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MATRIC NO: 13/SCI14/005
Sedimentary environment is a part of the earth surface which is physically,
chemically and biologically distinct from the adjacent terrain. Sedimentary environment
could be grouped into three types; Continental, Transitional and Marine environment.
The Alluvial fan environment can be grouped under the continental environment. An
alluvial fan is a fan or cone- shaped deposit of sediment crossed and built up by streams.
Alluvial fans are fan- shaped deposits of water transported material (alluvium). They are
typically formed at the base of topographic features where there is a marked break in slope.
Alluvial fans tend to be coarse grained, especially at their mouths. At their edges, however,
they can be relatively fine grained. Alluvial fans are best developed in dry (arid) climates like
California and Nevada where there is a sudden transition from a slep slope to a shallow
Depositionally, alluvial fans are complex and their character changes over time. The
major deposit types that make them up are;
DEBIS FLOW: are slurries of mud, rock debris, and just enough water to make the sediment
into a viscous flow. Due to the high velocity, the flow is laminar, like a glacier, and like a
glacier, there is no significant setting of grain sizes. Debris can transport very large blocks.
Debris flow continue to move until the internal the internal friction of the flow due to
viscosity exceeds the flow’s momentum when it freezes into place. This can occur due to
either the loss of water or lower slope. The resulting deposits show little sorting and would
be classified as a mud supported breccia or a diamictite. (Diamictities are defined as very
poorly sored sedimentary rocks with no grain size sorting within them; they are
characteristics of lamina flow deposits.). In most cases, debris flow are unsorted and lack
any form of stratification. They are laterally restricted because they do not spread out too
much, and they are commonly an even thickness throughout, with steep edges of flow.
A MUD FLOW: is a class of debris flow with mainly fine grained particles that can move at
rapid rates (up to 10km/hr.) also forming narrow lobes of matrix supported sediment.
SHEET FLOWS: are turbulent flows with significantly more water and less mud than debris
flow. Since the flows are turbulent, here is significant grain sorting and normally graded,
fining upward deposits are common. Once a flow reaches the mouth of a canyon, the flow
spreads out and the coarsest rocks are deposited first. Finer grins are deposited later and
farther down the fan and later in time. This produces normally graded beds, but deposition
is very rapid and he grading is commonly poor. The suspended load may make it to the toe
of the fan if water doesn’t filter into it first. Sheet flows deposits produce broad deposits
that are clast supported, with some imbrication of clasts. Sheet flows commonly cover 1/3
to ½ of a fan surface. Note: the transition between the canyon stream bed and the fan is the
INTERSECTION POINT of the stream channel and the fan surface.
BRAIDED STREAM DEPOSITS: The upper surface of an alluvial fan is dominated by braided
streams which typically have wide and shallow anastomosing channels that form in the
upper reaches of stream where slope is greater and where flowing water is often choked
with some sediment that the fluid can carry. During the high point in a flood water escapes
the main channel and creates a sheet flood of well sorted sand or fine gravel with little or no
silt or clay. During low flow or waning stage of flash floods when water is confined to stream
channels, braided stream deposits are laid down.
The size of an alluvial fan depends on the following;
Area of the drainage basin
Rock lithology in the drainage basin
Tectonic activity
Space available for fan growth
Lithology of rocks in the same area (drainage basin) determines the rate and character of
sediment supply. Coarse sediment loads build steeper fans; large sediment supplies build
bigger fans.