Une faune d’ammonites inédite du Kimméridgien inférieur à Molinges (Jura) et la limite Oxfordien-Kimméridgien dans le faisceau externe du Jura méridional

Abstract

Our study of some ammonites discovered near Molinges (Jura) in 1993, but so far never published, shows that they are apparently faunistically homogeneous and date from the lowermost Early Kimmeridgian, Platynota Zone, Desmoides subzone. A description of the Upper Oxfordian - Lower Kimmeridgian stratigraphy and palaeogeography reveals 1) the difficulty in realizing correlations and in defining the Oxfordian-Kimmeridgian boundary between the southeastern part of the Southern Jura, where the ages are well constrained by faunas, and the northwestern part where valuable data are lacking, and 2) the value of the Molinges fauna for resolving the Oxfordian-Kimmeridgian boundary problem, for which quite different interpretations have been put forward, as well as for explaining how this fauna came to be here. The exact locality from which the ammonites were collected is unknown, but the probable position in view of the other field data is in the uppermost beds of the Oolithe de Corveissiat (Corveissiat Oolite) Formation. We put forward an interpretation that explains how the Molinges ammonites came to be far beyond the known western boundary of ammonite-bearing beds of the same age; they were probably empty shells transported by storm-waves, the area being in a palaeogeographic location prone to the development of tropical storms. Abridged English version The major facies changes between the northwestern (or inner platform) and southeastern (or outer platform) parts of the Southern Jura Mountains has led to diverse correlations being proposed for the Lower Kimmeridgian. The studied ammonite fauna from near Molinges can help to determine the Oxfordian-Kimmeridgian boundary in the inner platform facies. The accepted Oxfordian-Kimmeridgian boundary is that defined for the Tethyan Realm by the 1st Jurassic Colloquium in Luxembourg (1962). However, new correlations with the Boreal Realm show that this boundary was placed too high and needs to be changed. To date, the working group of the International Subcommision on Jurassic Stratigraphy has not presented proposals and the Subcommission is not in a position to decide on this issue. Upper Oxfordian and Lower Kimmeridgian stratigraphy and palaeogeography The terms used for the lithological and sedimentological descriptions are the same as in the reference works on the studied area: Gaillard (1983), Bernier (1984) and Chevallier (1986, 1987, 1989). Upper Oxfordian: two sedimentary areas with a transitional zone In the southeast, the Calcaires pseudo-lithographiques (Pseudolithographic Limestone) Formation is a thick (70-80 m), well-bedded and monotonous limestone succession, the upper limit of which is defined by the first oncolitic beds of the Calcaires d’Aranc (Aranc Limestone) Formation. Bioclastic or lumachellic and oolitic intercalations are seen from southeast to northwest, beginning in the upper part, and becoming progressively lower in the succession. In the zone that is transitional to the Oolithe de Corveissiat (Corveissiat Oolite) Formation, oolitic facies gradually replace the whole Calcaires pseudolithographiques. At first, Riche (1900, 1904) assumed the replacement was only a vertical change, and this is also the opinion of Bernier (1984) and Cochet (1995). Later, Riche (1911, 1912) acknowledged the presence of lateral facies change and an age equivalence for the two facies, an opinion that is accepted by Enay (1966), Chevallier (1986, 1987, 1989) and Enay et al. (1988). In the northwest, beyond the Ain river and north of Cize-Bolozon, the Oolithe de Corveissiat is the only formation present. It overlies the oncoid facies of the Calcaires lités (Bedded Limestone) Formation which, in the southeast, also underlies the Calcaires pseudolithographiques (see Fig. 3). Another northern counterpart is the Couches du Morillon (Morillon Beds) Formation (Fig. 4), a bioclastic facies with predominant echinoderms and rare ooliths. Bernier (1984) and Cochet (1995), however, placed the Couches du Morillon below the Oolithe de Corveissiat, which is attributed to the Kimmeridgian. Farther to the northwest, the Oolithe de Corveissiat is replaced by low-energy deposits, the so-called “Calcaires fins séquaniens”, now known as the Calcaires de Besançon (Besançon Limestone) Formation, which ends with a hardground. Above it comes the Calcaires à ptérocères (Pteroceres Limestone) Formation, which has yielded an ammonite fauna from high in the Lower Kimmeridgian. The relationships and geographical extent of the Upper Oxfordian facies are discussed in Enay et al. (1988). Lower Kimmeridgian: different successions and palaeogeography In the southeast, the Lower Kimmeridgian is represented by the Calcaires d’Aranc and the Couches à céphalopodes (Cephalopod-bearing beds). The Calcaires d’Aranc extends from the southeast to the northwest and includes thick-bedded, bioclastic and oncolitic limestone; it ends with a hardground and is overlain by the Couches à céphalopodes, the age of which youngs from southeast to northwest (Fig. 4B). The Couches à céphalopodes overlies either the Calcaires pseudolithographiques in the southeast or the Calcaires d’Aranc in the northwest. It consists of alternations of well-bedded limestone (0.20-0.80 m) and marl that in places reveal a clear lithoclinal sequence. The greatest northwestward extension was reached during the late Early Kimmeridgian (Divisum Zone) showing that the base of the formation is heterochronous as well as the top [e.g. Calcaires de Tabalcon (Tabalcon Limestone Formation] (Fig. 4A) and the Couches à céphalopodes are seen to be wedge-shaped from southeast to northwest within the carbonate platform deposits. In the northwest we find the Couches (or Marnes and Calcaires) à ptérocères. The oncolite-bearing beds between the Oolithe de Corveissiat and the Couches à ptérocères, were interpreted quite differently, especially in the Molinges district. The Couches à ptérocères, not widespread in the studied area, is well developed in the Central and Northern Jura where the ammonite fauna proves a late Early Kimmeridgian age (Hypselocyclum Zone). Brachiopods and a few nautilids give the same age for the southern extension (Chevallier et al., 1987). The Couches à ptérocères documents a deepening of the water that is of about the same age as the greatest extension of the Couches à céphalopodes. This is probably related to a sea-level rise. But the drowning of the carbonate platform was not complete Carbonate platform deposits continued on what we have termed the Pont d’Ain-Oyonnax-Morez “calcareous axis” (Enay and Contini in Enay et al., 1984) dividing the inner (Couches à ptérocères) and outer (Couches à céphalopodes) platform facies (Figs. 2, 3). The Pont d’Ain-Oyonnax-Morez “calcareous axis” In the west and northwest of the area over which the Couches à céphalopodes was deposited, the Calcaires d’Aranc reaches its maximum thickness of 40-50 m (Fig. 4A). It is overlain by a thick carbonate succession with repetitive facies, the Calcaires d’Oyonnax (Oyonnax Limestone) Formation (Chevallier, 1986, 1989), which represents the “calcareous axis” (Fig. 3). The Oxfordian-Kimmeridgian boundary in the Southern Jura In the southeast, the Oxfordian-Kimmeridgian boundary is traced just below the Calcaires d’Aranc whose eastern extension is well dated as Early Kimmeridgian (Platynota Zone). Towards the northwest, however, oolitic and bioclastic facies become increasingly widespread and the thickness of the Calcaires d’Aranc increases. We have no data for assuming that the Calcaires d’Aranc is isochronous and more than likely it is not so. Moreover, the northwestern extension and correlation are still open to debate (Bernier, 1984; Chevallier, 1986, 1987, 1989; Enay et al., 1988; Cochet, 1995). The unpublished ammonites from near Molinges (Jura) The unpublished ammonite fauna from near Molinges consists of only eight specimens, some complete and some not, and two fossil impressions. The collector is unknown and it has not been established whether the handwritten labels were prepared by the collector or by the people who received the collection in the university. The probably oldest labels indicate only the date, “21 May 1933” and refer to the stratigraphic unit used at the time. The others add data on the formation, “Oolithe crayeuse” (“Chalky oolite”) (e.g. Oolithe de Corveissiat), and the locality, but the indication “bifurcation of the road from Molinges to Viry” is not clear and the exact locality remains uncertain. The matrix of all the specimens is the same: a white chalky limestone with small ooliths and pseudooliths that is also found inside the body chamber when this is preserved (O. enayi) (Plate 1). Septate inner whorls are preserved in crystalline calcite which does not fill in the chamber completely, leaving a large empty space. The Molinges ammonite fauna includes: Taramelliceras (Metahaploceras) rigidum (Wegele), 1 ex. (FSL 101 461), Pl. 1, Fig. 3a, b. Orthosphinctes (Ardescia) desmoides debelmasi Atrops, 1 ex. (FSL 101 462), Pl. 1, Fig. 2a, b. Orthosphinctes (Ardescia) enayi Atrops, 3 ex. (FSL 101 463 à 465), Pl. 1, Fig. 1a, b. Orthosphinctes (Lithacosphinctes) cf. stromeri (Wegele), 1 ex. (FSL 101 466), Pl. 1, Fig. 4a, b. Orthosphinctes sp., 2 ex. (FSL 101 467, 468). Discussion and interpretation The age of the fauna, well defined with reference to the range of the species in the Southeast Basin (Atrops, 1982), is Early Kimmeridgian, Platynota Zone, probably Desmoides Subzone. The Molinges section, which has been described twice by Bernier (1973, 1984), is instructive for the correlation and the ages that Bernier assumed. New and different interpretations were later proposed by Chevallier (1986, 1989) and Cochet (1995). Within this regional context, two further questions arise concerning 1) the exact position of the fauna in the Molinges section, and 2) the relationship with the few other Platynota Zone ammonite localities in the Southern Jura. Three different interpretations of the Oxfordian-Kimmeridgian boundary In the northwest area, the beds between the Oolithe de Corveissiat below and the Calcaires à ptérocères above, contain locally abundant oncolites and have been used for quite different correlations. Bernier (1984, and in Enay et al., 1984, fig. 5.20) attributed these beds to the Calcaires de la Semine (Semine Limestone), a new name for the Calcaire à momies intermédiaire (Enay, 1965) which, in the type-area, divided the Couches de Prapont of Late Kimmeridgian age into two members. As a result, 1) the “Couches à ptérocères du Bois de Trequin” are assumed to be different from the northern Calcaires à ptérocères, also dated Late Kimmeridgian, and 2) the Oolithe de Corveissiat is correlated with the lower member of the Couches de Prapont. Chevallier (1986, 1987, 1989) adopted the same view as Enay and Contini (in Enay et al., 1984, fig. 5.19) and considered the so-called Calcaires de la Semine as being a westward extension of the Calcaires d’Aranc. The microfauna is different from that of the Calcaires d’Aranc in the type area, being closer to the microfauna of the Calcaires de la Semine, which was an argument for Bernier’s correlation. Chevallier explains that the difference is related to the situation within the oncolitic bank, more distal for the typical Calcaires d’Aranc than for the so-called Calcaires de la Semine at Molinges (Fig. 3). Cochet (Cochet et al., 1993, 1994; Cochet, 1995) adopted a scheme close to that of Bernier. He also places the Oolithe de Corveissiat stratigraphically above the Couches du Morillon, but shifts it geographically down and south of a hypothetical “swell” that occurs only at one locality, i.e. Mt. Rivel, near Champagnole (Jura). The Oxfordian-Kimmeridgian boundary is placed below the Calcaires d’Aranc in the southeast area; it is below the Oolithe de Corveissiat where this formation occurs and at the top of the Couches du Morillon on the Mt. Rivel swell (Fig. 4B, C). Curiously, Cochet attributed the oncoid-bearing beds that everywhere overlie the Calcaires lités to the Couches du Morillon B (?). This is overlain by the Calcaires pseudolithographiques, the Oolithe de Corveissiat and the Couches du Morillon (Riche, 1902, 1912; Enay, 1966; Enay et al., 1988). The position of the Molinges ammonites in the Molinges section The Molinges fauna should be relocated in the section described by Bernier (1973, 1984). The ammonites are in the Oolithe de Corveissiat facies that crops out from the bifurcation of roads D.100 and D.63 for about 400 m along D.100. According to the labels, the ammonites would be from the base, or from near the base, of the formation that would be Kimmeridgian. This, however, disagrees with the comparative examination of the outcrops, and it is probable that the location indicated on the labels is only approximative. Bernier examined thin sections of the matrix of the ammonites and assumed it to be the microfacies of his beds 6 to 8 with inframillimetre-size ooliths. Nevertheless, beds with the same microfacies are also present higher up; for instance beds 11 to 20 and 22 of the Bernier section, the last lying just below the so-called Calcaires de la Semine. Although the section was visited several times, the exact location of the ammonites remains unknown. They were probably found at a single localized spot, and there is very little chance for the discovery to be repeated. The site of the fauna could be unit “l” of Cochet (probably bed 22 of Bernier) that has a sedimentary structure indicative of a tidal environment (Cochet, 1995), or unit “k” (bed 21 of Bernier) that contains numerous nerineids, accumulated without any order, with an internal calcitic fill. Relationship with the other known localities of the Platynota Zone The nearest outcrops of the Platynota Zone are in Monts Jura and Vuache. These units moved northwestward along the Vuache sinistral wrench fault and are situated beyond the eastward extension of the Calcaires d’Aranc (Enay, 1959, 1966, 1969). The greatest northwestern extension of the Couches à Céphalopode, at the end of the Early Kimmeridgian (Divisum Zone) did not reach the “calcareous axis”. So, whatever the exact site of the Molinges ammonites, it is not easy to explain why a fauna of the Platynota Zone is present on the inner side of the “calcareous axis”. Besides the Monts Jura and Vuache, the Platynota Zone has been characterized by a few perisphinctids from biodetrital limestone associated with the oncolitic facies (Calcaires d’Aranc) and interpreted as channel deposits within the oncolitic bank in the Ile de Crémieu (Enay, 1955, 1958, 1966). Northward, in the folded Jura, the Calcaires d’Aranc show great variation in oncolite size and abundance, and several types have been described by Dahanayake (1974). Non deposition or/and erosional surfaces occur through the formation, and Dahanayake (1974) subdivided the formation into sequences defined by the cement or by the types of oncolite. Both the data from Dahanayake and observations by the author at the eastern boundary of the Calcaires d’Aranc suggest a regime of elevated areas with oncolites and channels changing with time. The Molinges ammonites reached their present position as shells that drifted along such channels from the eastern areas where ammonites were living. The Molinges fauna is probably the result of a single exceptional event. Recently, a new interpretation of the Upper Jurassic brecciated limestone of the Southeast Basin was proposed by Raja Gabaglia (1995) and Séguret et al. (in press), who attach a prominent role to storm-waves generated by tropical hurricanes. For instance, a thick disturbed unit called “the great slide”, covers a large surface area in the Platynota Zone and the lower half (Hippolytense subzone) of the Hypselocyclum Zone. The Jura area on the northwestern margin of the Southeast (or Dauphinois-Helvetic) Basin was in the same palaeolatitude situation, prone to the development of tropical hurricanes, but at the distal part of the Late Jurassic carbonate ramp, far beyond the attenuation zone of storm waves. A combination of sea-level rise and storm waves could have caused empty shells to drift far away and be rapidly buried. This would well explain some aspects of the Molinges fauna (a single find that is faunistically homogeneous) and the ammonite preservation. Conclusion The Molinges ammonites are dated as Early Kimmeridgian Platynota Zone, probably Desmoides Subzone, but their exact location in the Molinges section remains unknown. The proposal that the fauna came from the highest beds in the Oolithe de Corveissiat agrees with 1) the field data on the lateral facies change and the same age of the Calcaires pseudolithographiques and Oolithe de Corveissiat (Riche, 1911, 1912; Enay, 1966; Gaillard, 1983; Chevallier, 1986, 1987, 1989; Enay et al., 1988), and 2) the late Early Kimmeridgian age of the Couches à ptérocères (Chevallier et al., 1987). The actual location of the Molinges ammonites so far away from the present outcrops of the Platynota Zone is explained by a drift of empty shells due to storm-waves, the area being in a palaeogeographic location prone to the development of tropical hurricanes.

Key words: Ammonites, Oxfordian-Kimmeridgian boundary, Southern Jura