36
Chapter 2 Continental Sediments
SIEVE DEPOSITS
STREAMFLOOD, OLDER CHANNEL
AND SHEET FLOOD DEPOSITS
SHEET FLOOD AND STREAM
DEPOSITS
General descriptions of alluvial fans include BuH (1977),
Gloppen and Steel (1981), Nilsen (1982). Current directions
are discussed, e.g., by Collinson and Thompson (1989),
Pettijohn et al. (1987). The various factors controlling the
size and sediment mass of alluvial fans are addressed, e.g., by
North et al. (1989), Allen and Hovius (1998). The greatly
varying conditions of fan deltas and their sediments are described in many articles and some special volumes (e.g.
Nemec and Steel 1988; several articles in Collela and Prior
1990, including Postma 1990; Blair and McPherson 1994).
Sedimentation on alluvial fans begins where the
strearns leave their confined valleys and lose some of
their transport efficiency. Basically, alluvial fans are
composed of two types of sediment: stream deposits
and sediment gravity jlows (Fig. 2.11a). Current-transported sediments usually predominate. They are depos- .
ited either from ephemeral or perennial water flow in
the channel system or, after extreme rain storms, from
sheet-floods inundating large parts of the alluvial fan.
Sometimes, gravel is concentrated locallyto formsieve
DEPRIS FLOW LEVEE
DEBRIS FLOW
(Gms)
b
Fig. 2.11. Simplified facies models
of a alluvial fan
(proximal to mid
fan region) and b
fan delta. See Table 2.1 for explanation of symbols
deposits (coarse gravel and boulders devoid of finergrained matrix). From time to time, large debris flows
with a muddy-sandy matrix reach the proximal and
mid-fan areas and bury part ofthe pre-existing, radi ating channel system. At their lower end, such debris
flows terminate in characteristic lobes, and they often
concentrate large boulders and gravel at their outer
margin, forming levees. Later, new channels cut into
the mass flow deposits and rework and redistribute
great proportions of their material.
Many alluvial fans are only a few kilometers long,
but some can reach a length of more than 50 km. The
sediment mass of alluvial fans deposited in a certain
time interval mainly depends on the size, relief, and
lithology oftheir source areas. The stream gradients of
fans in arid to semi-arid climates often decrease from
1 0 to 3 0 at their heads to 0.1 0 to 0.5 0 at their bases.
Similarly, grain size decreases down fan, and roundness of gravel increases.
Chapter 2 Continental Sediments
SIEVE DEPOSITS
STREAMFLOOD, OLDER CHANNEL
AND SHEET FLOOD DEPOSITS
SHEET FLOOD AND STREAM
DEPOSITS
General descriptions of alluvial fans include BuH (1977),
Gloppen and Steel (1981), Nilsen (1982). Current directions
are discussed, e.g., by Collinson and Thompson (1989),
Pettijohn et al. (1987). The various factors controlling the
size and sediment mass of alluvial fans are addressed, e.g., by
North et al. (1989), Allen and Hovius (1998). The greatly
varying conditions of fan deltas and their sediments are described in many articles and some special volumes (e.g.
Nemec and Steel 1988; several articles in Collela and Prior
1990, including Postma 1990; Blair and McPherson 1994).
Sedimentation on alluvial fans begins where the
strearns leave their confined valleys and lose some of
their transport efficiency. Basically, alluvial fans are
composed of two types of sediment: stream deposits
and sediment gravity jlows (Fig. 2.11a). Current-transported sediments usually predominate. They are depos- .
ited either from ephemeral or perennial water flow in
the channel system or, after extreme rain storms, from
sheet-floods inundating large parts of the alluvial fan.
Sometimes, gravel is concentrated locallyto formsieve
DEPRIS FLOW LEVEE
DEBRIS FLOW
(Gms)
b
Fig. 2.11. Simplified facies models
of a alluvial fan
(proximal to mid
fan region) and b
fan delta. See Table 2.1 for explanation of symbols
deposits (coarse gravel and boulders devoid of finergrained matrix). From time to time, large debris flows
with a muddy-sandy matrix reach the proximal and
mid-fan areas and bury part ofthe pre-existing, radi ating channel system. At their lower end, such debris
flows terminate in characteristic lobes, and they often
concentrate large boulders and gravel at their outer
margin, forming levees. Later, new channels cut into
the mass flow deposits and rework and redistribute
great proportions of their material.
Many alluvial fans are only a few kilometers long,
but some can reach a length of more than 50 km. The
sediment mass of alluvial fans deposited in a certain
time interval mainly depends on the size, relief, and
lithology oftheir source areas. The stream gradients of
fans in arid to semi-arid climates often decrease from
1 0 to 3 0 at their heads to 0.1 0 to 0.5 0 at their bases.
Similarly, grain size decreases down fan, and roundness of gravel increases.