(46) Earth Science

Sedimentary Rocks

ES44

Carbonate Sedimentary Rocks

Carbonate rocks all have carbon-related compounds in their composition. The two most important minerals found in carbonate rocks are:

* Calcite (CaCO3)

* Dolomite (CaMg(CO3)2)

Carbonate sedimentary rocks are formed through chemical and biochemical processes. They include the limestones, which contain over 80% of the carbonates of calcium and magnesium, and dolostones. Limestone is made up of calcium carbonate (CaCO3) from carbonate sands and mud, while dolostone is made up of calcium–magnesium carbonate (CaMg (CO3)2).

Dolomite formation is little different from some of the other evaporite and chemical sediments. Dolomite is formed by the reaction between sedimentary calcite or aragonite with magnesium ions in any seawater that trickles down through any sedimentary spaces. As the ions are exchanged, some of the calcium ions are switched with magnesium ions and calcium carbonate is then changed into dolomite.

Carbonate rocks are separated by their texture and contents. They include everything from fine mud to a mix-mash of fossils and debris.

Unlike igneous rock, carbonate sedimentary rocks have a fine-grained texture. There are a variety of different forms found. Some of these include the following:

* Micrite (microcrystalline limestone) – very fine-grained; may be light gray or tan to nearly black in color; made of lime mud (calcilutite),

* Oolitic limestone (look for the sand-sized oolites),

* Fossiliferous limestone (fossils in a limestone matrix),

* Coquina (fossil hash cemented together; may resemble granola),

* Chalk (made of microscopic planktonic organisms such as coccolithophores; fizzes readily in acid),

* Crystalline limestone,

* Travertine (evaporates of calcium carbonate, CaCO3) stalactites and stalagmites, and

  • Other – intraclastic limestone, pelleted limestone

Siliceous Rocks

This type of sedimentary rock is commonly formed from silica-secreting organisms such as diatoms, radiolarians, or some types of sponges. It is most commonly called diatomaceous earth. Many expert gardeners use high silica containing diatomaceous earth to aerate and balance the acidity in soil.

Siliceous (silica-containing) rocks are sedimentary rocks with high silica (SiO2) content.

Biologic sedimentary rocks form when large numbers of living organisms die, pile up, and are compressed and cemented to form rock. Accumulated and pressurized carbon-rich plant material may form coal. Deposits made mostly of animal shells may form limestone, coquina, or chert. Diatomite looks like chalk and fizzes easily in acid. It is made up of microscopic plankton (tiny plants) called diatoms. When the silica from diatom remains is dried and powdered, it is used as one of the main ingredients in dynamite.

Chert (also known as flint) is very different in appearance from diatomite. It is made of hard, extremely fine, microcrystalline quartz and can be dark or light in color. Chert is formed when silica in solution goes through chemical changes within limestone. It often replaces limestone and does not fizz in acid.

Flint was used by early hunters for spear and arrowheads. It was easily formed into points and sharp, cutting edges. Opal is a white or multicolored, less-developed crystalline form of chert used in jewelry. Opal has high water content.

COALS (ORGANIC SEDIMENTARY ROCKS)

Organic sedimentary rock is made up of rocks that were originally organic material (like plants). Because of this, they don’t contain inorganic elements and minerals. These organic sedimentary rocks are known as coals. Coals are usually described in the order of their depth, temperature, and pressure. They are made almost completely of organic carbon from the diagenesis of swamp vegetation. Coals contain the following types of materials:

* Peat (spongy mass of brown plant bits a lot like peat moss),

* Lignite (easily broken and black),

* Bituminous coal (dull to shiny and black; sooty; sometimes with layers), and

* Anthracite coal (very shiny and black, a bit of a golden gleam; low density; not sooty; could be a metamorphic rock from exposure to high temperatures and pressures).

Coal is formed from peat, which is a collection of rotting plants found in and around swamps. The conversion from peat to coal is called coalification.

In the various stages of coalification, peat is changed to lignite, lignite is changed to subbituminous coal, subbituminous coal is changed to bituminous coal, and bituminous coal is changed to anthracite coal.

In the United States, coal is found in areas of eastern and western Kentucky, where it is layered between shales, sandstones, conglomerates, and thin limestones. The time span from approximately 320 million years ago and until about 30 million years is commonly called the Coal Age.

Sedimentary Stratification

We saw how sedimentary layers can gather in one location, as a result of natural processes such as waves, currents, drying, wind, and other factors, when we looked at different stratas.

Geologists often use the words sedimentary bed and sedimentary layer to mean much the same thing. I have followed this pattern and will use both words to define sedimentary rock layers. The sedimentary rock strata are laid down in certain well-known structures such as:

* Lamination bedding,

* Uniform layers,

* Cross-bedding,

* Graded beds,

* Turbidity layers, and

* Mud cracks.

We will look at the differences between these types and how they give a different look to a variety of sedimentary rock layers.

BEDDING

Nearly all sedimentary rocks are laid down in layers or beds. Layers can be very thin, like a few millimeters or as thick as 10–20m or more. This sedimentary layering or bedding gives it the characteristic striped look seen in the Grand Canyon and deserts of the United States. The exposed mesas and arches are made of layered sedimentary rock.

A bedding plane is a specific surface where sediments have been deposited. Bedding most often happens in a flat plane as wind or water has layered it over and over onto the same area. When a bedding plane has a different color from surrounding rock, it makes it easier to spot one layer from another. Although we think of bedding as horizontal, this is not always the case.

Bedding is the formation of parallel sediment layers by the settling of particles in water or on land.

LAMINATION BEDDING

When an area has fine, thin (less than 1 cm in thickness) bedding layers, it is known as lamination or lamination bedding. Over millions of years, a single bed made up of very thin individual layers can be several meters thick. Different sedimentary lamination layers can be set apart by grain size and composition. These differences are caused by the different environments in place over long stretches of geologic time.

UNIFORM LAYER

A sedimentary rock layer, made up of particles, all about the same size, is known as a uniform layer. A uniform layer of clastic rock has particles of a single size that have been tumbled by a current of a constant speed. If a uniform bed is made up of layers of single particle sizes, it is thought that currents of different speeds caused the uniform layering of like particles at different times. When nonclastic minerals precipitate out of a solution, the crystals that form uniform layers are all the same size.

CROSS, GRADED, AND TURBIDITY BEDDING

Cross bedding happens when wind or water causes sedimentary layers to be laid down at inclined angles to each other. These can be up to 358 from the horizontal and are found when sediments are laid down on the downhill slopes of sand dunes on land or sandbars in rivers or shallow seas. Crossbedding of wind-deposited sediments can be beautifully complex with many changes in direction. Figure 7-6 gives you an idea of cross-bedding found in sandstone. Graded bedding comes about through the sorting of particles by a current. A graded or gradient bed is layered with heavy, coarse particlesat the bottom, medium particles in the middle layers, and fine particles on top.

A graded layer is made up of particles that are layered from coarse to fine with the heaviest particles on the bottom.

It’s like a jar full of beach sand, small shells, sea glass, and seawater. If you shake it up, everything swirls around for a few minutes before settling. When settling according to weight, the heavier glass pieces settle first, followed by the shells, before everything is coated finally by sand. Over geologic time, these graded beds are piled on top of each other, many meters thick, by deep ocean currents along the sea floor.

Turbidity bedding is found as ripples in the sedimentary rock record. Just as parallel lines of beach sand near the water line are caused by the constant pounding of the surf, sedimentary rock layers are hardened in these same patterns.

When bedding of sediments happens in water, it is almost always horizontal. But currents can affect the look of sedimentary rock as well, with constant wave action giving sedimentary rock layers a symmetrical look. Water currents making swirls and eddies cause permanent overlaying of sedimentary rock. Waves at the beach, constantly depositing sediment with a back-and-forth movement, produce bedding with evenly shaped (symmetrical) peaks. Sediments deposited by a current in only one direction cause sedimentary peaks to be tilted away (asymmetrical) from the direction of the current..

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