EXERCISE 15.8: Interpreting Unconformities on a Block Diagram

EXERCISE 15.8: INTERPRETING UNCONFORMITIES ON A BLOCK DIAGRAM. Indicate what type of unconformity is shown in each of the blocks in the figure on p. 387 of your lab manual. Part a.1. What is the type of unconformity shown in Block 1? Note: All the layers shown are sedimentary.

Disconformity FEEDBACK: The unconformity shown in Block 1 is a disconformity because the layers above and below the unconformity are parallel.

Part a.2. What is the type of unconformity shown in Block 2? Note: All the layers shown are sedimentary.

Disconformity FEEDBACK: The unconformity shown in Block 2 is a disconformity because the layers above and below the unconformity are parallel.

Part a.4. What is the type of unconformity shown in Block 4? Note: The basement layer is composed of metamorphic or igneous rock; the other layers are sedimentary.

Nonconformity FEEDBACK: The unconformity shown in Block 4 is a nonconformity because it is between a metamorphic/igneous basement layer and overlying sedimentary layers.

Part c. In Block 3, in which direction are the post-unconformity strata dipping?

Southeast FEEDBACK: The post-unconformity strata are dipping southeast in Block 3. If they were dipping straight south, the layers would be horizontal on the front face of the block. But because they dip gently to the east, the layers are dipping southeast.

Part a.3. What is the type of unconformity shown in Block 3? Note: All the layers shown are sedimentary.

Angular unconformity FEEDBACK: The unconformity shown in Block 3 is an angular unconformity because the orientation of the beds above the unconformity is different from that below.