Submarine Slope Systems
Characterisation of the Numidian flysch, western Mediterranean
PhD research by Myron Thomas
Introduction:
The Numidian Flysch is the most widespread tectono-stratigraphic unit in the western Mediterranean. It outcrops in the Alpine nappe belt, in southern Spain, Morocco, Algeria, Tunisia, Sicily and southern Italy. In Sicily and Tunisia it is an Oligocene to mid-Miocene flysch-type deposit sourced from the north-African passive margin and deposited into an east-west trending foreland basin. This study focusses on outcrops in northern Sicily and Tunisia, evaluating the sedimentology and provenance within the context of the basin as a whole. Special emphasis is placed upon the controls on deposition, which until now have remained largely unknown.
Aim and objectives:
The aim of the project is to re-evaluate the sedimentology and provenance of the Numidian flysch in the light of modern thinking on deep water systems.
Geological background:
The Numidian Flysch represent a late Oligocene to early Miocene foreland basin deposit. They represent a tectonically evolving submarine slope system with associates hemipelagics, channel systems, turbidites, and mass transport deposits (slumps, slides etc).
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As hard collision occurred between the African and European plates (starting around the upper Eocene) a complex subduction zone was developed. The collision generated thrust and fold belts which represent the accretionary prism, can be traced from the Betic and Riff sectors of south Spain and Morocco, through the Tellian Atlas sector of Algeria and Tunisia, across the Sicily straits, and forming the southern and northern Apennines of Italy. In north Italy the belt turns east in to the Alpine chain, and from there in to the Carpathian mountains and the Balkans (Guerrera et al 2005, Wezel 1970). Oucrops of Numidian Flysch follow the orogenic belt, and are seen within southward verging nappes.
The palaeo-environment is difficult to recreate due to dissection from the south eastwards migrating Mahgrebide-Sicillide fold and thrust belt. Classic examples of foreland basins however envisage a long trough basin parallel to the approaching/encroaching accretionary Prism, with multiple input sources producing fan systems. The floor of the basin, whose geometry is controlled by the tectonics and geodynamics of the subduction zone (rather than eustacy and climate), may contain a main axial sediment transport flux, reworking distal basin fan deposits. Most work on the Numidian flysch describes how deposition occurred in a west-north west to east-south east trending thin foreland basin, which splits in the south east in to the northeast trending Lucanian ocean, and the south east trending Ionian ocean.
The Numidian Flysch deposits were caught in the migrating thrust belt from the Miocene onwards, and tectonically transported south east. In Sicily, the Mahgrebian/Sicillide belt was superceded by a Pleiocene-Pleistocene foreland, which in turn was superceded by the present day foreland basin, both of which can be seen in the south east of Sicily. The Ragusa Plateau, made from platform carbonates represents the most northerly piece of African plate, and this is directly abutted to its north by the present foreland basin which cuts across the southern part of Sicily. The presently active portion of accretionary prism lies across the northern half of Sicily, accounting for the active seismicity observed.
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Hydrocarbon potential:
The Numidian Flysch is a potential hydrocarbon reservoir offshore Tunisia, which displays seismic anomalies (Direct Hydrocarbon indicators) offshore Tunisia. The Flysch is found offshore Northern Tunisia, below a gently sloping continental shelf and water depths of 200-400m. The Numidian Flysch is defined as one of 3 principle Hydrocarbon producing systems; the Albuan/Cenomanian/Maastrichtian fractured Carbonates; the Ypresian Marine Limestones; and the Oligocene-Miocene Numidian Flysch. The Flysch is a proven play in Sicily, and exploration is commencing offshore Tunisia.
Sedimentological studies:
Studies of channel complex outcrops in the region of Cefalu, Sicily, allow characterisation of a large channel complex set at least 5 Km in lateral extent, with 3.5 Km of downdip exposure and a minimum thickness of 430 m. 16 discrete channel complex bodies are mapped with dimensions of 50 to 280 m in width and 15 to 85 m thick. Current work is attempting to relate the timing of channel formation with local and regional allocyclic events, in order to constrain the controls upon this system. In terms of size and geometry, the studied channels are comparable with the well studies Green channel complex from offshore Angola.
Elsewhere in Sicily, a majority of outcrops demonstrate a slope lobe architecture, with thinning upwards cycles and minor distributory channels. Some evidence for coutouritic currents exists, reworking the tops of thin turbidite beds.
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Provenance studies:
The provenance of the Numidian flyschremains a controversial subject which has hindered understanding of this regionally widespread depositional system. This study has performed a wide ranging regional review of published evidence, including four main lines if evidence including structural position, petrology of Numidian sandstones, zircon analysis and palaeocurrent orientations. The results of this study are to be published shortly, however a southern, African source is strongly indicated with little evidence for a northerly source as suggested by a number of authors.
Downloadable files:
Conference abstracts |
Conference posters |
Papers/Abstracts |
Architecture and evolution of incisional, upper-slope submarine channel complexes within the Oligo-Miocene Numidian Flysch of Sicily and Tunisia. University of Liverpool 2008 Talk Abstract (pdf 7Kb) |
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International Association of Sedimentologists Conference 2009. Talk abstract (pdf 10kb) |
AAPG conference, Cape Town, South Africa 2008. Poster abstract (pdf 6Kb) |
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