ABSTRACT 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.

ABSTRACT The effects of bolide impacts on carbonate platform sedimentation and stacking patterns are poorly understood, partly because the geological evidence for marine impact sites is typically unavailable. Givetian–Frasnian carbonates in southern Nevada contain a continuous record of sedimentation before, during, and after the Devonian (Frasnian) Alamo impact event (382 Ma), evidenced mainly by the regional Alamo Breccia Member of the Guilmette Formation. Two transects arranged from seven stratigraphic sections measured through the lower ~300 m of the Guilmette Formation record environmental lithofacies deposited from peritidal to deep subtidal zones. Stacking patterns of peritidal and subtidal cycles indicate four relatively high-frequency sequences superimposed on the larger-magnitude eustatic Taghanic onlap of the Kaskaskia sequence. Sequences are interpreted based on facies proportions and cycle stacking trends because of a lack of prominent erosional surfaces developed on the Frasnian greenhouse shelf. Lateral correlation of facies and cycle stacking indicates that the Alamo impact took place during the late phase of sedimentation during deposition of “Sequence 3” in the Guilmette Formation. Underlying facies and surfaces were obliterated and excavated during the impact, resulting in truncated terminations of sequence boundary and maximum flooding zones. Eustatic sea-level rise during the late Frasnian resulted in an overarching shoreline backstep and deepening of vertical facies associations prior to the Alamo impact. Additional accommodation was gained instantaneously as a result of the Alamo impact, which formed a local, steep-sided basin and shifted the slope break of the platform margin. Postimpact sedimentation within the Alamo crater is characterized by condensed sections of continuously deposited thin-bedded mudstones with pelagic (tentaculites) fauna. Thick shoreface sandstones were deposited in a lowstand clastic wedge as the last phase of crater fill in the study area. While accommodation and depositional environment changed dramatically at the impact site, long-term sedimentation trends immediately outside of the impact site were unaffected by the Alamo event, demonstrating that the forces that control overall carbonate platform growth and evolution (tectonics, climate, oceanography, biology) are of far greater importance than even regional-scale physical perturbations such as meteor impacts.

For asteroid or comet impacts, the mass of the projectile or bolide and its velocity control the scale of damage and secondary catastrophes induced, and the impact flux can be used to determine whether such an impact was likely to occur at the time of interest. Impact cratering processes are still orders of magnitude more deadly than volcanism when considering the potential for atmospheric loading of deleterious particulate and gaseous materials, due to the extraordinarily rapid transfer of energy. Based on impact flux, there could have been sufficient large impactors to cause one or more of the “Big Five” mass extinctions in the last 300 m.y. The best contender so far is the Chicxulub event, but this did not trigger massive volcanism in situ, and the Deccan volcanism was not located correctly to be its antipodal pair. The combination of volcanism with impact cratering is a real possibility for the end-Cretaceous extinction, but there is no established connection. This contribution reviews the wider aspects of impact volcanism, including impact fluxes, impact melting, crater thermal anomalies, and secondary impact crises like antipodal volcanism in the context of Phanerozoic mass extinctions.