Boggs S. Principles of Sedimentology and Stratigraphy
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246
Chapter 8 I Continental (Terrestrial) Environments
Figure 8.1
Aerial view of a debris-flow
dominated alluvial fan at
the mouth of a canyon in
the steep east wall of Death
Va lley, California. The high
way gives the scale. [Photo
graph by john Shelton.]
(e.g., Collinson, 1996) and many subenvironments of the fluvial system can be rec
ognized, most ancient fluvial deposits can be assigned to one of two broad envi
ronmental settings: alluvial fan and river. These environments may be interrelated
and overlapping.
Alluvial Fans
Denition and Depositional Setting
Alluvial
fans are deposits with gross shapes approximating a segment of a cone
and exhibiting convex-up cross-sectional profile (Fig. 8.1). Many have fairly steep
depositional slopes. Sediments on alluvial fans are typically poorly sorted and in
clude abundant gravel-size detritus. Modern alluvial fans are particularly com
mon in areas of high relief, generally at the base of a mountain range, where an
abundant supply of sediment is available. In many cases, they form downslope
from major fault scarps. They occur both in sparsely vegetated arid or semiarid re
gions, where sediment transport occurs infrequently but with great violence dur
ing sudden cloudbursts, and in more-humid areas where rainfall is intense. arid
or semiarid settings, alluvial fans may pass downslope into desert-floor environ
ments with inteal drainage, including playa lake environments. In humid re
gions, they may merge downslope with alluvial or deltaic plains and beaches or
tidal flats, or they may even build into lakes or the ocean. Fans that build into
standing bodies of water are called fan deltas (Chapter 9). Along mountain fronts,
alluvial fans developed in adjacent drainage systems may merge laterally to form
an extensive piedmont, or bajada.
On the basis of depositional process, alluvial fans c be divided into debris
flow-dominated fans and stream-flow-dominated fans (Fig. 8.2). Although modem
alluvial fans are common, the features that characterize alluvial fan deposits and
that distinguish them from other fluvial deposits are controversial. Some authors
(e.g., Blair and McPherson , 1994a) regard alluvial fans as relatively small scale fea
tures with steep slopes (1.5°-25°) that were deposited may by sediment-gravity
flows, particularly debris flows, and upper-flow-regime fluid flows. Accordg to
this definition, many fluvial deposits originally considered to be fans are not true
alluvial fans. Instead, they would be called distributary fluvial systems or braid
8.2 Fluvial Systems
247
A
B
Pximal-fan
gravel
Older fan segment:
gullying, eolian
working,
.bioturbation, soil
formation
Recent debris-flow
levees & lobes
'Older'
debris-flow lobe
Active depositional lobe
fault
Abandoned lobe:
gullying, winnowing
of lines to pduce
gravel mantles
channel,
suicial
distributa
channels &
minor gullying
lJnn1nified
sheetflood
deltas (see discussion by Miall, 1996, p. 246). Stanistreet and McCarthy (1993) pro
a broader spectrum of fan types that include large fans with well-defined flu
vial channels, such as the ant Kosi Fan of India and the huge Okavango Fan of
Botswana (Africa), as well as smaller fans such as the Ya na Fan of Alaska (Fig. 8.3).
Sedimenta Processes on Fa ns
As flows emerge from confined channels in a mountain front onto a fan, they are
fe to spread out, and wa ter may infiltrate into the fan. Stream power is thus re
duced, leading to deposition. Sediment-gravity flows, including debris flows and
mudflows, are dominant transport and depositional processes on many fans in both
arid-semiarid regions and humid setting. Debris-ow deposits (e.g., Fig. 2.10) are
aracteristically poorly sorted and lacking in sedimentary suctures except possi
ble reverse graded bedding in their basal parts. They may conta blocks of various
sizes, including large boulders, and they are typically impermeable and nonporous
owing to their high content of muddy matrix. Both clast-rich and clast-poor debris
Figure 8.2
Schematic diagrams illus
trating the depositional fea
tures of (A) debris-flow and
(B) stream-flow-dominated
alluvial fans adjacent to ac
tive normal faults. [Modi
fied from Blair and
McPherson, 1994, Alluvial
fans and their natural dis
tinction from rivers based
on morphology, hydraulic
process, sedimentary
processes, and facies as
semblages: jour. Sedimen
tary Research, v. A34, Fig.
1, p. 455, reproduced by
permission of the Society
for Sedimentary Geology.]
248
Chapter 8 I Continental (Te rrestrial) Environments
Figure 8.3
Aerial view of the Yana outwash
fan, Chugach Mountains, south
eastern Alaska. [From ). C.
Boothroyd and G. M. Ashley,
1975, Process bar morphology,
and sedimentary structures on
braided outwash fans, northeast
ern Gulf of Alaska, SEPM Special
Pub. 23, Fig. 38, p. 196, repro
duced by permission.]
flows can be differentiated. Debris flows commonly "freeze up" and stop flowing
after relatively short distances of transport over lower slopes on the fan; howev
er, some flows have been reported to travel distances of up to 24 km (15 mi)
(Sharp and Nobles, 1953). Mudflows are similar to debris flows but consist ain
ly of sand-size and finer sediments. Landslides are commonly associated with de
bris flows, and in many cases landslide deposits form a source of sediment
for the
debris ows. The surface of debris-flow-dominated fans lends be steep with lit
tle vegetation (e.g., Fig. 8.1 ).
Stream-flow (fluid-flow) processes take place on all types of alluvial fans and
are the principal transport mechanism on stream-flow dominated fans. Tw o types of
stream-ow processes are operative: sheetflood and incised channel flow (Blair and
McPherson, 1994b). Sheetflood is a broad expanse of unconfined, sediment-laden
nmoff water moving downslope, cononly produced by catastrophic discharge.
Sediment concentraon in wa ter flows is typicaiJy about 20 percent; flows contain
ing between about 20 to 45 percent sediment are referred to as hyperconcentrated.
Incised-channel flow takes place through channels, 1 m high, incised into the
upper f. These channels facilitate downslope movement of sediment-gravity
flows and sheetfloods. After deposition by debris-flow or stream-flow processes
curs, subsequent surficial reworking can take pla by discharge from rainfa or
snowmelt, eolian (wind) activity, and bioturbation by plants and animals.
Distinguishing Characteristics of Alluvial Fa ns
Alluvial fans are cone-shaped to arcuate in plan view, with network of branching
distributary channels (Fig. 8.1, 8.2}. The long pmfile, from fanhead to fant, is
commonly concave upward; the greatest stope occurs at the fan apex and deoreas
es down the fan. The transverse or cross-fan profile is generally convex upward.
Alluvial fan sediments are dominated by graveUy deposits, which typically show
down-fan decease in grain size and bed thickness and an increase in ediment
sorting. Debris-flow-dominated fans are characterized by lobes of poorly sorted,
coarse sediment, commonly with a muddy matrix. Stream-flow sediments consti
tute more sheetlike deposits of gravel, sand, and silt that may be moderately we
sorted, cross-bedded, laminated, or nearly structureless.
Hooke (1967) suggested that runoff in the coarse deposits of the upper fan
may percolate through the subsurface and rapidly deposit a gravel lobe as a sieve
deposit. Presumably, highly permeable gravel deposits are generated that allow
8.2 Fluvial Systems
249
water to pass through rather than over the deposits, holding back only the coarser
material. Sieve deposits have long been considered to be distinguishing features
of alluvial fans; however, Blair and McPherson (1994b, p. 376) question the validi-
ty of the sieve concept, suggting nstead that most so-called sieve Jobes are actu-
ally debris-flow deposits. Roger Hooke (personal communication, 24) sees no
reason to change his mind about the concept of sieve lobes, and maintains that the
concept is still valid and useful.
Many individual beds alluvial fans may display no detectable vertical
grain-size trends; howeve1� others may become either finer or coarser upward.
Overall,
.
lluvial-fan deposits tend to be characterized by strongly developed
thickening- and coarsenmg-upward successions, caused by active fan prograda
bion or outbuildiltg. Nonetheless, some fans display thinning- and fining-upward
successions,
which illdicate relative inactivity of depositional processes or fan ret
rogradation (retreat) (Niln, 1982). The thickness of these fng- or coarsen.ing
upwrd sucessions may be htmdreds or even thousand of meters-for example,
Miene alluval-fan deposits of the San Onofre Breccia near Dana Point, southern
California, and Devonian aluvial-fan deposits along the northern margin of the
Hornelen Basin, Noay. Alluvial fan deposits grade laterally into nonfan deposits
such as uvial-plain sediments, windblown deposits, or playa-lake sediments.
Ancient Alluvial-Fan Deposits
Alluvial fans may have been particularly important in Precambrian and early Paleo
zoic me, before the appearance of land plants that could provide an adequate vege
taon cover to bit erosion; however, alluvial-fan deposits have been reported from
stratiaphic successions of many other ages. Reported occurrences include alluvi
fan deposi in the Devonian-Hornelen Basin of Norway, the Devonian-Carboniferous
of the Gaspe Peninsula, Canada, Permo-Carboniferous successions in England, the
Tr iassic Mount To by Conglomerate of Massachusetts, and the Jurassic Todos Santos
Formation of New Mexico, as well as numerous Te rtiary examples in the United
States and other parts of the world (see listing by Blair and McPherson, 1994a).
The Cannes de Roche Formation (Carboniferous) of the Gaspe Peninsula,
Canada, provides a Paleozoic example (Rust, 1981). A schematic depositional
model for this formation is shown Figure 8.4. The Lower Member of the forma
tion, interpreted as alluvial-fan deposits, consists of coarse red breccia interbedded
with silty sandstone and mudstone. The breccia clasts are predominately siliceous
limestone. These coarse breccia units are interpreted as debris-ow deposits on the
Upper Member
(buft-gray conglomerate,
sandstone, mudstone)
Low Memb
(r breia, sandstone,
mudste)
Figure 8.4
Alluvial fan depositional model, the
Cannes de Roche Formation (Car
boniferous), Gaspe Peninsula, Cana
da. [Redrawn from Rust, 1981,
Alluvial deposits and tectonic style:
Devonian and Carboniferous suc
cessions in eastern Gaspe, in Miall,
A. D. (ed.), Sedimentation and tec
tonics in alluvial basins: Geological
Association of Canada Special Paper
23, Fig. 12, p. 65, reproduced by
permission.]
250
Chapter 8 I Continental (Terrestrial) Environments
basis of poor sorting and lack of stratification. Interbedded, horizontally stratified
and cross-stratified red breccia, sandstone, and mudstone in the Lower Member, as
well as the Middle Member, are interpreted to have formed by stream-flow
processes. The Middle Member is the finer grained, down-fan equivalent of the
coarse proximal deposits of the Lower Member. The Upper Memr of the forma
tion consists of buff to gray conglomerate with ronnded cobbles, sandstone, and
mudstone contag abundant plant fragments. This member is considered to be
the deposits of a nearby river that flowed across
the alluvial plain.
ver Systems
River systems through e have been more important as sediment transport con
duits to lakes and oceans than as sites of deposition. Nonetheless, rivers deposit
sediment and some of this sediment is preserved under certain conditions to be
come part of the ancient sedimentary record. To recognize and nnderstand the de
posits of ancient river systems, it i:s useful lo exe the channel shapes, sediment
transport processes, and sediment characteristics of modern rivers.
Channel Form
According to Leeder (1999, p. 311), the cha1mel form of rivers can be described in
terms of the deviation of the channel from a straight path (sinuosity), the number
of channels (single or multiple), the degree of channel subdivision by large bed
forms (bars) and accreting islands aronnd which cha1mel reaches diverge and con
verge (braidilzg), and more permanent distributive channel subdivision into
stationary smaller channels (separated by floodplains) that each contain their own
cha1mels and point bars (anastomosi11g). Some of these features, such as the size
and shape of bars, vary as a fnnction of river levels; that is, they may appear dif
ferently at lm·v-water stage than at flood stage.
It has been common practice in the past to classify rivers into three main
types on the basis of chael form: meandering (single-channel) (e.g., Fig. 8.5),
Figure 8.5
Two small meandering rivers. in a broad alluvial valley, Brooks Range, northern Alaska.
Note numerous cuto meanders.
gure8.6
8.2 Fluvial Systems
251
Braid bars in a braided lower reach of the Kongakut River, Arctic National Wildlife Refuge,
northeastern Alaska.
braided (multiple-channel) (e.g., Fig. 8.6), and anastomosing (e.g., Fig. 8.7). Some
geologists now suggest that such rigid classification is oversimplified and unsatis
factory because the different classes of channel patterns are not mutually exclu
sive (e.g., many rivers show combinations of sinuosity and braiding in different
reaches of the river); also, diJferent parameters are used to define the different pat
terns (e.g., Bridge, 2003, p. 14; Leeder, 1999, p. 311). Even so, geologists continue
to refer to rivers by using these channel-form names.
The factors that ,inuence channel sinuosity and braiding have been suggest
ed o include the magnitude and variability of stream discharge, channel slope,
grain
size of sediment, bed roughnesss,
the amount and kind of sediment load
(bedload vs. suspend load), and the stability of the chael banks. These factors
are complex, interrelated, and not fully understood. The exact causes of meander
ing and braiding remain somewhat obscure.
Box 8.1 Suggested Causes of Sinuosity and Braiding
Bridge (2003, p. 153) suggests that the geometry of alluvial rivers is mainly
controlled by ow and sedimentary processes that operate during seasonal
floods when discharge is maximal. Grain size of transported sediment is pro
portional to channel slope. Jn turn, grain size affects channel roughness, which
increases with increasing grain size and stream power. The degree of braiding
apparently increases as water discharge increases for a given slope and bed
sediment s1ze, or as slope is increased for a given water discharge and bed
sediment size. Braiding occurs at lower slopes and/ or discharge as bed
material size decreases. Discharge variability has also been suggested to pro
mote braiding; however, discharge variability may not actually be a critical
factor. Many rivers with constant discharge display along-stream variations in
chael patte.
252 Chapter 8 I Continental (Terrestrial) Environments
Sinuosity of channels increases with their width/ depth for low-powered,
single-channel streams, but decreases with width/ depth for multiple-channel
rivers. Sinuosity of single-channel rivers also increases with decreasing bed
material size for single channel rivers with a given discharge and slope.
It has also been suggested that rivers that transport large amounts of
(coarse) bed load relative to suspended load tend to be associated with easi
ly eroded banks of sand or gravel and that these rivers have large channel
slopes and stream power. Such rivers have been assumed to be laterally un
stable and thus prone to braiding. By contrast, large suspended loads were
assumed to be characteristic of single-channel rivers of high sinuosity. Such
rivers are allegedly associated with stable, cohesive muddy banks and low
stream gradient and stream power. These generalities are not applicable in
many cases. For example, Bridge (2003, p. 157) reports that many braided
rivers are sandy and silty (e.g., Brahmaputra in Bangledesh, Yellow in China,
Platte in Nebraska), and many single-channet sinuous rivers are sandy and
gravelly (Madison in Montana, South Esk in Scotland, Yukon in Alaska).
Bridge also suggests that difficultly erodable banks, stabilized by vegetation
or early cementation, may not have an important influence on the equilibri
um channel pattern, as long as the flood flow is capable of eroding banks and
transporting sediment.
Sediment Transport Processes in Rivers
Channel ansport. Sediment transport (and erosion) in the higher gradient
proximal reaches of rivers occurs mainly within the river channels. Down
stream flow of water around channel bends leads to helical spiraling of flow,
out toward the surface and inward at the bed (Fig. 8.8). The channels are char
acterized by the presence of bars. Point bars (also referred to as side bars and
lateral bars) are attached to the river bank (e.g., Fig. 8.5). The basic dynamics of
flow around meanders leads to erosion on the outside parts of bends and deposi
tion on the point bars. Helical flow transports sediment, eroded from the cut
bank, across the stream along the bottom and deposits it by lateral accretion on the
point bar. The resulting point-bar sediments are characterized by cross-bedding
and general fining upward toward the top of the bar. In braided rivers, braid
bars (also called channel, mediat longitudinal, and transverse bars, as well as
sand flats) are present in midchannel position (e.g., Fig. 8.6). These braid bars
can be thought of as double-sided point bars. As the current splits around the
upstream end of the bar, helical flow causes lateral accretion on both sides of the
bar. Because braid bars are free to move, in contrast to point bars, scouring and
subsequent deltalike deposition takes place at the downstream end of the bar.
Thus, braid bars can migrate downstream. On the other hand, some braid bars
remain stable long enough to be colonized by vegetation, thus forming islands.
Floodplain Deposition. Floodplains are strips of land adjacent to rivers that are
commonly inundated during seasonal floods. Floodplains can be present along
both braided and meandering rivers, although they appear to be particularly
common along single-channel rivers. When the stream floods and overtops its
banks, deposition of fine sediment occurs on naral levees, in adjacent flood
basins, and in oxbow lakes (Fig. 8.9). Deposion from overbank waters results in
upbuilding of the sediment surface and is thus called vertical accretion, in contrast
to the lateral accretion that takes place on point bars. Natural-levee deposits form
primarily on the concave or steep-bank side of meander loops immediately adja
cent to e channel as a result of sudden loss of competence, and they typically
Figure 8.7
8.2 Fluvial Systems
253
Landsat photograph of the Bramaputra River immediately north of its
confluence with the Ganges, showing anastomosing, single-channel,
and braided channel patterns. [From Bridge, ]. S., 1993, The interac
tion between channel geometry, water flow, sediment transport and
deposition in braided rivers, in Best, ]. L., and C. S. Bristow (eds.),
Braided rivers, Geological Society London Special Publication No. 75,
Fig. 4j p. 21, reproduced by permission.]
contain horizontally stratified fine sands overlain by laminated mud. Floodpla
deposits are fine-grained sediments that settle out of suspension from floodwaters
carried into the floodbasin, which may be a broad, low-relief pla, a swamp, or
even a shallow lake. These thin, fine-grained deposits commonly contain consid
erable plant debris and may be bioturbated by land-dwelling organisms or plant
rts. Crevasse-s
p
la
y
deposits may also occur on floodplains where rising flood
waters breach natural levees (Fig. 8.9). Sedimentation from traction and suspen
sion occurs rapidly after breaching as water containing both coarse bedload
. sediment and suspended sediment debouch suddenly onto the plain, resulting
in graded deposits that may resemble a Bouma turbidite sequence (Walker and
Cant, 1979). A river may also abandon its channel and move, relatively suddenly,
o another position on the fldplain. This process is termed avulsion.
Characteristis of Fluvial Deposits
It is dear from the precedg discussion that sediments can be deposited in a variety
of subenvironments within the fluvial system: on pot bars and in channels of me
anderig
rivers, braid bars of braided rivers, and natural levees, floodbasins,
254
Chapter 8 I Continental e rrestrial) Environments
Figure 8.9
The morphological ele
ments of a meandering
river system. Note: A thal
weg is a line connecting
the
deepest points along
a
stream channel; it is com
monly the line of maxi
mum current velocity.
[From Walker, R. G., and D.
J. Cant, 1984, Sandy fluvial
systems, in R. G. Walker
(ed.), Facies models: Geo
science Canada Reprint Ser.
1, Fig. 1, p. 72, reprinted
by permission of Geological
A�sociation of Canada.]
Accretion
topography
Lateral accretion
suace
Decline of mean flow veloc ity,
bed shear stress, and stream
wer
cross-lamination
� cross-bedding
> 't
)
decreasing grain size
skin-friction line
---- constant stream power
Figure 8.8
Helical flow in a meander bend, leading to lateral accretion of cross-bedded, fining-up
ward deposits. [From Leeder, M., 1999, Sedimentology and sedimentary basins, Fig.
B1 7.1, p. 313. Reproduced by permission of Blackwell Science Ltd.]
(SYSTEM NOW
ABANNED)
DUNES
-�
8.2 Fluvial Syste ms
255
and oxbow lakes of floodplains. Therefore, it is difficult to generalize about the
characteristics of fluvial deposits. Nevertheless, fluvial sediments have some com-
mon properties. Most fluvial deposits consist of sand and gravel, although mud
may be common in floodplain deposits of meanderg streams. Some braided
channels may also have been formed in muddy sediments on floodplains; howev-
er, the mud was probably transported as sand-sized pellets (Bridge, 2003, p. 157).
Sorting of most fluvial sediments ranges from moderate to poor. The deposits of
int bars and braid bars generally display fining-upward gra size owing to the
helical nature of sediment transport on bars. Migration of meanders also produces
a general fining-upward succession as channel lag deposits are overlain by fining-
upward point bar deposits and, tu, silty and muddy floodplain deposits
(len, l970b). Multiple episodes of chael shifting and bar migration in braided
rivers produce ertical stackin of bar deposits, perhaps separated by thin mud-
stones {Fig. 8.l�A). Multiple episodes of meander migration produce vertical
stacking of fining-upward successions in meandering-river deposits (Fig. 8.108).
e Miall (16) for additional examples of vertical profiles in fluvial sediments.
Fluvial deposits commonly display abundant traction structures, including
planar and trough cross-bedding, upper-flow-regime planar bedding, and rip
ple-marked surfaces. Sedimentary structures yield unidirectional, downstream
0
�
t
t
t
t
t
superimposed
flood cycles
t
t
+
t
t
t
1
Trough cross
beaded sand
Planar cross
bedded sand
Planar laminated
sand
®
�andy meandering
nver
t
l crevass splay
1
r
channel fill
1 w;th potot b-.
Ripple-marked
sand
Mud
Figure 8.10
Examples of lithofacies and verti
cal profiles in sediment from a
sandy braided river (A) and a
sandy meandering river (B).
[Afte
r Miall, A. D., 1996, The ge
ology of fluvial deposits, Fig.
8.80, p. 205, and 8.8G, p. 204,
Springer-Verlag, reproduced by
perm iss ion.]