Question

Which one of the following is characteristic of a point bar succession?

• A. Thick homogenous sandstone
• B. Upward gradation from sandstone to shale
• C. Upward gradation from shale to sandstone
• D. Thick laminated shale

Hint:

Point bars typically show an upward transition from fine-grained materials like shale to coarser-grained materials like sandstone.

Answer 1

The Correct Option is C

Step 1: Understanding point bars.
Point bars are sedimentary deposits formed on the inner bend of a meandering river. They typically consist of layers of sand, silt, and clay deposited by the flow of water. As the river meanders, the point bar is formed by deposition of material on the inner side of the bend.

Step 2: Analyzing the types of sedimentation.
The point bar succession typically shows an upward change from finer to coarser materials due to decreasing water velocity as the river flow decreases. This leads to a gradation from shale (fine-grained material) at the bottom to sandstone (coarser material) at the top.

Step 3: Analyzing Option (A).
Thick homogeneous sandstone does not reflect the typical gradation seen in point bar successions, where finer materials are deposited at the bottom and coarser materials on top.

Step 4: Analyzing Option (B).
Upward gradation from sandstone to shale is more characteristic of a transgressive sequence where sea level is rising, which is not typical of a point bar.

Step 5: Analyzing Option (C).
Upward gradation from shale to sandstone is characteristic of a point bar, as it shows the typical coarsening upward sequence from fine-grained to coarser materials.

Step 6: Analyzing Option (D).
Thick laminated shale is more typical of quiet water environments like lakes or deep water deposits and is not typical of a point bar, which tends to have more coarser-grained materials.

Step 7: Conclusion.
The characteristic of a point bar succession is an upward gradation from shale to sandstone, as observed in Option (C).
\[ \boxed{\text{Upward gradation from shale to sandstone}} \]