Sedimentological and stratigraphic constraints on Oligo-Miocene deposition in the Mogod Mountains, northern Tunisia: new insights for paleogeographic evolution of North Africa passive margin

摘要

Understanding the sediment distribution and basin context of the different Oligo-Miocene successions in the eastern Maghrebides thrust belt of northern Tunisia is challenging and several contrasting models have been proposed. We present the results of detailed field mapping together with facies, microfacies and microfaunal analyses of the NE Mogod Mountains that allowed refining the Bizerte geologic map and reconstructing the Oligocene-early Miocene depositional environment of northern Tunisia. Facies characterization of the Jebel Sebaa, Cap-Blanc and Bizerte Town sections demonstrate their deposition by submarine gravity flows typical of the Numidian Flysch Formation, rather than the Fortuna Formation as previously proposed. This is supported by the occurrence of deep marine microfossils. The association of deepwater massive sandstone facies and a high proportion of shale-clast conglomerates in the Jebel Zoukar, Ras El Korane and Jebel Sebaa sections are interpreted as bypass channel facies, representing proximal deposition. The section at La Baie des Carrieres is distinguished by its high glauconite content, a rich microfauna of larger benthic foraminifera, Ditrupa and echinoids is typical of a shallow marine depositional environment and interpreted as part of the Gres de Bejaoua Formation. The Oligocene sections at Beni Aouf and Nefza also show Gres de Bejaoua affinities, but with some deepening of the marine environment represented by pelagic foraminifera-rich mudstones, and calcareous glauconitic sandstones. The facies distribution, depositional environments and the sediment routing of these time equivalent deposits appears to be dictated by topographic highs and inherited faults (e.g. salt dome paleohigh, El Hairech-Ichkeul ridge, Zaghouan Thrust fault and faults affecting the basin-floor). Within this framework, the northern part of the salt dome area acted as topographic high that guided the fluvial drainage pattern of the Fortuna Formation along a SW-NE trend. The absence of early Miocene deposits in the Tellian domain indicates that this area has been part of a forebulge setting since the late Oligocene. Corresponding foredeep subsidence led to submergence of the Bizerte-Nefza area and deposition of the deep marine Numidian Flysch Formation. The advance of the fold-and-thrust belt since the late Burdigalian-Langhian induced the vertical stacking of foredeep deposits over the forebulge succession.