Geoscience Reference
In-Depth Information
moved up and down by many centimeters, equivalent to tens of
thousands of years.
Reworking is a similar, but mechanical, process in which a layer
of sediment, and the biota it contains, is deposited in the sea or in a
streambed but, before it is hardened into rock, is stirred by waves or
currents, and redeposited. As with bioturbation, such disturbance
before sediments are consolidated mixes them up and causes some
of the temporal information to be lost.
Both bioturbation and reworking raise fossils from the dead, like
zombies, and redeposit them higher in the section than they deserve
to be, in younger rocks than those in which the organisms actually
lived. This makes it appear that the organisms lived longer than they
did and thus causes a sudden extinction to appear gradual, or not to
have happened at all. For example, Tertiary sedimentary rocks some-
times contain foraminifera that some specialists believe lived only in
the Cretaceous; to be present in Tertiary sediments, therefore, the
forams must have been reworked. Others believe that instead these
forams survived the K-T event and lived on, into the Tertiary, in
which case they were not killed off by an impacting meteorite and
the Alvarez theory is undercut. (In Chapter 9 we see how paleon-
tologists go about trying to solve this particular puzzle.)
CHANNEL CUTTING AND DEPOSITION
Rivers move back and forth across their floodplains, eroding
here and depositing there. They cut channels down into the rocks
beneath them at one time and later deposit fresh river sediment into
those channels. Rivers can dislodge fossils from the rocks along their
beds and banks and deposit them in their water-cut channels. This
effect, like bioturbation, juxtaposes material of different ages.
As long as the channel-deposited rocks can be distinguished
from those the channel is cut into, no one is led astray, but if, say,
both are sandstones, it may not be easy to tell them apart. When we
do not recognize the channel deposits for what they are, younger
fossils carried downward into older rock appear to belong there and
to have originated before they actually did. In the famous dinosaur
beds of Montana, for example, fossils of mammals that were to be
important in the Tertiary have been said to occur well down in Cre-
taceous rocks, suggesting that the replacement of dinosaurs by
mammals began well before K-T time, which likely means that the
dinosaurs were going extinct long before the boundary. But if the
mammal fossils were washed off a Tertiary landscape and deposited
into channels cut by Tertiary streams down into the Cretaceous
