High-Resolution Facies Characterization and
Metre-Scale Depositional Organization of
Palustrine-Lacustrine Carbonates

Thibaut Jamey, Gautier Laurent, Céline Mallet, Simon Andrieu, Kévin Moreau, E Husson, Mohamed Azaroual

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Thibaut Jamey, Gautier Laurent, Céline Mallet, Simon Andrieu, Kévin Moreau, et al. High-Resolution Facies Characterization and Metre-Scale Depositional Organization of Palustrine-Lacustrine Carbonates. 2026. hal-05502940

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# High-Resolution Facies Characterization and Metre-Scale Depositional Organization of Palustrine-Lacustrine Carbonates

T. Jamey<fnref n="1" />,<fnref n="2" />, G. Laurent<fnref n="1" />, C. Mallet<fnref n="1" />, S. Andrieu<fnref n="3" />, K. Moreau<fnref n="4" />, E. Husson<fnref n="2" />, and M. Azaroual<fnref n="1" />,<fnref n="2" />

## Highlights

* Detailed description of the Beauce Limestones leading to the identification of 11 palustrine-lacustrine facies distributed into 3 depositional environments.

* Reconstruction of the 3D architecture of the deposit with sequence stratigraphy along 2 cross sections.

* Role of autogenic processes in controlling short-term depositional cyclicity.

* Impact of the pedogenic overprint on the sedimentary record during regressive systems tracts.

## Graphical Abstract

<figure><img src="image_2.png" /></figure>

## Contents

1 Introduction 2

2 Geological Setting 3

3 Material and Methods 4

4 Sedimentary Facies Identification 4

4.1 Facies Association 1 (FA1): Lacustrine setting 4

4.2 Facies Association 2 (FA2): Lake-Margins setting 7

4.3 Facies Association 3 (FA3): Palustrine setting 7

5 Stratigrahic Sequences 12

5.1 Cross-sections 12

5.2 Description of the cycles 12

5.2.1 Medium and short-term cycles 12

5.2.2 Linking depositional landscapes to short-order cycles 12

6 Discussion 16

6.1 Scale of facies variations along a depositional profile 16

6.2 Control of the local and quick depositional shifts 16

6.3 Perspectives to improve meter-scale correlations 17

7 Conclusion 18

<footnote n="1">¹ISTO, UMR 7327, Univ. Orléans, CNRS, BRGM, OSUC, F-45071 Orléans, France</footnote>

<footnote n="2">²BRGM, F-45060 Orléans, France</footnote>

<footnote n="3">³Université de Lyon 1, ENSL, CNRS, LGL-TPE, 69622 Villeurbanne, France</footnote>

<footnote n="4">⁴Géosciences Rennes, CNRS, UMR6118, Univ Rennes, France</footnote>
# High-Resolution Facies Characterization and Metre-Scale Depositional Organization of Palustrine-Lacustrine Carbonates

T. Jamey<fnref n="1" />,<fnref n="2" />, G. Laurent<fnref n="1" />, C. Mallet<fnref n="1" />, S. Andrieu<fnref n="3" />, K. Moreau<fnref n="4" />, E. Husson<fnref n="2" />, and M. Azaroual<fnref n="1" />,<fnref n="2" />

**Abstract** Accurate 3D modelling of continental limestone reservoirs is still hindered by the high spatial variability of their flow properties. This variability results from the combination of heterogeneous and laterally discontinuous sedimentary facies with vertically stacked pedogenic overprints, which remain poorly constrained at the multi-meter scale. To address this issue, this study provides a sedimentological and stratigraphic characterization of the Aquitanian Pithiviers Limestones hosting the vadose zone of the Beauce aquifer (central France). This analysis takes advantage of a high-resolution dataset (eight 25-m-deep cored boreholes, spaced 5 to 8 m apart). Eleven facies were identified grouped into lacustrine, lake-margins and palustrine depositional environments. Two cross-sections (22 m long) reveal a heterogeneous architecture structured by three hierarchical orders of cyclicity: (1) medium-term cycles (several metres thick) record alternations between widespread lacustrine and palustrine conditions, (2) short-term cycles that document vertical and lateral facies shifts at the metre scale and (3) very-short-term cycles illustrating sub-metre to metre-scale facies variability driven by autogenic processes. Palaeogeographical reconstructions highlight three depositional organizations: lacustrine-dominated, palustrine-dominated, and mixed configurations. Mixed configurations exhibit pronounced spatial heterogeneity, with lacustrine, lake-margins, and palustrine facies coexisting laterally at the scale of the study site, over distances of a few metres. Lake-margins deposits are restricted in extent and frequently absent from vertical successions, indicating rapid pedogenic overprinting. These results demonstrate that autogenic processes dominate metre-scale facies architecture in lacustrine–palustrine carbonate systems and must be explicitly accounted for when modelling fluid flow and connectivity in continental carbonate aquifers.

**Keywords**

Continental Limestones

Lacustrine-palustrine carbonates

Lateral facies variations

Sequence Stratigraphy

Heterogeneities

## 1 Introduction

Carbonates are key reservoirs of freshwater resources for drinking water and agriculture, making their understanding crucial for the UN Sustainable Development Goal SDGs6 (Clean Water and Sanitation). However, their heterogeneity — particularly in continental deposits — hinders the construction of predictive models for fluid transfer. In such deposits, the superposition of facies and the lack of fine-scale sedimentological data prevent reliable lateral correlations [Moore, 2001; Lucia et al., 2003; Benavente et al., 2021].

Continental carbonate sediments encompass a wide range of lacustrine and palustrine facies, whose transformation is driven by processes such as pedogenesis, biological activity, desiccation or episodic flooding [de Wet et al., 1998; Dunagan and Driese, 1999; Méndez-Bedia et al., 2020]. These processes generate high spatial heterogeneity resulting from a diversity of structures such as root traces, desiccation cracks, microbial laminites or calcrete nodules, creating a continuum between lacustrine and palustrine environments. Syn- and post-depositional diagenesis further increases uncertainty regarding both depositional organization and lateral facies variability [Bowen et al., 2008; Armenteros, 2010; De Boever et al., 2017; Benavente et al., 2021].

As a result, distinguishing these environments and defining their depositional architecture remain challenging [Freytet et al., 1982; Alonso-Zarza, 2003; Brasier, 2011]. The depositional sequences of such environments are also challenging to correlate. They often telescope due to superimposed climatic or tectonic influences that tend to impact the sediment several times, producing discontinuous and laterally variable units [Changsong et al., 2001; Keighley et al., 2003; Roemers-Oliveira et al., 2024]. Consequently, most existing depositional models fail to understand the nature of lacustrine and palustrine carbonates lateral facies transitions and lack well-defined stratigraphic scales, although essential to build accurate sedimentary models for subsurface flow [Djamali et al., 2006; Coutinho et al., 2015; Vennin et al., 2021; Isch et al., 2022]. The characterization of continental carbonate heterogeneity requires observations at the metric and sub-metric scale, where individual facies and their lateral transitions can be documented in detail [Ahmed, 2009]. This is particularly relevant for aquifer formations whose hydraulic properties depend on fine-scale sedimentological architecture. This study focusses on a very

<footnote n="1">¹ISTO, UMR 7327, Univ. Orléans, CNRS, BRGM, OSUC, F-45071 Orléans, France</footnote>

<footnote n="2">²BRGM, F-45060 Orléans, France</footnote>

<footnote n="3">³Université de Lyon 1, ENSL, CNRS, LGL-TPE, 69622 Villeurbanne, France</footnote>

<footnote n="4">⁴Géosciences Rennes, CNRS, UMR6118, Univ Rennes, France</footnote>
<figure><img src="image_3.png" /><figcaption>**Figure 1** Left: Palaeogeographic map of western Europe during the Aquitanian [Prista et al., 2015]. Right: Current map of western Europe with main tectonic units [Torsvik and Cocks, 2017; Meschede and Warr, 2019]. The red pin represents the location of the study site and the black line the cross section of Fig. 2.</figcaption></figure>

local site within the Beauce aquifer in central France — one of the country's largest freshwater reservoirs extending over nearly 10,000 km². This aquifer is increasingly subject to quantity and quality issues, largely driven by climate change and intensive agricultural practices [Le Coz, 2000; Verley, 2020]. Its unsaturated zone, link between the surface and the aquifer, consists of the early Miocene-aged Pithiviers limestones (Section 2), a 20-35 m-thick lacustrine-palustrine unit [Lorain, 1972; Guillemin, 1976]. The Observatory of the Vadose Zone (in French, Observatoire des transferts dans la Zone Non Saturée: O ZNS) provides an exceptionally high-density coring data set (Section 3) enabling detailed documentation of small-scale heterogeneities [Isch et al., 2018; Azaroual, 2022; Abbar et al., 2023; Laurent et al., 2023].

This study aims (1) to provide a high-resolution sedimentological characterization of the facies and depositional sequences of the Pithiviers limestones using the O-ZNS cored boreholes (Sections 4 and 5), and (2) to propose a scaled lacustrine-palustrine depositional profile with tri-dimensional reconstruction of the associated facies through stratigraphic correlations (Section 5).

## 2 Geological Setting

The Paris Basin is a large intracratonic basin characterized by low subsidence rates (1 to 10 m/Ma), low-amplitude structural features [Berger, 1994] and a broad sedimentary infill composed of alternating marine and continental deposits [Coltorti et al., 2010]. Its evolution, recorded during its long-lasting history from the Triassic to the present [Brunet and Le Pichon, 1982], reflects a complex interplay of thermal subsidence inherited from aborted rifting phases and shorter-term tectonic pulses associated with intraplate deformation events [Guillocheau et al., 2000]. Episodic phases of uplift and unconformities punctuate the sedimentary record, resulting in condensed intervals and sedimentary gaps [Pomerol, 1989; Briais, 2015].

The Pithiviers Limestones – at the core of this study – represent one of the members of the Beauce Limestone formation deposited within a peculiar sequence of the Paris Basin's history, at the transition from Oligocene to Miocene. During the Rupelian (33.9–27.3 Ma, [Cohen et al., 2013]), the migration of the depocenter southward [Ménillet and Edwards, 2000], coupled with periodic interplate tectonic pulses causing localized uplift and basin flexure [Lenoir et al., 2014], led to the progressive isolation of the basin from the North Sea, favouring closed palustrine and lacustrine systems (Fig. 1). This transition also led to the Chattian (27.30–23.04 Ma) hiatus (Fig. 2), a significant break in sedimentation with non-deposition and erosion [Robin et al., 2000; Quesnel et al., 2014, 2016]. This sequence is closed by the deposition of the Burdigalian sands (20.45–15.98 Ma), interpreted as fluvial to marginal marine systems [Pomerol, 1989; Guillocheau et al., 2000]. The Beauce Limestone formation corresponds to a stack of Rupelian to Aquitanian subunits, whose thicknesses vary across the basin (Fig. 2). These include: the Étampes and Gâtinais Limestones, Rupelian in age and 20-50 m thick [Riveline, 1983; Escarguel et al., 1997], and the combination of the Orléanais Limestones (10 m thick), the Pithiviers Limestones (4-40 m thick) and the Blamont marls (0-20 m thick), all three Aquitanian in age [Ginsburg and Hugueney, 1980; Londeix, 2014]. Those carbonate units are separated from each other by the detritical formations of the Molasse du Gâtinais, 0-40 m thick and the Voise marls around 10 m thick [BRGM, 2011]. The total Beauce succession pile reaches up to 190 m around Orléans [SIGES, 2005].

At the time of deposition, the palaeogeography was dominated by a large lacustrine system filling a structural depression in the southern part of the Paris Basin [Guillocheau
<figure><img src="image_4.png" /><figcaption>**Figure 2** Wheeler Diagram of the Paris Basin formations from Late Oligocene and Early Miocene (adapted from Moreau [2023]).</figcaption></figure>

et al., 2000]. Fed by episodic fluvial inputs and experiencing a subtropical, seasonally variable climate, this lake system generated a mosaic of hydrologically isolated subbasins. Carbonate precipitation, microbialites formation, and palustrine processes led to distinct lacustrine and palustrine facies within the carbonate succession [Lorain, 1972; Ménillet, 1974; Guillemin, 1976; Ménillet and Edwards, 2000; Moreau et al., 2024].

Today, the Beauce Limestones host the Beauce aquifer, with an average storage capacity of 2.7 billion m3 [SIGES, 2005; Flipo et al., 2012]. It serves agriculture (55% of the withdraws in 2012 [SAGE, 2022], drinking water (38%) and industry (4%) [SAGE, 2003; Verley, 2020]. The Beauce aquifer is intensively exploited with over 4,000 wells, thereby exposing the water resource to quality and quantity issues [Chery, 1995; Schnebelen et al., 1999; Verley, 2020].

# 3 Material and Methods

The geological characterization is held at the Observatory of the Vadose Zone (O-ZNS) located near Orléans, in central France (Fig. 3, left). Within the 900 m² area, the O-ZNS aims to better understand the mass, flow, and heat transfers throughout the unsaturated zone thanks to eight cored boreholes spaced 5 to 8 m apart and associated logs (20-25 m long), and to the 6 m diameter and 20 m depth well, from which eight tonnes of bulk samples were extracted (Fig. 3, right). Complementary studies are being conducted on the O-ZNS site to provide a multidisciplinary understanding of the processes in the vadose zone [Aldana, 2019; Abbas et al., 2021; Isch et al., 2022; Mallet et al., 2022a,b; Abbar et al., 2022; Ryckebusch et al., 2025]. The eight cores were logged at a detailed 1:6 scale by characterizing their lithology, granulometry, sorting, sedimentary textures and structures, bioclastic and non-bioclastic features. Their descriptions are complemented by high-resolution photographs of each core and by the petrographic analysis of 22 thin sections to support the interpretation of facies. In addition, two cross sections are established (Fig. 3) by applying sequence stratigraphy model to continental carbonates [Hanneman and Widdman, 2010]. Regressive and transgressive systems tracts are bounded by Maximum Regressive Surfaces (MRS) and Maximum Flooding surfaces (MFS) with MRS placed after a succession of shallowing facies and MFS after a succession of deepening facies (see Section 5.1). The cross sections complement the vertical description of the Pithiviers unit along the boreholes and further constrain the 3D organization of the facies.

<figure><img src="image_5.png" /><figcaption>**Figure 3** Left: Location of the Beauce Aquifer (red borders) and the O-ZNS site (green dot) in France; Right: Positions at the O-ZNS site of the cored boreholes (B1 to B9, B4 was a destructive drillhole and is therefore not represented in the figure), position of the 6 m-diameter central well (red circle) and of the two cross sections used in Section 5 (yellow and pink lines).</figcaption></figure>

# 4 Sedimentary Facies Identification

This section presents the sedimentological analysis of the eight cored boreholes from the O-ZNS and of the 22 thin sections. Three main depositional environments are identified, each corresponding to a facies association (FA): lacustrine, lake-margins, and palustrine (Fig. 4). The lake-margins environment corresponds to the lacustrine-palustrine transition according to Alonso-Zarza [2003]. The three facies associations are based on eleven facies, each characterized by specific lithological features, sedimentary structures, textures [Dunham, 1962], or biogenic components. The following results provide a detailed description of the depositional settings and the facies, organized from the lacustrine domain toward the most proximal palustrine setting. Table S1 of supplementary materials provides a synthetic description of the eleven facies and their palaeoenvironmental interpretation, as well as the drawing patterns used to represent each facies in the figures.

## 4.1 Facies Association 1 (FA1): Lacustrine setting

The lacustrine setting corresponds to permanently submerged areas where water depth, hydrodynamic conditions, and aquatic biotic activity exert the primary control on sediment accumulation and fabric [Platt and Wright, 1991]. The facies associated with the lacustrine depositional environment are represented by polygenic and monogenic breccias, in which clast size, angularity, and sphericity vary according to the depth of deposit. These clasts are embedded in a micritic matrix and sometimes accompanied with oncoids, black pebbles, and microbial mats. Three facies were characterized: FA1.a, FA1.b, FA1.c.

### FA1.a – Thin polygenic Breccia

Description – This facies is characterized as a matrix-supported wackestone to packstone, with a predominance of millimetric rounded grains that mostly correspond to carbonate intraclasts reworked from previously deposited and hardened lacustrine-palustrine facies (e.g., FA2.a, FA2.b or
<figure><img src="image_6.png" /><figcaption>**Figure 4** Schematic representation of the three depositional environments or Facies Associations (FA): palustrine, lake-margins and lacustrine. The 11 facies described in the Pithiviers Limestones are placed in the different depositional environments based on their description in Section 4. The symbols in the legend represent the illustration of the features of the facies, as used in their drawing patterns (see Section 4 or Table S1 in supplementary material).</figcaption></figure>

FA3.a). Other millimetric scattered clasts are visible such as oncoids and carbonate black pebbles, which colour could indicate organic matter [Alonso-Zarza, 2003]. Thin-section observations additionally reveal the presence of ostracods dispersed within the micritic matrix. Facies FA1.a occasionally exhibits millimetric dissolution structures filled with a material similar in composition and texture to the surrounding matrix (Fig. 5, FA1.a).

Interpretation – The presence of small, rounded and moderately sorted grains in a carbonate mud matrix indicates the reworking and transport of material from the margins of the lake towards deeper, low-energy zones. The occurrence of rare dissolution features suggests short subaerial exposure phases, implying that the lake was relatively shallow — probably no more than a few metres deep even in its most distal areas.

### FA1.b – Monogenic breccia

Description – The monogenic breccia is characterized as a matrix-supported floatstone composed of monogenic rounded dark carbonate mudstone intraclasts. Grain size ranges from a few millimetres to about one centimetre, showing overall moderate sorting. The clasts are well rounded and embedded within a lighter micritic matrix, producing a clear contrast between the grains and the matrix. Occasionally, thin and laterally discontinuous stratiform mi-

crobial mats, corresponding to a succession of millimetric dark layers of a few millimetres thick, crosscut the matrix, as well as millimetric cemented vugs (Fig. 5, FA1.b).

Interpretation – The overall muddy texture and moderate sorting of the intraclasts indicate a deposit forming under low to moderate energy conditions, where intraclasts were likely reworked from close-by lake margins to slightly more distal lacustrine settings. The local development of microbial mats suggests episodic stabilization of the system in shallow water with limited agitation.

### FA1.c – Coarse polygenic breccia

Description – Facies FA1.c is a coarse, poorly sorted floatstone to rudstone. It is composed of a polygenic association of clasts ranging from granules to pebbles, with coarse clasts reaching several centimetres in diameter. The clasts are subrounded to angular and include black pebbles, breccia intraclasts, and oncoids. Black pebbles are a distinctive component and may be locally coated or encrusted by microbial mats. Millimetric rounded grains and carbonate mud occupy the interstices between larger clasts, resulting in variable packing from grain-supported to locally matrix-supported. No well-developed primary sedimentary structures are observed, although local weak imbrication of the largest clasts may occur. Secondary features include scattered millimetric voids and locally centimetric-deep chan-
<figure><img src="image_7.png" /><figcaption>FA1.b</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>FA1.c</figcaption></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>FA1.a</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>FA1.b</figcaption></figure>

<figure><img src="image_12.png" /></figure>
**Figure 5** Lacustrine facies association. Core photographs (top) – The white scale is equivalent to a length of 2 cm on each photograph – FA1.a: Thin polygenic breccia with oncoids, intraclasts and black pebbles. FA1.b: Monogenic breccia with rounded locally monogenic intraclasts. FA1.c: Coarse polygenic breccia with centimetric clasts and black pebbles. Thin sections photographs (bottom), a blue resin was used to colour the porosity. The grey scale is equivalent to a length of 200 m on each photograph – FA1.a: Ostracods in the micritic matrix, FA1.b: Micritic matrix and progressive filling of a vug with dolomite cement.

nels (Fig. 5, FA1.c).

**Interpretation** – The association of coarse, poorly sorted intraclasts of different origins, micritic matrix, and local mi- crobial encrustation indicates deposition in a proximal, shal- low lacustrine setting, close to the lake margins. This facies records sporadic, short-lived high-energy events (as storms) capable of mobilizing coarse material. Local intraclasts im- brication indicate short-distance transport by current. In- traclasts abundance, size, and shape, are highly variable in this facies, which is however consistently marked by the presence of reworked centimetric, angular clasts.

## 4.2 Facies Association 2 (FA2): Lake-Margins setting

The lake-margins setting represents a transitional zone between lacustrine and palustrine environments, where sed- imentary characteristics from both domains are expressed. This environment is influenced by alternating periods of emersion and submersion, which induce desiccation and in- situ brecciation. However, the former sediment is still recog-nisable. The associated facies are dominated by breccias formed by the desiccation of carbonate muds and filled cav- ities. Three facies were characterized: FA2.a, FA2.b, FA2.c.

### FA2.a – Floatstone with desiccation cracks

**Description** – This facies consists of fine-grained carbon- ate mudstone to wackestone with multi-centimetric desicca- tion cracks forming irregular patterns that fragment the sed- iment into jigsaw-puzzle-like breccias. The cracks delineate irregular intraclasts ranging from a few millimetres to sev- eral centimetres in size. The spacing between the fragments is typically millimetric but can locally reach one centimetre. The cracks are uniformly filled by a lighter, fine-grained car- bonate material similar in texture and composition to the host sediment, showing no distinct clastic input. Small dis- solution cavities, from a few millimetres to a centimetre in diameter, develop locally and preferentially along the cracks between the intraclasts (Fig. 6, FA2.a). No other clastic com- ponents, biogenic remains, or sedimentary structures were observed within this facies.

**Interpretation** – These features record subaerial exposure of the former carbonate mud, followed by flooding and rapid infilling of desiccation voids by suspended carbonate material. The combination of desiccation cracks, in-situ brecciation, and dissolution points to a lake margins envi- ronment influenced by fluctuating water levels and short- term emersions and immersions.

### FA2.b – Fragmented Breccia

**Description** – This facies consists of pluricentimetric clasts, ranging from rounded to angular, embedded in a fine mi- critic matrix. The main feature is the presence of irregular, centimetric clasts interpreted as fragments of the facies with desiccation cracks (Fig. 6, FA2.a) that were locally reworked and redeposited. The matrix is texturally similar to the in- filling observed within desiccation cracks of the previous

facies. In addition to these larger clasts, sub centimetric to millimetric rounded intraclasts are scattered within the matrix; they share the same composition as those observed within the desiccation cracks (Fig. 6, FA2.b). No other sedi- mentary structures, biogenic remains, or secondary features are observed.

**Interpretation** – This facies is interpreted as a possible reworked equivalent of FA2.a, as suggested by the cycle de- scriptions (see Section 5.2), formed under conditions of lim- ited transport along lake margins and with sufficient energy to fragment the desiccated sediment. Progressive reworking of this material toward the lake interior would ultimately produce the smaller, more rounded intraclasts characteristic of the monogenic breccia facies (FA1.b).

### FA2.c – Pseudomicrokarst Floatstone

**Description** – This facies is a floatstone composed of a mudstone matrix containing centimetric cavities filled with micrite and intraclasts similar in composition to the sur- rounding sediment. The intraclasts' size varies from milli- metric to centimetric. The cavity infill is commonly similar in colour to the host sediment, but can occasionally appear darker. Scattered millimetric rounded intraclasts and thin microbial mats are also observed (Fig. 6, FA2.c). No other sedimentary structure is present.

**Interpretation** – These cavities formed by karstification during subaerial exposure of the sediment, consistent with descriptions of pseudomicrokarst in continental carbonates [Freytet and Verrecchia, 2002; Alonso-Zarza and Wright, 2010; Moreau et al., 2024]. Subsequent flooding and sedi- ment reworking filled the cavities with intraclasts, preserv- ing evidence of alternating emersion and shallow submer- sion at the lake margins. This facies represents a near- surface diagenetic modification due to drying and flooding cycles, recording ephemeral karstification during droughts, and followed by re-deposition when the sediment is again submerged.

## 4.3 Facies Association 3 (FA3): Palustrine setting

Palustrine facies develop in low-energy, shallow, vege- tated wetlands, marshes, and ponded areas. In this setting, sediments are only weakly protected by the water column and frequently undergo subaerial exposure. As a result, the associated facies are highly heterogeneous and strongly in- fluenced by climatic events, vegetation, and pedogenic pro- cesses, which produce features such as local erosive discon- tinuities, root traces, calcrete, Microcadium or silicification. Five facies were characterized: FA3.a, FA3.b, FA3.c, FA3.d and FA3.e.

### FA3.a – Silicified Pedogenic Breccia

**Description** – This facies marks the complete silicification of the carbonate sediment. The primary fabric exhibits abun- dant evidence of pedogenesis, including root traces, erosive discontinuities, and a variety of clasts with sizes ranging from a few millimetres to several centimetres and display-
<figure><img src="image_13.png" /><figcaption>**Figure 6** Lake-margins facies association. Core photographs (top) – The white scale is equivalent to a length of 2 cm on each photograph – FA2.a: Floatstone with desiccation cracks forming jig-saw puzzle structures. Note the dissolution cavity following a crack. FA2.b: Fragmented breccia exhibiting centimetric desiccated fragments. FA2.c: Pseudomicrokarst floatstone with multi-centimetric cavities filled by micrite and intraclasts. Thin sections photographs (bottom), a blue resin was used to colour the porosity. The grey scale is equivalent to a length of 400 m on each photograph – FA2.b: Desiccation crack and circum-granular cracks around intraclasts, FA2.c: Micritic matrix and microscopic pseudomicrokarst.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>FA3.b</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>FA3.d</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>FA3.a</figcaption></figure>

<figure><img src="image_17.png" /></figure>
**Figure 7** Palustrine facies association. Core photographs (top) – The white scale is equivalent to a length of 2 cm on each photograph – FA3.a: Silicified pedogenic breccia with silicified intervals exhibiting discontinuities, root traces and intraclasts. FA3.b: Rhizoliths floatstone with millimetric root molds and mudstone intervals. FA3.c: Floatstone with Microcodium arranging in the matrix or at the edges of the larger centimetric intraclasts. FA3.d: Calcrete-nodule floatstone exhibiting large grains associated with microbial mats and root traces (rhizoliths). Thin sections photographs (bottom), a blue resin was used to colour the porosity – FA3.a: Silica cementing of the matrix around a centimetric cavity, FA3.b: Glaebule structures within the micritic matrix, FA3.c: Microscopic view in polarized light of the Microcodium (red star) in facies FA3.c.

ing diverse shapes and degrees of angularity (Fig. 7, FA3.a). Root traces appear in multiple forms: scattered millimetric tubular vugs (rhizoliths), grain-scale cleavage of centimetric grains forming pairs of fragments separated by a few millimetres, or alteration horizons developed beneath larger roots, forming localized bands of modified sediment (Fig. 7, FA3.a, right).

**Interpretation** – The silicification of this facies occurs as centimetric beds that preserve the primary sedimentary fabric and is consistently associated to the pedogenic features described above. In some instance, root traces seem to crosscut already silicified levels (Fig. 7, FA3.a, right), and empty root moulds are also preserved, both indicating an early silicification, contemporaneous with sedimentation. Therefore, it is interpreted as pedogenically enhanced, promoted by vegetation, microbial activity, decomposition of organic matter [Lorain, 1972; Knoll, 1985; Mahran, 1999; Bustillo et al., 2002] and the circulation of fluids under conditions that are transitional between lacustrine and palustrine environments [Bustillo, 2010]. This facies is interpreted as developing in low-energy, shallow palustrine environments at the transition from lake margins to vegetated wetlands, where frequent subaerial exposure, pedogenic activity, and minor sediment reworking shaped the silicified fabric and preserved root traces, discontinuities, and intraclasts.

### FA3.b – Rhizolith Floatstone

**Description** – This facies consists of homogeneous mudstone to wackestone intervals, with scattered millimetric rounded intraclasts. A distinctive feature is the occurrence of densely packed rhizolith network within the micritic matrix. Rounded sub-millimetric micritic aggregates formed around the intraclasts or associated to the rhizolith vugs, identified as glaebules, are also present in the thin sections (Fig. 7, FA3.b). Silicification typically occurring as discrete nodules or thin centimetric layers within the altered matrix, which can display a chalky appearance [Lorain, 1972; Menillet, 1980; Arribas et al., 2004; Moreau et al., 2024].

**Interpretation** – The facies reflects low-energy palustrine conditions at the lake margin–palustrine transition, where limited hydrodynamic reworking allowed preservation of the fine mud matrix and rhizoliths.

### FA3.c – Floatstone with Microcodium

**Description** – This facies is composed of multi-centimetric, subrounded intraclasts embedded within a micritic matrix. Millimetric elongated, tubular and radial calcite structures, interpreted as cauliflower-like shape Microcodium, are found both around the larger intraclasts, forming irregular nodular horizons along the edges, and dispersed within the matrix itself, forming isolated aggregates or nodular clusters (Fig. 7, FA3.c).

**Interpretation** – The facies reflects low-energy, subaerially influenced palustrine conditions, where biological activity

in the rhizosphere promoted the development and preservation of Microcodium [Wright, 1990; Košir, 2004; Kabanov et al., 2008].

### FA3.d – Calcrete-Nodule floatstone

**Description** – This facies is a floatstone containing irregular centimetric to multi-centimetric carbonate nodules and scattered rhizoliths within the matrix (Fig. 7, FA3.d). The nodules display diffuse, cloudy outlines with irregular boundaries and are commonly affected by desiccation cracks. Thin microbial mats, composed of multiple millimetric laminae coating the base of many nodules, are also observed. In thin section, laminar crusts and micro-scale desiccation cracks are observed in association with the nodules.

**Interpretation** – These features indicate repeated phases of exposure, drying, and carbonate precipitation affecting the host sediment and forming calcrete nodules resulting from pedogenic carbonate accumulation [Wright and Tucker, 2009]. The occurrence of these nodules, together with the microbial mats and rhizoliths, indicates deposition in a proximal palustrine settings, where strong modification of the primary sediment occurred during long and/or intense subaerial exposure phases [Alonso-Zarza, 2003].

### FA3.e – Microbial Laminite

**Description** – This facies is defined by centimetric microbial laminae forming laminated intervals (Fig. 8). These laminae consist of thin microbial mats organized into subhorizontal layers, generally 1-3 cm thick and only exceptionally reaching up to 10 cm in our observations. Their internal fabric is dominated by fine micritic lamination displaying undulatory geometries and strong thickness variations. This facies differs from the microbial crusts described in other facies: the microbial structures form true stratiform laminites rather than encrustations around the grains. The laminated intervals commonly exhibit intraclasts derived from the surrounding facies, reflecting local reworking or contemporaneous trapping. These include, for example, millimetric black pebbles or oncoids, which are embedded within or between the microbial laminae (Fig. 8). A notable characteristic of this facies is its limited lateral continuity. Even at the scale of the 8-cm-diameter cores, the thickness of successive laminae varies markedly from 4 to 2.3 cm within the core's radius (Fig. 8).

**Interpretation** – This facies has the same characteristics as the laminar calcretes defined by Wright et al. [1988]. When this laminated structure forms a discontinuous crust above a bed surface, it represents the formation of a composite of microbial and root mats associated with a significant emission phase. This facies can also be found on a smaller scale in other facies, as laminations can develop around clasts after their deposition (see Fig. 5, FA1.c).
<figure><img src="image_18.png" /><figcaption>**Figure 8** FA3.e: Microbial laminite. Core photographs (top) – The white scale is equivalent to a length of 2 cm on each photograph – Microbial Laminite exhibiting centimetric intervals of undulatory laminae whose outlines have been highlighted on the photographs with dotted lines. The microbial laminites locally trap black pebbles, intraclasts or oncoids. Thin sections photographs (bottom), a blue resin was used to colour the porosity – FA3.e : Laminar crust at the micro-scale and associated alveolar septal structures.</figcaption></figure>
# 5 Stratigrahic Sequences

## 5.1 Cross-sections

In order to investigate the lateral extent and the scale of facies variability within the Beauce continental carbonates, two cross sections were constructed, each correlating four wells of the O-ZNS over a lateral length of 22 m. Assessing the spatial continuity (or discontinuity) of the depositional units is essential, as it directly governs the connectivity between facies bodies and thus the potential pathways for fluid circulation. To reconstruct the 3D stratigraphic architecture and to enable time-slice palaeogeographical interpretations, the cross sections follow two perpendicular directions: a N-S orientation (Fig. 9) and a SW-NE orientation (Fig. 10). Three hierarchical orders of cyclicity can be recognized within the two cross sections: (1) a very-short-term cyclicity at bed scale, corresponding to the facies shifts (Fig. 10, B8, lake-margins beds in cycle 4), often not mappable between well and not represented in the cross sections; (2) a short-term cyclicity corresponding to depositional-environments shifts (Arabic numerals in the cross sections); (3) a medium-term cyclicity (Roman numerals in the cross sections). Three medium-term cycles (I, II, III) and six short-term cycles (from 1 to 6) are identified and traced between the wells. The short-term cycles are displaying very variable thicknesses, ranging from 10 cm to more than 5 m. The cycles are correlated exclusively by matching Maximum Regressive Surfaces (MRS) and Maximum Flooding Surfaces (MFS). Maximum flooding surfaces of short-term cycles are located either in beds recorded under the deepest or least hydrodynamic conditions, or in beds that record features associated with the least pronounced subaerial exposure and pedogenesis. On the other hand, subaerial discordances and beds rich in pedogenesis features are interpreted as maximum regressive surfaces [Mahran, 1999; Changsong et al., 2001; Bustillo et al., 2002].

## 5.2 Description of the cycles

### 5.2.1 Medium and short-term cycles

Medium-term cycles constitute the main vertical structuring of the Pithiviers continental carbonate succession and the longest-term (highest-order) cyclicity observed at the O-ZNS site. Each cycle is several metres thick (typically between 5 and 10 m) within the 25 m thick Aquitanian succession, that formed in approximately 2.6 Myr [Raffi et al., 2020]. These cycles therefore correspond to significant temporal and stratigraphic intervals (lasting possibly several hundreds of thousands of years), which could result from basin-scale long-term trends [Schlager, 2005].

Medium-term cycle I is only partially represented within the studied cores, as its lower boundary is absent. It includes short-term cycle 1 whose maximum flooding surface is marked by a lacustrine incursion (facies FA1.b) within a palustrine succession. The overall vertical evolution of cycle I is interpreted as a shallowing trend, although the incomplete nature of the cycle prevents a full reconstruction of its organization. At the scale of the study area, the maximum regressive surface (MRS) of cycle I corresponds to a phase during which palustrine conditions extended across the entire site (palustrine facies FA3.a, FA3.b and FA3.d).

Cycle II is the only complete medium-term cycle within the studied interval. It has a total thickness of 10 to 12 m

and comprises four short-term cycles (cycles 2 to 5). This cycle displays an overall regressive trend, expressed by a progressive transition from lacustrine-dominated (facies FA1.a, FA1.b and FA1.c) to palustrine-dominated configuration (facies FA3.b and FA3.c) (Fig. 9, 10). Each short-term cycle within cycle II begins with a relatively thin transgressive systems tract (generally less than 1.3 m thick and often only a few tens of centimetres), followed by a thicker regressive systems tract, locally reaching up to 5 m in thickness, particularly in cycle 5. The upper MRS of medium-term cycle II is marked by a fully palustrine-dominated configuration at the top of cycle 5 (Fig. 9, 10), resulting from the general shallowing trend. Medium-term cycle III is incompletely preserved due to recent alteration and truncation of the upper part of the succession. It includes short-term cycle 6 and is at least 6 m thick.

Cycle III expresses a clear deepening trend characterized by a 1 to 5 m thick transgressive systems tract. Transgression culminates with a maximum flooding surface (MFS) marked by lacustrine facies (facies FA1.a and FA1.b) in all boreholes (Fig. 9, 10). This lacustrine-dominated configuration indicates a renewed expansion of the lake system over the study area, interrupting the long-term regressive trend observed in cycle II.

While medium-term cycles document the long-term vertical evolution of the depositional system, they do not capture the strong spatial variability observed at shorter timescales. This variability is addressed through the analysis of short-term cycles and the depositional organizations they define at the scale of the study area [Fragoso et al., 2021; Tipper, 2000].

### 5.2.2 Linking depositional landscapes to short-order cycles

Short-term cycles are here grouped according to the type of depositional organization they define across the study area. Three recurrent depositional landscape are recognized: (1) palustrine-dominated, (2) lacustrine-dominated, and (3) mixed landscapes. To illustrate the spatial configurations of the depositional system at these key stratigraphic moments, palaeogeographical reconstructions of the study area at selected maximum flooding surfaces (cycles 3 and 5) and maximum regressive surfaces (cycles 2, 3 and 5) are shown in Figure 11.

Palustrine-dominated landscapes correspond to phases during which palustrine facies extend across the entire study area (Fig. 11). Such configurations are identified at the maximum regressive surface (MRS) topping short-term cycles 1, 5, and 6 (Fig. 9, 10). During these phases, all boreholes display palustrine facies, including silicified palustrine deposits (FA3.a, FA3.b), calcrete-nodule floatstone (FA3.d), and Floatstone with Microcadium (FA3.d). These facies indicate prolonged subaerial exposure and intense pedogenic modification affecting the former lacustrine sediments. The lateral continuity of palustrine facies during these intervals exceeds at least 30 m, corresponding to the full extent of the study site.

Lacustrine-dominated landscapes correspond to phases of maximum flooding, during which lacustrine conditions prevail across the entire study area (Fig. 11). They are systematically observed at maximum flooding surfaces (MFS) and are identified at the MFS of short-term cycles 2, 3, 4, and 6 (Fig. 9, 10), where all boreholes consistently record la-
<figure><img src="image_19.png" /><figcaption>**Figure 9** Twenty-two-meters-long cross section 1 oriented North-South and crossing boreholes B9, B2, B8 and B6. The location of the cross section in the O-ZNS site is represented at the top-left of the figure. The medium-term (I, II, III) and short-term (1 to 6) cycles are represented by their MRS, MFS and systems tracts at both sides of the cross section.</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>**Figure 10** Twenty-two-meters-long cross section 2 oriented Southwest-Northeast and crossing boreholes B7, B8, B5 and B1. The location of the cross section in the O-ZNS site is represented at the top-left of the figure. The medium-term (I, II, III) and short-term (I to 6) cycles are represented by their MRS, MFS and systems tracts at both sides of the cross section.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Figure 11** Reconstructed palaeogeographies of the O-ZNS site for cycles 2, 3 and 5. Each borehole is placed in a specific depositional environment corresponding to the facies at the end of the cycle (at the MFS or MRS). The interpolation around and beyond the wells is determined thanks to the cross sections, to the vertical succession of the facies inside the cycles and thanks to the determined relationships between the facies (see Fig 12).</figcaption></figure>
custrine facies, either thin polygenic breccia (FA1.a), mono-
genic breccia (FA1.b) or coarse polygenic breccia (FA1.c).
These configurations reflect periods during which the study
area was entirely submerged, either as a consequence of
lake-level rise leading to broader lacustrine expansion, or
through lateral migration of the depocenter towards the
study site, bringing lacustrine conditions at the location of
each borehole. The lateral homogeneity of facies at these
surfaces suggests a relatively uniform depositional environ-
ment over distances of at least 30 metres.

Mixed depositional landscapes are characterized by the
lateral coexistence of palustrine, lake-margins, and lacus-
trine facies within the study area in a given stratigraphic
interval or at a given surface (Fig. 11). These configura-
tions are observed at both maximum transgressive or regres-
sive surfaces, at the MFS of cycles 1 and 5, and at the MRS
topping cycles 2, 3 and 4 (Fig. 9, 10). The study site is
therefore located at the transition from lacustrine to palus-
trine environments. During these phases, facies transitions
occur over very short distances, locally between adjacent
boreholes separated by 5 to 8 m. The lake-margins environ-
ment appears to be particularly restricted spatially, typically
extending over only one or two boreholes (i.e., 5 to 10 m),
and rarely up to 15 m. The spatial distribution of facies
within mixed lacustrine-palustrine landscapes allows to de-
termine the orientation of the depositional system. For ex-
ample, the MRS of cycle 2 records a proximal-to-distal trend
from west to east, whereas the MRS of cycle 3 displays a
radial organization, with palustrine conditions transition-
ing toward lacustrine settings in several directions (west,
south and east, Fig. 11). At the MFS of cycle 5, the sys-
tem shows again a proximal to distal trend from west to
east. This illustrates both the strong vertical variability of
the system at short timescales, expressed by the stacking of
contrasted palaeogeographical configurations, and its pro-
nounced lateral variability at a given time, with transitions
between depositional environments occurring over only a
few metres, particularly for the lake-margins settings.

# 6 Discussion

## 6.1 Scale of facies variations along a depositional profile

The lateral transitions between lacustrine (FA1), lake-
margins (FA2), and palustrine (FA3) facies highlighted by
the cross sections provide the basis for constraining their
distribution over a depositional profile. Among the cycles
described before, the regressive systems tract of short-term
cycle 3 offers a particularly revealing example, as it pre-
serves a complete lateral transition of depositional environ-
ments from lacustrine (FA1.c) to palustrine (FA3.b) within
the study area (Fig. 9, 10). This configuration allows to
clarify the relative distality of the facies and serves as a ref-
erence framework for building the synthetic depositional
profile in Figure 12A, which integrates vertical stacking pat-
terns and lateral facies transitions. Based on the environ-
mental interpretation of the different facies established in
section 4, the cross-sections (Fig. 9, 10) and the resulting
palaeogeographical reconstructions (Fig. 11) we estimated
the relative distality of the eleven facies to propose a more
comprehensive depositional profile (Fig. 12B). This second
profile does not represent a configuration that was revealed
in its entirety at a specific time on the cross-sections, but

rather a conceptual tool that combines the lateral relation-
ship between facies evidenced during different depositional
stages.

The resulting depositional profile highlights the high-
resolution spatial variability of facies at the O-ZNS site, with
lateral facies transitions occurring over few meters. The con-
sistently limited lateral extent of the lake-margins facies sug-
gests that this environment represents a spatially restricted
transition zone, from a few meters to maximum 15 meters
wide, between lacustrine and palustrine settings. Such a
narrow transition implies that pedogenic processes could
significantly develop in palustrine setting, only ten meters
away from lacustrine environments. It also suggests that
even minor and local lake-level drops might allow vegeta-
tion colonization and pedogenic alteration of former lacus-
trine facies. This is consistent with the frequent absence of
lake-margins facies in vertical successions, where palustrine
deposits directly overlain lacustrine facies (see Fig. 9, 10:
cycle 2, B7; cycle 3, B2, B1; cycle 6, B1, B2, B5), indicating
that marginal deposits are either short-lived or rapidly over-
printed by pedogenesis. Comparable behaviour has been
documented in modern shallow lacustrine systems charac-
terized by low relief and strong hydrological variability, such
as the Las Tablas de Daimiel wetlands in Spain [Alonso-
Zarza et al., 2006] and the Everglades in Florida [Platt and
Wright, 1992; Gaiser et al., 2012]. These analogues illus-
trate how low-amplitude autocyclic water-level changes can
generate rapid lateral shifts of depositional environments
and promote early pedogenic modifications (Pederson et
al., 2018). In contrast, lacustrine-dominated and palustrine-
dominated landscapes correspond to intervals during which
a single depositional environment extends across the entire
study area, producing laterally homogeneous deposits over
distances exceeding 30 m. Such phases may be controlled
by longer-term trends rather than by purely autocyclic and
local reorganization [Alonso-Zarza et al., 2012]. The al-
ternation between these homogeneous and highly hetero-
geneous environmental organizations highlights the multi-
scale nature of facies variability in lacustrine-palustrine car-
bonate systems and provides the framework for discussing
the respective roles of local and broader-scale controls on
sedimentation in the following section.

## 6.2 Control of the local and quick depositional shifts

The recognition of several hierarchical levels of
transgression-regression cycles in the cross sections (Sec-
tion 5.2) highlights the coexistence of distinct possible con-
trols governing the organization of the lacustrine-palustrine
deposits. These levels are primarily distinguished based on
their lateral continuity and vertical thickness.

Very short-term cycles, which cannot be correlated be-
tween wells and display a lateral continuity limited to
only a few metres (Fig. 9, 10), reflect strictly autogenic
origin, driven by very local sedimentary reorganization,
without the influence of allocyclic or basin-scale controls
[Catuneanu et al., 2011; Xi and Burgess, 2022]. Such pro-
cesses likely include short-lived metric to sub-metric lake-
level fluctuations controlled by the ratio between water sup-
ply and evaporation, leading to rapid and short exposure
and flooding of marginal sediments, localized pedogenesis
and sediment reworking in shallow water bodies [Freytet
et al., 1982; Alonso-Zarza and Wright, 2010].

Short-term cycles, characterized by a dominance of re-
<figure><img src="image_22.png" /><figcaption>**Figure 12** A – Partial depositional profile built from the lacustrine to palustrine transition in short-term cycle 3. B – Global depositional profile including the 11 facies by order of distality from the deepest (left) to the shallowest (right). Each facies is represented by its drawing pattern, illustrating its main characteristics (see Section 4 or Table S1 in supplementary materials).</figcaption></figure>

gressive systems tracts (Fig. 9, 10, cycles 2 to 5) and likely corresponding to successive lake infilling phases [Bohacs et al., 2000], can be correlated between wells but remain highly variable in thickness and facies over short distances. This variability is particularly expressed during the hybrid eras, where transitions between depositional environments occur within a few metres to a few tens of metres (Section 5.2). Short-term cycles are no more than a few meters thick and, within these cycles, the facies units attributed to the same environment are about a meter to less than a metre thick (Fig. 9, 10 of cyclicity is largely controlled by low amplitude (e.g., about a meter) autogenic lake-level variations.

In contrast, the origin of the medium-term cycles remains more uncertain. Their decametric thickness, their continuity across the entire study site and the fact that they mark transitions from fully palustrine- to fully lacustrine landscapes suggest that they may reflect broader temporal and spatial controls that modulated the local depositional dynamics (Section 5.2) [Alonso-Zarza, 2003; Miall, 2010]. However, their origin cannot be confidently related to eustatic fluctuations or another specific allocyclic factor because of the lack of temporal constraint, but may still relate to longer-term controls superimposed on a depositional system otherwise dominated by autogenic dynamics [Carroll

and Bohacs, 1999). These medium-cycles are included in the long-term cycle (cycle A) identified by Moreau et al. [2024] in the Beauce Limestones, which spans the Rupelian to Aquitanian intervals and is characterized by a short transgressive phase followed by a prolonged regressive trend. Moreau et al. [2024] explicitly emphasize that no discernible link can be established between this long-term cycle and eustatic variations [Speijer et al., 2020], invoking the possible role of local and/or regional tectonic movements.

## 6.3 Perspectives to improve meter-scale correlations

environmental shifts, resulting in highly heterogeneous successions. Consequently, the identification — and more critically the hierarchical attribution — of stratigraphic surfaces such as maximum flooding surfaces (MFS) and maximum regressive surfaces (MRS) remains difficult [Roemers-Oliveira et al., 2024]. Correlating these surfaces between boreholes therefore involves a substantial degree of uncertainty, which is all the more important as the order of cycles decreases. To improve sequence stratigraphy correlations, the integration of complementary datasets such as geophysical well logs or seismic lines could help guide correlations between boreholes [Mallet et al., 2022b]. In addition, the analysis of metric to decametric lateral continuous outcrops
would allow direct observation of facies geometries and lateral transitions, providing critical constraints on sedimentary stacking patterns and systems tracts organization that cannot be fully resolved from borehole data alone [Laurent et al., 2023].

The second limitation to correlation is the pedogenic overprint that introduces an additional source of uncertainty, both for facies identification and, more importantly, for the positioning of MFS and MRS. Once sediments are exposed, pedogenic processes rapidly transform lacustrine and lake-margins facies into palustrine facies through the development of calcretes, microbial laminites, silicification, root traces, and Microcadium [Freytet and Verrecchia, 2002; Alonso-Zarza, 2003]. As a result, lake-margins facies tend to be poorly preserved in vertical successions, as they are progressively replaced by palustrine facies through downward penetration of pedogenic features into the sediment affecting underlying lacustrine deposits. Consequently, and as discussed by Roemers-Oliveira et al. [2024], the MFS may be positioned artificially deeper in the stratigraphic record, since originally lacustrine sediments above the MFS can be overprinted and newly identified as palustrine (Fig. 13). Maximum flooding surfaces of few meters-thick short-term

transgressive-regressive cycles could even be impossible to identify if lacustrine facies have been entirely altered through pedogenesis, which would make those cycles invisible to the sedimentologist's eye and could cause a series of cascading errors in sequence stratigraphy interpretations and lateral correlations.

Pedogenic overprint must be explicitly accounted for when constructing stratigraphic correlations in palustrine–lacustrine systems, and further quantification of the intensity and vertical extent of pedogenic processes would help refine the positioning of the stratigraphic surfaces.

## 7 Conclusion

The detailed description of eight closely spaced cores, separated by 5 to 8 meters each, allows the identification of eleven facies, grouped into three depositional environments (lacustrine, lake-margins, and palustrine), establishing a robust facies framework for interpreting metre-scale depositional variability in lacustrine–palustrine settings.

Stratigraphic correlations across two 22 meters-long perpendicular cross sections, each crossing four boreholes, reveal a strongly heterogeneous sedimentary architecture governed by three hierarchical orders of cyclicity. Three

<figure><img src="image_23.png" /><figcaption>**Figure 13** Progressive pedogenic overprinting of lacustrine and lake-margins facies over time (through representation n°1 to n°4) leading to the formation of sedimentary cycle 3 (see cross-sections on Fig. 9, 10). The inclination of the depositional profile is exaggerated for illustration purposes. The regressive and transgressive systems tracts on the right side represent the location of the Maximum flooding Surface (MFS) and Maximum Regressive Surface (MRS) determined from the sedimentary record (left) and from the theoretical real location of the MFS without considering the pedogenic overprint (right). In representation n°4, the sedimentary description of cycle 3 deposits in cores B7 and B1 are placed along the theoretical depositional profile to illustrate how pedogenic overprints affect the sedimentary record in the logs. In core B1, see how lake-margins facies have been overprinted by palustrine facies, resulting in palustrine facies directly overlaying lacustrine facies.</figcaption></figure>
medium-term cycles structure the evolution of the overall 25 m-thick sedimentary succession. These cycles record alternations between phases of widespread lacustrine and palustrine systems at the location of the study site. Within these 3 main cycles, six short-term cycles document environmental fluctuations expressed by metre-scale vertical shifts of depositional environments.

The lake-margins environment forms a narrow (from a few meters to 15 meters wide) and ephemeral transition zone between submerged and subaerial conditions, which show that intense pedogenesis could take place only a few meters far from the lake. The frequent absence of lake margin facies in vertical successions indicates that pedogenic processes can become effective rapidly following minor lake-level drops, overprinting marginal and even shallow lacustrine sediments. The resulting depositional system is therefore characterized by sharp environmental gradients, short-lived transitional settings, and strong lateral variability at a scale of a few meters. Comparable behaviour in modern hydrologically variable systems supports the interpretation that such architectures are primarily driven by autogenic processes, including lake-level oscillations, vegetation dynamics, microtopography and local hydrological conditions.

These results have important implications for the Beauce aquifer. The metre-scale facies heterogeneity in the Pithiviers Limestones, host of the unsaturated zone of the aquifer, imply that hydraulic properties and connectivity are likely to vary significantly over very short distances. Consequently, hydrogeological models of the Beauce aquifer should explicitly account for fine-scale sedimentological heterogeneity coupled to complementary approaches, such as geophysical well logs and the analysis of metric to decametric outcrops, to better constrain facies geometries and their scale of lateral continuity, particularly within the vadose zone and near-surface carbonate units where such variability is commonly underestimated.

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