Carnets Geol. 23 (7)  

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[1. Introduction] [2. Studied section] [3. Material and methods]
[4. Lithostratigraphy] [5. Biostratigraphy] [6. Discussion] [7. Conclusions]
and ... [Bibliographic references]

Hiatuses and redeposits in the Tithonian-Berriasian transition
at Le Chouet (Les Près, La Drôme, SE France):
Sedimentological and biostratigraphical implications

Bruno R.C. Granier

Dépt. STU, Fac. Sci. Tech., UBO, 6 avenue Le Gorgeu, CS 93837, F-29238 Brest (France)

Serge Ferry

6D avenue Général de Gaulle, F-05200 Briançon (France)

Mohamed Benzaggagh

Department of Geology, Moulay Ismail University, Meknes (Morocco)

Published online in final form (pdf) on June 27, 2023
DOI 10.2110/carnets.2023.2307

[Editor: Michel Moullade; language editor: Phil Salvador; technical editor: Bruno Granier]

Click here to download the PDF version!


Our new study of the Tithonian and lower Berriasian succession of Le Chouet (Les Près, La Drôme, France) better characterizes the lithological succession, the macro- and microfacies, and the stratigraphic ranges of some microfossils mostly calibrated on the calpionellid biozonation. On the lithological side, the Tithonian strata are dominantly characterized by thick-bedded breccias representing debris flows and related calciturbidites whereas the Berriasian strata are typically white limestones that also comprises scattered intercalations of thin-bedded breccias and calciturbitides (including cryptic mud calciturbidites). In thin sections, these white limestones display mud- to wackestone textures and their allochems are mostly tiny bioclasts (e.g., radiolarians, calpionellids, saccocomids). Breccias are lithoclastic rudstones and/or floatstones with a matrix similar to the calciturbidites. Their lithoclasts are either extraclasts sensu stricto (i.e., material derived from updip shallow-water areas) or pseudointraclasts, representing reworked subautochthonous material (i.e., mud- and wackestone lithoclasts with radiolarians, saccocomids and/or calpionellids). In addition to the erosional features observed at the bases of the gravity flows, these pseudointraclasts document the intensity of submarine erosion. Locally they help to estimate the depths of erosion updip of the deposit. A number of bioclasts are reworked from updip shallow-water areas; among them, it is worth mentioning the foraminifer Protopeneroplis ultragranulata (Gorbatchik), the first occurrence of which is dated to late early Tithonian. Saccocomids are part of the dominating pelagic biota reported from the lower and lower upper Tithonian interval whereas calpionellids replace them in the uppermost Tithonian to lower Berriasian interval. Intervals with saccocomids characteristic of zones 4-5 and zones 6-7 are respectively ascribed here to the lower Tithonian (4-5) and pro parte to the upper Tithonian (6-7). The biozonation of the calpionellid group sensu lato allows identification of the Boneti Subzone of the chitinoidellids, the Crassicollaria Zone with its four subzones (A0-A3), and the Alpina Zone with its first subzone (B1). On the basis of biostratigraphical and sedimentological data (including the rates of sedimentation), most zonal boundaries are located at the erosional bases of breccia or turbidite layers and thus coincide with hiatuses.


• Tithonian;
• Berriasian;
• calpionellids;
• saccocomids;
• erosion;
• debris flows;
• turbidites;
• lithoclasts;
• Vocontian Trough


Granier B.R.C., Ferry S. & Benzaggagh M. (2023).- Hiatuses and redeposits in the Tithonian-Berriasian transition at Le Chouet (Les Près, La Drôme, SE France): Sedimentological and biostratigraphical implications.- Carnets Geol., Madrid, vol. 23, no. 7, p. 123-147.


Lacunes et redéposition à la transition Tithonien-Berriasien au Chouet (Les Près, Drôme, SE France) : Implications sédimentologiques et biostratigraphiques.- Notre nouvelle étude de la succession du Tithonien au Berriasien inférieur du Chouet (Les Près, Drôme, France) précise la succession lithologique, les macro- et microfaciès, et les répartitions stratigraphiques de quelques microfossiles essentiellement calibrées ici sur la biozonation des calpionellidés. D'un point de vue lithologique, les dépôts du Tithonien sont principalement représentés par des brèches en bancs épais représentant des coulées de débris et par des calciturbidites associées, tandis que les dépôts du Berriasien sont typiquement constitués de calcaires blancs qui recèlent également des intercalations éparses de passées bréchiques et de calciturbitides (y compris des calciturbidites de boue, souvent cryptiques). En lames minces, ces calcaires blancs présentent des textures mud- à wackestone et leurs éléments figurés sont pour la plupart de minuscules bioclastes (comme, par exemple, des radiolaires, des calpionellidés, des saccocomidés). Les brèches sont des rud- et/ou floatstones lithoclastiques avec une matrice comparable aux calciturbidites. Leurs lithoclastes sont soit des extraclastes sensu stricto (c'est-à-dire du matériel remanié de zones peu profondes en amont), soit des pseudo-intraclastes, représentant du matériel sub-autochtone remanié (c'est-à-dire des lithoclastes à textures mud- à wackestone avec des radiolaires, des saccocomidés et/ou des calpionellidés). Outre les structures liées à l'érosion observées à la base des écoulements gravitaires, ces pseudo-intraclastes témoignent de l'intensité de l'érosion sous-marine. Localement, ils permettent d'estimer les profondeurs d'érosion en amont des coulées. Un nombre non négligeable de bioclastes sont remaniés de zones peu profondes en amont, parmi lesquels il convient de mentionner le foraminifère Protopeneroplis ultragranulata (Gorbatchik), dont la première apparition est datée ici du Tithonien inférieur terminal. Les saccocomidés font partie des organismes pélagiques dominants signalés dans l'intervalle Tithonien inférieur à supérieur, tandis que les calpionelles les remplacent dans l'intervalle Tithonien supérieur à Berriasien inférieur. Les intervalles à saccocomidés, caractéristiques des zones 4-5 ainsi que des zones 6-7, sont respectivement attribués ici au Tithonien inférieur (4-5) ainsi que pro parte au Tithonien supérieur (6-7). La biozonation du groupe des calpionelles sensu lato permet l'identification de la Sous-Zone à Boneti des chitinoïdellidés, la Zone à Crassicollaria avec ses quatre sous-zones (A0-A3) et la Zone à Alpina avec sa première sous-zone (B1). Sur la base de données sédimentologiques (y compris les taux de sédimentation) et biostratigraphiques, la plupart des limites de zones sont localisées aux bases érosives de couches de brèches ou de turbidites et correspondent donc à des lacunes sédimentaires.


• Tithonien ;
• Berriasien ;
• calpionellidés ;
• saccocomidés ;
• érosion ;
• coulées de débris ;
• turbidites ;
• lithoclastes ;
• Fosse vocontienne

1. Introduction

In parallel with recent investigations on the former Berriasian GSSP candidate at Tré Maroua, Le Saix, Hautes-Alpes (Granier et al., 2020b, 2022, 2023), our group studied another key section of the Vocontian Trough (SE France) for the Tithonian-Berriasian transition at Le Chouet, Les Près, La Drôme (Ferry & Granier, 2018). This second section (Figs. 1 - 2 ) was studied by Remane (1970: Fig. 6) and by the former Berriasian Working Group (Wimbledon et al., 2013: Fig. 4, 2020a: Supplement Fig. S1). Its ammonite fauna was first studied by Le Hégarat (1973) but it did not prove to be useful because the author's log (op.cit.: Fig. 25) cannot be correlated with any of the other logs. The Le Chouet section partly spans two lithostratigraphic units: 1) the uppermost part of the "brèches tithoniennes" (Tithonian breccias) below and 2) the lower part of the "calcaires blancs vocontiens" (Vocontian white limestones) above. However, as shown in the logs (Figs. 2 - 3 ), facies separation is no so clear cut: lime mudstone layers sporadically occur between conglomeratic beds of the Tithonian breccias whereas conglomeratic layers locally occur in the white limestones. This pattern merely reflects the decreasing intensity of the resedimentation over the Tithonian-Berriasian transition. Only the dominant type of macrofacies justifies this subdivision.

Contrary to Remane (1970), Wimbledon et al. (2013, 2020a) have mostly neglected the sedimentological aspects of the Le Chouet section. More specifically they underestimated the gravity-flow erosion and the redeposition of the derived material that obviously impacted the apparent continuity of the sedimentary record. As will be demonstrated below, although that is not always visible at the scale of temporal resolution of the ammonite zones or that of the calpionellids, the logged section is hiatal. It was already the case for the former Berriasian GSSP candidate at Tré Maroua, Le Saix, Hautes-Alpes (Granier et al., 2020b, 2022, 2023).

Fig. 1
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Figure 1: Topographic map of the Le Chouet area (A) and location of the sections studied (B).

Fig. 2
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Figure 2: Le Chouet log displaying the lithological succession, the stratigraphic distribution of the main (micro-) fossil groups of the Tithonian-lower Berriasian, both the calpionellid and saccocomid biozones, and the main reworked neritic elements. Autochthonous sedimentation (mud- and wackestones) is in white; cryptoturbidites (micropack- and micrograinstones) in grey; coarse grained turbidites (pack- and grainstones) in orange; debris flows (float- and rudstones) in blue-green. The red color in the distribution columns corresponds to reworked material. Њ: belemnite rostrum; ϡ: aptychus; grain size: s, silt; f, m, c, fine, medium, coarse sands; µbr, microbreccia; br, breccia.

Fig. 3
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Figure 3: Lithostratigraphic correlations of Remane's 1970 log (right column), Wimbledon et al.' 2013 log (left column), and this study (center; see caption in Fig. 2) with the various interpretations of the calpionellid biozones.

2. Studied section

The Le Chouet section is located some tens of meters southeast of the eponymic farm (Les Près municipality, La Drôme department, Fig. 1 ), in an area located in the center of the geological map at 1/50,000 scale of Luc-en-Diois (Flandrin, 1970). A composite section, more than 50 m long (Figs. 2 - 3 ), was measured along the tarred track from the D306 departmental road to the Le Chouet farm. It is split in three parts: 1) The lower part, which is sited on the left side of the stream, starts after a short tunnel section (GPS coordinates: 44°32'25.8"N 5°33'37.4"E). It corresponds to the first 10 m of the logged section. 2) The median part starts at the hairpin bend (GPS coordinates: 44°32'34.3"N 5°33'37.0"E) where the track crosscuts the Fournet stream, one of the small tributaries of La Drôme river. This median part, which is sited on the right side of the stream, ends with a fault at 46 m height on the log (GPS coordinates: 44°32'29.7"N 5°33'32.5"E). The outcrop conditions after the fault become poor; consequently, 3) the upper part of the section was not logged in detail. The whole section (Figs. 2 - 3 ) was measured with a Jacob's staff and later correlated with those of Remane (1970: Fig. 6) and Wimbledon et al. (2013: Fig. 4, 2020a: Supplement Fig. S1): Figure 3 . An eight meter offset exists between the log on Figure 2 and that on Figure 3 ; we shall refer to Figure 2 to give the elevations of the beds and samples on the logged section.

3. Material and methods

The Le Chouet section was sampled twice, the first 16 samples with labels ranging from Ch1 to Ch8c (collected on the occasion of a first visit on 2018/05/08 by the second author, S.F., see Fig. 2 ) and the second 64 samples with labels ranging from 41 to 10 (collected on the occasion of a second joint visit on 2019/05/12 by the first author, B.R.C.G, and the second author, S.F., see Fig. 2 ). The rock samples were cut and the derived slabs scrupulously examined in the search for mud turbidites, grainy turbidites and debris flows. In addition, sets of thin sections were successively prepared to validate the identification of mud turbidites, identify the source of the lithoclasts (parautochthonous or allochthonous), and study the microfossil contents.

4. Lithostratigraphy

4.1. Summary description

The lower part (up to 2 m on Fig. 2 ) is comprised of two massive breccia intervals of the uppermost Tithonian breccia. In the next interval (up to 14 m on the log of Fig. 2 ) the yellowish to light grey limestones that form the "background" basinal sedimentation are irregularly alternating with thin beds of fine-grained (Fig. 4H ) to mud (Fig. 4I ) turbidites. From 14 upward to 28 m on the same log, thick beds made of coarse-grained graded turbidites (Figs. 4B pars, C, E pars, F-G , 5G-I ) and breccias, often rudstones of pebbles and cobbles (Fig. 4B pars, D, E pars ), are common whereas the mud turbidites remain present (Figs. 4A , 6L ). The facies of the next and last interval, up to the fault (at 38 m), are characteristic of the "calcaires blancs" (white limestones) and include some cryptic mud turbidites (Fig. 5B ). Additionally, this interval still displays graded turbidites (Fig. 5C-D, F pars ) and breccias, often floatstones of pebbles and cobbles (Fig. 5A, E ).

Fig. 4
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Figure 4: Lower (I) and upper (A-H) Tithonian facies. A: pinkish cryptoturbidites 25; B: breccia 28 sandwiched between turbidites 27 (above) and 29 (below); C: breccia 35 and its basal erosion surface; D: breccia Ch7 and its basal erosion surface; E: breccia Ch7 (top) and graded turbidites Ch6 and Ch5 (bottom); F: enlargement of E, detail of the basal erosion surface of the graded turbidite Ch6; G: enlargement of E, detail of the graded turbidite Ch5; H: basal erosion surface of the turbidite 40; I: thin cryptoturbidite Ch4 with a silty beige base (m) indicating a faint grading. A, F-G, I: pencil for scale; B, D: Jacob's staff for scale; C, E: no physical scale; H: finger for scale.

Fig. 5
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Figure 5: Berriasian facies. A: floatstone breccia 10; B: pinkish cryptoturbidites 12 (m); C: graded turbidite 14; D: graded turbidite 16; E: enlargement of F, detail of floatstone breccia 18; F: turbidite 20 (bottom) and floatstone breccia 18 (top); G: graded turbidite 22; H: graded turbidite 24 (above) eroding fine-grained turbidite 25 (below); I: enlargement of H, detail of the contact of turbidites 25 (below) and 25 (above). A-B, D, F, I: hammer for scale; C, E: pencil for scale; H: no physical scale.

Fig. 6
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Figure 6: Reworked and in situ material from Le Chouet section. A-D: concentric radial ooids (a miliolid as a nucleus in D); E-F: biseriate foraminifers, Textulariidae; G: set of Globochaete alpina Lombard on a lamella (tangential section); H: extraclast of oolitic grainstone, reworked from a hardground as evidenced by its early marine fibrous cementation; I: Redmondoides lugeoni (Septfontaine) at the top and a biseriate foraminifer (Textulariidae) at the bottom; J: Koskinobullina socialis Cherchi & Schroeder at the top and Tubiphytes sp. at the bottom; K: incertae sedis; L: erosional surface at the bottom of a micrograinstone ("mud turbitidite", cryptoturbidite) with numerous calpionellid loricae in microlithoclasts; M: Mohlerina basiliensis (Mohler) at the left top and ammonite phragmocone in transverse section to the right; N: belemnite rostrum; O: aptychus; P: micrograinstone (fine-grained turbidite) with numerous calpionellid loricae in microlithoclasts. Photomicrographs A-K, N with the same scale (scale bar on J = 250 µm), photomicrographs L-M, O-Q with the same scale (scale bar on O = 500 µm). A: sample 14B; B, H: sample 15B; C: sample 34M; D: sample Ch8a; E, J: sample Ch6b; F, I: sample Ch8b; G: sample 31M; K: sample 40M; L: sample 30H; M: sample 16; N: sample 14M; Q: sample 22B; O: sample 31H; P: sample 25. Upper Tithonian: C-G, I-L, O; lower Berriasian: A-B, H, M-N, P.

4.2. Macrofacies and microfacies

In terms of macro- and microfacies, the rocks naturally fall into two categories: 1) the autochthonous facies and 2) the parautochthonous- and allochthonous-derived facies.

The first category corresponds to background facies related to the regular sedimentation of pelagic material falling and slowly accumulating on the seafloor. They consist of mud- and wackestone microfacies with saccocomids (which are here restricted to Tithonian strata), calpionellids (which are here restricted to upper Tithonian - lower Berriasian strata), and/or radiolarians (which may occur here in both Tithonian and lower Berriasian strata). Besides the pelagic groups of microorganims, which also comprise calcareous dinoflagellates (Fig. 7AH-AJ ), Globochaete alpina Lombard (Figs. 6G , 7Y-AB ) and the Iranopsis nov. group (Fig. 7AC-AG, AK-AN ), the thin sections contain various parautochthonous and autochthonous bioclasts, either smaller (e.g., sponge spicules) or larger ones [e.g., aptychii (Fig. 6O ), ammonite phragmocones (Fig. 6M pars ), belemnite rostra (Fig. 6N ) and jaws].

Fig. 7
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Figure 7: A-X: Calpionellid specimens from the Crassicollaria Zone (A3 subzone) and the Alpina Zone (B1 subzone) - reworked and/or in situ material - of the Le Chouet section. A-M: Brevis-Massutiniana Subzone (A3 subzone), N-T: Alpina-Parvula Subzone (B1 subzone), U-X: specimens from the top of the Crassicollaria Zone reworked in the Alpina-Parvula Subzone (B1 subzone). A-D: Crassicollaria massutiniana (Colom) (reworked specimens); E-G: Cr. brevis Remane (reworked specimens; G: specimen in a microlithoclast); H: Calpionella grandalpina Nagy (reworked specimen); I: C. elliptalpina Nagy (reworked specimen); J: small-sized C. alpina Lorenz (reworked specimen); K-L: medium-sized C. alpina Lorenz (reworked specimens); M: Tintinnopsella pseudocarpathica Benzaggagh et al. (reworked specimen); N-P: Calpionella alpina Lorenz with ovoid rounded loricae; Q: C. alpina Lorenz with ovoid elongated lorica; R-T: Crassicollaria parvula Remane; U-W: Calpionella elliptalpina Nagy (reworked specimens); X: Crassicollaria intermedia Durand Delga (reworked specimen). Y-AN: Various microfossils from Tithonian - lower Berriasian of the Le Chouet section. Y-AB: Globochaete alpina Lombard: Y-Z, as pairs; AA, isolated on a lamella; AB, set on a lamella (transverse section); AC-AG, AK-AN: Iranopsis nov. group; AH: Colomisphaera carpathica (Borza); AI: Parastomiosphaera tuberculata Benzaggagh et al.; AJ: Parastomiosphaera aff. malmica (Borza). Photomicrographs A-Z with the same scale bar = 100 µm, photomicrographs Y-AN with the same scale bar = 250 µm. A-B, D, AA: sample 22B; C, E-I, K, AB: sample 24B; J, L: sample 23H; M: sample 22H; N-O: sample 10; P: sample 18M; Q: sample 19B; R: sample 15H; S-T: sample 21B; U: sample 18M (reworked specimen); V: sample 21/1.0; W: sample 21B (reworked specimen); X: sample 16/1.2 (reworked specimen); Y, AM-AN: sample 36.5; Z: sample 36.4; AC: sample 40M; AD, AI, AL: sample 26/1.0; AE: sample 38B; AH: sample 18B; AF, AK: sample 19H; AG: sample 21/2.0; AJ: sample Ch2a. Lower Tithonian: AJ; upper Tithonian: Y-Z, AC-AE, AI, AL-AN; lower Berriasian: A-X, AA-AB, AF-AI, AK-AL.

The second category corresponds to gravity-flow deposition in the form of turbidites and associated debris flows. They consist of pack- and grainstones that commonly form the matrices of lithoclastic float- and rudstones. Basal surfaces of both turbidites and debris flows are commonly erosional (Fig. 8 ). Some lithoclasts have been created by the erosive processes that have affected more or less deeply the underlying strata. Subsequently they were incorporated into the turbidites and debris flows. These lithoclasts are not intraclasts, but pseudointraclasts, because they do not result from the in situ dismantling of the same and single layer but that of several discrete layers located updip on the slope. Accordingly, a single breccia layer contain several discrete types of pseudointraclasts (e.g., saccocomid wackestone lithoclasts, calpionellid wackestone lithoclasts, radiolarian wackestone lithoclasts, and even microlithoclastic grainstone lithoclasts). Granier et al. (2020b) referred to them as "extraclasts" whereas Granier et al. (2023) identified them as "pseudointraclasts" because their fabrics are similar to that of the background relatively deep-water sedimentation in contrast to the extraclasts sensu stricto that correspond to lithoclasts reworked from updip shallow-water areas.

Fig. 8
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Figure 8: Avalanche model for the formation of debris flows and related calciturbidites combining erosion and transportation modified from Postma et al. (1988).

Fig. 9
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Figure 9: Composite* debris flow microfacies. A: extraclast of hermatypic coral boundstone; B, F: extraclast of radiolarian wackestone; C: extraclast of oolitic grainstone; D: extraclast of microclastic grainstone (turbidite) with calpionellids; E: extraclast of saccocomid wackestone; G: extraclast of calpionellid wackestone; H: Tubiphytes sp. The matrix is a bioclastic and extraclastic (pack-) grainstone. The lithoclasts B and D-G, which should be derived from slope and/or basinal facies, are also referred to as pseudointraclasts. *: The background as well as clasts B and D-F are patches copied from another source and pasted here to illustrate a perfect example of calcareous debris flow (if all types of clasts may well occur in the same thin section, they hardly occur in the same field of view). The background is derived from thin section Ch8b-2 whereas the fake clast B is derived from 37H, D from 11, E from Ch1, and F from 34M. Genuine clast A is observed in thin section Ch6 whereas C is found in 14B, G in 18H, and H in Ch8b-2. All photomicrographs with the same bar = 500 µm.

The pseudointraclasts are usually larger and subrounded (Fig. 9B, D-G ) because they were made of mechanically abraded unlithified material whereas the extraclasts sensu stricto are commonly smaller and subangular (Figs. 6H , 9A, C, H ). Extraclasts sensu stricto but also skeletal grains (bioclasts) and concentric calcitic ooids (Fig. 6A-D, N pars ) are all reworked from shallow-water environments of a neighbouring platform edge. For instance, these extraclasts may consists of cemented oolitic grainstones (Fig. 9C ) or of microbial boundstones. Bioclasts are diverse comprising bryozoans, calcareous and siliceous sponges, hermatypic corals (Fig. 9A ), echinoderm remains, calcareous algae [among which Salpingoporella pygmaea (Gümbel) (Fig. 10F ), S. annulata Carozzi (Fig. 6N pars ) and Thaumatoporella parvovesiculifera (Raineri) (Fig. 11A-E ), as well as the problematic Iberopora bodeuri Granier & Berthou (Fig. 10S-U )], and foraminifers. The benthic foraminifers are the most diverse: ataxophragmiids (Fig. 12AH-AJ ), Redmondoides lugeoni (Septfontaine) (Fig. 6I pars ), textulariids (Fig. 6E-F, I pars ), lituolids (Fig. 10N-R, V ), miliolids (Fig. 6D ), lenticulinids, Frentzenella sp. (Fig. 11R-T ), Coscinoconus sp. (Fig. 10A-F ), Ichnusella spp. (Fig. 12A-AB ), Mohlerina basiliensis (Mohler) (Figs. 6M pars , 10J-M , 11O-Q, U ), Protopeneroplis ultragranulata (Gorbatchik) (Fig. 12AC-AF, ?AG ), as well as the incertae sedis Koskinobullina socialis Cherchi & Schroeder (Fig. 6J pars ) and Tubiphytes sp. (Figs. 6J pars , 9H ).

Fig. 10
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Figure 10: Reworked benthic foraminifers and calcareous algae from Le Chouet section. A-F: Coscinoconus sp.; G-I: ? Ataxophragmiidae; J-M: Mohlerina basiliensis (Mohler); N-O: cf. Anchispirocyclina lusitanica (Egger); P: sea urchin radiole (bottom right) and subepidermal meshwork of a large agglutinated foraminiferal test; Q-R: Nautiloculina sp.; S-U: Iberoporella bodeuri Granier & Berthou, with Calpionella alpina in U; V: large agglutinated foraminiferal test. All photomicrographs with the same scale (scale bar on R = 250 µm). A-B, L, S: sample 14B; C: sample 18H; D, I-K, M, U: sample Ch8b; E: sample 22M; F: sample 14M; G: sample 15B; H: sample Ch2a; N-O: sample Ch6b; P-R: sample Ch8a; T: sample 24B; V: sample 16. Lower Tithonian: H; upper Tithonian: D, I-K, M-R, U; lower Berriasian: A-C, E-G, F, S-T, V.

Fig. 11
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Figure 11: Reworked calcareous algae, sponges, and benthic foraminifers from Le Chouet (A-M, O-U) and Tré Maroua (N) sections. A-E: Thaumatoporella parvovesiculifera (Raineri); F-H, ?I: Salpingoporella sp. [F: S.pygmaea (Gümbel)]; J-K: ? Clypeina sp.; L-N: Perturbatacrusta leini Schlagintweit & Gawlick, 2011; O-Q, U: Mohlerina basiliensis (Mohler); R-T: Ichnusella sp.. All photomicrographs with the same scale (scale bar on R = 250 µm). A: sample Ch2b; B, H, J, L: sample Ch6b; C-D, G: sample Ch8b; E, T: sample 14B; F: sample 14M; I: sample 15B; K, R: sample Ch6a; M, S: sample Ch8a; N: Tré Maroua 58; O: sample 31M; P: sample 16; Q: sample 22M; U: sample 22b. Lower Tithonian: A; upper Tithonian: B-D, G-H, J-M, O, R-S; lower Berriasian: E-F, I, N, P-Q, T-U.

Fig. 12
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Figure 12: Reworked benthic foraminifers from Le Chouet section. A-AB: Ichnusella spp. including Ichn. infragranulata (Noth); AC-AF, ?AG: Protopeneroplis ultragranulata (Gorbatchik); AH-AJ: triseriate foraminifers, Ataxophragmiidae. All photomicrographs with the same scale (scale bar on AG = 250 µm). A, I: sample 14B; B, U, AA: sample Ch8a; C-D, H, S-T: sample 14M; E: sample 15B; F: sample 22B; G, AC: sample Ch2b; K-Q, V, X-Y, AB, AD-AE, AH-AJ: sample Ch8b; R, W, Z: sample Ch6b; AF-AG: sample 24B. Lower Tithonian: G, AC; upper Tithonian: B, K-R, U-AB, AD-AE, AH-AJ; lower Berriasian: A, C-F, H-I, S-T, AF-AG.

As in the Tré Maroua section (Granier et al., 2020b, 2023), the Le Chouet section contains a number of cryptoturbidites, i.e., well-sorted micrograinstones with a very finely lithoclastic composition. These mud turbidites are hardly detectable with hand lenses in the field (Granier et al., 2020b, Fig. 5). Their nature is fully revealed only with thin sections under a standard microscope (Fig. 6L, P ). Wimbledon et al. (2013, Fig. 8.2-4, 8.6) have identified similar microfacies as pelbiomicrosparites or pelbiosparites. As a matter of fact, most allochems that are less than 100 µm in diameter are not bioclasts or peloids but mostly small rounded pseudointraclasts (microlithoclasts, Fig. 6L, P ), commonly consisting of a calpionellid lorica filled and coated by micrite.

5. Biostratigraphy (M.B.)

5.1. Saccocomid biozones: For practical reasons, saccocomid sections have been named after their geometric shapes (Benzaggagh et al., 2015) as follows: biaxis with acute branches; biaxis with broad branches; 2prl.Br: two parallel branches; 2tn.prl.Br: two thin parallel branches; triaxis with acute branches; triaxis with broad branches; brd.2Ax: broad biaxis; broad molar tooth thin; concave molar tooth; convex molar tooth; div.elg.Br: divided elongated branch; elg.Br: elongated branch; elongated molar tooth; elg.psd-hxg.Hd: elongated pseudo-hexagonal head; elg.Tt: elongated tooth; flt.psd-hxg.Hd: flattened pseudo-hexagonal head; irg.Hd: irregular head; prp.Wg: propeller wings; srd.Wg: serrated wings; tn.Wg: thin wings; /2lat.apd:with two lateral appendices; / with axial tip; / with elongated axial tip; / with elongated tips; /int.cvt: with internal cavity; / with ovoid axial tip; / with pseudo-rectangular axial tip; / with pointed basis; / with short axial tip; / with simple tips; /tk.ts: with thick test; /tn.crw: with thin crown; /tn.flt.crw: with thin and flat crown; /trg.cvt: with triangular cavity.

This microfossil group largely dominates the pelagic assemblages of the Tethys realm at least from the earliest Tithonian (Hybonotum Zone) up to its disappearance in the latest Tithonian. However, it has been largely overlooked except in the outer Rif of Morocco. Five saccocomid zones (zones 3-7) have been defined for the Tithonian stage (Benzaggagh et al., 2015: Fig. 17). Four of them, i.e., the saccocomid zones 4 to 7, are identified in the Le Chouet section. The assemblages and successions at Le Chouet are very similar to those known in the outer Rif with a large dominance of thick sections in the lowermost Tithonian (Saccocomid zone 4), with highly diversified smaller sections in both the upper lower Tithonian (Saccocomid zone 5) and the lower upper Tithonian (Saccocomid zone 6), and finally with less frequent and undiversified sections dominated by biaxial sections in the middle upper Tithonian (Saccocomid zone 7). In the Le Chouet section, samples Ch1-Ch2b contain numerous saccocomid sections (Fig. 13A-CD ) dominated by large and thick forms (Fig. 13A-D, F-G ). The latter become scarcer in samples Ch3a-Ch4b with an assemblage dominated by sections typical of the upper lower Tithonian (Fig. 13E, H-T ). In the overlying samples (40M to 27), saccocomid sections become less and less abundant and are dominated by smaller forms (Fig. 13U-CD ). The group disappears at the base of the Crassicollaria subzone A3 above sample 27. Note that samples 29-27 were picked in a turbiditic interval and these specimens are possibly reworked. Sample 22B picked in the second debris flow level above the base of the calpionellid zone B (i.e., the former Tithonian/Berriasian boundary) also contains some reworked specimens.

5.1.1. Saccocomid zone 4 (equivalent to the Darwini and Semiforme zones, above the Hybonotum Zone) is characterized by an abundance of thick sections of the srd.Wg/tk.ts, irg.Hd, and flt.psd-hxg.Hd/2lat.apd types. At the base of the Le Chouet section (Ch1 and Ch2a-b), the typical saccocomid sections of the zone 3 are missing and these levels are ascribed to the saccocomid zone 4. The assemblage is dominated by large and thick sections, e.g., irg.Hd (Fig. 13A-C, H ), th.Wg/ (Fig. 13D, F ), and th.Wg/ (Fig. 13G ). It also comprises sections of the (Fig. 13E, R ), flt.psd-hxg.Hd/2lat.apd (Fig. 13N ), 2prl.Br/ (Fig. 13M ), div.elg.Br (Fig. 13I-J, LF ), elg.Tt (Fig. 13T ), and (Fig. 13P ) types.

5.1.2. In contrast to the previous zone, Saccocomid zone 5 (equivalent to the Fallauxi Zone) is characterized by the scarcity of thick sections and by the relative abundance of far less thick sections of the elg.psd-hxg, flt.psd-hxg.Hd, elg.Br, 2prl.Br/,, tn.Wg, elg.Tt, ml.Tt, and types. In levels Ch2a to Ch4b, the saccocomid assemblage is dominated by sections of the div.elg.Br (Fig. 13Q ), elg.Tt (Fig. 13K, S ), and /lrg.crw (Fig. 13O ) types.

5.1.3. Saccocomid zone 6 (equivalent to the Ponti Zone and, pro parte, the Microcanthum Zone) is rich in sections of tn.Wg, prp.Wg, elg.psd-hxg, 2prl.Br, 2tn.prl.Br,,, elg.Br,, and elg.Tt types, as well as the various sections of the ml.Tt type. In the Le Chouet section, the assemblage of samples 40M-36.1 is composed of sections of tn.Wg/ (Fig. 13BX, BZ ), flt.psd-hxg.Hd/2lat.apd (Fig. 13CA-CB ), elg.psd-hxg.Hd/2lat.apd (Fig. 13CC ), 2tn.prl.Br (Fig. 13BW ), brd.2Ax/trg.cvt (Fig. 13BK ), (Fig. 13BB ), 2prl.Br/ (Fig. 13BY ), (Fig. 13BD, BF-BG ), and (Fig. 13AN ) types, as well as frequent sections of (Fig. 13U1, X-Y1, W, AB, AD-AE, AL, AP, AU, AW ), (Fig. 13U2, Y2, AO, AR-AS, AV, AX-AY ), (Fig. 13AI, AK, AT ), (Fig. 13AF ), (Fig. 13BN-BQ, BT ), (Fig. 13BS ), (Fig. 13BL ), and elg.Br (Fig. 13BU ) types.

5.1.3. Saccocomid zone 7 (equivalent to the upper part of the Microcanthum Zone and the lower part of the Durangites Zone) contains fewer saccocomid sections. Its assemblage is largely dominated by sections of type, with a rounded (Fig. 13AA, AC, AG-AH, AM, BE ), elongated (Fig. 13BI ) or pseudo-rectangular (Fig. 13V, Z, AQ ) axial apex, and it contains rare sections of, elg.Br, 2prl.Br/, tn.Wg, (Fig. 13AZ, BA ), elg.Tt, and ml.Tt types. At Le Chouet, it also contains common type and rare sections of (Fig. 13BM ), (Fig. 13BR ), flt.psd-hxg.Hd/2lat.apd (Fig. 13CD ), (Fig. 13BC, BH ), and div.elg.Br (Fig. 13BV ) types.

Fig. 13
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Figure 13: Sections of skeletal segments of Tithonian saccocomids from the saccocomid zones 4 to 7 in the Le Chouet section. A-C, H: irg.Hd; D, F: th.Wg/; E, R, AN: 2Ax.act.Br/; G: th.Wg/; I-J, L, Q: div.elg.Br; K, S-T: elg.Tt; M: 2prl.Br/; N: flt.psd-hxg.Hd/2lat.apd; O-P:; U1, W, X, Y1, AA, AB, AC, AD, AE, AG, AH, AL, AM, AP, AU, AW, BE:; U2, V, Y2, Z, AO, AQ, AS, AV, AX, AY:; AF, AR, BI:; AI, AK, AT:; AJ:; AZ, BA, BB:; BC, BD, BF, BG, BH:; BK: brd.2Ax/trg.cvt; BL:; BJ: new unnamed morphotype; BM-BQ, BT:; BR:; BS:; BU: elg.Br; BV: div.elg.Br; BW: div.elg.Br; BX, BZ: tn.Wg/; BY: 2prl.Br/; CA-CB: elg.psd-hxg.Hd/2lat.apd; CC-CD: flt.psd-hxg.Hd/2lat.apd. All photomicrographs with the same scale bar = 250 µm. A-C: Ch2b; D-J, L-N, P, R, T: Ch1; K, O, Q, S: Ch4a; U, Y, AL, BD: 36.1; V, BC, BL-BM, BR, BV: 31H; W-X, AP, BE, BK: 38H; Z, AR, AZ, BG, BT, BW: 35M; AA, AM, AQ, BA, BI, BN, BO, CB: 34M; AB, AK, AV, AX, BX: 36.3; AC: 32B; AD: 36.2; AE-AF, BF: 37B; AG, AH, BH: 31B; AI, AO, AS, BP-BQ, BZ, BY, CC: 36.4; AJ: Ch8; AN, AU, BB: 37H; AT, AW: 36.5; AY, BS, CA: 38B; BE: 32/1.2; BJ: 36.6; BU: 40M; CD: 32H. Lower Tithonian: A-C, K, O, Q, S; upper Tithonian: D-J, L-N, P, R, T-CD. Lower Tithonian: A-T; upper Tithonian: U-CD.

5.2. Chitinoidella Zone (Boneti Subzone): Contrary to the Dobeni Subzone that to date has never been identified in any of the Vocontian Trough sections, the Boneti Subzone is characterized here by the occurrence of species belonging exclusively to the subfamily Bonetinae (Fig. 14A-F ), the assemblage of which is similar to those known from both margins of the Tethys realm (i.e., eastern and western Europe, Turkey, Iran, Cuba, Mexico, and recently reported from the Blue Nile Basin in Ethiopia, Jain et al., 2021) and outside it (e.g., in the Neuquén basin of the eastern Pacific margin of Argentina, Kietzmann, 2017; Kietzmann et al., 2021). In the Ardescian series, Cecca et al. (1989) have already reported the occurrences of "Chitinoidella sp.", "Ch. boneti Doben", and "Ch. cf. cubensis (Furrazola-Bermúdez)" whereas, in the Le Chouet section, samples 40M-37M contain Bonetilla boneti (Doben) (Fig. 14A ), Furrazolaia insueta (Řehánek) (Fig. 14B ), and F. cristobalensis (Furrazola-Bermúdez) (Fig. 14C-D ). Due to a discontinuous sampling from Ch4b to 40M, i.e., a nearly 5 meter interval, the base of the Boneti Subzone could not be accurately defined.

5.3. Calpionellid zones (Benzaggagh, 2020): Close-spaced sampling of the Le Chouet section allowed characterization of the four subzones of the Crassicollaria Zone and the first subzone of the Alpina Zone. The first specimens of calpionellids with microgranular loricae are reported from sample 40M whereas the first specimens of calpionellids with hyaline tests appear in sample 36.6. The latters are represented by primitive, often small loricae with undeveloped or poorly developed collars (Fig. 14G-X ). They are initially sparse to moderately abundant but they become abundant and dominate the pelagic microorganisms from the upper half of the upper Tithonian Crassicollaria Zone and throughout the lower Berriasian Alpina Zone.

5.3.1. Crassicollaria Zone (zone A): In the Vocontian Basin, as in most basins of the Tethys margins, the Crassicollaria Zone is dominated by the genus Crassicollaria. In the Le Chouet section, it spans the interval ranging from sample 36.6 (and probably Ch7) to sample 26B. Its base is marked by an interval containing the last chitinoidellids and the first primitive calpionellids (subzone A0), equivalent to the Remanei and Praetintinnopsella zones of some authors (e.g., Remane et al., 1986; Ölveczká & Reháková, 2022). The rest of the zone is subdivided into three successive assemblage subzones with from bottom to top: 1) one with numerous small to medium-sized Crassicollaria intermedia and some Calpionella grandalpina (subzone A1), 2) another with numerous regular Crassicollaria intermedia, larger in size than those of the underlying subzone, and with few Cr. colomi (subzone A2), and 3) a last one with numerous Cr. brevis and Cr. massutiniana but also with few Calpionella elliptalpina (subzone A3). The Chitinoidellid-primitive calpionellid Subzone (subzone A0) spans at least the sampling interval comprised between 36.6 and 36.1. It probably extends downward to the bottom of the breccia Ch7. It is characterized by the assemblage of the last chitinoidellids and the first calpionellids with hyaline tests, mostly primitive forms, with or without small collars. The chitinoidellids comprise representatives of the subfamily Bonetinae, among which Bonetilla boneti (Doben) and B. sphaerica Benzaggagh. Calpionellids are represented by primitive forms dominated by Crassicollaria aff. intermedia Durand Delga, small to medium in size (Fig. 14J-K, M-O, R ), and also comprise Calpionella aff. alpina Lorenz, small in size, with ovoid rounded (Fig. 14G ) or ovoid elongated (Fig. 14I ) loricae, and Tintinnopsella aff. carpathica (Murgeanu & Filipescu) (Fig. 14V-X ). We did not observe T. remanei Borza nor Praetintinnopsella andrusovi Borza. Both species are considered by several authors (e.g., Remane et al., 1986; Ölveczká & Reháková, 2022) as subzone indexes of the base of the Crassicollaria Zone, i.e., Remanei Subzone and Praetintinnopsella Subzone respectively. The Tintinnopsella-Intermedia Subzone (subzone A1) spans the sampling interval comprised between 35M and 32H. Calpionellids are larger in size than in the previous subzone. The first typical medium-sized Crassicollaria intermedia (Fig. 14AE-AF ) appears there. It also contains medium-sized Cr. aff. intermedia (Fig. 14AG ), as well as common Calpionella grandalpina (Fig. 14Y-AB ), small-sized C. alpina (Fig. 14AC-AD ), and Crassicollaria aff. massutiniana (Fig. 14AH-AK ), and few Tintinnopsella pseudocarpathica Benzaggagh et al. (Fig. 14U ) with smaller loricae than those of the genuine T. carpathica (Murgeanu & Filipescu) from the Berriasian-Valanginian. Bonetilla boneti (Doben) (Fig. 14F ), B. sphaerica Benzaggagh (Fig. 14E ), and primitive calpionellids (Fig. 14L, P-Q, S-T ) found in pseudointraclasts of breccia 35 are reworked from the underlying subzone A0. The Intermedia-Alpina Subzone (subzone A2) covers the samples 31B to 30H. Typical forms of Crassicollaria intermedia Durand Delga (Fig. 14AL-AO ) are dominating. Their sizes are often larger than those of specimens from the previous subzone. This subzone also commonly contains Cr. colomi Lorenz (Fig. 14AQ-AT ), Calpionella grandalpina Nagy (Fig. 14AY-BA ), small-sized C. alpina Lorenz (Fig. 14AW-AX), rare Crassicollaria massutiniana (Colom) (Fig. 14AP ), Cr. parvula Remane (Fig. 14AU ), and Cr. aff. brevis Remane (Fig. 14AV ). The Brevis-Massutiniana Subzone (subzone A3) comprises at least the samples 26/1.0 and 26B. It probably extends downward to the bottom of the breccia 29. Typical forms of Crassicollaria massutiniana (Colom) and Cr. brevis Remane dominate. It also commonly contains small-sized to medium-sized Calpionella alpina Lorenz, C. grandalpina Nagy, and few Tintinnopsella pseudocarpathica Benzaggagh et al..

5.3.2. The Alpina Zone (zone B), as originally defined by Remane (1963), corresponds to an "acmé" interval of the fossil index (op.cit., p. 62: "la prédominance du genre Calpionella"). The base of this biozone is not characterized by the first occurrence of Calpionella alpina but by the base of its first acme. Some followers of Remane have focused on identifying "explosions" of C. alpina. For instance, Wimbledon's summary Figure (Wimbledon et al., 2011, Fig. 1) identified not less than "three 'C. alpina' explosions". However, this "explosion" concept can be misleading as documented by Granier et al. (2020b, 2023). As a matter of fact, some cryptoturbidites (Fig. 6L, P ) are made of well-sorted calpionellid pseudointraclastic micrograinstones, a kind of dynamic accumulation that is not related to any biological bloom (Granier et al., 2023).

At Le Chouet, the Alpina Zone starts from breccia 24 (or possibly from turbidite 25 (Fig. 6P ), which is locally missing due to the subsequent basal erosion of debris flow 24) upward at least to breccia 10. The physical boundary is aligned with an important change in the calpionellid assemblage that corresponds to the disappearance of the main species of the Crassicollaria Zone, i.e., Crassicollaria brevis (Fig. 7E-G ), Cr. colomi, Cr. intermedia, and Cr. massutiniana (Fig. 7A-D ), which are still found in pseudointraclasts (Fig. 2 , red rectangles). Cr. parvula is the only representative of the genus Crassicollaria to persist in Berriasian times.

Only its first subzone, i.e., the Alpina-Parvula Subzone (B1 subzone), has been identified in the Le Chouet section. The abundance of calpionellid loricae contrasts with the low specific diversity of the whole assemblage that consists of -dominating- small-sized Calpionella alpina Lorenz (Fig. 7J-L, N-P ), -common- Crassicollaria parvula (Fig. 7S-T ), and -rare- Tintinnopsella pseudocarpathica (Fig. 7M ). The Alpina-Parvula Subzone (subzone B1) is largely dominated by small-sized Calpionella alpina, in particular the forms with ovoid rounded loricae (Fig. 7N-P ) and ovoid elongated loricae (Fig. 7Q ). It also contains common Crassicollaria parvula Remane (Fig. 7R-T ) and few Tintinnopsella pseudocarpathica Benzaggagh et al. Within the subzone B1 of the logged section, the breccias commonly contains lithoclasts with specimens of Calpionella elliptalpina Nagy (Fig. 7I, U-W ), C. grandalpina Nagy, Crassicollaria brevis and Cr. intermedia Durand Delga (Fig. 7X ), all reworked from the older strata of the Crassicollaria Zone.

Fig. 14
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Figure 14: Chitinoidellid and calpionellid specimens from the upper Tithonian - Boneti Subzone and Crassicollaria Zone (A zone) strata of the Le Chouet section. A-D: Boneti Subzone; G, I-K, M-O, R, V, X: Chitinoidellid - primitive calpionellid Subzone (A0 subzone); E-F, H, L, P-Q, S-T, U, W: material from the Chitinoidellid - primitive calpionellid Subzone (A0 subzone) reworked in the Tintinnopsella-Intermedia Subzone (A1 subzone); Y-AK: Tintinnopsella-Intermedia Subzone (A1 subzone); AL-AP, AU-BA: Intermedia-Alpina Subzone (A2 subzone); AQ-AT: Intermedia-Alpina Subzone (A2 subzone) or ? Brevis-Massutiniana Subzone (? A3 subzone). A: Bonetilla boneti (Doben); B: Furrazolaia insueta (Řehánek); C-D: F. cristobalensis (Furrazola-Bermúdez); E: Bonetilla sphaerica Benzaggagh; F: B. boneti (Doben); G-H: small-sized Calpionella aff. alpina Lorenz with rounded loricae; I: small-sized C. aff. alpina Lorenz with ovoid elongated lorica; J-T: Crassicollaria aff. intermedia Durand Delga; U: Tintinnopsella pseudocarpathica Benzaggagh et al.; V-X: T. aff. carpathica (Murgeanu & Filipescu); Y-AB: Calpionella grandalpina Nagy; AC-AD: small-sized C. alpina Lorenz; AE-AG: Crassicollaria aff. intermedia Durand Delga; AH-AK: Cr. aff. massutiniana (Colom); AL-AO: Cr. intermedia Durand Delga; AP: Cr. massutiniana (Colom); AQ-AT: Cr. colomi Doben; AU: Cr. parvula; AV: Cr. aff. brevis; AW-AX: small-sized Calpionella alpina Lorenz; AY-BA: C. grandalpina Nagy. All photomicrographs with the same scale bar = 100 µm. A: sample 40M; B-C: sample 37B; D: sample 38B; E-F, H, L, P-Q, S-U, W: sample 35M (reworked specimens from the subzone A0); G, I, N, V: sample 36.6; J-K, R: sample 36.4; M: sample 36.2; O: sample 36.5; X: sample 36.3; Y-AA, AF, AI-AJ: sample 32B; AB, AK: sample 32H; AC-AD, AH: sample 33; AE, AG: sample 34M; AL, AV, BA: sample 31H; AM-AO: sample 31B; AP, AU, AW-AZ: sample 30B; AQ-AT: sample 28 (reworked specimens from the subzone A2).

6. Discussion

6.1. Impact of sedimentation and erosion rates on the definition of biozonal boundaries

One could argue that a zone or subzone precisely starts with the first occurrence of the proxy (Fig. 15.1 ). This assumption can be valid when sampling is dense but it is debatable when the sampling frequency is low. For instance, in the illustrated model (Fig. 15.1-2 ), two samples A (below) and B (above) were picked 1 meter apart. Because we deal with a single lithology, it is assumed that the rate of sedimentation is constant. To simplify the graphical interpretation, the thickness scales (meters) on the left hand side of the columns and the time scales (ka) on their right hand side are congruent. The proxy B is observed in the red zone, not in the blue zone. In this simple model with one lithology (Fig. 15.1-2 ), the probability that B belongs to the red zone is 100% whereas the probability that A belongs to the red zone is nil. Symmetrically, the probability that A belongs to the blue zone is 100% whereas the probability that B belongs to the blue zone is nil. Depending of its relative sampling distance between A and B, the probability that a sample C picked between A and B belongs to zone B increases when C getting closer to B and decreases when C getting closer to A. On the basis of this approach, the zonal blue/red boundary is drawn as an oblique line, not as a horizontal line (Fig. 15.2 ).

For example, the base of the Chitinoidella Zone, i.e., the lower/upper Tithonian boundary, at Le Chouet is drawn as an oblique line from the location of sample CH4b up to that of sample 40M, both samples being picked more than 3 meters apart on the log. In alternative approaches, the boundary is commonly ascribed 1) either to the bottom of the B bed when the latter is found between samples B and A, 2) to a median location between samples B and A (that was the option chosen by Wimbledon et al., 2020a, Supplement Fig. S1), or 3) to a random location close to sample B (that was the option chosen by Wimbledon et al., 2013, p. 444: "The Crassicollaria-Calpionella" (Alpina) "zonal boundary may be placed between 101 and the next sample below", Fig. 4). All these non-probabilistic approaches (either the interpolation or the diagonal boundary) are misleading in the case of Le Chouet (an previously in that of Tré Maroua, Granier et al., 2020b, 2023) when a turbidite or debris flow bed occurs between samples B and A.

As discussed above, identifying a zonal boundary can be straightforward when dealing with a single facies because the main concerns will be the sampling frequency and because, after an initial screening, one can always return to the field and densify the sampling around the boundary to better circumscribe it. However, in the case for most of Le Chouet section, two discrete types of facies differ markedly in their net sedimentation rates, which implies adaptation of the simplistic model. The sedimentation rate of the "calcaires blancs" (saccocomid, calpionellid or radiolarian mud- or wackestones) likely represents a relatively constant background basinal sedimentation of a few cm/ka whereas that of the episodic turbidites and debris flows is often higher than several centimeters (or even tens of centimeters) per hour (see Granier et al., 2013: Fig. 7.B).

Figure 15.3 illustrates a case with two lithologies where a slice of debris flow occurs within the basinal facies and between the locations of samples A and B. The first column (Fig. 15.3a ) corresponds to the lithology versus the thickness; the second column (Fig. 15.3b ) corresponds to the lithology versus the time. In a practical case, to convert thickness into time, the thickness of each interval should be divided by the corresponding rate of sedimentation. As previously noted for the single lithology case, the thickness scales (meters) and the time scales (ka) are congruent. The graphical thicknesses remain the same for the "calcaires blancs" but the graphical thickness of the debris flow tends toward zero (Fig. 15.3b ), i.e., the graphical representation of a few hours on a ka scale would be a horizontal line. However, the basinal facies above and below the debris flow are not exactly superposed. There is a time gap between them that represents basinal sediment (and possibly turbidites and debris flows) that was removed by erosion at the bottom of the debris flow (Fig. 8 ).

As stated in Granier et al. (2023), "There is no relationship" "between the depth of erosion", i.e., the amount of material removed at the erosion surface, "at a location and the amount of material accumulated above the erosion surface at this same location". The duration of such hiatuses is commonly impossible to determine. Following the previously described simplistic approach, the zonal blue/red boundary is drawn as an oblique line (Fig. 15.3c ). In order to revert to the original figure (Fig. 15.3a ), i.e., to convert again time into thickness, the duration of each interval should be multiplied by the rate of sedimentation for the corresponding lithology. On the new figure, the blue/red boundary is not a straight line anymore (Fig. 15.3d ). This last graphical representation makes it clear how, in this case, the debris flow very highly probably belongs to the red zone and that the base of this red zone should be located on the erosional surface at the base of this debris flow. The probability that the red zone extends below the breccia is not nil but quite low. For instance, as documented by Granier et al. (2020b, 2023) for the Tré Maroua section, the base of the Ferasini Subzone of the Alpina Zone and the base of the M18r magnetozone should be located at the bottom of the uppermost debris flow, i.e., number 69, not at its top as shown in Wimbledon et al. (2020a: Fig. 4).

Fig. 15
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Figure 15: Models for a confident identification of zonal boundaries based on the lithologies and the corresponding sedimentary rates. Sampling rate is not negligible. Erosion rates at the bottom of debris flows and turbitides may vary significantly. See description in the text.

Our control on the saccocomid zones or on the Chitinoidella Zone is poor, mainly due to a low sampling frequency in the corresponding interval. For instance, the significant uncertainty on the location of the lower/upper Tithonian boundary is mainly driven by a poor sampling within the interval from sample Ch4b to sample 40M.

In contrast, the bases of the Crassicollaria Zone and subzones and those of the Alpina Zone, hence of its first subzone, are well identified. They systemically correspond to the bottoms of breccia or turbidite layers:

6.2. Discrepancies in the placement of zonal boundaries

Besides the probabilistic or non-probabilistic options and the sampling frequency, the other key factor that generates inherent discrepancies in the location of the zonal boundaries can be differences in the diverse acceptions of species or zones by the various authors. For instance, Reháková (in Wimbledon et al., 2013, 2020a) does not refer to Tintinnopsella pseudocarpathica Benzaggagh et al., 2012, or Bonetilla sphaerica Benzaggagh, 2021. The various authors do not necessarily use the same subzones (see Benzaggagh, 2020, for discussion).

As for the calpionellid biozones, all authors generally share the same view regarding the definitions of the Chitinoidella, Crassicollaria (zone A) and Alpina (zone B) zones. Due to poor sampling and facies that are not favorable to the preservation (or identification) of the chitinelloids, i.e, turbidites, the base of the Chitinoidella Zone cannot be accurately identified: It is expected to fall between 9 and 12 m on the log of Figure 2 whereas it would be near 5 m (below our turbidite Ch2) according to Remane's log (1970) and near 9 m according to Wimbledon et al.' (2020a) log. All the authors agree upon the base of the Crassicollaria Zone near 15.5 m (at the bottom of our breccia Ch7) on the log of Figure 2 . The main discrepancies occur at the base of the Calpionella alpina acme Zone: 1) Remane (1970) identifies it at the base of turbidite 22, near 27.7 m; 2) Wimbledon et al. near 30 and near 29.5 m in 2013 and in 2020 (2020a) respectively; 3) we assume that it should be placed at the base of turbidite(s) 24 (or 25), near 26.5 m on the log of Figure 2 . It is worth mentioning that the marker for this boundary (i.e., the base of the Calpionella alpina acme Zone) was the candidate proxy for the Berriasian GSSP proposed by the past Berriasian Working Group (Wimbledon et al., 2013, 2020a). There are also significant discrepancies regarding the subzones of the Crassicollaria Zone. The only agreements are found with 1) the Remanei Subzone of Wimbledon et al. (2020a) that matches our subzone A0 and 2) the base of the Intermedia Subzone of Wimbledon et al. (2020a) and that of our subzone A1 because both fall at the base of turbidite 35, near 18 m on the log of Figure 2 . The bases of Remane's (1970) subzones A2 and A3 respectively fall on top of turbidite 33, at 20.5 m, and above turbidite 27, at 25.5 m on the log of Figure 2 . The bases of the Intermedia and Colomi subzones of Wimbledon et al. (2013, 2020a) fall between turbidite 33 and breccia 31, at 22 m for the first one, and below turbidites 24-25, near 26.4 m for the second one. In contrast and as for our previous zones and subzones, the bases of our subzones A2 and A3 correspond to the erosional bases of turbidites or debris flows, i.e., to the base of breccia 31, at 23 m for the first one, and to the base of turbidite 29, at 24.5 m for the second one.

6.3. Significance of the local erosion

From the outcrops on the side of the tarred track, the erosional bases of the gravity flows (turbidites and debris flows) at all scales are well exposed (Figs. 4H , 5C, F, H ). In Figure 5H , turbidite 25 is partly eroded at the bottom of the debris flow 24. Many extraclasts and other erosional features such as microscopic erosional surfaces are also visible under the binocular microscope (see Fig. 6L here or see Granier et al., 2023, Fig. 8E at Tré Maroua).

According to Granier et al. (2023), "There is inherently no relationship between the thickness of a debris flow or a turbidite layer at a location and the amount of material that was eroded from this same location and that accumulated downdip". Due to the limited temporal resolution of the saccocomid and calpionellid biozones, estimating the depth of erosion is challenging, not to say almost impossible. However, there are a few exceptions. For instance, at Tré Maroua, Granier et al. (2020b, 2023) reported saccocomid-bearing lithoclasts more than 2 meters and even more than 10 meters above the last saccocomid zone. From the same section, Granier et al. (2020b, 2023) also documented Crassicollaria-bearing lithoclasts (Granier et al., 2020b, Pl. 3, figs. B-C) more than 5 meters and even more than 9 meters above the Crassicollaria Zone. At Le Chouet, there are fewer clues. In breccia 24, at 27 m on the log of Figure 2 , i.e., at the bottom of the Alpina Zone, Crassicollaria brevis Remane, Cr. intermedia Durand Delga (Fig. 7X ), and Cr. massutiniana (Colom) are found in extraclasts derived from the preceding zone (Crassicollaria Zone). This same Crassicollaria assemblage is also observed within the Alpina Zone in breccia 16, at 31.7 m, more than 5 meters above the Crassicollaria Zone.

7. Conclusions

The Le Chouet section spans a Tithonian-lower Berriasian interval. Saccocomid zones 4 to 7 as well as Chitinoidella, Crassicollaria and Alpina zones of the calpionellids have been documented. However, the Dobeni Subzone of the Chitinoidella Zone has not been identified in its lower part and only the first subzone of the Alpina Zone, i.e., the Alpina Subzone, has been documented at its upper part.

Whereas the location of the lower/upper Tithonian boundary remains uncertain (possibly close to Ch4, i.e., near 9 m on the log of Figure 2 ), the Tithonian/Berriasian boundary defined by calpionellid biostratigraphy coincides with the base(s) of turbidite 25 (or breccia 24 when the previous one has been eroded, i.e., near 26.5 m on the log of Figure 2 ): Figs. 4A , 5H-I , 6B . That pushes the Tithonian/Berriasian boundary down by almost 1 m relative to Remane (1970; note that most Crassicollarias found in the interval are here considered to be reworked) and some 4 m relative to Wimbledon et al. (2013, 2020a).

The foraminifer Protopeneroplis ultragranulata (Gorbatchik) is the index of the P. ultragranulata Subzone of the Anchispirocyclina lusitanica Zone (Tithonian-lower Berriasian). This microfossil was supposed to first occur in the late Tithonian (Bucur, 1997) and the eponymic subzone to span the upper Tithonian-lower Berriasian interval (Granier, 2019; Granier et al., 2020a). However, its find in upper lower Tithonian strata (Fig. 12AC , sample Ch2b) extends its subzone downwards. Granier (2019) stated that "it is impossible to distinguish the (upper) Tithonian from the lower Berriasian" in the shallow-water facies of the Tethys realm. This wider range for the index of the subzone drives the nail deeper into the coffin. This argument, together with the lack of biological crisis and the instability of the boundary definition and location, supports the assessment that the default Tithonian/Berriasian boundary (i.e., base of the Calpionella alpina "acme" Zone) is a poor system boundary. In contrast, as stated by Énay (2020), "the base Valanginian, which corresponds to biotic crises affecting the ammonites and other groups, is by far the better alternative" for "the Jurassic/Cretaceous system boundary".

The frequency of gravity-flow (debris flows and turbidites, including cryptoturbidites) led us to reassess the calpionellid stratigraphy with the eyes of sedimentologists. All bases of calpionellid zones and subzones (except those of the Chitinoidella Zone and its Boneti Subzone) are located at basal erosional surfaces of turbidites or debris flows. Accordingly, as in the Tré Maroua section (Granier et al., 2020b, 2023), all these zonal boundaries are probably hiatal here too. As already explained in Granier et al. (2023), not only these two localities but even "a wider region in the Vocontian Trough", which is affected by "paleotectonic instability", should enevitably fail to provide any suitable Berriasian GSSP candidate "in contradiction with the expectations of the 'Colloque sur la limite Jurassique/Crétacé' held in Lyon in 1973 (Flandrin et al., 1975)". It is worth mentioning that corresponding time gaps at erosional surfaces, even if short, artificially emphasise the contrasts between the successive calpionellid assemblages.

The depth of erosion at the bottom of any gravity-flow bed can hardly be estimated in any single site. However, because pebbles and cobbles sourced from the Crassicollaria Zone are reported at Le Chouet some 5 m above the local top of this zone, it is assumed that the depth of erosion could have reached more than 5 m in some updip locations. On the basis of data from the Tré Maroua site (Granier et al., 2020b, 2023), it can even be estimated that updip depth of erosion could have locally reached 10 m or more. Considering published regional data (Courjault, 2011; Courjault et al., 2011; Ferry et al., 2015; Ferry, 2017), such deep erosional features did not result from a single episode but probably from multiple, successive episodes digging each time deeper into the preexisting sedimentary substratum.


Field work was made possible thanks to a grant from the Foundation "Carnets de Géologie". Part of this work was presented on the occasion of JK2018 - International Meeting around the Jurassic/Cretaceous Boundary held in Genève (Switzerland) from December 5th to 7th, 2018 (Ferry & Granier, 2018), and of the 11th International Cretaceous Symposium held in Warsaw (Poland) from August 22th to 26th, 2022 (Granier et al., 2022). It is a contribution to Project no. 46908YB of the PHC ("Partenariats Hubert Curien") Polonium 2022. The manuscript was originally submitted to Geologica Carpathica and later withdrawn due to requests to remove or modify some key figures. However, the authors are grateful to the two reviewers and the editor for their comments and suggestions that helped improve the original manuscript. Special thanks are also due to Phil Salvador who kindly checked the final version of our English text.

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