Weathering and morphogenesis in a tropical plateau

CATENA Vol. 10, 237-251 Braunschweig 1983

WEATHERING AND MORPHOGENESIS IN A TROPICAL PLATEAU

M.Ch. Modenesi, S~o Paulo

SUMMARY

Weathering phenomena on the Campos do Jord~o Plateau have been analysed in order to show their relationship to geomorphic compartition and morphogenetic dynamics.

Correlations between weathering characteristics ofsurficial materials and geomorphic compartments are more conspicuous in saprolite than in soils.

On the plateau highest areas gibbsite is the most important secondary mineral; at lower altitudes kaolinite prevails. Thus the spatial distribution of secondary minerals seems to outline two weathering processes - alitization and monosialitization - defined and interpreted as two different stages within a general alitic weathering trend.

On hillslopes, bedrock weathering degree reflects the intensity ofmorphogeneticactions. Hillslope deposit profiles frequently show degrees and sequences of weathering related to downslope mouvement processes rather than to incipient post-depositional pedogenetic activity.

Red ferralitic materials overlaid by superficial podzolized ones point to the superposition of two pedogenetic trends linked to variations in environmental conditions.

"Ferralitic materials would be remnants of weathering processes active during the Tertiary at lower altitudes, prior to the plateau uplift.

1. INTRODUCTION

Weathering is closely related to the dynamics ofgeomorphology, as it drives and reflects morphogenesis all in one. The geochemical type of evolution and the degree of weathering of surflcial materials reflect environmental conditions and intensity of evolution. These premi- ses obviate the importance of weathering studies in geomorphology, for, as TRICART (1965) pointed out, they provide the elements for establishing geomorphological correlations. This paper is an attempt to identify weathering variations in its relations to morphogenesis by means of a spatial approach of the weathering phenomena occurring in the Campos do Jord~o Plateau.

The Campos do Jord~o Plateau (Fig. 1) is the uppermost surface of Southeastern Brazil block mountains. Towards the end of the Wealdian Reactivation (ALMEIDA 1976) it was uplifted by normal faults which followed the path set by transcurrent Pre-Cambrian/Cambrio- Ordovician faulting and by the lines of joints and foliation (HASUI et al. 1977). On its sum- mits, at 1850-2050 m, DE MARTONNE (1940) recognized the remnants of the "superficie dos altos campos", the oldest SE Brazil's planation surface, which probably goes back to the early Tertiary Age. FREITAS (1951) and ALMEIDA (1964) believe it to be younger- a result of Oligocene deformation of the Japi erosion surface (ALMEIDA 1976) which was accentuated in the Pliocene.

The uplift and subsequent climatic changes brought about an anomalous Quaternary evolution which resulted in an unique tropical landscape system. The "altos campos" (high grasslands) landscape has climate, vegetation cover and surficial materials which are charac-

238 MODENESI

I /3, .:-?A~ t..[//)(_ . 4 V ~ - . . , ~ ~_ o E3S~

\ ~ Camposdo _---~J" ~. ~ r ' -

0 I0 4 0 k m I I I I I

linguet9 .X~

25os J

i I

Fig. 1: Campos do Jord,~o, the Serra da Mantiqueira Scarpment and the Paraiba do Sul Rift Valley.

teristic. According to the IPT (1978) geological mapping, Upper Pre-Cambrian Agungui Group

migmatites (heterogeneous and homogeneous stromatites) with associated granitoids predominate in the plateau, between the Mantiqueira Scarpment and the Sapucai Guagfi Valley (Fig. 1). Schists, quartzites and metaconglomerates also occur.

The mean annual temperature in the Campos do Jordfio Plateau is 14,5" C. The war- mest months are January and February (17,5" C); maximum temperatures reach up to 30 ° C. The coldest months are June and July (11 ° C) with minimum temperatures of-3* C. Frost is very common from May till August. Rainfall is rather variable, ranging annually from 1,205 to 2,298 mm. Rainfall distribution shows a concentration of over 80% from October to March and a relatively dry period in autumn and winter however, the water balance shows no defi- ciency. In the highest part of the plateau, orographic influence causes a decrease of temperature and an increase of rainfall. The dry season is not well defined. Temperature analysis show an altitude climate with subtropical characteristics. The distribution of rainfall suggests a tropical rhythm.

In the"altos campos", vegetation distribution forms a typical forest-grassland mosaic or- ganized according to landforms, the drainage network and surficial materials. Grasslands prevail on hilltops and on the convex parts of hillslopes. Forests occupy the lower parts of convex hillslopes, rectilinear slopes and erosion amphitheaters (Fig. 2).

2. METHODOLOGY AND TECHNIQUES

Topographic compartition, as undertaken in the "altos campos", deals with minor landforms, fitting within the VI order of values in CaiUeux-Tricart Taxonomic Classification (in TRICART 1965). In Campos do Jord~o, these units are marked by differences in topography, hydrology, surficial materials and vegetation. They are true geomorphic compartments in which weathering-morphogenesis interactions are quite clear.

The field work and aerophotointerpretation techniques employed in the survey, as well as in the characterization of geomorphic compartments and their surficial materials, have

WEATHERING AND MORPHOGENESIS, TROPICAL PLATEAU 239

already been described in a previous paper (MODENESI 1974). The study of type and degree of weathering (MELFI & PEDRO 1977) was restricted to the

quartz-feldspar rocks of the orthosialferric sequence (homogeneous stromatites and granitoids). Materials from floodplains, which are related to hydromorphic conditions, were not appraised here. Degree of weathering was defined mainly by juxtaposition of the mineralogical sequences which are specific of feldspar, mica, quartz and newly formed minerals (WACKERMANN & MODENESI 1980). Weathering relative chronology was based upon the characterization of the maximum degree of weathering in each compartment.

Petrographic or pedologic analytical techniques were adopted, according to the different characteristics of materials: weathered rock, deposits and soils. Over 220 X-ray diffraction tests were performed in total samples, as well as in the day, silt and sand fractions of surticial materials. Clay minerals were identified following the procedures described by ROBERT (1975). Seventy-two thin sections of weathered rock and crusts were studied by optical micros- copy. Thermoponderal analysis, performed on just one profile per compartment, provided the percentages of kaolinite and gibbsite in total samples of weathered rock and of superficial horizons of soils.

3. GEOMORPHOLOGIC COMPARTITION AND MORPHOLOGICAL CHARAC- TERISTICS OF SURFICIAL MATERIALS

• At the level of minor landforms, a clear compartition characterizes the plateau's "altos campos" (Fig. 2). Topomorphologic differences correspond to changes in the hydrology of slopes, in surficial materials and in vegetation. Different surficial materials occur on hilltops, on convex or rectilinear hillslopes, in erosion amphitheaters and floodplains.

3.1. HILLTOPS

Between the Mantiquera divide (1900-2007 m) and the Sapucai Valley (1573 m in Vila Capivari), convex hilltops are leveled in decreasing altitudes. Three topographic levels suc- ceed each other at altitudes of 1800-1820 m, 1710-1740 m and 1640-1660 m. The two lower levels are underlain by migmatites; above them, granitoid rocks prevail (Fig. 3).

Soil - Within the shallow cover- 10 to 30 and exceptionally 50 cm- a black or dark brown friable A-horizon develops. Grains of clear, washed sand stand out within darker soils, which are quite common mainly above the 1800 m level. Fermginous crusts may outline the A-horizon basal contact with either the stone line or the weathered rock. Above 1800 rn, 10YR hues predominate; on lower hills, 7,5YR hues prevail. These soils match the ones described by OLIVEIRA et al. (1975) as being either lithosol or soils with a thin (under 20 cm) incipient B. Soil profiles on granite have quartz gravel and a higher percentage of coarse sand. It was noticed that as altitude decreases the prevailing texture changes from silty-sandy-muddy (above 1900 m) to muddy and clayey (1800 to 1820 m) and clayey (under 1740 m). A-horizon pH-values in water range from 4.7 to 5.2.

A stone line of quartzite and local rock pebbles usually outlines the contact soil- weathered rock. Above 1800 m, aluminous laterite blocks up to 40 cm may occur. Nodules (concentrations of non-differentiated structure) are common in such altitudes, as well as lamellar white pebbles (under 2 cm). In the 1710-1740 m topographic level, radiciform or

240 M O D E N E S I

tubular nodules with a medium diameter of 1 cm and up to 5 cm in length, are common. Weathered rock - Bedrock shows remarkable differences as to cohesion and color. The

most common hue is reddish. Above 1800 m light hues predominate; under this level they are always related to hydromorphic phenomena. Bedrock structure is usually preserved, except for discontinuous pockets where a C-horizon develops. Lack of good exposures hindered the observation of the depth of weathering. Fresh rock was never reached. In the highest areas of the plateau, rock surface might be coated by a thin ferruginous crust. Millimetric crusts ofgibbsite were found in altitudes around 1800 m. In the lower hills, weathering of homogeneous ophtalmitic stromatites show a typical vesicular structure.

In altitudes of 1930 and 1800 m, on the edge of hilltops (Fig. 3), remnants of old laterites (weathering formation, indurated, rich in aluminium and iron hydroxides) either outcrop or occur as blocks in the stone lines. Above 1900 m they have little density, are rugged, scoriaceous, with a ferruginous coating. They might present small vesicles, often with a white gibbsite lining. Laterite on migmatites have clearly preserved parent rock structures showing white hard bands rich in gibbsite and reddish or pink more friable bands with quartz grains. On granitoid rocks, the bands are replaced by irregular stains; no vestige of the original structure exists. Laterites on 1800 m hills, though more dense, have the same scoriaceous aspect of the previous ones. Its vesicles are frequently filled by red clay. Both on oriented and granitoid rocks, parent rock structure is preserved.

3.2. HILLSLOPES

In the "altos campos", mainly on granitoid rocks and stromatic migmatites, slopes are convex, frequently with a straight lower sector. Above 1800 m, in areas where erosion was less effective, convexities may reach the footslope. Colluvial deposits in contact with the alluvial plain determine concave footslopes. Rectilinear slopes are common on quartzites. They may also occur on other rocks in thoroughly dissected areas, being in this case frequently related to structural control.

Amphitheaters, deep hollows produced by important mass movements, are common in convex slopes. Correlative deposits may be preserved deep inside the basal part and top of amphitheaters. They consist of conglomeratic materials produced by reworking ofregolith or of regolith and previously pedogenized materials.

S o i l - Morphological characteristics of slope materials vary considerably. Convex slopes have soils equivalent to those of hilltops. Benches on slopes of all kinds have thicker materials ranging from 2 to 3 m. Soil profiles similar to the Campos do Jordao Soils (Photo 1) described by the COMISSAO DE SOLOS (1960) may occur, as well as cambisols and soils with textural B or latosolic B (OLIVEIRA et al. 1975). Complex profiles with buried A-horizons are quite frequent, mainly in amphitheaters. North of the Capivari Valley, reddish materials having morphological characteristics of latosolic B-horizons are overlain by yellowish soils, with brown or black A-horizons; these profiles are quite frequent on hillslopes or shoulders at altitudes of 1700-1780 m (Fig. 3). The transition between reddish and yellowish materials is gradual. Red materials usually show signs ofreworking, such as basal stone lines and/or inclusions of granules, pebbles and blocks of slightly weathered rock (Photo 2).

Slope soil superficial horizons show a tendency to have increasing average clay content as altitude decreases (23.5 to 46%); silt contents are varied (5 to 33%) and frequently higher than those found on hilltops. On the same topographical level, the A-horizon is usually less argil- lous on convex slope soils than on hilltop soils. A-horizon pH values in water range from 4.3 to


Photo 1: Slope soil profile (Campos do Jord~.o Soil) showing a sharp contrast between the A (black) and B (yellow) horizons and the weathered rock (red).

Photo 2: Reddish coluvial materials with morphological characteristics of a latossolic B horizon and inclusions of slightly weathered blocks, pebbles and granules.

Photo 3: Microfabric in alumino-ferruginous laterite. High porositywith preservation oforiginal structure. Fractured and corroded quartz with voids occupied by gibbsite. Biotites completely weathered to gibbsite and hematite. Macrocrystalline gibbsite concentrations and iron oxides in the periphery of voids with zoned clayey iron masses inside.

242 MODENESI

5.4. Stone lines, similar to that of hilltops, lie in the contact cover material-weathered rock. In

thicker profiles, they occur below the A-horizon or in contact with the rock and, at times, in both positions.

Weathered rock- Bedrock outcrops only in the steepest sectors of slopes and in the inner- most part of amphitheaters. In a general way, weathered rock retains its structure. As in hilltops, grayish hues may appear above 1800 m, but never under this level. In the Mantiqueira divide (Fig. 3), convex slopes show weathering processes with iron and aluminium individualization which characterize bauxitization of acid rocks.

In all types of slopes, profiles developed on saprolite and/or colluvium frequently show yellowish materials lying on reddish ones. In certain deposits, the abrupt color contrast corresponds to the superposition of different layers. Difference in color does not reflect degrees of richness in ferromagnesian minerals; yellowish hues occur in both biotite or muscovite rich materials.

3.3. VALLEY FLAT

Along the Sapucai-Guagu and its main tributaries, small fluvial plains are cut by entrenched river segments. These small basins ("alv6olos") have remnants of low gravel terraces overlain by slope deposits. Gravel deposits, mostly made up of subrounded quartz pebbles (up to 20 cm), are 40 to 50 cm thick, but were probably reduced by late colluvial processes. The underlying weathered bedrock shows light colors, a well preserved structure and some signs of hydromorphic conditions. Colluvial deposits vary considerably in thickness, composition and grain size. They frequently show layers rich in rudaceous materials and/or organic matter.

4. MINERALOGICAL CHARACTERISTICS OF SURFICIAL MATERIALS

Following is a concise minerological description of surficial material - soils and weathered rock - within each geomorphic compartment.

4.1. HILLTOPS

Soils - In a general way, hilltop soils are minerologically similar. However, materials lying above or below 1800 m seem to differ in some characteristics. Above 1800 m, quartz may represent only 30% of the coarse sand fraction of soils; rock remains and fledspars prevail, mostly already weathered into gibbsite. Below 1800 m quartz represents up to 92%.

In the medium sand fraction, quartz prevails (over 950/0). Exceptionally, between 1920 m and 1740 m altitude, materials having up to 54% of red clay and quartz aggregates (pseudo- sands) may occur. In the free sand fraction the amount of mica may be higher, depending on the underlying rock. The most important minerals within the silt fraction are gibbsite and quartz; traces of micas and microcline may occur in less evolved soils. Within the clay fraction, kaolinite is the main secondary mineral; gibbsite is also important and may prevail above 1900 m. In the two lower topographic levels, interstratified minerals and traces of micas are frequent.


Kaolinite and gibbsite make up 37 to 76% of the mineral fraction of hilltop soils. Kaolinite content varies from 18 to 570/0, and gibbsite's from 12 to 210/0. As altitude decreases, the total kaolinite plus gibbsite tends to increase parallel to the increase of the clay and silt fractions.

Nodules and small lamellar white pebbles in stone lines are mostly made up ofgibbsite. Tubuliform and radiciform nodules are mainly constituted of gibbsite and secondarily of kaolinite.

Weathered rock - Feldspar weathering is characterized by the formation of gibbsite; kaolinite only occurs in incipient weathering or, occasionally at 1710-1740 m. Both on the highest and on the lowest hilltops there are signs of total dissolution of feldspars. Above 1900 m quartz seems to be better preserved, even in presence of strong gibbsitization, at 1710-1740 m it is frequently fractured.

Weathering of muscovite is characterized by partial kaolinization. Above 1900 m, biotites are weathered into gibbsite, kaolinite only occurs in less evolved materials; at lower altitudes, they are transformed into pseudomorphic kaolinite often with gibbsite inclusions. Sericite may occur either intact or partially weathered into kaolinite. In less evolved materials 2:1 hidroxialuminous clay minerals are common.

Quartz and muscovite are the main residual minerals. However, in hilltops at 1800-1820 m, amphiboles, albite and opaque minerals may occur. In the two lower levels, sericite and epidote are frequent.

Iron hidroxides occur as peripheral impregnations in muscovites and as residues ofinter- lamellar remobilization in biotites. At 1710-1740 m, hematite and goethite may occur in amphiboles; ferruginization of quartz is rarely seen. Above 1700 m, small hematite granules, scattered within gibbsitic matrix, are frequent mainly in well crystallized zones. Between 1710 and 1820 m, hematite and also goethite are common as dispersed impregnations within cryp- tocrystalline kaolonitic matrix.

Gibbsite remobilization and reorganization phenomena are generalized, with the only exception of the weathered rock within 1640-1660 m. Reorganization of iron is typical of laterites, connoting in greater porosity and weathering.

Percentages of kaolinite plus gibbsite found in total samples of weathered rock, are seen in figure 3. Gibbsite prevails or occurs together with kaolinite in almost equal quantities. At 1710-1740 m, in stromatic migmatites with vesicular structure resulting from the total weathering of feldspar porphyroblasts or, more rarely, of other minerals (even quartz), kaolinite predominates.

Residual product ofgranitoid rock and migmatite weathering, the outcrops on the edge of the highest hilltops represent the maximum degree of evolution observed in the area (Photo 3 ). Gibbsite occurs either in feldspar and micas, or in a matrix of varied granulation, eventually with sparse nodules, streaks and geodes. Biotite lamellae are replaced with peripheral dis- placement of iron oxides and neoformation ofgibbsite. Micas are destroyed by exfoliation and buckling. The only residual mineral is quartz, either preserved or fractured, with fdling in of gibbsite and/or iron. Coating and lining by iron oxides were observed on cracks and voids often with formation of zoned clayey iron masses. Such materials may be considered as primary laterites, according to HARRISON (1933) and ERHART (1973) definitions. They are lateritic blanket bauxites (LELONG et al. 1976) characterized by general gibbsitization. Kaolinite plus gibbsite contents of 57 and 72%, both with predominance ofgibbsite (43 and 52%), were found in two samples oflaterite. These values, equivalent to alumina contents of 33 and 420/0, are lower than the ones quoted by various authors (in: LELONG et al. 1976) for aluminoferruginous laterites or impure bauxites; they probably reflect the importance of quartz as a residual mineral.

244 MODENESI

4.2. HILLSLOPES

Soils - Though morphologically and minerologically similar to materials from hilltops, the shallow soils of convex hillslopes have a slightly inferior degree of weathering. Quartz con- stitutes up to 70% of the coarse sand fraction; remnants of rock, 10 to 57% and weathered feldspar up to 33%. Less important (under 6%) are weathered opaque minerals, micas and pseudo- sands. In the medium sand fraction, quartz prevails (91 to 96%). As in hilltop soils, gibbsite occurs in the sand fraction. Within the silt fraction, quartz and gibbsite are the most important minerals; secondarily micas, microcline and interstratified minerals occur. In the clay fraction, eithergibbsite or kaolinite prevails; aluminous vermiculite is also quite frequent. Gibbsite and kaolinite total up to 43% of the mineral fraction of these soils; kaolinite (14 to 24%) or gibbsite (12 to 26°) may predominate.

In dissected hillslopes, mineralogical characteristics may change in depth, within the B or B/C horizons of thicker profiles. Clay and silt fractions show an increase of kaolinite content in relation to that ofgibbsite. Micas are less weathered, and interstratified minerals are rare. In the silt fraction, quartz is less important. Sand is richer in residual minerals (micas, feldspar, opaques). The A-horizon ofa hillslope soil shows a kaolinite plus gibbsite content of 13%, with 8% ofgibbsite.

Weathered rock - In the different sectors of hillslopes, characteristics of weathered rock mostly depend upon the intensity of variations of erosive phenomena and on the last fluvial incision, both determinant factors of slope materials permanence.

In convex slopes, primary mineral weathering is similar to that of the respective hilltops. Microfabric analysis often shows gibbsite remobilization and reorganization; concentrations of iron oxides and hydroxides only occur in the C-horizons. Amounts of kaolinite plus gibbsite are similar to those obtained at hilltops; kaolinite usually prevails (Fig. 2). At the plateau's edge, mainly above 1900 m, bauxitization phenomena were observed on granitoid rocks (Fig. 3). Individualization of iron and aluminum is achieved without loss of parent rock structure. Gibbsite is predominant and occurs in feldspar and micas and, at times, also as geodes and streaks. Intact qtiartz and muscovite, pited microclines and little weathered biotites remain. Primary bauxites in quartz-felspathic rocks are thus defined by the direct transformation of silicated minerals into gibbsite, probably due to intensive leaching. As LELONG et al. (1976) pointed out, solubility limits ofsilicated minerals are not necessarily reached under intensive leaching; evolution may directly lead to a bauxitic stage before feldspar and quartz are totally destroied.

In rectilinear hillslopes and in straight sectors of convex hillslopes, outcrops of fresh or little weathered rock are frequent. In further stages of weathering, feldspar is transformed into gibbsite or kaolinite. Quartz is usually well preserved; where cracked or corroded, it may present gibbsite filling. Muscovites and sericites are either intact or partially kaolinized. Bio- tites are weathered into kaolinite or gibbsite, always ferruginized. Residual minerals are: quartz, muscovite, sericite, epidote; in the less weathered materials: microcline, albite and biotite. There is no remobilization of iron, and there are only incipient signs ofgibbsite reorganization. The value for total kaolinite plus gibbsite is the smallest yet observed (Fig. 2); kaolinite prevails.

In erosion amphitheaters related to the two lower topographic levels, kaolinite is the main secondary mineral. Interstratified minerals occur in biotites. There is no remobilization of either gibbsite or iron. Rock partial weathering is defined by residues of primary, less resis- tant minerals and by frequent occurrence of micas or interstratified minerals. Gibbsite is more important above 1800 m; total gibbsitization of feldspars, quartz corrosion, remobilization


and reorganization ofgibbsite and iron, are observed beside little weathered biotites. Micas and, but seldom, microcline and albite may occur as residual minerals.

In amphitheater deposits kaolinite and micas, besides quartz, are the most common minerals. Gibbsite is only important above 1900 m. Traces of unstable minerals are rare. The weathering sequences observed in these deposits are normal, more weathered materials cover less weathered ones, as in soil profiles.

4.3. VALLEY FLAT

'Under the low terrace gravel deposits, feldspar weathering is characterized by kaolinization or by partial dissolution. Kaolinite and clay minerals, probably aluminous-vermiculites, occur on micas; biotites are impregnated by iron oxides. Quartz is always well preserved. Residues of primary minerals are frequent. There is no evidence of remobilization of either iron or gibbsite. Kaolinite is the prevailing secondary mineral (Fig. 2).

In colluvial deposits related to the highest hills, gibbsite is the most important mineral; under 1800 m, kaolinite may prevail. Micas occurrence is characteristicofall colluvia; in some layers, traces ofmicrocline and, more rarely, ofplagioclase and amphibole may still be present. Inverse or disordered weathering sequences are typical.

5. WEATHERING DEGREE OF SURFICIAL MATERIALS AND GEOMORPHIC COMPARTITION

All geomorphic compartments have highly evolved soils and weathered rock; quartz, gibbsite and kaolinite are their common constituents. Relationships between geomorphic compartition and degree of weathering are more obvious in weathered rock than in soils. Rock weathering is greater in the edge and top of hills. Weathering degree variations in different compartments, as well as within each one of them, reflect the intensity of erosive phenomena.

On the shallow hilltop soils, the amount of clay increases as altitude decreases; this is probably related to the stromatic migmatite substratum, which is poorer in quartz and richer in micas than granites from higher areas. In the clay fraction, kaolinite and gibbsite may predominate, regardless of the summit altitudes, but kaolinite and interstratified minerals are more frequent below 1800 m. No evidence of a relationship between weathering degree of soils and relative age of topographic levels exists. As already seen, one can only outline a separation between soils above and below 1800 m.

Bedrock structure is usually well preserved. Depth of weathering is considerable, and apparently, all the more above 1800 m. Kaolinite plus gibbsite contents are higher than 50% (Fig. 2 and 3). Below 1800 m signs of a weathering dichotomy seem to appear with the kaolinization of micas and gibbsitization of feldspar. Better preserved micas and the relative importance of kaolinite seem to indicate a lesser degree of weathering in the two lower topographic levels. Such a separation coincides with lithologic differences and small environmental ~,ariations. On the Mantiqueira divide, granitoid rocks prevail, temperatures are slightly lower and precipitation is higher. Towards the interior, decreasing altitude corresponds to migmatite occurrences, slightly higher temperatures and decreasing precipitation. Although quantitative data are not effective in separating the summital laterites from other hilltop materials, the absence of residual minerals - excepting quartz - and the characteristic

246 MODENESI

1 1 1 1 II

I vAlLEY FLAT ECT,L,NEAR sector I C O N V E X S L O P E H I L L T O P MPHITHEATRE floodplain T S L O P E

TOTAL KAOLINITE (K) + G IBBSITE (G}

37% 52 to 650/0 /'57",. ~r~to . . . . I I ~50/~ / / to 720/'0,~ . . . . I I - - " 1 1

MAIN SECONDARY MINERAL

I A 1 I K K , / G "" G K or G K

Fig. 2:Schematic cross section of the "altos campos" minor landforms. Rock weathering correlations.

MORRO DE ITAPEVA

L~ B R/betr6o Coplvot/ L J l I ~

o I a 3 4 5 6 -;,m I I m I

TOPOGRAPHIC LEVEL T 1640-1660m 1710-1740m 1800-1f120m Over ISOOm s ~

PREVAILING LITH OLOG Y M I G M AT I T E S / G RAN I TO I D S

INCREASING CONTENT ~ K A O L I N I T E G IBBSITE ~.

NEATHERING PROD.FROM FELDSPAR GIBBSITIZATION f~GIBBSITIZATION OF FELDSPAR AND MICAS FELO.AND MICAS MICAS KAOLINIZATION s " PREVAILING PEDOGEO- ~ ~" CHEMICAL PROCESS M O N O S I A L I T I Z A T ION ~ A L I T I Z A T I O N

DEGREE OF WEATHERING .

T - Terrace FC - Ferrahhc col luwa L - Alummo-ferr uginous Ioterites B - Primary bauxites

Fig. 3: Cross section of the Campos do Jord,~o Plateau and its relationship to weathering phenomena.

phenomena of iron remobilization witness their greater evolution. On hillslopes, essentially dynamic areas of landscape, erosion and deposition con-

siderably modify the distribution of surficial materials. An increase in soil thickness, a greater variation of morphological characteristics and a relative enrichment of the sand fraction are


observed. Superposition of soils is frequent. Relatively stable, convex slopes have soils and weathered rock both slightly less

weathered than on hilltops. In weathered rock, the relative importance ofgibbsite decreases, perhaps with the exception of bauxites observed above 1900 m in acid' rocks; this exception was not confirmed, due to lack of quantitative data. In dissected hillslopes, a lesser degree of evolution characterizes all surficial materials. Minimum kaolinite plus-gibbsite contents occur. However, in amphitheaters related to hills above 1800 m, bedrock may be as weathered as on hilltops - an indication of deeper weathering.

On the valley fiat, either the absence or small amounts ofgibbsite characterize weathered rock under the low terrace deposits. Weathering characteristics show a degree of evolution similar to that observed for straight slopes (Fig. 2). Degree of weathering ofcolluvial deposits depend on the mineralogy ofupslope materials. These deposits, as well as the weathered rock in amphitheaters, show an increased occurrence of micas.

Within different compartments, grassland and forest distribution reflects the morphologic characteristics and the weathering degree of bedrock and soils (Fig. 2).

6. WEATHERING AND ITS MORPHOGENETIC IMPLICATIONS

Within some hillslope deposits, the degree and sequence of weathering and its relationship to materials situated higher along the hillslope frequently reflect the processes responsible for slope erosion and debris transportation. This can be seen in colluvial deposits and in amphitheaters. In colluvial deposits, the shallow processes that rework the loose, more superficial and weathered materials, have slightly modified their mineralogy. Its weathering characteristics are similar to those ofupslope materials, and the weathering degree is practical- ly the same. Inverse or disordered weathering sequences suggest successive deposition, with superposition of stratigraphic bands. In erosion amphitheaters, deep mass movements reworked both the superficial and the less weathered profile zones, thus producing greater changes in the mineralogical characteristics. Weathering relations between deposits and up- slope surficial materials lessen; material renewal is more obvious. Amphitheater deposits, show normal weathering sequences due either to processes able to transport the whole profiles whilst preserving the original sequence, or, in older deposits, to post-depositional weathering.

Intense gibbsitization does not necessarily imply in a final stage of weathering. High gibbsite contents may occur in less weathered rocks still with feldspar residues, suggesting intense leaching processes in a relatively short time. Although not exclusive, gibbsitization is more intense and generalized above 1900 m (Fig. 3) on the Mantiqueira divide, in residues of the old "altos campos" erosion surface; the importance of gibbsite in weathered rock may be ex- plained by the presumed antiquity of exposure, by the predominance of feldspar rich granitoid rocks, by the gibbsitization tendency of feldspar, as well as by higher moisture and better drainage conditions.

Kaolinization coincides with the migmatite substratum of the more recent levels (Fig. 3). However, the evidence shows that lithology is not the determinant factor on the predominance of either gibbsite or kaolinite in weathered materials. Also, kaolinite's importance in the lower sector of convex hillslopes and in amphitheaters cannot be atributed only to the relatively poor drainage conditions of such sites, for they also are the most dissected areas of slopes. Hydromorphic conditions may only determine a clear preponderance of kaolinite in the low terraces. Kaolinite is always associated with less weathered rock. Gibbsitization, on the

2 4 8 M O D E N E S I

contrary, occurs in both more or less weathered materials, showing that other factors besides time may influence neoformation ofgibbsite.

Spatial distribution of kaolinite or gibbsite seems to outline two weathering tendencies (Fig. 3). At the Mantiqueira divide, intense gibbsitization characterizes alitization processes (PEDRO 1966), be it in highly evolved residual materials (aluminofermginous laterites), or in less weathered rocks (primary bauxites). Thus gibbsitization may be defined as a recurring tendency. Below 1800 m, the growing importance of kaolinite characterize sialitization processes - monosialitization (PEDRO 1966). These considerations suggest that monosialitization and alitization correspond to different stages of a relatively fast general aliticweathering trend. At the Mantiqueira divide, this trend was probably accentuated by the superposition of favourable factors.

Certain soil characteristics, more conspicuous in the highest part of the plateau, seem to show a tendency to podzolization. This tendency is in equilibrium with present temperature and moisture conditions, that favour iron reduction and the formation of organic complexing compounds, able to cause typical podzolic iron migration (SEGALEN 1964). At 1700-1780 m, morphological characteristics ofhillslope soil profiles suggest the superposition of two pedogeochemical trends, the oldest one being ferralitic and the more recent one tending towards podzolization. The underlying ferralitic materials are probably connected to a weathering environment- different from the present one- having good drainage conditions, oxidising at the surface, and with rapid decomposition of organic matter (SEGALEN 1964, MAIGNIEN 1966).

In hillslope deposits and soils, yellow colors commonly lie above of red ones. As many authors have shown (SEGALEN 1964, LAMOUROUX et al. 1977, VOLKOFF 1978, amongst others), such colors are related to the nature of ferruginous soil minerals: red hues correspond to predominance of hematite and yellowish ones ofgoethite. In Campos do Jor- dao slope deposits, deeper red hues abruptly change upwards to yellowish brown ones; soil profiles, on the other hand, show a gradual change which suggests transformation of hematite into goethite (SCHWERTMANN 1971), closely linked to changes in soil organic matter balance and consequently to climatic changes. Yellowish materials reflect present weathering conditions, which induce a slow evolution of organic matter. Acid hydrolysis or acidolysis would be responsible for the superimposition of goethite on the superficial layer of soils (SCHWERTMANN 1971).

These considerations about soil colors and the nature ofsecondaryiron compounds, plus morphologic evidences of superimposition of pedogenetic tendencies, suggest changes in environmental conditions and evolution mechanisms, probably related to past climates, different from the present one. Red latosols and laterites are probably remnants of hotter, humid

. or sub-humid climates, in which the rainy period was a warm season (SEGALEN 1964, MAIGNIEN 1966). The presence ofgibbsite in red latosols is characteristic of hot climates with alternate seasons and savannah vegetation because, according to many authors (in: CHATELIN 1972), such conditions would favour neoformation ofgibbsite in upper weathering horizons.

In the weathered rock, gibbsitization and "in process" bauxitization are not in conflict with the present climate, seeing that gibbsite formation is more related to local drainage conditions rather than being directly related to temperature and rainfall (SEGALEN 1973). Besides, as CHATELIN (1972) points out, neoformation ofgibbsite in the contact fresh rock- weathered rock seems to be encouraged by wet climates with regularly distributed rainfall.

The topographic position of remnants of laterite and red latosol, plus the fact that a generalized and important ferralitization, such as the one suggested by these remnants, take


long periods of time to be accomplished, suggest a considerable age for these materials. The occurrence of probable allochthonous latosols and evidences of intensive erosion on the higher hilltops (profiles with incipiently weathered rock) would suggest reworking and redis- tribution of previously weathered materials. Laterites and latosols would be the last remnants of hot humid, or more probably of hot subhumid weathering processes which were active during the Tertiary. These reworked materials would be related to past ferralitic weathering phenomena previous, at least, to the Pliocene accentuation of the plateau's upliffand to conse- quent climatic changes.

Such observations seem to confirm FREITA'S (1951) and ALMEIDA'S (1964) propositions on Japi's surface deformation and its tectonic unfold in Cristas M6dias and Cam- pos surfaces, both proposed by DE MARTONNE (1940) and considered by AB'SABER, in many of his works (amongst them, AB'SABER & BERNARDES 1958) as independent erosion surfaces.

7. CONSLUSIONS

1. At the Campos do Jord,~o Plateau, surficial materials are characterized by thin soils and deep rock weathering. Bedrock, deposits and soils are very weathered and almost exclu- sively constituted by quartz, gibbsite and kaolinite; aluminous vermiculites occur in incipient weathering ofgranitoid rocks and in some A-horizons of soils.

2. Above 1800 m, feldspar and micas are transformed in gibbsite; on convex hillslopes, "in process" bauxitization phenomena occur. From 1800 m downward, gibbsitization of feldspars and kaolinitization of micas outline a weathering dichotomy.

3. The relative importance of iron remobilization and reorganization in the weathered rock seems to be an effident criterion for differentiating degrees of weathering. Gibbsite reorganization is independent from the degree of evolution.

4. In the weathered rock, a general alitic weathering trend, is particularly clear above 1800 m. Under this altitude, an increase in kaolinite indicates the predominance of monosialitization processes. The two types of weathering seem to correspond to different stages within the same general tendency.

5. Relationships between weathering degree of surficial materials and geomorphic compartition are more conspicuous in weathered rock than in soils.

6. Mineralogical characteristics of the weathered bedrock define hilltop materials as the more evolved ones. The aluminoferruginous laterites that crop out above 1800 m represent the last stage of rock weathering.

7. In hillslopes, weathering chronology is related to morphogenetic dynamics; in the most dissected slope sectors, the degree of weathering of the bedrock decreases.

8. In hillslope deposits, degree and sequence of weathering are usually related to morphodynamics and not to incipient post-depositional pedogenetic activity. Weathering characteristics frequently reflect the type ofdownslope movement processes.

9. The superposition of colors which is frequent on hillslope surficial materials reflects two opposing pedogeochemical tendencies related to variations in environmental conditions. Red materials - mostly reworked latosols - are related to past favourable ferralitization conditions; the superficial yellow ones reflect present conditions of acid hydrolysis or acidolysis which are responsible for the podzolization tendency.

10. Alitization is a recurrent weathering trend, important in the past (aluminoferruginous laterites) and also at present (primary bauxites). Ferralitization is linked to past

250 MODENESI

environmental conditions. 11. Laterites and latosols contrast with the relatively low temperatures of the present

altitude climate, prevailing ever since the Pliocene accentuation of the plateau's uplift. These occurrences seem to confirm the tectonic unfold of the Japi or Cristas M6dias surface, thus allowing for the correlation of the "superficie dos altos campos" to degradational processes active at lower altitudes prior to the uplift of the plateau to its present level.

12. Surficial materials morphologic characteristics and weathering degree are important factors in the organization of the "altos campos" vegetation mosaic. Forests gather in amphitheaters and hillslope areas where soils are deeper and where incoherent regolith allows for the penetration of roots. Also where soils, but mostly the weathered rock, still have a supply of primary minerals.

ACKNOWLEDGEMENTS

Special thanks are due to Dr. J.M. Wackermann for his constant help in the mineralogical analysis and to Dr. A.J. Melfi for technical support, as well as for his constructive criticism.

REFERENCES

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ALMEIDA, F.F.M. de (1976): The System of Continental Rifts bordering the Santos Basin, Brasil. Anais Academia Brasileira de Ci~ncias 48 (supl.), 15-26.

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Geologia 6, 120 p. HARRISON, J.G. (1933): The Katamorphism of Igneous Rocks under Humid Tropical Conditions.

Imperial Bur. Soil Scien., Rothamted Exp. Stn., 79 p., Harpenden. HASUI, Y., PON(~ANO, W.L., ALMEIDA, M.A. & SANTOS, M.C.S.R. (1977): Compartimenta~o

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LELONG, E., TARDY, Y., GRANDIN, G., TRESCASES, J.J. & BOULANGE, B. (1976): Pedogenesis, Chemical Weathering and Processes of Formation of Some Supergene Ore Deposits. In: Hand- book of Strata-bound and Stratiform Ore Deposits. K.H. Wold (ed.), Elsevier Scient, Co., Cap. 3, 93-173. Amsterdam.

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PEDRO, G. (1966): Essai sur la caract6rization g6ochimique des differents processus zonaux de l'alt& ration surpeficielle des roches. Compt. Rend. Acad. Scien. Paris (292-d), 1828-1831.

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SCHWERTMANN, U. (1971): Transformation of Hematite to Geothite in Soils. NaCre 232, 624-625. SEGALEN, P. (1964): Le fer dans les sols. Initiation Documents T6chniques ORSTOM, 150 p. SEGALEN, P. (1973): L'aluminium dans les sols. Initiation Documents T6chniques ORSTOM 22, 23 p. TRICART, J. (1965): Principes et M6thodes de la G6omorphologie. 496 p., Masson et Cie. Edit., Pads. VOLKOFF, B. (1978): Os produtos ferruginosos que determinam a c, or dos latossolos da Bahia. Revista

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ficidas sob clima tropical de altitude no planalto de Campos do Jord~o. Estado de S~o Paulo (unpublished).

Address of author: May Christine Modenesi, Instituto de Geogratia - Cidade Universi~da Armando Sales de (Jliveira C.Posta120.715, 05508 S~o Paulo - SP - Brasil

Weathering and morphogenesis in a tropical plateau

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