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The third-order Highstand Systems Tract (HST) overlying the early Toarcian TST along the North-South Axis is generally represented by shallower deposits, such as the shallow marine, bioclastic carbonates of the Guemgouma Formation and equivalent units (eg., [49,64,68]). In Zaghouane region limestone with Figure 7. Interpreted Seismic sections located to the west of the North South Axis (NOSA) confirming the existence of the anoxic facies of the Toarcian strata (from [29]) belemnite, zoophycos, filaments of the Kef el Orma Formation belonging to the middle Toarcian HST was deposited on the Dorasale and northern Tunisian trough (eg. BKH and R sections), implying a pronounced diachroneity of this lithostratigraphic unit. Unfortunately, no data have been given in the offshore Gulf of Tunis or the Pelagian Sea or the Mediterranean. The Figure 9 represents NW-SE seismic line and seismic interpretation of the Gulf of Tunis, nearby the Dorsale and the Northern Tunisian trough and we tried inspection about the presence of the early Toarcian black shales within this basin.

Figure 7 The third-order Highstand Systems Tract (HST) overlying the early Toarcian TST along the North-South Axis is generally represented by shallower deposits, such as the shallow marine, bioclastic carbonates of the Guemgouma Formation and equivalent units (eg., [49,64,68]). In Zaghouane region limestone with Figure 7. Interpreted Seismic sections located to the west of the North South Axis (NOSA) confirming the existence of the anoxic facies of the Toarcian strata (from [29]) belemnite, zoophycos, filaments of the Kef el Orma Formation belonging to the middle Toarcian HST was deposited on the Dorasale and northern Tunisian trough (eg. BKH and R sections), implying a pronounced diachroneity of this lithostratigraphic unit. Unfortunately, no data have been given in the offshore Gulf of Tunis or the Pelagian Sea or the Mediterranean. The Figure 9 represents NW-SE seismic line and seismic interpretation of the Gulf of Tunis, nearby the Dorsale and the Northern Tunisian trough and we tried inspection about the presence of the early Toarcian black shales within this basin.

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to 19 wt% [47] indicating variation in redox conditions, eustatic level and primary productivity. However, recorded total organic carbon values (TOC) from T-OAE black shales range globally from 0.60 wt%; Figure 1. Paleogeographical reconstruction during the Toarcian (ca. 185 Ma), A. position of the southem Tethyan margin among the Tethys and Panthalassa oceans, B. Organic rich black shale accumulation during the early Toarcian in the Tethyan margins. The box outlines Tunisia in which are marked three organic-rich Toarcian sections (Bou Kornine, Zaghouan and Chaabet El Attaris). Paleogeographical reconstructions adapted from (127,32,37])
to 19 wt% [47] indicating variation in redox conditions, eustatic level and primary productivity. However, recorded total organic carbon values (TOC) from T-OAE black shales range globally from 0.60 wt%; Figure 1. Paleogeographical reconstruction during the Toarcian (ca. 185 Ma), A. position of the southem Tethyan margin among the Tethys and Panthalassa oceans, B. Organic rich black shale accumulation during the early Toarcian in the Tethyan margins. The box outlines Tunisia in which are marked three organic-rich Toarcian sections (Bou Kornine, Zaghouan and Chaabet El Attaris). Paleogeographical reconstructions adapted from (127,32,37])
Figure 2. The main Oceanic Anoxic Events (OAE’s) discussed in the text and identified worldwide as well as in Tunisia, a tentative correlation with th events which occurred in Tunisia during the Mezosoic (Jurassic - mid-Cretaceous). Timescale, global events and carbon isotopic data are compiler from [37] Figure 3. Lithostratigraphic reconstruction of the Jurassic system in different basins of Tunisia discussed in the text (Tunisian trough, Dorsale, Nortt South Axis and the Chott basin) and biostratigraphic bioszonation (ammonites) of the Toarcian. The upper box (Tunisian trough, Dorsale, North Soutt Axis) is adapted from [49] and the lower box is adapted from [51]
Figure 2. The main Oceanic Anoxic Events (OAE’s) discussed in the text and identified worldwide as well as in Tunisia, a tentative correlation with th events which occurred in Tunisia during the Mezosoic (Jurassic - mid-Cretaceous). Timescale, global events and carbon isotopic data are compiler from [37] Figure 3. Lithostratigraphic reconstruction of the Jurassic system in different basins of Tunisia discussed in the text (Tunisian trough, Dorsale, Nortt South Axis and the Chott basin) and biostratigraphic bioszonation (ammonites) of the Toarcian. The upper box (Tunisian trough, Dorsale, North Soutt Axis) is adapted from [49] and the lower box is adapted from [51]
near Gulf of Tunis through Jebel Bou Kornine of Fahs, and along the North South Axis from Loridga through Khechem El Kalb. These black shales are labeled Chaabet El Attaris Formation [49]. This sedimentation pass laterally towards the Saharan platform domain into neritic carbonates, evaporates and clastics ([50,51,52]). central Tunisian platform series which seemed to be deposited in proximal basin setting. The Figure 3 shows that Jurassic strata encompassing the organic rich Toarcian black shale accumulation are cropping out in the northern area from Jebel Ammar through Thuburnic, along the Dorsale from Jebel Boukornine of Hammam Lif,
near Gulf of Tunis through Jebel Bou Kornine of Fahs, and along the North South Axis from Loridga through Khechem El Kalb. These black shales are labeled Chaabet El Attaris Formation [49]. This sedimentation pass laterally towards the Saharan platform domain into neritic carbonates, evaporates and clastics ([50,51,52]). central Tunisian platform series which seemed to be deposited in proximal basin setting. The Figure 3 shows that Jurassic strata encompassing the organic rich Toarcian black shale accumulation are cropping out in the northern area from Jebel Ammar through Thuburnic, along the Dorsale from Jebel Boukornine of Hammam Lif,
Figure 4. Thickness and facies distribution of the Toarcian anoxic event in Tunisia, data compiled after ([48,49,51,57,66]). Note the position on the map of the A-A’, B-B’ and C-C’ sections (Figs. 6, 9 and 12 respectively) 4,1.1. Distribution and Characteristics of the Organic- Rich Toarcian Strata In central Tunisia, [55] reported that the Jurassic sedimentation in Majoura area, limited to the east by the NS Axis (Figure 5), is controlled by N120 strike slip corridor of Gafsa. The general facies trend was complicated by a complex tectonic palaeorelief configuration which resulted in dislocation into tilted blocks towards the south, individualizing deep half-graben systems towards the North, the NW and the West since the late Sinumerian-Carixian ([49,51,55-61]). = This configuration results in strong lateral thickness and facies variations of the early Toarcian strata (Figure 7).
Figure 4. Thickness and facies distribution of the Toarcian anoxic event in Tunisia, data compiled after ([48,49,51,57,66]). Note the position on the map of the A-A’, B-B’ and C-C’ sections (Figs. 6, 9 and 12 respectively) 4,1.1. Distribution and Characteristics of the Organic- Rich Toarcian Strata In central Tunisia, [55] reported that the Jurassic sedimentation in Majoura area, limited to the east by the NS Axis (Figure 5), is controlled by N120 strike slip corridor of Gafsa. The general facies trend was complicated by a complex tectonic palaeorelief configuration which resulted in dislocation into tilted blocks towards the south, individualizing deep half-graben systems towards the North, the NW and the West since the late Sinumerian-Carixian ([49,51,55-61]). = This configuration results in strong lateral thickness and facies variations of the early Toarcian strata (Figure 7).
A comparison with the palaeogeographic map (Figure 4) suggests that the distribution of the organic-rich Chaabet Attaris is restricted to intermediate to medium water depth and may represents the impingement of an OMZ onto the northern margin of Africa (Figure 6) The Chaabet A ttaris Formation constitutes generally few tens of meters thick (2 to 30m) in the Dorsale and the North-South Axis areas Figure 5. Jurassic outcrop showing early Toarcian black shale accumulation pertaining to the Chaabet El Attaris Formation and main sections discussed in the text. Note the structural control of the Jurassic trend. On the map is also shown the position of wells and half-graben systems
A comparison with the palaeogeographic map (Figure 4) suggests that the distribution of the organic-rich Chaabet Attaris is restricted to intermediate to medium water depth and may represents the impingement of an OMZ onto the northern margin of Africa (Figure 6) The Chaabet A ttaris Formation constitutes generally few tens of meters thick (2 to 30m) in the Dorsale and the North-South Axis areas Figure 5. Jurassic outcrop showing early Toarcian black shale accumulation pertaining to the Chaabet El Attaris Formation and main sections discussed in the text. Note the structural control of the Jurassic trend. On the map is also shown the position of wells and half-graben systems
Figure 6. A-A’ section of the Figure 4: organic rich black shales distribution through an Oxygen Minimum Zone Model (OMZ) and Horst & graber structuration which controlled the anoxic deposition during the Toarcian. (modified A fter [57])
Figure 6. A-A’ section of the Figure 4: organic rich black shales distribution through an Oxygen Minimum Zone Model (OMZ) and Horst & graber structuration which controlled the anoxic deposition during the Toarcian. (modified A fter [57])
aquatic depositional environment under reducing bottom conditions [77] and it is concomitant with lower Pr/n-C,7 and Ph/n-C;, ratios (0.28 and 0.34 respectively) reflecting probably the marginal and sufficient maturity degree (T max) since the values of these ratios decrease with increasing thermal maturity (e.g. [78,79]). amounts within these sediments, inferred from the Pr/Ph ratios (1.27) is indicative of a redox environment (anoxic condition) and reflect significant input from the chlorophyll of higher plants, algae and photosynthetic bacteria and from archaeobacteria [76]. This low Pr/Ph ratio in the early Toarcian black shales suggests also an Figure 8. Tentative correlation of the early Toarcian organic-rich strata in Central Tunisia (see Fig. 4 and Fig. 5 for location)
aquatic depositional environment under reducing bottom conditions [77] and it is concomitant with lower Pr/n-C,7 and Ph/n-C;, ratios (0.28 and 0.34 respectively) reflecting probably the marginal and sufficient maturity degree (T max) since the values of these ratios decrease with increasing thermal maturity (e.g. [78,79]). amounts within these sediments, inferred from the Pr/Ph ratios (1.27) is indicative of a redox environment (anoxic condition) and reflect significant input from the chlorophyll of higher plants, algae and photosynthetic bacteria and from archaeobacteria [76]. This low Pr/Ph ratio in the early Toarcian black shales suggests also an Figure 8. Tentative correlation of the early Toarcian organic-rich strata in Central Tunisia (see Fig. 4 and Fig. 5 for location)
Figure 9. B-B’ section: Early Jurassic half graben systems controlling deposition of Toarcian strata on the Tunisian shelf in the Gulf of Tunis. Note the Triassic structural style. Position of early Toarcian strata only approximated. For location, see Figure 4. Interpretation is based mainly on the existing well data and on the NW-SE regional seismic section
Figure 9. B-B’ section: Early Jurassic half graben systems controlling deposition of Toarcian strata on the Tunisian shelf in the Gulf of Tunis. Note the Triassic structural style. Position of early Toarcian strata only approximated. For location, see Figure 4. Interpretation is based mainly on the existing well data and on the NW-SE regional seismic section
Figure 11. Facies, TOC and Maturity distribution maps of the Callovian anoxic level recorded within the utilized petroleum wells in this study after [49,51,54] is shown in the same Figure 11, which also includes the distinction of an organic-rich and an organic-poor Smida facies trends. The organic-rich Callovian Smida is penetrated by several wells in the area. The Figure 12 illustrates the distribution of the organic facies through a NW-SE cross section (C-C’) displayed by the Gamma-ray events. However, the exact boundaries still remain unclear, especially the northern Gulf of Gabes and the eastern Algeria. Organic-rich Callovian strata occur in large part of the southern Chotts domain of Tunisia and are grouped into he Smida Formation that was defined by Marathon Oil company (Figure 11) and is considered as the equivalent yf both the Protoglobigerinids marls and Ramthia imestone Formations ([49,51,54]). The thickness listribution of the Smida Formation in Southern Chotts lomain of Tunisia is illustrated in Figure 11. The regional listribution of the main Callovian facies types, compiled
Figure 11. Facies, TOC and Maturity distribution maps of the Callovian anoxic level recorded within the utilized petroleum wells in this study after [49,51,54] is shown in the same Figure 11, which also includes the distinction of an organic-rich and an organic-poor Smida facies trends. The organic-rich Callovian Smida is penetrated by several wells in the area. The Figure 12 illustrates the distribution of the organic facies through a NW-SE cross section (C-C’) displayed by the Gamma-ray events. However, the exact boundaries still remain unclear, especially the northern Gulf of Gabes and the eastern Algeria. Organic-rich Callovian strata occur in large part of the southern Chotts domain of Tunisia and are grouped into he Smida Formation that was defined by Marathon Oil company (Figure 11) and is considered as the equivalent yf both the Protoglobigerinids marls and Ramthia imestone Formations ([49,51,54]). The thickness listribution of the Smida Formation in Southern Chotts lomain of Tunisia is illustrated in Figure 11. The regional listribution of the main Callovian facies types, compiled
Figure 10. Total Organic Carbon (TOC) and Maturity distribution maps of the Toarcian anoxic facies (T-OAE). Data are provided from [57] and [53 4.2.1. Distribution and characteristics of the organic- rich Callovian strata
Figure 10. Total Organic Carbon (TOC) and Maturity distribution maps of the Toarcian anoxic facies (T-OAE). Data are provided from [57] and [53 4.2.1. Distribution and characteristics of the organic- rich Callovian strata
Table 1. Abbreviation codes, thickness and references of Tunisian early Toarcian sections discussed in the text. Th is Thickness
Table 1. Abbreviation codes, thickness and references of Tunisian early Toarcian sections discussed in the text. Th is Thickness
Figure 12. Gamma ray anomaly recorded through the Callovian anoxic level recording the Callovian anoxic event (C-OAE). Tentative correlation within the south chott basin The onset of deposition of the Smida Formation in the Southem Chott and Gulf of Gabes domains generally coincides with the base of the Callovian Trangressive Systems Tract (TST) of a major third-order Tethyan eustatic sea-level ([71,72,73]). Particularly, the transgressive surface (TS) is situated generally underneath
Figure 12. Gamma ray anomaly recorded through the Callovian anoxic level recording the Callovian anoxic event (C-OAE). Tentative correlation within the south chott basin The onset of deposition of the Smida Formation in the Southem Chott and Gulf of Gabes domains generally coincides with the base of the Callovian Trangressive Systems Tract (TST) of a major third-order Tethyan eustatic sea-level ([71,72,73]). Particularly, the transgressive surface (TS) is situated generally underneath
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Oceanic Anoxic EventsBlack shalesToarcian OAE
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