Geology of Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 45

+ (H 2 CO 3 ) -

H 2 CO 3

H 2 O H +1

+ CO 2

(Water) (Carbon

dioxide)

(Carbonic acid) (Hydrogen

ion)

(Bicarbonate ion)

FIGURE 2.22
Formation of carbonic acid and hydrogen and bicarbonate ions.

Al 2 Si2O 5 (OH) 4 4SiO 2

+ +

2KAlSi 3 O 8 + 2H +1 + 2K +1

H 2 O

(Potassium
feldspar)

(Hydrogen
ions)

(Water) (Potassium

ions)

(Kaolinite) (Silica)

FIGURE 2.23
Formation of kaolinite through hydrolysis of potassium feldspar.

Hydrogen ions are extremely effective in decomposing minerals through a process of
hydrolysis. Hydrolysis is a chemical process in which a molecule is cleaved into two parts
by the addition of a molecule of water (Greenwood and Earnshaw 1997). The formation of
the clay mineral kaolinite from potassium feldspar is shown in the hydrolysis reaction in
Figure 2.23.
Clay deposits may be formed in place but usually are eroded and transported from their
original location to generally low-energy depositional environments such as large lakes
and marine deposits.

2.4.1.5 Thickness

Thicknesses of unconsolidated sedimentary deposits can vary enormously over short
distances. Just a few feet of separation can result in significant variation. The thickness of
unconsolidated sediments beneath many urban regions ranges from merely a few meters
to more than 305 m (1000 ft) (USGS 2009). The deposition of these materials forms a com-
plex interconnected web that may drastically change both vertically and horizontally. It
is not uncommon to observe changes in sediment type and origin of formation in only a
few centimeters in the vertical or horizontal directions. Figure 2.24 is an example of how
sediment layers can change over very short distances in both grain size and composition.

2.4.2 Bedded or Stratified Depositional Features

Depositional features provide clues about the life cycle of sedimentary deposits (Wicander
and Monroe 2007), and provide valuable information about the environment where the
deposits are formed. Some of the more commonly observed features include

• Bedding or stratification—defined as the layering of sediments as they are formed
and deposited (Wicander and Monroe 2007). Layering of sediments is the result
of vertical differences in composition, texture, and grain fabric. The most wide-
spread deposition structure of sedimentary deposits is their bedding or stratifi-
cation. Bedding layers may be very thin, on the order of less than a millimeter,
to very thick, on the order of several meters. One variety of bedding layers is
termed laminae , which are defined as very thinly bedded layers on the order of a
millimeter (Potter et al. 1980). Careful inspection of stratified sediments must be
conducted because compaction may enhance and distort bedding irregularities
of hydraulic origin. There are several different types of stratification, including
(Potter et al. 1980; Pettijohn et al. 1987)