Question

The schematic diagram represents thin section of a carbonate rock. The type of cement formed by large calcite crystals is known as: 

• A. Overgrowth cement
• B. Poikilotopic cement
• C. Isopachous cement
• D. Meniscus cement

Hint:

In carbonate petrography: large crystals engulfing multiple grains $\Rightarrow$ poikilotopic cement; uniform rim $\Rightarrow$ isopachous; bridge at contacts $\Rightarrow$ meniscus; syntaxial rim $\Rightarrow$ overgrowth.

Answer 1

The Correct Option is B

Step 1: Recall the different cement types in carbonates.
- Overgrowth cement: Forms as syntaxial rim cement, growing outward from the grain, usually seen in quartz arenites. Not applicable here.
- Isopachous cement: Layer of cement coating grains equally on all sides, usually fibrous aragonite or calcite, typical of submarine phreatic environments.
- Meniscus cement: Cement precipitated at grain–grain contacts in vadose conditions, forming curved bridges.
- Poikilotopic cement: Large, equant calcite crystals that enclose and engulf multiple grains, forming a distinctive texture in thin section.

Step 2: Interpretation of the figure.
The schematic shows grains floating within large crystals of calcite cement that extend across multiple grains, indicating that the cement crystals are much larger than the grains themselves. This is diagnostic of poikilotopic cement.

Step 3: Eliminate incorrect options.
- Overgrowth cement: not shown (no syntaxial rims).
- Isopachous cement: would be thin, equal rims around each grain, not engulfing.
- Meniscus cement: would be localized at contact points, not filling entire pores.

Final Answer:
\[ \boxed{\text{Poikilotopic cement}} \]