Walker J. Facies models, Canada, 1992

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ALLUVIAL MODELS

127

bounding surfaces (Figs.

12, 13).

This

assemblage of cross-bedded deposits

and their enclosing surfaces reveals

the former existence of an active,

non-

periodic, possibly irregular-shaped bar

form comparable in height and width

to the channel in which it formed.

The bounding surfaces are of

first-,

second-, and third-order type; they dip

gently

(clOO)

downstream, oblique to

---+

Surface current

---

Bottom current

flow, or gently upstream around and

over a low-relief bar core. Between

these surfaces are sets or cosets of

St, Sp, Sh,

SI, or Sr (see Figs. 5D-G,

for examples of these facies). The Sh

and

SI laminae are organized parallel

or subparallel to the internal bound-

ing surfaces. Detailed paleocurrent

studies show that the

bedforms ad-

vance generally down the slopes

defined by the first- to third-order sur-

faces, or oblique to the surfaces drap-

ing the bar cores. These data reveal a

picture of fields of

bedforms driving

across, around and down the bar

forms.

Cant and Walker

(1978)

referred to

the large, mid-channel bars in the

South Saskatchewan River as

sand

flats.

These evolve from large, simple,

Figure

Three-dimensional diagram that

illustrates the formation of lateral-accretion

deposits on the inside of a meander bend.

Surface flow around the bend impinges

against the outer bank, where it maintains

an active

cutbank. The flow turns down and

passes obliquely across the point-bar

surface, setting up a helical overturn

pattern. Sediment is carried up onto the

point bar, and sorted under conditions of

decreasing depth and velocity. This ac-

counts for the decrease in grain size and in

scale of hydrodynamic structures up the

point-bar surface. A vertical section through

the bar is bracketed at the right. It is

capped by fine-grained floodplain deposits

accumulated by vertical accretion following

continued channel migration. This model is

the basis for the classic

fluvial fining-

upward cycle of Bernard

a/.

(1 962), Allen

(1963), Visher (1965), and others.

Figure

Examples of lateral-accretion

elements, illustrating the wide range of

scales and facies (from

Miall, 1985). No

vertical exaggeration. All sections are ori-

ented approximately perpendicular to the

channel (comparable to the cross-section

shown in Fig.

9).

Facies codes are given in

Table 2. A) Conglomerate point bar (facies

Gm), with chute channels (facies Gt;

Quaternary alluvial fan gravels, Italy);

Element composed of medium-grained

sandstone, with abundant internal

planar-

tabular cross bedding (facies Sp; Penn-

sylvanian, Appalachian coal basin);

Fine- to very-coarse sandstone and pebbly

sandstone with cobble to boulder conglom-

erate lag. Abundant internal cross bedding

(facies Sp, St, Sh, and

SI; Devonian,

Wales); D) Small sandy point bar with

abundant dune and ripple cross bedding

(facies St, Sr; Miocene, Spain); E) Point

bar composed mainly of fine sandstone

and siltstone (facies

SI) with minor

medium- to coarse-grained, cross-bedded

sandstone (facies St) at base (modern

river, British Columbia);

Giant point bar

with thick, fine-grained trough

cross-

bedded sandstone at base (facies St)

passing up into accretionary sets of alter-

nating fine sandstone and argillaceous

silt-

stone showing evidence of tidal bundling

(facies Se; Athabasca Oil Sands, Alberta).

cross-bedded flow-transverse bed-

forms, termed cross-channel bars

by Cant and Walker (1978). Elevated

parts of these

bedforms become

emergent at low water and then form

the nuclei of new sand flats, which

anchor part of the bar in the middle of

the channel. Sediment is added to the

cross-channel

bedforms by the migra-

tion of fields of dunes and ripples.

Those parts of the crestline in deeper

water continue to advance more rapid-

ly than the sand flat nucleus, so that

the entire

bedform swings around

oblique to the channel direction (Cant

and Walker, 1978; Allen, 1983, his Fig.

20). The macroforms accrete sediment

partly by the process of

bedform

capture on the upstream or flanks, and

partly by rapid burial and preservation

of superimposed

bedforms on the ad-

vancing downstream face (Fig. 12).

Many of the variations in composi-

tion and geometry between described

macroforms probably reflect fluctua-

tions in

stage (the depth of water in

the river). The bar surfaces may be

cut by numerous erosional channels

during falling stage, and may be aban-

doned altogether at low stage, or as a

result of a shift in channel position,

leading to overall nondeposition or

erosion (Fig. 11). The minor channels,

and the cross-cutting erosion surfaces

that separate growth increments, are

defined as third-order surfaces.

Other architectural elements

Characteristics

the other main

element types are listed in Table

The channel element (CH) may be

defined for small channels within a

channel complex (the main channel-fill

deposits typically may be broken down

into component elements).

Sediment

gravify flow deposits (element

SG;

Fig.

5A) are common in some proximal al-

luvial fans. Here, abundant loose sedi-

ment may be available in the source

area, and can be transported by occa-

sional sudden run-off events related to

severe weather systems.

Sandy bed-

forms

(SB) are typical of crevasse

splays (mini-deltas which accrete into

the floodplain from breaks in the chan-

nel bank; see Fig.

I), bar tops, and

some sand sheets in shallow rivers

where major sand flats are not able to

develop. Commonly they show upward

fining.

Laminated sand sheets

(LS)

are

tabular units characterized by parting

Figure

Typical point bar deposit; an example of a lateral-accretion

(LA)

deposit. Each

accretionary unit consists of a resistant sandstone unit at the base, which becomes more

thinly bedded and more recessive laterally upward, along the bedding surface.

Carboniferous, Black Warrior Basin, Alabama. Thickness of point bar, between the two coal

beds, approximately

Figure

Generalized models of downstream-accretion

(DA)

architectural elements.

Scales are approximate. Internal geometry varies considerably, depending on channel

depth, grain size, discharge amount and variability. The upper diagram shows

deposit ac-

cumulated as three separate increments, separated by internal, dipping, third-order erosion

surfaces. Two minor channels formed by surface runoff at times of falling discharge, are also

shown

(Miall,

1985).

. -.

. .

.....

....

---

ALLUVIAL

MODELS

129

Figure

Example of a downstream-accretion element. Kayenta Formation, SW Colorado. Letters and numbers at left are numbers given to

bounding surfaces and individual elements at this outcrop. Numbers 3,

and 5 on bounding surfaces indicate the interpreted rank of the surface.

Individual paleocurrent readings are shown by arrows, oriented with respect to the outcrop face (horizontal arrows pointing to the left mean

flow

directions parallel to the face oriented to the left). Element 2-DA is a small downstream-accretion element, as shown by the third-order surfaces

dipping toward the left, and extending from the top-right of the element to the lower left, indicating accretion to the left. Paleocurrent measure-

ments on individual cross bed sets (arrows) indicate that they formed by

bedform migration in the same (leftward) direction.

Figure

The range of alluvial-channel patterns.

bedload channels,

mixed-load channels,

suspended-load channels (Schumm,

1981). The four major channel styles are meandering

(7,

8, 12, 13), braided

(3-5,

lo),

anastomosed (14), and straight (2). Straight chan-

nels (1,

11) are rare in nature.

s mg

cfc

LLIl_U

lineation (facies Sh; Fig.

5F),

and

develop by upper flow regime condi-

tions during flash floods in arid envi-

ronments.

The

overbank fines element

(OF;

Fig. 5H) is highly variable. Its charac-

teristics depend primarily on climate. In

humid-tropical regions coal may be im-

portant, although it may be necessary

for the

overbank region to develop as a

raised swamp, in order to prevent

clastic influxes infiltrating the accumu-

lating peat and downgrading the re-

sulting deposit to carbonaceous shale

(McCabe, 1984). In more arid regions

alternation of evaporation and rain infil-

tration concentrates dissolved carbon-

ates and silicates near the sediment

surface, leading to nodular

calcretes

(Fig. 6B) and

silcretes.

Commonly

these develop in association with soil

formation. Several authors have shown

how the structure and colour of cal-

cretes can be used to estimate the

time of maturation of these deposits

(Leeder, 1975; Bown and Kraus, 1987;

Kraus, 1987). Small, isolated nodules

and grey and orange colours indicate

(D)

immature soils, whereas layers of

large, possibly laterally coalesced

nodules, and red and purple colours

Figure

Comparison of four vertical profiles, showing how similar fining-upward cycles may

be produced in several different ways. A) Model cyclic sequence, Battery Point Formation

indicate greater maturation times.

(~elonian), Quebec, probably formed by vertical bar aggradation in a low-sinuosity (braided)

river. B) Sequence formed by lateral accretion of a point bar, Castisent Sandstone (Eocene),

Spain. C) Sequence formed by lateral accretion of

point bar, modem Amite River, Louisiana.

D) Sequence formed by vertical aggradation and progressive channel abandonment on an al-

luvial fan, under conditions of tectonic quiescence (allogenic control), Upper Carboniferous

coal measures, northern Spain. Columns A,

and

include fine-grained overbank deposits at

the top formed by vertical accretion on the floodplain following the abandonment of the bar.

Diagram from

Miall

(1980).

Figure

The Kosi fan, India. This is

probably the world's largest fan. The dates

occu~ied bv each distributaw channel are

Figure

Aerial view of alluvial fans along the flank of the Mackenzie Mountains, Canada.

shown

note the gradual westward mi-

Note the varying size of these fans, their tendency to merge laterally, and the fact that in most

gration of the main distributary. Sediments

instances only a small area of each fan is occupied by active distributaries. Field of view

range from boulder conglomerate near the

proximately

km wide.

apex, to fine sand and silt at the foot.

. .

ALLUVIAL MODELS

Thick, well-developed calcrete beds in-

dicate that the floodplain has been

abandoned in a state of nondeposition

for tens of thousands of years. Such

surfaces may extend for many

kilome-

tres, and may have significance as re-

gional sixth- or seventh-order bounding

surfaces indicating times of tectonic

quiescence and stable base level.

Apart from these chemical sedi-

ments the

overbank area may accu-

mulate thick muds in swamps or tem-

porary ponds, and may also contain

sandstone sheets (element

SB) de-

posited as crevasse splays.

CHANNEL STYLES

Four traditional channel patterns form

the basis of most textbook discussions

of alluvial depositional systems. There

are both scientific and historical rea-

sons for this; the straight, meandering,

braided and anastomosed patterns are

certainly common in nature, but these

patterns have been overemphasized in

the search for simple models by sedi-

mentologists and petroleum geologists

in the 1950s and 1960s. A full review

of the historical development of

fluvial

studies has been given by Miall

(1978a). It is now accepted that, al-

though these patterns are common,

there are many gradations between

them.

Areal variations in channel styles,

and their causes

Why are there so many different

fluvial

channels styles (Fig. 14)? This is an

extremely complex question, to which

there is no simple answer. Braided

channels (patterns 3-5, 9, 10 in Fig.

14) have numerous bars and islands

that represent temporary sediment

storage during downstream transport.

This indicates that the capacity of the

river has been exceeded for some or

most of the time. Thus abundant sedi-

ment supply, for example from easily

eroded banks, is one cause of braiding.

A second factor favouring a braided

channel pattern is a highly variable dis-

charge. Large volumes of sediment can

be carried during periods of high dis-

charge, but the river does not have the

competence or capacity to move the

load at other times, hence the storage

of sediment in braid bars and islands.

Rivers characterized by large dis-

charge fluctuations related to seasonal

variations in temperate, arctic, and

alpine climates, or the more random

discharge variations in arid regions,

therefore tend to be braided, given ad-

equate sediment supply.

Meandering (Fig. 14, patterns

12, 13) and anastomosed (Fig. 14,

pattern 14) rivers characteristically

occur on lower slopes and carry finer

grained sediment loads. Anastomosed

rivers have numerous channels of var-

iable sinuosity. They are rather stable

in position, and do not migrate later-

ally, as do the meander bends of me-

andering rivers. There is growing

evidence that the anastomosed pat-

tern develops where the river is af-

fected by a downstream damming or

backtilting process. This may occur

where a river crosses a tectonically

positive area, or flows toward an area

of postglacial rebound. Vegetation is

also an important control on channel

style. Dense vegetation along river

YALLAHS FAN DELTA

SKEIDARARSANDUR BRAIDPLAIN DELTA

FLUVIAL REGIME FLUVIAL REGIME

NATWlE

Braided

channels

wtth

l~ngit~dinal

bars

GRADIENT

Smlkm

SEDIMENT

GRADE

Sand

wlth

bars

SEDIMENT

GRADE

Sand

. .

. . . . .

BASINAL REGlM BASINAL REGIME

TIDAL

Microtidal

TIDAL

REGIME

Mesotidal

(spring

ranoe

2m)

WAVE

ENERGY

HIGH

maximum

WAVE

ENERGY

Summer:

breaker

maximum

breaker

height

1.2m

Winter:

deepwater

wave

DELTA

FRONT

180mlkm

heights

14m

SLOPE DELTA

FRONT

Bmlkm

Wave Energy (ergs)

Figure

Examples of modern coastal alluvial depositional systems. The Yallahs fan delta is in Jamaica, a tropical setting. The

Skeidararsandur braidplain delta

is on the south coast of Iceland, a cool-temperate setting. Diagram compiled by Orton

(1988).

banks, such as grasses or tropical-tree

roots, tends to inhibit bank widening

and prevent the development of

braiding. Vegetation is climatically de-

pendent, and has only been an impor-

tant control on channel style since the

Silurian or Devonian, when land plants

first became a significant component

of the subaerial landscape. Until that

time poorly consolidated banks were

probably the norm, with a consequent

tendency for braided-channel styles to

predominate

(Schumm, 1968).

Given these controls, there is a ten-

dency within most alluvial basins for

proximal rivers (including alluvial fans

if present) to be braided, whereas

more distal rivers, at least in perennial,

humid environments, are more likely to

be meandering. Anastomosed rivers

are also more likely to occur in distal

settings, but may also occur in prox-

imal settings where the river crosses

an area undergoing gentle tectonic

uplift, such as a fault.

Whatever the actual channel style,

channels are always characterized by

some sinuosity; meandering is a char-

acteristic of large-scale turbulence of

moving water bodies. Even in straight

channels there may be a meandering

thalweg

(line of deepest channel), with

bank-attached

side

bars

deposited on

alternate sides of the channel inside

each bend in the thalweg (Fig. 14,

pattern 2). Completely straight chan-

nels, such as patterns 1, 6 and

1 in

Figure 14, are rare.

Standardized facies models were

built during the 1960s and

1970s,

based on the basic depositional charac-

-teristics of the four main channel styles,

plus such variables as sediment-load

grain size and climatic criteria. A con-

siderable increase in the volume of de-

scriptive data during the last decade

or so has demonstrated that these

models are inadequate to convey the

full variability of

fluvial depositional

styles, and it has also been shown that

they contain some contradictory fea-

tures. For example, it is now known

that the process of lateral accretion,

which gives rise to the distinctive ar-

chitecture of the point bar (Fig. 9-11),

is not confined to meandering rivers,

as was once thought, but is common

in braided streams, and also occurs

in minor or modified form in

anasto-

mosed and straight channels.

In the past, considerable reliance

Figure

Block diagram of typical gravel-bed braided river. Element and facies codes for

this and subsequent diagrams are given in Tables

and

Figure

Block diagram of a large, perennial, sand-bed braided river, in which large sand

flats are developing. Note the internal complexity of the sand flats, which typically include

and

elements (Figs. 9-1

3).

SHEET

SANDSTONE

order surface

5th

order sul

order surface

MACROFORM

ASSEMBLAGE

A-8

Macroform scale fining upward cycle

C-D

Channellscour scale fining upward scale

E-F

Sheet

scale

fining upward cycle

Figure

The three scales of fining-upward cycle in a typical sandy-braided deposit, the

Westwater Canyon Member of the Morrison Formation

(Godin, 1991). The depositional envi-

ronment of this river was similar to that shown in Figure

20.

Figure

Block diagram of a typical coarse-grained (sand and/or gravel) meandering

river.

Figure

23 Diagrammatic cross section across a point bar in the Castisent Sandstone,

Spain, showing the architecture of the deposit, and the range of sedimentary structures.

(Modified from

Nijman and Puigdefabregas, 1978).

Cutbank Scour Lower 2-D dune Upper

pool

pointbar subfacies po~ntbar

Figure

Block diagram of anastomosed river. Horizontal ruling on the top surface of the

diagram indicates swamp vegetation, and encloses areas of open water comprising small,

shallow, floodplain ponds.

Accretionary bank

Sr, FI

Figure

An interpreted example of an anastomosed river deposit. The sandstone lens is

thick, and represents a channel deposit. It is encased in thick floodplain siltstones and

sandstones which, in this case, also include nodular calcrete deposits. Cutler Group,

northern New Mexico (Eberth and

Miall, 1991).

has also been placed on the charac-

teristics of vertical profiles as indica-

tors of channel style. For example, the

presence of fining-upward succes-

sions with their characteristic assem-

blage of sedimentary structures (Allen,

1963; Visher, 1965) was regarded with

near certainty as proof of a

mean-

dering-fluvial environment. We now

know that vertical profiles are unreli-

able diagnostic tools. For example,

Figure

shows four fining-upward

cycles, three produced by different

autogenic

(within basin) processes

and the fourth developed by a tectonic

(allogenic)

control.

Modern architectural studies of

flu-

vial deposits demonstrate an enor-

mous variety of

fluvial styles. Twelve

different styles were illustrated by

Miall

(1985), who emphasized that this was

not intended to provide a complete

coverage of the subject. Indeed, con-

tinued field research has provided the

basis for defining several additional

fluvial styles, none of them exactly

similar to the published twelve. How-

ever, each style is characterized by a

particular assemblage of the architec-

tural elements described in this chap-

ter. This suggests the basis for a

modified approach to facies analysis

for use in complex systems such as al-

luvial deposits, where a deposit may

be encapsulated by a unique

summary

of the environment.

It is the architec-

tural elements that provide the re-

cognizable, common thread running

through the analysis. In a subsequent

section a few common

fluvial styles

are selected, and it is shown how the

corresponding deposits can be ana-

lyzed in terms of the standard ele-

ments described in this chapter.

ALLUVIAL FANS AND FAN DELTAS

These environments have become

widely recognized by

sedimentolo-

gists, but the use of the terms may be

confusing. An

alluvial fan

is a distribu-

tary

fluvial system, formed where

rivers emerge from a confined, moun-

tain valley onto the basin floor, where

the channels are no longer confined

(Fig. 16). The

fluvial system fans out

from a point source (the feeder chan-

nel) into a series of distributaries, al-

though only one or two may be active

at any one time. Typically deposition

within a narrow channel belt builds up

the fan surface in one area and

reduces the slope. Eventually a chan-

nel-switching

(avulsion)

event occurs,

normally during a high-discharge event,

when flooding overtops the channel

banks, and discharge switches to a

region of the fan having a steeper

slope. In this way the water and sedi-

ment gradually shift across the entire

fan, building the typical fan-shaped de-

positional lobe (Fig. 17). In arid envi-

ronments the

fluvial discharge may .be

in the form of sheet floods or lobate

sediment-gravity flows that are not

confined to channels. Nevertheless,

the flow radiates from a point source,

and may therefore be said to be

formed within a fan-like distributary

system. A

fan delta

(Fig. 18) is an allu-

vial fan that progrades directly into a

standing body of water (sea, lake;

Nemec and Steel, 1988).

For most geologists the terms alluvial

fan and fan delta imply coarse-grained

sediments deposited in braided chan-

nels. There is a tendency for geologists

to use the terms in a descriptive sense

for any ancient basin-margin conglom-

eratic unit, hence the evolution of the

term

fanglomerate.

Such sediments

are assigned to element GB (Fig.

5C),

and may or may not contain inter-

bedded sediment-gravity-flow deposits

(element SG; Fig. 5A). The gravel

STRIKE-SLIP

v-,ysg,

facies illustrated in Figure

are typical

of those formed within fan environ-

ments. The

fluvial styles of gravel-dom-

inated braided channels are illustrated

in the next section. However, the terms

alluvial fan and fan delta do not, in fact,

have a unique facies sense. There are

giant modern fans, such as the Kosi, of

India (Fig.

17), which grade from

boulder conglomerate near the moun-

tains to fine sand-silt-mud 140 km

downslope at its distal end. Hirst and

Nichols (1986) described an ancient

example of a basin-margin distributary-

alluvial-fan system (documented by pa-

leocurrent data) which consisted

predominantly of sandstone organised

into ribbons (element

CH)

and sheets

(elements SB, LS).

Parkash

et al.

(1 983) described a modern

terminal

fan,

which is an ephemeral fluvial dis-

tributary system 12 km long that de-

posits fine sand, silt and mud at the

edges of playas or tidal flats in arid

regions (element SB).

There is some ambiguity in the terms

alluvial fan and fan delta. Not all basin-

margin talus prisms are built by dis-

tributary systems emanating from

point sources. Many, particularly in

arid environments, are deposited by

an array of individual, parallel streams.

Coastal glacial-outwash systems, such

Thrust fault Direction of

Normal fault

plate migration

. .

Strike-slip offset Alluvial deposits

. . . .

Figure

Tectonic setting of fluvial deposits. Note the distinction between

transverse

drainage, which is oriented perpendicular to structural grain, and

axial,

longitudinal

drainage,

which typically consists of trunk streams flowing along the basin axis.

as those on the south coast of Iceland

and Alaska consist of parallel braided

systems fed from several or many

meltwater sources (Fig. 18; see Chap-

ter 5). They have been termed fan

deltas by some writers, but the dis-

persal may differ from the classical

radial pattern. If the geomorphic impli-

cation of the two terms is considered

important, they cannot be used for

all basin-margin alluvial environments.

The alternative term

braidplain

may be

used for

fluvial systems consisting

of an array of parallel streams with

multiple sources. The corresponding

coastal system may be called a

braid-

plain delta.

In practice the difficulty of

distinguishing single from multiple

sources and parallel versus radiating

dispersion in the ancient record may

make these distinctions difficult to

apply.

Orton (1988) suggested that

most fans and fan deltas have a trans-

verse relationship to regional structural

grain, contrasting with the axial or

Ion-

Alluvial

valley

deposits

Channel

scour

Figure

Schematic block diagram of

the valley-fill deposits of the Mississippi

River. The basal bounding discontinuity of

the valley formed during the Pleistocene

lowstand of sea level, when the river

incised bedrock to adjust itself to a lower

base level. As sea level rose following

glaciation the valley filled first with coarse

sands deposited in a braided-stream envi-

ronment, and then with lower energy me-

andering-stream deposits. Many ancient

valley-fill alluvial successions formed in

this way. In some, the uppermost deposits

are estuarine to marine in depositional en-

vironment, and may be followed by

widespread transgressive-marine blanket

deposits (Weimer, 1986; based on the

work of

Fisk)

---

ALLUVIAL

MODELS

gitudinal orientation of typical braid-

plains. He also suggested that debris

flow deposits and sheetflood deposits

were more likely to occur on fans, as

opposed to braidplains.

In light of the problems discussed

above, the terms alluvial fan and fan

delta should clearly be used with ex-

treme caution when applied to the rock

record. Because the primary criterion

in modern environments is diverging

distributary patterns, these should be

demonstrated in ancient rocks if this

terminology is to be used meaning-

fully. Excellent descriptions of alluvial

fans and fan deltas are given by

Nilsen (1985) and Nemec and Steel

988), respectively.

EXAMPLES OF

FLUVIAL STYLES

Given the variability in nature of the

many controls that govern

fluvial style

(climate, discharge amount and vari-

ability, sediment size and amount, re-

gional slope, subsidence rate, bank

stability) it is not surprising that

fluvial

systems show a very wide range of de-

positional styles. The basic sedimen-

tary processes are comparable in all

rivers, which is why it is possible to use

a single facies classification for all allu-

vial deposits. The major architectural

elements are also comparable in all

rivers. Thus lateral-accretion deposits

occur in gravel- and sand-bed braided

rivers and all types of meandering

rivers and, to a minor extent, also in

anastomosing and straight rivers. But

the geometry of the channels and the

way by which the elements are stacked

on each other show a wide degree of

variability. A few of the better known

examples of

fluvial style are described

briefly in this section.

Gravel bed rivers

Gravel bed rivers occur as the distribu-

taries of many alluvial fans, and also

form the proximal reaches of many

flu-

vioglacial outwash plains. Many chan-

nels may be active within a single

broad valley. Element GB predomi-

nates (Fig. 19). This is built up by the

development of low gravel sheets

during high-discharge events (facies

Gm; Fig.

5B), and by the migration of

gravel

bedforms (Gp, Gt; Fig. 5B).

Typically gravel comprises up to 95 per

cent of the total thickness, the re-

mainder being sand deposited in minor

channels, during low-water stage.

A variation of this style includes

sheets or lobes of element SG (Fig.

5A), representing catastrophic run-off

events. This

fluvial style is most typical

of alluvial fans in unvegetated areas,

where there is little to inhibit rapid

runoff from sudden rain storms.

Sand bed braided rivers

There is a wide variety of styles of

sand bed braided rivers. Shallow

pe-

UTAH

COLORADO

WASATCH

BOOK

CLIFFS FRONT RANGE UPLIFT DENVER BASIN

PLATEAU

North

Horn

.....

......

Dakota

Sandstone

600

400

Nonmarine

rocks Calcareous shale

200

Marine

shale

and

siltstone

OO*

Conglomerate

----

Approximate

time

line

Vert.

Exag.

151

Figure

Stratigraphic cross section of Cretaceous-Tertiary rocks from central Utah to northeastern Colorado. Clastic wedges of this type

are very common in retroarc and peripheral foreland basins. This example is from the foreland basin of the North American Western Interior.

Note

interfingering of marine and nonmarine strata (Molenaar and Rice,

1988).

rennial rivers, such as much of the

Platte River of Nebraska, are charac-

terized by the migration of large,

lobate

3-D dunes, which deposit suites of

planar-tabular cross bedding and asso-

ciated minor structures (element SB;

Fig.

5E).

Braidplains in arid environ-

ments may be ephemeral, and are

then characterized by tabular sand-

bodies up to several metres thick con-

sisting of plane-laminated sand (ele-

ment LS; Fig.

5F), or by flood sheets

comprising thinning- and fining-upward

assemblages of cross bedding and

ripples (SB). The most complex sandy-

braided deposits are those developed

in large, perennial rivers, in which a

wide variety of large bar forms (macro-

forms) develops. Side bars, which

evolve in a similar style to point bars

and deposit element LA are common.

Mid-channel bars are also common,

and may accrete in any direction within

the channel, but most typically grow

laterally and downstream (Figs. 12,

13). The deposits then may consist of

elements LA and DA, and these may

grade into each other within the same

sand sheet. Figure 20 is a block

diagram of such a river.

Fining-upward cycles are common

in sandy-braided rivers

(Miall, 1977;

Fig. 15A of this paper). However, they

are generated by several different pro-

cesses.

Godin (1991), in a detailed

study of one ancient deposit defined

three types of cycle (Fig. 21): 1) cycles

1-6 m thick developed by the genera-

tion of a macroform

(e.g., sand flat) or

the fill of a large scour hollow;

stacked macroform cycles constituting

the fill of a channel, 1-10 m thick, and

cycles representing the thickness of

major sandstone sheets 4-16 m thick

and up to several or many kilometres

in lateral extent. Unlike the first two

types of cycle, sheet cycles may have

allogenic causes, reflecting gentle

basin tilting or base-level change. De-

finition and outcrop mapping of the

various ranks of bounding surface are

essential for the correct identification

of these various types of cycle.

Coarse-grained

meandering streams

The coarse-grained (sand andlor gra-

vel) meandering stream (Figs. 22, 23)

is characterized by point-bar deposits

(element LA; Fig. 10) that typically

have a lag deposit of caved bank ma-

terial, waterlogged plant material, or

calcrete pebbles and cobbles at their

base. The accretionary face of the bar

is crossed by numerous sandy bed-

forms, such as sand waves and dunes

(2-D and 3-D). The point bar may show

upward fining (Fig.

158,

C),

as illus-

trated in the early models of Allen

(1 963) and Visher (1

965), depending

on such factors as meander sinuosity

and flow patterns around the bend. In

the ideal case, the sediment trans-

ported across the bar is sorted in an

environment in which flow depth and

velocity decrease up the bar surface,

as a result of the helical flow patterns

that develop around the meander

bend (Fig.

9).

Meander scars and

abandoned channels (oxbow lakes)

are common in the floodplain.

There are several other classes of

meandering river, which differ from

each other primarily in the grain size of

the sediment load and the correspond-

ing sedimentary structures. They range

from vigorous, gravel bed rivers with

gravel point bars that develop in some

basin-margin environments, to slug-

gish, suspended-load streams in which

the dominant deposits are mud, silt

and very fine-grained sand.

Anastomosed rivers

Anastomosed rivers consist of several

or many active channels of low to high

sinuosity (Fig.

24).

They are relatively

Figure

lsopach map (contours in

metres) of valley-fill sandstone, Little Bow

Field, Mannville Group, Alberta.

The

cross

section is shown in Figure

(Wood and

Hopkins,

1989)

gamma ray

log

density log

Figure

Stratigraphic cross sections across the valley-fill at Little Bow field. Note the presence of three separate pools in the valley-fill,

separated by shale-filled channels. The location of the cross section is given in Figure 29 (Wood and Hopkins, 1989).