Ries Crater and Suevite Revisited: Part I Observations

Introduction: The Ries of Nördlingen (diameter: 26 km, age: 14.35 Ma) is the type locality of suevite which is the most characteristic rock type formed in impact craters. Though studied since 1834 [1], the formation and emplacement of suevite in the Ries and in impact craters in general is still enigmatic. As information from suevite can contribute significantly with regard to understanding the cratering process in general and the ejecta plume in particular, we have reevaluated suevite genesis in the context of modern modelling capabilities in conjunction with new petro-graphic studies [2] Geology of the Ries crater: The crater is formed in a 500 to 700 m thick layer of Mesozoic sediments underlain by crystalline basement [3]. It has 5 major structural elements (Fig. 1): (1) a central, ~ 700 m deep cavity (radius r = 6 km), (2) an uplifted inner ring and (3) a megablock zone from r = 6 to 13 km, (4) a " tectonic " rim at r = 13-15 km, and (5) an outer ejecta blanket from r = 13 to ca. 45 km. Types of suevite and their geological setting: At the Ries suevite occurs in three different geological settings [4, 5]: (1) a thick continuous layer in the central crater cavity inside the inner ring (" crater suevite " = CS), (2) thin isolated patches on top of the continuous ejecta blanket (previously called " fallout suevite " [5]; we propose to call it " outer suevite " = OS), and (3) dikes in the crater basement and in displaced megablocks. Crater suevite CS. The CS is exposed in several industrial and research drillings. Only the drill core Nördlingen 1973 (Fig. 1; [6]) provides a complete section. There, we distinguish severals subunits based on texture and modal composition: An upper unit A comprising ~ 60 m of " reworked " suevite underlain by ~ 17 m of " sorted " suevite (probably more than one cycle), a middle unit B of ~ 194 m of " high temperature " , i.e., melt-rich suevite, and a lower unit C of ~ 77 m of " low temperature " suevite characterized by a low melt content that decreases with depth. Units A and C display no or only extremely low reversed remanent magnetization, whereas unit B is strongly magnetized [7]. Except for unit A the crater suevite is not stratified and has a very …