Geoscience Reference
In-Depth Information
sent, none having been removed by erosion—was constructed. It is
the basis for the subdivisions shown in Figure 2. Since in a given
geographical area only a limited portion of the geologic column is
exposed, its fundamental units were sometimes situated in entirely
different countries, leaving no way to correlate them precisely. Near
the end of Cretaceous time, different types of rocks were being
deposited in different environments: a limestone on the undersea
shelf near one continent; a sandstone on a beach halfway around the
globe; a shale in a swamp on another continent. In the absence of a
worldwide, short, terminal event, these processes would not have
ended at exactly the same time, and thus the K-T boundary would
have a slightly different age at different places around the world.
Compare, for example, the K-T boundary at Gubbio with that
at Hell Creek, Montana, source of
Tyrannosaurus rex
and the best-
studied dinosaur fossils in the world. As shown in Figure 3, the K-T
boundary at Gubbio is easy to spot—you can place your finger right
on it. On the other hand, at Hell Creek the boundary is exceedingly
difficult to locate, or even to define (it is described in the literature
as "above the highest dinosaur fossil and just below the level of the
lowest coal bed," neither of which occurs at Gubbio). Let us sup-
pose, however, that we could find a boundary at Hell Creek that we
believe demarcates the K-T. How could we determine whether it is
of exactly the same age as the K-T boundary at Gubbio? We cannot
do so by comparing fossils, because those at Gubbio are marine
microfossils (foraminifera) whereas the rocks at Hell Creek were
formed in freshwater and contain dinosaur and mammal bones, but
no forams. One way to determine rock ages precisely is through the
use of a pair of elements in which one, the parent, decays radioac-
tively into the other, the daughter, as when uranium decays into
lead. If one knows how much uranium is present in a sample, and
how much lead, and one knows how fast uranium decays into lead
(the half-life), one can calculate how long the process of decay has
been going on in that sample, and thus derive the age of the rock.
But none of the rocks from Gubbio has enough of the parent ele-
ment or occurs in close association with the volcanic rocks that are
best suited for parent-daughter (radiometric) dating. Even if the
parent-daughter methods could be used, however, they are insuffi-
ciently precise for exact correlation.
Very well, if we cannot show that two rocks from the same sec-
tion of the geologic column on different continents are exactly the
same age, can we do the opposite and show that their ages differ
measurably? Not by using forams on one continent and dinosaurs on
another, nor by using radioactive parent-daughter ages, where the
