Author: Philip Allen
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Erosion rates in the source region of an ancient sediment routing system: Comparison of depositional volumes with thermochronometric estimates

Nikolaos A. Michael, Andrew Carter, Alexander C. Whittaker & Philip A. Allen

SUPPLEMENTARY MATERIAL 

Provenance Methods

In order to link sources and sinks in the Escanilla palaeo-sediment routing system a variety of provenance tools have been used. The tools used are palaeocurrent directions, U/Pb geochronology of detrital zircons, clast lithology counts of gravels, heavy mineral point counting and detrital apatite fission track analysis. 

The provenance analysis was focused on the mid-upper Eocene stratigraphy of three wedge top sub-basins in the South-Central Pyrenees (the Pobla, Tremp-Graus and Ainsa basins) comprising five outcrop areas: 1) Sis palaeovalley, 2) la Pobla de Segur and Gurp escarpment, 3) Viacamp, 4) Lascuarre and 5) Ainsa Basin. Apart from these five focus areas, published and unpublished information from the Jaca Basin was incorporated from other authors.

Clast Lithologies:
For the Escanilla and Antist/Collegats polymictic systems, we compiled clast lithology count data from proximal feeder points to depositional zones in the Tremp-Graus and Ainsa Basins to verify the connectivity between outcrop stations and to evaluate the evolution of the source and catchment areas through time. Table 2 in the Main Text summarizes the main clast types that have been recorded along the sediment routing system fairway and their possible provenance. The clast lithological types have been allocated to six groups of similar provenance. Clast lithology data were acquired using a random Wolman point counting technique, a methodology explained in detail in Wolman (1954), Whittaker et al. (2010, 2011) and Parsons et al. (2012). With this technique 100-200 clasts were counted and their lithology reported at a total of 117 localities. Data were collected from conglomeratic bars and conglomeratic sheet facies that span the three time intervals of the late Lutetian to Priabonian Escanilla paleo-sediment routing system, and covers 80 km of down-system distance. 

The dominant clast types found in the Escanilla palaeo-sediment routing system are Tertiary sandstone clasts and grey Mesozoic limestone clasts. Additionally, in some stations a large proportion of clasts is represented by Triassic red sandstone and conglomeratic clasts and Devonian/Palaeozoic limestone clasts (black and nodular limestones), which have a provenance in the Nogueres Zone (Vincent, 2001; Saura & Teixell, 2006; Whittaker et al., 2011; Parsons et al., 2012). Granitic clasts come from Hercynian plutons in the Axial Zone, and metasedimentary/phyllite and mafic igneous rocks also have source areas in the Axial Zone. These last two clast types are the most useful provenance indicators in the Escanilla palaeo-sediment routing system.

In the Sis palaeovalley granitic clasts are the characteristic clast type; granitic clasts are almost absent in the Gurb fans. For the Gurb region, the characteristic clast types are common green metasedimentary sandstone clasts, mafic and ultramafic clasts, phyllites and slates, which for this study have been grouped together. These clast types are completely absent in the Ecanilla-aged equivalents in the Sis palaeovalley and are only found in the Rupelian Antist or Collegats units.

Metasedimentary and granitic clasts are identified in the Escanilla Formation both in the Viacamp and Lascuarre localities and interpreted as due to the mixing of clasts from these two feeder points. The mafic and ultramafic igneous rocks, which are dominant at some sites in the Gurb area, are very sparse at Viacamp and almost absent at Lascuarre. From the group of igneous and metasedimentary lithologies, only metasedimentary green sandstone survives and is present in abundances of 5-10 %. Granitic clasts are present but make up only 2-5 % of the total clasts measured.

At Ainsa, granitic clasts are almost absent; this is interpreted as the result of abrasion, disintegration and selective extraction of the granitic clasts upstream of the Ainsa sites. Similarly, metasedimentary clasts are very few in number and only make up a few percent of the clasts recorded. In the Ainsa Basin, the dominant clast lithologies are grey Mesozoic limestones, Tertiary sedimentary clasts and Triassic red sandstones. This same mixture of clast lithologies is found in the Jaca Basin in the Lower Campodarbe (Upper Escanilla-equivalent) unit based on the provenance study of Jolley (1987).

In summary, clast lithology data show that there are two distinct signatures in conglomerates from Gurb and Sis, and we see a mixing of these two signals in the Tremp Basin. With distance, exotic clasts sourced from metamorphic and granitic basement massifs break down, representing few clasts in Ainsa, which is dominated by the presence of Triassic red-sandstones and more durable Mesozoic limestones and Tertiary sandstones. Similar abundances of clast types are found in the Jaca Basin (Jolley, 1987) - Triassic, Mesozoic limestones and Tertiary sandstone clasts. Overall, the clast lithology data in the Tremp Basin show that Viacamp and Lascuarre were teleconnected with the Sis and Gurp fanglomerates in the north of the Tremp Basin, which evidently acted as feeder points of sediment for the Escanilla units in Viacamp and Lascuarre. The abundances in the Ainsa and Jaca Basins indicate a teleconnection between the two basins, as indicated from previous sedimentological and sediment budget studies (e.g. Jolley, 1987; Bentham et al., 1992; Hogan & Burbank 1996; Teixell, 1998; Dreyer et al., 1999).

Palaeocurrents:
Palaeocurrent indicators in conglomeratic and sandstone-rich facies were measured. Clast imbrications and gutter casts were measured from conglomeratic bars and azimuths of planar cross-stratification, climbing ripples and plan view orientation of trough cross-bedded sets were measured from sandstone-rich facies. The palaeocurrent data were acquired from more than 150 stations with normally 30-60 measurements at each station along the fully fluvial segment of the fairway, from Gurb and Sis palaeovalleys to Ainsa (Olson) localities (Fig. 2B in Main Text). This fluvial system is thought to have sourced the Escanilla time-equivalent units in the Jaca Basin (Bentham et al., 1992; Nijman et al., 1998; Teixell, 1998; Dreyer et al., 1999; Remacha et al., 2005; Das Gupta & Pickering, 2008; Caja, et al., 2010; Whitchurch et al., 2011). In order to reconstruct the system in the Jaca Basin the provenance and palaeocurrent work of Jolley (1987) and Puigdefàbregas (1975) were incorporated.

Figure 2B in the Main Text summarizes the data from all time intervals combined in the localities of Gurb, Sis, Viacamp, Lascuarre and Ainsa. In Gurb, the palaeocurrent indicators are mainly towards the S-SSW and SW and similar directions are seen in the Sis palaeovalley, with palaeoflows ranging from SE to SW. The palaeoflow information from Viacamp shows a shift from the dominant SE-SW directions in the northern part of the Tremp Basin to predominantly to the NW and W and fewer towards the SW, S and SSE. In the Lascuarre region, the palaeoflow directions are also variable, with the strongest palaeoflow components towards the W and WNW, with significant components towards the SW, SSW, S and even SE. In the Ainsa Basin, the palaeoflows have a bimodal and highly variable pattern, with the majority of palaeocurrents towards the NE, N and even NW and some measurements towards the W and E, and some important components towards the SW. The palaeoflow directions towards the NE to NW are found in the Lower Escanilla (upper Lutetian and Bartonian) interval, following the same palaeogeography as the underlying Hecho group turbidites (Remacha et al., 2005; Das Gupta & Pickering, 2008; Caja et al., 2010; Whitchurch et al., 2011). In the Priabonian Upper Escanilla units a more highly variable palaeocurrent pattern is revealed with the majority towards the N, but also a strong component towards the SW and S, as also noted by Labourdette & Jones (2007).

In the Jaca Basin, the most comprehensive studies are the original work of Puigdefábregas, who mapped the area in 1975, and the doctoral work of Jolley (1987). The dominant palaeoflow is towards the W in the marine upper Eocene intervals of the Jaca Basin in the Belsué-Atarés Formations. In the Lower Campodarbe, which is time-equivalent of the Upper Escanilla (Hogan and Burbank, 1996; Costa et al. 2010) palaeoflows are towards the S and SW close to Santa Orasia alluvial fans and towards the WSW, W and NW in the rest of the basin.

The palaeocurrent data therefore show that the dominant palaeoslope was towards the W, in agreement with the prevailing palaeoflow seen in palaeogeographic reconstructions (Bentham et al., 1992; Nijman, 1998; Dreyer et al., 1999; Remacha et al., 2005; Das Gupta & Pickering, 2008; Caja et al., 2010; Whitchurch et al., 2011). Palaeocurrent data indicate that the Sis and Gurb palaeovalleys had palaeocurrents towards the S, swinging towards the W in the southern Tremp Basin influenced by the Montsec thrust system. In the Ainsa Basin, rivers flowed towards mostly the N, influenced by the Mediano and Boltaña anticlines, which were already palaeohighs at this time. Rivers must have gone around the northern limit of the Boltaña anticline to source sedimentary units in the Jaca Basin.

Heavy minerals:
Heavy mineral analysis is a well-established and standard provenance tool (Carter 2007). Heavy minerals are very helpful in pinpointing source areas and mapping out sediment trajectories. The heavy mineral analysis was performed by Dr. Maria Mange and was carried out in two out of the five localities that have been studied in detail. Samples were collected along mapped time-lines (Fig. 2A in Main Text) of upper Lutetian to Rupelian stratigraphy in the Sis palaeovalley area (32 samples) (Vincent, 2001; Beamud et al., 2003) and in the Lascuarre section (9 samples) (Bentham, 1992). Heavy mineral fractions of these samples were extracted from crushed sandstone samples using the heavy liquid bromoform (CHBr_3, s.g. 2.85-2.89). The extracted heavy mineral fraction was then mounted on glass slides using epoxy and these mounts were point counted by mineral identification to record the presence and abundance of the various heavy minerals. The detailed procedure explained in Mange & Wright (2007) was followed for the recognition and determination of the heavy minerals. Due to the large volume of data, mineral abundances were analyzed using cluster analysis in MATLAB and specifically the command “dendrogram”. Dendrograms are tree like diagrams that cluster together samples produced by a “linkage” or a hierarchical clustering. The results of the cluster analysis are given in the doctoral thesis of Michael (2013).

In the Sis palaeovalley there is a clear change in the heavy mineralogy with time from time-interval 1 (Cornudella and Lower Sis 1 samples) to time-interval 2 (upper Sis 1 and 2 samples) to time-interval 3 (Sis 3 and 4) and the overlying Oligocene Antist Group. The heavy minerals that dominate in the older units of the Sis palaeovalley-fill (Cornudella Formation and Sis 1) are zircon, tourmaline, apatite and garnet with small abundances of the epidote group. In Sis 4 there is a dramatic change in the abundance of the epidote group, which dominate the samples in Sis 4 and the Antist Group (20% to 85% abundance in some slides). In Sis 4 and Antist samples tourmaline is present, alongside smaller proportions of zircon, garnet and apatite. Sis 2 samples represent an intermediate condition between the Cornudella Formation/Sis1 and Sis4/Antist signals.

In the Lascuarre section, the heavy mineral signals are very similar to those of Sis 4 samples; they are enriched in epidote group minerals and contain garnet and some tourmaline, zircon and apatite. This spectrum persists throughout the deposition of the Escanilla and Collegats Formations. Groups of samples can be observed by dendrogram cluster analysis.  Samples fall into 2 main groups based on the distance (similarity-dissimilarity) between the various sampled localities. Group 1 comprises tourmaline-rich and epidote-poor samples of time-interval 1 of Sis, and a few samples of time-interval 2 and 3 of Sis. Group 2, on the other hand, represents the typical basin-fill heavy mineral abundances, which reflect the lithologies of the various source regions for the basin-fill in the Lascuarre region. An important observation is that with time (from time-interval 1 to 2) samples from the Sis palaeovalley-fill fall into Group 2 rather than Group 1.

The available heavy mineral data in combination with clast lithology data provide a broad idea of rock units that sourced the Escanilla system. The Sis palaeovalley clearly sourced the Escanilla system in Lascuarre (Vincent 2001), but an additional source region(s) is needed to explain the heavy mineral signals in time-intervals 1 and 2. We suggest that the Gurb catchment acted as this additional source region. The most abundant heavy minerals in the Sis and Lascuarre regions are zircon, apatite, tourmaline, and garnet and epidote group minerals. In general, zircon and apatite are very common and associated with a wide range of petrogenetic environments. Zircon can be found in magmatic, metamorphic, pegmatitic and alluvial rocks, apatite in calc-silicate rocks, igneous veins, sedimentary rocks and alkali-pegmatite. Tourmaline on the other hand indicates a rhyolitic, granitic, dioritic, pneumatitic or volcanic origin, as well as being found in contact metamorphism and skarn environments, hydrothermally altered basalt clasts, gabbro and gneisses. The epidote group contains a very broad spectrum of chemical end-members and indicate a wide range of environments, but is mostly found in mafic rocks that have experienced a greenschist and lower amphibolite facies of metamorphism. Garnet is found in ultrabasic rocks, metamorphic rocks, magmatic and pegmatites. In other words, whereas tourmaline indicates a more granitic origin, epidote group minerals indicate metamorphic source rocks.
 
The increase of epidote group abundance in the samples in the Sis palaeovalley-fill may therefore indicate a reorganization in the catchment region that sourced the Sis palaeovalley to include more metamorphic rocks. In relation with the U/Pb data, it is possible that these Palaeozoic metamorphic rocks are the source of both the recycled pre-Hercynian and pre-Cambrian zircon seen in the U/Pb geochronology results. If this is correct, then zircons with early Ordovician and Ediacaran to Early Cambrian ages do not come from the Eastern Pyrenees, but instead from catchment regions north of the palaeovalleys in the Central Pyrenees. 

U/Pb geochronology of detrital zircons:
U/Pb geochronology was performed on detrital zircon grains, which is a standard provenance tool in recent years (Whitchurch et al., 2011). Zircons were separated from crushed (coarse to medium sand grain-size) rock using standard procedures based on the following steps: 1) Density separation was undertaken on the samples using bromoform (CHBr_3, s.g. 2.85-89), to concentrate the heavy-mineral fraction; 2) Heavy mineral sinks were then passed through a Franz isodynamic magnetic separator to concentrate apatite and zircon, which are effectively non-magnetic at field strengths < 1 amp; 3) Final separation of the lighter apatite from the heavier zircons was carried out using the heavy liquid Diiodomethane (CH_2I_2, s.g. 3.3). After extraction the zircon samples were mounted on glass slides in araldite and after curing on a hotplate the grain mounts were polished using alumina pastes down to a 0.3µm grain size, to reveal internal surfaces for the analysis.

Only 18 samples were suitable for analysis. The suitability of each sample was judged by the need to meet criteria for statistical significance, which required at least 117 grains (Vermeesch 2004). The aim of the analysis was to cover as much of the geographical and stratigraphic extent of the palaeo-sediment routing system as possible (i.e. all the localities and all the time intervals at each locality). In most localities this was possible, but at Gurb only one sample was dated due to lack of suitable sample mounts.

The analysis of detrital zircons was carried out using LA-ICPMS with a New Wave Nd:YAG 213 nm laser ablation system, linked to an Agilent 7500a quadrupole mass spectrometer. At the start of each new sample, after every 30 detrital grain measurements and at the end of the analysis, a standard zircon Plesovice (TIMS reference age 337.13 ± 0.37 Ma, Slama et al., 2007) was analyzed along with the standard NIST 612 silicate glass (Pearce et al., 1997), which were used to correct for instrumental mass bias and for the depth-dependent inter-element fractionation of Pb, Th and U. After acquiring the data, Glitter v4.4 data reduction software was used (www.glitter-gemoc.com). Age estimations of the acquired signals were made using Isoplot v3.6 (Ludwig, 2003), which is a script and macro add-in for Microsoft Excel. Only ages that were between 15% and -10% discordant were accepted; the rest were discarded. The age estimations and errors from Isoplot were plotted using Density plotter v1.4 (Vermeesch 2012). 

A multidimensional scaling (MDS) technique was used in order to compare the stations statistically. A script written for MATLAB (Vermeesch 2013) gives indications of the proximity of the various samples or their dissimilarities. It compares pairs of samples and assigns values that are the two-sample Kolmogorov-Smirnov (K-S) statistic. For this multidimensional scaling and non-metric MDS was performed. 

The results of Whitchurch et al. (2011) have set up a U/Pb framework for the sedimentary fill of the South-Central Spanish Pyrenees and especially for the upper Eocene units that this study focuses on. Whitchurch et al. (2012) recognized three main age components recorded in the U/Pb geochronological age distributions: (a) A strong Hercynian (Carboniferous) age signal of ca. 310 Ma and two pre-Hercynian age components of (b) a late-Ordovician magmatic event at ca. 480 Ma and (c) a ca. 580 Ma (Ediacaran) to Early Cambrian magmatic event. These same age components are recorded in two other U/Pb studies in the region by Filleaudeau et al. (2012) and Roda-Boluda (2012).

U/Pb geochronological studies in catchment regions have aimed at dating igneous massifs in the Axial Zone of the Pyrenees (e.g. Deloule et al., 2002; Laumonier, 2004; Castiñeiras et al., 2008; Denele et al., 2009) and have established ages and distribution of zircon ages in these massifs. Based on these studies, Whitchurch et al. (2011) argued that all the pre-Hercynian signals are associated predominantly with the Eastern Pyrenean igneous massifs. Whitchurch et al. (2011) suggested that there are two typical modes of sediment dispersal, a transverse and a perpendicular one, and that pre-Hercynian age signals are associated with the transverse mode.

Details of the results of the zircon U/Pb geochronology of samples from the Escanilla palaeo-sediment routing system are given in the doctoral thesis of Michael (2013).

Almost all the samples have all the three peaks recognized by Whitchurch et al. (2011), which are both Hercynian and pre Hercynian (Early Ordovician and Ediacaran to Early Cambrian). In addition, grain-age mixture models reveal additional peaks (Filleaudeau et al., 2012). The most abundant 4th peak records a Tonian to Cryogenian event (Early to Middle Neoproterozoic) at ca. 738-995 Ma (from 2 to 17% abundance). Three samples record a peak of around 1.5 Ga (Early Mesoproterozoic), but most samples have peaks around 1.9-2.2 Ga, showing a clear Palaeoproterozoic event, with abundance ranging from 1 to 10%. The oldest systematic old zircon ages recorded have Neo-Archean ages (ca. 2.5-2.8 Ga). All the peaks described constitute the baseline U/Pb age distribution signal supplied by the catchments. 

There are clear similarities between the signals in the various localities and especially between the signals in the Ainsa and Lascuarre regions. In addition, there is a marked dissimilarity between samples of time-interval 1 and time-interval 2 at Sis, and in the Ainsa region for time-interval 1, which records a very strong Hercynian peak. This dissimilarity has implications for the catchment regions of the Sis palaeovalley during the Eocene, showing a clearly different signal from the Gurb palaeovalley in the same period (sample 19; Whitchurch et al., 2011) and the baseline signal discussed above. It also indicates that the proximity of the Maladeta and other Hercynian igneous massifs strongly influenced the Sis palaeovalley U/Pb signals.

In time-interval 2, however, a shift of U/Pb signals can be observed in the Sis palaeovalley, with all samples from this time-interval regarded statistically as one group, since samples from different regions become more and more similar. Finally, in time-interval 3 and during Antist times (Rupelian) all the samples from the various sub-basins have almost the same baseline U/Pb signal.

In summary, the U/Pb data reinforce the hypothesis that the Tremp and Ainsa Basins were connected during Escanilla Formation deposition (Bentham, 1992; Poblet et al., 1998; Dreyer et al., 1999) with very similar signals in both basins. Multidimensional scaling analysis reveals that the zircon U/Pb signals in the Sis and Gurb feeder systems become more homogenized, reflecting lithological changes in the catchments that sourced the Sis palaeovalley. It also shows that there is mixing in the Tremp Basin by the multiple signals of different source regions. This mixed signal was propagated into the Ainsa Basin.

To summarize the observations from the various provenance tools:
a) In the Sis palaeovalley, the heavy mineral signals indicate a change in provenance from a dominantly granitic source in time-interval 1 to a mixed granitic and metamorphic source in time-interval 2, suggesting that the catchment regions were at least as far back as the metamorphic units close to the Bielsa Massif (NNW of the Sis palaeovalley).
b) Heavy minerals and clast lithology data in both the Lascuarre and Gurb regions indicate granitic and metamorphic source regions throughout deposition of the Escanilla system, pinpointing sources of these rocks in the North Pyrenean Fault Zone and the nearby Aston (AS) and Hospitale (H) Massifs.
c) Throughout the Eocene the Gurb fans were sourced by Hercynian and pre-Hercynian granites. The Hercynian granites are abundant throughout the Pyrenees, whereas the pre-Hercynian granites are restricted close to the North Pyrenean Fault Zone and in the Eastern Pyrenees. If compartmentalization of the late Eocene Pyrenean fold-thrust belt prevented longitudinal flow (Whitchurch et al., 2011; Allen et al., 2013) from the Eastern Pyrenees to the Tremp Basin, the source of the pre–Hercynian zircons found at Gurb must have been close to the drainage divide of the central Pyrenees.
d) For the Sis feeder, the main source throughout time-intervals 1 and 2 was Hercynian granites, based on the U/Pb geochronological evidence. In time-interval 3 and onwards, however, catchments for the Sis deposits delivered pre-Hercynian aged zircons, indicating that the catchment must have expanded up to the North Pyrenean Fault Zone The provenance signals in both Sis and Gurp therefore imply catchment areas extending into the Axial Zone. The minimum dimensions of these palaeo-catchments are approximately 60 by 30 km for Sis and 65 by 30 km for Gurp. 

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