23 Jun 2023
 | 23 Jun 2023

Cosmogenic nuclide-derived downcutting rates of canyons within large limestone plateaus of southern Massif Central (France) reveal a different regional speleogenesis of karst networks

Oswald Malcles, Philippe Vernant, David Fink, Gaël Cazes, Jean-François Ritz, Toshiyuki Fujioka, and Jean Chery

Abstract. We present 27 new burial ages based on 26Al / 10Be ratios of terrestrial cosmogenic radionuclides measured in clasts and sediments deep within 12 caves in the southern Massif Central, France. Our results together with previously published burial ages, verifies that cave morphogenesis has been continuously active in this region for at least the past ~6 Myrs. Combining sample burial ages with their associated cave elevation above modern stream bed gives a mean regional incision rate of 88 ± 5 m/Ma for the Grands Causses area. South of the Cevennes Fault zone bordering the Grands Causses, the incision rate is 43 ± 5 m/Ma, suggesting that this difference might be accommodated by the fault zone. Sediment burial ages from caves which are not located on river valley flanks or cliff walls are surprisingly too young relatively to the expected age calculated using this regional average river incision rate. This suggests that the classical epigenic speleogenesis model that presumes a direct correlation between cave level development and regional base level lowering does not apply for the study area. Therefore, we propose that regional speleogenesis is mainly controlled by removal of ghost-rocks by regressive erosion from river canyons to central parts of the plateaus, emptying incipient primokarst passages to create cave systems. Our results suggest a continuum process from hypogene primokarst composed of ghost-rocks filled passages to epigene karst dynamic emptying these passages and creating cave networks. We propose this is a major process in the southern Massif Central that initiates the speleogenesis and control the geometry of the networks. In this region tiered karst cannot be associated with major incising rivers but must be explained by former ghost-rocks (or hypogene) processes.

Oswald Malcles et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-765', Fritz Schlunegger, 27 Jul 2023
    • AC1: 'Reply on RC1', Oswald Malcles, 08 Sep 2023
  • RC2: 'Comment on egusphere-2023-765', Philippe Audra, 28 Jul 2023
    • AC2: 'Reply on RC2', Oswald Malcles, 08 Sep 2023

Oswald Malcles et al.


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Short summary
In the Grand-Causses area (Southern France) we study the relationship between the evolution of the river, its incision through time, and the location of the nearby caves. It is commonly accepted that horizontal caves are formed during a period of river stability (no incision), at the elevation of the river. Our original results show that it is wrong in our case study. Therefore, another model of cave formation is proposed that do not rely on direct river control onto cave locations.