Stratigraphy of the Ries Suevite, Germany, from Stereometric Analysis (original) (raw)

Introduction: The suevite of the 14.35 Ma old, 25km wide Ries crater in southern Germany occurs in 3 different geological settings: 1) the crater suevite in the central crater cavity inside the inner ring, 2) the outer suevite on top of the continuous ejecta blanket, 3) dikes in the crater basement and in displaced megablocks [1]. The mechanisms of transport of particles in the suevite remains poorly understood. In [2] the following processes are discussed: 1) "aerial" transport in a gaseous medium, 2) ground surging in a turbulent flow, 3) interaction of a temporary melt sheet in the central crater with surface water leading to "phreatomagmatic" explosions and subsequent aerial transport. We measured the shape and size distribution of particles in several drill core sections (thickness of suevite in parentheses): Nördlingen, inside the inner ring (300m); Enkingen, at the inner ring (80m); Wörnitzostheim, between inner ring and crater boundary (80m); and Otting, outside the crater (9m). The drill cores were studied by digital stereometric analysis. Grain sizes and shapes of lithic clasts and melt particles were measured on the plane surface of the half cores and on thin sections of the same core sections. The following grain parameters were measured in the size range of +2 to-6 phi (0.25 to 63mm): 1) particle content, 2) aspect ratio = minor axis/major axis, 3) maximum grain size = mean of the ten largest particles, 4) particle size distribution represented as fractal dimension [3]. Observation and subdivision of the Ries suevite: The crater suevites can be divided into stratigraphic subtypes. They differ in their stereometric parameters and show distinct differences between the central part and the outer part of the inner crater. The Nördlingen drill core can be subdivided into four suevite units: 1) Upper redeposited suevite (296-314m), 2) "Graded" suevite (314-330m), 3) Melt-rich suevite (331-520m), and 4) Melt poor suevite (520-602m). The Enkingen drill core can be subdivided into four units: 1) Upper suevite (21-40m), 2) Middle suevite (40-66m), 3) Lower suevite with intersection of coherent melt layers (66-86m), and 4) coherent impact melt (below 86m). The Wörnitzostheim drill core can be subdivided into three units: 1) Upper suevite (19-25m), 2) Melt-rich suevite (25-80m), 3) Melt-poor suevite (80-100m). The Otting drill core is homogenous. A comparison of the characteristics of all drill cores can be summarized as follows: 1) The suevite of