TITLE:
Microspherules-Bearing White Sandstone: Implication of Cosmic Impact Event near Jurassic-Cretaceous Boundary in West Central Sinai, Egypt
AUTHORS:
Hanan S. Badawy, Ahmed M. Zayed, Mohamed G. Shahien
KEYWORDS:
Impact Glass, Microtektites, Ejecta, Nubian Sandstone, SiC
JOURNAL NAME:
Journal of Geoscience and Environment Protection,
Vol.4 No.7,
July
25,
2016
ABSTRACT: The presence of glass microspherules enclosing
relict grains, shattered quartz and silicon carbide in white sandstone beds
near the Jurassic-Cretaceous boundary in west central Sinai indicates a cosmic
impact event. Characterization of the impact microspherules and proposing a
reasonable scenario for their origin are the aims of this work. Field
observations, optical, binocular, scanning electron and high-resolution
transmitted electron microscopy investigations and chemical analyses were
carried out. The study revealed that glass microspherules have high Al2O3 and FeO contents and low CaO and MgO contents. The high content of Al2O3 indicates that the source of microtektite-like microspherules is attributed to
the melting of a clay-rich sandstone and carbonaceous matter, while the high
content of FeO indicates admixing with projectile matter. The reaction between
silica and carbon was carried out under conditions of high temperature (T >
1000°C) and carbon (C/Si > 1) which resulted in the production of silicon
carbide with microdiamond intergrowth. Consequently, this intergrowth is in
accordance with the impact origin via rapid condensation and growth within a
vapor phase. In spite of the fact that no source crater has been recognized to
date in the study area, the authors propose at least a single cosmic impact
event scenario for the recorded glass microspherules in west central Sinai. The
impact excavated the Paleozoic siliciclastic sedimentary rocks and then the
glass microspherules showered the area of study. The deposition of
microtektite-like glass particles within the white sandstone beds of the Malha
Formation took place in the fluvial plain terrestrial environment. This setting
precluded severe post-depositional reworking, yielding preservation of the
glass particles in a primary layer. Eventually, lateral migration of the
braided channels led to the reworking of the microspherules layer and the
spatial dispersal of the shattered quartz.