Sediments and Impact Rocks Filling the Boltysh Impact Crater

Abstract

Ar-Ar dating of impact melts extracted from boreholes into the crater floor indicate that the Boltysh impact crater formed on the Ukrainian Shield at 65.17±0.64 Ma, an age that is indistinguishable from that of the Cretaceous-Tertiary (K/T) boundary and formation time of the giant Chicxulub impact crater. Unfortunately almost all information relating to the drilling along with much of the actual core has been lost. We have studied the remaining core to in an attempt to illuminate the post-impact evolution of this critical crater. The Boltysh crater formed in sub-aerial conditions of the central part of the Ukrainian Shield. Immediately after the impact, unconsolidated clastic sediments derived from the crater walls were deposited, and succeeded by slow sediment accumulation at the bottom of an anoxic crater lake into the Paleogene and even the Neogene. The crater wall was breached during marine transgression in the mid-Eocene and there followed a period of marine sedimentation. Marine regression during the Late Oligocene/Early Neogene led to the return of sub-aerial conditions and later a shallow fresh water lake again filled the crater. A final covering of loess in the Quaternary erased all surface traces of the crater which remains completely buried apart from occasional exposures of the ejecta in river valleys. The crater sediments are rich in the remains of flora and fauna that teemed in the lake, and the excellent sediment preservation of sediments in the now buried crater provides an exceptional opportunity for the investigation of the burial history of a terrestrial impact crater and the evolution of the Ukrainian Shield area surrounding the crater area over a period of 65 million years.