Environmental Engineering Reference
In-Depth Information
• Raindrop prints—are small circular impressions formed from raindrops hitting
the soft surface of exposed sediment.
• Fossils—observing fossils within a sedimentary deposit offer geologists many
insights as to how the deposit formed, the age of the deposit, and many other
factors. When placed in a stratigraphic sequence with deposits from both above
and below, fossils are used to reconstruct the history of the region. For instance,
observing fossils or remnants of fish indicate the deposit formed under water, and
observing terrestrial fossils, such as a Mammoth or Mastodon indicates the land
surface was nearby.
• Bioturbation and tracks—bioturbation is the physical rearrangement of the soil
profile by soil organisms. The resulting features are also described as trace fossils
and can range in scale from worm burrows to animal tracks.
• Varves—form in a variety of marine and lacustrine depositional environments
from seasonal variations in clastic, biological, and chemical layering of sediments.
Varves are a special variety of stratification. The classic varve deposit appears as
alternative light and dark layering of sediment in a glacial lake. The light layers
usually are composed of silt and fine-grained sand, and are higher energy deposits
because they traveled farther, originating as glacial meltwater. These light layers
are also typically coarse-grained than the darker clay layers. In lakes, the layering
pattern is a function of their seasonal freezing and thawing, and/or the seasonal
fluctuations in sediment loading.
2.5 Sedimentary Depositional Environments
Sedimentary depositional environments represent a combination of physical, chemical,
and biological processes associated with the deposition of a specific type of sediment.
Grain size, shape, sorting, composition, thickness, anthropogenic disturbance, natural dis-
turbance, unconformities, and other features present clues as to how, where, and when
both clastic and chemical sedimentary deposits formed. Since sedimentary deposits form
at or very near the surface of the ground, they also communicate to geologists what the
climatic conditions were at the time of deposition (Wicander and Monroe 2007). This dem-
onstrates the interdisciplinary nature of geology since a significant portion of the forma-
tion and preservation of sedimentary deposits is atmospheric and weather-related. Figure
2.29 shows several sedimentary depositional environments.
An example of a depositional structure containing direct evidence about the environ-
ment of its deposition is an extensive cross-bedded sand deposit, indicating there were
either dunes or a beach nearby. Shell fragments embedded in this sand would further
indicate the sand was deposited in shallow water. This pattern may have significant impli-
cations because it could signal the historical climatic shifts and changes, and perhaps
present clues related to atmospheric-related hazards (e.g., floods) affecting the site and its
surrounding area. Table 2.3 lists the dominant types of clastic sedimentary deposits and
corresponding depositional environments in which they form.
Most urban areas are not located directly upon bedrock. Urban landscapes tend to be
located in continental sedimentary depositional environments including alluvial, beach,
eolian, fluvial, glacial, lacustrine, and volcanic-type deposits. Many cities are also located
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