Document sans titre

3. STOPS n° 3-6 : the Lodève basin ;

a. STOP n° 3 : the Saxonian Group near la Lieude

* lithostratigraphy of the Salagou Formation, Mérifons facies.

• sequential organisation, geochemistry : data from Cogema, ODIN (1986), NMILA et al. (I992). (GG, JG presentations).

During this stop, we will study the typical elementary sequence formed of red massive polyedric mud-siltstones and of white – yellow siltstones beds. The latter are used as markers (R, fig. 4B = ‘niveaux repères’ 100-1000). Some of them contain pyroclastic material referring to NMILA et al. (1992). Following is a part of their text-description only changed a little by using Saxonian instead of Thuringian.
“ …The second cycle is mostly composed of monotonous interbedded red shales, siltstones and a subordinate amount of very fine-grained sandstone (Saxonian).
“ Previously, we have obtained new evidence indicating that tuffaceous sandy-silt rocks are common in these Upper Permian red beds. In the present study, we found beds up to 1-20 cm thick of volcanic marker horizons (niveaux repères) composed of tuffaceous material mixed with small amounts of detrital sediments. These horizons were previously described as carbonated incrustations here, we demonstrate their volcanic origin by lithological, petrological and geochemical studies. In hand specimens, important characteristics are their constant thickness and lateral extension, the sharp contact with the enclosing formations, their strong induration, the fineness of grains and shapes of minerals.
“ In thin section, these marker levels display the following main characteristics : quartz typically of volcanic origin (isomorphous faces, volcanic glass inclusions), numerous stretched crystals of feldspar showing more or less advanced degrees of alteration, volcanic glass shards and pumice fragments with I, T, Y, X, etc. type perlite and oval glass stones formed at the same time showing a degree of alteration .
“ Analcime formed during diagenesis and developed from fragments of acidic volcanic glass.
“ These characteristics are typical of ash falls and provide evidence of’ the volcanic origin of these marker horizons (‘niveaux repères’).
“ The trace element geochemistry demonstrates the volcanic origin of marker levels (niveaux repères) and helps to define the nature of this volcanism. The global chemical composition of the marker levels is similar to cinerite. Moreover, their low concentrations of 3d transition elements (Co, Cr, Ni) and Al2O3, P2O5, TiO2 confirm their volcanic origin and their acidic composition.
“ In Th-La/2-Sc space we can observe a similar location for ash tuffs (Autunian cinerites) and marker horizons. Their intermediate location between Lodève basin sediments and Permian acidic volcanism of Esterel-Corsica confirm their volcanic origin.
“ In Ocean Ridge Granites (ORG) normalized spidergrams, cinerites and marker horizons of the Lodève basin compared with the early calc-alkaline cycle and the latter alkaline cycle of Permian acidic volcanism from Corsica-Esterel, show similar patterns (parallel and impoverished).
“ Moreover, marker horizons (niveaux repères) show Th/Ta ratios close to seven, similar to the Th/Ta ratios of the Permian alkaline volcanism of Corsica and Esterel. On the contrary, the Th/Ta ratio of the cinerites near ten is comparable with the first cycle Permian calc-alkaline volcanism of Corsica.
“ The typologic study of the zircon population from the cinerites and conglomeratic horizons of the Rabejac formation (level 100), compared with those of Permian acidic volcanism allow us to confirm the existence of two types of volcanism (calc-alkaline during the Autunian deposit and alkaline during the Saxonian deposit).
“ The origin of this volcanism is unknown. The grain-size of ash tuffs (Autunian cinerites) and their decreasing thickness to the North-West, suggest an origin from a volcanic area located around 100 km toward the South-East and now hidden under the Mediterranean Sea. ”
Up to 15 cm thick air fall tuff horizons, consisting of vesicular shards (fig. 7-11), have been mapped in the red beds of the Octon facies (KÖRNER 1999) ; zircon dating is currently in progress (Trapp, Munster).

Fig 7– Geochimical and chronostratigraphical data of the Lodeve permian serie after Körner et al.1999

• Sedimentology and geochemistry : new data (FK and JS ; cf. in CASSINIS 2001).
In the course of a project of the German Research Foundation (DFG), detailed sedimentological, geochemical, mineralogical and palaeoecological investigations were carried out in the Lodève basin, which is the most complete, well exposed higher Permian red bed sequence in Europe. The aim is to reconstruct the climatic processes which control the litho- and biofacies patterns during an Icehouse / Greenhouse transition, as well as the correlation of marine and continental climatic signals. Based on ODIN 1986, HENRIOT 1992, NMILA 1995 and supported by J. GARRIC and J. LAPEYRIE, 750 m of the Usclas – St. Privat, Tuilières – Loiras and Viala fm., that means sediments of the grey facies and the transition zone from grey to red beds, as well as 1600 m red beds of Rabejac, Salagou and La Lieude Fm. have been documented. In the Salagou fm. about 5.000 cycles have been counted and measured for cyclostratigraphy. Based on that, the climatic development is described briefly as follows.

Fig 10– Graphe Na2O / Al2O3 permettant d’estimer l’indice chimique de l’altération (CIA). Un rapport élevé signifie une faible importance de l’altération chimique de la région nourricière ; d’où l’évolution climatique suivante. La pluviosité la plus grande se réalise durant le dépôt de la Formation du Viala. Le climat devient légèrement plus sec pour la Formation de Rabejac avec un maximum d’aridité pour le faciès d’Octon. Il redevient légèrement plus humide pendant la sédimentation du faciès de Mérifons ; high Na2O/Al2o3 ratios.

In the upper Tuilières-Loiras fm. to the lower Viala fm. takes place the transition from warm-humid to semiarid conditions. Gypsum-pseudomorphs, desiccation cracks, xeromorphic calcisols and vertisols as well as mesophile fauna and flora are characteristic for the red brown flood plain deposits of Viala and Rabejac fm. Cycles of the following Octon facies consist of some metres thick, massive red brown claystones and beige-coloured, cm-thick carbonaceous siltstones. In the Lower Salagou Fm. a maximum of aridity was reached. Here the CIA (Chemical Index of Alteration) (NESBITT & YOUNG 1982) has the lowest values (69-70). For calcite the 18O curve shows a trend towards higher values. In contrast to the muddy playa sediments of Octon facies, prevailing facies types of the lower and middle Salagou fm., belong to a more central playa position. Cycles of the Merifons facies consist of cm to dm thick, often laminated, red brown claystones and grey-green, cm-thick carbonaceous siltstones. After periodic (? episodic) strong rain falls, temporary water filled channels contain mass occurrences of xerophile organisms (conchostracans, triopsids, insects). The same is known from observations in modern playa and sabkha environments, e.g. in Jordan and Tunisia.

Fig 8– Profile : eastside of Carols, between Cartels and Celles.

Typical for the grey facies are kaolinite and K-feldspar as well as Corg. For example K-feldspar is very common in the grey facies. It is absent in the whole red facies of Viala-, Rabejac– and Salagou facies. K-feldspar recurred only in the La Lieude facies. In contrast, analcime and albite are characteristic for the red beds, that means for semiarid conditions. Analcim is an authigen, by the process of evaporative pumping forming the mineral zeolithe. It appears for the first time in the Viala fm. and vanishes at the top of the profile with the first conglomerates of the La Lieude facies. The CIA, results of the clay mineral analysis and the distribution of Analcim point to a maximum of aridity in the higher Octon facies. During the Merifons facies, the rate of precipitation was increasing. The sedimentary architecture of the La Lieude fm. indicates a fast change of sediment supply. Coarse clastics appear suddenly above 2 000 m of nearly exclusive fine grained sediments! They are the result of rapidly increasing precipitation and consequently prograding alluvial fans. Linked to that is the appearance of very diverse tetrapod tracks and the re-occurrence of a mesophile flora and invertebrate burrows (Scoyenia). This climate change is possibly the effect of Upper Permian transgressions (Bellerophon-, Zechstein transgression). This interpretation is supported by first results of magneto-stratigraphic investigations (Bachtadse, München), which point to a position of the La Lieude Fm. in the range of resp. above the Ilawarra reversal in Lower Capitanian resp. Lower Tatarian.

Fig 9–Profile : river Rieupeyre at Montagne de la Boutine.

*Fauna.

• In F4 Rabejac Formation : little Branchiosauridae / Rachitoms has been mentionned by HEYLER 1969 and in Salagou Formation only bone beds (fig. 4 B).

* Palichnofauna (GG).

• Footprints (fig. 5).
Near ‘la Lieude’ more than a thousand footprints are distributed in twenty trackways amounting to 220 m in length. They have been found on calcareous siltstone level in the B site named also ‘Réserve Naturelle Volontaire’. This last is located in the Saxonian summit (Salagou Formation) (fig. 4).
GAND et al. (2000) have distinguished 4 following ichnotaxa : Lunaepes ollierorum, Merifontichnus thalerius, Planipes brachydactylus and Brontopus circagiganteus. All these traces are attributed with possibility or probability to Therapsida or to Therosauria, except Brontopus circagiganteus that could be due to Caseamorpha. All these animals whose sizes have been estimated between 1 and 5 m lived in a playa environment. The biological and sedimentological data from ‘la Lieude’ footprint levels compared with informations provided by the track orientations, suggest the following scenario. Animals coming from the North have crossed a sandy channel bank with plants zones by directing to the South for the majority. May be, they were going to the lacustrian part of the playa, close to ‘la Lieude’ footprints that they have just trampled on.

• Invertebrates tracks (fig. 12).
We have found numerous invertebrate traces (unpublished for a large part ; WALTER et al. under preparation). Some of them are Endichnia : endogenic traces (tubes and burrows) among them delicate Scoyenia gracilis common in the Rabejac Formation. They are probably due to burrowing insects.

Fig 12– Traces d’invertébrés ; A and B = fouissage Isopodichnus eutendorfensis LINCK, 1942 ; C-G = traces de repos avec C-D = Isopodichnus sp., E = Isopodichnus minutus DEBRIETTE & GAND, 1990 (échelle = 1 cm sauf pour F, G) ; H = trace de locomotion Trachomatichnus permultus MILLER, 1880 (échelle = 1 cm) ; I-M = trace de repos Isopodichnus furcosus GAND ; A and B = furrowing/burrowing tracks Isopodichnus eutendorfensis LINCK, 1942 ; C, D = Isopodichnus sp. ; E-G = resting coffee bean-shaped traces Isopodichnus minutus DEBRIETTE & GAND, 1990 (scale = 1 cm except F, G) ; H = walking/crawling Trachomatichnus permultus MILLER, 1880 (scale = 1 cm) ; I-M = resting track Isopodichnus furcosus GAND.

Others are Epichnia : exogene ichnites made on the surface in which are :
– furrowing bilobate fine striken tracks Isopodichnus eutendorfensis LINCK, 1942 from Usclas-du-Bosc Member levels (Fialhomme Wood, DEBRIETTE & GAND 1990) and Salagou Formation (near Octon, unpublished)
– resting ‘coffee-bean-shaped’ traces I. minutus DEBRIETTE & GAND, 1990 and I. furcosus GAND, 1993 from Rabejac and Salagou Formations ;
– walking/crawling bilobate homopod tracks (Trachomatichnus permultus MILLER, 1880 = Acripes MATTHEW, 1910) found in Fialhomme wood and common in the Salagou Formation. It seems likely that Triopsids are trackmakers of Isopodichnus eutendorfensis and Trachomatichnus. These animals are abundant in silstones of the Salagou Formation (GAND et al. 1997) sometimes with their body connected to their trackways (unpublished). Curiously, for this time, we have never found in the Saxonian Group Lithographus, a walking insect track although numerous wings and sometimes bodies fossils have been gathered in this Group (GAND et al. 1997, NEL et al. 1999).