Laboratory Manual for Introductory Geology 4e

EXERCISE 6.7
Recognizing Sediment Deposited by Streams, Wind, and Glaciers
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Fill in the following table to summarize the characteristics of sediment deposited by the transporting agents listed.
Agent of transportation
Textural feature Streams Wind Glaciers
Grain size
Sorting
Grain shape
called sediment maturity: the degree to which a sediment has changed during transportation
and deposition. The changes it can undergo include changes in grain
size and shape by the physical processes described above as well as the loss of easily
weathered minerals by chemical reactions.
For example, if sediment accumulates quickly and close to its source, it generally
contains a variety of grain sizes and is poorly sorted. Also, if chemical weathering has
not progressed to completion, relatively unstable minerals (e.g., feldspar, mica, and
amphibole) will be mixed with stable minerals (e.g., quartz). Finally, since grains
have not traveled far, they may retain their original angular shapes. Geologists refer
to a sediment with these characteristics as immature.
If, however, the sediment is carried a long distance by a river over a long time,
or is washed by wave action along a shore, and if the sediment has time to undergo
substantial chemical weathering so that unstable minerals transform into clay and
wash away, it will be quite different—better sorted, made almost entirely of stable
minerals, and with grains that are well rounded. Geologists refer to sediments with
these combined mineralogical and textural characteristics as mature (FIG. 6.8).
FIGURE 6.8 Sediment maturity. As sediments are transported progressively farther, weatherable minerals such as feldspar
break down and convert to clay, which washes away, so the proportion of sediment consisting of resistant minerals such
as quartz increases. Further, the physical bouncing and grinding that accompanies the transport of sediment progressively
rounds the quartz grains and sorts them.
Increasing distance of transport
Mountain stream
River
Beach
Lithic clast
Quartz sand grain
Silt grain
Feldspar
Clay flakes
Less mature
More mature
160 CHAPTER 6 USING SEDIMENTARY ROCKS TO INTERPRET EARTH HISTORY