Facies and depositional setting of the Lower Cambrian archeocyath–bearing limestones of southern Montagne Noire (Massif Central, France)

Abstract

Abstract Archaeocyath-bearing limestones of the Lower Cambrian Pardailhan Formation of southern Montagne Noire are restricted to few intervals (H1, H2, H3) of the mixed carbonate-detrital succession exposed in several superposed tectonic units affected by severe deformation. The comparative analysis of the archaeocyath assemblages together with a detailed sedimentological investigation leads to the understanding of the depositional settings and of the building style of the calcimicrobial-archaeocyath groups and to a better definition of the paleogeographic relationships of the different tectonic units. Interval H1: small, low-relief ‘pioneer reefs’ built by Epiphyton bushes and Girvanella crusts, associated with clusters of small stick-shaped archaeocyaths, colonized the detrital sand of the bottom. They are frequent in southern Minervois and more randomly distributed in northern Minervois and northern and southern Pardailhan and represent short-lived attempts of carbonate colonization on the sandy, mobile substrate of a wide continental shelf. Interval H2: Platy and bioclastic grainstones form the substrate and apron of small crust/cement reefs and associated clusters of ribbon-like and conical archaeocyaths in southern Minervois, or of mud-rich calcimicrobial mounds in southern Pardailhan. Displaced cups of stick-shaped archaeocyaths similar to those of Interval H1, occur in the granular facies, while in place saucer-like cups of Anthomorpha margarita are associated to the mud-rich mounds. This facies association records the repeated attempt at instauration and discrete development of a carbonate platform made of low-relief banks, the margins of which were colonized by Girvanella crust-buildups whereas in the more protected deeper zones Epiphyton/Renalcis mud-mounds dominated. Interval H3: Epiphyton/Renalcis, mud-rich mounds with solitary, large saucer-like cups of Anthomorpha margarita, represented by long, ribbon-like fragments, are dominant in the upper part of the platform. They rest on reduced lenses of grainstone and Girvanella crust boundstone and in southern Pardailhan are interbedded with nodular, marly mudstones containing bioclastic debris partly derived from the buildups. In this area they mark the transition from platform to shallow basin.