This section was not incorporated in the above study and resulting paper about the paleotopographic evolution of the Pyrenees but gives interesting insights about the Pyrenean Paleocene tectonic lull.
Using the same approach than the above study, and using already published data from the southern Pyrenees, we test is δ18O data can trace the Paleocene tectonic quiescence moment inferred by low South pyrenean shortening and Iberia-Europe convergence rates (Grool et al. (2018), Macchiavelli et al. (2017), Rosenbaum et al.
(2002)).
We use carbon and oxygen isotopes measured on pedogenic carbonate nodules sampled in the Ar´en section (Figure 4.7). The sample range from the Thanethian to the Early Illerdian, encompassing the PETM. Paleocene nodules samples (n = 93) come from Hunger (2018). Nodules encompassing the pre-onsetδ13C excursion (POE) and the PETM (n = 170) come from Khozyem Saleh (2013).
Cretaceous Cretaceous-Paleocene Tremp-Graus basin
N
La Puebla de Roda 5 km
Rio Noguera R.
Arén
Rio Isabena Puente de Montanyana
Rio Esera
Eocene
Eocene (Illerdian) - Alveolina lmst Eocene (Cuisian) - Castissent Fm Arén section France
Spain Atlantic Ocean
Pyrenees
study area study area
N
Figure 4.7: Geographical and geological position of the Ar´en section.
The Paleocene δ13C values vary between -10.1 and -5.6 hwith a mean value and 1 SD of -7.1± 0.9 h. δ18O values vary between -5.7 and -3.5hwith a mean value and 1 SD of -4.4±0.4 h. The PETM intervalδ13C values vary between -16.2 and
Pyrenees Eocene topographic evolution 75 -4.6h with a mean value and 1 SD of -11.2± 2.7 h. δ18O values vary between -7.7 and -4.0h with a mean value and 1 SD of -5.8 ± 0.8 h (Figure 4.8).
Carbonate nodules form in situ in the soil and are sensitive to the environmental conditions occurring during their formation (Milli`ere et al. (2011a), see chapter 3). Waters present in the soil are presumed to be in isotropically in equilibrium with precipitation. Therefore, because carbonate nodules precipitate from these waters, the nodules are assumed to preserve theδ18O signal of precipitation (Cerling (1984)). This approach ignores the effect of evapotransporation (see discussion in Caves Rugenstein and Chamberlain (2018)). Evapotransporation is the process by which moisture returns to the atmosphere through soil evaporation and plant transpiration. Evaporation processes favour the evaporation of16O thus maintaining the atmospheric δ18O values of vapour relatively high. For this reason, because evapotranspiration is not constrained in this study, the present δ18O data from paleosol nodules is not used to infer quantitatively the Pyrenean paleotopography.
However, because δ18O decreases with a rate of ∼0.28 h/100 m (Poage and Chamberlain (2001)), we assume that a relative change in paleotopography would still visibly affect theδ18O values of soil carbonates.
56 tie-points
Khozyem, 2013 tectonic quiescence
Figure 4.8: δ18O record from the Ar´en section from pedogenic carbonate nodules from Hunger (2018) and Khozyem Saleh (2013). Global curve from Cramer et al. (2009). Both curves have a 9 point moving average. Iberia-Europe convergence rate from Macchiavelli et al. (2017), southern Pyrenees shortening rates from Grool et al. (2018). POE: pre-onset
δ13C excursion.
The PETM minima was used as a tie-point to correlate the Ar´en section and the global isotope records using the AnalySeries software (Paillard et al. (1996)).
Unfortunately, no particular event occurs in the δ18O global record during the Paleocene, which would allow — similarly to the above chapter — a comparison between the local and global isotope curves. The δ18O values throughout the Paleocene show little variation (-4.4±0.4 h, Figure 4.8). And likewise to the δ18O values of soil carbonates from the Chiriveta section, the soil carbonates from the Paleocene are likely stabilized by the position close to the coast of the Ar´en section (Cerling (1984), Figure 3.6). Therefore, the stable δ18O values of pedogenic carbonates in the Ar´en section suggests no important changes in δ18O values of precipitation and river waters. This is in line with the tectonic quiescence inferred by low South pyrenean shortening and Iberia-Europe convergence rates (Grool et al. (2018), Macchiavelli et al. (2017), Rosenbaum et al. (2002)).
Chapter 5
Investigating the potential of small hyperthermals as
stratigraphic correlation tools in fluvial environments
In this chapter are addressed additional and more prospective questions to those already discussed in chapters 2 to 4. In particular the potential of hyperthermal
”U”, identified in chapter 3, as a correlation tool in fluvial environments. These issues were not submitted in a peer-reviewed journal.
5.1 Introduction
Correlation between stratigraphic sections are essential to geologists to apprehend the evolution in time and space of sedimentary systems. A correlation between two or more sections (from field logs or cores) allows a 2D and sometimes 3D view of past landscapes. Correlations along sedimentary sequences provide a ”film” of the evolution of landscape through time and space and is therefore the basis on which to build hypotheses about the internal/external drivers controlling sediment transport, deposition and preservation and the origin of stratigraphic sequences on geological timescales.
77
For instance, correlation methods such as sequence stratigraphy highlighted the changes in relative sea level as a main factor driving the sediment flux to the basins (e.g., Vail (1987)). Even if sequence stratigraphy can ”force” a correlation by imposing the concept over the observation (Miall (1992)), it has showed to be a powerful tool and still is extensively used in the oil and gas industry. First applied to the marine and deltaic environments it has been extended to the continental environment in order to better understand how sea level modulates fluvial architecture and channel amalgamation because of the importance of alluvial reservoir for the oil and gas industry (Shanley and McCabe (1994), Wright and Marriott (1993)).
On a small-scale and with optimal outcrop condition (i.e., the Book Cliffs, Mi-all and Arush (2001)) correlation can be achieved by field and aerial mapping.
However, when the outcrop conditions don’t allow the tracing of surfaces from a section to an other, different approaches have to be used to constrain a correlation between two (or more) sections. For instance, biostratigraphy — dating using fossil assemblages — can constrain a correlation assuming the change of fauna occurs simultaneously and that the fauna is present at all the sites. Because of the widespread and ubiquity of some marine fauna (i.e., foraminiferas), biostratigraphy is a well established tool for marine correlations (e.g., France-Lanord et al. (1993), Westerhold et al. (2017)). However it’s use is obviously limited for marine to continental environments correlations because the same fauna is not present in both environments. Magnetostratigraphy, based on the fact that Earth’s magnetic field changed through time from normal (i.e., like today with the north magnetic pole near the north geographic pole) to reversed (i.e., north magnetic pole near the south geographic pole), is also a widely used tool to date and correlate sections (e.g., Bentham and Burbank (1996)). However, these change occur randomly and few million years can pass without any change such as the Cretaceous normal superchron C34, which lasted about 40 Ma (Gradstein et al. (2012)).
During the Eocene, in the south Pyrenees, moments with no change in magne-tozone or biozone can make correlation challenging, especially in the continental environment where biozones are very scarce, for instance during the Castissent Formation (Fig. 5.1). The Castissent Formation is a major basin-axis oriented fluvial progradation of about 50 m thick coarse sandstone interbedded with vivid yellow to red paleosol corresponding to overbank deposits (Marzo et al. (1988), Mutti et al. (1988)). This fluvial formation is commonly divided in two or three
Hyperthermals as correlation tools 79
Figure 5.1: Eocene time-interval with no change in magnetozone or biozone. The time-interval in red corresponds to the Castissent Formation deposition time.
members: Castissent 1 and 2 (Mutti et al. (1988)) or A, B, and C members (Marzo et al. (1988), Nijman (1998)). See chapter 3 for a more detailed description of the Castissent Formation. Westwards, the fluvial character of the Castissent Formation gradually evolves to more littoral environment were it consists of distributary channel fills, mouth bars and crevasse deposits (Nijman and Nio (1975)). The Castissent finally becomes turbidites deposits in the Ainsa basin where it corre-sponds to the Fosado and Arro systems, which are separated by important thickness of hemipelagic marls (Mutti et al. (1988)). See chapter 2 for description and figures of the turbidites systems. The correlations between the Castissent continental and marine environments is well accepted and correlates the Castissent 1 member with the Fosado system and the Castissent 2 member with the Arro system (Mutti et al.
(1988)). Yet, a more high-resolution correlation focusing on when the fluvial and turbiditic sand bodies as well as the hemipelagic marles intervals of the Arro and Fosado systems and Castissent Formation are deposited respectively is still lacking.
While tracing the Castissent Formation as a whole is to a certain extent straight-forward due to it prominence in the landscape (e.g., Poyatos-Mor´e (2014)), high-resolution correlation between continental sections and Castissent members is more complex. Until now, correlations based on the mapping of marker beds and flooding surfaces (Marzo et al. (1988), Nijman (1998)) have been proposed.
However, due to the internal dynamics of fluvial system (i.e., erosion, non-uniform
deposition, Foreman and Straub (2017)), correlation based on the hypothesis that fluvial layers are continuous and traceable is critical. The Castissent Formation important sand progradation is interpreted to be triggered the combination of sea level fall and an important pulse of exhumation and thrust activity in the hinterland (Marzo et al. (1988), Puigdef`abregas and Souquet (1986), Castelltort et al. (2017)). However, in chapter 2, we discussed the non-uniqueness of the controls on the Castissent deep-marine time-equivalent clastic deposition. A robust correlation in the Castissent Formation would bring new indications to discuss the evolution of its fluvial architecture through time and space allowing to put forward the driver(s) controlling its continental and marine deposition and therefore address the question of the origin of this important sand deposition. This is crucial, for instance for reservoir purposes, to better understand what and to which extend controls the deposition of this important sand deposit.
The Eocene is a time-period with several climatic events (see chapter 1.2.2), among which short-lived global perturbation of the carbon cycles known as ”hyperthermal events” mark the sedimentary record with sudden decrease in global δ13C values (e.g., Westerhold et al. (2018)). The Paleocene-Eocene thermal maximum (PETM) is the most prominent of these events and because of its global impact, it has been use as marker for correlation between marine and continental sections (e.g., Koch et al. (1992), Duller et al. (2019), Figure 5.2). During the deposition of the Castissent Formation, a smaller-scale hyperthermal U occurs (Lauretano et al.
(2016), chapter 3) and can therefore be used as marker to correlated two continental, and one deep-marine sections encompassing the Castissent Formation time.
The aim of chapter 3 has been to identify hyperthermal U, within the Castissent Formation. The objective of the following is to establish a δ13C record in the continental and marine Castissent time-equivalent successions in order to determine if:
1. hyperthermal U can be used to correlate fluvial and marine sections.
2. hyperthermal U is preserved in an other Castissent continental section.
3. hyperthermal U can be used to correlate fluvial sections with each other.
Hyperthermals as correlation tools 81
Figure 5.2: Correlation between continental and marine section using the negative carbon isotope excursion of the Paleocene-Eocene thermal maximum (PETM) as a
marker. From Duller et al. (2019).