limestone consists of countless trillions of shells of very tiny organisms. these organisms have to grow, they need light and a food source and oxygen and so on. When they die, due to their small size, they fall only slowly to the bottom of the water. Chalk is a particular example of this. The numbers of microorganisms that can actually live in the water at any one time are limited quite significantly by available materials - note that an overabundance of sediments will stop the organisms from growing as there will be no light, and also any additional sediments would settle along with the shells - this has not happened, as we can see from the purity of the deposits.
The UK is a particularly good example of this, in which there are three primary chalk layers, each above the other. The Lower Chalk averages about 200 feet thick, and contains additionally fossils of ammonites and so on. The middle chalk averages about 200 feet in thickness, and is very sparse in additional fossils, and the upper layer averages about 300 feet thick. so what you need there is a single yearly event that can somehow deposit 700 feet of chalk (besides all the other layers) - which is deposited only in shallow waters. Remember also that this chalk is pretty pure stuff. The following feature, the White cliffs of Dover, is made of cocoliths: single celled algae.
This of course only shows a single cliffso far as we know, because of the conditions required for these cocoliths to grow, this sort of thickness of chalk would take millions of years to form. Your sources are making the claim that this feature developed in a few weeks to months, and that is a pretty extraordinary claim, and to be honest, requires extraordainary evidence that it can happen.
Answersingenesis attempt to answer this here:
http://www.answersingenesis.org/tj/v8/i1/chalk.asp
but of course they ignore many of the features of the chalk, but their flood scenario is rather thin on substance and constradicts itself; on one hand they comment on the high purity of the chalk and then on the other they claim the water is full of rotting fish and plant matter. They also ignore the oxygen and other chemical requirements which would be immense for this level of chalk production. they further make the claim that the additional fossils found in the lower chalk layer require rapid fossilization, however this is false. These are shell fossils, and shells can survive a long time. in the UK there are many beaches that consist almost purely of shells. I also have a paper somewhere about some whale skeletons that are partially embedded in currently settling chalk deposits. We can also actually see chalk beds of high purity in the process of deposition, which again goes against their claims, which in this case is just an argument from incredulity.