Alethéa Ernandes Martins Sallun et al. Geociências - SciELO · Alethéa Ernandes Martins Sallun...

343REM: R. Esc. Minas, Ouro Preto, 61(3): 343-356, jul. set. 2008

Alethéa Ernandes Martins Sallun et al.

AbstractSedimentologic characterization and provenance

studies of quaternary colluvial deposits (9±1 to 980±100ky B.P. - dated by luminescence) were based on texturaland mineralogical analyses. These deposits occurextensively between Marília and Presidente Prudente(São Paulo State, Brazil), superimposed on the BauruGroup sedimentary rocks or the Serra Geral Formationbasaltic rocks, both of Cretaceous age. They aredistributed irregularly throughout the study area andare studied by sedimentologic and morphologicmethods. These deposits are composed of very fine tocoarse-grained unconsolidated sands, mostlyconstituted by quartzose monocrystalline grainsreddened by iron oxide and hydroxide impregnation.Analyses of grain sizes indicated that totalgranulometric distributions are richer in the clayfraction than in the sedimentary rocks located. Non-micaceous transparent heavy minerals studiedmicroscopically in fine-grained as well as in very fine-grained sands showed that zircon, tourmaline, stauroliteand rutile are present in all the samples. Differences ingrain sizes among the colluvial deposits and the BauruGroup sedimentary rocks are attributable topedogenetic processes.

Keywords: Quaternary, colluvio-eluvial,sedimentological characterization, heavy minerals, SãoPaulo.

Geociências

Sedimentological characterization of thequaternary deposits in the region betweenMarília and Presidente Prudente, São Paulo

State, Brazil(Caracterização sedimentológica dos depósitos

quaternários da região entre Marília e Presidente Prudente,Estado de São Paulo, Brasil)

Alethéa Ernandes Martins SallunDoutora, Pesquisadora Científica do Instituto Geológico (IG/SMA-SP)

E-mail: [email protected]

Kenitiro SuguioProf. Emérito, Instituto de Geociências (IGc-USP), Prof. Titular, Universidade Guarulhos (UnG-CEPPE)

E-mail: [email protected]

ResumoPara caracterização sedimentológica e estudos de

proveniência de depósitos coluviais de idade quaternária(9±1 a 980±100 ka A.P. - datados por luminescência), foramrealizadas análises textural e mineralógica. Tais depósitosocorrem extensivamente entre Marília e PresidentePrudente (SP), superpostos às rochas sedimentares doGrupo Bauru ou rochas basálticas do Grupo Serra Geral(Formação Serra Geral), ambas de idade cretácica. Sãodistribuídos irregularmente na região e foram estudadospor métodos sedimentológicos e morfológicos. Osdepósitos coluviais são compostos de areias muito finasa grossas e inconsolidadas, em sua maioria formada porgrãos monocristalinos de quartzo, de coloraçãoavermelhada por impregnação de óxidos e hidróxidos deferro. As análises granulométricas mostraram que osdepósitos coluviais apresentam contribuição maior deareia argilosa do que as rochas sedimentares sotopostas.O estudo microscópico de minerais pesados transparentesnão-micáceos mostrou que, tanto nas areias finas, quantonas muito finas, zircão, turmalina, estaurolita e rutilo estãopresentes em todas as amostras analisadas. As diferençasgranulométricas constatadas entre os depósitos coluviaise as rochas sedimentares do Grupo Bauru sãoperfeitamente explicáveis por processos pedogenéticos.

Palavras-chave: Quaternário, colúvio, caracterizaçãosedimentológica, minerais pesados, São Paulo.

REM: R. Esc. Minas, Ouro Preto, 61(3): 343-356, jul. set. 2008344

Sedimentological characterization of the quaternary deposits in the region between Marília and ...

1. IntroductionQuaternary deposits related to

distinct geologic contexts can be foundat several locations in the State of SãoPaulo, Brazil. Due to their lack of structureand similarity with material resulting fromthe weathering of regional rock, colluvialdeposits are sometimes confused witheluvial deposits. In the studied areabetween Marília and Presidente Prudente(western portion of the State of SãoPaulo) there are Quaternary depositssuperimposed on Cretaceous sandstonesof the Marília and Adamantinaformations (Bauru Group) and onCretaceous basalts of the Serra GeralFormation (São Bento Group), bothbelonging to the Paraná Sedimentary

Basin (Dias Brito et alli, 2001, Renne etalli, 1992, Figure 1).

The Quaternary deposits weresubdivided into colluvial and alluvialdeposits, according to compositionalsimilarities and provenanceinterpretations compared to presentmorphologic features of the study area.Luminescence dating proved thedeposits to be from the Pleistocene agesfor the colluvial (9±1 to 980±100 ka B.P.)and the alluvial (14±2 to 240±30 ka B.P.)deposits (Tatumi et alli, 2006).

The colluvial deposits occur inpatches on the Western Plateau of theState. They consist of structureless

sands and rudites less than 10 m thick(Figure 2). When the contact with theunderlying sandstones or basalts isexposed, it is unconformable. Thepresence of ferricretes and/or stonelinesat the base of the colluvial depositssuggests the existence of a depositionalhiatus. The basal ferricretes constitutelocally interrupted, wavy beds, 2 to 40cmthick, that contain fragmented ferricretes,rounded centimetric quartz or quartzitepebbles. These horizons always occurat the contacts between colluvialdeposits and the Adamantina Formation,or as reworked and scattered fragmentswithin colluvial deposits superimposedon the Serra Geral, Adamantina or Marília

Figure 1 - Geographic and geologic location of the study area and sampling sites (geology according to Almeida et alli, 1981 andMilani and Ramos, 1998).



formations (Figure 3). The undulatingbeds or pseudofolding could representa simple adaptation to apaleotopographic surface, but it ispossible that the ferricrete bed faultingcould be atectonic or even related toeventual neotectonic activities (Sallun,2006).

The outcrop units are studied indetail from a sedimentologic viewpointto determine their characteristics andprovenance. The data obtainedcontributed to a better understanding ofthe region, the Quaternary geologicevolution of which remains largelyunknown (Sallun, 2006).

2. Materials andmethods

The field work consisted ofsampling and visual description ofgeologic outcrops, distributed all overthe studied area for sedimentologicanalysis (Figure 1).

To determine the textural andmineralogical characteristics, 112samples of colluvial deposits and 31samples of sandstones of theAdamantina and Marília formations werecollected. Where the colluvial depositsoccurred above these sandstones, theywere sampled for comparative texturaland mineralogical studies. Tocharacterize the textures, conventional

grain-size analysis was performed. Thefine (0.250 - 0.125 mm) and very fine(0.125 - 0.062 mm) sand fractions from 22samples were separated into light andheavy minerals (Rittenhouse, 1943)according to differences in their density(Parfenoff, et alli 1970; Carver, 1971) usingbromoform (CHBr3). After separation, thelight minerals were treated with 5% oxalicacid on hot aluminium plate for thelixiviation of ferruginous films found onthe sand grains.

The light minerals were mountedon thin glass board. One hundred quartzgrains were counted for morphologic andtypological purposes. To determinequartz/feldspar ratios semiquantitatively,X-ray diffraction analysis was performedafter the rest of grains had beenpowdered in an agate mortar.

The heavy minerals were weighedto obtain the amount within the totalsample. The heavy opaque minerals wereseparated using a hand magnet,according to Friedman (1958), 100 non-micaceous transparent heavy mineralswere identified and counted on the thinglass board. Remnant opaque mineralsand clay-limonite nodules were excludedfrom counting on the thin glass boards.

To better interpret the meanings ofnon-micaceous transparent heavyminerals, the found frequencies of theassemblages were classified accordingto Coutinho and Coimbra’s (1974)

method, and ZTR index values weredetermined. These reflected themineralogical maturity and correspondedto the content summation of theoccurrences of zircon, tourmaline andrutile among these minerals.

In this study, similarities betweenthe Quaternary and Cretaceous unitswere studied by comparing the grain-sizedata. Established statistical treatmentsfor determinate types of environmentswere used, without any intention ofdefining or considering depositionalenvironments for units, because thesediments can preserve the inherentcharacteristics of other depositionalcycles.

For grain-size distribution data, theFolk and Ward (1957) graphic methodwas used to characterize the colluvialdeposits and to enable comparison withthe underlying Cretaceous sandstonesof the Marilia and Adamantinaformations that constitute the substrate.The statistical parameters werecalculated for total and sandydistributions, because pelitic fractionscan be subjected to illuviation duringpedogenesis, which changes the originalgrain-size frequency.

After obtaining the Folk and Ward(1957) parameters, calculated for thesandy fractions of the samples, statisticalanalysis was used, as proposed byFriedman (1961, 1967) and Friedman andJohnson (1982).

Figure 2 - Field appearance of the massive colluvial deposit(site 154).

Figure 3 - Irregular ferricrete bed at the contact of Bauru Groupsediments (II) and colluvial deposits (I), sampling site 102.



For a more completecharacterization of the ferruginousconcretions, the following steps havebeen performed: chemical analysis(McFarlane, 1983) by X-ray fluorescence;petrographic study on a thin section; andsurface textural analysis by scanningelectron microscope (SEM). Chemicalanalyses of major, minor and traceelements have been performed onpowdered representative samples.

3. ResultsIn order to compare the differences

and similarities of the colluvial depositsand their underlying substrate,petrological and petrographic data wereobtained from both colluvial cover andthe Adamantina and Marília formations.Only a few outcrops of colluvial depositssuperimposed on the Serra GeralFormation were studied so there is nostatistical data; however, these depositswere subjected to mineralogical analysisand provenance studies.

3.1 Grain size characteristicsThe analysis of the distribution of

the grain-size fractions in the colluvialdeposits shows: clays and silts areabundant in the NW and SE portions ofthe studied area, very fine-grained sandin the NW, fine sand in the centre andmedium-grained sand in the SE. Thethickest colluvial deposits are associatedwith sediments with abundant fine-grained sand and scarce clays and silts.These differences can be observed alsoin the distribution of units in the BauruGroup, and might indicate transport ofcolluvial sediments from E to W.

The studied units constitute 13grain-size classes and the totalcumulative curves for grain-sizedistribution demonstrate that, in general,these deposits are homogenous (Figure4), consisting chiefly of sand, clayeysand and silty sand. Grain-size data forthe majority of the samples aresummarized in Table 1.

Figure 4 - The grain-size classification of the data obtained from colluvial depositssuperimposed on the Marília Formation (CDSMF), colluvial deposits superimposed onthe Adamantina Formation (CDSAF), Marília Formation (MF) and Adamantina Formation(AF), according to the triangular diagram proposed by Shepard (1954).

Table 1 - Summary of the data obtained for the majority of the studied units, colluvialdeposits (CD), colluvial deposits superimposed to the Adamantina Formation (CDSAF),colluvial deposits superimposed to the Marília Formation (CDSMF), Adamantina Formation(AF) and Marília Formation (MF) (S-sand, CS-clayey sand, SS-silty sand, FS-fine sand,VFS-very fine sand, CS-coarse silt, VPS-very poorly sorted, PS-poorly sorted,MS-moderately sorted, WS-well sorted, VP-very positive, P-positive, ASi-approximatelysymmetrical, VL-very leptokurtic, VPL-very platikurtic, M-mesokurtic, L-leptokurtic).



Figure 5 - Grain-size distribution cumulative curves of the data obtained from colluvial deposits superimposed on the MaríliaFormation (CDSMF), colluvial deposits superimposed on the Adamantina Formation (CDSAF), Marília Formation (MF) and AdamantinaFormation (AF).

Most of the analyzed samplespresented a fine grain size, from fine sandto coarse silt, with the finer portion beingthe worst sorted (Figure 5). The grain-size distributions were, in general,moderately sorted, which representspeculiarities inherited from most of theBauru Group samples studied in the area.

Comparison of the units shows thatthe colluvial deposits present grain-sizecharacteristics that are similar to theirclosely underlying rocks. The obtainedvalues indicate fluvial and beachenvironments, but the Marília Formationsamples and the colluvial depositssuperimposed on the Marília Formationalways indicate a fluvial environment(Figures 6A, B, E, F, G and J). The largerare the medium diameters, the smaller arethe degrees of skewness and thestandard deviations (Figures 6A, C, Fand H). The Adamantina Formation andthe colluvial deposits superimposed onthe Adamantina Formation exhibit highersorting degrees than the MaríliaFormation and the colluvial depositssuperimposed on the Marília Formation(Table 2).

Table 2 - Summary of the data obtained for total sand distribution of colluvial deposits(CD), colluvial deposits superimposed to the Adamantina Formation (CDSAF) and colluvialdeposits superimposed to the Marília Formation (CDSMF). av= average, Md= mediumdiameter, σI= standard deviation, KG= kurtosis and SKI= skewness.



Figure 6 - Representations of the data obtained from colluvial deposits superimposed on the Marília Formation (CDSMF), colluvialdeposits superimposed on the Adamantina Formation (CDSAF), Marília Formation (MF) and Adamantina Formation (AF), according tothe diagrams proposed by Friedman (1961, 1967).



For all units, the kurtosis values(Figures 6D and I) are concentratedaround 0.8 to 1 (platikurtic to mesokurtic),while the medium diameters (Figures 6A,C, F and H) plotted between 3 and 2 Φ(fine sand), and the degree of skewness(Figures 6B, C, D, G, H and I) lay between-0.1 and 0.1 (approximately symmetrical).

The comparison of results obtainedfor the total distribution showed thatclay contents are higher in the colluvialdeposits than in the Bauru Group. Thesand distributions of the colluvialdeposits and of Bauru Group rocks, whenoriginated from the same outcrop,exhibited a generally good correlationbetween superimposed and underlyingdeposits.

3.2 Mineralogicalcharacteristics

The heavy mineral contents of thecolluvial deposits vary between 0.03%to 0.66% and 0.50% to 4.86% of the totalweight in fine-grained and very fine-grained sands, respectively (Table 3).Their frequencies are higher in very finesands than in fine-grained sands. Themagnetically heavy minerals represent0% to 1% of the total heavy mineralswithin fine-grained and very fine-grainedsands, except in sample 48-2, wheremagnetically heavy minerals represent27.27% of the total heavy minerals. Thecolluvial deposits superimposed on theSerra Geral Formation exhibit a higherheavy mineral content than the otherunits studied.

3.2.1 Light minerals

In the analysis of light mineralspresent in the fine-grained sand fraction,only quartz grains are found in thecolluvial deposits, while in theunderlying Marília and Adamantinaformations, feldspars ranging in contentbetween 7% and 24% were found (Table 4).

As a sample, on site, 11 very fine-grained sands of the colluvial deposits,quartz grains (up to 100%), and feldspar(up to 4%) were found, while in the

underlying Marília and Adamantinaformations quartz (up to100%), feldspars(up to 30%) and calcite and dolomite (upto 4%) were found.

3.2.2 Heavy minerals

During the petrographic study ofnon-micaceous transparent heavyminerals separated from fine and veryfine-grained sands, the presence ofzircon, tourmaline, staurolite and rutilewas verified in all samples analyzed.Together with kyanite, they may be calledsuperabundant (Tables 5, 6 and 7). Theabundant minerals are sillimanita andgarnet, and pyroxene is common.

In grain-size distributions oftransparent heavy minerals in colluvialsediments, zircon and rutile are presentin higher frequencies in very fine-grainedsands, while tourmaline and staurolite rare most abundant in fine-grained sands(Tables 5 and 6), due to differences inthe specific gravities of these minerals(Tucker, 2001).

The distribution of heavy mineralsin the study area show that in fine-grained sand, tourmaline is the scarcestin the NW, while zircon and rutile arescarce in the E and S of the study area.In the very fine-grained sand, the amountof tourmaline is lower than in the fine-grained sand. Zircon is more abundant

Table 3 - Maximum/minimum of total masses (%) of heavy minerals for FS (fine-grainedsands) and VFS (very fine-grained sands) of the studied units, colluvial deposits (CD),colluvial deposits superimposed to the Adamantina Formation (CDSAF), colluvial depositssuperimposed to the Marília Formation (CDSMF), Adamantina Formation (AF) and MaríliaFormation (MF).

Table 4 - Light mineral contents in fine- and very fine-grained sands of the colluvialdeposits (CD) and the Bauru Group (BG) sedimentary rocks, that occur at the samelocality. Qz= quartz, Fd= feldspar and CD= calcite and dolomite.



Table 5 - Contents of non-micaceous transparent heavy minerals in fine-grained sands of colluvial deposits (CD), and Bauru Group(BG) sedimentary rocks occurring in the same locality. Tour= tourmaline, Zir= zircon, Rut= rutile, Sta= staurolite, Gar= garnet,Kya= kyanite, Sil= sillimanite, Py= pyroxene and NO= Bauru Group, which does not occur in this locality.

Table 6 - Contents of non-micaceous transparent heavy minerals in very fine-grained of colluvial deposits (CD), and Bauru Group(BG) sedimentary rocks occurring in the same locality. Tour= tourmaline, Zir= zircon, Rut= rutile, Sta= staurolite, Gar= garnet,Kya= kyanite, Sil= sillimanite, Py= pyroxene and NO= Bauru Group, which does not occur in this locality.



and is distributed more extensively.Rutile is the scarcest, but it is moreabundant in the very fine-grained sandthan in the fine-grained sand. Stauroliteis the scarcerin the fine-grained sands.

Only six minerals appear in morethan 25% of the studied samples ofcolluvial deposits: zircon, rutile andstaurolite are more frequent in very fine-grained sands, and tourmaline, kyaniteand sillimanite are more frequent in fine-grained sands. Seven minerals appear inmore than 25% of the studied samples ofBauru Group sedimentary rocks: rutile,kyanite, sillimanite, zircon (more frequentin very fine-grained sands), andtourmaline, staurolite and pyroxene(more frequent in fine-grained sands).

The most peculiar characteristics ofthese heavy minerals are as follows(Figure 7):

a) Tourmaline: the shapes are sphericaland well-rounded (Figures 7A andB), followed by rounded-edgeprismatic forms (Figures 7C, D andF), while little-rounded prismaticforms with euhedral extremities arerarely found (Figures 7J, I and G).Some grains exhibit inclusions(Figures 7C, E and J), and brownish(ferromagnesian) and greenish(Figure 7G) colored grains arefrequent but yellowish (magnesian -Figures 7C, E, B, F and I), bluish(indicolite - Figures 7A and H) andpinkish (Figures 7D and J) coloredgrains are less common.

b) Zircon: the grains are colorless(Figure 7K) to slightly pinkish.Equidimensional and well-roundedshapes (Figure 7M) are mostfrequent, followed by rounded-edgeprismatic (Figures 7L and N), andbipyramidal prismatic forms. Zoningand inclusions are observed (Figures7K and L).

c) Rutile: the grains are reddish (Figure7O), yellowish, yellowish red andalmost opaque. They exhibit

spherical and subrounded (Figure7O) forms, as well as prismatic formssuggestive of pyramidal terminations(Figure 7P). There are knees, heart orelbow-like twinnings withlongitudinal and oblique striations(Figure 7O).

d) Garnet: the most common grains arecolorless; few pinkish grains occuras well. Spherical and well-roundedshapes are dominant, but in BauruGroup sedimentary rocks there areelongated forms (Figure 7Q).Rectangular or losangular surfacealteration (perforation) patterns arecommon.

e) Kyanite and sillimanite: the colorlessgrains (Figure 7R) exhibit elongatedto spherical subrounded andeuhedral shapes and displayinclusions. Optical characteristicswere used to distinguish betweenthese polymorphs.

f) Staurolite: there are irregular (Figures7T and U) and rounded-edgeprismatic (Figure 7S) grains withyellowish and yellow-reddish colors.They often exhibit zigzag saw-likeedges (Figures 7T, U and V),providing the surface with a scale-like pattern (evidence of dissolution).They can present inclusions (Figure7S).

g) Pyroxene: the grains exhibitelongated and rounded shapes withweathering features caused byindentations. Greenish (probablyaugite) or emerald-greenish (probablyaegirine) colors are very common.

h) Epidote: the grains exhibit greenishcolor and are spherical.

i) Anatase: these grains occur only inthe Bauru Group sedimentary rocks,displaying yellowish, light-bluishtones. They have a square andeuhedral shape, which suggests thatthey are of authigenic origin.

Table 7 - ZTR index, contents of non-micaceous transparent heavy minerals zircon+tourmaline+rutile, in fine- and very fine-grained sands of colluvial deposits (CD), andBauru Group (BG) sedimentary rocks occurring in the same locality (NO= Bauru Group,which does not occur in this locality).



Figure 7 - Photomicrographs of non-micaceous transparent heavy minerals from fine- and very fine-grained sands of colluvialdeposits (1) and from the Bauru Group (2). Tourmalines: well-rounded (1A and 1B), with prismatic inclusions (1C), rounded-edge(1D, 2E, 2F and 2G), irregular blue (2H), with little rounded euhedral terminations (1I), prismatic pink with euhedral terminations andinclusions (2J); Zircon: prismatic colorless with zoning (1K), with little rounded-edges (1L) and spheric and well-rounded 1(M) andwith inclusions (2N); Rutile: prismatic red with subrounded edges (1O) and prismatic forms suggestive of pyramidal terminationswith striations (2P); Garnet: pink to colorless with inclusions (2Q); Kyanite: prismatic colorless (1R); Staurolite: prismatic withrounded edges and inclusions (1S) and irregular yellow with zigzag saw-like edges (1T, 2U and 2V).



3.2.3 Ferricretes

The ferricretes appear asaggregated blocks of concretions andnodules, concentric or nonconcentric,millimetric to centimetric, with irregularor spherical shapes and reddish colors(Figure 8). The aggregates contain fine-to coarse-grained sands, essentiallyquartzose, with ferruginous (goethite)cementation. According to Bourman(1993), in similar deposits in Australia,this material could be interpreted asclastic deposits transformed intopisolitic and nodular ferricretes. They arefound only overlying the colluvialdeposits superimposed on theAdamantina Formation in 14 localities(Points 19, 21, 28, 43, 45, 48, 64, 71, 102,108, 134, 156, 157 and 158) and asstoneline fragments at the base ofcolluvial deposits.

The ferricretes appear asaggregated blocks of concretions andnodules, concentric or nonconcentric,millimetric to centimetric, with irregularor spherical shapes and reddish colors(Figure 8). In thin sections, theaggregates contain fine- to coarse-grained sands, essentially quartzose,with ferruginous (goethite) cementation.According to Bourman (1993), in similardeposits in Australia, this material couldbe interpreted as clastic depositstransformed into pisolitic and nodularferricretes. They are only foundoverlying the colluvial depositssuperimposed on the AdamantinaFormation in 14 localities (Points 19, 21,28, 43, 45, 48, 64, 71, 102, 108, 134, 156,157 and 158) and as stoneline fragmentsat the base of colluvial deposits.

In general the ferricretes show highSiO2 contents and lower Fe2O3 and Al2O3contents (Table 8), or lower SiO2 contentsand high Fe2O3. This suggests that theyoriginated from sands or sandstones(Figure 9), and the high content of SiO2is due to the presence of quartz.

The quartz grains of the ferricretespresent corrosion features composed ofsuperficial microfissures that suggestdissolution, which probably occurreddue to strong weathering (Figure 10). The

Figure 8 - Ferricrete fragment from site 45.

Figure 9 - Triangular diagram of ferricrete classification (modified from Dury 1969,MacFarlane 1983), with samples analyzed by the X-ray fluorescence method.



Table 8 - Results of chemical analysis by X-ray fluorescence of selected elements, in percentages in weight of oxides. All Fe wascalculated as Fe2O3, DL= detection limit and IL= ignition loss.

dissolution is also responsible for theconcentration of iron hydroxides(ferralitization). Goethite is currentlypresent in the outer yellow or brown crustor within the nodules. The precipitationof iron hydroxides as concentric porefillings suggests that there is ironremobilization through alternatingdissolution and precipitation (Figure 11).

4. ConclusionsThere are few differences in the

grain-sizes between the Quaternarycolluvial deposits and the Cretaceoussedimentary rocks of the Marília andAdamantina formations. Excluding thepelitic fraction (clay+silt) of the samples,the variability in grain-size distributionis diminished. The processes active inthe formation of the colluvial depositsdid not modify the grain-size distribution

Figure 10 - Ferricrete photomicrographs obtained by SEM (scanning electronmicroscope), showing quartz grains with superficial corrosion features and surroundedby iron hydroxide cement; A, B and C - triangular geometric cavities “saw-teeth” typestructure originated by chemical alteration.



of the sand fractions; changes occurredonly in pelitic contents. The sortingdegrees of the sand fractions wereimproved during pedogenetic processesand subsequent colluviation, whichsuggests that the pedogenetic processesattenuated the variability.

In general, the colluvial deposits arecharacterized by high mineralogicalmaturity expressed by higher values ofthe ZTR index (76% to 97% and 79% to96% in fine-grained and very fine-grained sands, respectively). Thesevalues change from 53% to 100% and67% to 98% within the same fractions ofthe Bauru Group sedimentary rocks. Infine- and very fine-grained sandfractions, the mineralogical contents ofcolluvial deposits and of the Marília andAdamantina formations, when occurringat the same locality, show goodcorrelation. However, there is a generalenrichment of ultra stable minerals(zircon, turmaline and rutile), byelimination of less stable minerals(pyroxene and garnet), probably duringeluviation and colluviation processes.There are small variations in thefrequencies from the same outcrop, oftourmaline, staurolite, zircon and rutilein the fine-grained sand, and oftourmaline, staurolite, zircon, rutile andadditionally kyanite and sillimanite in thevery fine-grained sand fractions,

Moreover, when colluvial depositsand the sedimentary rocks of the Maríliaand Adamantina sandstones’ heavymineral assemblages from the sameoutcrop were compared, some mineralsof metamorphic origin appeared only incolluvial deposits, including thosesuperimposed on the Serra GeralFormation too. This fact could suggestan eventual contribution of eoliantransportation processes or by creepingof the Bauru Group and Serra Geralweathered rocks, which contained heavyminerals (such as sillimanita, stauroliteand pyroxene).

The ferricretes could have beenformed through remobilization processesand the concentration of iron hydroxidesby ancient groundwater level

fluctuations induced by paleoclimaticchanges and/or by relief uplift. The ironcould have been leached from upstreamand deposited in ancient vadose andphreatic zone interfaces, where wetterand drier conditions due to paleoclimateoscillations existed, which propitiatedthe precipitation of iron. Originally, theseconcretions might have exhibitedconsiderable lateral extension. Later, theprobable dismantling of the iron crustspromoted the transformation of hematitein goethite, which originated pisolitichorizons. The presence of quartz and thesandy composition of the ferricretessuggest a pedogenetic origin. The crustmust have originated on the Bauru Grouprock hills, which became resistant toerosion and, eventually, were molded toancient surfaces probably of atectonicorigin. The fragments could have beentransported over short distances as thelandscape evolved, and nowadays aremixed within the colluvial cover, asobserved by Roquim (1990) in Mali.

The obtained data will subsidizefuture decisions on anthropogenicoccupations, because this quaternarydeposit exhibits high susceptibility for

erosion and anthropogeniccontaminations of soil and groundwater.

5. AcknowledgementsTo FAPESP (Fundação de Amparo

à Pesquisa do Estado de São Paulo) forMaster and Doctoral scholarships(Processes 00/10672-6 and 03/01737-5)to the first author, and by a research grantto Kenitiro Suguio (Process 01/07469-7).To CNPq (Conselho Nacional deDesenvolvimento Científico eTecnológico) by research Grant (Process47404/2001-9 NV) and productivityscholarship to Kenitiro Suguio (Process304718/2003-3).

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Artigo recebido em 10/10/2007 e aprovado em 26/02/2008.

Alethéa Ernandes Martins Sallun et al. Geociências - SciELO · Alethéa Ernandes Martins Sallun...

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