Late Oligocene–Early Miocene paleosols of distal ... - Semantic Scholar

Palaeogeography, Palaeoclimatology, Palaeoecology 247 (2007) 220 – 235 www.elsevier.com/locate/palaeo

Late Oligocene–Early Miocene paleosols of distal fluvial systems, Ebro Basin, Spain J.M.M. Hamer ⁎, N.D. Sheldon, G.J. Nichols, M.E. Collinson Geology Department, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, UK Received 13 February 2006; received in revised form 12 September 2006; accepted 24 October 2006

Abstract The study of paleosols can be a vital tool in the paleoenvironmental interpretation of continental deposits because their physical and chemical characteristics are a reflection of variations in aspects of soil formation. Changes in paleosol characteristics can be used to determine the architecture and nature of parent material, paleosol maturity, topography, climate and soil-forming organisms (including vegetation cover) on the ancient floodplain, and can be utilized to recreate a palaeocatena. Furthermore, paleosols can provide quantitative data on paleoclimate where other sources, such as organic matter and pedogenic carbonate are absent. Late Oligocene–Early Miocene strata exposed in the Ebro Basin, Spain, have been identified as proximal alluvial fan, fluvial and lacustrine continental facies. In parts of the distal fluvial system pedogenically altered channel and overbank deposits interfinger with ephemeral lacustrine facies. Three main paleosol types developed in these sediments and have been described and compared to modern soils: Entisol-like (early successional soils), Inceptisol-like (young soils) and Alfisol-like (open woodland soils). Paleoenvironmental reconstructions suggest a mosaic of ecotypes with areas of open woodland composed of shrubs, herbs and small trees and other plants of low stature, and with each ecotype being controlled primarily by fluctuating paleohydrological conditions. From field observations and geochemical analysis the climate was found to be humid continental (mean annual temperature (MAT) 10–14 °C ± 4 °C, mean annual precipitation (MAP) 450–830 mm yr− 1 ± 200 mm yr− 1). These results indicate that climatic conditions were wetter than the present-day Ebro Basin (MAT 14 °C, MAP 320 mm yr− 1), and differ from previous interpretations of an arid to semi-arid environment based on sedimentological criteria alone. © 2006 Elsevier B.V. All rights reserved. Keywords: Paleosols; Ebro Basin; Oligocene–Miocene; Paleoclimate

1. Introduction Paleosols found in the vast continental deposits of the Ebro Basin, Spain, can provide a comprehensive paleoenvironmental interpretation of the controls on and landscape architecture that could be significantly applicable by analogy to both modern and ancient foreland ⁎ Corresponding author. Tel.: +44 1784 443 581; fax: +44 1784 471780. E-mail address: [email protected] (J.M.M. Hamer). 0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.10.016

basin settings. Previous research on paleoenvironmental and paleoclimatic reconstruction in continental settings has been focused primarily on qualitative sedimentological and paleontological studies (e.g., Retallack, 1983; Pierce and Rasmussen, 1992; Boucot and Gray, 2001) and the exciting and significant wealth of information that can be acquired from paleosols is only now starting to come to light (e.g., Retallack, 2004), allowing previous interpretations to be refined. Despite much research on the continental deposits of the Ebro Basin (e.g., Nichols and Hirst, 1998; Arenas et al., 2001; Cabrera et al., 2002),

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there has been little quantitative work on the paleoclimatic and paleoenvironmental architecture of the Oligocene– Miocene fluvial systems present. The spatial and temporal distribution of paleosols reflects different paleoenvironments, with numerous processes defining the characteristics of a single paleosol. The identification of a position on the alluvial paleo-landscape can be of great importance in both paleoenvironmental and economic context (e.g., Platt and Keller, 1992; McCarthy and Plint, 2003). In light of the above, it is important to understand fully not only the significance of a single paleosol, but also the distribution of different paleosols and the causal processes underlying their distribution within the particular paleoenvironment in question. Palynological studies have suggested that during the Early Oligocene there was a warm and dry climate in the Ebro Basin (Cavagnetto and Anadón, 1996), whereas during the Late Miocene a humid and seasonal climate is inferred from the study of fossil mammal assemblages (Alonso-Zarza and Calvo, 2000). Previous authors have contended that a semi-arid to arid environment existed between these times (e.g. García-Castellanos et al., 2003). The study of micromammal assemblages within the Luna and Huesca fluvial systems found that from the Late Oligocene to Middle Miocene the climate within the Ebro Basin was relatively humid (Álvarez Sierra et al., 1990),

Fig. 2. Three sections studied within the distal Luna and Huesca Distributary Systems. A) Channel cutting into alluvial deposits of the Erla and Bolea pedotypes, the channel shows weak pedogenic modification that is characteristic of the La Sotonera pedotype. B) Small channels and crevasse splays that show pedogenic alteration of the La Sotonera pedotype interbedded with mud and silt of Erla, Perdido, Sigena, Cinca and Bolea pedotypes. The dashed lines are lacustrine limestone beds (15 cm–25 cm thickness) deposited over the floodplain sediments. C) Interbedded limestones and mudstones of the Monegros, Cinca and Perdido pedotypes.

Fig. 1. Map showing the distribution of sediments within the Luna and Huesca Distributary Systems. Paleocurrent direction and the location of pedotypes is also shown. Grid references are UTM. Adapted from Nichols and Hirst (1998), Arenas and Pardo (2000).

similarly, stable isotope analysis of lacustrine sediments of the central basin lake record a slight decrease in aridity during the Early Miocene (Arenas and Pardo, 2000). Using a combination of detailed outcrop study and geochemical analyses, this paper describes, for the first time, the paleosols present and the paleoenvironment in which these soils developed during the Late Oligocene– Early Miocene of the Ebro Basin. Furthermore, this paper discusses the controls on the distribution of paleosol type by examining the factors that influence soil formation: parent material, topographic relief, time, climate, and organisms.

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2. Geological setting and paleogeography The Ebro Basin is situated on the north-eastern part of the Iberian Peninsula, and occupies an area of over 45 000 km2 (Puidefàbregas et al., 1992). Paleomagnetic data and palinspastic plate reconstructions indicate that the Ebro Basin had a paleolatitude slightly south (37° N) of its present latitude (40° N) (Van der Voo, 1993; Barberà et al., 2001). The formation of the basin began in the Paleocene by flexural subsidence related to the shortening within the Pyrenean orogenic belt to the north. Convergence between Iberia and Europe and tectonic shortening during the Late Eocene closed the connection to the Atlantic Ocean until the Late Miocene (García-Castellanos et al., 2003). During the Late Eocene–Late Miocene the Ebro Basin was endorheic and had no external drainage to the Mediterranean or to the Atlantic Ocean, and was therefore not subject to regional or global changes in sea level (Riba et al., 1983). Two fluvial systems that prograded into a central, ephemeral lacustrine environment were identified by Hirst and Nichols (1986), the Luna Distributary System and the Huesca Distributary System. The apex points of both fluvial systems are around 100 km apart and are sourced separately from the southern side of the Pyrenees, Fig. 1. The sediments studied form the Sariñena Formation within the Huesca Distributary System and the Uncastillo Formation within the Luna Distributary System and comprise predominantly alluvial mudstone and sandstone, with some thin marl beds towards the basin centre (Quirantes, 1978; Arenas et al., 1997, 2001). Age constraints within the Sariñena Formation are limited due to an absence of chronostratigraphic markers, lithostratigraphically diachronous boundaries,

and frequent lateral changes in facies. The paleosols within the Luna and Huesca distributary systems range from Chattian (Early Oligocene) to Aquitanian (Early Miocene) age based on magnetostratigraphic and biostratigraphic studies conducted in the area (Barberà et al., 1994; Arenas et al., 2001; Agustí et al., 2001). Proximal fluvial sediments within both systems are composed of cobble-sized conglomerates and coarse sandstone deposited from braided rivers. The medial sections are composed of single, meandering channels as well as braided stream deposits, with distal deposits being dominated by alluvial mudstone and sandstones deposited from unconfined flow to channelized flow (Nichols and Hirst, 1998; Fisher et al., in press). The central ephemeral lake was generally only a few meters deep and never exceeded 15 m in depth (Cabrera et al., 2002). It retreated and expanded, resulting in the interdigitation of lacustrine and alluvial deposits (Hirst and Nichols, 1986). The water salinity ranging from diluted freshwater to oligosaline, however, extreme euryhaline and hypersaline conditions were uncommon (Cabrera et al., 2002). Paleosols are identified within the distal sections of the Luna and Huesca distributary systems interbedded with and overprinting channel, overbank and lacustrine sediments, Fig. 2. These deposits are well exposed by areas of badlands topography, particularly at the margins of the fluvial systems. 3. Methods Twenty-three stratigraphic sections were selected and measured within different facies (channel, floodplain, palustrine) of the distal parts of the Luna and Huesca distributary systems. These sections were excavated to a

Table 1 Classification of pedotypes Pedotype

Diagnosis

Bolea

Some relict primary structure (laminations) cambic horizon, coloured (Bw) horizon, sandy A horizon, absence of pedogenic carbonate. Cinca Cambic horizon, some relict primary structure (laminations), coloured (Bw) horizon, absence of pedogenic carbonate. Erla Argillic (Bt) horizon, ustic moisture regime? Absence of pedogenic carbonate La Sotonera Relict bedding, sandy in all horizons, absence of pedogenic carbonate. Monegros Relict bedding, preservation of organic matter, drab colours, absence of pedogenic carbonate. Sigena Argillic horizon (Bt), redoximorphic features, absence of pedogenic carbonate. Perdido Argillic horizon (Bt), with a hue N 2.5YR, Ustic moisture regime? Absence of pedogenic carbonate. a b

FAO (2003). Soil Survey Staff (1999).

FAOa

Mack et al. (1993) USDAb

Cambisol Protosol

Inceptisol (Ochrept)

Cambisol Protosol

Inceptisol (Ochrept)

Luvisol

Argillisol

Alfisol (Ustalf?)

Fluvisol Fluvisol

Protosol Protosol

Entisol (Psammaquent) Entisol (Aquent)

Luvisol

Argillisol

Alfisol (Aqualf)

Luvisol

Argillisol

Alfisol (Rhodustalfs?)

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Fig. 3. A and B. The seven pedotypes representative of the three major soil types within the Luna and Huesca Distributary Systems. The Entisol-like (early successional soils) Monegros and La Sotonera pedotypes show little pedogenic alteration. The Cinca and Bolea pedotypes are Inceptisol-like (young soils), which represents a stage in soil formation beyond that of Entisols but lacks the development found in other soil orders. These Inceptisol-like palaeosols have a distinctive Bw (rubified) horizon but no (Cinca) or little (Bolea) clay enrichment. The Erla, Sigena, and Perdido pedotypes are Alfisol-like soils (open woodland) that are enriched in clays and bases within the Bt horizon.

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Fig. 3 (continued ).

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depth of 30 cm to provide a fresh surface for description and sampling. The description included accumulations of clays, fossil root traces, horizonization, mottling, Munsell colour from dry samples, profile thickness, structure and occurrence of ichnofossils (Retallack, 2001). Paleosols (n = N 150) were classified into pedotypes based on the field observations described above. Paleosol characteristics are comparable to those of modern soils and therefore pedotypes can be classified using the USDA taxonomic system (Soil Survey Staff, 1997). The top of paleosol profiles (paleosol thickness 0.3–1.2 m) was defined as the surface from which root traces emanate and the base, by the first occurrence of little altered parent material. Samples were obtained from profiles every 10–20 cm and sealed into labelled plastic bags. Geochemical analysis for major and trace elements using powdered whole-rock samples obtained from the pedotype profiles was undertaken using an X-ray fluorescence spectrometer (PW 1480) at Royal Holloway, University of London; results are collated as supplemental data. Five-hundred point-counts using a Swift counter on thin sections of each pedotype were used to determine the mineral assemblage and size fractions with an uncertainty of 2% (Murphy, 1983), and their micromorphology and ped structure were described using the methodology of Retallack (2001).

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experienced some burial compaction and for the vast majority, a dearth in organic matter indicates its post-burial oxidation (Retallack, 1991). From considerations of regional stratigraphic relationships and structure the original thickness of sediment overlying the modern day exposures was usually less than 1.3 km (Nichols and Hirst, 1998; Nichols, 2004). This overburden was minimal, with ichnofossils showing little if any evidence of compaction. However, minor increases in the density of the paleosols and associated sediments have been taken into account for use in mass balance calculation using an algorithm of decompaction (Sheldon and Retallack, 2001). Organic matter is present in paleosols that were affected by significant gleization and is found in the form of carbonaceous root traces and very small of woody fragments. However, most of the paleosols studied do not contain any organic material. This may be due to efficient decomposition from biota in an oxidising environment and/or efficient removal of organic matter by alkaline ground waters (Retallack, 2001). Diagenetic alteration is thought to be minimal as Ca and Sr within paleosol profiles were found to mirror each other, see Supplemental data. However, post-pedogenic groundwater movement found in the Bolea pedotype precipitated gypsum within burrows and other voids. 4.3. Factors of soil formation

4. Results 4.1. Field description of paleosols Pedogenic alteration of the Late Oligocene–Early Miocene alluvial, fluvial and lacustrine sediments in the Luna and Huesca Distributary Systems was indicated by a number of criteria including the occurrence of roots, peds and horizonization. Seven pedotypes were identified based on distinctive characteristics included in these criteria, Table 1, as well as the maturity and the relationship with stratigraphically adjacent paleosols (Kraus, 1999). The three principal paleosol types found within the distal fluvial systems are represented by the Monegros, Cinca and Erla pedotypes, Fig. 3A and B. The Sigena, Perdido, Bolea, and La Sotonera pedotypes are further subdivisions of these three paleosol types. The names of pedotypes have been chosen using local village and area names. 4.2. Paleosol post-burial alteration Alteration after burial can result in misinterpretation of data if it is not corrected for (e.g. Sheldon and Retallack, 2001). All the paleosols described here are likely to have

4.3.1. Parent material The influence of parent material on soil formation is expressed in terms of weathering as it determines the abundance of mineral material in a soil, as well as the subsequent weathering properties of the soil itself. The Bolea, Sigena, Perdido, Erla, and Cinca pedotypes have a parent material that was siltstone or mudstone deposited as alluvium. For the Monegros pedotype the parent material was a lacustrine wackestone and the La Sotonera pedotype had a parent material of fine-to coarse-grained sandstone deposited by crevasse splays and channels, Table 2. However, all of the distal fluvial deposits have a high clay content, typically no less than 25% (Fisher et al., in press).The molar ratio of the fairly immobile elements titanium and aluminum (Ti/Al) is useful as an indicator of uniform parent material within profiles and between pedotypes, Fig. 4. Constant ratios between these two elements, typically nearly immobile during weathering, indicate that the soil forming weathered material of the different pedotypes is sourced from the same area. The source area for both the Luna and Huesca distributary systems is likely to be from the Pyrenean Axial Zone and the more southern Pyrenean zones including the Eocene–Oligocene Jaca Basin and

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Table 2 Factors of soil formation Pedotype (USDAa)

Paleoclimatea

Former vegetation

Paleotopography

Parent material

Time (years)b

Bolea (Inceptisol)

Herbaceous vegetation, low stature plants

Most proximal to source area; well drained; proximal to active channel.

Siltstone

103

Herbaceous vegetation, low stature plants

Predominantly well drained, significant fluctuation in water table, distal to source area.

Mudstone

103

Open woodland — herbaceous vegetation, low stature plants and small trees Herbaceous vegetation

Well drained, distal to active channel.

Siltstone

104

La Sotonera (Entisol)

MAT Incpt 12 °C MAT 11 °C MAP 490 mm yr− 1 MAT Incpt16 °C MAT 10 °C MAP 460 mm yr− 1 MAT 11 °C MAP 880 mm yr− 1 NA

Fine-coarse sandstone

b102

Monegros (Entisol)

NA

Wackestone

b102

Sigena (Alfisol)

MAT 12 °C MAP 800 mm yr− 1 MAT 11 °C MAP 560 mm yr− 1

Subaerially exposed splay deposit; proximal to active channel. Well drained. Flood plain depression poorly drained. Fluctuating water table.

Mudstone

104

Well drained.

Mudstone

104

Cinca (Inceptisol)

Erla (Alfisol)

Perdido (Alfisol)

a b

Reed-like monocotyledons such as Typha angustata Open woodland herbaceous vegetation, low stature plants and small trees Open woodland herbaceous vegetation, low stature plants

Error MAT ± 4 °C MAT for Inceptisols ± 0.6 °C; MAP ± 196 mm yr− 1. Time taken for soil development estimated from degree of development of the argillic (Bt) horizon (Retallack, 1998).

the Tremp Graus Basin (Hirst, 1983; Hirst and Nichols, 1986; Yuste et al., 2005). 4.3.2. Topographic relief The minor disconformities resulting from the erosion of fluvial paleochannels into floodplain paleosols suggests that the channels are close to fluvial base level this is further supported by the occurrence of a salt crust in proximity to the Sigena and Cinca pedotypes.. The salt crust is interpreted as having formed on a mudflat (sabkka), which occurred only in the lowest parts of the landscape (Jenny, 1941). Previous estimates of slope gradient are around 0.1° or less (Hirst, 1983), and are likely to have a negligible impact on paleosol thickness. However, an increase in the base level of the lake by 10 m would have resulted in a transgression of over 5 km, resulting in the interfingering of fluvial and lacustrine deposits, Fig. 2B (Nichols, 2004). 4.3.3. Time The degree of development of an argillic B horizon (Bt) and the extent of chemical weathering can be used as a proxy for paleosol development. Point counts from thin sections show an argillic horizon in the Erla, Sigena and

Perdido pedotypes, (Fig. 3. and Supplemental data). Those pedotypes that have a distinct argillic horizon have a microfabric which ranges from insepic (weakly developed) to mosepic — an indication of moderate development (Retallack, 2001). However, the rate of

Fig. 4. Provenance as determined from the ratio of Ti/Al. Indicates a constant source area for all pedotypes in both the Luna and Huesca Distributary Systems.

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argillic horizon development can be affected by both climate and parent material and it is important to note that some paleosols may have been truncated at the top of the profile by erosively based paleochannel and overbank deposition. 4.3.4. Climate The modern climate of the Ebro Basin is Mediterranean continental with semi-arid characteristics; mean annual precipitation is 320 mm yr− 1 and a mean annual temperature of 14 °C. There is a seasonal range of 25 °C in July and 4 °C in January (Sirvent et al., 1997). Despite the abundance of detrital carbonate within the sediments studied, typically 15–30% (see Supplemental data) there is an absence of pedogenically formed carbonate. The lack of calcrete within all palaeosols of the Luna and Huesca distributary systems infers a paleoprecipitation in excess of 500 mm yr− 1 (Birkeland, 1999). The dearth of evaporites within the paleosol profiles themselves is not particularly significant because the sodium ion as well as calcium sulphate can only persist in a climate where evapotranspiration is always greater than precipitation and the drainage is not great enough to enable the evaporites to go into solution. With frequent freshwater lacustrine incursions it is likely that ground water movement would have removed any gypsum or halite that was present within a profile. However, the absence of hollow gypsum pseudomorphs in thin section suggests that evaporites were not present as primary deposits within the paleosols. The degree of chemical weathering of B horizons within the pedotypes studied can be used to estimate the mean annual precipitation (MAP) and mean annual temperature (MAT) using geochemical climofunctions derived from the Bt and Bw horizons of modern North American soils (Sheldon et al., 2002; Retallack, 2004). MAT is estimated using salinization; however this does not include calcium sulphate which is widespread as a secondary deposit: MATð-CÞ ¼ −18:5S þ 17:3

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MAP can be estimated using the chemical index of alteration minus potassium. This climofunction is calibrated for precipitation values between 200 and 1600 mm yr− 1, however there is no field evidence to suggest that the MAP was out of this range. MATðmm yr−1 Þ ¼ 221e0:0197ðCIAKÞ ð3Þ h i Al2 O3 where CIA ¼ 100  ðAl2 O3 þK2 OþCaOþNa2 OÞ and R 2 = 0.72 with an error of ± 196 mm yr − 1 (Sheldon et al., 2002). The climate was variable with a maximum possible precipitation range, inclusive of error, of 265 mm yr− 1 and 1025 mm yr− 1, Fig. 5. However, the absence of calcrete indicates that precipitation was probably never less than 500 mm yr− 1 (Birkeland, 1999), considerably wetter than the modern mean annual precipitation rate within the Ebro Basin. The calculated mean annual paleotemperature values range from 8 °C to 16 °C including potential error using the equation for Alfisols (Eq. (1)), and between 12 °C and 15 °C using the equation for Inceptisols on the Cinca and Bolea pedotypes respectively, (Fig. 5). This is comparable to modern MAT value of 14 °C, and is not surprising as the change in latitude of the Eurasian plate has been minimal since the Early Miocene (Van der Voo, 1993; Barberà et al., 2001). 4.3.5. Organisms A highly oxidising environment has resulted in a dearth of organic matter within the studied paleosols, with the exception of the Monegros pedotype. Here, fine

ð1Þ

where S = (mK + mNa)/mAl and R2 = 0.37 with a standard error of ± 4.4 °C. The error on this is substantial and a more precise estimation for MAT can be derived for the Inceptisollike paleosols using the following equation: MATð-CÞ ¼ 46:9C þ 4:0

ð2Þ

where C = mAl/mSi and R2 = 0.96 with an error of ± 0.6 °C (Sheldon, 2006).

Fig. 5. Estimates of mean annual temperature and mean annual precipitation from geochemical climofunctions (Sheldon et al., 2002). Dashed line represents modern MAT and MAP (Sirvent et al., 1997). ⁎ Estimation of MAT from the application of geochemical climofunctions for Inceptisols (Sheldon, 2006).

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carbonaceous roots are present as well as poorly preserved, narrow reed-like monocotyledons. In other pedotypes, root traces are preserved as drab haloes and are predominantly fine, b4 mm, and variably both shallowly and deeply penetrating, indicating a fluctuating water table. The depth to the Bt horizon in all the Alfisol-like pedotypes is closely related to the point of maximum density of roots and concurs with a mixed open woodland and herbaceous vegetation, with deeper Bt horizons forming under open woodland and shallower Bt horizons under herbaceous vegetation (Runge, 1973). Typha angustata (cat-tail or bulrush) autochthonous remains are recorded in the Late Oligocene central lake deposits of the Ebro Basin (Nishimoto, 1974). The proximity to the lake margin and the occurrence of poorly preserved reed-like monocotyledons make T. angustata a possible candidate for the development of the Monegros pedotype. Vertebrates such as turtle and freshwater gastropods such as lymnaeid and planorbid as well as charophytes occur in the ephemeral lake deposits, indicating that at least during periods of lake incursion the central basin lake was freshwater. The paleosols themselves exhibit bioturbation by both flora and fauna, (Fig. 6). Within the Entisol-like soils of the Monegros and Bolea pedotype, a diverse ichnofauna has burrowed into subaerially exposed lacustrine limestones, abandoned channel, and sandy

overbank deposits. Within the La Sotonera pedotype ichnofossils comprising of deeply penetrating ant nestssimilar in morphology to nests created by the ant Prenolepis imparis (Tschinkel, 2003), dung beetle burrows, spider burrows, rhizoliths, bee cells showing solitary to gregarious behaviour as well as various horizontal crawling traces of arthropods have been identified (Hasiotis, 2002). Dung beetle nests and boli are commonly found in the La Sotonera and Bolea Pedotypes and suggest a herbaceous vegetation (Retallack, 1990; Genise et al., 2000). Depth of penetration of ant and termite nests appears to be controlled by bed thickness, with ant and termite nests reaching a depth of up to 2.5 m in the distal-medial sectors of the Luna and Huesca fluvial systems. The Entisol-like paleosols of the Monegros pedotype contain abundant rhizolith traces as well as ichnofossils indicative of a high water table such as Scolicia isp — a gastropod locomotion trace (Hasiotis, 2002). 5. Pedogenesis 5.1. Weathering ratios The application of molecular weathering ratios to determine the degree of chemical weathering of paleosols has been useful in characterising variability between paleosols (Sheldon et al., 2002). One such

Fig. 6. Examples of ichnofossils found within the paleosols of the distal fluvial systems. A) Drab haloed root trace (Erla pedotype) B) carbonaceous root (Sigena pedotype), C) pedogenically modified limestone roots preserved as casts (Monegros pedotype), D) subterranean termite nest (La Sotonera pedotype), E) Subterranean social wasps nest (Erla pedotype) and F) ants nest (La Sotonera pedotype).

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ratio, alumina/bases, can be used to determine the extent of hydrolysis: Alumina=bases ¼

Al2 O3 ðCaO þ MgO þ K2 O þ Na2 OÞ

ð4Þ

The alumina/bases ratio varies greatly between soil types, and is useful for differentiating between highly weathered Ultisols and less weathered Alfisols (Sheldon et al., 2002). Within all of the pedotypes values are less than 2, supporting an Alfisol, Inceptisol and Entisol classification of the paleosols found. The Erla, Sigena and Perdido pedotypes show clear increases compared to the parent material in the ratio of alumina/bases as feldspars and mafic minerals are weathered to form alumina-rich clay. The less developed Cinca and Bolea pedotypes also show some minor increase in the weathering to clay within the Bw horizon. Only the most developed paleosols have an observable enrichment of clay in the Bt horizon, these are the Erla, Sigena and Perdido pedotypes (Fig. 3). Clay enrichment can be determined from both point-counting of mineral abundance and the ratio of aluminium/silica (Al2O3/SiO2), (see Supplementary data). Another measurement of chemical weathering is the chemical index of alteration, (CIA), which is useful in quantifying the alteration of feldspars to clay (Nesbitt and Young, 1982).  CIA ¼ 100 

 Al2 O3 Al2 O3 þ K2 O þ CaO þ Na2 O

ð5Þ

The most developed paleosols have the greatest CIA values and so can be used in combination with the values for clayeyness within B horizons as a proxy for paleosol development and maturity, (Fig. 7). The ratio of K2O + Na2O/Al2O3 can be used to determine the salinization of a paleosol and to denote whether the soil formed in an arid climate, with a ratio N 1 indicating an occurrence of evaporites within a profile and therefore arid conditions (Retallack, 2001). All values were found to be less than 1 indicating that salt accumulations did not precipitate and supporting field observations of an absence of in situ evaporites within the sections studied. However, the ratio for salinization does not include gypsum as an evaporite (CaSO4.2H2O), which is frequently observed as a secondary deposit within parts of the Luna and Huesca distributary systems. Leaching, the removal of bases from the A horizon downwards and out of the profile, and occurs where precipitation exceeds evaportranspiration. Leaching is evident within the Cinca and Bolea pedotypes from the accumulation of sesquioxides and their resulting rubefaction, but movement of water within these poorly

Fig. 7. Degree of development of paleosols within the Luna and Huesca fluvial systems. Based on the weathering ratios of clayeyness and the chemical index of alteration (CIA), paleosols show development from very weakly developed Entisol-like palaeosols to moderately developed Alfisol-like palaeosols.

developed soils is not sufficient or prolonged enough to cause clay illuviation as is apparent in the mineralogical data obtained from point counts. There are also poor accumulations of clay within the Bt horizon of the Perdido pedotype despite a high abundance of sesquioxides. The migration of clay may be inhibited by the presence of exchangeable Ca, which flocculates clay and prevents it being transported in suspension down the soil profile. If this were the case, only in situ weathering of feldspars, micas and ferromagnesium minerals would occur. 5.2. Mass balance The elemental gains and loses within a soil or paleosol profile can be determined using constitutive mass balance calculations by making a comparison of the elemental composition of collected samples within a paleosol profile and that of the least altered parent material (Chadwick et al., 1990). The parent material was identified as the first occurrence of sediments that do not exhibit pedogenic alteration (i.e., occurrence of roots, mottling) and that retain primary sedimentary structure. These changes can be calculated by identifying an “immobile” element, i, and relating the movement of a “mobile” element, j, to the immobile element. The mass transport function for a single element, τj,w, can be calculated as follows: " # ðqw Cj:w Þ  ½ei;w þ 1−1 sj;w ¼ ð6Þ ðqp Cj:p Þ ρw Cj.w

Density of weathered material wt.% composition of element j in weathered material

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ρp Cj.p

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Density of parent material wt.% composition of element j in parent material

calculations because it is more abundant in all of the samples analyzed. 5.3. Elemental gains and losses

For an immobile element (τj,w = 0), Eq. (6) can be rearranged to solve for εi,w, the strain (volume change) on the immobile element:   ðqp Cj:p Þ ei; ei;w ¼ −1 ð7Þ qw Cj:w For this paper, Ti was taken to be the immobile element, i. This immobility was found by means of plotting the transported mass fraction of Ti versus strain calculated assuming Zr is immobile, and by plotting the transported mass fraction of Zr versus strain calculated by assuming Ti is immobile (Egli and Fitze, 2000). Immobility is indicated by the element that plots closest to the origin. It was found that both elements were fairly immobile and Ti was selected for the subsequent

Gains and losses within each pedotype are shown in profile for selected elements in Fig. 8. (See Supplementary data for all other major elements.) From weathering ratios and point counts, weathering and clay enrichment is evident within 3 of the pedotypes, Erla, Perdido and Sigena. Weathering was most intense within the Erla and Sigena pedotypes, which show a marked loss of cations. The Erla pedotype shows high rates of weathering with an increase in the loss of Ca and Mg2 up profile. The overall movement of cations is greater in the more developed paleosol profiles and less so in the weakly developed profiles such as the Inceptisol-like Cinca pedotype as can be expected given the greater length of time for soil formation within the more mature paleosols. Large gains of Ca within the Perdido pedotype are

Fig. 8. Mass balance profiles for Inceptisol-like and Alfisol-like paleosols. The mass balance is calculated by comparing each sample in the profile to the least altered parent material. Each mass balance profile begins with the first altered sample collected. Note the overall greater movement of elements within the more developed Erla and Sigena pedotypes.

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attributed to a large influx of detrital calcite, probably associated with the overlying lacustrine incursion. 6. Discussion Both the Luna and Huesca distal fluvial systems show similar patterns of paleosol type and distribution indicative of a similar paleoenvironmental and paleoclimatic setting. The dominant soil type on the floodplain of both fluvial systems is Alfisol-like (open woodland soils represented by the Erla pedotype), however, proximity to the active channel and the central lake plays a dominant role in the determination of both soil type and maturity. Temporal shifts in lake and fluvial channel position correspond to geographical shifts in the position of soil types. Poorly developed Entisol-like soils (La Sotonera pedotype) develop on sandy, subaerially exposed crevasse splay and channel fill deposits, (Fig. 2A, B), where a temporary hiatus in sedimentation enables

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pedogenesis. These paleosols show very little pedogenic alteration and retain some original sedimentary structures, (Fig. 3A). Bioturbation is evident with the preferential development of complex termite and ant nests as well as beetle burrows. Burial gleization of the top surface of these sandstone beds is frequently evident as a reduced blue-grey band of approximately 2–4 cm thickness and is most likely to be the result of the anaerobic decay of organic matter buried there (Retallack, 1991). Similar gleization is evident within the O horizon of the Erla Pedotype where it is preserved. Entisol-like paleosols such as the Monegros pedotype develop in more reducing conditions and are associated with rapid fluctuations of the margin of the central lake with short periods of subarial exposure (b102 yr, Fig. 2C). The high water table contributed to the preservation of some carbonaceous roots, woody material, and leaves, (Fig. 9). Weakly developed compound paleosols indicate episodic sedimentation and minimal erosion in this topographic low (Kraus,

Fig. 9. Development of paleosols in different paleohydrological conditions. A) Well-drained, fluvial dominated setting and B) intermittentlypermanently water-logged soils associated with lacustrine deposition and the preservation of organics in reducing conditions.

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1999). The paleosols of the Monegros pedotype represent soils closest to a lake margin, where plant communities colonized carbonate-rich muds for example monocotyledons such as Typha angustata. Inceptisol-like paleosols (poorly developed soils) are uncommon and are characterised by the Cinca (lacustrinedominated) and Bolea (fluvial-dominated) pedotypes. The Cinca pedotype is found close to ephemeral lacustrine deposits and becomes more developed with distance from the lacustrine incursion forming a continuum into Alfisollike soils. The Cinca pedotype has abundant drab haloed root traces indicative of fluctuating hydrological conditions. Fairly low FeO/Fe2O3 ratios, ∼0.7, (see Supplemental data), indicate a predominantly oxidizing environment suggesting that despite proximity to the lake edge, soils were for the most part well-drained, although there were fluctuating hydrological conditions as seen from the abundance of drab haloed root traces. Within the fluvial system the Bolea pedotype is an intermediate soil type between the coarse-grained La Sotonera pedotype and the more developed Alfisol-like Erla pedotype, with both horizonization and mottles present but retaining some primary structure, (Fig. 3A). However, the lack of clay enrichment within the Bw horizon indicates that soil development was not long enough for lessivage to be of significance. Alfisol-like paleosols (open woodland soils) represented by the Erla, Sigena and Perdido pedotypes, develop where there are longer hiatuses of sedimentation and where sedimentation occurs in low energy conditions from unconfined flow on a floodplain (Fisher et al., in press). Scouring from overlying crevasse splay and channel deposits which has removed the O and upper part of the A horizon of these Alfisol-like paleosols is common. The maturity of these paleosols appears to be controlled predominantly by the frequency of overbank deposition, with the most developed paleosols established where the sedimentation rate was slow, and becoming less developed closer to the active channel. Paleosols found proximal to paleochannels formed compound soils, which were frequently truncated, indicating that sedimentation was rapid and often erosive. Sets of paleosols affected solely by the central lake, in the

most distal parts of the fluvial systems were also found to be compound in nature (e.g., Sigena), however the absence of any significant truncation reflects the lower energy within this part of the fluvio–lacustrine system. With increasing distance from the active channel and lacustrine incursions more developed composite paleosols form (e.g., Marriott and Wright, 1993; Kraus, 1999). The three main paleosol types within the Luna and Huesca distal fluvial systems form a catena. During periods of high discharge into the central lake, lacustrine incursions occurred over these Alfisol-like paleosols preventing further pedogenic alteration. As the lake margin retreated and became subaerially exposed, pedogenesis occurred and poorly developed Entisol-like paleosols form. With time and continued slow overbank sedimentation Alfisollike paleosols formed again. This anisotropic nature of soils means that variation between the soils of the paleofloodplain forms a continuum which can be described in a two-dimensional transect forming a catena, Fig. 10. (Sommer and Schlichting, 1997). Similarly to the palaeocatena developed by Bown and Kraus (1987), it was found that from the edge of the lacustrine incursion the paleosol development altered from Entisol-like to Inceptisol-like to Alfisol-like over a lateral distance of approximately 55 m at the Cinca pedotype locality. Lateral transitions between the different paleosol types are continuous over this distance with a few pioneer plants occurring on mud flats most proximal to the edge of the lake and progressing to open-woodland. This distribution of weakly developed paleosols on marginal channel deposits with increasing soil maturity away from the active channel which was controlled primarily by the rate of sedimentation. However, sedimentation rates are likely to have been affected by autogenic factors, for example, channel avulsion and vegetation as well as extrinsic mechanisms such as climate. 6.1. Paleoenvironment Evidence from the geochemical composition of paleosols indicates that the paleoclimate of the central Ebro Basin during the Early Miocene was probably wetter by over 300 mm yr− 1 in comparison to modern

Fig. 10. Model catena of the distal fluvial system during periods of high base level and low base level. For vegetation identification please see key in Fig. 2B.

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values. Although there is likely to have been seasonality, there has been very little change in paleolatitude and palaeotopography, so it is not possible to ascertain precisely whether the moisture regime would have been ustic (spring and summer/autumn wet) or xeric (winter wet). We speculate, however, that with the absence of calcrete and silica cemented rocks the moisture regime may be ustic. Similar modern seasonality patterns also support this contention, with highest precipitation occurring in the spring and autumn characteristic of continental rather than that of Mediterranean climatic patterns (Sirvent et al., 1997). Global temperatures are generally considered to have been warmer than present during the Late Oligocene–Early Miocene (e.g., Zachos et al., 2001), however, significantly in this context there is no evidence within the palaeosols studied that the mean annual temperature was considerably different from modern values. In addition the abundance of subterranean termite nests within the channel–levee complex suggests a mean annual temperature greater than 8 °C on the basis that no extant termites are known to survive in climates cooler than this (Retallack, 1991). The dominant soil type within the Late Oligocene– Early Miocene was found to be typical of open woodlands (Alfisols), which is further confirmed by trace fossil assemblages. Modern ants such as Prenolepis imparis, which have a similar nest architecture to those found in the La Sotonera pedotype (Tschinkel, 2003), thrive in open woodland environments, where a low water table permits them to construct deep nests. This allows the ant colony to move within the soil profile in response to changing ground temperatures caused by seasonal variation (Kubota, 1948). The occurrence of dung beetle nests and boli indicates that herbaceous vegetation was also present (Retallack, 1990; Genise et al., 2000). The absence of large rooting systems suggests that the vegetation is of low stature but the density of rooting systems indicate that they were abundant. Comparison of these paleosols and modern soils within the Ebro Basin suggests that the soil types present today are not wholly different from those of the Late Oligocene–Early Miocene. However, on the paleolandscape the dominant soils were Alfisols, whereas the modern landscape is dominated by Inceptisols. This may be a result of decreased precipitation resulting in reduced pedogenesis or anthropic changes to a highly degradational landscape that may also inhibit soil development. From field observations, soil type and Köppen's climate classification it can be concluded that the environment was a mid-latitude deciduous open woodland. During the Late Oligocene–Early Miocene the climate is likely to have varied, but without a good

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chronostratigraphic time frame for the Sariñena and Uncastillo formations it has not been possible to determine the exact nature of this variability. The plants present within the Late Oligocene–Early Miocene distal fluvial systems would have ranged from reed-like monocotyledons such as cat-tail, developing in water logged soils proximal to the lake edge, to open woodland composed of low stature plants, herbaceous vegetation and small trees in the distal floodplain and herbaceous vegetation, colonising abandoned channels and subaerially exposed splay deposits. 7. Conclusions Paleosols within the distal Luna and Huesca distributary systems are comparable to modern day Entisol, Inceptisol and poorly-moderately developed Alfisol soils. These represent an early successional landscape, immature landscape and open woodland of varying maturity. During the Late Oligocene–Early Miocene the paleoenvironment was considerably more humid than the modern dry, continental climate of the Ebro Basin, however, the overall mosaic of ecotypes of low stature plants and herbaceous vegetation is unlikely to have changed significantly. The development and maturity of paleosols within these systems forms a continuum, from early successional to moderately mature soils, and is a reflection of the amount of time of non-deposition. Paleosol type was determined principally by the paleohydrology (vegetation and climate being significant controls) with effects from both lake incursions and overbank flow. Acknowledgments The authors are very grateful for the helpful comments made by John Fisher, Matthew Thirlwall and Daren Marshall. This work was conducted during the tenure of a U.K. Natural Environment Research Council studentship at Royal Holloway, University of London. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j. palaeo.2006.10.016. References Agustí, J., Cabrera, L., Garces, M., Krijgsman, W., Oms, O., Pares, J.M., 2001. A calibrated mammal scale for the Neogene of western Europe; state of the art. Earth-Science Reviews 52, 247–260.

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