SHOCK DEFORMATION IN THE TRIASSIC SHINARUMP CONGLOMERATE NEAR METEOR CRATER, ARIZONA: RADIALLY FRACTURED AND DIMPLED COBBLES

Exposures of the Triassic Shinarump Conglomerate (basal member of the Chinle Formation) near Winslow and Holbrook, Arizona contain predominantly quartzite cobbles that exhibit pervasive dimples or craters, commonly with radial fractures. The dimples are small, millimeter- to centimeter-sized semicircular depressions that are surrounded by finely crushed and fractured material. On many of the cobbles, fractures radiate out from these depressions or concentric spallation fractures are present. In outcrop, these features are only found on cobbles in clast-supported beds of the Shinarump and are absent from cobbles in matrix-supported zones or on isolated cobbles in sandstone beds. These depressions also form only where cobbles are in close point-to-point contact. The deepest pits are where the cobbles actually touched while others are lined with layers of crushed, sandy matrix. Traditionally, features like these have been attributed to either powerful clast collisions during bedload transport, or to pressure solution during burial or tectonic compression. Comparison of these rocks to similarly ornamented cobbles in Triassic Buntsandstein conglomerates in northeastern Spain and impact studies by Ernstson et al, 2001, Geology, v.29, no. 1, p. 11-14, plus their proximity to the Barringer Meteorite Crater (Meteor Crater), Arizona suggests that these features are the result of clast impacts generated by the propagation of shock waves through the ground due to the impact of the Barringer Meteorite 49,000 years ago.