Professor Cam Nelson
Limestone is sedimentary rock, so it must have formed at the Earth’s surface.
It starts off as sediment, and it must have more than 50% calcium carbonate in it to qualify as limestone. Rock such as mudstone or sandstone – where the grains have come down rivers and been delivered to the sea – they can have a highish calcium carbonate content perhaps as well, but less than 50%, and so we could call those calcareous mudstones or calcareous sandstones to indicate that carbonate content.
So limestone specifically must have more than 50% calcium carbonate, and the little grains that give you that calcium carbonate are typically the smashed up shell remains of a whole variety of organisms. These can be molluscs, gastropods, echinoderms, bryozoans, brachiopods, worm tubes – most invertebrate animals secrete some type of skeleton, and very often that’s a calcium carbonate skeleton.
So in New Zealand, they are the main contributors to the formation of carbonate sediments, which, once they are changed into rock, become limestones.
McDonald’s Lime Limited
Dr Roger Grace
National Institute of Water and Atmospheric Research (NIWA)