Middleton G.V. (Ed.) Encyclopedia of Sediments and Sedimentary Rocks
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MELANGE; MELANGE
435
presence of blocks of varied lithologies and sizes supported by
a fine-grained matrix."
Interpretations
Proposed melange origins fall into three large categories. In the
first group are interpretations that appeal to gravitationally
driven movement on slopes to disrupt bedding in sedimentary
components and to mix the blocks and matrix. Such processes
include slides, slumps, and debris-flows. The term "olistos-
trome" (Flores, 1955, 1956), literally "slide layer," describes a
melange, with or without fabric, interpreted to be formed by
slope processes. In the second category are processes, such as
fluid overpressuring and diapirism, that occur within a pile of
sedimentary rocks, and result in disruption and mixing without
necessarily penetrating to the Earth's surface. Thirdly, there
are processes related to faults that are rooted at depth in the
lithosphere, including subduction zones and transform faults.
In attempts to distinguish between these potential origins,
investigators have typically appealed to a variety of lines of
evidence, studying both the internal fabric and structure of
melanges, and the external contacts and geometry of mapped
melange bodies.
Features and origin of blocks
Observations of the internal fabric of melanges can provide
information on the rheology of the blocks and matrix and may
therefore help to constrain the mode of formation.
Many melanges preserve evidence for the process of
fragmentation that has produced blocks. In some cases it is
possible to observe partially fragmented beds cut by brittle
surfaces. These surfaces may be either extensional or shear
fractures, that produce irregular shapes and trains of
fragments derived from an original block or bed (e.g.. Cowan,
1982).
If fractures cross-cut grains, cements or veins in the
bloeks, this may indicate a post-lithification origin for the
structures, and that fragmentation occurred at depth. Where
extensional fractures occur, matrix may penetrate into the
blocks producing veins that are partially or completely filled by
shale. This has been interpreted as an indication of the
presence ofa fluid matrix, but in other cases it has been shown
that matrix was already in a compacted state, with strong
fissility, before it penetrated into veins (e.g., Waldron et al.,
1988).
Whatever the state of lithification, it is clear that
elevated fluid pressures can play a role in promoting brittle
fracturing by reducing the effective normal stress on potential
fracture planes.
Not all blocks are produced by fracturing. In some cases,
disruption of bedding in sedimentary protoliths leads to the
dispersion of individual grains or highly attenuated beds into
the melange matrix. Fragments produced in such circum-
stances have less angular outlines, typically appearing as lens
shapes or "phacoids." The presence of phacoids is an
indication of deformation that was ductile at mesoscopic
seale; however, phacoids in unmetamorphosed melanges do
not typically show evidence for intracrystalline plastic defor-
mation at microscopic scale. In some melanges, blocks are
connected by highly thinned "wisps" representing attenuated
beds (e.g., Byrne, 1984); microscopic examination indicates
that such wisps are typically formed by disaggregation, and
not by grain breakage or plastic flow (Brandon, 1989).
Some melanges contain blocks from diverse sources,
including intrusive igneous and metamorphie lithologies (e.g.,
ophiolites and blueschists) that cannot have formed in
proximity to a lower grade or purely sedimentary matrix.
Cloos (1982) suggests that such "exotic" blocks are incorpo-
rated by large-scale fiow within an accretionary wedge.
However, incorporation as large sedimentary clasts derived
from fault scarps or mass flows that incorporate older rocks at
the Earth's surface is a possibility that is difficult to eliminate,
especially in tectonically active environments such as subduc-
tion complexes and collision zones.
Fabric and origin of matrix
Matrix fabrics in melanges are extremely variable. Some
melanges lack matrix fabric; in these cases it is generally clear
that the matrix formed by flow of wet sedimentary material. In
most cases some fabric is present, and typically this takes the
form ofa scaly foliation (Moore etal., 1986), characterized by
anastomosing planes surrounding lenticular bodies of fine-
grained, fissile material. Foliation surfaces are typically
polished and may show sliekenside striations that indicate
sense of shear. Some such surfaces may be traceable into faults
or shear zones that dismember blocks. Larger scale fabrics may
be present: in many melanges the matrix is heterogeneous, and
can be divided into domains of varying color, texture, etc.,
reflecting different protoliths. The domains are typically highly
elongated in map outline, and may contribute an overall
foliation to the map pattern, and an overall stratification to the
melange. Such stratification has been used as an indicator of
sedimentary origin (Steen and Andresen, 1997), but it is
quite possible to envisage the development of large-scale
foliation during accretion and deformation in a subduction
zone,
so such inferences are probably unwarranted. Other
melanges display more continuous fabrics such as slaty cleavage
or metamorphic foliation; in many cases it can be shown that
these fabrics post-date the mixing and fragmentation processes
and may be unrelated to the formation of the melange.
Overall geometry of melange units
Given the difficulty of using arguments based on internal
fabric to distinguish between possible origins, arguments based
on contacts and the overall geometry of melanges often
provide a better basis for distinguishing between possible
modes of origin.
Most helpful are clear stratigraphic contacts either at the
base or top of a melange body. Brandon (1989) emphasizes
evidence from depositional basal stratigraphic contacts in
attributing an olistostromal origin to melanges of the Pacific
Rim Complex of Vancouver Island. Swarbrick and Naylor
(1980) describe interbedded sediments between melange units
in Cyprus, that indicate that the melange is clearly stratified.
Pini (1999) describes tabular bodies with fabric that form part
of a stratigraphic succession and are therefore identified as of
sedimentary origin, despite the presence of scaly fabrics.
Melanges of diapiric origin may also be identified based on
large-scale geometry. Orange (1990) describes the variations in
fabric within the Duck Creek melange of the Olympic
Peninsula, Washington, tracing a transition from margins
with pervasive scaly foliation and inequant phacoidal blocks,
to a core with poorly developed foliation, and equant, angular
blocks. Using fold asymmetries. Orange (1990) is able to
436
MICRITIZATION
demonstrate opposing senses
of
shear
on
opposite sides
of the
melange, further supporting
an
origin
by
diapirism.
Melange formation
by
tectonic processes
in
fault
or
shear zones
is
most easily demonstrated
by
regional seale
cross-cutting relationships along fault zones. Pini (1999)
was
able
to
show such relationships
for
units
in the
Apennines,
tenned tectonosomes. Orange (1990) described
the
Hogsback
melange
of
the Olympic Peninsula, concluding that shear-zone
origin
is
indicated
by a
combination
of:
consistent foliation
in
map-pattern, consistent fold vergence, increase
in
foliation
intensity toward
the
center
of a
melange,
and
tight clustering
of clast long-axis directions throughout
the
unit.
Conclusion
Units described
as
melange undoubtedly have
a
variety
of
origins. Field
and
thin-section evidence
may
provide informa-
tion about
the
lithification state
of the
material
at the
time
of
fragmentation and/or mixing,
but
only
in
cases where there
is
clear evidence
for
prior burial
and
lithifieation
can a
deep-
seated, tectonic origin
be
inferred. Many melanges involve
some process
of
deformation
of wet or
incompletely lithified
material, either within
a
gravity-driven slump
or
debris flow,
within
a
diapir,
or
within
a
teetonic accretionary terrain
in
which effective stresses
are
reduced
by
high fluid pressures.
The
products
of
these processes (boudinage, brittle fracturing, scaly
i"abrics) appear similar
in all
three settings. Under such
circumstances, determining
the
origin
of a
melange depends
on careful recording
of
variations across
the
melange body
and
documentation
of
contact relationships.
John W.F. Waldron
Bibliography
Brandon,
M.T., 1989.
Deformational styles
in a
sequence
of
olistostromal melanges, Pacific
Rim
Complex, western Vancouver
Island, Canada.
Geological
Soeiety of Ameriea Bulletin, 101:
1520-
1542.
Byrne,
T.,
1984. Structural geology
of
melange terranes
in the
Ghost
Rocks Formation, Kodiak Islands, Alaska.
In
Raymond,
L.A.
(ed.),
Melanges. Their Nature, Origin
and
Signifieanee. Geological
Society
of
America, Special Paper, 198,
pp.
21-52.
Cloos,
M., 1982.
Flow melanges: numerical modeling
and
geological
constraints
on
their origin
in the
Franciscan subduction complex,
California.
Geological
Soeiety of Ameriea Bulletin, 93: 330-345.
Cowan, D.S., 1982. Deformation
of
partly dewatered
and
consolidated
Franciscan sediments near Piedras Blancas Point, California.
In
Leggett,
J.K.
(ed.).
Trench
andForeare
Geology.
Geological Society
of London, Special Publication,
10, pp.
439-457.
Flores,
G.,
1955.
Les
resultats
des
etudes pour
la
recherche petrolifere
en Sicilie: discussion.
In
Proceedings, Fourth World Petroleutn
Congress. Section l/A/2. Rome: Casa Editrice Carlo Columbo,
pp.
121-122.
Flores,
G.,
1956.
The
results
of the
studies
on
petroleum exploration
in Sicily: discussion. Bollettino
Del
Servizio Geologieo d'ltalia.
78:
46-47.
Greenly,
E.,
1919. TheGeologyof Anglesey, Geological Survey
of
Great
Britain, Memoir,
I.
Hsu,
K.J.,
1968.
The
principles
of
melanges
and
their bearing
on the
Franciscan-Knoxville paradox. Geologieal Soeiety
of
America
Bulletin.
79:
1063-1074.
Hsu,
K.J.,
1974. Melanges
and
their distinction from olistostromes.
In
Dott,
R.H., and
Shaver,
R.H.
(eds.). Modern and Aneient Geosynet-
inal Sedimentation. Society
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and
Mineralogists, Special Publication,
19, pp.
321-333.
Moore,
J.C,
Roeske,
S.,
Lundberg,
N.,
Schoonmaker,
J.,
Cowan,
D.S.,
Gonzales,
E., and
Lucas,
S.E.,
1986. Scaly fabrics from Deep
Sea Drilling Project cores from forearcs.
In
Moore,
J.C.
(ed.).
Structural Fahrie
in
Deep
Sea
Drilling Project Cores frotn Foreares.
Geological Society
of
America, Memoir, 166,
pp.
55-68.
Orange,
D.L., 1990.
Criteria helpful
in
recognizing shear-zone
and
diapiric melanges; examples from
the Hoh
accretionary complex,
Olympic Peninsula, Washington. Geological Society
of
America
Bulletin.
102:
935-951.
Pini,
G.A., 1999.
Tectonosomes
and
Olistostromes
in
the Argite Seagliose
of the Northern Apennines. Geological Society
of
America, Special
Paper,
335.
Raymond,
L.A.,
1984. Classification
of
melanges.
In
Raymond,
L.A.
(ed.).
Melanges: Their Nature, Origin
and
Signifieanee, Geological
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America, Special Paper,
198, pp. 7-20.
Silver,
E., and
Beutner,
E., 1980.
Melanges—Penrose Conference
Report. Geology,
8:
32-34.
Steen,
0., and
Andresen,
A., 1997.
Deformational structures
associated with gravitational block gliding: examples from
sedimentary olistoliths
in the
Kalvag Melange, western Norway.
Ameriean Journatof Seienee, 297: 56-97.
Swarbrick,
R.E., and
Naylor,
M.A., 1980. The
Kathikas melange,
southwest Cyprus: Late Cretaceous submarine debris fiows. Sedi-
mentology,
27:
63-78.
Waldron, J.W.F., Turner,
D., and
Stevens,
K.M., 1988.
Stratal
disruption
and
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861-873.
Cross-references
Debris Flow
Deformation
of
Sediments
Gravity-Driven Mass Flows
Mass Movement
MICRITIZATION
Bathurst (1966) coined
the
term "micritization"
in
reference
to
a process
of
alteration
of
original skeletal grain fabric
to a
cryptocrystalline texture
by
repeated algal microborings
and
subsequent filling
of the
microborings
by
micritic precipitates.
Bathurst
was
simply applying Folk's (1959) term
for
crypto-
crystalline carbonate (<4nm) mierite—a contraction
of the
words microcrystailine calcite—to describe
a
process
of
textural diminution
in
carbonate grains.
In
fact. Wolf (1965)
used
the
term "grain-diminution"
to
describe
the
early
alteration
of
crustose coralline algal cellular skeletons
to
mierite. Bathurst, however,
did not
accept recrystallizated
mierite
in his
definition
of
"micritization", which
he
wanted
to
restrict
to
referring
to the
process
of
microboring
and
subsequent precipitation
of
mierite infill. Bathurst's idea
became very popular (e.g., Lloyd,
1971;
Gunatilaka,
1976;
Kobluk
and
Risk, 1977). Subsequently, Friedman (1985),
pointed
out
that Folk (1959, 1962) introduced
the
term mierite
to describe microcrystailine ooze matrix,
not
textural altera-
tion.
He
felt that Bathurst
and
others were misusing Folk's
terminology.
Despite
the
concerns about
the
original intentions
of
authors when they introduced
the
terms mierite
and
micritiza-
tion, Alexandersson (1972) indicated that there
was a
need
for
the broadest
use of
both terms.
He
proposed that
the
term
mierite should
no
longer have
"any
mineralogical implica-
tions"
(p.206)
and it
should
be
used simply
as a
textural term.
Alexandersson further suggested that micritization should
refer
to an
alteration
"of a
pre-existing fabric
to
mierite" with
"no genetic implications" (p.206).
To him,
micritization
MICKlTIZATiON 437
included all proeesses involving the destruction of an ordered
crystallographic fabric to a disordered mieritie texture, with
either a decrease or increase of crystal size. Most reeent studies
of the alteration of carbonate grains have accepted Ale.\an-
dersson's broad defuiitions (e.g.. Land and Moore. 1980; Reid
elul.. 1992: Reid and Macintyre. 1998).
As mcniioncd earlier. Bathurst (1966) suggested that the
process responsible for micritization of carbonate grains was
algal mieroboring and subsequent precipitation of mierite in
open borings. Earlier studies based on light microscopy had
concluded that the cryptocrystalline textures in many shallow-
water carbonate sediments were the result of reerystallization
(e.g., Illing. 1954; Purdy. 1963, 1968; Pusey, 1964; Winland.
1969).
However, for almost two decades after Bathurst's initial
report, shallow-water marine carbonates were generally
thought to be stable, with micritic textural alterations formed
by boring and infilling.
More recently, porc-watcr chemical studies have indicated
that extensive alteration of carbonate sediments is taking place
in shallow tropical waters (e.g.. Morse eta!,. 1985; Walter and
Burton. 1990; Rude and Aller. 1991: Walter etal.. 1993;
Patterson and Walter, 1994). In addition a series of detailed
petrographie and mineralogical sttidies have illustrated that
micriti/ation by recrystallization is widespread in carbonate
grains fotind on the seafloor of tropical shallow waters (Reid
etal.. 1992; Reid and Macintyre. 1998). Macinytre and Reid
also discovered that the original skeletal needles of the green
alga Hatitm-ilaincrassata (Maeintyre and Reid. 1995) and the
porcelaneous foraminifera Archaiasati^ntlatis (Macintyre and
Reid, 1998) are recrystallizing to titiiiiiiiierlte (crystals <1 nm
after Folk, 1974) while these organisms are still alive
(Figure Ml 1(A),(B)). In vivo micritization of carbonate
skeletons is a textural alteration only and does not involve
changes in mineralogy. In both of these studies the micritiza-
tion process uas thought to be related to the breakdown of the
skeletal needles into their original polycrystalline aggregates or
domains, which were deposited in organic envelopes that
controlled the needle shape. Degradation of these organic enve-
lopes and alternating conditions of precipitation and dissolu-
tion caused by reversals on CO2 partial pressures associated
with changes in photosynthesis and respiration in Halitneda
(Macintyre and Reid. 1995) and algal symbionts in the
foraminifera (Macintyre and Reid. I99S). may have resulted
in the micriti/ation of the skeletons of these living organisms.
Further studies of Haliinctla and poreelaneotis foraminifera
grains deposited on the shallow tropical sealioor indicate that
the micritization of skeletal needles continues after death, with
extensive formation of minimicritc (Reid and Maeintyre.
1998).
As the micritization process continues, the anhedral
equant minimicrite tends to increase in size to form subhedral
blocky crystals of mierite (Figure MI2). This continuing
micritization of carbonate sediment grains can involve miner-
alogical changes and "(ogether with concomitant cavity filling
results in the gradual obliteration of skeletal elements and the
formation of micritized grains wilh little evidence of original
skeletal structure" (Reid and Macintyre. 1998, p.945). The
processes responsible for this mieritization of carbonate
sediment grains may include organic alteration of pore-water
chemistry., or reduction of high free stirface energies associated
with the increase in crystal size.
The role of microborers in the micritization process of
sediment grains was re-introduced when lightly etched
polished sections of mieritized grains examined using scanning
electron microscope (SEM) revealed the presence of multi-
cyclic boring and concurrent filling of bore holes (Macintyre
er al., 2000; Reid and Macintyre. 2000). The endolithic
cyanobacterium responsible for this boring and infilling
activity is Solentia sp. This microborer docs not limit its
aetivity to the edge of grains but eventually reuorks the entire
grain with minimal damage the grain's outer walls
(Figure MI3(A)). The infilling cement consists of fibrous
minimicrite crystals, whieh form banded patterns extending
across the 5 lO)jm diameter boreholes (Figure MI3(B))
(Macintyre
('/(//..
2000). With the lack of open boreholes and
the well preserved grain boundaries, it is very diffieult in SEM
and light petrographic microscope studies to distinguish bet-
ween concurrent infillings of mieroboHngs and recrystallization
Figure Mil Original skelefdl needles being micritized to minimfcrile
in
livin^^
organisms. lAl The green jiga Hiilimech incr^is^tn. (Bl The
|)(ir(cll.ine()us foraminiler.i Arch.ii.is n
Figure Ml2 Suhhedral micrilc in an area (if minimi( rite in
<in Arc
h.
sediment grain. Great Bahama Bank, Bahamas.
438
.MICRITIZATION
(A)
Figure
M13
Scanning electron microscopL' photomicrographs
of
etched ihin sections showing grains micritized
by the
cndulrthic
cyanobaclerium Solenlij
sp. (A)
Grain almost completely micritized
by ixiring
and
infilling action
ot
Solentia
sp.
Note that ihcre
is
very
little damaRG
to the
origin,il grain boundaries.
iB)
Detail
of
cyanob.ictcri.il mirrobnrings showing banded minimicrile infillings.
in the formation of micritized carbonate grains (Reid and
Macintyre, 2000).
Summary
(1) Micritization of carbonate sediment grains in shallow-
water tropical seas is widespread forming micriiic grains,
which commonly show no evidence of original structure.
(2) Micritization of carbonate skeletons can be initiated when
the organism is still alive.
(3) The breakdown of original skeletal needles to form
minimicritc in living organisms likely involves both the
destruction of the organic envelopes, in which the needles
are originally formed and changing internal chemical
conditions. No mineralogicai changes of the skeleton
occur during this in vivo micritization.
(4) The micritization of skeletal grains continues after
skeletons are deposited on the seafloor. Postmortem
mieritization involves the formation of anhedral mini-
mierite and subhedral mierite and may include changes in
mineralogy.
(5) Driving forces for the micritization of sediment grains may
include biologieal alteration of pore-water chemistry and
inorganic reduction in free energy of crystal surfaces.
{()} Microboring activity can also be an important factor in the
micritization of carbonate grains. This can involve (a)
formation of open boreholes and later precipitation of
infilling cement, as described by Bathurst. or (b) con-
current infilling with little evidence of open boreholes. In
the latter ease, micritization by microboring is difTicult to
distinguish from mieritization by recrystallization.
Ian G.Macintyre and R. Pamela Reid
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J.E.. 1992.
Micritized skeletal
grains
in
northern Belize lagoon:
a
major source
of
Mg-calcite
mud. Journal of .Sedimentary Petrology. 62: 145-156.
Reid. R.P..
and
Macintyre,
I.G.,
1998. Carbonate reerystallization
in
shallow marine environments:
a
widespread diagenetic proeess
forming micritized grains. Journal
of
Sedinwntarv Research.
68:
928
946.
Reid. R.P..
and
Macintyre.
I.G..
2t)()0. Microboring verses reerystaffi-
zation: further insight into
the
micrili/ation process. Jottrnal
of
.Sedimentary Research.
70: 24 28.
Rude.
P.O.. and
Aller.
R.C..
I99L Fluorine mobility during early
diagenesis
of
earbonate sediment:
an
indieator
of
mineral
transformation. Geochinucaet Cosmoehimiea
.Acta.
55:
249f -251)9.
Walter, L.M..
and
Burton.
E.A.,
1990. Dissolution
of
recent piatform
carbonate sediments
in
marine pore fluids. Ameriean Journal
of
.Science. 290: 601-643.
Walter,
L.M..
Bischof.
S.A.,
Patterson. W.P..
and
Lyons. T.W.,
f993.
Dissolution
and
reerystallization
in
modern shelf carbonates:
evidenee from pore water
and
solid phase chemistry. Philosophical
Transactions
oj
the
Royal Soeiety (Londom.
344:
27-36.
Whinland.
H.D., 1969.
Stability
of
carbonate polymorphs
in
warm,
shallow seawater. Journal
of
Sedimentarv Petroh>i;y.
39:
1579-1587.
Wolf,
K.H., 1965.
"Grain-diminution"
of
algaf colonies
to
mierite.
Journal ol Sedimentary Petrology. 35:
420 427.
MICROBIALLY fNODCED SFDIMFNTARY STRLiCTURFS
MICROBIALLY rNDUCED SEDIMENTARY
STRUCTURES
Coastal sediniLMitary systems ol' moderate climate zones are
governed tiot only by physical dynamics like erosive tidal
flushing or reworking by wave action, but also by biotic factors
like sediment stabilizing or accumulating by epibenthic
bacterial commiinilies. The shallow-tmirinc environments are
coloni/ed by benthic microorganisms, amongst those photo-
iuitotroph cyanobacteria are an abundant gn)up (ecology:
Whitton and Potts. 20()()). With the aid of their adhesive and
slimy "extracellular polymeric substances"" (EPS. see for
introduction Decho. I99U), epibenthic species attach to
mineral grains, and they can even form thick, tissue-like,
organic layers covering extensive areas of the sedimentary
surface. Such bacterial "carpels'" are termed "mierobial mats""
(definition of term see Krumbein. 1983: overview also in
Gerdes and Krumbein. 1987; Cohen and Rosenberg, 1989; Stal
and Caumette, 1994; Stolz. 2(K)0).
Colonizing at the interf^ice between sediment and water,
cyanobacteria are able to react to sediment-affect ing physical
agents in difTerent ways.
Responsive behavior of epibenthic cyanobacteria to
physical sedimentary dynamics
[during periods of no or low rates of sediment deposition, thick
mierobial mat layers develop. By growth, they smoothen out
or level the original tidal surface morphology, and form flat
bedding surfaces (compare Noffke and Krumbein, 1999). In
close up on vertical sections through thick mat layers, the
growing biomass drags upward mineral grains from the
sedimentary substrate. The grains are separated from each
other, atid become oriented with their long-axes perpendicu-
larly to the loadini! pressure (mierobial grain separation:
Noffke £'/«/.. 1997).'
Slow moving, suspension-rich bottom currents induce a
vertical orientation of cyanobacterial filaments that trigger
fall-out of sediment by "baffling, trapping, and binding"*
(Black, 193.1). Over time, the sediment agglutinating mierobial
community grows upward (Gerdes cl al., 1991). In vertical
section, this produces laminated intrasedinientary patterns.
In depositional areas of higher hydrodynamic energies, only
thin mierobial mats ean grow. They cover surface structures,
like ripple marks, without altering the original shape of the
relief.
Buried depositional surfaces are "imprinted"" by the
thin, former mat layers (Gerdes ei al.. 2000. and references
therein).
Figure M14 Microbi.illy induced sedimentary slruclures in physical depositional areas of moderate tlimate zones—examples. (Al Sedimentary
smoothened by mierobial mat cover; (B) "Oriented grains" within mierobial mat layer; thin section; (C) "Blol.iminiie"; thin section;
"Miiltidiretted rippie marks".
440 MICROBIALLY INDUCED SEDIMENTARY STRUCTURFS
Only when subjected to strong bottom currents, mat-secured
sedimentary surfaces are eroded, because the dense and
coherent mierobial mats seal efTectivety their sitbstrate and
increase resistance against erosion. Additionally, cyanobacterial
layers prohibit exchange of gas between sediment and water.
These effects are known as "biostabilization"" (Paterson. 1994;
Krumbein eUtl.. 1994; Paterson. 1997; Noffke
ctaf.
2001).
Microbially induced sedimentary structures
The responsive behavior of the mat-constructing micro-
epibenthos generates a great variety of characteristic sedi-
mentary structures. The structures differ significantly IVom
biogene stromatolites we are familiar frotn chetnical litho-
logies, and mineral precipitation plays no role in their
formation.
Flat bedding planes originated by immense mat growth are
termed -'leveled surfaces""^(Nofl"ke. 2000; NoflTce ctai.. 2001).
Figure M I4(A). In the fossil record, they can be recognized as
"wrinkle structures" (Hagadorn and Bottjer, 1997), where they
document pauses in sedimentation (Noffke, in press). Char-
acteristically, thin-sections ihrough leveled surfaces reveal
"tnat layer-bound oriented grains'", that is mineral particles
floating indepetidently from each other in the organic tnatrix
like a chain of pearls. Figure M14(B). They document former
mierobial grain separation, and pressure-related orientation
(Noffke
('/(//..
1997).
Laminated intrasedimentary pattern caused by baffling.
trapping, and binding arc well-known as "biolaminites"". or,
after consolidation, as "planar stromatolites"" (discussion of
tenns by Krumbein. 1983). Figure M14(C).
Typical surface morphologies that are shaped by biostabil-
ization interfering with erosion are "erosional remnants and
pockets"" (NoflTie. 1999; fossil example; MacKeti/ie. 1968). or
"multidirected ripple marks'" (Noffke, 1998; fossil counter-
parts:
Pllueger, 1999). Figure M14(D). Internal features are
"fenestrae fabrics" (Gerdes etal.. 2000). "Sinoidal structures"",
visible in vertical sections through the sediment, itnprint
former ripple mark valleys (Gerdes etal.. 2000; Noffke etat..
2001;
fossil example: Nofi^ke. 2000).
Because the structures are formed syndepositionally by
biotic-physical interference, they were grouped as an own
category "tnicrobtatly induced sedimentary structures"" into
the Classiflcation of Primary Sedimentary Structures (Noffke
etal.. 2001).
Nora Noffke
Bibliography
Black.
M., 193.1 The
afgal scdimcrtts
of
Andros Islands. Bahamas.
Royal Society oj London Phiiosophieal
Transaetion.\.
Scries.
B, 222:
165-192.
Cohen.
Y.. and
Rosenberg,
E.. 1989.
Microbiai Mats. Physiological
Ecology
oj
Benthic Microhial Communities. Wasfiingtoii,
DC:
American Society
ol'
Microbiologists. 494pp.
Decho.
A.W.. 1990.
Microbiai exopolymer secretions
in
ocean
environments: their role(s)
in
food webs atid tnarine processes.
Oceanographic Marine Biology Annual Review.
28:
7."^-1
Sy.
Gerdes.
G..
atid Krumbein,
W.E., 1987.
Biolaminated Deposits,
Springer. 1X3
p.
Gerdes,
G.,
Krumbein,
W.E.. and
Rdncck.
H.F.. 1991.
Biolaniina-
tions eeofogical versus depositional dynamies.
In
Einsele,
G..
Ricken.
W., ;ind
Seilachcr.
A.
(eds.).
Cycles
and
Events
in Stratigr(t-
phy. Springer-Verlag,
pp. 592 607.
Gerdes.
G..
Klenke, Tli.,
and
Noffke. N.. 2000. Microbiai signatures
in
peritidal silicitlastic sediments;
a
eataloguc. Sedimentology.
47:
279-308.
Hagadorn, J.W..
and
Boltjer. D.. 1997. Wrinkle structures: microbially
mediated sedimentary structures common
in
subtidal siliciclastic
settings
at the
Proterozoic Phanerozoic trtinsilion. Geoloi'v.
25:
1047-1050.
Krumbein.
W.E,. 1983.
Stromatolites—The challenge
of a
term
in
space
and
lime. Precambrian Research. 20:
493-531.
Krumbein. W.L., Paicrson. D.M..
and
Sial.
L.J..
1994. Bio.stahilization
of Sediments.
BIS,
University
of
Oldenburg, 526pp.
MacKen^^ic. D.B.. 1968. Sedimentary features
of
.Alameda A\enue
cut.
Denver. Colorado. Mintntain Geologist.
5: 3
-13.
Non"ke.
N., 1998.
Multidirected ripple marks rising from biological
and sedimentological processes
in
modern lower supratidiil
deposits (Mellum Island, southern North
Sea).
Geology.
26:
879
882.
Nol'fke.
N., 1999.
Erosional remnants
and
pockets evolving from
biotic-physieal interaetions
in a
Recent lower supratidal environ-
ment. Sedimentary Geology. 123:
175 181.
Noffke.
N..
2000. Extensive microbiai mats atid their influences
on the
erosional
and
depositionai dynamics
of a
siliciclaslic cold water
environment (Lower Arenigian. Motitagne Noire. France). Sedi-
tnenlary Geology. 136: 207-215.
Nt^ffkc.
N..
2003.
Epibenlhic cyanobacterial conitnunities
in the
conte.xt
of
sedimentary processes within siliciclastic depositional
systems (present
and
past).
In
Patterson.
D..
Zavarzin,
G., and
Krumbein.
W.E.
(eds.). Biojilms Through Spaee
and
Time. Kluwer
Academic Publishers
(in
press).
Noffke,
N.. and
Krumbein.
W.E.. 1999. A
quantitative approaeh
to
sedimentary surface structures contoured
by the
interplay
of
tnicrobial colonization
and
physical dynamies. Sedimentology.
46:
4!7
426.
NofllL-.
N..
Gerdes.
G..
Klenke.
Th.. and
Krumbein,
W.E.. 1997. A
microscopic sedimentary sueeession indicating
the
presetice
of
microbiai mats
in
silieielastic tidal fiats. Sedimentary Geology.
110:
16.
Noffke,
N.,
Gerdes,
G.,
Klenke.
Th.. and
Krumbein.
W.E.. 2001.
Mierobially indueed sedimentary strnciurcs—a
new
category
within
the
classification
of
primary sedimentary struetures. .(our-
iuili>l
.Sedimentary Research. 71: 649-656.
Paterson.
D.M.. 1994.
Microbiological mediation
of
sediment struc-
ture
and
behaviour,
hi
Slal.
L.J.. and
Caumette.
P.
(eds.). Micro-
hial Mats, Springer-Verlag.
pp.
97-109.
Paterson.
D.M.. 1997.
Biological tiiedialion
of
sediment erodiability:
ecological
and
physical dynamics.
In
Biirt.
N.,
Parker,
R.. and
Watts.
J.
(eds.). Cohesive Sediments. Wiley,
pp.
215-229.
Peuijohn.
E.J.. and
Potter,
P.E,, 1964.
Atlas
and
Glossary
of
Primary
Sedimentary Structures. Springer-Veriag.
Pfiueger.
F.,
1999. Maturound struetures
and
redox facies. Palaios.
14:
25
.19.
Stal,
L.J.. and
Caumette.
P.. 1994.
Microbiaf mats. Strueture,
development
and
environmental significance. NATO
AS/
Series
Ecological Seiences,
35,
Berlin, Heidelberg,
New
York: Springer
Verlag. 462pp.
Slol/:,
.I.F..
2000. Structure
of
mierobial mats
and
biofilms.
In
Riding.
R.. and
Awramik,
S.M.
(eds.). Microhial Sediments.
Springer Verlag.
pp. I 8.
Whitton.^B.A..
and
Potts.
M..
2000.
The
Ecology
of
Cyanohaeteria.
Their Diversity
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Time
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Spaee. Kltiwer Academie Publishers.
669p.
Cross-references
Algal
and
Baeterial Carbonate Sediments
Bacteria
in
Seditnents
Bedding
and
IiUeinal Structures
Biogenie Sedimentary Slruelures
Surface Forms
MILANKOVITCH CYCLES
441
MILANKOVITCH CYCLES
Variations
in the
Farth's orbital parameters eause quasi-
periodic.
10 -lO''
year scale changes
to
occur
in ihe
incoming
solar radiation,
or
insolation. These insolation changes
are
commonly known
as
Milattkoviieh cvcles. after
the
Yugoslav
malhenialician
who
first described
the
cycles (Milankovitch,
IWl).
Milankovitch cycles have been linked
to
large-scale
climate cycles
and, in
turn,
to
climate-mediated sedimentary
cycles (e.g.. Berger tV///..
1^84;
Einsele (7<//.. 1991; Shackleton
ctai. 1999).
Earth's orbital parameters
The Earth's rotating axis
of
figure
is
tilted with respect
to the
Sun. Moon
and the
other planets. Consequently,
the
axis
precesses mainly
in
response
to the
luni-solar gravitational
attraction
on the
Earth's equatorial bulge,
at a
rate governed
primarily
by the
Earth's shape, rotation rate,
and
Earth Moon
distance.
In
addition,
the
axial precession experiences gravita-
liona! perturbations from
the
motions
of the
other planets
acting
on the
Earth's orbit (e.g.. Laskar
19H8.
Berger
and
Loulre. 1990). Some perturbations modulate
the
distance
and
timing
of the
Earth's closest annual approach
to the Sun
(orbital eccentricity
and
longitude
of
perihelion), which
together affect
the
year-to-year duration
and
intensity
of
seasonal insolation,
or
clit)iaticprecession. alsoealled preeession
parameter, precession index,
or
precession eccentricity
syn-
drome. Variations
in the
Earth's orbitaleccetitrieity. generated
by interactions among these perturbations, give rise
to
small
changes
in the
Earth's total annual insolation. Other
perturbations cause
the
Earth's orbital plane
to tip and
precess
(orbital inclination
and
longitude
of the
ascending node),
thereby modulating
the
Earth's axial tilt angle,
or
ohliquity.
relative
to the Sun. and
consequently,
the
Earth's laiiludinal
distribution
of
insolation.
The evolution
of the
Earth's orbital parameters over
the
past lOmyr
is
shown
in
Figure
MI5. The
parameters exhibit
quasi-periodic behavior with many closely spaced frequencies
that
are
traceable
to
modulating effects from
the
planetary
perturbations discussed above
(see
also Laskar,
1988.
1990).
The relatively short time spans
(<10
million years) covered
by
most stratigraphic seqtienccs will usually involve analysis that
averages over some
of
the more closely spaced modes. Current
astrodynamical theory provides
an
accurate ephemeris back
to
cu.
16 Ma;
for
times prior
to
this,
the
theory
can be
used only
in limited ways
and
then with extreme caution (Laskar, 1999).
Geophysical phenomena related
to the
Earth's tidal dissipa-
tion
may
have played
a
significant role
in the
long-term
evolution
of the
orbital parameters. Specifically,
the
faster
rotation rates, closer Earth Moon distances,
and
higher
ellipticities predicted
for
remote geologic ages would have
involved faster preeession rates, hence shorter periodicities
for
the climatit: precession
and
obliquity (e.g., Berger ctai.. 1992).
On
the
other hand, some
of
the orbital eccentricity modes,
for
example, the 404 kyr cycle,
are
thought
to
have remained stable
over the past 200 Ma. raising the possibility that they can be used
as high-precision geochronometers (Shackleton etal.. 1999).
Earth's orbitally forced insolation
The insolation received
at a
specific terrestrial horizontal
surface
is
determined
by the
Earth
Sun
distance
and
elevation
of
the Sun in the sky.
which together depend upon
geo-
graphical latitude, time
of
day
and
position
of
the Earth along
its annual orbit. From year
to
year,
the
orbital parameters
alter
the
Earth
Sun
distanee
and
solar elevation angle
on any
given calendar
day. The
relative eontributions
of the
para-
meters that force
the
insolation depend upon latitude
and
effective time(s)
of
forcing. This
was
first recognized
by
Milankovitch (1941).
who
showed that summer-time "caloric
insolation"
in the
tropics
is
driven tnainly
by
preeession.
but
6
8 Ma
0.015
0.025
l/kyr
0.035
l/kyr
02-
01
0
24.
23
IS
\
.67
22
•4
.37
23
19
12
IS
10
1
.96
16
,47
F
0.03 0.04
0.05 0.06 l/kyr
Figure
Ml5
Earth's orbital |),ir,jn)elt'rs, ()M.3
to
10Ma, attofdinji
to the
LA90 model (Paillard
cl JI.
19961.
(A)
eccenlricity stTios;
(Bl
eccentricity
spci irum;
(Ci
obliquity series:
ID)
oblic]uity spectrum;
lEl
climatic prt'ct'ssion series;
(F)
tlimatit" precession spectrum. Labels inciicate periodicity
in
kyr ot
significant harmonic components.
442
MILANKOVITCH CYCLES
poleward mainly by obliquity. In fact, there are an infinite
number of ways to compute orbitally forced insolation. For
example, in the illuminated latitudes of Northern summer
solstice, noon-time instantaneous insolation is characterized
intcrannually by dominant precession forcing (Figure MI6(A)).
In contrast, insolation integrated over all daylight hours
has increased obliquity forcing toward the North Pole
(Figure M16(B)), and insolation Integrated over rnidday solar
elevations only results in dominant obliquity forcing every-
where (Figure M16(C)). These and other insolation calcula-
tions for equinoctial times and over other time intervals are
discussed in Berger eral, (1993). How the orbital parameters
are ultimately captured in the sedimentary record depends on
what form of orbitatly forced insolation drives the climatic
variables that control sedimentation.
Milankovitch cycles in stratigraphy
Evidence for orbital forcing in the stratigraphic record is found
in the geochemistry and lithology of sediment in both marine
and continental deposits, in Milankovitch studies, the most
intensely researched sedimentary parameter is Ihe oxygen
isotope ratio (i:>"*O) in foraminifera. which is thought to track
global ice volume and. to a lesser extent, ocean temperature
and salinity. The Plio-Pleistocene benthic ()"^O record in
sediments cored from the tropical eastern Atlantic
seafloor shows predominant obliquity forcing that would be
expected of high-latitude insolation-forced glaeiation eyeles
(Fiiiure MI7(A)). Eolian dust variations in the satne sediment
(A) ^r^o
precession
S |-4000 precessiou
750 1\
obliquity
Figure M16 Global varitibility of the interannual insolation on 21
[line,
(1
Ka to
1
50
ka,
in spectral amplitude, as a function of latitude i/i.
(A) instantaneous insolation at noon (zenith angle z =
()l;
(Bl d.iily
integrated insolation; (Cl noon zenith-class (z ~
\4)-i>.
90' -(p], where
(5 is 5olar declintition on 2J |une) integrated insolation. Locations
0>9O'
S—(i experience pular winter night and receive no insolation.
Redrawn from Berger ef
JI.
(199.i, Figure 14).
are dominated instead by climate precession (Figure MI7(B)).
which is interpreted as evidence for low-latttude insolation
forcing of the monsoon over Africa. At same time, continental
loess sequences of north central China developed with an
oscillating concentration of iron-rich minerals linked to
pedogenesis, detected as a tnagnetic susceptibility signal. At
the Xifeng-1 locale, the signal exhibits a strong obliquity mode
up to ca. 600 ka, when it abruptly gains an extremely high
amplitude -- 100 kyr mode—possibly a response to the rapid
expansion of the Northern continental glaciations causing
the power spectrum to be heavily weighted in the eccentricity
band (Figure MI7(C)). These records represent only three of
many scores of Plio-Pleistocene sedimentary sequences
colleeted over the past 20 years that have yielded signals
consistent with Milankovitch forcing.
While knowledge about the orbital parameters prior to
-l6Ma is uncertain, all indications are that the parameters
were operating in a recognizable form at least as long ago as
the Triassic. For example, the Ca/Fe ratio of Late Eocene
Eccentricity Obliquity Climatic Precession
1 Periodicity
in kvr
, Plio-Pleistocene
I Benthic
6'**0
! 18"N
Plio-Pleistocene
I Aeolian dust
Pho-Pleistocene
I Magn. susc.
Late Cretaceous
I (
Magn. .sust.
JO
0.01 0.02 0.03 0.04 0.05 0.06 cvcles/kvr
Figure MM Power spectra of sedimentary [i,ir<imeters frcim marine
and continental stratigraphy. Vertical axes indicate spectral density.
(Al Plio-Plcistt)cene benthic d'"O, ODP site 659, orbitally tuned, 0 Ma
to 2.5 Ma (Tiedemann et ai., 1994|. IB) Piio-Pleistocene aeolian dust
concentration, ODP site 659, orbitally tuned, OMa to 2.5 Ma
(Tiedemann cl
sL,
19941.
IC) Magnetic susceptibility of loess at Xifeng-
1 (Kuklaef.3/., 1990), untuned, OMa to 2.6Ma, referred lo the GPTS of
Berggren et dl,
119951.
(D) Late Eocene Ca/Fe series. ODP site 1052,
orbitally tuned, 35 Ma to 38
Ma,
(Palike el ni, 2001). (E) Late
Cretaceous magnetic susceptibility series, DSDP site 516F, untuned,
-81 Ma lo82Ma, referred to the GPTS of Cande and Kent (19921
(Park et ai,
19931.
(Fl Late Triassic depth rank series, Lockatong
formation,
Newark Basin, ca.
.5
myr
long,
untuned (Olsen and Kent,
19961.
MIXED SILICICLASTIC AND CARBONATE SEDIMENTATION 443
sediments from offshore Florida tracks carbonate versus
terrigenous content, and shiows a mixed obliquity-precession
response consistent witti middle latitude insolation forcing
(Figure M17(D)). Late Cretaceous carbonate-rich sequences
from the Brazil Basin were deposited with a strong climatic
precession signature (Figure Mt7(E)). Other basinat sequences
extending back into the Early Cretaceous have also been found
to contain depositional patterns with orbital modes (e.g.,
Herbert et al., 1995). Likewise, evidence for orbital forcing
occurs in Jurassic stratigraphy (Shackteton et al., 1999).
Finally, one of the most striking examples of Milankovitch
cycles occur in the Triassic continental deposits of eastern
North America. t4ere, lacustrine, cycles occur as facies
alternating between deep perennial lake to ptaya, and have
been interpreted as flooding-drying responses to climatic
precession. The spectrum of the depth rank series constructed
from the facies logs indicates that the cycles were strongly
modulated by the orbital eccentricity (Figure Mt7(F)).
Summary
The Earth's orbital parameters dictate the amount of
insolation received at the Earth's surface. The orbitally forced
insolation that comes to affect climate change can take on
many different forms that depend on the latitude and time of
forcing of the responding climate; these are the Milankovitch
cycles. Geochemical and lithotogical variations of many
ancient cyclic sedimentary sequences have signals that are
consistent with Milankovitch cycles.
Linda Hinnov
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Cross-references
Climatic Control of Sedimentation
Cyclic Sedimentation
Isotopic Methods in Sedimentology
Neritie Carbonate Depositional Environments
MIXED SILICICLASTIC AND CARBONATE
SEDIMENTATION
Introduction
Every student of the sedimentology and stratigraphy of
carbonates early on appreciates the unique aspects of
carbonate deposition and the fundamental differences between
carbonate and siliciclastic (or clastic) deposition (e.g., Wilson,
1975;
Watker and James, 1992). The majority of carbonate
deposits forming today in tropical and sub-tropical settings
and their ancient counterparts (e.g., rimmed shelves, ramps,
etc.) require warm, shallow (within the photic zone), clear
water (low turbidity) of near normal salinity which is well
circulated by currents, waves and tidal processes to insure
proper oxygenation and nutrient levels. The requirement of
iow turbidity, clear water generally means that where clastic
sediment is supplied coevally to a carbonate environment,
deposition of the two sediment types is generally mutually
exclusive, such that the "carbonate factory" favors or is
restricted to areas free of "polluting" ciastic materials.
However, the mixing of clastic and carbonates occurs today
in several Holocene settings, including the Belize
shelf,
the
northeastern shelf of Queensland, Australia (landward of the
Great Barrier Reef), and along coastal environments within
the Persian Gulf (Wantland and Pusey, 1975; Maxwell and
Swinchatt, 1970; Purser, 1973). tn addition, the mixing and
interbedding of carbonate and clastic sediments is extremely
common throughout the stratigraphic record (Mount, 1984;
Budd and Harris, 1990; Doyle and Roberts, 1988; Lomando
and Harris, 1991; Dolan, 1989), where mixing occurs in a
444 MIXED SILICICLASTIC AND CARBONATE SEDIMENTATION
variety of environments: coastat and inner
shelf;
middle and
outer shelf (reef); slope to basin environments. Mixed deposits
occur in temperate as well as tropical shelf environments.
Late Paleozoic mixed clastic-carbonate successions have
probably received the most study of all mixed deposits. For
example Carboniferous "cyclothems" of the Midcontinent and
southwestern USA and Europe (e.g., Weller, 1930; Duff etal.,
1967;
Wilson, 1967; Rankey, 1999) provide the underpinnings
of models for cyclic stratigraphy at all scales. From these and
additional studies of mixed clastic-carbonate strata of the
Permian Basin in the New Mexico and Texas, the concept of
"cyclic and reciprocal sedimentation" (Van Siclen, t958;
Wilson, 1967; Silver and Todd, 1969; Meissner, t972) was
introduced to explain the origin of mixed clastic-carbonate
shelf-to-basin cyclic deposits. This critical concept has evolved
to provide the foundation upon which present-day "sequence
stratigraphy" is predicated (e.g., Sarg, 1988). From an
economic viewpoint, mixed deposits are significant reservoirs
of oil and gas (e.g., the Permian Basin, USA), and studies of
the high-resolution stratigraphic architecture of mixed clastic-
carbonate sequences has lead to the discipline of reservoir
characterization (e.g., Kerans and Tinker, 1997).
An insightfut approach to the problem of mixed deposits
has been set forth by Budd and Harris (1990) who recognize
basically two types of mixed deposition: (1) mixtures due to
spatial variability, where mixing occurs principally by lateral
facies mixing of coeval sedimentary environments (see also
Mount, 1984); and (2) mixtures due to temporat evolution in
sedimentation, induced by sea-level changes and/or variations
in sediment supply, causing a vertical variation in the
stratigraphic succession. Mixing can occur through a wide
range of scales, from millimeters to kilometers, respond to a
wide range of processes, and can be influenced by all orders of
eyclicity. Trends of mixing can vary along both depositional
strike and dip, ranging from coastal environments to deep
basinal settings.
Important factors that influence mixing include: (i) climate:
humid (fiuvial-deltaic clastic input onto a carbonate
shelf;
e.g.,
the Belize shelf) versus arid (eolian influx; e.g., the Persian
Gulf);
(ii) sediment supply of clastic material and source
terrain (proximal clastic marine shelf with a linear source
versus distal tectonically active, uplifted source terrain with a
fluvial, point-sourced input); (iii) transport mode (episodic
storm mixing versus lowstand-induced sediment by-pass of a
carbonate shelf); (iv) shelf width and paleobathymetry
(carbonate ramp, distally-steepened carbonate ramp, reefal
rimmed high-relief platform, etc.) (v) oceanographic factors
such as prevailing wind- and storm-driven currents, tidal
range, etc.; (vi) relative changes in sea-level which will act to
either trap clastic sediments updip on the shelf during relative
highstands or promote clastic influx onto and across a
carbonate shelf during relative lowstands (Van Siclen, 1958).
The rates and magnitudes and of such relative sea-level
changes will determine the amount and style of mixing, such
that mixed clastic-carbonate cycles and sequences will vary
markedly from "ice-house" periods in geologic history to
"greenhouse" periods.
Styles of clastic and carbonate mixing
Depositional mixing and stratigraphic partitioning occurs
at all scales and to all degrees. For instance, such mixing
may occur: (t) within a single bed whereby minor amounts of
clastic material are dispersed within a carbonate bed with
a low degree of "lithologic separation" (e.g., Cambro-
Ordovician peritidal facies of North America, Read and
Goldhammer, 1988; Goldhammer el al., 1993); (2a) within a
single heterolithic bed ("ribbon rocks") whereby carbonate
material (ooids, peloids, etc.) comprises coarser-grained
planar and ripple lamination which alternates with fine-
grained clastic material (shale; e.g.. Cowan and James, 1993,
1996);
(2b) within a single heterolithic bed ("ribbon rocks")
whereby clastic material (mature quartz sand) comprises
coarser-grained planar laminations and ripple lamination
which alternates with thin intercalations of finer-grained
carbonate mud (e.g., Demicco and Hardie, 1994); (3a) between
vertically juxtaposed beds within high-frequency cycles
(meter-scale; "fifth"- and "fourth"-order) with a high-degree
of lithologic separation (e.g., Pennsylvanian "cyclothems";
Wilson, 1975; Driese and Dott, 1984; Atchley and Loope,
1993;
Yang, 1996; Rankey etal., 1999); (3b) between vertically
juxtaposed bedsets within tow-frequency sequences (deca-
meter-scale, "third-order") with a high-degree of lithologic
separation (e.g., basinal portions of "composite sequences" of
the Permian of west Texas; Silver and Todd, 1969; Meissner,
1972;
Tinker, 1998); (4a) between vertically juxtaposed beds
within high-frequency cycles (meter-scale; "fifth"- and
"fourth"-order) with a low degree of lithologic separation,
such that vertical contacts between elastics (typically) shale
and carbonates are gradational within a cycle (e.g., Grotzinger,
1986;
Osleger, 1991; Cowan and James, 1993); (4b) between
vertically juxtaposed beds within low-frequency sequences
(decameter-scale, "third-order") with a low degree of lithologic
separation, such that vertical, as well as lateral contacts
between elastics (typically shale) and carbonates are grada-
tional within a cycle (e.g., Grotzinger, 1986; Chow and James,
1987;
Sonnenfeld and Cross, 1993); (5) laterally within a single
bed as time-equivalent facies changes (spatial mixing; Houlik,
1973;
Shinn, 1973; Mount, 1984; Brett and Baird, 1985;
Grotzinger, 1986; Calvet and Tucker, 1988; Adams and
Grotzinger, 1996; Cowan and James, 1996); and (6) as
regionally extensive km-scale depositional systems that can
replace one another in time and space (Aitken, 1978; Byers and
Dott, 1981).
Models of clastic and carbonate mixing
Spatial variations in coeval environments
In his summary of mixed clastic-carbonate systems. Mount
(1984) highlighted four types of mixing of spatially separated
carbonate and clastic environments for shallow Holocene shelf
settings: (i) punctuated mixing—where sporadic storms and
other extreme, high-intensity periodic events transfer sediment
across highly contrasting environmental boundaries separating
elastics from carbonates; (ii) facies mixing—where sediments
are mixed along diffuse borders between contrasting facies, for
example nearshore clastic belts and offshore carbonate reefs or
ooid shoals; (iii) in situ mixing—where the carbonate fraction
consists of the authochthonous death assemblages of calcar-
eous organisms that accumulated on or within clastic
substrates, for example foram-mollusc assemblages within a
subtidal clastic
shelf;
and (iv) source mixing—where admix-
tures of carbonates into a clastic-dominated setting are
generated in response to uplift and erosion of proximal
carbonate terrains.