Middleton G.V. (Ed.) Encyclopedia of Sediments and Sedimentary Rocks

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CHLORITE IN SEDIMENTS

125

metamorphic rocks. In stimmary, the conditions necessary to

supply detrital chlorite to sediments in great quantity arc

probably only rarely established. Therefore, the simple

presence of ahundant chlorite in sediments is more often than

not an indication of its diagenetic fonnation, or at least that

such an origin should be seriously investigated. Often it is

assumed, quite correctly, thai the presence of the Mb polytype

in un-metamorphosed sediments indicates a detrital origin for

chlorite. Catitioti must be applied, however, since the polytypic

fortTts of chlorite arc not entirely indepetident of compositional

controls, for example Mg-rich chlorites formed at diagenetic

temperatures, such as those in the evaporiie/doloniite associa-

tion described below, often crystallize as the lib polytype

(Hillier, \99i: Hillier t7(//., 19%).

Odinitc-borthieritie-chamosite shallow

marine association

The Fe-rich variety of chlorite known as chamosite is named

after its occurrence in an oolitic ironstone, near the town of

Chamoson in the Krench speaking part of the Swiss Alps. The

ironstone at Chatnoson has been affected by low-grade

mctamorphistn atid sitnilar ironstones frotn other localities

not so affected arc characterized itistead by ihe presence of the

0.7 nm (7A) serpentine mineral berthierine (e.g.. Hughes.

1989).

Oolitic ironstones arc deposited in shallow marine

etivit'ontncnts and increasingly the presence of Fe-rich chlorite

(chamosite). often as a clearly authigenic isopachous pore-

lining (Figure C3I), has been recognized in sandstones

deposited in similar shallow marine settings. Indeed, iron

ooids are frequently present in small quantities in sueh sand-

stones (bhrenberg, 1993). This evidence is enough to suggest

that the gcochctnical conditions thai lead to chloi ite fortnation

tnust be similar in both cases. In modern environments

examples of berthierine formation in shallow trtarine tropical

deltaic environtnenis have been known for some time

(Porrenga, 1967). although they have more recently been

rcinicrprctcd and the 0.7ntn (7.4) mineral renamed odinitc

(Odin. 19St^: Bailey, 1988b). Reintcrptctation rests largely on

Figure C31 DijgonL'tic pure-lining thiorite from a shallow

sandstotie (Dogger heta Haupsandstein, soe Hillier, 1994).

the basis that tnuch of iron present is ferric rather than ferrous

as in berthierine/'i^/.vc. Pore-lining ehatnosites frotn sandstones

often show evidence for interstratification with a 0.7 nm (7A)

mineral, probably berthierine. Thus it has been suggested that

odinite and/or berthierine tnay represent the precursors of

diagenetic chamosite, since all appear to share a common

depositiotial/geochemical emironmetit characterized by a high

supply of colloidal iron frotn a tropically weathered landscape.

Taken together one tnay thus define an odinite-berthierine

chamosite association. Although many questions have yet to

be answered coneeniing ihe precise genetic links amongst these

minerals and the precise conditions lhat promote their

formation, the alteration of syn-scdimentary Fe-rich 0.7 nm

(7A) minerals to Fe-rich chlorite as a result of diagenetic

processes is a comtnon hypothesis.

Diagenetic alteration of chlorites iVotn shallow marine

sandstones has recently been studied in some detail for (Hillier.

1994) including the well-known Tuscaloosa sandstone of the

US Gulf Coast (Ryan and Reynolds. 1996). Thus polytypes are

known to change from Ib to

lb with increasing temperature

of alteration, with the la polytype an intermediate step.

Additionally. ().7nm (7A) layers are eventually, if erratically,

lost and crystal size may increase with temperature.

Mg-rich chlorites and the evaporitc/dolomite

association

It has long been known thai Mg-rich chlorites and corrensite,

{RI trioetahedral mixed-layer chlorite(51))-smectite. sec

mixed-

layer clays) are comtnon in tnany dolotnite and/or evaporite

bearing rocks. Early work in both Europe and North America

was sutnmarized by Millot (1964) and later by Hauff (19f^l)

who focused on occurrences of corrensitc. F.xamplcs from

Permo-Triassic rocks are well-known almost worldwide and it

is probably fair to say that some of them may represent the

most chlorite-rich sediments known. Generally, there is a

consensus that occurrences of chlorite and corrensite in this

category represent authigenic minerals, bul ihere are still two

schools of thought concerning the timing o\' the authigenesis.

One hypothesis is that these minerals are syn-seditncntary. the

other that they fortn as a result of burial diagenesis. Hillier

(1993) summarized the arguments in favor of the role of burial

diagenesis, such as the absence of any modern syn-seditnenlary

analogues. He also proposed that diagetietic reactions amongst

dioctahedral clay minerals (the most comtnon detrital elay

mineral assemblage) and dolomite, may be an itnportant but

hitherto overlooked pathway for the fortnation o\' Mg-rich

chlorites and corrensitc in rocks that fall into this category. A

further pathway is by the diagenetic alteration of precursor

Mg-rich stncctitcs that may be altered to corrensite and

subsequently to chlorite with increasing diagenesis. This is

analogous to the reaction pathways that have now been well

docutncntcd from andesitic and basaltic volcanic rocks

affeeted by low grade metamorphism (Frey and Robinson.

1999).

The Mg-rich chlorites of the evaporite/dolomite

association are rarely restricted to one lithology atid tnay he

as comtnon in sandstones (Hillier

ciaf.

1996) as they are in

mudstones.

Aluminous dioctahedral chlorites red beds association

In most types of sediments and sedimentary rocks dioctahedral

chlorites are rarer than trioetahedral types. In some red beds.

126

CHIOKITF IN SFDIMENTS

however, this trend may be reversed. In fact the consistent and

pervasive occurrence of dioctahedral chlorites in some red

beds,

as well as the allied mixed-layer dioctahedral chlorite-

smectite mineral called tosuditc. indicates a clear association

which hitherto has not generally been recognized. Examples

include the Triassic ofthe Colorado Plateau (Schiiltz. 1963).

some Permo-Triassic sandstones of Germany e,g,. Kiilkc

(1969).

and parts of the Old Red Sandstone of the United

Kingdom (Garvie, 1992). One way to recognize dioetahedral

chlorite is by an intense 003 peak in an X-ray powder

diffraction pattern; this feature is directly is related to the

aluminous composition. Indeed, other features such as

abundant kaoiinite or dickite in association with dioclahedral

ehlorites indicates quite clearly that the type ol' reds beds in

which they occur have alnminons diagenetic mineral assem-

blages. In red beds this might be related to widespread leaching

and flushing by meteoric water during early diagenesis, with

consequent alteration of feldspars and micas, Dioctahedral

chlorites are also common in hydrothermal alterations of

aluminous rocks and in sedimentary basins they can occur as a

result of loealized hydrothermal activity associated with fault

systems. However, the pervasive nature of their distribution in

some red beds argues against a localized hydrothermal origin.

Pathways for their diagenetic formation have not yet been

adequately established but might involve either a kaolin

precursor, or perhaps a sequence beginning with aluminous

(beidellitic) smectite that evolves to dioctahedral chlorite via

tosudite as an intermediate step. Such a sequence would be

entirely analogous to the well-documented saponite to

corrensile to trioctahedral chlorite sequence in Mg-rich

environments. Indeed, tosudile may be easily misidentllied as

corrensite when it occurs in red beds simply beeause the

association of eorrensite with red beds is more widely known.

Any clay mineral identified as "corrcnsiic" but associated wiih

abundant kaolinite (aluminous system) in a red bed is certainly

worthy of close scrutiny.

Lithic and volcaniclastic sediment association

One of the well-known pathways by which chlorite may form

diagenetically is by the alteration of detrital ferromagnesian

minerals, for example biotite is commonly altered to chlorite

during diagenesis (Jiang and Peacor. 1994), Clearly, this

process may operate in any sediment where detrital ferro-

magnesium minerals arc present, but is particularly common in

sediments that are inineralogically immature. Volcaniclastic

sediments also are prone to the diagenetic formation of

chlorite, often with associated formation of corrensite (Almon

etal., 1976: Stalder, 1979), In volcaniclastic sandstones there

arc undoubtedly many parallels with the low temperature

formation of corrensite and ehlorite in altered voleanic roeks

(Frey and Robinson. 1999).

Shale diagenesis/low grade-metjmorphism

association

The smectite to itlite reaction, or more generally illilization. is

one of the most important reaetions that occur during

diagenesis. Although there are different ways to write the

overall reaction, it is generally agreed that magnesium and

especially iron are liberated and become available lor chlorite

formation. Thus in the classic study of Hower ctat. (1976) of

shales from the Gulf Coast chlorite was first observed at

2500ni, increased in abundance to 3700m and thereafter

remained constant to the greatest depth examined at 5500m,

Hower etal. (1976) interpreted all ehlorite they observed as

diagenetie and later TEM work by Ahn and Peacor (I9S5)

supported the hypothesis that chlorite was formed diageneti-

cally in Gulf Coast shales as a result of the itlitization of

smectite. Recalculation of the data of Howei" et at. (1976)

indicates the formation of between 5-7 percent chlorite by

weight in the shales they studied. Formation of chlorite as a

consequence of the processes of illitization is probably

common place in mudrocks. Furthermore it has been

suggested that this process may account for the observation

that older rocks (c,g,. Paleozoic shales) tend to have higher

chlorite eontents than younger rocks (e.g.. Weaver 1989).

simply because they are more likely (longer thermal

histories) to have been affected by advanced diagenesis. This

seems a logical explanation since sedimentary rocks that

have entered the realm of low-grade metamorphism are

typified by a phyllosilieate assemblage of white mica and

chlorite, culminating in the greenschist facies. It is unhkely.

however, that lale diagenetic chlorite formation in mudrocks is

entirely a consequence of illitization processes. More likely

there is a spectrum of reactants that may be involved in

chlorite forEiiation depending on mudrock composition

(Burton etal., 1987). This spectrum may encompass many of

the processes that characterize any of the different associa-

tions outlined above thereby resulting in Fe-rieh, Mg-rich or

Al-rich chlorite.

Stephen Hillier

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slLidy of diaijcnclic chlorile in Gulf-Coast argillaceous sediments,

Chn-sdiutChiv Miiwrah. 33: 228-236,

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Biiin. D.C.. 1977, Wealhering of chlorite minerals in some Scottish

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JoiiriiiiloJ Soil Science. 28: 144 164,

Barnhisel. R,I,. and Bertsch, P,M,. 1989, Chlorites and hydroxy-

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S,B, (eds,) Miiicruls in Soil Envirouments. Soil Science Society of

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kaolinite and ehlorite in Texas Gull-Coast sediments, Clavs and

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Chamley. H,, 1989, Clav Seiiimeniology. Springer Verlag.

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CLASSIFICATION

SEDIMENTS

AND

SEDIMENTARY

K(K KS

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Hillier. S.. 1993. Origin, diagenesis. and mineralogy of chlorite

minerals in Devoniiiii laciislrine mudrocks, Orcadian basin. Scol-

htnd. a<n.uindC!iiy\Jini-nil.s. 41: 240 259.

Millier. S., 1994. Porc-liniiig chlorites in siliciclastic reservoir sand-

sloncs - elcclron-micropmbi.'. SLM iind XRD data, and implica-

tions lor thcif oiigin. Clay Miiivrah. 29: (i(i5 679.

Hillier, S.. Fallick. A.E.. and Matter. A.. 1996. Origin of pore-lining

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grain-c-oating serpentine-chlorite in Tuscaloosa formation sand-

stone. L'S Gulf Coast, .'imvricuii Mitieralo^isi. 81: 213 225.

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1147-C: Cl C71.

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the Taveyannaz Sandstone of the Swiss Alps and equivalent

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463 4S2.

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A/(>i<™/,v. 41: 260-267.

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AL-ademie Press, pp. 103-135.

Cross-references

[Jerihierinc

Clay Mineralogy

Glaueony and Verdine

liliic Ciroiip Clay Minerals

Kaolin Group Minerals

Mi.\ed-Layer Clays

Smectite (Montmorillonite) Group

Talc

Vermieulite

CLASSIFICATION OF SEDIMENTS AND

SEDIMENTARY ROCKS

Sediment.s and sediiTietitary rocks mny be divided into two

kinds,

inlrttbasinal or autochthonous and extrabasinal or

Lillochthonotis. Intrabasinal sediments and sedimentary rocks

or iiiitochthonoLis deposits are those whose particles were

derived from within the basin of deposilion. Most carbonate

sedimetits and rocks (including limestones and dolomites or

dolostoncs) were precipitated within a basin of deposition.

Terrigenous particles or sedimentary rocks belong to the

extrabasinal or allochthonous grotip and were derived from

outside the basin of deposition: examples are sandstones and

shales (Lajoie, 1979; Friedman etal.. 1992).

Intrabasinal or autochthonous deposits

Intrabasinai or autochthonous particles include various solids

that grew biochemically or chemically in the waters of the

depositional basin. These include carbonate biocrystals and

other carbonate particles, silica biocrystals. particles cotnposed

of evaporite minerals, and certain authigenic minerals, such

as glaueony (glauconite), that grew at the water/sediment

interface.

Carbonate particles, sediments and rocks

Carbonate seditnents and rocks are composed o\' carbonate

particles and cctnent. They grew as solids in the depositional

basin and include skeletal and non-skeletal calcium-carbonate

materials. Carbonate skeletal debris includes whole skeletons

of calcium carbonate-secreting organisms, as well as broken

pieces of the hard parts secreted by organisms. Ordinarily,

organisms must die to contribute their skeletal material to

sediments. However, several exceptions arc known. Ostracodes

and trilobites discard their shells during moiling and coccoliths

shed plates during life.

The clay- and silt-size particles of carbonate sediments are

collectively designated as lime nittd. Lime mud for (he most

part consists of tiny needles and platelets ofcarbonatc crystals.

Organisms secrete the tiny solids that compose lime mud. Lime

mud may form by the accumulation of tiny skeletal

components secreted by algae. During the Cretaceous pelagic

algae, known as coccoliths. were abundant enough to form an

oo/e that, when lithified. became chalk {qv).

The tninerals secreted by organisms form biocrystals, that is,

solids having the lattice properties of minerals but distinctive

shapes that are not crystal faces. In the modern marine

environment, widely varying groups of organisms produce

skeletal debris. In the past, some groups of organisms that are

now important sediment producers were insignificant, and

some once-important groups ate now minor or no longer exist

at all.

In modern carbonate sediments the two principal minerals

are calcite and aragonite. This fact was established in 1859 by

Henry Clifton Sorby (1826 1908; sec Sedinientologists). who

also showed that calcite tends to predotninate in sand-size

skeletal debris, whereas aragonite predominates in lime mud.

Although the chemical formulas of both aragonite

and calcite are the same. CaCOi, these minerals differ in

128

CLASSIFICATION OF SEDIMENTS AND SEDIMENTARY ROCKS

crystallographic arrangment. in density, in hardness along

certain crystallographic directions, in their contents of trace

elements, and in their solubilities in various fluids (see Carbo-

nate Mineralogy ami Gcochcmisty). One variety of calcite has

been termed low-magnesian calcite and the other high-

magnesian calcite. Modern skeletal carbonate particles consist

mostly of high-magncsian calcite and aragonite. whereas

ancient limestone consists o'i low-magncsion calcite. Low-

magnesian calcite has less than 8 niol percent MgCOi. A

calcite containing 8mol percent to lOmol percent MgCOi

may be considered either high or low-magnesian calcite. The

composition of the mineral calcite, which represents both

kinds,

can be given by the formula (Cai ^Mg^jCO^. where

0<x<n.30 (Bathtirsi. 1975; Sanders and Friedman, 1967;

Tucker and Wright. 1990).

Lime mud:silt- and clay-size

The silty-and clay-size components of carbonate sediments arc

colleclively designated as lime mud. Most lime mud consists of

tiny needles and platelets of carbonate crystals that are too

small to resolve uith binocular- or petrographic microscopes.

Accordingly, interpretations ofthe particles of lime mud have

been based on the notion that they shared a common origin,

namely inorganic precipitation from saline water. Studies with

the sccinning-electron microscope have shown that, among ihe

supposedly uniform constituents of lime muds, great variety

exists.

Three mechanisms for forming lime mud include:

(1) inorganic precipitation. (2) biochemical secretion, and

(3) breakup of sand-size (or larger) aragonitic skeletal debris.

Inorganic precipitation may be an important source of lime

mud. According to an estimate of Shinn etal. (1989). the

amount o\' lime mud precipitated inorganically on the Great

Bahama Bank may be much greater than that secreted by

algae. The tiny needles of aragonite precipitated inorganically

arc dispersed in the water and form drifting "clouds" of milky-

white water called whitings. Sedimentation rates from whitings

arc high, yet the whitings appear to persist for months. This

implies that, despite the losses from the falloul to the bottom,

the components ofthe whitings arc being renewed. A plausible

mechanism for such renewal is rapid precipitation in the water

column of aragonite needles (and possibly small particles of

high-niagnesian calcite). The tiny individual aragonite crystals

precipitated by whitings are indistinguishable from aragonite

biocrystal needles secreted by codiacean green algae.

Within their cells, certain eilgae secrete solid skeletal

components that consist of tiny biocrystals of aragonite.

Much lime mud forms by the accumulation of such tiny

biocrystals, which arc released onto the bottom when the

algae die.

Codiacean or green algae, especially the alga Pcnicilhis sp.

which resembles a shaving brush and hence is popularly

known as the shaving-bruch alga, produce tine biocrystals

(needles) of aragonite (<l5nm) within the sheaths of their

filaments. When the organism dies, the filaments disintegrate

and release the needles to the sea, where they accumulate as

lime mud. The modern-day rate of production of aragonite

needles by Pcnicilhis sp. alone can account tor all the mud in

the inner Florida Reef tract and one third of the mud in

northeastern Florida Bay. Two more-or-less-equally abundant

green algae (Udotea and Rhipocephatus) are likewise active

contributors of lime mud in this area. Codiacean algae arc

known in rocks as old as Ordovician, and they may have

contributed aragonite needles to ancient hmc muds.

Coralline red algae secrete skeletons composed of high-

magnesian calcite. High-magnesian calcite. probably derived

from the breakdown of red-algal skeletons, is abundant in

carbonate muds in F-lorida Bay and on the Great Bahama

Bank west of Andros Island. Red algae and scrpulid worms

living on leaves of the marine grass

Tlutias.sia

may produce as

much as or more lime mud than docs Penicilhis.

Some lime mud results from the physical breakage of sand-

size-and larger aragonitic skeletal material. Such mud can

originate by (1) decomposition of organic matter, such as

conchiolin, which binds the small hard parts together: (2)

weakening or comminution ofthe skeletal material by boring

organisms such as fungi, algae or sponges. (3) feeding activities

of predatory organisms, and (4) physical breakage by abrasion

in agitated waters.

Intraclasts

The term intraclast refers to sand-size or larger particles,

lexturally analogous to rock fragments, broken from con-

solidatcd-or hardened materials in one locality and redeposited

at another locality within the basin of deposition. "Intra"

means within, in this context within the basin of deposition,

and "clast" denotes broken. Many intraclasts are recycled

fragments of coherent sediment. Intraclasts are of various sizes

and shapes. Many are angular and their diameters may

exceed 2 mm.

Micrite

Lithtied lime mud that was deposited mechanically, and which

may form a matri.x among sand-size particles in limestone or

be the only particle in a fine-textured limestone is known as a

micrite. This term is a shorthand expression for microcryslal-

line calcium carbonate.

Pellets

Spherical- or ellipsoidal sand-size particles of calciimi carbo-

nate arc called pellets. Internally, pellets arc commonly

homogeneous. Although they are of sand size, most pellets

may be compared to tiny mudballs. and commonly consist of

aragonite. Most pellets arc formed by deposit-feeding organ-

isms that eat the mud. These organisms digest organic matter

from the mud and excrete the undigested lime mud in (he form

of fecal pellets.

Peloids

Pcloids arc particles that resemble pellets but for which no

particular origin is implied. Not all pelletlike particles arc of

fecal origin; some arc lime-mud aggregates that originated

when lime mud dried out on exposure to the atmosphere.

When lime mud is so exposed, desiccation cracks (the so-called

nitid cracks) and small chips of dried-out mud spall off. These

panicles may be larger than sand-size pellets, but they are very

fragile, and are quickly comminuted to rounded, sand-size,

pellet like particles. Many peloids are sand-size, rounded

intraclasts. Fecal pellets and pcllet-shapcd mud aggregates

commonly cannot be distinguished.

CLASSIFICATION OF SEDIMFNTS AND SPOIMFNTARY ROCKS 129

Ooids

The sand-size coated particles known as ooids derive their

name from a Greek word which means egg or egglike because

under the microscope these particles resemble the roe of fish.

Ooids usually arc spherical or elliptical.

The rims of most modern marine ooids consist of aragonite:

a few are composed of high-magnesian calcite. In most modern

aragonitic ooids the long axes of the individual aragonite

crystals ;tre tangential to the rims. This is known as concentric

tabric. In some ooids the iabric is radial; the long axes of the

aragonite crystals are normal to the rims and therefore diverge

away frotn the center o\' the ooid. Modern ooids having radial

fabric (mostly composed of high-niagncsian calcite) occur in

hypcrsaline waters, such as in the Great Salt Lake of Utah; in

Baflin Bay. otf Laguna Madre. Texas; or in thick accumula-

tions of algal mats as found in sea-marginal ponds of the

modern Red Sea and along the west coast of Australia. In

addition to higher salinity, locations of formation of radial

ooids are characterized by Icss-cncrgctic water movement. It

appears, therefore, that low-energy conditions favor growth of

radial oold fabric whereas high-energy conditions favor

growth of tangential (concentric) ooid fabric. However, this

applies only lo aragonitic ooids; modern marine high-

magnesian calcite ooids are exclusively radial (see Ootitcsaiul

other caatcil

grains).

On the basis of their internal fabric, most ancient marine

ooids may be classified into two groups; (I) Group one consists

of ooids having either (a) concentric aragonitic tabrlc or (b)

coatings consisting of crystalline mosaics of calcite (or

dolomite), commonly lacking any vestige of tangential or

radial structure. These mosaics may consist of minute euhedral

rhombs only a few micrometers across or of anhedral spar

crystals nearly as large as the ooids themselves. The structures

of these ooids have been substantially altered by diagenesis.

The ooids in algal mats may be precipitated by the algae

themselves, yet the origin of most ooids is still problematic.

Most modern ooids occur in or close to the intertidal position

in a zone in which the waves break and pound. Many

geologists consider that in this turbulent, shallow-water

environment, ooids may form inorganically as the cooler

water from adjoining deep zones spreads across the shoals,

gives up some of its CO:, 'I'td becomes warmer.

Pisolites

Spherical- or elliptical coated particles that exceed 2 mm in

diameter are known as pisolites. The division between pisolites

and ooids is one of size; ooids arc smaller than 2 mm. Despite

this seemingly arbitrary size differentiation, the origins ol'

ooids and pisolites are not the same. Pisolites that arc merely

oversized ooids are not common, either in modern carbonate

sedimentary environments or in ancient limestones. Three

common kinds of pisolites are known: oncolites, caliche or

vadose pisolites, and cave pearls or cave pisolites.

Oncolites (also known as algal pisolites) very closely

resemble vadose pisolites. Their origins are, however, quite

different. Oncolites consist of cncrustralions on various

particles. When particles, commonly skeletal, roll about

intermittently on the sedimentation surface, microorganisms,

especially cyanobacteria, but also green algae, bacteria, and

diatoms, attach themselves to and repeatedly coat these

particles with concentric laminae by trapping and binding

sediment. The microorganisms coat the exposed portions of

the oncolites during periods of quiescence, and may be

partially abraded from the surfaces of the oncolites during

the intervening periods of rolling. Thus oncolites commonly

are composed of irrcgular-and incotnpletc laminae.

The most common pisolites are known as caliche- or vadose

pisolites. Vadose pisolites form in the weathering zone as

caliche

((/.

v.).

Cave pearls, or cave pisolites, arc pisolites that

are true particles; they form in a manner analogous to ooids.

but in pools of water in caves (Bathurst, 1975; Friedman, 1985;

Friedman etal., 1992: Shinn etal.. 1989; Tucker and Wright.

1990).

Limestones

Fxcept for reefs which framework builders construct, for

consolidated lime mud (micrite) and for limestones formed by

the bacterial breakdown of calcium sulfate. the basic

constituents of most limestones are recognizable sand-size

particles and the spaces between these particles. The inter-

particles spaces may be occupied by (1) micrite, (2) optically

clear spar (calcite), and (3) void space, which may be filled with

fluids,

such as water or petroleum (oil or gas) (Friedman eta!..

1992;

ScolTm, 1987; Jones and Goodbody, 19S5: Tucker and

Wrighl, 1990).

Dunham classification

Dunham (1962) recognized iwo first categories among lime-

stones: (I) those in which the original particle components

were bound together (named collectively as boundstoncs) and

(2) those in which the original components were nol bound

together (no collective name) (Figure C.'^2).

Boundstoncs are limestones showing evidence that particles

being deposited were bound by organisms or that they consist

frameworks constructed by organistns. This group includes reefs,

stromatolites, and travertine. Embry and Klovan (1971)

have divided Dunham's boundstone group into subgroups

according lo the nature ofthe binding. Thus limestones bound

together

ith microbiat laminae arc bindstones. Limestones that

originated ill place (autochthonous limestones) as framc-buiti

reefs are framestones. Limestones that consist predominantly of

.\ediment trapped by baffling organisms are bafflestones. This

expansion of the Dunham classification is congruent with

ecological classifications of dominant reef organisms into

guilds (constructors, binders, bafflers).

Particle-supported rocks are subdivided into two further

classes: (I) particle-supported linivstoncs devoid of

tithijii-d

lime

mud. known as grainstones. and (2) particle-supported limc-

stoni'S

containing

.some

lithified lime-mud matrix, but not ciicugh

to keep the sand-size particles from tcucliing one aunt her: know

as packstoncs.

Matrix-supported limestones are also subdivided into two

classes: (1) nutd-supported

lime.stones

containing at most

per-

cent sand-size-or larger particles, known as mudstones. and

(2) mud-supported limestones containing at least Id percent

sand-

size-

larger particles, known as wackestones. Notice that in

the Dunham classification the term micrite is not used. Instead.

the tine- size carbonate rock is called mudstotie (better, hmc

mudstone) (Figure C32).

The rationale of this classification is that it allows one to

map gradients in rate of production of sand-size particles

relative lo rate of accumulation of lime mud. In calm waters.

130

CLASSIFICATION OF SEDIMENTS AND SFDIMENTARY ROCK.S

aASStFtCATION ACCOItDINO TO

DEPOSIT lONAL

DiPOMTIONAL TEXTURE

Ofiginal componenli not bound together during depasiiion

Contains mud

ica'ttcle? ai ciay-

and (tie siil si/el

Mud suppof'ed

Lsss ih»r

T0%

particles

Mo'e If an

10"'.

particles

Particle supported

Pockstone

PO-

TEXTURE

Lacks mud

ond IS

tide supported

Original componerta

were bound logeth^r

as showi bf

i"ie'g'o*n

lafnirahon co'ilraiy lo gravity

Ol sediment tioofed c.aviiies thai

a'S 'CO'ed ovf-r Oy

r>'Qariii;

- IT

questtoraDly organsc rFiai!e< a"(J

are too la'ge to be ioie'S!>ces

Figure C32 CLissitying and naming limestones according lo deposilion.il lexture; the scheme of Dunham n*'f)2l.

Allochemical

Rocks

Spaify Calcile

Cement

Inlraclasts

Ooids

Fossils

Pellets

Orthochemical

Hock

MIcflte

Reel

Rock

Figure C33 Classifying and naming limestones according to the

scheme of Folk M9S'f) imodified by Friedman ef a/., 1992).

lime mud, if present, settles on the bottom and remains there.

Hence liineslones derived by lithification of an original lime

mud deserve to be contrasted wilh those derived by lithifica-

tion of carbonate sediment devoid of lime mud. This relation-

ship between satid-size partieles and lime mud generally

distinguishes an original sediment deposited in calm water

from a sediment deposited in agitated water. This distinction is

fundamental (Dunham, 1962).

Folk classification

R.L. Folk (1959) subdivided the constituents of limestones into

two categories: (I) allochemical constituents or alloehcms

(partieles. meaning sand-size or larger), and (2) orthochemical

constituents (collectively designated by the word orthoehems

and eonsisting of micrite. presumably lithified original time

mud; and cement, sparry calcite: Figure C33). Folk's

classification lists four kinds of allochems (partieles):

(1) intraclasts. (2) ooids. (3) skeletal partieles and (4) pellets

(including peloids).

The interstices among the allochems are filled with the

orthochemical constituents (micriie or sparry calcite cement).

Combinations of allochems and orthoehems provide a basis

for recognizing eight kinds of limestones. In addition, micrite

may lack sand-size particles and henee stand alone as a ninth

kind of limestone. Finally, a tenth kind is a reef rock

(biolithite) (Figure C33).

Folk assigned names to eight of these ten kinds of limestone

by using composite words consisting of two pans: (a) an initial

abbreviated expression for the itllochems. and (b) a word

designating the orthoehemical constituents, based on one of

the two groups listed above. These words for the orthoehems

are (I) micrite, and (2) sparite (limestones having a cement con-

sisting of sparry calcltc]. The prefixes for the allochems

are abbreviated as follows: intraclasts = intra: ooids = oo:

fossils —bio; pellets = pel.

Thus the names of these eight kinds of limestone are

intrasparite. intramierite. oosparite. oomierite. biosparite.

biomicrite. pelsparite, and pelmierite. If several kinds of

particles are important constituents of a limestone, their

abbreviations are strung together in order of increasing

abundance. For example, a limestone composed of micrite

and having allochems composed of 10 percent intraclasis.

20 percent skeletal debris, and 70 percent peloids would be an

intrabiopelmicrite.

The Folk classilieation allows for particles larger than sand

size,

which are however, not particularly common in lime-

stones. For partieles of pebble size, such as the intraclasts of

flat-pebb!c conglomerates, the shells of lag concentrates, or the

pellets of organisms having large anal diameter. Folk

continued the tradition of one of Grabau"s terms, rudite. A

limestone eomposed of large pellets, if accumulated in a

micrite, is known as pelniicrudiie: Hat pebbles in a micrite

would be intramicrudite. If sparry cement occurs between

these same pebble-size partieles, the limestones would be

designated pelsparrudite and nitritsparudite.

The distinctive aspect of Folk's classification (Folk. 1959) is

its use of names for eight major kinds of limestone based on

what lies between the particles (spar or micrite).

(lASSIHICATION OF SFDIMFNTS AND SEDIMENTARY ROCKS

131

Dolostones

DolosiDiies are carbonate rocks consisling of the minertil

dolomite CaMg (COi):. Two kinds of dolostone occurs: (I)

Calcareous dolostones. carbonate rocks containing 50 lo 10

percent ol'the mineral dolomite, and (2) Oolostones. carbonate

rocks containing 90 percent or more ot~ the mineral dolomite.

Dolostones may or may not preser\e the original texture of the

antecedent limestone, such as skeletal particles or ooids. which

they have replaced

A dolomite fabric in which the predominant constituents are

euhedral crystals is known as idiotopic. Idiotopic dolostone is

one of the most-impoi-tant carbonate reservoirs tor oil and gas

because it is abundant, porous, and permeable. Dolomite

crystals partly bounded by crystal iaces are siibhedal. A

dolomite iabrie in whieh most of the constituent crystals are

subhedral is hypidiotopic. Dolomite crystals bounded by

anhedral crystal facies constitute a fabric known as xenotopic

(Friedman. 1965).

Otlier crystals may be added to the name as moditiers. Such

modifiers for crystal size include: micron-sized: O^un to l()|.mi

(0.001 to 0.01 nim): decimicron-sized: lO^im to 100^m (0.010

toO.lOO mni):centimicron-sized: 100pm lo 1000).ini (O.IOOmm

to l.OOOmm): millimeter-sized:

mm to 10mm: centimeter-

sized: 10mm to 100mm.

A further breakdown of the centimicron-size range may be

made by using the terms tine crystalline (IO()|.im to 250).mi).

medium crystalline (250jun to 500|jm). and coarse crystalline

(500 to 1000 |.im). This further breakdown into three

iiddilional size classes is based on empirical observations of

the distribution of crystal sizes in dolostones and recrystallized

limestones and is Iherctbre useful: it does not. however, tbilow

the metric framework ofthe above-given size classitication.

The naming and elassitying of these various fabrics and size

classes ean also be extended to limestones that have undergone

neomorphism or to precipitates, sueh as evaporites.

Dolostones may also be classified genetically into Ihree

major groups: (1) syngenetic dolostone. (2) diagenetie dolos-

tone.

and (.1) epigenetie dolostone. Syngenetic dolostone is

here defined as dotostonc that has formed pcnecontempora-

ncously in

irs

environment of

clcposiiion.

This kind ot dolostone

contrasts with diagenetic dolostone. which is dolostone formed

by replacement of calcium-carbonate .sediments or of limestones

during

or following

consolidation,

such as within beds ofcarbonatc

sediments or limestones. In borderline cases, the distinction

between syngenetie and diagenelie becomes difficult or

impossible to make. Epigenetie dolostone is here detined as

dolostone that has formed by replacement of limestone

along

po.st-

dcpo.-iitional structural elements, such as faults and fractures.

Many, but not all. epigenetie dolostones are genetically

associated with metallic ore deposits, iiolably of lead and zinc

minerals. These dolostones have recently become known as

hydrothermal dolostones (Friedman. 1965; Friedman and

Sanders 1967: Bathursl. 1975: Baker and Burns. 1985: Land,

19S5:

Hardie l^ST; Tucker and Wright. 1990: Friedman etal..

1992).

Authigenic rocks

Authigenic is a term designating rocks that have grown in

place subsequent to the formation of the sediment or rock of

whieh they constitute a part. The most-important kinds of

authigenic rocks are (1) chert. (2) phosphate rock, and (3)

sedimentary iron ores. Evaporites formed by preeipitation in

the interstitial waters ofa preexisting sediment likewise quality

as being authigenie.

Chert

Chert is a tough, brittle siliceous rock exhibiting a spUnlery-lo

eonehoidal fracture and a vitreous luster. The silica to form

most cherts is thought to come from the tests of siliceous

organistns.

The tests of siliceous organisms, such as those of radiolar-

ians and diatoms, as well as the spieules of sponges consist of

opal. Likewise, siliceous shells in cherts of Tertiary age are

mostly opaline. In contrast, nearly all silica in Paleozoie cherts

oeeurs as quartz and chalcedony.

Silica minerals present in ehert include qtiartzinc and

lutccine, which are tibrous varieties of quart/., and cristobalite

(opal-CT). Quari/ine and lutecine may form at the expense of

sultate minerals in evaporite deposits. These silica minerals

may be the only testimony to the former presenec of such

sulfates. now replaced and vanished. These silica minerals may

therefore be useful indicators of ancient environments.

The silica of chert may be (1) an alteration produet of

\olcanic roek, such as smectite and voleanie glass, or (2) a

precipitate derived from the dissolution of tests of silieeous

organisms. The most-probable source of silica in the eherts of

the eciitral Pacific Ocean is dissolution of radiolarian tests. The

silica in most oceanic cherts probably is of biogenie origin.

Biogenie opal is dissolved and rcprecipitated as tinely erystal-

line cristobalite or opal-CT. which inverts to quartz. The end

produet is a classic dense, vitreous chert.

Chert oeeurs (1) in nodules, and (2) in strata (bedded or

ribbon cherts). Common synonyms tor cherl or varieties of

chert include jasper. Hint, novaculite. and poreellanite.

Although the source of silica tor many cherts evidently is the

dissolved silieeous tests of marine organisms, many ehert

nodules are inferred to have grown by the simultaneous action

of (a) dissolution of carbonate, and (b) preeipitation of silica in

thin tilms of water at the boundaries of the carbonate being

dissolved (Jones and Segnit, 1971: Laschet. 1984: Tada and

lijima. 1983: Williams and Crear. 1985).

Phosphate rock

Sedimentary phosphate deposits usually consist ofa carbonate

fluorapatite [Ca5(PO4)i(F. COU]: a variety of apatite known as

francolite. When the apatite-like phase eannot be identitied.

the name collophane is commonly applied. Sueh phosphates,

or phosphorites (the two terms are synonymous) are here

referred to as phosphate roek.

Phosphate rock oeeurs on the modern sea bottom and in

aneient deposits. Ancient deposits of phosphate roek tbrm

extensive phosphosenic provinces (Bentor, 1980: Ciermann

et al.. 1984: Kolodny. 1981; Riggs, 1986; Sheldon, 1987;

Notholt elal.. 1989; Notholt and Jarvis. 1990; ).

Sedimentary iron ores

Valuable iron-rich sedimentary rocks, true ores in the fullest

sense of the term, have influenced modern history. England

achieved greatness through exploitation of her sedimentary

iron ores, while at the same time mining her rich eoai deposits.

During the nineteenth- and early part of the twentieth

132

CLASSIFICATION OF SEDIMFNTS AND SEDIMFNTAKY RCK"KS

centuries Germany and lYance were al each other's throats

over possession of the vital Lorraine deposits, the famous

oolitic minette ores of Jurassic age. Diseovery of the vast Lake

Superior iron ore deposits in 1844 ushered in the industrial age

for the United States. In Europe and in Norlh America, where

sedimentary iron ores and coal met, large industrial centers

arose, particularly in England. France. Germany, and the

United States.

The term sedimentary iron ores designates those eonsisting

predominantly of iron minerals, notably oxides, hydroxides,

carbonates, silicates, and sultides. The oxides are hematite and

magnetite; the hydroxide mineral is goethite (including

limonite): Ihe earbonate mineral is siderite; the silicate minerals

are ehamosite. greenalite, and glaueonite; and the sulfide

minerals are pyrite and pyrrhotite.

Sedimentary iron ores are classitied into three broad groups:

(1) bog-iron deposits.

(2)

ironstones,

and (?)

iron formations.

The basis ofthe classification combines mineral composition,

occurrence, and geologic age. In addition, sedimentary iron

ores may be named aeeording to their dominant iron mineral;

e.g., siderite rock, hematite rock, or magnetite rock (James,

1954;

Maynard. 1983; Guilbert and Park, 1986; Youne and

Taylor, 19*89; Van Houton, 1990).

Evaporites

Evaporites form by precipitation from brines whose salinity

values have been greatly increased by evaporation. Evapora-

tion may take place in elosed or in open systems in numerous

environmental settings.

Although almost forty different preeipitate-type minerals

have been recorded from evaporite deposits, only about twenty

are present in more than traee amounts. Of these, only two

kinds are common, sulfales and halides, both of which

form extensive deposits of sedimentary rock. Two kinds of

sulfate minerals are common in sedimentary roeks: cypsum

(CaSOj

•

2H:O) and anhydrite (CaSOj).

Among the halides, halite rock (rock salt) is the most

common. It (brms successions up to I.()(JO meters (m) thick

(Schreiber and Friedman, 1976: Warren. 1989: Tueker and

Wright. 1990).

Carbonaceous sediments and rocks

Carbonaceous sediments and roeks. primarily coal, have been

classified in two ways: by rank (eontent of earbon and caloric

value) and by petrographic characteristics, in order of

increasing rank, the classes of eoal are (I) lignite: (2)

subbituminous coal: (3) bituminous coal; and (4) anthracite.

The progression of carbonaceous material through the

continuous series of lignite (or its precursor, peat) through

bituminous coal to anthracite is known as coalification.

Coal has been recorded in strata ranging in age from

Precambrian to Holocene. However, only after the Late

Silurian Period, when land plants had become established,

was it possible for plant material to accumulate on a scale to

form large deposits of eoal. In the history of the earth, two

particularly rich coal-forming periods are known, the Carbo-

niferous (principally Pennsylvanian) and Early Tertiary

Periods. Most coals form from aecumulated plant debris in

sea-marginal swamps or in closed fluvial basins (Sackett etal..

1974;

Hunt, 1979).

Exfrabasinal or allochthonous deposits

Extrabasinal or allochthous rocks consist mainly of one group.

These are the terrigenous rocks.

A fundamental basis for classification is particle size. On the

basis of increasing particle size, we recognize three main kinds

of terrigenous rocks: (1) shales, (2) sandstones, and (3)

eonglomerates and sedimentary breccias. According to the

Grabau system of classification, we use the terms lutite for

shale, arenile for sandstone, and rudile for conglomerates.

Some geologists recognize a fourth kind of terrigenous rock.

siltstone. For it the term siltite has been introduced.

Shales

Fine-textured terrigenous rocks compose about two thirds of

the sedimentary roek record, yet because their partieles are so

small, they are incompletely understood. Various bases for

naming these roeks have been proposed, including (1) partiele

size,

(2) proportion of clay minerals, and (3) fissility, the prop-

erty of splitting easily into thin layers parallel

the bedding.

Probably the most-satisfactory definition of shale is accord-

ing to particle size. In terms of size, the name shale is the

lithified equivalent of mud. Mud is a sediment eonsisting of

clay- and silt-size particles. Likewise, shales are terrigenous

rocks composed of clay- and silt-size particles.

The proportion of clay minerals in shales varies widely. In

most shales, quartz may compose between one quarter and one

half of the total; in some shales, quartz together with feldspar

generally contain greater proportions of quartz and feldspar,

whereas fine shales contain less of these and a greater

proportion of clay minerals. Fine-textured terrigeneous

sedimentary roeks having abundant quartz (and eommonly

also feldspar) partieles of silt size and that as a result, elosely

resemble sandstones.

On surface exposures, where weathering has taken place, it

is easy to distinguish fissile from nonfissle fine-textured rocks.

The terms mudrock. mudstone, claystone, and argillite have

been employed for shales lacking fissility: argillites are more-

indurated shales than mudstones and claystones. In the

Dunham classitication of carbonates, the term mudstone refers

to tine-textured limestone (in which case the roek should be

called lime mudstone to avoid confusion with fine-textured

siliciclastic rocks).

A dark, thinly laminated carbonaceous shale, exceptionally

rich in organic matter (5 percent or more of total organic

carbon) and sulfides (FeS2), is known as black shale. It forms

by partial anaerobic decay of organic matter in a reducing

setting in which water circulation is restricted and deposition is

slow (Chamley, 1989: Dunbarand Rodgers. 1957; Fiichtbauer

and Leggewie. 1984; Heling. 1988; Pettijohn, 1975; Potter

etal.. 1980; Weaver, 1989).

Sandstones

Sandstones are terrigenous sedimentary rocks in which sand-

size particles predominate. As with limestones, sandstones ean

be classified on the basis of (1) particles. (2) matrix, and

(3) cement. Particles are mostly derived from the lands, hence

are terrigenous, but pyroclastic and even elastic-carbonate

particles may be abundant. The essential particles of sand-

stones are (I) quartz, (2) feldspar, and (3) roek fragments.

Many other kinds of partieles occur in sands and sandstones.

End-nicmber sand-size particles

Qu;irlz

feldspar

Rock tragments

CLASSIFICATION OF SEDIMRNTS AND StiDIMFNTARY ROCKS

Idcali/L'd L'lid-inenihLT R)t:k iiatiics

15"'^!

malri\

Quartz sandstone

Feldspar sandslone

Rock-lragmeiil sundstonc

;.' \S"/u niiitrix

Argilkiccous Quartz sandstone

Argillaceous feldspar sandstone

Argillaeeous roek-fragment sandstone

Figure C34 C!assityin}> .ind naming sandstones.

but those other are not common and are not considered

essential in naming and classifying sandstones. The matrix is

the mud: physically deposited material that eonsists mostly of

elay minerals and quartz. Cement is the ehemieally preeipi-

tated filling of original void spaces. Some sandstones lack any

matrix: only mineral cement occupies the spaees among the

framework partieles. The argillaceous matrix of sandstones

implies (I) availability of mud, and (2) a low-energy regime in

the depositional environment.

Depending on the presence or absence of argillaceous

matter, we recognize two end-member groups of sandstones:

(1) argillaceous sandstones (sandstones eontaining about 15

percent or more of clay-size material), and (2) ordinary

sandstones, which contain less than about 15 percent clay-

size material. The value of 15 percent is arbitrary; in a

qualitative study, the obvious presence ofa fair amount of clay

suftiees to classify a sample as argillaeeous.

Based on the proportions of quartz, feldspar, and rock

fragments, we can divide each of these two first-order groups

(argillaceous sandstones and ordinary sandstones) into three

seeond-ordcr groups. Thus, as indicated in Figure C34, we ean

reeognize six sandstone end members: (1) quartz sandstone.

(2) feldspar sandstone, (3) rock-fragment sandstone. (4)

argillaceous quartz sandstone, (5) argillaceous feldspar sand-

stone, and (6) argillaceous rock-fragment sandstone.

The sand-size debris of most sandstones includes ehiefly

quartz, but with the quartz may be variable amounts of

feldspar or roek fragments or feldspar and rock fragments.

These mixtures are named by incorporating the names of all

kinds of sand-size debris present, listed in increasing order of

abundance. Thus if a sandstone contains 70 pereent quartz. 20

percent feldspar, and 10 percent rock fragments, we would

name it a roek fragment-feldspar-quartz sandstone.

The kinds of feldspars or rock fragments reflect (1) the

parent terrain or proveiiance from which the particles in

a sample of sandstone have been derived, and (2) their

preservation during weathering, transportation, and

diagenesis.

The pioneer sedimentary petrographer Paul D. Krynine

(1902 1964; see Sedimentologists) provided the modern

impetus in naming and classifying sandstones. Krynine's

(1948) original article is a classic in geology; it combines

astute observation in the field and of samples with a deep

insight and appreciation for the fundamental principles of

geology. In his elassilieation of sandstones. Krynine intro-

duced triangular diagrams for plotting his three fundamental

end members: orthoquartzite. graywacke. and arkose

(Figure C.15).

The earliest description of graywacke dates from 1789. and

the term arkose was introdueed in 1823. Aeeording to

Krynine, a graywacke Is a sandstone composed of angular

quartz {and chert) particles and abundant metamorphic rock

Figure C35 TrianguNir didj^ram; Krynine's classification of sandstones.

Uitlor P.D. Krynine, 1948, Figure Ci5. p.l37).

fragments, with little or uo cement and feldspar, and containing

more than 12 percent to 17 percent micas and

chlorite

(either in the

clay matrix or as metamorphic rock franwntsj. Krynine felt that

the rnica and chlorite between the partieles were a mechani-

cally deposited matrix and represented poor sorting; we now

know that this can no longer be assumed.

When deeply buried and subjected to appropriate geother-

mal gradients and migrating solutions, framework particles

may be dissolved and new clay minerals, such as chlorite and

illite.

may form. This diagenetic neoformation of clay-mineral

cements within sandstones may generate a rock composition

fitting the term graywacke. Yet Krynine and many others use

this term to refer to a rock having a mechanically deposited

matrix and poor sorting. To avoid the term graywaeke. some

authors substituted the term waeke. In his classitieation,

Krynine used the term arkose for a .sandstone with more than

30 percent feldspar.

A rock defined by Krynine as a graywacke would be named

argillaceous rock fragment-quartz sandstone if it contained no

feldspar, and an argillaceous roek fragment-feldspar-quartz

sandstone if if contained feldspar in greater abundanee than

rock fragments, or an argillaeeous feldspar-rock fragment-

quartz sandstone if the rock fragments were more abundant

than the feldspar. The arkose of Krynine would be named a

feldspar-quartz sandstone if the quartz is more abundant than

the feldspar, but a quartz-feldspar sandstone if feldspar

abundanee exceeds quartz abundance (Folk. 1956; McBride,

1962;

Klein, 1963; Sanders. 1978: Fuchtbauer

(•/(//.,

1988).

134 CLASSIFICATION

SEDIMENTS

AND

SHDIMFNTARY ROCKS

Conglomerates

and

sedimentary breccias

Coarse terrigenous rocks

are

formed

by the

lithification

gravel.

The

degree

rounding

of the

partieles defines

two

categories. Conglomerate

defined as

a coarse tcrrigcntnis

.sedi-

mentary rock formed

by the

lithification

rounded gravel.

Sedimentary breccia

is (/

coarse terrigenous sedimentary rock

formed by the lithification of angular gravel-size fir larger particles.

In conglomerates

and

sedimentary breccias more than

percent

of the

large partieles exceed

2 mm in

diameter.

The

particles

may

consist

pebbles, cobbles, boulders,

mixtures

of these sizes.

Conglomerates

and

sedimentary breccias

may be

named

and

classified

by (1) the

proportion

gravel

si/e

particles.

(2) the

kind

matrix,

and (3) the

kinds

gravel-si/e particles.

According

to the

proportion

gravel-si/.e particles

and the

kinds

matrix,

(1) a

sample containing

percent pebbles,

cobbles, and/or boulders

termed

conglomerate proper,

and

(2)

one

having between

pereent

and 80

pereent

such

coarse partieles

is (a) a

sandy

arenaeeous conglomerate

(b)

shaly

argillaceous conglomerate.

The

matrix between

the coarse particles

in a

conglomerate

may

also

calcareous

or sideritic.

Based

on the

variety

pebbles, cobbles, and/or boulders

composing them,

we can

classify conglomerates into

two

kinds:

(1)

conglomerates consisting only of pebbles,

cobbles,

and/

or boulders of a single kind

rock (sueh

as one or

various

varieties of chert

and

quartziteor

other rock),

oligomictic

conglomerates;

and (2)

conglomerates containing pebbles,

cobbles, and/or boulders

many kinds

roek. known

polymietie conglomerates.

The term diamictite

has

been proposed

for a

non.sortednon-

calcareous, terrigenous sedimentary rock composed

sandsize

and/or larger particles dispersed through

fme-tcxtured matrix.

Many,

if not

most, diamictites

are

conglomerates. Diamictites

originate

landslides, earth fiows, mudflows, solifluction,

ice

rafting, subaqueous slumping

and

sliding, and/or

glaeiers.

A noniithified diamictite

is a

diamicton (Flint etal..

1960:

Pettijohn,

1975:

Friedman

and

Sanders.

1978:

Fuchtbauer

etal., 1988).

Pyroclastic rocks

Pyroelastie rocks

are

lithified tephra. Their partieles originate

as explosive igneous niaterial

but are

deposited

sediments.

Voleanoes

and

their products, both

land

and in the sea.

are

the

major source

particles

for

pyroelastie rocks,

espeeially

areas

teetonie activity

and in

island arcs.

Particles originating

explosive ejection from volcanic vents

spread

out

over great areas.

On the

eontinents. pyroclastic

rocks

far

exceed

the

volume

extrusive igneous rocks.

Tcphras

are

chemically

and

texturally

di.stinctive.

ancient

tephras

may be

used

for

eorrelation. Mapping

particle size

and thickness

tephra deposits assists

loeating

the

parent

volcano.

Figure

C36

shows

how

sizes

tephra particles

can be

used

to classify pyroelastie rocks.

Pyroclastic rt)cks composed mo.stly

blocks

are

volcanic

breccia: pyroclastic roeks eomposed mostly

lapilli

are

lapilli

tuffs:

i.ind

pyroclasticrockscompo.wdofash

are

tuffs

in the

strict

sense

of the

word.

The three

end

members

pyroclastic rocks

are (I)

voleanie

rock fragments,

(2)

erystals,

and (3)

glass.

by size

rock

is a

Lirniling Particle

Diameter

units

Siandatd

Size Classes

Sediments

CoDbies

Pebb'es

Sand

Size Classes

Tepnra Panicles

LapiMi

Coatse

ash

Fine

ash

SJ'e Classes

Pyroclastic

Rocks

Aqgiometate,,,,,*^

^..X^oicanic

^^ btecc;a

LapiUi

lulls

TuKs

Figure

C36

Size classes

and

terms

for

tephra particles

and

pyroclastic

rocks (modilled from

R.V.

Fisher, 1966).

tuff

and

rock fragments exceed glass, this tuff

may be

described

as a

glass, rock-fragment

tuff. F.nd

members

are

listed

order

increasing abundance.

this approach

one

can recognize erystal-, rock-fragment-,

and

glass tuffs.

Adjectival moditiers

may be

added.

Beeausc

of the

chemical reactivity

and

instability

their

constituents, pyroelastie roeks

are

highly susceptible

diagenetic alteration. Glass alters

clay minerals, especially

to smectite, zeolites, chalcedony, opal, quartz,

or to a

microcrystaliine material, which

in a

thin slice under

the

petrographic microscope resembles chert.

alteration

product

tephra

bentonite.

plastic clay

shale composed

for the most part of ihe clay mineral motumorillonitc

variety

.smectite)

that formed by the alteration of tephra.

One kind

tuff that carries

speeial name

ignimbrite

welded

tuff, a

non.sorted pyroclastic rock deposited from

nuee

ardente while

the

particles were .still

in a

pta.stic condition.

lgnimbrites

may

occupy large areas

and

attain considerable

thicknesses (Fisher.

1961,

1966; Lajoie, 1979: Sehmincke.

1988;

USDOE, 1988).

Gerald

Friedman

Bibliography

Baker.

P.A..

Ltnd Burns.

S.J..

1985.

The

occurrence atid formalion

dolomite

organic-rich continental miirgin sediments: Btdk'linnf

llw Anicrt'ain

.•issociiiriun

ofPftrolciim

Geoloi^ist.s.

69:

1917-1930.

Balliur^t, R.G.C..

I^VS.

Carhomtiv Sedimenis tnitt Their Diagenesis.

2nd

cdn.

Elsevier Scientific Publishing Company.

Bentor.

Y.K.

(ed.).

1980.

Muiiiie

Piio.splutriic:.

Society

Economic

Pa Icon otologists

and

Mineralogists. Special Publication,

29.

Chamley.

H.. 1989.

Clay Settinwrtlolo^y. Springer-Verlag.

Dunhar.

CO.. and

Rodgers.

J., 1957.

Principles

of Slrati^nipliv. John

Wiley

Sons.

Dunham.

R.J.. 1962.

Classification

earbonate rocks according

depositional texture.

In Ham. W.E.

(ed.). Clu.ssificutionofCcirho-

luite Rocks. Tulsa.

OK:

American Association

Petroleum

Geologists Memoir

1. pp. 108 121.

Embry.

A.F., and

Kiovan. .I.E..

1971. A

late Devonian reef iracl

Northeasicrn Banks Island. N.W.T. Ccniiuliati Peiioteum Geology

Bitlh'iin.

19: 730

Fisher,

R.V..

1961. Proposed classification

voleaniclastic sediments

and rocks, deoloi'ii til Society of America Bulletin.

72:

1409-1414.