Surface and subsurface architecture of the Kasseb structures: implications for petroleum exploration beneath the Tellian allochthon, the easternmost portion of the Maghrebides
Résumé
In this study, we present field investigations based on structural analyses and interpretations in the onshore part of the Tunisian Tell, at the front of the Numidian Flysch units. Field observations allow the deciphering of the main structural styles and tectonic events in the Tell domain of Northern Tunisia, especially at the type locality of the Kasseb units. New field data collected in the Kasseb area, a key area to understand the structures of the Tunisian Tell, outline a major north-dipping normal fault located between the Numidian Flysch and the underlying imbricated fan of the Kasseb basinal units. Detailed observations allow the viewing of the Numidian allochthon as a passively transported domain at the top of the underlying duplexes, which implies an overall duplex configuration within the Tellian thrust wedge in the Kasseb area. The Kasseb basinal units represent the southernmost extent of pelagic carbonate series issued from a paleo-basinal domain situated farther north. These pelagic carbonate units have been progressively tectonically accreted within a dominantly submarine environment since the Oligocene, before reaching their current position just after the Langhian. Currently, shallow marine Oligocene-Langhian siliciclastic series rest unconformably on top of these Late Cretaceous-Eocene pelagic carbonate units. The post-Langhian compression events induced a progressive inversion of the underthrust foreland platform domain, as well as the re-folding of the overlying thin-skinned structures of the Tell domain as observed today. The Kasseb units show duplications with duplex configurations with a sole thrust located in the Paleocene shales and synsedimentary thrust tectonics active presumably during the Eocene, as the Oligocene to Early Miocene Numidian Flysch rests unconformably on top of the underlying thrust structures. The tectonic style is therefore a combination of thin-skinned and thick-skinned deformations: the thin-skinned structures correspond to the nappe structures and Tellian units thrust contacts. By contrast, the thick-skinned deformational structures correspond to the rejuvenation of deeper structures accounting for the inversion of underlying foreland structures with deep-seated thrust faults, pop-up and pop-down folded structures. For the oil/gas exploration, we propose a tectono-stratigraphic and oil occurrence conceptual model for the Tell, outlining the structural control on the hydrocarbon occurrences in different potential structural traps associated with (1) ramps and duplexes anticlines within the thin-skinned allochthonous structures as well as (2) inversional thick-skinned structures such as pop-up anticlines in the subsurface of the Tellian allochthon. We discuss potential traps associated with the nappes anticlines which should be viewed as important petroleum structural prospects emphasizing their future exploration and drilling.