Cyclic hierarchy and depositional sequences of the Middle-Upper Eocene ramp facies: An example from Beni Suef area, east Nile Valley, Egypt

摘要

The present work addresses the cyclo- and sequence stratigraphy of the Middle-Upper Eocene ramp facies outcropping on the eastern side of the Nile Valley in the vicinity of Beni Suef City. The Eocene rocks in the considered area have been subdivided into three rocks units; El Fashn Formation (Late Lutetian-Bartonian), Beni Suef and Maadi formations (Priabonian). Nineteen microfacies types were identified; their depositional environments show a gradual change from the inner to outer ramp depositional settings. The studied mixed siliciclastic-carbonate deposits are organized in small-scale cycles (i.e. peritidal, shallow subtidal, deep subtidal and exposed subtidal cycles), which are packaged into medium-scale cycles (cycle sets) and large-scale cycles (depositional sequences). These cycles demonstrate the interplaying relationship between the accommodation space, sediment supply and the small-scale sea-level fluctuations. The studied sedimentary record displays a conspicuous facies variation that is produced by both the auto- and allocyclic mechanisms. The autogenic cycles are generated by processes acted within the basin of deposition (tidal flat progradation, water energy and change in the rate of carbonate production), while the allogenic cycles are controlled mainly by both the sea-level eustasy and local tectonic activities. Four discontinuity horizons implying sea-level falls are recognized. These are the Late Lutetian-Early Bartonian/Early Bartonian (SB-1), Early Bartonian/Late Batronian (SB-2), Late Bartonian/Early Priabonian (SB-3) and Early Priabonain/Late Priabonian (SB-4). These discontinuites bracket four third-order depositional sequences (S-1 to S-4). These depositional sequences comprise transgressive and highstand systems tracts. They are correlated together and with their time equivalents in the neighboring areas, as well as, with the global regimes of Haq et al. (1987) in order to acquire an abridged picture of their regional and global distribution.