Termite nests, rhizoliths and pedotypes of the Oligocene fluviomarine rock sequence in northern Egypt: Proxies for Tethyan tropical palaeoclimates

The present study gives an account of the development of several associations of the continental trace fossils within a particular type of palaeosols on the Oligocene fluviomarine rock sequence that is equivalent to the Jebel Qatrani Formation and their palaeoclimatic and palaeoenvironmental implications in the northern Western Desert of Egypt. This study made use of field observations, binocular and polarizing microscopy investigation of isotropism and anisotropism of the materials and the birefringence fabric of the finest materials. Also, the study carried out scanning electron microscopy investigation and X-ray diffraction analysis of termite nests, rhizoliths and palaeosols. The present contribution recognises four pedotypes represented by Oxisols, Aridisols, Entisols and Spodosols based on the macro- and micromorphologic attributes, mineralogy and by comparison with extant soils. The four pedotypes bear similar types of rhizoliths and termite nests, but with variable abundance and depths of emplacement. The pedogenic features associated with the studied continental trace fossils overprint the marine trace fossils and the sedimentary facies of the studied fluviomarine rock sequence. The features of the developed associations reveal that during the Oligocene, the northern Western Desert terrain of Egypt was closely similar to the Fayum Depression where the lowland coastal plain was characterised by wet soil and seasonal rainfall. The developed associations on the coastal plain represent in situ proxies for the palaeoclimatic setting under which they were formed. Their attributes indicate that tropical wet-dry conditions with the fluctuating moisture regime were prevalent during the deposition of the studied rock sequence.