Abstract The implementation of new groundwater tracer tests to the chalky karst plateau of the northwestern end of the Paris Basin, combined with a critical review of previous tracer test investigations, makes it possible to characterize the role of karsts in relation to many structural features. The impact of tectonic structures and lithology on the development and evolution of the karst networks is analysed. The consequences for drinking-water supplies and its protection are examined.

The field of volcanology has greatly changed during the last half century. The profession is now much more diverse and interdisciplinary, even including collaborating researchers from the social and medical sciences. This new mode of cooperation and working has been more successful in mitigating volcanic hazards and risks. There are fewer of the strong-willed lone rangers of the past and more of those who work with teams to more effectively understand how volcanoes work to protect those living on or near active or potentially active volcanoes. Moreover, there are more university departments with volcanology in their curricula and more international symposia and workshops focusing on mitigation of risk posed by volcano-related hazards. We all have respected colleagues and volcano observatories in many countries. The importance of understanding explosive volcanic eruptions and tracking of eruption plumes involves volcanologists, atmospheric physicists, and air-traffic controllers and is of great interest to the aviation industry. We now have the links in place between great science and practical applications.

The Wetumpka impact structure (near the town of Wetumpka, Alabama) has a semicircular crystalline rim that is ~5 km in diameter. This marine-target impact structure developed in both poorly consolidated, water-saturated sediments and underlying crystalline basement. Previous studies have described a semicircular, crystalline rim, an interior structure-filling unit, and an exterior disturbed terrain developed within the sedimentary target sequence outside the southwestern part of the central basement crater. Based on new field and drill-core observations, we recognize the following specific structural and lithological features: overturned crystalline rim flap; slumped interior megablock terrain; central polymict breccia (originating as near-field ejecta); interior marine chalk deposits and reworked glauconitic sands (formed by resurge and postimpact deposition); and a collapsed southern part of the rim with overturned flap (mainly developed within the sedimentary target rocks). In this paper, we describe the origin of these features and present a new reconstructed sequence of events.

Water resurge into newly excavated impact craters causes both erosion and conspicuous graded deposits in those cases where the water is deep enough to overrun the elevated crater rim. We compare published information on resurge deposits from mainly the Lockne, Tvären, and Chesapeake Bay structures with new results from low-velocity impact experiments and numerical simulations. Notwithstanding the limitations of each of the analytical methods (observation, experiment, and simulation), we can visualize the resurge process for various initial impact-target configurations, for which one single method would have been insufficient. The focus is on the ways in which variations in impact angle and target water depth affect water-cavity collapse, the initiation and continuation of the resurge, its transformation into a central water plume, and subsequent antiresurge, as well as tsunami generation. We show that (1) the resurge at oblique impacts, as well as impacts into a target with a varied water depth, becomes strongly asymmetrical, which greatly influences the development of the central water plume and sediment deposition; (2) the resurge may cause a central peak–like debris cumulate at the location of the collapsing central water plume; (3) at relatively deep target waters, the resurge proper is eventually prevented from reaching the crater center by the force of the antiresurge; (4) the antiresurge is separated into an upper and a lower component; (5) the resurge from the deep-water side at an impact into water of varied depth may overcome the resurge from the shallow-water side and push it back out of the crater; and (6) contrary to rim-wave tsunamis, a collapse-wave tsunami requires deeper relative water depth than that of Lockne, the crater-forming impact event with the currently deepest known target water depth.

A ground-penetrating radar (GPR) survey was carried out in order to characterize reflection patterns at Paluküla Hill, at the NE portion of the rim of the 4-km-diameter, Upper Ordovician Kärdla impact crater. The results allow us to distinguish between the Quaternary overburden and layered postimpact marine sedimentary rocks that cover the crystalline rim. The bedrock is indicated by reflections parallel to bedding that are tilted inward and outward with respect to the crater along the inner and outer rim slope, respectively. It is obvious that the uppermost part of the Paluküla crystalline rim was eroded and leveled in the Late Ordovician prior to having been covered by subhorizontally layered sediments. The unexpected position and low height of the crystalline rim at the northernmost edge of Paluküla Hill indicate that the rim of the Kärdla structure has collapsed to different degrees. This is consistent with the different heights of the crystalline rim. Our results favor the interpretation that the observed geology is not due to erosion by sea resurging into the crater but is rather a result of collapse of the crater rim.

Volcanic activity on the island of Ischia in the past 10 k.y. has included both effusive and explosive eruptions, mainly in the eastern sector of the island. Vent location, eruption dynamics, transport mechanisms, and depositional processes have been reconstructed for each recognized lithostratigraphic unit. Periods of quiescence have alternated with periods of very intense volcanism, mainly concentrated at ca. 5.5 ka and over the past 2.9 k.y. Volcanism has not been continuous, but it has been strongly influenced by the mechanism of a resurgence phenomenon that has affected the island since ca. 33 ka. Therefore, it has been hypothesized that magma intrusion and uplift events have occurred intermittently. In the past 5.5 k.y., volcanic activity has been invariably accompanied by the emplacement of slope instability–related deposits, illustrating that the slope instability was also induced by reactivation of vertical movements, likely related to resurgence.

The late Eocene Chesapeake Bay impact structure, located on the Atlantic margin of Virginia, may be Earth's best-preserved large impact structure formed in a shallow marine, siliciclastic, continental-shelf environment. It has the form of an inverted sombrero in which a central crater ∼40 km in diameter is surrounded by a shallower brim, the annular trough, that extends the diameter to ∼85 km. The annular trough is interpreted to have formed largely by the collapse and mobilization of weak sediments. Crystalline-clast suevite, found only in the central crater, contains clasts and blocks of shocked gneiss that likely were derived from the fragmentation of the central-uplift basement. The suevite and entrained megablocks are interpreted to have formed from impact-melt particles and crystalline-rock debris that never left the central crater, rather than as a fallback deposit. Impact-modified sediments in the annular trough include megablocks of Cretaceous nonmarine sediment disrupted by faults, fluidized sands, fractured clays, and mixed-sediment intercalations. These impact-modified sediments could have formed by a combination of processes, including ejection into and mixing of sediments in the water column, rarefaction-induced fragmentation and clastic injection, liquefaction and fluidization of sand in response to acousticwave vibrations, gravitational collapse, and inward lateral spreading. The Exmore beds, which blanket the entire crater and nearby areas, consist of a lower diamicton member overlain by an upper stratified member. They are interpreted as unstratified ocean-resurge deposits, having depositional cycles that may represent stages of inward resurge or outward anti-resurge flow, overlain by stratified fallout of suspended sediment from the water column.

Based on evaluation of past results and new research, we have partitioned the distribution of the Alamo Breccia in southeastern Nevada and western Utah into six genetic Realms that provide a working model for the marine Late Devonian Alamo Impact Event. Each Realm exhibits discrete impact processes and stratigraphic products that are enumerated here. The first five form roughly concentric semicircular bands across the Devonian shallow-water carbonate platform. These are: (1) Rim Realm, where a newly defined impact stratigraphy includes both autogenic and allogenic breccias associated with the crater rim; (2) Ring Realm, where breccias are now interpreted to have formed sequentially by seismic shock, passage of the ejecta curtain, tsunami waves or surge, and runoff that accumulated over tilted terrace(s) bounded by syn-Event, ring-forming, listric faults; (3) Runup Realm, where graded breccias were stranded by tsunami surge or waves; (4) Runoff Realm, where sheet-floods carried traces of impact debris across the distal platform beds and channels filled with impact debris; (5) Seismite Realm, where near-surface beds far across the platform were uniquely deformed; and (6) Runout/Resurge Realm, where offshore channels of thick off-platform Alamo Breccia, together with large-scale olistolith(s), signal contemporaneous massive collapse of the platform margin, possibly into the central crater. Five breccia Units characterize the newly interpreted Rim Realm, in ascending order: (1) deformed target rocks, (2) injected dikes and sills, (3) chaotic fallback, (4) smeared fallback, and (5) resurge. This succession is covered by deepwater limestones deposited inside the crater rim, or across a new slope created after platform margin collapse. Unit 1 exhibits shatter-cone-like structures interpreted as impact products. Newly discovered Ordovician and probable older meter-scale clasts in Unit 3 confirm a minimum excavation depth of 1.5 km. Microscopic components in Units 3 and 4 indicate high pressures (>10 GPa), probable quenched carbonate melt, and accreted particles that may be new kinds of impact products. Postimpact tectonics and other factors obscure the full panorama, including the location and character of the missing central crater, but the assemblage of Realms offers a working model to compare with expected impact paradigms.