Middleton G.V. (Ed.) Encyclopedia of Sediments and Sedimentary Rocks
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SUBMAKINF FANS AND CHANNFLS
695
Chann«l With Lavaai
Locilly
} Abandoned Lobe
Slomp
' Small Unlavead Channel
(Channel
Pattern Unknown)
Small Channal
to Daposltlonal Lobe?
Channel DImanalona
Height Varaua
Width tn m
I " I DebrU Flow
All Channel VaHaya Shown In
Eaalarn And Waatarn Lavaa
Complaxai Ara Lavaad
Saa Mounta or
Topographic Higha
Figure S48 Map of the Am.i/on fan. from
Pi<
kerinfi ct .il. (l'IH<), their Figure 7.21 on
p.
179).
696
SUBMARINE FANS AND CHANNELS
SEISMIC CHARACTERISTICS
ENTIRE 'UPPER SERIES"
FIXED MAIN CHANNEL WITH
AGGRADATION BY STACKING
OF ACOUSTIC UNITS.
TOTAL THICKNESS
ca.600ms
SINGLE ACOUSTIC UNIT
MAXIMUM DIMENSIONS
THICKNESS 100ms
WCTH ca.45km.
ZONE OF MINIMUM THICKNESS
LATERALLY (wHGRATlNG MAIN
CHANNEL WITH OFFSET
ACOUSTIC UNITS.
TOTAL THICKNESS
ca.SOOms
MIGRATING RAMFIED
CHANNELS WITH COMMON
SHIFTING OF CHANNELS
THICKNESS DECREASES
FROM 500ms TO 0
MAXIMUM DIMENSIONS
THICKNESS 150m5
WOTH ca. 70km.
THICKNESS O-SOms
WIDTH 0-BOkm.
SCHEMATIC GEOMETRY OF
A SINGLE ACOUSTIC UNIT
I
1
0-50 ms
50-100
too-150
>150
Figure S49 Sthemalic diagram 'showing ch.innel-levee systems on Ihe Rhone E.in. Thiknesses are in two-way seismic travel limes, where
J
ms is
about 0.75 m. From Pickering et jl. (1989, iheir Figure 7,19 on
p.
1771.
MORPHOLOGY
AND
SEDIMENTATION
UPPER
FAN
OVERSPILLED
CHANNELIZED TURBIDITY
TURBIDITY ,— CURRENT
CURRENT
SAND
MIDDLE
FAN
LOWER
FAN
UNCHANNELIZED
TURBIDITY
CURRENT
CLAYS
Figure S50 Model tit seclimenlatidn lor the Indus fan channels, proposed by Kolla andCoumes (1986, their Figure 18 on p,f>74). The decreasing
int'lucuLL' of channelized turbidity current down channel is shown in the sections on the
leti,
along with the increasing importance of overbank
flows,
and apparent branching of <:hannels shown on the plan on the right.
SUBMARINE FANS AND CHANNELS
697
MAJOR
MUD-niCH
RIVER DELTA
ABANDONED
CHANNEL LEVEE
SYSTEM IN
SUBSURFACE
CHENIERS, BARRIEHS
HUCVSAND-RICH
DELTA COAST & SHELF
Figure
S51
Two "end member" models for large fans (from Readinji^nd Rithards, l')94: to[X their Figure
.i,
on
|).80l;
bottom, their Figure 5 on
p.803). The upper figure shows a model fora "point soune mud-rich submarine fan" and the lower shows a model tor
a
"point-source mud/sand-
rich submarine fan." Note ihe difference in scale, and in the abundance of levees, channels, and lobes.
oramalg;miLUion of beds, and the types of internal structures
displayed by the sand beds, to identify not only channels and
lobes,
but also ancient levee deposits, and to distinguish fan
deposits frotn the sediment gravity flow deposits of other
environments, such as basin plains. Though the tnodels are still
useful, sequence stratigraphic concepts, developed since the
1980s, show that they need to be modified to include the effects
on submarine systems of tectonics and changes in relative sea
level, both of which can alVect the nature and tVeqiiency of the
events supplying sediment into the feeder channel, and
therefore the facies and stratigraphic sequences that are
produced by the fan.
698
SUBSTRATE-CONTKOLLED ICMNOFACIES
Models
for
large fans
Thus far, few attempts have been made to develop getieral
models for very large fans, such as the Mississippi and
Amazon fans. Individual modern fans, however, are discussed
and analyzed in Kolla and Coumes (1987). Weitner and Link
(1991), Droz
(•/(//.
(1996), Bouma and Stone (2000). and Piper
and Normark (20(10). Figure S4S shows a map of the Amazon
fan. Figure S49 shows a "'model" based on the Rhone fan. and
Figure S50 shows a "model" for the deposits of an active
channel on the Indus fan. Reading and Richards (1994) have
presented a spectrum of models, two of which re shown in
Figure S51. Larger physical scale implies that large scale
controls (clitnate. tectonic, and sea-level change over long
periods of titne) become tnore itnportant. So sequence
stratigraphic concepts are just as crucial as seditnentological
processes, and are more useful when outcrop Is not available
and only large features can be observed, using seismic methods
in the subsurface.
Sorne authors prefer to abandon the idea of "facies models."
and instead prefer to use the concept of "architectural
analysis." In this approach, each fan. or larger "'turbidite
system" is unique, but is constructed from recognizable
"architectural clernents" of several different scale (e.g.. in
order of increasing scale: beds, facies associations, stages,
systems: see Mutti and Normark. in Weimer and Link. 1991).
Thus there are not just a few "models" but an almost limitless
number of possible '"complexes" that may be constructed
using the various "elements." In assessing the recent literature,
the reader should keep in mind the difficuUy in observing the
smaller-scale "elements" in deep water, and the present limited
understanding of the physical processes involved (turbidity
currents, debris flows, etc.) at very large scales.
Gerard V, Middleton
Bibliography
A.H.,
and
Stone.
C.G.
(cds.). 2000, Fiw-gniincd
Systems. American Association
of
Petroleum Geologists Memoir.
72.
Briilin. C.IJ.L.. atid Walker.
R.G.. 1997.
Internal architcclure
and
sedimentary evokition
of
coarsc-gramed. turbidite channel-levee
complexes. F.iirly Eocene Rcycncia Canyon. Espiritii Santo Basin.
Brazil, Sccitim-uiolo^y.
44
(1): 17-46.
Carlson.
J.. and
Grotzinger.
J.P,.
2(K)1.
Submiirine
tan
environment
inferred from turbidite thickness distributions.
Scc!ii>ii'iir(jlof;v.
48:
1331
1351.
Clark.
J.D.. and
Pickering.
K.T.. 1996.
Arehitectural elements
and
growth patlenis
of
submarine channels: application
to
hydro-
carbon exploration. .Aiueriaiii
.A.ssociatiaii
d
I'ciniU'iiiii
Cn'dlci^ist.s
Btillcltn.
80:
194-221.
Dro:^.
L..
Rigaut. F,. Cochonat.
P., and
Tofatii.
R,.
l^'id.
Morphology
and recent evolution
of
the Zaire turbidite system (Gulf
of
Guinea).
GcoloiiicalSdcifiyal
Ai>u'fita
Bulk'liii. 108:
253 269.
Hartley.
A.J.. and
Prosser.
D.J.
(eds.). 199^.
C'htinit
lcriMiii(i)uif
Deep
Marine Clastic Systems. Getilogical Society
[of
London]. Special
Publication
No. 94
Heller. P.L..
and
Dickinson. W.R., 19S5. Submarine ratiip facies model
for dcltii-ted sand-rich tiubidite systems, .imeriian
.AssDcititii'ii
nj
Peiroh-wuGcohgi.slsHulk'iui.
69: 960 976.
Hesse.
R.. and
Rakofsky. A.. 1992. Deep-sea ehannel/submarine ya/oo
system
of
the Labntdor Sea:
a neu
deep-water facics model, Aim-i-
iian
Assiiciutiiiij
of
fctrolcum
Gvchii^ists
Bulletin.
76:
(i^O
707.
Kollii.
V., and
Coumes.
F,. 1987.
Morphology, internal structure,
seismic stratigraphy,
and
sedimentation
of
Indus
Fan.
American
As.soeiationof Petroleum
Geologists
Bulletin. 71:
650 677.
McHargue.
T.R.. and
Webb.
J.F,. 1986.
Internal geometry, seismic
tacies.
and
petroleum poteniial
of
canyons
and
inner
fan
channels
of
the
Indus Submarine
Fan.
American As.wciaiion
of
Peirok-um
GcnhfiistsBiilk-iin.lQ:
161 180.
Mulder.
T.. and
Syvitski. J.P.M.. 1995. Turbidity currents generated
at
m{>uths
of
rivers durint! exceptional discharges
to
the world oceans.
.hiinial of Geology. 103: 285-299.
Mutti,
E.. and
Ricei Lucchi.
F., 1972. Le
torbiditi dell'Apennino
settentrionale: introduzione alfanalisi
di
facies.
Mi-Diorietlel/a
So-
(ifhi
Cieoloaica
Ikiliana.
11: 161 199.
Fnglish translation
by T.H.
Nilsen. Iniemaiional
Geology
Review.
20. 125 160.
1978).
Normark.
W.R..
Posamentier.
H.. and
Mutti,
E.,
t99_l. Turbidite
systems: state
of the an and
future directions. Reviews
ot
Geophv-
.vfcv.
32: 85 113.
Pickering. K.T.. Hiscott,
R.N.. and
Hein.
FJ..
1989. Deep Marine En-
viroiiinenis:
Cla.slie
Sedimentation
and
Tectonics. London: Unwin
Hynian.
Piper.' D,J.W..
and
Normark. W.R,. 2001. Sandy fans from Amazon
10 Hueneme
and
beyond. American Association
of
Petr<ileiim
Geolo-
iiisls
tSulk-lin.
85: 1407 1438.
Reading.
H.G.. and
Richards.
M.. 1994.
Ttirbidite systems
in
deepwater basin margins classified
by
grain size
and
feeder system.
American Assoeialion oj Petroleum
Geolofiists
BtiUdin.
78: 792 822.
Walker.
R.G..
1992. Turbidites
and
submarine fans.
In
Walker.
R,G..
and James.
N.P.
(eds.). Fades Models: Response
lo Sea
Level
Ckaniie. Geological Association
of
Canada,
pp. 239 263.
Weimer.
P., and
Link.
M,H.
(eds.). 1991. Seismic Fades and Salimen-
tarv
Pn>ces.ses
of
Submarine Fans
and
Turhidite Syswms. Springer-
Verlag
Weimer.
P..
Slatt.
R.M..
Dromgoole.
P..
Bowman.
M.. and
Leonard.
A.. 2000, Developing
and
managing turbidite reservoirs: ease
histories
and
experiences: results
of the 1998
EAGE/AAPG
Research Conference. American
.Assoealion
ofPcirok'imi Gcoloiiisis
Bulletin.
84: 453 465.
Whitakcr, J,H.McD. (ed.).
1976.
Submarine Canyons
and
Dcep-Seti
Foils. Dowden: Hutcbinson
&
Ross.
Cross-references
Alluvial
Fan
Climatic Control
of
Sedimentation
Cyclie Sedimentation
Debris
Mow
Facies Models
CJravityT)riven Mass Flows
Liqucfiietion
and
Fkiidization
Mass Movement
Slide
and
Slump Structures
Slope Sediments
Slurry
Tectonic Controls
of
Sedimentation
Turbidites
SUBSTRATE-CONTROLLED ICHNOFACIES
Three substrate-controlled ichnofacies have been established
(Fkdale etal.. 1984); Glossifiiiii^ites (tirmground). Trypanitcs
(hardgrotmd). and Tcredolhes (woodground).
The
Glossifungites
ichnofacies
The
Gli>s.-iifuiii;iti'.s
ichnofacies is environmentally wide ranging,
but only develops in lirm. unlithified substrates such as
dewatered muds. In clastic settings dewatering results from
burial and tlie substrates are made available to tracemakers if
SUIiSTRATE-(:(.)NTROLL[D KHNtJFACIES
699
(A)
Diplocmimnn
Substrate Conlrolkd Ichnofacies
Glossifungites: Semi Cohessive
Trypanites: Hardgrounds
Teredolites:
Woodgnmnds
(C)
Figure S52 Substrate controlled ichnotac
ies;
lA) Tlie
Glossifungiles
icbnofacies i^ indicative uf semi cohesive substrates like dewatereri muds or
I (impacted sands; iBl The Tcrt'dolitcs ichnofncies is indicative of a woodground like a peal deposit; and (C) The
Tryp.inites
ichnof,K.ies is
indicative of fuily cemented substrates like hardgrounris, disconformities, anrl beathrock.
b> kster et"(.)sioti (Pcnibcrloii ;ind Frey, 1985).
hxluiination ciiii occur in terrestrial cnviroiinienls. as a result
of channel meandering or valley incision, in shallow-wiitcr
environments as a result of meandering tidal channels, coastiil
erosion,
erosive shorelace retreal, or as a result of submarine
channels cutting through previously deposited sediments. Such
horizons commonly form at bounding discontinuities and may
be critical in the evolving concept of sequence stratigraphy
(Pcmberlon
ctal..
1992: Petnberton and Mactachern. 1995).
Ihe (ilossifiiniiitvs ichnofacies (Figure S52(A)) is character-
ized by: (1) vertical, cylindrical U or tear shaped pscudo
borings, sparsely lo densely branching dwelling bnrrows, and/
or mixtures of burrows and pseudo borings; (2) protrusive
spreiten in some burrows ihat develop mostly through animal
growth (funnel shaped Rhizoamilliutn and Diplocratc-rion
[formerly Glt>.\siliinf'itc.-i]): i^) animals that leave the burrow
to feed
(e.g.,
crabsj as well as suspension feeders; and (4) low
diversity, but commonly abundant individual structures.
The Teredolites ichnofacies
The Tcn'dolitfs ichnofacies (Figure S52(B)) consists of a
characteristic assemblage of borings or burrows in woody or
highly carbonaceous substrates. Woodgrounds differ from this
substrates in three main ways: (1) they may be flexible instead
of
rigid:
(2) they are cotnposed of carbonaceous material
instead of mineral matter; atid (3) they are readily biodegrad-
able (Bromley
('/(//..
1984). Such differences dictate that the
means by which, as well as the reasons fur which these two
types of substrates are penetrated are different. Because
currents can raft woody substrates, it is imporumt to
determine whether the borings are autochthonous or al-
lochthonous. Only the autochthonous forms arc true members
of the
Teredolites
ichnofacies. These assemblages may also be
important in defining sequence and parasequence boundaries
(Savrda, 1991). The use of bored logs to define bounding
surfaces as in the concept of log-grounds
(i.e..
Savrda
etal..
700
SUBSTRATE-CONTROLLED ICHNOFACIFS
1993) should be avoided,
as it is
very difficult
to
ascertiiin when
the logs were bored. Siieli logs
are
clasts tind should
not be
treiited
in lhe
siime manner
as the
TcrecioHle.s
ichnofiicies.
The
Tcri'dolih's
ichnofiicies
is
characterized
by; (I)
sparse
to
profuse ekih shaped horings;
(2)
horing walls thai
are
generally
orniimcnted with
the
texture
of the
host substrate (i.e.. tree
ring impressions):
(3)
stumpy
to
elongate subcylindrical
excavations
in
marine
or
marginal marine settings;
and
(4) shallower, sparse
to
profuse nonclavate etchings (isopod
borings)
in
freshwater settings.
The
Trypanites
rchnofacies
The
rrrpciniic.s
ichnofacics (Figure S52(C)) develops
in
fully
lithified substrates such
us
hardgrounds, reels, rocky coasts,
beachrock
and
other oinission surfaces.
As
such, development
of this ichnofacies also corresponds
to
discontinuities that have
major sequence stratigraphie significance. Bromley
and
Asgaard (1993) subdivided
the
Trvpaiiiics ichnofiicies into
two ichnofacies:
the
Eiuohiu ichnofiicies
for
rocky shorelines
(also
see De
Gilbert
el al.. 1998) and the
O'nailiirliiiii.s
ichnofacies
for
bored shells
and
houMcrs found further
ofTshore. Bored shells
and
boulders, however,
are not
substrates that
can be
correlated because
it is
difficult
to
ascertain when
the
boring activity
was
initiated.
The traces
in the
Trypiniitc.s
ichnofacies are characterized
by:
(1) cylindrical
to
vase, tear
or U
shaped
to
irregular domiciles
of suspension feeders
or
passive carnivores:
(2)
raspings
and
gnawings
of
iilgal grazers
and
similar organisms (mainly
ehilons. limpets,
and
eehinoids):
(3)
moderately
low
diversity,
although
the
borings
and
scrapings
of
individual iehnogenera
may
be
abundanl:
and (4)
borings oriented perpendicular
to
the substrate which
may
include large numbers
of
overhangs.
In contrast
to the
Glos.sifungiics ichnofacies,
the
walls
of the
borings
cui
through hard parts
of the
substrate rather than
skirting around them.
Significance
of
substrate controlled ichnofacies
In clastic settings, most
of
these (race assemblages
are
associated with erosionally exhumed (dewatered
and com-
pacted
or
cemented) substrates,
and
hence, eorrespond
to
erosional discontinuities. Woodgrounds consist
of
xylic elasts
or interwoven mats
of
vegetation, forming resilient substrates
that
are not
necessarily erosionally exhumed.
In
carbonate
settings, firmground
or
hardground surfaces
may
occur
at the
sediment wiiter interfaee.
due
either
to
erosional exhumation
or non-depositional breaks with associated submarine eemen-
tation (Bromley, 1975). Depositions! breaks,
in
particular
condensed sections,
may
also
be
semilithified
or
lithified
presumiibly
at the
upper contact
(or
downlap surface)
and
may
be
colonized without associated erosion.
In
general,
however,
the
recognition
of
suhstrate-eontrolled ichnofaeies
may
be
regarded
as
equivalent
to the
recognition
of
discontinuities
in the
stratigraphic record.
Although certain insect
and
animal burrows
in the
terrestrial realm
may be
properly regarded
as
firmgound
(e.g.. Fursich
and
Mayr.
1981) or
more rarely, hardground
suites,
they have
a low
preservational potential
and
constitute
a relatively minor component
in the
geologic record
of
such
associations.
The
overwhelming majority
of
these assemblages
originate
in
marine
or
margin marine settings.
A
discontinuity
may
be
generated
in
either subaerial
or
submarine settings:
but
colonization
of the
surface
may be
regarded
to be
marine
influence, particularly
in pre
Tertiary intervals. This circum-
stance
has
important implications,
so far as
interpretation
of
the origin
of the
discontinuity
is
concerned.
The substrate conlrolled ichnoeoenose typically cross euts
a
preexisting softground suite
and.
hence, refieets eonditions
post dating both initial deposition
of the
unit
and
erosion
of
that unit.
The
suite also corresponds
to a
hiatus between
the
erosion event (which exhumes
the
substrate)
and
deposition
of
the overlying unit. During this time
gap.
organisms colonize
the substrate.
By
observing
(1) the
soflground ichnofacies
(contemporaneous with deposition
of the
unit):
(2) the
ichnofacies
of the
exhumed substrate:
and (3) the
iehnofaeies
ofthe overlying unit,
it is
possible
to
make some interpretation
regitrding
the
origin
of the
surfaee
and the
allocyclic
or
autocyclic mechanisms responsible.
S. George Pemberton
Bibliography
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R.G.. and
Asgaard.
U.. 1993. Two
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produced
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early
and
late burial associated with
sea
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Pembcrkon.
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htmahf-y:
The
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iif
True
f
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Eursich.
F.R,. and
Mayr.
H..
1981. Non-marine Rhizocoralliiim (trace
fossils) from
lhe
Upper Freshwater molasse (Upper Mioeene)
of
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fi'ir
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Paltioiiloloi^ic,
MoihtrsiiefK:
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321-333.
Pombcrton.
S.G.. and
Frey.
R.W.. 1985. The
Glosslfungites ichno-
facies:
modern examples from
lhe
Georgia coast. USA,
In
Curran.
H.A. (ed.). Biogenic Sinicliirvs: Their
Use in
Iiitcrptvlin,i;
Depo\iiiaiiiii Eimronmenis. Society
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and Mineralogists. Special Piiblicaiion.
35. pp.
237-259.
Pembcrion.
S.G.. and
MacEachern.
J.A.. 1995. The
sequence
straligraphic significance
of
trace fossils: examples from
the
cretaceous foreland basin
of
Alberta. Canada.
In Van
Wagoner.
J.C.
and
Berlram.
G.
(eds.). Seqiieitve Slruiigruphy
oJ
ForekuulBit-
siit Deposits- OuhTtipauit Suhsurfiice E.xiimplesjhitn the Oviaceuus
(if
iVorili.-inwricd.
Tulsa: American Association Petroleum Geologists.
Memoir 64.
pp.
429-475.
Pemberton.
S.G..
MacEachern.
J.A.. and
Frey.
R.W.. 1992.
Trace
fossil faeies models: environmenlal
and
allostratigraphic signifi-
cance.
In
Walker.
R.G.. and
.lames.
N.
(eds.).
Fu(i(:\
Models:
Response tn
.Sea
Level Change, Gcoloiiical Assoeiation
of
Canada,
pp.
47 72.
Savrda.
C.E.. 1991.
Tercdoliies. wood substrates,
and
sea-level
dynamics. Ceology.
19:
905-90S,
Savrda.
C.E..
Ozalas.
K..
Demko.
T.H-.
Hutchinson.
R.A.. and
Scheiwe.
T.D.. 1993. Log
grounds iind
the
ichnofossil Teredoliles
in transgressive deposits
of the
clayton formation (Lower
Paleocene). western Alabama, Pahtins.
8: 311 324.
Cross-references
Biogenic Sedimentary Struclures
Taphonomy: Sedimentological Implications
of
Fossil Preservation
SULFIDE MINEKALb IN SEDIMENTS
701
SULFIDE MINERALS IN SEDIMENTS
The fortnatioti processes of metal suUidcs in scditnents.
cspcciLtlly iron stillides. have been the subjects of intense
scientific research because of linkages to the global biogeo-
chcmical cycles of
iron,
sulfur, carbon, and oxygen. Metal
siillidc precipitation can occur during several stages of
diagenesis and burial of continental-margin, pelagic, and
non-marine sediments. During early stages of diagenesis,
anaerobic sull'ate-reduciny bacteria consume organic carbon
and sultate and produce hydrogen sullide and bicarbonate.
Pyrite (FeS^) is formed by the reaction of this biogenic sultide
wiih reactive, detrital iron-bearing minerals near the sediment-
water interlace (Kaplan
etal..
1963; Berner. 1970. I9S4). The
initial reaction products are iron monosultide minerals that
may subsequently transform to pyrite by reaction with
polysiillitlcs or other reactive oxidtmts
(e.g..
Morse et al..
1987). Pyrite formation in sediments is thought to be limited
either by the tnicrobial production of sulfidc (organic carbon
or sulfiite limitation) or by the availability of reactive iron
minerals (iron limitation). During later stages of diagenesis
and deep sediment burial, thermochemical sulfate reduction
tnay becotiie important and more refractory iron oxides,
typically unreactivc during early diagenesis, are sources of iron
in SLillidation reactiotis. Studies of metal sulfides in sediments
and seditnenlary rocks etnploy analytical techniques such as
microscopy, sulfur isotope ratio measurements, and trace
element measurements. Relationships between the eoncentra-
tions and partitioning of sedimentary Fe, S, and C provide
important tools for reconstructing paleoenvironments over a
ranae of temporal and spatial scales
(e.g,.
Goldhaber and
Kaplan.
1974),
Transition metal sullides
(e.g..
NiS, CuS, ZnS. CdS, HgS)
have exceedingly low solubility products and might be
expected tti form during early diagenesis. However, with (he
exception of
iroti.
d-transilion metals are typically present in
trace amounts in sediments, which does not allow for any
signiticant accutnulation of sullide minerals other than those of
iron.
Consequently, iron sulfides are the only metal sulRdes
commonly recognized in tnarine and non-marine sediments. In
contaminated sediments or systerns with high tiietal loadings,
snllides of Zn. Cd. and Cu have been reported
(e,g..
Luther
et al.. 1980), Trace metals are frequently found associated
with pyrite and include Co, Ni. Cu. Zn. As, Mo, Cd. Sb. Hg.
and sometimes Mn, In particular. .As. Hg. and Mo have a liigh
alTinily for pyrite and are generally enriched in pyrite relative
to bulk sediments (Raiswell and Plant. 1980: Huerta-Diaz and
Morse.
1992), Questions remain as to whether the trace metals
found associated with the sullide fraction are as discrete phases
or as coprecipitates with the abundant iron sulfides.
Several iron suHide phases have been synthesized in the
laboratory, either as transient intermediates or as stable end
products, and are therefore likely to form in seditnentary
environments. These phases are: disordered maekinawite.
FeuvS;
maekinawite. Fei-^S; cubic iron sulfide. FeS: hex-
agonal pyrrhotite. Fe] ^S: greigite. Fe,S4: smythite. Fe<,S|i;
niarcasite. orthorhombie FeS^; and pyrite. cubic FeS:.
Pyrrhotite and pyrite represent the thermodynamically stable
phases at the temperatures and pressures of early diagenesis
(Berner. 1967), Disordered mackinawiie, maekinawite. and
Figure S5'i Ciruup ol pvrite I'r.imhoicis iniodern slopu scdimenls, Black
Seal.
Figure S54 Fr.imlK)ici with pyritohedral mitrotrystals (Cretaceous
sediments}.
greigite are tnetastable with respect to pyrite and/or stoiehio-
metric pyrrhotite but are considered to be the principal
precursor phases to pyrite
(e.g,.
Schoonen and Barnes. 1991).
Smythite and cubic FeS are exceedingly rare (Krupp. 1994).
Marcasite has not been identified in modern sedimentary
environments but is present in some ancient sedimentary
rocks.
Marcasite formation is thought to require a low pH and
high concentration of polysullide species. Greigite was
originally reported from Tertiary laeustrine deposits bni has
been more recently docutiiented in sediments as old as Upper
Cretaceous siliciclastics (Reynolds ct al., 1994). Despite its
predicted stability, hexagonal pyrrhotite is apparently very
rare as an authigenic phase in sediments and sedimentary
rocks.
In marine and non-marine sediments, pyrite is the
normal end product of sulfate reduction and iron mineral
sulfidation.
The kinetics of transformation from preeursor iron
sullides to pyrile tnay be slow (years) or fast (days) depending
on factors such as pll, redox state, and temperature.
702
SUIFIDE MINI-KALS IN SEDIMENTS
Pyrite is the most common authigenic mineral in recent
marine and laeustrine sediments, as well as in sedimentary
rocks,
especially gray- to blaek-colored shales (Arthur and
Sageman, 1994), It occurs typically as single grains
1
|.mi to
!()|.mi in size, as framboids mainly <2()|.itn in diatneter. as
groups of framboids termed polyframboids. and as replace-
ments of organic matter (Figures S5I-S53). Microcrystals and
framboids are the basic textures that form during sediment
deposition and diagenesis (Raiswell. 1982). Microcrystals are
usually subhedra! to euhedral; octahedrons and pyritohedrons
are cotntnon. The lenn Jhiiiihoid retcrs to a unique raspberry-
like microtexture. which has been the subject of numerous
studies. The ordered, cell-like arrangement of pyrite micro-
crystals in framboids has resulted in a long-lived debate on
whether they are organic or inorganic in origin (see Wilkin and
Barnes. 1997 and references therein). These textures ean
survive to low-greenschist facies metamorphism. Pressure
solution and overgrowth may occur during low-grade meta-
morphism and deformation. With progressive defortnation.
pressure solution produces flattened grains perpendieular to
the tnaximutn compressive stress that results in the develop-
ment of foliations and lineations (McClay and Ellis. 1983).
Mass transfer during pressure solution has important itnplica-
tions with respeet to the distribution of trace elements and the
sulfur isotopic systematics of suilide ores.
There are other examples of localized, macroscopic to
microscopic varieties of pyrite found in modern and ancient
sediments. Some examples are pyritizcd organic tnatter.
pyritized burt"ow tubes, shell infillings of pyrite, carbonate
shell replacetnent by pyrite. pyrite concretions and nodules,
and perhaps most spectacularly, the pyritization of soft-bodied
materials (e.c. Briggs ef al.. 1991; Canfield and Raiswell.
1991).
A second setting where rnetal sulfides accumulate in
sedimentary systems is in areas such as mid-ocean ridges
where hydrothermal lUiids discharge into seawater. Rapid
cooling of the hydrothermal fluids as they inleract with eold
ocean water causes the precipitation of dissolved solutes, often
as very-fine grained sullide minerals ("black smokers"). Metal
sulfides may also precipitate on the walls of the vents or
in vertical tubular structures ("ehirnneys"). Pyrite again
Figure S55 Pyritohedral grain of pyritu (Crctjceuue-
dominates the sulfide mineralogy of these submarine volcano-
genie deposits although sulfide precipitates can also inelude
those of Cu and Zn. Precious metals such as Ag and Au may
also be enriched in the prodticts of these hydrothermal
processes. Volcanogenic massive sulfide deposits have been
important sources of base metals; notable are the deposits in
Japan (Kuroko-type) atid the Iberian Peninsula (Pyrite Belt).
Such volcano-sedimentary deposits grade into bedded rnetal
sulfide ores that often show no clear associations with
volcanism or hydrotherrnal activity. The origin of these
sediment-hosted sulfide ores historically has been controver-
sial; however, this important class of mineral deposits includes
some of the most famous ore districts (e.g.. Broken Hill.
Australia: Sullivan. North America; Kupfersehiefer. Furope:
Zambian Copper Belt. Africa), In these deposits, metal sulfides
are thought to have tbrmed syngenetically. precipitated frotn
hydrogen sulfide of hydrothermal or microbiological origin.
Geologic evidence suggests the persistence for over
3
Ga of
deep-ocean hydrothermal systems. The discovery of life and
diverse ecosystems that have adapted to the warm, anoxic
environments of seafioor hydrothermal systems, along
w
ith the
discovery of ancient sulfur-metabolizing bacteria has spawned
hypotheses that sueh environments could be the sites for the
origin of life on earth. Metal sulfides in sediments may have
played a role in catalyzing reactions and/or providing energy
sources through redox reactions that ultimately led to the
buildup of complex organic molecules, thereby guiding the
transition frotn a prebiotic earth to a biotic earth (e.g..
Wachterhiiuser. 198S).
Richard T. Wilkin
Bibliography
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dcpositioiuil mechanisms and environments of ancient deposits.
Annnal Review of Earihand Fkitieiary Seienees. IT.
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Berner, R.A., 1967. Tlicrmodvnamic stability of sedimentary iron
siiltides. American Journal of Scieiue. 265: 773 785.
Bonier. R.A.. 1970. Sedimentary pyritc formation. American Journal
oJ
Science. 268: I 23.
Berner. R.A.. 1984, Sedimentary pyritc formation: an update. Geochi-
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Cdsm<ieltimieit
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New York Stale. Geolof;y. 19: 1221-1224.
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iltc Data Locked in the
Fos.sil
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lolimte 9 of
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Wiley, pp. 569 655.
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Acui. 56: 2681-2702,
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distribution and isotopie abundanee of siilTur in reeeni sediment
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297-331.
Krupp, R.H.. 1994. Phase relations and phase transrormations between
the low-temperature iron sulphides maekinawite. greigite. and
sm>thitt'.
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Journal of Xfincralogy. 6: 265 278.
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1980.
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larv
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MeClay. K.R.. and Ellis, P.G.. 1983. Deformation and rccrystalliza-
tion of pyrite. Mineralo^icul Magazine. 47; 527 538.
SURFACE FORMS
703
Morse. J.W.. Millcro. F.J.. Cornwell. J.C and Rickard. D.. 1987. The
chemistry ol" the hydrogen sulphide and iron sulphide systems in
niiliiriil Witters. Larih-Scienee Reyietv^. 24: I 42.
KiiiswcIL R.. I9S2, Pyrite lexture. isotupic composition and the
ii\LiilLihility of iron,
.American
Journalof Seietue. 282: I244-I26.V
RJls\^L•ll, R.. and Plant.
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\'-M).
The incorporation of trace metals
into pyrite during diagonesis of black shales. Yorkshire. England,
Economic Geohigv. 75: 684-A89,
Reynolds. R,L.. Tiittle. M.L,. Rice. C.A.. Fishman. N.S., Karachcuski,
J.A., and Sherman. D.M.. 1994, Magnoii/alion and geochemistry
of greigitc-bcaring Cretaceous strata. North Slope liaNin. Alaska,
Americati .lournai of Science. 294: 485
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528.
Schooncn. M.A.A.. and Barnes. H.L.. IWl. Reactions lorming pyrite
and marcasite from solution. II Via leS precursors below 10(1 C,
(ieoehimicaet Cosmoehimiea.-Uta, 55: 1.505-1514,
Wachterhauser, G,. 1988. Pyritc formation, the first energy source
for life: a hypothesis, .Syslematie anil Applied Mierohiologw 10:
207-210.
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framboidal pyrite. Geoehimieaet Cosmoehimiea Acta. 61: 323 .I.'^y.
Cross-references
liacloria in Sediments
Diagenesis
He;uy Minerals
Isolopic Methods in Sedimentology
Toxicity
ol"
Sediments
SURFACE FORMS
The nature of surface forms
Surface forms are geometrical features that develop on a
surface of cohesive or noneohesive sediment by the action ofa
How of fluid over that surface. Surface forms are commonly
produced by flows of vvatei" in streatns and rivers, by tidal
currents in shallow coastal areas, by continental-shelf bottom
ilows.
and by various kinds of flows in the deep ocean. The
generating flows on continental shelves tnay be purely
unidirectional, or purely oscillatory as a result of the passage
of surface waves, or combinations of unidirectional and
oscillatory flows. They are also common in environments ow
land where winds are strong enough to move sand along the
surface. Sotne surface forms arc generated solely by localized
erosion of the sediment bed: others are generated by
transportation of sediment along and over the bed in a pattern
of local erosion and deposition without overall net erosion of
the surface, and perhaps with overall net deposition.
The range in both the seale and the geometry of surface
forms is extremely wide. The scale oi comtnon surface forms
ranges from centimeters to kilometers horizontally and from
eentimelers to at least tens oi meters vertically. Most surfaee
Ibrnis show more or less elongation in their geometry, either
trans\erse to the flow or parallel to the flow, although some are
oblique to the flow direetion and others are approximate!)
equiditnensional rather than elongate. In part this wide range
of diversity is owing to a wide range in eonditions over which a
generating process acts, but in part also beeause the proeesses
that generate surfaee forms are diverse. It is clear that surface
forms are a polygenetie phenomenon: a number of disparate
effects of fluid flow give rise to them.
Sui-f"ace forms may or may not exist in a state of equilibriutii
uith the generating flow. Equilibrium oi surface forms is
eommon when the flow persists in a nearly unchanging state
for a time long enough that the surface forms ean adjust their
characteristics fully in response to the flow. In other cases, the
deveiopmenl of the surfaee forms lags behind changes in the
flow. Such disequilibrium makes for even greater variety in
surface forms, and it is a complicating factor when attempts
are made to use the characteristics of preserved surface forms
for making interpretations of the former flow that generated
the forms.
The size of sediment in the bed of seditnenl on whieh surface
forms can exist ranges widely, from cohesive or semieohesive
elays and muds to gravels. Distinctive erosional forms, like
flutes and flow-parallel furrows, are characteristic of flne.
eohesive .sediment surfaces. Rugged flow-transverse fortns, like
ripples and dunes, are characteristic of coarse silts, sands, and
(with lesser frequency) gravels.
The significance of surface forms
Surface forms are of interest to a wide range of specialists in
the natural scienees. ineluding geologists, geomorphologists.
geographers, and oceanographers. beeause they are so varied
in their seale, geometry, and origin and are so characteristics of
natural flow environments, both subaqueous (under water)
and subaerial (under air). Moreover, even apart from their
intrinsic interest, surface forms are of interest tor praetieal
reasons as well. Large subaqueous surface forms many meters
high can be obstacles to navigation, and their movement can
be a threat to submarine structures. The rugged topography oi
surface forms in rivers and tidal channels causes Ilow
separation at the crests and therefore large values of form
drag. Surfaee forms are thus the most important determinant
of resistance to channel flow, and hydratilic engineers have
expended tnneh effort on the development of depth discharge
predictors based on the hydraulic relattonships of the surlaee
forms (Vanoni, 1975, p. 114-152). The nature of the surfaee
forms is also cioseiy bound up with the sediment transport rate
in unidirectional flows, in that the downcurrent movement of
rugged flow-transvcrsc surface fortns largely involves recycling
of bed load within the fortns. Sedimentologists have given
much attention to certain kinds oi surface forms because of
their role in generating stratification, which is one oi the tnost
useful tools available for interpreting ancient sedimentary
environments (see BeddiitgaudIttterttal Structures).
Terminology and classification of surface forms
Terminology for surface forms is diverse and can be confusing.
Nonspecialists are confronted with a wide variety of terms.
among the most common being ripples, dunes, antidunes,
sand waves, sediment waves, furrows, and flutes. The genera!
term surface form is nearly synonymous with the common
terms bed form or bedform. Some workers use the term bed
tbrm for an individual, distinctive geotnetrical element that is
formed on a sediment bed by u flow of overlying fluid. In that
context, the individual bed forms eonstitute a bed configura-
tion, which is the aggregate of all of the bed forms that are
present in a given area of the sediment bed at a given time.
Others use the term bed fortn (more commonly, bedform) for
both the individual eletnent and the entire configuration., with
the intended sense derived froin (he context o!" the deseription
or discussion.
704 SURFACE FORMS
Even under an unchanging flow, and with unchanging
sediment charaeteristies, the partienlar geometry of ihe bed
contigttration that is actively coupled with the Ilow at a given
time differs in detail
["'rom
that of the eonllgnration at any
other titne. It Is sotnelimes useful lo think of the average
eharacteristics of such a collection of equivalent configurations
as the bed state. Bed states with closely related geometry and
kinematics, formed under a particular range of flow and
sediment characteristics and readily distinguishable from those
formed under different ranges of eonditions, are sotnetimes
called bed phases, in loose aitalogy with thermodynatnic
phases. Some exatnples of sueh bed phases are current ripples,
subaqueous dunes, subaerial dunes, flutes, or erosional
l"urrows (see classification below).
Surface fortns can be classified in a variety of ways. An ideal
classification would distingtiish atnong genetieally different
kinds of forms, and within those kinds, characteristic
subclasses based on differences in geometry and kinematics
as a function of differences in flow and sediment char-
acteristics. There is no universally accepted classification
scheme for surfaee f"orms. Widely recognised categories of
surf"aee forms are shown in Table I. Owing lo the complexity
and diversity of processes that generate surfaee forms, such a
classification is bound to be inadequate to express the full
range of the phenomenon, but it can serve as a guide to the
principal kinds of surface forms discussed in later sections of
this article.
Flows that generate surface forms
Flows of water that generate surface forms range from
unidirectional to oscillatory. In unidirectional flows, water
veloeity is always in the same direction, although speeds tnay
vary on timescales ranging from seconds to tidal periods and
longer. In purely oscillatory flows, water movements range
from back-and-forlh movetnent in a single orientation to more
eomplex oseillations in which water tnotions change direction
seemingly at random owing to the superposition of more than
one oscillatory eotnponeni. as for exatnple at the bed of an
ocean or a lake where ihc water surface is characterized by a
cotnplex multidirectional wave speetrum. In most subaerial
environments, the wind is notoriously changeable in both
speed and direction, although in some areas of the world,
sand-moving winds blow very consistently from a narrow
range of direetions.
Subaqueous surface forms generated by
unidirectional flows
Flow-transverse surface forms
Flows of water over noneohesive beds of silt, sand, or gravel
generate a variety of rugged surface forms. These forms are
most cotnmon in sands. It has been customary to think in
terms of a progression of regimes of surfaee fortns with
increasing flow strength: eurrent ripples, dunes, plane-bed
transport, and antidunes. The picture is actually tnore
eotnplieated, however, because the succession of dif"ferent
kinds of surface forms varies not only with flow strength but
also with sediment size.
Flow-transverse surfaee t"orms have been studied in water
channels and flumes, by both hydratilic engineers and earth
scientists, sinee the beginning of the twentieth eentury.
Partiettlarly notable siudies are those by Gilbert (1914) and
Sitnons and coworkers (Simons and Richardson,
\96?<;
Guy
etal.. 1966). For a sumtnary of laboratorv data, see Southard
(1991).
Theoretical studies oi the existence of rugged flow-
transverse surface forms have had a long and varied history. A
long-standing approaeh has been throtigh stability analyzes, in
which the equation of tnoiion, suppletnented with a relation-
ship between flow strength and sediment transport, is
appropriately linearized and then a small perturbation is
introduced. If the perturbation is damped, then a planar
transport surface is stable: if the perturbation grows, then ihe
bed develops into rugged surface forms. The most suecessful of
stich approaches has been that of Richards (1980).
Graphical representation
There have been many attempts to represent the range of
eonditions that produce each kind of surface form in graphs.
Sueh graphs are loosely analogous to the thermodynamic
phase tiiagrams. Sueh graphs are useful ways of portraying the
relationships among the variotis kinds of surface fortns, to the
extent that ihe existenee Delds in the graphs do not overlap.
Beeause of the effeet of water temperature on water viscosity.
ihese variables must be in nondimensionalized form in order
to represent the existence of the various kinds of surfaee forms
in a rational way. In the graphs presented below, these
nondimensionalized variables have been eonverted to their
10 C equivalents, to make comparison with actual tlow
conditions more readily apparent.
There have been two main approaches to the construction of
graphs oi the existenee fields oi flow-transverse subaqueous
surf^aee forms. In one approach, the shear stress the flow exerts
on the sediment bed is used to represent the strength of the
(low. frotn the standpoint that it is this shear stress that
aetually moves the sediment. The problem with this approach
is thai when the bed is roughened with rugged surface forms,
tnost oi the bed shear stress is from drag rather than skin
friction, which is the part of the bed shear stress that actually
moves the sediment. Such graphs are successful only to the
extent that the skin friction can somehow be separated out
from the total bed shear stress. The most recent exatnpie of
such a graph is by Van den Berg and Van Gelder (1993).
shown here as Figure S56. The graph in Figure S56 uses a
nondimensionalized form of the skin friction, eotnputed
according to a rational method, and nondimcnsional tnedian
sediment si/e.
The other approaeh to graphs of the existence fields is to use
the tnean veloeity of the flow as a sutTogate for the flow
strength. The problem introduced by using the mean flow
velocity is that the flow depth must be introduced as a leading
variable as well, because various combinations of flow depth
and tlow velocity correspond to the same bed shear stress. The
advantage of sueh a representation, however, is that it
sidesteps the problem of partitioning the bed shear stress and
leads to unambiguotis charaeterization of [he existence fields.
The tnost recent example of" such a graph is by Southard and
Hoguchwal (1990), shown here as Figure S57. The graph in
Figure S57. which shows surfaee fortns in tenns of nondimen-
sional mean flow veloeity and nondimensional median sedi-
ment size, is drawn for just ouc value of nonditnensional mean
flow depth. For greater (lesser) flow depths, ihe existence fields
shif!
their position up (down), and the shapes of the fields
change somewhat as well.