Alluvial fan sediments

Alluvial fan sediments

Alluvial fan sediments are deposits of sediments in regions of high relief, generally where streams issue from mountains onto a level plain. The fan starts at the apex, the source of sediments from regions of higher relief, as shown in figure below, sediment transport from the apex tends to follow the steepest slope downward, and the sediments, therefore, spread out in a fan.

The largest boulders or pebbles are deposited near the apex. Meanwhile downslope, the fan channel splits up into a number of smaller channels. This reduces the velocity of the water flow, and capacity for carrying sediment is down. Therefore, sediments become finer-grained down-slope, even if there is no reduction in steepness of the slope. Given this points, further downslope it becomes braided streams and lake deposit. In front of the apex the large faulting sedimentary basin usually are in form. This phenomenon is quite common in the Mesozoic coal-bearing strata in the eastern China.

alluvial fan illustration
Schematic sediments of an alluvial fan

Near the apex the sediments are primarily consists of conglomerates, and together with their main components are coarse gravels and boulders. The gravels are full with clay, silt, and sand. The conglomerate layer of course distributes in strips and is parallel to the direction of the water flow.

alluvial fan example
Example of alluvial fan in real condition

Geomechanical features of the alluvial fan sediments

As the explanation above, we can expect the downslope to the middle fan. In the same way also the sedimentary rocks are mainly consists of sandstone with some gravels. The fan tail of the sediments are much finer. In summary, the sedimentary rocks are consisted of sandstones, siltstones, claystones, and coal seams (Meng et al. 1999). The main geomechanical features of the alluvial fan sediments are as follows:

  •  The rocks are blocky with great thickness and high strength.
  •  The sediment lithology is complex with coarse granularity, and the rocks are easily weathered.
  •  On the end of an alluvial fan, the coarse rocks have high porosity and it may be a good aquifer.


Engineering Geology for Underground Rocks by Suping Peng, Jincai Zhang

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