Shock-Metamorphic Features in Two Meteorite Impact Structures, Southeastern Libya

Abstract

Shock-metamorphic effects in sandstone samples from two circular structures about 80 km apart in southeastern Libya demonstrate that these structures formed by meteorite impact. The smaller structure is about 2.8 km in diameter, and the larger is 11.5 km in diameter. Both structures occur in flat-lying Nubian Sandstone (Early Cretaceous) that is intensely disturbed within the structures. Sandstone specimens from both show extensive microscopic shock effects in quartz, including cleavage and planar features (shock lamellae), with prominent orientations in the {0001}, {1¯122}, and {1¯011} planes. A reduced development of planes parallel to {1¯013} and {1¯012}, also typical of shocked Coconino Sandstone from Meteor Crater, Arizona, indicates that slightly porous or cemented sandstone shows significantly different responses to shock waves than massive crystalline rocks show. Both Libyan structures are post-Nubian in age. Their similar degree of erosion suggests a simultaneous origin by a double impact.