The Metlaoui Group was deposited on a broad ramp that deepened to the northeast into the Tethyan Sea. The ramp contains a series of broad facies belts. The inner-ramp facies belt is composed of three facies tracts: (1) Faid sabkha (most landward) characterised by skeletal-poor dolomudstones and wackestones and interbedded evaporites, (2) Ain Merhotta restricted shallow-lagoon facies composed of sparse skeletal wackestones, packstones, and cross-bedded gastropod grainstones; (3) updip El Garia high-energy shoal complex (most seaward) composed of red algal-discocyclinid grainstones and packstones which are locally cross-bedded. The El Garia carbonates in the mid-ramp setting are predominantly composed of large, flat nummulites. Dominant textures are burrowed, mud-poor packstones and poorly sorted or bimodal grainstones. The accumulation was located below fair-weather wave base and above storm wave base, at a water depth estimated to have ranged from 30 to 60 m (middle neritic). The deep-water (deep neritic), outer-ramp Ousselat Member consists of coarse silt- to medium sand-sized nummulithoclastic debris that grades from coarser to finer debris down the ramp. The Bou Dabbous Formation is composed of basinal foraminiferal mudstones, wackestone and packstones deposited by suspension and less commonly by gravity-flow deposition in upper bathyal water depths. Reservoir quality is variable, with most lime packstones and grainstones having moderate to high porosity (related to intraparticle and microporosity), but only poor to fair permeability (generally less than 10 mD). The higher-quality reservoirs have preserved interparticle porosity with permeabilities ranging from tens of millidarcys to several darcys. Permeable nummulitic packstones and grainstones are favoured by the following factors: (1) low abundance of lime mud, (2) low abundance of nummulithoclastic debris, (3) low abundance of echinoderm fragments, (4) moderate sorting, (5) minor precipitation of late burial cements, (6) dolomitization.
Acid preparation, at the Natural History Museum, London, of a turtle skull and jaw presented to the Museum of Geology, Sandown, Isle of Wight, England, by R. W. Harris in 1931, has revealed extremely derived palatal morphology previously undescribed in turtles. The specimen is described as a new genus and species, Sandownia harrisi, and is superficially similar to certain soft-shelled turtles (family Trionychidae) although the extensive secondary palate and extensive skull roof are previously unknown for this family. Other features of the skull suggest that it is not a trionychid but can be assigned to the Trionychoidea and that its closest affinities are with the members of the Trionychia (Carettochelyidae and Trionychidae). This specimen is one of the oldest known cryptodiran turtles that nests within the living families (the Polycryptodira). It suggests that the deep divergences among the living families of the Cryptodira occurred more than 110 million years ago.
From Middle Jurassic to Late Cretaceous time the African-European Rift Zone (AERZ), a seaway connecting the western part of the Tethys Ocean to the embryonic Atlantic Ocean, was characterized by sinistral transtension. On the African margin of the AERZ this caused break-up of Tethyan Triassic and Lower Jurassic evaporite and carbonate platform sequences by linked, strike-slip and normal-slip fault displacements that delineated a system of sedimentary basins separated by horsts. The Zaghouan-Ressas Structural Belt (ZRSB) in northern Tunisia was initiated as a north-tapering horst in this system, bounded to the northwest by a pelagic basin (the Tunisian Trough) in which thick Lower Cretaceous sequences were deposited and to the east by a north-south trending system of reactivated Tethyan margin faults. Thickness variations in syn-rift stratigraphy led to lateral flow in underlying Triassic evaporitic sequences and the initiation of pillows and, perhaps, piercement diapirs. Mid- to latest Cretaceous post-rift sequences onlapped syn-rift fault blocks, but the post-rift period is complicated by a reversal in the displacement sense across the AERZ leading to dextral transpression and local fault inversion. In Paleocene and Eocene time, northern Tunisia was characterized by northeast-southwest extension accommodated by displacements on linked systems of reactivated AERZ-related and Tethyan margin faults at the southern margin of Mesogea. This was associated with drift of the Apulian microplate into eventual collision with the European margin to form the Western Alps. Further west, convergence of Africa relative to Europe was initially taken up by subduction of oceanic lithosphere in the remnant AERZ. The Oligo-Miocene evolution of the western Mediterranean reflects the destruction of this oceanic lithosphere and its successor oceanic basin, the Proto-Mediterranean. The Atlassic orogeny in northern Tunisia began in Oligocene time as a result of collision of microplates rifted off the European margin with the North African margin, and coincided with the progressive elimination of Proto-Mediterranean lithosphere from west to east along the African margin. Evidence of the contractional deformation is the development of an Oligocene-Miocene foreland basin in northern Tunisia and its deformation in the Atlas fold-thrust belt of mid- to Upper Miocene age. Within this tectonic framework, two models for the structural evolution of the ZRSB during the Atlassic orogeny are evaluated. The first recognizes the importance of facies variation in controlling thrust geometry, but is essentially a thin-skinned model in which detachment on incompetent Triassic strata forms the main control of structural style. The second model emphazises reactivation of AERZ-related basin margin faults during contraction and accounts for the major folds in the ZRSB at Djebel Zaghouan and Djebel Ressas as forced folds formed by fault inversion. Anticlines at Hamman Zriba, and east of Grombalia, are also interpreted as fault-inversion folds formed in normal sequence on the external side of the ZRSB. Flow of Triassic strata into the cores of these folds may have been assisted by tectonic loading during fault inversion along the ZRSB. Subsequently, structures in the ZRSB were dissected by northeast-southwest-trending faults that propagated through the post-rift sequence during post-Miocene reactivation of syn-rift extensional faults. These faults accommodated dextral oblique-slip displacement and were linked to extension in northwest-southeast-trending graben that cut the ZRSB and the Intermediate Atlas Zone.
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