Question

In an undeformed and normal stratigraphic succession, a dolerite dyke was emplaced before deposition of sandstone B. The difference between the maximum ages (in Myr) of deposition of sandstone A and sandstone B is ............... 

Hint:

In stratigraphy, the principle of superposition helps determine the relative ages of rock layers. The maximum age of a layer is typically the age of the layer above it, unless other dating methods are used.

Answer 1

Correct Answer: 57

Stratigraphic sequence analysis:

Given information:

• Dolerite dyke age: 75 Ma
• Volcanic ash bed 1 age: 132 Ma
• Volcanic ash bed 2 age: 62 Ma
• The succession is undeformed and normal (no overturning)
• Dolerite dyke was emplaced before deposition of sandstone B

Applying stratigraphic principles:

From bottom to top:

1. Volcanic ash bed 1 (132 Ma) - oldest
2. Dolerite dyke (75 Ma) - cuts through volcanic ash bed 1 and sandstone A
3. Sandstone A - deposited after volcanic ash bed 1 but before the dyke
4. Sandstone B - deposited after the dyke (given condition)
5. Volcanic ash bed 2 (62 Ma) - youngest

Age constraints:

Sandstone A: Must be younger than volcanic ash bed 1 (132 Ma) and older than the dyke (75 Ma)

• Maximum age of sandstone A: 132 Ma

Sandstone B: Must be younger than the dyke (75 Ma) and older than volcanic ash bed 2 (62 Ma)

• Maximum age of sandstone B: 75 Ma

Difference between maximum ages:

$$\text{Difference} = 132 - 75 = 57 \text{ Myr}$$

Answer: 57 Myr