Geoscience Reference
In-Depth Information
In order for the new magnetic time scale to be used to date
rocks, it had to be tied into the standard geologic time scale that had
been built up through the decades based on the diagnostic fossils
contained in sedimentary rocks. To do so scientists needed to find a
cross section of fossil-bearing rock of known age that had been de-
posited steadily and slowly, allowing the magnetic minerals in the
parent sediment to capture the fine details as the earth's magnetic
field repeatedly reversed itself. Gubbio was ideal. In a 400-m gorge
outside the town, rocks of middle Cretaceous age, 100 million years
old, are exposed at the bottom and are successively overlain by
younger beds that reach well up into the Tertiary, to an age of about
50 million years. Especially prominent are thick beds of a beautiful
rosy limestone—
scaglia rossa
—a favorite Italian building stone.
These were exactly the kinds of rocks required by Walter Alvarez
and his colleagues, for such limestones build up slowly on the deep
ocean floor and their magnetism would have captured each change
in the earth's magnetic field.
Not only did the team find the reversals in the rocks of the
gorge, they were expressed so intricately that the geologists proposed
the Gubbio section as the "type"—the world standard—for the
Cretaceous-Tertiary part of the magnetic reversal time scale.
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Walter Alvarez and his co-workers had succeeded in their effort
to fill an important hole in geological knowledge. Were it not for
the unique coincidence of scientific and paternal circumstances
described in the Prologue, that likely would have been that.
The K-T boundary in the rocks of the Gubbio gorge can be
spotted just with the naked eye (Figure 3). The white limestone
below the boundary is rich in sand-sized fossils of a one-celled
organism, a kind of plankton called foraminifera, many belonging to
the genus
Globotruncana.
In the red limestone above the boundary,
however,
Globotruncana
completely disappears, replaced by a much
more scarce and much smaller foraminifer with the awkward name
of
Parvularugoglobigerina eugubina.
Clearly, at this boundary some-
thing happened that killed off almost all of the "forams," as the
micropaleontologists call them. Exactly at the boundary, between
the two units, lies a 1 -cm-thick layer of reddish clay, without fossils.
Walter brought home to Berkeley a polished specimen from
Gubbio that included each of the three layers at the K-T bound-
ary—the K, the T, and the clay in between—showed it to his father,
and explained that it captured the time of the great mass extinc-
tion and marked the disappearance not only of most forams but of
the dinosaurs as well. Although most nongeologists viewing this
chunk of rock would have registered at most a polite curiosity (did
