At Wetumpka impact structure, the surficial polymict impact breccia crops out discontinuously over a relatively small area within the intrastructure terrain. This polymict breccia unit contains a significant number of shocked quartz grains that represent a slightly lower shock pressure regime than the previously documented shocked quartz population obtained from a separate subsurface impact breccia unit. Specifically, the shocked quartz grains found in the polymict breccia unit display two distinct types of planar microstructures (named herein P1 and P2), and the host grains range in size from fine sand to pebbles and cobbles. P1 elements closely resemble planar fractures (PFs) or planar cleavage in quartz, which occur in multiple sets of open, parallel, fl at to curviplanar planes aligned with distinct crystallographic orientations. P2 elements are much shorter, much thinner, and more closely spaced than P1 planes, and they resemble in part closed, partly decorated to nondecorated, classic, longer planar deformation features (PDFs). However, these P2 elements are not PDFs. Sets of P2 elements are commonly developed off of, or are crosscut by, through-going P1 planes, which form “feather features” (FFs). P1- and P2-type planar microstructures are associated with low shock pressures (~7–10 GPa), whereas more typical planar deformation features previously described in Wetumpka's subsurface impact breccias are associated with shock pressures of 10–16 GPa. We attribute the difference in Wetumpka's shock levels (i.e., between quartz grains in the polymict impact breccia and previously described quartz grains in the deeper, subsurface impact breccias) to differences in provenance of these grains from within the impact structure's transient crater. Proximal ejecta deposits, which were derived from more shallow reaches of the target materials, are the most likely candidate for sources for the P1- and P2-bearing grains in the surficial polymict impact breccia unit. We interpret the present distribution and occurrence of the surficial impact breccia unit to be best explained by late-modification-stage slumping of proximal ejecta from the impact structure's rim.

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.

Utah offers spectacular geologic features and valuable analog environments and processes for Mars studies. Horizontal strata of the Colorado Plateau are analogous to Mars because the overprint of plate tectonics is minimal, yet the effects of strong ground motion from earthquakes or impacts are preserved in the sedimentary record. The close proximity of analog environments and lack of vegetative cover are advantages for field and remote-sensing studies. Dry, desert climate and modern wind processes of Utah are comparable to Mars and its current surface. Analogs in Utah include eolian, sabkha and saline bodies, glacial, lacustrine, spring, alluvial, fluvial, delta, and outflow channel depositional environments, as well as volcanic landforms and impact craters. Analogous secondary processes producing modification features include: diagenetic concretions, weathering and soils, sinkholes, sapping, knobs and pinnacles, crusts and varnish, and patterned grounds. Utah's physical and chemical environments are analogous to conditions on Mars where water existed and could support microorganisms. The development of Mars includes: ancient and modern depositional records, burial and diagenesis, uplift and tectonic alteration, and modern sculpting or weathering of the surface exposures. Recent satellite images are providing unprecedented details that rival the outcrop scale. Analogs in Utah are prime field localities that can be utilized in planning future robotic and human missions to Mars, and for teaching the next generation of planetary explorers.

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.

Collapse and inward slumping of unconsolidated sedimentary strata expanded the Chesapeake Bay impact structure far beyond its central basement crater. During crater collapse, sediment-loaded water surged back to fill the crater. Here, we analyze clast frequency and granulometry of these resurge deposits in one core hole from the outermost part of the collapsed zone (i.e., Langley) as well as a core hole from the moat of the basement crater (i.e., Eyreville A). Comparisons of clast provenance and flow dynamics show that at both locations, there is a clear change in clast frequency and size between a lower unit, which we interpret to be dominated by slumped material, and an upper, water-transported unit, i.e., resurge deposit. The contribution of material to the resurge deposit was primarily controlled by stripping and erosion. This includes entrainment of fallback ejecta and sediments eroded from the surrounding seafloor, found to be dominant at Langley, and slumped material that covered the annular trough and basement crater, found to be dominant at Eyreville. Eyreville shows a higher content of crystalline clasts than Langley. There is equivocal evidence for an anti-resurge from a collapsing central water plume or, alternatively, a second resurge pulse, as well as a transition into oscillating resurge. The resurge material shows more of a debris-flow–like transport compared to resurge deposits at some other marine target craters, where the ratio of sediment to water has been relatively low. This result is likely a consequence of the combination of easily disaggregated host sediments and a relatively shallow target water depth.