GEOL 115: The Earth’s Energy Resources

If you are looking for energy resources you need to find thick sections of sedimentary rock. That’s a gross oversimplification. But its worth remembering that oil, gas, coal, and many of the largest types of uranium deposits are confined to thick accumulations of sedimentary rocks.  We will discuss why we need thick sections a little later, for now lets just accept that we do.

It is difficult to preserve thick sections of sedimentary rock.  The forces of nature will conspire to wear away any significant thicknesses of rock that extend above sea level. The only way to accumulate and preserve thick sections of sedimentary rock is to depress the crust (make a hole) and to fill that depression with sediment. These depressions are called sedimentary basins.

Lets define the term

sedimentary basin - A depression in the crust of the Earth formed by tectonic activity in which sediments accumulate.

The tectonic (the means relating to, causing, or resulting from structural deformation of the earth’s crust) activity that causes the depression of the earth’s crust can be largely thermal or mechanical in origin or some combination of the two.

Examples of sedimentary basins that form in response to thermal effects are

Intracratonic basins form in center of continents after periods of continental rifting (when supercontinents get torn apart).  They have subsidence histories that last for hundreds of millions of years.  The Michigan basin is a classic intracratonic basin.  Note the classic “bull-eye” pattern with youngest rocks in the center of the map surrounded by older rocks.

Passive margin basins develop after continents have been pulled apart.  When continents are pulled apart new oceanic crust is generated as the continents drift away from each other.  The edge of the continental crust will sag as it cools after the rifting.  Sedimentary basins can form in these depressions.  Many formed along the Atlantic margin when Europe and North American split apart.

One of the best examples of a basin-type formed in response to mechanical stress is the foreland basin.  Foreland basins form along mountain belts during continental collisions.  They tend to be elongate and  have subsidence histories measured in 10’s of millions of years.  Foreland basins can be remarkably effective at generating petroleum accumulations.  They can also be quite effective in destroying these accumulations.

Other basins may be associated with both thermal and mechanical effects.  Examples of the are the basins associated with strike-slip faulting in California (the Los Angeles basin for example).  Some of these basins will have rapid subsidence histories, high geothermal gradients and will generate prolific amounts of oil.  These basins are destroyed easily when continents collide because they form along the margins of continents.

Its important to realize that basins can start forming from one mechanism and be overprinted by another.

One good way to study basins is to look at the old exams.