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 analyzed 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.

Smit, Walter Alvarez, and others attribute the two layers to two trajectories of impact explosion. Immediately after impact, the explosive fireball, composed mostly of superheated vapor and mineral dust, was lofted high above the stratosphere. Like the volcanic dust from Krakatoa, it took several months to settle back to the earth; arriving last, it became the upper layer. The coarser material, ejected next from the crater, traveled on ballistic trajectories that did not carry it to high altitudes. Thus it settled out first and became the lower layer. The presence of the double layer only in North America was yet another indication that the crater is located there.

The upper stratum shown in Figure 13 is the K-T clay layer on which we have focused so far—the one marking the boundary at Gubbio and the other sites outside North America. It occurs sandwiched within a variety of marine and nonmarine sediments that were deposited at greatly differing rates. Yet regardless of how rapid the rate of sedimentation of the rocks on either side, the clay layer always has the same thickness: 2 mm to 4 mm. This can only mean that it was deposited independent of normal sedimentation processes and at a much faster rate than that of any of the surrounding sedi-ments.3 Comparison with the settling rates of atmospheric dust suggests that deposition of the upper layer took place over only two to three months.

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