Burgess Shale. What events lead to the fine preservation?

Question

Ok, what events lead to the fine preservation of fossils in Burgess Shale? Why doesn't the soft part of the organism just rot away like usual??

Can anyone help me!!!!

Answer 1

The Burgess Shale deposit was formed in a very large lagoon around the edge of which were a number of explosive volcanoes. When there was an eruption the lagoon was inundated with huge quantities of very fine ash which sank to the bottom instantly covering all life forms in an oxygen free blanket. The combination of the very fine grain of the ash and the instant covering of the organisms resulted in the high level of preservation that is seen in these fossils. The lagoon quickly recovered as fauna returned from the unaffected areas around filling it with life ready for the next eruption to create a fresh supply of fossils. These cyclic conditions have made a very rare fossil site which has no exact equal elsewhere although soft part fossils are not as uncommon as you might think.

Answer 2

Decomposition requires a supply of oxygen to occur efficiently. If an organism dies in or is rapidly moved after death to an area with severe anoxic conditions then the soft parts will be preserved. That is the case with the Burgess shale.

Edit: it is highly likely that the creatures found within the shale were swept over the reef front present at that time by periodic mudflows.