Boggs S. Principles of Sedimentology and Stratigraphy
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4.6 Paleocurrent Analysis from Sedimentary Structures
115
coosiderable scatter. Therefore, directional data must be treated statistically some
manner to veal primary and secondary directional tr ends. For example, the dip di-
dion of cross-bed fo resets the ancient deposits of a meandering river system
y range from N 20° W.to N 20° E because of variations in flow direction of the
stream different parts of the meanderg river system. By examg the orienta-
on data statistically, we may be able to determine that the primary flow direction of
seam was approximately due north. Because all of the cross-bed forets ts
example indicate ow in the same general direction, spite of some scatter, we say
thaH low was unidirectional. By contrast, the cross-bed foresets in sandy deposits of
mare tidal chnels may display two opposing dip directions owing to formation
ot cross-beds during both incoming and outgoing tides. This type of opposing flow
is rred to as bidirectional. some environments, such as e eolian environ-
ment, depositing currents may flow in several directions (polydirectional) at vari-
ous es during deposition of a particular sedimentary unit.
e paleocurrent data collected from stratigraphic units that have under
gone little or no tectonic deformation or tilting can be compiled and summarized
dittly. If the rocks have undergone considerable tilting, it is necessary to correct
the measured orientation by restoring directions to their original attitude be
fore Hltfng, A simple procedure using a stereogram can be used to reorient di
ctional data collected from tilted stratigraphic units (Collinson and Thompson,
1989,
p
. 200). After any reorientation of data has been done, the data are common
plotted as a circular histogram or "rose diagram" (Fig. 4.43). (Commercial com
puter software programs are available for plotting rose diagrams, e.g., Rose, from
Rkware, Wheat dge, Colorado.) Such diagrams show the prcipal direction
of
p
alurrent flow and any secondary or tertiary mod of flow. If the paleocur
nt flow as revealed by the rose diagram is domantly a single direction, the
paleocurrent vector is said to be unimodal. If two principal directions of flow are
indicated, it is bimodal, and if three or more directions of flow are revealed by the
directional data, the paleocurrent flow is called polymodat
Local paleocurrent directions may have environmental significance. For ex
am
p
le, sediments from alluvial and deltaic environments tend to have unimodal
paleocurrent vector patterns, whereas bimodal paleocurrent patterns are more
coon in shoreline and shelf sedents. Paleocurrent data have their greatest
usefulness when plotted on a regional scale to reveal regional paleocurrent pat
te (discussed Chapter 16).
Bimodal
pattern
Polymodal
pattern
Figure 4.43
Hypothetical paleocurrent
data plotted as rose dia
grams: unimodal, bimodal,
and polymodal patterns of
paleoflow directions are
shown. N = number of
measurements (directions)
taken in the field.
116
Chapter 4 I Sedimentary Structures
FURTHER READING
Mainly Physically Formed Structures
Allen, J. R L, 1982, Sedimentary structures: Their character and
physical basis, v. 1-2: Elsevier, Amsterdam, 664 p.
Collinson, J. D., and D. B. Thompson, 1989, Sedimentary struc
tures, 2nd ed.: Chapman and Hall, London, 207 p.
Harms, J. C, J. B. Souard, and R G. Wa lker, 1982, Structures
and sequences in clastic rocks: Soc. Econ. Paleontologists and
Mineralogists Short Course No. 9, Tu lsa, Okla.
Pettijohn, J. , and P. E. Potter, 1964, Atlas and glossary of prima
ry sedimentary structures: Springer-Verlag, New Yo rk, 370 p.
Ricci Lucchi, E, 1995, Sedimentographica: A photographic atlas
of sedimentary structures, 2nd ed.: Columbia University
Press, New York, 255 p.
Biogenic Structures
Bromley, R. G., 1996, Tra ce fossils: Biology, taphonomy and ap
plications: Chapman and Hall, London, 361 p.
Curran, H. A (ed.), 1985, Biogenic structures: Their use in inter
preting depositional environments: Soc. Econ. Paleontolo
gists and Mineralogists Special Publication No. 35, Tulsa,
Okla., 347 p.
Donovan, S. K., 1994, The paleobiology of trace fossils: e Johns
Hopkins University Press, Baltimore, 308 p.
Ekdale, A A., R G. Bromley, and S. G. Pemberton, 1984, Ichnolo
gy, trace fossils in sedimentology and stratigraphy: Soc. Econ.
Paleontologists and Mineralogists Short Course No. 15, Tulsa,
Okla., 317 p.
Hasiotis, S. , 2002, Continental trace fossils: SEPM (Society for
Sedimentary Geology), Tulsa, Okla., 130 p.
Maples, C. G., and R. R. West (eds.), 1992, Trace fossils: Paleonto
logical Society Short Courses in Paleontology . 5, 238 p.
Miller, M. , A A. Ekdale, and M. D. Picard (eds.), 1984, Trace
fossils and paleoenvironments: Marine carbonate, marginal
marine terrigenous and continental terrigenous settings: Jour.
Paleontology, v. 58, p. 283-597.
·
Pemberton, S. G., J. A. MacEache, and R. W. Frey, 1992, Trace
fossil facies models: Environmental and allostra
-
tigraphic sig
nificance, in Walker, R. G., and N. James (eds.), Facies mod
els: Response to sea level change: Geological Association of
Canada, p. 47-72.
Composition, Classcation,
and Diagenesis of
Sedimentary Rocks
To p: Microquartz (arrows)
cementing quartz and
chert grains in sandstone,
jeerson City Formation
(Ordovician), Missouri
Bottom: Ooids in lime
stone (Devonian), Canada
117
118
I
n Chapters 3 and 4 we examed sedimentary textures and structures and dis
cussed some of the ways that these physical properties are used to interpret the
genesis of sedimentary rocks. Particle and chemical composition are also fun
damental properties of sedimentary rocks, properties that allow us to distinguish
one kd of sedimentary rock from another and that provide additional informa
tion about the history of the rocks. Geologists commonly use the term mineralogy
to refer to the identity of all the particles or grains in rocks. When we talk about
the mineralogy of a sandstone, for example, we mean quartz, feldspar, micas, and
other particulate mineral grains that make up the sandstone. Bulk chemistry is
anoer aspect of the overall composition of sedimentary rocks and is directly re
lated to the mineralogy of the rocks. Tr aditionally, sedimentologists have been in
terested more in mineralogy than in bulk chemistry because they consider
mineralogy to be more useful than chemical composition in characterizing and
classifying sedimentary rocks and in intereting their geologic history. That per
ception is changing, and we are now seeing an increasing number of published
papers that are conceed in some way with sedimentary geochemistry.
The next three chapters deal with the composition of siliciclastic sedimentary
rocks (sandstones, conglomerates, shales), carbonate rocks (limestones and
dolomites), other biochemical/ chemical sedimentary rocks (evaporites, cherts,
iron-rich sedimentary rocks, phosphorites), and carbonaceous sedimentary rocks
(oil shales and coals). Among other things, we will examine the ways that the
characteristic, observable, and measurable properties of these rocks are used to
classi them into different kinds. We will also examine the various ways that min
eralogy and chemical composition are used to interpret sediment provenance
(source), depositional process, depositional environments, and diagenesis (burial
alteration). Many of e biochemical/ chemical sedimentary rocks, such as dolomite,
iron-rich sedimentary rocks, and phosphorites, are enigmatic rocks whose origins
are still poorly understood. This section discusses ideas conceing the origin of
these rocks and analyzes controversial and conflicting hypotheses. Study of the
mineralogy of sedimentary rocks is commonly referred to as sedimentary petrog
raphy. The next three chapters provide an introduction to sedimentary petrogra
ph Additional details are available a host of papers and monographs, several
of which are listed under "Further Reading" at the ends of the chapters.
Siliciclastic Sedimentary Rocks
5.1 INTRODUCTION
A
s discussed in Chapter 1, sedimentary rocks composed mainly of sicate
particles derived by the weathering breakdown of older rocks and by py
roclastic volcanism are called siliciclastic sedimentary rocks. Sandstones,
nglomerates, and shales compose the members of this group. The siliciclastic
sedimentary rocks make up roughly three-fourths of all sedimentary rocks in the
geologic record, and they are present in sedimentary successions ranging in age
om Precambrian to Holocene. They are of special interest to geologists as indica
of Earth history. Many of the textures and structures discussed in Chapters 3
and 4 are particularly well developed in these rocks. These textures and structures
pde important information about ancient sediment transport and depositional
conditions. addition, the minerals and rock fragments in siliciclastic sedimenta
rocks fuish our most definitive clues to the natu and location of vanished
ancit mountain systems such as the ancestral Rocky Mountains and Appalachi
Mountas of the United States. Petroleum geologists are especially interested
sandstones because more than half of the world's reserves of oil and gas occur
rocks. Shales are likewise of great interest because the organic matter con
tained in shales is believed to be the source material of oil and gas.
this chapter, we focus on the particle composition (kinds of minerals and
rk agments) of sandstones, shales, and conglomerates and explore ways that
composition can be used to classify these rocks and interpret aspects of their ori
g. We will also briey examine changes in composition and texture that take
place owing to burial diagenesis.
5.2 SANDSTONES
ndstones make up 20-25 percent of all sedimentary rocks. They are common
rks geologic systems of all ages, and they are distributed throughout the con
tinents of Earth. Sandstones consist mainly of silicate grains ranging in size from
1/16 to 2 mm. These particles make up the framework fraction of the sandstones.
dstones may also contain various amounts of cement and very fine size
( < -0.03 mm) material called matrix, which are present within interstitial pore
space among the framework grains. Because of their coarse size (relative to the
119
120
Chapter 5 I Siliciclastic Sedimentary Rocks
sizes of particles in shales), the amework mieralogy of sandstones can general
ly be determined with reasonable accuracy with a standard petrographic micro
scope or by backscattered electron microscopy (e.g., Krinsley et a!., 1998). Bulk
chemical composition can be measured by instrumental techniques such as X-ray
fluorescence and inductively coupled argon plasma emission spectrometry (ICP).
e chemistry of individual mineral grains is commonly determined by use of an
electron probe microanalyzer or an energy dispersive X-ray detector (EDX) at
tached to a scanning electron microscope.
In this section, we examine the mineralogy and chemical composition of
sandstones, discuss the classification of sandstones on the basis of mineral compo
sition, and evaluate the usefulness of particle and chemical composition in inter
preting the genesis of sandstones.
Framework Mineralo
e particles that make up sandstones are main! y sand-size and coarse silt-size sil
icate minerals and rock fragments referred to as framework grains, as mentioned.
Only a few principal kinds of minerals make up the bulk of all sandstones. These
common merals and rock fragments are shown in Table 5.1 and are discussed in
greater detail in the following paragraphs.
Quartz
Quartz (Si02) is the dominant mineral in most sandstones, making up on average
about 500 percent of the framework fraction. It is a comparatively easy mineral
to identify, both megascopically in hand specimens and by petrographic examina
tion in thin sections, although it can be confused with feldspars. Because of its su
perior hardness and chemical stability, quartz can survive multiple recycling. The
quartz grains in many sandstones display some degree of rounding acquired by
abrasion during one or more episodes of transport, particularly by wind.
Quartz can occur as single (monocrystalline) grains (Fig. 5.1A) or as compos
ite
(polycrystalline) grains (Fig. 5.1B). When examined under crossed polarizing
prisms with a petrographic microscope, many quartz grains display sweeping
pattes of extinction as the stage is rotated. This property is called undulatory ex
tinction. Some authors (Folk, 1974; Basu et a!., 1975) suggest that the properties of
polycrystallinity and undulatory extinction can be used to distinguish quartz de
rived from different sources. Quartz is derived from plutonic rock, particularly
felsic plutonic rocks such as granites, metamohic rocks, and older sandstones.
Feldspars
Feldspar minerals make up about 120 percent of the framework grains of aver
age sandstones. They are the second most abundant mineral in most sandstones.
Several varieties of feldspars are recognized on the basis of differences in chemical
composition and optical properties. They are divided into two broad groups: alka
li feldspars and plagioclase feldspars.
Alkali feldspars constitute a group of minerals in which chemical composi
on can range through a complete solid solution sees from KA1Si308 through
(K, Na)AISi30s to NaA1Si308. Because potassium-rich feldspars are such com
mon members of this group, it has become widespread practice to call the alkali
feldspars potassium feldspars, often shortened to simply K-spars. Common
members of the potassium-feldspar group include orthoclase, microcline (Fig.
5.1C), and sanidine. Plagioclase feldspars form a complex solid solution series
ranging in composition from NaA1Si308 (albite) through CaA12Si208 (anorthite).
A general formula for the series is (Na, Ca) (AI, Si )Si208. Plagioclase feldspars can
Major minerals (abundance >-1-2%)
Sble minerals (gatest resistance to chemical decomposition):
rtz-makes up approximately 65% of framework grains in average sandstone,
% of minerals in average shale.
Less stable minerals:
Feldspars-include K-feldspars (oroclase, microcline, sanidine, anorthoclase) and
plagioclase fe ldspars (albite, oligoclase, andesine, labradorite, bytownite, anor
thite); make up about 1-15% of amework grains in average sandstone, 5% of
average shale.
Clay minerals and ne micas-day minerals include the kaoline group, illite group,
smecte group (montmorillonite a principal variety), and chlorite group; fine
micas are principally muscovite (sericite) and biotite; minor abundance in sand
stones,
where they occur as matrix minerals, but compose >60% of the minerals in
average shale.
Accesso minerals (abundances < -1-2%)
Coarse micas-principally muscovite and biotite.
Heavy minerals (specific gravity >-2.9):
Stable nonopaque minerals-zircon, tourmaline, rutile, anatase.
Metastable nonopaque minerals-amphiboles, pyroxenes, chlorite, gaet, apatite,
staurolite, epidote, olivine, sphene, zoisite, clinozoisite, topaz, monazite, plus about
100 others of minor importance volumetrically.
Sble opaque minerals-hematite, limonite.
Metastable opaque mineral-magnetite, ilmenite, leucoxene.
Rk Fragments (make up about 10-15% of the siliciclastic grains in average sandstone
and mt of the gravel-size particles in conglomerates; shales contain few rock frag
men)
Igneous rock fragments-clasts of any kind of igneous rock possible in conglomerate;
however, fragments of fine-crystalline volcanic rock are most common in sand
stones.
Memorphic rock fragments-may include clasts of any kind of metamorphic rock;
however, meta quartzite, schist, phyllite, slate, and argillite clasts are most common
in sandstones.
Sedimentary ck fragmentslasts of any kind of sedimentary rock possible in con
glomerates; clasts of fine sandstone, siltstone, shale, and chert are most common in
sandstones; limestone clasts are comparatively rare in sandstones.
Chemical Cements (abundance variable)
Silate minerals-predominantly quartz; others may include microquartz (chert),
opal, feldspars, and zeolites.
Caonate minerals-principally calcite; less commonly aragonite, dolomite, siderite.
In oxide mineral-hematite, limonite, goethite.
Sulfate minerals-anhydte, gypsum, barite.
No: e terms stable and metastable refer to chemical stability.
only be distguished from potassium feldspars on the basis of optical prop
eres such as twng (compare Fig. 5.1 D with Fig. 5.1 C) by examination with a
graphic microscope. Unfortunately, some K-feldspars (e.g., orthoclase and
sa
nidine) and some plagioclase are untwinned, making them difficult to dier
enate from each other and from quartz. Potassium feldspars are generally con
sidered to be somewhat more abundant overall sedimentary rocks than
plagilase feldspars; however, plagioclase is more abundant in sandstones de
v from volcanic rocks.
5.2 Sandstones
121
122
Chapter 5 I Siliciclastic Sedimentary Rocks
Figure 5.1
Principal kinds of framework grains in sandstones. A. Monocrystalline quartz, Roubidoux
Fm. (Ordovician), Missouri. B. Polycrystalline quartz, Miocene sandstone, japan Sea.
C. Potassium feldspar (microcline), Bateman Fm. (Eocene), Oregon. D. Plagioclase
feldspar, Bateman Fm. (Eocene), Oregon. E. Mica (muscovite), Bateman Fm. (Eocene),
Oregon. F. Heavy minerals (Z = Zircon; M = magnetite; P = pyroxene), India. Crossed nicol
photomicrographs.
Feldspars ar chemically less stable than quartz and are more susceptible to
chemical destrudion during weathering and diagenesis. Because they are also
softer than quartz, feldspars become more readily rounded during transport. They
also appear to be somewhat more prone to mechanical shattering and breakup
owg to their deavage. Feldspars are less likely than quartz o survive several
episcdes of recycling, although they can survive o1e than one cycle if weather
ing occurs in a moderately arid or cold climate. Owing to this possibility of recy
cling, the presence of a few feldspar grains in a sedimentary rock does not
necessarily mean that the rock is composed of first-cycle sediments derived direct
ly om crystalline igneous or metamorphic rocks. On the other hand1 a high con
t
of ldspars, particularly on the order of 25 percent or more1 probably
dicas derivation directly from crystalline source rocks.
Access
Framework Minerals
erals at have an average abundance in sedimentary rocks less than about
1-2 percent are called accessory minerals. These minerals include the common
cas, muscovite (white mica) and biotite (dark mica), and a large number of so
called
heavy minerals, which are denser than quartz.
The average abundance of coarse micas in siliciclastic sedimentary rocks is
ls about 0.5 percent, although some sandstones may contain 2-3 percent.
are distinguished from other merals by their platy or flaky habit (Fig. 5.1E).
Muscovite is chemically more stable than biotite and is commonly much more
abdant in sandstones than biotite. Micas are derived particularly from meta
mohic source rocks as well as from some plutonic igneous rocks.
Minerals that have a specific gravity greater than about 2.9 are called hea
nerals. These minerals include bo chemically stable and unstable (labile) va
es as shown in Tab le 5.1. Stable heavy minerals such as zircon (Fig. 5.1F) and
le c survive multiple recycling episodes and are commonly rounded, indi
tg at e last source was sedimentary. Less stable minerals, such as mag
nete, pyroxenes1 and amphiboles, are less likely to survive recycling. They are
cooy rst-cycle sediments that reflect the composition of proximate source
s. us, heavy minerals are useful indicators of sediment source rocks be
cause different types of source rocks yield different suites of heavy merals.
Hea minerals are derived from a variety of igneous, metamorphic, and sedi-
ry rocks.
Because of their low abundance in sandstones, heavy minerals are common
concentrated for study by separating them from the light mineral fraction by
usg heavy liquids such as bromoform or sodium polytungstate (e.g., Lindholm/
1987, p. 214). this separation process, disaggregated sediment is stirred into a
hvy liquid contained in a funnel placed inside a fume hood. (Many heavy liq
uids a toxic and must be handled with extreme care.) e light minerals float on
e surface of the heavy liquid, but the heavy minerals gradually sk to the
stem of e fnel where they can be drawn off and separated from the light frac
on. After the heavy liquid has been washed off in a suitable solvent, these heavy
eral concentrates can then be mounted on a glass microscope slide (Fig. 5.1F)
d studied with a petrographic microscope.
ck Fragments
s of ancient source rocks that have not yet disintegrated to yield individual
meral grains are called rock fragments or clasts. Rock fragments make up about
120 percent of the framework grains in the average sandstone; however, the
k-agment content of sandstones is highly variable and ranges from zero to
more than 95 percent. Fragments of any kind of igneous, metamohic, or sedi
mtary rock can occur in sandstones (Fig. 5.2); however, clasts of fine-grained
ce rocks are most likely to be preserved as sand-size fragments (Boggs, 1968).
V coarse grained source rocks such as granites tically yield clasts of coarse
d size or larger (e.g. Fig. 5.2A); however, such clasts are not common because
ites commonly disintegrate to yield individual minerals raer than clasts.
most common rock fragments in sandstones are clasts of volcanic rocks (Fig.
5.2B), volcanic glass (in younger rocks), and fine-grained metamohic rocks such
slate, phyllite, schist (Fig. 5.2C), and quartzite (Fig. 5.2D). Sand-size fragments
of sica-cemented siltstone, fine-grained sandstone (Fig. 5.2E), and shale are less
5.2 Sandstones
123
124
Chapter 5 I Siliciclastic Sedimenta Rocks
Figure 5.2
Common kinds of rock fragments in sandstones. A. Plutonic (granite; arrows), Bateman
Fm. (Eocene), Oregon. B. Volcanic, Miocene sandstone, Japan Sea. C. Metamorphic
(schist; arrows), Pottsville Group (Pennsylvanian), eastern Pennsylvania. D. Metamorphic
(quartzite; arrows) Pottsville Group (Pennsylvanian), eastern Pennsylvania. E. Sedimentary
(sandstone;
arrows; note rounding of
grains within this clast), Bateman Fm. (Eocene),
Oregon. F. Sedimentary (chert; arrows), Otter Point Fm. (Jurassic), Oregon. Crossed nicol
photomicrographs.
common. Clasts of limestone or other carbonate rocks are alsg less common, prob
ably in part because they do not survive weathering and transport. Composite
quartz grains that consist of exceedingly small quartz crystals, referred to as mi
crocrystalline quartz, are called chert. Chert gras are actually rock fragments,
derived by weathering of bedded chert or chert nodules in limestone, and can be
abundant in some sandstones (Fig. 5.2F).