Page 158 - Sedimentology and Stratigraphy

Alluvial Fans 145

Fig. 9.23 Sheetflood deposits on an alluvial fan showing
Fig. 9.22 A debris flow on an alluvial fan: the conglomer- well-developed stratification.
ate is poorly sorted, with the larger clasts completely sur-
rounded by a matrix of finer sediment.
but individual beds often have a sheet geometry, the
result of lateral amalgamation of channel deposits.
3 imbrication of clasts is common, and up-stream Beds are sharp-based, with clast-supported conglom-
cross-stratification formed by antidunes may also be erate fining up to sandstone: sedimentary structures
preserved; are those of a braided river, including imbrication and
4 the sediment is poorly sorted, but silt and clay sized cross-stratification in gravels and cross-bedded sand-
material is largely absent; stone (Fig. 9.21).
5 beds may show normal grading due to waning flow.

9.5.3 Modification of alluvial fan deposits
Fluvial deposits forming alluvial fans
The river emerging from the feeder canyon may con- Deposition on alluvial fans in arid regions occurs very
tinue to flow as a confined channel on the alluvial infrequently (on a human time scale). The sheetfloods
plain. The abrupt reduction in gradient as flow occurs and debris flows that deposit sediment normally last
on the low slope of the plain promotes deposition of only a matter of hours and these events are separated
gravel on bars within the channel to create a braided by tens or hundreds of years. Between depositional
depositional form. Deposition in the channel by a episodes, less intense rainfall events in the catchment
number of high-discharge events will eventually will result in water flowing on the fan as superficial,
cause the channel to become choked with sediment, non-depositing streams. These flows can locally win-
and the active flow will move, either by a process of now out sand and mud from between the gravel
gradual lateral migration or by avulsion. Through clasts, removing the matrix of the deposit, and if the
time the position of the braided river channel migrates spaces are not filled in later an open framework or
over the whole fan surface, depositing a more-or-less matrix-free conglomerate may be preserved. A
continuous sheet of gravel. The radius of the fan more significant modification of the alluvial fan sur-
formed will be determined by the length over which face is by streams that become established on the fan
the channel is depositing gravel: sand and finer sus- surface between depositional episodes. These rework
pended load will be carried further out onto the allu- debris flow and sheetflood deposits, form a channel
vial plain. and remove some material from the fan surface and
The overall shape of the sediment body formed will on many modern alluvial fans this is an important
be similar to that of a fan formed by sheetflood depos- process (Blair & McPherson 1994). These steep chan-
its, but the radius is not limited by the extent that nels have the form of a braided river with bars of
unconfined sheetflood processes can transport sedi- gravel redeposited or left uneroded within the chan-
ment, and these fans can therefore be over 10 km nel. Alluvial fan surfaces are also subject to modifica-
from apex to toe. Distinct channels may be preserved, tion by soil processes, and aeolian processes can