Resumen de Les lahars du strato-volcan du Cantal (Massif central, France); stratigraphie, modes de mise en place et implications paleo-geomorphologiques
David Huguet, Jean-Claude Thouret, Pierre Nehlig, Jeannine Raffy
Introduction: The study of lahar (Lh) deposits (a) describes sedimentary facies associations in a volcano-sedimentary system, (b) establishes the identification of criteria to recognize epiclastic deposits in fossil volcanic successions [Thouret, 1999] and (c) reconstructs paleo-landforms (stratocones, paleovalleys, and volcaniclastic fans) in an old volcanic massif. Lh deposits form the "complexe conglomeratique superieur" ("upper conglomeratic complex") [Brousse et al., 1972, 1975, 1977, 1980, 1989] associated with pyroclastic deposits and streamflow deposits above the "breche inferieure" ("lower breccia"), reinterpreted as debris-avalanche (DAv) deposits. From 9.5 to 6.5 Ma, a trachyandesitic stratovolcano has been built up. Several sector collapses generated DAv and an explosive activity produced pyroclastic-flow deposits. Pyroclastic deposits and both Lh and DAv deposits built up volcaniclastic fans. The study aims (a) to determine Lh deposit generations associated with paleo-landforms and (2) to use Lh deposits as landmarks to recognize some geomorphologic stages in the history of Cantal volcano (45 degrees N-2.5 degrees E; 2500 km 2 , approximately 380 km 3 , 1 855 m). Lahar generations: Lh deposits (9 km 3 ) cover 280 km 2 (fig. 1). They show two facies, clast-supported and matrix-supported debris-flow deposits (fig. 2 and 3), located as far as 20 km from the geographic centre (Puy Griou). Firstly, field observations and geochronological data enable us to distinguish as much as five Lh deposit generations. Secondly, geometric and stratigraphic relations, between Lh deposits and both pyroclastic and DAv deposits, allow us to decipher the genetic relations between distinct volcaniclastic formations. The Cere valley shows three Lh generations. The "Faillitoux" generation is interbedded with the schistose basement and the lava and pyroclastic deposits of the Elanceze massif (1571 m) (fig. 4). An ankaramitic lava (9.53+ or -0,5 Ma, K/Ar) [Nehlig et al., 1999], fitting into Lh deposits of the Elanceze massif post-dates the apparition of the first lahar generation. The "Curebourse" generation was emplaced above DAv deposits (fig. 2A and 5). Both DAv and Lh deposits of the "Curebourse" generation filled the paleo-Cere valley about 7.1 Ma. The "Thiezac" generation (>6.7 Ma, K/Ar) [Nehlig et al., 1999] (fig. 2C) overlies a thick pyroclastic deposit (fig. 4) and is not related to the DAv and the "Curebourse" generation. The fourth generation (Impradine valley) is stratigraphically and genetically associated with pyroclastic deposits located in the upper Impradine valley (fig. 5). These pyroclastic deposits are older than 7.96 Ma (K/Ar age on a trachyandesitic lava overlying Lh deposits) and result from pyroclastic deposits removed as Lh deposits downvalley. The fifth identified lahar generation is located in the Petite-Rhue valley, to the north of the volcano, where a 5-m-thick pumiceous pyroclastite (7.6+ or -0.03 Ma; 40 Ar/ 39 Ar) [Platevoet, 2000] is interstratified with Lh deposits in Cheylade. Genetic relations with pyroclastic deposits: To determine the nature of the relationships between Lh deposits and DAv deposits, we observed geometric relationships between both formations. Some Lh deposits of "Curebourse" generation filled paleothalwegs (fig. 6) cut into DAv deposits suggesting a remission stage after emplacement of DAv deposits. We did not identify sedimentologic features such as dewatering structures indicating that lahars evolved from the top or the front of DAv deposits. Thus, no obvious genetic link was clearly determined between Lh and DAv deposits. In the Impradine valley, we observe the transformation of these pyroclastic deposits in Lh deposits. A proximal pyroclastic facies (upper Impradine) (fig. 5), intruded by numerous dykes and intercalated with trachyandesitic lava, shows the proximity of a stratocone located 1,5 km to the South-East. Field observations indicate a stratigraphic link between pyroclastic and Lh deposits. Debris flows have removed pyroclastic deposits over a 6 km distance. Lh deposits are ungraded or inversely graded and show matrix- or clast-supported facies. About 50% of dense subrounded to rounded clasts were incorporated during the flow. The remaining 50% are dense trachyandesitic juvenile clasts derived from primary pyroclastic-flow deposits. Geomorphological implications: Determinations of five Lh deposit generations and observations of geometrical relations with volcaniclastic deposits (DAv and pyroclastic deposits) enable us to reconstruct paleo-landforms and some stages of the geomorphic evolution of Cantal. In this way, the Impradine volcaniclastic unit is a fragment of a volcaniclastic fan facing north-east (fig. 7). In the Cere valley, the "Faillitoux" generation is the remnant of a proximal section of a volcaniclastic fan facing south-west. These lahars flowed from a trachyandesitic stratocone located close to the Elanceze massif about 9.5 Ma ago (fig. 7). These paleo-stratocones were eroded and are no longer visible in the present geomorphic landscape. Lh deposits allow us to determine geomorphic inheritances, contemporaneous with the activity of the stratovolcano from 9.5 to 6.5 Ma. About 7.1 Ma, the paleo-Cere valley was filled with DAv and Lh deposits of the "Curebourse" generation. The "Curebourse" generation formed a volcaniclastic fan on the top of DAv deposits. DAv and Lh deposits, that are less resistant than the trachyandesitic Elanceze massif and Plomb-du-Cantal range, have been eroded. Accordingly, the Cere valley is being exhumed. The present-day drainage pattern occupies the paleothalweg. However, in distal positions, paleo-landforms are not as well preserved. The current drainage pattern does not use any more paleothalwegs in contrast to what is seen in proximal position (fig. 8).
