# Imaging Bajocian coral ridges in the Paris Basin and deciphering their origin

Benjamin Brigaud, Benoît Vincent, Marc Pessel, Albane Saintenoy, Hermann

Zeyen, Christophe Durlet, Jessica Saïag, Michel Hayet

► To cite this version:

Benjamin Brigaud, Benoît Vincent, Marc Pessel, Albane Saintenoy, Hermann Zeyen, et al.. Imaging Bajocian coral ridges in the Paris Basin and deciphering their origin. Basin Research, 2025, 37 (3), pp.e70034. 10.1111/bre.70034. hal-04635083v3

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# Imaging Bajocian Coral Ridges in the Paris Basin and Deciphering Their Origin

Benjamin Brigaud<fnref n="1" />,² | Benoît Vincent³ | Marc Pessel<fnref n="1" /> | Albane Saintenoy<fnref n="1" /> | Hermann Zeyen<fnref n="1" /> | Christophe Durlet⁴ |
Jessica Saïag<fnref n="1" /> | Michel Hayet⁵

**Correspondence:** Benjamin Brigaud (benjamin.brigaud@universite-paris-saclay.fr)

**Received:** 18 June 2024 | **Revised:** 31 January 2025 | **Accepted:** 25 April 2025

**Funding:** This work was supported by ANDRA and Centre National de la Recherche Scientifique.

**Keywords:** Bajocian | carbonate | coral | facies | geophysics | reef

## ABSTRACT

Enigmatic N120° ridges have been identified from 3D seismic reflection imaging of the Bajocian limestones of the eastern Paris Basin. These features may impact flows within the active Middle Jurassic aquifer beneath the Callovian–Oxfordian claystones and marls that host the Underground Research Laboratory (URL) where the Andra (French National Agency for Radioactive Waste Management) is studying the feasibility of a deep repository for radioactive waste. Worldwide, there are numerous other examples where carbonate buildups form ridges in 3D, which are more or less interconnected, laterally amalgamated, or bifurcating, with no clear reason for these geometric features. It is consequently of paramount importance to understand the nature and origin of these ridges, and an integrated study combining (1) well logs, (2) new 3D seismic acquisitions, (3) classical field sedimentology and stratigraphy, (4) near-surface geophysics including ground penetrating radar (GPR), electrical resistivity tomography (ERT) and frequency-domain electromagnetics (FDEM) and (5) seismic refraction has been developed to investigate them. Facies analysis and a regional sequence stratigraphy interpretation, integrating near-surface geophysical imaging performed on time-equivalent outcrop sections, demonstrate that elongated mounds of hermatypic scleractinian corals developed during the early Bajocian (*Humphriesianum* chronozone) in shallow, warm oligotrophic seawater. These buildups nucleated as patches on hard-ground atop giant subaqueous dunes composed of peloidal and bioclastic grainstones, dipping mostly N30° with N120°-oriented crests. Some of the coral reefs form buildups up to 15 m high and several hundred meters wide. They are elongated in the main N120° direction of the underlying dunes, although the dispersion of measurements illustrates the complexity of the interfingered structures observed in 3D seismic images. The coral buildups are progressively onlapped and draped by oncoid-rich alternating marl-limestones that may result from a shift from oligotrophic to mesotrophic conditions probably brought about by a rise in relative sea level. Near-surface geophysics provide insightful supporting evidence to supplement the field work, particularly by imaging the roots of several coral reefs, their internal structures and the infill of the inter-reefal troughs. This work demonstrates the critical importance of outcrop analogue studies for resolving subsurface problems and also shows how near-surface geophysical methods can usefully supplement direct classical field geology investigations. This new characterisation suggests that coral reefs forming ridges, structures that have often remained enigmatic, could develop by directly settling at the top of early cemented giant oo-bioclastic dunes. Submarine early diagenesis generates hardgrounds that constitute stable substrates for nucleation and growth of coral reefs, especially when they cap prominent submarine reliefs.

This is an open access article under the terms of the [Creative Commons Attribution License](https://creativecommons.org/licenses/by/4.0/), which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2025 The Author(s). *Basin Research* published by International Association of Sedimentologists and European Association of Geoscientists and Engineers and John Wiley & Sons Ltd.

<footnote n="1">¹Université Paris-Saclay, CNRS, GEOPS, Orsay, France | ²Institut Universitaire de France (IUF), Paris, France | ³Cambridge Carbonate Ltd., Marey-sur-Tille, France | ⁴Biogéosciences, CNRS, Université de Bourgogne, Dijon, France | ⁵Andra, Bure, France</footnote>
## Summary

* Seismic reflection surveys identified enigmatic ridges elongated N120° in Bajocian limestones.

* Classical geological fieldwork suggests that coral reefs developed on giant subaqueous dunes.

* Near-surface geophysical methods support fieldwork, imaging coral reefs and dunes.

* Ridges observed in 3D seismic images correspond to the coral buildups, probably nucleated from the crest of dunes.

* Early diagenesis generates hardgrounds that constitute stable substrates for nucleation and growth of coral reefs.

# 1 | Introduction

Small-scale (infra-kilometre) carbonate buildups are very common in the sedimentary record through several periods of the Phanerozoic worldwide (e.g., Fournier et al. 2005; Kiessling et al. 2002; Koša 2015; Mancini et al. 2004; Olivier et al. 2008). Coral reefs or patch reefs produce marked heterogeneities in carbonate platforms and within reservoirs together with anisotropic fluid flows (e.g., Howarth and Alves 2016). It is therefore essential to better constrain the origin and development of such coral reefs in order to improve the characterisation of subsurface carbonate prospects (oil and gas exploration and production, geothermal prospects, CO₂ capture and storage, nuclear waste repository, etc.) (Vennin et al. 2003; Wolpert et al. 2022).

At this scale, the emplacement and development of hermatypic scleractinian corals depend on several key factors including eustatism, salinity, temperature, clastic fluxes, turbidity, light and nutrient supply (Pomar 1991, 2001). Their emplacement is also highly dependent on the inherited topography of the substratum (Vennin et al. 2004; Bourillot et al. 2009; Goldstein et al. 2013), which may be driven by (1) synsedimentary folds or faults (Durlet et al. 1997, 2001; Quesne et al. 2000; Fournier et al. 2005), (2) halokinetic movements (Poprawski et al. 2016; Martín-Martín et al. 2017; Teixell et al. 2017), (3) karstic reliefs (Schlager and Purkis 2015) and (4) former sedimentary structures of diverse origins (Macintyre 1972; Nichol et al. 2012; Droxler and Jorry 2013; Riera et al. 2023). The latter aspect is probably particularly prevalent on platform morphologies where seafloor topography contacts are usually limited (Riera et al. 2023). However, this influence of topographic relief derived from sedimentary structures on the development of coral reefs, although well appraised in modern or recent examples (Droxler and Jorry 2013), is still poorly documented in the ancient depositional record (> 1 Ma) using geophysical data.

In the eastern Paris Basin, the Andra (French National Agency for Radioactive Waste Management) is operating an Underground Research Laboratory (URL) to study the feasibility of subsurface storage of radioactive waste (Landrein et al. 2013; Pagel 2014). It has conducted a wide spectrum of geological investigations for more than 20 years, including 2D

and 3D seismic reflection surveys, well drilling with coring and the acquisition of complete high-resolution sets of well logs. The multidisciplinary geological explorations have focused at depth on a 150-m-thick marl formation of Callovian–Oxfordian age (Pagel 2014), where the URL is located, and to a lesser extent on the surrounding limestone formations (Vincent et al. 2007; Brigaud, Vincent, Durlet, et al. 2014; Carpentier et al. 2014). These bounding formations, respectively of Bajocian–Bathonian and Oxfordian–Kimmeridgian ages and ranging from 250-m to 300-m thick (Landrein et al. 2013), represent two active aquifers whose characterisation is obviously critical for safety reasons. Within the Bajocian limestone, 3D seismic reflection surveys conducted in the vicinity of the URL have revealed some peculiar ridges several kilometres long and less than 1-km wide, trending mainly N120° (Bergerat et al. 2007). A deviated well targeted one of these elongated mounded structures and revealed coral-bearing facies (Brigaud et al. 2009, 2010). The N120° direction is not a frequent structural direction in the Jurassic limestones of the Paris Basin (Bergerat 1987; André et al. 2010; Blaise et al. 2022), and as a consequence, the origin of these ridges remains enigmatic. This direction is notably different from the N10°–N20° and N70°–N80° synsedimentary faults that influenced the nucleation of early Bajocian coral reefs in the SE Paris Basin and in Burgundy (Durlet et al. 1997, 2001). Elsewhere in the world, there are many examples from 3D seismic data revealing that coral reefs form long ridges, as documented by Rafaelsen et al. (2008) in the Barents Sea or Bachtel et al. (2010) offshore Australia. However, the origin of these ridges remains enigmatic, with no clear reason for this phenomenon.

This paper sets out the results of a close integration between deep subsurface geology, based on previously published core data and logs (Brigaud et al. 2009, 2010) and geophysics, including seismic reflection, where the 1999 seismic cube is already interpreted in Bergerat et al. (2007), unlike the latest 2010 seismic cube, which is not interpreted. It also incorporates new near-surface geophysics, including ground penetrating radar (GPR), electrical resistivity sections and seismic refraction sections, together with high-resolution sedimentological field work on outcropping Bajocian limestone in quarries of the eastern edge of the Paris Basin. The first objective will be to image the coral reef buildup geometries, their shape, or size using near-surface geophysics directly on selected quarries, coupled with sedimentological observations. To achieve this, the near-surface geophysical methods will be carefully calibrated along outcrops where sedimentary structures and facies are clearly visible. Once the calibration is completed, and the demonstration is made that we can image the coral reefs along the outcrops, we will proceed to image the structures beneath the quarries where nothing is visible. Our second objective is to characterise the N120° ridges as precisely as possible in order to determine whether this direction is really present on this platform and whether it is reproducible on a regional scale. This second objective will provide insights and constraints for further static and dynamic modelling of the Middle Jurassic aquifer, a major concern for the geological characterisation of a radioactive waste storage site. A third objective is to define the dynamics of the emplacement and development of Bajocian coral reefs within a detailed
paleo-environmental context, in order to determine the main factors controlling the origin of these coral ridge structures in a carbonate platform.

## 2 | Geological Setting

During the Jurassic, the Paris Basin was an epicontinental sea open to the Tethys ocean to the East and bordered to the West by the possibly exposed landmasses of the Armorican

massif and located at tropical to subtropical latitudes (25°–35°N; Figure 1A,B). A wide SE–NW intracratonic carbonate platform developed over a vast area of Western Europe, extending for more than 2000 km from Spain in the SW to Poland in the NE. In France, this platform encompassed the Aquitaine in the SW, the Jura mountains in the East and Normandy in the West (figure 1B, Brigaud, Vincent, Carpentier, et al. 2014).

Bajocian limestones crop out on the eastern side of the Paris Basin, along a NE–SW zone some 5 km wide (Figure 1C). At

<figure><img src="image_3.png" /><figcaption>**FIGURE 1** | (A) Paleogeographic map (late Jurassic) from R. Blakey's maps, https://deetimemaps.com. (B) Paleogeographic map (Callovian) from Atlas Peri-Tethys (Dercourt et al. 2000). (C, D) Geological maps with the location of the Andra Underground Research Laboratory (URL) site and associated wells and seismic surveys, and positioning all the studied outcrop sections.</figcaption></figure>
present, these limestones dip gently (1°–2°) towards the NW and consequently occur at a burial depth of around 700 m below the URL (Figure 1D). A set of 12 outcrops, including quarries and natural outcrops, was selected and studied for the Bajocian series (Figure 1C).

Below the Bajocian limestone and prior to the development of the Jurassic carbonate platform, the *Marnes à Voltzi* Formation of Late Toarcian age is made up of siltstone and claystone with thin micaceous sandstone beds. It is overlain by the still Late Toarcian *Grès supraliasiques* Formation (Figure 2), which displays a vertical succession of (1) silty claystone, (2) alternating centimetre-thick beds of siltstone and sandstone and (3) sandstone with sigmoidal cross-bedding (Guillocheau et al. 2002). The *Grès supraliasiques* facies corresponds to deposition in a tide-dominated distal deltaic system with migrating tidal sand dunes. The overlying *Formation ferrifère* or ‘Minette de Lorraine’ (Guillocheau et al. 2002) displays similar facies to the *Grès supraliasiques*, together with iron ooids with a few calcite bioclasts and a carbonate matrix, appearing as lenses within the dominant clastic facies, submitted to tidal dynamics (Teyssen 1984). Attributed to the Late Toarcian to Aalenian, this formation is present to the North of the study area (in Lorraine, Audun-le-Roman well; Figure 2) and in the South (around Nogent for instance, figure 1C, Maubeuge 1956), but is not documented in the centre of the study area (Andra well EST-433; Figure 2A). The overlying *Marnes micacées* Formation, dated through the *Concavum* to *Discites* ammonite chronozones of the Aalenian to Bajocian transition (Guillocheau et al. 2002; Thiry-Bastien 2002), is probably indicative of the incipient return of a carbonate factory prior to the development of the Jurassic platform. These deposits thicken significantly from the study area, where they are less than 10-m-thick, northwards in Lorraine where they reach about 30-m thick (Figure 2B). The *Calcaires d'Ottange* Member is composed of bioclastic grainstones with undulated bed planes (Lathuilière 2005), which are the earliest evidence of the establishment of the Middle Jurassic carbonate platform in the eastern Paris Basin. This member occurs in Lorraine and passes laterally to the South, in the study area, into sandy limestones interbedded with thin marls locally named *Calcaires sableux de Haye* and dated from the *Laeviuscula* ammonite chronozone of the early Bajocian (Lathuilière et al. 2003), which are, however, not always recorded. The *Calcaires à Entroques* Member comprises cross-bedded, crinoid-rich bioclastic limestones (Guillocheau et al. 2002; Lathuilière et al. 2003; Lathuilière 2005), dated through the *Propinquans* to *Humphriesianum* chronozones of the early Bajocian (Durlet and Thierry 2000; Lathuilière et al. 2003). Although relatively thin in the study area, and locally absent, the crinoid-rich packstones to grainstones of this formation are strongly anisopachous, sometimes reaching up to 30-m thick in Lorraine (*Calcaire de Haut-Pont*, Figure 2B) or in the Burgundy High where crinoid fragments accumulate in large sandwaves or in wide prograding slopes sheltered behind pre-existing reliefs (Durlet 1996; Durlet et al. 1997, 2001; Durlet and Thierry 2000). The *Oolithe à Clypeus angistoporus* is a Member consisting of a relatively thin array of peloidal and bioclastic limestone, subsidiary oolitic, that is sometimes difficult to differentiate from the *Calcaires à Entroques* Formation, and also laterally equivalent northwards to the *Calcaire de*

*Haut-Pont*. The *Calcaires à polypiers inférieurs* Member contains coral buildups from several meters to a maximum of 15 m-thick, with an overall low diversity of corals (Geister and Lathuilière 1991; Lathuilière 2000a, 2000b; Lathuilière and Marchal 2009). The *Oncolithe cannabine* Member, which corresponds to two separate members in Lorraine to the North (Figure 2B), is dominated by argillaceous limestones with thin marly interbeds, containing oncoids encrusted by nubecularid foraminifers, along with bivalves and rarer belemnites and ammonites. The *Calcaires à polypiers supérieurs* Member consists of cross-bedded bioclastic grainstones with bivalves and crinoids and abundant superficial ooids, exhibiting small (less than 1 m) bioherms and biostromes. The *Calcaires à polypiers inférieurs*, the *Oncolithe cannabine* and the *Calcaires à polypiers supérieurs* Members are dated from the *Humphriesianum* chronozone of the early Bajocian (Mangold et al. 1994; Durlet 1996; Lathuilière 2000b). All these units are members of the *Calcaires à polypiers* Formation (Figure 2B).

The *Marnes de Longwy* Formation is characterised by alternating marls and decimetre-thick beds of argillaceous limestones, and varies widely in thickness from a few metres around Neufchâteau to more than 30 m in the EST-433 well (Figure 1C). It is locally very rich in brachiopods and also contains some nubecularian oncoids, bivalves and ammonites indicative of the late Bajocian *Niortense* chronozone (Mangold et al. 1994).

Structurally, the study area is located South and North of the main E–W Vittel fault, bounding the southern Morvan-Vosgian and Northern Saxon-Thuringian Hercynian terranes, which form the substratum of the Mesozoic series in the eastern Paris Basin. This fault line acted as a normal fault in the Hercynian substratum, and also as a strike-slip fault during the Permian, marking the southern boundary of the Sarre-Lorraine coal basin (Elion et al. 2005). It was probablyreactivated during the Jurassic, especially during the Toarcian and Aalenian (Durlet et al. 1997; Jacquin and de Graciansky 1998; Jacquin et al. 1998; Vincent 2001; Thiry-Bastien 2002; de Graciansky and Jacquin 2003; Collin and Courville 2006). The Metz fault, located to the North of the study area, is another NE–SW Hercynian feature separating two terranes, the Saxon-Thuringian block to the South and the Rheno-Hercynian block to the North. It probably impacted sedimentation during the Jurassic, including the Oxfordian (Carpentier et al. 2007). Closer to the URL, within the study area, the main structural feature is the Gondrecourt-le-Château graben (Figure 1C). It is a 30 km-long NNE–SSW structure up to 1500 m thick indicating an atypical sedimentary succession and with vertical displacements of up to several tens of meters (Pagel et al. 2018). Recent U–Pb dating of calcite breccia and veins of the SE Gondrecourt-le-Château graben indicates a two-stage development of the trough, first during the Lutetian and then during the Rupelian (Pagel et al. 2018). The Joinville graben is a very similar structure to the Gondrecourt-le-Château graben, trending in the same direction and of similar, although slightly shorter, dimensions and reflecting the same dynamic evolution (Figure 1C). U–Pb analyses of tension gashes in the Jurassic limestones, including the *Calcaires à polypiers* Formation, yield the same ages from Lutetian to Rupelian (Blaise et al. 2022). A first cluster of ages records the
<figure><img src="image_4.png" /><figcaption>**FIGURE 2** | (A) Log of the EST-433 borehole, with calibration of the main formation/member tops on a regional 2D seismic line. (B) Lithostratigraphy at regional scale along a N–S trend between the EST-433 and Audun-le-Roman 101 boreholes. EST-433 borehole stratigraphy modified and redrawn from Landrein et al. (2013) and Brigaud, Vincent, Carpentier, et al. (2014) and Audun-le-Roman 101 borehole stratigraphy modified and redrawn from Guillocheau et al. (2002).</figcaption></figure>
far-field onset of the Pyrenean compressional phase during
the Lutetian–Bartonian. A second cluster of ages around
33 Ma is related to extensional deformation and the opening
of the European Cenozoic Rift System (Blaise et al. 2022). A
number of NNW–SSE faults also form a kind of relay between
the above-mentioned grabens (Elion et al. 2005). Close to the
study area, the N170° Marne faults (Figure 1C) extend over
about 100 km and show a cumulated vertical displacement of
about 100 m within the Mesozoic sediments.

3 | Methods

**3.1 | Sedimentology and Sequence Stratigraphy**

Facies were classically described in the field where outcrop
sections were built identifying lithology, textures, allochems
and sedimentary structures. These macroscopic descrip-
tions were supplemented by microscopic investigations of 51
carbonate-stained and blue epoxy-impregnated thin sections.
Depositional environments were interpreted through the in-
tegration of macro-and microfacies information on textures,
skeletal and non-skeletal grains, sedimentary structures and
ichnofacies.

The vertical transformations in depositional environments
along the sections analysed illustrate changes in the accom-
modation/sedimentation (A/S) ratio (Homewood et al. 1999).
Terminology and interpretation of the sequence stratigra-
phy follow the concept of transgressive-regressive hemicy-
cles, which together constitute one cycle (T-R; Embry and
Johannessen 1992). Cycles or sequences of A/S ratio variations
are bounded by sequence boundaries (SBs). Within the cycles
or sequences, maximum flooding surfaces (MFS) separate a
transgressive hemicycle (A/S > 1, indicated conventionally
by a blue triangle) from a regressive hemicycle (A/S < 1, red
triangle).

In addition to sedimentological sections and sequence stratig-
raphy correlations, high-resolution photographs of the quar-
ries were acquired in order to build photo-panoramas that
were interpreted using the established sequence stratigraphic
scheme.

**3.2 | Near-Surface Geophysics**

Four near-surface geophysical techniques were used in se-
lected quarries (Attignéville, Beaufremont, Rouvres-la-Chétive,
Sommerécourt; Figure 1C) in order to characterise as far as
possible the ridges under investigation. The techniques were (1)
GPR, (2) electrical resistivity tomography (ERT), (3) frequency-
domain electromagnetics (FDEM) and (4) seismic refraction. All
the instruments used belong to the GEOPS laboratory at Paris-
Saclay University.

GPR is increasingly used in carbonate successions (Dagallier
et al. 2000; Jorry and Biévre 2011) for investigating the physico-
chemical properties of rocks at depths between 0 and 5 m. It re-
cords the travel time in nanoseconds (ns) between emitters and
receivers of electromagnetic pulses. A total of 19 sections were

acquired with a RAMAC Mala using shielded antennas centred
at 250 MHz.

ERT sections were acquired with an IRIS Instruments multi-
electrode system (Syscal Pro), using 48 electrodes at 1 m spacing.
The Wenner protocol was used in order to optimise both acqui-
sition for an investigation depth of about 10 m and the signal/
background noise ratio. Five sections were acquired for which
the apparent resistivity data were inverted with ResIPy (Blanchy
et al. 2020).

Five FDEM profiles were produced with a Slimgram multi-
frequency system (110 Hz–56 kHz; PROMIS, IRIS instruments),
and a distance of 20 m between receiver and transmitter.
Measurements of high frequencies (> 14 kHz) included substan-
tial background noise rendering them useless. FDEM soundings
were inverted separately and integrated to obtain a pseudo-2D
section with the IX1D Interpex software.

Five seismic sections were acquired using DMT-Summit 36
traces equipment. Seismic waves were generated on each line
using a 4-kg hammer striking a steel plate. Shot point spacing
was 4 m and propagation of P waves was recorded with 60-Hz
geophones laid out every 2 m with a sampling frequency of
8 kHz. The data were treated and inverted using PyRefra soft-
ware (Zeyen and Léger 2024).

**3.3 | 3D Seismic Reflection**

The 3D seismic reflection data that first revealed the Bajocian
ridges below the URL were acquired in two phases. A first
phase in 1999 was performed by CGG over 4.35 km², in the im-
mediate vicinity of the URL (Figure 1D). The source was a vi-
broseis source generating a signal in the 19–120 Hz bandwidth.
Recording time was 1.5 s with a sampling step of 1 ms, the inter-
trace distance was 25 m, and the bin size was 12.5 × 12.5 m. The
ridges appeared at the bottom of the Middle Jurassic limestone
as N120° elongated antiforms about 10 m in height, and with an
approximate wavelength of 300 m (Cuilhé and Bruneton 2003;
Dépré et al. 2003; Drouiller and Guillemot 2005; Elion et al. 2005;
Bergerat et al. 2007).

A second, larger 3D cube of 37km² was acquired in 2010 by
CGG, slightly to the North, targeting a zone for a more in-depth
investigation to define the location of the potential repository
site. The active spread was composed of 12 receiver lines with
120 stations on each line. The receiver and source lines were
perpendicular, and their relative spacing was respectively 80 m
and 120 m. The receiver and source point spacing was 20 m. The
source was a vibroseis source generating a sweep signal in the
12–140-Hz frequency range, logarithmically ascending with a
slope of 6-dB per octave and upper and lower cosine tapers of
500 ms long. The bin size was 10 × 10 m.

There is no overlap between the two blocks which are, however,
linked by a 2D seismic line (10EST01; Figure 1D). A spectral
analysis of the seismic data was conducted to check the consis-
tency of the data content between the two cubes. The dominant
frequency varies between 75 and 93 Hz, which corresponds
to a vertical resolution of 13–16 m (assuming that the vertical
resolution is approximately $\lambda/4$, with $\lambda = v/f$), for an average carbonate velocity within the Middle Jurassic limestone of 5100 m/s (Brigaud et al. 2010).

## 3.4 | Well Data

The EST-433 well is a deep vertical well drilled about 2 km to the NW of the main 3D seismic block (Figure 1D). It is a sedimentological reference that has been extensively studied and described (Landrein et al. 2013), in particular for the Middle Jurassic carbonates (Brigaud et al. 2009; Brigaud, Vincent, Durlet, et al. 2014).

Deviated by about 60° within the Bajocian limestone, another well (EST-210) was designed and drilled to target the ridges (figure 1D; Rebours et al. 2004). A vertical seismic profile (VSP) was acquired along the well by SCHLUMBERGER in 2004. This VSP was acquired with sources at the surface (vibrating Merte M22 apparatus) and three receivers within the well, spaced 15 m apart, which were shifted along the well trajectory by 5 m for each shot. The recording time was 1900 ms with a sampling step of 2 ms, for a total of 354 shots and 1105 used traces. Brigaud et al. (2010), Brigaud, Vincent, Carpentier, et al. (2014) and Brigaud, Vincent, Durlet, et al. (2014) published the sedimentological descriptions and a full set of petrophysical analyses for the Middle Jurassic limestone penetrated by this well.

The main seismic horizons were tied to well EST-433 (figure 2A; Landrein et al. 2013; Mari and Yven 2014). Several attributes were used in order to image the N120° ridges as clearly as possible, such as azimuth, dip, dip azimuth and discontinuity. Isochron and isopach maps were constructed and used together with attribute maps.

# 4 | Results

## 4.1 | Depositional Environments: Descriptions and Interpretations

The sedimentological work made it possible to identify six facies that were grouped into two facies associations interpreted in terms of depositional environments. The entire Lower Bajocian succession could be observed in Attignéville quarry (Figure 3A).

The first facies association FA1 comprises three facies (Figure 3B–F; Table 1): (1) black silty claystone to siltstone (facies F1a), (2) alternating packstone or grainstone with marls and oncoidal/coral fragments (facies F1b) and (3) alternating marls and bioclastic packstone or grainstone (facies F1c). Facies F1a displays several kinds of sedimentary structures such as thinning-upward thin layers of siltstone or very fine sandstone and current ripples. Bioturbation is also common with *Planolites* and Skolithos ichnofacies in particular. The grainy facies F1b is composed of oncoids with encrusting *Nubularia* foraminifers in their cortex (Figure 3E), but it also contains bivalves, crinoid ossicles, rare brachiopods, rarer ooids and/or peloids, silt-sized quartz grains and, locally, coral fragments. Grainstone within the alternating marls-limestones of facies F1c is composed of bivalves, echinoderms and silt- to sand-sized quartz grains (Figure 3F). It appears

locally as more or less discontinuous lenses with erosional bases, and with internal Hummocky-Cross-Stratification (HCS; Figure 3C,D). Facies association FA1 is definitely characteristic of upper offshore environments, influenced by episodic, but common, storm-related hydrodynamics, with the individual facies indicating proximal-distal and lateral variations within this environment (Brigaud, Vincent, Carpentier, et al. 2014).

The second facies association FA2 comprises three facies (Figure 3G,H; Table 1) including: (1) coral reefs (facies F2a), (2) bioclastic grainstone to rudstone (facies F2b) and (3) cross-bedded peloidal/oolitic and bioclastic grainstone (facies F2c). The reefal facies of F2a consist of dome-shaped bioherms reaching 15 m in thickness and a maximum lateral extent of more than 100 m (Figure 3A), where scleractinian corals are the main building organisms (Geister and Lathuilière 1991; Lathuilière 2000a, 2000b). Lamellar/tabular (*Isastrea* genus) and platy forms dominate in the coral reefs, but domal and even branching forms are present as well (Figure 3G). Lens-shaped biostromes dominated by platy colonies are also observed. The faunal assemblage also contains crinoids, bivalves, bryozoans and gastropods. Corals are encrusted by micritic microbialites, displaying planar laminae composed of dark micrite with undetermined micro-encrusting organisms (Olivier et al. 2006, 2007) and a few serpulids. The F2b bioclastic grainstone to rudstone contains oncoids, peloids, intraclasts, fragments of corals, echinoderms, bivalves, bryozoans, brachiopods and rare serpulids and gastropods. Locally, this facies exhibits sigmoidal cross-bedding forming dunes up to several decimetres high. This facies is coarse (rudstone) when close to the coral reefs, where it results from the dismantling of reefs, and where it is arranged in prograding bodies fining away from the reefs. The F2c grainstone contains ooids and overall abundant peloids, as well as various bioclasts including fragments of echinoderms, corals, bivalves and bryozoans (Figure 3H). It invariably occurs in large cross beds corresponding to giant subaqueous dunes (Dalrymple and Rhodes 1995). Facies association FA2 is characteristic of relatively high-energy wave-dominated conditions in a shoreface environment located on the platform (Brigaud, Vincent, Carpentier, et al. 2014).

## 4.2 | Regional Stratigraphic Architectures: Outcrop Correlations

A lithostratigraphic correlation of nine selected outcrops was constructed and interpreted using a sequence stratigraphic approach (Figure 4). Above the Early Bajocian *Marnes micacées* Formation, within which is located a 3rd order MFS, the echinoderm-rich bioclastic grainstone to rudstone facies (Facies F2b, *Calcaires à entroques*) and cross-bedded oo-bioclastic grainstone (Facies F2c, *Oolithe à Clypeus angustiporus*) constitute the first significant shallow marine carbonate deposits of the Jurassic in the study area (Brigaud, Vincent, Carpentier, et al. 2014). The top of these shallow-water facies is marked by a hardground surface with borings (Figure 4), where early cements form isopachous fibrous cements typical of a marine environment (Figure 3H). It could be a transgressive surface, forming at the very beginning of the transgressive hemicycle. These are followed, in places, by the development of coral boundstones appearing as meter-scale
<figure><img src="image_5.png" /><figcaption>**FIGURE 3** | Legend on next page.</figcaption></figure>
**FIGURE 3** | Early Bajocian outcrop photographs and thin section photomicrographs illustrating facies variability. (A) View of Attignéville quarry showing the carbonate succession from the *Marnes micacées* Formation (Fm.) to the *Marnes de Longwy* Fm. (B) Silty claystone and marls (facies F1a) and bioclastic packstone or grainstone alternations (facies F1c) in Rouvres-la-Chétive quarry. (C) Marls and bioclastic packstone or grainstone alternations (facies F1c) in Attignéville quarry, showing hummocky-cross-stratification (location of AT1 sample of Figure 3F is marked). (D) Zoom on four superimposed generations of HCS-like structures in the marls and bioclastic packstone or grainstone alternations (facies F1c) in Attignéville quarry. In HCS1, the hummock layer (H) appears above the swaley-fill layer (S). HCS2 is composed of a hummock layer (H) and two swaley-fill layers (S). Note the two opposite directions of migration of the hummock (H) and Skolithos ichnofacies (Ic) in HCS3 structure. (E) Packstone to grainstone (facies F1b, Attignéville quarry, sample AT8), composed of echinoderms (ech), Nubecularia (nu) oncoids (on), ooids (oo) and superficial ooids (s. oo) with late sparite cement. This facies is found in marl-limestone alternations where limestone beds are tens of meters thick. (F) Bioclastic packstone to grainstone alternations (facies F1c, *Calcaires à entroques*, Attignéville quarry, sample AT1), composed of echinoderms (ech), ophiorids (op), crinoids (cri) and detrital quartz (qz) cemented by sparite and altered Fe-rich dolomite. This facies is found in marl-limestone alternations where limestone beds are tens of centimetres thick and contain HCS. (G) A boundstone with recrystallised branching corals (br.) in a muddy and micro-bioclastic matrix (facies F2a, *Calcaire à polypiers inférieur*, borehole EST210, sample E160, measured depth of 1079 m). (H) Bioclastic grainstone (facies F2c, *Oolithes à Clypeus angustiporus*, Rouvres-la-Chétive quarry, sample Rou3), composed of coral fragments, echinoderms (ech), crinoids (cri) and bryozoans (bryo). Note the early cementation (ec) consisting of isopachous bladed cements.

biostromes or bioherms with dominantly platy corals, as at Marnay or Attignéville (Figure 4), or larger coral reefs as in the Sommerécourt, Rouvres-la-Chétive, Bourg-St-Marie and Viterne outcrops (Figure 4). In the Rouvres-la-Chétive quarry, the coral reefs are emplaced on the top of giant oo-bioclastic cross-bedding (*Oolithe à Clypeus angustiporus* Member), a system described in more detail hereafter. Laterally to the coral facies, some interdigitation of open marine echinoderm and oncoidal-rich limestones and marl interbeds (Facies F1b) is also recorded. These facies form the transgressive hemicycle. In between the coral reefs, a continuous record of locally cross-bedded, echinoderm-rich, grain-supported bioclastic facies (facies F2b) is observed such as in the Ozières outcrop (Figure 4). This 12–20 m-thick succession illustrates an overall 3rd order regressive systems tract, which ends with a net maximum regressive surface, with no evidence of subaerial exposure (Figure 4). This surface is characterised in many places by a submarine hardground, which may correspond to a sedimentation hiatus and early cementation at the very early part of the transgressive hemicycle, considered as a transgressive surface (Figure 4).

Above that surface, bioclastic oncoid-rich packstone with marl interbeds (Facies F1b) are deposited across the entire study area, but are isopachous, ranging from 4 m (e.g., Viterne) to about 10 m (e.g., Rouvres-la-Chétive; Figure 4). These deposits record a 3rd order transgressive hemicycle, whose MFS is almost impossible to accurately spot within the succession of oncoid-rich facies. The regressive hemicycle is characterised by another system of echinoderm-rich grainstone (facies F2b) with superficial ooids and local cross-bedding. Meter-scale biostromes with platy corals (facies F2a) are interbedded in these grain-supported facies, capped in most of the quarries by another hardground with no evidence of subaerial exposure. This hardground is also interpreted as a transgressive surface (e.g., Jainvilotte; Figure 4). The latter is overlain by the fine-grained limestones and marls (facies F1a) of the *Marnes de Longwy*, recording a pronounced transgression (e.g., Attignéville; Figure 4).

The marl facies (F1a, F1b and F1c) and part of the coral reef facies (F2a) are transgressive, while the carbonate intervals (F2a, F2b, F2c) represent the regressive portions of the cycles. The cycles end with a hardground, with no indication of emersion,

featuring marine organism borings and early marine cements, indicating that these surfaces are good candidates for placing the SB. These surfaces correspond to a transgressive surface.

## 4.3 | Near-Surface Geophysical Responses of Facies Associations and Facies: A Review From Key Sites

### 4.3.1 | Attignéville Quarry: Facies Association FA1

Facies association FA1 has been investigated in Attignéville quarry where the *Marnes micacées* Formation crops out at the bottom, displaying 2.5 m of facies F1a claystone to siltstone (Figure 5A,B). These deposits are overlain by the 2-m-thick *Calcaires sableux de Haye*, corresponding to bioclastic grainstone (facies F1c). ERT and FDEM profiles were acquired just above the outcrop in the second floor, along the same section (Figure 5).

The ERT section acquired in the quarry shows that the low resistivity values, mainly ranging between 30 and 500 Ohm.m, correspond to facies F1a, whereas slightly higher values from 500 to 1000 Ohm.m correspond to F1c (Figure 5A–C).

An FDEM profile is used to measure resistivities at greater depth (Figure 5D). From 0 m to 5 m depth, resistivity values range between 50 and 400 Ohm.m and correspond to facies F1a and F1c. Even lower resistivity values of 10–70 Ohm.m are measured below, from 5 m to 13 m, and could correspond to facies F1a of the *Marnes micacées* Formation (Figure 5D; Table 1). Below this and down to 20 m, the resistivity of 50–500 Ohm.m could correspond to the *Grés supraliasiques* facies according to regional well data (Guillocheau et al. 2002).

### 4.3.2 | Signature of Coral Reefs and Surrounding Sediments: Facies Association FA2

Facies association FA2 was studied in three different quarries.

#### 4.3.2.1 | Beaufremont Quarry.
Beaufremont quarry displays a coral reef (facies F2a) of a minimum thickness of 9 m and 40–50 m in width (Figure 6A,B). The top
<table><caption>TABLE 1 | Facies descriptions, facies associations and interpretation of depositional environments.</caption><thead><tr><th>Depositional environment</th><th>Facies</th><th>Grains 1- Non-skeletal 2- Skeletal</th><th>Sedimentary and biogenic structures</th><th>Ground Penetrating Radar (GPR)</th><th>Resistivities for Electrical Resistivity Tomography (ERT) and frequency-domain electromagnetics (FDEM); Seismic velocities (S)</th></tr></thead><tbody><tr><td rowspan="3">Upper offshore, Storm-dominated environment (Facies Association FA1)</td><td rowspan="2">Facies F1a—Silty claystone to siltstone</td><td>1. Silt-sized quartz (C)</td><td rowspan="2">Storm-graded layers<br>Bioturbations (Planolites)</td><td rowspan="2"></td><td rowspan="2">ERT: 30–500 Ohm.m<br>FDEM: 10–70 Ohm.m</td></tr><tr><td>2. Bivalves (R)</td></tr><tr><td rowspan="2">Facies F1b—Marls and oncoidal packstone or grainstone alternations</td><td>1. Oncoids (C/F), superficial ooids/ peloids(R), silt-sized quartz (C)</td><td rowspan="2">Storm-graded layers<br>Rare bioturbations</td><td rowspan="2"></td><td rowspan="2">ERT: 200–800 Ohm.m<br>FDEM: 300–1100 Ohm.m<br>S: 1–1.8 km s<sup>-1</sup></td></tr><tr><td>2. Echinoderms (F), bivalves (F), brachiopods (R), bryozoans (R)</td></tr><tr><td rowspan="2">Facies F1c—Marls and bioclastic packstone or grainstone alternations</td><td>1. Silt to sand-sized quartz (C), peloids (C)</td><td rowspan="2">Storm-graded layers<br>Hummocky-cross-stratification</td><td rowspan="2"></td><td rowspan="2">ERT: 300–1000 Ohm.m<br>EM: 50–400 Ohm.m</td></tr><tr><td>2. Echinoderms (F), bivalves (F), brachiopods (R)</td></tr><tr><td rowspan="2">Facies F2a—Coral reef facies</td><td>1. Peloids (R/C), intraclasts (R)</td><td rowspan="2">Dome-shaped bioherms or lens-shaped biostromes</td><td rowspan="2"></td><td rowspan="2">ERT: 600–2000 Ohm.m<br>EM: 400–1000 Ohm.m<br>S: 2–3 km s<sup>-1</sup> at Beaufremont and Rouvres-la-Chétive<br>4–5 km s<sup>-1</sup> at Sommerécourt</td></tr><tr><td>2. Platy colonies, predominantly Isastrea genus (A), branching colonies (branching phaceloid) (C), crinoids (C), microbial encrustations (C), bivalves (R), gastropods (R), bryozoans (R)</td></tr><tr><td rowspan="2">High-energy wave-dominated conditions in a shoreface environments (Facies Association FA2)</td><td>1. Peloids (A), oncoids (R/C), intraclasts (R)</td><td rowspan="2">Sigmoidal cross-bedding</td><td rowspan="2"></td><td rowspan="2">ERT: 500–1200 Ohm.m<br>S: 3–3.5 km s<sup>-1</sup></td></tr><tr><td>2. Bivalves (A), coral fragments (A), echinoderms (C), bryozoans (C), brachiopods (C), gastropods (R), serpulids (R)</td></tr><tr><td rowspan="2">Facies F2b—Bioclastic grainstone to rudstone</td><td>1. Superficial ooids (C), peloids (F)</td><td rowspan="2">Large-scale cross-bedding</td><td rowspan="2"></td><td rowspan="2">S: 4–5 km s<sup>-1</sup></td></tr><tr><td>2. Echinoderms (C), bivalves (A), bryozoans (C), coral fragments (F)</td></tr></tbody></table>

Note: R = Rare < 10%; C = Common 10%–20%; F = Frequent 20%–50%; A = Abundant > 50%. Color code by facies used in the figures; Facies F1a - grey, F1b - green, F1c - light green, F2a - orange, F2b - yellow, and F2c - pink. In Ground Penetrating Radar (GPR) column, a typical radargram for three facies is shown, with a black-to-white gradient indicating signal intensity.
<figure><img src="image_6.png" /><figcaption>**FIGURE 4** | Sequence stratigraphy correlation between the key outcrop sections.</figcaption></figure>

of the coral reef is a zone of intercalation between true coral facies and grain-supported facies F2b of the *Oncolithe cannabine* (Figure 6A,B), illustrating a progressive demise of the coral reef. Reworked coral rubble is observed in the onlap zone (F2b; white arrows—Figure 6A). Alternating marl-limestone of the *Oncolithe cannabine* Member (facies F1b) onlaps the coral reef and ultimately drapes it continuously.

On the GPR profile 6 acquired at the top of the coral reef, the *Oncolithe cannabine* (facies F2b) is characterised by chaotic reflectors with internal diffraction hyperbolas on the GPR sections, whereas the surrounding and overlying marl-limestone alternations (facies F1b) are imaged as very continuous parallel reflectors (Figure 6C; Table 1). The onlaps observed in the quarry are clearly revealed by the GPR (Figure 6C; white arrows; Table 1).

ERT Section 2, also acquired at the top of the coral reef, shows very high-resistivity values of 1000–2000 Ohm.m in the coral reef facies (F2a), and lower values of 200–400 Ohm.m in the surrounding F1b facies (Figure 6D; Table 1). The resistivity of the grainy facies F2b, including material dismantled from the coral reef, ranges from 500 to 1200 Ohm.m.

Another coral reef is present in this quarry, but only 2m of it crop out (Figure 7A,B). Two ERT sections (profiles 3 & 4 Figures 6E and 7C) were realised on the quarry floor, showing that the base of the coral reef is located approximately 5m below this floor and is characterised by high-resistivity values up to and around 1000 Ohm.m (Figures 6C and 7C). The surrounding relatively resistive succession most probably corresponds to the F1b facies (Figure 7C). The uppermost 2–3m

of the sections display very low resistivity values below 100 Ohm.m, probably corresponding to a bench floor (Figure 7C).

In this site, the FDEM section on the quarry floor does not allow us to distinguish between the coral reef facies F2a and the surrounding marl-limestone alternations of facies F1b, both of which display a wide range of overlapping resistivity, from 300 to 1000 Ohm.m (Figure 7D; Table 1). The uppermost part of the FDEM section also exhibits very low values, below 100 Ohm.m, confirming the embankment layer. On the contrary, the GPR section allows us to clearly distinguish the coral buildup from the surrounding interbedded marls and limestones (Figure 7E).

A seismic refraction profile was acquired on the floor of this quarry, between the two faces illustrated in the panoramas of Figures 6 and 7 (Figure 8A,B). The uppermost meter of the profile, showing low velocities (< 1000 m/s), confirms a thin bench floor. Below, three distinct patches of higher velocities (2000–3000 m/s) definitely illustrate some coral buildups (Facies F2a; Figure 8A,B). The largest high-velocity patch on the SSE of the seismic profile (A in Figure 8B) corresponds to the rooting of the coral buildup illustrated in Figure 6, whereas a smaller patch on the NNW side of the profile (C in Figure 8B) likely corresponds to the base of the small coral buildup illustrated in Figure 7. The third patch (B in Figure 8B) is not visible on the quarry faces. The base of the three coral buildups is confirmed to be 4–5m below the quarry floor (Figure 8B). A 20m-wide trough is imaged on the profile between the high-velocity patches A and B, and is filled with a lower-velocity material (1000–1500 m/s; Figure 8B), probably corresponding to facies F1b of the *Oncolithe cannabine*.
<figure><img src="image_7.png" /><figcaption>**FIGURE 5** | (A) Photograph of the lower part of the *Marnes micacées* Formation, *Calcaires Sableux de Haye* and *Calcaires à entroques* Members of the *Calcaires à polypiers* Formation, Attignéville quarry. (B) Sketch from photo A showing the facies and lithostratigraphic units. (C) Electrical resistivity tomography profile (ERT profile1) with facies interpretations (see discussion in the text). (D) FDEM profile with facies interpretations (see discussion in the text).</figcaption></figure>

### 4.3.2.2 | Rouvres-la-Chétive Quarry.
In this quarry, a first face (NE-SW) displays a lenticular-shaped coral buildup that developed above a sedimentary body composed of oobioclastic facies F2c (*Oolithe à Clypeus angustiporus* Member), displaying large-scale cross-bedding with inclined sets dipping steeply to the NE with a N30° direction (Figure 9A-C). In front of the coral reef, a trough some 10 m deep with an erosional base is filled with alternating fine limestones and marls of facies F1b, also showing beds of bioclastic sediments containing components derived from the dismantling of the coral reef and downlapping on the basal surface (Figure 9A-D). The large-scale cross beds are capped by a hardground surface with annelid perforations (Figure 9E) and early cements lithified the last decimetres below the top of the *Oolithe à Clypeus angustiporus* Member (Figure 9C-E). The coral buildup is progressively onlapped by the same alternations of the *Oncolithe cannabine* (Figure 9A-D). On an NW-SE face of the quarry, the same coral reef also appears on top

of prograding sets which dip N30° at a lower angle (Figure 9F,G). The sets here show a complex internal organisation with cross-bedding and erosional surfaces between and within sets. On this face, the coral buildup does not evolve laterally towards other facies but pinches out downward on the slope of the topmost set (Figure 9F,G). It is also onlapped and finally draped by the fine planar stratified sediments of the *Oncolithe cannabine*. This succession is overlain by the cross-bedded grain-supported facies of the *Calcaires à polypiers supérieurs* Member.

Two seismic refraction profiles were acquired in the quarry, behind the SW-NE face illustrated in Figure 9A,B (Figure 10A). Profile S3 oriented SW-NE shows a clear boundary between relatively high-velocity facies (2000–3000 m/s) and the overly-ing lower-velocity facies (<1800 m/s) at around 10–12 m depth (Figure 10B). This boundary plunges significantly to the NE between 45 m and 55 m along the profile, which corresponds to
<figure><img src="image_8.png" /><figcaption>**FIGURE 6** | (A) Photograph of the lower part of the *Calcaires à polypiers* Formation in Beaufremont quarry, showing a coral reef. All locations of near-surface resistivity and GPR acquisition profiles are indicated. (B) Sketch from photo A with interpreted facies. (C) GPR profile 6 acquired on the top floor of the quarry with interpreted facies; the chaotic reflectors illustrate the coral reef, with onlaps and pinch-outs on its edges (arrows). (D) Electrical resistivity tomography profile (ERT profile 2) acquired on the top floor of the quarry (see discussion in the text). (E) Electrical resistivity tomography profile (ERT profile 3) acquired on the ground floor of the quarry, in front of the main faces of photo A (see discussion in the text).</figcaption></figure>

the dip of the large cross beds observed on the SW-NE quarry face (Figure 9C). The velocity contrast boundary consequently corresponds to the contact between facies F2c of the cross beds and the overlying facies F1b of the *Oncolithe cannabine*. Profile S4 reveals the same velocity contrast boundary, but gently dipping from N to S from 4-m deep to about 20-m deep (Figure 10C). This could be interpreted as the image of the stoss side of a giant dune, the cross beds of which are observed on the quarry faces. Higher velocity patches (2500-3000 m/s) are visible, deforming the velocity boundary and forming positive mounded 3-4 m high reliefs (A and B in Figure 10C). They could likely be interpreted as coral reefs (facies F2a) developing on the cross beds, as they are observed on the quarry faces (Figure 9).

4.3.2.3 | Sommerécourt Quarry. This quarry reveals a main coral reef some 10-m high with many corals in place, extending over more than 100-m along an SSW-NNE oriented face (Figure 11A). On the flanks of the major coral reef (facies F2a), as well as laterally, smaller meter-scale buildups are observed (Figure 11C,D). The coral reefs are separated by troughs filled with bioclastic facies F2b (Figure 11C-F). The coral reef is built from platy and branching corals, with subsidiary encrusting micritic microbialites which include microfabric composed of planar micritic laminae (sensu Vennin et al. 2021) and serpulids. A smaller, less accessible reefal buildup is also visible on the NNE extreme part of the face (Figure 11A-D). Another part of the coral reef is observed in the SE part of the quarry, in
<figure><img src="image_9.png" /></figure>

**FIGURE 7** | (A) Photograph of the *Calcaires à polypiers inférieurs*, *Oncolithe cannabine* and *Calcaires à polypiers supérieurs* members in Beaufremont quarry, showing the top of a coral reef at the base of another face than Figure 6. (B) Sketch from photo A with facies interpretation. (C) Electrical resistivity tomography profile 4 acquired on the ground floor in front of the face (see discussion in the text). (D) FDEM profile 2 acquired on the ground floor in front of the face (see discussion in the text). (E) GPR profile 14 acquired on the ground floor in front of the face; note the chaotic reflectors of the coral reef (orange) and the parallel continuous reflectors of the facies infilling the topography adjacent to the coral reef (green).

another cliff (Figure 11A,B). The line between the occurrences of the coral reef high along the two cliffs is oriented N120°, giving a possible direction of the coral ridge (Figure 11A,B). A second view of the SSW-NNE face allows us to clearly observe the coral ridge on the first cliff while, on the second cliff, triangular fractures appear at the apex of the coral ridge, highlighting its N120° orientation.

A GPR section was acquired on the second floor of the quarry, parallel to the main stratigraphic photopanorama (Figure 11C,D). The GPR image is very similar to the interpreted stratigraphic panorama, with two positive mounded structures corresponding to the observed coral buildups (facies F2a), characterised by

chaotic reflectors on either side of the section and showing a set of strong high-amplitude reflectors at their top (Figure 12A,B). They are separated by an inter-reefal trough with moderate amplitude parallel reflectors, some onlapping the coral reefs. The base of the trough seems to show slightly higher amplitude reflectors than the upper part, which respectively correspond to facies F1b and F2b filling the trough, observed in the panorama (Figure 11C,D).

The FDEM section, conducted on the quarry floor and investigating the 20-m depth below, reveals three different zones, here named boxes (Figure 12C). Boxes 1 and 1', from 2 to 8-m deep, display the lowest resistivities between 20
<figure><img src="image_10.png" /><figcaption>**FIGURE 8** | (A) Location on the Beaufremont quarry satellite photo of the three electrical resistivity tomography (ERT) profiles ERT2, ERT3, ERT4, the frequency-domain electromagnetic (FDEM) profile FDEM2, the two ground penetrating radar (GPR) profiles GRP6, GPR14, and the seismic profile S1. The two panoramas illustrated in Figures 6 and 7 are also indicated. (B) NW–SE seismic profile S1 acquired on the ground floor of Beaufremont quarry revealing three patches of high velocity, A, B and C, between 1 m and 6 m below the surface; note the trough of low velocity between patches A and B.</figcaption></figure>

and 80 Ohm.m. Boxes 2 and 2' display intermediate resistivities ranging from 100 to 200 Ohm.m. Finally, boxes 3 and 3' have the highest resistivities reaching up to 1000 Ohm.m, with the highest values being at the base of the boxes. These latter zones could be interpreted as tight oo-bioclastic grain-supported and cross-stratified facies F2c, similar to that observed in Rouvres-la-Chétive quarry. Thus, these two patches could correspond to large cross beds possibly serving as a substratum for the coral reefs.

The seismic section (Figure 12D), investigating 20-m depth below the quarry floor, identifies a thin 0–2-m-thick uppermost layer with velocities below 1.5 km/s, probably corresponding to either a bench floor and underlain by some 5 m of porous, altered and fractured limestone probably of facies F2b, with velocities between 3 and 3.5 km/s (Figure 12D). Velocities between 4 and

5 km/s most likely correspond either to reefal facies F2a or to tightly cemented oo-bioclastic facies F2c of the *Oolithe à Clypeus angustiporus* Member. These velocities are found at shallower depths (below 4–6 m) near the edges of the transect and at greater depths (more than 8–10 m) in the middle of the section, illustrating an undulation of up to 4 m.

The ERT section reaches a depth of 8 m below the quarry floor (Figure 12E). The first meter displays low resistivity values between 100 and 300 Ohm.m, probably corresponding to a bench floor, as suggested by the seismic profile (Box 1). Two patches of higher resistivity occur below on either side of the section, with values ranging between 600 and 1000 Ohm.m that could correspond to reefal facies F2a (Boxes 2 and 2'). Between the high-resistivity patches, a lower resistivity zone is visible, with values ranging between 300 and 400 Ohm.m (Figure 12E, Box 3). This
<figure><img src="image_11.png" /><figcaption>**FIGURE 9** | (A) NE–SW panoramic view of Rouvres-la-Chétive quarry. (B) Sketch from photo A with facies interpretation. (C) Focus on the SW corner of the quarry showing a coral reef installed on top of large cross beds of the *Oolithe à Clypeus angustiporus*; the crest of the cross beds is oriented N120° and the main dip direction of the sets is N30°. (D, E) Zoom on the same corner showing the slope of the sets of the large cross beds capped by a hardground with annelid perforation (perf.) in thin section photo (E) the facies of the *Oncolithe cannabine* fill the topography in front of the coral reef and cross beds. (F) NW–SE panoramic view of the quarry showing lower angle slopes of the sets of cross beds, with a coral reef installed on top. (G) Sketch from photo F with facies interpretation.</figcaption></figure>

zone could correspond to bioclastic grainstone to rudstone, with
interbedded coral debris (F2b).

4.4 | Deep Geophysics and Well Data

This section describes some of the findings from the 3D seis-
mic reflection and well data acquired near the URL of Bure. The

dip attribute extracted at the top of the Middle Jurassic does not
display any specific features (Figure 13A), indicating that the
interval is not affected by mounded features at this scale of ob-
servation. The same attribute extracted at the top of the *Calcaire
à polypiers* Formation reveals a number of elongated kilometre-
scale N120° ridges (Figure 13B). Isochron maps for the same for-
mation also show N120° elongated but more rounded ‘thicker’
structures (Figure 13D). The azimuthal dip attribute map of
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /><figcaption>**FIGURE 10** | (A) Location on the Rouvres-la-Chétive quarry satellite photo of the seismic profiles S3 and S4 and of the two panoramas, illustrated in Figure 9 for comparison. (B) SW-NE seismic profile acquired behind a face of Rouvres-la-Chétive quarry and showing a boundary of velocity contrast, first slightly dipping from SW to NE and plunging towards the NE between 45 m and 55 m (dotted line). (C) N-S seismic profile acquired slightly farther behind the same face of Rouvres-la-Chétive quarry, also revealing the same boundary of velocity contrast although in a more complex arrangement (see dotted line and the text for explanation).</figcaption></figure>

the top of the *Calcaire à polypiers* Formation indicates that, for the eastward dipping class, the dip is greater towards the NE (Figure 13C).

The Est-210 deviated well was designed to cut through one of the 3D seismic-imaged N120° lineaments in the laboratory zone (Figures 13 and 14A,B). A full set of logs was acquired along this well, supplemented with a VSP (Figure 14A) revealing a mounded structure within the *Calcaire à polypiers* Formation below the well trace (Figure 14C). Below the *Marnes de Longwy* Formation, the cores clearly cut through reefal limestone (Figure 14D), described by Brigaud et al. (2010). In summary, from base to top, between 770 m and 760 m (TVD), boundstones with dominantly platy and subsidiary phaceloid branching corals are observed (facies F2a). From 760 m to 750 m, cross-bedded

peloidal and bioclastic packstones to grainstones with fragments of echinoderms, corals and bivalves (facies F2b) are present. Above, a 2 m-thick interval of marls with a few limestone beds is recorded; the limestone beds are wackestones to packstones with nubecularian oncoids and fragments of echinoderms, corals and bivalves (facies F1b). Between 748 m and 739 m, another interval of facies F2b is observed, encompassing a meter-thick zone of small bioherms/biostromes with platy corals.

This cored succession, together with the analogous outcropping levels presented previously, demonstrates that the positive mounded features observed both on the VSP and on seismic reflection lines (Figure 14B), and which correspond to the N120° lineaments of the 3D-seismic image, are elongated coral reef ridges probably formed by stacked patch reefs.
<figure><img src="image_15.png" /><figcaption>**FIGURE 11** | Legend on next page.</figcaption></figure>
**FIGURE 11** | (A) Location on the Sommerécourt quarry satellite photo of electrical resistivity tomography (ERT) profile ERT5, the frequency-domain electromagnetic (FDEM) profile FDEM5, the ground penetrating radar (GPR) profile GRP2 and seismic profile S2, and the panoramas. (B) 3D block diagram of Sommerécourt quarry displaying the possible N120° orientation of the coral ridge. (C, D) Large-scale panoramic view and facies-interpreted sketch of Sommerécourt quarry showing large coral reefs; troughs between buildups are filled with well-bedded facies, including some products of dismantling of coral buildups, see eye in A for location of the observation point. (E, F) Second panoramic view and facies-interpreted sketch of Sommerécourt quarry, from another observation point (see eye in A for location of the observation point). The coral reefs high is oriented N120°.

## 5 | Discussion

Through some direct field observation and constrained indirect near-surface geophysical interpretations, it is clear now that the coral reefs forming the ridges developed on giant subaqueous dunes of the *Oolithe à Clypeus angustiporus* Member, with a N30° progradation and with a crest orientation of N120° (Figure 15A), guiding the development of the seismic-scale ridges (Figure 13D). Despite this overall constant direction, the structures appear more interfingered than initially thought based on the 1999 URL 3D seismic block alone (Figure 15B,C). The good quality of the 2010 larger seismic block still reveals elongated ridges, but of a more massive aspect (Figure 15B,C), corresponding to positive mounded features located in the lower part of the *Calcaires à polypiers* Formation (Figure 15A). This is consistent with the outcrops, some 30 km away (Figure 1C), where all the larger-scale reefs are located in the *Calcaires à polypiers inférieurs* Member. The coral reefs mainly form ridges, with the crests oriented at N120°, and these crests are separated by distances ranging from about 300 m to 1 km. They extend over a kilometre-long stretch, reaching up to 6 km (Figure 13B-D). The height of the coral reefs can reach up to 15 m. They are separated by troughs, which are more or less filled simultaneously with the growth of the corals or during the final stages of coral reef development. Importantly, the 2010 seismic block also shows that none of these structures is traceable within the underlying Liassic or Rhaetian series (Figure 16), the basal contact of the *Calcaires à polypiers* Formation corresponding to a net plane reflector (Figure 16), allowing us to rule out any tectonic rooting. This goes against reefs aligned above synsedimentary faults, as described for example 70 km further South, in the Marne Spring quarries (Durlet et al. 1997). These near-surface techniques on outcrops are likely to be valuable complements for studies of carbonate coral reefs in geothermal reservoirs, such as those of the Upper Jurassic in Germany (Wolpert et al. 2022), or carbonate coral reefs observed on outcrops (Dagallier et al. 2000; Matyszkiewicz et al. 2006).

### 5.1 | Origin of the Giant Dunes

In Rouvres-la-Chétive quarry, the main dip direction of the sets of giant subaqueous dunes is N30°, although it is not really possible to determine whether these are 2D or 3D dunes (Figure 9), and the main direction of the crest is around N120°. The dune height is estimated to be between 8 m and 10 m.

It is tempting to compare these Bajocian dunes morphologically with some sedimentary features of the Tongue of the Ocean shoal complex in the Bahamas (Harris et al. 2011; Rankey and

Reeder 2012; Purkis and Harris 2017). The latter complex consists of a vast number of tidal dunes of variable geometries, including giant longitudinal and interfingered systems several kilometres long (Figure 17A,B). In the southern and western parts of this modern analogue, the average dune length is around 10 km, and their average width several hundreds of meters, and height estimated at 6 m, for bathymetry ranging from 2 m to 8 m (Figure 17B; Harris et al. 2011; Purkis and Harris 2017). In these places, dunes are separated by deeper troughs of about 1-km wide and up to 8-m deep (Harris et al. 2011; Rankey and Reeder 2012; Purkis and Harris 2017). The bathymetry of the dune crest is about 2-m deep (Harris et al. 2011; Purkis and Harris 2017). Such dimensions are similar to the estimated sizes of the giant Bajocian structures visible in the 3D seismic image (Figure 16B,C) and on the outcrops.

Modern and ancient giant dunes and tidal channels are often interpreted as resulting from tidal currents (Malikides et al. 1988; Fenster et al. 1990; Anastas et al. 1997; Bastos et al. 2003; Todd and Shaw 2009; Harris et al. 2011; Purkis and Harris 2017). In the geological record, these dunes and tidal channels would be mostly recorded in low-stand or early transgressive periods due to the local constriction of tidal currents (Reynaud and Dalrymple 2012), the slow increase in accommodation space preserving them from systematic erosion. The giant dunes observed in the Lower Bajocian succession under study were deposited as part of a regressive systems tract. Detailed regional paleogeographic reconstructions (Thiry-Bastien 2002) show that, during the early Bajocian, the study area overall corresponded to a shallow-water area of a large carbonate platform, with open ocean zones situated to the South (Jura, Bresse graben), to the West (Paris area), and for the closest one to the NE (Souabe basin in Germany). It is likely that such a geographically restricted shallow-water zone was subject to strong tidal currents bridging the open zones, which may have given rise to the large subaqueous dunes and tidal channels.

### 5.2 | Forcing Parameters on the Development and Evolution of the Bajocian Coral Reefs

In the study area, the Aalenian is missing or limited to lenticular deposits, which remain undocumented because of poor outcrop conditions. This is probably due to a second order regressive hemicycle recorded in numerous European basins during the Aalenian (Jacquin et al. 1998). In the eastern Paris Basin and in Burgundy, accommodation was very low during the Aalenian and the earliest Bajocian, preventing significant sedimentation and/or sediment preservation (Durlet and Thierry 2000). Continuous sedimentation resumed from *Pseudoradiosa* to *Discites* chronozones, firstly with clay-rich sediments of the
<figure><img src="image_16.png" /><figcaption>**FIGURE 12** | Legend on next page.</figcaption></figure>
**FIGURE 12** | (A) Facies-interpreted sketch of photo C of Figure 11, Sommerécourt quarry. (B) GPR profile 2 acquired on the intermediate floor of the quarry (aligned blocks of rocks in photo A); note the clear imaging of coral reefs with chaotic reflectors (orange), and the more parallel reflectors of the sediments filling the inter-reefal trough (yellow and green). (C) FDEM profile 5 acquired on the ground floor of the quarry (see discussion in the text). (D) Seismic profile 2 acquired on the ground floor of the quarry; velocities increase with depth, but an undulation of the top of the high-velocity zone (> 4 km/s) is visible (see discussion in the text). (E) Electrical resistivity tomography profile 5 acquired on the ground floor of the quarry (see discussion in the text). (F) Facies-interpreted sketch of photo A, extended into shallow subsurface by an interpretation of near-surface geophysical results (0 m is the ground floor of the quarry).

<figure><img src="image_17.png" /></figure>

**FIGURE 13** | (A) Dip attribute maps from 3D seismic image extracted on Top Dogger. (B) Dip attribute maps from 3D seismic image revealing some elongated N120° structures on the top of the *Calcaires à polypiers* Formation. (C) Azimuthal dip attribute map at the top of the *Calcaires à polypiers* Formation; the main dip direction of the N120° structures is NE. (D) Isochron map of the *Calcaires à polypiers* Formation; the N120° structures correspond to thicker time intervals.
<figure><img src="image_18.png" /><figcaption>**FIGURE 14** | (A) Well logs of the EST-210 borehole with a vertical seismic profile (VSP) illustrating positive mounded features in the *Calcaires à polypiers* Formation. (B) Crossline 255 (2010 3D seismic) and trajectories of EST-210, EST-211 and EST-212 boreholes; the red interval is the cored interval in EST-210 (section in D). (C) Close-up of the VSP of the EST-210 well showing that the positive mounded features are situated in the lower part of the *Calcaires à polypiers* Formation. (D) Sedimentological log of the cored section of the EST-210 borehole with sonic log and P-wave velocities on core plug samples (Brigaud et al. 2010).</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**FIGURE 15** | (A) Sketch of the early Bajocian platform sedimentary architectures, built from the direct and indirect (near-surface geophysics) observations in various outcrops; two main seismic reflectors correspond to the base and top of the *Calcaires à polypiers* Formation, but internal architectures are below the resolution. (B) Isochron map of the *Calcaires à polypiers* Formation, with contouring of the N120° elongated reefal ridges. (C) Interpretation of the isochron map in terms of facies; reefal ridges are more massive and interfingered than previously thought on the basis of the small 1999 URL bloc to the S.</figcaption></figure>

*Marnes micacées* Formation, probably under mesotrophic conditions and temperate surface seawater temperatures of 15°C-20°C (Dera et al. 2011). During the *Laeviuscula* and *Propinquans* chronozones, various limestone units (*Calcaires sableux de Haye*, *Calcaires de Ottange*, *Calcaires de Haut-Pont*, *Calcaires à entroques*) usually rich in crinoids, bryozoans and pelecypods were deposited, indicating slightly warmer waters but still nutrient-loaded and with few distal detrital inputs. During the *Laeviuscula* or the early *Humphriesianum* chronozones, more arid and warmer conditions promoted deposition of pure bioclastic and oobioclastic grainstones (*Oolithe à Clypeus angustiporus* Member), sometimes structured with large cross beds corresponding to the N120° subaqueous giant dunes (Figure 18A). These dunes form during a regressive hemicycle.

Later, in the early Bajocian *Humphriesianum* chronozone, installation of durable oligotrophic conditions and higher surface temperatures reaching 20°C-30°C (Brigaud et al. 2009; Dera et al. 2011) favoured the growth of coral reefs (Piuz 2008).

Coral reefs first nucleated on the crests of the giant dunes during the beginning of the transgressive hemicycle (Figure 18B) and then developed to form high relief buildups of up to 15 m (Figure 18C), while the inter-reefal troughs were filled with bioclastic sediments including material from the dismantling of coral buildups (*Calcaires à polypiers inférieurs* Member, Figure 18C). When the available space was reduced, reefs extended laterally (progradation), which is observed in quarries and partly explains the massive interfingered aspect of the
<figure><img src="image_20.png" /></figure>

**FIGURE 16** | (A) Crossline 270 (2010 3D seismic) illustrating the undulating aspect of the top reflector of the *Calcaires à polypiers* Formation. (B) Zoom on the Bajocian/Bathonian interval showing that the positive mounded features responsible for the undulation of the top reflector are situated at the base of the formation (coral reefs). (C) Same profile with interpreted coral reefs; the upper half of the *Calcaires à polypiers* Formation exhibits continuous parallel reflectors, levelling the topography.
<figure><img src="image_21.png" /><figcaption>**FIGURE 17** | (A) Satellite view of part of the Great Bahama Bank showing a large shoal complex with giant tidal carbonate sand dunes (image Landsat 2014). (B) Zoom on the eastern part of the shoal complex showing longitudinal dunes and tidal channels on a bathymetry map, map extracted from Purkis and Harris (2017). (C) Dip attribute map from 3D seismic image at the top of the *Calcaires à polypiers* Formation, rescaled as in B (see the text for comparison and discussion).</figcaption></figure>

structures (Figure 18C). The paleotopographic lows were simultaneously filled with bioclastic sediments during the regressive hemicycle (Figure 18D). In the absence of structural roots, in a tectonically quiet period of the intracratonic Paris Basin, the sedimentary-inherited topographies (i.e., subaqueous dunes) played a key role in the emplacement and development of these seismic-scale coral systems.

Another factor favoured the coral larvae settlement and the progressive coral growth on dune crests in the absence of tectonic rooting or synsedimentary deformation. In outcrops and cored wells, the top of the *Oolithe à Clypeus angustiporus* facies usually corresponds to a hardground (Figures 4 and 9) with pelecypods and worm borings that cross-cut isopachous calcite cements. Evidence of early calcite development, just below the coral reefs, is present at Marnay/Marne Poulangy, Rouvre-la-Chétive, Attignéville and

Viterne (Figure 4). These marine cements precipitated during the hiatus linked to the discontinuity, as documented further South below many other perforated surfaces in the Lower Bajocian limestones (Durlet et al. 1992; Loreau and Durlet 2000). These early cements lithified the last decimetres below the top of the dunes. Without such early lithification, the heavy coral buildups would have likely progressively deformed the underlying calcarenites and would have sunk into them. As for the chemical and biochemical production of relatively pure oo-bioclastic facies (facies F2b and F2c) constituting the dunes, an arid climate with warm oligotrophic seawater probably promoted the early calcite cementation, among other factors. Submarine hardgrounds are commonly stable substrates for nucleation and growth of coral buildups or reefs, especially when they cap prominent submarine reliefs where higher photic levels favour photosynthesis (for hermatypic corals) and where currents bring nutrients to the colonies.
<figure><img src="image_22.png" /><figcaption>**FIGURE 18** | Legend on next page.</figcaption></figure>
**FIGURE 18** | Conceptual sketch of development and evolution of the Bajocian carbonate platform, illustrating the variations of some key controlling factors on the carbonate factory(ies). Topographic undulations were created in the early stages of the platform with migrating giant subaqueous dunes that served as nucleation points for the growth of coral reefs; the undulations were levelled progressively as the platform evolved. P: Packstone. G: Grainstone.

The topography was levelled slightly later with oncoid-rich fine limestone and marl alternations (Oncolithe cannabine Member) onlapping and draping the coral reefs. This facies probably indicates a return to mesotrophic conditions that were less favourable for scleractinian corals. Later, as evidenced by the regional and worldwide Jurassic series, it was only from the Late Oxfordian that ecosystems were to allow the development of new big coral reefs (Carpentier et al. 2007; Kiessling 2009).

**6 | Conclusions**

An integrated study associating classical field sedimentology and stratigraphy, near-surface geophysics, including GPR, resistivity (ERT and FDEM) and seismic refraction, has been developed to investigate the Bajocian limestones of the eastern Paris Basin. In order to determine the nature, characteristics and origin of enigmatic N120° ridges identified at a present-day depth of about 750 m from 3D seismic reflection by the Andra (French National Agency for Radioactive Waste Management), a comparison between new data and available deep geophysics and well data has been performed.

Classical facies analysis and a regional sequence stratigraphy interpretation have been performed on time-equivalent outcrop sections (quarries), demonstrating that elongated mounds of corals with hermatypic scleractinian corals developed during the early Bajocian (*Humphriasianum* chronozone) in warm oligotrophic and shallow seawater. Some of the coral reefs form buildups reaching 15 m in height and hundreds of meters in width. They are elongated in the main N120° direction, with, however, a significant spread of measurements illustrating the complexity of the interfingered structures observed in 3D seismic images. The coral reefs mainly form ridges, with the crests oriented at N120°. Coral ridge crests are separated by distances ranging from about 300 m to 1 km. They extend over a kilometre-long stretch, reaching up to 6 km. The height of the coral reefs can reach up to 15 m. The coral reefs were progressively onlapped and draped by alternating oncoid-rich marls-limestones that probably resulted from a change from oligotrophic to mesotrophic conditions possibly associated with a relative sea-level rise, corresponding to a third order transgressive systems tract. Near-surface geophysical data supplement the limited direct observations on quarry faces, with GPR especially providing excellent quality images of the internal reefal structures and of the infill of the inter-coral reef troughs.

In several outcrops, but especially in Rouvres-la-Chétive quarry, it is observed that coral reefs nucleated on large-scale cross beds of peloidal and bioclastic grainstones, corresponding to giant subaqueous dunes. Before coral reef development, the tops of these dunes were lithified during early diagenesis by isopachous calcite cement, forming submarine hardgrounds and thus stable substrates for buildup growth. The main dip direction of the dune sets is N30°, with a crest direction of around N120°.

Near-surface geophysics supplements imaging of the roots of several coral reefs that only crop out in part.

This work demonstrates the critical importance of outcrop analogue studies for resolving subsurface problems, and it also shows how near-surface geophysical methods can supplement classical field geology investigations. From a more applied perspective, the position and geometry of the dune or coral reef complexes also shed new light on the constraints to be applied to static and dynamic modelling of the Bajocian aquifer. These near-surface techniques on outcrops are likely to be valuable complements for studies of coral reefs in geothermal reservoirs.

**Acknowledgements**

This work is a contribution to project No 2011-XV 'Origin of N120° structures detected within the 3D subsurface seismic cube: insight from the geophysical and sedimentological study of outcrops of the Middle Jurassic Limestones' of the CNRS Programme sur l'Aval du Cycle et l'Energie Nucléaire (PACEN), and to collaborative project No N87890 'Surface geophysical study of the Bajocian outcrops of the eastern Paris Basin: Contribution to the understanding of structures detected in 3D seismics' between the University Paris-Saclay and the Andra (French National Agency for Radioactive Waste Management). This study has benefited greatly from CNRS and Andra funding. We thank Professor Maurice Pagel for promoting this work on the geology of this area. The authors would like to thank Philippe Courville (Univ. Rennes) for determinations of brachiopod fauna, Thierry Albertin, Christophe Pouzot (BSc students) and Frédéric Hirn (MSc student). They are grateful to Philippe Blanc (Lithologie Bourgogne) for the high-quality thin sections. Bernard Lathuilière provided valuable and constructive feedback that improved an earlier version of this manuscript. We thank one anonymous reviewer and Luigi Spalluto for their helpful comments.

**Conflicts of Interest**

The authors declare no conflicts of interest.

**Data Availability Statement**

The data that support the findings in this study are openly available in Recherche Data Gouv repository at https://doi.org/10.57745/TMB0HR.

**Peer Review**

The peer review history for this article is available at https://www.webofscience.com/api/gateway/wos/peer-review/10.1111/bre.70034.

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