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The Oligocene-early Miocene paralic to nonmarine Fortuna Formation in east-central Tunisia accumulated in a cratonic basin on adjacent parts of the Tunisian Atlas province and the Pelagian block. This coarsening-upward detrital sequence was derived from west and northwest uplands and reached a maximum thickness of 1,100 to 1,200 m near the northeastern end of its northeast-trending depocenter (graben?) along the boundary between the two crustal blocks.
After early Cenozoic culmination of the Alpine orogeny the Fortuna basin and source area were created during a new phase of extensional deformation that affected the western Mediterranean region. Continued differential uplift and subsidence produced an increasingly sandy Oligocene lower member composed of shallow-marine to deltaic mudstone and fine-grained sandstone, a maximum of 400 to 600 m thick along the central part of the axial trough. Detritus from the uplift was dispersed east and southeast, and the sandstone grades into a marine carbonate facies on the Pelagian shelf on the east.
Accentuated vertical displacement of basin and source area produced the increasingly coarser grained fluvial upper member (lower Miocene) that was dispersed mainly east and northeastward. This member has a maximum thickness of 850 m near the northeastern end of the trough. Stringers and lenses of conglomerate in the upper part contain well-rounded pebbles of quartz, chert, and quartzite, as long as 4 cm near the northwestern border and 2 cm along the axial depocenter. Accumulation of the Fortuna Formation terminated abruptly, followed by a widespread late early Miocene marine transgression.
Fortuna basin and its northwestern upland on the unstable cratonic margin of North Africa responded to remote effects of late Alpine activity. Early Oligocene to Pliocene sediments in the Fortuna basin area reflect each episode of deformation, regression, and transgression that dominated the western Mediterranean.
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