Bill writes:
rox writes:
Fossils from different time periods are not found in the same
'depositional stratum.' Meaning that you will never find
Jurassic forams being deposited with forams only found in
Tertiary rocks.
You might find them in the same vicinity, but they will be
separated by either other strata or unconformities (e.g.,
erosional).
In case of articulated skeletons, ichnofossils, and fragile fossils, this is true. However, in case of smaller shells, bones, teeth, and microfossils, this isn't true. Such fossils, like any sedimentary particle, can be eroded out of older rocks, transported, and redeposited in younger sediments, which can eventually become new sedimentary rocks, by a process called "reworking."
Yes, I thought about adding that, but sometimes I don't want to confuse people who don't have much geologic knowledge. Also, I guess I'm also waiting for people to ask questions before giving a more detailed answer. I've written long posts only to have them completely ignored or dismissed with a handwave before and I just don't feel like wasting anymore time with people who are uninterested in learning. I think I should probably not do that anymore, however.
I've seen reworking in action on the shores of Pyramid Lake, Nevada. Some beaches on the lake are white because they are composed of tiny gastropod shells being eroded [by wave action] out of localized fossiliferous outcrops. The fossils are not that old, probably several thousand years.
My problem is that I'm either too simplistic or get too complex in my explanations. I just can't seem to find that happy medium - especially if I'm rushed.
However, you always do a great job explaining geologic concepts.
[This message has been edited by roxrkool, 02-16-2004]