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AAPG Bulletin, V.
A transition from carbonate shelf to pelagic basin environments of deposition: Rifting and depositional systems in the Jurassic of northeastern Tunisia
1Unite de Recherches Appliquee aux Mines et aux Materiaux, Universite Tunis El Manar, Faculte des Sciences de Tunis, 1060 Tunis, Tunisia; [email protected]
2Unite de Recherches Appliquee aux Mines et aux Materiaux, Universite Tunis El Manar, Faculte des Sciences de Tunis, 1060 Tunis, Tunisia; [email protected]
3Faculte des Sciences et Techniques, Departement des Sciences de la Terre, Beni Mellel, Morocco; [email protected]
4Entreprise Tunisienne des Activites Petrolieres, 27 bis, Avenue Kheireddine Pacha, BP 367, 1002 Tunis, Tunisia; [email protected]
Jurassic deposition in the Maghrebian tethys was governed by eustasy and rifting. Two periods were delineated: (1) a carbonate shelf (Rhaetian–early Pliensbachian) and (2) a platform-basin complex (early Pliensbachian–Callovian). The carbonate shelf evolved in four stages, generating three sedimentary sequences, J1 to J3, separated by boundary sea level falls, drawdown, exposure, and local erosion. Sediment facies bear evidence of sea level rises and falls. Lateral changes in lithofacies indicate shoaling and deepening upward during the Sinemurian. A major pulse of rifting with an abrupt transition from carbonate shelf to pelagic basin environments of deposition marks the upper boundary of the lower Pliensbachian carbonate shelf deposits. This rifting episode with brittle fractures broke up the Rhaetian–early Pliensbachian carbonate shelf and has created a network of grabens, half grabens, horsts, and stacked ramps. Following this episode, a relative sea level rise led to pelagic sedimentation in the rift basins with local anoxic environments that also received debris shed from uplifted ramp crests. Another major episode spanning the whole early Pliensbachian–Bajocian is suggested by early brecciation, mass flows, slumps, olistolites, erosion, pinch-outs, and sedimentary prisms. A later increase in the rates of drifting marked a progress toward rift cessation during the Late Jurassic. These Jurassic carbonates with detrital deposits and black shales as the source rocks in northeastern Tunisia may define interesting petroleum plays (pinch-out flanking ramps, onlaps, and structurally upraised blocks sealed inside grabens). Source rock maturation and hydrocarbon migration began early in the Cretaceous and reached a maximum during the late Tortonian–Pliocene Atlassic orogeny.
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