Structure: Stratigraphy and Folds

A. Stratigraphy is the study of layers of sedimentary rock in the field and their age relationships.

B. There are three basic laws of stratigraphy.

1. Original horizontality -- sediments deposited in water form horizontal or near-horizontal layers.

2. Law of superposition -- in rocks that haven't been deformed, the layers get older going from top to bottom.

3. Law of cross-cutting relationships -- "a disrupted pattern is older than the cause of disruption" (pg. 160, Plummer & McGeary). Examples: faults, folds, dikes.

C. When physical forces move rocks from a horizontal orientation, we need to have a way of describing the orientation of those beds in the field. We describe the orientation by strike and dip.

1. Strike measures the orientation of the intersection of the bed of rock with a horizontal surface. We measure the orientation in compass direction.

2. The dip measures the angle the bed makes with a horizontal surface. The dip will be less than or equal to 90 degrees.

3. A tool called a transit compass or Brunton measures strike and dip. A transit compass is a regular compass with a levelling bubble and an inclinometer.

A. Ductile behavior is found when rocks bend or behave plastically. Rock folds are an example of ductile behavior. Ductile behavior often takes place deep in the subsurface rather than at the surface.

B. Brittle behavior is found when rocks break, crack, or fracture. Faulting in rocks is an example of brittle behavior. Brittle behavior is typical at Earth's surface.

III. Rock units that bend in response to deformational conditions form folds.

A. What are the basic types of folds?

1. Rock beds that fold upward form a syncline. Younger rocks lie toward the center of a syncline.

2. Rock beds that fold downward form an anticline. Older rocks lie toward the center of an anticline.

3. The way to remember which name goes with which fold is to look at the second letter of each type of fold

a. The second letter of syncline is "y". The letter y opens upward, much like the actual fold, syncline.

b. The second letter of anticline is "n". The letter n opens downward, much like the actual fold, anticline.

B. What are the different parts of a fold?

1. The sides of a fold are called the limbs.

2. The line along which rock units fold is called a fold axis. This may also be called the hinge of the fold.

C. How may synclines and anticlines be found in the field?

1. Folded rocks usually alternate between forming synclines and anticlines.

2. Folds that tilt into the ground are called plunging folds.

3. If pressures intensify the folding, then the existing folds will usually become steeper and steeper. As the folds become steeper, they are more likely to become asymmetrical. If the limbs of a fold dip in the same direction, then the fold is overturned.

4. Folds in the field are sometimes less obvious than they are on the chalkboard.

a. Folds can often be identified by repeating beds.

b. In the case of identifying folds by repeating beds, it is often more important to determine which beds are older, rather than which way they are dipping. Think of the case of overturned folds.