Insights into the Cenozoic geology of North Beirut (harbour area): biostratigraphy, sedimentology and structural history

Germaine Noujaim Clark,Marcelle Boudagher-Fadel

doi:10.14324/111.444/ucloe.000004

Germaine Noujaim Clark, Marcelle Boudagher-Fadel

Open Access

https://doi.org/10.14324/111.444/ucloe.000004

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Abstract

The biostratigraphy and sedimentology of the outcrops and bedrock recently exposed in archaeological excavations around the harbour area of Beirut (~5 km²) unlock the geological and structural history of that area, which in turn are key to understanding the hydrocarbon and hydrogeological potential of the region. A key location (Site 2) of a studied outcrop section and newly uncovered bedrock is on the northern foothill cliff of East Beirut (Achrafieh). The outcrop section of carbonates is of Eocene beds overlain by conformable Miocene beds. The excavation of the slope bordering the outcrop uncovered a bedrock section of an early Pliocene shoreline of carbonate/siliciclastic sands at its base and topped by a beach-rock structure. The early Pliocene age of the shoreline section is dated by an assemblage of planktonic foraminifera that includes Sphaeroidinellopsis subdehiscens, Sphaeroidinella dehiscens and Orbulina universa. The Eocene carbonates of Site 2 extend the coverage of the previously reported Eocene outcrops in the harbour area. They form a parasequence of thin-bedded, chalky white limestones that includes the youngest fossil fish deposits in Lebanon (Bregmaceros filamentosus). The deposits are dated as early Priabonian by their association with the planktonic foraminiferal assemblage of Porticulasphaera tropicalis, Globigerinatheka barri, Dentoglobigerina venezuelana, Globigerina praebulloides, Turborotalia centralis and Borelis sp. The Middle Miocene carbonates that conformably overlie the early Priabonian, parasequence include a planktonic foraminiferal assemblage of Globigerinoides trilobus, Orbulina universa and Borelis melo. Elsewhere, in the harbour area, the preserved Eocene limestones are also overlain by conformable Miocene carbonate parasequences of Langhian-Serravallian age. Younger argillaceous limestone beds of the Mio/Pliocene age occur in the eastern central part of the harbour area and enclose an assemblage of Truncorotalia crassaformis, Globorotalia inflata and Orbulina universa. The three markers of old and recently raised structural blocks in the harbour area are a Lutetian/Bartonian marine terrace in the south west corner, a lower Pliocene shoreline carbonate section in the north east side and a Holocene raised beach of marine conglomerates in the north east corner of the area. The locations of these paleo-shorelines, less than 2 km apart, indicate a progressive platform narrowing of North Beirut since the Paleogene. This study underpins the geological complexity of the region and contributes to understanding the underlying geology, which will be needed for future regional archaeological, hydrocarbon and hydrogeological exploration.

Highlights

Lebanon abuts an active transform plate boundary, and its geology reveals a correspondingly complex tectonic history
The newly revealed excavated sites of the Beirut harbour area tie the occurrence of the Paleogene and Neogene rock formations to the differentially preserved coastal ribbon of Cenozoic outcrops
At Site 2 the younger Priabonian thin beds of the inner platform carbonates occurred in a minimal accommodation space near a shoreline. This occurrence is in contrast to the shelf edge occurrence of their contemporaneous chalk deposits in south coastal Lebanon [1]. As these two deposits are the only evidence of the Late Eocene platform sedimentation along maritime Lebanon, their occurrences in the inner shelf and shelf edge environments follow the paleogeography drawn in BouDagher-Fadel and Noujaim Clark [1] with an interpreted and drawn shoreline around the Beirut harbour area

Summary

Introduction

Lebanon abuts an active transform plate boundary, and its geology reveals a correspondingly complex tectonic history. In Beirut, many reconstructions and infrastructural renewal projects in the harbour and downtown areas have been excavated before construction got underway These works have given a unique opportunity to sample, and study the newly uncovered geological sections of bedrock, and to expand our understanding of the Cenozoic geology of maritime Lebanon [1]. The newly revealed excavated sites of the Beirut harbour area tie the occurrence of the Paleogene and Neogene rock formations to the differentially preserved coastal ribbon of Cenozoic outcrops.

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