Geology of the Morasko craters, Poznań, Poland—Small impact craters in unconsolidated sediments

Abstract

Confirmed small impact craters in unconsolidated deposits are rare on Earth, and only a few have been the subjects of detailed investigations. Consequently, our knowledge of indicators permitting unambiguous identification of such structures is limited. In this work, detailed geological mapping was performed in the area of the Morasko craters, of which the largest crater is of about 96 m diameter. These craters were formed in the mid‐Holocene (~5000 yr ago) in unconsolidated sediments of a glacial terminal moraine. Fragments of the impactor—an iron meteorite—have been found in the craters’ vicinity for many decades. Despite numerous studies of the meteorite, no detailed research concerning the geological structure around the craters and of the ejecta deposits has been undertaken. The new data, including evaluation of over 52 sediment cores and 260 shallow drillings, permit the identification of four main sediment types: Neogene clays, diamicton with Neogene clay clasts containing charcoal pieces, diamicton without clasts, and sand with locally preserved paleosoil and charcoal pieces. Based on sedimentological properties, the ejecta deposits are mainly identified as diamicton with Neogene clay clasts, described as lithic impact breccia, covering locally preserved pre‐impact soil. Moreover, crater sections characterized by inverse stratigraphy of sediments are identified as belonging to overturned flaps.

Determining the age and possibility for an extraterrestrial impact formation mechanism of the Ilumetsa structures (Estonia)

“…The structures are listed as proven impact craters in the Impact Earth database, despite lack of commonly accepted, unequivocal proof of extraterrestrial collision identified at this location. We excavated trenches though the Ilumetsa Large and Ilumetsa Small structures and found small pieces of charcoal within the putative proximal ejecta in both structures, in a similar geological setting as previously identified charcoal in Kaali (Losiak et al 2016) and Morasko craters (see Szokaluk et al 2019). Our 14 C dating of charcoal allowed us to conclude that these crater-like features formed simultaneously between 7170 and 7000 cal.…”

“…), the Kaali 2/8 double crater (unpublished data based on 2016 fieldwork), and at the Morasko crater (Szokaluk et al. [, ] and field visits by AL) have been found to have ages consistent with independently determined ages of the formation of these craters. Despite the lack of obvious meteroritic evidence found in the region of the Ilumetsa structures, we show that charcoal is similarly distributed within proposed ejecta blanket of both Ilumetsa structures.…”

“…Within these eject a blankets, we found charcoals in the same geomorphological and stratigraphical setting in relation to the respective crater size and rim location. Additionally, the charcoals found at the Kaali Main crater (Losiak et al 2016), the Kaali 2/8 double crater (unpublished data based on 2016 fieldwork), and at the Morasko crater (Szokaluk et al [2015(Szokaluk et al [ , 2019 and field visits by AL) have been found to have ages consistent with independently determined ages of the formation of these craters. Despite the lack of obvious meteroritic evidence found in the region of the Ilumetsa structures, we show that charcoal is similarly distributed within proposed ejecta blanket of both Ilumetsa structures.…”

“…), and the Morasko ejecta blanket (Szokaluk et al. ). Second, we attempt to determine if the Ilumetsa structures were indeed created by a collision of an extraterrestrial body with the Earth surface by means of looking for meteorite fragments in the field and for geochemical traces of extraterrestrial material in the proposed proximal ejecta, and by searching for additional structures that may be related to this potential strewn field.…”

“…The aim of this study is to: First, assess the ages of both Ilumetsa structures by 14 C dating of organic material covered by what we believe to be a layer of proximal ejecta. In order to achieve this, we have searched for possible impact-related charcoals because this approach has recently been successfully applied to the dating of three other craters found in Estonia; the Kaali Main crater (Losiak et al 2016), the Kaali 2/8 crater (Losiak et al 2018), and the Morasko ejecta blanket (Szokaluk et al 2019). Second, we attempt to determine if the Ilumetsa structures were indeed created by a collision of an extraterrestrial body with the Earth surface by means of looking for meteorite fragments in the field and for geochemical traces of extraterrestrial material in the proposed proximal ejecta, and by searching for additional structures that may be related to this potential strewn field.…”