Grain size, mineralogical and geochemical studies of coastal and

Revista Mexicana de Ciencias Geológicas, 24,inland núm. 3, 2007, p. 423-438 Coastal v. and dune sands from El Vizcaíno Desert, Mexico

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Grain size, mineralogical and geochemical studies of coastal and inland dune sands from El Vizcaíno Desert, Baja California Peninsula, Mexico

Juan José Kasper-Zubillaga1,* and Hugo Zolezzi-Ruiz2 1 Instituto de Ciencias del Mar y Limnología, Geología Marina y Ambiental, Universidad Nacional Autónoma de México, Circuito Exterior s/n, 04510, México D.F., Mexico. 2 Posgrado en Ciencias del Mar y Limnología, Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México, Circuito Exterior s/n, 04510, México D.F. , Mexico. * [email protected]

ABSTRACT A sedimentological, petrological and geochemical research work was carried out in order to find out the origin and provenance of coastal and inland desert dunes from El Vizcaíno Desert, northwestern Mexico. Fifty four sand samples were collected from the windward, crest and slip face of coastal and desert dunes (barchan, transverse, aeolian sand sheets). Onshore winds generates fine, well sorted, near symmetrical dune sands with mesokurtic distributions in the El Vizcaíno Desert inherited from beach sands from the Vizcaíno bay. The coastal and inland dune sands are derived from nearby sand sources like the beach sands and also from alluvial deposits originated from sedimentary-volcanic and schists, granitic and granodiorite sources. This is evidenced by the presence of high quartz content, shell debris, carbonates, mica and hornblende that are constituents of the both coastal and inland dune sands and are probably derived from the action of longshore drifts and onshore winds. The El Vizcaíno coastal and inland dune sands are placed in the craton interior and recycled orogen fields in the Q-F-L diagram suggesting intrusive, sedimentary and partly metamorphosed sources in the composition of the sand. The geochemistry of the sands supports also the maturity process of the sands mainly associated with the presence of alluvial deposits and marine-aeolian action. Additionally, the El Vizcaíno dune sands are chemically related to acid rocks, felsic-plutonic detritus source rocks, which are associated to an active continental margin. The low chemical index of alteration (CIA) values in the dune sands suggest that dryness of the area plays a role in the preservation of labile minerals. The presence of volcanic, metamorphic and plutonic rock around the El Vizcaíno desert basin might contribute to the higher content of plagioclase and mica in the sands when compared to other North American deserts. Key words: grain size, mineralogy, geochemistry, provenance, coastal and inland dune sands, Vizcaíno Desert, Mexico.

RESUMEN Se realizó un estudio sedimentológico, petrológico y geoquímico en arena de dunas para establecer el origen y procedencia de las dunas costeras y continentales del Desierto de El Vizcaíno, Noroeste de México. Cincuenta y cuatro muestras se colectaron del barlovento, cresta y sotavento de dunas costeras y continentales (barjan, transversales, depósitos eólicos arenosos). Vientos hacia la costa generan arenas finas, bien clasificadas, casi simétricas, con distribuciones mesocúrticas en el Desierto de El Vizcaíno heredadas de la arena de playa de la Bahía del Vizcaíno. Las dunas costeras y continentales se derivan de fuentes cercanas como las playas, pero también de depósitos aluviales originados a partir de rocas sedimentarias, volcánicas y esquistos, graníticas y granodioritas. La evidencia está en la presencia de fragmentos de conchas, carbonatos, mica y hornblenda que componen las dunas costeras y continentales y que se derivan por transporte litoral y vientos hacia la costa. Las dunas costeras y continentales del

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Kasper-Zubillaga and Zolezzi-Ruiz Vizcaíno se clasifican dentro del cratón interior y orógeno reciclado en el diagrama C-F-L sugiriendo fuentes intrusivas, sedimentarias y parcialmente metamorfoseadas en la composición de la arena. La geoquímica respalda el proceso de madurez composicional de la arena de duna asociada a la presencia de depósitos aluviales y acción marina-eólica. Adicionalmente, las dunas del Vizcaíno están químicamente relacionadas a rocas ácidas, fuentes félsicas-plutónicas asociadas a una margen continental activa. El bajo índice de alteración química (IAQ) en las dunas indica que el clima seco del área juega un papel importante en la preservación de minerales inestables. La presencia de rocas volcánicas, metamórficas y plutónicas alrededor del Vizcaíno contribuye a la presencia de plagioclasa y mica en comparación con otros desiertos de Norteamérica. Palabras clave: tamaño de grano, mineralogía, geoquímica, procedencia, dunas costeras y continentales, Desierto de El Vizcaíno, México.

INTRODUCTION Coastal and inland dune sands in desert environments are compositionally and texturally controlled by physical and chemical processes such as the wind action, marine/ fluvial processes, weathering, air temperature and precipitation ( Pye and Mazzullo, 1994; Lancaster 1995; Livingstone et al., 1999; Muhs and Holliday, 2001; Garzanti et al., 2003; Muhs et al., 2003, Honda et al., 2004). Grain size variations in coastal and desert dune sands have been widely used to infer transport and depositional mechanisms (Bagnold, 1941; Khalaf, 1989, Pye and Tsoar, 1990; Lancaster, 1992; Wang et al., 2003; Kasper-Zubillaga and Carranza-Edwards, 2005). For example, size coarsening of the dune sands may be due to wind deflation of fine grains leaving behind the coarse fraction in the sands (Khalaf, 1989). It has been also observed that moderately to poorly sorted dune sands occur with short transport from the source of sediments to the dune systems (Blount and Lancaster, 1990). In contrast, longer aeolian transport produces better sorted and fine-grained dune sands (Leeder, 1982; Kasper-Zubillaga and Carranza Edwards 2005). In addition, mineralogical and geochemical studies of dune sands provide new insights into the origin and evolution of aeolian sand bodies (Muhs, 2004). Quartzrich sand dunes are mineralogically mature and they might have inherited their composition from quartz-rich sandstones and weathered plutonic and metamorphic rocks. Maturity of the sands might also be related to losses of labile minerals like feldspar grains due to ballistic impacts in high energy aeolian environments, chemical weathering of feldspar in soils, and fluvial size reduction of feldspars (Dutta et al., 1993; Muhs et al., 2003; Muhs 2004). In contrast, feldsparrich dune sands might be derived from feldspar-rich sources (arkosic sources) but also by little chemical weathering and short aeolian transport (Muhs, 2004). In this paper, we establish the provenance of coastal and inland dune sands from El Vizcaíno Desert, Baja California Peninsula, Mexico. The specific aim of this paper is to observe the grain size attributes, mineralogical and geochemical differences between the dune fields close to the beaches of the Vizcaíno Bay and the inland

dune fields to interpret the processes (i.e., fluvial, aeolian, chemical) that dominate the grain size characteristics and composition of both dune fields. Furthermore, this study provides information on dune sands probably derived from a mix of sedimentary, volcanic, metamorphic and plutonic rocks. Our hypothesis states that the El Vizcaíno dune sands are probably influenced by more than one source rock compared to other North American desert dunes (i.e., Altar Desert, Mexico, Algodones Dunes, California, and Parker Dunes, Arizona) (Muhs et al., 1995; Winspear and Pye, 1995; Zimbelman and Williams, 2002; Muhs et al., 2003; Kasper-Zubillaga et al., 2006b, in press) but still with mature composition despite the complex lithology surrounding the dune fields. STUDY AREA The study area is located in the Baja California Peninsula, Mexico between 26º 29’ and 28º 30’ N and 112º 15’ 45” and 115º 15’ W. (Figure 1a). The low elevations of the central and western parts of the reserve receive constant coastal winds and intense solar radiation. Altitudes range from 0 m at the coast to 1,985 m above sea level at the highest peaks in the mountains. According to Köeppen (1948), climate in the El Vizcaíno Desert is arid (Bw) with an average annual rainfall between 10 to 25 mm (Tamayo, 2000). Onshore winds are northerly, westerly and northwesterly measured at the Vizcaíno Bay (Pérez-Villegas, 1989). Northerly and westerly winds occur 10 % to 30 % of the time in one month with velocities between 2 to 4 m·s-1, whilst northwesterly winds occur 40 % of the time in one month with velocities between 4 to 6 m·s-1 (Pérez-Villegas, 1989). Longshore current comes from the north with average velocities from 6 to 12 cm·s-1 (Fernández-Eguiarte et al., 1992). Average wave height is 2.4 m near Guerrero Negro and further north (Buoy Weather, 2005). The geomorphological unit in the western coastal area of the Baja California Peninsula is the Western Californian Plain (WCP) (Tamayo, 2000). Average slope in Guerrero Negro and northern beaches is

Coastal and inland dune sands from El Vizcaíno Desert, Mexico

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Figure 1. a) Simplified geological map of the studied and surrounding areas (Padilla y Sánchez and Aceves-Quesada, 1992). Sedimentary rocks: lu : shale; ar: sandstone; cg: conglomerate; cz: limestone; al: alluvial; Sd: sand dunes. Metamorphic rocks: pz: slate; E: schist; met: metamorfic complex. Extrusive igneous rocks: Igea: acid; Igeb: basic. Intrusive igneous rocks: Igia: acid; Di; diorite; Gd: granodiorite; cbu: ultrabasic complex. b) Sampling sites of the study area. See Table 1 for sampling keys and coordinates. Asterisks represent the fluvial sand sampling site. Arrow shows the prevail longshore drift and dotted arrows show the prevail onshore winds (Fernández-Eguiarte et al., 1992; Pérez-Villegas, 1989). Rivers: ET: El Tomatal, SP: San Pablo, PO: El Porvenir.

4.3º (Carranza-Edwards et al., 1998). Coastal dunes are mobile and semimobile, vegetated dune types, and morphologically they are barchan, transverse and linear types. Desert dunes are vegetated, semimobile, linear and transverse dune types (Inman et al., 1966; Zolezzi-Ruiz, 2007).

The El Vizcaíno Desert is surrounded by shales, sandstones, conglomerate, and limestones present mainly in the southern part of the desert basin. Slate and schists are also present in the north, and basalts, rhyolite, granites, diorites and granodiorites in the northern and the eastern part of the basin (Figure 1a).

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Kasper-Zubillaga and Zolezzi-Ruiz

MATERIALS AND METHODS A systematic dune sand sampling was performed on coastal and inland dune sands during May-June 2005 (Figure 1b). Samples were collected from the windward, crest and slip face of coastal and desert barchan dunes and transverse sand sheets from El Vizcaíno Desert. This was done because in some cases such as in certain linear and crescent dunes, morphology might control the grain size parameters and mineralogy of the coastal (Kasper-Zubillaga and Dickinson, 2001) and desert dunes (Lancaster 1983; Watson; 1986; Livingstone et al., 1999; Wang et al., 2003). Fifty four sand samples were placed in plastic bags, labeled

and separated for grain size, thin sectioning and geochemical determinations. A Global Positioning System (GPS) was used to locate sampling sites and to measure dune heights above sea level. Approximately 1 to 2 g of sand samples were used for grain size analysis after storing 10 g of each sand sample to ensure repeatability in the grain size analysis. The grain size analysis was performed with a Laser Particle Size Analyser (Model Coulter LS230) that determines the particle sizes between –1.0 φ and 14.6 φ. Particle size distributions were given in μm and converted into φ units to calculate the grain size distribution parameters with the formula Log2 (mm) and percentiles utilized in Folk´s formulae (Folk, 1980) (Table

Table 1. Grain size parameters of the coastal dune sands from the El Vizcaíno Desert (n= 41). Location and sampling site

Mz

σ

Ski

KG

1. Playa Pacheco Norte. Transverse dunes PPN1F 114º 03’; 28º 25’ 2.596 PPN1C 114º 03’; 28º 25’ 2.536 PPN3F 114º 04’; 28º 26’ 2.561 PPN3C 114º 04’; 28º 26’ 2.569 Average 2.566 Standard deviation 0.025

0.397 0.391 0.391 0.376 0.389 0.009

0.069 0.053 0.061 0.057 0.060 0.007

0.990 0.987 0.985 0.997 0.990 0.005

2. Playa Pacheco Sur. Transverse dunes PPS1F 114º 05’; 28º 18’ 2.359 PPS1C 114º 05’; 28º 18’ 2.314 PPS7F 114º 06’; 28º 19’ 2.446 PPS7C 114º 06’; 28º 19’ 2.483 Average 2.401 Standard deviation 0.078

0.391 0.400 0.400 0.405 0.399 0.006

0.013 0.007 0.036 0.052 0.027 0.021

0.969 0.965 0.984 1.005 0.981 0.018

3. Laguna Manuela. Batchan Dunes LM1W 114º 02’; 28º 13’ 2.486 LM1C 114º 02’; 28º 13’ 2.537 LM1S 114º 02’; 28º 13’ 2.407 LM5C 114º 03’; 28º 12’ 2.580 LM7W 114º 03’; 28º 12’ 2.554 LM7C 114º 03’; 28º 12’ 2.587 LM7S 114º 03’; 28º 12’ 2.586 Average 2.534

0.436 0.462 0.432 0.394 0.403 0.392 0.379 0.414

-0.039 -0.024 -0.051 0.045 0.013 0.037 0.025 0.001

1.029 1.016 1.040 0.977 0.978 0.970 0.962 0.996

4. La Golondrina. Barchan Dunes LG1W 114º 02’; 28º 07’ 2.674 LG1C 114º 02’; 28º 07’ 2.659 LG3W 114º 03’; 28º 06’ 2.622 LG3C 114º 03’; 28º 06’ 2.528 LG3S 114º 03’; 28º 06’ 2.533 LG7F 114º 03’; 28º 06’ 2.710 Average 2.621 Standard deviation 0.076

0.362 0.374 0.392 0.433 0.397 0.364 0.387 0.027

0.025 0.024 0.017 -0.024 -0.005 0.033 0.012 0.022

0.952 0.960 0.978 1.009 0.971 0.957 0.971 0.021

5. Isla de Arena. Barchan Dunes IA1W 114º 07’; 28º 02’ 2.094 IA1C 114º 07’; 28º 02’ 2.451 IA1S 114º 07’; 28º 02’ 2.028 IA3W 114º 08’; 28º 01’ 2.403 IA3C 114º 08’; 28º 01’ 2.498 IA3S 114º 08’; 28º 01’ 2.593 IA5W 114º 08’; 28º 01’ 2.623 IA5C 114º 08’; 28º 01’ 2.591 IA5S 114º 08’; 28º 01’ 2.619 Sverage 2.433 Standard deviation 0.225

0.904 0.543 1.001 0.636 0.466 0.400 0.400 0.416 0.397 0.574 0.230

-0.455 -0.184 -0.491 -0.269 -0.089 -0.009 0.000 -0.025 -0.004 -0.170 0.195

0.991 1.259 0.847 1.424 1.084 0.985 0.987 1.006 0.987 1.063 0.174

Location and sampling site

Mz

σ

Ski

KG

6. Exportadora de Sal. Transverse dunes ES1F 114º 05’; 27º 55’ 2.500 ES1C 114º 05’; 27º 55’ 2.530 ES3F 114º 05’; 27º 54’ 2.475 ES5F 114º 05’; 27º 54’ 2.412 ES5C 114º 05’; 27º 54’ 2.442 ES6C 114º 05’; 27º 54’ 2.688 Average 2.508 Standard deviation 0.098

0.395 0.383 0.398 0.390 0.393 0.493 0.409 0.042

0.029 0.029 0.032 0.016 0.031 0.164 0.050 0.056

0.985 0.968 0.974 0.956 0.964 1.205 1.009 0.097

7. Puerto Chaparrito. Barchan Dunes PC1W 114º 07’; 27º55’ 1.875 PC1C 114º 07’; 27º 55’ 2.281 PC3C 114º 08’; 27º 54’ 2.026 PC3S 114º 08’; 27º 54’ 2.251 PC4C 114º 08’; 27º 54’ 2.506 PC4S 114º 08’; 27º 54’ 2.413 Average 2.225 Standard deviation 0.237

0.776 0.555 0.716 0.515 0.387 0.383 0.555 0.164

-0.240 -0.132 -0.222 -0.145 0.009 0.009 -0.120 0.108

0.936 1.170 1.147 1.163 0.981 0.954 1.059 0.112

8. El Vizcaíno. Transverse dunes V1F 113º 50’; 27º 30’ 2.646 V1C 113º 50’; 27º 30’ 2.702 V5F 113º 49’; 27º 29’ 2.619 V5C 114º 49’; 28º 29’ 2.632 V9F 114º 49’; 28º 29’ 2.677 V9C 114º 49’; 28º 29’ 2.632 V13F 114º 48’; 28º 28’ 2.679 V13C 114º 48’; 28º 28’ 2.743 Average 2.666 Standard deviation 0.042

0.395 0.399 0.382 0.392 0.378 0.413 0.395 0.404 0.395 0.011

0.051 0.067 0.053 0.037 0.046 0.047 0.057 0.072 0.054 0.011

0.991 0.984 0.975 0.984 0.959 1.000 0.976 0.993 0.983 0.013

9. La Bombita. Transverse dunes LB1F 113º 46’; 27º 53’ 1.539 LB1C 113º 46’; 27º 53’ 2.536 LB3C 113º 47’; 27º 52’ 2.694 LB5F 113º 47’; 27º 52’ 2.498 LB5C 113º 47’; 27º 52’ 2.769 Average 2.407 Standard deviation 0.498

1.005 0.945 0.450 0.497 0.398 0.659 0.291

0.160 0.111 0.023 0.041 0.055 0.078 0.056

0.950 1.582 1.012 1.018 1.002 1.113 0.264

Mz: mean graphic size, σ: sorting, Ski: skewness, KG: kurtosis. See text for formulae used to determine grain size parameters. F: dune flank, C: dune crest, W: windward, S: slip face. The data were not tested for possible outliers although suggested by Verma and Quiroz-Ruiz (2006).

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Coastal and inland dune sands from El Vizcaíno Desert, Mexico

1). Graphic mean represents the average grain size and it was calculated using Mz= (φ16 + φ50 + φ84)/3. Sorting represents the degree in which the sediment is mixed with coarse and fine sizes. It is computed with (φ84 - φ16)/4 + (φ95 - φ5)/6.6. Skewness is a measure of symmetry in a grain size distribution. Its values can be obtained with (φ16 + φ84 - 2φ50)/(φ84 - φ16). Kurtosis is the degree of peakedness in the graphic distribution (Folk, 1980). Fifty four thin sections of bulk composition were prepared to analyze the dune sands. Point counting was carried out using the traditional standard method of 250 grains

for the major compositional framework of quartz, feldspar and lithics in 54 dune sand samples and three river sand samples (Franzinelli and Potter 1983) (Tables 2 and 3). This was done because quartz enrichment of the sands and little dispersion in the size fractions has been observed in dune sands (Livingstone et al., 1999; Wang et al., 2003; Honda et al., 2004; Muhs, 2004; Kasper-Zubillaga and CarranzaEdwards, 2005). Additionaly 50 grains were counted for minor components like opaque minerals (magnetite)(Op), translucid heavy minerals (pyroxenes, hornblende, apatite, ilmenite, magnetite)(Hm), mica (biotite, chlorite)(Mc) and

Table 2. Point counts of dune sands from the El Vizcaíno Desert. Sample

Qm

Qp

Coastal dunes PPN1F PPN1C PPN3F PPN3C Average Standard deviation PPS1F PPS1C PPS7F PPS7C Average Standard deviation LM1W LM1C LM1S LM5C LM7W LM7C LM7S Average Standard deviation LG1W LG1C LG3W LG3C LG3S LG7F Average Standard deviation IA1W IA1C IA1S IA3W IA3C IA3S IA5W IA5C IA5S Average Standard deviation

194 192 201 210 199 8.14 201 214 190 198 201 9.98 200 196 213 201 205 197 191 200 7.07 195 212 198 194 215 213 205 9.81 190 190 199 189 192 195 197 187 183 191 5.02

2 8 2 4 4 2.83 2 0 7 5 3.5 3.11 3 3 3 6 2 7 3 3.86 1.86 7 2 4 5 4 4 4.33 1.63 18 14 11 12 15 14 13 12 18 14.1 2.52

Fk

0 3 1 3 1.8 1.50 3 1 0 0 1 1.41 1 2 1 0 1 0 2 1 0.82 0 1 1 1 1 1 0.8 0.41 1 2 0 4 0 2 0 2 0 1.2 1.39

P

Lv

22 11 23 9 26 4 12 5 21 7.25 6.08 3.30 19 10 16 6 23 7 22 6 20 7.25 3.16 1.89 22 2 17 7 14 5 17 5 20 2 25 4 26 1 20 3.71 4.45 2.14 18 4 22 1 22 2 19 6 15 2 11 3 18 3 4.26 1.79 20 2 25 3 17 4 22 4 21 7 19 4 21 3 18 8 26 6 21 4.56 3.00 2.01

Ls

Lm

Lp

Total

Op

10 8 12 12 10.5 1.91 12 9 18 16 13.8 4.03 15 21 13 19 16 12 22 16.9 3.89 23 12 19 24 10 14 17 5.87 14 16 15 17 13 16 14 19 15 15.4 1.81

6 4 2 2 3.5 1.91 2 1 3 1 1.75 0.96 3 0 0 0 0 1 4 1.14 1.68 2 0 1 0 1 2 1 0.89 0 0 3 0 2 0 2 3 1 3 1.30

5 3 2 2 3 1.41 1 3 2 2 2 0.82 4 4 1 2 4 4 1 2.9 1.46 1 0 3 1 2 2 1.5 1.05 5 0 1 2 0 0 0 1 1 1.1 1.62

250 250 250 250 250 0.00 250 250 250 250 250 0.00 250 250 250 250 250 250 250 250 0.00 250 250 250 250 250 250 250 0.00 250 250 250 250 250 250 250 250 250 250 0.00

7 8 6 6 6.8 0.96 8 0 3 3 3.5 3.32 7 6 5 8 5 5 2 5.4 1.90 5 4 13 12 5 5 7.3 4.03 0 0 2 1 1 0 3 6 4 1.9 2.09

Hm

Mc

2 1 1 5 2.3 1.89 0 4 2 4 2.5 1.91 0 2 3 4 4 4 3 2.9 1.46 0 0 2 0 1 3 1 1.26 0 0 0 0 1 0 0 0 1 0.2 0.44

40 36 42 38 39 2.58 36 35 37 34 36 1.29 32 21 34 32 31 34 38 32 5.25 33 37 27 27 34 35 32 4.22 5 7 4 8 7 12 6 11 11 8 2.85

Bg+C Total

1 5 1 1 2 2.00 6 11 8 9 8.5 2.08 11 21 8 6 10 7 7 10 5.16 12 9 8 11 10 7 9.5 1.87 45 43 44 41 41 38 41 33 34 40 4.21

50 50 50 50 50 0.00 50 50 50 50 50 0.00 50 50 50 50 50 50 50 50 0.00 50 50 50 50 50 50 50 0.00 50 50 50 50 50 50 50 50 50 50 0.00

Qt (%)

Ft (%)

Lt (%)

78.4 80 81.2 85.6 81.3 3.09 81.2 85.6 78.8 81.2 81.7 2.84 81.2 79.6 86.4 82.8 82.8 81.6 77.6 81.7 2.77 80.8 85.6 80.8 79.6 87.6 86.8 83.5 3.51 83.2 81.6 84 80.4 82.8 83.6 84 79.6 80.4 82.2 1.71

8.8 10.4 10.8 6 9.0 2.18 8.8 6.8 9.2 8.8 8.4 1.08 9.2 7.6 6 6.8 8.4 10 11.2 8.5 1.82 7.2 9.2 9.2 8 6.4 4.8 7.5 1.71 8.4 10.8 6.8 10.4 8.4 8.4 8.4 8 10.4 8.9 1.34

12.8 9.6 8 8.4 9.7 2.18 10 7.6 12 10 9.9 1.80 9.6 12.8 7.6 10.4 8.8 8.4 11.2 9.8 1.79 12 5.2 10 12.4 6 8.4 9.0 3.01 8.4 7.6 9.2 9.2 8.8 8 7.6 12.4 9.2 8.9 1.46

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Kasper-Zubillaga and Zolezzi-Ruiz Table 2. (Continued).

Sample ES1C ES3F ES5F ES5C ES6C Average Standard deviation PC1W PC1C PC3C PC3S PC4C PC4S Average Standard deviation V1 F V1 C V5 F V5C V9F V9C V13F V13C Average Standard deviation LB1F LB1C LB3C LB5F LB5C Average Standard deviation

Qm

Qp

189 4 204 4 214 4 200 2 213 2 204 3.2 10.27 1.10 199 1 212 3 196 3 199 7 185 12 189 5 197 5.17 9.40 3.92 211 3 201 3 214 6 201 1 216 1 215 1 198 3 207 3 208 2.63 7.14 1.69 192 3 207 4 223 8 213 4 213 9 210 5.6 11.39 2.70

Fk 1 2 0 2 0 1 1.00 4 1 1 5 2 2 2.5 1.64 3 4 7 2 6 2 1 2 3.4 2.13 5 1 1 1 8 3.2 3.19

P

Lv

Ls

23 21 16 28 22 22 4.30 26 11 19 20 27 31 22 7.15 22 23 19 31 14 18 30 28 23 6.10 20 22 15 20 16 19 2.97

6 0 2 2 3 2.6 2.19 3 4 4 3 3 2 3.17 0.75 0 4 1 1 4 2 4 1 2.13 1.64 5 4 0 2 2 2.6 1.95

22 15 11 10 8 13.2 5.54 14 17 24 13 17 18 17.2 3.87 11 8 2 5 6 8 4 4 6 2.88 3 5 3 10 0 4.2 3.70

Lm

Lp

2 3 2 2 1 2 4 2 2 0 2.2 1.8 1.10 1.10 0 3 0 2 1 2 1 2 1 3 2 1 0.83 2.2 0.75 0.75 0 0 2 5 0 1 3 6 1 2 1 3 4 6 3 2 1.75 3.1 1.49 2.30 7 15 1 6 0 0 0 0 0 2 1.6 4.6 3.05 6.31

total

Op

250 250 250 250 250 250 0.00 250 250 250 250 250 250 250 0.00 250 250 250 250 250 250 250 250 250 0.00 250 250 250 250 250 250 0.00

4 8 2 3 15 6.4 5.32 0 0 2 0 1 0 0.5 0.84 2 0 0 1 5 7 5 11 3.9 3.87 6 8 5 13 11 8.6 3.36

Hm

Mc

1 3 1 0 0 1 1.22 1 0 1 1 0 0 0.5 0.55 3 6 3 0 1 4 0 5 2.8 2.25 2 4 3 1 2 2.4 1.14

28 24 20 21 25 24 3.21 6 6 7 8 17 11 9 4.26 33 34 29 32 38 31 44 30 34 4.94 37 34 36 33 33 34.6 1.82

Bg+C total 17 50 15 50 27 50 26 50 10 50 19 50 7.31 0.00 43 50 44 50 40 50 41 50 32 50 39 50 40 50 4.26 0.00 12 50 10 50 18 50 17 50 6 50 8 50 1 50 4 50 9.5 50 6.00 0.00 5 50 4 50 6 50 3 50 4 50 4.4 50 1.14 0.00

Qt (%)

Ft (%)

Lt (%)

77.2 83.2 87.2 80.8 86 82.9 4.03 80 86 79.6 82.4 78.8 77.6 80.7 3.03 85.6 81.6 88 80.8 86.8 86.4 80.4 84 84.2 2.95 78 84.4 92.4 86.8 88.8 86.1 5.38

9.6 13.2 9.2 7.6 6.4 6.4 12 7.2 8.8 5.2 9.2 7.9 2.00 3.09 12 8 4.8 9.2 8 12.4 10 7.6 11.6 9.6 13.2 9.2 9.9 9.3 3.09 1.69 10 4.4 10.8 7.6 10.4 1.6 13.2 6 8 5.2 8 5.6 12.4 7.2 12 4 10.6 5.2 1.92 1.91 10 12 9.2 6.4 6.4 1.2 8.4 4.8 9.6 1.6 8.7 5.2 1.43 4.38

Qm: monocrystalline quartz; Qp: polycrystalline quartz; Fk: potash feldspar; P: plagioclase; Lv: volcanic lithics (basalt, andesite ?); Ls: sedimentary lithics (sandstone, siltstone, chert,); Lm: metamorphic lithics (schists); Lp: plutonic lithics (granite); Op: opaque minerals (mainly magnetite); Hm: heavy minerals (pyroxenes, hornblende, apatite, ilmenite), Mc: mica (biotite, chlorite); Bg+ C: biogenic debris (mainly shell fragments, foraminifera, calcareous algae) and carbonates (limestone, calcite, dolomite). Qt: total quartz; Ft: total feldspar; Lt: total lithics.

biogenic (broken shells, foraminifera, calcareous algae) plus carbonates (limestone, calcite, dolomite) (Bg + C). In addition, three river samples were also collected near the sites 1, 2 and 9. River sands were collected from the uppermost centimeter in the bed of dry streams at sites close to the main road. The whole bulk sediment was used for point counting of 250 grains. In the case of the dune and river sands, point counts were normalized to 100 % and ternary diagrams for mineralogic (n= 54) and geochemical data (n=24) were plotted for the dune sands data only using confidence regions of the population mean (CRPM) at 95 % confidence level around the mean population of samples. These regions were constructed with the algorithm developed by Weltje (2002) and converted into ellipses by using the Sigma Plot software. The ellipses represent the area in which samples

might have variations in relation to the mean. This implies that the CRPM define rigorously if two mean populations are significantly different (Weltje, 2002). Sand samples (n= 24) were dried at 110º C and treated with lithium metaborate and lithium tetraborate to make pressed powder pellets. They were analysed with a X-ray fluorescence Siemens SRS 3000 equipment for major and trace elements (Table 4). For major and trace elements, precision is valuated in terms of relative standard deviation being