OH! I understand that...it seem the weathering would have stop the fossilization and the mineralization would have taken longer than 50k years...hopefully someone on FR can help.
Fossilization could happen in a thousand different ways - as would the time it takes. In the context of a footprint, if you stepped in wet clay and later built a fire, "fossilization" would be near instantaneous. Normally fossilization is the replacement of one material by another - carbon based materials are not normally replaced (skin and muscle) but the calcium and bone minearals can be replaced by ion exchange or a lot of other processes. There are so many chemicals and minerals that can affect the replacement, that the range could be from a hundred years to infinity (depending on acidity, salinity, moisture content, etc.). But it is stretching credulity to believe their is nothing older than 6000 years given the depth of stratigraphic layers that can be found, and the range of fossil types.