Transport-Limited Denudation Regime Inferred from Sand Petrography and Chemical Composition : Cenozoic Sediments from the Guadiana Basin ( SW Spain )

The Guadiana Basin is an E-W intraplate Cenozoic basin located in SW of the Iberian Peninsula, covering an area close to 6000 km2. NE-SW Alpine faults divide the basin into two subbasins: the Vegas Altas (VA) to the East, and the Vegas Bajas (VB) to the West. Sedimentary record consists of <200 m thick of sediments, arranged in two main depositional sequences, DS1 and DS2. Petrographic analysis of sediment sandy-fraction combined with geochemical analysis of <2 mm fraction permit to infer provenance and factors controlling sediment composition. Petrographic analysis of sandy-fraction reveals provenance from basin margins lithologies. In VA subbasin, quartzolithic petrofacies characterized eastern subbasin infill related to the erosion of Neoproterozoic and Palaeozoic metasediments. To the West quartzofeldspathic petrofacies predominates, associated with the erosion of coarse-grained plutonites and short distance transport of their products. Intermediate petrofacies characterizes clastic deposits from the VB. Chemical analysis of clastic deposits corroborates a main felsic origin of sediments with a composition that fits well with the upper crust compositional norm and with the Neoproterozoic Iberian Average Shale (NIBAS). In addition, minor contributions from mafic sources can be identified in DS1 from VB, manifested by anomalies in the content of Co, Ni, Cr, V and Sc. Different stages of weathering (from intermediate to intense) can be deduced by the analysis of both, sandy-fraction petrography and bulk sediment geochemistry. This fact is related to the great hiatus at the base of the Cenozoicbasin infill. Transport-limited denudation regimes can be inferred by the presence of high and intermediate weathered and texturalyinmature sediments. In addition, trace element ratios suggest local enrichment in dense minerals, attributable to recycling processes from metasediments. Finally, the sedimen-


Introduction
In the W of the Iberian Plate, Cenozoic basins developed during the Alpine Orogeny over Variscan basement (Iberian Massif), infilling E-W elongated troughs.These basins control and lodge the main modern Atlantic river basins (Duero, Tajo and Guadiana rivers) (Figure 1).
The Guadiana Basin is located in the SW Spain, occupying their sediments an area close to 6000 km 2 , from Madrigalejo to Badajoz localities (Figure 1).Two geographic sectors have been classically considered: 1) Vegas Altas (VA), at the East of Mérida locality; and 2) Vegas Bajas (VB), located at the West of Mérida locality.
Sedimentary record of this basin is Palaeogene to Pliocene in age and consists of a thin pile of sediments (<200 m) constituted mainly by complex clastic lithologies as mixtures of gravels, sand, silt and clay particles, due to a prevalence of mass transport processes associated to alluvial fan systems ([1] [2]).Detailed stratigraphy of the basin is not well known mainly due to a discontinuous presence of outcrop hindering correlations of sedimentary units.In addition, many problems exist in relation to the origin and detailed chronology of these deposits.The great hiatus between ancient basement rocks and basinal sediment infill (at least of 220 Ma), suggests long periods of weathering at the source, modifying the provenance signals in sediments.Recently, two main stratigraphic sequences have been defined ( [3]) and differences in clay mineralogy between those sequences were pointed ( [4]).Petrographic data from sandy deposits (e.g.[5]- [10]), together with geochemical data (e.g.[11]- [13]) has demonstrated as a very useful tools to decipher aspects related to source lithology, source weathering, transport and geotectonic setting in clastic deposits.
This paper aims to throw light to the origin and provenance of Guadiana sedimentary record, based on petrographic and geochemical analysis, and as a consequence, infer the denudation regime.These data may help in the understanding of the origin and evolution of these particular intraplate basins developing significant hiatus between basement and infill sediments.

Geological Setting and Stratigraphy
The Guadiana Basin corresponds to a Cenozoic Basin developed on the SW of the Iberian Massif.Alpine compression events in this area produced an undulatory pattern of low and highs corresponding to sedimentary basins and mountain range alignments ( [14]- [16]).Thus, the Guadiana Basin is bounded by Alpine E-W folds and thrusts, and reactivated inherited left-lateral NE-SW strike-slip faults.These structures divide the basin in two small subbasins (VA and VB), (Figure 1).
The basement of the Guadiana Basin consists of Precambrian to Upper Palaeozoic metasediments and igneous rocks.Two of the major tectonostratigraphic divisions of the Hesperian Massif make up the basement of the area: the Centro Iberian Zone (CIZ), to the east, and the Ossa Morena Zone (OMZ), to the West (Figure 1).Structures present in the basement show NW-SE and E-W trends.In the CIZ, a thick succession of Upper Neoproterozoic greywackes and slates occupy the anticline cores whereas Ordovician and Silurian quarzites and slates form the interior parts of synclines ( [17]).The OMZ represents the inner zone of the Variscan orogen.It includes a complete succession from Neoproterozoic to Upper Paleozoic of more than 10,000 m in thickness of metasediments (slates, sandstones, conglomerates and carbonates) of low to very low grade of metamorphism ( [18] [19]).Locally, medium grade metamorphic lithologies (metabasites and anfibolites) are also present.Batholiths appear widespread in both zones, dominating a granitic composition.However, basic igneous rocks are also present in the OMZ, associated to the VB area of the basin.
The sedimentary record of the Guadiana Basin infill is composed of a succession of variable lithologies as red shales, sporadic thin layers of sandstones and poorly sorted quartzite and slate pebbles, gravels and arkosic sands with red and tan clay matrix.Tractive sedimentary structures are very scarce.The thickness of the succession is extremely variable (20 to 200 m) and have an irregular distribution from west (VB) to east (VA) ( [20] [21]).These deposits have been considered as fluvio-lacustrine sediments related to alluvial fan systems ( [20]).
Based on geomorphological, sedimentological and compositional criteria, two main Depositional Sequences (DS1 and DS2) separated by a regional scale unconformity, have been distinguished ([3] [20]).This unconformity is manifested by the presence of a carbonated unit constituted by quartzite and dolomite polimictic conglomerates and dolomite layers located at the base of DS2.DS2 is finally covered by a thin layer (<1 m) of carbonates.
Recently, a detailed study of clay mineral composition (<2 μm) ( [4]) has revealed a different mineral association for the two depositional sequences.DS1 is characterized by an illite (predominant)-kaolinite-smectite assemblage.By contrast, clastic sediments from DS2 exhibit a smectite (predominant)-illite-kaolinite mineral assemblage.In addition, carbonated levels at the base of DS2 show a dolomite-palygorskite assemblage, whereas carbonates from top of DS2 are characterized by a calcite-palygorskite assemblage.

Methods
A total of 32 selected samples of clastic and carbonate deposits were collected from the two depositional sequences (DS1 and DS2) in both VA and VB subbasins from the Guadiana Basin.A group of 11 samples were used to extract the medium sand fraction (0.25 -0.5 mm) by wet sieving to be investigated following classic methods in petrographic analysis (e.g.[5] [6]).Thin sections were performed on artificially cemented medium sandy-fraction with epoxy resin.Selective feldspar stain methods were used to facilitate mineral identification ( [22]).Four hundred grains were counted per thin section to achieve statistical data about the distribution of the main petrographic classes of detrital grains (Table 1).Ambivalent point counting method and petrographic criteria from ([6] [23]) have been used to elaborate the petrographic data set for medium-grained sandy-fraction (Table 1).
Finally, chemical analysis of the <2 mm fraction was carried out on 32 selected samples, avoiding gravel

Grain Types
Following criteria in ( [24]), sand grain population is mainly constituted by non-carbonate extrabasinal (NCE) grains.Locally, sparitic carbonate extrabasinal (CE) grains have been identified.Quartz is the main detrital component exhibiting a wide spectrum of types (monocrystalline, polycrystalline and chert).Feldspars population, if present, is dominated by K-feldspars over plagioclases.Rock fragments are mainly fine-grained low grade metamorphic (shales), and quartzose metasandstones.Frequently, quartzofeldspatic coarse-grained rock fragments (plutonic) appears as a significant detrital population.

Petrofacies
Petrographic composition of medium sand fraction is highly variable, as arkosic, subarkosic, litharenitic and sublitharenitic compositions (Figure 2(a)).Two main petrofacies have been defined: Petrofacies A and B. Petrofacies A, exhibits a litharenitic composition, with a quartz grain content greater than 50% and a lithic population that varies from 8% to 38%, dominated by low grade metamorphic fragments (fine-grained shales).Quartzose coarse-grained rock fragments (metasandstone) are very common.Quartz population is mainly constituted by polycrystalline and undulatory monocrystalline types (Figure 2(b)).Chert and alterite grains (rounded Fe-oxidiced clasts) are also present.Feldspars are absent or appear as trace amounts.This petrofacies characterizes the eastern part of the VA subbasin (VAE in Figure 1 and Figure 2(a)).
Petrofacies B, is constituted by a quartzofeldespathic composition with a prevalence of K-feldspar over plagioclase (Figure 2(d)).Quartz grain population (60% to 90%) consists mainly by monocrystalline types.Rock fragments are scarce (<20%) and are mainly coarse-grained quartzofeldspathic (plutonic) grains.Low-grade metamorphic fine-grained lithics (shales) have been also identified.Locally, monocrystalline and polycrystalline calcite and minor dolomite grains are also common.This petrofacies B characterizes the western area of VA and the entire sedimentary record in VB (VAW in Figure 1 and Figure 2(a)).
Intermediate petrofacies between A and B frequently develop in VB.This intermediate petrofacies shows a greater quartz population where the amount of polycrystalline types increases noticeably (Figure 2(b)).

Grain Size and Geochemical Composition
Grain size is a significant factor that produces variations in chemical composition from the bulk clastic sediments.The great mixing clastic size texture manifested by the wide range of fine-clastics (<0.062 mm) content in samples (Table 2) prevent the analysis of pure clastic intervals.However, a contrast between significant elements content (SiO 2 , Al 2 O 3 , Na 2 O and Rare Earth Elements, REE) and fine-fraction content (Figure 3), reveals broad correlations.Decrease in SiO 2 correlates with an increase in fine-fraction content as a function of lost of quartz and an increase of phyllosilicates in this fraction (Figure 3(a)).This last increment is also manifested by the  The low rank of correlation between composition and grain size also denotes that variations in composition are also attributable to other sedimentary processes, as nature of source and grade of weathering.

Major Elements
Values in SiO 2 vary greatly from 37.35% to 80.88%, as a result of the presence of other elements, as Al 2 O 3 , car-  2).Lower values (<6%) are recorded in the DS2 from the VB.
SiO 2 , Al 2 O 3 and Fe 2 O 3 contents are very low in carbonate deposits from DS2.However, some samples show significant contents in SiO 2 , rising 37.75%.This high value in silica is attributable to silicification processes, manifested by the presence of chert nodules.
The content in alkaline elements (Na 2 O and K 2 O) is lower than 3.4% and reflects the presence of feldspars or its weathering products (clay minerals).A very low content in Na 2 O (<1%) is observed in the sediments from the eastern part of VA, associated to the quartzolithic petrofacies.Alkaline earth elements (CaO and MgO) show variable values, from 0.56% to 6.91% (MgO), and from 0.03 to 12.86% (CaO) (Table 2).These cations show the grade of occurrence of carbonate phases in analyzed samples.
Harker diagram for SiO 2 versus Al 2 O 3 shows a negative correlation (Figure 4(a)).All samples fit well with this correlation if the content in other major cations (i.e., CaO or MgO) is low.The presence of carbonate minerals in clastic deposits as detrital components or as autigenic phases (cements) reduces SiO 2 and Al 2 O 3 content, producing a shift of plots from the negative correlation toward the origin (Figure 4(a)).Loss On Ignition (LOI) values positively correlates with CaO plus MgO, being a clear index of carbonate content in samples (Figure 4(b)).
No clear correlations (neither positive nor negative) exists between SiO 2 and other major elements as Fe 2 O 3 , K 2 O, MgO, Na 2 O or TiO 2 (Table 2), suggesting that these elements are not clearly related with minerals from the sand fraction.Only a slight positive correlation exists with the content of TiO 2 in samples from DS2 in VA.In this case, TiO 2 content could be associated to sand sized heavy minerals.
Harker diagrams for Al  Ti-oxide as the source of this element.

Trace Elements
Sediment contents in Cs (mean 7 ppm), Rb (mean 85 ppm), Ba (mean 660 ppm) and Sr (mean 132 ppm) show considerable scatter values (Table 3) mainly as a function of grain size of the sediment.Concentration of Large Ion Lithofile Elements (LILEs) is favoured by the presence of the fine-fraction.In spite of this fact, average values are comparable with North America Shale Composite (NASC; [25]) and Average Upper Crust (AUC; [26]).A good positive correlation in K vs. Rb (r 2 : 0.85) exists.Potassium versus the other elements shows broader positive correlation, which suggests that concentration of these elements occurs mainly in K-bearing clay minerals ( [13] [27] [28]).In carbonate levels, very low content in Cs (mean 2 ppm), Rb (mean 15 ppm) and Ba (mean 206 ppm) occurs.By contrast, higher concentrations in Sr are registered in these levels (mean 339 ppm).Values of ferromagnesian trace elements contents (Co, Cr, Ni, V, and Sc) are similar to AUC ( [26]).However, greater values are observed in sediments from the DS1, suggesting a slight mafic contribution from the source.

Rare Earth Elements (REE)
The main elements from La to Lu have been considered to obtain several parameters involving main light and heavy REE distribution (La/Yb, La/Sm, and their corresponding condrite-normalized parameters) (Table 4).Carbonate levels show very low contents in REE.No clear differences between VA and VB, neither between DS1 and DS2 sediments has been found.Thus, the analysis of the distribution of REE has been performed considering as a unique sample population of clastic and carbonate sediments (Figure 5).The most significant signatures of REE distribution in clastics are: 1) High values of La/Sm ratio ((La/Sm) N = mean 4.45) that suggests a steep decrease in Light Rare Earth Elements (LREE); 2) high values in La/Yb ratios ((La/Yb) N = mean 7.94) pointing on a flat distribution of Heavy Rare Earth Elements (HREE) (Figure 5(a)).Very low concentrations in HREE (Eu to Lu) can be deduced.As could be expected, the pattern of REE distribution is similar to that of the

Source and Tectonic Setting
The presence of two main sandstone petrofacies in the sedimentary record of Guadiana Basin suggests significant differences concerning the lithology at the source.Quartzolithic character of petrofacies A corresponds to the supplies from the erosion of metasediments.These metasediments can be correlated with the Neoproterozoic, Cambrian and Ordovician sequences of the Central Iberian Zone from the Variscan Chain.These sedimentary sequences constitute the basement and surrounding areas of the eastern part of VA subbasin.
The quartzofeldspathic signal of petrofacies B is acquired by the erosion of plutonic terrains.The greater amount in feldspars took place in the southwestern area of the VA subbasin and can be related to the erosion of local variscan granitoids cropping close to this part of VA and northward VB.Intermediate petrofacies can be related to the mixing of petrofacies A and B and are developed mainly in DS1 and DS2 from VB.
Geochemical analysis confirms a general felsic origin of sediments that correlates with upper continental crust composition, as revealed the distribution of REE (Figure 5), and the specific La/Th vs. Hf ratios ( [26] [29]).In addition, data from Guadiana Basin sediments correlate with geochemical data of Neoproterozoic Iberian Average Shale (NIBAS), in ( [30]).
The spatial distribution of petrofacies can be related directly with the composition of lithologies at their sources, close to the site of deposition.This fact suggests that the sedimentary record of the basin corresponds to local supplies with very low degree of maturation.This fact is also evidenced by the low rank correlations between the bulk chemical composition and grain size (Figure 3).Thus, sediments from the eastern area of VA show a composition similar to a "recycled orogenic" provenance type ( [5]) (Figure 2(c)).Western VA sediments and the main VB sediments manifest a composition similar to a "craton interior" and transition to "basement uplift" provenance types (Figure 2(c)) ([5]).Major element relationships ( [31]) concur with a geotectonic setting composition associated mainly to passive continental margins (Figure 6).Quartzofeldspathic sediments from petrofacies B display lowest contents in SiO 2 /Al 2 O 3 and K 2 O/Na 2 O values.

Weathering
Intensity of weathering processes acting during the generation of sediments can be analyzed using concentration of immobile Al 2 O 3 and the loss of mobile cations as Ca, Na, and K ( [11]) (Figure 7).Chemical Index of Altera-  tion (CIA) values vary from close to 60 to close to 90, indicating a wide spectrum of weathered samples.Lower CIA values may correspond with samples with significant content of carbonates, increasing the content in CaO + Na 2 O (CN).In addition, main sample population plots following a line trend from ideal plagioclase to ideal illite composition, manifesting a progressive lost in CN as weathering proceeds.This trend correlates with the Zone III of a weathering profile in ( [32]), and suggests a relative equilibrium between weathering and erosion at the source.Samples from DS1 at VA plot on the A-K line (Figure 7), showing a total loss of CN and a partial loss of K (CIA values close to 90).These sediments correspond to a Zone II of a weathering profile ( [32]), representing a greater stage of weathering.Variation in the grade of weathering suggests changes in the type of denudation (transport limited or weathering limited, [33]).The loss of Na and K is evident if compare their contents in Cenozoic sediments with their potential Neoproterozoic sources (NIBAS in [30]).Differences in clay mineral contents between DS1 (illite dominated) and DS2 (smectite dominated) exist, suggesting changes in weathering conditions ([4]).

Transport
Th/Sc values close to 1 and relatively low La/Sc ratios reveal that sediments are not mature ( [13] [26]) (Figure 8).In addition, significant variations in Zr/Sc ratios with low Th/Sc values can be interpreted as a significant enrichment of zircon in sediments ( [34]) (Figure 8).This suggests that sediments have not acquired maturity during transport, but enrichment of specific dense minerals by recycling processes from metasedimentary rocks.In addition, relative contents in specific trace elements (La, Th, Co, Sc, and Zr) ( [35]) show a clear fractionation of these elements due to sorting effect from sandy to silty-clay deposits (Figure 9).As expressed earlier, composition varies as a function of grain size, but sorting does not produce a clear and net compositional separation.

Conclusions
The sedimentary record that infill the intraplate Guadiana Basin (SW Spain) is a Cenozoic succession of continental, mainly siliciclastic sediments, with <200 m thick.Commonly, deposits are constituted by complex textural mixtures of clasts of variable grain size (gravel, sand and mud) generated mainly by mass transport processes.These deposits outcrop in isolate areas, of difficult sedimentary correlation.Classic petrographic techniques and bulk chemical analysis (major, trace elements, and principal REE) are used to characterize these sediments and infer deductions about its genesis, nature of source areas, type of transport, and intensity of weathering, and as a consequence, the involved denudation regime.
From the petrographic analysis of the sandy-fraction, two main sources can be inferred from the presence of  two petrofacies.These sources can be associated to the erosion of: 1) the Neoproterozoicand Palaeozoic succession which denotes a recycled sedimentary character, generating a quartzolithic petrofacies (petrofacies A); and 2) Coarse-grained plutonites that produces a quartzofeldspathic petrofacies (petrofacies B).Local distribution of these petrofacies in VA east and VA west, respectively, suggests short distance transport and low maturation of sediments.Intermediate petrofacies between A and B, mainly characterizes clastic deposits from the VB, as a result of mixing deposits from commented sources.Chemical analysis (including REE distribution) corroborates a main felsic origin of sediments with a composition that fits with the upper crust compositional norm and NIBAS.However, minor contributions from mafic sources can be identified in DS1 from VB (compositional anomalies in Co, Ni, Cr, V and Sc).The analysis of weathering from both sandy-fraction petrography and bulk sediment geochemistry (loss of mobile cations), show different stages of weathering (from intermediate to intense), suggesting variation in its duration and intensity at the source and pointing different denudation regimes ( [33]).This fact can be associated to the great hiatus at the base of the sedimentary record (DS1 in VA), being this record mainly the result of a collection of deeply weathered materials from their sources.Thus, transport-limited denudation regimes can be deduced from petrography and geochemical character of clastic deposits.A more detailed stratigraphic analysis is needed to establish levels of sediments from different stages of weathering.
Analysed sediments are not texturally mature by the presence of a wide range of clastic size detritus.Transport has manifested as not effective process to sort clasts.In addition, sandy composition and bulk sediment geochemistry denote low grades of compositional maturation.Several geochemical parameters show relative increases in specific dense minerals not by maturation during transport, but as a recycling process from metasedimentary sources (i.e., Neoproterozoic).Trace element fractionation can be related mainly to variations in grain size and the abundance of silty-clay fractions in sediments.

Figure 1 .
Figure 1.Location of the study area in the Iberian Peninsula and simplified geological map of the Guadiana Basin and the related substratum lithology.Location of analysed samples is also shown.Zones of the Iberian Massif: CZ, Cantabrian Zone; WALZ, Western Asturian Leonese Zone; CIZ, Central Iberian Zone; OMZ, Ossa Morena Zone; SPZ, South Portuguese Zone.

Figure 3 .
Figure 3. Diagrams showing the correlation between the content in fine-fraction (<0.062 mm) and the content in different and significant chemical elements in samples from the Guadiana Basin.slightpositive correlation between Al 2 O 3 and fine-fraction contents (Figure3(b)).In addition, labile cations as Na are progressively removed from the sediment when fine-fraction increases (Figure3(c)).This fact can be related to plagioclase decay.Rare Earth Element (REE) content shows positive correlation with clastic finefraction content, as a consequence of REE lodging in phyllosilicate structure (Figure3(d)).The low rank of correlation between composition and grain size also denotes that variations in composition are also attributable to other sedimentary processes, as nature of source and grade of weathering.

2 O 3 versus
Na 2 O and K 2 O show positive correlations in samples from both DS1 and DS2 (Figure 4(c) and Figure 4(d)), suggesting that these elements constitute parts of alumino-silicate minerals (feldspars, micas and clay minerals).Other elements show scatter distributions with respect Al 2 O 3 , as MgO and CaO suggesting a main prescence of these elements in carbonate minerals (Figure 4(e) and Figure 4(f)).Broad negative correlation between Al 2 O 3 and TiO 2 in DS2 samples from VA (Figure 4(g)) suggests a detrital

Figure 4 .
Figure 4. Haker diagrams showing major element variations in samples from the Guadiana Basin.

Figure 8 .
Figure 8. Th/Sc versus Zr/Sc and La/Sc trace element indices for analysed samples from the Guadiana Basin.See Figure 4 for symbols.

Figure 9 .
Figure 9. Trace element relations for analysed samples from the Guadiana Basin.See Figure 4 for symbols.

Table 1 .
Petrographic data base of medium (0.25 -0.5 mm) sandy-fraction of selected samples from the Guadiana Basin.

Table 3 .
Trace element concentrations in ppm of samples from the Guadiana Basin.

Table 4 .
Rare earth element compositions in ppm and related parameters of samples from the Guadiana Basin.

Eu Tb Yb Lu ΣREE LREE HREE La/Yb (La/Yb) N La/Sm (La/Sm) N
bonates (CaO, MgO), or Fe-oxides (Fe 2 O 3 ).Deposits dominated by the presence of sands, mainly show SiO 2 values greater than 60% (Table2, and Figure3(a)).By contrast, lower SiO 2 contents are observed in silty-clay deposits, with higher absolute concentrations in other major elements.Thus, percentage of Al 2 O 3 varies from 8.53% to 21.23%, and can be interpreted as a result of fine-fraction contents (clay minerals) (Table2).However, high values in Al 2 O 3 (greater than 15.6%) have been also recorded in sandy deposits mainly from the western part of VA and the entire VB, associated to the quartzofeldspathic petrofacies.Fe 2 O 3 content varies from low (3.00%) to high values (8.48%), mainly associated to fine-grained (silty clay) detrital deposits (Table