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Messinian RST's: The Key to Exploration Future Success in Nile Delta (Egypt) Incised Valleys

Abstract

The Egyptian Late Miocene stratigraphy directly records the fluvial channel cut and fill stages which bracket the Messinian crisis. High resolution sequence stratigraphy (detailed seismic facies analysis coupled with depositional log motifs) allows the identification of downstepping fluvial terraces within incised valleys. These fluvial terraces represent the preservation of Regressive Systems Tracts (RST) which record the fall of relative fluvial base level (sea level) on a coastal plain. During the early Messinian, owing to a global sea level fall, the Qawasim sequence records the Nile delta progradation and channel downcutting on the coastal plain with deposition of fluvial RST terraces. At its lowest level, Lowstand Systems Tract (LST) channel lags occur. With a subsequent global sea level rise, as the Mediterranean is still connected, the Nile delta retrogrades with subsequent channel flooding with Transgressive Systems Tract (TST) shales culminating with a Highstand Systems Tract (HST) shale. However, around 5.95 Ma, the global and Egyptian sea level curves begin to diverge with the Egyptian Sea Level beginning to fall which is recorded by the Late Messinian Abu Madi Sequence at 5.59 Ma. As the Mediterranean Sea continued to lower Nile fluvial base levels, channels downcut with Abu Madi terraces ending with channel lags. Suddenly at about 5.33 Ma, the Egyptian sea level curve begins a rapid rise with the Nile Delta rapidly retrograding and the deposition of transgressive prodelta shales which culminate with widespread marine shale deposition accompanying the reconnection of the Mediterranean with the global ocean. These TST events accompanying a catastrophic flooding event end with widespread HST shales which provide excellent seals for hydrocarbon entrapment in the preserved fluvial terraces as evidenced by commercial gas production within. The construction of an Egyptian Messinian sea level curve which brackets the Messinian Crisis provides potential utility for correlation to other Mediterranean depositional systems if similar high resolution sequence defined relative sea level curves are constructed. However, the exploration of the Messinian Nile Delta suffers from a complexity of stratigraphic relations within two major Messinian formations (Qawasim and Abu Madi). The identification of the fluvial sequence stages represents the key for optimizing future exploration in Mediterranean fluvial systems during the Messinian Crisis.