Bergman: The silt itself on the bottom is a pretty good ... The culprit was a thick layer of toxic hydrogen sulfide spanning the width of the entire sinkhole like a floating blanket.

They unearthed the fossil on Soom Shale, a band of silt and clay about 250 miles north ... but a “stinky,” colorless gas called hydrogen sulphide dissolved in the water and may have helped ...

For example, the deep red stripes are rich in iron oxides, the yellow layers contain abundant iron sulfide and the green ... in the rock while the sand and silt grains were still cementing together.