Petrography of the impact breccias of the Enkingen (SUBO 18) drill core, southern Ries crater, Germany: New estimate of impact melt volume
A new drill core (SUBO 18) that intersects a continuous sequence of impact breccias has been obtained at the town of Enkingen at the southern edge of the Ries inner crater. Detailed petrographic analysis of this core provides important new information regarding the nature and origin of within-crater suevite, which is not only relevant for the understanding of the formation of the Ries impact structure but has strong implications for impact breccia deposition in general. In the Enkingen core, below 21 m of crater sediment, ∼90 m of impact breccia have been sampled. Most of this material is what in the past has been referred to as suevite—defined as groundmass of clastic material entraining cogenetic melt fragments and target rock clasts. Only the bottommost 10 m of core are melt-dominated impact breccia. Any relatively thick melt rock intersection along this core is revealed to be composed of individual centimeter- to decimeter-sized melt bodies best described as "agglomerate" due to its close resemblance to volcaniclastics of that type. Detailed macro-, meso-, and microscopic petrographic analysis has established that the SUBO 18 impact breccias cannot be divided into distinct units. The groundmass is extensively altered to phyllosilicate (smectite or chlorite) or carbonate minerals. Results of a detailed scanning electron microscopic investigation of the groundmasses of samples from all parts of this sequence emphatically support the traditional definition of suevite, irrespective of scale. Modal analysis at macroscopic to submicroscopic scales has shown that the overall, average melt content of Enkingen suevite is on the order of 48 vol%. This exceeds by far previous estimates and brings the Ries crater melt volume in line with values traditionally suggested from crater and melt volume scaling for similar-sized impact structures in crystalline and mixed targets.