versión impresa ISSN 1026-8774
Rev. mex. cienc. geol v.24 n.3 México dic. 2007
Grain size, mineralogical and geochemical studies of coastal and inland dune sands from El Vizcaíno Desert, Baja California Peninsula, Mexico
Estudios granulométricos, mineralógicos y geoquímicos de arenas de dunas costeras e interiores del Desierto de El Vizcaíno, península de Baja California, México
Juan José KasperZubillaga1* and Hugo ZolezziRuiz2
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., México. * email@example.com
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., México.
Manuscript received: May 17, 2007
Corrected manuscript received: September 7, 2007
Manuscript accepted: September 9, 2007
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 sedimentaryvolcanic 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 QFL 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 marineaeolian action. Additionally, the El Vizcaíno dune sands are chemically related to acid rocks, felsicplutonic 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.
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 Vizcaíno se clasifican dentro del cratón interior y orógeno reciclado en el diagrama CFL 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 marinaeólica. Adicionalmente, las dunas del Vizcaíno están químicamente relacionadas a rocas ácidas, fuentes félsicasplutónicas 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.
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; KasperZubillaga and CarranzaEdwards, 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 finegrained dune sands (Leeder, 1982; KasperZubillaga 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 quartzrich 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 feldsparrich 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; KasperZubillaga et al., 2006b, in press) but still with mature composition despite the complex lithology surrounding the dune fields.
The study area is located in the Baja California Peninsula, Mexico between 26° 29' and 28° 30' N and 112° 15' 45" and 115° 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 (PérezVillegas, 1989). Northerly and westerly winds occur 10 % to 30 % of the time in one month with velocities between 2 to 4 m•s1, whilst northwesterly winds occur 40 % of the time in one month with velocities between 4 to 6 m•s1 (PérezVillegas, 1989). Longshore current comes from the north with average velocities from 6 to 12 cm•s1 (FernándezEguiarte 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 4.3° (CarranzaEdwards 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; ZolezziRuiz, 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).
MATERIALS AND METHODS
A systematic dune sand sampling was performed on coastal and inland dune sands during MayJune 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 (KasperZubillaga 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 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 5)/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; KasperZubillaga 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 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 110° C and treated with lithium metaborate and lithium tetraborate to make pressed powder pellets. They were analysed with a Xray 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 <1 % (Sutarno and Steger, 1985). The Chemical Index of Alteration (CIA) values, based on the equation CIA= 100• [Al2O3/(Al2O3+CaO*+Na2O+K2O)] (Nesbitt and Young, 1982), were obtained using the CaO* values present only in the silicate fraction (Honda, electronic communication). No geochemical data were available for surrounding/parental rocks. Correlations were established between textural, mineralogic and geochemical parameters. However, we should mention that discordancy tests for the correlations were not carried out, which implies that rejection or identification of outliers of special interest could not be established (Barnet and Lewis, 1994, Verma, 1997; Verma and QuirozRuiz, 2006).
El Vizcaíno dune sands characteristics
According to the field observations and satellite images, coastal and desert dunes were separated on the basis of the physiography of the area. Coastal dunes are located mainly near beaches of the Vizcaíno Bay (sites 17). They are of transverse (sites 1, 2, 4, 6) and barchan types (sites 3, 4, 5, 7) Inland desert dunes are vegetated sand sheets with little morphological definition but probably parallel to the dominant wind direction (sites 89) (Figure 1B). Mobile dunes are located in sites 3, 5 and 7 whilst the rest of the dune sites are semimobile vegetated systems. The average heights for the coastal and desert dunes are 7.7 m and 14.0 m above sea level respectively.
Grain size distributions
Average grain size parameters for each sampling site are presented in Table 1. The average grain size for the coastal desert sands is 2.434 Φ, sorting is 0.466 Φ, skewness is 0.040, and kurtosis is 1.025. The average grain size for the inland dune sands is 2.520 Φ, sorting is 0.476 Φ, skewness is 0.028, and kurtosis is 1.015 (Table 1). The grain size correlations among textural parameters for the El Vizcaíno Desert dune sands show that there are only two significant correlations between grain size, sorting and grain size and skewness (Khalaf, 1989; KasperZubillaga and CarranzaEdwards, 2005) (Figure 2).
The compositional framework of the sands consisted of monocrystalline (Qm) and polycrystalline quartz (Qp), potassium feldspar (Fk), plagioclase (P), volanic (basalt, andesite) (Lv), sedimentary (sandstone, siltsone, chert) (Ls), metamorphic (schist) (Lm) and plutonic lithics (granite) (Lp). The El Vizcaíno Desert coastal dune sands are quartzolithic sands (Qt82 Ft14 Lt4) (Figure 3). Average lithic percentages are 67 %, 18 % and 15 % for Ls, Lv and Lm+Lp respectively. Mica and biogenic detritus plus carbonates are abundant with 47 % and 42 % content in relation to opaque and heavy minerals (2 % to 9 %, respectively) (Table 2).
The El Vizcaíno inland dune sands are quartzolithic sands (Qt85 Ft10 Lt5)(Figure 3). Average lithic percentages are 43 %, 20 % and 37 % for Ls, Lv and Lm+Lp respectively. Point counts of accessory minerals show an increase in the mica content compared to the rest of the trace components (Table 2). Mica and biogenic detrirtus plus carbonates content is 68 % and 15 % content, respectively, whereas opaque and heavy minerals are 11 % and 15 %, respectively.
The QFL diagram (Dickinson et al., 1983) shows that the coastal and inland dune sands plot in the craton interior and recycled orogen fields (Figure 4).
Similar major elements values are presented for coastal and inland dunes sands (Tables 5, 6). However there are higher values of trace elements like Rb, Sr, Y, Co, Ni, Cu and Zn for coastal dune sands compared to the inland dune sands (Table 6).
The ABC ternary diagram with CRPM, where A is SiO2, B is K2O+Na2O+Al2O3, and C is Fe2O3+TiO2+MgO, shows a dispersal towards the AB poles for the coastal dune sands (Figure 5a). Average content of SiO2 is 76 %. In the CaONa2OK2O ternary diagram, the content of CaO, Na2O and K2O is 53% and 34 % and 13 %, respectively. The slight dispersal of the CRPM is towards the K2O pole (Figure 5b).
The ABC ternary diagram with CRPM shows a dispersal towards the AB poles with similar percentages in ABC for the inland dune sands compared to the coastal dune sands. (Figure 5a). The CaONa2OK2O ternary diagram shows a slight increase in K2O and abrupt dispersal of the CRPM towards the K2O pole compared to the coastal dune sands (Figure 5b).
The CIAACNK triangle (Nesbitt and Young, 1996) shows that most of the coastal and inland dune sand samples tend towards the A pole, representing the Al2O3 concentration, with relatively low CIA values (Figure 6)
The K2O/Na2O vs. SiO2/Al2O3 diagram (Roser and Korsch, 1986) indicates that some coastal and inland dune sand samples plot in the evolved arc setting felsicplutonic detritus field and in the active continental margin arc, and passive margin fields (Figure 7).
The Ni vs. Ti plot (Floyd et al., 1989; Nagarajan et al., 2007) shows that the overall of the coastal and inland dune sand samples are placed in the mature sediments fields with only three samples in the acidic source field (Figure 8)
Grain size distribution
The coastal and inland dune sands are fine, well sorted, near symmetrical sands with mesokurtic distributions. Similar patterns in coastal and desert dune sands have been reported in dune systems from five continents where dune sands are well sorted fine sands with symmetrical distributions and mesokurtic curves (Ahlbrandt, 1979).
The El Vizcaíno coastal and inland dune sands suggest that the sands have experienced an aeolian process due to westerly and northerly onshore winds, which might have caused that the dune sands have retained some of the beach sands textural characteristics as it is observed by the finegrained and wellsorted values (average= 2.6 Φ; sorting= 0.42 Φ) (CarranzaEdwards et al., 1998). The onshore wind patterns with velocities of 2 to 6 m•s1 and 40 % of frequency might control the finesized and wellsorted distributions of the dune sands. This can be supported by the fact that threshold velocities in fine to medium sands are above 4 m•s1. This suggests that the finesized beach sand grains might have experienced a short transport after their removal from the beach into the dune systems, leaving their textural characteristics similar to those observed for the beach sands. The grain size distributions of coastal and inland dunes indicate that the dune sands are not grain size selective when the beach sediments are fine grained and well sorted (Pye 1991).
The CRPM shape of coastal dune sands, compared to the inland sands, suggests more concentration of coastal sand samples near the mean population compared to the inland dune sands, but this is also due to the higher number of samples for the costal dune sands (Figure 3). These shapes are associated with the amount of dispersal of data plotted in the ternary diagram (Weltje , 2002). The CRPM quantitative approach reveals that coastal and inland dune fields are not significantly different. The enrichment of quartz in both dune systems within the El Vizcaíno desert basin probably resulted from the maturity process, where the source rock might provide quartzrich sediments. This interpretation is supported by 1) the presence of the alluvial deposits in the El Vizcaíno basin underlying the dune fields; 2) the composition of some of the rivers draining throughout the basin with moderately high quartz content; 3) the high content of sedimentary lithics probably derived from sedimentary outcrops in the area, but also from the alluvial deposits that provides quartz detritus to the coastal and desert dune sands; and 4) the presence of quartzrich beach sands (Qt90 Ft9 Lt1) near sites 1 to 7 that might contribute to the landward transport of beach sands by the wind (CarranzaEdwards et al., 1998). Moreover, the QFL plot (Dickinson et al., 1983) indicates a craton interior and recycled orogen with tectonic fields suggesting intrusive, sedimentary and partly metamorphosed sources (Dickinson et al., 1983; AmstrongAltrin et al., 2004) (Figure 4). Maturation of the sands may be related to secondary processes as wind action leads to quartzrich dune sands (Muhs, 2004; KasperZubillaga et al., 1999). Furthermore, some samples from the inland dune sands (V9C, V13C, LB5F) have been probably influenced by some eroded acid, volcanic, and metamorphic (schist) rocks, as well as by acidintermediate plutonic rocks like the granitic and granodiorite outcrops in the north of the El Vizcaíno Desert basins. The alluvial deposits in the El Vizcaíno Desert were mainly derived from the above mentioned rock types. This is especially observed for the volcanic lithic fractions. From the alluvial deposits, the wind transports sediment towards the inland dune sands. Also, the release of mica and hornblende in the inland dune sands might support the influence of granitic, granodiorite and schist sources to the alluvial deposits, and consequently to the dune sands during short aeolian transport. This is especially observed in the San Pablo (SP) river, where mica is a relatively important constituent of fluvial sands and where plutonic and metamorphic fractions are depleted (Table 3).
The presence of carbonate shells in the coastal dune sands indicates the beach sand influence in the composition of these sands. It is likely that the northwesterly and northerly winds are capable of transporting shell fragments onto the dune fields of the coast. The amounts of broken shells and carbonate minerals like calcite are depleted landward as result of their softness and long transport by the wind.
In the ABC ternary diagram (Figure 5a), it is observed that the compositions of the coastal and desert sands overlap, which can be attributed to the maturity of the dune sands near and away from the coast. This can be visualized by the high SiO2 contents and relatively similar mineralogical composition of feldspar and lithic fractions for both dune types. The CaONa2OK2O diagramm shows no significant differences between the coastal and desert dune sands (Figure 5b). The K2O content in the desert dune sands might be associated with the presence of mica, and dispersal of the CRPM towards the K2O pole may be due to the different amount of mica in the samples. Furthermore, low concentration of potassium feldspar in the El Vizcaíno dune sands (Table 2) might be associated with the composition of the beach and alluvial deposits that provide little Kfeldspar. This is also related to the chemical composition of the northern beach sands that in the overall have more than 60% of CaO, derived from plagioclase, and less than 30 % of K2O, associated with potassium feldspar (CarranzaEdwards et al., 1998).
In addition, the Chemical Index of Alteration (CIA) values obtained for the sand samples indicate low chemical weathering for the coastal and inland dune sands (Figure 6). The ACNK diagram indicates that, in the overall, El Vizcaíno sand samples have experienced low chemical weathering probably because of the dryness of the area. El Vizcaíno dune sands have similar CIA values to those observed in arkose and litharenites from ancient sandstones in India (ArmstrongAltrin et al., 2004).
The presence of Rb, Sr and Ba in both dune systems is probably associated with the presence of mica and, in lesser extent, of potash feldspars. This is because Rb, Sr and Ba are trace elements that may substitute for K in the lattice of mica and potash feldspar (Sawyer, 1986; Gallet et al., 1996; Canfield, 1997; Muhs et al., 2003). Yttrium may be associated with the presence of basaltic and metamorphic sources in the coastal dune sands (Hawkesworth and Morrison, 1978; Hill et al., 2000).
The enrichment in Co in the coastal dunes may be related to the recycling of sedimentary lithics within sand samples near marine environments, where correlation between Co and Ls is relatively significant (r= 0.72). This is because some sedimentary lithics can be potential carriers of some accessory minerals like opaques and heavy minerals that may concentrate Co under marine weathering conditions because of the immobility of Co in aqueous conditions (ZolezziRuiz, 2007). Furthermore, Co may be associated with the presence of titanium and iron oxides (Krupka and Serne, 2002) in the sedimentary lithics composed of accessory minerals like rutile, sphene, Ilmenite and magnetite. This is evidenced by 1) the high concentrations of Co in sand samples coming from the sites 2, 3, 4, 5, 6, and 7, near the coastal areas with marine influence; and 2) the presence of titanium and iron minerals in the El Vizcaíno dune sands. Ni, Cu and Zn may be associated with the presence of sedimentary lithics in the coastal dune sands, which may contain some opaque and heavy minerals probably associated to ultramafic rocks (Lee, 2002; ZolezziRuiz, 2007).
Correlations between Fe2O3, TiO2, MgO and V (Figure 9) indicate the following: 1) The positive correlation between Fe2O3, TiO2, and MgO suggests the presence of heavy minerals associated with sedimentary sources mainly ilmenite and magnetite (Basu and Molinaroli, 1989; CarranzaEdwards et al., 2001); 2) the positive correlations between TiO2, Zr and V suggests that the sands are influenced by magnetite, ilmenite and zircon minerals associated with some other heavy minerals probably derived from sedimentaryvolcanic sources. In both cases, similar correlations have been observed in dune and beach sands in the Gulf of Mexico coast, the Mexican western coast and northwestern Mexico (KasperZubillaga et al., 1999; CarranzaEdwards et al., 2001; KasperZubillaga et al., 2006b). In the El Vizcaíno dune sands, velocity and frequency of the winds might transport magnetite and some other heavy minerals landward. This process has been also observed for the Altar Desert dune sands close to the Colorado Delta River in northwestern Mexico, where short aeolian transport enables the transport of heavy minerals from the source sediment to the dune system (KasperZubillaga et al., in press).
Comparisons of dune fields from El Vizcaíno Desert with other North American Deserts
Quartz percentages in the El Vizcaíno dune sands are similar to those found in the Altar Desert dune sands in northwestern Mexico (KasperZubillaga et al., 2006b) with a slight enrichment in plagioclase minerals (Figures 10a, b). The data plotted in a QtFtLt ternary diagram with the CRPM show that the El Vizcaíno sands tend slightly towards the total feldspar pole. The CRPM for both sites do not overlap, which suggests that dune sands from both areas are significantly different (Figure 10b).
One source of the El Vizcaíno dune sands are probably beach sands in turn derived mainly from sedimentary (alluvial) and, in lesser extent, by metamorphic and plutonic sources exposed near the El Vizcaíno Desert. Some volcanic lithics also contribute to the dune composition but the release of monomineralic crystals of plagioclase, some pyroxenes, hornblende and opaque minerals are important constituents of the dune sands despite the short aeolian transport, especially in the coastal dunes (Akulov and Agafonov, 2007). The slight enrichment in plagioclase and lithics in the El Vizcaíno dune sands might derive from the alluvial deposits. This is evidenced by the large content of sedimentary lithics in the El Vizcaíno dune sands and also by the extensive area in which the alluvial deposits outcrop north of the El Vizcaíno. When comparing the Vizcaíno dune sands with the Altar Desert dune sands in Sonora, the CaONa2OK2O diagram ( Figure 11) shows that El Vizcaíno dune sands follow the CaONa2O line, whereas the Altar Desert dune sands follow slightly the CaOK2O trend and approaches towards the K2O peak. High content of CaONa2O in the El Vizcaíno Desert dune sands is again probably due to the slight enrichment of plagioclase feldspar.
An AB diagram was plotted for different dune systems of North America where A= SiO2 and B= K2O+ Na2O+Al2O3. The El Vizcaíno Desert dune sands are higher in B content compared to the Altar Desert dune sands, the Algodones and Parker dunes and the Colorado River sediments (Muhs, 2004; KasperZubillaga et al., 2006) (Figure 12). Likewise, the El Vizcaíno dune sands lay within the range of A and B content (with the exception of sample ES5F) of the Rice Valley dunes in California (Muhs, 2004). In the case of the El Vizcaíno dune sands, high B content is probably associated with the presence of mica in the sands High mica content in the El Vizcaíno dune sands indicates that wind velocity exerts a control in the composition of the sands carrying beach and alluvial sands into the dune systems. perhaps enhanced by the platy morphology of the micas.
In addition, the low chemical index of alteration values in the El Vizcaíno Desert, similar to those in the Rice Valley dunes (Muhs 2004), indicate that the plagioclase content might be preserved in the sands. This is evidenced by the content of Na2O. In contrast, dune sands from the Altar Desert and Algodones have inherited their maturity from the silicarich Colorado River Delta sediments (Muhs 2004; KasperZubillaga et al., 2006, in press).
1. The El Vizcaíno dune sands are fine, well sorted, near symmetrical sands with mesokurtic distributions. This reflects that onshore wind frequencies and velocities produce finesized and wellsorted dune sands inherited from beach sands during short aeolian transport into the coastal and inland dunes.
2. The El Vizcaíno coastal and desert dune sands are placed in the craton interior and recycled orogen fields in the QFL triangle, which suggest intrusive, sedimentary and partly metamorphosed sources have contributed to the composition of the sands. The presence of minerals like mica and hornblende in the bulk composition of the dune sands supports the influence of granitic, granodioritic and schistose sources. The presence of biogenic detritus and carbonates in the coastal dune sands suggests that they are derived from beach sand sources mixed with alluvial deposits, whereas the inland desert dune sands are derived from alluvial deposits derived from sedimentary, volcanic, schists and granitic and granodiorites.
3. Maturity of the El Vizcaíno dune sands is inherited from the alluvial and beach sands of the Vizcaíno Bay and the desert basin, despite the complex lithology surrounding the dune fields. Maturity of the dune sands is related to aeolian/marine processes.
4. The geochemistry of the El Vizcaíno dune sands show that the dune sands are associated with acid, felsicplutonic detritus linked to an active continental margin. Low CIA values are related to the dry climate of the area. The high concentrations of Rb, Sr, Ba, Y, Co Ni, Cu and Zn in the coastal dune sands are probably associated with the presence of mica, basaltic and metamorphic sources, sedimentary lithics containing titanium and iron minerals, opaques and heavy minerals. Correlations between Fe2O3, TiO2, MgO, Zr and V in the dune sands indicate the presence of ilmenite, magnetite and zircon in the sands.
We are indebted to the Instituto de Ciencias del Mar y Limnología, UNAM, for the financial support (internal project 109, Marine processes in coastal sedimentary environments). We are also grateful to Eduardo Morales de la Garza for his invaluable assistance during the particle size analysis. We are indebted to Dr. Thomas E. Gill and Dr. J. Madhavaraju for their invaluable review of this paper.
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