Page 156 - Sedimentology and Stratigraphy

Alluvial Fans 143

9.5.2 Processes of deposition on 3 the clasts may show a crude alignment parallel to
alluvial fans flow in the basal sheared layer but otherwise the beds
are structureless with clasts randomly oriented;
The processes of deposition on an alluvial fan will be 4 outsize clasts that may be metres across may occur
determined by the availability of water, the amount within a debris flow unit (Fig. 9.22);
and type of sediment being carried from the feeder 5 beds deposited by debris flows are tens of centi-
canyon, and the gradient on the fan surface metres to metres thick.
(Fig. 9.20). Where there is a dense mixture of
water and sediment, transport and deposition are Sheetflood deposition
by debris flow (4.5.1), a viscous slurry of material
that spreads out on the fan surface as a lobe. Debris When the catchment area of an alluvial fan is inun-
flows do not travel far and a small, relatively steep, dated with water by a heavy rainstorm, the loose
alluvial fan cone is built up if this is the dominant detritus is moved as bedload and in suspension out
process. With more water available, the mixture of onto the fan surface. The flow then spreads out over
sediment and water is more dilute: deposition will be a portion of the fan as a sheetflood, a rapid, super-
either by unconfined sheetfloods (see below), or critical, turbulent flow that occurs on slopes of about
flow will be constrained to channels on the surface. 38 to 58 (Blair 2000b). Under these upper flow regime
Dilute, water-lain fan deposits form fans with shal- conditions (4.3.6) most of the pebbles, cobbles and
lower slopes and greater radial extent (around boulders are carried as bedload, but finer pebbles and
10 km). granules may be partially in suspension along with
sand and finer sediment. These flows usually only last
for an hour or so, and standing waves intermittently
Subaerial debris flows
form in the flow, creating antidune bedforms (4.3.5)
A mixture consisting of a large amount of detritus and in the gravel bedload. Cross-stratification dipping
a small quantity of water flows as a dense slurry with up-flow generated by the antidune bedforms may be
a consistency similar to a wet concrete mix. Due to preserved, but more often the bedform is washed
the high density and viscosity the flow will be laminar out as the standing wave breaks down. The most
and it will continue to flow over the land surface as a common style of bedding seen in sheetflood facies
viscous mass until it runs out of momentum, usually are depositional couplets of coarse gravel deposited
when the gradient decreases or the flow loses water as bedload when standing waves are forming, over-
content. Beds deposited by debris flows may be tens of lain by finer gravel and sand deposited from suspen-
centimetres to metres thick and will show very little sion as the wave is washed out. The formation and
thinning in a downflow direction. Clasts of all sizes, destruction of standing waves occurs repeatedly
from clay particles to boulders, can be carried in a during a sheetflood event. These couplets are typically
debris flow and because of the lack of turbulence there 5–20 cm thick and occur in packages tens of centi-
is no sorting of the grain sizes within the flow. The metres to a couple of metres thick formed by indivi-
clasts are also commonly randomly oriented, with the dual flow events. Individual sheetflood deposits may
exception of some elongate clasts that may be re- be hundreds of metres wide and stretch from the apex
aligned parallel to the flow, and clasts in the basal to the toe of the fan, but individual couplets within
part of the flow where friction with the underlying them are typically only a few metres across. There
substrate results in a crude horizontal stratification appears to be little difference between sheetflood
and parallel alignment of clasts. deposits on proximal and distal parts of a fan, and
The characteristics of a bed deposited by a debris most of the fine sediment is carried in suspension
flow are (Fig. 9.21): beyond the fan (Blair 2000b).
1 the conglomerate normally has a matrix-supported The characteristics of a sheetflood deposit on an
fabric – the clasts are mainly not in contact with each alluvial fan are (Fig. 9.21):
other and are almost entirely separated by the finer 1 sheet geometry of beds that are tens of centimetres
matrix; to a couple of metres thick;
2 sorting of the conglomerate into different clast sizes 2 beds are very well stratified with distinct couplets of
within or between beds is usually very poor; coarser gravel and sandy, finer gravel (Fig. 9.23);