On the southeastern Tunisian coastline, very diverse living microbial mats colonize the lower supratidal and intertidal zones, and locally may extend into the upper infratidal zone. The interaction between the benthic cyanobacteria and their siliciclastic substratum leads to the development of several types of microbially induced sedimentary structures (MISS). The mapping of the microbial mats has allowed the identification of the types of MISS that characterize the different segments of the coastal environment. The modern microbial mats have been compared with those recorded at the top of the Holocene deposits, which are composed of biodegraded microbial black mats alternating with white laminae made of clastic and evaporitic sediments, indicative of very high frequency cycles of flood and drought. A hypothetic profile showing their occurrences along the different areas bordering the coastline is proposed as a guide for the reconstruction of the ancient depositional environment. The roles of tidal dynamics, storms, and climate in controlling their genesis and spatial distribution, are discussed and highlighted. The modern MISS of southeastern Tunisia are compared with their equivalents that are well documented through the different geological eras.
The Bir M'Cherga basin (North-east Tunisia), with about 600 km 2 area, had recorded four Middle Cretaceous source rocks well stratigraphically correlated with the four known oceanic anoxic sub-events: OAE1a, OAE1b, OAE1c, and OAE1d. Variety of lithology, thickness and organic richness had characterized these source rocks. The sedimentary tectonic analysis, the petrology and geochemistry study established the petroleum system of these source rocks. Basin formation began early in the Barremian-Aptian interval by synsedimentary tectonics reactivating old basement faults. During the Aptian-Albian, the formed basin had a depocenter that recorded thicker black shales while its NW and SE edges remained raised under the Triassic halokinetic activities. The evolution of the sedimentary filling during this period generated two sedimentary cycles corresponding to two filling second-order fining and thickening upwards sequences. The black shales that constitute these source rocks are formed between subtidal and external platform environment and are interbedded with juxtaposed high organic rich layers and poor ones. The rich organic facies consists of dark shale that constitutes the source rock. The poor organic beds formed by light grey and nodular limestones constitute an intra host reservoir. Thereby, petroleum system consists in an "unconventional oil shale hybrid systems with a combination of juxtaposed organic-rich and organic-lean intervals associated to open fractures". The kerogen is essentially amorphous, with marine planktonic origin and low ligneous organic matter contribution. This organic material of dark facies had been well preserved in an anoxic environment with little or no energy. Light grey limestones were of oxic-to-sub-oxic environment. The stage of the thermal evolution for these source rocks provided by IH/T max diagram is of the "oil window". The average transformation ratio (TR) is estimated as 45% suggesting thus black shales are oil shale resources which still close about untransformed 55% of its hydrocarbon generating potential. The expulsion and release of oil into these source rocks are proven by the observation of hydrocarbons filling micro-cracks and by the variable values of the oil saturation index OSI ranging from 0 to 138%. The latter exceeds 100% near the paleo-high reliefs indicating two "oil crossover" areas attributed to the high degree of oil source rock saturation and accumulation of hydrocarbons considered ideal for hydraulic fracturing. This oil crossover is a consequence of secondary migration into black shale source rock, achieved by various faults created during the distensive phase that were reactivated again several times.
En Tunisie, le golfe de Tunis qui est une zone semi fermée commence à manifester des signes de dégradation. Le long de l'oued Méliane, principal oued de la région et ses affluents. Le débit le long de l'oued est faible et ponctuel sauf en cas de pluviométries importantes ou de crues. Les S.T.E.P localisées dans la région sont la station de prétraitement à El Fahs qui déverse dans l'oued et la grande station de Rades ben Arous déversant dans la mer. Les dégâts environnementaux qui commencent à être de plus en plus visibles et saillants dans les parties côtières montrent l'importance d'étudier les rejets liquides et solides. Cette étude met l'accent sur l'influence des activités anthropiques sur un tronçon d'environ 2,5 km linéaires par une étude granulométrique, minéralogique et géochimique des sédiments de la plage entre Rades-Ezzahra. Ces polluants sont essentiellement drainés par les eaux et les sédiments de l'Oued Méliane et son affluent Oued Maezete. Les résultats montrent que cette plage est polluée par le Cd et la matière organique qui sont en partie d'origine anthropique. Cette pollution n'est pas très ancienne et la situation globale s'est améliorée par rapport à 1993.
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