Two continental sections of the Castissent Formation, the Chiriveta and La Roca sections, were sampled for correlation (Figure 5.3). Hereafter only the La Roca section is detailed. The reader is refereed to the Material and Methods of Chapter 3 for the Chiriveta section description, material, and methods not described hereafter.
Figure 5.3: A - Geological map of the study area. B - transect of the Castissent Formation showing members A to C and the correlation based on field mapping and
tracing of marker beds. From Nijman (1998)
A total of 37 samples were collected from the early Eocene La Roca section for geochemical studies. All samples consist of floodplain material and were taken below the weathering depth (∼50 cm). When important sandbodies occurred, lateral equivalent floodplain material or intercalated palaeosol horizons were sampled. The carbonate nodules were extracted from the bulk palaeosol material by sieving and then cleaned by repeated washes with deionized water in an ultrasound bath. From each cleaned nodules set, subsamples of one to four nodules were taken, leading to a total of 85 subsamples of pedogenic carbonate nodules.
Hyperthermals as correlation tools 83
Marine sands and marls Fluvial channels Paleosol Red bed Mottled silts/sands Dark gray paleosol Nodule rich layer hyperthermal U
Figure 5.4: Isotope data from the Chiriveta and La Roca sections. For the isotope dataset, the curves passes through the mean values at each sample position. In the La Roca section, carbon isotope excursion over 2 standard deviations are named 1 and 2.
5.4 Results
5.4.1 Sedimentology of the Castissent Formation at La Roca
We describe here the La Roca section logged and sampled in this work. The interpretation of the La Roca section is based on the mapping by Marzo etal. (1988) who traced four marine incursions (M0 to M3) throughout the basin (Figure 5.3).
These marine incursions are more distinct in La Roca than in the Chiriveta section because of the more distal paleogeographical position of the section. The Castissent Formation channel-fills and paleosols in the La Roca section are interbedded with alluvial fan deposits with northern provenance (Marzo et al. (1988)). At the base of the section, marine incursion M0 is characterized by a coarse sandy deposit with cross-stratifications and oyster shells. In the La Roca section, the Castissent Member A is a∼20 m thick interval comprising coarse-grained channel-fill deposits characterized by important basal erosion with a thick central part (up to 7 m) and thinner (2 m) sheet-like edges (Marzo et al. (1988)). A 4-m-thick red bed overlies the Castissent A member and several iron-oxydes nodules are found in a 30-cm, strongly mottled interval above the red bed deposits. A 30-m-thick succession of
paleosols follows until marine incursion M1. The second marine incursion M1 is located at 70 m and consists of a 8-m-thick grey sandy interval with oyster shells.
The Castissent B Member is only represented by 14 m-thick floodplain deposits interpreted as lateral floodplain deposits of Castissent B channels not visible in the La Roca section. A laterally extensive northern alluvial fan deposits separates the Castissent B floodplain deposits and the marine incursion M2 which is expressed as two gray paleosols. Castissent C member is expressed as as sheet like channel-fill with erosive base and mottled tops. A thick alluvial fan related succession precedes a marly interval with bivalve shells debris interpreted as marine incursion M3.
5.4.2 Stable isotope record
Carbon and oxygen isotope ratios from the carbonate nodules are presented in Fig.
5.4 and in Table B.9. The δ13C values vary between -12.2 and -2.9 hwith a mean value and 1 SD of –8.4± 1.6h. Three carbon isotope excursions (CIE, at 38, 110 and 124 m) are more negative than -10.04 h (i.e., the mean value – 1 standard deviation) amongst which two are below 2 SDs named CIE 1 and 2 respectively in Figure 5.4. The values are –11.9, 10.8, and 12.2 h for CIEs from bottom to top respectively. At the bottom of the section, the first δ13C values are relatively constant around the mean value. Just above a thick red bed, CIE 1 occurs and is again followed by a relatively constant interval of mean δ13C values. The second part of the section is also marked by an important negative CIE (CIE 2), and it is followed by a strong δ13C shift toward more positive values. The last samples are relatively constant around the mean δ13C value. Both CIE are of similar amplitude.
The δ1(O values vary between –7.0 and –4.8 h with a mean value of –6.0 ± 0.4 h, which makes them less dispersed than the δ13C record. Four negative oxygen isotope excursions are more negative than the mean value – 1 SD, amongst which one is below 2 SD reaching a minimum value of –7.0 h at 24 m. The oxygen isotope excursions do not correspond with CIEs described above.
5.4.3 Automated petrography (QEMSCAN)
Iron nodules found in the sampled of CIE 1 from the La Roca section were mounted in epoxy plugs, polished and carbon-coated. Composition of the iron nodules was assessed by QEMSCAN® (Quantitative Evaluation of Materials by Scanning
Hyperthermals as correlation tools 85 Electron Microscopy) imagery at the University of Geneva. QEMSCAN associate scanning electron microscope (SEM) imaging and elemental analyses to create mineralogical maps of studied samples (Allen et al. (2012)). Analyses were carried out under high vacuum conditions (10−6 mbar) and at an acceleration voltage of 15 kV with a probe current of 10 nA. The QEMSCAN® processing point-spacing between two analyses is 2 µm and detection limit is 2 %.
Figure 5.5: QUEMSCAN®image of the iron-barite nodule found at La Roca section in sample ICS9 in the Isabena valley. QUEMSCAN® detection limit is 2 %. Point-spacing between two analysis is 2µm. A- Back-Scattered Electrons (BSE) image. Lighter gray shades indicate denser minerals. B - Iron map of the nodule. At least three phased of iron precipitation can be observed. C - Barium map. Barium precipitation is only
observed in the external rim of the nodule. D - Sulfur map.
5.5 Discussion
5.5.1 Hyperthermal U record in La Roca
In order to estimate its capacity to record a hyperthermal event of 40 kyr, we estimate the compensation timescale (Tc) in the La Roca section. In fluvial
environments, Tcgives an estimate of a timescale below which stratigraphic signals may be of autogenic origin and should therefore be interpreted carefully (Wang et al. (2011), Foreman and Straub (2017), Trampush et al. (2017)). It is given by the maximum channel depth divided by the average subsidence rate Wang et al.
(2011). Assuming a sedimentation rate of 0.34 mm/yr (i.e., twice the sedimentation rate as in the Chiriveta section because the section is twice as thick) and a mean and maximum channel depth of 4 and 7 m respectively, we obtain Tcvalues ranging between 10 and 20 kyr. A hyperthermal events with a duration of about 40 kyr has therefore the potential to be recorded in the La Roca section despite the fluvial system internal dynamics.
Based on previous mapping (Marzo et al. (1988), Nijman (1998)) and isotope analyses (c.f., Chapter 3) hyperthermal U was identified in the Castissent A floodplain deposits in the Chiriveta section. In this section, the isotopic record is marked by a stepped negative δ13C excursions which peak with a cluster of 5 samples with values lower than the mean minus 1 standard deviation. The minimum value of this group of samples is -10.9 h representing a value above two standard deviations and is interpreted as the climax of the hyperthermal U event.
The isotope record of the La Roca section has a lower resolution and clear stepped δ13C excursions are not observed. This detailed isotope evolution preceding hyper-thermal U observed in the Chiriveta section and lacking in the La Roca section can be attributed to the sampling path going through the Castissent member A channels which show highly erosive bases. No other lateral floodplain equivalent was available to obtain a more continuous section.
However, in the same interval as hyperthermal U in the Chiriveta section, a sample shows a clear negative δ13C excursion (CIE 1, -12.2 h) at 38 m (Fig. 5.4). In the Chiriveta section, the low δ13C values during hyperthermal U are interpreted as being caused by an increase in soil respiration triggered by warmer temperatures coupled with high primary productivity. This is interpreted to lead to a greater input of carbon to the soil ultimately leading to a higher soil respiration of organic matter (see Chapter 3 and reference therein). If this is correct, the δ13C negative excursion of CIE 1 would imply warmer temperatures consistent with a hypethermal event. Moreover, the sample showing the negative δ13C excursion also contains several iron oxides nodules. One of these nodule analysed by QEMSCAN imagery and elemental maps of iron, barium and sulfure are shown in Figure 5.5. Both barium and sulfur maps overlap as well as the back-scattered electrons (BSE)
Hyperthermals as correlation tools 87 image indicating a denser mineral. Altogether, it suggests authigenic barite crytals (BaSO4) (Figure 5.5). Iron-barite nodules can be characteristic of distinctive environmental deposition conditions. According to Jennings et al. (2015), these nodules form in a stable low-gradient landscape with reducing conditions that last long enough to release barium from barium-rich parent material into the soil solution. For Retallack and Kirby (2007), barite nodules can be related to some green-house climatic spikes, which could have produced soil leaching waters enabling barium mobility, or widespread waterlogging and anoxic conditions in soils, which may have accelerated bacterial barite accumulation. Abnormal barite accumulations have likewise been found in deep-sea sediments related to methane discharge from the ocean floor or through increased fluvial barium input during the PETM (Dickens et al. (2003)). Iron-barite nodules do indicate peculiar climatic and/or landscape conditions and may be related to hyperthermal events, making it a potential correlation point with the Chiriveta section. The upper part of the δ13C record in La Roca, although of lower resolution shows a similar trend as the Chiriveta section.
If the data is correct, hyperthermal U is recognized in both continental sections and also in the same interval (i.e., Castissent A floodplain). This result would confirm the mapped based correlation using the marine incursions (Marzo et al.
(1988), Nijman (1998)). However, the individual red beds used as correlation by Marzo et al. (1988) do not seem to be traceable through the sections although the red bed as an whole interval seems to be a consistent marker (Fig. 5.4). This result suggests that hyperthermal events, even of small scale regarding the PETM, are a potential tool that can be used for correlation in continental environments despite the natural dynamics inherent to these environments. Yet, the lack of data, especially in CIE 1 (only constrained by 1 sample), makes the correlation point of CIE 1 and hyperthermal U uncertain. A higher resolution and/or another δ13C record in a Castissent Formation section are needed to confirm this peak.
5.5.2 Influence of hyperthermal U on fluvial architecture
It is unlikely that the hyperthermal U had an influence on the origin of the Castissent Formation. Firstly, because the duration of the hyperthermal event (∼
40 kyr) is significantly smaller than the Castissent duration (∼ 0.6 Ma). Secondly, situated between the Castissent A thick channels and marine incursion M1, the
hyperthermal event does not seem to be responsible of an important depositional event.
valley incision
Castissent A ideal high-frequency sequence
red beds
late valley aggradation floodplain
aggradation
δ13C trend
decreasing
minimum
returning to background
values initial valley aggradation
background values
Figure 5.6: Influence of hyperthermal event ”U” on high-frequency stacking sequence of the Castissent A member. Modified from Marzo et al. (1988)
However, Marzo et al. (1988) describe several aggradational-degradational cycles within the Castissent A member (Figure 5.6), which are attributed to an unnamed high frequency allocyclic control (Emery and Myers (1996)). Regarding the stack-ing model (Figure 5.6), low rates of sedimentation in the floodplain favour the development of widespread red beds. Red beds are condensed surfaces marking the beginning of a fluvial sequence in the floodplain. The yellow floodplain deposits overlying the red beds, record the initial and late aggradation fluvial sequence.
This alternation of red bed and yellow paleosol succession is clearly observed in the member A of the Castissent in the Chiriveta section (Figure 3.5). Interestingly, each red bed is equivalent with aδ13C minima, suggesting a link between theseδ13C pulses are the fluvial amalgamation sequence. Hyperthermal events are interpreted to increase the hydrological cycle and enhancing seasonal precipitation (Slotnick et al. (2012)) ultimately influencing river transport capacities through an increase water flux (Armitage et al. (2015)). Moreover, models have shown that rivers are sensitive to changes in water discharges (Simpson and Castelltort (2012)). We suggest, based on the model by Marzo et al. (1988) that during each negative δ13C pulse, the increase sediment transport capacity induced by the increased water flux allows valley incision and sediment transport to the distal as well as the development of red beds because of the low sedimentation rate in the floodplain. During times
Hyperthermals as correlation tools 89 of reduced hydrological cycles (i.e., when the δ13C increases), water flux decreases together with sediment transport capacity, which allows a steady aggradation in the channel and floodplain. If this can be proven correct, it would suggest that hyperthermal events can influence fluvial architecture at high-frequency leading to potential predictive model in sandbody occurrence and connectivity.