Structural and stratigraphic relationships between the Numidien formation and underlying formations in the westernmost Mogod Mountains, northern Tunisia
Miller, Elizabeth Louise
Burchfiel, B. C.
Master of Arts
The Numidien formation in the westernmost Mogods overlies a sequence of Upper Cretaceous to Eocene limestones and marls which exhibit a shallowing event beginning prior to the lowermost Eocene. Water depths of deposition based on faunal assemblages indicate that the Upper Cretaceous limestones and marls were deposited in water depths greater than 1 meters. Middle and Upper (?) Eocene marls were deposited in water depths less than 2 meters. A probable Oligo-Miocene (?) shelf facies sandstone sequence overlies the Middle to Upper (?) Eocene marls in places. The Oligo-Miocene Numidien formation is part of a deep sea fan complex deposited further to the north and was derived from the African craton. Presence of MeIonis pompilioides (Fitchel and Moll) indicates that this sequence was deposited in water depths greater than 2 meters, and as it overlies a partially time-equivalent shelf sequence, stratigraphic considerations permit one to conclude that the Numidien is allochthonous in this area. A minimum of 15 kilometers of southeastward tectonic transport may be demonstrated. The structural style of deformation within the Numidien thrust and that within the underlying rocks indicate that the two tectonic units have been deformed and have been moved separately. Gypsiferous Triassic rocks have been tectonically injected along the detachment horizon under the Numidien. A detachment horizon also must exist in the marls beneath the Upper Cretaceous limestones and there must be one at the Upper Triassic level as well in order to allow the tectonic injection/diapirism of these rocks into higher tectonic levels. The timing of deformation is bracketed by the age of the youngest Numidien units involved in the thrusting (14-15 m.y.) and the age of a post-tectonic volcanic stock that has been dated at 8.3-.8 m.y. by Vass and others (1974) Timing of deformati onal events in this area coincides with major tectonic events occurring in the Western Mediterranean. Sardinia separated from Corsica along the straights of Bonifacio in the Early Burdigalean and continued to rotate southeastwards (Alvarez, 1974). Its subsequent collision with the North African plate caused the thrusting and deformation in the Western Mogods. The collisional event could not have been very late in the Miocene as a period of extensional faulting caused minor separation of the two continental masses previous to the Messinian salinity crisis. Later minor movement of the Numidien thrust post-dates volcanics dated at 6.6 m.y. by Glaçon and Rouvier (1972) and was presumably caused by renewed compression between the European and North African plates.