Fossil natural glasses composed of ferric oxyhydroxides: impactites of the 35.5 million year old Chesapeake Bay crater

Abstract Many of the pebbles and cobbles found in the ∼5000-km 2 ‘upland deposits’ of eastern Virginia and Southern Maryland (USA) are decorated with external flanges of hard red–brown material with uniform thicknesses ∼5 mm terminated by crisp conchoidal fracture surfaces. Thirteen high-grade sandstone (quartzite) pebbles and cobbles exhibiting such features were cut with rock saws, revealing that the external flanges are contiguous with undulating red–brown bands of about the same thicknesses extending all the way through their interiors and, in several cases, that these interior bands are approximately parallel to spalled surfaces and/or internal fractures. These rocks have been studied by thin-section photomicroscopy, scanning electron microscopy, extended X-ray absorption fine structure (EXAFS), X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron-spin-resonance imaging, and Mossbauer spectrometry. The EXAFS, XRF, XRD, and Mossbauer data combine to demonstrate that the hard red–brown materials contain goethite (α-FeOOH) in particle sizes ∼100–150 A with impurity contents ⩽5%. It is not ruled out that these materials may also include an amorphous-ferric-oxyhydroxide component with short-range order constrained by the EXAFS to be similar to that in goethite. The origin of the upland deposits has long posed a riddle to geologists. And the geometry of the red–brown bands internal to the present rocks (not explainable by aqueous diffusion) now confronts materials scientists with a puzzle. We propose that the solutions to both the riddle and the puzzle may be linked to the effects of the known impact of an extraterrestrial object 35.5 million years ago into the area that is now the Chesapeake Bay (CB) on the US Atlantic Coastal Plain. We strengthen this proposition with (1) critical reconsideration of the previously accepted geological explanation of the upland deposits, (2) an assessment of the probable natures of the materials present in the CB-crater target area that would have become ejecta, (3) a review of the pertinent physics of impact cratering, (4) the detailed studies of the rocks mentioned above, and (5) a preliminary study of a sample of goethite subjected to experimental shocking. Based on these considerations, the hard red–brown materials of the upland deposits are proposed to have been quenched from molten sheets of Fe-oxyhydroxides generated by impact-induced shock waves passing through water rich in suspended ferric-oxyhydroxide particles. A model for the physics of this process is presented. It is reasonably assumed that iron-rich ground waters were present in aquifers extending into the ∼500-m-deep accumulation of quartz-based sediments known to have been in the target area of the CB impactor. Penetration of the molten Fe-oxyhydroxide sheets into the quartzite cobbles is ascribed to opening of interstitial spaces between the constituent sand grains by action of reverberating pressure and rarefaction components of the same shock waves that boosted these rocks to their present-day upland locations.