CHICXULUB AND SUDBURY: COMPARISON OF IMPACT MELT ABUNDANCES AND PROPOSED EXPLANATION

We compare the structure and stratigraphy of the Chicxulub crater in Mexico and the Sudbury crater in Canada, taking advantage of the well-preserved morphologic state of Chicxulub and the highly-exposed (~8 km erosion) state of Sudbury. We identified five structural rings with similar character and dimensions at both craters. These rings have diameters of about 85, 120, 150, 200, and 250 km. A sixth ring, the peak ring (~80 km), is present at Chicxulub, but apparently has been eroded away at Sudbury. The structural analysis indicates that both Sudbury and Chicxulub are 200 km diameter craters. While the structure of the two craters is similar, the stratigraphy is not. For example, Sudbury has seven times more suevite in the central basin than Chicxulub.

To better quantify these differences, we reconstructed the impact deposits at Sudbury using the structural similarities with Chicxulub as a guide. We conclude that much more impact melt was produced at Sudbury compared to Chicxulub: 31,000 km³ vs. 18,000 km³. To explore this difference we revised the Kieffer-Simonds (1980) analytical cratering model to account for shell-like rather than spherical expansion of the shock waves (suggested by Melosh, 1989); this model gives excellent agreement of shock melt volume relations with finite difference supercomputer models. After accounting for digestion of clasts into the shock melt to produce impact melt, we find that a 45^o impact of asteroids in the range of 11.4-14.4 km diameter at velocities of 20-30 km/s produce the correct size crater and ~18,000 km³ of melt, in good agreement with the melt abundance at Chicxulub. A 30-45^o impact of comets of diameters 14.3-16.3 km and velocities of 40-50 km/s produces the correct size crater and ~28,000-45,000 km³ of impact melt, in good agreement with the melt abundance at Sudbury. Thus, we conclude that the velocity contrast between a comet and asteroid impact is sufficient to explain the difference in melt volumes.