Microfacies, diagenesis, and depositional environments of the Tertiary carbonates of Usfan Formation in Haddat Ash Sham area, Western Arabian Shield, Saudi Arabia

Abstract

The studied carbonates are present within the middle part of the Tertiary siliciclastic dominated succession in Haddat Ash Sham area. This succession consists of conglomerates, sandstones, siltstones, mudstones, and ironstones arranged in upward coarsening and fining cycles of intermittent depositional environments ranging from fluvial coastal plain to shallow marine. The carbonates represent deposition in slightly restricted depositional environments during periods of very low clastic input. The studied two carbonate sections show a pronounced lateral and vertical changes from predominantly dolostones in the west to mixed dolostones and limestones in the east. Both sections began by egg yellow dolomitic lime mudstones and fine crystalline dolomite which indicate the formation of dolomite by late depositional to early diagenetic recrystallization of precursor Fe and Mg lime mud. During this time period, the western part of the study area continues in the deposition of Fe and Mg lime mud in slightly restricted depositional environments and formation of interbedded fine and coarse crystalline dolostones in coarsening-upward cycles. In the eastern part, the depositional environments become deeper and more circulated with short-lived periods of fluctuation in sea level and hence the deposition of foraminiferal wackestone, bioclastic packstones, and bioclastic rudstone in coarsening and shallowing-upward cycles. Both the western and eastern parts of the study area are terminated by characteristic yellow coarse crystalline phosphatic dolomite which indicate ultimate stages of shoaling and increasing the level and concentration of phosphorous in the restricted water and hence the formation of in situ and reworked phosphatic components. The diagenetic processes affected the dolostones and limestones microfacies associations include: dolomitization of Fe and Mg-rich lime mud, hematitization, calcitization, gypsification, and phosphatization of dolomite. Neophormation and recrystallization of micritic matrix and micritic intraclasts as well as shell fragments are the main diagenetic processes affected the limestones.