On the outer parts of shelves carbonate sedimentation is dominated by fine-grained deposits.
These carbonate mudstones are composed of the calcareous remains of planktonic algae and other fine grained biogenic carbonate.
This facies is found in both modern and ancient outer platform settings and when lithified the fine-grained carbonate sediment is called chalk. Similar facies also occur in deeper water settings.
Chalk deposited in shallower water may contain the shelly remains of benthic and planktonic organisms and there is extensive evidence of bioturbation in some units.