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Types of Sedimentary Rock Help (page 3)

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By — McGraw-Hill Professional
Updated on Aug 31, 2011

Ironstone

Iron oxide sediments are sedimentary rocks containing more than 15% iron in the form of iron oxides, iron silicates, and iron carbonates. Geologists think that most sedimentary iron was formed early in the Earth’s history when there was less oxygen in the air and iron was more soluble. When soluble iron was carried to the ocean, it formed iron oxides and other compounds that then settled in layers to the bottom.

Ironstone , heavy, compact fine-grained stone is found mostly in nodules and in uneven beds with carboniferous and other rocks. It has 20–30% iron content and a clay-like texture, with large amounts of iron oxide, mostly limonite , in nodular form. Much of the iron produced in the United Kingdom is made from ironstone.

Evaporites

This group includes the evaporites , the carbonates (limestones and dolostone), and the siliceous rocks . Evaporites form from chemical elements dissolved in seawater. These compounds can be removed from saltwater and crystallized into rock by chemical processes or through biological processes (such as shell growth). Sometimes it’s tough to sort between the two (carbonates and siliceous rocks), so evaporites are commonly grouped as chemical/biochemical.

Evaporites that form as elements become more and more concentrated in an evaporating solution (usually seawater).

Marine evaporites are the sediments and sedimentary crystalline rocks formed as seawater becomes more and more salty through evaporation. Some marine evaporites are hundreds of meters thick. Huge amounts of seawater would have to evaporate for this amount of crystal formation to have been possible. Figure 7-5 illustrates how crystallization of evaporites happens in a shallow area with little freshwater input.

Sedimentary Rock Evaporites

 

Fig. 7-5. Shallow basins allow the evaporation of crystals from super-saturated seawater.

The most common types of evaporites include the following:

  • Carbonates – mostly calcite and dolomite by diagenesis,
  • Gypsum – made up of calcium sulfate (CaSO 4 and water),
  • Halite ( rock salt ) – made up of sodium chloride (NaCl), and
  • Travertine – made up of calcium carbonate (CaCO 3 ) (forms in caves and around hot springs).

Geologists have found that three things must happen in a bay for large amounts of evaporites to form. They are: (1) freshwater that flows into the bay from rivers and streams is limited, (2) connections to the open sea are constricted, and (3) the climate is parched and dry. In these bays, seawater evaporates constantly, but is replenished at a steady rate remaining supersaturated all the time. Evaporite minerals then settle steadily to the floor of the bay in sedimentary layers.

Phosphorite is another marine evaporite formed from chemical and biochemical sediments. It is made up mostly of calcium phosphate from places along the continental margins where ocean water is cold and deep. The phosphorite forms from an interaction between phosphate-rich seawater and muddy or carbonate-containing sediments.

Land ( nonmarine ) evaporites form in areas usually far from the sea. These are found in desert-region lakes with little or no river outlet. In these places, minerals come into the lake from chemical weathering and erosion, but without water current can’t move on. One of the best known examples of this is the Great Salt Lake in Utah in the western United States. Rivers bring ions into the lake, but there they stay when the water evaporates. The concentrated dissolved ions in the Great Salt Lake make it one of the saltiest places on Earth, with levels eight times saltier than seawater.

Practice problems of this concept can be found at: Sedimentary Rock Practice Test

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