1. Biogenic Origin:
* Fossil-rich sedimentary rocks: Limestone often forms from the accumulation of the shells, skeletons, and other hard parts of marine organisms like coral, mollusks, and foraminifera. These organisms extract calcium carbonate from seawater and build their structures, which eventually become cemented together to form limestone.
* Biogenic chert: This is a type of silica-rich sedimentary rock. While not directly a "parent", it can be a source of calcium carbonate if dissolved silica reacts with calcite.
2. Chemical Origin:
* Evaporites: Limestone can form through chemical precipitation of calcium carbonate from seawater or other bodies of water. This often occurs in shallow, warm environments with high evaporation rates.
* Other carbonate rocks: Limestone can also form by the alteration of other carbonate rocks, like dolostone (calcium magnesium carbonate).
3. Weathering and Transportation:
* Other sedimentary rocks: Limestone can be formed by the breakdown and re-deposition of other sedimentary rocks, especially those rich in calcium carbonate.
In summary:
Limestone is a diverse rock, and its formation depends on the specific geological conditions. The "parent rock" can be anything from shells to other carbonate rocks to even silica-rich rocks.
