Microfossils and facies analysis of the neogene deposits of the Western Bekaa valley (Lebanon) : paleoecology, paleobiogeography and paleoclimate.

Abstract

The initiation of the Dead Sea Fault system during the Neogene and subsequent transpression in Lebanon caused maximum uplift of Mount Lebanon and Anti Lebanon ranges and maximum sedimentation within the Bekaa continental basin. Continental sedimentary rocks from the margins of the Central Bekaa Valley (Lebanon) have been studied from the sedimentological and micropaleontological viewpoints. Six closely-spaced stratigraphic sections were raised near the town of Zahle, and two other sections northwards near the villages of Niha and Cherara. Three facies assemblages has been characterized (1) a massive conglomerate unit interpreted as alluvial fan deposits, (2) a fossiliferous yellowish marls and marly limestones interpreted as perennial lacustrine deposits, and (3) an organic-rich marls rich in edaphic structures and lignites intercalated with volcanic ash horizons interpreted as a palustrine interval. These lacustrine-palustrine deposits provided a diverse microfossil assemblage composed of 5 species of stoneworts (charophytes), 14 species of terrestrial and aquatic snails (gastropods), 1 species of clamshell (bivalve), 5 species of crustaceans (ostracods) as well as other vertebrate remains such as bone fragments and teeth of fishes, amphibians, reptiles and mammals. These microfossils are here described and illustrated for the first time, and discussed in terms of their present and past ecology and geographical distribution. The paleoenvironmental characteristics and evolution of the lacustrine deposits are inferred through facies analysis and through comparison of microfossil tolerances with the ecological requirements of their nearest living relatives. Results suggest that during the late Miocene, the Bekaa Valley was occupied by a relatively shallow, stable, oligotrophic freshwater or slightly oligohaline lake that evolved to a very shallow eutrophic lake with a dense palustrine vegetation belt. The paleolake ultimately regressed to the south due to climatic changes and tectonic stresses leaving the lacustri