Richard Grieve

Large meteorite impact structures on the terrestrial bodies of the Solar System contain pronounced topographic rings, which emerged from uplifted target (crustal) rocks within minutes of impact. To flow rapidly over large distances, these target rocks must have weakened drastically, but they subsequently regained sufficient strength to build and sustain topographic rings. The mechanisms of rock deformation that accomplish such extreme change in mechanical behaviour during cratering are largely unknown and have been debated for decades. Recent drilling of the approximately 200-km-diameter Chicxulub impact structure in Mexico has produced a record of brittle and viscous deformation within its peak-ring rocks. Here we show how catastrophic rock weakening upon impact is followed by an increase in rock strength that culminated in the formation of the peak ring during cratering. The observations point to quasi-continuous rock flow and hence acoustic fluidization as the dominant physical p...

The approx. 7.5 diameter Wanapitei impact structure (46 deg 45 min N; 80 deg 45 min W) lies entirely within Lake Wanapitei in central Ontario, Canada. Impact lithologies are known only from glacial float at the southern end of the lake. Over 50% of the impact lithologies recovered from this float can be classified as suevite, less than 20% as

... Again, the calculated slump volume, bulked by 10% for brecciation (Innes, 1961), corresponds well with the observed breccia lens volume (Table 3 ... 0.91 1.16 0.310 0.160 0.047 0.051 0.069 0.055 0.74 1.16 0.310 0.160 0.067 0.051 0.045 0.063 1.13 Dimensions from Roddy et al. ...

... Design Engineers (Utilities). Biozeen; Bangalore India. More science jobs. Post a job for free. The age of the Popigai impact event and its relation to events at the Eocene/Oligocene boundary. Richard Bottomley 1 , Richard Grieve 2 , Derek York 3 & Victor Masaitis 4. ...

ABSTRACT An oval structure in southwestern Nova Scotia measuring 350 x 420 m has been identified as a possible Late- Glacial age impact crater. Evidence for an impact origin is based on integrated analysis of geomorphic, magnetic, petrographic, ground penetrating radar and stratigraphic data. A magnetic survey of the site indicates that the regional linear magnetic pattern is interrupted and distorted within the raised crystalline crater rim, due either to shock remagnetization or reorientation of broken blocks. Probable shock-metamorphic features in rim rocks, not present in unaltered rocks outside the structure, include common single and multiple sets of closely spaced (4-15 μm) planar microstructures in quartz and feldspar, kink- banded feldspar and biotite, reduced mineral birefringence, rare diaplectic feldspar and rare melt veinlets with flow textures. Fresh grain comminution, grain mosaicism and other lattice distortion features are pervasive. Ground penetrating radar shows that the crater has a depressed inner floor that is sharply ringed by a 10-m-high buried scarp. Heterogeneous material under the floor, interpreted as ejecta fallback or slumpback deposits, is overlain by stratified and faulted lacustrine sediment. A Late-Glacial age is inferred through similarities in sedimentation rates to nearby bogs with well-constrained ages and the lack of any evidence of ice-sheet reworking and associated glacial deposits. Strata within the structure appear to be wedge-shaped, indicating post-glacial differential subsidence and compaction. In addition to the main crater, a cluster of arcuate, rimmed scarps 1 km north of the structure may record additional smaller impact sites, suggesting the impactor fragmented upon entry into the atmosphere producing a crater field. The oval shape of the main crater may also indicate an oblique impact or impact doublet. Continuing research focuses on identifying ejecta material in lake sediments from southwestern Nova Scotia in order to elucidate any potential link to Late-Glacial environmental change.

Cratering mechanics suggests that if the proposed Cretaceous-Tertiary impact event occurred in the ocean, it may have been able to locally excavate the oceanic crust and bring upper mantle material to the surface, thereby creating a geophysical anomaly that has yet ...

ABSTRACT Sedimentary rocks are present in the target sequence of approx. 70% of the world's known impact structures. One of the outstanding questions in impact cratering studies is: do sedimentary rocks undergo impact melting? This question cannot be addressed through experimentation in the laboratory, which is limited to impact velocities generally below that required for wholesale melting. Numerical and computer-based modeling may offer some important information, however, as Pierazzo et al. note, "there is no good model for melt production from impact craters in sedimentary targets". Studies of naturally shocked rocks, therefore, offer the only true ground-truth data on the response of sedimentary rocks to impact. We have carried out detailed field and analytical studies of naturally shocked sedimentary rocks that will hopefully provide constraints for future modeling.