ABSTRACT The rare earth element–bearing phosphate xenotime (YPO 4 ) is isostructural with zircon, and therefore it has been predicted that xenotime forms similar shock deformation microstructures. However, systematic characterization of the range of microstructures that form in xenotime has not been conducted previously. Here, we report a study of 25 xenotime grains from 10 shatter cones in silicified sandstone from the Spider impact structure in Western Australia. We used electron backscatter diffraction (EBSD) in order to characterize deformation and microstructures within xenotime. The studied grains preserve multiple sets of planar fractures, lamellar {112} deformation twins, high-angle planar deformation bands (PDBs), partially recrystallized domains, and pre-impact polycrystalline grains. Pressure estimates from microstructures in coexisting minerals (quartz and zircon) allow some broad empirical constraints on formation conditions of ~10–20 GPa to be placed on the observed microstructures in xenotime; at present, more precise formation conditions are unavailable due to the absence of experimental constraints. Results from this study indicate that the most promising microstructures in xenotime for recording shock deformation are lamellar {112} twins, polycrystalline grains, and high-angle PDBs. The {112} deformation twins in xenotime are likely to be a diagnostic shock indicator, but they may require a different stress regime than that of {112} twinning in zircon. Likewise, polycrystalline grains are suggestive of impact-induced thermal recrystallization; however, in contrast to zircon, the impact-generated polycrystalline xenotime grains here appear to have formed in the solid state, and, in some cases, they may be difficult to distinguish from diagenetic xenotime with broadly similar textures.

“…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.

Cerro do Jarau is a prominent, ~13.5-km-wide, circular landform rising >200 m above the plains of the “pampas” in southern Brazil. The name (meaning Jarau hills) comes from the prominent crests of silicified sandstones, which form a semiring of elevated hills in the northern part of the structure. The origin of this structure has been debated for decades, and varied suggestions of its formation include either endogenous tectonic processes or large meteorite impact. However, no conclusive evidence to support either hypothesis has been presented to date. This structure was formed in Mesozoic volcano-sedimentary rocks of the Paraná Basin and consists of the Jurassic-Cretaceous Guará (sandstones), Botucatu (sandstones), and Serra Geral (basalts) formations. The Botucatu Formation sandstones are intensely silicified and deformed, and were subject to radial and annular faulting. Our investigations at Cerro do Jarau identified the occurrence of parautochthonous monomict lithic breccia and polymict breccias resembling suevite and striated joint surfaces resembling crude shatter cones in sandstones and basalts. In addition, our first mineral deformation studies show the presence of rare planar features in quartz clasts in polymict breccias. The identification of these features at Cerro do Jarau, for the first time, is suggestive of an impact origin for the structure. If confirmed by further investigation of possible shock features, Cerro do Jarau would become the sixth known impact structure in Brazil, as well as the fifth basalt-hosted impact structure on Earth.

The recently discovered 6-km-diameter impact structure Jebel Waqf as Suwwan of Jordan (31°02.9′N, 36°48.4′E) has a prominent outer rim and a well-exposed central uplift of ~1000 m diameter, which provides a section through the entire target stratigraphy. The impact occurred into sedimentary rocks of considerable competence contrast. The innermost area of the central uplift exposes Lower Cretaceous sandstones, the oldest strata of the crater. Limestones and marly limestones surround this core and are dismembered into competent blocks that are internally folded. The limestone blocks, in turn, are encircled by a sequence of incompetent marls and chalks. These weak beds accommodated space incompatibilities during block deformation of the competent beds beneath and above. Thick chert beds form the prominent outer collar of the central uplift. Radial folding and faulting are the most conspicuous structural attributes of this sequence. In the southwestern part of the collar, normal layering dominates, and fold axes plunge outward, whereas overturning of strata and fold axes is the rule in the northeastern part. This indicates a top-to-NE shearing component that is explained by an oblique impact scenario with an impact from the southwest. The inferred trajectory runs parallel to the SW-NE axis of symmetry of the central uplift defined by the exposure of strata. Block sizes in limestones and cherts of the central uplift increase with increasing radial distance; however, block sizes are also influenced by the different strength properties of limestone and chert. Shatter cones are abundant throughout the Waqf as Suwwan central uplift, but they also occur prominently along its periphery. Other shock features, such as planar deformation features, planar fractures, and feather features, occur exclusively in Lower Cretaceous sandstones; limestone and microcrystalline chert—the dominant lithologies—are devoid of such effects. The moat between the central uplift and crater rim is largely covered by alluvial wadi sediments. The crater rim is composed of white marls and massive chert beds of Eocene age, the youngest strata of the crater, which also provide a maximum age for the cratering event. Both antithetic and synthetic block slumping are common along the uplifted crater rim.

The 200 Ma, 24-km-diameter Rochechouart impact structure was formed in granitic intrusive and metamorphic rocks of Variscan age (400–300 Ma) close to the margin of the Mesozoic sea. Fractured basement and autochthonous breccias form a several-decameter-thick semicontinuous zone over an 18–20-km-diameter zone. Impact melt rocks, suevite, and polymict lithic breccia are spread over an ~15 km inner zone, forming a centro-symmetric deposit inclined 0.6°N. No topographic expression of the central uplift exists. The crater floor is at the same elevation (~±50 m) over a zone at least 20 km in diameter, corresponding to the central part of the original crater. The pre-erosional diameter of the crater is probably larger than previously thought and possibly reached 40–50 km. The structure appears much less eroded than previously thought, as the sequence of crater fill is complete as exposed near Chassenon. The suevite in Chassenon is capped by an ash-like horizontal deposit of very glass-poor, fine-grained, lithic debris derived from basement rocks. Material with similar grain size and composition is observed in centimeter- to meter-thick multilayered glass-bearing intercalations (dikes) cutting through the suevite. The integrity of the Chassenon sequence strikingly contrasts with the age and morphology of the structure, implying that a rapid and thick sedimentary deposit has covered the crater to protect it from erosion. The impactoclastic top deposit also firmly constrains the thickness and volume of the initial crater fill, which appear extremely depleted (by a factor of 5 or more) compared with similar-sized impact structures and model-based calculations. This anomaly remains unexplained. All the impactites, including the glass-poor and glass-free impactites, are characterized by a prominent K-metasomatism signifying pronounced postimpact hydrothermal activity. Exposed in isolated occurrences from the center to the periphery of the inner 15-km-diameter zone, impact melt rocks are extremely unlikely to have formed a continuous sheet. They display a large variety of textures, grading from pure melt rock into basal suevite, which are distinct in composition, texture, and setting from the main suevite body forming the top of the impact deposit. Heterogeneity and relative inefficiency in mixing are characteristic of the whole impact deposit, resulting in heterogeneous melts at the scale of hand specimens, but also at the kilometer scale, as suggested by close ties between the composition of melt-bearing rocks and the subjacent target rocks.

The Carswell structure in the western Athabasca Basin, northern Saskatchewan (Canada), has previously been interpreted as an eroded impact structure with a minimum diameter of ~36 km, the outer margin of which is broadly defined by an outer ring of sediments composed of the only algal reefs observed in the Athabasca Group, the Carswell Formation. This ring surrounds an 18-km-wide uplifted basement core composed of gneiss units of Archean to Paleoproterozoic age that display shatter cones, planar deformation features (PDFs), pseudotachylyte veins, and impact melts and breccias (Cluff melt sheet, Cluff breccias) indicating that pressures and temperatures locally exceeded 60 GPa and 1500 °C. Detailed analysis of the basement–Athabasca Group contact from field and drill core samples indicates that shock features are not present in the Athabasca Group sediments in direct contact with the highly shocked basement gneisses. This pattern is inconsistent with a post–Athabasca Group age for the impact. Moreover, our study has revealed PDF-bearing quartz grains in basal units of the Athabasca Group 14 km south of the southern edge of the basement core (outside the estimated outer ring). The new proposed model suggests that the impact event is of pre-Athabasca (Proterozoic) age and that it produced a multiring structure that controlled the paleogeography of the Athabasca Group units in the western part of the basin. The model is well supported by basin analysis and gravity data. The Carswell Formation is the result of algal reefs building on peak-ring–related seamounts at the end of Athabasca Group deposition. The overturned bedding observed locally adjacent to the basement core is interpreted as the result of gravity-driven readjustment of the central uplift.