Cratered cobbles in Triassic Buntsandstein conglomerates in northeastern Spain: An indicator of shock deformation in the vicinity of large impacts

Quartzite cobbles in Lower Triassic Buntsandstein conglomerates from northeastern Spain display unusual millimeter- to centimeter-sized circular craters, commonly having central mounds and surrounded by radial fractures. The conglomerates are also marked by intense fracturing down to microscopic scale. These features have traditionally been attributed to tectonic compression and pressure dissolution at cobble contacts. Sections through the cratered cobbles reveal pervasive internal fracturing, segments detached along concave spall fractures, and zones marked by quartz grains with planar deformation features. Comparison with results of impact experiments on artificial conglomerates suggests that these features were produced by internal accelerations, grain collisions, and spallation related to shock-wave propagation through inhomogeneous deposits. The proximity of the outcrops to the Azuara and proposed Rubielos de la Cérida impact structures suggests that shock deformation of conglomerates can provide an easily recognizable regional impact signature.