkaline (pH > 9)
and concentrated with magnesium. Delicate rosettes and
cornflake aggregates of talc in salt beds of the Paradox
Formation, Paradox Basin, Utah, indicate an authigenic origin
for the mineral (Weaver, 1989, p,413), Droste (1963) studied
phyllosilicates in a variety of North American salt deposits and
found talc, corrensite, serpentine, and other clays commonly
present. Talc also is present in the Permian Zechstein salt
deposits of Germany (Braitsch, 1971) and the Silurian
evaporite deposits of New York (Bodine and Standaert,
1977),
among others, Serivenor and Sanderson (1982) reported
that isolated flakes of talc in Triassie halite deposits in
Somerset, England, likely formed from the reaction of
dolomite + silica + water during burial diagenesis. In some
evaporite deposits, talc may exist within a mixed-layer clay
structure, such as talc-saponite or talc-chlorite. Tale in these
sedimentary rocks is often assoeiated with other syngenetic
and epigenetic Mg-rich phyllosilicates such as saponite,
stevensite, corrensite, trioetahedral chlorites, sepiolite, and
serpentine, to name but a few.
Talc is found in soils derived from the weathering of
uitramafic rocks and rocks containing Mg-rich minerals such
as enstatite and hornblende (Nahon and Colin, 1982; Proust,
1982;
Zelazny and White, 1989). The mineral may form both
by transformation of preexisting silicates and by neoformation
involving congruent dissolution of the parent mineral and
subsequent precipitation of talc from solution (Noack et al.,
1986).
Talc occasionally is present in soils as a minor inherited
component.
Finally, tale is present in oceanic surface waters, but it is
rarely found as a signifieant component of modern-day
marine sediments (Hathaway, 1979), The mineral has been
identified in the suspended Joad of the Atlantic Ocean, the
Caribbean and Mediterranean Seas, and in the Amazon
River (Gibbs, 1967; Jacobs and Ewing, 1969; Poppe etal.,
1983),
tt has also been found in small quantities in Upper
Plioeene-Holoeene marine sediments from the Alboran Basin
(Skillbeek and Tribble, 1999), and in beach sands along the
coast of the Gulf of Mexico and Florida (Griffin, 1963),
Because talc is widely used commercially, especially as a carrier
of pesticides in crop dusting, it is suspected that some talc in
the suspended load of surface waters derives from anthro-
pogenic dust and industrial effluents, A seeond important
source is likely long-range transport by atmospheric winds and
dust storms,
Richard H, April
Bibliography
Bailey, S,W,. 1980, Structures of layer silicates. In Brindley, G,W,,
and Brown, G, (eds,). Crystal Structures of Clay Minerats and
Their X-ray Identifieation. Mineralogical Society Monograph 5,
pp,
1-123,
Blount, A,M,, and Vassiliou, A,H,, 1980, The mineralogy and origin of
the talc deposits near Winterboro, Alabama, Eeonomie Geology,
75:
107-116,
Bodine, M,W,, and Standaert, R,R,, 1977, Chlorite and illite
compositions from Upper Silurian rock salts,
Retsof,
New York,
Clays
and Clay Minerats, 25;
57-71,
Braitsch, O,, 1971, Salt Deposits, Their Origin and Composition.
Springer-Verlag,
Droste, J,B,, 1963, Clay mineral composition of evaporite sequences.
Northern Ohio
Geological
Soeiety Monograph, 1: 47-54,
Evans, B,W,, and Guggenheim, S,, 1988, Talc, pyrophyllite, and
related minerals. In Bailey, S,W, (ed,). Hydrous Phyllosilicates (ex-
clusiveofmica). Reviews in Mineralogy 19, Mineralogical Society of
America, pp, 225-294,
Gibbs, R,J,, 1967, The geochemistry of the Amazon River system; part
I, the factors that control the salinity and the composition and
concentration of the suspended solids. Geological Societyof America
Bulletin, 78: 1203-1232,
Giese, R,F,, 1975, Interlayer bonding in talc and pyrophyllite. Clays
and Clay Minerats, 23: 165-166,
Griffin, G,M,, 1963, Occurrence of talc in clay fractions from beach
sands of the Gulf of Mexico, Journal of Sedimentarv Petrology, 33:
231-233,
Hannington, M,, Scholten, J,, Botz, R,, Garbe-Schonberg, D,,
Jonasson, I,R,, Roest, W,, Herzig, P,, and Stoffers, P,, 2001, First
observations of high-temperature submarine hydrothermal vents
and massive anhydrite deposits off the north coast of Iceland,
Marine Geology, \71: 199-220,
Hathaway, J,C,, 1979, Clay minerals. In Burns, R,G, (ed,). Marine
Minerals. Reviews in Mineralogy, 6, Mineralogical Society of
America, pp, 123-150,
Jacobs, M,B,, and Ewing, M,, 1969, Mineral source and transport in
waters of the Gulf of Mexico and Caribbean Sea, Science, 163:
805-809,
Marumo, K,, and Hattori, K,H,, 1999, Seafloor hydrothermal clay
alteration at Jade in the back-arc Okinawa Trough: mineralogy,
geochemistry, and isotope characteristics, Geochimica et Cosmoehi-
miea Acta, 63: 2785-2804,
McKinley, J,M,, Worden, R,H,, and Ruffell, A,H,, 2001, Contact
diagenesis: the effect of an intrusion on reservoir quality in the
Triassie Sherwood Sandstone Group, Northern Ireland, Journalof
Sedimentary Researeh, A7t: 484-495,
Nahon, D,B,, and Colin, F,, 1982, Chemical weathering of ortho-
pyroxenes under lateritic conditions, American Journalof Science,
282:
1232-1243,
Nesbitt, B,E,, and Prochaska, W,, 1998, Solute chemistry of inclusion
fluids from sparry dolomites and magnesites in Middle Cambrian
carbonate rocks of the southern Canadian Rocky Mountains,
Canadian Journal of Earth Sciences, 35: 546-555,
Noack, Y,, Decarreau, A,, and Manceau, A,, 1986, Spectroscopic and
oxygen isotope evidence for low and high temperature origin of
talc.
Bulletin de Mineralogie, 109: 253-263,
Noack, Y,, Decarreau, A,, Boudzoumou, F,, and Trompette, R,, 1989,
Low-temperature oolitic talc in Upper Proterozoic rocks, C:ongo,
Journatof Sedimentary Petrotogy, 59: 717-723,
Poppe, L,J,, Hathaway, J,C,, and Parmenter, CM,, 1983, Talc in the
suspended matter of the northwestern Atlantic, Clays and Clav
Minerals, 31: 60-64,
Proust, D,, 1982, Supergene alteration of hornblende in an amphibolite
from Massif Central, France, In Van Olphen, H,, and Veniale, F,
(eds,).
Proceedings of the International Clay Conference. 1981,
Elsevier, pp, 'i(>l-'i(A.
Sandier, A,, Nathan, Y,, Eshet, Y,, and Raab, M,, 2001, Diagenesis of
trioctahedral clays in a Miocene to Pleistocene sedimentary-
magmatic sequence in the Dead Sea Rift, Israel, Clay Minerals,
36:
29-47,
Serivenor, R,C,, and Sanderson, R,W,, 1982, Talc and aragonite from
the Triassie halite deposits of the Burton Row borehole. Brent
TAPHONOMY: SEDIMENTOLOCICAL IMPLICATIONS OF FOSSIL PRESERVATION
723
Knoll, Somerset, Report. UK Institute of
Geologieal
Sciences 82 (1)-
58-60,
Shau, Y-H,, and Peacor, D,R,, 1992, Phyllosilicates in hydrothermally
altered basalts from DSDP Hole 504B, Leg 83; a TEM and AEM
study. Contributions
to
Mineralogy and
Petrotogy,
\\2\ 119-133,
Skillbeek, CC, and Tribble, J,S,, 1999, Description, classification, and
the origin of Upper Plioeene-Holoeene marine sediments in the
Alboran Basin,
Proeeedings
of the Ocean Drilling Program: Scientific
Results, 161: 83-97,
Weaver, CE,, 1989, Clays, Muds, and Shales. Developments in
sedimentology 44, Elsevier,
Zelazny, L,W,, and White, G,N,, 1989, The pyrophyllite-talc group. In
Dixon, J,B,, and Weed, S,B, (eds,), Minerats in Soil Environments,
2nd edn, Madison: soil science society of America, pp, 527-550,
Cross-references
Anhydrite and Gypsum
Aulhigenesis
Berthierine
Chlorite in Sediments
Desert Sedimeniary Environments
Diagenesis
Dolomites and Dolomitization
Evaporites
Mixed-Layer Clays
Mudroeks
Porewaters in Sediments
Smectite Group
Weathering, Soils, and Paleosols
TAPHONOMY: SEDIMENTOLOGICAL
IMPLICATIONS OF FOSSIL PRESERVATION
Taphonomy is the study of processes that influence preserva-
tion of potential fossils; for general reviews, see Schafer (1972),
Muller (1979), Seilacher et al. (1985), Allison and Briggs
(1991),
Donovan (1991), Kidwell and Behrensmeyer (1993),
Martin (1999), This field encompasses two major aspects:
biostratinotny, the study of processes affecting organism
remains or their traces, prior to their final burial, and fossil
diagetiesis, investigation of phenomena affecting potential
fossils after burial (Figures TI-T3), Recently, taphonomy has
developed, both as a means of assessing bias in the fossil
record, and more positively, as a critical tool for paleo-
environmental analysis, Taphonomie analyzes compliment
sedimentological study and yield critical information on the
physico-chemical parameters of ancient environments, Uni-
formitarianistic assumptions are applicable in taphonomy
because the physical and chemical properties of organism
skeletons have probably been relatively constant throughout
geologic time, despite the nonuniformity imparted by
evolution.
Biostratinomy: fossils as sedimentary particles
Disarticulation and rates of burial
Certain fossils, including bivalved shells and, especially, multi-
element skeletons, are sensitive indicators of rapid, episodic
sediment accumulation. The rate at which organism skeletons
disintegrate after death is a function of their delicacy.
environmental energy, temperature, oxygen levels, and resi-
dence time on the seafloor.
Articulated, closed bivalve shells indicate burial that was
rapid enough to prevent the shells from gaping open at the
hinge (Schafer, 1972), Conversely, abundant "butterflied"
(splayed) bivalve shells indicate moderate rates of sedimenta-
tion. Hinge ligaments may remain intact for periods of up to
months, but will ultimately decay and allow the valves to
become disassociated, Brachiopod shells tend to remain
closed after death. Closed shells filled with calcite spar,
pyrite, or other minerals, as opposed to sediment, indicate
pulses of burial that covered the sfiells while interiors were still
occupied by soft parts, preventing infiltration of sediment.
Tissues subsequently decayed, leaving a void space within the
shells.
Ratios of articulated to disarticulated shells of bivalves or
braehiopods may be an important indicator of the extent of
time-averaging in a deposit (Kidwell and Bosence, 1991; Brett
and Baird, 1993), Deposits containing very high proportions of
articulated specimens, will be minimally time-averaged.
Organisms with multielement skeletons, composed of multi-
ple articulated ossicles, e,g,, trilobites, crabs, eehinoderms, and
vertebrates, are only rarely preserved intact (Figure TI), Field
and experimental studies indicate that degradation of muscle
tissues occurs within hours to days after death, while
destruction of eollagenous ligaments ensues very rapidly, such
that most skeletons are completely disarticulated within days
to months (Schafer, 1972; Donovan, 1991; Plotniek, 1986;
Brett etal., 1997), Not all multielement skeletons are equally
subject to disarticulation, Starflsh and crinoid crown skeletons
disintegrate within days of death (Figure TI), Likewise, most
arthropods disintegrate very rapidly following death (Schafer,
1972;
Plotniek, 1986), but some barnacles, remain intaet for
much longer periods, Echinoid tests have interlocking sutures
that may remain intact for periods of several years (Figure TI),
depending on energy levels, oxygenation, and water tempera-
ture (Allison, 1990; Kidwell and Baumilier, 1990), Crinoid
columns may disarticulate into more durable increments
(pluricolumnals) of approximately equal length because of
slight differences in the ligamental articulation at certain joints
within the column (Baumilier and Ausich, 1992),
Some beds of intact pelagic fossils, such as fish, squids, or
marine reptiles may represent carcasses that settled onto
anoxic seafloors (Martill, 1991), Furthermore, Allison's (1988)
calculations demonstrate that larger organism bodies become
anoxic microenvironments internally during early phases of
decay. Inhibition of scavenging in these environments may
prolong the association of skeletal elements and even preserve
traces of soft parts (Seilacher et al., 1985), However, even
under conditions of anaerobiosis, bacterial decay of ligaments
is rapid and the slightest currents will serve to disarray pieces
(Allison, 1988), implying that burial is a critical prerequisite to
articulated preservation.
Groupings of well-preserved, articulated, multi-element
skeletons occur on certain planes that would not otherwise
be recognizable as event-beds. Because such skeletons can not
be reworked without dissociation, even a single intact speci-
men of a trilobite, crinoid crown or vertebrate skeleton,
provides unambiguous evidence that the enclosing sediment
accumulated rapidly and was not subsequently disturbed.
Under extraordinary conditions entire bodies with soft
parts may be preserved (see Seilacher etal., 1985), Such conser-
vation Lagerstatten deposits reflect combinations of rapid
724
TAPHONOMY: SEDIMFNTOLOGICAL IMPLICATIONS OF FOSSIL PRESFRVATION
Category
examples
Conditions with Varying Burial Rate
Hours to 1 Day 1 Day to 2 Weeks 2 Weeks to 1 Year More Than
1
Year
TYPE 1:
ophiuroids,
asteriods,
"carpolds,"
paleoechinolds,
eocrlnolds,
edrloasteroids
TYPE 2:
most crinoids,
"cystoidB,"
regular echlnolds
TYPE 3:
robust camerates
microcrinoids,
blastolds,
irregular
echlnolds
Figure TI Presorvational condition (gradesi ot three taphonomie categories ot echinoderm fossils for varying limes of post-mortem
exposure. From Brett et
at.
11997].
burial, anoxic sediments and early, baclcrially-mediated
tnineralization.
Life orientations and m situ burial
Examples oi in siiu preservation, including trace fossils, are
common among burrowing ;ind hard substrate encrusting/
endolithic organisms. Some assemblages ot^ well preserved
fossils, such as cemented oysters, tube worms, bryozoans. and
rooted crinoids are dcmonstrably in situ and mark colonized
discontinuity surfaces, such as hardgrounds.
Organisms, preserved in unstable orientations, such as
ceratoid corals, or pedically-altached braehiopods preserved
in upright position are very unlikely to have rolled into this
position accidentally. Such life orientations record episodes of
sediment input within low-energy settings,
Braehiopods and clams, preserved in life positions, may
provide indicators of sedimentation events. Burrowing bi-
valves may die within their burrows: however, experimental,
and observational studies have shown that, in most cases,
when bivalves are stressed, they rise from their burrows
(Schiifer, 1972). Thus, large numbers of articulated infaunal
shellfish on bedding planes may be associated with deposi-
tional events.
Another sensitive indicator is provided by associated,
though disarticulated arthropod (e.g., trilobite) molt parts. It
is virtually impossible for diflbrcnt portions of the skeleton to
be transported any distance and remain assoeiated. Hence,
their occurrence proves insitu biologieal activity in low energy
environments.
Transport and reorientation
Fossils also provide evidence for reworking and/or transport
of skeletons that would otherwise remain unsuspected (Allen,
1992).
Articulated fragile fossils, e.g.. crinoids or trilobites, arc
commonly thought to have been buried insitu in low energy
environments. However, Allison (1986) demonstrated in
tumbling barrel experiments that even soft-bodied organisms
may be potentially moved considerable distances without
disarticulating, providing that this happens very shortly
(hours) after their death. Such tnay apply to articulated
aligned skeletons found at the bases of turbidites or storm beds.
Many skeletal elements, including valves of most braehio-
pods,
osiracodes, and clams, and trilobite cephala and pygidia,
have approximately concavo-convex or dish-shapes. Random
orientations, including some lateral or edgewise shells, occur
primarily in heavily bioturbated sediments or low energy traps.