Granier et al., 2023 - T/B boundary, Tre Maroua

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Granier B., Benzaggagh M., Ferry S. (2023) Revised holostratigraphy of the Tithonian-Berriasian transition at Tré Maroua (Le Saix, Hautes-Alpes, SE France): Study of a rejected Berriasian GSSP candidate // Volumina Jurassica. Vol. XXI. P. 1–18. DOI: 10.7306/VJ.21.1

The study of the Tithonian and lower Berriasian succession of Le Saix (Hautes-Alpes, France) has made it possible to better characterize the lithological succession at a former Berriasian GSSP candidate, its set of microfacies, the stratigraphic ranges of the main groups of marine plankton and therefore the calpionellid and saccocomid biozonations. On the lithological level, the Tithonian strata are characterized by thick-bedded breccias representing debris flows and related calciturbidites, whereas the Berriasian strata are typically white limestones. The lower part of the Berriasian is comprised of scattered intercalations of thin-bedded breccias and calciturbitides (including cryptic mud calciturbidites). In thin sections, the white limestones display mud- to wackestone textures and their allochems are mostly tiny bioclasts (e.g., radiolarians, calpionellids, saccocomids). Calciturbidites have wacke- to grainstone textures and their allochems are mostly pseudointraclasts and extraclasts, comprising various bioclasts and some ooids. Mud turbidites are made of micrograinstones some yielding almost exclusively well-sorted calpionellids, which were previously erroneously interpreted as the signature of “explosions” or “blooms” of Calpionella alpina. Breccias are mostly lithoclastic floatstones with a matrix similar to that of 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. Radiolarians are common over the whole studied interval. Saccocomids are part of the dominating biota reported from the lower and lower upper Tithonian interval whereas calpionellids replace them in the uppermost Tithonian to lower Berriasian interval. Minor planktonic groups comprise calcareous dinoflagellates and Globochaete alpina; Iranopsis nov. group is also present. Intervals with saccocomid sections characteristic of zones 4–5 and zone 6 are respectively ascribed here to the lower Tithonian (4–5) and the lower upper Tithonian (6). The biozonation of the calpionellid group sensu lato allows identification of i) the Boneti Subzone of the chitinoidellids, ii) the Crassicollaria Zone, more specifically its Tintinnopsella-Intermedia (A1), Intermedia-Alpina (A2) and Brevis-Massutiniana (A3) subzones, and iii) the Alpina Zone, with its Alpina-Parvula (B1) and its Alpina-Remaniella (B2) subzones. On the basis of biostratigraphical and sedimentological data, most zonal boundaries prove to be hiatal, located at the erosional base of breccia or turbidite layers whereas the Tithonian/Berriasian stage boundary appears to be located at a strike-slip fault plane in the Tré Maroua section.