BP and Oasis impact structures, Libya, and their relation to Libyan Desert Glass

Abstract

We have conducted petrographic, geochemical, and isotopic studies on a suite of target rock samples from two impact structures, BP and Oasis, in southeastern Libya. Both structures occur in Lower Cretaceous sandstone of the Nubia Group and are deeply eroded. Earlier microscopic thin-section studies provided evidence for the impact origin of the structures by demonstrating the presence of shock-characteristic planar deformation features in quartz. No absolute ages have yet been determined for these structures. From the proximity of these structures to the occurrence of the enigmatic Libyan Desert Glass (LDG), previously interpreted as impact glasses formed from a mature sandstone, and from the absence of disturbed strata of the sandstone of the Nubia Group in the area of the occurrence of the LDG, a possible relation was suggested between BP and Oasis impact structures and the LDG. Most of the target rocks at both structures have somewhat lower SiO2 and higher contents of other major oxides than LDG, but this distinction disappears when the composition of the sandstones is recalculated to a water-free basis. Also, there is a good correlation between the major and trace element compositions of samples from the target rocks of BP and Oasis structures and samples of LDG, but the refractory trace element content of the LDG is generally somewhat higher than that of the target rocks of the two Libyan craters. Rare earth elements (REE) from the BP and Oasis impact structures and from LDG have similar abundances and display a similar chondrite-normalized pattern. The isotopic ratios of Nd and Sr for samples of the two structures and for the LDG are in a similar range. They are characterized by negative eNd values and positive eSr values, which are characteristic of upper continental crustal rocks. In an eNd vs. eSr diagram, LDG values plot within the field defined by the BP and Oasis rocks, and in a 1/Sr vs. eSr plot LDG is within the range defined by BP and Oasis target rocks. From the available petrographic, geochemical, and isotopic data, we conclude that the target rocks of the Libyan structures could represent the parent material for LDG. However, without further age information it is not possible to conclude unambiguously that the BP or Oasis structures are the source craters for the LDG. Abate, B., Koeberl, C., Kruger, F. J., and Underwood, J. R., Jr., 1999, BP and Oasis impact structures, Libya, and their relation to Libyan Desert Glass, in Dressler, B. O., and Sharpton, V. L., eds., Large Meteorite Impacts and Planetary Evolution II: Boulder, Colorado, Geological Society of America Special Paper 339. 177 * Present addresses: (Abate) Ethiopian Institute of Geological Surveys, P.O. Box 30389, Addis Ababa, Ethiopia; (Underwood) 9518 Topridge Dr. #3, Balcones Place, Austin, TX 78750-3500, USA.