Science, Maths & Technology

Soupy sediment

Updated Monday 15th November 2004

Taphonomy - the process of fossil preservation in soupy sediment

Floating carcass

Dead animals often float to the surface. The reason for this is a build up of gases released inside the carcass during decomposition by bacteria. Eventually the gas is released and the carcass would have sunk at quite a speed towards the soft soupy seabed.

Falling fossil fish-reptiles

When the animal impacted with the surface of the sediment, it penetrated into it – this ‘instant burial’ encourages good preservation. However, not all of the animal would be buried in the soup – some parts would still be exposed on the surface, prone to being eaten by scavengers. As the exposed parts of the body decomposed, the bones fell about, whereas the buried parts were protected and supported by the sediment – these parts were destined to become perfectly preserved fossils, as the soupy sediment solidified over time to become solid rock.

But how do we know the sediment was soupy?

Sedimentary rocks are built up in layers. Thick layers are called ‘beds’ and layers less than one centimetre are called ‘laminations’. Laminations can show us many things about how rocks formed. For example, laminations around nodules may appear to be draped over them. This indicates that the rock has been compacted and proved that the hard nodules formed early, well before the sedimentary rock was squashed.

Laminations in soupy sediments

The laminations in soupy sediment are particularly delicate and prone to displacement, and this is exactly what we see in the rocks. Some fossils of bullet-like belemnites have been found vertical in the sediment, at 90 degree angles to the laminations - where the laminations meet the fossil, they bend downwards. Basically, the belemnite looks like is has been shot into the seabed. The belemnite must have dropped from the dead animal and penetrated the seabed – this would only be possible if the seabed was particularly soft and soupy. Fish are also found preserved at significant angles to the laminations – these too must have penetrated the particularly soft layers. Some of these fish even have their mouths full of sediment, so their mouths must have been open as they hit the soup.

 

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