Sedimentary Rocks--Text

Earth Science Essentials

by Russ Colson

 

As we explored in the previous reading, sedimentary rocks are ones that have formed by the weathering of preexisting rocks (chemical and mechanical breaking up of sedimentary, igneous, or metamorphic rocks) to form sediments that are transported (moved by wind, rivers, ice, or other means), and deposited (for example, when the wind or river becomes too slow to carry the sediment further and it settles out), and then lithified (minerals, often deposited out of pore water, cement the sediment grains together to make a new rock).  

 

Key Story of Sedimentary Rocks :   Sedimentary rocks tell us about past environments at Earth's surface.   Because of this, they are the primary story-tellers of past climate, life, and major events at Earth's surface.   Each type of environment has particular set of processes that occur that cause a particular type of sediment to be deposited there.  

 

This can be illustrated by a simple experiment (Ideally, you should do this experimental simulation yourself: you will need dirty sand with some fine silt and small gravel mixed in, a plastic basin such as a kitty-litter tray, and water.   If these aren't available, at least do a "thought" experiment).   Think about putting a pile of dirty sand and gravel in a pile on one side of a plastic basin, and putting water at the other side.   Gently slosh the water against the "shoreline" of the mixture of mud, sand and gravel (making waves).   Do this for a while.   What happens?

Answer the 2-point essay question

 

 

 

 

This simple experiment illustrates the key way that many sedimentary rocks tell stories:   The size of particles reveals the energy present in the environment of deposition.   Moving water or wind sort the sediment, that is, they wash away some sizes and leave other sizes behind, causing the sediment to become more "single sized".  

 

Think of another example:   Fast mountain rivers have gravel in them because smaller sand and mud is washed away.   Slow meandering rivers often have mud in them because the slow water allows the mud (clay and silt) to settle out.   Water, ice, and wind each carry sediment in a different way, with water and wind sorting the sediment out by size according to the speed (energy) of the water or wind, whereas ice carries sediment indiscriminately, resulting in an unsorted mixture of gravel, sand, and mud which we call glacial till.   Finding an unsorted sediment or rock can therefore be a clue to a long-vanished glacier.   Finding shale (made of tiny clay and silt particles--try scraping them off with a knife to see the powder that forms) is a clue to a body of very quiet water with little movement, like a deep lake, swamp, or an ocean bottom distant from the waves.

         

We can also interpret sedimentary rocks by considering the concept 'The Present is the Key to the Past'.   For example, if you find a sandstone, in what type of environment might that rock have formed?   Where do you find sand today? (beaches, rivers, dunes for example).   If you find coal, made of abundant plant material that has accumulated, consider what kind of environment we find plant material accumulating in today (swamps and bogs for example).   By comparing the rocks we see with modern environments, we can figure out what type of environment that rock formed in.   By considering details of the rock, such as the rock texture (how the grains fit together and their size) and structures (cross-bedding of layers, ripple marks, etc) and comparing these to the textures and structures in modern sediments, we can identify more details about the environment that a rock formed in.

 

Reading Graphs :   Understanding graphs is an important skill in science, and earth science is no exception.   The relationships among particles size, stream velocity (or other measures of the energy of an environment), and whether a sediment will be eroded or deposited can best be shown graphically (If you are taking this class for advanced credit, making measurements related to construction of a graph like this is the goal of our experimental investigation).

 

Let's make a prediction of a relationship between size of particles and the speed of a river. Suppose that we draw a line on a graph to represent the set of points where a particle is just about to start moving (transport) in the stream, but not quite. If the stream were to increase velocity just a little, the particles of a particular size would start moving. Which one of the lines on the graph below best shows the relationship between stream velocity and the size of sediment where the particles are just about to start moving?

 

 

A classic thesis published in 1935 on a study of the River Fyris gave us the key graph that illustrates the how particle size and stream velocity influence whether a sediment will be deposited in an area or will erode.   Before looking at this graph, see if you can predict the general form of the graph, using what we have learned about sediment depositional environments.

 

 

Here is the actual graph similar to that published in 1935.

Graph adapted from 'Studies of the Morphological Activity of Rivers as Illustrated by the River Fyris', 1935, dissertation by F. Hjulström, published by Almqvist & Wiksell in Uppsala.

 

This graph splits the zone of erosion and non-deposition into two parts.   The zone in which we get neither deposition nor erosion (called zone of non-deposition) is partly a consequence of the fact that tiny particles tend to clump together (or flocculate), and thus once they are deposited they will not erode until a higher-than-expected velocity is reached.   However, in reading the stories that rocks tell, we are most interested in deposition, and so you should take note of the zone where particles are not deposited.   Notice that the smaller particles are not deposited until the velocity is very low; larger particles will only erode when the velocity is much higher.

 

Try another exercise to strengthen both your graph reading abilities and your conceptual understanding of deposition and erosion.   Start with the known correct graph reproduced below:

 

Now, try to draw in a new correct graph, but with the axes labels changed!

 

  

 

 

 

Non-Detrital Sedimentary Rocks :   Some sedimentary rocks form not by deposition of particles carried by water, wind, ice etc. (called detrital rocks) but rather by crystallization from water (chemical sedimentary rocks).   For example, table salt is mined from rock halite, a rock that was deposited from an ancient sea as the water became very salty (such as when sea water evaporates faster than new rain falls).   Such a rock tells the story of a body of sea water that has restricted access to fresh sea water (which would dilute the water and prevent salt from crystallizing) and possibly arid or hot conditions.

 

Some chemical rocks are formed through the action of living things.   For example, coral and many shelled creatures make their hard-parts out of CaCO₃ (either Calcite or Aragonite), which they extract from the sea water.   Algae and other living things can sometimes alter the chemistry of the water to cause the CaCO₃ to precipitate out.   This material becomes the substance for making the rock that we call limestone.   Limestone often tells the story of a former environment where there was not much supply of detrital sediment (such as a marine environment quite a ways offshore but still on the continental shelf or in a place like the offshore areas of Florida that are a long ways from eroding mountains).

 

A Few Sedimentary Rock Names:

 

Detrital sedimentary rocks:

particle size too small to see             sand sized particles       some pebble-sized particles

              shale                                      sandstone                               conglomerate

 

Chemical sedimentary rocks:

              rock halite (NaCl), rock gypsum, limestone

 

Biochemical sedimentary rocks (Chemical formation aided by living things):

              limestone (mostly CaCO₃), coal (mostly C).