Answer:
a quick burial
The most important part: the body has to be buried suddenly, which is rare. Rapid burial can happen due to natural effects, including volcanic eruptions, which bury things in ash, or dying near a flooding stream, which rapidly covers the body in sediment.
Step-by-step explanation:
Water seeps into the remains, and minerals dissolved in the water seep into the spaces within the remains, where they form crystals. These crystallized minerals cause the remains to harden along with the encasing sedimentary rock.