Sedimentological characteristics and genetic pathways of the nummulithoclast facies within the Middle Eocene rocks of Egypt

• Published in: Arabian journal of geosciences 2023, Vol.16 (12), Article 639
• Main Author: El Ayyat, Abdalla M.
• Format: Article
• Language: English
• Subjects: Bioerosion; Carbonate sediments; Carbonates; Climate change; Diagenesis; Earth and Environmental Science; Earth science; Earth Sciences; Eocene; Euphotic zone; Fragmentation; Gene mapping; Global warming; Graben; Grain size; Hydrocarbons; Nummulites; Offshore; Original Paper; Petrography; Petrology; Platforms (geology); Sea surface; Sea surface temperature; Sediment traps; Sedimentary facies; Sedimentary structures; Sediments; Silt; Storm surges; Storms; Surface temperature; Winnowing
• ISSN: 1866-7511; 1866-7538
• DOI: 10.1007/s12517-023-11757-8

In Egypt, the Middle Eocene time represents a prolific interval for the evolution of Nummulite carbonate platforms on the southern continental paleo-margins of the Tethys. The prevalence of Nummulites has produced a significant piles of carbonate sediments represented mainly by many different facies belts interbedded with silt-sized nummulithoclastic packstones facies. These carbonates are considered as important reservoirs of hydrocarbons in the Mediterranean area, as both in offshore Tunisia and Libya. In Egypt and especially in the target area, these carbonates have been carefully studied in the field in both central Eastern Desert and west-central Sinai. On the other side, the petrography of these facies shows an exceptional richness in nummulithoclasts facies. The nummulithoclasts are spreading vertically and horizontally within parautochthonous to allochtonous Nummulite banks with a clear decrease in grain size from the shallowest (south) to the deepest (north) areas of the carbonate platform. The studied Nummulite build-ups exhibit several physically controlled sedimentary structures and bio-fabrics that could be detected on micro- and macro-scales. These structures highlight the roll of abiotic and biotic agents as triggering mechanisms in fragmentation of Nummulite tests leading to silt-sized bioclasts (nummulithoclasts). Candidates of such triggering mechanisms are storm surges and compaction with diagenetic processes (i.e., pressure solution and bioerosion). Storm currents swept away sediments from shallow near-shore to outer-ramp deeper environment. They were accumulated in a coevally formed small half-graben basins (sediment traps). Microfacies data proved that Nummulite banks were accumulated within the back-bank/lagoonal, bank-core, and fore-bank open marine settings. The small Nummulites (A-forms) with Alveolines were occupying shallow, euphotic waters, but ovate and robust tests lived and accumulated in middle waters indicating the influence of currents with wave-winnowing processes. The fore-bank (distal assemblages), were colonized by nummulitid with orthophragminid shells in the open marine deeper portion of the photic zone. Palaeogeographic reconstruction maps during the Middle Eocene indicate that Egypt was situated in tropical, storm-dominated paleo-latitudes, which were controlled by a global warming. This global warming enhanced the sea surface temperature. Comparison between nummulithoclasts and other bio-sedimentary facies in Alg