Shock metamorphism in samples from the Shili impact structure (Kazakhstan) and discussion of its size and age

Four impact structures are known from the Republic of Kazakhstan, most of which have been poorly studied. This includes the Shili impact structure, an ~1.5-km-wide circular feature visible in satellite imagery. It is located in the western part of Kazakhstan, in the Aktobe Region, where the structure is centered at 49°10.5′N and 57°50′E. While the structure was first considered to be a salt diapir, its impact origin was confirmed in 1989 based on the findings of rare shocked quartz grains and a few poorly developed “shatter cones.” In this contribution, we report the results of a field campaign and a detailed petrographic investigation of 15 quartz sandstone samples. We confirm the presence of rare shocked quartz grains with planar fractures (PFs) and planar deformation features (PDFs). The characterization of shocked quartz allows us to not only confirm the impact origin of the structure, but also to estimate a shock pressure of at least 16 GPa (with a local peak-shock pressure of at least 20 GPa) for some of the rocks now outcropping at the surface. Signs of postimpact hydrothermal alteration include the decoration of many of the PDFs and the occurrence of fractures filled with secondary silica in a few samples. The name and some statistics commonly reported for this structure are also discussed. We suggest the structure be referred as “Shili,” after the name of a nearby river and also that of a phytonym. The minimum original diameter of the Shili impact crater is estimated at ~4–5 km based on a minimum central uplift diameter of 1 km. An early Eocene to Pliocene age for the formation of the Shili impact structure is inferred based on stratigraphy.