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Lower Toarcian ammonite biostratigraphy and sedimentary evolution of the Causses Basin (S France)

100%

Morard A.

tom Vol. 4, no. 1

193-194

Detailed palaeontological sampling of numerous sections in the Schistes Carton Formation from the Causses Basin (S France) revealed several important aspects for the biostratigraphy and palaeogeography of the Lower Toarcian in this epicontinental setting. First of all, a major hiatus is documented at the Domerian (Upper Pliensbachian) – Toarcian transition, spanning most of the NW European Hawskerense Subzone and Tenuicostatum Zone. A regressive trend is clearly documented in the Upper Domerian, capped by a red pyritic layer, interpreted as a hard-ground (Guex et al. 2001). Sedimentation resumes under transgressive dysoxic conditions at the very end of the Semicelatum Subzone. Further upward the ammonite succession is very similar to that of other NW European basins (e.g. Yorkshire), with the following recognizable faunal horizons: Semicelatum, Elegantulum, Exaratum, Elegans, Pseudoserpentinum, Falciferum and Douvillei. A stage of relatively low sea-level is still recorded up to the so-called “Fish-Bed” (Exaratum Horizon), which we interpret as an episode of episodic water column stirring due to tempests. The dysoxic conditions progressively disappear at the base of the Middle Toarcian, with the couches en échellons (Sublevisoni Subzone) where oxic/dysoxic conditions alternate. It should be noted here that the Lower-Middle Toarcian transition occurs within the Schistes Carton Formation. This boundary, as defined by the first occurence of the genus Hildoceras, does not exactly correspond to the Falciferum- Commune zone boundary as used in the British Isles (Howarth 1992). In our view, the top of the Bituminous Shales and the Ovatum Band already belong to the Sublevisoni Subzone, and hence to the Middle Toarcian. At the basin scale, the Lower Toarcian deposits are characterized by a wide and almost uniform spread of the Schistes Carton Formation, with thicknesses ranging from 50 cm to more than 32 m. This uniformity is due to the rapid and large trangression. Other sedimentary facies are only present in the most proximal areas, mainly as sands and ferrugineous oolites, with intercalated bituminous limestones. Very interesting faunas are in course of study from these localities, as they may represent records of uppermost Domerian and/or lowermost Toarcian limited to some few lenses.

Variability changes in the ammonite family Dactylioceratidae during the Lower Toarcian

63%

Morard A. , Guex J.

tom Vol. 4, no. 1

194-195

The comparison of successive dactylioceratid samples collected in the Moroccan Middle-Atlas, the Spanish Betic Cordillera, the Lusitanian Basin and the Causses Basin, allowed us to better characterize the range of variability of these forms from the very base of the Toarcian to the Lower-Middle Toarcian transition. From a biostratigraphic point of view, the Tethyan subgenus Eodactylites (Schmidt-Effing 1972) clearly precedes the NW European subgenus Orthodactylites – as recorded in the Lusitanian Basin succession for example, the latter then giving rise to the Dactylioceras s.s. Both early subgenera can be distinguished morphologically, and separated in a number of “species”, although isolated specimens may be difficult to identify. The main point is that Eodactylites, as well as early Orthodactylites form complete series of continuous covariation, the latter progressively branching into two main lineages in the Semicelatum Subzone. Following the occurence of some rare forerunners, known from the Middle-Upper Domerian, the evolution of the macroconch representatives of this ammonite family can be summarised in three main steps: 1. A sudden “mass apparition” of Eodactylites defines the base of the Toarcian. Their variability spectrum is immediately quite large, with a covariation series between a densely ornamented, somewhat involute and compressed pole (E. mirabile) and a more robust very evolute form with distant primary ribs and possible tuberculation (E. pseudocommune). 2. Ornamental variability tends to disappear in the outer whorls of Orthodactylites, whereas their inner whorls still display a wide covariation series (from slightly compressed forms with dense simple ribs to stout pantuberculate ones), as already noted by Howarth (1962). Intermediate forms seem to disappear in the Semicelatum Subzone. 3. Two lineages are then clearly distinct, one leading from isocostate to variocostate non tuberculate serpenticones (Dactylioceras s.s.), the second developing depressed whorls prone to tuberculation, particularly in inner whorls (Nodicoeloceras). A further diversification occurs at the Lower-Middle Toarcian transition (Guex 1971). In this new evolutionary frame, divergent views on the taxonomy of the Toarcian Dactylioceratidae can eventually be reconciled, as different authors in the past based their classification schemes only on a limited portion of the succession presented here.