Study of the erosion rates in the upper Maracujá Basin (Quadrilátero Ferrífero/MG, Brazil) by the...

Erosion rates in the upper Maracujá Basin 905

Copyright © 2006 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 32, 905–911 (2007)DOI: 10.1002/esp

Earth Surface Processes and LandformsEarth Surf. Process. Landforms 32, 905–911 (2007)Published online 10 October 2006 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/esp.1448

Study of the erosion rates in the upper MaracujáBasin (Quadrilátero Ferrífero/MG, Brazil) by thein situ produced cosmogenic 10Be methodAndré Salgado,1,2* César Varajão,1 Fabrice Colin,3 Régis Braucher,4 Angélica Varajão1 andHerminio Nalini Jr1

1 Universidade Federal do Ouro Preto, Geology, Brazil2 Université Aix-Marseille III, CEREGE, France3 IRD, Geology, France4 CNRS, CEREGE, France

AbstractThe present work quantifies the erosive processes in the two main substrates (schists–phyllitesand granites–gneisses) of the upper Maracujá Basin in the Quadrilátero Ferrífero/MG, Bra-zil, a region of semi-humid tropical climate. Two measuring methods of concentration wereused: (i) in situ produced 10Be in quartz veins (surface erosion rates) and (ii) 10Be in fluvialsediments (basin erosion rates). The results confirm that (i) erosion tends to be more aggres-sive close to the headwaters than in the lower parts of the basin and (ii) the region is nowaffected by dissection. Copyright © 2006 John Wiley & Sons, Ltd.

Keywords: 10Be cosmogenic nuclides; Quadrilátero Ferrífero, Brazil; erosion rates

*Correspondence to: AndreSalgado, Geography Department,Minas Gerais Federal University,Brazil.E-mail: [email protected]

Introduction

The Quadrilátero Ferrífero/MG, located on the southern border of the São Francisco Craton, with tropical semi-humidclimate and an area of 7200 km2, is one of the most important Brazilian mineral provinces. The geology of this regionis described as follows (Alkmim and Marshak, 1998) (Figure 1): (i) the Crystalline Complex (Bação Complex),composed of Archean granites, gneisses and migmatites; (ii) the Rio das Velhas Supergroup, which constitutes anArchean greenstone belt sequence and is composed of quartzites, schists and phyllites; (iii) the Minas Supergroup,constituted by a Proterozoic metasedimentary sequence, and (iv) the Itacolomi Group, constituted by Proterozoicquartzites.

Its landforms have been the object of several studies since the work of Hader and Chamberlin (1915), whoconcluded that the regional relief results from structure and differential erosion, quartzites and itabirites constitutingthe highland, the schists–phyllites the medium-high land and granites–gneisses the lowland substrates. Spatially, thehighlands constitute a group of crests with a roughly quadrangular shape (Figure 1), forming the perimeter of theQuadrilátero Ferrífero, surrounded by the lowlands of the crystalline complex.

Similar observations on the role played by differential erosion in regional land shaping have been presented in thefollowing works: Ruellan (1950); King (1956); Barbosa and Rodrigues (1965, 1967); Dorr (1969); Maxwell (1972);Lichte (1979) and Varajão (1991). Tricart (1961), however, affirms that the schists–phyllites and the granites–gneissesare similarly resistant to the erosion processes. To Tricart (1961), the schists–phyllites are topographically higher inrelation to the crystalline basement because quartzites and itabirites, which are more resistant rocks, are usually foundbetween them. Quartzites and itabirites, as well as a relief shaped on an inverted syncline, would prevent schists–phyllites from being completely eroded (Barbosa, 1980). Therefore, the method to measure long-term (1·5 Ma) ero-sion rates based on cosmogenic 10Be production could constitute a useful tool to quantify the effect of this process ondifferent types of rock.

Received 28 March 2006;Revised 22 August 2006;Accepted 7 September 2006

906 André Salgado et al.


Figure 1. Quadrilátero Ferrífero geology by Alkmin & Marshak (1998).

The method to calculate erosion rates using cosmogenic 10Be is based on the measurement of the 10Be production inquartz veins, soils, rocks and sediments. 10Be is produced in the Earth’s atmosphere as the result of the interaction ofcosmic-ray primary (α particles and protons) and secondary particles (neutrons, slow and fast muons) with atmo-spheric 14N and 16O nuclei (Siame et al., 2000). Traces of beryllium are also produced inside lithospheric materials(sediments, rocks, soil etc.) at the crust’s shallower depths (Brown et al., 1995; Braucher, 1998). These traces areformed by interaction between cosmic rays and 16O, 27Al, 28Si and 56Fe (Siame et al., 2000) present in these materials.The intensity of such production varies according to the intensity of the cosmic radiation that has affected thelithospheric material. This intensity varies as a function of altitude, latitude, depth, amount of shade produced by therelief and exposure time. Thus, knowing the first four variables and measuring the 10Be production, it is possible tocalculate the exposure intensity of the lithospheric material to the cosmic radiation. The erosion rate of a surface(soils, rocks and veins) or the erosion rate of a hydrographic basin (fluvial sediments) is calculated by means of theexposure intensity.

In fluvial sediments, this method measures erosion rates of the hydrographic basin because the sediments have beenexposed to cosmic radiation not only during the whole surface erosion process, but also during the transport process.Thus, the in situ-produced 10Be method applied to fluvial sediments records the average erosion rate (rates of erosionsurface and transport sediment) of the hydrographic basin upstream of the point where these sediments were collected(Brown et al., 1995; Granger et al., 1996; Von Blanckenburg, 2005).



The upper Maracujá Basin

The upper Maracujá Basin, with an area of 15 368 km2, is located in the Quadrilátero Ferrífero, south of the BaçãoComplex depression, where the granitic–gneissic–migmatitic basement crops out (Figure 1). The crystalline basementis the substrate downstream, whereas upstream the Maracujá Basin flows on phyllites that compose the PiracicabaGroup – Minas Supergroup (Figure 2). Between these two compartments quartzite and itabirite scarps occur (Figure 2).These scarps mark differences not only in altitude but also in landforms. Savanna vegetation predominates in bothcompartments, and is used as pasture for extensive cattle breeding.

The landscape of the compartment downstream (granites–gneisses) is composed of half-orange shaped hills. Duringthe Quaternary their slopes were covered by colluvium from the interfluves (Valadão and Silveira, 1992). Theseslopes, including the colluvial areas, are covered by latosols. In medium and low slopes, the C horizon crops out,which demonstrates that the latosols and the colluvial covers were eroded while the drainage net was evolving(Figueiredo et al., 2004). One significant characteristic of the Bação Complex landscape is the large number of gullies.The gullies predate anthropic interference (Bacellar et al., 2005) and can reach significant sizes, of the order of 400–500 m length, 150 m width and 50 m depth.

The compartment upstream is composed of elongated hills that present dissected slopes where schists–phyllites cropout. Lenticular quartzite and itabirite intercalations are common, and gullies do not occur in this compartment.

Methods

The samples collected for the calculation of the erosion rates are from quartz veins found on the top of two hills, onevein crosscutting schists–phyllites and the other granites–gneisses. Quartz veins were chosen because quartz is easilypurified and dissolved for 10Be extraction in the laboratory, and its structure is such that contamination of in situ-produced 10Be by atmospheric 10Be and 10Be loss to the atmosphere are minimized (Braucher, 1998; Brown et al.,2003). The samples were collected on the top of the hills in order to avoid possible interference by colluvium, whichwould affect the cosmic radiation incidence on the sample.

In the Maracujá Basin, no quartz veins were found crosscutting the granite–gneissic substrate on top of the hills.Therefore, a quartz vein sample was collected on the top of a hill located some 6 km from the upper Maracujá Basinoutlet, inside the Bação Complex, and in an area with similar rock, geomorphology, climate, vegetation and soil use.The fluvial sediments sampled for the calculation of the hydrographic basin erosion rates were collected at four pointsin the upper Maracujá Basin (Figure 2).

Quartz was isolated from crushed and sieved (250–1000 µm) sediment samples by dissolving all other mineralswith mixtures of HCl and H2SiF6. Atmospheric 10Be was then eliminated by successive HF sequential dissolutions.The purified quartz was finally dissolved in Suprapur HF and the resulting solution was spiked with 0·3 mg of 9Becarrier (Merck Titrisol). Beryllium was separated by successive solvent extractions and precipitations. All 10Be meas-urements were performed by accelerator mass spectrometry at the Tandétron AMS facility, Gif-sur-Yvette. Measured10Be/9Be ratios were calibrated directly against the National Institute of Standards and Technology (NIST) standardreference material SRM 4325 using its certified 10Be/ 9Be ratio of (26·8 ± 1·4) × 10−12. 10Be uncertainties (1σ) includea 3% contribution conservatively estimated from observed standard variations during the runs, a 1% statistical error inthe number of 10Be events counted, uncertainty on the blank correction and a 6% uncertainty on in situ 10Be produc-tion rate within quartz (Stone, 2000).

To determine the long-term erosion rate (surface and hydrographic basin), the following equation (Lal, 1991;Braucher, 1998) was used:

C x tP p x P p x P p xo n

n

n

o s

s

s

o f

f

f

( ; ) .

.exp

.

.exp

.

.exp=

+−

++

−

+

+−

ε λ ε λ ε λ

µ

µ

µ

µ

µ

µΛ

ΛΛ

ΛΛ

Λ

where C (at /g) is the concentration of a radioactive cosmogenic nuclide produced by spallation reactions in a rock orsediment exposed; x is the depth (g/cm2); t is the time (years); Po is the production rate (atom/g/year); pn, pµs and pµf

refer to the neutron and slow and fast muon contributions (these are 97·85, 1·5 and 0·65%, respectively, in quartz), Λn,Λµs and Λµf are the neutron and slow and fast muon attenuation lengths, which are 150, 1500 and 5300 g/cm2 (Braucheret al., 2003) respectively; λ (1/year) is the radioactive decay constant and ε (g/cm2/year) the erosion rate.



Figure 2. High Maracujá Basin geology by Johnson (1962).



Table 1. Erosion rates in the hill slopes

Rock Depth 10Be erosion ratePoint Geology1 density (m) 10Be (at /g) (surface) (m/My)

Br5A P 2·81 0·0 1 172 615 ± 169 523 4·32 ± 0·68Br5B P 2·81 0·6 530 754 ± 114 272 3·93 ± 0·88Br5C P 2·81 1·8 168 229 ± 24 557 4·92 ± 0·78Br5D P 2·81 2·1 74 957 ± 10 836 14·45 ± 2·26Br5E P 2·81 3·0 78 392 ± 14 056 10·71 ± 2·03Br6A Gr and Gn 2·73 1·5 906 160 ± 274 567 0·16 ± 0·05Br6B Gr and Gn 2·73 2·5 305 688 ± 34 032 0·30 ± 0·04Br6C Gr and Gn 2·73 4·0 189 481 ± 33 975 0·39 ± 0·07

1 Gr, Gn and P refer to granite, gneiss and phyllite.

Results and Discussion

The total long-term granite–gneiss surface erosion rate is lower (0·16 m/My at 1·5 m deep) than the schist–phyllitesurface erosion rate (4·32 m/My at surface) (Table I; Figure 2). The difference between the total long-term erosionrates obtained for fluvial sediments from sub-basins installed on schists–phyllites (9·21 and 13·65 m/My) and ongranite–gneiss substrates (14·91 m/ My) is not significant (Table II). Therefore, in this study it is not possible to provethe existence of differential erosion (between schists–phyllites and granites–gneisses) in the Quadrilátero Ferrífero.

The results show that the smaller the sub-basin and the closer the sample is to the headwaters, the more intense theerosion process tends to be (Figure 2, Table II). The basin outlet yields erosion rates (4·31 m/My) that are much lowerthan those corresponding to the tributary sub-basins or than that corresponding to the main sub-basin, in a regioncloser to the headwaters (values varying between 9·21 and 14·91 m/My) (Table II). The results obtained in other works(Stallard et al., 1991; Milliman and Syvitski, 1992; Howard et al., 1994; Edmond et al., 1995) are then confirmed: theerosive processes are more aggressive in the portions located more upstream in the hydrographic basins.

The erosion rates measured in the fluvial sediments represent the average erosion rate in the upstream portion of thehydrographic basin (high, middle and low slope) from the point where the sediment was collected (Brown et al., 1995;Granger et al., 1996; Von Blanckenburg, 2005). Therefore, the results obtained in this study suggest that the relief ofthe Maracujá Basin is being dissected. This interpretation is supported by the fact that the upper slopes are beingeroded more slowly than the average erosion rate of the basin (Tables I and II). Such variation in the erosion rate canonly be explained by higher erosion in the middle and lower slopes than in the upper slopes, and characterizes adissection relief process. The quaternary colluvium (medium and low slope) erosion described by Valadão and Silveira(1992), Bacellar (2000) and Bacellar et al. (2005) is probably a consequence of this relief dissection process.

Table 1I. Erosion rates in the studied basins

10Be erosion rateRock Position in Basin area (hydrographic basin)

Point Geology1 density the relief (km2) 10Be (at /g) (m/My)

Br1 P, S 2·81 Tributary 0·88 578 200 ± 130 448 9·21 ± 2·15(1146 m)

Br2 P, S 2·81 Middle 4·98 402 996 ± 152 797 13·65 ± 5·24(1146 m)

Br3 Gr and Gn 2·73 Tributary 1·15 371 289 ± 52 203 14·91 ± 2·28(1072 m)

Br4 Gr, Gn, I, S, P, 2·73 Outlet 15·37 1 139 204 ± 404 217 4·31 ± 1·55M, D and Q (1072 m)

1 M, D, Q, Gr, Gn, S, P and I refer to marbles, dolomitic phyllite, quartzite, granite, gneiss, schist, phyllite and itabirite respectively.



Conclusion

This work shows that the 10Be method can be a very useful tool for geomorphological study in inter-tropical and intra-cratonic areas. This method does not differentiate between the role of geochemical and mechanical erosion and that oftotal erosion. However, the method has further application as a means of obtaining the measurement of quaternaryerosion rates in hydrographic basins.

The results show that the differential erosion between schists–phyllites and granites–gneisses–migmatites is notevident in the studied area, as claimed in previous works. The higher quaternary erosion rate in the upper portionbasin, as well as the slope dissection process, constitute the main controls for the relief evolution of the MaracujáBasin.

AcknowledgmentThe authors would like to present their sincere thanks to CAPES-COFECUB (Brazil–France) for the financial support (Project 406).

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Study of the erosion rates in the upper Maracujá Basin (Quadrilátero Ferrífero/MG, Brazil) by the...

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