Origin of compound biogenic sedimentary structures in Eocene strata of Wadi El-Hitan universal heritage area, Fayum, Egypt: Mangrove roots or not?

摘要

Ichnological data are integrated with sedimentologic data and body fossil content to reconsider the origins of the biogenic sedimentary structures in upper middle Eocene strata of Wadi El-Hitan (UNESCO universal heritage area) in Egypt. Three types of rhizoliths (Type 1 to 3) are recognized, in addition to co-occurrence of six common marine ichnogenera (Ophiomorpha, Skolithos, Thalassinoides, Palaeophycus, Teichichnus and Taenidium). Fossil content comprises bivalves (Ostrea sp., Cardia sp., Cardium sp., Carditopsis sp. and Pinna sp.), gastropods (Turritella sp.), scaphopoda (tusk shells), arthropods (crabs and Balanus sp.) and the larger foraminifer Nummulites sp. as well as bony fossils of Basilosaurus isis, Dorudon atrox, crocodiles, sharks, rays and saw fish. Facies characteristics, fossil content, rhizoliths and invertebrate trace-fossil suites imply a bay-margin origin of the plant-and animal-associated biogenic sedimentary structures. Importantly, characteristics such as downward tapering and/or branching into smaller branches in addition to external and internal ornamentations are used to distinguish rhizoliths from burrows. Comparison between modern mangrove environments (i.e., substrates, fauna and flora) with the examined biogenic sedimentary structures can be used to support the marginal marine mangrove-swamp origin. Importantly, living mangrove roots are characteristically associated with marine bioturbation, which can be compared to the structures reported in the Eocene bioturbated bed. Large vertical Type1 rhizoliths are interpreted as mangrove tree trunks. Type-2 rhizoliths correspond to prop/stilt roots similar to those of living Rhizophora sp. and locally to pneumatophore / anchor roots of living Avicennia sp. Type-3 rhizoliths are ascribed to sea-grasses roots and / or tertiary roots and root hairs. The examined compound biogenic sedimentary structures represent an excellent example for distinguishing previously misinterpreted plant roots (rhizoliths) from animal trace fossils (burrows) in marine setting as well as recognizing mangrove rhizoliths in the rock record.