| Summary: | Middle-Upper Eocene successions were studied in the Fayum Depression in order to establish depositional and paleoenvironmental models that link the ichnological and sedimentologic data to relative sea-level changes in a sequence stratigraphic framework. Five facies associations (FA1- FA5) are identified. The facies depositional models show overall progradation from quiescent open-marine bay (FA1-2: Gehannam and Birket Qarun formations) to lagoon/distributary channel/estuary sedimentary environments (FA3-5: Qasr El-Sagha Formation). The facies successions and their stratigraphic evolution are controlled by a regional, second-order cycle associated with the northward regression of the Tethys, which is overprinted by subordinate third- and higher-order cycles.
Whale-bearing FA1 and FA2 are subdivided into five sedimentary facies. Seventeen ichnospecies belonging to thirteen ichnogenera, as well as rhizoliths are observed within these facies. Facies Association 1 accumulated in a low-energy fullymarine bay, whereas FA 2 represents a bay margin / supratidal paleoenvironments. Clastic point-sources are dominantly hypopycnal although eolian sand may represent an important source locally. The quiescent marine bay is a typical environment and biome for the Eocene whales. Preservation of these fossil whales must occur in association with rapid sedimentation rates, but sufficiently that bioturbation eradicates the physical
sedimentary structures.
Unusual, large-sized sedimentary structures are examined along the parasequence-bounding surfaces of the Birket Qarun Sandstone. Ichnological data, petrography and stable-isotope analysis are integrated to propose a bio-sedimentologic/diagenetic model, interpreting the origin of these structures as concretion growths around ichnofossils. The marine pore-water carbon was influenced by organic carbon and mixing of meteoric groundwater under eodiagenetic conditions. These conditions led to the precipitation of pervasive authigenic calcite-dominated cement in and around the burrows.
More than twenty-five Glossifungites Ichnofaciesdemarcated discontinuities are examined in the study area. These surfaces are grouped into those of autocyclic and those of allocyclic origin. Occurrences of the allocyclically significant Glossifungites Ichnofacies can be classified into sequence-bounding, systems tract-bounding and parasequence-bounding surfaces. Sequence-bounding Glossifungites Ichnofacies-demarcated surfaces divide the studied successions into four third-order sequences. Systems tract-bounding and parasequence-bounding Glossifungites Ichnofacies-demarcated surfaces display higher-order cycles, overprinting the third-order cycles.
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