Syn-rift diagenesis of Middle Miocene carbonate platforms on the north-western Red Sea coast, Egypt

Abstract

The mid-Miocene carbonates exposed along the north- western coastal region of the Red Sea and southern Gulf of Suez, although relatively thin (less than 150 m), are highly modified by a series of Miocene diagenetic phases. Developed under near-surface conditions, the general diagenetic evolution began with marine cementation of slope deposits. This was followed by massive dolomitization and dissolution of aragonitic debris. The geometry of the dolomitized strata does not appear to be related to specific sequence boundaries or faults and its pervasive nature, negative oxygen isotopes and associated hydrocarbons all suggest dolomitization from heated basinal sea-waters. A later diagenetic phase relating to the penetration of sulphate-rich brines into the dolomite substratum is expressed by a polyphased diagenesis involving dolomite dissolution, aragonite precipitation and dolomitization, celestite cementation and dissolution. Large volumes of secondary sulphate have replaced the dolomite substratum.

The phases of diagenesis, most of which were related to the lateral and vertical movement of large volumes of marine waters, were probably stimulated by variations in density aided by a relatively high temperature gradient. The delivery of these marine waters to the interstitial pore-system was facilitated by the inclined nature of the sediment/water interfaces. These factors, and the resultant diagenesis, are a natural consequence of rift basins.