Biostratigraphic Indications of the Age of the Boltysh Impact Crater, Ukraine

Abstract

The Boltysh meteorite crater (diameter approximate to 23 km) on the Ukrainian shield was formed in Precambrian (similar to 2.1 Ga) granites and has a classical complex structure with a central uplift (average diameter similar to 4 km). The annular trough around the uplift is filled with predominantly glassy impactites (impact melt rocks = tagamites) that are up to 230 m thick. The crater lake sediments overlie impactites and are represented by laminated siltstones, sandstones, claystones. These sediments have a maximum thickness of nearly 400 m in the center of the crater. Previous data about the geological age of the lowermost crater-fill sediments were contradictory (K-1-P-2), as weere the K-Ar whole rock ages of the tagamites ( 177-56 Ma). The main subject of the present paper is the study of the ejecta of the Boltysh impact crater, relics of which exist outside of the crater proper. We studied the ejecta breccia from outcrops at distances of 25 - 30 km NW from the center of the crater. Two types of breccia exist: breccia composed of disintegrated basement granites (lower unit), and breccia with different rock fragments cemented by sand-like and more fine-grained material (upper unit). The granites and other basement rocks are predominant among the fragments in the upper unit, but a minor amount (near I vol%) of carboniferous sandstones, siltstones, cherts, as well as glassy impactites, also exist. Some mineral grains of the cement have a distinct shock metamorphic features, such as planar deformation features (PDFs) in quartz, deformation bands in feldspar, and kink-band in micas. In sedimentary dykes in the lower breccia unit, from an outcrop near the village of Lebedivka (similar to 25 km NW of the crater center), small fragments of cherty rocks (up to I cm in size) with upper Maastrichtian foraminifera were found. From these data the time range of the crater formation in comparison with resent chronobistratigraphic scale was determined to be 66.8 - 65 Ma. Near the village of Luzanivka (similar to 30 km NW of the crater center) an abundant lower Paleocene fauna, including nannoplankton of the NP1-NP4 zones, occurs in clayey and carbonate sands that overlie the upper unit of distal ejecta mentioned before. Thus, the upper limit of the age of the deposition of the Boltysh ejecta can be set at 65 Ma. This age agrees with recent precise Ar-Ar radiometric ages. A more precise biostratigraphic age determination could be obtained from further biostratigraphic and litholological studies of the ejecta, in particular in the adjacent areas of the Dnieper-Donets depression and within the sedimentary filling of the older Rotmistrovka crater.