I've been doing a little research into one of my local bodies of limestone. I didn't quite realize that limestone has more than one way to form. We're all taught that limestone forms in shallow seas. Most of the time during geologic history, the sea has sat much higher than it does today (when the Antarctic and Greenland ice caps hold large amounts of water out of the ocean). And so it is that huge tracts of the continents are covered with flat beds of limestone, laid down in the wide, shallow epicontinental seas of past times.
We're taught that shallow tropical nearshore environments like the Bahamas and the Great Barrier Reef are where limestone is forming today. Thus it's easy to picture every body of limestone that way. But my local limestone turns out to be pelagic limestone, which forms way out in the open ocean.
Deep-sea limestone is rare because the tiny calcium carbonate shells, or tests, of plankton don't make it down to the deep sea; they dissolve at what's called the carbonate compensation depth. That's why most of the deep seafloor is covered with clay or bare rock. The only ways to form a pelagic limestone are around an ocean islandyour typical coral atollor on large volcanic plateaus that lie scattered below the ocean.
My limestone is the plateau kind. It has no corals, no crinoids, none of that stuff, only plankton tests with some diatom tests, which are silica, mixed in. This is not the kind of thing that's obvious to the eye. It took the attentions of microfossil experts to figure this out.
How such a limestone makes its way to the continents is a separate story. As part of plate tectonics, the seafloor is carried one way or another until eventually it runs into a continent and is subducted. My plateau rode from the tropical Pacific north and east as far as California, where it got stuck. Eventually it became part of the Coast Range, where it caught the attention of quarriers, and now it's gradually being mined away and turned into concrete.