Types of Sedimentary Rock Help (page 2)

Types Of Sedimentary Rocks

Unlike igneous rock, most sedimentary rocks have a fine-grained texture. Since a lot of the reason they have layered or settled in one place is due to water or wind, the particles of sediment are usually small and fine.

The way that sedimentary rock is deposited can also be related to size. Since wind can’t blow or carry away boulders (well, maybe tornadoes can), generally it is the lighter, finer grains of silt that are transported by the wind.

In contrast to that, water tumbles rocks of different sizes. With the water deposit of sedimentary rock, current plays a big part. The stronger the current, the larger the rock and the farther it is carried. The relationship between current and particle size is the reason why many beds have the same types of particles. They sort and group according to size when flowing in the same current stream. So you see sand together with sand, river pebbles with other river pebbles.

Clastic

Clastic or detrital sedimentary rocks are formed from the weathering of existing rocks, which have been carried to a different spot from where they were originally and then turned into rock. They have a clastic (broken) texture made up of clasts (bigger pieces, like sand or gravel) and are grouped according to their grain size. Table 7-1 lists the various clastic particles and their associated sizes.

Table 7-1 Clastic sediments and rocks are named by size and shape.

Detritus is igneous, sedimentary, or metamorphic rock that has been moved away from its original location.

Clastic sedimentary rocks are made up of pieces of other rocks. These pieces of rock are loosened by weathering , and then carried to some low area or crack where they are trapped as sediment. If the sediment gets buried deeply enough, it becomes compacted and cemented, forming sedimentary rock.

Clastic sedimentary rocks have particles ranging in size from microscopic clay to huge boulders. Their names are based on their clast or grain size. Beginning with the smallest grains, there are clay , then silt , then sand . Grains that are larger than 2 mm are called pebbles .

Shale is a rock made mostly of clay, siltstone is made up of silt-sized grains, sandstone is made of sand-sized clasts, and a conglomerate is made of pebbles surrounded by a covering of sand or mud. Figure 7-4 compares the different sedimentary rock types and their different proportions.

Fig. 7-4. Siltstone, mudstone, and shale are found in much higher amounts than other sedimentary rock types.

• Coarse-grained clastics

Gravel (grain size greater than 2 mm; rounded clasts = conglomerate; angular clasts = breccia)

• Medium-grained clastics

Sand (grain size from 1/16 to 2 mm)

Sandstone (mostly quartz grains = quartz sandstone (also called quartz arenite ); mostly feldspar grains = arkose ; mostly sand-sized rock fragment grains = lithic sandstone (also called litharenite or greywacke ))

• Cement (the glue that holds it all together) like calcite, iron oxide, silica
• Fine-grained clastics

• Silt and siltstone (grain size from 1/16 to 1/256 mm)

  Mud (clay), mudstone (claystone), and shale mud (grain size <1/256 mm)

The deposit of these sedimentary rock types by different currents is as you might guess. The larger gravel, rocks, and pebbles are only carried along by strong currents. These are rushing mountain streams, rocky beaches with high waves, and glaciers’ melt water. Strong glacial currents also carry sand. That is why you usually see sand between the gravel and pebbles. Pebbles and small stones are tumbled along and become smooth very quickly while bouncing along the land or in the water. Beach gravels and broken bits of glass, constantly rolled back and forth in the surf, also get smooth and rounded.

The coarse-grained clastic rock that doesn’t easily smooth or erode is not a conglomerate, but instead a breccia . These sharp-edged rock fragments are found close to their source where sedimentary rock has been layered on top of them before they travel very far. Although some breccias are sedimentary in origin, others come from igneous rock and volcanic beginnings. They were deposited onto a sedimentary rock layer after first being shot out during an eruption or broken away from igneous rock along a fault during an earthquake.

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.

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