Geoscience Reference
In-Depth Information
FIGURE I 3
The K-T boundary in the Raton Basin. [Photo courtesy of
Glenn Izett.]
found on continents. The chemistry of the Haitian glasses, as ana-
lyzed by Glenn Izett and his colleagues, and the ubiquitous grains of
shocked quartz in the boundary clay, also pointed landward. Quartz
is an essential mineral in granites but is absent in the basalts of the
ocean floors. By the mid-1980s, quartz grains in the boundary clay at
North American sites were recognized to be both larger and more
abundant than those found elsewhere, suggesting that the crater was
to be found on or near North America. Based on the evidence from
the first reports of shocked quartz in rocks from Montana, in 1984
impact specialist Bevan French made the bold prediction that the
target must lie no more than 3,500 km from the Montana site.
2
The North American K-T boundary sites proved to be different
in another way, for at them not one but two boundary layers are
often found, as shown in Figure 13. The upper layer, about 3 mm
thick, contains the iridium spike, shocked quartz, and spherules; the
lower layer, about 2 cm thick, carries much less of all three. This
double layering, first recognized by Jan Smit, turns out to occur in
many of the K-T sections from North America, but nowhere else.
