Siliciclastic-carbonate ramp, a hostile world for carbonated fauna? The example of a Middle and Upper Devonian platform

ARNAUD BIGNON; CATHERINE CRÔNIER

Mendoza

Congreso; The 4th International Palaeontological Congress; 2014

During the Middle and Upper Devonian, the sedimentation on the continental platform of Ardenne Massif has changed from a mixed siliciclastic-carbonate ramp (Eifelian), through a carbonate shelf (Givetian), to a detritic influx with local mud-mounds (Frasnian). Due to such modifications, this ancient massif represents an open-air laboratory for understanding the workings of the carbonate factory. In this study, we want to illustrate if the modification of sedimental influx constitutes a deep change of the ecosystem and thus implies an important reorganizing of the biodiversity. We studied the biodiversity fluctuations in trilobites using multivariate analyses. Four communities were identified, one restricted to the Eifelian, another to the Givetian and two to the Frasnian. The Eifelian community occurs both in the ramp or carbonated (local reef developed on the ramp) facies without any significant difference in its composition. The Givetian community occurs preferentially close to barrier reefs, but persists during short periods of detrital input. The Frasnian communities show a strong relationship with their environment, one is only found in the reef systems, whereas the other flourishes exclusively in the lateral facies. During the Givetian, brief comebacks of the mixed ramp characterize the Ardenne platform. In order to highlight the effects of such environmental changes on the biodiversity, we analysed the distribution of megaguilds in the Mont d?Haurs section (Lower Givetian) through four successive formations. From studied microfacies, seven megaguilds and their environment associated are identified. The results show that the reef is well demarked from others carbonate shelf environments whereas fore-reef, back-reef and ramp environments no show significant differentiation. Moreover, the distribution of megaguilds does not seem being affected by the comeback of the siliciclastic-carbonate input. Indeed, the multivariate analyses show no particular distinction between the megaguild distribution associated to the carbonate shelf and those corresponding to the mixed ramp. These studies illustrate that the biodiversity fluctuations during the Middle Devonian of Ardenne is mainly lead by fluctuations of the sea-level more than modifications of the platform morphology. In the present case, changes of sedimentary supply do not imply a structural modification of the ecosystem and affect the biodiversity only at the community-type level.