Ries Crater and Suevite Revisited : Part Ii Modelling

Introduction: At the 26-km-diameter Ries impact structure (Germany) suevite occurs in three different geological settings [1]: (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 (" outer suevite " = OS), and (3) dikes in the crater basement and in displaced megablocks. In this paper we will discuss the first two types of suevite. Previous concepts of the genesis of suevite types: A first concept for the Ries suevite genesis was presented in 1977 [2], with some later modifications in [3, 4]. There was agreement that the OS is derived from the ejecta plume and deposited " non-ballistically " as fallback material from the plume. The interpretation of the CS, subdivided from top to bottom into units A, B, and C (see [1]), was less straightforward. In [4] it was suggested that units B and C had never been ejected from the transient crater cavity (TC), whereas [3] interpreted unit B as fallback material from the plume and unit C as material ground-surged along the floor of the transient cavity (TC). The graded suevite of unit A was seen as true fallback material [2, 5]. Kieffer and Simonds [6] suggested that impacts into volatile-rich material may result in the dispersion of impact melt, the formation of suevite, and, finally, in impact melt deficiency in comparison with craters in crystalline rocks. With reference to this idea, Newsom et al. [7] pointed out a striking similarity between de-gassing pipes in Ries suevite with features observed in ignimbrites and pyroclastic flows. They argued that the Ries suevite represented fluidized ejecta with volatiles released from molten basement rocks (0.8 wt% of water in igneous rocks and 2.5 wt% of water in metamor-phic rocks). Such a water content in the flow allowed " fluidization " but not sorting of suevite. At a later stage, suevite had been subjected to substantial hydro-thermal alteration. Water derived from underlying Bunte Breccia (continuous ejecta) was suggested as an alternative source of volatiles [8]. Recently it was proposed [9,10] that surficial (outer) suevites of the Ries had been emplaced as ground-hugging impact melt flows during the modification stage of crater formation. This hypothesis is consistent with the fact that these rocks overlie ballistic ejecta (Bunte Breccia), without any mixing between them. However, the substantial clastic component in suevite …