Megaturbidites and the late Quaternary regional sedimentology of the eastern and central Mediterranean Sea

Abstract

The late Quaternary sedimentology of the Mediterranean is controlled primarily by fluctuations in global climate and sea level, and by an overriding compressive regime between the Eurasian and African plates. Local variations in this convergence have led to marked differences in the two regions studied for this thesis, as reflected in their dominant sedimentological processes, sediment distribution and architecture. This present research assesses the sedimentology of the deep (>3000 m) Herodotus Basin situated between the Nile Cone and the deformed Mediterranean Ridge, focusing on sediment sources and influences on emplacement of the many turbidity current deposits (turbidites) noted in the stratigraphy. Radiocarbon dating and geochemical fingerprinting of turbidite muds has allowed construction of an accurate stratigraphy and correlation for the Herodotus Basin, with the recognition of climatically-controlled cyclic patterns of sedimentation (cyclothems). The Sicilian-Tunisian Platform of the Central Mediterranean has a contrasting late Quaternary sedimentology, due to complex uplift and rotation of the region. Deep troughs (>1000 m) incise the broad, shallow platform creating smaller, localised sedimentary environments. Climate-controlled bottom currents scour the region, creating sediment drifts (contourites) with heightened sedimentation rates via a process herein termed 'bottom current flow lofting' and interaction with downslope processes. Local volcanism, relatively undisturbed hemipelagic sediment accumulation and sapropel formation provide dateable horizons, allowing calculation of sedimentary budgets and insights into controls and influences on the many sedimentary processes noted in this region. Across the Mediterranean, distinctive, thick turbidites with large volumes (megaturbidites) are noted as acoustically transparent layers (ATLs) on shallow seismic profiles and as graded sand and mud beds in long-piston cores. Of particular interest are 1) the Herodotus Basin Megaturbidite (HBM), dated at approximately 27,000 years and with a volume in the order of 400 km³ and 2) the bi-partite Pantelleria Trough Megabed of the Central Mediterranean, emplacing circa 30 km³ of volcanic and carbonate material approximately 35,000 years before present. This study analyses these events and other global occurrences of megaturbidites, evaluating their recorded geometries, seismic and sedimentologic characteristics, proposed triggering mechanisms, flow properties and the interaction between sea floor and flow during mega-turbidity current flow. A new classification, using all case studies cited in the geological literature, has been constructed to characterise megaturbidites by their mode of deposition and their association with other mass flow deposits.

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