Diagenesis and Porosity Evolution of Upper Cretaceous Dolomites, Northern Egypt

Hanafy M. Holail, Kyger C Lohmann

Porosity patterns in the Upper Cretaceous marine carbonate sequence of northern Egypt record a complex history of diagenesis in marine, hypersaline, and meteoric waters. Depositional facies and fabrics suggest an overall upward-shallowing sequence which, in turn, is bounded above by a major pre-middle Eocene unconformity. Associated with this unconformity, subaerial exposure resulted in a regional influx of meteoric waters. In this context, analysis of the paragenetic relationships and geochemical character of diagenetic phases has allowed for unique determination of the timing and environments of porosity evolution within this sequence.

Shallow shelf sequences have been partially to pervasively dolomitized. On the basis of petrographic, cathodoluminescence, minor element, and isotopic criteria, three distinct dolomite types are present. Within the red bed series, microcrystalline dolomite formed syndepositionally from hypersaline marine fluids. This dolomite is distinct from zoned, Fe²⁺-rich dolomite which comprises the bulk of dolomite within the Cenomanian and Campanian strata. Isotopic and minor element compositions of coarse, zoned dolomites implicate an early replacement of limestones in the presence of marine fluids during burial. This is followed by a later stage of dolomite which is superimposed on early marine dolomite cores as (1) overgrowth rims, (2) corrosive, partial replacement of dolomite co es, and (3) inter-rhomb cements. On the basis of stratigraphic distribution and geochemistry, this episode of dolomitization and a related calcite cement are due to an early influx of meteoric waters. Moreover, diagnostic variations in isotopic and minor element compositions indicate a mixed, meteoric-marine diagenetic setting for this dolomite phase. Progressive dominance by meteoric waters is evidenced by a late calcite stage which occurs both as replacive dedolomite and as intergranular sparry cements.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.