High Mg-glauconite in the Campanian Duwi Formation of Abu Tartur Plateau, Egypt and its implications

Abstract

This study presents a detailed crystallo-chemical characterization of glauconites occurring at different stratigraphic intervals within the phosphorite-bearing Campanian Duwi Formation. While authigenic glauconites occur within the shallow marine, transgressive deposits of Abu Tartur Phosphate and Maghrabi Shale Members in the Campanian Duwi Formation, allogenic variety occurs within the regressive deposits at the top of the Liffiya Shale Member. Nascent to slightly evolved, shallow-marine originated authigenic glauconites are distinct by a high content of MgO. A good correlation between K2O and Fe2O3(total)contents of glauconites in the Maghrabi Shale supports ‘layer lattice’ theory while a combination of ‘layer lattice’ and ‘verdissement’ theories explain the origin of other glauconites within the Duwi Formation. X-Ray Diffraction studies reveal a ‘true’ di-octahedral nature and 1M ordering of the glauconite pellets with smectite inter-stratification. Mössbauer spectroscopic study reveals Fe3+ as the dominant cation in the octahedral site that preferentially occupies the less distorted cis M(2)sites. Glauconites acquire a high Mg-content because of the passage of Mg- and Ca-rich fluid that leads to extensive dolomitization. Glauconites form at the transition zone between black shale and phosphate, representing oxygen-depleted conditions. Similar shallow marine glauconites occur all along the paleo-Tethyan margin during the Late Cretaceous time.