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

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.

The role of bacterial sulphate reduction in carbonate replacement of vanished evaporites: examples from the Holocene, Jurassic and Neoarchaean

“…The absence of dolomite fabrics crosscutting facies boundaries argues against a seepage reflux origin for these dolostones. It is more likely that the pyrite is related to sulphate reduction of sulphate dissolved in the hypersaline, ambient waters and on discrete gypsum and/or anhydrite crystals in association with abundant organic matter in the form of degraded cyanobacteria (Berner et al, 1979;Kirkham, 2004). BSR operating in a hypersaline, microbially-dominated environment can drive dolomite formation (e.g.…”

“…BSR operating in a hypersaline, microbially-dominated environment can drive dolomite formation (e.g. Gunatilaka et al, 1984;Vasconcelos & McKenzie, 1997;Wright, 1999;Wright & Wacey, 2004, and the patterned dolomite may thus represent vanished evaporites interbedded with microbial carbonates (Kirkham, 2004).…”

“…Selenite crystals in nucleation cones form clusters oriented normal and oblique to bedding. Bacterial sulphate reduction and vanished evaporites 305 ductile and soft-sediment deformation of laminae (Kirkham, 2004).…”

“…Taberner et al, 1999;Kirkham, 2004;Gandin et al, 2005), especially in the case of endoevaporitic microbial sediments. Although SRB can exploit solid as well as dissolved sulphate (e.g.…”

Facies stacking patterns in a modern arid environment: a case study of the Abu Dhabi sabkha in the vicinity of Al‐Qanatir Island, United Arab Emirates

“…However, it is important to note that most of the thick ancient anhydrite deposits, like those of the Upper Permian (Khuff and Zechstein formations) and the Upper Jurassic (including the Arab Formation), mostly represent salina-or saltern-type (Warren, 1983(Warren, , 1989(Warren, , 1999Warren & Kendall, 1985;Schreiber, 1988;Kendall, 1992) rather than sabkha-type deposits (Al- Silwadi et al, 1996;Strohmenger et al, 1996;Steinhoff & Strohmenger, 1999;Kirkham, 2004).…”