Walker J. Facies models, Canada, 1992

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Figure

Idealized vertical and lateral facies variations in subaerial and subaqueous environments for an explosive island volcano.

REFERENCES

Basic

sources

of information

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Lipman, P.W. and Mullineaux, D.R., 1981,

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and classify deposits.

Williams, H. and

McBirney, A.H., 1979,

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Cooper and Company, 387

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STlX

Walker, G.P.L., 1973, Explosive volcanic

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62,

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characteristics of different fallout de-

posits and proposes a

dispersal-frag-

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Wilgus, C.K., Hastings,

B.S., Posamentier,

H.W., Ross, C.A. and

Kendall, C.G.St.C.,

eds., 1988, Sea level changes: an inte-

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plus numerous case studies of sea level

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Booth, B., Croasdale, R, and Walker, G.P.L.,

1978, A quantitative study of five thou-

sand years of volcanism on Sao Miguel,

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Pelean deposits in the Fort Cemetery of

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and Lajoie, J., 1990, The May 1902

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Brissette, F.P. and Lajoie, J., 1991,

Depositional mechanics of turbulent

nubes ardentes (surges) from their grain

sizes: Bulletin of Volcanology, v. 53,

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Discussion of settling velocities of pyro-

clasts of different grain size parameters

(mean, coarse, fine) in suspensions of

varying densities. A method to evaluate

mean-flow velocity in low concentration,

turbulent nudes ardentes, independently

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H.,

1986, The

1982 eruptions of El

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Carlisle,

D.,

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Chough, S.K. and Sohn, Y.K., 1990, Deposi-

tional mechanics and sequences of

base surges, Songaksan tuff ring, Cheju

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Fink, J., 1980, Surface folding and viscosity

of rhyolite flows: Geology, v. 8, p.

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volcani-

clastic rocks, in Kokelaar, B.P. and

Howells,

M.F.,eds., ~ar~inal Basin

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mentary and tectonic processes in

modern and ancient marginal basins:

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D.A., 1990, Eruption terms: Nature,

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I.B. and Atkins,

F.B., 1980, Subglacial volcanics-on the

formation of acid

hyaloclastite: Journal of

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Honnorez, J. and Kirst, P., 1975, Submarine

basaltic volcanism: morphometric param-

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from

hyalotuffs: Bulletin Volcanologique,

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Kittleman, L.R., 1973, Mineralogy, correla-

tion, and grain-size distributions of

Mazama tephra and other post-glacial

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Lanti, E., Lanzafame, G., Rossi, P.L.,

Tranne, C.A. and Calanchi,

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Moore, J.M. and Sisson, T.W., 1981, Depos-

its

and effects of the May 18 pyroclastic

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St.

Helens, Washington: United States

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1250,

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Murai,

I.,

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Japan: Bulletin of the Earthquake Re-

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Sarna-Wojcicki, A.M., Shipley, S., Waitt,

R.B., Dzurisin,

D. and Wood, S.H., 1981,

Areal distribution, thickness, mass,

volume, and grain size of air-fall ash

from six major eruptions of 1980, in

Lipman, P.W. and Mullineaux, D.R.,

eds., The 1980 eruptions of Mount St.

Helens, Washington: United States

Geological Survey, Professional Paper

1250,

p. 577-600.

Self,

S. and Sparks, R.S.J., 1978, Charac-

teristics of widespread pyroclastic de-

posits formed by the interaction of silicic

magma and water: Bulletin Volcano-

logique, v. 41, p. 196-21 2.

Sigurdsson, H., Sparks, R.S.J., Carey, S.N.

and Huang, T.C., 1980, Volcanogenic

sedimentation in the Lesser Antilles Arc:

Journal of Geology, v. 88, p. 523-540.

Sohn,

Y.K.

and Chough, S.K., 1989,

Depositional processes of the

Suwol-

bong tuff ring, Cheju Island (Korea):

Sedimentology, v. 36, p. 837-855.

Tass6, N., Lajoie, J. and Dimroth, E., 1978,

The anatomy and interpretation of an

Archean volcaniclastic sequence,

Noranda region,

QuBbec: Canadian

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Walker, G.P.L., 1971, Grain-size character-

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Walker, G.P.L., 1980, The Taupo Pumice:

product of the most powerful known

(ul-

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Two plinian-type eruptions in the

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1980, Tarawera 1886, New

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a basaltic plinian fissure eruption:

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Wohletz, K.H. and Sheridan, M.F., 1979, A

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Yamada,

E.,

1984, Subaqueous pyroclastic

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M.F., eds., Marginal Basin Geology.

Volcanic and associated sedimentary

and tectonic processes in modern and

ancient marginal basins: Geological

Society of London, Special Publication,

29-35.

INTRODUCTION

Alluvial deposits are an important

component of the stratigraphic record.

They occur in a wide range of tectonic

settings, and are sensitive indicators

allogenic

(extrabasinal) controls such

as tectonism and sea level change.

They host major nonrenewable re-

sources such as petroleum, placer gold

and uranium. Much of the oil in the

giant fields at Prudhoe Bay (Alaska),

Brent (North Sea), and Daqing (China)

occurs in

fluvial reservoirs. Mineral de-

posits include gold, uranium and dia-

monds in the Witwatersrand Super-

group (South Africa). Uranium is found

in the Huronian Supergroup (Ontario),

and in the Morrison Formation (Col-

orado Plateau). Many important coal

deposits are of

fluvial origin, including

some of the Pennsylvanian coals

of West Virginia and Kentucky, and

some Tertiary coals of the Western

Interior Basin in Alberta, Wyoming and

Colorado.

Alluvial sediments are predominantly

clastic, ranging in grain size from the

Alluvial

Deposits

Andrew

Miall, Department of Geology,

Toronto, Ontario

M5S

381

finest mud to the coarsest boulder con-

glomerates. Minor chemical sediments

are formed in floodplain environments.

They include nodular carbonate de-

posits formed in fossil soils

(paleosols),

some coals, and minor evaporites de-

posited in nonmarine sabkhas.

SUBDIVISION OF ALLUVIAL

DEPOSITS IN

THIS CHAPTER

On a small scale, various types of strat-

ification and

bedforms can be recog-

nized in alluvial deposits. Larger

depositional units are termed

bars

(Fig.

1).

Channels are filled by a wide

variety of bars and bedforms. There

are some common patterns of ge-

ometry and composition in these de-

posits, whatever the channel style.

This has enabled sedimentologists to

erect standardized classifications of

the deposits at two distinct scales. The

smaller scale includes the individual

building-blocks, or

facies,

of the de-

posit. At a somewhat larger scale,

these facies assemblages can be

grouped into

architectural elements

Thalweg (line of

peepest channel)

Chute channel

SAND

FLAT

(downstream- accretio

element)

Crevasse-splay

(sand

bedform element)

Sand sheet

Figure

Block diagram of a hypothetical river, to illustrate some of the terms used in this

chapter.

University of Toronto,

characterized by distinctive shapes

and internal geometries that repre-

sent some of the major types of bar

and channel formation within

fluvial

systems.

On a still larger scale, four styles of

fluvial channels have traditionally been

recognized,

meandering, braided, ana-

stomosed

and

straight.

Most of the

coarse, porous beds that host petrol-

eum and mineral deposits are formed

within the channels and their associ-

ated bar deposits, and so an under-

standing of the architecture of the

various types of channel fill is critical.

In nature, there are many variations

between the four basic end member

channel styles.

On a much broader scale, important

aspects of alluvial sediments concern

their location on the earth's surface, and

the control of their large-scale architec-

ture. The study of alluvial deposits plays

a key role in the reconstruction of the

plate-tectonic evolution of sedimentary

basins, and in deducing the history of

relative changes in sea level within

sed-

Figure

An exposed bar surface at the

edge of a river channel (Congaree River,

South Carolina). A field of dunes

(10-20

high) was deposited on this surface when

the water level was higher, and has now

been abandoned. The crests are oriented

perpendicular to flow (which is toward the

viewer) and extend under water to the right,

where the dunes may still be actively ad-

vancing. Photo courtesy of

M.O.

Hayes.

boundina surface

(el

Figure

The scales of depositional elements in a fluvial

system, showing the bounding-surface hierarchy described in

this paper (see also Table 1). Circled numbers indicate the

ranks of the bounding surfaces. In diagram

the two-letter

codes indicate lateral accretion

(LA),

downstream accretion

(DA) and channels

(CH).

In diagram D the sand flat is shown as

being built up by migrating "sand waves".

Foreset terminations

of these are shown on the top of the diagram, but internal cross

bedding that results has been omitted for clarity

(Miall, 1988).

Table

Hierarchy of depositional units in alluvial deposits. Modified from Miall

(1991

b).

Group Time scale of Examples or Instantaneous Fluvial, deltaic

Rank and

processes (a) processes sedimentation rate depositional

units characteristics of

(mlka) bounding surfaces

lo-e

burst-sweep cycle

lV5-

lo4

bedform migration

lo3

bedform migration

1V2-

lo-'

bedform migration

lamina

0th-order, lamination

surface

ripple (microform) 1st-order, set bounding

surface

diurnal dune increment, 1st-order, set bounding

reactivation surface surface

dune (mesoform) 2nd-order,

coset bounding surface

lo0-

lo1

seasonal events, 10'-

lo3

macroform growth 3rd-order, slipping 5-20"

10-year flood increment in direction of accretion

lo2

lo3

100-year flood,

bar migration

lo2-

lo3

macroform, e.g., point 4th-order, convex-up

bar, levee, splay macroform top

lo3- lo4

long term geomorphic

lo0- lo1

channel, delta lobe 5th-order, flat to concave-

processes up channel base

lo4-

lo5

5th-order (Milankovitch)

10.'

channel belt sequence 6th-order, flat, regionally

cycles

extensive

lo5- lo6

4th-order (Milankovitch)

lo-'

1W2 depositional system 7th-order, sequence

cycles alluvial fan, sequence boundary; flat, regionally

extensive

lo6-

lo7

3rd-order cycles,

10.'-

1C2 basin-fill complex 8th-order, regional

Tectonic and eustatic disconformity

processes

---

-----

7. ALLUVIAL MODELS 121

imentary basins. Generally, alluvial sedi- traction-current deposits, as noted in

ments occur in two main settings, thick the next section.

clastic wedges resulting from tectonic Most alluvial sediments are depos-

uplift of a major source area, and thin-

ited from traction currents, but in

ner, pod- or sheet-like bodies deposited certain settings sediment gravity flows

on regional unconformity surfaces. may be important. These occur when

large masses of loose sediment are

SEDIMENT TRANSPORT

mobilised by liquefaction on a sloping

Sediment is transported in rivers by surface. Such occurrences are

particu-

two mechanisms, traction currents and

larly common under water, but also

sediment gravity flows. Traction cur-

occur subaerially, typically at times of

rents are those which transport cohe- heavy rainfall. The flow may start as a

sionless sediment as dispersed grains, landslide, and develop into a moving

each moving individually. Large grains

body of wet, unsorted sediment termed

are moved by sliding or rolling along a debris flow. Movement ceases when

the bed (bedload), smaller grains

the flow loses momentum on a flat

bounce along the bed or are swept

basin floor, and when the lubricating

along for short distances in suspen- effect of the pore waters has been lost

sion (bed contact load). Grains moved by the water draining out into the sub-

bedload and by intermittent sus- strate. The result is an apparently cha-

pension typically accumulate in a vari-

otic deposit of poorly sorted debris,

ety of

bedforms (Fig.

2).

Migration of

with large pebbles, cobbles, even giant

the

bedforms produces the sedimen- boulders, mixed in together and usually

tary structures seen in ancient separated from each other by a finer

deposits. The structures are charac- grained matrix of sand, silt and mud.

terized by their surface form or their in- Close examination may reveal a subtle

ternal stratification in cross section. sorting (grading) of the grains or the

The smallest grains, including the clay

matrix, and a regular orientation of the

fraction, remain in suspension unless

larger clasts.

the water body comes to a complete

Two terms, competence and

ca-

rest, as in a floodplain pond or aban-

pacity, are used to describe the ability

doned channel. At this time the grains

of a river to transport sediment. Com-

will slowly settle out. The style in which

petence indicates the grain size of sed-

the grains accumulate to form stratifi-

iment that can be transported, and

cation and sedimentary structures is

relates to the strength of the flow (velo-

the basis for the definition of facies in

city, shear stress). Capacity indicates

Megascoprc heterogenelty

Mesoscoplc

heterogenelty

Macroscopic heterogeneity

EXPLANATION

Drarned reservoir

Non-reservo~r Untapped

Residual

od lntrareservolr

compartment

rocks

moblle 011

between grams seals

Figure

Schematic cross section through a meander belt sandstone, showing the various

scales of heterogeneity, and the relation of these heterogeneities to the distribution of reser-

voir fluids. Note that much of the potential production will remain unproduced unless the

geologist can reconstruct the architectural complexities of the reservoir, and position wells

accordingly (Tyler,

1988).

the total volume of sediment that can

be moved, and is a reflection of the

magnitude of the discharge.

A HIERARCHY

SCALES

DEPOSITIONAL UNITS

Geologists describe clastic deposits in

terms of their natural depositional units,

such as (in order of increasing scale)

the

bedform, bed, stratigraphic unit

(member, formation, group), and basin

fill complex.

Fluvial depositional units

can be subdivided into ten natural

groups, based on their physical dimen-

sions, their sedimentation rate, and the

time scale represented by each type of

unit (Table

Fig.

3).

At each of these

scales rock bodies are enclosed by

bounding discontinuities. These include

the contacts between beds and sedi-

mentary structures, channel scours, the

base and top of stratigraphic units, and

the surfaces which define the major

al-

logenic subdivisions of alluvial succes-

sions, such as regional unconformities.

A sedimentological classification of

these surfaces (Table

Fig.

is a

useful tool for field studies, facilitating

field description and leading to more

comprehensive interpretations.

Petroleum reservoir geologists recog-

nize at least five scales of hetero-

geneity for purposes of calculating

volumes and rates of production. Four

of these are shown in Figure

Micro-

scopic heterogeneity is concerned with

porosity variations at the scale of indi-

vidual sand grains (within group 1 of

Table

1).

Mesoscopic heterogeneity is

that of bedding units and sedimentary

structures (groups 1-4). Macroscopic

heterogeneity includes the variability

associated with the deposition of chan-

nels and bars (groups 5-7). Megascopic

heterogeneity deals with the variations

across major sedimentary units and

entire basins (groups 8-10).

A wide variety of sedimentation rates

are shown in Table 1. At each level of

the hierarchy, instantaneous rates of

sedimentation can be estimated from

measurements in the laboratory, or

determined from study of modern river

sediments and ancient deposits

(Miall,

1991b). However, rates measured

from modern sediments usually bear

little relation to geologically meaningful

accumulation rates calculated from

ancient rock units. Depositional rates

in modern sediments can be estimated

using

l4C techniques, or by studying

the evolution of meanders and bars on

maps and aerial photographs. These

rates are typically far higher than

those estimated from ancient geolog-

ical units. This is because individual

depositional units have a limited

preservation potential, and any given

sedimentary structure, bar or channel

may be removed by erosion rather

than become buried and preserved.

The preservation potential becomes

successively lower for smaller-scale

units (smaller group numbers in Table

1). Long-term geological sedimentation

rates necessarily include numerous

episodes of nondeposition and erosion.

The contrast between the average sed-

iment accumulation rates in the upper

and lower parts of Table

emphasizes

how many depositional units and how

much sediment may be deposited, only

to be removed before it can be per-

manently preserved by burial.

FACIES

The study of sedimentary structures in

modern river deposits played an impor-

tant part in the development of our

understanding of their hydrodynamic

formation. Experimental work has

recently been summarized by Ashley

(1990) in a series of

phase diagrams.

These set out the flow depths, flow ve-

locities and grain sizes that control the

formation and stability of the various

bedforms.

The recognition of sedimentary struc-

tures, and the ways in which they group

together, is one of the main means by

which geologists define facies (Chapter

1).

Many years of study by fluvial sedi-

mentologists have shown that a limited

number of facies occur in alluvial envi-

ronments. A classification erected by

Miall (1977, 1978b; Table

is now in

widespread use for surface outcrop

studies and subsurface core analysis.

Examples of facies types are given in

Figures 5 and

In gravels, an important distinction is

that between matrix-supported (facies

Gms; Fig. 5A) and clast-supported

types (Figs.

58,

C).

Matrix support indi-

cates that clasts and matrix were de-

posited together, a characteristic of

debris flows. The other facies are those

deposited by traction currents.

Sand facies are characterized by a

range of sedimentary structures indi-

cating various conditions of deposition

(Fig.

5D-G). The vertical succession of

such facies is commonly repetitive,

or cyclic, and indicates progressive

change in depositional conditions. For

example, a vertical decrease in grain

size and in scale of cross bedding is

a feature of the classic fining-up-

ward point-bar succession (Allen,

1963), which is discussed in a later

section of this chapter.

ARCHITECTURAL ELEMENTS

Definitions

A river consist of various straight and

curved channel reaches, with large

areas of exposed gravel, sand, or

mud, termed

bars

(Fig.

7).

The devel-

opment and distribution of these fea-

tures follows certain relatively pre-

dictable patterns that leave their

record in the resulting deposits. The

channels and bars are the basic depo-

sitional elements of the river, and can

be subdivided into

architectural

Table

Facies classification. From Miall

(1978b).

Facies Facies Sedimentary Structures Interpretation

code

Gms massive, matrix supported gravel grading debris flow deposits

Gm massive or crudely bedded gravel horizontal bedding, imbrication longitudinal bars, lag deposits,

sieve deposits

Gt gravel, stratisfied trough cross beds

minor channel fills

Gp gravel, stratisfied planer cross beds

longitudinal bars, deltaic growths

from older bar remnants

sand, medium to very coarse,

solitary or grouped trough cross beds dunes (lower flow regime)

may be pebbly

sand, medium to very coarse,

solitary or grouped planer cross beds linguoid, transverse bars, sand

may be pebbly

waves (lower flow regime)

Sr sand, very fine to coarse ripple cross lamination ripples (lower flow regime)

Sh sand, very fine to very coarse horizontal lamination planer bed flow (upper flow

may be pebbly parting or streaming lineation regime)

sand, very fine to very coarse

low angle

(clOO)

cross beds

scour fills, washed-out dunes,

may be pebbly antidunes

Se erosional scours with intraclasts crude cross bedding scour fills

sand, fine to very coarse, may be pebbly

broad, shallow scours

scour fills

FI sand, silt, mud deposits

fine lamination, vey small ripples

overbank or waning flood

Fsc silt, mud

laminated to massive

backswamp deposit

Fcf mud

massive, with freshwater molluscs backswamp pond deposits

Fm mud, silt

massive, desiccation cracks

overbank or drape deposits

coal, carbonaceous mud plant, mud films

swamp deposits

carbonate

pedogenic features

paleosol

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elements (Allen, 1983; Ramos and

Sopetia, 1983). These are components

of a depositional system equivalent in

size to, or smaller than a channel fill,

and larger than an individual facies

unit. They are characterized by a dis-

tinctive facies assemblage, internal ge-

ometry, external form and (in some

instances) vertical profile (Fig.

1).

Work

fluvial architectural elements has

been expanded and synthesized by

Miall (1985, 1988), who proposed eight

basic types (Table 3; Fig. 8). Many ele-

ments are

macroforms, or large-scale

components of the

fluvial landscape.

They represent the cumulative effects

of depositional and erosional pro-

cesses over periods of tens to thou-

sands of years

(e.g., the sand flats in

Fig.

7B), and are group

deposits in

the classification of

able

1. in fluvial

systems, macroforms include major

channels and bars, such as point bars,

side bars, sand flats and islands (Fig.

7). In the rock record, bars are typi-

cally bounded at the top by convex-up

fourth-order surfaces (although many

bars are truncated by a succeeding

sand sheet). The bars are cut internally

by third-order surfaces, indicating the

presence of separate growth incre-

ments. Each depositional unit bounded

by a third-order surface represents a

period of active bar growth, for exam-

ple, during seasonal runoff peaks or

during flash floods in a desert. Inter-

vening periods of low water are repre-

sented by mud drapes. Periods of

erosion are represented by low-angle

disconformity surfaces that commonly

truncate underlying bedding at a low

angle. Comparable cross-cutting ero-

sion surfaces occur within individual

cross bed units, where they are termed

reactivation surfaces. They represent

surfaces where the

bedform became

reactivated following a pause in depo-

sition (Collinson, 1970). The same term

can be used for the third-order surfaces

that separate macroform growth incre-

ments.

Following Ashley

(1990), the term bar

is now restricted to these large forms.

The simple cross-bedded bars that

have been termed linguoid,

lobate or

transverse bars in earlier literature, are

now recognized to be simple dunes.

They are two-dimensional or

three-di-

Lateral-accretion deposits

Point bars (Fig. 7A) are examples of

the lateral-accretion architectural

element (LA in Table 3; Figs.

7A, 8),

so termed because the direction of ac-

cretion is at a high angle to the main

channel trend. The term is now pre-

ferred to "point bar" for descriptive pur-

poses, because lateral-accretion

deposits occur in many

fluvial settings

that are not point bars, such as in

mid-channel settings. In meandering

streams they develop on the insides of

meander bends as the bend widens or

migrates downstream. Surface flow

impinges against the outer bank,

where it maintains a

cutbank by active

erosion (Figs.

7A, 9). The flow turns

downward, developing a helical over-

turn pattern. The return flow, at depth,

passes obliquely over the point-bar

surface (Fig. 9). The helical flow pat-

tern decays as the flow emerges from

the bend, and is replaced by a helical

overturn in the opposite direction, as

the flow impinges on the

cutbank of

the next bend downstream.

Sediment removed from the

cutbank

is incorporated in the overall sediment

load of the river. Large slump blocks

may accumulate in the deepest part of

the channel as a lag deposit. Material

broken down into individual grains is

incorporated into the bedload and sus-

pension load, and is swept down-

stream. Much of it becomes deposited

in bedforms and bars. Sediment is

added to the point-bar surface at a

rate comparable to that at which it is

removed by erosion along the outer

mensiOnal depending

whether

they

Figure

Floodplain deposits. A) Close-up view of floodplain siltstones (facies

Frn)

con-

are simple, straight-crested forms

(24%

taining rootlets; Triassic, Spain.

A well-developed calcrete (facies

P);

Buntsandstein

or sinuous crested (Fig. 7B).

(Triassic), Spain.

bank of the meander. The resulting

lateral growth can easily be detected

in many modern bars by the curved

ridges and depressions (swales) on

the bar surface. In Figure 7A from the

Milk River, the centre of each point bar

is covered by old trees, with the size

and density of vegetation decreasing

outward to the vegetation-free bar

surface at the edge of the channel.

This spectrum of vegetation illustrates

7. ALLUVIAL MODELS 125

the bar evolution and indicates the

time available for growth on each part

of the bar. Variations in depth and ve-

locity over the point-bar surface result

in sediment sorting and variations in

the assemblage of sedimentary struc-

tures, with a tendency for finer grain

sizes and structures of lower flow re-

gime to occur in the upper part of the

bar. Figure

shows the edge of a

modern point bar, with the active de-

Figure

Examples of different styles of river channels and bars. A) The Milk River,

southern Alberta, an example of a highly sinuous,

meandering

river. The bare sand surfaces

at the edge of the channel on the inside of each meander bend are good examples of point

bars (PB). Channel is about

m wide.

The William River, northern Saskatchewan, an

example of a multiple-channel,

braided

river. The large pale-coloured exposed areas in the

distance are examples of what are commonly termed sand flats. They are compound bars

built by depositional and erosional processes over many years. In the foreground is a series

of large,

lobate, three-dimensional dunes (formerly termed linguoid bars) advancing toward

the viewer. Channel is about

wide. Both photos courtesy of

Smith.

positional surface covered with two-

dimensional dunes.

Continued enlargement of a mean-

der loop results in increased sinuosity.

Eventually a

cutbank may incise into

an adjacent reach of the channel. This

typically occurs at the neck of a

meander (Fig.