SEDIMENTARY ENVIRONMENT AND PETROLOGY OF THE AIN TOBI FORMATION, TRIPOLITANIA, LIBYA

The Aptian to Cenomanian aged Ain Tobi Formation of northwest Libya and adjacent Tunisia is a shallowing upward platform deposit. It has been studied in detail long its outcrop in the Jebel Nefusa of Tripolitania and with well control in adjacent areas north and south. The Ain Tobi was deposited in five depositional environments which roughly parallel the present coastline. The three southernmost depositional environments extend for about 400km. from the present coastline south. Two northern environments are in offshore areas where little data are available. The sedimentary environment of the Ain Tobi is similar to Cenomanian platform deposits of southern France and Italy. The southernmost restricted platform environment consisted of shallow lagoons--sites of evaporite precipitation, intervening pellet mud shoals, and rare ooid-bioclastic sand shoals. Rocks include bedded gypsum, dolomitic limestone, marl, and shale. Mudstones and wackestones are the dominant textures. To the north a slightly restricted to open platform environment consisted of clean to muddy carbonate sand flats, ooid-bioclastic sand shoals, and deposits which graded into the restricted environment represented by algal laminated sediments and evaporites. Rocks from the slightly restricted to open platform include dolomite, sandy and/or argillaceous dolomite, siliceous dolomite, quartz sandstone, shale, and chert. Wackestones and packstones are the dominant textures. The platform margin environment lay farthest north approximately coincident with the present coastline. The Ain Tobi Formation had a ramp type margin consisting of sand flats, ooid shoals, and oyster and rudist biostromes. Dolomite is the dominant lithology, wackestones to grainstones are the dominant textures. Cenomanian time equivalents of the Ain Tobi in the offshore Gabes-Sabratha Basin include rocks consisting of dolomitic marls with planktonic foraminifera and platform margin detritus indicating a basin-slope environment. In addition, a paleotopographic high capped by rudist biostromes similar to those of the platform margin environment exists here. Dolomitization of the Ain Tobi is probably the result of a seepage reflux mechanism which acted at the time of deposition. A dorag mechanism may have been of secondary importance in dolomitizing the Ain Tobi, but of major importance in dolomitizing the overlying section