a In the space provided below each block in the figure that follows, indicate wh

Question

a In the space provided below each block in the figure that follows, indicate what type of unconformity is shown. Block diagrams of unconformities. Block 1: Block 2: en Block 4 Block 3: (b) The sedimentary rocks in Block 2 were deposited as horizontal layers. What has happened to the layers? Was the unconformity originally horizontal or tilted? Explain. continu (c) In Block 3, in which direction are the post-unconformity strata dipping? (d) In Block 4, a gray area at the north edge of the block appears on the map surface. What geologic obs ervation(s) cou ld prove that the contact between the gray area and the sedimentary beds is an unconformity and not an intrus contact?

Explanation / Answer

A) Block 1: Local unconformity. Because the upper and lower layer contact is not irregular. The contact is smooth. Time involved is also short, so the age difference between the overlying and the underlying beds is very less.

Block 2: Disconformity. Because the unconformity plane is irregular.

Block 3: Angular unconformity. Because the lower beds are anular, not [arallel with upper beds.

block 4: Non-conformity. Because lower beds are igneous in origin.

B) The unconformity originally tilted. It is identified by uppwer and lower beds of unconformity is parallel. After deposition of upper and lower layer of unconformity, the whole sequence become tilted.

C) The post-conformity starta dipping towards south-east direction. Post-conformity beds represents those beds which are deposited above unconformity.

D) If it is intrusion, not unconformity then it shows sharp contact boundary with surrounding rocks. Even, intrusion can form contact effect (heating effect) to the surrounding rocks. As in picture, the contact boundary between gray area and surrounding rock shows irregular boundary (no sharp contact) without contact effect to the surrounding rocks. So, it is unconformity. It is exposed to the suface due to erosion.

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