Question

A well-developed succession of laminated shale is bound by two volcanic ash beds that were precisely dated as shown in the schematic diagram given below. Assuming a constant sedimentation rate, the age of the fossiliferous limestone bed 65 m above the basal volcanic ash bed is ............ Ma. (Round off to nearest integer) 

Hint:

The sedimentation rate can be calculated by dividing the age difference between two layers by the distance between them. Then, use this rate to find the age of any intermediate layer.

Answer 1

Step 1: Understand the information.
- The distance between the two volcanic ash beds is \( 195 \, \text{m} \).
- The age of the volcanic ash bed at the bottom is \( 109 \, \text{Ma} \), and the age of the volcanic ash bed at the top is \( 96 \, \text{Ma} \).
- The fossiliferous limestone bed is \( 65 \, \text{m} \) above the basal volcanic ash bed.
Step 2: Calculate the sedimentation rate.
The age difference between the two volcanic ash beds is: \[ 109 \, \text{Ma} - 96 \, \text{Ma} = 13 \, \text{Ma} \] The sedimentation rate is the age difference divided by the distance between the beds: \[ \text{Sedimentation rate} = \frac{13 \, \text{Ma}}{195 \, \text{m}} = 0.06667 \, \text{Ma/m} \] Step 3: Calculate the age of the limestone bed.
Now, calculate the age of the limestone bed, which is \( 65 \, \text{m} \) above the basal volcanic ash bed: \[ \text{Age of limestone bed} = 96 \, \text{Ma} + \left( 65 \, \text{m} \times 0.06667 \, \text{Ma/m} \right) = 96 \, \text{Ma} + 4.33 \, \text{Ma} = 100.33 \, \text{Ma} \] Step 4: Round to the nearest integer.
The age of the fossiliferous limestone bed is approximately: \[ \boxed{100} \, \text{Ma} \]