SEDIMENTOLOGY OF THE PALEOCENE – EARLY EOCENE SEQUENCE, SOUTHWESTERN IRAQ

Abstract

Facies analysis, depositional environments and diagenesis of the Paleocene – early Eocene sequence at southwestern Iraq is studied in ten boreholes. The whole sequence is carbonate – evaporite bounded by unconformities and incorporates Umm Er Radhuma, Rus and Jil Formations. The sequence comprises grey clayey dolomite, dolomitic marl, nodular or bedded evaporites, and crystalline limestone representing vanished evaporites and dedolomite. Six lithofacies are recognized in the studied sequence extending from shore towards the basin, these lithofacies are: 1) Evaporites Lithofacies; 2) Laminated Dolomite Lithofacies; 3) Non-laminated Dolomite Lithofacies; 4) Sucrose Dolomite Lithofacies, 5) Massive Fossiliferous Dolomite Lithofacies and (6) Phosphatic Arenite Lithofacies. The evaporites are formed in a lagoon as sabkha cycles and subaqueous shallow marine evaporite. Possibly the evaporite nodules grew under the sediment/water interface under the lagoon. The sulphate could have grown by mixing of waters of different salinities. The evaporitic basin seems to have shifted its geographic position with time. Several diagenetic processes affected the sequence, such as neomorphic replacement, dissolution, dolomitization, sulphate development, vanished evaporites and dedolomitization. Some of these processes obliterated the primary textures.