Boggs S. Principles of Sedimentology and Stratigraphy
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5.3 onglomerates
1 35
ensands (glauconitic sands), phosphatic sandstones, and calcarenaceous
dstones (composed of sand-size carbonate grains). These rocks are not te sand-
stones (siliciclastic rocks) but rather are chemical/biochemical sedimentary rocks
(Chapters 6 and 7).
5.3 CONGLOMETES
The te onglomerates is used in this book as a general class name for sedimen
tary that contain a substantial fraction (at least 30 percent) of gravel-size
(>2 mm) particles (Fig. 5.8A). Breccias (Fig. 5.88), which are composed of very
angular, gravel-size fragments, are not distinguished from conglomerates in the
succeeding diussion. Conglomerates are common in stratigraphic successions of
all ages but probably make up less than l percent by weight of the total sedimen
tary ck mass (Garrels and McKenzie, 1971, p. 40). They are closely related to
ndstones in terms of origin and depositional mechanisms, and they contain
some of the·same kinds of sedimentary structures (e.g., tabular and trough cross
bding, g�'aded bedding).
Paicle Composition
Conglomerate may contain gravel-size pieces of individual minerals such as
quartz; however, most of the gravel-size framework grains are rock fragments
(clasts). Individual sand- or mud-size mineral grains are commonly present as a
Figure 5.8
A. Clast-supported conglomerate underlying laminated
sands. Terrace deposits (Holocene) of the Umpqua River,
southwest Oregon. B. Large breccia clasts (dark) cement
ed with calcite. Nevada Limestone (Devonian), Treasure
Peak, Nevada. Photograph courtesy of Walter
Yo ungquist.
136 Chapter 5 I Siliciclastic Sedimentary Rocks
matrix. Any kind of igneous, metamorphic, or sedimentary rock may be present in
a conglomerate, depending upon source rocks and depositional conditions. Some
conglomerates are composed of only the most stable and durable kinds of clasts
(quartzite, chert, vein-quartz). Stable conglomerates composed mainly of a single
clast type are referred to by Pettijohn (1975) as oligomict conglomerates. Most
oligomict conglomerates were probably derived from mixed parent-rock sources
that included less stable rock types. Continued recycling of mixed ultrastable and
unstable clasts through several generations of conglomerates ultimately led to se
lective destruction of the less stable clasts and concentration of stable clasts. Con
glomerates that contain an assortment of many kinds of clasts are polymict
conglomerates. Polymict conglomerates that are made up of a mixture of largely
unstable or metastable clasts such as basalt, limestone, shale, and metamorphic
phyllite are commonly called petromict conglomerates (Pettijohn, 1975). Almost
any combination of these clast types is possible in a petromict conglomerate. The
matrix of conglomerates commonly consists of various kinds of clay minerals and
fine micas and/ or silt- or sand-size quartz, feldspars, rock fragments, and heavy
minerals. The matrix may be cemented with quartz, calcite, hematite, clay, or other
cements.
Classification
Conglomerates can originate by several processes, as shown in Table 5.3. We are
interested most in epiclastic conglomerates, which form by breakdown of older
rocks through the processes of weathering and erosion. Epiclastic conglomerates
that are so rich in gravel-size framework grains that the gravel-size grains touch
l
e
5.3
fd en
l
genefk pes of co
n
glo��
d
�
fdas
�
�
�
Major typ es
Epiclastic conglomerate
and breccia
Vo lcanic breccia
Cataclastic breccia
Solution breccia
Meteorite impact breccia
Subtypes
Extraformational
conglomerate
and breccia
Intraformational
conglomerate
and breccia
Pyroclastic breccia
Auto breccia
Hyaloclastic breccia
Landslide and slump
breccia
Tectonic breccia: fault,
fold, crush breccia
Collapse breccia
Origin of clasts
Breakdown of older rocks of any kind through the processes of
weathering and erosion; deposition by fluid flows (water, ice)
and sediment gravity flows
Penecontemporaneous fragmentation of weakly consolidated
sedimentary beds; deposition by fluid flows and sediment
gravity flows
Explosive volcanic eruptions, either magmatic or phreatic
(steam) eruptions; deposited by air-falls or pyroclastic flows
Breakup of viscous, partially congealed lava owing to continued
movement of the lava
Shattering of hot, coherent magma into glassy fragments owing
to contact with water, snow, or water-saturated sediment
(quench fragmentation)
Breakup of rock owing to tensile stresses and impact during
sliding and slumping of rock masses
Breakage of brittle rock as a result of crustal movements
Breakage of brittle rock owing to collapse into an opening
created by solution or other processes
Insoluble fragments that remain after solution of more soluble
material; e.g., chert clasts concentrated by solution of
limestone
Shattering of rock owing to meteorite impact
Source: Modified from Pettijohn, F. J., 1975, Sedimenta
rocks, 3rd ed., Harper & Row, New York, p. 165.
5.3 Conglomerates
137
and form a supporting framework are called clast-supported conglomerates.
Clast-poor conglomerates that const of sparse gravels supported in a mud/ sand
matrix are called matrix-supported conglomerates. Boggs (1992, p. 212) suggests
that clast-supported conglomerates be referred to simply as conglomerates and
at matrix-supported conglomerates be called diamictites. Although the term di-
ictite
is often used for poorly sorted glacial deposits, it is actually a nongenetic
rm that can be applied to nonsorted or poorly sorted siliclastic sedimentary
cks that contain larger parcles of any size in a muddy matrix.
Conglomerates and diamictites can be further divided on the basis of clast sta
bility into quartzose (oligomict) conglomerate/diamictite and petromict conglom
ate/diactite on the basis of relative abundance of these clast types (Table 5.4).
Furer classification on the basis of clast type (igneous, metamorphic, sedimenta
) can be made if desired (Fig. 5.9); however, such classification may not be nec
essary in many cases.
Origin and Occurrence of Conglomerates
Quarose (oligomict) conglomerates are derived from metasedimentary rocks
containing quartzite beds, igneous rocks containing quartz-filled veins, and sedi
mentary successions, particularly limestones, containing chert beds. As men
oned,
less stable rock types must have been destroyed by weathering, erosion,
and sediment transport, perhaps through several cycles of transport, to produce a
siduum of stable clasts. Because quartzose clasts represent only a small fraction
of a much larger original
body of rock, the total volume of quartzose conglomer
ates is small. They tend to occur as thin, pebbly layers or lenses of pebbles in dom
inantly sandstone units. They may be either clast-supported or matrix-supported.
Although their overall volume is small, quartzose conglomerates are common in
the geologic record ranging from the Precambrian to the Tertiary. Most quartzose
conglomerates appear to be of fluvial origin (Chapter 8) and are probably deposit
ma ly in braided streams. Marine, wave-worked quartzose conglomerates
that were deposited in the littoral (beach) environment are also known.
Most petromict conomerates are polymict conglomerates that consist of a
vaety of metastable clasts. They can be derived from many kinds of plutonic
neous, volcanic, metamorphic, and sedimentary rocks, although the clasts in a
particular conglomerate may be dominantly one or another of these rock types.
Thus, a particular conglomerate may be a limestone conglomerate, a basalt con
glomerate, a schist conglomerate, and on. Conglomerates composed dominantly
of plutonic igneous clasts appear to be uncommon, probably because plutonic
rs such as granites tend to disintegrate into sand-size fragments rather than
larger blocks. The volume of ancient petromict conglomerates is far greater
than that of quartzose conglomerates. They form the truly great conglomerate
bodies of the geologic record and may reach thicknesses of thousands of meters.
Preservation of such great thicess of conglomerate implies rapid erosion of
sharp
ly elevated highlands or areas of active volcanism. Petromict conglomerates
may be transported by fluid-flow and sediment gravity-flow mecnisms; they
Percentage of
ultrastable clasts
>90
<90
Ty pe of fabric support
Clast-supported
Quartzose conglomerate
Petromict conglomerate
Matrix-supported
Quartzose diamictite
Petromict diamictite
.
w
Granoblastic and hornfelsic clasts
QUARTZ ITE, MARBLE, HORNFELS, ETC .
CONGLOMERATE
OR DIAMICTITE
GNEISS CONGLOMERATE
OR DIAMICTITE
SCHIST, PHYLLITE, SLATE, ETC.
CONGLOMERAT E OR DIAMICTITE
GRANITE, DIORITE,
GABBRO CONGLOMERATE,
OR DIAMICTITE
RHYOLITE, DACITE,
CONGLOMERATE,
OR DIAMICTITE
Felsic volcanic 50
%
Intermediate and mafic
\
Che clasts
Fire 5.9
clasts
volcanic clasts
ANDESITE, BASALT,
CONGLOMERAT E,
OR DIAMICTITE
50
%
M = META MORPHIC CLASTS
= IGNEOUS CLASTS
S = SEDIMENTARY CLASTS
LIMESTONE, DOLOMITE
CONGLOMERATE, OR
DIAMICTITE
SANDSTONE, SHALE ETC.
CONGLOMERATE, OR
DIAMICTITE
Siliciclastic clasts
Classification of conglomerate on the basis of clast lithology and fabric support. [From Boggs, 1992, Petrology of sedimenta rocks,
Fig. 6.3, p. 220: Macmillan Publishing Co., New York, reproduced by permission of Prentice
5.4 Shales (Mudrocks)
139
are deposited in environments ranging from uvial rough shallow marine to
deep marine. Deep-marine conglomerates are so-called resedimented con
g
lomer-
ates that were retransported from nearshore areas by tbidity currents or other
sediment gravity-ow processes. The bulk of the truly thick conglomerate bodies
(>20 m) were probably deposited in nonmarine (alluvial fan/braided river) set-
gs or deep-sea fan settings.
Intraformational conglomerates are composed of clasts of sediments be
lieved to have formed within depositional basins, in contrast to the clasts of ex
trarmational conglomerates that are derived from outside e depositional
basin. Intraformational conglomerates originate by penecontemporaneous defor
mation of semiconsolidated sediment and redeposition of the fragments fairly
dose to the site of deformation. Penecontemporaneous breakup of sediment to
form clasts may take place subaerially, such as by drying out of mud on a tidal flat,
or
under water. Subaqueous rip-ups of semiconsolidated muds by tidal currents,
waves, or sediment-gravity ows are possible causes. In any case, sedimen
on is interrupted only a short time ding this process. The most common
types of fragments found in traformational conglomerates are siliciclasc mud
as and lime clasts. The clasts are commonly angular or only slightly rounded,
suggesting little transport. In some beds, attened clasts are stacked virtually on
edge, apparently owing to unusually strong wave or current agitation, to form
what called ed
g
ewise con
g
lomerates (Pettijohn, 1975, p. 184).
Intraformational conglomerates commonly form thin beds, a few centime
rs to a meter in thickness, that may be laterally extensive. Although much less
abundant than extraformational conglomerates, they nonetheless occur in rocks of
many ages. So-called flat-pebble conglomerates composed of carbonate or limy
siltone clasts are particulay common in Cambrian-age rocks in various parts of
North America. They also occur in many oer early Paleozoic limestones of the
Appalachian region. Intraformational conglomerates composed of shale rip-up
asts embedded in the basal part of sandstone units are very common in sedi
mentary successions deposited by sediment gravity-flow pcesses.
5.4 SHALES (MUDROCKS)
Shales are fine-grained, siliciclastic sedimentary rocks, that is, rocks that contain
more an 50 percent siliciclastic grain less than 0.062 (1 /256) mm. Thus, they are
made up domantly of silt-size (1/16-1 /256 mm) and clay-size (<1 /256 mm)
pacles. Shale is an historically accepted class name for this group of rocks
(Toulot, 1960), equivalent to the class name sandstone, a usage accepted by Pot
ter, Maynard, d Pryor (1980, p. 12-15). These authors use the term shale as the
class name for all fine-grained siliciclastic sedimentary rocks, but they divide
ales into several kinds, such as mudstones and mudshales, depending upon the
percentage of day-size constituents and the presence or absence of lamination
(discussed subsequently under classification).
the other hand, some auors prefer to use the class name mudrock,
rather than shale, for all fine-graed rocks (e.g., Blatt, Middleton, and Murray,
1980, p. 382). They divide mudrocks into shales (if laminated) or mudstones (if
nonlaminated). Thus, they restrict the usage of shale to fine-grained rocks, such as
ose in Figure 5.10A, that display lamination or fissility (the ability to split easi
ly into thin layers). Fine-grained, nonlaminated rocks such as those shown in
Figure 5.108 are, according to this usage, mudstones. In this book, l follow the
usage of Potter, Maynard, and Pryor and apply the general class name shale to all
e -grained siliclastic sedimentary rocks. Clearly, however, the usage of shale or
mudrock as a class name for ne-grained siliciclastic rocks is a matter of personal
pce.
140
Chapter 5 I Siliciclastic Sedimentary Rocks
Figure 5.10
A. Laminated (red) shale (pre-Mississippi
an), Arctic National Wildlife Refuge, Alas
ka. Note binoculars for scale. B.
Lacustrine mudstones (nonlaminated),
Furnace Creek Formation
(Miocene/Pliocene), Death Va lley, Califor
nia. Photograph by james Stovall.
Shales are abundant in sedimentary successions, making up roughly 50 per
cent of all the sedimentary rocks in the geologic record. Historically, shales have
been an understudied group of rocks, mainly because their fe grain size makes
them difficult to study with ordinary petrographic microscope. This perspec
tive is changing, however, as instruments are developed such as the scanning elec
tron microscope and electron probe microanalyzer that allow study of fine-size
grains at high magnification (e.g., Fig. 5.11).
Composition
Mineralogy
Shales are composed primarily of clay minerals and fine-size quartz and feldspars
(Table 5.5). They also contain various amounts of other minerals, including car
bonate minerals (calcite, dolomite, siderite), sulfides (pyrite, marcasite), iron ox
ides (goethite), and heavy merals, as well as a small amount of organic carbon.
Figure 5.11 is a high-magnification photograph, taken by use of backscattered
scanning electron microscopy, which allows both the meralogy and texture of
this laminated shale to be examined. [See Krinsley et al., 1998, for discussion of the
use of backscattered electron microscopy the study of sedimentary rocks.] The
5.4 Shales (Mudrocks)
141
Figure 5.11
Backscattered scanning electron mi
croscope (BSE) photograph of a lami
nated shale. The elongated, pla, or
flaky minerals are clay minerals (illite)
and fine micas. Other coarser miner
als are quartz (Q), feldspar (F), calcite
(C), mica (M), and pyrite (very bright
mineral). Note that orientation of
clay minerals and fine micas creates
the lamination. Whitby Mudstone
(Jurassic), Yorkshire, England. Scale
bar = 50 pm. Photograph courtesy
of David Krinsley.
data in Table 5.5 show mineral composition as a ftmction of age. No discernible
nd of mineralogy vs. age is evident from this table, except possibly a slight
nd of decreasing feldspar with increasing age. Many factors affect the composi
tion of shales, including tectonic setting and provenance (source), depositional en
vironments, grain size, and burial diagenesis. Some minerals, such as carbonate
minerals and sulfides, form in the shales during burial as cements or replacement
minerals. Quartz, feldspars, and clay minerals are mainly detrital (terrigenous)
5.5
Av erage percent composion of ss of different ages
Age
Quaternary
Pliene
Miene
Oligene
Eene
Cretacus
Jurassic
Triaic
Permian
Penylvanian
ssissippi
vo
dovici
. ages
5
4
9
4
11
9
10
9
1
7
3
22
2
29
�
l .5
0 s
29.9
56.5
25.3
33.7
40.2
27.4
34.7
29.4
17.0
48.9
57.2
41.8
44.9
47.8
42.3
14.6
34.1
53.5
34.6
52.9
21.9
45.9
28.0
32.6
29.1
47.1
32.2
33.1
12.4
5.7
7.4
3.0
2.0
3.6
0.6
10.7
4.0
0.8
0.4
0.6
<1.0
<1.0
11.9
11.7
8.1
1.6
4.4
0.7
8.0
6.2
2.9
6.3
5.5
6.6
3.2
14.6
5.5
3.8
2.9
14.6
3.7
1.4
2.0
9.8
5.2
�
.
'§
0
0
2.4
1.2
4.6
7.9
1.6
4.1
1.0
2.1
1.3
0.5
2.3
�
·
2.9
1.7
0.1
0.4
5.1
3.4
0.6
0.3
0.5
0.8
r: Brien, N. R., and R. M. Sial I, 10, Argillaceous rock atlas, Sprger-Verlag, New York, Ta ble I, p. 12125.
Valu adjust lo 1 for shales of each age.
5.6
1.8
1.9
1.6
1.6
10.9
3.5
5.1
3.3
3.4
3.1
:
.5
0 s
<1.0
2.4
4.0
42.0
u
·
�
:
: 0
l �
l
0 u
0.9
2.6
1.4
0.4
3.5
2.0
10.9
0.3
0.2
1.0
4.7
3.7
1.5
4.5
142
Chapter 5 I Siliciclastic Sedimenta Rocks
Fire 5.12
minerals, although some fraction of these minerals may also form during burial
diagenesis. particular, clay minerals appear to be strongly affected by diagenet
ic processes. As shown in Ta ble 5.1, the principal clay mineral groups are kaolinite,
illite, smectite, and chlorite. The relative proportions of these day-mineral groups
have been reported to change systematicaUy with age (Fig. 5.12). With time, par
ticularly in rocks older than the Mesozoic, the proporti011 of illite an chlorite in
creases at the expense of kaolinite and smectite. These trelds are attributed to the
diagenetic alteration of kaolinite and smeotte to form illite and chlorite.
Chemical Composition
The chemical composition of shales is a direct h.mction of their mineral composi
tion. Compositions of some average North American and Russian shales are
shown in Ta ble 5.6. Si02 is the most abundant chemical oonsttuent in these shales
(578 percent), followed by Al203 (16-19 percent). The Si02 content of shales is
affected by all silicate minerals prent but particularly by quartz. Thus, shales
tend to contain less Si02 than do sandstones, which commonly are enriched
quartz. Al203 is derived mainly from clay minerals and feldspars. It is more abun
dant in shales than in sandstones because of the greater clay mineral content of
Cenozoic
Mesozoic
Paleozoic
� smectite
illite
20
40
� kaolinite
chlorite
60 80
Systematic changes in relative abundance of
major clay minerals as a function of geologic
age. [After Singer, A., and G. Muller, 1983,
Diagenesis in argillaceous sediments, in Lar
son, G., and Chilingarian, G. V. (eds.), Dia
genesis in sediments and sedimentary rocks,
v. 2, Fig. 3.28, p. 176, reproduced by per
mission of Elsevier Science Publishers, New
York.]
Proterozoic
5.4 Shales (Mudrocks)
143
1
2
3 4
5
6 7 8 9
10
Si02
60.65
64.80
59.75
56.78
67.78
64.09
66.90 63.04 62.13
65.47
Al203
17.53
16.90
17.79
16.89
16.59
16.65
16.67
18.63
18.11
16.11
Fe203
7.11
F
5.66
5.59
6.56
4.11
6.03
5.87
7.66
7.33
5.85
MgO
2.04 2.86
4.02
4.56
3.38
2.54
2.59
2.60
3.57
2.50
CaO
0.52
3.63
6.10 8.91
3.91
5.65
0.53
1.31
2.22
4.10
NazO
1.47
1.14
0.72
0.77
0.98
1.27 1.50
1.02
2.68
2.80
KP
3.28
3.97
4.82
4.38
2.44
2.73
4.97
4.57
2.92
2.37
Ti0
2
0.97
0.70
0.98 0.92
0.70 0.82
0.78
0.94
0.78
0.49
P
2
0
s
0.13
0.13 0.12
0.13
0.10 0.12
0.14
0.10
0.17
MnO
0.10
0.06
0.08
0.07
0.06
0.12
1.10
0.07
u:
(1) Moore, 1978 (Pennsyh-anmn shale, lHinois Basin)
(2) Gromet et aL, 184 (North American shale composite)
{3) Ronov and .1igdisov, 1971 (average orth American Paleozoic shale)
(4) Ronov and Migdisov, 1971 (average Russian Paleozoic shale)
(5) Ronov and Migdisov, 1971 (average North American Mesozoic shale)
(6} Ronov and igdisov, 1971 (aYerage Russian Mesozoic shale}
(
71
Camer and Garrels, 1980 (averag(• Canadian Proterozoic shale)
{8) Ronov and Mtgdisov. 1971 (average Russian Proterozoic shale)
() Cameron and Garrels, 1980 (average Canadian Archean shale)
{10) Ronov and Migdisov, 1971 (average Arhean shale}
(11) Clarke, 1924 (average shale)
(l2i Shaw, 1956 (compilation of 155 analyses of shale)
(13) Av erage of alues in columns 1 through 12
shales. Fe in shales is supplied by iron oxide minerals (hematite, goethite), biotite,
and a few other minerals such as siderite, ankerite, and smectite day minerals.
K20 and MgO abundance is related mainly to day mineral abundance, although
some Mg may be supplied by dolomite and K is present in some feldspars. Na
abundance is related to the presence of clay minerals (e.g., smectites) and sodium
plaoclase. Ca is supplied by calcium-rich plagioclase and carbonate minerals
(calte, dolomite).
Classification
Because special analytical techniques are required to determine the mineral com
posion of shales, and because such techniques are time consuming and expen
sive, many geologists do not routinely determine the mineral composition of
shales (mudrocks). erefore, most classifications that have been proposed for
shales
have not been based on mineral composition, or at least not entirely on
meral composition. ese classifications, none of which has been widely accept
, commonly emphasize the relative amounts of silt and day, the hardness or de
e of induration of the rocks, and the presence or absence of fissile lamination.
(Fissility is defined as the property of a rock to split easily along thin, closely
spaced, approximately parallel layers.) Exceptions to this general practice of clas
sificaon are the classifications of Picard (1971), which emphasizes mineral com
position of the silt-size grains in mudrocks (shales), and the classification of
Lewan (1978), which requires semiquantitative X-ray diffraction analysis to deter
me mineralogy.
11
12
13
64.21
64.10
63.31
17.02 17.70
17.22
2.70
0.82
6.71
4.05
5.45
2.70
2.65
3.00
3.44
1.88
3.52
1.44
1.91 1.48
3.58
3.60
3.64
0.72
0.86
0.81
0. 10
0.05
0.06
144
Chapter 5 I Siliciclastic Sedimentary Rocks
�
�
B
z
�
z
�
�
�
:
�
�
The classification of Potter, Maynard, and Pryor (1980), shown in Table 5.7, is
based on grain size, lamination, and degree of induration. It is similar to the field
classification of shales proposed by Lundegard and Samuels (1980). This classifi
cation emphasizes the importance of day-size constituents and bedding thickness,
that is, whether bedded or laminated. Depending upon these variables, shales can
be divided into mudstone (33-65% day-size constituents and bedded) or
mudshale (33-65% clay-size constituents and laminated), and claystone (66-100%
clay-size constituents and bedded) or clayshale (66-100% day-size constituents
and laminated). Fine-grained siliciclastic rocks that contain less than 33 percent
day-size constituents are siltstones.
Additional informal terms can be used with this classification to provide fur
ther information about the properties of the shales. These may include terms that
express color, type of cementation (calcareous, or limy; ferruginous, or iron-rich;
siliceous); degree of induration (hard, soft); mineralogy if known (e.g., quartzose,
feldspathic, micaceous); fossil content (e.g., fossiliferous, foram-rich); organic mat
ter content (e.g., carbonaceous, kerogen-rich, coaly); type of fracturing (con
choidal, hackly, blocky); or nature of bedding (e.g., wavy, lenticular, parallel).
Origin and Occurrence of Shales (Mudrocks)
Shales form under any environmental conditions in which fine sediment is abun
dant and water energy is sufficiently low to allow settling of suspended fine silt
Pe rcentage day-size
constituents
Field adjective
Beds
>lOmm
Laminae
<lOmm
Beds
>lOmm
Laminae
<lO mm
Low
e
.�
j
O
��
g�
I
e
High
32
Gritty
Bedded
silt
Laminated
silt
Bedded
siltstone
Laminated
siltstone
Quartz
Argillite
Quartz
Slate
335
66-100
Loamy
Fat or slick
Bedded
Bedded
mud
claymud
Laminated
Laminated
mud
claymud
Mudstone Claystone
Mudshale
Clays hale
Argillite
Slate
Phyllite and/ or Mica Schist
eo Potter. P E.,). B. Maynard, and W. . Pryor, 1980, imentology of sh aleso Springer-Ve rlag, New York, Ta ble 1.2, p. 14.