Fossils are only found in sedimentary rocks. Does the reverse also apply?
I do suspect that there are plenty of layers that don’t contain fossils, but I’d like to know for sure (if there’s a sure answer to that).
Answer
Dear Lies,
The occurrence of fossils in a sediment layer depends on two factors. Organisms must have lived in the vicinity of the deposit of the sediment and they must also fossilize. If we limit ourselves to fossils visible to the naked eye, such as shells, trilobites and ammonites, then sedimentary rocks older than 600 million years (slightly older than the latest Precambrian) do not contain fossils. Some sedimentary environments of a younger date also appear to be unsuitable for fossilizing shells, for example because calcareous shells sooner or later completely dissolve in the rock.
So yes, there are sedimentary rocks in which you will never find a fossil. However, if we include the microscopic fossils, such as foraminifera, radiolaria, acritarchs, spores, etc., then many apparently ‘sterile’ rocks turn out to be very rich in fossils; even sediments as much as a billion years old.
Answered by
prof. Robert Speyer
Geology – Paleontology – Paleoclimatology. You study geology in Leuven!
Old Market 13 3000 Leuven
https://www.kuleuven.be/
.