ABSTRACT: Diagenesis of Diagenetic Quartzarenites, Gebel El-Zeit Area, Gulf of Suez, Egypt

Antar A. Abdel-Wahab, Earle F. McBride

At the northern corner of El Zeit Range, 370 m of sandstone overlie Precambrian granite and underlie Cretaceous marine strata. The sandstones include the marine Lower Cambrian Araba Formation and the overlying dominantly fluvial Upper Cambrian(?) Naqus Formation. The framework composition of both sandstones is almost entirely quartz with trace amounts of muscovite, K-feldspar, and heavy minerals. Up to 18% oversize pores, some filled with younger cements, attest to extensive dissolution loss of detrital grains. Because the final mineralogical maturation of these quartzarenites was through diagenesis, they are diagenetic quartzarenites.

During burial diagenesis, the introduction of 5% quartz cement (unhomogeneously distributed) was followed by locally pore-occluding calcite cement. The sands were then compacted sufficiently to develop slightly sutured quartz grain contacts. These events were followed by extensive dissolution loss of carbonate cement, detrital feldspars, micas, and heavy minerals; by the formation of local patches of kaolinite (average = 6%); and by the formation of extensive iron-oxide cement, including specular hematite. These features suggest extensive invasion of meteoric water and exposure to oxidizing conditions. The time of this event can only be dated as post-Cambrian(?) and pre-Cenomanian. Some outcrop samples contain pore-occluding gypsum cement or mixtures of gypsum and halite cement. Sr87/Sr⁸⁶ ratios of four samples of gypsum cement have two modes equal to 0.7079 and 0.7085. These values are those of Miocene and slightly younger seawater. We suggest that evaporites were leached by modern meteoric water from nearby exposed Miocene and younger marine evaporite-bearing strata, transported in surface and groundwater to the topographically low terrain where the Cambrian sandstones crop out, and were precipitated by evaporation of surface water. The possibility also exists that some salt was precipitated during flooding from the Gulf of Suez in the Pleistocene or Holocene.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990