It is believed that limestone has existed for more than 3 billion years, since the Precambrian times. Shallow seas provided the ideal breeding ground for calcium carbonate, which begins life under water as a result of an accumulated biological deposit.
The bio-accumulation of calcium carbonate happens in (preferably sea) waters at 25-30C°, which comprise corals, algae and shells. Waters cannot be too deep to allow the organisms to benefit from sun light. Tropical shallow seas are therefore ideal.
As a consequence of continental drift, carbonate deposits of different ages are now found all around the globe.
Hurricanes, strong currents and waves destroy corals, algae and shells, milling them naturally. The resulting sand is deposited onto the sea bed and moves horizontally over time. It takes thousands to millions of years for limestone to be created from these deposits, as it needs to be compacted and further cemented via pressure from deposits above.
Bedded deposits can be recognized by their prominent bedding planes. Reefal deposits are distinguishable by their ‘mound’ shape and the usually massive size of the rock.
Just as mountains are formed by the movement of tectonic plates, so too have carbonate deposits been transported from the sea bed or even deeper settings to the Earth’s surface, a phenomenon that has made them not only visible, but mineable.
The formation of carbonates began billions of years ago and it is still occuring today.
See also pictures in ROCK PHOTO GALLERY