If the Noah flood happened, how did limestone deposits formed?

Much limestone is made of the skeletons of zillions of microscopic sea animals. Some deposits are thousands of meters thick. Were all those animals alive when the Flood started? If not, how do you explain the well-ordered sequence of fossils in the deposits? Roughly 1.5 x 1015 grams of calcium carbonate are deposited on the ocean floor each year. A deposition rate ten times as high for 5000 years before the Flood would still only account for less than 0.02% of limestone deposits.