Geoscience Reference
In-Depth Information
The basalt flows that make up the traps alternate with layers of sed-
iment called
intertrappeans.
The alternating stratigraphy of the basalts
and the intertrappeans tells us that volcanism began, stopped long
enough for sediments to accumulate, resumed, stopped again while
more sediments collected, and so on until a layer cake of basalt and
sediment built up.
The Indian geologists sampled in the Anjar region of Gujurat
State, where seven basalt flows are recognized, each separated from
the next by intertrappean sedimentary layers several meters thick.
The third intertrappean bed from the bottom, ITIII, contains bones
and eggshells of dinosaurs. The Indian geologists used the argon-
argon method to date the lava beds designated Fill and FIV, which
lie above and below ITIII, at 65.5 ± 0.7 and 65.4 ± 0.7 million years,
identical to the 65.0-million-year date for the K-T boundary. (This
evidence also shows that dinosaurs were still alive up to the very
end of the Cretaceous.) Since the third lava bed is 65 million years
old, the two earlier ones must be older, as the magnetic reversal
results indicated they are, confirming that Deccan volcanism started
well before the K-T boundary.
Within sedimentary layer ITIII, just above the highest dinosaur
fossil, there are three chocolate-brown layers each less than a cen-
timeter thick. In one of these thin layers, Bhandari and colleagues
found a sharp iridium peak reaching 1,271 ppt, compared to a back-
ground of less than 10 ppt in the basalts (the low levels in the basalt
were later confirmed by Schmitz and Asaro) and to less than 100 ppt
in the nearby intertrappean sediments. Osmium levels are high in the
layer and the osmium:iridium ratio is the same as in meteorites and
the mantle. The remarkable perseverance and skillful detective work
of the Indian geologists clearly confirm that this thin, unremark-
able layer is the K-T boundary clay. Even when you know where to
look and have good age control, finding an iridium-rich ejecta layer is
difficult.
The exemplary findings of the Indian geologists and the accu-
mulated knowledge of the chronology of the Deccan traps lead to
several conclusions:
• The magnetic results obtained by the French geologists show
that the Deccan eruptions began at least 1 million years before
K-T time and lasted for at least 1 million years after it, far too long
an interval to be consistent with the considerable evidence that the
K-T event was rapid.
• Contrary to the claims of some paleontologists, and others who
have opposed the Alvarez theory, the dinosaurs did not die out
