congl omerates: The Significance of Texture and Petrology
hum id setting. The Devonian Malbaie Formation of
Quebec contains no mass-flow deposits (Rust 1984) ,
whereas in a study of Carboniferous fan and paleovalley deposits in Spain, Heward (1978b) reported
su bstantial accumulations of debris-flow deposits.
The paleovalleys are filled with debris-flow conglomerates. The deposits of fan channels and the
overbank areas of fans consist of interbedded debris-flow and stream-flow deposits, with debris
flows more important in proximal reaches. Rust
(1984) reported an unusual abundance of cross-bedded gravels (lithofacies Gp) in the Malbaie Formation, and attributed this to the presence of deep
braided channels and perennial discharge in a tropical, hu mid setting. In another study in Spain, Iwaniw
( 19 84) reported that debris-flow deposits are hnportant, whereas in a study of a Paleogene fan (braiddelta) complex in Washington state it was found that
debris-flow deposits are of minor importance and
can be related to volcanic infl uence (Evans 19 91).
Late Cenozoic deposits described by Darby et al.
(1990) contain no evidence of mass-flow processes.
7 2.5. 7.4 Boreal-Paraglacial Climates
It is not suprising that mass-flow deposits have been
found to be a dominant facies in ice-margin settings,
where abundant gl acial debris is available to be
transported by large volumes of glacial meltwater.
Reports of Late Cenozoic deposits from various Alpine environment S around the world are consistent
on this point (Church and Ryder 19 72; Wasson 1977;
Derbyshire and Owen 1990). An interesting exception is the complex of Quaternary fan deposits in
Crete constituting l
in origin, yet were deposited almost entirely by
stream-flow traction-current processes.
Glacial outwash deposits provided the first model
for humid fans (Boothroyd and Ashley 1975; Miall
1977; Sect. 8.2.2), although some have objected to the
use of the term "fan" for su ch deposits (McPherson
and Blair 1993; Blair and McPherson 1994). The term
"sandur" is to be preferred. The deposits are normall y characterized by stream-flow traction-current
deposits (Boothroyd and Ashley 1975; Boothroyd
and Nummedal 19 78 ; Leblanc Smith and Erikkson
1979; Miall 19 83b; Ashley et al. 19 85) , although massflow processes are important in the event of the
catastrophic floods -jOkulhlaups - that characterize
many outwash plains. These floods are triggered by
the failure of ice dams or by other sudden glacial
431
movements that release large volumes of meltwater.
In Iceland jokulhlaups are also triggered by volcanic
eruptions under ice caps. These floods may do little
more than erode and rework pre-existing fluvioglacial sands and gravels, moraine, or till. Rounded
till ball s are common in some outwash deposits
(Ashley et al. 19 85). However, jokulhlaups may trigger mass flows, and in Iceland such deposits
consititute a significant proportion of the outwash
succession (Maizels 1993) . Another product of catastrophic floods may be the deposition of trains of
giant bedforms, as in the floods that affected the
Columbia River valley du ring the draining of Lake
Missoula (Baker 19 78 b) .
12.5. 1.5 Summary and Co nclusions
To summarize, it is clear that while mass-flow deposits, particularly debris-flow conglomerates, and associated upper flow�regime stream-flow deposits are
very common in arid environments (Trollheim fan
being the type example) , they are not always generated in this climatic setting and can occur under
other climatic regimes, ranging from paraglacial to
tropical (Table 12.1). The association with an arid
climate is a reflection of the requirement for abundant loose debris, mobilized by rapid ru noff, uninhibited by a vegetation cover. However, catastrophic
floods can occur in all environments. They are triggered by cyclonic storms in temperate and tropical
settings and by jokulhlaups in glacial settings. Climates characterized by seasonal rainfalls and seasonal dry periods are candidates for flash floods
because of the lower density of vegetation in such
areas than in humid settings. Floods need not be very
frequent to impose a dramatic change on the depositional system that takes a long time to eradicate
under ('normaF' weather conditions.
Many of the studies reported here indicate a con�
siderable variation in the frequency and importance
of mass-flow events and their deposits between one
fan and another within the same study area. The
steepness of the relief in the drainage basin and the
nature of the bedrock are important controls. Mass
fl ows are favored by steep slopes and by bedrock that
weathers to produce large volumes of fine-grained
sediment, especially clay. Therefore, within the same
sedimentary basin, under constant tectonic and
cl imatic conditions, the composition of the basinmargin conglomerate wedge may vary along strike
because of controls imposed by the character of the
fan drainage basins.
hum id setting. The Devonian Malbaie Formation of
Quebec contains no mass-flow deposits (Rust 1984) ,
whereas in a study of Carboniferous fan and paleovalley deposits in Spain, Heward (1978b) reported
su bstantial accumulations of debris-flow deposits.
The paleovalleys are filled with debris-flow conglomerates. The deposits of fan channels and the
overbank areas of fans consist of interbedded debris-flow and stream-flow deposits, with debris
flows more important in proximal reaches. Rust
(1984) reported an unusual abundance of cross-bedded gravels (lithofacies Gp) in the Malbaie Formation, and attributed this to the presence of deep
braided channels and perennial discharge in a tropical, hu mid setting. In another study in Spain, Iwaniw
( 19 84) reported that debris-flow deposits are hnportant, whereas in a study of a Paleogene fan (braiddelta) complex in Washington state it was found that
debris-flow deposits are of minor importance and
can be related to volcanic infl uence (Evans 19 91).
Late Cenozoic deposits described by Darby et al.
(1990) contain no evidence of mass-flow processes.
7 2.5. 7.4 Boreal-Paraglacial Climates
It is not suprising that mass-flow deposits have been
found to be a dominant facies in ice-margin settings,
where abundant gl acial debris is available to be
transported by large volumes of glacial meltwater.
Reports of Late Cenozoic deposits from various Alpine environment S around the world are consistent
on this point (Church and Ryder 19 72; Wasson 1977;
Derbyshire and Owen 1990). An interesting exception is the complex of Quaternary fan deposits in
Crete constituting l
in origin, yet were deposited almost entirely by
stream-flow traction-current processes.
Glacial outwash deposits provided the first model
for humid fans (Boothroyd and Ashley 1975; Miall
1977; Sect. 8.2.2), although some have objected to the
use of the term "fan" for su ch deposits (McPherson
and Blair 1993; Blair and McPherson 1994). The term
"sandur" is to be preferred. The deposits are normall y characterized by stream-flow traction-current
deposits (Boothroyd and Ashley 1975; Boothroyd
and Nummedal 19 78 ; Leblanc Smith and Erikkson
1979; Miall 19 83b; Ashley et al. 19 85) , although massflow processes are important in the event of the
catastrophic floods -jOkulhlaups - that characterize
many outwash plains. These floods are triggered by
the failure of ice dams or by other sudden glacial
431
movements that release large volumes of meltwater.
In Iceland jokulhlaups are also triggered by volcanic
eruptions under ice caps. These floods may do little
more than erode and rework pre-existing fluvioglacial sands and gravels, moraine, or till. Rounded
till ball s are common in some outwash deposits
(Ashley et al. 19 85). However, jokulhlaups may trigger mass flows, and in Iceland such deposits
consititute a significant proportion of the outwash
succession (Maizels 1993) . Another product of catastrophic floods may be the deposition of trains of
giant bedforms, as in the floods that affected the
Columbia River valley du ring the draining of Lake
Missoula (Baker 19 78 b) .
12.5. 1.5 Summary and Co nclusions
To summarize, it is clear that while mass-flow deposits, particularly debris-flow conglomerates, and associated upper flow�regime stream-flow deposits are
very common in arid environments (Trollheim fan
being the type example) , they are not always generated in this climatic setting and can occur under
other climatic regimes, ranging from paraglacial to
tropical (Table 12.1). The association with an arid
climate is a reflection of the requirement for abundant loose debris, mobilized by rapid ru noff, uninhibited by a vegetation cover. However, catastrophic
floods can occur in all environments. They are triggered by cyclonic storms in temperate and tropical
settings and by jokulhlaups in glacial settings. Climates characterized by seasonal rainfalls and seasonal dry periods are candidates for flash floods
because of the lower density of vegetation in such
areas than in humid settings. Floods need not be very
frequent to impose a dramatic change on the depositional system that takes a long time to eradicate
under ('normaF' weather conditions.
Many of the studies reported here indicate a con�
siderable variation in the frequency and importance
of mass-flow events and their deposits between one
fan and another within the same study area. The
steepness of the relief in the drainage basin and the
nature of the bedrock are important controls. Mass
fl ows are favored by steep slopes and by bedrock that
weathers to produce large volumes of fine-grained
sediment, especially clay. Therefore, within the same
sedimentary basin, under constant tectonic and
cl imatic conditions, the composition of the basinmargin conglomerate wedge may vary along strike
because of controls imposed by the character of the
fan drainage basins.