The Dolomitization in the Deltaic System of Meloussi Formation (Upper Berriasian-Valanginian) in Jebel Meloussi (Central Tunisia)

Abstract

The present work aims at studying the dolostones of the Meloussi Formation (Upper Berriasian-Valanginian) in Jebel Meloussi (Central Tunisia) which is considered as a paleodeltaic system M’Rabet [1]. The study is based on the analysis of dolostones sampled along the Khanguet Zebbag section. Various analyses have been achieved on the dolomite samples in order to reconstruct the environment and the mechanisms of dolomitization. The dolostones include alternating regressive sandy dolomites and transgressive fossiliferous dolomites. The optic microscope and SEM observations show three main petrotypes of dolomite: replacive dolomicrite (P1) associated to the sandy dolostones beds, recrystallized subhedral dolomicrosparite (P2) and cementing euhedral dolosparite (P3) associated to the fossiliferous dolostones. The P1 petrotype resulted from mixing waters dolomitization during the regressive intervals and delta progradation as shown by the significant concentration of Fe and Mn and the negative values of δ¹⁸O and δ¹³C. The P2 resulted from a late burial recrystallization but it should have been initially developed in a rather marine environment during the transgressive intervals and the delta retrogradation as testified by the high amount of Sr, Na and Mg versus low amount of Fe and Mn and by the slightly negative δ¹⁸O and δ¹³C. The P3 represents late burial dolomite cement resulting from dolomite precipitation within marine water as attested by the very high concentration of Sr and Na in comparison with those of Fe and Mn and the moderately negative to positive δ¹⁸O and δ¹³C.