Modes of origin and genetic pathways of peloids within the Duwi Formation (late Cretaceous) in Eastern and Western deserts of Egypt: A petrologic perspective

Abstract

Peloids are considered as major constituents of modern and ancient sedimentary rocks, although their origins and pathways of formation are not yet comprehensively understood. Based on field and petrologic investigations, peloids have been recorded in the Duwi Formation (late Cretaceous) in both Eastern and Western deserts of Egypt. The Duwi Formation represents the first onset of fully marine conditions in Egypt accompanying the major late Cretaceous marine transgression. It comprises a heterogeneous suite of shallow marine sediments (phosphorites, oyster limestones, shales, glauconites and dolomite). According to the hosted rock, peloids are classified into two types: peloids in phosphorites and peloids in limestones, which are characterized by different shapes and sizes. Systematic study indicates that peloids resulted from the interplay between physico-chemical and microbial processes. Accordingly, the more important modes of origin are peloids derived from coprolites; by micritization of bivalve shells; fragmentation and abrasion of intraclasts; by phosphomicritization of bone fragments; peloids derived from internal molds; intraskeletal and interskeletal peloids; nucleated peloids; and peloids within matrices. The size and morphology of the studied peloids are mainly controlled by the parent material.