ALBERT

Notes: In the uppermost Jurassic of the central part of the South-East Basin of France, an association of lime mudstone beds, calcarenite beds and coarse carbonate breccia bodies form an informal stratigraphical unit called the ‘Barre Tithonique’. In the ‘Barre Tithonique’, gradual transitions from lime mudstone or calcarenite to breccia show different stages of deformation leading to progressive brecciation of the original lithologies. The study of the breccia facies, and the observed gradual transitions as a whole, document a new early diagenetic process in carbonate environments, resulting from water-wave and seabed interaction. Water-wave induced brecciation and its abundance in the ‘Barre Tithonique’ indicate that sea–seabed interaction was significant. Comparison with modern studies of the mechanics of wave–seabed interaction suggests that water depth was less than 200 m. It is demonstrated that sedimentary features such as channel-like structures, previously interpreted as being the result of erosion and deposition of mud-flows, were in fact produced by wave-induced, in situ reworking of lime mud, without any significant unidirectional flow or gravity induced displacement.

Notes: Uppermost Jurassic limestones of the South-East Basin (France) are organized into four facies associations that were deposited in four distinct zones: (1) peritidal lagoonal limestones; (2) bioclastic and reefal limestones; (3) pelagic lime mudstones; (4) lime mudstones/calcarenites/coarse breccias. Calcarenite deposits of zone 4 exhibit sedimentary structures that are diagnostic of deposition under wave-induced combined flow. In subzone 4a, both vertical and lateral transitions from lime mudstone/calcarenite to breccia indicate in situ brecciation under wave-cyclic loading. Breccias were produced by heterogeneous liquefaction of material previously deposited on the sea floor. Deposits in subzone 4a record relatively long periods (〉400 kyr) of sedimentation below wave base, alternating with periods of deposition under wave-induced currents and periods of in situ deformation. In this zone, storm waves were attenuated by wave–sediment interaction, and wave energy was absorbed by the deformation of soft sediment. With reference to present-day wave attenuation, water depths in this zone ranged between 50 and 80 m. Landwards of the attenuation zone, in zone 3, storm waves were reduced to fair-weather wave heights. Storm wave base was not horizontal and became shallower landwards. As a consequence, water depth and wave energy were not linearly related. On a small area of the seaward edge of subzone 4a, cobbles were removed by traction currents and redeposited in subzone 4b. There, they formed a 100-m-thick wedge, which prograded over 3 km and was built up by the stacking of 5- to 20-m-thick cross-stratified sets of coarse breccia. This wedge records the transport and redeposition of cobbles by a high-velocity unidirectional component of a combined flow. The increase in flow velocity in a restricted area is proposed to result from flow concentration in a channel-like structure of the downwelling in the gulf formed by the basin. In more distal subzone 4c, the hydrodynamic effect of wave-induced currents was quasi-permanent, and brecciation by wave–sediment interaction occurred only episodically. This indicates that, seawards of the attenuation zone, hydrodynamic storm wave base was deeper than mechanical storm wave base. Uppermost Jurassic carbonates were deposited and soft-sediment deformed on a hurricane-dominated ramp of very gentle slope and characterized by a zone of storm wave degeneration, located seawards of a zone of sedimentation below wave base.

Notes: N–S to NW–SE-trending faults and reverse faults occur within the Palaeozoic Ahnet–Mouydir Basin of the Algerian Saharan Platform, located to the east of the West African Pan-African suture zone. Deformation and stratigraphic analysis show that this basin underwent a NNE–SSW to ENE–WSW shortening at about the Carboniferous/Permian transition or, more probably, during the Early Permian. A brief review of the more or less synchronous deformations registered in the neighbouring regions, and more specifically around the West African Craton, illustrates the mechanical coupling between intraplate tectonics and the Hercynian plate margins orogenies.