Sedimentary Rocks

Geologists Start With Rocks

And, the main type of rocks they study are called sedimentary (sed-a-mentree) rocks.  Most sedimentary rocks are formed in lakes, rivers, or oceans.

Rivers and streams carve out tiny bits of solid rock and carry them downstream.  If the rock bits are fairly coarse (about the size of salt grains, or larger), they are called "sand".  If they are a little finer, they are called "silt".  If the rock bits are really fine (like flour) they may be called "mud".  Remember, "sand", "silt", and "mud" refer to the size of the grains, not what they are made of.

At some point in their travel, the rivers slow down.  This may be because the surrounding land is very flat, or the river may enter a lake, or (eventually), the river enters the ocean.  When the water slows down,  the grains of sand, silt, or mud being carried by the river drop to the bottom and form layers of sediment.  Usually a layer will be mostly sand, mostly silt, or mostly mud, but they are often mixed up.  

Take a look at the satellite photo of the Mississippi River Delta.  A "delta" is a fancy word for a big pile of sand that forms in an ocean or lake at the end of a stream or river.

 In this case, the Mississippi River is bringing down a HUGE amount of sediment that has been scoured from all over eastern North America, and is forming new land (many miles long) right before our eyes, south of the City of New Orleans.  

Note the main channel of the Mississippi River snaking down through the delta.  The darker areas on the picture show where land sticks up (just barely, no more than a foot or two) above the surface of the ocean and allows plant life to grow.  The lighter, whitish areas show sediments (sand, silt, and shale) that are just under the surface of the water.  The darker blue to the left shows deeper water.

A Natural Sandbox

If you have been to a beach, you stood on a pile of sand that was eroded by the forces of rain and wind from rocks many hundreds of miles away, was transported by a stream or river for a long distance, was broken into TINIER and TINIER bits as it traveled,  and was then then spread out in a long, continuous bar by the work of waves, tides, and wind.  

Click here to see a neat animation of the sedimentary process.

A beach is just one type of many sand deposits that may become deeply buried and later become an excellent oil or gas field.

As the sediments pile up, the oldest ones are buried deeper and deeper.  When they are buried deep enough, heat and pressure and other workings of the earth make the soft sediments hard, and turn them into rock.  This is called "lithification", and sediments that have become hard are said to be "lithified".
 
SANDSTONE is formed when sand-sized sediments are turned into rock, SILTSTONE is formed when silt-sized sediments are turned into rock, and SHALE is formed when the tiniest mud-sized sediments are turned into rock.  See the grain size chart for more information.

Sandstone is a rock made up of grains that are 1/16 millimeter to 2 millimeters in size.  The largest sand grains would be about 1/3 the size of a grain of rice.  Grains larger than this are called PEBBLES, or if bigger,  COBBLES.

Even though sandstone is hard, and appears very solid, it is really very much like a sponge (a very hard, uncompressible sponge).  Between the grains of sand, enough space exists to trap fluids like oil or natural gas!  The "holes" in sandstone are called "porosity" (from the word "porous").

The picture at the right shows a very thin slice (thinner than a human hair) of actual sandstone as seen through a microscope.  The larger brown and yellow pieces are grains of "quartz", a common mineral.  Between the grains, you can see the "holes", or porosity, in the rock; it shows up as black.

Look at the picture below.  This is is a drawing of a typical sandstone under a microscope.

The porosity is shown as black.  Oil or gas could fill these holes in the rock.  What percentage of this sample could be filled with oil or gas?  I would estimate at least 15%.  What do you think?

The ocean, lake, and river environments were ideal for the formation of sedimentary rocks like sandstone, but they also supported a very abundant and diverse collection of living things. 

These animals, whether smaller than a grain of sand or larger than the biggest whale alive today, eventually died and contributed their bodies to the sediments forming below their habitat.  When these animals were buried beneath thousands of feet of sediments, heat and pressure in the earth "cooked" their bodies into oil and natural gas.  These hydrocarbons either  became trapped in the sediment layers that the animal died in, or moved some distance and ended up in a hydrocarbon trap.

Look again at the photo of the Mississippi River Delta near  the top of this page.  Imagine if that gigantic pile of sand and silt were eventually buried thousands of feet deep.  We would have the potential for a tremendous oil or gas field!

Sandstone hand sample photo: Laboratory Manual for Physical Geology

Sandstone photomicrograph:  AAPG Color Guide To Sandstones