Evolution of a Late Oxfordian: early Kimmeridgian carbonate platform, French Jura Mountains

Abstract

A detailed facies analysis and interpretation of the evolution of depositional environments along a northsouth transect of the Late Oxfordian-Early Kimmeridgian French Jura carbonate platform highlights hierarchically stacked depositional sequences. The identified small-and medium-scale depositional sequences are matched with the precise cyclostratigraphic framework initially established for the Swiss Jura platform. The superimposition of a longterm (2nd order) sea-level rise with long (400 kyr) eccentricity cycles explains most of the French Jura platform architecture. During the Bimammatum and Planula zones, the climate became progressively warmer and more arid, allowing enhanced carbonate production. This resulted in a strong progradation of the French Jura platform that progressively evolved from a ramp to an oolitic rimmed shelf. A brief interval characterized by a more humid climate during the upper Bimammatum Zone caused an increase in siliciclastic and nutrient input, leading to a reduction in carbonate production and strong retrogradation of the platform. During the Platynota and lower Hypselocyclum zones, the shallow inner shelf carbonate production once more exceeded the accommodation, leading to a general aggradation of the platform. From the upper Hypselocyclum Zone, with a more humid climate, the carbonate production did not outweigh the accommodation creation and the platform evolved to a flat-topped shelf. Thus, sealevel changes and climatic conditions (temperature and humidity) are the key factors controlling the nature of the sedimentation and the depositional profile of the French Jura platform during the Late Oxfordian-Early Kimmeridgian.

Distribution of coral-microbialite reefs along the French Jura platform during the Bimammatum Zone (Oxfordian, Late Jurassic)

“…These localities are in the department of the Jura and are aligned along a north-south axis between the towns of Nantua and Besançon. During the Late Jurassic, there were three paleogeographical sectors (northern, central and southern parts) along the French Jura platform (Enay et al 1988, Cochet 1995, Cariou 2013, Olivier et al 2015. Mont Rivel (46° 45´ 36.91˝ N, 5° 55´ 7.26˝ E), Loulle (46° 43´ 0.41˝ N, 5° 53´ 32.65˝ E) and Morillon (46° 37´ 56.48˝ N, 5° 56´ 50.54˝ E) localities belong to the Central Jura, and Molinges (46° 21´ 6.24˝ N,5° 45´ 59.46˝ E) to the Southern Jura.…”

“…Mont Rivel (46° 45´ 36.91˝ N, 5° 55´ 7.26˝ E), Loulle (46° 43´ 0.41˝ N, 5° 53´ 32.65˝ E) and Morillon (46° 37´ 56.48˝ N, 5° 56´ 50.54˝ E) localities belong to the Central Jura, and Molinges (46° 21´ 6.24˝ N,5° 45´ 59.46˝ E) to the Southern Jura. Detailed information concerning stratigraphical and sedimentological contexts of these different reef localities can be found in the works of Olivier et al (2011), Cariou (2013), Cariou et al (2014), and Olivier et al (2015). These studies have notably established highresolution biostratigraphical and sequential frameworks to accurately position the coral reefs sampled along the French Jura platform.…”

“…These reefs are dated to the Late Oxfordian, and more precisely to the Bimammatum Zone (Tab. 1; Olivier et al 2015).…”

“…During the Bimammatum Zone, the Mont Rivel, Loulle and Morillon reefs developed in shallow and relatively high-energy shoal setting (Couches du Morillon Formation), at the south of more inner-platform marls of the Marnes de Besançon Formation (Fig. 1;Gaillard 1983, Bernier 1984, Enay et al 1988, Olivier et al 2015. Further south, the Molinges reefs developed in a mid-shelf setting with oncoidal limestones (Troisième niveau à concretions Member) of the Calcaires lités Formation (Tab.…”

“…While the stratigraphic framework is relatively well constrained for sponge bioherms (Enay 1966, Gaillard 1983, the nature and the distribution of the coral reefs along the French Jura platform are only partially known (Beauvais & Bernier 1981, Bernier 1984, Fookes 1995, Habrant & Lathuilière 2000, Olivier et al 2011. Recent sedimentological work increased our knowledge of the French Jura platform during the Oxfordian-Kimmeridgian interval (Cariou 2013, Olivier et al 2015. This highresolution sequence-stratigraphy framework makes it possible to analyse the composition and distribution of the coral reefs along this platform during a short time interval, the Bimammatum Zone.…”