Ejecta of the Boltysh Impact Crater in the Ukrainian Shield

Abstract

The Boltysh crater is an about 24-km-diameter complex impact structure, which is situated in the central part of the Ukrainian Shield. The crater is surrounded by an ejecta blanket represented by a polymict breccia layer that is preserved over an area of similar to6500 km(2). The later fall-back or suevite ejecta are preserved only in the crater overlying an impact-melt sheet. The ejecta blanket outside the crater is extensively eroded and varies in thickness from tens of meters at a distance of about 2-3 crater radii to 1-4 meters at 4-5 crater radii from the center of the structure. The farthest recorded distance of the ejecta is ca. 66 km or 5.5 crater radii to the W-N-W of the Boltysh structure. Fall-back ejecta from the Boltysh impact event include a suevite breccia layer, which now occurs only within the crater rim, where it overlies an impact melt sheet and the top of the central uplift in the crater. The thickness of the suevite layer varies from a few meters to up to 97 m in the eastern part of the crater. The Boltysh ejecta were deposited on top of Precambrian crystalline basement of the Ukrainian Shield over almost the entire extent of the ejecta blanket. Only within the nearby, about 140-Myr-old Rotmistrovka impact structure, 45 km NW of the Boltysh structure, do the Boltysh ejecta overlie older sediments, which comprise the post-impact sediments of that crater. There, up to 18 m of Boltysh ejecta overlie marls and chalk of Cenomanian-Turonian age, causing brecciation of the underlying chalk. Outside the crater rim the Boltysh ejecta layer is overlain by Middle Eocene sediments, but deposition of these sediments was preceded by extensive erosion of the Boltysh breccias. The post-impact sediments in the Boltysh structure are up to 550 m thick in the deepest parts of the crater. The lower series are sandstones and siltstones of closed freshwater lake origin, without any visible floral and faunal remains. The overlying series of shales and oil shales contain abundant remains of the Paleocene and Eocene paleoflora. These sediments are, in turn, covered by deposits of early to mid Eocene rocks and by more recent sediments of Neogene-Quaternary age. Thus, the stratigraphic age of the Boltysh impact crater, and its ejecta, is constrained to be between the Cenomanian-Turonian and the Paleocene. The most commonly quoted age for the Boltysh age is 88 +/- 3 Ma years, based on whole rock K-Ar ages of impact-melt rocks. However, our new data, derived by laser stepped heating and Spot Ar-40/Ar-39 dating of impact melt rocks, yields an age of 65.17 +/- 0.64 Ma. This age is in agreement with an earlier fission track age, and some recent biostratigraphic studies, indicating that the Boltysh crater was formed simultaneously with, or within a few hundred thousand years of, the Cretaceous-Tertiary boundary age Chicxulub impact structure.