Boggs S. Principles of Sedimentology and Stratigraphy
Подождите немного. Документ загружается.
0
POROSITY .
METEORIGZONE
CEMENTATION
START OF
OVERPRESSURING
-
DISSOLUTION
-,
"NORMAL" BURIAL CURVE
HYPOTHETICAL
POROSITY-DEPTH
CURVES
6.8 Diagenesis
19
Figure 6.17
Hypothetical curves illustrating (1) a
"normal" porosity-depth relationship
for fine-grained sediments with marine
pore waters; (2) cementation in the me
teoric zone (horizontal segments) alter
nating with burial in marine pore
waters; (3) reversal of normal-porosity
depth trend owing to dissolution in the
deep subsurface, followed by resump
tion of normal burial; and (4) arrested
porosity reduction owing to abnormally
high pore pressure. [From Choquette,
P. W., and N. P. james, 1987, Diagenesis
12. Diagenesis in limestones-3. The
deep-burial environment: Geoscience
Canada, v. 14, Fig. 33, p. 23, reprinted
by permission of Geological Association
of Canada.]
by brittle fracturg and breaking and by plastic or ductile squeezing. Even at bur
ial depths as shallow as 100 m, compaction can reduce the depositional thickness of
carbonate sedents by as much as one-half, with accompanying porosity losses of
to 60 percent of original pore volumes (Shinn and Robbin, 1983).
At burial depths ranging from about 200 to 1500 m, chemical compaction of
carbonate sediments is also initiated. As discussed under "Siliciclastic Diagene
sis," pressure solution at grain-to-gra contacts can result in interpenetrating or
sutured contacts between grains (Fig. 6.14F). a larger scale, pressure-solution
seams called stylolites develop. Stylolites are particularly common in carbonate
rocks. The stylolite seams are marked by the presence of clay minerals and other
fine-size noncarbonate minerals (commonly referred to as an
i
n
soluble
residue)
that accumulate as carbonate minerals dissolve. Stylolites range in size from mi
crostylolites between grains, in which the amplitude of the interpenetrating con
tacts between grains is less than 0.25 mm, to stylolites with amplitudes exceedg
1 em (e.g., Fig. 6.16). Pressure solution, with accompanying stylolite formation,
causes significant loss of porosity (perhaps as much as 30 percent of original pore
volume) and thinning of beds.
Summary Results of Carbonate Diagenesis
The combined effects of organic, physical, and chemical diagenesis produce signifi
cant changes in the depositional characteristics of carbonate sediments. Organisms
196
Chapter 6 I Carbonate Sedimentary Rocks
desoy primary sedimentary structures such as lamination and attack carbonate
grains by boring and sediment ingestion. Physical and chemical compaction re
sulting from overburden pressure cause extensive porosity reduction and bed
thinning. Cementation further reduces porosity. On the other hand, dissolution
processes in the meteoric realm, and to a lesser extent the subsurface realm, create
new porosity. Even previously deposited cements may be dissolved during tela
genesis.
The various ways by which diagenec processes can combe to affect the
final porosity of carbonate sediments are illustrated in Figure 6.17. Finally, calciti
zation (neomorphism) converts most aragonite and high-magnesian calcite to
calcite, and subsurface dolomitization can bring about pervasive replacement of
carbonate minerals by dolomite. The photomicrographs offered in this chapter
illustrate many of the common diagenetic textures of carbonate rocks. See
Scholle and Ulmer-Scholle (2003) for numerous additional examples of diage
netic textures.
FURTHER READING
Adams, A. E., and W. S. MacKenzie, 1998, A color atlas of carbon
ate sediments and rocks under the microscope: John Wiley &
Sons,
New York, 180 p.
Baurst, R. G. C., 1975, Carbonate sediments and their diagene
sis, 2nd ed.: Elseveir, Amsterdam, 658 p.
Carozzi, A., 1993, Sedimentary petrography: PT R Prentice Hall,
Englewood Cliffs, New Jersey, 263 p.
Demicco, R. V., and L.A. Hardie, 1994, Sedimentary structures
and early diagenetic features of shallow marine c
a
rbonate de
posits: SEPM Atlas Series No. 1, Society for Sedimentary Ge
olo
gy, Tulsa, Okla., 265 p.
Grotzinger, J. P., and N. P. James (eds.), 2000, Carbonate sedimen
tation and diagenesis in the evolving Precambrian world:
SEPM
Special Publication o. 67, Tulsa, Okla., 364 p.
James, . P. , and J. A. D. Clarke (eds.), 1997, Cool-water carbon
ates: SEPM Sp ecial Publication o. 56, Society for Sedimenta
ry Geology, Tulsa, Okla., 440 p.
Montanez, I. P., J. M. Gregg, and K. L Shelton (eds.), 1997, Basin
wide diagenetic patterns: Integrated petrologic, geochemical,
and hydrologic considerations: SEPM Special Publication o.
57, Society for Sedimentary Geology, Tulsa, Okla., 302 p.
Moore, C. H., 1989, Carbonate diagenesis and porosity: Elsevier,
Amsterdam, 338 p.
Morse, J. W., and Mackenzie, 1990, Geochemistry of sedi
mentary carbonates: Elsevier, Amsterdam, 707 p.
Purser, B., M. Tucker, and D. Zenger (eds.), 1994, Dolomites: A
volume in honor of Dolomieu: International Association of
Sedimentologists, Special Publication No. 21, Blackwell Sci
entific Publications, Oxford, 451 p.
Rezak, R., and D. L. Lavoie (eds.), 1993, Carbonate microfabrics:
Springer-Verlag, New York, 313 p.
Scholle, P. A., and D. S. Ulmer-Scholle, 2003, A color guide to the
petrography of carbonate rocks: Grains, textures, porosity, di
agenesis: AAPG Memoir 77, American Association of Petrole
um
Geologists, Tulsa, Okla., 474 p.
Scoffin, P., 1987, Carbonate sediments and rocks: Blackie, Glas
gow, 274 p.
Stanley, S. M., and L.A. Hardie, 1999, Hypercalcication: Paleon
tology links plate tectonics and geochemistry to sedimentol
ogy: GSA Today, v. 9, p. 1-7.
Tucker, M. E., and P. Wright, 1990, Carbonate sedimentology:
Blackwell Scientific Publications, Oxford, 482 p.
Other Chemical/Biochemical
and Carbonaceous
Sedimentary Rocks
1 INTRODUCTION
I
n addition to the carbonate rocks discussed in Chapter 6, the chemical/bio
chemical sedimentary rocks include a diverse group of rocks of which some
(evaporites and iron-rich sedimentary rocks) form mainly by chemical process
es and others (cherts and phosphorites) form largely through biogenic processes.
Although volumetrically less significant than carbonate rocks, these sedimentary
rocks are extremely important both as economic resources and as indicators of
specialized paleoenvironments. Evaporite deposits such as gypsum, halite (rock
salt), and trona (sodium carbonate) are mined for industrial and agricultural pur
poses. The iron-rich sedimentary rocks are iron ores that have enormous world
wide economic significance as a source for most of our iron. Sedimentary
phosphorites are the major source of commercial phosphates used for fertilizers
and chemical purposes. Even the siliceous sedimentary rocks may have some eco
nomic value in the semiconductor industry.
Aside from their economic value, these chemical/biochemical rocks are ex
emely interesting because they indicate past environmental conditions that ap
pear to be uncommon on Earth today-or perhaps we simply don't recognize the
modern counterparts of these ancient environments. For example, we know of no
place on Earth where massive sedimentary iron deposits, of the type common in
late Precambrian rocks, are forming today. Nor do we fully understand the mech
anism of iron deposition or the source of the enormous amount of iron present in
iron-rich sedimentary rocks, Likewise, the mechanisms by which low levels of
phosphorus ocean water become concentrated a millionfold to form phospho
te deposits is only partially understood, this chapter, we look at the characteris
cs of these enigmatic sedimentary rocks and discuss some of the more interesting
aspects of their origin.
We also briefly exine the characteristics and origins of the carbonaceous
sedimentary rocks-rocks that contain significant amounts ( > � 10 percent) of or
ganic carbon. Together with petroleum and natural gas, these organic-rich rocks,
which include coals and oil shales, are the source of our fossil fuels. Fossil fuels
currently supply most of the world's energy needs; therefore, commercial interest
exploiting carbonaceous sedimentary rocks is understandably high, Geologists
are further interested in the origin of these rocks, particularly the processes and
197
198
Chapter 7 I Other Chemical/Biochemical and Carbonaceous Sedimenta Rocks
conditions that allow preservation of such high levels of organic matter. The aver
age organic content of other sedimentary rocks is only about 1.5 percent.
7.2 EVAPORITES
Introduction
The term evaporites is used for all deposits, such as salt deposits, that are com
posed of minerals that originally precipitated from saline solutions concentrated
by solar evaporation. Because of their generally high solubility and their suscepti
bility to deformation, many of these evaporites have subsequently been diagenet
ically or secondarily altered in various ways during burial. Thus, there are few
completely "primary" evaporite beds older than about twenty-five million years
(Warren, 1999, p. 1 ). Evaporites occur in rocks of most ages, including the Precam
brian, but they are particularly common in Cambrian, Permian, Jurassic, and
Miocene successions (Ronov et al., 1980). Although the total volume of evaporites
in the geologic record is much less than that of carbonate rocks, some individual
evaporite deposits, such as the Miocene Messinian of the Mediterranean region,
reach thicknesses exceeding 1 km. Evaporites form under both marine and non
marine conditions; however, marine evaporites tend to be thicker and more later
ally extensive than nonmarine evaporites and are of greater geologic interest.
Evaporite deposits are composed dominantly of various proportions of halite
(rock salt), anhydrite, and gypsum. Although approximately eighty minerals have
been reported from evaporite deposits (Stewart, 1963; Warren, 1999), only about a
dozen of these merals are common enough to be considered important evapor
ite rock formers. Evaporite minerals are commonly classified into those of marine
origin and those of nonmarine origin, although Hardie (1991) suggests that identi
fying the marine or nonmarine origin of evaporites on the basis of their mineralo
and chemistry may not be entirely justified. If carbonate minerals, most of
which are not evaporites, are excluded, the most common merals generally con
sidered to characterize marine evaporites are the calcium sulfate minerals gypsum
and anhydrite. Halite is next in abundance, followed by the potash salts, sylvite,
camellite, langbeinite, polyhalite, and kainite, and the magnesium sulfate,
kieserite (Table 7.1 ). The marine evaporite minerals can be grouped by chemical
composition into chlorides, sulfates, and carbonates. Marine evaporites commonly
contain mixtures of minerals, although gypsum (or anhydrite) and halite predom
inate in most horizons. Deposits may range from those that are composed almost
entirely of anhydrite or gypsum to those that are mainly halite. Gypsum is more
abundant than anhydrite in mode evaporite deposits, but anhydrite is mo
abundant in ancient deposits, owing to diagenetic alteration of gypsum to anhy
drite. Marine evaporite deposits may also contain various amounts of impurities
such as clay minerals, quartz, feldspar, and sulfur.
Nonmarine evaporites are characterized by evaporite minerals that are not
common in mare evaporites because the water fm which nonmarine evaporites
precipitate generally has proportions of chemical elements different from those of
marine water (e.g., more bicarbonate and magnesium and little or no chlorine).
These nonmarine minerals may include bloedite (Na2S04 • MgS04 ·4H20), borax
[Na2B40s(OH)4 • 8Hz0], epsomite (MgS04 · 7H z0), gaylussite (NazC03 ·CaC03 ·
SHzO), glauberite [Na2Ca(S04 )], magadiite (NaSiPB(OHh" 3H20), mirabilite
(NazS04 ·lOHzO), thenardite (NaS04), and trona [Na,H(C03h ·2H20]. Nonma
rine deposits may also contain anhydrite, gyspum, and halite and may even be
dominated by these merals.
Evaporites may be classified as chlorides, sulfates, or carbonates on the
basis of their chemical composition (Table 7. 1); however, few rock names have
le
1.1
Cl
s
caon � evapo on e
basis or
comsion
. .
Mineral class
Mineral name
Chemical composition
Rock name
lorides
Halite
NaCI Halite; rock salt
Sylvite
KCI
Carnallite
KMgCI3 · 6Hz0
Langbeinite
KzMgz
(
S04
h
Potash salts
I
Poly halite
KzCazMg(S04)6
· HzO
Kainite
KMg(S04)Cl· 3H20
I
Sulfates
Anhydrite
CaS04
Anhydrite
I
Gypsum
CaS04·2HzO Gypsum
Kieserite MgS04·HzO
Calcite
CaC03
Limestone
Carbonates
Magnesite
MgC03
-
Dolomite
CaMg(C03)z
Dolomite; dolostone
been applied to evaporite deposits. Rocks composed predominantly of the miner
al halite are called halite or rock salt. Rocks made up predominantly of gypsum or
hydrite are simply called gypsum or anhydrite, although some geologists use
the names rock gypsum or rock anhydrite. Few evaporite beds are composed
predominantly of minerals other than the calcium sulfates and halite. Specific
names have not been proposed for rocks enriched in other evaporite minerals, al
ough the term potash salts used informally for potassium-rich evaporites.
Evaporite deposits can display textures that range from primary deposition
al features to diagenetically produced features, depending upon their ages and
deformation histories. Primary features can include a variety of crystal textures
and bedding features that reflect chemical precipitation processes (e.g., crystal set
tling, bottom nucleation). Also, structures such as cross or graded bedding and
ripple marks may reflect physical processes that indicate traction-current or tur
bidity-current transport. As mentioned, many ancient (subsurface) evaporites
have undergone physical and chemical diagenetic modifications that have altered
primary textures to secondary textures such as nodules and crystal pseudo
morphs. Most buried gypsum and anhydrite deposits provide good examples of
such diagenetically altered features.
Kinds of Evaporites
Gypsum and Anhydte
Calcium sulfates are deposited dominantly as gypsum. Gypsum can be altered
into, and be pseudomorphed by, anhydrite while the sediments are still in their
general depositional environments. Gypsum is also dehydrated to anhydrite
upon burial to a few hundred meters, and this loss of water is accompanied by a
38 percent decrease in solid volume of the gypsum. Because of this rapid dehy
dration with burial, most ancient calcium sulfate deposits are composed of anhy
drite. Anhydrite can be hydrated back to gypsum after uplift and exposure to
low-salinity surface waters, with an accompanying increase in volume. These
volume changes resulting from dehydration and hydration can distort original
dositional structures and textures, and many calcium sulfate deposits are char
acterized by distorted fabrics. Three fundamental structural groups of anhydrite
2 Evaporites
199
200 Chapter 7 I Other Chemical/Biochemical and Carbonaceous Sedimentary Rocks
Figure 7
Nodular anhydrite with an abundant ca lcare
ous (dolomicrite) matrix. Falces Formation
(Oligocene), Ebro Basin, Spain. Photograph
courtesy of joseph M. Salvany, Universitat
Politecnica de Catalunya, Barcelona.
are recognized on the basis of fabric, bedding, and the presence or absence of dis
tortion: nodular anhydrites, laminated anh ydrites, and massive anhydrites.
Nodular anhydrites are irregularly shaped lumps of anhydrite that are part
ly or completely separated from each other by a salt or carbonate matrix (Fig. 7.1).
The term chicken wire structure is used for a particular type of nodular anhydrite
that consists of slightly elongated, irregular polygonal masses of anhydrite sepa
rated by thin dark stringers of other minerals such as carbonate or clay minerals
(Fig. 7.2).
The formation of nodular anhydrite is initiated by displacive growth of gyp
sum in carbonate or clayey sediments. Gypsum crystals subsequently alter to an
hydrite pseudomorphs, which continue to enlarge by addition of Ca2+ and SO/
from an external source and ultimately grow displacively into anhydrite nodules.
Chickenwire anhydrite forms when, with increasing size, the nodules ultimately
coalesce and interfere. Most of the enclosing sediment is pushed aside and what
remains forms thin stringers between the nodules.
Nodular anhydrites have been observed in many modern coastal sabkha en
vironments (see Fig. 6.11). Nodular anhydrites c also form in deeper water en
vi
ronments. fact, all that is needed for formation of nodular anhydrite is growth
of crystals in mud in contact with highly saline brines, which can cur in deep or
shallow standing water as well as in a sabkha environment.
Laminated anhydrites consist of thin, nearly white, anhydrite or gypsum lam
inations that alternate with dark gray or black laminae rich in doromite or organic
matter (Fig. 7.3). These laminae are commonly only a few millimeters thick and
rarely reach 1 em. MaRy of the thin laminae are remarkably wu form, with sharp pla
nar
contacts. Many laminae can be traced laterally for long distances-some mo
than 100 (Dean and Anderson, 1978). Also, they may compose vertical succes
sions htmdreds of meters thick in which htmdreds of thousands of laminae may be
Figure 7.2
2 Evaporites
201
Chickenwire structure in anhydrite. Evaporite series of the
Lower Lias Uurassic), Aquitaine Basin, southwest France. [From
Bouroullec, j., 1981 , Sequential study of the top of the evapor
itic series of the Lower Lias in a well in the Aquitaine Basin
(Auch 1 ), southwestern France, in Chambre Sydnical de Ia
Recherche et de Ia Production due Petrole et du Gaz Naturel
(eds.), Evaporite deposits: Illustration and interpretation of
some environ mental sequences, PI 36, p. 157, reprinted by
permission of Editions Te ch nip, Paris, and Gulf Publishing Co.,
Houston, Tex. [Photograph courtesy of ). Bouroullec.]
present (e.g., the Permian Castile Formation, southeast New Mexico and west
Te xas). Pairs of alteating light and dark bands have been suggested to be annual
varves resulting from seasonal changes water chemistry and temperature; how
ever, they might equally well represent cyclic changes or disturbances of longer
duration. Lamae of anhydrite can also alternate with thicker layers of halite, pro
ducing laminated halite.
Because of the lateral persistence of laminated evaporites, which dicates
f
o
rm depositional conditions over a wide area, laminites are commonly interpreted
to form by precipitati of evaporites in quiet water below wave base. They could
pr
e
sumably form either in a shallow-water area protted in some manner from
202
Chapter 7 I other Chemical/Biochemical and Carbonaceous Sedimentary Rocks
gure 7. 3
laminated anhydrite from the Prairie Evaporite (Devonian),
Canada.
strong bottom currents and wave agitation or in a deeper water envent. Lam
inated anhydrites in ancient sedimentary successions such as the Permian Casle
Formation indicate that these deposits have largely eaped physical deformation,
although original gypsum minerals have en altered to anhydrite.
Some laminated anhydrite may form y coalescing of anhydrite nodules,
which by continued growth in a lateral direction merge into one another to pro
duce a layer. Layers formed by tl1is mechanism are thought to be thicker and less
distinct and continuous than laminae formed by precipitation. A special type of
contorted layering that has resulted from coalescing nodules has been ob
served in some modern sabkha deposits where continued growth of nodules
creates a demand for space. The laeral pressures that result from this demand
cause the layers to become contorted, forming ropy bedding or enterolithic
structures (Fig. 7.4).
Massive anhydrite is anhydrite that lacks rceptible inteal structures. True
massive anhydrite appears to be less oommon than nodular and laminated ay
drite, and little formation is available regarding its formation. Presumably, it repre
sents sustained, uniform conditions of deposition. Haney and Briggs (19) suggest
that massive anhydrite forms by evaporation at brine salities of approately
200 to 275 £ (parts per thousand), just below the salinities at wch halite begins
to precipitate. (awater has an average salinity of 35 £.)
Halite
Halite forms as crusts, presumably in shallow water, and as very finely laminated
deposits (deep water?) that may reach thicknesses of as much as 1000 m. Laminated
halite deposits commonly include anhydrite-carbonate laminae. Anhydrite along
with other minerals such as dolomite, calcite, quartz, and clay minerals may also
Figure 7. 4
2 Evaporites
203
Enterolithic structure in nodular gypsum of
the Grenada Basin, southern Spain. [Photo
graph courtesy of ]. M. Rouchy.]
be present in halite as inclusions. Dark, inclusion-rich laminae may alternate with
light, in clusion-poor or inclusion-free laminae. Originally formed halite crystals
can be chevron, coronet, or hopper shaped; however, halite that has been recrys
tallized and subjected to movement (salt flowage) may display highly interpene
trating, cen timeter-size crystals. Halite deposits may also display sedimentary
structures such as ripples and cross-beddg.
Origin of Evaporite Deposits
Evaporation Sequence
When ocean water is evaporated in the laboratory, evaporite minerals are precipi
tated in a definite sequence that was first demonstrated by Usiglio in 1848 (reported
in Clarke, 1924). Minor quantities of carbonate minerals begin to form when the
original volume of seawater is reduced by evaporaion to about one-half. Gypsum
appears when he original volume has been reduced to about 20 percent, and
halite forms when the water volume reaches approximay 10 percent of the orig
inal volume (see also discussion in Kendall and Harwood, 1996). As discussed in
Chapter 6, the precipitation of gypsum increases the Mg/Ca ratio in maining
wa ter, which favors the process of dolomitization. Dolomite occurs in assiation
with evaporites in many ancient sedimentary successions as well as in some mod
ern environments. Magnesium and potassium salts are deposited when less than
about 5 percent of the original volume of seawater remains. The same general se
quence of evaporite minerals occurs in natural evaporite deposits, although many
discrepancies exist between the theoretical sequences predicted on the basis of
laborato ry experiments and the sequences actually observed in the rock record. In
general, e proportion of CaS04 (gypsum and anhydrite) is greater and the pro
portion of Na-Mg sulfates is less in natural deposits than predicted from theoreti
cal considerations (Borchert and Muir, 1964).
Depositional Models for Evaporites
Modern evaporite deposits accumulate in a variety of subaerial and shallow
subaqueous environments, as illustrated in Figure 7.5. Subaerial environments
include both coastal and continental sabkhas, or salt ats, and interdune environ
ments. Shallow subaqueous environments are present mainly saline coastal
lakes called salinas. With the possible exception of the Dead a in the Middle East,
mo modern examples of a deep-water evaporite basin exists; however, geologists
204
Chapter 7 I Other Chemical/Biochemical and Carbonaceous Sedimentary Rocks
Figure 7. 5
Principal settings in which
modern evaporite deposits
are accumulating. [From
Kendall, A. C., 1984, Evap
orites, in R. G. Walker (ed.),
Facies models: Geoscience
Canada Reprint Ser. 1, Fig.
1, p. 260, as modified
slightly by Warren, 1989;
reprinted by permission of
Geological
Association of
Canada.]
BEACH RIDGE
(buried)
CONTINENTAL
SABKHA-PLAYA
(fluvial-lacustrine)
INTERDUNE
(eolian domited)
-
•
�
-
COASTAL SABKHA
(marine dominated)
TIDAL DELTA
MARINE
believe that many of the thick, laterally extensive ancient" evaporite deposits did
accumulate in deep-water basins.
Some ancient evaporite deposits such as the Permian Zechstein of the North
Sea area exceed 2 km in thickness, yet evaporation of a col of seawater 1000 m
thick will produce only about 15 m of evaporites. Evaporating all the water the
Mediterranean Sea, for example, would yield a mean thickness of evaporites of
only about 60 m. Obviously, special geologic conditions operating over a long pe
riod of time are required to deposit thick successions of natural evaporites. The
basic requirements for deposition of marine evaporites are a relatively arid cli
mate, where rates of evaporation exceed rates of precipitation, and partial isola
tion of the depositional basin from the open ocean. Isolation is achieved by means
of some type of barrier that restricts free circulation of ocean water into and out of
the basin. Under these restricted conditions, the brines formed by evaporation are
prevented from returning to the open ocean, causing them to become concentrated
to the point where evaporite minerals are precipitated.
Although geologists agree on these general requirements for formation of
evaporites, considerable controversy still exists regarding deep-water vs. sham!ow
water depositional mechanisms for many ancient evaporite deposits. Figure 7.6
shows three possible models for deposition of thick successions of marine evapor
ites. The deep-water, deep-basin model assumes existence of a deep basin separated
from the open ocean by some type of topographic sill. The sill acts as a barrier to
prevent free interchange of water in the basin with water in the open ocean, but it
allows enough water into the basin to replenish that lost by evaporation. Seaward
escape of some brine allows a particular concentration of brine to be maintained
for a long time, leading to thick deposits of certain evaporite minerals such as gyp
sum. The shallow-water, shallow-basin model assumes concentration of brines in
a
shallow, silled basin, but it allows for accumulation of great thicknesses of evap
orites caused by continued subsidence of the floor of the basin. The shallow-water,
deep-basin model r
.
equires that the brine level the basin be reduced well below
the level of the sill, a process called evaporative drawdown; recharge of wa ter