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.

“…the frustration of discovering an unusually good exposure or feature only to have it quarried or covered in succeeding weeks is disheartening. On the other hand, quarry advance enables one to project the geology from time to time, which helps one to fill in the three-dimensional puzzle.” — Gutschick (1972) ABSTRACT We summarize and then build on the three decades of geological mapping and analyses done by Ray Gutschick at the Newton County (Kentland) quarry. We present our own new data and ideas on the kinematics and significance of radial faults, shock metamorphism, petrography and diagenesis of impact breccia dikes, impactite geochemistry, and a preliminary new paleomagnetically determined Jurassic age for the crater. We list and describe the stops for this field excursion.

ABSTRACT The Flynn Creek impact structure was originally recognized in 1968 by David Roddy as one of the original six confirmed impact structures on Earth. The Flynn Creek impact structure is also the first recognized marine-target impact structure. Exposure at Flynn Creek varies, as there is no obvious rim and the geological map of the area does not look like a crater. But, there is an impact breccia unit dominated by two classes of breccia—the lower, chaotic, slump breccia and the upper graded resurge breccia. The post-impact unit is Chattanooga Shale, of which one facies is present only in the crater itself. Participants will visit historical outcrops identified by Roddy, including both the breccia units and the central uplift. New results from ongoing reinvestigations of a drill core from Flynn Creek, as well as insight from other marine-target impact structures in the southeast, will add to lively discussions.

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.

Abstract The impact of the asteroid (KPg impact event) at Chicxulub is now a well-documented geologic event which took place at the Cretaceous/Paleogene (KPg) boundary ( Schulte et al. , 2010 ). However, the effect of this event is relatively unknown in the Tampico-Misantla Basin, which is only about 900 km to the west of the impact site. In the detailed well reports in the Tampico-Misantla Basin, it is noted that a “brecha” (breccia) is often described at the top of the Cretaceous in many of the wells.. The breccia is described as being gray or white, containing mudstone clasts, having a sandy matrix, recrystallized Globotruncana , and traces of chert, amber, and bentonite (for example, in the Marques-1 well). Early geologists thought the breccia had been deposited in response to the Laramide uplift of the Sierra Madre Oriental. To our knowledge, none of the 100 project wells cored the breccia. The same KPg breccia crops out in the southern part of the Tampico-Misantla Basin to the southwest of the town of Martinez de la Torre ( Figs. 1 and 2 ). Here, the breccia is a clast-supported conglomerate consisting of cobbles and boulders of limestone, sandstone (medium to coarse grained), and quartz. The matrix is a medium- to coarse-grained sandstone. The KPg contact has been documented just to the west of this outcrop (Mark Bitter, personal communication). The limestone clasts are thought to have been derived from the Tuxpan platform to the northeast, and the sandstone clasts are thought to have been derived from the Sierra Madre Oriental to the west by the backwash of the tsunami generated by the impact event. In many of the well reports, the wellsite geologists also note that the “Velasco Formation” overlies this breccia. The Velasco Formation is always described as a shale, red, gray, or brown, and compacted. The Velasco Formation has been cored in the Entabladero-101 well from 1140-1149 m and the core has been described as compacted grey/brown shale ( Fig. 3 ). It is devoid of sand. In this study, the presence and thickness of both the breccia and the Velasco Formation were noted and mapped from the well reports. The wells were drilled between 1936 and 2010 and the early wellsite geologists were probably not always aware of the detailed stratigraphic sequence and certainly not aware of the relevance of the breccia in the basin. The thicknesses of both the breccias and Velasco Formation were estimated from the cuttings descriptions in the wells. The breccia is absent in the northern third of the Chicontepec Basin. The thickness of the breccia deposit varies between 4 m and 38 m, and is generally about 10-15 m thick. The deposit is fan-shaped, the source is interpreted to be from the northeast (Tuxpan platform), and it pinches out to the southwest ( Fig. 4 ). The only carbonate source area that is present to the northeast is the Faja de Oro atoll, an Albian age reef complex. Additionally, the distribution of the Velasco Formation seems to mimic the distribution of the breccia, albeit covering a slightly larger area. The Velasco Formation varies in thickness between 16 m and 145 m, but it is generally in the range of 30 to 40 m thick. It is proposed that the breccia plus the Velasco Formation are actually a “megabed” created by the huge tsunami (estimated by some authors to have been over 300 m high) from the KPg impact event ( Fig. 5 ). Many other megabeds around the world show these same characteristics ( Cossey and Ehrlich, 1979 ). The breccia would represent the basal Bouma “A,” or graded division. The Velasco Formation would represent the muddy top, or Bouma “E” division. A good analog for this megabed is from the Jurassic of northern Tunisia ( Cossey and Ehrlich, 1979 ) where a carbonate megabed up to 90 m thick is exposed. Prior to the KPg impact event, the Tampico-Misantla Basin is primarily a site of carbonate deposition throughout the Cretaceous. Afterwards, the Chicontepec Basin forms as a foredeep in front of the rising Sierra Madre Oriental to the west. The overlying Paleocene Chicontepec Formation consists of turbidite sands composed of over 50% carbonate material ( Bitter, 1993 ) derived from the uplift and erosion of the Sierra Madre Oriental.

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 45 km 2 map area is situated at the south end of the Hot Creek Range in central Nevada, ~16 km east of the buried leading edge of the Mississippian Roberts Mountains thrust. Three eastward-trending left-slip faults divide the area into four structural blocks. The southernmost block is occupied solely by upper Oligocene volcanic rocks. The narrow northernmost block, now occupied surficially by valley fill and volcanic rocks, represents the south end of the main part of the Hot Creek Range, from which the study area is offset. The middle two blocks display different aspects of the eastward-traveled outer crater rim created by the ca. 382 Ma (early Late Devonian, middle Frasnian) Alamo impact. The Alamo impact was produced by a 5-km-diameter bolide, most likely a comet, which excavated a transient submarine crater 44–65 km in diameter. Comparison of thin (8–12 m) Alamo Breccia deposits in the northern of the middle two blocks with a more easterly, thick (35–42 m) Alamo deposit in the main Hot Creek Range, 4 km north of the map area, suggests that these blocks traveled many kilometers eastward. The northern of the middle two blocks contains a large olistolith capped by the thin breccia, whereas the southern block contains a larger olistolith lacking an Alamo Breccia cap. Three Devonian pulses of the Antler orogeny are better documented in the chapter on the Bisoni-McKay area. Here, the first Antler pulse in latest Middle Devonian time is obscured within an ~9 m.y. hiatus enlarged by excavation of the Alamo impact crater. The second Antler pulse is recorded by the ~4 m.y. hiatus produced by the regional unconformity between the lower and upper members of the Woodruff Formation. The third Antler pulse is documented by an ~8 m.y. regional hiatus between the Mississippian Webb Formation and Upper Devonian Woodruff Formation. In previous papers, we had interpreted this pulse to initiate the Antler orogeny.