Geoscience Reference
In-Depth Information
14
Dryland alluvial fans
Adrian Harvey
14.1
Introduction: dryland alluvial
fans - an overview
14.1.1
Definitions, local occurrence, general
morphology
the mountain catchments, may have multiple sources (Fig-
ure 14.2(c)). Basin-margin fans may coalesce down-fan
to form a depositional apron, known as a bajada (Fig-
ure 14.2(d)). Distally, fan deposits may interdigitate with
river, lake or windblown deposits and may form part of
a continuous suite of deposits extending from mountain-
front, basin margin locations to basin centre. Fans that
terminate in standing water, lakes or the sea are known
as fan deltas. They may include a substantial subaqueous
portion and their distal facies may interact with lacus-
trine or marine coastal sediments. The long profiles of
alluvial fans tend to be more or less planar or exhibit a
slight upwards concavity and the cross-profiles tend to be
upwardly convex (Hooke, 1967; Bull, 1977).
Within the mountain catchment the stream channels
would have a conventional tributary pattern, but on the
fan the feeder channel may become a multithread dis-
tributary system. On fully aggrading, nondissected fans
this transition may take place at the fan apex, but on many
fans the feeder channel may be incised into the proximal
fan surfaces as a fanhead trench (Figure 14.2(e)), emerg-
ing on to the fan surface mid-fan at an intersection point
(Hooke, 1967). On some fans the channel may by trenched
below the fan surface throughout the fan, in which case
the fan surface is essentially a fossil surface receiving no
further sedimentation. The main channel may be an axial
fan channel running more or less down the centre of the
fan, but may switch to run down the steeper flanks to be-
come or join an interfan channel draining the depression
between two adjacent fans (Figure 14.1(b)). As the re-
sult of such switching the focus of deposition will switch
from one part of the fan to another (Denny, 1967)(Fig-
ure 14.1(c)). Other small channels may form on the fan
surface as fan surface washes, draining parts of the fan
surface, but having no mountain source areas.
Alluvial fans are depositional landforms that occur where
confined stream channels emerge from mountain catch-
ments into zones of reduced stream power. The abrupt
reduction of stream power results in the deposition es-
pecially of the coarse fraction of the sediment load.
The result is progressive deposition to form a fan-like
body of sediment. A common situation where this oc-
curs is at mountain fronts (Bull, 1977), either at ero-
sional, pediment-controlled mountain fronts or at fault-
bounded mountain fronts (Figures 14.1(a) and 2(a)), often
at the margins of a subsiding sedimentary basin. Alterna-
tively, such situations occur within intermontane basins
or at tributary junctions, where steep confined tributary
stream channels join a more open, lower-gradient main
valley (Figures 14.1(a) and 2(b)). In these cases the fans
may be confined by the valley walls, whereas mountain-
front fans may only be confined by adjacent fans. Fans
at faulted mountain fronts and at the margins of sedi-
mentary basins may involve a thick sequence of deposits,
whereas those in erosionally controlled situations will be
thinner.
Alluvial fans generally have a conical surface form,
modified by whatever confinement may be present, with
slopes radiating away from an apex at the point where
the channel issues from the mountain catchment (Figure
14.1(b)). Some fans, especially along active faults, may
have a single stream source at the apex; others, especially
where pedimentation has occurred prior to burial by fan
deposits or where the fan sediments have backfilled into
