Identification and dating of small impact crater ejecta deposits, case of Morasko craters, Poland

Impact cratering is probably the most ubiquitous geological process in the Solar System. However, most of the studies on impacts on Earth so far focus on the large impacts in hard rock substrate although smaller impacts in... [ view full abstract ]

Impact cratering is probably the most ubiquitous geological process in the Solar System. However, most of the studies on impacts on Earth so far focus on the large impacts in hard rock substrate although smaller impacts in areas covered by unconsolidated sediments or weathering covers are much more frequent. The smaller impact craters, the more likely they are altered by various Earth surface processes. Thus, only few of them are relatively well known. The existing record of small, well-preserved crater structures, as the Morasko (Poland) crater field, give an unique opportunity to extend our knowledge on the relatively frequent small to medium size impacts and associated processes.
Around 5,000 years ago the largest known iron meteorite shower in Central Europe took place nearby the present-day Morasko district of the city of Poznań, western Poland. Evidences obtained thus far indicates that the impact included thousands of iron meteorite pieces distributed over an area of
approx. 3 x 1 km and at least 7 crater-like depressions with diameters of up to about 90 m.
The aim of the project presented here is to extend the previous investigations and to undertake field and laboratory investigations, as well as to perform numerical modeling to date the time of the meteorite fallout and test the hypothesis of the impact origin of the nearby depressions and provide a tool to identify ejecta deposits.
During the field studies we have encountered a clear evidence of partly preserved paleosoil covered by ejecta layer. The datings of the palesoil are in agreement with the age of the oldest organic sediments filling the craters and provide robust evidence of the impact origin of the depressions. We have studied the degree of preservation of the paleosoil and concluded that it is preserved only in some regions close to the crater rim. The numerical modeling confirmed that the landing velocity of the ejecta was the slowest close to the crater rim and thus enhanced the chance for preservation of the paleosoil, which served as the main evidence for ejecta deposits. It is likely that in case of small craters formed in unconsoildated sediments the paleosoils may be the key to identify their impact origin.
The work was supported by National Science Center (Poland), grant no. 2013/09/B/ST10/01666.