Université Claude Bernard Lyon 1

PhD offer in carbonate sedimentology and diagenesis : “PREdicting the lateral evolution of sedimentary DIScontinuities”

Location and context of the PhD project:
This PhD project is part of the PREDIS project, funded by the French research agency (ANR). It will take place in France, in the “Laboratoire de Géologie de Lyon (LGL TPE)” (University Claude Bernard Lyon 1) under the supervision of Simon ANDRIEU and Guillaume SUAN. The PhD candidate will interact with a wide team of European and Moroccan researchers that are part of the PREDIS project.

Profile and skills required:
With a Master’s degree or an engineering diploma in Geosciences/Earth Sciences, the candidate we are looking for is a field sedimentologist and petrographer, if possible, with experience of carbonate systems. Experience or interest in carbonate diagenesis and isotope geochemistry would be welcome. In terms of interpersonal skills, we expect the candidate to demonstrate an aptitude for field and laboratory work, teamwork and communication. Scientific curiosity and motivation for the subject will be essential. The candidate may be fluent in at least English or French. Good written and spoken English will be valued.

Context and problematic:
Ager’s (1973)1 statement (“More gap than record!”) summarizes well that much of Earth’s history is not recorded in sediments, but “lost” in sedimentary discontinuities, those surfaces forming when sedimentation ceases due to environmental or climatic changes2-3. The morphology, biological activity and diagenesis associated with discontinuities depend on their environment of formation3. Early calcite cements for instance record paleoenvironmental evolution when sediments are lacking4,5. Discontinuities are hence of primordial importance for fully apprehending the geological record, although they have received far less attention than the sedimentary rocks surrounding them. In parallel to that, discontinuities are essential to better exploit and preserve underground resources since they are a main regulator on fluid flows6. Discontinuities often correspond to cycle boundaries and are then fundamental to constrain sequence stratigraphy models, instrumental for correlation at basin scale and beyond6-8. Nonetheless, most of the knowledge gathered so far about discontinuities is based on the interpretation of vertical stratigraphic successions, leaving large uncertainties about their lateral change from proximal to distal settings. One major technical obstacle to achieve this is the reduced horizontal continuity of most outcrops, rarely exceeding a few hundreds of meters. It implies an insufficient understanding of the environmental processes controlling the formation and lithification of discontinuities. While sequence stratigraphy models provide a precise description of system tracts, they do not include the typology of discontinuities marking sequence stratigraphy surfaces, that are features easily recognizable on the field. This lack of knowledge calls into question the assumptions made by sedimentologists when they laterally extrapolate discontinuities to correlate sedimentary sections at various scales (100s of meters to 10s of kilometers)6,7, which is essential to reconstruct the evolution of past sedimentary systems.

Scientific hypotheses to be tested and objective: The following research hypotheses remain to be confirmed or rejected: (1) Subaerial exposures connect to marine discontinuities laterally7. (2) In the marine setting, a unique hardground correlates offshore to several firmgrounds and softgrounds that are interspersed by periods of sedimentation recovery9. (3) The depth and intensity of lithification below marine discontinuities decreases distally and early cements development does not exceed one meter below those surfaces7,9. (4) Discontinuities related to global climatic perturbations present “diagenetic signatures”10 (e.g., geochemistry of early cements) that allow inter-basins correlations. Hence, the purpose of this work is to document at various scales of investigation (102 m to 106 m) the proximal to distal evolution of discontinuities in limestones, where early lithification makes them easier to identify than in siliciclastic rocks. More precisely, we aim at assessing the spatial distribution of processes (i.e., cementation, bioturbation, dissolution…) affecting sediments under discontinuities, both vertically beneath and laterally along them. The PhD student will shed light on this question by studying three prominent surfaces in the Lower to Middle Jurassic outcrops of the Moroccan central High Atlas, where outstanding exposure allows us to track surfaces over tens of kilometers from subaerial exposure to offshore. The inter-basin variability of the Lower Jurassic discontinuities will be determined by comparing the Moroccan surfaces with those of the same age in France and Italy.

Scientific strategy and methods to be applied by the PhD student:
This PhD project aims at documenting the lateral variability of sedimentary discontinuities at various scales (102 m to 106 km) through an inter-disciplinary approach (sedimentology, stratigraphy, petrography, geochemistry). It will partly rely on previously acquired data and, during the first month of the project, the PhD student will have to summarize of the available data in order to better target the new acquisitions required. The PhD work will be organized along three axes: fieldwork, petrography and geochemistry.

• Fieldwork: First, lateral variability of sedimentary discontinuities will be documented on the field. Fieldwork aims at (1) documenting the lateral evolution of the surface morphology, facies changes, mineralization and bioturbations associated to targeted sedimentary discontinuities and (2) sampling for future petrographic and geochemical analyses. The study will focus on three discontinuities (D1, D2 and D3) documented from the Toarcian and Aalenian-Bajocian limestones. The main study area will be the central High Atlas in Morocco (Fig.1 A, B), where the targeted discontinuities can be followed on tens of kilometers from subaerial exposure to offshore (Fig. 1). In addition to Morocco, our study will extend to Toarcian sections from France and Italy where D1 and D2 have been identified in previous studies, while not characterized in details11,12 (Fig. 1). This comparison will attempt identifying large-scale “diagenetic signatures” of discontinuities resulting from global perturbations that could help for inter-basins correlations.
• Petrography: The PhD candidate will study about 200 thin sections from sample taken below and above discontinuities. Observations will firstly be conducted by optical microscopy to characterize microfacies and early diagenetic features (cements, dissolution, mineralogical changes). Particular attention will be paid to early cements morphology that can be characteristic of specific diagenetic environments as the vadose zone (meniscus and pendant cements, geopetal vadose silt) or the marine phreatic zones (e.g., isopachous fibrous cements, botryoidal cements). Thin sections presenting well-developed early cements will be studied using the Cathodyne (Newtec) cold-cathode luminescence apparatus coupled to a microscope NIKON E600 available at the LGL-TPE. Point counting method will be applied through the “JMicroVision” or “Rock.AR” open-source softwares to assess early cements proportion. This method consists of randomly positioning a pointer on an image (in this case a thin section scan) to which the user must assign a category (e.g., fibrous cement, meniscus cement, ooids, etc.).
• Geochemistry: the PhD student will carry measurements of carbon and oxygen isotope compositions of bulk carbonate rock below discontinuities to try identifying subaerial exposure surfaces (see the work of Christ et al., 201213). Similar isotopic measurements will be carried out on organic matter by master students, under the supervision of the PhD student. The resulting data will aim to perform chemostratigraphic correlations between sedimentary sections. For the tracing of cementation fluids, the C and O isotopes of early cements will be measured in situ, directly on polished thin sections, with a Secondary Ion Mass Spectrometer (IMS 1280-HR2, LG-SIMS, CAMECA; Instrument National de l’INSU) at the “Centre de Recherches Pétrographiques et Géochimiques” (CRPG) in Nancy, France. Major and minor elementary concentrations will be measured with a handheld X-ray fluorescence spectrometer along and below discontinuities to reconstruct red-ox conditions (e.g., Fe and Mn contents).

References
[1] Ager D. V., 1973. The Nature of the stratigraphical record. Wiley, London. [2] Clari, P.A. et al., 1995. Discontinuities in carbonate successions: identification, interpretation and classification of some Italian examples. Sedimentary Geology. [3] Hillgärtner, H., 1998. Discontinuity surfaces on a shallow-marine carbonate platform (Berriasian, Valanginian, France and Switzerland). Journal of Sedimentary Research. [4] Andrieu, S. et al., 2018. The complex diagenetic history of discontinuities in shallow- marine carbonate rocks: New insights from high-resolution ion microprobe investigation of δ18O and δ13C of early cements. Sedimentology. [5] Brigaud, B., Andrieu, S., et al., 2021. Calcite uranium-lead geochronology applied to hardground lithification and sequence boundary dating. Sedimentology. [6] Christ, N., et al., 2015. Petrography and environmental controls on the formation of Phanerozoic marine carbonate hardgrounds. Earth Science Reviews. [7] Andrieu, S., et al., 2017. Linking early diagenesis and sedimentary facies to sequence stratigraphy on a prograding oolitic wedge. The Bathonian of western France (Aquitaine Basin). Marine and Petroleum Geology. [8] Andrieu, S., et al., 2022. Anatomy of a platform margin during a carbonate factory collapse: implications for the sedimentary record and sequence stratigraphic interpretation of poisoning event. Journal of the Geological Society. [9] Christ, N., et al., 2012. Characterization and interpretation of discontinuity surfaces in a Jurassic ramp setting (High Atlas, Morocco). Sedimentology. [10] Durlet, C., Loreau, J.P, Pascal, A., 1992. Signature diagénétique des discontinuités et nouvelle représentation graphique de la diagenèse. Comptes Rendus de l’Accadémie des Sciences de Paris 314, 1507-1514. [11] Bodin, S., 2023. More gaps than record! An updated scenario for the Pliensbachian/Toarcian boundary event guided by coupled chemo-sequence stratigraphy. Palaeogeography, Palaeoclimatology, Palaeoecology. [12] Trecalli, A. et al., M., 2012. Carbonate platform evidence of ocean acidification at the onset of the early Toarcian oceanic anoxic event. Earth Planetary Science Letters. [13] Christ, N., et al., 2012. Triassic Latemar cycle tops – subaerial exposure of platform carbonates under tropical arid climate. Sedimentary Geology

Send the following document to the email address written in the contact details section :

• Curriculum vitae
• Cover letter
• Master’s degree or equivalent: detailed transcript of marks obtained in each subject and ranking
• Details of professional contacts and/or letter of recommendation