Patterned dolomites: microbial origins and clues to vanished evaporites in the Arab Formation, Upper Jurassic, Arabian Gulf

Abstract

Abstract The common occurrence of patterned dolomites in the upper Arab Formation of the Arabian Gulf is highlighted. Their mottled appearance is due to concentrations of microcrystalline iron sulphide. The dolomites are typically devoid of bioclastic debris and are usually interbedded with stromatolites and thin carbonates, both of which commonly contain anhydrite nodules. Previous workers have interpreted the patterned appearance as originating from birds-eye porosity, burrowing activity or plant roots, but their mottled appearance, and possibly also the dolomite, probably originated as a by-product of the activities of sulphate-reducing bacteria in water-logged or subaqueous sediments on the floors of salinas or very highly saline lagoons from which dissolved sulphate and/or gypsum had been removed. This implies that the Arab Formation was originally locally even more gypsiferous than is evident from its existing bedded anhydrites. The related processes of sulphate removal and sulphide formation were penecontemporaneous, as shown by occurrences of the mottled fabrics as intraclasts. Contorted lamination within the patterned dolomites and their frequent involvement with sediment injection structures are evidence of fluidization. Reasons for the contortion and fluidization are considered with respect to compatible Holocene depositional and diagenetic environments. In addition to the comparatively simple depositional models of extensively prograding supratidal sabkhas, which are traditionally applied to facies associations of stromatolites and bedded anhydrites in the Arab Formation, the presence of patterned dolomites demands consideration of less continuous and static facies belts with reduced correlatability that imply higher levels of reservoir heterogeneity.