Reappraisal of strain estimations and measurement methods in the Hammamat Group sediments: Comparison of primary and secondary grain fabrics with new data from Wadi Zeidun and Wadi Arak basins, CED, Egypt

Abstract

In this study, pebble and sand grain shape and orientation data from undeformed and deformed conglomerates and sandstones in the Egyptian Central Eastern Desert (CED) Wadi Zeidun and Wadi Arak basins were collected, with the aim of providing more accurate tectonic strain estimations for deformed Hammamat sediments by comparing their primary and secondary sedimentary fabrics. Common assumptions about primary pebble fabrics and pebble deformation behaviour in previous strain studies of the region are shown to be invalid. This raises doubts about the accuracy of earlier reported strain estimates for the Hammamat that were based on such assumptions. For low strains in the Hammamat sediments, the best strain measurement method is normalized Fry (NFry) analysis of sand grain centre-to-centre distance data. An alternative is conventional Fry analysis of pebble ellipse centre-to-centre distance data, provided that large numbers of pebbles (∼300) are used, and the pebbles form closely packed aggregates with high degrees of anticlustering. This study also provides systematic guidelines for pebble axial length and orientation measurements in the field. The progressive changes in the mechanical behaviour of pebbles with increasing bulk strain are discussed.