Boggs S. Principles of Sedimentology and Stratigraphy
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Rock fragments are particularly important in studies of sediment source
ks. They are moderately easily identified, and they are more reliable indicators
of source rock types than are individual merals such as quartz or feldspar, which
n derived from different types of source rocks.
Mineral Cements
fra mework gras in most siliciclastic sedimentary rocks are bound together
by some type of mineral cement. These cementing materials may be either sili
cate minerals such as quartz and opa� or nonsiicate minerals such as calcite and
dolomite. Quarz is the most common silicate miner!l that acts as a cement. In
most sans tones, the quartz cement is �heically attached to the crystal lattice of
isg quartz grains, forming rims of cement �aled overgrowths (Fig. 5.3A).
Such overgrowths bhal retain crystallographic continuity of a grain are said to be
syntaxiaJ. Because syntaxial overgrowths are optically continuous with the origi
nal
grain, they go fa extinction the same position as the origal grain when ro
tated on the stage of a polarizing microscope. Overgrowths can be recognized by a
line
of impurities or bubbles that mark the surface of the origal grain. Quartz
overgrw
ths are particularly common in quartz-rich sandstones. Less commonly,
q
uartz cement is present as microcrystalline quartz, which has a fine-grained,
crystalline texture similar to that of chert. When silica cement is deposited as mi
crrystalline quartz, it forms a mosaic of very tiny quartz crystals that fill the in
te rstitial spaces among framework silicate grains (Fig. 5.38). Not uncommonly, the
Fire 5.3
Common cements in sandstones. A. Quar ove rgrowths (arrows), Lamotte Sandstone
(Cambrian), Missouri. B. Microquartz (chert; arrows) cementing quartz and chert grains,
fn City Fm. (Ordovician), Missouri. C. Calcite, Kayenta Fm. (Triassic), Utah. D. Clay
neral (chlorite; arrows), Miocene sandstone, Japan Sea. Crossed nicol photomicrographs.
5.2 Sandstones
125
126
Chapter 5 I Siliciclastic Sedimentary Rocks
crystals next to the framework grains are smalt slightly elongated, and are orient
ed normal to e surfaces of the framework grains. More rarely, opal (an isotropic
mineral) occurs as a cement in sandstones, particularly in sandstones rich in vol
canogenic materials. Like quartz and microcrystalline quartz (chert), opal is also
composed of Si02, but, unlike these minerals, opal contains some water and lacks
a definite crystal structure. Thus, it is said to be amorphous. Opal is metastable
and crystallizes in time to microcrystalline quartz.
Carbonate minerals are the most abundant nonsilicate mineral cements in
siliciclastic sedimentary rocks. Calcite is a particularly common carbonate cement.
It is precipitated in the pore spaces among framework grains, typically forming a
mosaic of smaller crystals (Fig. 5.3C). These crystals adhere to the larger framework
grains and bind them together. Less common carbonate cements are dolomite and
siderite (iron carbonate). Oer minerals that act as cements in sandstones include
the iron oxide minerals hematite and limonite, feldspars, anhydrite, gypsum,
barite, day minerals (Fig. 5.3D), and zeolite minerals. Zeolites are hydrous alumi
nosilicate minerals that occur as cements primarily in volcaniclasc sedimentary
rocks (discussed in a subsequent section).
All cements are secondary minerals at form in sandstones after deposition
and during burial. Details of the cementation process are given in Section 5.5
(Diagenesis).
Matrix Minerals
Grains in sandstones smaller than about 0.03 mm, which fill interstitial spaces
among amework grains, are referred to as matrix minerals. Matrix minerals may
include fine-size micas, quartz, and feldspars; however, clay minerals make up the
bulk of matrix grains. Because of their small size, day minerals are difficult to
identify by routine petrographic microscopy. They must be identified by X-ray dif
fraction techniques, electron microscopy, or other nonoptical methods. Clay min
erals are compositionally diverse. They belong to the phyllosilicate mineral group,
which is characterized by two-dimensional layer structures arranged in inde
nitely extending sheets.
The most common clay mineral groups are illite [K2(Si�l2)Al4020(0H)4],
smectite (monorillonite) [(Al, Mg)8(Si4010)3(0H)10 · 12H20], kaolinite [Al2Si2
05(0H)4], and chlorite I (Mg, Fe )5( AI, Fe
3
+)zSi3010( OH)8]. Kaolinite is a
layer clay; the others are three-layer clays. Smectite is a day-mineral group, of
which monorillonite is a principal variety. Clay minerals form prcipally as
secondary minerals during subaerial weathering and hydrolysis, alough they
can also form by subaqueous weathering in the marine environment and during
burial diagenesis.
Chemical Composition
Sedimentologists have traditionally placed relavely little emphasis on study of
the chemical composition of siliciclastic sedimentary rocks, in sharp contrast to
geologists who study igneous and metamorphic rocks. This lack of interest is
mainly attributed to the common belief that chemical composition of siliciclasc
rocks is less useful than mineral composition for interpreting depositional history
and provenance. Also, the present chemical composition of these rocks may not
accurately reflect their composition at the time of deposition, because crystalliza
tion of new minerals during sediment burial and diagenesis can change the origi
nal chemical composition. e formerly high cost of doing chemical analyses is an
addional factor that helped to discourage extensive chemical studies of sedimen
tary rocks. Several new tools, such as the electron probe microanalyzer and X-ray
fluorescence equipment, now enable rapid and comparatively inexpensive chemi
cal analyses of rock composition. These new tools, as well as changing attitudes
garding the significance of chemical composition, are causing sedimentologist&
develop a deeper terest in the chemistry of sedimentary rocks. For example,
emical data, both bulk chemical composition and the trace-element composition
of individual minerals, have been applied to provenance studies, that is, studies
linking sedimentary materials to their source rocks (e.g., Morton et aL, 1991;
Johnsson and Basu, 1993).
Because most grains in siliciclastic sedimentary rocks are derived from vari
ous types of igneous, metamorphic, and sedimentary rocks, the mineralogy and
chemical composition of siliciclastic rocks are clearly a function of parent rock
composition. Nonetheless, sedimentary rocks display distinct chemical differ
ences from parent source rocks owing to chemical changes that occur during
weathering and diagenesis. For example, they tend to be enriched in silica and de
pleted in iron, magnesium, calcium, sodium, and potassium compared to the par
ent rocks. Enrichment in silica occurs because siliceous minerals resist chemical
weathering; thus, silica is concentrated in weathering residues with respect to
more soluble cations. Also, the superior chemical stability of Si02 minerals such as
quartz and chert causes sedimentary rocks to become progressively enriched in
ese minerals during multiple recycling events. Thus, overall, silica content in
cases at the expense of less stable iron- and magnesium-rich minerals.
The average chemical composition of some sandstones reported in the litera
ture is shown in Ta ble 5.2. I stress that this table shows average composition of a
few sandstones. Specimens of sandstone from other formations may have chemi
cal compositions that deviate considerably om these average values. Silicon, ex
pssed as Si02, is the most abundant chemical constituent in all types of
sandstones because of the abundance of quartz in sandstones and the presence of
silicon in all silicate minerals. Aluminum (A1203) is moderately abundant in
sandstones containing abundant feldspars or in rock-fragment-rich sandstones
at contain a matrix of clay minerals. It is much less abundant in quartz-rich
sandstones, which commonly do not have a clay matrix. On average, iron, magne
sium, calcium, sodium, and potassium are all less abundant in sandstones than is
aluminum. Relative concentrations of these elements vary as a function of the
mineralogy of the sand-size grains and the types of matrix clay minerals and dia
getic cements in the rock. For example, sandstones with abundant calcium car
bonate cement or carbonate fossils may have anomalously high calcium content.
Classification of Sandstones
Descriptive classification of sandstones is based fundamentally on framework
mineralogy, although the relative abundance of matrix plays a role in some classi
fications. Although mineralogy is the principal basis for classifying sandstones,
nding a classification that is suitable for all types of sandstones and acceptable to
most geologists has proven to be an elusive goaL In fact, more than fifty different
classifications for sandstones have been proposed (Friedman and Sanders, 1978),
but none has received widespread acceptance. Classifications that are all-inclusive
tend to be too complicated and unwieldy for general use, and classifications that
are oversimplified may convey too little useful information.
Textural Nomenclature of Mixed Sediments
Unconsolidated siliciclastic sediment is called gravel (dominance of >2 m-size
grains), sand (1/16-2 mm), or mud ( <1/16 mm ), depending upon gra size. The
lithified rock equivalents of these sediments are conglomerate, sandstone, and
shale (mudrock). Because many siliciclastic sedimentary rocks a composed of
5.2 Sandstones
127
128 Chapter 5 I Siliciclastic Sedimentary Rocks
otto
$� �
(
1
)
(2)
(3)
(4
)
(5
)
(6)
(7)
(8)
(
9
)
n 11
30
16 18
12
119 12
59
Si02 86.5
67.8 65.6
56.9
56.2 68.4 70.6
37.3
50.3
Ti02
0.53 0.95
0.91 1.42
0.89 0.69
0.64 0.34
0.64
z
03
5.71 15.4
15.1
12.3
15.3
13.5
12.6
7.91
14.0
Fe203(t)
2.69 6.46
6.09 6.18 6.48
5.30
4.97
3.18
6.40
MnO 0.02 0.07
0.15
0.11
0.07
0.09
0.08
0.10 0.13
M
0.69
1.73
1.82
4.20
2.35
1.68
1.51
1.07
3.25
CaO
0.05
0.42
1.94
5.82
5.74
2.38 1.61 26.0
9.90
Na20
0.02 1.07
0.87
1.92 1.28
3.15
2.76
0.92
KzO
1.55
2.74
3.03
1.90
2.80
2.62 2.20 0.51
2.09
Pz
O
s
0.02
0.16
0.17 0.17 0.17
0.18 0.02
0.10
0.21
V (ppm)
51
123
159
100
126
71
79
103
Cr
55
82 88 225
71
55
44
31
Ni
19
231
58
130 49
30
8
5
49
29
52
104
84 114
69
66
91
Rb 12.1
133
125
93
10
79
Sr
29
1
113
233
168
310
110
879
267
y
17
31
40
21
35
36 37
15
29
Zr
417
238
260
191
187
333
413
58
118
Sourceo Argas! and Donnelly, 17, four. Sed. Petlo, v. 57, p. 813-823: Society Economic Mineralogists and Paleontologists, Tulsa, Okla.
Note: Iron is reported as total Fe203; n is the number of sample in each average; f indicat total; a dash indicates no reported value.
*(1) Shawangunk Formation near Ellle New York (quartz arenite}
(2) Millport Member of the Rhinestreet Formation, Elmira, New York (lithic arenite/wacke)
(3) Formaon, Unadilla, York (lithic arenite/wacke)
(4) Cloridorme Formation, St on and Gr Mome, Quebec (lithic arenite/wacke)
(5) Austin Glen Member of Normans kill Formation, Poughkeepsie, New York (lithic arenite /wacke)
(6) Renessalaer Member of the Nassau Formation, near Grafton, New York (feldspathk arite/wacke)
(7) Renesselaer Member, averages of analyses from drick and Gnffiths (1969) (feldspathic arenite/wacke)
(8) Rio Culebrinas Formation, La Tasca, Puerto Rico (fossiliferous volcaniastics)
(9) Turbidites from DP site 379A (lithic arites/wacke)
grains of mixed sizes, it is not always easy to decide whether a rock should
called a conglomerate, a sandstone, or a shale. It might be diicult to decide, for
example,
whether a sedimentary rock composed of nearly equal portions of sand
size and mud-size particles should be called a sandstone or a shale. Various classi
fication schemes have been devised for naming texturally mixed sediments and
sedimentary rocks, most of which make use of triangular texture diagrams such
those shown in Figure 5.4.
e textural classification illustrated in Figure 5.4A includes parcles rang
ing
in size from mud (clay and fine silt) to gravel. Note that the textural bound
aries in this classificaon scheme are not entirely symmetrical. Ideally, we might
expect the boundary between gravel and mud-sand to be set at 50 percent; how
ever, this is not always the case, as Figure 5.4A shows. Because particles of gravel
size are commonly less abundant than sand and mud particles, my geologists
consider a sediment with as little as 30 percent gravel-size fragments to be a grav
el. If sediments contain only particles of sand size and smaller, a textural classi
cation scheme such as Figure 5.4B or 5.4C that uses sand, silt, and clay as end
members of the classification is more appropriate. Once the textural nomenclatu
G
ravel
Mud
Figu 5.4
1:1
A
Sand Clay
2:1
1:2
B
Silt
Clay
Sand
50%
c
Silt
Nomenclature of mixed sediments. A, B. Simplified from Folk. C. After Robinson. C = clay,
CS clayey sand, G = gravel, M = mud, MG = muddy gravel, MS = muddy sand, S
sand, SG = sandy gravel, Z
silt, Zs silty sand. [A and B after Folk, R. L., 1954, The dis
nction between grain size and mineral composition in sedimentary rock nomenclature:
)our. Geology, v. 62, Fig. 1 a, p. 346, and 1 b, p. 349, reprinted by permission of University
Chicago Press. C after Robinson, G. W., 1949, Soils, their origin, constitution, and clas
sication, 3rd ed.; Murby, London.]
of siliciclastic sediment or sedimentary rocks has been established, rocks within
ea textural group can be further classified on the basis of composition.
Mineralogical Classifi cation
Most sandstones are made up of mixtures of a very small number of dominant
framework components. Quartz, feldspars, and rock fragments such as chert and
volcanic clasts are the only framework constituents that are commonly abundant
enough to be important in sandstone classification. In addition to framework
grains, matrix may be present in interstitial spaces among these grains. In spite of
the very simple composition of sandstones, geologists have not been able to agree
on a single, acceptable sandstone classification. Published classifications range
m those that have a strong genetic orientation to those based strictly on observ
able, descriptive properties of sandstones. Most authors of sandstone classifica
tions use a classification scheme that involves a QFR or QFL plot. These plots are
iangular diagrams on which quartz (Q), feldspars (F), and rock fragments (R or
L) are plotted as end members at the poles of the classification triangle. There are
numerous possible ways that such a triangle can be subdivided into classification
fields, and geologists have explored the full range of these possibilities (see re
views by Klein, 1963; Boggs, 1967b; Okada, 1971; Yanov, 1978).
One of the simplest and easiest classifications to use is that of Gilbert
(Williams, Turner, and Gilbert, 1982), shown in Figure 5.5, which is based on an
earlier classification by Dott (1964). In this classification, sandstones that are ef
ctively free of matrix ( <5 percent) are classified as quartz arenites, feldspath
ic arenites, or lithic arenites depending upon the relative abundance of QFL
constituents. If matrix can be recognized (at least 5 percent), the terms quartz
wacke, feldspathic wacke, and lithic wacke are used instead. A principal differ
ence between Gilbert's (1982) and Dott's (1964) classification is that Dott sets the
bodary between arenites and wackes at 15 percent matrix. Sandstone classifica
ons that include more classification "pigeonholes" than Gilbert's, and that have
bn rather widely used by American geologists, include those of McBride (1963)
and Folk, Andrews, and Lewis (1970). These classifications do not include matrix
as part of the classification scheme.
The name arkose is often used informally by geologists for any feldspathic
arenite that is particularly rich ( >�25 percent) in feldspars. Another term in
5.2 Sandstones
129
130
Chapter 5 I Siliciclastic Sedimenta Rocks
gure 5.5
Classification of sandstones on the basis of three mineral components: Q quar, chert,
quarite fragments; F = feldspars; L = unstable, lithic grains (rock fragments). Points
within the triangles represent relative proportions of Q, F, and Lend members. Percentage
of argillaceous matrix is represented by a vector extending toward the rear of the dia
gram. The term arenite is restricted to sandstones containing less than about 5 percent
matrix; sandstones containing more matrix are wackes. [After Williams, H. F., F. J. Turner,
and C. M. Gilbert, 1982, Petrography, an introduction to the study of rocks in thin sec
tion, 2nd ed., W. H. Freeman and Co., San Francisco, Fig. 13.1, p. 327. Modified from
Dott, R. H., jr., 1964, Wacke, graywacke, and matrix-what approach to immature sand
stone classification: jour. Sed. Petrology, v. 34, Fig. 3, p. 629, reprinted by permission of
SEPM, Tu lsa, Okla.]
general use is
g
raywacke. This name is commonly applied to matrix-rich sand
stones of any composition that have undergone deep burial, have a chloritic ma
trix, and are dark gray to dark gen, very hard, and dense. is term has been
much misused, and its contued used is controversial (see Boggs, 1992, p.
185-186). Some geologists think that the term should be abandoned entirely and
that we should substitute the word wacke for graywacke. at is probably good
advice. In any case, the name is best restricted to field use and should not be used
as a petrographic term.
Sandstone Maturity
The term maturity is applied to sandstones in two different ways. Compositional
maturity refers to the relative abundance of stable and unstable framework grains
in a sandstone. A sandstone composed mainly of quartz is considered composition
ally mature, whereas a sandstone that contas abundant unstable minerals (e.g.,
feldspars) or unstable rock fraents is compositionally immare. Textural matu
rity is determined by the relative abundance of matrix and the degree of rounding
and sortg of framework grains, as illustrated Figure 5.6. Textural maturity c
range from nature (much clay, framework grains poorly sorted and poorly
rounded) to supermature (little or no day, framework grains well sorted and well
rounded). Textural maturity allegedly reflec the degree of sediment transport and
reworkg; however, it may also be aected by diagenetic processes (i.e., clay min
erals may form in pore spaces during burial diagenesis).
Stage of textural maturity
Much
··--- -- c ·-
-
-� -
Li
ttle or no
clay
l
�---- Grains well sorted �-----
'
,
Process
1
Grains rounded
entirely l���j--
:=
--=
:
::;
completed
Process
largely
complet
re 5.6
Total input modifying kinetic eney
Te
xtural maturity classification of Folk. Te xtural maturity of sands is shown as a function of
input of kinetic energy. [From Folk, R. L, 1951, Stages textural maturity in sedimentary
c: jour. Sed. Petrology, v. 21, Fig. 1, p. 128, reprinted by permission of SEPM, Tulsa,
Okla.]
General Characteristi of Major Classes of Sandstones
The preceding discussion indicates that sandstones can be divided on the basis of
amework mineralogy into three major groups: quartz arenites, feldspathic aren
is, and lithic arenites (these groups include wackes). Some general characteris
tics of each of these major sandstone clans are discussed below.
Quaz Arenites
artz arenites are composed of more than 90 percent siliceous grains that may
clude quartz, chert, and quartzose rock fragments (Fig. 5.7 A). They are com
monly white or light gray but may be stained red, pink, yellow, or brown by iron
oxides. They are generally well lithified and well cemented with silica or carbon
a cement; however, some are porous d friable. Quartz arenites typically occur
association with assemblages of rocks deposited in stable cratonic environ
men such as eolian, beach, and shelf environments. Thus, they tend to be in
terbedded with shallow-water carbonates and, in some cases, with feldspathic
sandstones. Most quartz arenites are texturally mature
to supermature (Fig. 5.6);
quartz wackes are uncommon. Cross-bedding is parcularly characteristic of
ese sandstones, and ripple marks are moderately common. Fossils are rarely
abun
dant, possibly owing to poor preservation or to the eolian origin of some
quartz arenites, but fossils may be psent. Also, trace fossils such as burrows of the
Slithos facies may be locally abundant in some shallow-marine quartz arenites.
ar arenites are common in the geologic record. Pettijohn (1963) estimates that
they make up about one-third of all sandstones.
Quartz arenites can originate as first-cycle deposits derived from primary
ystalline or metamorphic rocks, but they are more likely to be the product of
5.2 Sandstones 131
132
Chapter 5 I Siliciclastic Sedimentary Rocks
Figure 5.7
Representative photomicrographs illustrating major classes of sandstones. A. Quartz aren
ite (>95% quartz), Roubidoux Fm. (Ordovician), Missouri. B. Feldspathic arenite
(F = feldspar; Q = quartz ), Belt Group (Precambrian), Montana. C. Lithic arenite
(V
= volcanic rock fragment; Q = quartz; Ch = chert-some apparent chert grains may
actually be silicified volcanic rock fragments), Otter Point Fm. (Jurassic), Oregon. D. Vol
caniclastic sandstone (V = volcanic rock fragment; P = plagioclase; M = matrix),
Miocene sandstone, japan Sea. Crossed nicols.
multiple recycling of quartz gras from sedimentary source rocks. If they are
first-cycle deposits, they must have formed under weathering, transport, and de
positional conditions so vigorous that most grains chemically less stable than
quartz were eliminated (e.g., Johnsson, Stallard, and Meade, 1988). Conceivably,
extensive chemical leaching under hot, humid, low-relief weathering conitions;
prolonged transport by wind; intensive reworking in the surf zone; or a combina
tion of these processes might be adequate o generate a fiFSt-cycle quartz arenite.
Most quartz arenites are probably polycydi, and their history may have included
at least one episode of eolian transport, although not necessarily during the last
depositional cycle.
Quartz arenites are very common rocks in the geologic record, particularly in
Mesozoic and Paleozoic stratigraphic successions. Some well-known examples of
quartz arenites North America clude the Ordovician St. Peter Sandstone
the midcontinent United States, the Jurassic Navajo Sandstone of the Colorado
Plateau, the Ordovician Eureka Quartzite in Nevada and California, parts of the
Cretaceous Dakota Sandstone in the Colorado Plateau and Great Plains, and many
Cambro-Ordovician sandstones the Upper Mississippi Va lley. Numerous exam
ples of quartz arenites are also known from other continents. Pettijolm, Potter, and
Siever (1987, p. 179-184) list additional examples of quartz arenites from North
America, Europe, and other parts of the world.
Feldspathic Arenites
Feldspaic arenites (Fig. 5.7B) contain less than 90 percent quartz, more feldspar
than unstable rock fragments, and minor amounts of other merals such as micas
d heavy minerals. Some feldspathic arenites are colored pink or red because of
the presence of potassium feldspars or iron oxides; others are light gray to white.
They are typically medium to coarse grained and may contain high percentages of
subangular to angular grains. Matrix content may range from trace amounts to
more
than 15 percent, and sorting of framework grains can range from moderate
ly well sorted to poorly sorted. Thus, feldspathic sandstones are commoy textu
rally immature or submature, Le., wackes.
Feldspathic arenites are not characterized by any particular kinds of sedi
mentary suctures. Bedding may range om essentially structureless to parallel
lanated or cross laminated. Fossils may be present, especially in marine beds.
Feldspathic arenites typicay occur in cratonic or stable shelf settings, where they
may be associated with conglomerates, shallow-water quartz arenites or lithic
arenites,
carbonate rocks, or evaporites. Less typically, they occur in sedimentary
successions that were deposited in unstable bass or other deeper water, mobile
belt settings. Feldspathic arenites of the latter types, which are commonly matrix
rich and well indurated owing to deep burial, are often called feldspathic
graywackes. The abundance of feldspathic arenites in the geologic record is not
well established. Pettijohn (1963) estimates that arkoses make up about 15 percent
of all sandstones. If feldspathic graywackes are included, feldspathic arenites are
probably more abundant than 15 percent.
Some arkoses originate essentially in situ when granite and related rocks dis
tegrate to produce a granular sediment called gs. These residual arkosic mate
rials may be shifted a short distance downslope and deposited as fans or aprons of
waste material, commonly referred to as clasc wedges. These fans may extend
into basins and become intercalated or interbedded with better stratified and bet
ter sorted sediments. Other feldspathic arenites undergo considerable transport
and reworking by rivers or the sea before ey are deposited. These reworked
sandstones commonly contain less feldspar than do residual arkoses, and they are
better sorted and gras are better rounded.
Most feldspathic sandstones are deved from granitic-type primary crys
talline rocks, such as coarse granite or metasomatic rocks containing abundant
potassium feldspar. Feldspathic arenites containing feldspars that are dominantly
plagioclase, derived from igneous rocks such as quartz diorites or om volcanic
cks, are also known. The preservation of large quantities of feldspars during
weathering appears to requi that fe ldspathic arenites originate either (1) in very
cold or very arid climates, where chemical weathering processes are inhibited, or (2)
warmer, more humid climates where marked relief of local uplifts allows rapid
erosion of feldspars before they can be decomposed. Although some feldspars may
sive recycling from a sedimentary source, it appears unlikely that sedimentary
source rocks can fush enough feldspar to produce a feldspathic arenite or arkose.
Feldspathic arenites occur in sedimentary successions of all ages, although
ey appear to be particularly abundant in Mesozoic and Paleozoic strata. Some
common examples include the Old Red Sandstone (Carboniferous) in Scotland,
e Triassic Newark Group in the New Jersey area, the Pennsylvanian Fountain
and Lyons formations of the Colorado Front Range, and the Paleocene Swauk For
maon of Washington. The Swauk Formation is particularly interesting because it
a plagioclase arkose.
Lithic Arenites
ic aretes are an extremely diverse group of rocks that are characterized by gen
elly content of unstable rock agments such as volcanic and metamorphic
5.2 Sandstones
133
1 34
Chapter 5 I Siliciclastic Sedimentary Rocks
clasts; however, lithic arenites may contain some stable clasts such as chert (e.g.,
Fig. 5.7C). ey contain less than 90 percent quartzose grains and contain more
unstable rock fragments than feldspars. Colors may range from light gray, salt
and-pepper to uniform medium to dark gray. Many lithic arenites are poorly sorted;
however, sorting ranges from well sorted to very poorly sorted. Quartz and many
other framework grains are generally poorly rounded. Lithic arenites tend to con
tain substantial amounts of matrix, much of which may be of secondary origin.
Thus, most lithic sandstones are texturally immature to submature (lithic wackes ).
Lithic arenites may range from irregularly bedded, laterally restricted, cross-strat
ified fluvial units to evenly bedded, laterally extensive, graded, marine turbidite
units. They may occur in association with fluvial conglomerates and other fluvial
deposits or in association with deeper water marine conglomerates, pelagic
shales, cherts, and submarine basalts. Liic arenites include sandstones at
many geologists connue to refer to as graywackes. Graywackes differ from "nor
mal" lithic arenites in that ey are dark gray to dark green, are well indurated or
lithified, and commonly have a matrix consisting of secondary chlorite. The term
graywacke is used so loosely, however, that it might be best to simply drop it, as
mentioned. Pettijohn (1963) estimates that lithic arenites and graywackes together
make up nearly one-half of all sandstones.
Lithic arenites are typically compositionally immature sandstones that origi
nate der conditions favoring the production and deposition of large volumes of
relatively unstable materials. The mechanically weak character of many of the
liic fragments in these sandstones suggests that they are probably derived from
rugged, gh-relief source areas. Liic arenites may be deposited in nonmarine
settings in proximal alluvial fans or other fluvial environments. Alteatively, they
may be deposited in marine foreland basins (Chapter 16) adjacent to fold-thrust
belts, or they may be transported by large rivers off the continent into deltaic or
shallow shelf environments. Lithic sediments deposited in coastal areas may be
retransported into deeper water by turbidity currents or by oer sediment gravity
flow mechanisms. These deeper-water sedents are particularly likely to undergo
deep burial and incipient metamorphism, leading to development of characteris
tics generally ascribed to graywackes.
Common examples of lithic sandstones include the Paleozoic sandstone suc
cessions of the central Appalachians in e eastern United States (e.g., Ordovician
Juniata Formation, ssissippian Pocono Formation, Pennsylvanian Pottsville
Formation); many sandstones associated with the Coal Measures throughout the
world; many Jurassic and Creceous sandstones of the U.S. and Canadian Rocky
Mountains and the U.S. We st Coast (e.g., Cretaceous Belly River Sandstone of
Canada, Jurassic Franciscan Formation of California); and Tertiary sandstones of
the Gulf Coast, the We st Coast, and the Alps.
Vo lcaniclastic sandstones are a special kind of litc arenite composed pri
marily of volcanic detritus (Fig. 5.7D). Vo lcaniclastic sandstones may be made up
largely of pyroclastic materials that have been transported and reworked, or they
may conta volcanic detritus derived by weathering of older volcanic rocks. ey
are
especially characterized by the presence of euhedral feldspars, pumice frag
men, glass shards, and volcanic rock fragments, and they generally have a very
low quartz content (e.g., Boggs, 1992, p. 197-209).
Other Sandstones
The sandstones discussed above are composed of constituents derived primarily
by weathering of preexisting rocks or by explosive volcanism. A few less abun
dant types of "sandstones" are known whose constituents formed largely within the
depositional basin by chemical or biochemical processes. These rocks, called hybrid
sandstones by some authors, include such common varieties of sandstones as