After minerals such as kaolinite are eroded and enter the sediment transport pro

Question

After minerals such as kaolinite are eroded and enter the sediment transport process the mobility of an individual particle is a function of its grain size and the energy of the transport medium. Describe, using diagrams as appropriate, the relationship between the transportability, transport distance, sorting and maturity of sediments as a function of grain size and energy in a transport system, such as a river? When some sedimentary grains, especially phyllosilicates, reach a certain minimum grain size their mobility and entrainment in a transport medium decreases in a relationship that is described on a Hjulstrom diagram - effectively, the transportability of grains decreases the smaller their grain size. Why is this relationship observed, and why is it particularly common for phyllosilicate minerals?

Explanation / Answer

3. Sediment maturity is a measure of distance or time from the source area to the depositional site. A lot of factors influence sediment maturity, especially the climatic condition of weathering and transport and the mineralogical make up of the source area rock. Maturity can be gauged in terms of texture (textural maturity), mineralogy (minerologic maturity), and composition (compositional maturity). The terminology used to describe maturity is relatively simple: immature, submature, mature, and supermature.Textural maturity is gauged largely in terms of grain size, grain sorting, and grain roundness. At a source site the weathering process tends to generate a wide range of grain sizes but generally big chunks. And these tend to have a rough or angular exterior. The maturation process makes big things smaller up to a point that is. Grains of medium sand size tend to resist further size reduction. The maturation process also knocks of the rough edges reducing the grain exterior to a smooth or rounded surface. Therefore

Immature

Submature

Mature

Supermature

Sorting

Extremely poorly sorted to very poorly sorted

poorly sorted to moderately sorted

moderately well sorted to well sorted

very well sorted

Grains Size

very coarse or bigger

coarse

medium

Medium

Roundness   Angular subangular rounded   well rounded

The inherent assumption in using sediment maturity in interpreting depositional environments is the following:  “The greater the transport distance and length of time in the transport medium, the more mature a resultant sediment becomes”. Fundamentally, glacial sediment is extremely heterogeneous, reflecting the fact that glacial erosion (e.g., plucking, abrasion) produces clasts of all sizes, from boulders the size of houses to rock flour smaller than 1 micron.  However, the mode of transport and time spent in transport is substantially different between the two locations.

It has been observed that there are the following trends in texture as the time in transport increases:
– Grain size (see Wentworth grain size scale).
• Cobble, boulder, pebble, sand, silt and clay.
• Grain size tends to decrease as the time in transport increases.
– Grain shape
• Roundness: Degree of angularity of grains
• Sphericity: How well the shape approaches a sphere
• Both roundness and sphericity tend to increase with increasing as the time in transport.
– Grain sorting:
• Sorting refers to the range of particle sizes in a rock. Sorting tends to increase as the time in transport by water or wind increases.

4. The plot shows several key concepts about the relationships between erosion, transportation, and deposition. For particle sizes where friction is the dominating force preventing erosion, the curves follow each other closely and the required velocity increases with particle size. However, for cohesive sediment, mostly clay but also silt, the erosion velocity increases with decreasing grain size, as the cohesive forces are relatively more important when the particles get smaller. The critical velocity for deposition, on the other hand, depends on the settling velocity, and that decreases with decreasing grainsize. The Hjulstrom curve shows that sand particles of a size around 0.1 mm require the lowest stream velocity to erode.

Immature

Submature

Mature

Supermature

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