Origins of thousands of small enclosed depressions in Southwest France: an integrated geomorphological, geostatistical and geophysical study

• Main Authors: Bussière, Léa, Schmutz, Myriam, Dupuy, Alain
• Other Authors: Environnements et Paléoenvironnements OCéaniques (EPOC), Observatoire aquitain des sciences de l'univers (OASU), Université Sciences et Technologies - Bordeaux 1 (UB)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Sciences et Technologies - Bordeaux 1 (UB)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-École Pratique des Hautes Études (EPHE), Université Paris Sciences et Lettres (PSL)-Université Paris Sciences et Lettres (PSL)-Centre National de la Recherche Scientifique (CNRS), Université Laval Québec (ULaval), Universität Heidelberg Heidelberg = Heidelberg University, Bureau de Recherches Géologiques et Minières (BRGM), Contrat doctoral n°1-2017 International IDEX (Université Bordeaux Montaigne), Bourses d’excellence doctorales Sentinelle Nord (Université Laval)
• Format: Report
• Language: English
• Published: HAL CCSD 2024
• Subjects: GIS; Spatial analysis; Ground penetrating radar GPR; Electrical resistivity tomography ERT; Deflation; Doline; Thermokarst; [SDU.STU.GM]Sciences of the Universe [physics]/Earth Sciences/Geomorphology; [SDU.STU.GP]Sciences of the Universe [physics]/Earth Sciences/Geophysics [physics.geo-ph]
• Online Access: https://hal.science/hal-04615553; https://hal.science/hal-04615553/document; https://hal.science/hal-04615553/file/Preprint_Bussiere_et_al.pdf

Small topographic depressions can provide information on past and present groundwater recharge processes. The Landes Triangle (SW France) is dotted with more than 2,400 of them, valued for their ecological significance. Yet, their origin and hydrogeological implications remain poorly understood. This paper aims to elucidate their formation process, with special emphasis on the Villagrains-Landiras (VL) anticline sector: a key recharge zone for regional aquifers, which also features the highest density of depressions. We first assess the likelihood of three historical hypotheses with open GIS data, then detail the near-surface structure of a depression in the VL sector with electrical resistivity tomographies and ground penetrating radar. We identify two distinct groups of depressions in terms of morphology, distribution and geological context (SE1 and SE2). SE2 shows characteristics consistent with aeolian origin. SE1 includes 97% of the VL anticline depressions, and exhibits circular morphology and proximity to karst features, natural streams, faults, and neighboring depressions, suggesting a karst origin. However, 18% of SE1 may rather be of periglacial origin, as they do not lie in the crypto-karst area derived from GIS references. Yet, the geophysical survey specifically designed to target periglacial features in this context showed only evidence of subsidence, and none of cryogenic sediment or solifluction. We conclude that a karst origin remains the best hypothesis in the VL anticline area, and highlight that sole reliance on GIS references may underestimate crypto-karst extent. Further geological investigation is therefore needed to fully capture its extent near the VL anticline, and map related water pathways.