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Sedimentary Stratification Help

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By — McGraw-Hill Professional
Updated on Sep 1, 2011

Sedimentary Stratification

We saw how sedimentary layers can gather in one location, as a result of natural processes such as waves, currents, drying, and wind, 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 10–20 m 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 35° 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. Cross-bedding 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.

Sedimentary Rock Cross, Graded, And Turbidity Bedding

Fig. 7-6. Sandstone cross-bedding shows wind direction at the time sediments were deposited.

Graded bedding comes about through the sorting of particles by a current. A graded or gradient bed is layered with heavy, coarse particles at 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. An example of a graded sedimentary rock bedding sequence is shown in Fig. 7-7.

Sedimentary Rock Cross, Graded, And Turbidity Bedding

 

Fig. 7-7. Bedding gradients reflect the size sorting of layered sediments.

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. Figure 7-8 shows the differences between these two peak forms.

Sedimentary Rock Cross, Graded, And Turbidity Bedding

Fig. 7-8. Different sandstone ripple forms are caused by waves versus current.

Biotubation

Sometimes you will see sedimentary rock with tubes crossing vertically or at an angle through several layers. This is known as biotubation . These fossilized sedimentary structures are the remains of burrows and tunnels made by worms, clams, and other marine organisms. These primitive ocean bottom-dwelling residents burrow through sedimentary layers in search of organic material. Geologists study their vacant homes and waste for clues to the ancient environment during the time they lived.

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

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