PROfile Thierry Gaillard and Jean-Luc Mari A new concept of karst development based on hydrogeology and geophysics The Poitou-threshold example
A new concept of karst development based on hydrogeology and geophysics The Poitou-threshold example Thierry Gaillard and Jean-Luc Mari QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE
DOI: 10.1051/978-2-7598-3934-6 ISBN(print): 978-2-7598-3934-6 – ISBN(ebook): 978-2-7598-3935-3 This book is published under Open Access Creative Commons License CC-BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/en/) allowing non-commercial use, distribution, reproduction of the text, via any medium, provided the source is cited. © EDP Sciences, 2026
5 Contents Prefaces 9 Foreword 11 T. Gaillard and J.-L. Mari The authors 13 Introduction 15 T. Gaillard and J.-L. Mari Chapter 1 • The Poitou Threshold 19 T. Gaillard and P. Branger Location and definition 19 Tectonic structure of the Poitou Threshold: a brief history of knowledge 21 Sedimentary coverage 28 Poitou threshold definition 31 References 32 Chapter 2 • The stratigraphy of the Middle Jurassic 37 P. Branger, T. Gaillard and H. Geairon Paleontological division of the Middle Jurassic 37 Deposit sequences 38 Stratigraphy of the Middle Jurassic on the Poitou Threshold 40 Aalenian 40
6 A new concept of karst development based on hydrogeology and geophysics D6 41 D7 – Lower Bajocian 41 D8 – Upper Bajocian 42 D9 – Lower Bathonian 43 D10 – Middle and upper Bathonian 43 Stratigraphy of the Bajocian south of Poitiers: the cliffs of Passelourdin 44 Conclusion 51 References 51 Chapter 3 • Hydrogeology of the Poitou Threshold 59 T. Gaillard and M. Moreau Aquifers of the Poitou threshold 59 Two main aquifers 59 Piezometric map 60 The supra-Toarcian aquifer 64 Groundwater flow patterns in the supra-Toarcian aquifer 64 Comparison between lineaments and speleological networks 65 Typology of voids in Poitou threshold limestones 68 Pumping tests 71 Conclusion 77 Acknowledgements 78 References 78 Chapter 4 • The Hydrogeological Experimental Site of Poitiers University (France) 81 T. Gaillard Origin of the hydrogeological experimental site (HES) 81 HES description 82 Scientific studies 84 References 85 Chapter 5 • Geophysical methods 87 J.-L. Mari 5.1 Seismic and acoustic methods 89 5.2 3D seismic imaging 92 5.3 Full waveform acoustic logging 98 5.4 Vertical seismic profiles versus full waveform acoustic logging 103 5.5 Electrical methods 107
7 Contents 5.6 Conclusions 111 Appendix: Static corrections Acknowledgements 116 References 116 Chapter 6 • Borehole electrical panels: an experiment 119 M. Moreau, P. Brunel and J.-L. Mari Material and method 120 Acquisition 123 Geological Interpretation 125 Summary 129 Conclusions 130 References 131 Chapter 7 • Hydrogeological flow logging and dye tracer tests on the Hydrogeological Experimental Site of Poitiers University (France) 133 A. Boulais, T. Gaillard and H. Geairon Introduction 133 Borehole flow logging acquisition 135 Principle of flow logging 135 Downflow and upflow in the borehole 136 Dynamic flow logging 136 Flow logging with pumping in MP6 (crossed dynamics MP6) 141 Tracer tests 144 Tracer test protocol 144 Results 144 Conclusion and interpretation of pathways in karst horizons 148 References 149 Chapter 8 • Hydro-stratigraphic study of the Hydrogeological Experimental Site of Poitiers, France 151 T. Gaillard Objectif and method 151 Aquifer stratigraphy of the HES 152 Stratigraphic context 152 Data available 153 Lithostratigraphy of the HES 154
8 A new concept of karst development based on hydrogeology and geophysics Occurrence ok karst 161 Fracture inventory 161 Stratigraphy versus seismic and acoustic data 162 Stratigraphy versus borehole electrical methods 164 Stratigraphy and hydrogeological data 167 Conclusion 168 References 169 Chapter 9 • The Deffend hydrogeological model 173 T. Gaillard Hydrogeological knowledge before Hydrogeological Experimental Site (HES) of Poitiers 173 Historical groundwater flow model of the Poitou Threshold 173 The Poitiers HES groundwater model 175 Karst dating 176 Syngenetic karstification hypothesis 178 Conclusion 182 References 182 Conclusion 187 J.-L. Mari and T. Gaillard Acknowledgements 191 J.-L. Mari and T. Gaillard
9 © EDP Sciences, 2026 DOI: https://doi.org/10.1051/978-2-7598-3934-6.c901 QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE Prefaces In 1995, the DATAR (French Delegation for Regional Planning and Development) commissioned researchers from the University of Poitiers to design scientific activities related to the planned underground laboratory for radioactive waste storage, located in the southern part of the Vienne department. Drawing on the expertise I had acquired during my doctoral research at the Béthune study site—developed under the supervision of Professor Norbert Crampon—and on the data already collected in southern Vienne, I naturally proposed the creation of an ambitious hydrogeological study area. However, due to the high degree of fracturing in the granitic bedrock, the National Evaluation Commission issued an unfavorable opinion regarding the siting of the underground laboratory. It was within this context that the Hydrogeological Experimental Site (HES) project was born. Submitted as part of the 2000–2006 State–Region planning contract to the Ministry of Research, the project was accepted and received funding of 13 million French francs. The first borehole was drilled in July 2002. By 2025, the HES comprised 45 boreholes spread over 32 hectares, traversing fractured and karstified limestone aquifers of the Lower and Middle Jurassic, with a thickness of 160 meters. Today, the HES stands as a unique experimental platform in France, open to the entire scientific community. It supports research projects, both initial and ongoing education programs, and serves as a testing ground for various disciplines, including pedology, geology, geophysics, hydrogeology, hydrogeochemistry, and numerical modeling. For over twenty years, this infrastructure has enabled significant advances in the understanding of the structure, behavior, and dynamics of carbonate reservoirs. I am particularly pleased by the fruitful collaboration between Thierry—a passionate geologist and Jurassic specialist—and Jean-Luc, a geophysicist involved with the HES from its inception. Together, they have produced a major work that skillfully combines their respective expertise to improve our understanding of the structural organization of the limestone reservoir. The detailed characterization of Middle
10 A new concept of karst development based on hydrogeology and geophysics Jurassic lithology, combined with a diverse geophysical approach, made it possible to identify the karstic levels responsible for the rapid flows observed at the site. Throughout the nine chapters, readers are guided toward a coherent and wellsupported representation of the reservoir structure, where variations in carbonate chemistry (pure limestone vs dolomitized limestone) emerge as key drivers of karstification. Within the 100-meter-thick section under study, three horizontal productive levels have been clearly identified, representing priority targets for the sustainable extraction of groundwater resources. Pr. Gilles POREL Head of the hydrogeological experimental site at the University of Poitiers It is with genuine satisfaction that I have the privilege, less than a year after the publication of the AGAP notebook “Geophysics in Geothermal Exploration: a review”, to write the preface to this new work: “A New Concept of Karst Development Based on Hydrogeology and Geophysics.” Once again, the term “notebook” hardly does justice to the content, as this is far more than a simple collection of summaries. It is a work of undeniable scientific merit, enriched by clear and visually compelling figures of exceptional quality. I would like to extend my sincere congratulations to the authors of the nine chapters and express my gratitude to Jean-Luc Mari and Thierry Gaillard. Their respective expertise in geophysics and in the geology of the Poitou-Charente Threshold has ensured a vital coherence in a work that demands true multidisciplinary mastery. I am highly confident that this “notebook” will be well received by its readers— whether geologists, hydrogeologists, karstologists, geophysicists, or geoscientists in general. Michel HAYET President, AGAP Quality
11 QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE © EDP Sciences, 2026 DOI: https://doi.org/10.1051/978-2-7598-3934-6.c902 Foreword T. Gaillard and J.-L. Mari Based on their experience in geophysics as applied to the oil and gas industry and in the geotechnical and hydrogeological fields, the authors have set out to explain how conventional seismic methods used in exploration or in reservoir geophysics for imaging can be applied to hydrogeological surveys and to site characterizations in the framework of karstified geological formations, using for example the Dogger limestone of the Poitou threshold. After reviewing the current state of geological knowledge of the Poitou threshold, the book aims to describe how 3D seismic surveying, implemented with light seismic spreads, combined with vertical seismic profiles and full waveform acoustic logging, can be used to obtain a very highresolution 3D block in depth, which points out karstic levels. The book highlights how seismic imaging, complemented with both a geological study at a regional biostratigraphic scale and sequential sections established from drilling data, can lead to a stratigraphic characterization of the Dogger limestone. The authors propose a coherent karstogenesis scheme, without involving tectonic constraints, of karstogenic horizons in a sequence of carbonate deposits. In addition to the field study of the Dogger limestone of the Poitou threshold, the authors provide readers with guidelines to carry out a seismic and stratigraphic characterization methodology that can be applied to hydrogeological investigations and reservoir studies. The authors thank the University of Poitiers for permission to use all the data sets available on the Hydrogeological Experimental Site (HES) of Poitiers, and more specifically Gilles Porel, who promoted their research project.
13 QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE © EDP Sciences, 2026 DOI: https://doi.org/10.1051/978-2-7598-3934-6.c903 The authors Thierry Gaillard Thierry Gaillard is a hydrogeologist trained at the Universities of Bordeaux (ENSEGID) and Jussieu (Paris-Sorbonne). He has spent his career in consulting companies, and since 2017, he has been the CEO of CPGF-HORIZON. Alongside his professional activities, he is an active member of the French chapter of the International Association of Hydrogeologists (IAH) and of several other associations (Association des Géologues du Bassin Parisien, Aquassistance, Société géologique de France). He has worked in France and in several African countries as a consultant or with NGOs (Bénin, Togo, Ivory Coast, Senegal, Niger, Morocco). Email: tgaillard@cpgf-horizon.fr Jean-Luc Mari Jean-Luc Mari is a geophysicist with a distinguished career in applied geosciences. A graduate of the Institut de Physique du Globe de Strasbourg and IFP School (Petroleum Geosciences, major in Geophysics, 1978), he joined IFP Energies Nouvelles in 1979 as a research engineer in the Geophysics Department. He led and contributed to key research projects in high-resolution seismic imaging, reservoir monitoring, and borehole tool development, in collaboration with industrial partners such as GDF-Suez, CGG, Total, and ELF Aquitaine.
14 A new concept of karst development based on hydrogeology and geophysics In 1986, he was seconded to ELF Aquitaine, where he focused on reservoir geophysics, before returning to IFPEN in 1987 to join the Reservoir Department. His work notably explored the application of geophysical methods in horizontal wells. In 1994, he became a professor at IFP School and earned his Habilitation à Diriger des Recherches (HDR) in Earth Sciences. He also served as a geophysics expert for IFPEN. A member of the European Association of Geoscientists and Engineers (EAGE), Jean-Luc has been an associate editor for Near Surface Geophysics. Now retired from IFPEN, he works as an independent researcher and consultant. He sits on the board of the Association for Quality in Applied Geophysics (AGAP – Association pour la Qualité en Géophysique Appliquée). Jean-Luc Mari is the author and co-author of numerous scientific papers, patents, and educational resources, including textbooks and digital learning tools. In 2010, he was awarded the title of Chevalier in the Ordre des Palmes Académiques for his contributions to science and education. Email: jeanluc90.mari@gmail.com
15 QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE © EDP Sciences, 2026 DOI: https://doi.org/10.1051/978-2-7598-3934-6.c904 Introduction T. Gaillard and J.-L. Mari Geophysical and hydrogeological investigations play a pivotal role in aquifer characterization. Here, we focus on a carbonate limestone aquifer in the Poitou region (France). We show how hydrogeology and geophysics have contributed to revising regional groundwater flow models and have provided significant new insights. The study highlights how a multidisciplinary geoscientific approach, integrating geophysical, hydrogeological, and stratigraphic analyses, can refine and strengthen a groundwater flow model. The study area is located at a geological transition zone between the Aquitaine and Paris basins in France, known as the Poitou threshold. Chapter 1 of the book provides a geological overview of the threshold. Chapter 2 contains a detailed synthesis of the middle Jurassic limestone stratigraphy, which forms the basis for defining the regional geodynamic context. Particular attention is given to the Poitiers region, including newly developed geological cross-sections that delineate key stratigraphic features in the central part of the threshold. From a hydrogeological perspective, the Poitou threshold exhibits notable karst flows, despite its low hydraulic gradient, the relatively limited limestone thickness compared to the Périgord and Causses regions of the Aquitaine basin, and an altitude that does not exceed 150 m above sea level. Geologists of the 19th and 20th centuries unanimously attributed the origin of these karst systems to fissures formed under tectonic stress. The concepts underpinning the hydrogeology and karst morphology of the Poitou threshold are explored in detail in Chapter 3 entitled “Hydrogeology of the Poitou Threshold.” The traditional understanding has been challenged by recent research conducted on a hydrogeological platform established near Poitiers under the leadership of
16 A new concept of karst development based on hydrogeology and geophysics Gilles Porel: Le Deffend Hydrogeological Experimental Site (HES). Spanning an area of 12 hectares and comprising approximately 35 systematically distributed boreholes, the site represents a significant research initiative. The platform and its associated research objectives are detailed in Chapter 4, dedicated to the description of the experimental hydrogeological site: Le Deffend. Among the investigative techniques implemented at the HES, geophysics—particularly seismic methods—holds a central role, offering a renewed perspective on groundwater flow geometry. In exploration geophysics and reservoir studies, seismic methods are primarily used for constructing subsurface models. These techniques are increasingly significant in geotechnical, hydrogeological, and site characterization studies, particularly in the context of seismic hazard assessments. Surface seismic reflection provides a three-dimensional (3D) representation of subsurface acoustic impedance contrasts, either in terms of time or depth. The resulting 3D seismic blocks generate interpretable images that can be used to develop porosity and karst models. When combined with borehole measurements such as vertical seismic profiles (VSP) in low-frequency ranges (5–200 Hz) and full-waveform acoustic logging in very high-frequency ranges (1–25 kHz), seismic data enable robust estimation of petrophysical parameters, such as seismic porosity. These methods also facilitate the characterization of specific geological attributes, including karstic features. More recently, electrical resistivity tomography (ERT), conducted both at the surface and in boreholes, has been deployed at the Le Deffend site, with findings that corroborate the seismic results. The results of the geophysical investigations are presented across two chapters. Chapter 5 highlights the contributions of various geophysical techniques, including seismic, acoustic, and electrical methods. Chapter 6 is dedicated to a borehole experiment conducted using Electrical Resistivity Tomography (ERT). Based on the geophysical data, a revised geometry of the karstic horizons has been established. Notably, the karstic horizons within the Middle Jurassic limestones were found to be sub-horizontal rather than vertical, challenging hydrogeological interpretations from the 19th and 20th centuries regarding the karstification process. To assess the implications for groundwater flow, logging and tracer tests were conducted alongside initial pumping tests. Pumping and hydraulic slug tests were used to evaluate the interference geometry between boreholes. These tests were initially aimed at identifying a preferential fracturing pattern shaped by the tectonic constraints of the Poitou threshold. However, the three-dimensional seismic block revealed the sub-horizontal geometry of the karst levels, leading to a re-evaluation of hydrogeological flow patterns and the development of a new conceptual model, referred to as the “Le Deffend model.” Details of the logging and hydrogeological tests performed at the site are presented in Chapter 7 entitled “Hydrogeological flow logging and dye tracer tests. The origin of the karstic horizons was also reexamined. While the distribution of karst horizons in limestone massifs is often considered random, a detailed stratigraphic
17 The authors analysis was conducted at this site. The analysis included core sampling and optical televiewer (OPTV) imaging from open-hole drilling. These efforts, outlined in Chapter 8 entitled “Hydro-Stratigraphic study,” made it possible to clarify the stratigraphic controls on the distribution of karstic horizons in the Le Deffend model. The Le Deffend model was subsequently integrated into a geodynamic framework. The results suggest that the distribution of karstified strata is closely tied to low stand system tracts (LST) within a broader depositional sequence affected by sealevel fluctuations. This geophysical and stratigraphic reinterpretation has significant implications for groundwater resource exploration and preservation. These findings are detailed in Chapter 9, “Hydrogeological Model of the Le Deffend site (HES).” The book aims to demonstrate that seismic attributes derived from 3D seismic blocks and other geophysical methods can effectively characterize karstic horizons within carbonate sequences through stratigraphic interpretation. Beyond advancing geophysical methods, the research conducted at the Le Deffend site over two decades provides actionable insights for preserving drinking water resources in the region. It underscores the necessity of controlling water well stratigraphy and borehole completion to optimize groundwater resource management. This collaborative effort between scientists and engineers serves as a model for how applied geophysics can support hydrogeologists, engineers, and policymakers in sustainably managing groundwater resources amidst challenges such as scarcity and pollution.
19 © EDP Sciences, 2026 DOI: https://doi.org/10.1051/978-2-7598-3934-6.c001 QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE The Poitou Threshold T. Gaillard and P. Branger Location and definition The Poitou region is a natural area located between the Paris and Aquitaine basins, and between the Massif Central and the southeastern terminus of the Armorican Massif (France). This unique geographical configuration has led Poitou to be alternatively conceptualized as a strait (Longuemar, 1870; Welsch, 1892; Fournier, 1888) or as a threshold (Welsch, 1892; Gabilly, 1962). The term “strait” refers to the marine channel that once connected the Paris and Aquitaine basins, while the term “threshold” denotes a region of intermediate elevation located between higher and lower elevations. From a geological perspective, the Poitou region is characterized as a strait, whereas geographers consider it a threshold. Both terms describe the same physical reality. “Geologists refer to this region as the Poitou Strait to indicate its role in connecting the sedimentary formations of the Paris Basin with those of the Aquitaine Basin. This concept of a strait stems from the interpretation that the Jurassic deposits of the Poitou Threshold represent sediments from a marine channel or strait extending between the Paris and Aquitaine basins” (Welsch, 1903). Welsch further elaborates: “Since the region around Poitiers is, in fact, a zone of lower elevation relative to the Limousin and Vendee areas that it separates, it is appropriate to designate it as the Poitou Threshold”. The threshold is thus defined by its intermediate elevation, its position between low-lying sedimentary basins and higher ancient massifs (Fig. 1). This geographical definition was adopted by Welsch, who described Poitou as “a vast plateau 1
20 A new concept of karst development based on hydrogeology and geophysics Figure 1 Poitou threshold location.
21 1. The Poitou Threshold encompassing most of the Vienne department, the southern part of Deux-Sèvres, and the northern part of Charente. Its average elevation is 146 meters above sea level, with a gradual rise to the east and southeast toward Limousin, reaching 200 meters at Lathus and 225 meters beyond Isle-Jourdain. To the west, the elevation against the Vendée Massif reaches altitudes of 160 to 190 meters” (Welsch, 1892). The adoption of these concepts likely reflects the influence of geographical studies, particularly the maps produced by Vidal-Lablache, which were widely employed in educational settings during the French Third Republic. But the topographic approach suffers from the fact that no elevation was selected to define the threshold’s limits/boundaries. Tectonic structure of the Poitou Threshold: a brief history of knowledge Welsch (1846, 1892) proposed the first tectonic structure of the Poitou Threshold, including two cross-sections: one cross-section that links the old Hercynian Mountain ranges and another that links the sedimentary basins (see Fig. 2 and 3). In the first cross-section (Fig. 2), Welsch noted only one fault: the Montalembert Fault, located south of the Deux-Sèvres department. The bedrock includes two major anticlines that connect ancient massifs: the Champagné-Saint-Hilaire anticline and the Ligugé anticline. The Poitou area was described as a strait (isthme in French on Fig. 2). The second cross-section (Fig. 3) links the Armorican and Limousin massifs, showing sub-horizontal sedimentary layers between Ménigoute (West) and AvaillesLimouzine (East). The bedrock crops out at both ends. In this figure, the Poitou area was identified as a threshold (Seuil in French in Fig. 3). Glangeaud’s observations refined earlier interpretations by identifying faults that isolate the Champagné-Saint-Hilaire anticline from “collapsed layers” (Glangeaud, 1895). Fournier (1903) later produced a geologic map of the Poitou Threshold. After World War I, Mathieu (1937) conducted additional structural studies for his thesis on Paleozoic terrains in the Vendée region. He identified structural links between the Limousin and Gâtine Hills, revisiting the mapped features of Welsch. Mathieu described four major structural axes, listed from north to south (Fig. 4): A4 – the Ligugé Anticline; A3 – an anticline extending from Le Fouilloux, through Lusignan and Champagné-Saint-Hilaire to Availles-Limouzine; A2 – The Rouillé-Couhé-Civray Axis (also called the SaintSauvant Anticline), and A1 – a complex anticline connecting Mervent to the Montalembert Horst. Axes A5 and A6 were not traced further east due to a lack of structural markers. The major axes were later integrated into the regional geological guide (Gabilly et al., 1978) under the names Essarts-Mervent-Melle-Montalembert Anticline (A1)
22 A new concept of karst development based on hydrogeology and geophysics Figure 2 SSW-NNE cross-section connecting the Aquitaine basin to the Paris basin (Welsch, 1892). Figure 3 NW-SE cross-section connecting the Armorican and Limousin massifs (Welsch, 1892).
23 1. The Poitou Threshold L1-p: Triassic formations; o-e: “siderolic” formation (clay with ferruginous pisolite), e = lacustrine marl (Eocene); JIV-p: Mesozoic limestones, L1-IV: lower Jurassic formations; h: “houiller” = coal formations; R: rhyolite of Cholet; x: shists; ε: amphibolite; γ1: granulite; γ2: granite; δ: diorite; ξ: gneiss; π porphyry; Q: quartz - A: anticline, S = syncline. Figure 4 Structural axes of the Poitou threshold by Mathieu (1937, p. 289).
24 A new concept of karst development based on hydrogeology and geophysics and Champagné-Saint-Hilaire Anticline (A3). This structural framework, based on a model of parallel fault alignments extending from the Limousin to the Vendée along a south-Armorican trend N120°E/N140°E (Fig. 5), prevailed until the early 1990s. In the 1980s, this structural model was reinforced by the identification of the Limousin Tonalitic Line (LTL) linking the Parthenay Massif to Limousin granites and the Availles-Limouzine Fault (Dhoste, 1983; Peiffer, 1987; Cuney et al., 2001). North of the threshold, the Marche Fault line is well documented only in the southern Berry area. This fault system between the Massif Central and the Paris basin was long unknown westward beneath the Poitou Threshold. Early indications came from gravimetric anomalies (Goguel, 1954) and a magnetic anomaly map of the Paris Basin (Debeglia, 1980). Figure 6 shows the correspondence between mapped faults and the gravity map of Martelet et al. (2009). The light gravimetric anomaly indicates the Eo-Variscan suture between Gondwana (or Central Armorica) and Laurussia and appears to cross the threshold without faulting in the sedimentary coverage. This contact is also highlighted by a negative Bouguer anomaly (Baptiste, 2017). The Nort-surErdre Fault marks the Ligérienne Province boundary and coincides with the light Bouguer anomaly. This fault is interpreted as an Eo-Variscan suture (Dercourt, 1998) and extends towards the Loudun fault. Goguel’s (1954) heavy gravimetric anomaly north of Poitiers is aligned with the eastern extension of the Thouars fault (Weber, 1973). This anomaly corresponds to a contact of Jurassic and Upper Cretaceous sediments, suggesting that the Thouars fault is a heritage that gave rise to the Thouars granitic intrusion. The Mirebeau fault is aligned with the Thouars fault. However, no faults are mapped east of Mirebeau on the Vouneuil-sur-Vienne geological map (Bourgueil et al., 1976). Seismic activity in this area (e.g., a magnitude 3.2 earthquake in Saint-Léger-La-Pallu on June 14, 2019) reinforces the hypothesis of a fault that extends from the Armorican Massif and is masked by Eocene formations. Further studies, including a study on the Parthenay Fault (Poncet, 1993) and ANDRA’s recognition of the Charroux Granite in the late 1990s, revised the structural map of the bedrock of the Poitou Threshold. These studies revealed three main structures originating from the Armorican Massif (Fig. 7): (i) the Vasles-Availles Fault Zone, passing through Champagné-Saint-Hilaire (formerly the Pouzauges-Oradour axis of Cariou et al., 1989); (ii) the Parthenay Fault, trending N160°E, which connects to the Saint-Maixent and Lezay grabens rather than the Vasles Fault and Champagné-Saint-Hilaire Horst (Poncet, 1993; Cuney et al., 2001; Rolin and Colchen, 2001); (iii) the Basse Marche Fault, south of Poitiers, is linked to the Ligugé Granite and extends to the Thouars Fault (Rolin and Colchen, 2001) via the Mirebeau fault. Rolin et al. (2009) consider these fault systems to be shear zones. Figure 7 is a schematic map of Paleozoic faults and granites from Rolin and Colchen (2001) and
25 1. The Poitou Threshold Figure 5 Poitou threshold structure (from Gabilly and Cariou, 1974)
26 A new concept of karst development based on hydrogeology and geophysics Figure 6 Fault system and gravimetry map (from Martelet et al., 2009).
27 1. The Poitou Threshold Figure 7 Schematic map of Paleozoic faulting and calc-alkaline magmatism.
28 A new concept of karst development based on hydrogeology and geophysics Rolin (2001). Granites intruded the Paleozoic sediments during the Cambrian, Ordovician, Devonian-Visean, and Namurian-Westphalian (Carboniferous). The primary granitic intrusion activity occurred between the Devonian and the Westphalian (Moscovian). Calc-alkaline magmas may be the result of the fusion of a mantle layer intruded into the tectonics of the threshold between the Parthenay fault and the Vasles-Availles fault (Rolin et al., 1999). Sedimentary coverage Granites and Paleozoic metamorphic formations make up the bedrock that is covered by Jurassic formations. Triassic sediments are located only at the threshold border (Infra Lias clay in the Aquitanian basin and arkose in the Paris basin). During the Early Jurassic (Sinemurian and Hettangian), shallow marine environments dominated the Poitou Threshold. The sediments consist primarily of shallow marine carbonates. During this period, three sedimentation areas were active: (i) the Atlantic area west of Fontenay-le-Comte, (ii) a Vendean area between Fontenayle-Comte and Thouars, and (iii) the “Pictave” area around Poitiers (Gabilly and Cariou, 1974). The Toarcian is marked by the deposition of dark, organic-rich marls and shales, indicative of deeper, more anoxic conditions associated with the global Toarcian Oceanic Anoxic Event (TOAE). Between the Aalenian and the Callovian, sedimentation transitioned to shallow marine carbonate platforms. These include micritic limestones, bioclastic limestones, oolitic limestones, and occasional reefal buildups. The Oxfordian is represented by limestones in the Pictave area. The sedimentary facies are marly from the Vendean to the Atlantic area. During the middle Jurassic, the sedimentation area was limited by the fault(s) inherited from Paleozoic tectonics (Mourier and Gabilly, 1985). Fossils, particularly ammonites and brachiopods, are critical for establishing relative ages due to their stratigraphic distribution and global correlation. While ammonite dating is accurate towards the basins, the scarcity of pelagic fauna in the middle of the Poitou threshold makes dating a delicate matter. For this reason, Gabilly studied signs of exposure and erosion registered in limestones and the biozone gap (Gabilly, 1962). This evidence is used to make correlations between the Poitou threshold and the Aquitanian basin margin (Gabilly et al., 1985). Gabilly et al. (1978) proposed a cross-section of the Poitou threshold, which is still valid (Fig. 8). The threshold affects sedimentation in several ways. The first is a thinning of the geological layers. The second is a facies variation, with carbonate facies on the threshold and marly facies towards the basins. The anticlines and synclines of the cross sections are associated with the fault system shown in Figure 8. No faults were identified as being related to the Paris Basin. As a result of the new fault pattern shown in Figure 7, the facies distribution of the middle Jurassic deposits could be revised. For example, Figure 9 shows the facies distribution during the upper Bathonian, as mapped by Gabilly et al. (1978). With
29 1. The Poitou Threshold Figure 8 Cross-section of the Poitou threshold (Gabilly et al., 1978).
30 A new concept of karst development based on hydrogeology and geophysics Figure 9 Facies distribution during the upper Bathonian (from Gabilly et al., 1978).
31 1. The Poitou Threshold the proposed schematic fault map, the various areas of sedimentation are superposed onto tectonic blocks. High-energy facies (oolitic limestones) were placed between the faults that delimit the Pictave block. Gravelly limestones that extended to the west were pinched out against the Vasles-Availles fault. Bioclastic limestone with sponge beds was confined to the area between the Vasles-Availles fault and the Parthenay fault. To the west of the Parthenay fault, sedimentation was marly. Poitou threshold definition Gabilly et al. (1978) mapped the facies distribution for the Toarcian, lower Bajocian, and Callovian. Mourier and Gabilly (1985) mapped it in the Vienne and Charente valleys. The pattern of the fault system is similar to Figure 9. The northern axis, which better limits the extension of high-energy facies, corresponds to the gravity anomaly parallel to the Thouars-Mirebeau fault. Sedimentation north of the Vasles-Availles-Limouzine axis is a carbonate platform environment, while to the south it becomes openly marine; this is the Vendean domain (Gabilly and Cariou, 1974; Mourier, 1983; Branger, 1989). The VaslesAvailles axis (roughly equivalent to the Pouzauges-Oradour axis of Gabilly, 1962; Mourier 1983; Branger, 1989) marks the end of the Poitevin Strait and the beginning of the Aquitaine Basin. Defined in this way, the Poitou threshold appears to be a mid-Jurassic shoal, with the northern and southern limits of the platform aligned with the structuring axes inherited from the basement. The shorelines are controlled by these granitic horst sets from the Toarcian to the Bathonian. The platform continues into the Bajocian and Bathonian towards Berry. In the Callovian, however, the inherited higher elevations no longer seem to constrain sedimentation, and the gap in some Callovian ammonite zones extends over a vast plateau (Cariou, 1980). Although some authors have sometimes compared the Poitou threshold to a vast NW-SE anticline with a large radius of curvature (Gabilly, 1978), the geometry of the strata is sub-horizontal and is offset by normal faults. In fact, the geometry of the limestone strata on the Poitou platform conforms to cycles of sea-level variation and the creation of available space for proximal and distal depositional facies (Mourier, 1983; Branger, 1989; Gonnin et al., 1992). Based on structural maps, it is possible to give a purely geological definition to the Poitou Threshold as the region between the Thouars-Mirebeau fault to the north and the Vasles-Champagné-Saint-HilaireAvailles-Limouzine axis to the south (Fig. 10). The advantage of this proposal is that it also represents the platform of the Pictave domain during the Jurassic (Gabilly and Cariou, 1974; Mourier and Gabilly, 1985).
32 A new concept of karst development based on hydrogeology and geophysics Figure 10 The Poitou threshold defined by Dogger basement structure and platform facies. References Baptiste J. (2017). Cartographie structurale et lithologique du substratum du Bassin parisien et sa place dans la chaîne varisque de l’Europe de l’ouest: approches combinées géophysiques, pétrophysiques, géochronologiques et modélisations 2D [Structural and lithological mapping of the bedrock of the Paris Basin and its place in the Variscan chain of Western Europe: combined geophysical, petrophysical, geochronological and 2D modelling approaches]. Ph.D. thesis, University of Orléans, France. Bourgueil B., Cariou E., Moreau P., Ducloux J., Teissier J.-L. (1976). Carte géologique de la France au 1/50000: feuille de Vouneuil-sur-Vienne [1:50,000 geological map of France: Vouneuil-sur-Vienne sheet], BRGM, n° 567. Branger P. (1989). La marge Nord-Aquitaine et le seuil du Poitou au Bajocien: stratigraphie séquentielle, évolution biosédimentaire et paléogéographie [The North Aquitaine margin and the Poitou Threshold in the Bajocian: sequence stratigraphy, biosedimentary evolution and palaeogeography]. Ph.D. thesis, University of Poitiers, France.
33 1. The Poitou Threshold Cariou E. (1980). L’étage Callovien dans le centre-Ouest de la France, première partie: stratigraphie et paléogéographie [The Callovian stage in central-western France, part 1: stratigraphy and palaeogeography]. Ph.D. thesis, University of Poitiers, France. Cariou E., Hantzpergue P., Jan Du Chene R., Vail P. (1989). Excursion in the Jurassic of the Charentes and Poitou area (France). University of Poitiers, France, STATOIL. Cuney M., Brouand M., Stussi J.-M. (2001). Le magmatisme hercynien en Vendée. Corrélations avec le socle du Poitou et l’ouest du Massif Central français [Hercynian magmatism in the Vendée. Correlations with the Poitou basement and the west of the French Massif Central]. Géologie de la France, 1/2: 117-142. Debeglia N. (1980). Carte du socle, écorché anté-triasique [Base map, AnteTriassic crust], in Synthèse géologique du Bassin de Paris, sous la direction de C. Mégnien (Vol. III). Mémoire BRGM, n° 102. Dhoste M. (1983). Prolongement en Poitou de la ligne tonalitique limousin [Extension of the Limousin tonalite line into Poitou]. Compte Rendu de l’Académie des Sciences, 2(296): 1659-1662. Fournier A. (1888). Documents pour servir à l’étude géologique du détroit poitevin [Documents for the geological study of the Poitevin Strait]. Bulletin de la Société Géologique de France, 3e série (XVI): 113-182. Fournier, A. (1903). Les maladies typhoïdes, L’hygiène et le sol en Poitou [Typhoid diseases, Hygiene and soil in Poitou]. Blais et Roy, Ed. Gabilly, J. (1962). Les variations de sédimentations du Lias et du Jurassique en relation avec le seuil du Poitou [Variations in Lias and Jurassic sedimentation in relation to the Poitou Treshold]. 87e Congrès des Sociétés Savantes, colloques sur les seuils en géologie (Poitiers): 679-699. Gabilly J., Cariou E. (1974). Groupe Français d’études du Jurassique: Journée d’études et excursion en Poitou 14-15-16 et 17 octobre 1974. Livret guide. University of Poitiers. Gabilly J., Brillanceau A., Cariou E., Ducloux J., Dupuis J., Hantzpergue P., Moreau P., Santallier P., Ters M. (1978). Guides géologiques régionaux: PoitouCharentes-Vendée (1re édition). Masson, Ed. Glangeaud P. (1895). Le Lias et le Jurassique moyen en bordure à l’ouest du plateau central [The Lias and Middle Jurassic on the western edge of the central plateau]. Bulletin de la Société Géologique de France, 3e série (XXIII): 10-54. Goguel J. (1954). Levé gravimétrique détaillé du Bassin parisien [Detailed gravity survey of the Paris Basin]. Mémoire BRGGM n° 12, BRGGM Ed. Gonnin C., Cariou E., Branger P. (1992). Les facteurs de contrôle de la sédimentation au début du Jurassique moyen sur le seuil du Poitou et ses abords [Factors
34 A new concept of karst development based on hydrogeology and geophysics controlling sedimentation at the beginning of the Middle Jurassic on the Poitou Treshold and its surroundings]. Compte Rendu de l’Académie des Sciences, 135(II): 853-859. Longuemar (de) A. (Le Touzé) (1870). Études géologiques et agronomiques sur le département de la Vienne [Geological and agronomic studies on the Vienne department]. Conseil Général de la Vienne, Ed. Martelet G., Pajot G., Debeglia N. (2009). Nouvelle carte gravimétrique de la France, RCGF09 [New gravity map of France, RCGF09]. Réseau et Carte Gravimétrique de la France. BRGM/RP-57908-FR. Mathieu G. (1937). Recherches géologiques sur les terrains paléozoïques de la région vendéenne. 1er fascicule: stratigraphie et tectonique [Geological research on the Palaeozoic terrains of the Vendée region. 1st part: stratigraphy and tectonics]. Ph.D. thesis, University of Lille, France. Mourier J.-P. (1983). Le versant Parisien du seuil du Poitou de l’Hettangien au Bathonien. Stratigraphie, sédimentologie, caractères paléontologiques, Paléogéographie [The Parisian slope of the Poitou Treshold from the Hettangian to the Bathonian. Stratigraphy, sedimentology, palaeontological features, palaeogeography]. Ph.D. thesis, University of Poitiers, France. Mourier J.-P., Gabilly J. (1985). Le Lias et le Dogger au sud-est du seuil du Poitou: tectonique synsédimentaire, paléogéographie [The Lias and the Dogger southeast of the Poitou threshold: synsedimentary tectonics and palaeogeography]. Géologie de la France, 3: 293-310. Peiffer M.T. (1987). La ligne tonalitique du Limousin, sa contribution à la connaissance de la géologie régionale [The Limousin tonalite line and its contribution to our knowledge of regional geology]. Annales scientifiques du Limousin, 3: 3-15. Poncet D. (1993). Le Cisaillement sud-armoricain dans le Haut-Bocage vendéen: analyse pétrostructurale et étude de la déformation dans les granitoïdes et leur encaissant métamorphique [The South Armorican shear in the Vendean HautBocage: petrostructural analysis and study of deformation in the granitoids and their metamorphic setting]. Ph.D. thesis, University of Poitiers, France. Rolin P., Stussi J.M., Colchen M., Cuney M. (1999). Structuration et magmatisme hercyniens post-collisionnels dans le Confolentais (Ouest du Massif Central) [Post-collisional Hercynian structuring and magmatism in the Confolentais (western Massif Central)]. Géologie de la France, 3: 11-31. Rolin P., Colchen M. (2001). Carte structurale du socle varisque Vendée-Seuil du Poitou-Limousin [Structural map of the Variscan basement Vendee-Poitou Threshold-Limousin]. Géologie de la France, 1-2: 3-6. Rolin P., Marquer D., Colchen M., Cartannaz C., Cocherie A., Thiery V., Quenardel J.-M., Rossi P. (2009). Famenno-Carboniferous (370-320 Ma) strike
35 1. The Poitou Threshold slip tectonics monitored by syn-kinematic plutons in the French Variscan belt (Massif Armoricain and French Massif Central). Bulletin of the Société Géologique de France, 80(3): 231-246. https://doi.org/10.2113/gssgfbull.180.3.231 Weber C. (1973). Le socle antétriasique sous la partie sud du bassin Parisien d’après les données de géophysique [The anteriassic basement beneath the southern part of the Paris Basin based on geophysical data]. Bulletin du BRGM, II(3): 293-343. Welsch J. (1903). Étude des terrains du Poitou dans le détroit poitevin et sur les bordures du massif ancien de Gâtine [Study of the Poitou terrain in the Poitevin strait and on the edges of the ancient Gâtine massif]. Bulletin de la société géologique de France, 3(4): 798-881. Welsch J. (1892). Essai sur la géographie physique du seuil du Poitou [Essay on the physical geography of the Poitou threshold]. Annales de Géographie, 2(5): 53-64.
37 © EDP Sciences, 2026 DOI: https://doi.org/10.1051/978-2-7598-3934-6.c002 QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE 2 The stratigraphy of the Middle Jurassic P. Branger, T. Gaillard and H. Geairon Paleontological division of the Middle Jurassic Since the end of the Toarcian period, the Poitou threshold and its Parisian and Aquitaine slopes have corresponded to a carbonate platform that persisted throughout the Middle Jurassic (Aalenian, Bajocian, and Bathonian). At that time, Poitou was located at the edge of the Tethys, a vast ocean between Gondwana and Laurasia (Fig. 1). The first sediments deposited on this platform date back to the Aalenian (174 Ma± in ICS, 2024). Two ammonite lineages are commonly used to date the sediments of this stage: Leioceratinae and Graphoceratinae. The Toarcian-Aalenian boundary is difficult to define paleontologically because the last Pleydellia have affinities with the first Leioceras (Cariou and Hantzpergue, 1997). In platform regions such as Poitou, the Toarcian-Aalenian boundary is linked to an ecological change with a drop in sea level. The first ammonite zone corresponds to the appearance of L. opalinum. This is followed by two species of ammonites that are widespread in Western Europe: Ludwigia murchisonae and Brasilia bradfordensis, whose horizons determine the Middle Aalenian and are separated by a discontinuity. In Poitou, the Middle Aalenian ends with the Brasilia gigantea horizon. The Upper Aalenian corresponds to the appearance of Graphoceras concavum.
38 A new concept of karst development based on hydrogeology and geophysics A major renewal of marine fauna marks the first Bajocian transgression at the boundary between the Concavum and Discites zones. The carbonate platform is well dated by the Tethyan ammonite fauna in Western Europe, as the boreal fauna did not penetrate the epicontinental seas of the western Tethys. The ammonite zones of the Lower Bajocian are defined by Hyperlioceras discites, Witchellia laeviuscula, Sonninia propinquans, and Stephanoceras humphresianum. The Upper Bajocian transgression lies between the Humphriesianum zone and the Niortense zone. The epicontinental seas underwent a reactivation of the basement faults and an oolitic barrier extending along the Massif Central to the Pyrenees, marking the sedimentation in the Aquitaine Basin (Cariou and Hantzpergue, 1997). The zonation of the Upper Bajocian is based on the ammonites Strenoceras niortense, Garantiana garantiana, and Parkinsonia parkinsoni. The appearance of Zigzagiceras zigzag marks the base of the Bathonian stage. In Poitou, boreal ammonites (Gonolkites convergens) coexist with Mediterranean ammonites (Morphoceras parvum). This mixture of fauna is a distinctive feature of this stage. The Lower Bathonian ends with the Procerites aurigerus zone. The Middle Bathonian includes the Procerites progracilis, Tulites subcontractus, Morrisiceras morrisi, and Cadomites bremeri zones. The Upper Bathonian includes the Prohecticoceras retrocostatum zone and the Clydoniceras discus zone. Deposit sequences The dating of carbonate deposits using ammonite fauna has enabled a detailed reconstruction of the sequence of deposits in the Paris Basin (Rioult et al., 1991). On the Poitou threshold, dating using ammonites is hampered by the rarity of pelagic fauna. Furthermore, the study of Middle Jurassic terrains on the edge of the Figure 1 175 My paleogeographic map of the western Tethys with the Poitou area (GPlates 2.5).
39 2. The stratigraphy of the Middle Jurassic Massif Central, particularly in Haut-Poitou, is difficult due to lateral variations in facies (Glangeaud, 1895). Under the impetus of Professor Gabilly, the stratigraphy of the Poitou threshold was revised in the 1970s with the definition of 107 ammonite biozones from the Sinemurian to the Oxfordian (Gabilly and Rioult, 1971; Gabilly and Cariou, 1974). This zoning was clarified using a lithological and sedimentological approach. Even before the advent of sequential stratigraphy, Gabilly understood the importance of taking into account major sedimentary discontinuities and deposit facies (Gabilly, 1962), which were often ignored by earlier authors (Welsch, 1895). The Poitou threshold series was thus revised and divided into 13 sequences separated by discontinuities and presented in 1974 during the excursion organized by Gabilly and Cariou on the Poitou threshold. The comparison of pelagic ammonite zones (Deux-Sèvres and Charente) with the discontinuities separating each sequence made it possible to define a sequential stratigraphy of the Jurassic platform on the threshold where ammonites are rare or even absent in the outcrops studied (Gonnin et al., 1993; Branger and Gonnin, 1994). Table 1 shows the initial stratigraphic division by Gabilly and Cariou (1974), which was subsequently used in the work of the Faculty of Geology at Poitiers and in the notes on the 1:50,000 geological maps. The numbers of the discontinuities (D6 to D11) are borrowed from the scale used in the work of the University of Poitiers (Mourier, 1983; Gabilly et al., 1985) and follow the logic of the 1974 sequences. Table 1 Sequence stratigraphy of the Poitou Threshold.
40 A new concept of karst development based on hydrogeology and geophysics With this sequential approach, the deposit sequences of the Poitou threshold can be precisely defined (Table 1). Discontinuities are associated with low sea levels on the eustatic curve. The sedimentary bodies correlate well with the discontinuities for the Aalenian and Bajocian. In the Bathonian, the deposit sequences follow a regressive trend. On the Poitou threshold, the determination of sequences with discontinuities is less precise. Stratigraphy of the Middle Jurassic on the Poitou Threshold Starting in the Early Jurassic period, the Poitou Threshold and its surroundings were gradually invaded by an epicontinental sea connected to the western Tethys Sea to the southeast. Throughout the Middle Jurassic, the region was a depositional area whose general physiography, inherited from the Permian-Triassic differential erosion of its Hercynian basement, allows several distinct areas to be identified. In simplified terms, an axis running from Pouzauges to Oradour-sur-Glane (Gabilly et al., 1978) separates, to the northeast, a shallow domain with abundant carbonate sedimentation and benthic fauna from a deeper domain to the southwest, where sedimentation was less abundant but often accompanied by a rich pelagic fauna of ammonites. This Pouzauges-Oradour-sur-Glane axis corresponds fairly closely to the Vasles fault line, which runs along the Nantes-Parthenay granite axis towards the Champagné SaintHilaire horst and the Availles-Limouzine fault (see southern limit in Gaillard and Branger, 2026). In detail, the shallowest deposits were formed on the northwestern edge of the Massif Central, which at that time formed a carbonate aureole bordering the central platform. The eastern edge of the Vendée Massif, also shallow, appears as a shoal backing onto the Armorican Landmass. These two areas are connected by the Pictavian domain. To the south of the paleogeographic axis lies the Melusine trough (see Gaillard and Branger, 2026), a depressed area linking the northern edge of the Aquitaine Basin, which can be traced as far as the Vendée coastline. The remarkable permanence of these sedimentary domains can be explained by low accumulation of deposits and the stability of the geological bedrock. Indeed, during this interval, the remobilization of the main Hercynian tectonic axes according to a tilted block model, which was paleogeographically decisive during the Callovian and Upper Jurassic periods, appears to have been very limited. The deposit sequences are labeled D in reference to the discontinuities described by Gabilly et al. (1985). Aalenian Throughout the region, the sedimentary evolution that began in the Upper Toarcian with a reduction in clay inputs continued during the Aalenian (bodies labeled 1 and 2 in Fig. 2). The Lower Aalenian (Opalinum zone), which is very thin (2 to 3 m),
41 2. The stratigraphy of the Middle Jurassic shows stratigraphic growth of fine clayey limestone beds. This is where the Gryphaea beaumonti lumachels are found, which constitute a regional stratigraphic marker. D6 From the Middle Aalenian onwards (units 3-4-5-6), the paleogeography changes and two areas can be distinguished: • a distal platform domain extending along the northern Aquitaine margin, characterized by highly condensed and lacunar sedimentation of calcareous biomicrites with ferruginous oolites, where nektonic fauna (ammonites, belemnites) and benthic fauna (mollusks) are equally abundant; • a proximal carbonate platform edge domain, with much thicker sedimentation, consisting of fine dolomitic limestones with flints and predominantly benthic fauna, from a shallower environment with moderate energy. In the basin area, from Niort to Saint-Maixent, the Middle Aalenian (Murchisonae zone) consists of thin marl beds and a more massive bank of clayey limestone. Above a marked discontinuity (D5bis?) is a level of reddish clayey limestone with ferruginous oolites and numerous fossils, gastropods, bivalves, belemnites, and ammonites characterizing the upper part of the zone. The whole is less than 1 m thick. The Upper Aalenian, Concavum zone, is only represented from the Saint-Maixent graben and also at Vitré, north of Celles-sur-Belle, in the form of reddish, sometimes purplish, highly fossiliferous clayey limestones with ferruginous oolites and not exceeding 10 cm in thickness (unit 7). In the Couhé-Vérac-Lusignan region, the Middle Aalenian thickens considerably (approximately 20 m) and occurs as dolomitic limestones with flints attributed to the Murchisonae subzone. The next 6 meters, Bradfordensis subzone, consist of often dolomitic limestones with flints alignments. The Upper Aalenian and the extreme base of the Bajocian (Concavum and Discites zones), nearly 5 m thick, correspond to pseudo-oncoides limestones. From Poitiers to Chauvigny, the Aalenian has the same facies with even greater thicknesses, from 19 to 27 m, for the whole. Islands dotted the platform with mangrove flora, giving rise to polyp limestones and driftwood. Continuing eastward into the Gartempe Valley, the Aalenian thins again as it approaches the northern edge of Limousin, with the return of ferruginous oolites in the Bradfordensis subzone and abundant pelagic fauna. D7 – Lower Bajocian Everywhere, the base of the Lower Bajocian is indicated by levels rich in ammonites, numerous Sonniniidae, and evidence of a major transgressive event (units 8 and 9). Along the entire northern Aquitaine border, there is a layer of gray limestone with ferruginous oolites, generally about 20 centimeters thick, rich in fossils, including
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