Environmental controls on the development of Mississippian microbial carbonate mounds and platform limestones in southern Montagne Noire (France)

Abstract

Late Mississippian carbonates in southern Montagne Noire are dominantly domical to laterally‐accreted microbial mounds in some formations, as well as stratiform microbial limestones occurring in hundreds of olistoliths within a flysch basin, constituting pieces of a giant puzzle that are used to help reconstruct a platform in a region that is no longer preserved. Petrographic data of limestone samples from 14 continuous long sections of olistoliths have been analyzed statistically, using multivariate clustering (Q‐mode) of the components/matrix/cement and canonical correspondence analysis that allow the reconstruction of the environmental parameters of carbonate microbial communities in space and time. Clustering analysis separated microbial and non‐microbial facies. The calculation of indices along the various axes from canonical correspondence analysis allows recognition of the controlling factors of the mounds and microbial growth as being turbidity, light penetration, bathymetry and storms. Turbidity and light penetration are the primary factors controlling the morphology of the microbial limestones. Representation of the light penetration and bathymetry indices on the stratigraphical sections defines two vertical environmental gradients. Light penetration can be subdivided into euphotic, euphotic–dysphotic and dysphotic‐aphotic conditions. The representation of the bathymetry allows the subdivision of samples into a deeper outer ramp, external mid‐ramp and internal mid‐ramp. The curve distance from the section base = f (index) suggests a cyclicity for the platform that cannot be compared with the onlap curve defined from other cratonic areas (Moscow Basin), and thus the cyclic succession of the Montagne Noire is interpreted to have been mostly tectonically‐controlled. Integration of the data allowed the reconstruction of the original Mississippian carbonate platform, where, up to the Mikhailovian, it appears to correspond to a platform morphology, with narrow shallow water facies and wide turbiditic systems, whereas the width of shallow‐water settings expanded during the Venevian to the Protvian, forming a ramp or distally‐steepened ramp with widespread microbial limestones.

Development of a Mississippian–Lower Pennsylvanian isolated carbonate platform within the basinal griotte facies of the Cantabrian Mountains, NW Spain

“…These mounds are only locally interrupted by capping beds in the upper part of the buildups (rudstones), when the bioconstructions grew-up into more turbulent water conditions, above the storm wave base. The occurrence of sparse cyanobacteria suggests at least dysphotic conditions (Lees and Miller 1995;Cózar et al 2019). Due to the relative abundance of bioclasts, a proximal outer platform setting is interpreted for this interval, like similar interpretations of microbial mounds of equivalent age in southern France (Cózar et al 2019).…”

“…The occurrence of sparse cyanobacteria suggests at least dysphotic conditions (Lees and Miller 1995;Cózar et al 2019). Due to the relative abundance of bioclasts, a proximal outer platform setting is interpreted for this interval, like similar interpretations of microbial mounds of equivalent age in southern France (Cózar et al 2019).…”

“…The green algae are as abundant in the mounds (in situ) as in the grainstone beds (transported). Such abundance of green algae in Mississippian microbial facies is only known in the Montagne Noire of France (Cózar et al 2018b(Cózar et al , 2019. The inferred shallow-water conditions, even with some levels reaching the wave-base, suggests a distal inner platform setting.…”

“…These boundstones contain poorly represented bioclastic facies and seem rather different from the microbial mounds observed in the platform facies of the Saigu subunit. This poverty of bioclasts is interpreted to characterize the deepest part of the outer platform, or possibly development on the uppermost slope (Cózar et al 2019). In the upper part of the section, capping beds are present, as well as the slightly richer bioclastic content, which are more typically developed on distal outer platform settings.…”

Demise of the Middle Jurassic carbonate platform: Responses to sea‐level fall in the Qiangtang Basin, Eastern Tethys

“…Gypsum and gypsum pseudomorphs represent an evaporative environment (Figure 3d,e). Micritic dolomite has been observed at the top of the Buqu Formation (Figure 3f), which may be penecontemporaneous with calcium carbonate mud in a supratidal environment (Cozar et al, 2019; Reolid, Betzler, & Ludmann, 2019; Warren, 2000). The upper Buqu limestones were deposited in a restricted platform environment, indicating a shallowing‐upward succession.…”