the supposedly Thanetian deposits at the northern flank of the Ager basin, and are shown and discussed in paragraphs below.
A comparison with Paleocene global trends could be speculative, primarily because of the data gap from the middle portion of the succession, and because of the lack of well-constrained chronostratigraphic data from the Paleocene of the Ager basin; a comparison can be made with carbonate isotopic data from the Esplugafreda section (northern Tremp basin) by Schmitz & Pujalte (2003). Features in common with the present work are the relatively-low variability of oxygen data, and the lighter values in the lower Thanetian with respect to the late Thanetian ones.
These results are opposite to the global trend shown in the late Paleocene data from Zachos et al. (2001), and are likely to represent a local feature limited to the southern Pyrenees foreland.
5.5.3 – Northern Ager basin, Danian – lower Thanetian (?)
Section [12] is the only section where inorganic and OM isotope data were studied altogether.
Figure 5.3 shows the vertical variations of inorganic ∂18O and ∂13C, and organic ∂13C, that are plotted against the stratigraphic log. Data from bulk micrites at the base of the succession are plotted for completeness (see 5.4.1.2).
The succession records an overall positive shift in stable isotopes values by a ~1‰, with values ranging in a wider interval (up to ~2,5‰ in carbon data). Where the record is better resolved (i.e., in the interval rich in pedogenic carbonate nodules), data show some major shifts, two of which are very well coupled in carbonate data (in concomitance with units tagged AF1 and AP2); OM carbon data seemingly follow the same pattern.
Figure 5.3 (next page) - Stratigraphic log and isotope trends in the continental deposits of section [12]. Color shades are provided in order to highlight relevant horizons discussed in the text.
If compared with the rock record, this progressive enrichment in heavy isotopes shows up in a coarsening succession (from mud-dominated to sandy and pebbly paleosols; see 4.2.1.2);
conversely, the deposition of a coarse unit (AF2 in figure 5.3) does not occur together with a significant change in isotopic values; this may reflect a sharp increase in sedimentation rate in that specific interval, up to two or three times with respect to the mud-dominated portions of the succession. This inference, anyway, assumes that no significant breaks occur during the deposition of the alluvial plain facies, but at least one break occurs in concomitance with the formation of the yellow paleosol horizon just below the AF1 unit. This is testified by field observations, since this horizon crops out as an hardened crust, probably the effect of a relatively prolonged interruption of sedimentation – and associated early diagenesis. A potential trace of this process stands in the strong negative shift in organic carbon isotopes measured in this horizon (notice that carbonate soil nodules are here absent).
5.5.4 – Southern Ager basin Paleocene
Figure 5.4 shows the vertical variations of organic and inorganic carbon along section [4].
Since sampling resolution was variable because of different outcrop conditions, the organic and inorganic record cannot be properly compared, but a better insight can be achieved by considering some sedimentological features.
Figure 5.5 displays in better detail the lower portion of the alluvial succession, including organic carbon data and a comparison with section [12].
No petrographic or compositional studies have been performed on these strata, but a detailed case-study from Kraus and Riggins (2007) on late Paleocene – early Eocene continental deposits from Bighorn basin (Wyoming) has been used as a guideline for field interpretation. The main difference from the chosen model lies in the definition of the “light red” paleosols, probably a facies different from the one found associated with the pseudoconglomeratic micrites within Unit URG 2;
the latter may be more closely related with the “purple” facies.
Kraus & Riggins's model offers an interpretation of paleosol facies based on physical and geochemical data, displaying a relation between drainage/moisture conditions and paleosol color;
this relation is synthesized in the legend of figure 5.5.
Figure 5.4 – Stratigraphic log and carbon isotope trends in the continental deposits of section [4].
5.5.4.1 – Unit URG 1
Despite the lower frequency of sampling in section [4], lower red paleosols are seemingly characterized by a small range of organic carbon isotopic compositions between – 23 and – 24 ‰ vs V-PDB. The same range applies for most of the samples collected in section [12].
A couple of yellow paleosol horizons develop near the base and the top of this interval, showing antithetic shifts in CORG isotopic values. The lower one shows a slightly lighter composition with respect to the red paleosols, and it hosts a thin layer of OM-rich blue-green mudstone presenting a peak light carbon isotopic composition. Conversely, the upper yellow horizon shows a composition heavier than the ones from red paleosols. This horizon, anyway, develops directly above a sandy layer of fluvial origin, and the color and isotopic composition might reflect different conditions from paleosols forming in interfluves.
The lower red paleosols are further characterized in their upper portions by color mottling and gypsum nodules, both scattered in the sediment and organized in layers. The presence of these concretions, described by Colombo and Cuevas (1993) as rhizomal features, is not apparently accompanied by relevant changes in CORG isotopic composition.
5.5.4.2 – Unit URG 2
The facies association is mainly represented by yellow, intensely mottled paleosols, together with pink layers commonly underlying or including pseudoconglomeratic limestones (see 4.2.1.2);
some layers of fluvial sandstones are intercalated as well.
This interval roughly corresponds to the lower portion of Sequence 2 of Rossi (1997) that correlates the palustrine deposits with coastal lakes in the western portion of Ager basin; all these observations agree with relatively wetter conditions with respect to the lower red paleosols.
Isotopic data initially display very negative values associated with a yellow-brown horizon particularly rich in gypsum concretions. Two sharp positive peaks are then recorded in concomitance with the main layers of palustrine/pedogenic limestones; values then drop significantly just above the limestone layers.
The pink mudstones commonly grade to blue-grey mudstones, following former papers (Rossi, 1997; Kraus & Riggins, 2007), are interpreted as gley paleosols, possibly representing the horizon that formed under peak moisture conditions.
5.5.4.3 – Unit URG 3
Unlike Tremp basin, whose late Thanetian deposits are primarily represented by red paleosols (cf. Rosell, 1967, 2001; Cuevas Schmitz & Pujalte, 2003; see 4.3.1.2), yellow paleosols dominate during this period in the Ager basin. Unfortunately, the study of this interval is hampered by a bad exposure, and a red/yellow color banding observed at the base and at the top of this interval might actually represent its original aspect.
Pedogenic facies suggest a relatively wet or poorly drained environment, eventually shifting to drier conditions in the subordinate red paleosol horizons.
CORG isotopic data are very scarce in this interval, and are apparently limited in the range of -24‰.
5.5.5 – Ager basin Paleocene N-S correlation
Although a paleosol facies change from the north to the south cannot be excluded (cf. Oms et al., 2007), red paleosols to the north are more likely to be correlated with a similar unit to the south;
actually, major changes should be expected to occur in the E-W direction, i.e., from the inner basin to the continental-marine transition: instead, pedogenic facies belts appear laterally continuous over many kms in this direction.
Organic matter carbon isotopic data suggest an equivalence between the northern red paleosols and the lower exposed interval of the red paleosols of the southern succession, because they share a similar range of isotopic compositions, including a negative excursion occurring along a yellow paleosol horizon, implying a Thanetian age for the northern paleosols; this correlation is presented in figure 5.5.
Figure 5.5 – Stratigraphic logs and organic carbon isotope trends in sections [12] and [4]. A tentative correlation is proposed (see text).