Geology of Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 39

• Chert—composed of silica (SiO 2 ) and may form from microscopic organisms that
secrete silica shells around their bodies or as a secondary precipitant.
• Halite—a mineral composed of sodium chloride (NaCl), and is formed from the
evaporation of concentrated seawater in a restricted basin.
• Gypsum—a sulfate mineral (CaSO 4 · 2H 2 O), and is an evaporite deposit formed in
a similar manner as halite.
2.3.2.1 Limestone and Dolostone
Limestone and dolostone are the dominant chemical and biochemical sedimentary depos-
its formed in marine environments. Limestone found on the continental landmasses was
deposited in shallow seas that transgressed and regressed across the continents during
times of rising and falling sea levels. Limestone may also form as reef deposits, such as the
440 million year old Niagara Escarpment that surrounds parts of the Great Lakes Basin. In
general, significant limestone deposition requires a marine environment with three charac-
teristics: clear water, warm water, and shallow water (Bathhurst 1975; Wilson 1975). It should
be noted that karst is a landscape formed by groundwater solution—it is not a deposit.
Deposition of limestone sediments now occurs within the equatorial belt of warm ocean
currents between latitudes of 30° north and south, but deposits as far as 40° north and
south have been observed (Wilson 1975). As noted above, warm tropical waters are only
one of three criteria for limestone deposition—clear water is also required, particularly for
limestone reefs. As a result, significant limestone production does not occur in environ-
ments located near significant sources of clastic sediments such as a river delta (e.g., the
Mississippi) because significant clastic sediment influx tends to choke limestone sedimen-
tation. In addition, severe storms such as hurricanes may interrupt limestone sediment for-
mation, or in some cases destroy the entire production of limestone sediment in local and
regional areas. Large storms and hurricanes have much higher wave action and potentially
stronger water currents. The waves from these events erode and destroy portions of near-
surface reefs and also transport significant amounts of suspended sediments. As the water
turbidity increases from these suspended sediments, interruption of limestone formation
may occur. With respect to limestone formation in shallow water, the controlling factors
are the availability of sunlight and warmer water located near the surface. Limestone will
not form in the deeper parts of the ocean because CaCO 3 dissolves in cold water.
Throughout geologic time, limestone production has been significant. Plate tectonics
coupled with the rise and fall of sea levels throughout the Paleozoic and Mesozoic have
resulted in thick sequences of limestone deposits throughout the world, including a sig-
nificant portion in the United States as a result of the transgressing seas. In fact, more than
10% of bedrock in the United States is composed of limestone, with significant deposits
located within the eastern United States (USGS 2003).
Formation of dolostone typically occurs as a slow replacement process and generally
follows limestone deposition (Wilson 1975); although primary dolomite formation has
also been documented. About half of the calcium in the CaCO 3 molecule is replaced with
magnesium forming dolostone (CaMg[CO 3 ] 2 ). Because of the addition of magnesium in its
molecular structure, dolostone is more resistant to chemical erosion than limestone.
2.3.2.2 Evaporites
An evaporite forms as a result of the evaporation of water (Blatt and Tracey 1996). As
the list at the beginning of the section shows, common evaporite deposits include the