International Geologiical Congress - Oslo 2008

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HPF-12 Environmental micropaleontology: Past, Present, future

 

Benthic biofacies eco-succession and paleoenvironmental changes in Rhodes island, (Late Pliocene, Kritika Member, Rhodes Formation) Greece

 

Efterpi Koskeridou, University of Athens (Greece)
Hara Drinia, University of Athens (Greece)
Pierre Moissette, Universite Lyon 1 (France)
Jean-Jacques Cornee, Universite Lyon 1 (France)
 

 

The late Pliocene-early Pleistocene succession of Kritika section in Rhodes island comprises brackish water gravel and nearshore sand to relatively deep-water clay, assigned to the Kritika Member of Rhodes Formation.
Detailed analysis of the benthic foraminifera and mollusc content of the section allowed its paleoenvironmental reconstruction. Variations in the abundance and distribution of the studied fauna led to the recognition of five sample associations-biotopes, each one consisting of one or more samples with similar faunas and dominant taxa.
Biotope 1 is defined by Ammonia tepida (>90%), and rare lucinids, indicative of a central-lagoonal estuarine environment influenced by the input of freshwater and/or organic material.
Biotope 2 also exhibits a restricted affinity characterised by high frequency of Haynesina depressula (60%), a very strong development of plano-convex foraminifera and small numbers of Miliolidae. The co-occurrence of a Posidonia-meadow association of molluscs (Jujubinus exasperatus, Tricolia sp. and Gibbula sp.) indicates mixing of hyposaline water to normal marine water.
Benthic foraminifera from Biotope 3 are very abundant and display a considerable diversity of species, characterized by remarkable percentages of Miliolidae, Ammonia beccarii and Elphidium crispum. The more diverse character of this association coincides with an upwards increase in the amount of silt and clay, and reflects increasing water depths. Partly, the relatively high abundance of Buliminidae and Bolivinidae indicates influence from deeper water, high food input and low oxygen levels. Mollusc assemblages are represented by Ostrea lamellosa, Spisula subtruncata and Timoclea ovata, all indicating an inner to middle shelf environment.
Biotope 4 is found within offshore-transition clay-sand alternations and transgressive barrier sands. The dominating epiphytal assemblage, mainly represented by Cibicides lobatulus points to a more wide-spread occurrence of submarine vegetation in the littoral realm. The mollusc association is characteristic of gravely bottom settings of Photophilous algae (Cardita calyculata, Arca sp., Bittium spp., and Acinopsis cancellata). Partly, the sandy marls which contain in situ rhizomes of the seagrass Posidonia oceanica, are dominated by an association of bivalves such as Arca noae, Striacra lactea, Nucula nucleus and gastropods.
Biotope 5 consists of the benthic foraminifera Fursenkoina acuta, Bolivina sp., Bulimina sp., Cassidulina carinata. Molluscs are represented by rare Loripes lacteus. The normal marine Fursenkoina species and Loripes lacteus are highly indicative of increased amounts of organic matter and decreased oxygen concentrations, associated with the influence of fresh-water (fluvial) discharge. The comparison with the other organisms also studied in the same section, evidences that foraminifera and molluscs have a similar behaviour in respect to the environmental stress factors of this area.

 

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