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.

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 (round- ed 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 is a significant factor that produces variations in chemical composition from the bulk clastic sediments.

*Not considered by the high content in carbonate.

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 (SiO₂, Al₂O₃, Na₂O and Rare Earth Elements, REE) and fine-fraction content (Figure 3), reveals broad correlations. Decrease in SiO₂ 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

slight positive correlation between Al₂O₃ and fine-fraction contents (Figure 3(b)). In addition, labile cations as Na are progressively removed from the sediment when fine-fraction increases (Figure 3(c)). This fact can be related to plagioclase decay. Rare Earth Element (REE) content shows positive correlation with clastic fine- fraction content, as a consequence of REE lodging in phyllosilicate structure (Figure 3(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.

Values in SiO₂ vary greatly from 37.35% to 80.88%, as a result of the presence of other elements, as Al₂O₃, car-

bonates (CaO, MgO), or Fe-oxides (Fe₂O₃). Deposits dominated by the presence of sands, mainly show SiO₂ values greater than 60% (Table 2, and Figure 3(a)). By contrast, lower SiO₂ contents are observed in silty-clay deposits, with higher absolute concentrations in other major elements. Thus, percentage of Al₂O₃ varies from 8.53% to 21.23%, and can be interpreted as a result of fine-fraction contents (clay minerals) (Table 2). However, high values in Al₂O₃ (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.

Fe₂O₃ content varies from low (3.00%) to high values (8.48%), mainly associated to fine-grained (silty clay) detrital deposits (Table 2). Lower values (<6%) are recorded in the DS2 from the VB.

SiO₂, Al₂O₃ and Fe₂O₃ contents are very low in carbonate deposits from DS2. However, some samples show significant contents in SiO₂, 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 (Na₂O and K₂O) is lower than 3.4% and reflects the presence of feldspars or its weathering products (clay minerals). A very low content in Na₂O (<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 SiO₂ versus Al₂O₃ 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 SiO₂ and Al₂O₃ 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 SiO₂ and other major elements as Fe₂O₃, K₂O, MgO, Na₂O or TiO₂ (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 TiO₂ in samples from DS2 in VA. In this case, TiO₂ content could be associated to sand sized heavy minerals.

Harker diagrams for Al₂O₃ versus Na₂O and K₂O 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 Al₂O₃, 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 Al₂O₃ and TiO₂ in DS2 samples from VA (Figure 4(g)) suggests a detrital

Ti-oxide as the source of this element.

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 (r²: 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.

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

Upper Continental Crust ( [26] ) (Figure 5(b)).

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 SiO₂/Al₂O₃ and K₂O/Na₂O values.

Intensity of weathering processes acting during the generation of sediments can be analyzed using concentration of immobile Al₂O₃ 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 + Na₂O (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] ).

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.