Boggs S. Principles of Sedimentology and Stratigraphy
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6.3 Limestone Te xtures
165
are formed (Chapter 4). Spheroidal stromatolites that reach a size exceeding 1 to
2 em are called oncoids.
Peloids
P�1oid is a nongenetic term for carbonate grains that are composed of microcrys
talline or cryptocrystalline calcite or aragonite and that do not display distinctive
inteal structures (Fig. 6.1F). Peloids are smaller than ooids and are generally of
si[t o fine-sand size (0.03-0.1 mm), although some may be larger. The most com
mon kd of peloids are fecal pellets, produced by organisms that ingest caldum
carbonat muds and extde undigested mud as pellets. Fecal pellets tend to be
small, oval to rounded, and uniform in size. They commonly contain enough fine
organic matter to make them appear opaque or dark colored. Pellets can be differ
entiated from ooids by their lack of concentric or radial inteal structure and
from rounded intraclasts by their uniformity of shape, good sorting, and small
ize. Because they are produced by organisms, their sizes and shapes are not relat
ed fo current transport, although pellets may be transported by currents and rede
posited fter initial deposition by organisms.
Peloids may also be produced by other processes, such as micritization of
small ooids or rotmded skeletal fragments caused by the boring activities of certain
oani�ms, particularly endolithic (boring) algae. These boring activities convert
e original grains into a nearly uniform, homogeneous mass of microcrystae
cal�ite. Some marine peloids may form by precipitation around active clumps of
pcteria (e.g., Chafetz, 1986). Other peloids may simply be very small, well-rounded
Intraclasts formed by reworking of semiconsolidated mud or mud aggregates .
.
A
gg re gate Grains
Aggregate grains are irregularly shaped carbonate grains that consist of two or
more carbonate fragme[lts (pellets, ooids, fossil fragments) joined together by a
carbonate-mud matrix that is generally dark colored and rich in organic matter.
Because the shapes of the aggregate grains in some modern carbonate-forming en
vironments, such as the Bahama Banks, resemble a bunch of grapes, they are com
monly called grapestones (llling, 1954). Other aggregate grains with a somewhat
smoother appearance have been referred to by the rather inelegant name of
lumps. Tu cker and Wright (1990, p. 12) suggest that lumps evolve from grape
stones by continued cementation and micritization of the grains (Fig. 6.2). Aggre
gate grains in modern carbonate environments are composed mainly of aragonite,
Figure 6.2
Schematic representation of lumps formed by micritization and cementation
of ooids.
166
Chapter 6 I Carbonate Sedimentary Rocks
Figure 6.3
but such grains in ancient limestones are domantly calcite. Aggregate grains
mode environments can commonly be recognized by their botryoidal shapes
and lack of internal structures; however, they can be confused with intraclasts.
fact, they are considered a type of intraclast by some geologists (e.g., Scholle and
Ulmer-Scholle, 2003, p. 246). Agggate grains are oy rarely reported in ancient
limestones, possibly because their shapes become distorted beyond recognition
owing to compaction during diagenesis.
Microcrystalline Calcite
Carbonate mud composed of very fine size calcite crystals is present in many
an
cient limestones in addition to sand-size carbonate grains. Carbonate mud or lime
mud occurs also in modern envonments where it consists dominantly of needle
shaped crystals of aragonite about 1 to 5 microns (0.001-0.005 mm) long. The car
bonate mud ancient limestones is composed of similar-size crystals of calcite.
Lime muds may also contain small amounts of fine-grained detril minerals such
as
clay merals, quartz, feldspar, and fine-size organic matter. They have a grayish
to brownish, subtranslucent appearance under the croscope (Fig. 6.3), and they
are easily distinguished from carbonate grains and sparry calcite crystals (dis
cussed below) by their extremely small crystal size. Folk (1959) proposed the con
traction micrite for microcrystalline calcite, a term that has been universally
adopted to signify very fine grained carbonate sediments.
Micrite may be present as matrix among carbonate grains, or it may make up
most or all of a limestone. A limestone composed mostly of micrite is analogous
texturally to a siliciclastic mudrock or shale. The presence of micrite in an ancient
limestone is commonly interpreted to indicate deposition under quiet-water oon
ditions where little winnowing of fine mud took place. By contrast, carbonate sed
iments deposited in environments where bottom currents or wave energy
strong are commonly mud-free because carbonate mud is selectively moved n
these environments. On the basis of purely chemical considerations, carbonate
mud or micrite can theoretically form by organic precipitation of aragonite, later
converted to calcite, from surface waters supersaturated with calcium bicarbon
ate. Geologists are uncertain, however, about how much aragonite is actually
being generated by inorganic processes in the modern ocean . Much modern car
bonate mud appears to originate through organic processes (Section 6.7). These
processes include breakdown of calcareous algae in shlow water to yield arago
nite mud, and deposition of carbonate nannofossils ( <35 in size) such as coc
coliths deeper water to eld calcite muds (chalks) .
Sparry Calcite
Many limestones contain large crystals of calcite, commonly on the order of 0.02 to
0.1 , that appear clear or white when viewed with a hd lens or in plane light
Micrite-rich limestone containing a few skeletal grains. Plattin
Limestone (Ordovician), Missouri. Crossed nicols.
Figure 6.4
6.4 Oolomlte Te xtures
167
Sparry calcite cementing rounded intraclasts. Note that the ce
ment displays drusy texture: small calcite crystals, oriented with
their long dimensions perpendicular to the clast surfaces, grade
outward from the margins of the clasts into larger, randomly ori
ented calcite crystals. Devonian li mestone, Canada. Crossed
nicols.
under a polarizing microscope. Such crystals are called sparry calcite. They are
distguished from micrite by their larger size and clarity and hom carbonate
grains by their crystal shapes and lack of internal texture. Some spany calcite can
be seen tmder the microscope to fill interstitial pore spaces among grains or to l
solution cavities as a cement (Fig. 6.4). The presence of sparry calcite cement in
tergranular pore spaces indicates that grain framework voids were empty of lime
mud at the time of deposition, suggesting deposition under agitated-water condi
tions that removed fine mud, as mentioned.
Sparry calcite can also form in ancient limestones by recrystallization of pri
mary depositional grains and micrite during diagenesis (Section 6.8). Sparry cal
cite fo rmed by recrystallization may be very difficult in some cases to differentiate
from sparry calcite cement (Boggs, 1992, p. 549). It is importt to distinguish be
tween the two types of sparry calcite because incorrectly identying recrystallized
spar as sparry calcite cement can cause errors both environmental terpreta
tion and limestone classification.
6.4 DOLOMITE TEXTURES
Dolomite (dolostone) is composed maly of the mineral dolomite [CaMg(C03)2].
Unlike limestone, which is characterized by the presence of grains, micrite, and/ or
sparry cement, dolomite has a largely crystalline (granular) texture. On the basis of
crystal shape, two kds of dolomite are recognized. Planar (or idiotopic) dolomite
(Fig. 6.5A) consists of rhombic, euhedral (well-formed) to aedral (poorly
Figure 6.5
Dolomite cstals: A. Planar dolomite exhibiting euhedral crystals with planar faces
(arrow), Bonneterre Formation (Cambrian), Missouri. B. Nonplanar dolomite, with curved
or irrular faes, Davis formation (Cambrian), Missouri. Crossed nicols.
168
Chapter 6 I Carbonate Sedimentary Rocks
Figure 6.6
Classification of dolomite
textures. [After Gregg, ]. M.,
and D. Sibley, 1984, Epi
genetic dolomitization and
the origin of xenotopic
dolomite texture: jour. Sed.
Petrology, v. 54, Fig. 6, p.
913, and Sibley, D. , and
Gregg, ]. M., 1987, Classifi
cation of dolomite rock tex
tures: Jou Sed. Petrology,
v. 57, Fig. 1, p. 968; figures
reprinted by permission of
Society of Economic Paleon
tologists and Mineralogists,
Tulsa, Okla.]
CRYSTAL BOUNDARY SHAPE
PLANAR (IDIOTOPIC) DOLOMITE
rhombic, euhedral to anhedral crystals
NONPLANAR (XENOTOPIC) DOLOMITE
nonrhombic, commonly anhedral dolomite
PNAR-EUHEDRAL; most
dolomite crystals are euhedral
rhombs; crystal-supported with
intercrystalline areas filled with
another mineral or unfilled
(porous)
� PNAR-SUBHEDRAL; subhedral
to anhedral dolomite crystals with
low porosity and/or low
intercrystalline matrix; straight
comprcmise boundaries are
common and many of the crystals
have
crystal-face
PLANAR VOID-FILLING; euhedral
dolomite crystals lining large pores
and vugs or surrounding patches of
another mineral such as gypsum or
calcite
PNAR-PORPHYROTOPIC;
euhedral dolomite crystals floating
in a limestone matrix; crystals are
matrix-supported
NONPLANAR-ANHEDRAL; tightly
packed anhedral dolomite crystals
with mostly curved, oblate, serrated,
indistinct or otherwise irregular
intercrystalline boundaries;
pre-crystal-face junctions rare;
crystals commonly have undulatory
extinction
NONPNAR VOID-FILLING; pore
lining, saddle-shaped, or baroque
(irregular) dolomite crystals
characterized by scimitar-like
termination. as viewed in thin
section; sweeping extinction
NONPLANAR-PORPHYROTOPIC;
single anhedral dolomite crystals or
patches of anhedral dolomite
crystals floating in a limestone
matrix; crystals commonly have
undulatory extinction
PRESERVATION STATE OF RECOGNIZABLE FABRIC ELEMENTS
Unreplaced
I
Molds
I
Partially replaced
I
Replaced
Mimic Nonmimic
Unreplaced
I
Partially replaced
I
Replaced
Unrel'aced
Partially replaced
I
Replaced
Mimic Nonmimic
formed) crystals. Nonplanar (or xenotopic) dolomite (Fig. 6.5B) is made of non
planar, commonly anhedral crystals (Sibley and Gregg, 1987). Each of these major
kinds of dolomite can be divided into subtypes as shown in Figure 6.6. Many
dolomites form by replacement of a precursor limestone. Original limestone tex
tures
may be preserved in such dolomites to various degrees, ranging from virtu
ally unreplaced to totally replaced (Fig. 6.6). That is, the replacing dolomite may
preserve the original textu as a "ghost" (mimicking replacement) or the original
texture may be completely destroyed (nonmimicking replacement).
6.5 STRUCTURES IN CARBONATE ROCKS
Car
bonate rocks contain many of the same kinds of sedimentary structures as
those present in siliciclastic rocks (Chapter 4). These structures include cross
stratification, laminated bedding, lenticular bedding, convolute lamination, flame
structures, load casts, flute casts, groove casts, and mudcracks, as well as trace fos
sils such as tracks, trails, and burrows (Demicco and Hardie, 1994). They may also
6.6 Classification of Carbonate Rocks
169
contain stromatolites and cryptoalgal structures and less common structures such
as teepee structures {arched or uptued polygons), solution cavities, and stro-
matactis (irregularly-shaped masses of sparry calcite and inteal sediments rang-
ing in shape from elongate to globose).
6.6 CLASSIFICAT ION OF CARBONATE ROCKS
Attempts to classify carbonate rocks date back to at least 1904 with the publication
of Grabeau's classic textbook on the classification of sedimentary rocks. Addition
al claifications were proposed by other authors in the 1930s, 1940s, and 1950s.
Most
of these early classifications were basically genetic schemes in which names
such as "fore-reef talus limestone" or "low-energy limestone" were used to identi
limestones according to their presumed environment of deposition (Ham and
Pray, 1962). These classifications failed to recognize the clear distinction between
carbonate grains and carbonate mud or to exploit difference in identity of the var
ious kinds of carbonate grains. Publication in 1959 of Folk's largely descriptive
"Practical Petrographic Classification of Limestones" marked the beginning of the
modem period of limestone classification. In 1962, several additional classifica
tions appeared (Ham, 1962) which, with one exception, are mainly descriptive
assifications. Unlike the confusion attending the proliferation of sandstone clas
sificaons, the appearance of several descriptive limestone classifications seems to
have had a largely positive effect because it forced geologists to become more
keenly aware of the varied constituents that make up limestones, as well as the en
vironmtal signicance of these constituents.
Mineralogy plays only a small role in classification of carbonate rocks because
most carbonate rocks are essentially monomineralic. Mineralogy is used primarily
dierentiate dolomite from limestone or carbonate rocks from noncarbonate
. e principal constituents or parameters used in carbonate classification are
e types of carbonate grains or allochems and the grain/micrite ratio. e nature of
e grain packing or fabric also used in some classifications, which the fabric is
rerred to as eier grain-supported or mud-supported. A grain-supported fabric
one in which grains are in contact, creating an intact gra framework in which
voids may or may not be filled with mud {matrix). a mud-supported fabric, most
grains do not touch, and they appear to "float" in the carbonate mud.
Folk's {1959, 1962) classification has probably been the most widely accepted
limestone classification because of its applicability to a wide range of carbonate
rock types and the ease with which its terms can be understood and utilized. The
assification is based on the relative abundance of three major types of con
stuents: {1) carbonate grains or allochems, (2) microcrystalline carbonate mud
(micrite), and (3) sparry calcite cement. As illustrated in Table 6.2, classification is
made by first determining the relative abundance of total allochems vs. micrite
plus sparry calcite cement. Further subdivision is then made on the basis of the
lative abundance of the various types of carbonate grains (Fig. 6.7) and the rela
tive abundance of micrite compared to sparry calcite cement. is classification
approach yields a bipartite name that reflects both the major type of carbonate
grain the limestone and the relative abundance of micrite and sparry calcite ce
ment. Thus, an oosparite is an ooid-rich rock cemented with sparry calcite that
:ontains little micrite, whereas an oomicrite is an ooid-rich limestone in which mi
:rite is abundant and sparry calcite is subordinate. Additional textural informa
tion can be added by use of the textural maturity terms shown in Figure 6.8. us,
1 packed oomicrite indicates a grain-supported oolitic limestone, and a sparse
>omicrite is an oolitic rock with a mud-supported fabric. Note that Folk's dassifi
:ation can also be used to classify dolomite rock, if "ghosts" of the original al
hems are still identifiable in the dolomite.
0
Table 6.2 · Classication of carbonate r
Limestones, partly dolomitized limestones
He
placement dolomites
,-----
�
-.-
-
-
�
0
·
'§
.
§
u
m
�
]
"
E
2
u
0
N
1-
8
?
N
�
·
N 0
AO
0
0
·
E o
-
E
0
·
-
> 10% Allochems
Allochcmical rocks
�1
c
�
� � �
A
!\
- u
Sparry calcite
cement > micro
crystalline ooze
matrix
Sparry allo
chemical rocks
lntrasparrudite
Jntrasparite
m
-�.
JO�dill
�
A
Biosparite
-
I
Biopelsparite
Microcrystalline
ooze matrix
> sparry calcite
cement
Microcrystalline
allochemical rocks
icrudite'
h
Oomicrud
Oomicrite*
Biomicru dite
Biomicrite
Biopelmicrite
-- -+
Pelsparite
Pel micrite
<% Allochems
Microcrystalline rocks
1 °/o-10�1;f
Allochems
traclasts:
intraclast
bearing
E
1
micrite*
2
Ooids:
ooid-bearing
I nticrite*
�
;
Fossils:
fossiliferous
micrite
Pellets:
pelletiferous
micrite
<lu,
Undis
turbed
Allochems I
bioherm
rocks
"
2
·
'
u
u
·
-
·
-
E
E
o
,_
·
-
0
, '
-�
E
0 0
Q
�
"
.:
E
·
u
·
-
.
§
0
A
l
lochem
Finely crystallin
e
intradasc
dolomite, etc.
61 Coarsely crystal
line oolitic
] dolomite, etc.
:
I
Aphanocrystalline
biogenic dolomite,
etc.
I
Very
fi
talline
•
dolomil
crys-
etc.
No
alloche
ghosts
Medium
crystalline
dolomite
Finely
crystalline
dolomite
etc.
un:e: Folk, R. L., 1962, Spectral subdivision of limestone types, in W. E. Ham (ed.), C1assihcation of carbonate rocks: Am. Assor. Petroleum Grolosits Mem. L Ta ble 1r p. 70, rep rinted by permission of AA, Tu lsa,
Okln.
Note: Names <.md symbols in the bty of the table refer to limestones. If the rock contains more than 10 percent replacement doloit1
refers to calcirudites {median allodwm size larger than LO mm); the lower name refers to ll rocks with medi.m
grains (1 iored.
* Deign.1tes rare rock types.
the term N dolomitidu to the rock name. The upper name in each bo
1.0 mm. Grain size and quantity of ooze matrix, cemt�nts, or h:r rigenous
6.6 Classification of Carbonate Rocks
171
Allochemical
Rocks
Ohochemical
Rocks
II m
SPARRY CALCITE
CEMENT
MICROCRYSTALLINE MICROCRYSTALLINE CALCITE
INTRACSTS
�
(i)
�
OOIDS
(
o
)
FOSSILS
(b)
PELLETS
(p)
Flgure 6.7
/NASPARITE(Ii)
•
'
'
.
'
OOSPARITE(Io)
8/0SPAR/TE(/b)
PELSPARITE(Ip)
CALCITE MATRIX
CKING ALLOCHEMS
•
.
.
INTRAMICRI
OOMJCRITE(o)
8/0M/CR/TE(//b)
·;::�:< .
·
.�
-
: �·
·
·
·
·
�:
:
� ���:::::�:<�
.
'
.
. · . . .
.
·
.
·
·
·
-
·
.
-::,.;·:- .·.-.-�-.-
.
·
.
PELMICRITE(IIp)
Sparry calcite
e
· .
.
.
.
MICR/TE(/1/m)
DISMICR/TE(/1/m
Autochthonous
Reef Rocks
IV
E
.
.
.
.
.
.
8/0LITH/ TE(/
[ : J
Microcrystalline Calcite
Schemtic representation of the constituents that form the basis for Folk's classification of
carbooate rocks (Table 6.2). (After Folk, R. l., 1962, Spectral subdivision of limestone
pes, in Ham, W. E. (ed.), Classification of carbonate rocks: Am. Assoc. Petroleum Geolo
gists Memoir 1, Fig. 3, p. 71, reprinted by permission of MPG, Tu lsa, Okla.]
OVER 3 LIME
MUD
Percent
Allhems
0-1%
1-10%
10-50%
Rentative MICRITE &
FSi/-
SPARSE
k FES
Terms
DISMICRITE
MICRITE
BIOMICRITE
Te rminology
Tergenous Sandy
SUB EQUAL
SPAR &
OVER SO%
LIME MUD
PACKED
POORLY
WASHED
BIOMICRITE
BIOSPARITE
Clayey or
Immature Sandstone
SORTING
SORTI NG
ROUNDED &
POOR
GOOD
ABRADED
UNSORTED
SORTED ROUNDED
BIOSPARITE BIOSPARITE BIOSPARITE
aues Clayston
e
�
• LIME MUD MATR IX
SPARRY CALCITE CEMENT
Figure 6.8
Te xtural classification of carbonate sediments on the basis of relative abundance of lime
mud matrix and sparry calcite cement and on the abundance and sorting of carbonate
grains (allochems). [After Folk, A. l., 1962, Spectral subdivision of limestone types, in
Ham, E. (ed.), Ciassificatn of carbonate roc: Am. Assoc. Petroleum Geologis Mem
oir 1, Fig. 4, p. 76, repn ted by permiion of MP Tu /, Okla.]
172
Chapter 6 I Carbonate Sedimentary Rocks
The terms used by Folk (1959, 1962) to dierentiate depositional textures are
purely descriptive; however, they also have environmental significance. The term
biomicrite, for example, conveys an interpretation of deposition under quiet
water conditions where micrite is abundant and winnowing of the lime mud is
minimal. Thus, micrite accumulates along wi skeletal particles. On the oer
hand, the term biosparite suggests deposition in a wave-agitated environment
where micrite is removed by winnowing currents, allowg mud-free carbonate
grains to accumulate. These grains are subsequently cemented with sparry calcite
during burial (diagenesis).
(1962) has a somewhat dierent te of classification (Table 6.3A)
that sesses the relative abundance of allochems and micrite but does not consider
the identity of different kinds of carbonate grains. Dunham's classification is
based solely upon depositional texture and considers two aspects of texture:
(1) grain packing and the relative abundance of grains to micrite and (2) deposi
tional binding of grains. Depositional bding means whether or not carbonate
grains show evidence of having been bound together at the time of deposition, as
in a colonial reef complex, a stromatolite (cyanobacteria) bed, or calcareous algae
mat. Dunham's classification separates components that were not bound together
at the time of deposition into those that lack lime mud and those that contain lime
mud.
Rocks that contain no mud are obviously grain-supported. Rocks that con
tain mud may be either grain-supported or mud-supported. Note, however, that
grain support does not depend upon the absolute grain-to-mud ratio, because grain
support is also a function of shapes of the carbonate grains. Platy or elongate
grains such as bivalve shells may form a grain-supported fabric at much lower
grain abundances than more spherical particles such as ooids. Therefore, Dunham's
boundary between grain-supported and mud-supported limestones is not based
on a fixed grain/micrite ratio. Dunham's classification was modified, with addi
tion of two new names (oatstone, rudstone), by Embry and Klovan (1972) to bet
ter reflect the presence of gravel-size (>2 mm) carbonate grains (Table 6.3B).
These authors also divided Dunham's boundstone into three types (framestone,
bindstone, bafestone) on the basis of the presumed kinds of organisms that
bound the sediment together. Wright (1992) offers a classification with names de
rived maly from the Embry-Klovan classification but which also takes into con
sideration features resultg from diagenetic processes.
Because Dunham's classification does not consider the identity of the carbon
ate grains, it may be desirable to use it in conjunction with another classification
such as Folk's. Thus, a limestone identified as a packed oomicrite using Folk's
classification could alteatively be called an oomicrite packstone using a combi
nation of Folk's and Dunham's classifications. Additional limestone classifications
are discussed in the symposium volume edited by Ham (1962), and classificatio
for mixed carbonate and siliciclastic sediment are discussed by Mount (1985) and
Zuffa (1980).
The terms coquina, chalk, and marl are commonly used as informal names
for carbonate rocks. A coquina is a mechanically sorted and abraded, poorly con
solidated carbonate sediment consisting predominantly of fossil debris; coquinite
is the consolidated equivalent. Chalk is soft, earthy, fine-textud limestone com
posed mainly of the calcite tests of floating microorganisms, such as foraminifers.
Marl is an old, rather imprecise, term for an earthy, loosely consolidated mixtu
of siliciclastic clay and calcium carbonate.
6.7 ORIGIN OF CARBONATE ROC
The chemical weathering processes discussed in Chapter 1 release chemical ions
from source rocks that eventually make their way, dissolved in groundwater and
surface water, to lakes and the oce. Bicarbonate ions (HC03 - ) may be added
-
w
ble 6.3
Clascaonoflimtones accog to deposi x
A
TEXTURE RECOGNIZABLE
DEPOS!110NAL TEXTURE
NOT RECOGNIZABLE
sition
Contains mud
(particles of clay and e silt size)
Mud-supported
Less than
10% grains
More than
1 0(}�) grains
Grain
supported
Lacks mud and is
MUDSTONE
WACKESTONE
PA CKSTONE
B
ALLOCHTHONOUS LIMESTONE
iginal components not organically
bound during deposition
Less than 10% >2 components
Contais lime n1ud ( <0.03 m.�
No lime mud
Mud-supported
Less than
I
Greater than
I
Grain-
10%
! 10% grains
I
supported
>0.03
<2 mm
MUD-
WACKE-
PACK-
GRAIN-
STONE STONE
STONE
STONE
·-
__
_
. . . as shown
by intergrown skeletal matter, lamina
tion contrary to gravity, or sediment
floored
cavities that are roofed over by
organic or questionably organic matter
and are too large to be interstices.
---
CRYSTALLINE
CARBONATE
(Subdivide according to cl assifications
designed to bear on physal texture or
diagenesis.)
AUTOCHTIIONOUS LIMESTONE
Original components organically
bound during deposition
Greater than
By organisms
By organisms
By organisms
10% >2 mm
that build that
that act as
components
a rigid
encrust
baffles
framework
and bind
>2 mm
Matrix-
component-
supported
supported
BOUNDSTONE
FLOAT-
RUD-
FRAME-
BIND-
BAFFLE-
STONE
STONE
STONE
STONE
STONE
urce: A, Af·r Dunham, R. f., 1962, Classification of carbonate rocks accord ing to depositional textures, in Ham, W. E., ed., Classication of carbonate rocks: Am. Assoc. Petroleum Geolo
g
ists Mem. 1 Ta ble 1, p. 117,.
reprinted by rmission of AAPG, Tuba, Okla. B, after Dunham, R. L 1962, as modified by Embry, E. , III and j. E. Klovan, 1972, Absolute wter depth limits of late Devonian pleoecological zones: GL Rund
hau, v. 61. Fig. 5, p. 676, reprinted by permission.
174
Chapter 6 I Carbonate Sedimenta Rocks
also by interaction of water with atmospheric and soil C02• Most dissolved ions
end up in the ocean, where they remain dissolved in seawater for periods ranging
from hundreds to millions of years. e average time at a particular chemical el
ement remains in solution in the ocean before precipitating is called its residence
time. Calcium ions (Ca
2
+) have short residence times ( �1,000,000 years) and car
bonate ions {Cacol-l have even shorter residence times ( �110,000 years).
The importance of residence times is indicated by comparing the abundance
of calcium and carbonate ions in river water to that of ocean water. Bicarbonate ions
(HC03 -) and carbonate is (C03
2
-) together make up almost 49 percent of the
total dissolved solids in average river water but less than 1 peent of e dissolved
solids in ocean water (Livingston, 1963; Mason, 1966). Calcium ions (Ca
2
+) make
up about 12 percent of dissolved solids in river water but only about 1 percent of
the dissolved solids in ocean water. The explanation for these differences in rela
tive abundance of carbonate, bicarbonate, and calcium ions in river water and
ocean water is provided by examining the relative abundance of various kinds of
nonsiliciclastic sedimentary rocks preserved in the geologic record. Measuremen
of the thickness and volume of different types of sedimentary rocks show that e
chemically and biochemically deposited sedimentary rocks make up nearly one
four of all sedimentary rocks. The majority of these rocks are carbonate rocks,
composed of calcium carbonate and calcium-magnesium carbonate minerals. Car
bonate ions, bicarbonate ions, and calcium ions have been preferentially removed
from the oceans throughout geologic time to form carbonate rocks, thus account
ing for the low relative abundance of these ions in the modern ocean and for their
short residence times. In this section, we explore the chemical/biochemical
processes that control the removal of dissolved ions from the ocean to form car
bonate rocks.
Limestones
Chemist of Calcium Carbonate Deposition
The dissolution and precipitation of the calcium carbonate (CaC03) minerals cal
cite and aragonite are controlled chiefly by pH, which in turn is largely controlled
by the partial pressure of dissolved carbon dioxide in water. When carbon dioxide
is dissolved in water, the following reactions can occur:
COz + HzO � H
2
C03 (carbonic acid)
H2
C03 H+ + HC03- (bicarbonate ion)
HC03 - H+ + C03
2
- (carbonate ion)
(6.1)
(6.2)
(6.3)
ese reactions show that the dissociation of carbonic acid to hydrogen ions and
bicarbonate ions (Equation 6.2) and the further dissociation of bicarbonate ions to
hydrogen ions and carbonate ions (Equation 6.3) release free hydrogen ions, thus
lowering the pH of the solution. The reaction shown in Equation 6.3 suggests that
adding C02 to water increases the amount of co/- in the water. The reaction
Equation 6.2 generates H ions at a rate so much faster than the reaction indicated
by Equation 6.3 generates carbonate ions, however, that adding C02 actually
causes dissolution of carbonate ions by lowering pH (increasing acidity).
If calcite or aragonite crystals are allowed to react with a carbonic acid solu
tion, these minerals are readily dissolved. This reaction can be summarized as
H20 + COz + CaC03 Ca
2
+ + 2 HC03
(calcite or
aragonite )
(6.4)