Cyclostratigraphy of the Middle to Upper Ordovician successions of the Armorican Massif (western France) using portable X-ray fluorescence
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Geochronology, 4, 251–267, 2022
https://doi.org/10.5194/gchron-4-251-2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.
Cyclostratigraphy of the Middle to Upper Ordovician
successions of the Armorican Massif (western France)
using portable X-ray fluorescence
Matthias Sinnesael1,2,3 , Alfredo Loi4 , Marie-Pierre Dabard5, , Thijs R. A. Vandenbroucke2 , and Philippe Claeys1
1 Analytical,
Environmental and Geo-Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
2 Department of Geology, Ghent University, Krijgslaan 281/S9, 9000 Ghent, Belgium
3 IMCCE, CNRS, Observatoire de Paris, PSL University, Sorbonne Université,
77 Avenue Denfert-Rochereau, 75014 Paris, France
4 Department of Chemical and Geological Sciences, University of Cagliari, Cittadella Universitaria,
Blocco A – 09042, Monserrato, Italy
5 Géosciences UMR6118 CNRS/Université Rennes, Campus de Beaulieu, 35042 Rennes CEDEX, France
deceased
Correspondence: Matthias Sinnesael (matthias.sinnesael@obspm.fr)
Received: 22 December 2021 – Discussion started: 12 January 2022
Revised: 5 April 2022 – Accepted: 14 April 2022 – Published: 16 May 2022
Abstract. To expand traditional cyclostratigraphic numer- ical cyclostratigraphic techniques on this siliciclastic storm-
ical methods beyond their common technical limitations dominated shelf environment, a non-traditional sedimento-
and apply them to truly deep-time archives, we need to re- logical setting for cyclostratigraphic analysis. In the parts of
flect on the development of new approaches to sedimentary the section with a relatively homogeneous lithology, spectral
archives that are not traditionally targeted for cyclostrati- power analyses and bandpass filtering hint towards a poten-
graphic analysis but that frequently occur in the impover- tial astronomical imprint of some sedimentary cycles, but this
ished deep-time record. Siliciclastic storm-dominated shelf needs further confirmation in the absence of more robust in-
environments are a good example of such records. Our case dependent age constraints.
study focuses on the Middle to Upper Ordovician siliciclas-
tic successions of the Armorican Massif (western France)
that are well-studied examples in terms of sedimentology
and sequence stratigraphy. In addition, these sections are pro- 1 Introduction
tected geological heritage due to the extraordinary quality of
the outcrops. We therefore tested the performance of non- Astronomical climate forcing is a major driver of natural cli-
destructive high-resolution (centimeter-scale) portable X-ray mate change and its sedimentological expressions can of-
fluorescence and natural gamma-ray analyses on an outcrop ten be identified in the stratigraphic record (Hinnov, 2018;
to obtain major and trace element compositions. Despite the Meyers, 2019; Sinnesael et al., 2019; Laskar, 2020). Study-
challenging outcrop conditions in the tidal beach zone, our ing past astronomical imprints is informative of past climate
geochemical analyses provide useful information regarding dynamics and sedimentological processes. Moreover, due to
general lithology and several specific sedimentary features the almost semi-periodic nature of the astronomical cycles
such as the detection of paleo-placers or the discrimination of eccentricity, precession and obliquity, the identification of
between different types of diagenetic concretions such as theses cycles (i.e., cyclostratigraphy) also provides highly re-
nodules. Secondly, these new high-resolution data are used solved temporal constraints to sedimentary sequences (i.e.,
to experiment with the application of commonly used numer- astrochronology). Cyclostratigraphic analyses are performed
on a wide range of sedimentological settings (Meyers, 2019;
Published by Copernicus Publications on behalf of the European Geosciences Union.
252 M. Sinnesael et al.: Ordovician cyclostratigraphy of the Armorican Massif
Sinnesael et al., 2019). Nevertheless, most of the currently nae and lenticular coquina beds (Fig. 3, Dabard et al., 2015).
used techniques for numerical analysis imply stringent un- Sedimentary features like HCS and lenticular coquina beds
derlying assumptions regarding the nature of the record, i.e., reflect storm wave action, and overall these sediments were
complete continuous records without major changes in accu- deposited in storm-dominated shelf environments.
mulation rates or depositional environments (Weedon, 2003; The biostratigraphic calibration is based on a standard
Hilgen et al., 2015). Typically, open marine pelagic deposits Gondwanan chitinozoan biozonation (Fig. 2; Paris, 1981,
are considered the ideal stratigraphic record for this type of 1990; Dabard et al., 2007, 2015). Desmochitina ornensis sup-
work (e.g., Westerhold et al., 2020). However, further back ports a Dapingian position for the top of the Grès Armoricain
in geological time, fewer of these records are preserved, Formation and the base of the Postolonnec Fm., while the
and they are virtually absent in the pre-Cretaceous record. succeeding Belonechitina henryi Biozone of the lower Dar-
Therefore, there is a need to explore new ways of identi- riwilian is found several meters above the base of the Pos-
fying astronomical cycles in less traditional archives using tolonnec Fm. Other chitinozoan biozones found in the Cro-
a broader range of analysis techniques (Noorbergen et al., zon area supporting a lower Darriwilian position are Desmo-
2018; Lantink et al., 2019; Montanez, 2021). chitina bulla, Cyathochitina protocalix, and Siphonochitina
This study compares numerical time series analysis (e.g., formosa. Specimens of Linochitina pissotensis are recorded
Weedon, 2003) with an alternative approach based on relative in the Morgat Member (Mb.) and indicate the upper Darri-
sea-level interpretations (e.g., Cataneanu et al., 2009) applied wilian Stage. The base of the Sandbian is correlated with
to a siliciclastic storm-dominated shelf environment, a set- the Lagenochitina ponceti Biozone and the Veryac’h Mem-
ting not traditionally targeted by cyclostratigraphic studies. ber, which also yields two younger Sandbian biozones: the
The Ordovician sections of the Crozon Peninsula (France; Lagenochitina deunffi Biozone and the Lagenochitina dal-
Fig. 1) have been studied in detail for cyclic sedimentary byensis Biozone. Overall, the Postolonnec Fm. spans roughly
expressions by interpreting stacked stratigraphic sequences 14 million years, starting close to the start of the Darriwilian
(Fig. 2; Dabard et al., 2015, and references therein). Pre- Stage (∼ 467 Ma) and ending close to the end of Sandbian
vious work has resulted in detailed sea-level change recon- Stage (∼ 453 Ma). No radioisotopic dating, chemostratigra-
structions based on sedimentological interpretations and low- phy, or any other further constraints are currently available.
resolution (meter-scale) natural gamma-ray data (Dabard et
al., 2007, 2015). Backstripping was applied to identify sub-
sidence and several orders of sea-level change (Fig. 2). 3 Methods
The third- to fifth-order eustatic sea-level changes are hy-
pothesized to correspond to various frequencies related to The coastal sections of Postolonnec are protected as valu-
astronomical forcing. Here, we expand the existing data able geological and natural heritage. Therefore, we investi-
set with new medium-resolution (decimeter-scale) natural gated the potential use of non-destructive NGR and pXRF
gamma-ray (NGR) data and high-resolution (centimeter- measurements on outcrops for stratigraphic and geochemi-
scale) portable X-ray fluorescence (pXRF) data for two se- cal characterizations. Next to common challenges of using
lected stratigraphical intervals with contrasting lithofacies. pXRF on outcrops (surface weathering and need for flat sam-
The high-resolution pXRF data can be used for detailed ple surface), a specific challenge for this beach section is that
time series analysis and provide additional geochemical in- it is subject to large tidal ranges, which makes the outcrops
sights into specific sedimentological features like nodules wet for most of the day and sometimes covered by organisms
and paleo-placer beds (Loi and Dabard, 2002; Dabard and with carbonate shells (e.g., barnacles). Measurements were
Loi, 2012; Pistis et al., 2008, 2016, 2018). always performed during dry weather, after low tide, and on
fresh uncovered outcrop surfaces. The potential added value
of the pXRF compared to the handheld NGR is that pXRF
2 Geological setting and biostratigraphy gives a wider range of elemental analyses and has a finer
spatial resolution regarding spot size (∼ 1 cm vs. ∼ 10 cm
The Middle to Upper Ordovician sections of the Crozon resolution). Moreover, the unshielded NGR measurement de-
Peninsula area, Armorican Massif (Paris et al., 1999; Vi- vice integrates the natural radioactive field beyond its actual
dal et al., 2011a), western France (Fig. 1a), were deposited spot size (sphere with radius of several decimeters), which
on the continental terrigenous platform of Gondwana, lo- is consequently influenced by the geometry of the outcrop at
cated at high paleo-latitudes (∼ 60◦ S). This study focuses the point of measurement. Elemental variations measured by
on the Postolonnec Formation (Fm.) (Fig. 1b) as it out- both instruments (e.g., potassium) can also be compared to
crops at the coastal cliffs of Postolonnec (48◦ 140 18.5000 N, test the robustness of the signals. In contrast to the NGR, the
4◦ 270 59.7000 W; see Fig. 3 in Dabard et al., 2015). The section pXRF centimeter-scale resolution allows us to measure fine-
is characterized by siliciclastic facies ranging from medium- scale features such as individual nodules or paleo-placers
to coarse-grained sandstone beds with hummocky cross strat- (Loi and Dabard, 2002; Dabard and Loi, 2012; Pistis et al.,
ification (HCS) to silty mudstones containing siltstone lami- 2016, 2018). Conversely, one has to keep in mind that be-
Geochronology, 4, 251–267, 2022 https://doi.org/10.5194/gchron-4-251-2022
M. Sinnesael et al.: Ordovician cyclostratigraphy of the Armorican Massif 253 Figure 1. (a) Location of the Postolonnec section on the Crozon Peninsula of the Armorican Massif. (b) Lithostratigraphic column of the Ordovician succession on the Crozon Peninsula. Modified from Dabard et al. (2015). cause of this fine spot size the result of the measurement The stratigraphically higher ∼ 14 m thick interval in the Ker- is very sensitive to the actual spot of analysis. For example, armor Mb. is characterized by larger and more frequent vari- in a coarse-grained heterogenous sandstone one could imag- ations in lithology, ranging from mudstones to medium-sized ine hitting a different mineralogy for different measurements sandstones (Figs. 2–3). Both sections were logged at a 1 : 20 within the same bed. The comparison of the geochemical scale and later transformed into a 1 cm resolution detailed composition of the nodules and their immediately surround- sedimentary log (presented in simplified forms in the lithol- ing matrix, for example, could be useful to gain insights into ogy columns of Fig. 4). The approach used in this work for their genesis. For our study, we compiled the acquired pXRF, the depositional sequence analyses and sea-level reconstruc- NGR, and lithology data; evaluated how they compared to tions are the same as those used in Dabard et al. (2015) with each other; and tested if the pXRF indeed reflects lithological the only difference being greater detail and a higher density and geochemical changes in these challenging measurement of NGR measurements in both sandy and clayey homolithic conditions. facies in this study. Successively, we measured the sections Two field surveys were carried out analyzing two different in high resolution with handheld NGR (10–20 cm resolution) stratigraphic intervals with contrasting facies at centimeter and pXRF (1–10 cm resolution) (Fig. 4). Dabard et al. (2015) resolution. The stratigraphically lower ∼ 10 m thick inter- measured NGR for the whole Postolonnec Fm. at a varying val spans the transition from the shoreface facies Kerarvail 10–100 cm resolution according to the thickness and homo- Member (Mb.) into the deeper marine Morgat Member that geneity of the facies. Table 1 summarizes all available data. is dominated by a deeper marine clay–silt facies (Figs. 2–3). All data and detailed logs are provided in the Supplement. https://doi.org/10.5194/gchron-4-251-2022 Geochronology, 4, 251–267, 2022
254 M. Sinnesael et al.: Ordovician cyclostratigraphy of the Armorican Massif
Figure 2. Stratigraphy of the Postolonnec Formation, including chitinozoan biostratigraphy, lithofacies interpretation, sequence stratigraphy
(very high-frequency sequences, purple and red curves; high-frequency or fourth-order sequences, blue curve; and third-order sequences,
pale blue curve), transgressive ravinement surfaces (dotted blue line), sharp-based surfaces (dotted red lines), and gamma-ray spectral logs
(cpm, count per minute). Orange arrows and shading indicate sections studied at high resolution in this study in the Morgat and Kerarmor
members. Modified from Dabard et al. (2015).
Table 1. Overview of available numerical data for the Postolonnec Formation. Stratigraphic intervals and members follow Dabard et
al. (2015). NGR stands for natural gamma-ray measurements, and pXRF stands for portable X-ray fluorescence.
Stratigraphic Member Data type Resolution points Source (mm/yyyy)
Interval (m) (cm) (N)
0.0–409.0 Kerloc’h–Veryac’h NGR 10–100 883 Dabard et al. (2015)
204.5–214.5 Morgat Lithology ∼1 991 This study (09/2017)
204.5–214.5 Morgat NGR 10–20 69 This study (09/2017)
204.5–214.5 Morgat pXRF 1–10 194 This study (09/2017)
324.6–338.6 Kerarmor Lithology ∼1 1371 This study (04/2016)
324.6–338.6 Kerarmor NGR 10–20 107 This study (04/2016)
324.6–338.6 Kerarmor pXRF 1–10 335 This study (04/2016)
Geochronology, 4, 251–267, 2022 https://doi.org/10.5194/gchron-4-251-2022
M. Sinnesael et al.: Ordovician cyclostratigraphy of the Armorican Massif 255 Figure 3. Postolonnec Formation outcrop images. (a) Overview basal Morgat Mb. (b) Overview of the upper half of the detailed logged section in the Kerarmor Mb. (c) Detail of the condensed lithofacies of the Morgat Mb. with dark nodules and shell beds. White chalk numbers indicate locations of pXRF analyses. (d) Detail of Kerarmor sandstones with white chalk circles indicating locations of NGR analyses. (e) Modus operandi of the use of pXRF and NGR on the outcrop surface. (f) Detail of Fig. 3b illustrating the high-resolution measurement strategy. The gamma-ray data were obtained using a portable spec- The fundamental parameters method makes use of the theo- trometer RS-230 (Radiation Solutions, Inc., Canada), the retical relationship between X-ray fluorescence and material same instrument used by Dabard et al. (2015). The measure- composition as determined by Sherman (1956). The factory- ments were taken with a stratigraphic interval of approxi- calibrated quantification method of the pXRF uses this fun- mately 10–20 cm and a counting time of 120 s. The counts damental principle with a correction based on the matrix ef- per minute in the selected energy windows are converted fect observed in soil and rock samples. All pXRF measure- to concentrations of K (%), U (ppm), and Th (ppm). The ments are carried out by putting the pXRF nozzle directly on pXRF measurements were done using a Bruker Tracer IV the dry and clean outcrop surface. All analyses had a mea- handheld portable XRF device equipped with a 2WRh an- surement time of 45 s (de Winter et al., 2017). The pXRF ode X-ray tube and a 10 mm2 silicon drift detector with a measurements were not calibrated using external reference resolution of 145 eV (Mn–Ka). The X-ray beam is focused materials. As such it is more relevant to consider relative on a 6 mm by 8 mm integrated area using a Pd collimator. variations in the elemental concentrations data rather than in- X-ray spectra from the pXRF are deconvoluted and quanti- terpreting absolute concentrations (Sinnesael et al., 2018a). fied using the standard factory “soil fundamental parameters” As a general principle the pXRF is also less suitable to mea- method. With this technical setting, we could not reliably sure very light (< Al) or heavy elements (> Ba) or elements measure Si, as in the soil fundamental parameters method the that only occur in trace amounts; this also depends on the SiO2 mass fraction is used to sum the total weight to 100 % instrument used (Beckhoff et al., 2006). (this is explained in more detail in Sinnesael et al., 2018a). https://doi.org/10.5194/gchron-4-251-2022 Geochronology, 4, 251–267, 2022
256 M. Sinnesael et al.: Ordovician cyclostratigraphy of the Armorican Massif Figure 4. Sequence stratigraphy and geochemical results from the portable X-ray fluorescence (pXRF) and natural gamma-ray (NGR) measurements. Several orders of sea-level change are interpreted in a sequence stratigraphic framework (very high-frequency sequences, purple and red curves; high-frequency or fourth-order sequences, blue curve). The association with respective possible astronomical cycles of precession, obliquity or eccentricity is only suggestive, sensu Dabard et al. (2015). Note that the pXRF concentrations are reported in weight percent (wt %) as measured by the internal instrumental calibration (soil method), and are thus not accurate in the absence of an additional external calibration. The pXRF measurements are more meaningfully interpreted in function of relative changes. The black stars indicate the stratigraphic occurrence of paleo-placers. The sequence stratigraphic interpretations follow the Formation (Fig. 1b). Despite their variability, all facies can method outlined in Dabard et al. (2015) and are based on be ascribed to depositional environments of a terrigenous the study of depositional sequences at different frequencies platform dominated by storms and tides (Guillocheau, 1983; in combination with NGR measurements (e.g., Fig. 2). There Guillocheau and Hoffert, 1988; Loi et al., 1999; Botquelen is a variety of sedimentary and condensational facies that can et al., 2006; Dabard et al., 2007, 2015; Pistis et al., 2008; be encountered from the upper part of the Grès Armoricain Vidal et al., 2011a, b). Analysis of sedimentary and conden- Fm., throughout the Postolonnec Fm., and into the Kermeur sation facies allowed the identification of genetic sequences Geochronology, 4, 251–267, 2022 https://doi.org/10.5194/gchron-4-251-2022
M. Sinnesael et al.: Ordovician cyclostratigraphy of the Armorican Massif 257 by identifying the maximum regressive surface (MRS) and 4 Stratigraphic and geochemical results maximum flooding surface (MFS) for all sequences of the different hierarchical orders (Catuneanu et al., 2009; Dabard The first type of data contains the facies analyses consider- et al., 2015). Due to the high lithological contrast of the sed- ing both sedimentary structures and variations in grain size imentary facies in the upper offshore marine environment, (Fig. 4). The Kerarmor Mb. is characterized by a much larger it is easy to identify the MRS and MFS and its correspond- stratonomic variation in grain size throughout the section ing genetic units. In contrast, in sedimentary environments than the Morgat Mb., with a large range of facies corre- where facies are homogeneous, such as the shoreface and sponding to various paleoenvironments, which makes it more lower offshore, sequence recognition was mainly based on straightforward to differentiate the different depositional se- the identification of condensation facies. In deeper environ- quences related to several orders of relative sea-level change. ments, clayey successions are monotonous, and condensa- The Morgat Mb. essentially consists of a continuous silty tion facies, such as diagenetic nodular concretionary levels mudstone facies, overlying the top of a thick sandstone unit (Loi et al., 1999; Loi and Dabard, 1999, 2002; Dabard et al., of the upper Kerarvail Mb. The more homogeneous distribu- 2007; Dabard and Loi, 2012) or shell beds levels (Botquelen tion of lithological compositions makes facies analysis and et al., 2004, 2006), are the main markers that allow effective high-frequency interpretation of possible changes of relative sequential partitioning especially when coupled with NGR sea level more difficult. In the lower offshore (
258 M. Sinnesael et al.: Ordovician cyclostratigraphy of the Armorican Massif For both high-resolution stratigraphic logs, the first-degree particularly useful. For example, combined elevated concen- variations in lithology are well reflected in the K curves mea- trations of U (NGR), Th (NGR), Zr (pXRF), and Ti (pXRF) sured by both pXRF and NGR (Fig. 4). The mudstone inter- are a very strong indicator of a paleo-placer. Around 6.5 m vals have a larger clay content and correspond with higher in the Kerarmor section there is an elevated Ti (and Ce) peak K values and vice versa for the sandstone intervals. There is for measurements performed at the base of a thick sandstone also an excellent agreement between the low-resolution NGR (Fig. 4). This level does, however, not show higher concen- data published in Dabard et al. (2015) and the new high- trations of Th, U, or Zr. Possibly the small spot of the pXRF resolution NGR data (Supplement). While the relative varia- hit a specific mineralogy, or represents “an outlier”. All data tions in K for both the pXRF and NGR agree well, there is an points measured in the field are reported in Fig. 4, without offset in the reported absolute concentrations because of the arbitrary screening for potential “outliers”. The pXRF ele- lack of absolute concentration calibration for the pXRF mea- mental analysis can thus be a fast and useful tool to per- surements. The same pattern in mudstone vs. sandstones is form multi-elemental analysis in the field and detect potential also visible in the pXRF Rb profile, which again follows the paleo-placers. clay content (Supplement). For both stratigraphic profiles, Various types and forms of concretions can be found variations in NGR U and Th concentrations behave similarly throughout the Postolonnec Fm. (e.g., Loi and Dabard, 2002; overall and seem to follow the first-degree features of the Dabard and Loi, 2012). The most common concretions are NGR K record. However, the relative variations in NGR U cemented by silica, phosphorus, or carbonate. Unfortunately, and Th concentrations are less pronounced in amplitude and due to the technical settings used for the pXRF measure- show a less consistent relationship with the measured lithol- ments, Si was not reliably reported (see Methods). Never- ogy. For example, while U and Th are generally expected to theless, the pXRF analysis still reveals useful information. be enriched in shales and clays (Adams and Weaver, 1958), Some concretions in the Morgat Mb. 8–10 m interval have the thick sandstone unit on the top of the Kerarvail Mb. has high Ca values and are most probably carbonate-cemented the highest measured U and Th values (Fig. 4) due to the concretions (Fig. 4). Some measured shell beds also show el- presence of paleo-placers. The term “paleo-placer” refers to evated Ca values, reflecting a preserved fraction of carbonate heavy mineral-rich laminae that often occur on the top of mineralogy of the original shells. These Ca-enriched nodules thick sandstone units of the Postolonnec Fm. and other sim- also have very low concentrations of K, Rb, and Ti. Another ilar paleoenvironmental depositional settings (Pistis et al., group of concretions have elevated Ce values (although cau- 2008, 2016, 2018). In addition, the thick sandstones (> 12 m) tion is required with respect to the quality of the measure- at the top of the Kerarmor Mb. have elevated U and Th values ment for this element) and K, Rb, and Ti values that are sim- (with paleo-placers), while the sandstone unit around 11 m ilar to their surrounding sediments. This group corresponds has low U and Th values (without paleo-placers). with the phosphatic concretions as Ce is one of the elements The pXRF measurements also yielded two other groups that is typically enriched in the phosphatic apatite. The nod- of elements, whose variations in concentrations seem to be ules can also be enriched in U (NGR), where it is linked driven by sedimentological or diagenetic processes, i.e., as- to the presence of organic matter, and at the same time of- sociated with paleo-placers and concretions (nodules). A first ten display low Th (NGR) concentrations. More systematic group of elements measured with the pXRF relates to paleo- work is needed, where the same nodules are analyzed with placers (Zr, Ce, and Ti, Fig. 4). An example of such a paleo- both pXRF and more robust analytical techniques (e.g., in- placer-rich stratigraphic interval is the top of the thick Kerar- ductively coupled plasma mass spectrometry), but these first vail Mb. sandstone unit, which is characterized by high levels results demonstrate the potential as a useful non-destructive of natural radioactivity due to the presence of certain min- screening tool usable in the field. erals. Certain elements associated with this mineral assem- blage (e.g., zircons, monazite, and titaniferous minerals, as described by Pistis et al., 2016, 2018) can be detected by the 5 Cyclostratigraphic analysis pXRF. This is most clearly reflected in the Zr and Ti profiles. Our “single spot per bed” measurements do not allow for spe- The cyclicity analysis of Dabard et al. (2015) was based on cific mineralogical identifications, but an approach that com- the identification of several orders of sea-level fluctuations. bines multiple pXRF measurements for the same sample for The most pronounced order of sea-level variations is the coarse-grained igneous rocks has shown promising results meter-to-decameter-thick “high-frequency cycle” (Fig. 2). when extracting mineralogical information (Triantafyllou et By taking the ratio between the number of identified high- al., 2021). In theory, Ce should also show elevated values in frequency cycles (n = 36) and the GTS2012-estimated dura- the paleo-placers, but this element is potentially too heavy tion for the Darriwilian (8.9 Myr, Cooper and Sadler, 2012; (and can occur in too low concentrations) to be reliably de- Cohen et al., 2013), a maximal duration estimate of 410 kyr tected by the pXRF analysis. Some elements like Ti occur per cycle was obtained. Therefore, this cycle was ascribed in elevated concentrations in both paleo-placers and other fa- to the ∼ 405 kyr long eccentricity cycle, which is theoreti- cies. Considering both NGR and pXRF data together can be cally the most stable astronomical cycle that is also recorded Geochronology, 4, 251–267, 2022 https://doi.org/10.5194/gchron-4-251-2022
M. Sinnesael et al.: Ordovician cyclostratigraphy of the Armorican Massif 259 throughout the Phanerozoic (Laskar, 2020). The higher-order ing whether the use of traditional spectral analysis tools is frequency cycles were then respectively ascribed to the ∼ valid and of added value. 100 kyr eccentricity and ∼ 20 kyr precession cycles. The ab- To investigate the most condensed part of the Morgat Mb. sence of more precise temporal constraints (other than chiti- high-resolution profile, we focus on the interval that does nozoan biostratigraphy) prevents further robust testing of this not contain any silt or sandy layers (from 3.5 m above the hypothesis. Furthermore, it makes it difficult to clearly dis- base of the profile up to the top at 10.0 m). The main sed- tinguish a potential short-term Ordovician precession cycle imentological information that could discriminate potential (∼ 20 kyr) from an obliquity (∼ 30 kyr) cycle (Berger and sequences here are the occurrence of shell beds, nodules (Loi Loutre, 1994; Laskar et al., 2004; Waltham, 2015). Consid- and Dabard, 2002), or variations in elemental concentrations ering a potential obliquity cycle is especially relevant given as measured by NGR and pXRF. that the Darriwilian is also hypothesized to have featured Let us first consider the medium-resolution NGR K signal some of the earliest Ordovician glacial episodes (e.g., Van- (Fig. 5b). The signal shows some fluctuations in concentra- denbroucke et al., 2010; Dabard et al., 2015; Pohl et al., tions, varying between 5 % and 6.5 % K. Here, we use evolu- 2016; Rasmussen et al., 2016), with high-latitude glacial dy- tive harmonic analysis (EHA) to evaluate the spectrum of the namics typically being more sensitive to an obliquity forcing signal as it evolves throughout the stratigraphy (Thomson, (Hinnov, 2018). The long-term 1.2 Myr obliquity cycle has 1982). The frequencies that explain more variation within been hypothesized to play a role in the Ordovician icehouse the moving window will have higher spectral power and are stratigraphic structures (Turner et al., 2012; Ghienne et al., shown by redder colors (Fig. 5b). This approach has the ad- 2014; Dabard et al., 2015). An important nuance is that the vantage over a single periodogram or multi-taper spectrum so-called very long 2.4 Myr eccentricity and 1.2 Myr obliq- that it can also be used to evaluate the stratigraphic consis- uity cycles do not necessarily have the same duration during tency of a certain period. Spectral analyses indicate two main the Ordovician due to the chaotic behavior of the solar sys- periodicities: a longer one of ∼ 1.5–2.0 m (0.5–1.0 cycles per tem (Laskar, 1989; Olsen et al., 2019; Hoang et al., 2021). meter) and a shorter one around ∼ 0.5 m (1.8–2.2 cycles per The highly variable sedimentation rates and potential (small) meter) cycle thickness (as indicated by the dotted lines on hiatuses also make it challenging to test for potential am- Fig. 5a). The rest of the EHA shows little elevated spectral plitude modulation patterns that could identify a unique as- power for other frequencies. The data set is relatively short tronomical imprint throughout the entire stratigraphy (Mey- (6.5 m), with only 46 data points and an average sample res- ers, 2015, 2019; Sinnesael et al., 2021). A further challenge olution of 14 cm, and thus one must consider that there are to consider is that the Ordovician geological timescale is only few of the 1.5–2.0 m cycles in the record and that the continuously evolving, with the most recent duration esti- 0.5 m cycle is close to the theoretical Nyquist frequency (stat- mates for the Darriwilian being 11.2 Myr (GTS2020 grapto- ing that you need at least two data points per cycle to be lite composite) or 11.6 Myr (GTS2020 conodont composite) able to detect it; in practice, it is often better to have even vs. the GTS2012’s much shorter 8.9 Myr estimation (Gold- more, e.g., Martinez et al., 2016). However, the bandpass fil- man et al., 2020). These duration estimates vary up to 30 %, ters of both periods show a good agreement with the raw sig- which might not change the order of magnitude of the cy- nal (Fig. 5c). Let us now consider the pXRF K signal that cle duration estimates that much but certainly has implica- has a higher average sampling resolution (6 cm) and more tions for the potential completeness of the record. More- data points (n = 106) for the same investigated stratigraphic over, one has to consider the difficulty of precisely identi- length (Fig. 5e). The spectral analysis of the pXRF K signal fying the stage boundaries based on chitinozoan biozonation again shows the ±1.5 m cycle, a less pronounced 0.5 m cycle, solely. A last limitation relates to the difficulty to clearly dis- and a new dominant cyclicity around 0.25 m (Fig. 5f). Com- tinguish stacked patterns in the very condensed parts of the pared to the NGR EHA, the pXRF EHA suggests additional Postolonnec Fm. (Dabard et al., 2015). Rather than suggest- frequencies with lower spectral power. These seem, however, ing a number of cycles that agrees with certain stage duration to be less stratigraphically continuous (Fig. 5). The bandpass estimates, there is much more value in trying to identify the filters of the NGR and pXRF signals demonstrate a similar actual number of cycles clearly distinguishable and consider- number of cycles, but they are not perfectly in phase with ing the possibility of missing cycles. Especially in the shal- each other (Fig. 5d). The pXRF 0.25 m periodicity bandpass low marine shelf environments, it is reasonable to, for exam- filter shows a pronounced amplitude modulation, which is ple, consider periods of non-deposition or erosion. Although consistent with the amplitude modulation in the 1.5 m peri- many of the discussed challenges are inherent to the nature odicity for the pXRF signal – although this is again not per- of the record and lack of independent constraints, we con- fectly in phase (Fig. 5c and d). The pattern of the amplitude sider two additional elements to the cyclostratigraphic anal- modulation in combination with the number of shorter cy- ysis of the Postolonnec Fm.: (i) investigating whether new cles (5–6) in the longer cycles is suggestive of a precession– high-resolution pXRF data in the highly condensed interval eccentricity signal. This signal is consistent with the ratio fit- (Morgat Mb.) are complementary to the NGR data and can ting of the 1.5, 0.5, and 0.25 m periodicities to the predicted help with clearly distinguishing sequences and (ii) determin- Ordovician duration of the astronomical cycles of short ec- https://doi.org/10.5194/gchron-4-251-2022 Geochronology, 4, 251–267, 2022
260 M. Sinnesael et al.: Ordovician cyclostratigraphy of the Armorican Massif Figure 5. Spectral analysis of the Morgat Member potassium records showing the measured data (b and e, orange lines), several bandpass filters (c and d, black lines), and evolutive harmonic analysis (a and f, spectral plots with ranges of bandpass filters are indicated by dotted rectangles). Panels(a–c) are based on the natural gamma-ray record and panels (d–f) are based on the portable X-ray fluorescence data. EHA stands for evolutive harmonic analysis. centricity, obliquity, and precession, respectively (Berger and acterized by a larger variation in facies (Fig. 6). The power Loutre, 1994; Laskar et al., 2004; Waltham, 2015), as well as spectrum is dominated by a ∼ 0.5–2.0 m (0.5–2.0 cycles per with the order of magnitude sedimentation rates estimated by meter) cyclicity, apparently with thicker cycles in the more Dabard et al. (2015) for this interval. In fact, it is of interest sandstone dominated intervals (below 5 and above 10 m) and that Dabard et al. (2015), independently of this study, inter- thinner cycles within the clay and siltstone facies. One logi- preted 405 kyr eccentricity minima roughly at the same strati- cal interpretation could be that the same dominant sequences graphic positions as we identify small amplitude variations get thinner within the finer fractions. The bandpass filter of in a potential precession signal. A precession–eccentricity this frequency range does not show any pronounced ampli- signal can also be statistically tested with the TimeOpt ap- tude modulation. The bandpass filter output does show some proach (Meyers, 2015, 2019). The TimeOpt results are close resemblances to the independently constructed relative sea- to our interpretation with the nominal TimeOpt outcome (as- level reconstructions. The thick sandstone beds are identified suming constant sedimentation rate over the whole record) by both methods as individual cycles, while they probably that suggests the presence of five short eccentricity cycles represent a single (or a few) depositional event(s). The se- or a sedimentation rate of 12 m Myr−1 , whereas the evolu- quence stratigraphic interpretation suggests the presence of a tionary eTimeOpt analysis suggests slightly higher sedimen- larger number of high-frequency cycles in the finer fractions tation rates (15 m Myr−1 ) with a small increase in sedimen- compared to the bandpass approach, which is limited by its tation rate up section (R script for TimeOpt analyses pro- user-defined frequency width. In the Dabard et al. (2015) in- vided in the Supplement). It is hard to further demonstrate terpretation, the highest-frequency relative sea-level changes an astronomical origin of these variations in the absence of were interpreted to correspond with the precession (or obliq- more precise stratigraphic constraints. However, our obser- uity) cycles and smoothed higher-order cycles with eccen- vations based on a numerical approach using high-resolution tricity cycles. Again, in the absence of better age control geochemical data were consistent with those of Dabard et this hypothesis is difficult to test further. The numerical cy- al. (2015), suggesting our new approach can be an additional clostratigraphic analyses did not provide further indications useful tool. The occurrences of the shell beds (mainly be- of a potential astronomical origin of these sequences but did tween 6.5 and 9.5 m) and nodules are not so clearly cyclically offer another way of describing the variations within the sig- distributed, making it difficult to robustly assess their usabil- nal. ity as tool for the assessment of “condensed sequences”. That The detailed study of these two short sections indicates nodules occur less systematically might be related to the in- that the commonly used numerical cyclostratigraphical ap- terplay of various boundary conditions that can lead to a var- proach may work for the selected short and relatively litho- ied sedimentological expression of a condensation interval logically homogenous interval but might be challenged when (as discussed in Loi and Dabard, 2002, and Dabard and Loi, applied to intervals with more contrasting lithologies. To test 2012). this idea further, we investigate the spectral analysis proper- We can now apply a similar approach to the pXRF K ties of the whole Postolonnec Fm. (11–393 m on the depth record of the shallower marine Kerarmor section that is char- scale of Dabard et al., 2015), based on the available low- Geochronology, 4, 251–267, 2022 https://doi.org/10.5194/gchron-4-251-2022
M. Sinnesael et al.: Ordovician cyclostratigraphy of the Armorican Massif 261 Figure 6. Comparison of sequence and numerical cyclostratigraphic analyses for the Kerarmor Member based on the facies, potassium from portable X-ray fluorescence, and bandpass filtering. EHA stands for evolutive harmonic analysis. Several orders of sea-level change are interpreted in a sequence stratigraphic framework (very high-frequency sequences, purple and red curves; high-frequency or fourth-order sequences, blue curve). resolution (meter-scale) NGR data. The importance of sam- mial trend or low-pass filtering. The challenge is finding the pling resolution to potentially pick up smaller-scale cycles delicate balance between removing what is long-term trend was already illustrated by the difference in outcomes for the and the signal to recover. Due to the potential large variation NGR vs. pXRF data for the Morgat Mb. analysis. With an in sedimentation rates (up to an order of magnitude) and pa- average sample rate of ∼ 0.5 m, it might be expected that leoenvironments, this is a difficult balance to strike for this shorter-term astronomical cycles such as precession or obliq- data set. We have applied various levels of detrending using uity cannot be recovered by numerical analysis of the low- a low-pass filter to assess the robustness of the consecutive resolution NGR record. The expression of such potential cy- spectral analyses (evaluated between f = 0.01–0.05 m−1 or cles in a relatively coarse siliciclastic facies may be very dif- periods > 100–20 m). In Fig. 7 we compare the EHAs for ferent (e.g., they might be much thicker and detectable with a simple linear detrending and a low-pass filtered detrend- a relatively low sampling rate). A first complication is that ing (f = 0.033 m−1 or periods > 30 m). The detrending has the amplitude of the variations of the NGR signal is much decreased the difference in spectral power between different larger in the sandstone-rich intervals (i.e., Kerarvail Mb. and lithologies, but the overall pattern stays the same. Both the Kerarmor Mb.) compared to the lithologically more homoge- Kerarvail and Kerarmor sandstone members show a consis- nous intervals, which makes the spectral power much higher tent 4 and 2.5 m cycle. The longer-term cycles are less robust in the sandstone intervals, masking potential smaller ampli- to identify and partially depend on the level of detrending. tude changes in the other intervals (Fig. 7). An additional Applying the respective sedimentation rates of the Dabard complication is that the very variable lithological succession et al. (2015) interpretations for these stratigraphic intervals results in quite a complex long-term trend in the NGR signal would result in a ∼ 100 kyr (short eccentricity) duration for that cannot be fully removed by subtracting a simple linear the 4–2.5 m cycles. Interestingly, sometimes a 12 m thick cy- trend (the most common and basic form of detrending a sig- cle (∼ 400 kyr) also appears in the spectral analysis results nal prior to spectral analysis). It is possible to perform more but only in the most heavily detrended signals. Overall, this complex ways of detrending, for instance by using a polyno- is a more tentative and not fully independent interpretation, https://doi.org/10.5194/gchron-4-251-2022 Geochronology, 4, 251–267, 2022
262 M. Sinnesael et al.: Ordovician cyclostratigraphy of the Armorican Massif
Figure 7. Spectral analysis results based on the potassium natural gamma-ray record for the whole Postolonnec Formation, demonstrating
the influence of detrending.
as Dabard et al. (2015) made their interpretations assuming such cases where there are tens of millions of years worth of
an astronomical origin of their signal. Striking the balance stratigraphy with poor chronological constraints.
between removing long-term trends and what is a poten- Alternatively, the “spectral moments” approach is de-
tial astronomical signal proves to be particularly challenging signed to pick up potential first-order changes in pronounced
for this long record with a pronounced change in sedimen- sedimentation rate changes in astronomically forced signals
tary environment. One could additionally analyze every sub- (Sinnesael et al., 2018b). The main underlying idea of the ap-
section individually, for example with high-resolution pXRF proach is that some of the main characteristics of the evolv-
data, but the absence of independent age controls remains. In ing spectra (e.g., mean frequency and bandwidth) shift fre-
the absence of datable volcanic ash beds, one way forward quencies with changing sedimentation rates. For the same
to obtain some numerical age constraints could be the dating astronomical imprint under lower sedimentation rates (corre-
of detrital zircons. Even though detrital zircons do not give a sponding to a sedimentary sequence evolving into more dis-
depositional age, recent technological advances make it pos- tal sedimentary environments), the same cycles are respec-
sible to, for example, date a large number of detrital zircons tively less thick and corresponding frequencies are higher.
(with laser ablation inductively coupled plasma mass spec- In general, there is also a higher occurrence rate of event
trometry, LA-ICP-MS), after which the youngest zircons can beds in more proximal and sandstone-dominated intervals,
be very precisely dated (with chemical abrasion isotope di- which are most probably not astronomically influenced. In
lution thermal ionization mass spectrometry, CA-ID-TIMS) our case, the appearance of additional higher-frequency cy-
to come to an informative “maximal depositional age” (e.g., cles in the sandstone intervals of the Postolonnec Fm. shifts
Karlstrom et al., 2019; Landing et al., 2021). A conceptu- the spectral characteristic values to higher values, implying
ally related approach concerns the dating of prismatic zir- lower sedimentation rates. This is the opposite signal than
cons in a Darriwilian limestone bed in Sweden (Lindskog et expected (higher sedimentation rates for sandstone intervals
al., 2017; Liao et al., 2020). For both the detrital zircon and compared to mudstone intervals) and can be explained by the
non-bentonite-associated prismatic zircon dating approaches extra components that appear in the higher-frequency ranges
it is crucial to keep in mind that the numerical age from the (e.g., around 0.55, 0.7 and 0.9 cycles m−1 in Fig. 7) compared
dating comes with an additional (larger) uncertainty on its to the more condensed intervals, rather than changes in sed-
depositional age. Even when such uncertainties would be on imentation rate. It is important to consider that the sedimen-
the order of millions of years, they can still be valuable in tation rate of sandstone is very likely to be higher than con-
densed mudstones, but sandstone intervals equally represent
Geochronology, 4, 251–267, 2022 https://doi.org/10.5194/gchron-4-251-2022M. Sinnesael et al.: Ordovician cyclostratigraphy of the Armorican Massif 263
a less complete or continuous sedimentary record. Therefore, surements trace lithological changes and several crucial sed-
a purely numerical approach to formulate a duration estimate imentary features such as the occurrence of paleo-placers or
for the studied section, for example, might be invalid and not condensation horizons, as reflected by the presence of differ-
necessarily more robust than the more qualitative approach ent types of diagenetic concretions such as nodules.
of Dabard et al. (2015), which is sometimes also applied in The relative variations in the pXRF K and NGR K are very
similar studies (e.g., Loi et al., 2010). Another consideration similar in the measured logs. A comparison with the NGR
is indicating a degree of (un)certainty: some intervals might high-resolution logs of this study and the low-resolution logs
show clearly distinguishable sedimentary cycles, while oth- presented in Dabard et al. (2015) shows that these NGR mea-
ers are less clearly interpretable. By explicitly formulating surements are robust and reproducible. An important reason
these types of uncertainties one can inform potential future for this reproducibility is the fact that the NGR measure-
studies and come to more informative duration estimations ments average out natural radioactivity within a radius of
(Cramer et al., 2015; Sinnesael et al., 2019). at least 10 cm. The pXRF analyses have a ∼ 1 cm spot size.
Formulating cyclostratigraphic uncertainties in a Paleo- This allows for a much higher sampling rate, the measure-
zoic integrated stratigraphic framework is not an easy task ment of individual nodules or paleo-placer beds, but this also
(e.g., Sinnesael et al., 2019; Ghobadi Pour et al., 2020). Stud- means that the measurements pick up more detailed small-
ies that similarly target less conventional facies in younger scale variations and are harder to reproduce over different
stratigraphical intervals might in general have more robust field visits. Another advantage of the pXRF is the multi-
independent age constraints (e.g., Noorbergen et al., 2018) or elemental nature of the analysis.
more reliable astronomical parameters like insolation curves The use of high-resolution pXRF measurements and tra-
available (e.g., Vaucher et al., 2021). This is much less the ditional cyclostratigraphic tools delivered promising results
case for the Paleozoic (e.g., Laskar, 2020), often resulting in to distinguish sedimentary cycles in the relatively homo-
looser temporal constraints on astronomical interpretations. geneous condensed mudstone facies. Application of com-
For example, Sinnesael et al. (2021) reinterpreted the expres- monly used spectral analysis tools for the whole record with
sion of astronomically forced Upper Ordovician sedimentary strongly varying facies is shown to be challenging, with it
cycles on Anticosti Island (Long, 2007; Elrick et al., 2013), having particular difficulty distinguishing longer-term trends
resulting in a different interpretation of the duration of the and potential astronomical signals. In the absence of precise
cycles by an order of magnitude. The use of correlations and independent age constraints, it is not possible to fully con-
ages that are only loosely constrained in order to imply astro- firm the astrochronological framework suggested in Dabard
nomical origins of sedimentary sequences is not uncommon et al. (2015). A potential astronomical signal is suggested in
when interpreting lower Paleozoic records (e.g., Sutcliffe et the more homogenous mudstone facies, while reliable cycle
al., 2000; Gambacorta et al., 2018). Another common prac- identification in the more proximal sandstone-dominated in-
tice is the application of spectral techniques on stratigraphic tervals proves to be challenging. More work is needed to ex-
records that might not be ideal for such type of analysis be- pand our toolboxes to also study non-traditional sedimentary
cause of, e.g., their variable lithologies and the associated archives in terms of astronomical climate forcing to advance
variable expression of the proxies used (e.g., Zhong et al., our knowledge of deep-time cyclostratigraphy.
2018). These challenges accentuate the need for tailored cy-
clostratigraphic methodologies that are not simply a copy of
what has been shown to work well for younger stratigraphic Code and data availability. All produced R code, sedimentary
intervals; instead we need techniques that are adapted to the logs, and geochemical data used for spectral analyses are provided
reality of both the more limited availability of accurate in- in the Supplement.
dependent age constraints and the absence of well-preserved
open marine pelagic sections that characterize the Paleozoic
sedimentary record. Supplement. The supplement related to this article is available
online at: https://doi.org/10.5194/gchron-4-251-2022-supplement.
6 Conclusions
Author contributions. MS, TRAV, and PC conceptualized the
research. MS, AL, and MPD performed the field surveys and
Using pXRF measurements directly on outcrops can be chal- geochemical measurements. AL performed sequence stratigraphic
lenging because of superficial weathering of the rocks and analyses. MS performed spectral analyses. MS wrote the original
the need for flat measurement surfaces for XRF. The pro- manuscript with contributions from all co-authors to the revised
tected Postolonnec beach section is characterized by strong drafts.
tidal ranges and coastal erosion. This creates fresh and
smooth surfaces but they are also prone to be covered by bi-
ological material. Despite the challenging nature of the out- Competing interests. The contact author has declared that nei-
crop, we demonstrate that the non-destructive pXRF mea- ther they nor their co-authors have any competing interests.
https://doi.org/10.5194/gchron-4-251-2022 Geochronology, 4, 251–267, 2022264 M. Sinnesael et al.: Ordovician cyclostratigraphy of the Armorican Massif
Disclaimer. Publisher’s note: Copernicus Publications remains et du Dévonien du Massif armoricain, C. R. Palevol., 3, 353–360,
neutral with regard to jurisdictional claims in published maps and https://doi.org/10.1016/j.crpv.2004.06.003, 2004.
institutional affiliations. Botquelen, A., Gourvennec, R., Loi, A., Pillola, G. L., and
Leone, F.: Replacements of benthic associations in a se-
quence stratigraphic framework, examples from the Up-
Acknowledgements. We would like to dedicate this study to per Ordovician of Sardinia and Lower Devonian of the
Marie-Pierre Dabard, whose knowledge and passion for the Pos- Massif Armoricain, Palaeogeogr. Palaeocl., 239, 286–310,
tolonnec was unprecedented and was taken away too soon. Matthias https://doi.org/10.1016/j.palaeo.2006.01.016, 2006.
Sinnesael thanks the Research Foundation – Flanders (PhD fel- Catuneanu, O., Abreu, V., Bhattachary, J. P., Blum, M., Dalrym-
lowship FWOTM782) and is funded by the European Research ple, R. W., Eriksson, P. G., Fielding, C. R., Fisher, W. L., Gal-
Council (ERC) under the European Union’s Horizon 2020 research loway, W. E., Gibling, M. R., Giles, K. A., Holbrook, J. M.,
and innovation programme (advanced grant no. AstroGeo-885250). Jordan, R., Kendall, C. G. S. C., Macurda, B., Martinsen, O. J.,
This work was supported by the Fondazione Banco di Sardegna Miall, A. D., Neal, J. E., Nummedal, D., Pomar, L., Posamen-
(grant nos. F74I19000960007 and F75F21001270007). Thijs Van- tier, H. W., Pratt, B. R., Sarg, J. F., Shanley, K. W., Steel, R. J.,
denbroucke thanks the Koning Boudewijnstichting (Professor T. Strasser, A., Tucker, M. E., and Winker, C.: Towards the stan-
Van Autenboer Fund) and the Bijzonder Onderzoeksfonds (BOF dardization of sequence stratigraphy, Earth-Sci. Rev., 92, 1–33,
– UGent-BOF17/STA/013) for funding. Philippe Claeys thanks https://doi.org/10.1016/j.earscirev.2008.10.003, 2009.
the VUB Strategic Research Program for funding and the FWO Cohen, K. M., Finney, S. M., Gibbard, P. L., and Fan, J.-X.: The ICS
Hercules foundation for financing the XRF analytical platform at International Chronostratigraphic Chart, Episodes, 36, 199–204,
the VUB. This work contributes to International Geoscience Pro- https://doi.org/10.18814/epiiugs/2013/v36i3/002, 2013.
gramme projects IGCP 652 (Reading geologic time in Paleozoic Cooper, R. A. and Sadler, P. M.: The Ordovician Period, in: The Ge-
sedimentary rocks) and IGCP 753 (Rocks and the Rise of Ordovi- ologic Time Scale 2012, edited by: Gradstein, F. M., Ogg, J. G.,
cian Life). Schmitz, M. D., and Ogg, G. M., Elsevier, Amsterdam, 489–523,
https://doi.org/10.1016/B978-0-444-59425-9.00020-2, 2012.
Cramer, B. D., Vandenbroucke, T. R. A., and Ludvigson, G. A.:
Financial support. This research has been supported by the High-Resolution Event Stratigraphy (HiRES) and the quantifica-
Fonds Wetenschappelijk Onderzoek (grant no. FWOTM782), tion of stratigraphic uncertainty: Silurian examples of the quest
the European Research Council (ERC) under the European for precision in stratigraphy, Earth-Sci. Rev., 141, 136–153,
Union’s Horizon 2020 research and innovation programme https://doi.org/10.1016/j.earscirev.2014.11.011, 2015.
(grant no. AstroGeo-885250), FWO Hercules foundation, the Dabard, M.-P. and Loi, A.: Environmental control on concretion-
Fondazione Banco di Sardegna (grant nos. F74I19000960007 forming processes: Examples from Paleozoic terrigenous sed-
and F75F21001270007), the Koning Boudewijnstichting (Pro- iments of the North Gondwana margin, Armorican Mas-
fessor T. Van Autenboer Fund), the Universiteit Gent (grant sif (Middle Ordovician and Middle Devonian) and SW Sar-
no. BOF17/STA/013), and the Vrije Universiteit Brussel (VUB dinia (Late Ordovician), Sediment. Geol., 267–268, 93–103,
Strategic Research Program). https://doi.org/10.1016/j.sedgeo.2012.05.013, 2012.
Dabard, M.-P., Loi, A., and Paris, F.: Relationship between phos-
phogenesis and sequence architecture: Sequence stratigraphy
and biostratigraphy in the Middle Ordovician of the Armori-
Review statement. This paper was edited by Peter Abbott and
can Massif (NW France), Palaeogeogr. Palaeocl., 248, 339–356,
reviewed by two anonymous referees.
https://doi.org/10.1016/j.palaeo.2006.12.011, 2007.
Dabard, M.-P., Loi, A., Paris, F., Ghienne, J. F., Pistis, M., and Vi-
dal, M.: Sea-level curve for the Middle to early Late Ordovi-
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