The Obolon impact structure, Ukraine, and its ejecta deposits

Abstract

The Obolon impact structure, 18 km in diameter, is situated at the northeastern slope of the Ukrainian Shield near its margin with the Dnieper-Donets Depression. The crater was formed in crystalline rocks of the Precambrian basement that are overlain by marine Carboniferous and continental Lower Triassic deposits. The post-impact sediments comprise marine Middle Jurassic (Bajocian and Bathonian) and younger Mesozoic and Cenozoic deposits. Today the impact structure is buried beneath an about 300-meter-thick sedimentary rock sequence. Most information on the Obolon structure is derived from two boreholes in the western part of the crater. The lowest part of the section in the deepest borehole is composed by allogenic breccia of crystalline basement rocks overlain by clast-rich impact melt rocks and suevites. Abundant shock metamorphic effects are planar deformation features (PDFs) in quartz and feldspars, kink bands in biotite, etc. Coesite and impact diamonds were found in clast-rich impact melt rocks. Crater-fill deposits are a series of sandstones and breccias with blocks of sedimentary rocks that are covered by a layer of crystalline rock breccia. Crystalline rock breccias, conglomeratic breccias, and sandstones with crystalline rock debris have been found in some boreholes around the Obolon impact structure to a distance of about 50 km from its center. Those deposits are always underlain by Lower Triassic continental red clay and overlain by Middle Jurassic marine clay. The K-Ar age of impact melt glasses is 169 Ma, which corresponds to the Middle Jurassic (Bajocian) age. The composition of crater-fill rocks within the crater and sediments outside the Obolon structure testify to its formation under submarine conditions.