Global and Planetary Change 213 (2022) 103813
Available online 6 April 2022
0921-8181/© 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by- nc-nd/4.0/).
Towards the steady state? A long-term river incision deceleration pattern during Pleistocene entrenchment (Upper Ebro River, Northern Spain)
Alfonso Benito-Calvo
a,*, Davinia Moreno
a, Toshiyuki Fujioka
a,b, Gloria I. L ´ opez
a,c, Fidel Martín-Gonz ´ alez
d, Adri ´ an Martínez-Fern ´ andez
a, Isabel Hernando-Alonso
a, Theodoros Karampaglidis
e, Jos ´ e María Bermúdez de Castro
a, Francisco Guti ´ errez
faCentro Nacional de Investigaci´on sobre la Evoluci´on Humana (CENIEH), Paseo Sierra de Atapuerca 3, 09002 Burgos, Spain
bAustralian Nuclear Science and Technology Organisation, Lucas Heights, 2234 Sydney, Australia
cLeon Recanati Institute for Maritime Studies RIMS, University of Haifa, 199 Aba Khoushy Ave, Mt Carmel, Haifa 3498838, Israel
dArea de Geología-ESCET, Universidad Rey Juan Carlos, C/ Tulip´ ´an, S/n, M´ostoles, Madrid 28933, Spain
eMONREPOS, Archaeological Research Centre and Museum for Human Behavioural Evolution, Schloss Monrepos, 56567 Neuwied, Germany
fDepartment of Earth Sciences, University of Zaragoza, 50009 Zaragoza, Spain
A R T I C L E I N F O Editor: Dr. Liviu Matenco Keywords:
Fluvial terraces ESR dating TCN dating OSL dating Incision deceleration Steady state
A B S T R A C T
Pleistocene fluvial incision acceleration resulting in narrow and deeply entrenched valleys has been widely described and is generally attributed to uplift rate increase or greater climatic severity. In this paper, the long- term downcutting pattern of the Upper Ebro River and driving mechanisms are assessed, and we reconstruct the valley incision recorded by an outstanding sequence of 22 river terraces. Dating of 8 fluvial levels by means of the ESR, TCN, and OSL techniques, spanning the last 1.2 Ma, reveals a long-term incision deceleration pattern. The estimated age-incision model indicates a decrease in the incision rates, showing a long-term deceleration during the final Early Pleistocene (from 0.42 to 0.18 m/ka), and a tendency towards a steady state or base level sta- bilization from the Middle Pleistocene (0.15–0.03 m/ka) to the Late Pleistocene (0.03 m/ka). This incision pattern does not support climate change as a long-term incision-acceleration driver by itself, demonstrating the need for base level lowering effects to have operated. Upper Ebro deceleration incision is explained by the headward attenuation of the incision wave induced by the opening of the Ebro Cenozoic Basin in a geodynamic context characterized by an absence of significant uplift over the last million years. This trend could have changed the aggressor-victim roles of the rivers involved in fluvial captures at the drainage divide. The docu- mented incision deceleration pattern differs from that reported for other valleys in the Iberian Peninsula, sug- gesting that the degree of maturity of the fluvial systems depends on their relative position with respect to the capture point, in addition to tectonic and lithostructural factors. In the Late Pleistocene-Holocene, a short-lived incision rate increase is recorded, corresponding to the last terrace dissection phase, during the transition from the last glacial MIS 2 to the interglacial MIS 1.
1. Introduction
Fluvial terraces record a cyclic evolution controlled by climatic fluctuations and base level lowering (Bridgland and Westaway, 2008;
Pazzaglia, 2013; Stokes et al., 2012). The latter driver can be related to one or more factors such as uplift, eustatic changes, basin opening, or fluvial capture (Stokes et al., 2002; García-Castellanos and Larrasoa˜na, 2015; Karampaglidis et al., 2020). A base level drop produces a perturbation or disequilibrium in the drainage network (Stokes et al.,
2002), which responds with the upstream propagation of an incision wave (Whipple, 2001; Zaprowski et al., 2001; García et al., 2004). This geomorphic adjustment tends to restore the equilibrium profile of the stream, with a trend to a steady-state fluvial network with stationary drainage divides (Ahnert, 1994; Struth et al., 2019). Incision waves combined with climatic oscillations lead to the development of terrace staircases, which record the overall entrenchment of valleys punctuated by episodic interruptions (Finnegan et al., 2014), with base level sta- bility (strath terraces) or aggradation (fill terraces).
* Corresponding author.
E-mail address: [email protected] (A. Benito-Calvo).
Contents lists available at ScienceDirect
Global and Planetary Change
journal homepage: www.elsevier.com/locate/gloplacha
https://doi.org/10.1016/j.gloplacha.2022.103813
Received 11 November 2021; Received in revised form 29 March 2022; Accepted 3 April 2022
Whereas climatic fluctuations (temperature, precipitation) can cause changes in slope stability, vegetation cover, discharge, and sediment flux, resulting in the development of an individual alluvial level, base level lowering is postulated as a necessary requisite for long-term valley incision and the formation of a sequence of inset fluvial terraces (Bridgland, 2000; Hu et al., 2012; Stokes et al., 2012). In addition, cli- matic changes have been suggested to modify the incision capability of
the fluvial systems (Gibbard and Lewin, 2009; Bender et al., 2020).
Thus, global uplift acceleration and the cyclic climatic changes have been proposed as the main drivers for the general incision acceleration pattern observed in many entrenched valleys since the Early-Middle Pleistocene (Bridgland and Westaway, 2008, 2014; Gibbard and Lewin, 2009). Valleys incised in this period tend to be particularly narrow and deep, and accelerated or constant incision rates have been
Fig. 1.Geographic and geological setting of the Upper Ebro valley. A) Location of the Ebro River catchment in the Iberian Peninsula. B) The main drainage network of the Ebro catchment highlighting the general study area. Na: Najerilla River; Ja: Jal´on River; Hu: Huerva River; Gu: Gualadope River; Ga: G´allego River; Se: Segre River; Al: Alcanadre River; Ci: Cinca River; Ar: Aragon River; Za: Zaragoza city; Lo: Logro´ ˜no city. C) Geological framework of the study area marking the three main study areas discussed in this paper. VG: Vitoria-Gasteiz; EM: Espinosa de los Monteros; Re: Reinosa; Vi: Villarcayo; MP: Medina de Pomar; O˜n: Ona; Br: Briviesca; ME: ˜ Miranda de Ebro; Ha: Haro; Be: Belorado; SDC: Santo Domingo de la Calzada; Na: N´ajera. SUP-FIG2: Location of Supplementary Material Fig. 2.
described worldwide (Granger et al., 2001; Bridgland and Westaway, 2008, 2014; Hu et al., 2016; Silva et al., 2016; Cunha et al., 2019;
Benito-Calvo et al., 2020; Rixhon et al., 2020). Nonetheless, the appli- cation of dating techniques that allow determining the age of the oldest terrace levels (e.g., TCN, ESR) is starting to provide a different and broader picture of the long-term incision pattern. For instance, long- term incision deceleration and stabilization in Pleistocene entrenched valleys are also being reported in recent years using water-table cave sequences (Sart´egou et al., 2018; Nehme et al., 2020). In the present study we document the fluvial terrace sequence of the Upper Ebro River in northern Spain (Pyrenean-Cantabrian orogen, Fig. 1), which shows a long-term fluvial incision deceleration pattern tending towards a steady state. This incision trend has important implications for understanding the interplay between geomorphological evolution, climatic changes and tectonic controls in valley incision.
The Upper Ebro catchment has been described as an ideal natural laboratory for investigating long-term fluvial incision and water divide migration (Vacherat et al., 2018). Here, a number of numerical models and morphometric studies have been carried out to elucidate the fluvial capture landform assemblages associated with the divide between the two largest drainage basins in the Iberian Peninsula, the Duero and the Ebro (Pineda, 1996; Vacherat et al., 2018; Struth et al., 2019). The Ebro catchment began to shift from endorheic to exorheic conditions during the Late Miocene (García-Castellanos and Larrasoa˜na, 2015), and this was followed by widespread erosion and an advanced phase of drainage network adjustment (Soria-J´auregui et al., 2018). In the mountain areas at the north (Pyrenees) and south (Iberian Chain) of the catchment, the Middle Ebro tributaries display sequences of six to twelve terraces (Supplementary Material Section 1), recording an incision accelera- tion from the Middle Pleistocene (incision rates between 0.1 and 0.76 m/ka) (Guti´errez and Pe˜na, 1994; Fuller et al., 1998; Macklin et al., 2002; Benito et al., 2010; Stange et al., 2013; Lewis et al., 2017; Benito- Calvo et al., 2020). In the upper catchment to the west, a similar incision pattern was previously reported, with five to fourteen fluvial terraces, and incision rates varying from 0.09 to 0.07 m/ka from the Early Pleistocene to 0.81–1.15 m/ka for the Late Pleistocene-Holocene (Soria- J´auregui et al., 2016; Par´es et al., 2021). In this study, we present a detailed geomorphological and geochronological analysis of fluvial terraces in three sectors of the Upper Ebro valley. This provides solid evidence based on the dating of 7 fluvial terraces for long-term incision deceleration, eliciting a different valley entrenchment pattern. This pattern suggests a trend towards a steady state or base level stabilization over the last million years in the absence of significant tectonic deformation.
2. Geological setting
The Ebro Cenozoic Basin developed between the Pyrenean orogen (Pyrenees and Cantabrian Mountains) to the north, the Iberian Chain to the south, and the Catalonian Coastal Ranges to the southeast (Fig. 1B).
It corresponds to the southern foreland basin of the Pyrenees and is related to the convergence and collision between the Iberian and Eurasian plates which started in the Upper Cretaceous. The Pyrenean orogeny began during Santonian times and continued until the Miocene (Sibuet et al., 2004; Barnolas and Pujalte, 2004). It involved the tectonic inversion of the Cretaceous basins (Quintana et al., 2015) and resulted in the development of a double-vergence orogen (Mu˜noz, 2002). The Cantabrian Mountains overthrust the Cenozoic sediments of the Ebro and Duero basins (Bureba or La Rioja corridor). The Sierra de Cantabria thrust is the southernmost thrust and accommodated a southward tec- tonic transport of around 15 km. In the central part of the hanging wall of this thrust, folded Mesozoic and Cenozoic rocks form the Miranda and Villarcayo synclines (i.e., piggy-back basins) (Martínez-Torres, 1993;
Quintana et al., 2015) (Fig. 1B). The Ebro foreland basin was filled under endorheic conditions by continental detrital, evaporitic and carbonate sediments from the Late Eocene to the Late Miocene.
During the Late Miocene, the Ebro basin was captured by the external drainage network and was opened to the Mediterranean Sea.
This major paleogeographic change prompted the development and incision of the current exorheic drainage system (Guti´errez and Pena, ˜ 1994; Soria-J´auregui et al., 2018). The Ebro River drains the Cantabrian Mountains in its upper course, flows longitudinally from NW to SE along the Ebro depression in its middle course, and in its lower reach traverses the Catalonian Coastal Range before flowing into the Mediterranean Sea (Fig. 1B).
The Ebro River drainage divide reaches its highest elevation at the peak Tres Mares (2175 m, Pe˜na Labra Range). In its headwaters, the Ebro flows southeastwards through the Cantabrian Mountains, cutting across several intramountainous basins and Alpine structures with a prevalent NW-SE trend (Fig. 1C), such as the Villarcayo Syncline and the Miranda Syncline (Martín Alafont et al., 1978; Oliv´e Dav´o and Ramirez del Pozo, 1979, Fig. 1). Just before entering the Ebro Cenozoic Basin, the Upper Ebro cuts through folded Mesozoic formations and an erosional mountain front associated with the E-W-trending Cantabrian Thrust (Fig. 1C). Along its route, the Upper Ebro has carved its valley into Triassic, Jurassic and Cretaceous formations of the Basque-Cantabrian Mesozoic basin (Abalos, 2016). The Paleogene sediments comprise ´ Eocene marine carbonate-rich sediments and Oligocene evaporites to the north, as well as outcrops of continental detrital deposits to the south. The Neogene exposures occur in the core of synclinal intra- mountainous basins (Villarcayo and Miranda synclines) and in the Ebro Cenozoic Basin. These late Cenozoic formations mainly consist of folded conglomerates, sandstones and claystones (Portero García and Ramírez del Pozo, 1979; Portero García et al., 1977), and include lacustrine limestones and marls in the Miranda depression. During the Miocene, the Ebro and Duero Cenozoic basins used to be connected at the Bureba trough (Pineda, 1996), which includes the youngest preserved sedi- ments of the endorheic succession, capped by lacustrine deposits with an estimated Late Miocene age (Benito and P´erez-Gonz´alez, 2005).
3. Methodology
3.1. Geomorphology
The fluvial terrace study was carried out by combining remote sensing datasets and techniques, and fieldwork including stratigraphical and sedimentological descriptions (Stokes et al., 2012; Pazzaglia, 2013).
Detailed terrace maps were produced in the reaches of the Upper Ebro valley carved in easily erodible Cenozoic sediments (Fig. 1C). The terrace mapping was conducted by means of thorough field surveys, and the interpretation of aerial photographs and Digital Elevation Models (DEM05) derived from LiDAR data (IGN, 2021). Maps were digitized and processed using ArcGIS 10.7. The numerical ages and the relative- height data of the terraces were used to estimate an age-incision model for the Upper Ebro River, using statistical regressions and several algebraic functions (Silva et al., 2016; Cunha et al., 2019; Benito- Calvo et al., 2020). These functions and their 95% confidence interval were applied to reconstruct the long-term fluvial incision and its trend.
We used the positions and ages obtained for 7 fluvial terrace levels distributed along the whole sequence (106 years timescale) to explore the long-term incision pattern, reducing the uncertainties related to: (1) extrapolation; (2) short sequences; and (3) the time dependence of incision related to interruptions and minor oscillations in the fluvial incision pattern (Finnegan et al., 2014). An exponential function pro- vided the best fit (r2 =0.98), and this was applied to model terrace ages and calculate incision rates between consecutive terraces, providing a continuous dataset of terrace-to-terrace incision rates.
3.2. Geochronological methods
A total of 14 samples from 8 different fluvial levels were collected for Electron Spin Resonance (ESR) and Optically Stimulated Luminescence
(OSL) dating, following the methodology described in Moreno et al.
(2017). In addition, two samples were collected for Terrestrial Cosmo- genic Nuclide (TCN) burial dating (Table 1 and 2; Figs. 2, 3, 4 and 5). In situ gamma spectrometry for ESR and OSL dating was performed using a LaBr3(Ce) probe connected to an Inspector1000 multichannel analyzer (Canberra) and inserted directly at the exact sampling spot.
3.2.1. Electron spin resonance (ESR)
The preparation of twelve samples from six different fluvial levels was carried out at the CENIEH’s ESR dating laboratory, in accordance with the ESR dating protocol described in (Moreno et al., 2021). Full details are given in Supplementary Material Section 3.1.a. The Mul- tiple Aliquots Additive (MAA) dose and the Multiple Center (MC) ap- proaches (Duval et al., 2015; Tissoux et al., 2007; Toyoda et al., 2000) were applied to all samples. ESR measurements were taken at low temperature (90 K), using a nitrogen gas flow system connected to an EMXmicro 6/1 Bruker X-band ESR spectrometer. The angular depen- dence of the ESR signal on sample heterogeneity was taken into account by measuring all aliquots three times after successive ~120◦rotations in the cavity. Furthermore, the data reproducibility was verified by running ESR measurements over different days. This procedure was carried out for both the Aluminium (Al) and Titanium (Ti) centers.
Further details can be found in Supplementary Material Section 3.1.b.
The equivalent dose (DE) values were calculated with the software Microcal Origin 8.5 using the Levenberg-Marquardt algorithm, by chi- square minimization. For the Al center, a single saturating exponential +linear function (SSE +LIN) (Duval et al., 2009) was fitted through the experimental points. For the Ti centers, the Ti-2 function initially pro- posed by (Woda and Wagner, 2007) was used. With the SSE + LIN function, data were weighted by the inverse of the squared ESR intensity (1/I2), whereas with the function Ti-2, data were weighted by the in- verse of the squared error (1/s2) (Duval et al., 2015) (Supplementary Material Section Table 2). The ESR fitting results obtained for the Al and Ti centers are explained in detail in Supplementary Material Section 3.1.b. The ESR Dose Reconstruction Curves (DRC) derived from the evaluation of the Al, Ti–Li and Ti–H centers are provided in Sup- plementary Material Sections 3.1.d, e, f and g.
Dose rate (D) values were obtained from a combination of in situ and
laboratory analyses. ESR age calculation was performed using non- commercial software based on DRAC (Durcan et al., 2015), which takes into account the uncertainties derived from U, Th and K concen- trations, depth, water content, in situ gamma dose rate, attenuations and DE values. The errors associated with total doses (D), equivalent doses (DE) and ESR age results are given at 1σ. Full details about the meth- odology applied can be found in Supplementary Material Section 3.1.
c and Moreno et al. (2021).
3.2.2. Terrestrial cosmogenic nuclide (TCN) burial dating
Two samples (G-MIR-1813 and G-MIR-1807; Table 2) were collected and prepared for cosmogenic 10Be and 26Al dating to determine burial ages (Supplementary Material Section 3.2). Burial ages were calcu- lated following (Granger and Muzikar, 2001; Granger, 2014). The method assumes a simple exposure-burial scenario, where quartz exposed to cosmic rays at the steadily-eroding surface is later buried instantaneously. The 26Al/10Be ratio will decrease after burial as 26Al decays faster (half-life t1/2 =0.705 Ma; Norris et al., 1983) than 10Be (t1/
2 =1.387 Ma; Korschinek et al., 2010; Chmeleff et al., 2010), and this ratio is used as a burial clock. There are then two approaches to calcu- lating burial ages (Granger, 2014). One of them assumes no post-burial production, i.e., a scenario where quartz is buried deeply and reached its current depth recently due to rapid erosion at terrace surface. In this case, calculated burial ages are regarded as minima. The other approach estimates the maximum amount of post-burial production, assuming that samples remained at the same depth for the whole time. The approach provides a maximum age. In this study, the second approach indicated that the maximum estimate of post-depositional production exceeds the measured nuclide concentrations, and thus it does not allow us to calculate the maximum burial age. Having no independent mea- surements of terrace erosion rates in the region, we report only the minimum age (Table 4).
3.2.3. Optically stimulated luminescence (OSL)
The sample preparation details and final DE distribution graphic results (histograms and radial plots) for samples G-MIR-1804 and G- MIR-1805 are provided in Supplementary Material Section 3.3. For the alluvial, fluvial and colluvial sediments, it is quite surprising to
Table 1
Sequence of terraces mapped in the studied reaches and their relative elevations in meters above the Ebro River. *Terraces sampled for geochronological dating. The MM1 lower fluvial unit was also sampled (Tables 2, 3, 4 and 5, Figs. 2, 3 and 5).
UPPER EBRO VALLEY MIDDLE EBRO VALLEY
Tobalina valley Miranda depression Haro sector
Level Terraces Rock-cut terraces Tufa deposits Terraces Alluvial fan Terraces
T1 203
T2 190
La Varga I 190
T3 166 166 *
T4 146
San Miguel alluvial fan (MM1 and MM2 upper sequences) 141 T5
La Varga II 130 133
T6 118–123 123
T7 111–115
T8 107 98–105
T9 95 89–95
T10 80–84 85
Frías 61–80
83 *
T11 76 72
T12 60–65 * 59–62
T13 50–56 48–54 48–54
T14 39–44 45 45–46
T15 37–43 * 37–40
T16 33–36 34 34–36 34–36 *
T17 28–32 27–32 28–32
T18 24–25 20–25 21–25
T19 19–22 * 20 17 17–20
T20 13–16 12–16 11–16
T21 10–12 8–11
T22 6–9 5–10 * 4–9
Flood-plain 3–5 2–5 2–5
obtain Gaussian-type DE distributions with small OD values (Supple- mentary Material Section 3.3). In these environments, particularly colluvial ones, it is very common to observe skewness towards high DE
values, most probably due to incomplete bleaching and/or lithological (dosimetric) heterogeneity. For these samples, low OD values (<20%;
Duller, 2008a, 2008b) were obtained, most probably indicating that the sediment was bleached homogenously at the time of transport/deposi- tion, hence allowing for a fairly smooth growth of the OSL signal over time. The low OD values allowed the use of the central age model (CAM) to obtain the OSL ages for all four samples (Table 5).
4. Results
4.1. Geomorphology
The geomorphological sequence of the Upper Ebro River was investigated in the intramountainous basins of the Cantabrian Moun- tains (Villarcayo and Miranda synclines) and in the initial reach of its middle sector within the Ebro Cenozoic Basin (Haro sector) (Fig. 1). The analysis of LiDAR data supported by field checking yielded a sequence comprising at least 22 fluvial paleobase levels (Table 1; Figs. 2, 3 and 4), mainly corresponding to fluvial terraces, usually with 2–10 m thick fluvial deposits, and rock-cut surfaces. Terraces range from +203 m to +2 m above the modern Ebro River, and their relative heights show a reasonable correlation between the study areas in the Upper Ebro valley (Villarcayo and Miranda depressions) and the initial middle Ebro valley (Haro sector) (Table 1). The relative elevation difference between suc- cessive terrace treads varies from 13 to 25 m between T1 and T3, 13–8 m between T6 and T12, and 8–3 m between T13 and the floodplain, evincing a declining trend during the episodic valley entrenchment, as observed in other valleys.
Geomorphological mapping in the Ebro valley at the Villarcayo depression (the so-called Tobalina valley), revealed a sequence of 17 fluvial paleobase levels (Table 1, Fig. 2A). The highest terrace bearing fluvial deposits lies at +80–84 m (T10) above the current Ebro channel (Figs. 2A and 3). It includes rounded quartzite and quartz clasts, indi- cating a source related to the Ebro River. Terraces at +50–56 m (T13) and +39–44 m (T14/15) have been also mapped, although the younger levels are better preserved (Fig. 2A). The terrace at +19–22 m (T19) shows a fluvial deposit >4 m thick, comprising rounded gravel bars and interbedded clayey sands, where an ESR sample was collected (G-FRI- 1801, Figs. 2A and 5A).
Apart from these levels, other landforms in the Tobalina valley could indicate the existence of older fluvial base levels (Table 1). Towards the south of the town of Frías, several subhorizontal benches carved in the folded Cretaceous limestones were interpreted as rock-cut terraces formed at the confluence between the Molinar and Ebro rivers (Figs. 2B and 3). In this area, several fluvial tufa units are preserved. Tufa de- posits located around Frías (Figs. 2B and 3) are described as cascade facies with prograding architecture towards the Ebro valley, with no relation to fluvial base levels (Gonz´alez-Amuchastegui and Serrano Ca˜nadas, 1996; Gonz´alez Amuchastegui and Serrano Canadas, 2014). ˜ Mantled pediments connecting with the fluvial terraces are also common.
In the Miranda Depression, 12 fluvial terraces were mapped (Table 1, Fig. 2C). The oldest fluvial terrace lies to the south of the city of Miranda de Ebro. It can be recognized by a tread lying +76 m (T11) above the current Ebro River (Figs. 2C and 3). Towards the south, Pleistocene deposits with minimum visible thickness between 60 m and 36 m (MM1 and MM2, respectively; Figs. 2, 3S6, S8, 5E and F) occur in the Monte Miranda area (MM) on both margins of the San Miguel stream. On both sides the sequences are similar, consisting of a lower fluvial unit of well-sorted, well-rounded, planar and trough cross- bedded sands and pebbles, which were sampled for ESR and TCN dating at the site MM1 (G-MIR-1807 and 1808, Figs. 2C, 3 and 5E–F).
The lower fluvial unit is overlain by an upper unit mainly consisting Table 2 List of samples collected for geochronological analyses from the Upper Ebro fluvial deposits in the study area. Terrace Level Terrace relative elevation Sample Dating method Geographic location (UTM30N ETRS89) Altitude m a.s.l. Depth below ground surface (m) T3 (+166 m) 166 m G-MIR-1814 ESR X: 515664 Y: 4708799 585 6.30 G-MIR-1813 ESR, TCN 7.00 T10 (+83 m) 83 m G-MIR-1812 ESR X: 514398 Y: 4712770 516 1.20 G-MIR-1811 ESR 1.90 MM1 lower fluvial unit G-MIR-1808 ESR X: 53235 Y: 4724037 540 19.0 G-MIR-1807 ESR, TCN 20.0 T12 (+60–65 m) 63 m G-MIR-1806 ESR X: 498052 Y: 4730579 524 3.00 T15 (+37–43 m) 42 m G-MIR-1802 ESR X: 0495567 Y: 4732917 502 5.00 G-MIR-1803 ESR 3.50 T16 (+34–36 m) 36 m G-MIR-1809 ESR X: 513872 Y: 4713647 467 1.70 G-MIR-1810 ESR 0.90 T19 (+19–22 m) 22 m G-FRI-1801 ESR X: 478046 Y: 4735081 530 0.80 T22 (+5–10 m) 6 m G-MIR-1804 OSL X: 498577 Y: 4730891 466 2.1 G-MIR-1805 OSL 3
of massive mudstones, sandstones, and breccias. The latter are composed of poorly sorted, subangular-to-subrounded boulder-to- pebble-sized limestone clasts, grading into limestone conglomerates in more distal areas. These sediments, interpreted as alluvial fan deposits, underlie elongated geomorphic surfaces on both sides of the San Miguel stream, sloping towards the Ebro valley. The lower fluvial unit is
situated at 534–514 m a.s.l. in MM1 and at 515–504 m a.s.l. in MM2. In view of its position, the lower fluvial unit sampled in MM1 could correlate to the terrace level lying at +76 m (T11), whose tread runs at 526 m a.s.l. just to the south (Fig. 3S6), or to the terrace at +80–85 m (T10) in the Villarcayo depression and in the Haro sectors. The contact between both units in MM is also sloping towards the east, indicating Fig. 2.Terraces mapped throughout the Ebro valley in the Villarcayo depression (A and B), the Miranda depression (C), and the Haro sector (D).
that the Ebro River was the corresponding base level (Figs. 3S6, S8 and 5E). The position of both units is higher on the left margin (MM1) than on the right margin (MM2) of the San Miguel valley, suggesting that the sequence could be older in MM1. However, this elevation difference could be related to the eastward syndepositional inclination of the detrital successions, implying that MM1 and MM2 might correspond to the same level and have similar ages. The terrace lying at +62–65 m (T12) displays gravel bars and sands with a petrocalcic horizon showing laminar and massive secondary carbonate accumulations (sample G- MIR-1806, Figs. 2C, 3 and 5B). Lower levels lie at +48–54 m (T13) and +37–43 m (T15), the latter being composed of cross-bedded gravels, massive gravels, laminated sands and massive silts locally overprinted by pedogenic carbonates (ESR samples G-MIR-1802 and 1803, (Figs. 2C, 3 and 5C). Level T22 (+5–10 m) was sampled for OSL dating in a lenticular bed of cross-bedded sands interbedded between gravels (samples G-MIR-1804 and 1805, respectively, Figs. 2C, 3 and 5D).
In the Ebro Cenozoic Basin (Haro sector), just downstream of the Cantabrian thrust, we recognized 19 fluvial terraces, lying between +166 m and +4–9 m above the modern Ebro channel, in addition to the floodplain level (Table 1, Fig. 2D). Terraces between +166 m (T2) and +89–95 m (T9) are restricted to the right margin of the Ebro valley and associated with the southern Ebro tributaries flowing from the Demanda Range (Fig. 1C).
The sediments observed in the terrace at +166 m (T3) are >7 m thick and are dominated by well-rounded, trough cross-bedded quarzitic pebbles and sands, and horizontally bedded quarzitic pebbles with sandy matrix. Sandy beds were sampled for ESR and TCN dating (G-MIR- 1814 and G-MIR-1814, Figs. 2D, 3 and 5G). On the left margin of the Ebro valley, the highest level identified corresponds to a terrace lying at
+83 m (T10), which displays a layer of massive subrounded and rounded quarzitic gravels 2 m thick, including sand layers that were sampled for ESR (G-MIR-1811 and G-MIR-1812, Figs. 2D, 3 and 5H). The terrace situated at +34–36 m (T16) is composed of clast-supported and matrix-supported gravels, together with massive and laminated sands.
The silty sand facies in this terrace were sampled for ESR dating (G-MIR- 1809 and G-MIR-1810, Figs. 2D, 3 and 5I).
4.2. Geochronology
4.2.1. Electron spin resonance (ESR)
The ESR fitting results obtained for the Al and Ti centers are explained in detail in Supplementary Material Section 3.1.b. The ESR Dose Reconstruction Curves (DRC) derived from the evaluation of the Al, Ti–Li and Ti–H centers are provided in Supplementary Material Sections 3.1.d, e, f and g. In our study, DE values derived from Ti–Li option D have been considered as the best estimate for seven of the twelve samples analyzed (Table 2). As expected, Ti–Li option A and Al centers show much higher DE values than Ti–Li option D center. DE
derived from the Ti–H center have been discarded because their reli- ability can reasonably be questioned, taking into account the poorness- of-fit found for most of them (r2 >0.87 to 0.96) and the scattered values obtained for a single terrace. For samples G-MIR-1813, G-MIR-1808, G- MIR-1807, and G-FRI-1801, the DE values obtained from Ti–H option C have been considered more reliable for calculating the final age. In this case, DE values calculated from the Al and Ti–Li centers (options A and D) were significantly higher than the DE value calculated for Ti–H (option C), by a factor between 1.5 and 2.2. This may be interpreted as evidence of incomplete reset of those ESR signals during sediment Fig. 3. Geomorphological cross-sections showing sequences of terraces in the Upper Ebro valley. See position of cross-sections in Fig. 2.
transport, as observed in Guti´errez et al. (2020). It should be noted that the Ti–H center is not usually used for Lower Pleistocene samples as it saturates faster than the other centres and thus the age could be underestimated. However, samples G-MIR1813, G-MIR1808, and G- MIR1809 show consistent data suggesting that dose estimates from the Ti–H center in this study are reliable from a methodological point of view. Finally, in the case of sample G-MIR1812, the DE value obtained from Ti–Li option A has been selected instead of DE values from Ti–Li option D or Ti–H option C because of the better adjusted r2 (>0.98), and the consistency of this DE with that obtained from Ti–Li option D in sample G-MIR1811, both samples being from the same outcrop.
The gamma dose rates derived from laboratory measurements are between 11% and 42% higher than those obtained from in situ mea- surements (Supplementary Material Section 3.1.c), with the excep- tion of sample G-MIR-1809 with an in situ gamma dose rate 23% higher than that measured in the laboratory. Such a difference is very likely due to the heterogeneity of the sedimentary environment in the vicinity of the samples. Total dose rate (D) values are based on alpha and beta dose rates derived from high resolution γ-spectrometry (HRGS) and gamma dose rates from in situ measurements. D values differ slightly from one terrace to another but they are similar for samples from the same outcrop (Table 3).
The ESR chronology obtained for the six different terrace levels, in correct morpho-stratigraphic order, can be summarized as follows:
terrace T3 (+166 m) ranges from 1186 ±46 to 1221 ±167 Ma (G-MIR- 1814 and G-MIR1813); terrace T10 (+83 m) from 945 ±118 to 966 ± 133 ka (G-MIR-1812 and G-MIR-1811); MM1 lower fluvial unit from 804 ±90 to 852 ±129 ka (G-MIR-1808 and G-MIR-1807); terrace T12 (+60–65 m) yields an age of 716 ±71 ka while 780 ±123 ka should be considered as a maximum estimate (G-MIR-1806); terrace T15 (+37–43 m) ranges from 660 ± 75 to 682 ±79 ka (G-MIR-1803 and G-MIR- 1802); T16 (+34–36 m) yielded ages between 630 ±59 and 644 ±81 ka (G-MIR-1810, G-MIR-1809) and, finally, terrace T19 (+19–22 m) pro- vides an age of 387 ±45 ka (G-FRI-1801) (Table 3).
4.2.2. Terrestrial Cosmogenic Nuclide (TCN) burial dating
10Be and 26Al concentrations from the two samples, G-MIR-1813 at T3 (+166 m) and G-MIR-1807 at MM1 lower fluvial unit, are shown in Table 4. 10Be concentrations are 1.24 and 0.79 ×105 atoms/g(Qz) with
~4% errors and 26Al concentrations are 1.70 and 2.88 ×105 atoms/g (Qz) with 6–8% errors. The simple burial ages (minimum age) assuming no post-burial production are calculated as 3.18 ±0.19 Ma and 1.26 ± 0.16 Ma for the samples 1807 and 1813, respectively.
4.2.3. Optically stimulated luminescence (OSL)
Two samples from two different terraces were OSL-dated as sum- marized in Table 5. Gamma dose rates derived from high resolution γ-spectrometry (HRGS) laboratory measurements were almost twice those obtained from in situ measurements, most probably confirming a lithological heterogeneity in the sampled sedimentary sequence. Total dose rate (D) values are based on beta dose rates derived from direct beta laboratory measurements and gamma dose rates from in situ measure- ments. D values differ slightly from one terrace to another, regardless of dating method, but are similar for samples from the same outcrop (Table 5). OSL ages appear in stratigraphical order.
5. Discussion
5.1. Fluvial terraces in the Upper Ebro valley
Geomorphological and Quaternary studies along the Upper Ebro valley have traditionally recognized a small number of fluvial terraces (Gonz´alez Amuchastegui and Serrano Canadas, 2014; ˜ Perucha et al., 2015; Soria-J´auregui et al., 2016) (Supplementary Material Section 1). Taking advantage of new LiDAR data and conducting detailed fieldwork, we have mapped a more complete terrace sequence for the Upper Ebro valley, consisting of at least 22 fluvial terraces represented by treads underlain by around 2–20 m thick alluvial cover and rock-cut surfaces (Table 1, Figs. 2, 3 and 4). Terrace mapping has been carried out in two intramountainous basins of the Cantabrian Mountains Fig. 4. Ebro River longitudinal profiles. A) Longitudinal profile of the Ebro River along its course from its headwaters to the Mediterranean Sea. B) Longitudinal profile of the Upper Ebro in the study area and altitudinal distribution of the terrace treads.
(Villarcayo and Miranda synclines) and in the initial reach of its middle sector within the Ebro Cenozoic Basin (Haro sector) (Fig. 1C). The ter- races display parallel longitudinal profiles with similar sequences in the three sectors (Fig. 4). The highest Ebro terrace reaches up to +203 m above the modern river (Figs. 2 and 3), similar to the older levels described in the Pyrenean valleys (Pe˜na and Sancho, 1988), and in the Middle Ebro valley (Guti´errez and Pena, 1994; Guerrero et al., 2013; ˜ Guti´errez et al., 2015), but lower than alluvial levels described in the Demanda piedmont at relative elevations between +245–320 m (Ib´a˜nez et al., 1986).
The number of terraces observed in the Upper Ebro valley contrasts with the sequences documented in the rest of the catchment, that consist of seven to twelve terraces (Supplementary Material Section 1). The Upper Ebro valley has as many as 21–23 preserved terraces, a similar
number to those reported in other Iberian catchments (Tagus and Duero rivers), although their chronology is markedly different (P´erez- Gonz´alez, 1994; Silva et al., 2016; Karampaglidis et al., 2020).
The Upper Ebro catchment is a deeply dissected basin with a mature drainage network (Soria-J´auregui et al., 2018), where incision rates of 0.98 m/ka were initially proposed from the Late Pleistocene (Soria- J´auregui et al., 2016). These values are similar to, or higher than, the long-term incision rates reported for the Ebro tributaries draining the Pyrenees (Benito et al., 2010; Stange et al., 2013; Sancho et al., 2016;
Lewis et al., 2017; Benito-Calvo et al., 2020) and the Iberian Chain (Macklin et al., 2002; Whitfield et al., 2013; Guti´errez et al., 2020).
Nonetheless, the new chronological framework based on 14 samples collected from eight fluvial levels spanning from the Early Pleistocene to the Late Pleistocene (Tables 1 and 2; and Figs. 2, 3, 4 and 5), and Fig. 5. Images of exposures of terrace deposits at sampling sites in the Upper Ebro (A-F) and in the initial Middle Ebro (G-I). Red dots indicate position of samples.
Lithofacies: Gm, massive o crudely bedded gravels; Gt, trough cross-bedded gravel; Sm, massive sand; St, cross-bedded sand; Sh, laminated sand; SLm, massive silty sand; Fm, massive mud; P, pedogenetic carbonate. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
combining several dating techniques (ESR: Electron Spin Resonance, TCN: Terrestrial Cosmogenic Nuclide, OSL: Optically Stimulated Lumi- nescence) (Tables 3, 4 and 5) suggests a very different scenario.
5.2. Chronology of the Upper Ebro fluvial terraces
The highest dated fluvial terrace is the Early Pleistocene level T3 (+166 m) for which ESR ages range between 1.18 ±0.05 and 1.22 ± 0.17 Ma (Tables 1 and 3). Its minimum age of 1.26 ± 0.16 Ma is
determined by TCN simple burial dating (assuming infinite erosion at the terrace surface) (Table 4). This TCN age must be strictly considered as a minimum value, whereas calculating the maximum age estimate (assuming zero erosion at the terrace surface) is not possible where post- burial production would overwhelm the measured nuclide concentra- tion (cf. Section 4.2.2). We have not observed any evidence of significant erosion at the terrace surface, but assigning the terrace erosion rates reported for other terraces of the catchment (3–8 m/Ma, Stange et al., 2013) would result in a burial age ~ 1.6–2.8 Ma. Although these Table 3
ESR results obtained on quartz grains. (Dint: internal dose rate; Dα: alpha dose rate; Dβ: beta dose rate; Dγ: gamma dose rate; Dcos: cosmic dose rate; D: total dose rate; DE: equivalent dose). Final age results are in bold.
T3 (+166 m) T10
(+83 m) MM1
lower fluvial unit T12 (+60–65 m)
T15 (+37–43 m) T16
(34–36 m) T19
(+19–22 m) G-MIR-
1814 G-MIR-
1813 G-MIR-
1812 G-MIR-
1811 G-MIR-
1808 G-MIR-
1807 G-MIR-
1806 G-MIR-
1809 G-MIR-
1810 G-MIR-
1802 G-MIR-
1803 G-FRI- 1801 Dint (μGy/a) 50 ±30 50 ±30 50 ±30 50 ±30 50 ±30 50 ±30 50 ±30 50 ±30 50 ±30 50 ±30 50 ±30 50 ±30 Dα (μGy/a) 15 ±4 14 ±3 19 ±5 16 ±4 14 ±4 13 ±3 37 ±9 16 ±4 25 ±6 26 ±6 25 ±6 14 ±3 Dβ (μGy/a) 310 ±6 293 ±6 492 ±10 413 ±9 285 ±5 285 ±6 989 ±22 316 ±5 623 ±13 808 ±19 727 ±16 447 ±10 Dγ (μGy/a) 145 ±44 148 ±45 196 ±46 205 ±49 155 ±47 186 ±51 521 ±68 294 ±55 306 ±53 315 ±54 403 ±61 230 ±49 Dcos (μGy/a) 127 ±13 114 ±11 187 ±19 166 ±17 26 ±3 24 ±2 140 ±14 170 ±17 195 ±19 103 ±10 129 ±13 200 ±20 D (μGy/a) 647 ±55 619 ±56 944 ±59 850 ±60 530 ±56 558 ±59 1737 ±80 847 ±65 847 ±65 1302 ±
66 1334 ±
71 941 ±62
DE (Gy) Al 1085 ±
85 852 ±
146 1054 ±
89 1845 ±
199 747 ±
120 884 ±
105 1355 ±
204 887 ±86 1275 ±
143 1023 ±
112 1410 ±
157 1138 ±
DE (Gy) Ti–Li D 113
option 734 ±65 1168 ±
89 683 ±
104 804 ±83 951 ±76 1159 ±
128 1243 ±
110 578 ±50 559 ±47 820 ±65 859 ±98 709 ±34 DE (Gy) Ti–Li A
option 851 ±98 1090 ±
75 912 ±
112 1096 ±
87 937 ±59 1113 ±
88 1149 ±83 666 ±37 647 ±37 812 ±47 1083 ±
78 717 ±41
DE (Gy) Ti–H C
option 395 ±
117 756 ±78 549 ±70 705 ±
298 426 ±16 476 ±51 460 ±158 609 ±
147 184 ±52 435 ±81 341 ±
157 364 ±35 Age (ka) Al 1677 ±
194 1376 ±
266 1116 ±
117 2168 ±
28 1410 ±
271 1583 ±
252 780 ± 123 1047 ±
130 1505 ±
205 786 ±95 1057 ±
131 1209 ±
Age (ka) Ti–Li 144
D option 1186 ±
146 1887 ±
222 723 ±
119 945 ±
118 1801 ±
239 2075 ±
318 716 ± 71 682 ±
79 660 ±
75 630 ±
59 644 ±
81 753 ±61 Age (ka) Ti–Li
A option 1315 ±
189 1761 ±
199 966 ±
133 1288 ±
137 1769 ±
218 1993 ±
264 662 ±57 786 ±75 764 ±73 624 ±48 812 ±73 762 ±67 Age (ka) Ti–H C
option 610 ±
188 1221 ±
167 581 ±82 828 ±
355 804 ±
90 852 ±
129 265 ±92 719 ±
182 217 ±64 334 ±65 256 ±
118 387 ± 45
Table 4
Cosmogenic nuclide concentrations and burial age calculations.
Sample ID Terrace Latitude
(◦N) Longitude
(◦W) Elevation
(m) Depth
(m)
10Be (105 atoms/g)
26Al
(105 atoms/g) Min age a (Ma)
G-MIR-1813 T3 (+166 m) 42.5314 2.8094 585 7 0.792 ±0.035 2.884 ±0.185 1.26 ±0.16
G-MIR-1807 MM1 lower fluvial unit 42.6684 2.9605 540 20 1.238 ±0.052 1.699 ±0.140 3.18 ±0.19
Errors denote 1σ.
aBurial ages calculated assuming a simple exposure history with no post-burial nuclide production (see text for details).
Table 5
Optically Stimulated Luminescence (OSL) ages on quartz grains from samples collected in Ebro fluvial terraces. The size range of quartz grains used for the OSL measurements was 90–125 μm in both cases.
Terrace Sample No.
a Water Content
(%) c ḊBeta (Gy/
ka) ḊGamma (Gy/
ka) ḊCosmic (Gy/
ka) Total Ḋ (Gy/
ka) DE
(Gy) Aliquots
(N) d Age (ka) e OD (%)
f Age Model
g
(T22 +5-10 m)
G-MIR1804 19.86 ±1.07 1.21 ±
0.05 0.51 ±0.07 0.16 ±0.02 1.44 ±0.07 32.2 ±
0.9 23 22.43 ±
1.28 12 CAM
G-MIR1805 16.02 ±0.59 0.55 ±
0.02 0.30 ±0.06 0.15 ±0.02 0.81 ±0.06 19.7 ±
0.5 23 24.48 ±
1.85 12 CAM
aOriginal sample number/nomenclature.
cAssumed to be 60% of the maximum saturation value measured in the laboratory.
dNumber of aliquots used for DE calculation based on a total of 24 aliquots measured.
eReference year for ages is 2019. Reported errors are at 1σ and incorporate systematic uncertainties of dose and water content, as well as errors associated with the determination of DE.
fOver-dispersion reflects precision beyond instrumental errors; values of ≤20% indicate low dispersion in DE values and unimodal distribution.
gCAM: central age model.
estimates could indicate an older chronology for T3 (+166 m), TCN simple burial ages could be overestimated by the presence of detrital particles, as suggested by the non-consistent Pliocene ages obtained for the MM1 lower fluvial unit (Table 4). Additionally, other terraces comparable to T3 (+166 m) were also dated at about 1.2 Ma in the Ebro catchment (Duval et al., 2015). Based on these data, Ebro terrace T3 (+166 m) could have formed around 1.2 Ma, perhaps related to the Marine Isotope Stage (MIS) 36 cold stadial.
Ebro terrace T10 (+80–85 m) has been dated by ESR at 945 ±118 and 966 ±133 ka (Tables 3 and 6), with central values and the statistical estimation falling between MIS 24 and MIS 26. Par´es et al. (2021) recently calculated an ESR Ti–Li option D age of 1579 ±198 ka and obtained reverse polarities for a putative terrace lying +85 m above the
Ebro River, which was sampled in a pit to the east of the Miranda Airfield (Aer´odromo de Miranda) (Fig. 1C, Supplementary Material Section 2 and Fig. 2). Nevertheless, this terrace and its age could have been misinterpreted since the detailed geomorphological mapping car- ried out in our study through fieldwork and using 2 and 5 m resolution LiDAR DEMs provided the following evidence about this landform (Supplementary Material Section 2 and Fig. 2): 1) rather than a terrace, the dated sediments are located in the distal area of an alluvial cone without a clear correlation to any base level; 2) the distal sector of the cone where the pit is located is inset around 18 m with respect to the nearest base level landform lying at +84 m; and 3) the fluvial sandstone located at the base of the dated sequence could correspond to or be influenced by the Cenozoic bedrock, since it shows clear lithostrati- graphic similarities with the Oligocene quartz-rich sandstone with conglomerate layers found in this sector of the Miranda basin (Portero García et al., 1977).
The lower fluvial unit underlying the San Miguel alluvial fan deposits in MM1 could have formed between 804 ±90 and 852 ±129 ka ac- cording to our ESR results (Table 3). This is supported by an ESR Ti–Li option D age of 856 ±83 ka reported by Par´es et al. (2021) in this same MM1 location. Nevertheless, these authors also report an ESR Ti–Li option D age of 1531 ±155 ka for a second sample collected in the same position, and two ages of around 1.2 Ma (ESR Ti–Li option D) in MM2 sequence, which ought to be correlative or younger than the MM1 sequence. Moreover, our sample G-MIR-1807 (MM1) provided a much older TCN burial age of 3.18 ±0.19 Ma (Table 4). The latter age esti- mate suggests that these deposits can be significantly influenced by the presence of detrital particles recycled from previous formations, which would provide ages older than the true burial age of the MM1 sequence.
Although Pliocene deposits are not preserved in the study area, sedi- mentary series with similar chronologies have been reported in the re- gion (Pueyo Morer et al., 1996). These results reveal significant uncertainties regarding the chronology of the MM sequences and their geomorphological significance. Par´es et al. (2021) assigned the entire MM1 and MM2 sequences to fluvial terrace aggradation, which would imply significant valley excavation and aggradation phases, not observed in the fluvial terraces of the region. However, previous studies (Soria-J´auregui, 2016) and our own study interpret the MM upper units as alluvial fans that prograded and buried the lower fluvial units. This evolutionary model explains the anomalous thickness observed in the MM sequences and relates the MM lower fluvial units to fluvial terraces.
The geometry and position of the MM1 lower fluvial unit allows pro- posing a preliminary correlation with terrace levels T10 and T11.
Terrace level T11, at +76 m, is located just south of the MM1 sequence and, based on the relative elevations and chronologies of the fluvial terrace sequence, yields a modelled age of 863 ka (Fig. 6, Table 6), equivalent to the three ESR ages reported for the MM1 lower fluvial unit.
Thus, terrace T11 (+76 m) could have formed around 860 ka (MIS 21–MIS 22), with a similar age to terrace Qt2 (+100–190 m) dated in the Alcanadre valley at 817 ± 68 ka by ESR and magnetostratigraphy (Duval et al., 2015; Sancho et al., 2016).
The development of T12 (+60–65 m) can be placed around the Early- Middle Pleistocene transition, since ESR analyses indicate an estimated age of 716 ±71 ka (Ti–Li center-option D) and a maximum age of 780
±123 ka (ESR Al center), probably falling between MIS 18 and 20 (Table 6). This chronology is significantly older than the MIS 6 age previously proposed for similar levels (Soria-J´auregui et al., 2016). ESR ages for terraces T15 (+37–43 m) and T16 (+33–36 m) were estimated at between 682 ±79 to 660 ±75 ka and between 630 ±59 to 644 ±81 ka, respectively, showing a wide statistical overlap. The age-incision trend considering the whole terrace sequence suggests an age of 649 ka for T15 (+37–43 m) and 601 ka for T16 (+33–36 m), between MIS 14 and MIS 16 (Table 6, Fig. 6). Terrace T19 (+19–22 m) yielded a probable ESR age of 387 ±45 ka (Ti–H center-option C) (Table 3), which also differs notably from previous estimates using OSL (Soria-J´auregui et al., 2016). ESR ages are significantly older than the OSL values previously Table 6
Chronomorphological sequence and incision rates for the terrace sequence of the Upper Ebro Valley. ESR-dateda, TCN-datedb, OSL-datedc. Modelled ages and Pleistocene incision rates were calculated from the age-incision exponential model of Fig. 6. The Holocene incision rate was calculated from T22 OSL age and its main relative elevation.
Terrace
level Relative elevation range
Mean relative elevation
AGE (measured) AGE
(modelled) Incision rate
m m ka ka m/ka
T3 166 166 1186 ±46a,
1221 ±167a,
≥1260 ± 160b
1167 0.42
T4 146 141 1107
0.38
T5 133 133 1086
0.37
T6 118–123 119 1049
0.28
T7 111–115 111 1020
0.32
T8 98–105 101 986
0.24
T9 89–95 92 952
0.24
T10 80–85 82 966 ±133a,
945 ±118a 910
0.21
T11 72–76 72 863
0.18
T12 59–65 62 716 ±71a,
780 ±123a max.
809
0.15
T13 48–56 51 738
0.11
T14 44–46 45 684
0.14
T15 37–43 40 682 ±79a,
660 ±75a 649
0.10
T16 33–36 35 630 ±59a,
644 ±81a 601
0.09
T17 27–32 30 545
0.07
T18 20–25 22 432
0.06
T19 17–22 20 387 ±45a 397
0.05
T20 11–16 14 268
0.03
T21 8–12 11 180
0.03
T22 4–10 8 22.43 ±
1.28c, 24.48
±1.85c 64
0.34
Channel 0 0 0
reported for terraces at relative elevations between +65 m to +20 m above the modern Ebro River. Nevertheless, the OSL ages are considered to represent minimum ages for the formation of these deposits because of their proximity to dose saturation (Soria-J´auregui et al., 2016).
Quartz OSL dating is widely used for establishing numerical chronolo- gies in Late Pleistocene sedimentary contexts, but is of limited applica- bility for covering Middle and Early Pleistocene stratigraphic records (Arnold et al., 2015). This could explain the differences observed be- tween ESR and OSL ages in the Middle Pleistocene terraces reported in this work.
OSL results from terrace T22 (+5–10 m) yielded chronologies in stratigraphical order of 22.4 ±1.3 and 24.5 ±1.9 ka (Table 5), indi- cating a clear correlation with stadial MIS 2. Such ages are significantly older than the OSL chronologies reported for the low terraces in previous studies, estimated within the range 11.5–8.5 ka; MIS 1 (Soria-J´auregui
et al., 2016). These authors report an almost identical OSL age for two different terrace levels lying at +5–10 m and +15–20 m (11.5–8.5 ka and 12.9 ka, respectively), which suggests chronological and geomor- phological inconsistencies. Our analyses also showed that gamma dose rates derived only from high resolution γ-spectrometry laboratory measurements (Soria-J´auregui et al., 2016) were almost twice those obtained in this study from in situ measurements, which could have led to much lower age calculations. In addition, a terrace similar to T22 (+5–10 m) lying at +8–10 m in the Ebro headwater was OSL-dated to between 32 and 67 ka B⋅P (Perucha et al., 2015), suggesting that its development occurred between MIS 2 and 3, rather than during MIS 1.
5.3. Incision pattern in the Upper Ebro
The age-incision model constructed for the Upper Ebro valley using Fig. 6. Upper Ebro River age-incision model, compared to models published for other northern Iberian rivers (Cunha et al., 2019; Benito-Calvo et al., 2018; Silva et al., 2016; Moreno et al., 2012; Lisiecki and Raymo, 2007). Relative elevation of terraces was measured in the sampled terraces from DEM05 (5 m resolution DEM, derived from LiDAR data of 2 m resolution, X-Y RMSE =0.3 m, Z RMSE =0.2–0.4 m; IGN, 2021). Ages show horizontal error bars at 1σ. The incision rate curve was constructed from terrace ages derived from the regression curve (Exponential 1) and from terrace relative elevation for Pleistocene times, and from the T22 OSL age and its relative elevation for the Holocene (see numerical data in Table 6). The locations of the different rivers are shown in Fig. 1.
