OF THE EVENT
Figure 37 illustrates our concept of how Sierra Madera formed.
Immediately after impact (figure 37A), the crater grew by low-angle jetting (Gault and others, 1968), while a shock wave passed outward and downward.
Figure 37B shows the central uplift forming during excavation of the crater, in accordance with the Canadian high-explosive cratering experiments and with the sequence inferred at Gosses Bluff (Milton and Brett, 1968). Outward thrusting may have dragged or injected slices of dolomite from the Tessey up around the periphery; the slices were then left isolated by subsequent normal faulting related to crater wall collapse. Withdrawal of material moving into the uplift may have caused collapse of the crater walls, or inward movement of collapsing wall blocks may have been responsible for the uplift. Either way, the adjustments may have continued past the excavation stage. Injection of sandstone and of highly shocked, shattered rock in mixed breccias probably began early in the deformation. If analogous to sand flows around the Snowball TNT crater (Diehl and Jones, 1967), some of the fluidized injections may have continued to flow for days.
The freshly formed crater is shown complete in figure 37C. Figure 37D represents the present eroded form of the structure.
from Wilshire, et al (1972).
Courtesy of the United States Geological Survey