Limestone may have been deposited by microorganisms in the Precambrian, prior to 540 million years ago, but inorganic processes were probably more important and likely took place in an ocean more highly oversaturated in calcium carbonate than the modern ocean.

How was limestone first formed?

Limestone is formed in two ways. It can be formed with the help of living organisms and by evaporation. Ocean-dwelling organisms such as oysters, clams, mussels and coral use calcium carbonate (CaCO3) found in seawater to create their shells and bones. ... The water pressure compacts the sediment, creating limestone.

How old are limestone rocks?

In large parts of the United States there are extensive deposits of marine limestone of various ages from a few thousand to more than 350 million years old.