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Nummulite banks which are common in neritic and shelfedge facies in many parts of the Tethyan Eocene have been mainly regarded as reef-type buildups so far. However, stratification and biofabrics of such banks in the middle Eocene around Cairo demonstrate the importance of physical processes in molding nummulitic sediment bodies.
Initiation of a nummulite bank at the Giza Pyramids Plateau is localized by a preexisting paleohigh, inherited from Late Cretaceous tectonism. On this "submarine swell" (about 1 × 1.5 km wide), ecological conditions were optimal for a flourishing Nummulites gizehensis-community, resulting in greater sediment production than in adjacent environments. Growth of the nummulite bank into a sediment body over 30 m (98 ft) in thickness and more than 1 km (.62 mi) in length is strongly enhanced by mechanical concentration of nummulite tests into coquinal packstones. These are interpreted to be a product of storm-generated winnowing. Paleoecological evidence shows that nummulite banks are largely an in-situ lag deposit. Periods of nummulite settlement are episodically disturbed by "catast ophic" storm events, which result in winnowing and local accumulation of the heavier bioclasts. Upward growth of the banks into shallower water is reflected by an increase in winnowed fabrics and by a cap of shoal calcarenites. During shallowing, patch reefs and a back-bank lagoon formed on the landward side of the bank.
This facies association may be regarded as a model for hydrocarbon reservoirs. The high intraparticle porosity in nummulite tests (54%) makes the banks a potential reservoir, while adjacent and overlying lagoonal mudstone and wackestone may serve as source and cap rocks.
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