Cenomanian–Turonian facies and sequence stratigraphy, Bahloul Formation, Tunisia

Abstract

The Cenomanian–Turonian Bahloul Formation in Tunisia consists of well-bedded dark grey limestones, with common local intercalations of marls and argillaceous limestones. In the Oued Bahloul section, the Bahloul Formation (28 m in thickness) directly overlies a massive bed (4 m), consisting of conglomeratic sandy limestone showing an incised base. In terms of sequences, the Oued Bahloul section shows that the Bahloul Formation is composed of deepening upward metric cycles, each comprising three units. Each cycle starts with a laminated dark grey limestone, progressively changing to argillaceous limestone, and terminates with bioturbated marls containing microfauna of the Whiteinella archaeocretacea biozone. This deepening upward cycle package, constituting the Bahloul Formation, is interpreted to be the expression of a transgressive interval directly overlying, in the Oued Bahloul area, a shelf margin wedge formed by the conglomeratic limestones. The transgressive interval ends with a maximum flooding surface (mfs) included within the filaments-rich facies. This mfs represents the upper boundary of the Bahloul Formation, which is directly overlain by a marly unit, the Annaba Member of the Kef Formation, assigned to the Helvetoglobotruncana helvetica biozone, indicating an early Turonian age. The filament event is one of four biosedimentary event horizons identified within the late Cenomanian–early Turonian interval in the Oued Bahloul section. From base to top, these four event horizons correspond to (1) the extinction of Rotalipora, (2) the Heterohelix shift, (3) the Whiteinella proliferation, and (4) the filament event. These events are recognized also in other areas in Tunisia and abroad, and we identify them here within sections preferentially located on the platform (to the south), in the basin (to the north) and especially within the transitional platform-basin environments (in Central Tunisia). Toward the SW, in Jebel Bireno, the Bahloul Formation is thinner (6 m) than in Oued Bahloul and wholly carbonate, and it was deposited in relatively shallower conditions (probably in an outer platform environment). In fact, organic-rich facies constituting the Bahloul Formation change to the south to open platform facies, in which planktonic microfauna are associated with benthonic foraminifera and rudists. However, toward the north (in the Bazina area, for example), the Bahloul facies changes to basinal marls rich in ammonites and planktonic foraminifera. Isotope analyses show that the main δ 13C positive excursions coincide with transgressive and maximum flooding surfaces. The latter are commonly used in regional and global correlations.