Boggs S. Principles of Sedimentology and Stratigraphy

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3.3 Particle Shape

3.3 PARTICLE SHAPE

The shapes of minerals and clasts (rock fragments) in sedimentary rocks are deter

mined by a variety of factors: the original shapes of mineral grains in the source

rocks; the orientation and spacing of fractures in bedrock that inuence e shapes

tt clasts (rock fragments) take on when they weather from exposed rock; the na

 and intensity of sediment transport, which can abrade grains and change

original shapes; and sediment burial processes such as compaction, which can

also change original shapes. Thefore, sedimentary particles may display a wide

range of shapes, depending upon their history.

Particle shape is defined by three related but different aspects of grains.

Fo refers to the gross, overall configuration (outline) of particles and ret1ects

variations in their proportions. The form of some particles sembles that of a

sphere; other particles may have a platy (t1attened) or rodlike form (Fig. 3.8A).

Fo should not be confused with roundness, which is a measure of the sharp

ness of grain coers. Well-rounded grains have smooth comers and edges; poorly

unded grains have sharp or angular corners and edges (Fig. 3.8B). Surface tex

ture refers to small-scale, microrelief markings such as pits, scratches, and ridges

at occur on the surface of grains (Fig. 3.8C). Form, roundness, and surface tex

ture are independent properties, as illustrated in Figure 3.8, and each can theoret

ically vary without aecting the other. Actually, form and roundness tend to be

positively correlated  sedimentary deposits; particles that are highly spherical in

shape also tend to be well rounded. Surface texture can change without signifi

cantly changing form or roundness, but a change in form or roundness will affect

surface texture because new surfaces are exposed. e three aspects of shape can

be thought of as constituting a hierarchy, in which form is a first-order property,

und

ness a second-order property superimposed on form, and surface texture a

i-order property superimposed on both the corners of a grain and the surfaces

beeen coers (Barrett, 1980).

Paicle Form (Sphericity)

Form ret1ects variations in the proportions of particles, that is, the relative lengths

of eir three major axes (long, intermediate, short). The term s

hericity was in

oduced by Wadell (1932) to express this relationship (Appendix A). If all three

axes have about the same length, a particle has high sphericity. If the axes differ

markedly in length, e particle has low sphericity. A formula for expressing this

relationship mathematically was developed by Krumbein (1941), which yields a

maematical value of 1 for a perfect sphere; less spherical particles have lower,

actional values (see Appendix A).

A. Form

re 3.8

Equant

Platy

Deeasing sphericity

B. Roundness

(angularity)

Decreasing roundness

(increasing angularity)

C. Suace

texture

Pitted

Smooth

Decreasing surface

texture

Schematic representation of

the principal aspects of parti

cle shape: form, roundness,

and surface texture. Note

that sphericity and roundness

are independent properties.

For example, a highly spheri

cal (equant) particle can be

either well rounded or poorly

rounded, and a well-rounded

particle can have either high

low sphericity.

Chapter 3 I Sedimentary Te xt ures

Oblate

0.8



C::�

�

0.6

Bla

�

0.4



0.2

0.4

gure 3.9

1.0

oA\

Equant

0.8

0.3

Prolate

(Roller)

0.4

0.6 � 0.8



5

�

�.7 0�



.



.

\ 





.





\ '

















-

.



0.2

0.4

0.6 2

0.8

1.0



A. Classification of shapes of pebbles after Zingg (1935). B. Relationship between mathe

matical sphericity and Zingg shape fields. The curves represent lines of equal sphericity.

[A, from Blatt, H., G. V. Middleton, and R. Murray, 1980, Origin of sedimentary rocks, 2nd

ed., Fig. 3.20, p. 80, reprinted by permission of Prentice-Hall, Englewood Cliffs, N.j.; B,

from Pettijohn, F. ]., 1975, Sedimentary rocks, 3rd ed., Fig. 3.19, p. 34, Harper and Row

Publishers, New York.]

Zgg (1935) took a somewhat different approach by proposing the use of

two shape dices DJ IDL and D5/D1 (Appendix A) to define four shape fields on a

bivariate plot: oblate (disc), equant (spheres), bladed, and prolate (rollers) (Fig.

3.9A). Lines of equal intercept sphecity (Krumbein) can be drawn on the Zingg

shape fields (Fig. 3.98), illustrating that parcles of quite dierent form can have

the same mathematical sphericity. See also Sneed and Folk (1958, 11150).

Paicle Roundness

Particle roundness refers to the degree of sharpness of the corners and edges of a

grain. If the corners and edges are quite smooth, the grain is said to be well round

ed (Fig. 3.88). If the coers and edges are sharp and angular, the grain is poorly

rounded. Wa dell (1932) developed a formula that expresses mathematically the

roundness of a particle {Appendix A). Like the sphericity formula, the roundness

formula yields a mathematical value of 1 for perfectly rounded particles and

smaller, fractional values for less well rounded particles. Because of the laborious

process necessary to measure and express roundness mathematically, most work

ers are content to estimate roundness by comparison with a visual grain round

ness scale (Fig. 3.10).

Fourier Shape Analysis

Sphericity and roundness are rather general descriptors of particle shape. More re

cently, attempts have been made to describe the two-dimensional shape of parti

cles more rigorously by using a method based on Fourier analysis, which is a

method of representing periodic mathematical functions as an infinite series of

summed sine and cosine terms (Ehrlich and We inberg, 1970). If the outline of a

grain is "cut" and "unrolled," the unrolled outle is a periodic function somewhat

resembling the shape of a sine wave. The shape of this unrolled grain can be pre

sented by a series of terms called harmonics. Harmonics are periodic functions

HIGH

SPHERICITY

l

SPHERICITY

VERY

ANGUR

ANGULAR

SUB

ANGULAR

SUB

NDED. -RouNOED

WELL

ROUNDE .

3.3 Particle Shape

Figure 3.10

Powers' grain images for esti

mating roundness of sedimen

tary particles. [After Powers,

M. C., 1953, A new roundness

scale for sedimentary par�iies:

jour. Sed. Petrology, v. 23,

Fig. 1, p. 118, reprinted by per

mission of Society of Economic

Paleontologists and Mineralo

gists, Tulsa, Okla.]

(e.g., a sine wave) that can have various shapes owing to differences  wave am

plitude and frequency.

By adding together (graphically) the shapes of various harmonics (done by

computer), we can faithfully reproduce the shape of any periodic function. The

method involves digitizing the periphery of grains by projecting grains onto a

grid and recording, either manually or with an automatic digitizer, intercepts of

the grain outline with the grid. Digitized data are reduced by computer to obtain

the

harmonics and produce computer-generated grain outlines. Presumably, the

results of Fourier analysis reflect both sphericity and rodness of a particle.

Fourier analysis has been used both to study the source of sediment grains and to

characterize gras from particular depositional environments. See Boggs (1992, p.

520) for additional review of Fourier grain-size techniques.

Sfgnificance of Paicle Shape

Spheci

The sphericity of particles in sedimentary deposits is a function maly of the orig

inal shapes of the grains, although the shapes of gravel-size particles can be mod

ied somewhat by abrasion and breakage during transport. Sphericity affects the

settling velocity of small particles (spherical particles settle faster than nonspheri

cal

particles) and the transportability of gravel-size particles, which move by trac

on (spheres and wller-shaped pebbles roll more readily than do pebbles of other

sha). Although sphericity is thus known to affect particle transport, it has not

yet

been demonstrated that he sphericity of ptides can be used alone as a reli

able tool for interpreting depositional envimnments. Although empirical studies

show �me relative differences in sphericity of grains from different environ

ments, these differences have not proven to be sufficiently distinctive to permit

environmental discrimination.

icle Roundness

The roundness of grains in a sedimentary deposit is a function of gra composi

tio, grain sizer type of transport process, and distance of transport. Hard, resis

tant gras such as quartz and zircon are rounded less readily during transport

than are weakly durable grains such as feldspars and pyroxenes. Pebble- to cobble

size grains commonly are more easily roded by abrasion during transport than

aFe sand-size grains. Resistant mineral grains smaller than 0.00.1 mm do not ap

ar to become rounded by any transport process. Because of these factors, it is al

ways necessary to work with particles of the same size and composition when

doing roundness studies.

Chapter 3 I Sedimentary Te xtures

Experimental studies in flumes and wind tunnels of the effects of abrasion

on transport of sand-size quartz grains show that transport by wind is 100 to 1000

times more effective in rounding these grains than transport by water (Kuenen,

1959, 1960). In fact, almost no rounding occurs in as much as 100 km of ansport

by water. Most roundness studies of small quartz grains in vers have corroborated

these experimental results. For example, Russell and Taylor (1937) observed no in

crease in rounding of quartz grains throughout a distance of 1100 mi (1775 km) in

the Mississippi River between Cairo, Illinois, and the Gulf of Mexico. The effec

tiveness of surf action on beaches in rounding sand-size quartz grains is not well

understood.  general, surf processes appear to be less effective in rounding

grains than wind transport but more eective than river transport.

Once acquired, the roundness of quartz grains is not easily lost and may be

preserved through several sedimentation cycles. We ll-rounded quartz grains in an

ancient sandstone may well indicate an episode of wind transport in its history,

but it may be difficult or impossible to determine if rounding took place during

the last episode of ansport or during some previous cycle.

The roundness of transported pebbles is strongly related to pebble composi

tion and size (Boggs, 1969). Soft pebbles such as shale and limestone become

rounded much more readily than quartzite or chert pebbles, and large pebbles

and cobbles are commonly better rounded than smaller pebbles. Although

stream transport is relatively ineffective in rounding small quartz grains, pebble

size grains can become well rounded by stream transport. Depending upon com

position and size, pebbles can become well rounded by stream transport in

distances ranging from 11 km (7 mi) for limestones to 300 km (186 mi) for quartz

(Pettijohn, 1975).

Well-rounded pebbles in ancient sedimentary rocks generally indicate flu

vial transport. The degree of rounding cannot, however, be depended upon to

give reliable estimates of the distance of transport. The greatest amount of round

ing takes place in the early stages of transport, generally within the first few kilo

meters. Also, the roundness of pebbles is not an unequivocal indicator of fluvial

environments because pebbles can also become rounded in bea environments

and possibly on lake shores. Furthermore, rounded fluvial pebbles may eventually

be transported into nearshore marine environments where they may be reen

trained by turbidity currents and resedimented in deeper parts of the ocean.

Suace Te ure

The surface of pebbles and mineral grains may be polished, frosted (dull, matte

texture like frosted glass), or marked by a variety of small-scale, low-relief features

such as pits, scratches, fractures, and ridges. ese surface textures originate in di

verse ways, including mechanical abrasion during sediment transport; tectonic

polishing during deformation; and chemical corrosion, etching, and precipitation

of authigenic growths on grain surfaces during diagenesis and weathering. Gross

surface textural features such as polishing and frosting can be observed with an

ordinary binocular or petrographic microscope; however, detailed study of sur

face texture requires high magnifications. Krinsley (1962) pioneered use of the

electron microscope for studying grain surface texture at high magnificaons.

Most investigators who study the surface texture of sediment grains carry

out their studies on quartz grains because e physical hardness and chemical sta

bility of quartz grains allow these particles to retain surface markings for geological

ly long periods of time. Through study of thousands of quartz grains, investigators

have now been able to fingerprint the markings on grains from various mode

depositional environments. More than twenty-five dierent surface textural fea

tures have been identified, including conchoidal fractures, straight and curved

scratches and striations, upturned plates, meander ridges, chemically etched V's,

Figure 3.11

3.3 Particle Shape

Electron micrograph of a quartz grain from unconsolidated

Pliocene-Pleistocene sand, Louisiana salt dome edge, southern

Louisiana, showing details of the surface texture. The grain has

been well rounded by wind transport and contains tiny

"upturned plates" characteristic of dune sands. Photograph

courtesy of David Krinsley. Scale bar = 25 microns.

mechanically formed V's, and dish-shaped concavities (e.g., BuU, 1986). Examples

of some of these markings are shown in Figure 3.11, illustrating the kinds of mark

ings that can be seen at gh magfication. Many other excellent electron micro

raphs of quartz surfaces may be found in Krinsley and Doornkamp's (1973) Atlas

of Quartz Sand Surfa ce Te xtures.

Surface texture appears to be more susceptible to change during sediment

transport and deposition than do sphecity and roundness. Removal of old sur

face textural features and generation of new features are more likely to occur than

marked changes in sphericity and roundness, and surface texture is more likely to

cord the last cycle of sediment transport or the last depositional environment.

Thefo, geologists are interested in surface textural features as possible indica

tors of ancient transport conditions and depositional environment. The usefulness

d surface texture  environmental analysis is limited, however, because similar

types of surface markings can be produced in different environments. Also, the

markings produced on grains in one environment may be retained on grains that

are transported into another environment. Although less abrasion is required to

remove surface markings from grains than is required to change roundness or

sphericity, markings inherited from a previous environment may remain on

grains for a long time before they are removed or replaced by different markings

produced in the new environmertt. For example, grains on an arctic marine shelf

may still retain surface microrelief features acquired during glacial transport of

e grains to the shelf.

With care and the use of statistical methods, it has proven possible on the

basis ef surface textures to distinguish quartz grains from at least three major

modern environmental settings: littoral (beach and nearshore), eolian (desert),

and glacial. Quartz gras from littoral environments are characterized especially

y V-shaped percussion marks nd conchoidal breakage pattes. Grains deposited

 eolia environments show surface smoothness and rounding, irgular upturned

plas,  silica solution and precipitation featus. Grains om glacial deposits

hav� coidal fracture atterns and parallel to semi parallel striations. Te chniques

for studying the surface texture of quartz gras in modem environments have also

Chapter 3 I Sedimentary Te xtures

Figure 3.12

been extended to study of ancient sedimentary deposits. Interpretation of pale

oenvironments on the basis of surface texture is complicated by the fact that

surface microtextures can be changed during diagenesis by addition of cement

ing overgrowths or by chemical etching and solution. Krinsley and Trusty

(1986) and Marshall (1987) provide additional details and discuss applications

of the study of quartz grain surface textures by using the scanning electron mi

croscope (SEM).

3.4 FA BRIC

The fabric of sedimentary rocks is a function of grain orientation and packing and

is thus a property of grain aggregates. Orientation and grain packing in turn con

trol such physical properties of sedimentary rocks as bulk density, porosity, and

permeability.

Grain Orientation

Particles in sedimentary rocks that have a platy (blade or disc) shape or an elon

gated (rod or roller) shape commonly show some degree of preferred orientation

(Fig. 3.12). Platy particles tend to be aligned in planes that are roughly parallel to

the bedding surfaces of the deposits. Elongated particles show a further tendency

to be oriented with their long axes pointing roughly in the same direction. The

preferred orientation of these particles is caused by transport and depositional

processes and is related particularly to flow velocities and other hydraulic condi

tions at the depositional site. Most orientation studies have shown at sand-size

particles deposited by fluid ows tend to become aligned parallel to the current

direction (Fig. 3.12A; Parkash and Middleton, 1970), although a secondary mode

of grains oriented normal to current ow (Fig. 3.12B) may be present. If the grains

have a streamline or tear-drop shape, the blunt end of the grains commonly poin

upstream because this is the most stable orientation wiin a current. Sand grains

can also show well-developed imbrication with long axes generally dipping up

current at angles less than about 20°. Imbrication refers to the overlapping

arrangement of particles like that of shingles on a roof (Fig. 3.12C). Particles of

Current

Quiet water

Schematic Illustration of the orientation of

elongated particles in relation to current

flow. A. Particles oriented parallel to current

flow. B. Particles oriented perpendicular

to current flow. C. Imbricated particles.

D. Randomly oriented particles, characteris-

tic of deposition in quiet water.

sandy

sediment deposited by turbidity currents and grain-flow or sandy debris

flow processes als0 tend to be aligrted praUel to ow directions and display up

tream imbrication t angles exceedg 20° (Hiscott and Middleton, 1980); however,

 some gravity·flow deposits, orientation and imbncation directions can be vari

able or polymodal. Particles deposited under quiet waer conditions may also

show

various orientations and a lack of imbrication (Fig. 3.120). Fabric inconsis

ncies or bimodality appear to be related mainly to very rapid deposition from

suspension or from sandy debris ow 

Pebbles in many grav] deposits and ancient congtomerates also display pre

fe rred orientation and imbticaion. River-deposited pebbles are coonly oriented

with their long axes normal to ow direction (Fig. 3.12C} and may display up

seam imbrication of up to 10 to 15° (Fig. 3.13). Orientation can also be parallel to

flow, or even bimodal. Increasg ow tensity appears to favor orientation with

long

axes pamllel to current flow rather than normal to flow (Johansson, 1976).

Pebbles deposited by turbidity currents or other gravity-flow processes also be

come oriented with their long axes maly parallel to flow direction, although ori

en tation in some deposits can be random. Pebbles in glacial tills show preferred

orientation paraUel to flow and a minor mode oriented normal to flow.

Grain Packing, Grain-to-Grain Relations, and Porosity

Grain packing refers to the spacing or density patterns of gras in a sedimentary

rock and is a function mainly of gra size, shape, and the degree of compaction of

e sediment. Packing strongly affects the bulk density of the rocks as well as their

porosity and permeability. The effects of packing on porosity can be illustrated by

considering the change in porosity that takes place when even-size spheres are re

arranged from loosest packing (cubic packing) to tightest packing (rhombohedral

packing) as shown in Figure 3.14. Cubic packg yields porosity of 47.6 percent,

whereas the porosity of rhombohedrally packed spheres is only 26.0 percent. The

packing of nah1ral particles is much more complex because of variations in size,

shape, and sorting and is further complicated in lithified sedimentary rocks by the

effec ts of compaction.

Poorly sorted sediments tend to have lower porosities and permeabilities

than well-sorted sediments because grains are packed more tightly in these sedi

ments owing to finer sediment filling pore spaces among larger grains. Petroleum

and groundwater geologists are especially concerned with the porosity of sedi

mentary rocks because porosity determines the volume of fluids (oil, gas, ground

ater) that can be held within a particular reservoir rock. Compaction causes

major red uction in porosity. For example, a sandstone having an original porosity of

about 40 percent may have porosity reduced durg burial to less than 10 percent

Figure 3.13

3.4 Fabric

Well-developed imbrication of river cobbles, Kiso River, japan.

Imbrication was produced by river currents flowing from left to

right (arrow). Note hammer for scale.

Chapter 3 I Sedimentary Textures

Figure 3.14

Progressive decrease in

porosity of spheres owing to

increasingly tight packing.

[After Graton, L. C., and

H. J. Fraser, 1935, Systemat

ic packing of spheres with

particular relation to porosi

ty and permeability: Jour.

Geology, v. 43, Fig. 3,

p. 796.]

Figure 3.15

C"blo pao�og -�

�



(47.6% po""lty)

�

Rhombohedral

packing

(26.0% posity)

owing to compaction resulting from the weight of overlying sediment. Com

paction forces grains into closer contact and causes changes in the types of grain

to-grain contacts. Taylor (1950) identified four types of contacts between grains that

can be observed in thin-sections: tangential contacts, or point contacts; long con

tacts, appearing as a straight line in the plane of a th-section; concavo-convex

contacts, appearing as a curved line in the plane of a thin section; and sutured

contacts, caused by mutual stylolitic interpenetration of two or more grains

(Fig. 3.15). In very loosely packed fabrics, some grains may not make contact with

other grains in the plane of the thin-section and are referred to as "floating

grains." Contact types are related to both particle shape and packing. Tangential

contacts occur only in loosely packed sediments or sedimentary rocks, whereas

"floating" grain

�

tangential

contact

concave-convex

contact

Diagrammatic illustration of principal kinds of grain

contacts. A. Ta ngential. B. Long. C. Concavo-convex.

D. Sutured. [Based on Taylor, ]. M., 1950, Pore-space

reduction in sandstones: Am. Assoc. Petroleum Geolo

gists Bull., v. 34, p. 701-716.]

� sutured contact

concavo-convex contacts and sutured contacts occur in rocks that have undergone

considerable compaction during buriaL The relative abundance of these various

types of contacts can be used as a rough measure of the degree of compaction and

packing and thus the depth of burial of sandstones. Several other methods for ex

pressing the packing of sediment have been proposed; see Boggs (1992, p. 68-69)

for details.

The sand-size grains in sandstones are commonly in continuous grain-to

grain contact when considered in three dimensions; thus, they form a grain

supported fabric. Conglomerates deposited by uid flows also generally have a

grain-supported fabric. On the other hand, conglomerates  glacial deposits,

mud-flow deposits, and debris-ow deposits commonly have a matrix-supported

fabric. In this type of fabric, the pebbles are not in grain-to-grain contact but

"float" in a matrix of sand or mud. Matrix-supported conglomerates indicate de

position

under conditions where fine sediment is abundant and deposition occurs

by mass-transport processes or by processes that cause little reworking at the de

posional site.

FURTH ER READING