The global bio-events at the Cenomanian-Turonian transition in the reduced Bahloul Formation of Bou Ghanem (central Tunisia)

In central Tunisia, the thickness of the Bahloul Formation varies generally between 20 to 40 m such as for example, respectively, at wadi Smara near Kalaat Senan and at wadi Bahloul near Maktar. The thickness is also of several tens metres at Koudiat el Azreg near Jerissa and at Tajerouine. In these sites where the thickness of the Bahloul Formation is dilated, in addition to five geochemical events (δ¹³C), nine bio-events were defined near the Cenomanian-Turonian transition, successively in ascending order: 1. LO Rotalipora cushmani, 2. Heterohelix bio-event, 3. FO Pseudaspidoceras pseudonodosoides, 4. LO P. pseudonodosoides, 5. LO Thalmanninella multiloculata, 6. “filaments” bio-event, Cenomanian-Turonian transition, 7. FO Watinoceras sp., 8. FO Pseudaspidoceras flexuosum, 9. FO Helvetoglobotruncana helvetica. Even though the Bahloul Formation in the Bou Ghanem site is only 7.7 m thick (from which only 2.4 m of laminated black limestones), all nine marker bio-events were recorded, almost in the same order except some small differences. A good number of these bio-events are global as they are also present at Pueblo (Colorado, USA), the stratotype for the base of the Turonian stage. Consequently, at a global scale, it is possible to place the Cenomanian-Turonian boundary with one or several of these bio-events, with a good level of confidence

The filament event near the Cenomanian-Turonian boundary in Tunisia: filament origin and environmental signification

Near the Cenomanian-Turonian boundary, the filament event corresponds to the massive occurrence of thin elongate shells, probably bivalves which constitute, in the Bahloul Formation, about 40% of allochems. This biosedimentary event was described in several areas, such as in Tunisia, Algeria, Morocco, North America, Oman and Tibet. In Tunisia, filaments are well represented within well laminated black shales constituting the Bahloul Formation, late Cenomanian – early Turonian in age. These black shales were deposited under anoxic conditions developed in an outer ramp environment. Filaments are particularly abundant in the upper part of the Bahloul Formation, which is topped by a condensation level exhibiting encrusted ammonites associated with frequent burrows, and phosphatic and glauconitic grains. In terms of depositional environments, the filament concentrations announce an evolution towards “normal”-marine conditions, which followed the anoxic conditions favouring black-shale formation. This trend coincided with a deepening that led to maximum flooding at the condensation surface. The petrographic study of samples taken along a N-S transect shows a clear evolution of filament structures from distal to proximal ramp. Limestones deposited in the outer ramp environment mainly contain well preserved “pelagic” bivalve shells showing a thin-shelled structure composed of elongated filaments. Laterally, toward the proximal ramp, the bivalve shells are progressively disintegrated, the filaments being arranged parallel to bedding. In an even more proximal setting, filaments appear finer, highly fragmented and arranged without preferential orientation. The Cenomanian-Turonian “pelagic” bivalves thus probably lived in an outer ramp environment and their thin shells disintegrated progressively into filaments. This happened during high energy conditions generated by storms.The filament event, which is associated to other bio-events expressed by the extinction and the first occurrence of some pelagic species, could be used to establish local to global correlations.When the ammonite Watinoceras, the main early Turonian marker, is rare or absent, the filament-rich facies could indicate the proximity of the Cenomanian-Turonian boundary.At a global scale, the filament event illustrates a sea-level rise in relation with eustatic and climatic changes. The maximum flooding of this transgressive event was identified in several localities and dated about 93 Ma. The very high sea-level conditions are favourable to accumulation and preservation of organic-rich facies such as black shales, which illustrate the anoxic event OAE2.The abundance of filaments near the Cenomanian-Turonian boundary is an indicator of high organic productivity following a climatic change to warmer conditions. The latter have clearly influenced oceanic water-masses circulation, water column stratification and nutrient partitioning

Interaction morphodynamique d’une plage sableuse dans une conjoncture d’élévation du niveau marin ; exemple du littoral de Trab el Makhadha dans le golfe de Gabes-Tunisie

Le littoral de Trab el Makhadha (golfe de Gabès) est caractérisé par une évolution morphologique rapide et réversible à l’échelle saisonnière et évènementielle. Un suivi quotidien de l’évolution du profil transversal durant deux années (septembre 2007-novembre 2009) a mis en évidence les facteurs hydrodynamiques à l’origine du transport sédimentaire ainsi que le temps d’ajustement morphologique. Le changement morphologique du littoral de Trab el Makhadha, à l’échelle événementielle, est contrôlé par l’action du vent et des courants locaux ayant le temps d’ajustement morphologique le plus court. La cyclicité saisonnière entre accrétion estival et érosion hivernale du profil transversal de la plage est dictée par l’action de la houle. Le bilan sédimentaire, au cours des trente années précédentes, est excédentaire. La progradation du littoral est matérialisée par la mise en place, depuis les années 1980, d’un cordon sableux de 2 m de hauteur. La genèse du cordon côtier est contemporaine à l’accélération de l’élévation récente du niveau marin

Deep and machine learning methods for the (semi-)automatic extraction of sandy shoreline and erosion risk assessment basing on remote sensing data (case of Jerba island -Tunisia)

International audienceAgainst the backdrop of the environmental crisis, the socioeconomic , ecological and cultural importance of the coastal zone calls for greater awareness of how coastal resources function, evolve, are managed and enhanced. This study aims to develop a high-performance (semi-) automatic coastal monitoring method based on Landsat-5 and Sentinel-2 multispectral satellite images for spatiotemporal analysis of shoreline changes and erosion risk assessment along Jerba Island (Tunisia) using remote sensing data and geospatial tools. A comparative study between the band ratioing (BR) method and the pixel-based image analysis (PBIA) and object-based image analysis (OBIA) methods has led to the development of machine learning (ML), random forest (RF), deep learning (DL) and convolutional neural network (CNN) algorithms. Using these classification methods, 15 different shorelines were successively detected in 1989, 2015 and 2023 and then compared with a digitized reference shoreline from the Landsat-5 and Sentinel-2 images. Following a quantitative evaluation, the accuracy of the classification model shows that the combined CNN-OBIA approach provided the least accurate results, with an overall accuracy (OA) index of 67%, while the OBIA-RF classification method provided the most accurate results (OA of 95%). This comparative study identified an accurate and improved extraction method for quantifying changes in the position of the shoreline on the east coast of Jerba Island, enabling managers to make better decisions on coastal protection and adaptation to climate change

Spatio-temporal analysis of shoreline changes and erosion risk assessment along Jerba island (Tunisia) based on remote-sensing data and geospatial tools

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Assessment of Shoreline Change of Jerba Island Based on Remote Sensing Data and GIS Using DSAS Tools

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Assessment of Coastal Vulnerability to Erosion Risk Using Geospatial and Remote Sensing Methods (Case of Jerba Island, Tunisia)

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Properties Evaluation of Composite Materials Based on Gypsum Plaster and Posidonia Oceanica Fibers

Estimating the amount of material without significant losses at the end of hybrid casting is a problem addressed in this study. To minimize manufacturing costs and improve the accuracy of results, a correction factor (CF) was used in the formula to estimate the volume percent of the material in order to reduce material losses during the sample manufacturing stage, allowing for greater confidence between the approved blending plan and the results obtained. In this context, three material mixing schemes of different sizes and shapes (gypsum plaster, sand (0/2), gravel (2/4), and Posidonia oceanica fibers (PO)) were created to verify the efficiency of CF and more precisely study the physico-mechanical effects on the samples. The results show that the use of a CF can reduce mixing loss to almost 0%. The optimal compressive strength of the sample (S1B) with the lowest mixing loss was 7.50 MPa. Under optimal conditions, the addition of PO improves mix volume percent correction (negligible), flexural strength (5.45%), density (18%), and porosity (3.70%) compared with S1B. On the other hand, the addition of PO thermo-chemical treatment by NaOH increases the compressive strength (3.97%) compared with PO due to the removal of impurities on the fiber surface, as shown by scanning electron microscopy. We then determined the optimal mixture ratio (PO divided by a mixture of plaster, sand, and gravel), which equals 0.0321 because Tunisian gypsum contains small amounts of bassanite and calcite, as shown by the X-ray diffraction results