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Article

Peatland-Type Sediment Filling in Valley Bottoms at the Head of Basins in a Stream Capture Context: The Example of the Bar and Petit Morin Peatland (Grand-Est, France)

by
Olivier Lejeune
1,*,
Jérémy Beucher
1,
Alain Devos
1,
Julien Berthe
1,
Thibaud Damien
2,
Delphine Combaz
1,
Nicolas Bollot
1 and
Théo Krauffel
1
1
Laboratory, UR 3795 GEGENA, University of Reims Champagne-Ardenne, 57 rue Pierre Taittinger, 51571 Reims CEDEX, France
2
Service Etudes et Prospective sur le Patrimoine, Direction de l’Eau et de l’Assainissement, Communauté de Commune du Grand Reims, CS 80036, 51722 Reims CEDEX, France
*
Author to whom correspondence should be addressed.
Geographies 2025, 5(3), 34; https://doi.org/10.3390/geographies5030034
Submission received: 17 May 2025 / Revised: 9 July 2025 / Accepted: 9 July 2025 / Published: 14 July 2025
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Abstract

The Quaternary saw numerous reorganizations of the hydrographic network, greatly modifying the hydrological network of these rivers. Eastern France is well known for many stream captures, described as early as the late 19th century. The oldest of these have been dated to the Middle Pleistocene. It is interesting to note, however, that these sites, located in the heart of vast limestone plateaus, have systematically become peatland zones, and understanding their functioning is fundamental to wetland restoration and renaturation programs. In addition to serving as biodiversity reservoirs, these peatlands also represent substantial carbon storage potential in the context of global climate change. Using two examples—the Marais de Saint-Gond and the Bar peatland—we propose to provide the key to understanding the origin of their sedimentary filling and the consequences of their current hydrogeological functioning.

1. Introduction

French and European national agencies responsible for water and wetland management have implemented a restoration/renaturation program for watercourses and peat bogs to preserve these areas so that they retain and recover their ecosystem functions, including acting as buffer zones against flooding, regaining their purification functions, and enabling maximum carbon storage [1].
Wetlands are ecosystems whose formation, processes, and characteristics are controlled and dominated by water. Wetland ecosystems are among the richest in terms of both diversity and quantity, and are of great economic, cultural, scientific, and recreational interest [2]. Wetlands are often referred to as “the kidneys of the planet” due to their ability to purify potential contaminants from the water that feeds them [3]. Among the various types of wetlands, peatlands are particularly well-known and studied for their capacity to store carbon through the accumulation of peat, or conversely, to release carbon when they are degraded [4,5,6,7]. Indeed, although peatlands represent only 3% of the world’s land surface, their carbon reserves account for nearly 25% to 30% of global stocks, depending on the source [2,4,8,9,10,11]. The majority of these peat reserves (35%) are located in the Northern Hemisphere, in boreal and subarctic regions [10]. Like wetlands in general, peatlands have important roles in groundwater and surface water control and recharge, water quality improvement, and ecosystem balances [12,13].
It is therefore essential to have a good understanding of the sedimentary structure of these peatlands in order to assess their hydrogeological functioning and their capacity to capture greenhouse gases (GHGs). However, in Europe, and more particularly in France, there is a lack of knowledge regarding the location of these peat bogs and their main structural and functional characteristics. An initial inventory was carried out in 1949 [14,15], with a view to exploiting peat resources. However, a new inventory did not appear until 2021 [16,17,18].
In Champagne-Ardenne, two major peat bogs are located in distinct geological contexts (Figure 1) and are situated on land that has undergone past reorganizations of the hydrographic network. These captures were already described by Davis [19] and are textbook cases of this phenomenon, although less studied than those of the Moselle [19,20,21,22,23,24,25].
The aim of this contribution is threefold: first, to demonstrate the influence of hydrographic captures on the morphogenesis of peatland sites; second, to characterize their potential hydrodynamic characteristics; and, finally, to date their morphogenesis.

2. Study Area

This study is based on investigations carried out at two of the main hydrographic capture sites in eastern France: the Petit-Morin capture site (Saint-Gond marshes) and the Aire-Bar capture site (Germont marshes). They are part of eastern France and are marked by a cuesta relief with a general dip of around 1–2% towards the west of the Paris Basin.
  • The Saint-Gond marsh
The capture site is located on the Campanian chalk (Upper Cretaceous) [26,27] at the foot of the Ile-de-France cuesta, at the cataclinal breakthrough of the Petit-Morin, which flows in the direction of the dip at the head of its watershed. The Saint-Gond marshes are located 25 km south of Epernay in the Marne département, in the Champagne chalk region, which is a vast 10 km long wetland that covers almost 2500 ha (Figure 2).
The marsh is organized into three successive depressions, separated by chalk ridges, the deepest of which reaches 10 m in places and is half-filled with peat resting on ancient alluvial formations. Damien [28] has provided a better understanding of the hydrodynamics of the marsh, as well as data on the nature and age of the sedimentary fill in this sector. It revealed two levels of peat interspersed with gray clays resting on chalk gravel (graveluches).
The presence of this vast marsh in this precise location is linked to the particular local hydrogeological conditions in the context of a cataclinal breakthrough, enabling the chalk water table to form a piezometric dome in this sector and supply the peat bog with water (minerotrophic peat bog). The bog is also fed by runoff from the cuesta face (topogenic bog, particularly on the tributary arms of the main bog).
However, the local geomorphological history is of great importance. Indeed, the Saint-Gond marsh is situated precisely on a former hydrographic capture site, first observed in the late 19th [1] and early 20th centuries [29]. Using hydrographic, morphometric, and sedimentary arguments, these authors demonstrate the capture of the head of the Petit Morin by the Somme-Soude, a tributary of the Marne basin.
The peat bog rests on a coarse alluvial stratum that has been connected to the alluvial terraces of the Marne, dating from around 100 ka [30]. Capture would then have occurred during the Weichselian (between isotopic stages 2 and 5d). Human forcing on the marsh is evident in the drainage carried out in the 17th and 18th centuries and in the exploitation of peat in pits that were reconceived as ponds. Using the diachronic approach based on archival documents shows that the surface area of the wetland shrank between 1832 and 2012, from 3484 ha to 2079 ha, respectively [28].
  • The Germont-Buzancy marsh
The Germont-Buzancy marshes lie at the foot of the Argonne cuesta (Figure 3), reinforced by gaize (siliceous sandstone) from the Lower Cretaceous (Albo-Cenomanian) resting on the Gault Clay (Albian) [26]. They lie on alluvium overlying Jurassic (Kimmeridgian) limestone in the upper valley of the Bar, a left-bank tributary of the Meuse between Sedan and Charleville-Mézières, bordering on the basin of the Agron, a right-bank tributary of the Aisne. This valley crosses the Argonne cuesta through an anaclinal breakthrough. The Bar flows in the opposite direction to the dip, crossing the Germont marshes.
The structure of the Bar’s alluvial fillings is poorly understood and is the subject of ongoing research by the Argonne workshop zone of the CNRS (National Center for Scientific Research). The first results of this research are included in this contribution. A level of peat, exploited in the 20th century, rests on a clay formation that overlies a layer of alluvial limestone gravel. The Germont-Buzancy marsh is only a small part of this alluvial continuum, located on the interfluve between the basins of the Bar, a tributary of the Meuse, and the Agron, a tributary of the Aisne (Seine basin).
Like the Saint-Gond marshes, this wetland occupies a capture site described more recently [22,31,32,33,34], although it was mentioned in the 19th century [35,36]. It concerns two catchments: the Aisne-Bar and the Aire-Bar catchments, which are dependent on the Meuse and benefit the Oise basin. The former corresponds to the paleo-Aisne, which flowed through the Bar valley to join the Meuse. Based on geomorphological arguments about the alluvium of the Meuse [37,38,39,40], the date of capture would be between 890 ka and 1.05 Ma [39,40,41]. However, another more recent study [42] does not confirm the capture of the Aisne-Bar or the possible paleo-outflow towards the Meuse. The latter concerns the capture of the Aire-Bar by the Aisne, the dating of which remains to be confirmed.
The slightly domed marsh appears to be fed by a combination of precipitation (umbrogenic), lateral inputs (tributaries of the Bar), and the Jurassic limestone water table at the limit of the captive water table (minerotrophic).
However, the study area is restricted to land managed by the CENCA (Conservatoire des Espaces Naturels de Champagne-Ardenne), which considerably limits its spatial scope, with a surface area of 99 ha. It is crossed by the river Bar in its northern part and by the Germont ditch in its southern part. The peat was mined between approximately 1835 and 1840, and more particularly after 1946 and especially after 1970 in peat pits that are now underwater.

3. Materials and Methods

A combination of approaches was used to address the issue, including the use of available databases, land surveys, and laboratory analyses.

3.1. Available Databases

Concise geomorphological mapping was carried out for each site to represent the ancient and modern alluvial formations (pre- and post-capture) of the nearest watercourses, and to determine the relative altitudes of the terraces. It was based on field survey boreholes from BSS (subsoil database) and 1/50,000 geological maps, both made by the BRGM (French Geological and Mining Research Institute) (Table 1). However, this information is likely to be heterogeneous given the disparity between the authors of geological maps and the degree of conservation of alluvial formations in contrasting environments (white chalk for the Saint-Gond marshes, and siliceous sandstone, clay, and limestone for the Germont-Buzancy marshes).
This approach discriminates only three levels of terraces: Holocene re-centric alluvium (Fz), ancient Lower Pleistocene alluvium (Fy), and Upper Pleistocene alluvium (Fx and Fw).

3.2. Land Surveys

In order to supplement the existing databases, a mapping of the alluvium was produced on a regional scale. This mapping covers the studied areas and adjacent valleys, and gives a global view of the alluvial terraces and their connections.
Field surveys enabled us to identify more terrace levels than the bibliographical approach. Each alluvial terrace is designated by the first letters of the river’s name (Ma for Marne, Som for Somme, So for Soude, SS for Somme-Soude, PM for Petit Morin, B for Bar, Ag for Agron), followed by a number correlative to the level of incision of the watercourse. The lowest number (0) corresponds to the youngest level, i.e., that of the minor bed incised in modern formations, while the highest number designates the oldest incision level, corresponding to ancient alluvial formations.
Longitudinal profiles of the alluvial terraces not only provide a better understanding of the methods of capture during the Pleistocene, but also enable us to characterize the wetlands in these singular contexts of capture sites. They can also be used to distinguish between pre- and post-capture alluvial formations.
ESR (Electronic Spin Resonance) dating of fluvial quartz was carried out in the alluvium of the upper terrace of the Aire-Bar [43,44,45] near the town of Verpel (Figure 1 and Table 2). L’ANDRA (French agency for radioactive waste management) had the samples analyzed by the MNHN (Museum National d’Histoire Naturelle) and provided us with the results. Other samples were dated using ESR, but these results are trade secrets.
On a local scale, focusing on the two studied areas, Auger samplings were carried out in modern alluvial formations (peat) to determine the structure of the fill (Figure 4 and Figure 5) and propose isopach maps produced using QGIS software (version 3.38.0). Eight core samples were taken in the Germont marsh, over a 99 ha area, whereas in Saint-Gond, about a hundred samples were taken over a 2500 ha area. Local boreholes are accompanied by a non-destructive geophysical approach to characterize the various levels in terms of their resistance to sinking. The procedure involves using a variable-energy dynamic penetrometer based on a PANDA® (Pénétromètre Automatique Numérique Dynamique Assisté par ordinateur) type device from Sol Solution (Riom, France) [46,47]. This penetrometer was used only in Germont on eight occasions. It was not yet available during the study of the Saint-Gond marshes.
Peat samples were collected for 14C dating and sent to the Poznan Radiocarbon Laboratory. Twelve samples were taken from four boreholes at various depths (from 30 to 325 cm) downstream of the Saint-Gond marshes (Figure 4). In the Germont sector (Figure 5), a single borehole (F1) was the subject of seven datings conducted at depths ranging from 25 to 180 cm.
A palynological study was carried out at two sites (Oyes and Fausse Rivière in Figure 4) in the lower part of the Marais de Saint-Gond to understand the morphogenetic conditions of the peat fills [28,48]. A similar study is currently underway in Germont. The results are not yet available.

3.3. Laboratory Analyses

The core samples taken at the two study sites were analyzed at the GEGENA laboratory (Table 3) in order to retrieve the sedimentary information essential to the characterization of the fill.
The amount of organic matter was measured in peat samples using loss of ignition (drying of the sample in an oven, followed by calcination at 550 °C for four hours).
The nature of the clay fills is characterized by their granulometries analyzed with the Mastersizer 3000 laser granulometer (Malvern compagny, Malvern, UK) and by their geochemical spectra in crushed core samples (granulometry < 100 µm) using the Niton Xl3t 980 X-ray fluorescence analyzer (Thermo Fisher Scientific, Billerica, MA, USA). The granulometry of the clays provides information on their depositional conditions via Passega diagram analysis, enabling us to determine whether they are of lacustrine or fluvial origin, while their geochemical spectrum enables us to determine whether they are of autogenic or allogenic origin.
Cross-referencing the results not only enables us to reconstruct the paleoenvironmental conditions of the fill materials, but also to understand the hydrographic capture modalities that contributed to the genesis of these wetlands/mudflats.

4. Results

4.1. Spatial Pattern of Alluvial Terraces on a Regional Scale

The mapping of alluvial formations shows similarities and specificities for both sites (Figure 6 and Figure 7).
On the one hand, it highlights several levels of alluvial terraces, testifying to the multiphase incision of rivers during the Pleistocene. Staged terraces can be found on the slopes of the Bar and Petit Morin rivers.
In the studied wetlands, a capture bend and an alluvial continuum between various watersheds are evidence of a different regional organization of Pleistocene flows, as they are part of ancient alluvial formations connecting the Somme and Petit Morin watersheds in the Champagne region, and the Bar and Aire watersheds in the Argonne region. We also note the absence of high terraces downstream of capture sites for conquering rivers such as the Somme before its confluence with the Soude and for the Aire downstream of its capture bend.
While geological maps only distinguish between ancient (Fy, Fx) and modern (Fz) alluvium, field surveys reveal more incisional stages underlined by alluvial flats more or less covered by slope colluvium.
In Argonne, around Germont, there are five stepped terraces on the Bar (B.1 to B.5), and other stepped levels on the Aire and Aisne. In Champagne, the Somme-Soude and its tributaries connect their terraces to those of the Marne, which has three tiered terraces (Ma.1 to Ma.3). However, at the headwaters of the Bar and Petit Morin rivers, alluvial formations are superimposed or interlocked at peat bog level. This can be seen in the longitudinal profiles of the alluvial terraces at both sites (Figure 8 and Figure 9), which enable us to distinguish between pre- and post-capture formations. However, at the headwaters of watercourses affected by catchment, this relative altitude decreases, as does the slope of the watercourse’s longitudinal profile. Conversely, the slopes of other streams (Somme, Soude, Agron), particularly those of the conquerors, accelerate, underlining an increased incision that contrasts sharply with those of the peat bog sites. But the relevant information from this approach lies in the alignment of the profiles of the ancient Somme catchment terraces (Som.1) with the ancient alluvial formations of the Petit Morin beneath the modern alluvium (Figure 8). The same arrangement is found with the Aire-Bar (Fw) terraces, which are connected to the Bar valley floor water table (Figure 9).
There are other similarities between the two sites, particularly along the upper part of the pre-capture paleo-section of the Somme and Aire rivers. Finally, the alluvial terraces are not only discontinuous, as they are cut by numerous periglacial valleys, but the highest ones are located at the top of the interfluves, in a position of relief inversion, testifying to a strong ablation/erosion of the surrounding relief. This is undoubtedly linked to the porosity of the bedrock, which was sensitive to cryoclastization under the Pleistocene periglacial climate.

4.2. Nature and Thickness of Alluvial Formations

4.2.1. In the Germont Marsh Area

The nature of the alluvial formations on the terraces is identified by auger drilling coupled with penetrometry (PANDA) for the valley bottoms, and by outcrops for the Pleistocene alluvium (Figure 10).
In the Bar valley, these are represented by coarse-grained, poorly classified limestone pebbles and cobbles with sandy matrix (according to Wentworth classification). These pebbles are the result of the cryoclastic dismantling of Jurassic limestone formations, which are well represented in the upper Aisne basin (Thinonian, Kimmeridgian). Preserved in the form of relictual shreds, their calcareous nature contrasts with the sandy-clay formations of the Lower Cretaceous.
These coarse formations are also found at the bottom of the valley in front of the studied peat marshes, but are covered by more recent and, above all, finer alluvial deposits.
In the valley floor of the Bar, the alluvial fill is characterized from bottom to top by three sedimentary units (Table 4).
US.1 is represented by a layer of coarse-grained, heterometric limestone gravels. It corresponds to rolled cryoclasts resulting from the dismantling of Jurassic geological formations. Under the marsh, this layer has not been observed. However, its presence has been confirmed by two boreholes (BSS000HMHQ and BSS000HMKW, Figure 9) at Harricourt. The material is identical to that found on the Verpel Fw terraces. Furthermore, they are located in the extension of this terrace on the longitudinal profile (Figure 9).
This level is overlain by compact gray sandy-clay silts (US.2), the strength of which is unknown in the Germont marshes, but which are intersected in two nearby upstream boreholes at Harricourt (BSSHMHQ) and Buzancy (BSSHMKW) (Figure 9) with a thickness of 7.5 m. The granulometric characteristics (median of 50 microns) of the silts indicate conditions of deposition by uniform, low-energy suspension, which may be associated with lacustrine environments. The crossing of this level is more resistant to penetration, with values exceeding 20 MPa, which helped limit the depths reached by drilling. Geochemical analysis of the sandy-clay silts (US.2) using X-ray fluorescence shows a homogeneous spectrum from top to bottom, with a predominance of Calcium (Ca > 100,000 ppm) assimilated to the dismantling of Jurassic limestone formations, the presence of Potassium (K > 10,000 ppm), Iron (Fe > 10,000 ppm), and finally Titanium (Ti > 1000 ppm), most likely derived from the erosion of the Albo-Cenomanian gaize with a similar geochemical spectrum and a predominance of these 3 elements.
A peaty level (US.3) less than 2.7 m thick outcrops on the surface. At this level, the resistance to penetration on the penetrogram is particularly low, with values systematically below 5 MPa. The thickness of this level is not homogeneous across the marsh, ranging from less than 0.3 m to 2.7 m (Figure 11), with maxima in the center, between the Bar and Fossée de Germont, and downstream of the marsh. However, the isopaque map of this sedimentary unit is skewed by the heterogeneous density of the boreholes.

4.2.2. In the Saint-Gond Marsh Area

In the Saint-Gond marshes, five sedimentary units (Table 5) are visible in the cores. Sedimentary unit US.1 takes the form of a chalky gravel with the largest particle size fraction (median diameter 0.352 mm, mean diameter 0.197 mm). It is a poorly sorted, frankly mineral (<1.5% OM) level deposited by graduated suspension in an energetic mode [30,50,51,52], which not only has all the characteristics of the ancient alluvium of the paleo-Somme, but is also aligned with these formations on the longitudinal profile (Figure 8). As such, it can be likened to the ante-capture formations. In Champagne, the Pleistocene alluvium corresponds to white chalk gravel or “graveluches”, more or less rolled, as the studied rivers are autogenic to the Upper Cretaceous chalk [53].
The overlying sedimentary unit US.2 (Table 5) corresponds to a frankly fine sandy-clay mineral level with a median diameter of 0.005 mm (5 μm), a mean diameter of 0.015 mm (15 μm), and a unimodal particle size distribution. It is assimilated to a lacustrine silt in the image of US.2 of the Germont marshes and is found higher up in the sedimentary unit (US.4) whose particle size fraction is slightly coarser with a median diameter of 0.010 mm (10 μm) and a mean of 0.016 mm (16 μm). This sedimentary unit was clearly formed in an anaerobic hydromorphic environment, as evidenced by the presence of very little poorly decomposed, rather reductive organic matter, as well as the grayish, even blue-gray color.
A peaty level (US.3) lies between US.2 and US.4 (Table 5), and above US.4 to outcrop on the subsurface. These two levels of peat (US.5), which are frankly organic, contrast sharply with the mineral facies of the other levels. They characterize anaerobic conditions that are conducive to poor plant decomposition and the development of a plant cover. Unlike the Germont marshes, the Saint Gond marshes feature two levels of peat.
This alluvial thickness is not uniform throughout the Saint-Gond marsh, varying from 0 to 10 m. In places, it forms a barrier between basins and drier areas where the chalk is sub-flush (Figure 12).

4.3. Alluvial Formations Dating

Samples were taken from both sites to date the peat sediments. Twelve 14C dates were obtained from the Saint-Gond marsh and seven from the Germont-Buzancy marsh. These dates are presented in the tables below (Table 6 and Table 7).
The 14C dates concern the peat levels of the Saint Gond (US 3 and 5) and Germont (US 3) marshes, from base to summit. They show many similarities with continuous peatification during the Holocene, beginning around 12 ky (Pre-Boreal) and ending during the medieval historical period around the year 1000. For the Saint Gond marshes, peatification appears to be sequenced with two levels of peat. The first (US 3) is dated on four samples from three boreholes, between 12,440 BP and 11,030 BP, corresponding to the Bölling and Alleröd chronozones. The second subsurface level (US 5) is the most represented, with seven dates on three samples, dated between 9910 BP and the present day. Based on these results, the age of US 4, a gray-blue mineral intercalated between the two peaty levels, would be between 11,030 BP and 9910 BP, which would place it during the Recent Dryas, the last chronozone of the Tardiglacial. On the other hand, the same mineral level in the Germont marshes would be slightly older (>12 ky), and therefore probably pre-Boreal or even correlative to the cataglacial stage of the LGP. This implies that these clay-silty levels would be synchronous with climatic weightings, with a colder climate under uniform lake-type sedimentation conditions.
In the Germont area, the only dating of the ancient terraces concerns the alluvial formations (Fw) of the paleo Aire-Bar immediately upstream of the capture bend of the Aire at Verpel, which connects in the longitudinal profile to the sedimentary unit US1 observed in the Germont marshes. As such, it predates the capture. ESR dating determines an age of 0.45 Ma corresponding to the isotopic stage 11 (Table 8), which would place the other stepped terraces of the Bar downstream from the site at much earlier ages (Middle Pleistocene).
In the Saint-Gond marshes area, unit US1 is attached to the Marne terraces, numbered Ma.2 on the longitudinal profile (Figure 8). This level is dated, using the ESR method (Table 9), at around 100 ka (isotopic stage 5a–d).

5. Discussion

The initial results of this geomorphological approach to the two study sites show a number of similarities, enabling us to propose a novel morphogenetic hypothesis for peatlands on former hydrographic catchment sites. Figure 13 shows a working conceptual model for the Saint-Gond and Germont marshes.
Both sites feature valley-bottom peat bogs at the head of catchment areas immediately downstream of capture bends, set against a backdrop of tiered low plateaus dissected by a cataclinal or anaclinal hydrographic network with openings through cuestas.
The incision of this hydrographic network is not uniform. Morphological and sedimentary indicators of stream captures have been observed during the Pleistocene. The traces are represented by characteristic changes of direction (right-angled watercourses) in enclosed meanders and an original spatial organization of ancient and modern alluvial formations. In the upper part of the valleys, the ancient alluvial deposits prior to paleodrain capture are relictual, layered, and perched. They are located on the tops of reliefs, generating a shape inversion. At the capture sites, these alluvial formations sink beneath the more recent alluvium in a context of superimposed nappes. Downstream of these sites, a spatial organization of stepped terraces marks out the slopes.
The ancient alluvium shows the same coarse, poorly striated grain spectrum, with chalk or limestone gravels resulting from the transport of cryoclasts synchronous with periglacial climatic conditions. These formations testify to energetic hydrodynamic conditions linked to the Pleistocene lowland nival regime of rivers during glacial periods, specifically during cata and anaglacial phases with multi-channel fluvial styles (Figure 13a).
The layering of surfaces in this cuesta context also favors that of watercourses, whose tributaries retreat by way of regressive erosion (Figure 13a) before capturing a larger drain. This episode is fundamental in explaining the genesis of wetlands, as it modifies the hydrodynamics of watercourses, either by considerably increasing the catchment area for conquering drains, or, on the contrary, by amputating a large part of their catchment area for captured drains. During the Pleistocene, after these episodes, the incision of the hydrographic network is maintained and mainly affects the lower reaches of rivers according to eustatic, isostatic, and climatic variations [55,56,57], generating concave long profiles according to an equilibrium profile. However, regressive erosion does not systematically move upstream in the middle or upper reaches of rivers [58,59,60,61], which retain their original gradient at capture sites, explaining the slope breaks and, above all, the low-slope reaches on the longitudinal profiles of the Petit Morin and Bar rivers. These conditions are conducive to strong aggradation, as can be seen in the isopaque maps of the alluvial formations (Figure 11 and Figure 12).
After the last glacial period, global warming has diffused hydrodynamic energy, favoring infiltration and underground flow in these chalky or limestone plateaus. This led to a significant drop in river capacity (single-channel river, Figure 13b) and fine, uniform sedimentation, probably lacustrine, on these gently sloping stretches, generating the gray sandy-clay formations. Finally, during the Holocene, climatic optima are conducive to lake peatification with organic deposits, which appears to have been continuous for 12 ky in the Germont marshes and sequenced in the Saint-Gond marshes, with a thermal weighting (Recent Dryas) that is less favorable to peat formation and a return to low-energy mineral sedimentation (clay, silt and sand) conditions resulting from the dismantling of surrounding reliefs. These optima are likely to be associated with forest colonization, as revealed by the palynological approach [28,62].
This conceptual model of the peaty hydrosystem genesis specific to hydrographic catchment sites still raises a number of questions. The presence of a water body, as demonstrated by fine, uniform sedimentation, remains to be explained.
The argument of the low slopes of the longitudinal profiles, linked to regressive erosion of the lower reaches, which has not yet affected the upstream, is not sufficiently satisfactory. Subterranean flow patterns also play a key role in the construction of these peat bogs, which developed during the Holocene. They are located at the edge of a captive water table, which determines the emergence and overflow sources that sustain their recharge during the Pleistocene. This is the case of the Saint-Gond marshes in the cataclinal breakthrough funnel: the chalk water table is free upstream, whereas it is captive under the tertiary formation downstream. The Germont marshes are also located in the Bar valley, which, like a hydrogeological window [61], allows the Jurassic limestone water table to outcrop (free water table), while its two banks border on the captive water table beneath the Lower Cretaceous formations.
In fact, exchanges between the water table and the river take place favorably in the hyporheic zone structured by alluvial formations, which maintains the wetland (fluviogenic peat bog). However, the latter condition can limit these exchanges with fine impermeable clay sediments, such as those observed at the two sites. In this case, the watercourses are no longer hydraulically connected to the bedrock aquifers (independent watercourses), resulting in topogenic or limnogenic peatlands [59,63].
Other peatland morphogenetic scenarios can also be proposed, such as those associated with dammed water bodies linked to lateral alluvial fans generated by tributaries, or with land movements affecting the slopes in the Argiles du Gault in the Argonne, or Tertiary formations in the Saint-Gond marshes.
However, this hydrodynamic functioning of the marshes remains poorly understood, as it is currently being studied as part of the CNRS Argonne Workshop Zone (ZARG) for the Germont marshes. Studies carried out on the earlier Saint-Gond marshes [28,60] show that the original evolution of these capture sites during the Pleistocene determines today’s spatial organization of the groundwater that feeds them. Victims of the peripheral incision of the conquering hydrographic network, these sites are perched in relation to the conquering rivers and are generally located on piezometric domes with diverging streamlines, as indicated in the presentation of the study sites. This results in the transfer of groundwater masses to neighboring catchments [64,65] at the expense of upper catchments, which are affected by capture.

6. Conclusions

This preliminary work is based on geomorphological approaches at different spatial (regional and local) and temporal (Pleistocene and Holocene) scales. Two specific peatland sites (Saint-Gond and Germont marshes) in the context of a stream capture site in the middle of a limestone plateau show many geographical similarities.
The mapping of alluvial terraces, their longitudinal profiles, dating, and sedimentary characterization have enabled us to understand their spatial organization on a regional scale. On a local scale, drilling and sediment sampling campaigns, particularly in peat, have enabled us to understand the geometry of the fillings. Cross-referencing the results obtained at these different scales enables us to reconstruct the pre- and post-capture palaeogeographical context.
This work demonstrates the structuring control of hydrographic captures over the genesis of wetlands, and more specifically peatlands, making it possible to pinpoint the date of capture of the Aire-Bar (between 450 ky and 12 ky). Conversely, the date of capture of the Petit Morin occurred during the last Weichselian glacial period (isotopic stages 2 to 5a–d). They also show that Holocene peatification is generally synchronous with climatic optima associated with forest colonization [66]. These sites, located at the head of the upper basins of rivers affected by capture on gentle longitudinal slopes, are located on the piezometric domes of the water tables that feed them, which determines water mass transfers to the neighboring catchment areas of the conquering rivers. They are characterized by low hydrodynamic energy and aggradation, which contribute to their compartmentalization. However, their main challenge lies in their ecosystem services, particularly in carbon sequestration driven by human (marsh management) and natural (Global Change) forcings. To address these issues, a high-definition lidar flight is planned for the Germont sector to map natural and man-made morphologies, which are likely to reveal these forcings. Studies are underway as part of the CNRS Argonne Workshop Zone (ZARG), with the contribution of participatory science (public participation in water level gauge readings of water bodies) and the marsh management plan, which will enable us to measure the impact of beavers and their dams on the re-watering of the peat bog.
Studying these peatlands is therefore essential, as it will also enable us to develop new strategies for restoring these wetlands, which have been altered for several centuries by anthropization and agricultural drainage [67,68,69,70,71,72]. In addition to the risks associated with flooding, due to the reduced capacity of peatlands to retain water, these areas, because of their seasonal drying, are re-emitting more and more GHGs [73,74]. New remediation techniques will be based on airborne imaging technologies [75,76,77].

Author Contributions

Conceptualization, O.L. and A.D.; methodology, O.L., J.B. (Julien Berthe) and T.D.; software, J.B. (Julien Berthe) and J.B. (Jeremy Beucher); validation, N.B., D.C. and T.K.; formal analysis, O.L. and A.D.; investigation, O.L., J.B. (Jeremy Beucher), A.D. and T.D.; resources, O.L. and N.B.; data curation, J.B. (Julien Berthe); writing—original draft preparation, O.L.; writing—review and editing, D.C.; visualization, J.B. (Jeremy Beucher); supervision, A.D.; project administration, O.L.; funding acquisition, O.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

As the research is still ongoing, the full dataset is not currently available. Interested parties may contact the corresponding author directly via email at olivier.lejeune@univ-reims.fr.

Acknowledgments

We would like to thank the local environmental players at the 2 sites, and in particular the staff at CENCA (Concervatoire d’Espaces Naturels de Champagne-Ardenne) and the “Communauté de Commune de l’Argonne Ardennaise” (2C2A) for their administrative support and help in the field. We also thank ANDRA (Agence Nationale pour la Gestion des Déchets Radioactifs) for kindly granting permission to use the ESR dating results. Finally, we would like to thank the CNRS Argonne workshop zone and the Champagne-Ardenne region for their support.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Location map of river catchments in eastern France and study sites A. Petit Morin stream capture, B. Ornain-Saulx stream capture, C. Aire-Bar stream capture, D. Aisne-Bar stream capture, E. Moselle stream capture, F. Meuse stream capture.
Figure 1. Location map of river catchments in eastern France and study sites A. Petit Morin stream capture, B. Ornain-Saulx stream capture, C. Aire-Bar stream capture, D. Aisne-Bar stream capture, E. Moselle stream capture, F. Meuse stream capture.
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Figure 2. The Saint-Gond marshes photographed by drone in March 2025 (photo: O. Lejeune).
Figure 2. The Saint-Gond marshes photographed by drone in March 2025 (photo: O. Lejeune).
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Figure 3. The Germont marshes photographed by drone in December 2024 (photo: O. Lejeune).
Figure 3. The Germont marshes photographed by drone in December 2024 (photo: O. Lejeune).
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Figure 4. Maps of instrumental devices in the Saint-Gond marshes.
Figure 4. Maps of instrumental devices in the Saint-Gond marshes.
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Figure 5. Maps of instrumental devices in the Germont-Buzancy marshes.
Figure 5. Maps of instrumental devices in the Germont-Buzancy marshes.
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Figure 6. Map of alluvial formations at the Saint-Gond marshes site.
Figure 6. Map of alluvial formations at the Saint-Gond marshes site.
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Figure 7. Map of alluvial formations at the Germont-Buzancy marshes site.
Figure 7. Map of alluvial formations at the Germont-Buzancy marshes site.
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Figure 8. Longitudinal profile of the alluvial formations in the Saint-Gond marsh area [28,30].
Figure 8. Longitudinal profile of the alluvial formations in the Saint-Gond marsh area [28,30].
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Figure 9. Longitudinal profile of the alluvial formations in the Germont marsh area [49].
Figure 9. Longitudinal profile of the alluvial formations in the Germont marsh area [49].
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Figure 10. Photographs of stratigraphic units encountered in boreholes and outcrops in the Germont marsh area: (a) Pleistocene alluvium of the Fw level assimilated to US.1 at Verpel (Photo: Lejeune O.), (b) black peaty level (US.1) on compact gray sandy-clayey silt (US.2) on drill F3.
Figure 10. Photographs of stratigraphic units encountered in boreholes and outcrops in the Germont marsh area: (a) Pleistocene alluvium of the Fw level assimilated to US.1 at Verpel (Photo: Lejeune O.), (b) black peaty level (US.1) on compact gray sandy-clayey silt (US.2) on drill F3.
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Figure 11. Isopaque peat map of the Germont marshes.
Figure 11. Isopaque peat map of the Germont marshes.
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Figure 12. Alluvial filling isopaque map of the Saint-Gond marshes.
Figure 12. Alluvial filling isopaque map of the Saint-Gond marshes.
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Figure 13. Conceptual modeling of peatland morphogenesis at hydrographic capture sites. (a) Pre-capture situation; (b) post-capture situation.
Figure 13. Conceptual modeling of peatland morphogenesis at hydrographic capture sites. (a) Pre-capture situation; (b) post-capture situation.
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Table 1. Map database table.
Table 1. Map database table.
Mapping TypeScaleAuthorNoteReferences
Geological map1/50,000BRGMBd charm (National geological database)https://infoterre.brgm.fr/formulaire/telechargement-cartes-geologiques-departementales-150-000-bd-charm-50 (accessed on 6 June 2025)
Drilling/BRGMBSS (subsoil database)https://infoterre.brgm.fr/page/banque-sol-bss (accessed on 6 June 2025)
Geomorphological map1/10,000Gegena labGeomorphological map of the Marne valley[30]
Geomorphological map1/10,000Gegena labGeomorphological map of the Saint-Gond marsh area[28]
Geomorphological map1/10,000Andra and
Personal data		Geomorphological map of the Germont marsh area and nearby valleysStudy in progress
Table 2. List of field manipulations and analyses.
Table 2. List of field manipulations and analyses.
Analysis TypeCategoryOperatorNoteCompletion Date
ESR (Electronic spin resonance)DatingMNHNDating alluvial terraces2005–2009
14C (Carbon 14)DatingPoznan Radiocarbon LaboratoryPeat dating2017 (Saint-Gond marshes)
2022 (Germont marshes)
Core drillingReconquering the subsoilGegena labRussian core drilling2014–2018 (Saint-Gond marshes)
2022–2025 (Germont marshes)
PANDA®Geotechnical investigationGegena labRapid reconnaissance of the subsoil2022–2025 (Germont marshes)
Palynological analysisPaleoenvironmental analysisGegena lab
Chrono-environment lab		 2016 (Saint-Gond marshes)
Analysis in progress (Germont marshes)
Table 3. List of laboratory analyses.
Table 3. List of laboratory analyses.
Analysis TypeCategoryOperatorNoteEquipment
GranulometrySediment analysisGegena labFor analyses under 1.6 mm
For analyses above 50 µm		laser granulometer (Mastersizer 3000)
Mechanical particle sizing
X-ray fluorescenceGeochemical analysisGegena lab32 chemical parametersNiton Xl3t 980
loss on ignition (LOI)Organic matter analysisGegena lab550 °C for four hoursMuffle furnace
Table 4. Summary of sedimentary units intersected in the Germont marshes.
Table 4. Summary of sedimentary units intersected in the Germont marshes.
Sedimentary Unit (Germont)Type of
Sediment		Organic Matter (%Min, %Max)Thickness (m)DatingGranulometry
US 3Peat40–90% 1.3 m (min); 2.7 m (max) (14C method)
1070 ± 30 BP (subsurface)
12,000 ± 60 BP (base)		/
US 2Sandy clay loam0–2%2 m (min); 7 m (max)/Clay and silt: 49.6% ± 13.9
Sand: 50.4% ± 14.1
Pebbles and cobbles: 0%
US 1Pebbles and cobbles0%10 m (max)(ESR method)
450 ky ± 100		Clay and silt: 5.36% ± 4.52
Sand: 50.21% ± 29.65
Pebbles and cobbles: 44.17% ± 31.66
Table 5. Synthesis of sedimentary units in the Saint-Gond marshes.
Table 5. Synthesis of sedimentary units in the Saint-Gond marshes.
Sedimentary Unit (Germont)Type of
Sediment		Organic Matter (%Min, %Max)Thickness (m)DatingGranulometry
US.5Peat65–80%1.9 m (max)(14C method)
895 ± 30 BP (subsurface)
9910 ± 50 BP (base)		/
US.4Lacustrine silty clay1% (max)1 m (max)/Clay and silt: 71.2% ± 13.2%
Sand: 28.8% ± 11.4%
Pebbles and cobbles: 0%
US.3Peat80–95%0.9 m (max)(14C method)
11,030 ± 50 BP (subsurface)
12,440 ± 50 BP (base)		/
US.2Lacustrine silty clay0.5%1.6 (max)/Clay and silt: 77.3% ± 9.9
Sand: 22.7% ± 18.2
Pebbles and cobbles: 0%
US.1Pebbles «graveluches»1.5%5 m (max)(ESR method)
93 ky ± 9
112 ky ± 13		Clay and silt: 29.9% ± 9.44
Sand: 29.1% ± 14.5
Pebbles and cobbles: 41% ± 17.12
Table 6. 14C dating data for the Saint-Gond marshes [28].
Table 6. 14C dating data for the Saint-Gond marshes [28].
Drilling NameDepth (cm)Sample NumberAge 14C BP
Voisy 2198Poz-748733355 ± 30
Vielle Tourbière204Poz-7487411,910 ± 60
Oyes 230Poz-86929895 ± 30
Oyes 270Poz-870121725 ± 30
Oyes 2110Poz-870142265 ± 30
Oyes 2160Poz-870137740 ± 35
Oyes 2190Poz-746149910 ± 50
Oyes 2228Poz-8701511,030 ± 50
Oyes 2266Poz-7461311,700 ± 60
Oyes 2301Poz-7461212430 ± 60
Fausse Rivière 1148Poz-748758490 ± 50
Fausse Rivière 1325Poz-7461512,440 ± 60
Table 7. 14C dating data for the Germont marshes [54].
Table 7. 14C dating data for the Germont marshes [54].
Drilling NameDepth (cm)Sample NumberAge 14C BP
F1C1E125Poz-156461 1070 ± 30 BP
F1C1E375Poz-1562471685 ± 30 BP
F1C1E498Poz-156996 3630 ± 30 BP
F1C1E5122Poz-156998 6560 ± 40 BP
F1C1E6148Poz-1569998530 ± 50 BP
F1C1E7167Poz-157094 10,800 ± 60 BP
F1C1E8180Poz-15709012,000 ± 60 BP
Table 8. ESR dating of the terrace associated with the Aire-Bar capture [44].
Table 8. ESR dating of the terrace associated with the Aire-Bar capture [44].
Sedimentary Section NameDepth (cm)Sample NumberESR Age (Ma)
Verpel18009VER01 0.45 ± 0.1
Table 9. ESR dating of the terrace associated with the Saint-Gond area [30].
Table 9. ESR dating of the terrace associated with the Saint-Gond area [30].
Sedimentary Section NameDepth (cm)Sample NumberESR Age (Ma)
Donjeux2102003DON070.112 ± 0.013
Loisy14001LOI01 0.093 ± 0.009
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Lejeune, O.; Beucher, J.; Devos, A.; Berthe, J.; Damien, T.; Combaz, D.; Bollot, N.; Krauffel, T. Peatland-Type Sediment Filling in Valley Bottoms at the Head of Basins in a Stream Capture Context: The Example of the Bar and Petit Morin Peatland (Grand-Est, France). Geographies 2025, 5, 34. https://doi.org/10.3390/geographies5030034

AMA Style

Lejeune O, Beucher J, Devos A, Berthe J, Damien T, Combaz D, Bollot N, Krauffel T. Peatland-Type Sediment Filling in Valley Bottoms at the Head of Basins in a Stream Capture Context: The Example of the Bar and Petit Morin Peatland (Grand-Est, France). Geographies. 2025; 5(3):34. https://doi.org/10.3390/geographies5030034

Chicago/Turabian Style

Lejeune, Olivier, Jérémy Beucher, Alain Devos, Julien Berthe, Thibaud Damien, Delphine Combaz, Nicolas Bollot, and Théo Krauffel. 2025. "Peatland-Type Sediment Filling in Valley Bottoms at the Head of Basins in a Stream Capture Context: The Example of the Bar and Petit Morin Peatland (Grand-Est, France)" Geographies 5, no. 3: 34. https://doi.org/10.3390/geographies5030034

APA Style

Lejeune, O., Beucher, J., Devos, A., Berthe, J., Damien, T., Combaz, D., Bollot, N., & Krauffel, T. (2025). Peatland-Type Sediment Filling in Valley Bottoms at the Head of Basins in a Stream Capture Context: The Example of the Bar and Petit Morin Peatland (Grand-Est, France). Geographies, 5(3), 34. https://doi.org/10.3390/geographies5030034

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