Evaporitic and biosiliceous cyclic sedimentation in the Miocene of the Gulf of Suez—Depositional and diagenetic aspects

Abstract

The Middle to Late Miocene evaporite-bearing formations which outcrop along the Gebel Zeit and Gems highs in the southwestern Gulf of Suez (Egypt), consist of thick calcium sulphate beds rhythmically interbedded with marlstones, siltstones and claystones containing various amounts of biogenic silica, as well as pure diatomites. The calcium sulphate beds are composed mainly of gypsum and anhydrite. Subaqueous crustallisation in subpermanent brine ponds of marine origin predominated during deposition of the selenite and laminated gypsum. Early diagenetic interstitial growth of nodular anhydrite occurred episodically when the water level dropped, in response to increasing salinity associated with the precipitation of halite in the deeper troughs. Nevertheless, most of the nodular facies resulted from the burial conversion of gypsum to anhydrite which has been sometimes rehydrated into gypsum. A last phase of gypsum dehydration has occurred at the surface, related to the present hot and dry climatic conditions. The biosiliceous deposits contain either well preserved (opal-A) or diagenetically altered (opal-CT, clinoptilolite, quartz and even clays) diatom frustules. Such diagenetic changes are commonly described in the oceanic biosiliceous deposits. The fossil content of these diatomitic layers implies shallow-marine conditions on the shelves of highs induced by block faulting. The cyclic succession of evaporited and biosiliceous deposits recorded the alternation of high sea level stands associated with high organic productivity, probably enhanced by seasonal input of nutrients and low sea level stands associated with hypersaline conditions and evaporite deposition. Locally, the sharp contact between the diatomites and gypsum argues for a rapid evolution of salinity towards high concentration (up to gypsum saturation and more). The formation of finely laminated diatomitic sediments is known to require either high organic productivity or anoxic conditions in bottom/intermediate waters, and eventually both processes. In the Gebel Zeit and Gemsa areas, a local association of the biosiliceous sediments with diagenetic carbonates and native sulphur, resulting from processes of bacterial sulphate reduction, confirms that anoxic conditions occurred in the deep-water body or in the sediments. Large amounts of organic compounds were involved in these diagenetic processes. Immature organic matter was consumed during an early diagenetic stage of transformation. It is assumed that this organic matter was supplied either by the biosilica-rich interbeds or the organic-rich laminated carbonates. The initial organic content of the biogenic deposits was impoverished as early as the first stages of sedimentation and diagenesis. Although a large part of the organic matter could have been destroyed by these early diagenetic processes and further oxidation in outcrops, these sediments can be considered as potential source rocks in nearby less exposed sequences.