There are a number of problems with Carter’s answer, but here is one I can correct off the top of my head. Calcium carbonate deposits ARE being formed today. The conditions are usually right in warm, shallow seas, e.g., in the Bahamas. If you want more information (and pictures!) ask Scott Ritter in the BYU Geological Sciences department. He has done a lot of work on carbonate deposition in the Bahamas.
Professor of Geological Sciences
Brigham Young University