Document sans titre

2. STOP n° 2 : Triassic of Dio ; (GG, J Ga).

* Stratigraphy (GG, fig. 6).
In the Lodévois area, Triassic series is about 250 m thick. Near Dio village it overlays the Permian (Saxonian) with some degrees of unconformity. Beneath the contact, in Permian red pelites, one can see numerous carbonate nodules (dolocretes). These structures result from pedogenesis occurring before the Triassic deposits.
According to ALABOUVETTE et al. (1982, 1987), the Triassic series consists of the following formations from the base to the top (fig. 3).
– t3 :‘Grès de base’ (Lower Sandstones = 30-50m) : alternation sandstones/red mudstones with sometimes pebbles-conglomerats coming from the basement.
– t4-5 : ‘Argilites inférieures’ (Lower Mudstones = 20-30m) with a succession of variegated mudstones (containing NaCl pseudomorphose) / sandstones / dolostones yielding numerous footprint levels (see below) and Voltzia type macroflora alike to the Lower Triassic macroflora of the Vosges.
– t6 : ‘Grès médians’ and ‘Barre carbonatée’ (Upper Sandstones and Conglomerates + Laminated Dolomite = 80 m). Successively, this formation consists of dolomite / sandstone, massive sandstones banks with conglomerates (50m) then dolomite.
Grès de base – Grès médians (Upper Sandstones and Conglomerates) are the ‘Lower Formation’ sensu Lopez & Mader (1992). This latter is dated Middle Anisian-Ladinian and contains footprints, paleoflora and Myophoria sp. It indicates a playa-sabkha environment with occasional fluvial channels.
– t7-9 : ‘Argiles Supérieures’ (Upper Mudstones =100m). They are multicoloured with successively : carbonate interbedding, Cavernous Dolomite, mudstones with anhydrite and gypsum.
The Formation t7-9 = former Keuper = Middle and Upper Formation sensu Lopez & Mader (1992). It is dated Carnian (by palynoflora) and Norian.
– t10 : Rhétien = Rhaetian Formation. There are 2 Members. The lower, 30 m thick, has sandstones, dolomites with fishes scales, pelecypodes and Avicula contorta. The Upper Member (15 m) contains above all carbonates.
In summary the Formations ‘Grès de base – Argilites inférieures’ (Lower Sandstones-Lower Mudstones) form the lower megacycle meaning a vertical environmental evolution from a fluvial system to a playa via a flood plain. The ‘Barre Carbonatée’ and ‘Argiles Supérieures’ (Upper Mudstones) Formations are the upper megacycle. It was deposited in an littoral environment with supratidal sabkhas. Rhaetian is the last fluvial system before the inter-subtidal marine hettangian sedimentation.

Fig 6– A = Triassic serie after Alabouvette et al 1987 (CB : carbonated bank, F : footprints). F = traces de pas) ; B à I = ichnites avec B et D = piste Rotodactylus bessieri DEMATHIEU, 1984, C = Squelette pédieux de Macrocnemus bassanii, E = Brachychirotherium gallicum (WILLRUTH, 1917) : 2 couples pied (P)-main (M), F = squelette pédieux reconstitué, G = pistes partielles de Brachychirotherium gallicum, H = Chirotherium barthii, I = piste Rhynchosauroides lutevensis DEMATHIEU, 1984 ;

* Palaeontology

• Palichnofauna (GD, fig. 6)
Vertebrate fossils have not been discovered into these Triassic layers. In contrast numerous ichnofossils have been mentioned and some of them, believed to be derived from vertebrates, have been known since 1857 and first described by P. GERVAIS. They include casts on the lower surface of thick slabs that come from the neighbourhood of Lodève, the village of Fozières. No one of these slabs, supplied to different Museums, Paris, Lodève, Genève, Lyon etc., has been founded in situ. Nevertheless it seems probable that their stratigraphic position lies into ‘t.4-5’. These levels contain the following footprints : Chirotherium barthii Kaup 1835, Brachychirotherium gallicum (WILLRUTH, 1917), Brachychirotherium circaparvum DEMATHIEU, 1971, Rotodactylus bessieri DEMATHIEU, 1984, Rhynchosauroides peabodyi FABER 1958, Rhynchosauroides lutevensis DEMATHIEU, 1984.

1 – Chirotherium barthii (221 x 155 mm) is represented by only a few footprints and was probably derived from a quadrupedal and pentadactyl Thecodont whose foot structure was somewhat advanced in the evolution of the Archosaurs by the high value of the cross axis (=90°), a very narrow trackway, and a high value of the pace angulation (about 180°).

2 – Brachychirotherium gallicum (WILLRUTH, 1917) (B.g, fig. 6A) is smaller (175 x 110mm) and the pes shows other characters : cross-axis is ≤ 70° and so indicates a more primitive organization of the basipode of the foot. Cross-axis lies between 70-80° – a common value for the ichnospecies of this genus. The fifth toe is short and directed straight to the outer part of the pes. Small and round scales are visible on a very well preserved imprint. A manus is always present and is smaller than the pes, by a quarter. It has about the same shape but its width is greater than the length.
3 – Brachychirotherium circaparvum DEMATHIEU, 1971 (B. C fig. 6A). Like the other pes imprints of this genus, the digits are rather thick with claws directed toward the outer side of the pes, except the fourth for which it seems it has no claw. Among digits, II ≤ III. The manus is somewhat rotated toward the outer side of the trackway. The characters of these footprints lead us to think they were made by a thecodont sphenosuchien.
4 – Synaptichnium sp. NOPCSA, 1923. This ichnogenus is known here from only two specimens ; its main characters are : quadrupedy, pentadactyly, subparallelism of the four first toes. The IVth is the longest. The fifth is right and in the continuity of the metatarsal. The cross-axis is low : 60-70° which points out a primitive feature of the basipode .
5 – Rotodactylus is a very peculiar pentadactyl imprint showing the fifth. Digit V impressed behind the set of the four first digits. Rotodactylus bessieri DEMATHIEU, 1984 has a medium size for the ichnogenus. The trackmaker is not well known ; Dinosaur or Lepidosaur according to different authors.
6 – Rhynchosauroides lutevensis DEMATHIEU, 1984 is a very frequent species in the lower and middle Triassic. The imprint of the pentadactyl foot shows only three toes, the II, III ,IV, the others being too short to mark the soil .The pentadactyl manus lies behind the pes and shows four sub-parallel straight digits, the fifth is located behind them. The trackmaker is of small size (60-70cm), and was probably a Lepidosaur. The manus lies behind the pes and indicates long limbs. These footprints appear in the red-violet horizon.
7– Rhynchosauroides peabodyi, FABER 1958.
This ichnospecies is large and presents some peculierities : manus and pes have the same shape ; digits are well separated, long, curved with accurate claws. Like the other ichnospecies the manus is put behind the pes.
Because the homopody and the presence of round scales under the toes, it is probable that the trackmaker was an archosaur like Chasmatosaurus
8– Rhynchosauroides cf. petri DEMATHIEU, 1966. This small ichnospecies is very common in the Triassic of the eastern border of the Massif Central. It is very small and its characters are the same as those of other ichnospecies. It is possible that these footprints, which have the same shape, are the result of animals of the same species at different stages of growth ?

• Paleoflora (JGa, JB)

DOUBINGER & CHEYLAN (1964) described a small plant assemblage with Voltzia heterophylla, Albertia elliptica, Equisetites mougeottii, Schizoneura cf. paradoxa, Endolepis vogesiaca and Aethophyllum sp. with a microflora characterized by Illinites kosankei, Chordasporites singulichorda and Triadispora falcata suggestive of a lower Anisian age. A more rich microflora allowed DOUBINGER & ADLOFF (1981) to propose a middle Anisian age ; this was confirmed by the recent study of DIEZ FERRER (2000) in the area north of La Lieude where a rich microflora with the association Illinites kosankei, I. chitonoides, Concentricisporites nevesi, Microcachryidites sittleri indicates an early Pelsoniense age.

Between stops 2 and 3 after the Merquière pass, a general view allows us to see the Triassic series resting on the Permian red pelites dipping at about 15° to the East.