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The extremely important role of groundwater has been largely overlooked in studies of meteorite and comet impact processes. Beyond the radius of plasma generation, impacts can produce massive shattering in saturated porous rocks. Fluid pressure rise reduces rock strength and facilitates hydrofracture, to produce intraformational monomict breccias, faulting, and generation of mobile polymict breccia slurries. Decompression of a deep “transient” crater accounts for complex central uplift and gravitational collapse of tremendous slide blocks that in turn cause injection and ejection of fluidized breccia. As pore fluid pressures equilibrate, frictional strength increases, and the structural form is locked into stability. Evidence is reported here for Kentland, Indiana, where quarry rocks display relatively low pressure-temperature (elastic to ductile transition, 100 kb–100 °C) impact phases of the model of D. Stöffler. Breccias include monomict, polymict, mixed polymict-fault, and conventional fault types. The monomict breccias are associated with aquifer beds and formed by pervasive shockwave transmission on impact. Polymict breccias are derived from all rock types and formed from late stage injection-ejection pseudoviscous slurries. These processes can apply to similar impacts like Wells Creek, Flynn Creek, Decaturville, Sierra Madre, and many others.

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