Middleton G.V. (Ed.) Encyclopedia of Sediments and Sedimentary Rocks
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OOl ITF AND COATED GRAINS
505
Figure O19 Thin section view of a Mississippian radial ooid, Ste.
Gcnt'vieve Limestone, Illinois. The once aragonitic gaslropod in the
nucleus ol the ooid has been rcpLiced by clear calcite cement. The fad
thiit no such replacement is seen in the cortex suggests strongly that
the cortex was originally calcile. Scale = 0.1 mm.
has shoun that these ooids were originally calcitic (Sandberg,
1975) (Figure 019). Originally aragonilie ooids have textures
similar lo that resulting from replacetnenl of known aragonitic
skeletal grains (Sandberg. 1985). These ooids contain coarse
neotnorphic spar, commonly with relic iticlusions of aragonite.
Aragonitic ooids are susceptible to dissolution in meteoric
waters and also tend to develop ootnoldic porosity that tnay
later be tilled with clear calcite cetnent.
Detailed petrographic studies have shown Ihat mineralogy
of ooids has oscillated throughout the Phaneroi^oic (Sandberg.
1983).
Ooids that were originally cotnposed of aragonite and
possibly Mg-ealeite occur primarily in rocks deposited during
"icehouse"" titnes (Late Precambrian Early Cambrian, Middle
Carboniferous Triassic. and Tertiary-Recent). Ooids that
were originally composed of calcite occur primarily in
rocks deposited during "greenhouse"" titnes (Ordovieian-
Mississippian and .lurassie-Cretaeeous) (Sandberg. 1983). A
few atiomalies have been reported (e.g.. Upper Jurassic
Stnackover Formation of the Gulf Coast) where aragonitic
ooids occur during a calcitc-facilitaling episode. This may be
due to local eonditions such as evaporite deposition, leading to
an increase in Mg/Ca ratio (Heydari and Moore. 1994). The
oseillalory trend in the mineralogy of ooids has been linked to
tluctiiations in ocean-atmosphere/7CO1 level (Sandberg. 19X3),
although more recent investigations suggest that fluctuations
in marine Mg/Ca ratios driven by plate tectonic processes are
the governing facu*r (Stanley and Hardie, 1998). The general
secular trend in the mineralogy of ooids correlates with the
(Irst-ordcr. global sea-level, whieh is also tied to plate tectonic
processes. High sea-level stands or submergent/greenhouse
episodes are associated with high rates of sea-fioor spreading
and low Mg/Ca ratio. Such conditions favor the precipitation
of low Mg-calcite ooids. Low sea-level stand.s or emergent/
icehouse episodes are associated with low rates the sea-floor
spreading rate and a relatively high Mg/Ca ratio. These
eonditions favor the precipitation of aragonite and high Mg-
calcite ooids (Wilkinson el a!.. 1985: Stanley and Hardie,
1998).
There were apparently four peak periods of ooid
deposition during the Phanerozoic, including the Late
Cambrian, Late Mississippian. Late Jurassic, and Holocene
(Wilkinson el al., 1985). Peak ooid production occurred
during times when overall sea-level was either rising or falling,
rather than at maximum (highstands) or minimum (lowstands)
sea-levels.
In addition to variations in the cotnpositioti of ooids with
time,
recent stitdies have shown that ooid size has varied over
geologic time, particularly during the Precambrian. Most
Phanerozoic ooids are less thati
2
tnm, whereas those from the
Precambrian have sizes up to
18
mm (Sumner and Grotzinger,
1993).
Neoproterozoic ooids are particularly large, a phenom-
enon attributed to low nuclei supply, high growth rates and
high average water veloeity during that titne (Sutnner and
Grotzinger, 1993).
Oolitic limestones fortn proiitic petroleum reservoirs (see
Keith and Zuppann. 1993). Inlergranular porosily is preserved
during greenhouse periods because of the ealeitic mineralogy
of the ooids. Caleitic ooids are resistant to dissolution
cotnpared to their aragonitic counterparts. This resistance
limits the formation of oomoldic porosity and the occlusion of
primary pores by secondary cements. Examples of petroleum
reservoirs in oolitic rocks include the Jurassic Arab sequence in
the Middle Past, the Jurassic reservoirs of the Paris Basin, and
the .lurassic Smackover reservoirs of the Gulf of Mexico
(Moore. 2001. p.271). Significant reservoirs occur in Mis-
sissippian oolitic rocks as well. These include the Ste,
Genevieve Limestone of the Illinois Basin, several Middle
and Late Mississippian fortnations in the Anadarko Basin and
Hugoton Etnbayment, the Greenbrier and Monleagic litne-
stones of the Appalachian Basin, and the Madison Group of
the Williston Basin (Keith and Zuppann, 1993. p.5).
Fredrick D, Siewers
Bibliography
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Uon of microbial mats. Palaios. 7;
227-29.1.
Davaud. li.. and Ciirardclos. .S.. 2001. Recent freshwater ooids and
oncuids from western Lake Geneva (Swilzerland): indications of a
common organically mediated orisiin. Journal ot Sedimentary
Research. 71:423 429.
Davies. P.J.. Bubela, B.. and Ferguson. J.. 1978. The formation of
ooids.
Sedimentologv. 25: 70.1 IM).
Davies. P.J.. and Martin. K.. 1976. Radial aragonite ooids. Lizard
Island. Great Barrier Reef Geology. 4: 120-122.
Fabricius. F.H., 1977. Origin of Marine Ooids and Grapestonc. Con-
tributions to Scdimcntolo{;y, Volume 7. Stiittgait: E. Schweizer-
bart'sche Verlagsbiichhandltmg.
Flijgcl, E.. iyS2. Mienifdeies Analysis of Limestones. Springer-Verhtg.
Fergtison. J.. Btihela. B.. iind Davies. P.J.. 197X. Synthesis and possible
mechanisms of formation of radial carbonate ooids. Chemieal
Geotogy. 22: 285-308.
Folk. R.I.... and Lynch. F.L.. 2001. Organic matter, ptiiative
namicibiictcria and the formation of ooids and liardgrounds.
Sedimentology. 48: 215 229,
506
OPHICALCITES
Gaffey,
S.J., 1983.
Formation
and
infilling
of
pits
of
marine ooid
surfaces. Journat of Sedimentary
Petrotogy,
53: 193-208.
Heydari,
E., and
Moore, C.t-I.,
1994.
Paleogeographic
and
paleo-
climatic controls
on
ooid mineralogy
of the
Smackover Forma-
tion, Mississippi salt basin: implications
for
Late Jurassic seawater
composition. Journatof Sedimentary Researeh,
64:
101-114.
Keith,
B.D., and
Zuppann,
C.W., 1993.
Mississippian Ootites
and
Modern Anatogs. AAPG Studies
in
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American Association
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E., 1908.
Oolith
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Land,
L.S.,
Behrens,
E.W., and
Frishman,
S.A.,
1979.
The
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Baffin
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Loreau, J.-P.,
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Purser,
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1973. Distribution
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Davies,
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1975. High magnesium calcite ooids
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Moore,
C.H., 2001.
Carbonate Reservoirs, Porosity Evotution
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E.G., and
Imbrie,
J., 1960.
Bahamian oolitic
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T.M.,
1983. CoatedGrains. Springer-Verlag.
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B.N., and
Wilkinson,
B.H.,
1983. Holocene lacusterine ooids
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T.M.
(ed.), CoatedGrains,
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oolite. American Geotogist,
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P.A.,
1975.
New
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Great Salt Lake ooids
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of
ancient non-skeletal carbonate mineralogy. Sedimentotogy,
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phanerozoic non-skeletal
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P.A., 1985.
Recognition criteria
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ealcitized skeletal
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272-281.
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L.,
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S.M., and
Hardie,
L.A., 1998.
Secular oscillations
in the
carbonate mineralogy
of
reef-building
and
sediment-producing
organisms driven by tectonically forced shifts in seawater chemistry.
Pateogeography,
Pateoctiniatotogy,
Pateoecotogy,
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E., and
Fiitterer,
D., 1972.
Aragonite ooids: experimental
precipitation from seawater
in the
presence
of
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Sedimentotogy, 19: 129-139.
Sumner, D.A., and Grotzinger, J.P., 1993. Numerical modeling
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ooid
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problem
of
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Sedimentary
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C, 1970.
Oolite, oolith, ooid: discussion. American
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Buttetin,
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1748-1749.
Tucker,
M.E., and
Wright,
V.P., 1990.
Carbonate Sedimentology.
Blackwell.
Wilkinson, B.H., Popp. B.N.,
and
Owen, R.M., 1980. Nearshore oolite
formation
in a
modern temperate-region marl lake. Journal
of
Geotogy, 88: 697-704.
Wilkinson,
B.H.,
Owen,
R.M., and
Carroll,
A.R.,
1985. Submarine
hydrothermal weathering, global eustasy,
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in
phanerozoic marine oolites. Journat
of
Sedimentary
Researeh, 55: 171-183.
Young,
T.P., 1989.
Phanerozoic ironstones:
an
introduction
and
review.
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Young,
T.P., and
Taylor, W.E.G. (eds.), Phanerozoic
Ironstones. Geological Society
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London, Special Publication,
46,
ix-xxv.
Cross-references
Algal
and
Bacterial Carbonate Sediments
Bacteria
in
Sediments
Bioerosion
Caliche-Calcrete
Carbonate Mineralogy
and
Geochemistry
Cave Sediments
Classification
of
Sediments
and
Sedimentary Rocks
Diagenesis
Neomorphism
and
Recrystallization
Neritic Carbonate Depositional Environments
Seawater: Temporal Changes
in the
Major Solutes
Tidal Flats
Tidal Inlets
and
Deltas
Tufas
and
Travertines
OPHICALCITES
Introduction
Serpentinites and ophiolitic melanges {q.v.) locally contain
carbonatized ophiolitic breccias known as ophicalcites. The
best-documented cases are from the Jurassic of the Swiss Alps
(Weissert and Bernoulli, 1984; Bernoulli and Weissert, 1985;
Friih-Green et at., 1990) and of the Italian Apennines (Folk
and McBride, 1976; Barbieri et at., 1979), and from the
Cretaceous of the USA. Rockies (Carlson, 1984). Paleozoic
ophiealeite is only known from the Ordovician of the
Canadian Appalachians (Lavoie and Cousineau, 1995).
Hypothesis
of
origin
The origin of ophicalcites is controversial (Folk, 1986;
Bernoulli and Weissert, 1986). In the second edition of the
AGI Glossary of Geology (1980, p.437), ophicalcites (or
ophicarbonates) are described as "a recrystallized meta-
morphic rock composed of calcite and serpentine, commonly
formed by dedolomitization of a siliceous dolostone." Before
ophiolites were interpreted to represent oceanic crust, ophi-
calcites were assumed to result from contact metamorphism
between intrusive ultramafics and older sedimentary hosts
(Peters, 1963) or from regional metamorphism (Trommsdorff
et al., 1980). Since plate tectonics, near-to-seatloor high
temperature processes have been suggested: hydrothermal
weathering (Barbieri etal., 1979) and combined tectonosedi-
mentary and hydrothermal events (Lemoine, 1980; Bernoulli
and Weissert, 1985; Friih-Green et al., 1990; Lavoie and
Cousineau, 1995). Cold processes have also been suggested:
carbonate precipitation from methane seepage on serpentine
seamounts (Haggerty, 1991; Fryer, 1992) and pedogenic
alteration of ophiolite (Folk and McBride, 1976).
Modern occurrences
of
ophiealeite
Carbonatized serpentine fragments have been dredged from
hydrothermally active transform-fault settings along the
mid-Atlantic ridge (Bonatti et al., 1974) and were also
recovered from serpentine seamounts in a forearc setting
(Haggerty, 1991). Void-filling pelagic and coarser-grained
carbonates are locally interbedded with lithic (ophiolite-
derived) laminae. In voids, carbonate cements consisting of
magnesium calcite or aragonite are present. A near-to-seat!oor
OPHICALCITES
507
origin was proposed: water reached the ultramafic part of an
oceanic crust and transformed it to serpentine resulting in
density inversion and diapiric rise of the serpentinite. Breccia-
tion and carbonatization follow as the serpentine diapir
reaches the ocean floor (Bonatti etal.,
\911).
Rock record of ophiealeite
Paleozoic ophiealeite
In the Canadian Appalachians, ophicalcites have been
reported in Ordovician ophiolitic melanges located along
major terrane-bounding faults (Cousineau, 1991). The
melanges consist of serpentinized ultramafics locally brecciated
and carbonatized (ophicalcites) as well as metasedimentary
and igneous rocks fragments. An oceanic transform-fault
setting has been proposed for the formation of the Ordovician
ophiolitic melange (Cousineau, 1991).
The ophiealeite is a breccia; it is composed of angular,
millimeter- to meter-size, clast- to matrix/cement-supported
carbonatized ultramafic blocks. The ultramafics were first
intensely serpentinized and variably deformed and carbonati-
zation of ultramafic material followed. The degree of
carbonatization ranges from faintly fractured serpentinite with
carbonatized phenocrysts and some micrite infills, up to
intensely brecciated material showing various carbonates with
few preserved indicators of the protolith including chromite
grains and serpentinite minerals. The relative chronology
between the various void fillings is obscured by complex
crosscutting patterns of fractures. In many cases, fractures
with the complete spectrum of sediments and cements (see
below) cut through older ones totally occluded by a similar
succession. Meter-sized fragments of well-cemented ophical-
cites are found in the micrite. All these suggest a complex
history of sedimentation—cementation—fracturing.
Carbonate sediments in the ophiealeite
Besides the pervasive initial metasomatic carbonatization of
brecciated ultramafics, discrete cavity-filling carbonates
are recognized. The most important consists of laminated
micrite interlayered with graded coarser-grained lithic (serpen-
tinite, chromite, mafic minerals) calcarenite. In the most
carbonatized cases, the sediments can constitute up to
50 percent of total carbonates, tn the least altered ultramafics,
it commonly is the only carbonate phase.
Carbonate eements in the ophiealeite
Isopachous layers of calcite cement coat micrite and were
precipitated either over undisturbed infills or on clasts of
indurated ^sediment, suggesting synchroneity with some frac-
turing. Remaining void spaces are healed, as seen under
cathodoluminescence, by multiple generations of blocky
carbonate cements (calcite and minor dolomite). These
cements are volumetrically important in large fractures and
voids that were not totally occluded by the previous
carbonates.
Geoehemistry of earbonate elements
For the Appalachian ophiealeite, the oxygen and carbon stable
isotope ratios of the micrite fall in the range of values for
Ordovician marine carbonates. Based on oxygen stable isotope
and fluid inclusions microthermometry, following cements
were precipitated by hot saline fluids. The carbon isotopic
values for cements indicate marine waters with input of
hydrothermal-derived fluids as also suggested from low radio-
genic strontium isotopic ratios of cements (Lavoie and
Cousineau, 1995; Lavoie, 1997; Chi and Lavoie, 2000).
Proposed origin for Paleozoie ophiealeite
Crosscutting relationships between sediments, cements, and
clasts of cemented ophiealeite together with carbonate
geochemistry provide evidence for complex early seatioor
f'racturing, sedimentation/cementation, and tluid circulation.
For the Paleozoic ophiealeite, a tectonically active seafloor
hydrothermal vent system was proposed to explain the
synchroneity of micrite sedimentation, fracturing, and cemen-
tation (Lavoie and Cousineau, 1995).
The Mesozoie ophiealeites
Detailed studies of Mesozoic ophicalcites have documented a
similar complex pattern of crosscutting relationships between
micrite, carbonate cements, and ophicalcite/ophiolite clasts
altogether with multiple fracturing events. Repetitive micrite
and serpentinite arenite infills coeval with diverse carbonate
cements is documented. Based on these observations and
stable isotope geochemistry, a similar scenario of early
tectonosedimentary origin on the deep seafloor was also
proposed for Jurassic ophicalcites (Friih-Green etat., 1990).
Life on the deep seafloor
Methane-based bacterial chemosynthesis associated with
ophicalcites is reported from the Mesozoic of North America
(Carlson, 1984). Limited biological activity occurred around
the hydrothermal vents responsible for the formation of the
Paleozoic ophiealeite (Lavoie, 1997). Peloidal mats coated
clasts and are followed by bacterially mediated botryoidal
calcite precipitation. Carbon stable isotope ratios of peloidal
mats and botryoidal cements indicate that the exhalatives were
hydrocarbon-free. The lack of corrosion of carbonates
suggests minimal rate of H2S venting and indicates that the
venting system was similar to H2S-poor white smokers.
Thermophilic bacterial sulfate reduction was active around
these Ordovician vents.
Denis Lavoie
Bibliography
Barbieri, M., Masi, U., and Tolomeo, L., 1979. Stable isotope evidence
for a marine origin of ophicalcites from the north-central
Apennines (ttaly). Marine
Geology,
30: 193-204.
Bernoulli, D., and Weissert, H., 1985. Sedimentary fabrics in Alpine
ophicalcites, south Pennine Arosa zone, Switzerland. Geotogy, t3:
755-758.
Bernoulli, D., and Weissert, H., 1986. Sedimentary fabrics in
Alpine ophicalcites, south Pennine Arosa zone, Switzerland—
reply. Geology, 14: 637-638.
Bonatti, E., Emiliani, C, Ferrara, G., Honnorez, J., and Rydell, H.,
1974.
tjltramatic carbonate breccias from the equitorial Mid-
Atlantic Ridge. Marine Geotogy, 16: 83-102.
Bonatti, E., Sarnthein, M., Boersma, A., Gorini, M., and Honnorez, J.,
1977.
Neogene crustal emersion and subsidence at the Romanche
508
OPHICALCITES
fracture, equatorial Atlantic. Earth and Planetary Science Letters,
35:
369-383.
Carlson, C, 1984. Stratigraphic and structural significance of foliate
serpentinite breccias, Wilbur Springs, tn Carlson, C. (ed.), Deposi-
tionat
Faeies
of Sedimentary Serpentinite: Setected Fxamptesfrom ttie
Coast Ranges, Catifornia, SEPM, Pacific Section, Field Trip
Guidebook No. 3, pp. 108-112.
Chi,
G., and Lavoie, D., 2000. A combined fluid-inclusion and stable
isotope study of Ordovician ophiealeite units from southern
Quebec Appalachians, Quebec, tn Geological Survey of Canada,
Current Research, 2000-D5, 9p.
Cousineau, P.A., 1991. The Riviere des Plantes ophiolitic melange;
Tectonic setting and melange formation in the Quebec Appala-
chians. Journatof Geotogy, 99: 81-96.
Folk, R.L., 1986. Sedimentary fabrics in Alpine ophicalcites, south
Pennine Arosa zone, Switzerland—discussion. Geotogy, 14: 636.
Folk, R.L., and McBride, E.F., 1976. Possible pedogenic origin of
Ligurian ophiealeite: a mesozoic calichified serpentinite. Geotogy,
4:
327-332.
Fruh-Grecn, G.L., Weissert, H., and Bernoulli, D., 1990. A multiple
fiuid history in Alpine ophiolites. Geotogieat Society of London
Journal, 147: 959-970.
Fryer, P., 1992. A synthesis of Leg 125 drilling of serpentine seamounts
on the Mariana and Izu-Bonin forearcs. In Fryer, P., Pearce, J.A.,
Stokking, L.B. et at, (eds.), Proeeedings of the Oeean Dritting
Program, Seientific Resutts, Volume 125, pp. 593-614.
Haggerty, J.A., 1991. Evidence from fiuid seeps atop serpentine
seamounts in the Mariana Forearc: clues for emplacement of the
seamounts and their relationships to forearc tectonics. Marine
Geology, 102: 293-309.
Lavoie, D., and Cousineau, P.A., 1995. Ordovician ophicalcites of
southern Quebec Appalachians: a proposed early seafioor tecto-
nosedimentary and hydrothermal origin. Journat of Sedimentary
Researeh, A65: 337-347.
Lavoie, D., 1997. Hydrothermal vent bacterial community in
Ordovician ophiealeite, southern Quebec Appalachians. Journatof
Sedimentary Research, 67:
47-53.
Lemoine, M., 1980. Serpentinites, gabbros and ophicalcites in the
Piedmont-Ligurian domain of the western Alps: possible indicators
of oceanic fracture zones and of associated serpentinite protrusions
in the Jurassic-Cretaceous Tethys. Archives des Sciences
Geneve,
33:
103-115.
Peters, T., 1963. Mineralogie und petrographiedes totalpserpentins bei
davos.
Sehweizerisehe Mineratogisetie Und Petrographisctie
Mitteitungen, 43: 529-686.
Trommsdorff,
V., Evans, B.W., and Pfeifer, H.R., 1980. Ophiearbo-
nate rocks: metamorphic reactions and possible origin. Arctiivesdes
Sciences Geneve, 33: 361-364.
Weissert, H., and Bernoulli, D., 1984. Oxygen isotope eomposition of
ealcite in Alpine ophicarbonates: a hydrothermal or Alpine
metamorphic signal? Eelogae
Geotogieae
Helvetiae, 77:
29-43.
Cross-references
Bacteria in Sediments
Cathodoluminescence
Cements and Cementation
Diagenesis
Fluid Inclusions
Isotopic Methods in Sedimentology
Melange; Melange
PALEOCURRENT ANALYSIS
Introduction
The configuration
of
sedimentary bodies, frotn
the
smallest
patch
of
satid
or
gravel
to the
deposits
of
entire depositional
systems,
is
determined
in
part
by the
pattern
of air or
water
curretits prevailing during deposition. Evidenee
of
the orietita-
tion
and
strength
of
these currents
can
comtnonly
be
dedueed
from evidence preserved within
the
rocks.
There
are two
types
of
paleoeurrent evidence:
(1) Sedimentary structures
and
fabrics that preserve
a
record
of the direction
and
orientation
of
the eurrent that formed
them:
(2) Rock properties that ehange
in the
direction
of
flow
as a
result
of the
transport process:
Paleocurrent analysis
may
provide inlbrtnation
on one or
more
of the
following:
1I)
the
direction
of
local
or
regional paleoslope, which refiects
tectonic subsidence patterns;
(2)
the
direction
of
seditrtent supply
(in
unimodal systems.
only);
(3)
the
geometry
and
trend
of
lithologic units;
and
(4)
the
depositional environment.
Paleocurrent indicators
Seditnentary structures that
may
contain useful paleocurrent
infot"tnalio[i include:
(I) Ripplenuirks
(c/,
r,) amicro.sshcdding
(see
Cross-laminutiori).
Dune
and
ripple crests
are
typieally oriented transverse
to
fiow, while
the
loresets oferossbed structures normally
dip
in
a
downstreatn direction. Beeause
of
local turbulence
of
How.
the
direction indicated
by
Ibreset
dip
tnay
not
always
eortespond e.\actly
to
local fiow direetion.
and so it is
usually advisable
to use a
large number
of
orientation
tneasurements
to
ensure reliable results. This
is the
only
type
of
seditnentary structure from whieh paleocurrent
dala
may be
obtained from
the
subsurface using petro-
physieal techniques such
as the
dipmeter
or
tnicroresistiv-
ity scanning tools.
(2) CluitJtielsatutseour.s occur
in
many environments
and may
indicate
the
orientation
of
major erosive currents, such
as
those generating river
or
tidal channels
and
delta
or
submarine
fan
distributaries.
The
larger channels, whieh
are those most likely
to be of
regional significance,
are
usually
too
large
to be
preserved
in
outcrops,
but may be
spectacularly displayed
by
refiection-scismic data, espe-
cially
in
horizontal (titnc-slicc) sections.
(3) Partittg litietttinn ii/.v.)
or
prituary eitrretit linealion,
the
product
of
pUuie-bed fiow eonditions,
is
only visible
on
bedding-plane exposures.
It
usually yields directional
readings
of low
variance because
it
forms during high-
energy flow
in
river, delta,
or
tidal ehannels. when bars
or
other obstructions
lo
flow
are
under water
and
fiow-
sinuosity
is
low.
The
structure indicates orientation
but not
direction
of
fiow, because
of the
ambiguity between
two
equally possible readings
at 180 lo
each other. Usually,
this
can be
resolved with reference
to
other structures
nearby.
(4) Clast transport
by
traction
or in
sediment-gravity tlows
commonly produces
a
measurable gravel fahric. Imbrica-
tion (q.v,)
in
traction current deposits occurs where platy
clasts
are
stacked
up in a
shingled pattern, with their
fiatiest surfaee dipping upstream
and
resting
on the
next
clast downstream. Because gravel
is
only tnoved under
high-energy fiow conditions,
it
tends
to
show
low
directional variance, like parting lineation.
(5) Soleittarkitig.s
are
typically associated with
the
deposits
ol'
turbidity currents
and
fiuidized
or
liquified tlow, where
vortex erosion
may
occur
at the
base
of a
flow
{see
Scour,
Seour Marks),
and
"loots," such
as
pebbles, twigs
or
bone
tVagments.
can be
swept down
a
bedding surface
(see
To(d
Marks).
They also oeeur less eotnmonly
in
other clastic
environments. Their greatest
use,
however,
is in the
investigation
of
submarine-canyon
and fan
deposits. They
are best seen
on the
undersides
of
bedding surfaces, where
sandstone
has
tbrmed
a
cast
of the
erosional feature
in the
510
I'AI.EOCURRENT ANALYSIS
underlying bed. Tool markings yield information on
orientation but not direction, like parting lineation; tlute
marks are longitudinally asymmetrie. with their deepest
end lying upstreatn.
(6) Orie/ttedplatiis.
hatie.s.
shells, ete. do not respond system-
atically to the aligning effects of currents unless they are
elongated.
There tnay be ambiguity as to whether they are
oriented transverse or perpendicular to current patterns,
and there are other difficulties, tor example, the tendency
of fossils such as high-spired gastropods to roll in an are.
Fossils are usually only useful for
local,
specialized
paleoeurrent studies.
(7) Oriented sandstone samples may be studied in the
laboratory to e.\plore such parameters as sand-grain
orientation and magnetic anisotropy.
Paleocurrent patterns may also be dedueed from regional
changes in roek properties such as clastic grain size and clast
roundness, the downstream decrease in thickness of sediment
gravity flows, or the change in proportions of detrital
components.
Data collection
and
processing
Paleocurrent data should be carefully documented in the
field,
including the following information:
1I) location and (if relevant) precise position in a stratigraphic
section;
(2) structure type;
(3) indicated current direction;
(4) seale of structure (thickness of crossbed, depth and width
of channel, mean or maximum clast size in imbricate
gravels): and
(5) local structural dip.
Current directions should be measured to the nearest 5" with
a magnetic or sun-compass and corrected to true north
wherever necessary. In the case of parting lineation and tool
markings, the correct orientation of two possibilities at 180'
can usually be identified by referring to other types of current
structures tiearby, Measitretnent aeeuracy greater than +5 is
difficult to achieve and is, in any case, rarely important.
DifticLtlties in field tneasurement commonly arise because of
incomplete exposure of crossbed sets, f-or example, two-
dimensional exposures in flat outcrop faces cannot provide
precise orientation information.
Indicated eurrent directions may need to be corrected for
structural dip and fotd plunge, otherwise significant directional
distortions may result. For linear structures such as parting
lineation or sole tnarkings, structural dips as high as 30 can be
safely ignored, as they result in errors of less than 4 . However.
the foreset dip orientation of crossbedding is significantly
affected by structural dip. and should be corrected wherever
the structural dip exceeds 10 . This subject was discussed
further by Potter and Pettijohn (1977. chapter 10), who
illustrated a correction technique using a stereonet, Ramsay
and Huber (1983) provided graphical solutions. Middleton
(2000) referred to compuler routines for the necessary
tfigonometrie calculations. Wells (1988) and Miall (1999)
provided practical guides to field procedures, and recom-
mended the use of a pocket ealeulator to perfonn struetural
corrections and data synthesis in the
field.
Sooner or later the question will arise as to how tnany
readings should be tnade'.' There is no single or simple answer
to this problem beeatise it depends on how many measurable
current indicators are available for observation and what the
objeetives of the study are. Olson and Potter (1954) discussed
the use of grid-sampling procedures and randotn selection of
stritetures to measure, followed by calculation of i"eliability
estitnators to determine how many readings were necessary in
order lo be sure of determining eorrect directional trends. In
this study, they were concerned only with determining regional
paleosiope. We now understand a great deal about air and
water fiow patterns in different depositional environments, and
a case could be made for measuring and recording every visible
seditnentary structure. Sueh detailed data can be imtnensely
useful in amplifying environtnental interpretations and clarity-
ing local problems. Some selection may have to be made in
areas of particularly good exposure. A practical comprotnise is
to t"ecord every available structure along measured strati-
graphie sections and to fill in the gaps between sections with
spot (gridded or random) samples. This procedure permits the
elaboration of both local and regional paleocurrent trends.
When constructing lateral profiles of large outcrops, it is
essential to docutncnt these with abundant paleoeurrent
determinations. Such data provide the third ditiiension to a
two-dimensional outcrop and yield essential information
regarding the shape and orientatioti of architectural elements.
If the irend itself is important. 25 readings per satnple
station is commonly regarded as the minimum necessary for
statistically sigtiificant small samples. However, the same or
fewer readings plotted in map or section form can yield a great
deal of environmental detail, whether or not their mean
direction turns out to be statistically significant. Several
hundred or a few thousand readings may be necessary for a
thorough analysis of a complete basin.
A variety of statistical data-reduction and data-display
techniques is available for paleocurrent work. The commonest
approaeh is to group data into subsets according to
stratigraphie or areal distribution eriteria. display them
visually in current-rose diagrams, and calculate their mean
and standard deviation {or varianee). The method of grouping
the data into subsets has an impotnant bearing on the
interpretations to be made from them, as diseussed in the
next section. A current mse diagram is simply a histogratn
converted to a circular distribution. The compass is divided
into 20. 30. 40. or 45' segments, and tbe rose is drawn with the
segment radius proportional to number of readings or percent
of total readings, but this exaggerates the importance of
modes. A visually tnore correct procedure is to draw the radius
proportional to the square root of the percent number of
readings, so that the segment area is proportional to pet"cent.
Fxamples are given in Figure PI. Wells (2000) warned that the
choice of origin tor rose diagrams and the method of
subdivision of the segtnents can affect the visual appearance
of the resulting diagram and the orientation of the modes. The
calculation of means, which is done from raw data, is not
affected,
but Wells" warning indicates the need for caution in
the visual interpretation of rose diagrams.
Arithmetic and vector methods may be used for calculating
mean,
variance, vector strength, and statistical tests Ibr
randomness (Currity. 1956: Potler and Pettijohn. 1977: Wells,
I98S).
Statistical procedures, sueh as moving averages and trend
analysis, are available for smoothing local detail and
determining regional trends, A moving average map is
constructed from gridded data. An arbitrary grid is drawn
PALEOCUKKtNl ANALYSIS
4A
Figure PI Tint' use ot' paleocurrenl rose di.igrjms to iilustrate the
spread ot" readings <it a given locilion. In this case (a Devonian fluvial
system in the Cinadiiin Arctic; from Miali and Gibiing,
19781,
each
sejiment of each rose is an art of 25 , and mean flow directions for
e.Hh bcition are shown by arrows. The arrow corresponding lo each
rose diagram is indkated by the location number.
on the map: the mean current direction for the data in each
group of four squares is then calculated and shown by an
arrow at the center of this area. Eaeh data point is thus used
four times (except at the edges of the map).
The basin analyst should be wary of becoming too deeply
enmeshed in the refmetnents of statistical methods. The use ot"
probability tesls is a useful curb on one's wilder flights of
interpretive fancy, but there is a vast literature on the statistics
of the circular distribution that seems to detract attention from
very simple questions: do the data make geologieai sense? Can
they be correlated with trends deri\ed ftom other methods,
such as lithofacies mapping or petrographic data? Moving
average and trend analyzes are useful techniques for reducing
masses of data lo visually appealing maps, bul they inevitably
result in a loss of much interesting detail.
Environment
and
paleoslope interpretations
A recommended first level of analysis is to plol current-rose
diagrams and tnean veetors for each outcrop or local outcrop
group (Figure PI), separating the readings aecording to major
facies variations. If contemporaneous basin shape and
orientation can be deduced from these data or from other
mapping techniques, the paleocurrenl data ean be used
interactively with iithofacies and biofacies criteria to interpret
the depositional etivirotiment and to outline major deposi-
tional systems, such as deltas or submarine fan cotnplexes. The
geologist should examine the relationships between mean
current directions in different lithofaeies and in different
outcrops. The number of modes in the rose diagrams
(modality) and their orientation with respect to assumed
shoreline or lithofacies contours are also important informa-
tion.
Another useful approach is to plot individual readings or
small groups of readings collected in measured sections at the
correct position in a graphic section.
Pateocurrent distributions are commonly categorized as
unimodal.
bimodal. trimodat and polymodal. Eaeh refiects a
partieular style of current dispersion. For example, the rose
diagrams in Figure PI are unimodal to weakly bimodal. and
vector mean directors are all oriented in easterly directions,
roughly normal to the basin tnargin. with the exception of
station 11C(G). The data uere derived mainly from trough and
planar erossbedding and parting lineation, and the envirun-
tnent of deposition is interpreted as braided fiuvial (Miall and
Gibiing.
1978). In some areas, for example, stations 6A. B, and
C, there is a suggestion of
a
fanning out of the current systems,
indicating possible deposition on large, sandy alluvial fans.
The anomalous data set of station 11C(G) was derived from
giant crossbed sets ranging from ! m to 6m in thickness. They
may be eolian dunes or large sand waves in a trunk river
draiiting the basin cast of Boothia Uplift. Field evidence is
inconclusive, bul the paleocurrent data ean be reconciled with
either interpretation.
Coastal regions where rivers debouch into an area affected
by waves and tides can give rise to very complex paleocurrent
patterns (Figure P2). Bimodal, trimodal. or polymodal
distributions may result. However, time-velocity asymmetry
of tidal currents can result in local segregation of eurrents, so
that they are locally unimodally ebb- or flood-dominated. This
could cause confusion at the outcrop level, because the tidal
deltas showing such paleocurrent patterns tnay consist of
lithofaeies that are very similar to some fluvial deposits. In
wave-dominated environments, eurrent reversals generate
distinctive internal ripple-lamination patterns and herringbone
crossbedding. Paleocurrent analyzes of these bimodal eross-
beds may yield much useful information on the direction of
wave attack and lienee on shoreline orientation. The term
paleoslope means little in this environment because there are a
diversity of local slopes and because waves and tides are only
marginally infiuenced by the presence and ofientatioti of
bottotii slopes. Paleocurrent data cannot be used lo itifer
provenance in such situations.
Paleocurrent data tbrmed part of Ihe basis lor a
long-
standing debate regarding the formation of hummoeky cross-
stratification in shallow-marine environtnents. Some workers
attributed this structure in part to geostrophic eurrents fiowing
obliquely across the continental shelf: others argued that
shore-normal paleocurrents indicate an origin as a result of
wave-generated oscillatory and return ilow across the shelf
fioor (the debate is sumtnarized in Walker and Plint. 1992),
Subtnarine-fan and other deep-marine deposits show three
main paleocurrent patterns: (1) Individual subtnarine fans
prograde out from the eontinental slope and therefore show
radial paleocurrent patterns with vector tnean direetions
5)2 PAKTING I.INEATIONS AND CURRENT CRESCENTS
Flood tidal currents
Ebb tidal currents
Waves
Longshore drift
Figure P2 The complex flow patterns around a tidal inlet. Waves
typically approach J shoreline obliquely, but are deflected to arrive at
the shoreiint' with a nearly perpendicular orientation. Tidal currents
are locally ebb- or flood-dominated. In such cases recorded paleoflow
may be nearly iinimodal. However, partial preservation of all wave
and tide influences is typical, and can generate very complex
paleocurrent pattertis. Longshore currents are generated by wave and
tide pressures
<U
the shore, and may be particularly active during
storms.
orienled perpendicular to the regioniil basin strike. This
pattern is typical of many eontinental margins, partieularly
divergent margins. (2) ln narrow oceans and many are-related
basins, deep-water sedimentation lakes plaee in a trough
oriented parallel to tectonie strike. Sediment-gravity flows,
particularly low-viscosity turbidity currents, emerge from
submarine fans and turn 90'. to flow longitudinally down-
slope, possibly for hundreds of kilometers. Current directions
may be reversed by tilting of the basin. (3) Contour currents or
boundary undercurrents flow pai-ullcl to continental margins
and generate paleocurrent patterns oriented parallel to basin
margins.
On a basinal seale paleocurrent trends may reveal large-
scale paieogeographie features. For example, the east to
northeastward-directed flow indieated by all the rose diagrams
in Figure PI (except liC{G)) iiidieates a regional paleoslope
dipping in that direction, a fact confirmed by other strati-
graphic and sedimentologic data, in fluvial environments, flow
direetions transverse and parallel to basin axis may indicate
the presence of tributary and trunk river systems. In coastal
regions paleoeurrents may indicate the orientation of shor-
elines,
and deep marine paleoeurrents may be used to
determine the orientation of the eontinental slope and
submarine canyons.
Andrew D. Miall
Bibliography
CuriLty. J.R.. 1956. The imalysis of two-dimensional orienUUion duta.
Jdiirnal of Geology. 64:
117-131.
Miaii. A.D.. 1999. Prim
tple.'i
oj Sedimcimiry Basin Analysis. 3rd edn.
Springer-Verlag.
Miatl. /Cn.. and Gibling. . M.R.. 1978. The Siluro-Devonuin clastic
wedge of Somerset Island, Arctic Ciinadii. and some regional
paletigeogriipliic implications. Sccliiui-nlaiy Geolngy. 21: 85-127.
Middletou. G.V., 20U0. Dala Analysis iiulu- tiarib Sciences. Prentiee
Hall.
Olson, J.S., and Potter. . P.E.. 19.54. Variance components of
crossbedding direction in some basal Pennsylvania!! sandstones
of the eastern Interior Basin: statistical methods. Journalof Geol-
ogy. 62: 26-49.
Parks.
J.M.. 1970. Compiitciized trigonometric solution for rotation of
structurally lilted sedimentary directional features. Geological So-
ciely of America Bitlleiin. 81: 537-540.
Potter. P.E.. anci Pettijohn, K.J,, 1977.
Palc<icurrcnts
and Basin Analy-
sis.
Academic Press.
Ramsay. J.G.. and Hubcr, M.L. 1983. Tln-Tcehniqiie.sofMmternSiriic-
liiralGcoloiiy. Academic Press.
Walker. R.CJ.. and Plint. A.G., 1992. Wave- and slorm-dominatcd
shallow marine systems. In Walker. R.G.. and James. N.P. (eds,).
Fades
Moilels:
Rc.spon.su lo
Sea-level
Change.
Geological Association
or Canada. Geotext I. pp. 219 238.
Wells.
N.A.. 198S. Working with paleocurrents. Journal of Gculoiiieal
Edmuiioiu 35:
39-43.
Wells,
N.A.. 2000. Are there better alteniatives to standard rose
diagrams? Journalof Sedimentary
Re.search.
70: 37 46.
Cross-references
Bedding and Internal Structures
Cross-sl ratification
Fabric. Porosity, and Permeability
Imbrication and Flow-Oriented Clasts
Parting Lincations and Current Crescents
Ripple. Ripple Mark. Ripple Structure
Scour. Scour Marks
Surface Forms
Tool Marks
PARTING LINEATIONS AND CURRENT
CRESCENTS
Introduction
Parting lineations and current crescents are structures that are
eommonly preserved on the bedding surfaees of generally
planar bedded sandstones and siltstones. Such structures are
useful in determining paleocurrent directions and the nature of
the current that transported the sediment. The structures can
form in response to unidirectional or oscillating (long and
short period) currents and they may be present in the deposits
of a range of environmental settings, including fluvial, beaeh.
shallow marine, and deep marine (turbidites) deposits.
Parting lineations
Parting lineations inelude two related struetures that may be
commonly seen when a planar laminated (horizontal or
inelincd) rock is split, or parted, along internal bedding planes.
This general term refers to any low
relief,
linear structure that
is present on such surfaees (Crowell, 1955). Two common
forms of parting lincation are termed "parting-step lineation"
and "current lineation" and both may be present on the same
bedding surface.
Parting-step lincation.s (MeBride and Yeakel. 1963), are
structures that reflect the manner in which a rock breaks,
parallel to original bedding. Indurated stratifled sandstones
will commonly split parallel to bedding but not perfectly so.
When sueh rocks arc split, steps form on the top bedding
surface which are remnants of inmiediately ovci-|ying bed that
have not separated from its basal surface. These remnants
I'ARTING LINEAT1(.)NS AND CURRfNT
C
DESCENTS 513
form latnina-lhick (a millimeter or less) steps on the surface
thai arc lonjzcr than they are wide; these elongated steps
iiKikc up parting-stL'p lineation (Figure P3{A)). McBride and
Ycaklc (1963) demonstrated that the orientation of the trend
of the length o\' these steps is approximately parallel to the
oricnlation of the long axes of sand grains within the roek.
Due lo the relatively strong preferred alignment of the sand
grains, when laminae break as a rock is parted, the broken
laminae form elongate ridges, or steps, that are longest in the
direction parallel to the grain long axes. The orientation oi'
sand grains in such rocks is well-known to be determined by
the direetion of the depositing llow; the grains are aligned
parallel lo the eurrent. Therefore, the orientation of parting
step lineation similarly trends parallel to the depositing
current. It is important to note that parting-step lineation is
not a primary structure but is a reflection of the primary
internal fabric which, in
turn,
reflects the sense (but not the
absolute direction) of the depositing eurrent.
Current lineations are primary sedimentary structures that
are made up of more-or-less parallel, linear mounds (a few
grain diameters high) extending parallel to parting-step
lineations. internal fabric and. therefore, the depositing eurrent
(Figures P3(A) and P4(B)). The lineations may extend from a
few centimeters lo several tens of centimeters along their length
and their transverse spacing ranges from a few millimeters to
over a centimeter. Allen (1964) explained the formation of
current lineation by the action of flow-parallel rotating
vortices near the boundary of the flow, which sweep grains
laterally into the linear mounds that charaeterize this
structure. More recently. Allen (1982) suggested that eurrent
lineations Ibrm as sand grains are swept into low speed streaks
within the viscous sublayer of a current. Weedman and
Slingerland (19S5) found that the spaeing of "sand streaks"
(active current lineations) was eomparable to that of viscous
sublayer streaks when line and medium sand is in transport
and that spacing increased as the tlow strength and sediment
transport also inereased. Current lineations are most com-
monly reported in the deposits of unidirectional Hows
(e.g..
fluvial deposits, B-division of turbidites) but have also been
reported on bedding surfaees in hummocky cross-stratified
sandstones.
Both current and parting-step lincalion are useful indicators
of the sense of the palcocurrent direction. They ean be used in
conjunction with other direetional indieators to infer the
absolute paleoeurrent direetion. Thin sections, cut perpendi-
cular to bedding and parallel lo the lineation. will show the
imbrication direction of grains which, pro\iding the absolute
paleoeurrent direction. In the
field,
heavy mineral shadows and
current crescents can indicate the absolute paleoeurrent
direction along the lineations.
Figure P3 (Al Parting-step (P-SL) and current liiie<ilions (CLi on .i
bedding surltice from the Silurian Whirlpool S.indstone of soulhern
Ont,.irio. Note that one ot each structure is labeled whereas there are
sevLTdl examples of each on in ihe photograph. The white scale bar is
approximately "icm
long.
(B) Current line.ition on a fine sand liedding
siirfdtf that was deposited under upper flow regime plane bed
(iinditions. The white scale bar i^ approximately flcm
long.
higure 1'4 i--^' .T Luiuiii i
n-niiii
i..iiiii()
in a Hume on a bed of fine
sand under flow strengths just last enough to move sand over the bed;
the ripples are the stable bedform for the flow t onditions; this crescent
formed after one minule of flow. The object wa^ a metal sphere but it
bas been removed for ihe photograph. Note tbat only !be scour along
the loading edge and sides of the crescent are well developed
whereas the depositional lobes (labeled "L") are poorly developed.
(B) A current crescent on the same sand bed after approximately 2 min
of flow. Note tbe pair of well-developed lobes separated by a medial
ridge (labeled
"MR"I,
The lobes extending from the scour (black "L")
bave generated a second set of rip|)le-like lobes (white "L").
5T4
PEAT
Current crescents
Current crescents are arcuate scours that may be preserved on
bedding surfaces (Figure P4), The scours form as a current
flows around an obstacle on the bed (e.g., a single particle,
shells,
debris). The form of the crescent reflects the pattern of
flow around the obstacle. At the leading edge of the obstacle, a
rolling vortex develops which produces a curved scour with
"arms"
extending downstream from the sides of the obstacle
(Figure P4). Further downstream sediment that has been
eroded from the bed is deposited in two lobes, on either side of
the object (but downstream from it) forming areas of higher
relief than the ambient bed. In well-developed crescents a
medial ridge separates the two lobes (Figure P4(B)). The
combination of erosional and depositional features collectively
make up the current crescent.
The form of a current crescent appears to depend on the size
and shape of the obstacle, the flow conditions and the duration
of time over which the structure forms. In general, when flows
are of short duration or if the obstacle is moved away from
the scour shortly after the current becomes active the structure
may resemble a small, crescent-shaped scour immediately
around the upstream edge and sides of the object; there may be
little deposition such that the depositional lobes are difficult to
recognize (Figure P4(A)). In contrast, when the current is of
significant duration and the obstacle remains stationary the
depositonal lobes may generate two trains of low ripples that
can extend for decimeters to meters downstream (Figure P4B),
depending on the size of the obstacle.
Unlike lineations, current crescents may be used to infer the
absolute paleoflow direction and their orientation can be
measured with a compass in the field. Current crescents are
most commonly unidirectional (pointing in one direction, the
paleoflow direction), particularly when found in the deposits
of unidirectional currents (e.g., rivers, turbidity currents).
However, it is a common structure on beach faces (foreshore)
where they can form in response to both the swash and
backwash. In this setting the structures may be bidirectional.
Furthermore, swash- and backwash-oriented crescents may
not be at 180° to each other, depending on the angular
relationship between incoming waves and the orientation of
the slope of the beach face.
Rick Cheel
Bibliography
Allen, J.R.L., 1964. Primary current lineations in the Lower Old
Red Sandstone (Devonian), Anglo-Welsh Basin. Sedimentology, 3:
89-108.
Allen, J.R.L., 1982. Sedimeniary Structures: Their Charaeter and
Physical Basis. Volume II: Elsevier.
Crowell, J.C, 1955. Directional-current structures from the Prealpine
Flysh, Switzerland. Geological Society of America Bulletin, 66:
1351-1384.
McBride, E.F., and Yeakel, L.S., 1963. Relationship between parting
lineation and rock fabric. Journal of Sedimentary Petrology, 33:
779-782.
Weedman, S.D., and Slingerland, R., 1985. Experimental study of sand
streaks formed in turbulent boundary layers. Sedimentology, 32:
133-145.
Cross-references
Fabric, Porosity, and Permeability
Heavy Mineral Shadows
Paleocurrent Analysis
Planar and Parallel Lamination
Scour, Scour Marks
Surface Forms
PEAT
Peat is a highly variable substance. It is highly variable in space
because there are a diverse number of plant taxa characteristic
of the different geographic biomes that contribute to peat
formation. Peat, ranging in age from less than a year to
hundreds and thousands of years, is also highly variable in
time because it changes character as it ages.
It is difficult to consider peat without considering how it
originates and the landform in which it occurs. Functional
criteria must be considered, especially hydrological conditions
and associated biological features that give rise to peat and
peatland formation (Ingram, 1978). Not doing so has led to
much confusion and misunderstanding of the nature of peat
and peat lands. Von Biilow (1929) and Overbeck (1975) are
good examples of attempts to characterize peat and peatlands.
Peat has different meanings to different people in many
disciplines, which has also confounded the problems. Peat, as a
fresh material or as a source of various chemical and biotie
compounds extracted from it, has a number of industrial
applications in horticulture, energy generation, medicine,
mining, and specialized product industries (Fuchsman, 1980).
Pristine and managed peatlands are of interest in ecology, soil
science, biogeochemistry, hydrology, archaeology, cultural
histories, agriculture, horticulture, forestry, and engineering.
All of these have developed, to varying degrees, a discipline-
specific and sometimes conflicting vocabulary to describe peat.
Functional criteria
After plants and animals living in waterlogged soils complete
their life cycle, their dead remains become peat. Given that
plants are the dominant component of the living community
and its litter, peat is considered to be largely plant matter. The
newly dead plant matter undergoes a series of changes, the
onset of which is variously referred to as decay, decomposi-
tion, humification or breakdown processes. The processes
involve: (a) weakening of plant structure and loss of organic
matter, as gas or in solution, as a result of leaching and of
attack by living aerobic soil microfauna and bacteria; (b) loss
of physical structure of broken plant parts; and (c) change of
chemical state (Clymo, 1983). This occurs in a near-surface
layer, or acrotelm, which is characterized by oscillating water
table, variable water content, high hydraulic conductivity,
periodic oxygen entry, and de-watering. Not all of the plant
matter decays before new plant matter from the following year
is added to that of the previous year. The addition of new peat
to the older leads to accumulation. The addition of peat year
after year gives rise to thick accumulations and to the buildup
of peat landforms or peatlands (Clymo, 1984). Eventually the
deeper and older peat will become completely engulfed by
the water table and will no longer be influenced by oscilla-
tions in the water table. It will have a more or less constant
water content and negligible oxygen entry, small hydraulic