Many animals lived in the shallow marine bays. More plants and
animals lived in the swampy areas along the rivers. As animals in
the bay died they fell to the bottom of the bay. They were joined
by plants and animals that died along the river and were washed
into the bays.
When the remains of these plants and animals sank to the bottom of
the bays, they were rapidly buried by the mud washing in from the river(s).
This process protected the remains from being destroyed. Bacteria began
to decompose the plant and animal remains in the mud. The action of
these bacteria produced carbon dioxide in the sediments around the remains.
The carbon dioxide combined with iron from the groundwater around the
remains forming siderite (ironstone). The siderite protected the remains
from further damage.
The combination of rapid burial and rapid
formation of siderite resulted in excellent preservation of the many
animals and plants that ended up in the mud.