#Out of sequence fossils
8 messages in this thread
"Do fossils from one time period ever appear preserved in sediment of a later
time period as the rock in which they were originally present erodes or
dissolves and is redeposited as sediment? Or is the erosion process so
complete that this doesn't happen."
I should answer that question the way I answer my students' questions, "We'll
have to find out the answer to that one. Where do you think we could start?"
But I'll hazard a speculation. A fossil might, through erosion, be transported
lower to an older formation, but such out of sequence fossils would probably be
easily recognized as such. Fossils are unlikely to "wash down" to younger
layers because the younger sediments overlay the older ones. Only in an area
where a tremendous upheaval has occurred and inverted the stratigraphy (like
the rim rocks around Meteor Crater, AZ) or where faulting has aligned the
sequence of adjacent rocks out of order would you be able to find older fossils
washing down into younger fossil beds.
That's my best guess but if someone knows more (which should be quite a few
people here) I'd like to know the answer too.
jf
>> Fossils are unlikely to "wash down" to younger layers because the younger
sediments overlay the older ones. Only in an area where a tremendous upheaval
has occurred and inverted the stratigraphy (like the rim rocks around Meteor
Crater, AZ) or where faulting has aligned the sequence of adjacent rocks out of
order would you be able to find older fossils washing down into younger fossil
beds. <<
My wife and her former advisor are both taphonomists and have studied this
issue. Also, I've worked on sites relevant to this question as well. Fossils
from earlier time periods very often end up in later strata. For example, in
the western rivers, like the Snake and it's tributaries, pleistocene fossils
are today eroding out of the banks and being redeposited in sand bars that also
contain bones of recently dead critters. So, for example, extinct camel bones
are being buried together with regular cow bones!
As you suggest, it is often easy to tell when this happens. For example, the
fossils are rocks, not bones, so they are eroded, scraped, oriented, etc as
rocks and not as bones, and thus show up as being very different. The bones
that I have found fossilized from two dramatacially different time periods were
very obviously fossilized in different ways, and although it was difficult to
work out (took years of research and a bunch of geologists, well, really one or
two really smart geologists and about a half hour, but those guys didn't show
up for the frist two years <g>) to figure out the exact (more or less) sequence
of events. Those fossils were from about 2.5 million years and about 6,000
years in age!
Cheers,
GTL
– Greg Laden, Written 19-Aug-1994 @ 14:11:14
Using: Windows NavCIS PRO 1.25
Oops. I think I forgot erosion is an ongoing process. Thanks for straightening
me out.
"…but those guys didn't show up for the frist two years…."
Sort of like how the scissors are always in the last place you look for them.
jf
There are some unusuall occurrences such as mentioned in Greg Laden's message,
but generally fossils to do not get redeposited out of sequence. Techinically
they oculd, older rocks are continuously being uplifted and eroded away and
redposited as younger sediments. There is one big problem though–this tends to
destroy the fossils.
Probably the biggest exception would be if there is rapid erosion and cobbles
and bolders of the fossil bearing sediment are carried off and deposited as a
conglomerate (or breccia), this would be evident.
Lanny
A little expansion and further comment:
>> There are some unusuall occurrences such as mentioned in Greg Laden's
message, but generally fossils to do not get redeposited out of sequence.>>
Right. It is especially important to note what we mean by "out of sequence".
It is very very hard to get a deposit to form where the fossils are inverted
systematically across several layers. Getting a few older ones mixed up with
newer ones happens now and then, and is expected to be quite obvious most of
the time. But a true inversion is extraordinarily rare. Can't think of any
real good examples at the moment, as a matter of fact.
<< Techinically they oculd, older rocks are continuously being uplifted and
eroded away and redposited as younger sediments. >
Right again. This is especially true in certain environments, such as a
forming rift valley (where we had so much trouble in Zaire, and as happens now
and then in the East African rift as well). In the rift valleys, you can get
sedimentary basons formd on grabens that are faulted, so there is, say, half
the graben sitting up hill pouring stuff down onto the surface of the other
half, down faulted, in a lake or stream bed.
But again, this is obvious when it happens, fossils get destroyed in the
process as you say, and it does not result in a true inversion.
Cheers,
GTL
– Greg Laden, Written 20-Aug-1994 @ 19:01:50
Using: Windows NavCIS PRO 1.25
<< Fossils are unlikely to "wash down" to younger layers because the younger
sediments overlay the older ones. Only in an area where a tremendous upheaval
has occurred and inverted the stratigraphy (like the rim rocks around Meteor
Crater, AZ) or where faulting has aligned the sequence of adjacent rocks out of
order would you be able to find older fossils washing down into younger fossil
beds. >>
I guess I didn't make clear my term "washing down". I was attempting to
describe a process in which, for example, fossils from an exposed Pennsylvanian
bed would be freed from the matrix by erosion or solution and re-deposited more
or less intact along with other sediments into a bed of Permian or later age.
I didn't mean "washing down" in the sense of "up" and "down" vertically, but
rather in the sense of older material being eroded and "washing down" as
sediment which would be incorporated into younger beds.
Picture a conglomerate of a given age containing within its pebbly matter
transported fossils from the older eroding highlands.
>>
I guess I didn't make clear my term "washing down". I was attempting to
describe a process in which, for example, fossils from an exposed Pennsylvanian
bed would be freed from the matrix by erosion or solution and re-deposited more
or less intact along with other sediments into a bed of Permian or later age.
…
Picture a conglomerate of a given age containing within its pebbly matter
transported fossils from the older eroding highlands. <<
Ed:
This is *EXACTLY* what happened to us in Zaire. An ancient fossil bed,
roughly 2.5 million years old, was partly exposted and rubblized by a stream
bed. The stream stopped cutting down just at this point, shifing it's channel
and leaving a terrace that intersected perfectly with the fossil bed. Then,
newer bones and stuff were deposited in among the older fossils. The sediment
re-cemented, looking a lot like (at first glance) the original bed, owing to
the presence of the appropriate materials for reformation.
The deposit is indeed a conglomerate, but it is cemented with materail very
similar to the rubble bits of the ancient rock…
Similar, but not exact. Some 6,000 years ago or so a very unusual volcano
exploded (Katwe, Uganda) leaving a distinctive trace of certain elements
througout the region. One of the main tests of the hypotheses that the
recemented conglomerate was recent was to search for this mineral in the
conglomerate componants (not found there) and the conglomerate matrix
(unfortunately, found there in abundance!).
So the answer is, yes, it does happen. But it can be detected, and at least
in this case, we knew something was strange the whole time, but it just took us
a while to figger it all out!
Cheers,
GTL
– Greg Laden, Written 20-Aug-1994 @ 19:06:27
Using: Windows NavCIS PRO 1.25
Another example: During the Jurassic the West Coast (North America) was
subjected to much folding and twisting, jumbling formations of varying earlier
ages, including some fossil bearing strata. This is the Franciscan melange.
Within this much disturbed terrain occasional, evidently Cambrian, fossils turn
up, mixed in with much later sediments. It is not difficult to realize what
happened, however, and I am unaware of this ever causing confusion as to the
ages of the fossils. B.