Stratigraphy--Text

Earth Science Essentials

by Russ Colson

  

Introduction :

As we have learned in previous units, individual rocks tell the story of how that rock formed, of what a place was like at some point in the past. But, to be complete, stories must tell how things change through time. This aspect of geological story-telling is found not in individual rocks but in the layers of rock in the Earth. A single rock gives us a snapshot. Layers of rock give us the full story in motion picture!

 

Nearly everyone has heard how the layers of rock are like the pages of a book telling Earth's story. However, I believe that there is no major concept in all of science that is so ill understood has how we read Earth's story. I have yet to meet a college student who encountered the principles of stratigraphy in middle or high school. I've asked teachers about it, and they've told me that "those ideas are too advanced for our students." Nonsense. What they mean is that they weren't taught that in high school and so, by golly, they aren't going to teach it either!

 

Alas.

 

There is little in all of science so elegant and wonderful as how we have pieced together the vast story of Earth's deep past. In the next three lessons, we will learn the key story-telling elements of reading the Earth--Earth's grammar and vocabulary--starting with how rock layers tell stories.

 

 

First, let's make sure you've understood the lecture.

 

 

 

 

 If you didn't get this question right, you might want to go back and watch the lecture again.

 

A Few Key Ideas of Stratigraphy:

On key idea of stratigraphy is the Principle of Superposition--the idea that later rocks deposited at a particular location will form on top of earlier rocks.  

A stratigraphic column always shows the oldest rocks on the bottom and younger rocks at the top, even if the rocks as they exist today have been tilted or turned upside down. .   The stratigraphic column is an illustration of the sequence with its original superposition intact.

 

This means that to construct a stratigraphic column from rocks as they exist in the real world, we need to know which was way originally up.   We need to find 'up arrows' in the rock.   Examples of up arrows can include fossils in their original growth position (like a rugose coral, or a tree).   Other up arrows are structures in the sediment that have an 'up' direction, like raindrop imprints, or symmetrical ripples in sand, or cross-bedding in sand.   Cross-bedding forms when a ripple or dune in sand migrates through as a layer forms.   Cross-bedding can look in concept like the following, with the tops of the crossbeds truncated and the lower part approaching a tangent:

 

 

The following picture shows cross-beds in the ancient Sioux Quartzite (metamorphosed sandstone) from a Precambrian shoreline in SW Minnesota.   Can you tell which way was originally up?

 

 

 

 

 

Another concept in stratigraphy is the concept of Facies.   Facies are different kinds of rocks that formed at one particular time, but in different adjacent environments.   For example, suppose we have sand along a beach.   As we walk offshore, and the water becomes deeper and quieter, the size of particle deposited decreases, eventually becoming fine silt and clay.   Thus, a sandstone and shale formed along this shore would be facies--formed from sediments deposited at the same time, but in different adjacent environments.

 

To summarize a key point:   Stratigraphic columns show things at different times but at one location.   Facies show things at one time, but in different locations.

 

Walther's law expresses the idea that only those rocks can be superimposed in a stratigraphic column that can be found beside each other in modern environments (in other words, facies become superimposed in stratigraphic columns).

 

A few thought puzzles

In the lecture, we learned about correlation—connecting matching rock layers together. Often, intermediate rocks have been eroded away, or covered by later rocks, and so correlation becomes a way of visualizing what the rock layers might look like if connected.

Likewise, people studying subsurface rocks will take samples at drill wells—but can't possibly take samples at all locations. Geologists then correlate the different stratigraphic columns in order to visualize how the rock layers might be related underground.

 

Try to correlate the two stratigraphic columns below that represent deposition of three types of rocks deposited in different adjacent environments that moved over time.

 

 

 Sketch out your answer to this puzzle before continuing.

 

 

 

 

 

Considering the Principle of Superposition discussed in the lecture, identify the correct age-relationships among X, Y, and Z below-- note: for this puzzle, X > Y means that the rocks at point X are older than rocks at point Y.

 

 

 

 

Here is a somewhat more challenging correlation problem.   Consider that the sequence below resulted from three environments that first migrated one way across the region, then migrated back the other direction.   Sketch out these columns and draw lines to show how the layers might look in the region between and on either side of the stratigraphic columns. Scan or take a picture of your result for later submission by dropbox before continuing.

 

 

 

 

Considering Walther's Law, propose a reasonable set of environments that might have existed in adjacent regions at one time, based on the stratigraphic column below (which represents how environments changed over time at one location). Ls=limestone, Sh=shale, Ss=sandstone.   Sketch out these environments in cross-sectional view.   Complete your cross-section before continuing.

 

 

 

Real geological environments are usually more complex, and not so idealized, as the simplified examples I give you in my thought puzzles.   To get a feel for the variety and complexity that exist in the real-life examples of the puzzle above, you might do an internet search on 'barrier island facies' and see what you get.

 

Testing a Story—Ancient mountains and plate tectonics

A theory is tested when the theory makes predictions that are shown to be true.   The claim has been made by geologists that plate convergence took place in the New England area in the Ordovician Period (meaning that North America was colliding with another plate). Compression and thickening of the crust during convergence produces mountains.   Make a prediction of the type of stratigraphy that would result from this collision for areas to the west of the collision such as New York.   Presume that New York was covered by sea before the beginning of plate convergence and the sea retreated and then sediment from the mountain flanks was deposited over the area.   A common sediment deposited on the flanks of mountains is a red-colored and immature shale or sandstone sometimes called redbed or arkose.   Include this rock in your prediction as well as other rocks you expect with a retreat of the sea—in proper order.

 

Draw your predicted stratigraphic column before continuing.

 

To compare your prediction to reality, examine the actual stratigraphy of Northwestern New York (source: Earth Through Time by Hal Levin).   Ls = limestone, Sh = shale, Ss = sandstone (almost illegible Oswego), Congl = conglomerate, Grp = group of various lithologies.   Does the actual stratigraphy support the claim that convergence occurred?   How?

 

 

 

 

 

Last updated 2/6/2015.   All text and pictures are the property of Russ Colson, with credit to Earth Through Time by Hal Levin.

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