Coherently overturned flaps surrounding craters

Abstract

MANY large impact and explosion craters exhibit coherently overturned flaps in the form of ejecta blankets^1–7 in which the pre-existing stratigraphy of the cratered ground is preserved in detail, but inverted. Sequential ejection of material into ballistic trajectories could produce a coarsely graded inversion of the material, but would not retain detailed stratigraphic coherence. The field evidence from fully excavated craters^3–7 shows that even totally incompetent strata composed of free flowing sand may be traced continuously through a hinge region below the crater rim outwards to at least three times the crater rim radius. The strata retain their integrity and relative position despite being reduced by stretching from an original thickness of some tens of centimetres to less than a centimetre at the extreme limit of coherent overturning. Figures 1 and 2, a photograph and a stratigraphic plot³ of the hinge region of the Prairie Flat crater at the Defence Research Establishment, Suffield, illustrate the phenomenon. Similar examples are given in the reports of the excavation^3–4. The effect was present in all the Suffield trials, and was inconsistent with a free flight mechanism in which discrete elements of sand and clay would follow independent trajectories. However, in very small scale experiments in sand⁸, which would be most likely to exhibit a free flight phenomenon, the coherence in the overturned flap increased markedly when the sand was saturated with water, as compared to the case with dry sand. In this note I suggest that such coherent overturning may be produced by a wave of hydrodynamic type, which becomes a ‘plunging breaker’ of the type described by Longuet–Higgins and Cokelet⁹. Evidence to support this suggestion is given below, and consequences of the mode of formation are predicted and compared with the field data.