Asteroids - Extreme Events: A Physical Reconstruction and Risk Assessment - page 238

and sediment gravity flows. These processes are unusual for deep shelf, quiet
waterenvironments which further suggests that the spherule layers resulted
from extraterrestrial impacts. The most common explanation for generation of
large waves in such circumstances is the collapse of the transient water crater
generated during an oceanic impact, leading to the formation of tsunamis.
However, impact-triggered seismic activity leading to submarine landsliding and
water displacement by ejecta may also have been responsible. It is not possible
to determine which of these processes triggered the large wave action, because
there is a lack of modern analogues of impact-generated tsunamis.
Impact ejecta is found globally in outcrops and drill cores at the K--T boundary
(Smith and Ward, 1998). Although not necessarily universally accepted, the K--T
boundary impact ejecta is widely believed to have been derived from the Chicx-
ulub impact crater, Mexico. Evidence of a mass extinction at the K--T boundary
is still, however, somewhat controversial. The most widely held view is that a
mass extinction event, including the demise of the dinosaurs, immediately fol-
lowed deposition of the K--T boundary layer sequence. Alternative views suggest
that a gradual change in the composition of flora and fauna can be found in
theupper Cretaceous record, indicating that the extinctions were the result of
adeteriorating climate. However, it might be difficult to distinguish between a
mass extinction resulting from climate deterioration due to an extraterrestrial
impact and climate change from other causes. There is little doubt, however,
that the K--T boundary layer was deposited following a large asteroid impact.
The K--T boundary layer is associated with iridium anomalies, shocked minerals
and impact spherules. Four types of ejecta deposits can be distinguished at the
K--T boundary. Smith and Ward (1998)statethat these are:

(1) an ejecta layer, a few millimetres thick, which is found globally;

(2) one to two centimetre thick ejecta layers are visible in the stratigraphic

record of the western interior of the USA;

(3) high-energy (tsunami) clastic deposits are found in Cretaceous to earliest

Cenozoic rocks in the Gulf of Mexico region; and

(4) ejecta blanket deposits are found up to 3.5 crater radii away from the

crater rim (Gulf of Mexico).

Volcanism has been suggested as an alternative cause of the Cretaceous--
Tertiary extinctions. However, Smit and Kyte (1985) suggest that it is unlikely
that a volcanic event is capable of a worldwide distribution of spherules as well
as numerous examples of shocked quartz such as that found in K--T bound-
ary sediments. Although volcanism cannot be excluded as a possible source of
these features, Smit and Kyte (1985)state that it is statistically more probable
foranimpact to be a more likely explanation than a poorly defined mantle