Triassic palaeogeography of Tunisia

Abstract

A stratigraphic, palaeogeographic and palinspastic synthesis of the Triassic successions in Tunisia is herein documented from a SSE–NNN oriented profile (Saharan Platform, Gulf of Gabes offshore, Tunisian Atlas, Tellian Units) across the northern boundary of the Gondwana Plate and the future South-Tethyan Margin. It is principally based on a reinvestigation of old data and on recent results, particularly related to new lithostratigraphic subdivisions and palaeontologic datings obtained from assemblages of benthic foraminifera, algae, palynomorphs, ammonites and brachiopods. The various Triassic successions of Tunisia are compared at the scale of the formations and sequences in the most possible precise biochronologic framework. The corresponding chart is used as reference to seven maps of the dominating depositional environments, during the following time intervals: Induan/Olenekian, Anisian, Lower Ladinian, Middle–Upper Carnian, ‘Norian’, ‘lowermost part of Rhaetian’ and ‘uppermost part of Rhaetian’. In a Peri-Tethyan domain controlled both by eustasy and extensional tectonics, the proposed maps show the alternation of regressions (marked by fluvio-deltaic siliciclastics and paralic sebkha-type evaporites) and transgressions (marked by foraminifera-bearing limestones/dolomites of internal/proximal platform, sometimes open to pelagic influences). The transgressions were particularly widespread during Carnian and ‘Rhaetian’ times and probably came from a deeper trough situated towards ENE. During a first rifting phase, the motion of normal crustal faults, strongly inclined towards the north, induced well-marked differential subsidences. In conclusion, a new palinspastic reconstruction along a south–north/SSE–NNW oriented profile is proposed. It shows a succession of shoals (flanked by lateral steps), such as the Tebaga of Medenine shoal, sub-basins (Atlas) and tilted blocks (Hairech–Ichkeul) heralding the structuring of the South-Tethyan Margin during Jurassic times.