What is the dark, organic-rich material that develops when the decay of plant material is slowed by the of lack of oxygen under waterlogged conditions?

This is what is commonly know as 'peat'.  Peat is found in peatlands, bogs, mires and moors, and is an accumulation of partially decayed vegetation or organic matter. The decay is slowed down by a lack of oxygen due to waterlogged conditions. These areas are very important carbon sinks as the CO2 released by the decaying matter is trapped within the peat. It requires thousands of years for peatland to develop. Peatlands are very important as they provide a record of past vegetation and climate within the preserved plant remains.