SPALL AT LONAR CRATER, INDIA

Spall is defined as “a chip of a material broken off a larger solid body, and can be produced by projectile impact”. In planetary science, spall is often cited as the likely mechanism that delivers meteorites from their parent bodies to Earth (Melosh, 1984; Melosh, 1985). Most spall is likely noted and labelled by planetary mappers as discontinuous ejecta deposits surrounding the continuous ejecta blanket of impact craters. Here, a case is made for a terrestrial spall deposit.

Three piles of ~5-10 cm-sized basalt cobbles and boulders with a visibly fractured appearance and totaling an area of ~5-10 m²were found ~600 m south of the southern edge of the Lonar Crater, India ejecta blanket. They appear as if one to three much larger (~1-2 m diameter) boulders were broken up in place. These materials are off of the continuous ejecta blanket defined by Maloof et al., 2010; Nayak, 1972; Milton, 1979, and others.

Petrography of the samples revealed a fractured texture at the microscopic level corresponding to Class 1 shocked basalt classified by Kieffer et al., 1976. This shock level Class 1 has fractured grains, including laths of labradorite and groundmass augite and labradorite grains, suggesting a shock pressure < 20 GPa. Twinning in labradorite laths is highly disturbed.

The interpretation, supported by their location ~600 m past the distal edge of the continuous ejecta blanket and their classification as Class 1 shocked basalt, is that these three piles were one to three masses spalled from at or near the initial impact point, and were jettisoned to their current position. Field maps of the location in relation to the CEB, field images of the spalled boulders, petrography showing Class 1 shocked basalt, and calculations showing their parabolic path and constraints on the velocity of these fragments will be shown.

Whereas there were likely many more spall created at the time of the Lonar impact event (~570 ka, Jourdan et al., 2011), the emplacement and flow of the lithic layer of the ejecta blanket likely incorporated and “covered” any spall that was closer to the rim into this breccia unit.

At least 23 distal ejecta layers, including impact lapilli, have been identified in the literature (Glass et al., 2012). This discovery at Lonar is likely the first reported occurrence of terrestrial spall.