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Table 1
Descriptions of the various dark levels.
Descriptions des différents niveaux riches en MO.
| Organic-Rich Levels (OL) | Description |
|---|---|
| The Goguel Level | The Goguel Level is made up with six horizons of dark, marly, laminated shales (GO 1 to GO 6) amounting to a thickness of about 3.4 m in the Sauzeries section, with each horizon being multi-cm to multi-dm thick (Bréhéret, 1997). However, the laminated horizons are often interbedded with thin turbidites in numerous locations (Friès, 1987; Bréhéret, 1997; Friès and Parize, 2003; Caillaud et al., 2020). For example, the Goguel Level reaches a thickness of 17 m in the Saint-Jaume section, where it is interbedded with thin turbidites and one slump. This OL records a firstly negative and then positive δ13C excursion in the Vocontian Basin (Herrle et al., 2004; Westermann et al., 2013), as is the case for the OAE1a in the Tethyan basins (Menegatti et al., 1998). The Goguel Level contains a total organic carbon (TOC) of 1 to 5 wt% with hydrogen index (HI) values up to 500 mgHC/gTOC, which corresponds to Type-II kerogens, dominated by marine algal-bacterial OM (Bréhéret, 1994, 1997; Heimhofer et al., 2004, 2006; Westermann et al., 2013; Ando et al., 2017; Caillaud et al., 2020). This level was the time-equivalent of the OAE1a (Bréhéret, 1994, 1997) and, therefore, it has many equivalent levels in other basins: the Livello Selli in Italian basins, the Fischschiefer in the Lower Saxony Basin, and some other black shales in the Tethyan basins and Atlantic and Pacific Oceans (Föllmi, 2012). The astronomical calibrations allowed to determine a duration of ∼ 1.1 Ma for the OAE1a (Malinverno et al., 2010). The Goguel Level was deposited during a major transgression recorded in the Tethys Ocean (Haq et al., 1987; Hardenbol et al., 1998), and coincided with a MFS (Rubino, 1989; pers. com. in 2017; Bréhéret, 1997) or even a “flash” transgression in the Vocontian Basin (Ferry, 2017). In the Tethyan realm, the deposition of the black shales was accounted for by intense surface-water productivity at the onset of the OAE1a (Weissert, 1990; Föllmi, 1995; Sanfourche and Baudin, 2001; Westermann et al., 2013). In the Vocontian Basin, the model was more specific: the Goguel Level was deposited during a period of sedimentary condensation (Bréhéret, 1997; Heimhofer et al., 2006) in a stratified basin (Caillaud et al., 2020), but, the efficient preservation of OM was depending on the possible occurrence of thin turbidites. Therefore, where thin turbidites were interbedded within the Goguel Level, they favored dilution and remineralization of OM, whereas where the Goguel Level was deposited without turbidites, the concentration and preservation of OM was furthered (see Caillaud et al., 2020 for details). |
| The Niveau Noir Interval | The Niveau Noir Interval underlines a basin-wide color change from pale to dark marlstones (Rubino, 1989). Over about 8 m in the Saint-Jaume section, this interval is composed of one pair of dark marly shale horizons and three pairs of slightly lesser dark marly shale beds (NN1 to NN4, Bréhéret, 1997) interbedded within grey marlstones. In the Vocontian Basin, this OL coincides with a positive δ13C excursion (Herrle et al., 2004,) and its duration was estimated to ca. 300 kyr using cyclostratigraphy (Dauphin, 2002). The Niveau Noir Interval is termed “black shales” or “organic-rich level” in the literature (Herrle et al., 2003, 2010; Föllmi, 2012; Ghirardi et al., 2014); however TOC contents are comprised between 0.7 and 1.2 wt% only (Bréhéret, 1997). This interval shows very low HI (< 100 mgHC/gTOC), corresponding to Type-III kerogens (terrestrial or degraded OM). The Niveau Noir Interval was deposited at a 3rd-order MFS (Rubino, 1989; Friès and Parize, 2003) and was ascribed the “productivity-driven model” by Herrle et al. (2010). This model involves a period of strong surface-water productivity and evaporation, enhancing oxygen depletion in the deep basin and therefore the establishment of a water-column stratification (based on a study of calcareous nannofossil, Herrle et al., 2010). |
| The Fallot Level | Throughout an interval of about 21 m in the Serre-Chaitieu section, the Fallot Interval corresponds to numerous bundles of two or three horizons each (FA 1 to FA 4, Bréhéret, 1997) of dark marlstones within grey marlstones, with multi-cm to m thickness for each bundle (Bréhéret, 1997, Dauphin, 2002, Friedrich et al., 2003). Based on astronomical calibration performed on the Piobbico core in Central Italy, the duration of the Fallot Interval was estimated to be ∼ 360 kyr (Huang et al., 2010), that is, very close to the duration of ∼ 350 kyr proposed by Friedrich et al. (2003). TOC values are ranging between 1 to 2 wt% and HI values are highly variable (Bréhéret, 1997). Thus, the kerogen-type varies according to each bundle of horizons: Type-II, II-III or III: according to Rock-Eval data, the origin of OM is mixed. At the top of the Fallot Interval, a negative δ13C excursion is observed in the Vocontian Basin (Herrle et al., 2004), which can be correlated with other levels in Tethyan basins (“Livello Renz”, Coccioni et al., 2006) and in the Atlantic and Pacific Oceans (“Tahlmann” black shales, Herrle et al., 2003). The Fallot Interval was deposited during a HST (Rubino, 1989; Friès and Parize, 2003). Based on foraminifera, palynomorphs, and kerogen data, two depositional models were proposed by Friedrich et al. (2003). Some of bundles were formed by the “productivity-driven model” (see above) and show Type-II kerogens (marine OM). The other bundles correspond to the “preservation-driven model”, which implies a limited renewal of bottom-water in the basin coupled to an increase of continental runoff and fresh water inputs. Consequently, a water-column stratification developed in the Vocontian Basin, favoring the formation of dysoxic bottom-waters. |
| The Jacob Level | The OAE1b was deposited during the Upper Aptian to Lower Albian and is composed of several horizons of black shales in some basins in the Atlantic Ocean and Tethyan Realm (Arthur et al., 1990; Bralower et al., 1993, 1999; Erbacher et al., 2001; Leckie et al., 2002). In the Vocontian Basin, the Paquier Level was originally considered to correspond to the OAE1b (Bréhéret, 1994, 1997; Herrle et al., 2003), but, currently, the Jacob, Kilian, Paquier, and Leenhard Levels are now gathered in the OAE1b set (Leckie et al., 2002; Trabucho Alexandre et al., 2010; Hu et al., 2014; Bodin et al., 2015; Sabatino et al., 2015, 2018; Ando et al., 2017). The Jacob Level corresponds to two horizons (35 and 25 cm thick, respectively) of dark marly laminated shales within grey marlstones, over an interval 1 m thick in the Tarendol section (Heimhofer et al., 2006). On the basis of astronomical calibrations, a duration of ca. 40 kyr was proposed for this OL (Huang et al., 2010). TOC values are of ∼ 2 wt% on average and Rock-Eval data indicate Type-III kerogens (HI of ∼ 200 mgHC/gTOC; Bréhéret, 1997; Heimhofer et al., 2006). The strong contribution of terrestrial OM is confirmed by palynofacies observations (Heimhofer et al., 2006). Contrary to the other OL, neither positive nor negative δ13C excursions have been observed for the Jacob Level (Herrle et al., 2004). The regional equivalent of the Jacob Level is the “Level 113” in the Umbria-Marche Basin (Erba et al., 1989; Herrle et al., 2004) and there would exist another equivalent in the Central Atlantic Ocean (Sabatino et al., 2015). The “preservation-driven model” was proposed for the deposition of the Jacob Level, which is coherent with: 1) the presence of Type-III kerogens (Bréhéret, 1997); 2) palynofacies being dominated by terrestrial particles (Heimhofer et al., 2006); and, 3) foraminiferal data (Erbacher et al., 1998). The Jacob Level is interpreted to be a 3rd order MFS (Rubino, 1989; pers. com. in 2017 but see also Friès and Parize, 2003). |
| The Kilian Level | The Kilian Level is present as a 80 cm-thick horizon of dark marly shales within grey marlstones in the Tarendol section (Bréhéret, 1997). The TOC can reach up to 3 wt% with very low HI values (∼ 100 mgHC/gTOC), corresponding to a terrestrial origin of the OM (Type-III kerogen; Bréhéret, 1994, 1997). According to astronomical calibration, the duration of this thin horizon was estimated to be of ca. 120 kyr (Huang et al., 2010). In the Vocontian Basin, the Kilian Level corresponds to a strong negative δ13C excursion (Herrle et al., 2004). There are some equivalents of this level, sometimes also called “Kilian Level” in the Umbria-Marche Basin and Central Atlantic (Sabatino et al., 2015, 2018). The depositional model proposed by Herrle et al. (2003) is very close to the “preservation-driven model”. On account of a monsoon-type forcing, enhanced warm and humid conditions implied increased fresh-water inputs into the Vocontian Basin, and, as a result, the reduction of bottom-water ventilation (Herrle et al., 2003). The Kilian Level was formed during a TST (Rubino, 1989; pers. com. in 2017). |
| The Paquier Level | The Paquier Level consists of dark, marly, laminated shales, 1.6 m-thick, within dark marlstones in the L’Arboudeysse section (Herrle et al., 2003). This OL is observed in numerous locations with an equal thickness (Bréhéret, 1997). TOC values are ranging between 3 to 8 wt%, with HI values up to 500 mgHC/gTOC (Tribovillard, 1989; Bréhéret, 1994, 1997). Type-II kerogens indicate a marine OM, however, palynofacies observation put forward the significant contribution of terrestrial OM (Tribovillard and Gorin, 1991). In the Vocontian Basin, this OL coincides with a strong negative δ13C excursion (Herrle et al., 2004,) and it was estimated to have a ca. 44 kyr duration (Huang et al., 2010). The Paquier Level corresponds to the “Livello Urbino” in the Umbria-Marche Basin, and has equivalents in the Central and North Atlantic (Coccioni et al., 2006; Föllmi, 2012). In the Vocontian Basin, the Paquier Level was interpreted as a 3rd order MFS (Rubino, 1989; Friès and Parize, 2003), and, at larger scale, it was contemporaneous of a major transgression in the Tethyan realm (Haq et al., 1987; Hardenbol et al., 1998). Moreover, it was deposited during a period of warm and humid conditions with strong fresh-water inputs into the basin. These conditions were similar to those interpreted for the Jacob and Kilian Levels (“preservation-driven model”; OAE1b), but they were more intense and led to anoxic conditions in the deep basin (Tribovillard and Gorin, 1991; Bréhéret, 1997; Erbacher et al., 2001; Herrle et al., 2003; Ando et al., 2017). Owing to strong continental-water inputs that have furthered nutrient inputs, surface productivity increased in the basin. |
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