Compared to breccias in other impact craters, the Chicxulub breccias are incredibly rich in silicate melt fragments (up to 84% versus 30 to 50%, for example, in the Ries). The sequence and composition of impact melts and breccias are grossly similar to those in the Yucatán-6 hole. The impact melt and breccias in the Yax-1 borehole are 100 m thick, which is approximately 1/5 the thickness of breccias and melts exposed in the Yucatán-6 exploration hole, which is also thought to be located between the peak ring and final rim of the Chicxulub crater. It is assumed that the impact had considerable influence on the Mid-Tertiary regional geology of the Iberian System, and we suggest that respective geologic models which have so far not considered this peculiar and far-reaching event, need considerable revision.Ībstract- The Chicxulub Scientific Drilling Project (CSDP), Mexico, produced a continuous core of material from depths of 404 to 1511 m in the Yaxcopoil-1 (Yax-1) borehole, revealing (top to bottom) Tertiary marine sediments, polymict breccias, an impact melt unit, and one or more blocks of Cretaceous target sediments that are crosscut with impact-generated dikes, in a region that lies between the peak ring and final crater rim. Glassy amorphous carbon particles in a solid C-O compound may be related with fullerenes and may originate from a quenched melt of extremely shocked coal or from extremely shocked limestones. The most striking impact evidence for both structures is given by strong shock metamorphism, including melt and diaplectic glass, planar deformation features (PDFs), different kinds of impact melt rocks (from former silicate melt, carbonate melt, carbonate-phosphate melt) and suevite breccias. Geological mapping has established abundant geologic impact evidence in the form of monomictic and polymictic breccias and breccia dikes, megabreccias, dislocated megablocks, remarkable structural features, extensive impact ejecta and impact signatures even in distant autochthonous deposits. From stratigraphic considerations and palaeontologic dating, an Upper Eocene or Oligocene age is very probable. Both structures have diameters of roughly 35 - 40 km and they have been formed in a purely sedimentary target. We report on the Azuara impact structure and its Rubielos de la Cérida companion crater, which establish the largest terrestrial doublet impact structure presently known. It is proposed that ramparts may result from enhanced shear localization and a stacking of ejecta material along internal glide planes at decreasing flow rates when the flow begins to freeze below a certain yield stress. In analogy with Martian fluidized ejecta blankets, it is suggested that the large runout was related to subsurface volatiles and the presence of basal glide planes, and was influenced by eroded bedrock lithologies. Systematic measurement of motion indicators revealed that the flow was deviated by a preexisting karst relief. Abundance of glide planes within the ejecta and particle abrasion both rise with crater distance, which implies a ground-hugging, erosive, and cohesive secondary ejecta flow. The ejecta blanket of the Chicxulub crater was identified on the southeastern Yucatán Peninsula at distances of 3.0–5.0 crater radii from the impact center. Detachment faulting may also occur in the periphery of Martian impacts and could be responsible for the formation of lobe-parallel ridges and furrows in the inner layer of double-layer and multiple-layer ejecta craters. Both processes are enhanced in rheologically layered targets and in the presence of fluids. Their formation and associated radial outward shearing was caused by weak spallation and subsequent dragging during deposition of the ejecta curtain. Recent field analysis of the Ries crater has revealed the existence of subhorizontal shear planes (detachments) in the periphery of the crater beneath the ejecta blanket at 0.9–1.8 crater radii distance. Here we discuss two examples that may yield implications for Martian craters: 1. Terrestrial impact structures provide field evidence for cratering processes on planetary bodies that have an atmosphere and volatiles in the target rocks.
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