During decomposition of a plant or animal, sometimes carbon is the only element left behind in the burial sediment, leaving a carbon film that preserves the details of tissues and soft body parts. Carbonization, also called coalified impression, occurs when the volatile gasses that comprise the plant (Nitrogen, Hydrogen and Oxygen) are "squeezed out" leaving behind the non-volatile residue, i.e., carbon.
This is an example of carbonization from the fossils we looked at in class.
You can tell that by the black, coal looking imprint of the plant.
"Picture of Carbonized Fossil." Taken by Melissa Rogalla. Web. 2 Feb 2011.