A review of the distribution of the Nördlinger Ries distal impact ejecta and its chronological constraint for the formation of the Middle Miocene Steinheim event

Abstract

Since the 1970s, it has been widely accepted that the Nördlinger Ries and the Steinheim impact structures represent a crater doublet formed by the simultaneous impact of a binary asteroid in the Middle Miocene. From a biostratigraphic point of view, however, the lowermost crater-lake sediments deposited in the drainless morphological depressions differ in age by ~ 0.5 to 1 Myr. Recent work additionally questioned the double-impact theory due to the occurrence of two vertically separated seismite horizons in North Alpine Foreland Basin deposits, interpreted to result from two different impact-induced seismic events. A continuous double-layer ejecta blanket originally surrounded the Ries crater within a minimum distance of 45 km from its center. Distal Ries ejecta consist of sedimentary and shocked basement rock fragments of the Ries area. The Steinheim crater is located 41 km WSW of the Ries crater and filled by a ‘basin breccia’ that consist of Middle and Upper Jurassic rock fragments. Most parts of the breccia and overlying crater-lake deposits are preserved. If both craters formed simultaneously, Ries ejecta would have reached the Steinheim area and should be incorporated in the Steinheim breccia or intercalated between the basin breccia and crater-lake deposits. However, no sedimentary or basement rock fragments derived from the Ries crater have ever been found in outcrops or drillings into the Steinheim crater. We conclude the Steinheim impact crater did not exist at the time of the Ries impact and the Steinheim asteroid rather impacted into the outer continuous distal Ries ejecta blanket some 0.5 to 1 Myr after the Ries impact.

Graphical abstract

Geological map of the Ries crater with the present distribution of its ejecta blanket and the geographical position of the Steinheim crater ~41 km WSW of the Ries crater. The Ries ejecta blanket consists of the more proximal type of impact breccia (Bunte Breccia) and the more distal type of impact breccia (Bunte Trümmermassen)