Relief-Rock-Soil relationship in the transition of Atlantic Plateau to Peripheral Depression, Sao Paulo, Brazil

Abstract

This paper presents an atlas of physical geography at a detailed scale for an area located between two large morphostructural units: the Atlantic Plateau, a crystalline shield of rough landforms, and the Peripheral Depression, a transition unit from Plateau to the Parana Sedimentary Basin, whose smooth landforms are generally sculpted over sedimentary terrain. The transition between these two land systems creates contrasting landforms as a function of bedrock structure and soil type. Smooth landforms underlain by sedimentary rocks and reddish clayey soils alternate with small hills with abrupt slopes sustained by igneous or metamorphic rocks. These soils, developed from weathering of siltstones can rapidly change to brownish and pale colors indicating leaching and weathering of stratified sandstones or massive structured quartzites. A geomorphopedological classification at 1:10,000 scale was performed initially by delimiting units according to landforms. Secondly, other themes were added by combining them with different types of rocks and soils. The geomorphological map was produced by using thematic maps including a topographic base map, hypsometric map, slope map and morphological map. Also the geological and pedological maps were created based on field data collection, morphological analysis of soil profiles and physical, chemical and mineralogical laboratory analysis. Finally, the geomorphopedological units were defined using all the mapped data. Each of these units has peculiar features concerning developed or undeveloped soil formation, with mineralogy and structure very often linked to underlying rock. The landform shape is either controlled by geologic structure or originated from weathering processes that make the relief flatter.

Keywords:

• geomorphopedology
• geomorphological mapping
• landforms
• Peripheral Depression

1. Introduction

In the State of Sao Paulo (Brazil), it is possible to observe a unique geomorphological framework. In Sorocaba County there is a landscape, due to its position between two morphostructural units (Ross & Moroz, 1997), that reflects its geological history. The South-Southeast portion of the county is in a crystalline, Pre-Cambrian region, connected to Atlantic Plateau genesis. In the North-Northwest portion, sedimentary rocks are linked to the Phanerozoic prograding of Parana's Basin, in a flat leveled surface known as the Peripheral Depression (Ross & Moroz, 1997). With respect to topography and soil, there is a very clear contrast in the characteristics of both areas. Smooth broad hill landforms northwards alternate with steep lower hills southwards. Also reddish oxidized soils change to pale colors, indicating chemical alteration in situ and leaching.

In order to better understand the relationships between rock, soil and relief, the production of a map was set as the main goal. The use of thematic maps, field data collection, desk work and laboratory analysis resulted in a geomorphopedological subdivision of the study area, presented as an Atlas in 14 plates. To achieve this goal, altimetry and slope were analyzed, as well as landform characteristics and pedology, linking all these features to the geological survey.

Different methodologies have been used for this type of analysis: one is the Tricart and Kilian approach (1982), for the division of geomorphological units and corresponding soils, defining the construction of morphopedological units. Another is Ross's methods (1991, 1992, 1994), looking at taxonomic cartography of geomorphic factors. Finally, there is Castro and Salomão's work (2000), for specific morphopedological subdivisions.

2. Research area

The research area is located in the southeast of Sao Paulo State (SP), southeastern Brazil (see Figure 1, Plate 1), between latitudes 23° 34′ 1″ to 23° 36′ 4″ S and longitudes 47° 32′ 3″ to 47° 29′ 2″ W, corresponding to an area of 25,070 km², situated 87 km West of Sao Paulo city. This area comprises three counties: Sorocaba to the North and Northwest, Salto de Pirapora in the South Central, West and Southeast and a small part of Votorantim county to the East and Northeast.

Sitting in the Morphosculptural Unit of the Peripheral Depression (Ross & Moroz, 1997) (see Figure 2, Plate 1), between the crystalline Atlantic Plateau and the sedimentary Western Plateau, the study area varies between smooth and broad tops of the Paleozoic sedimentary lithologies (sandstones and siltstones) and rougher landforms linked to crystalline complexes, such as granitic and quartzitic bodies (see Figure 3, Plate 1). The area is characterized by tropical climate, mean temperature ∼ 22°C and annual rainfall up to 1500 mm, where vegetation reflects the transition from moist montane forest to mixed grassland formations. The soils also vary, from red-clayey Oxysols on elevated areas to Cambisols or altered rocky soils on the steeper slopes (see Figures 4 and 5, Plate 1).

3. Materials and methods

Aiming at making the geomorphopedological map, a desk analysis of topographic maps was carried out (IGC, 1979a, 1979b, 1979c and 1979d) at 1:10,000 scale. This resulted in the topographic base map of the study area. All other data, including geology, hypsometry, slope, morphology, pedology, geomorphology and geomorphopedology were made based on the topographic base map. In addition, aerial photographs and geomorphological/geological/pedological field surveys were augmented. Here are the steps:

a.	Base Map (Plate 2): topographic maps that included the study area were scanned, georeferenced and digitized manually, choosing as elements contour lines, altimetric elevations (spot heights), hydrography and roads;
b.	Hypsometry (Plate 3): this thematic map was produced with altimetric ranges that better represents the variation of topography, graduating from pale colors, showing lower altitudes, to bright colors, showing higher altitudes (Libault, 1975) to demonstrate altitude increase. The hypsometrical limits were digitized manually;
c.	Slope (Plate 4): after printing the digitized base map, cartographic techniques were used to show slope inclination in the study area (De Biasi, 1992). Statutory percentage limits (0–5%, 6–11%, 12–19%, 20–30% and more than 30%) were used as criteria for classification. Colors were chosen according to increasing value (yellow, orange, red and brown);
d.	Geology (Plate 5): using aerial photographs (Base Aerofotogrametria, 1962a, 1962b, 1962c and 1962d) and the geological photo-interpretation sensu Soares and Fiori (1976), it was possible to manually delimit pre-Cambrian metasediment boundaries, Cambrian granites, Paleozoic sediments and Quaternary deposits, as well as photolineaments and attitudes (dips and strikes of bedrock). The color and nomenclature of lithologies were adapted from Almeida et al. (1981) and Teixeira et al. (2009a and 2009b);
e.	Morphology (Plate 6): landforms (morphology and morphometry of terrain) and morphological elements (relief features) were identified stereoscopically on aerial photographs (Base Aerofotogrametria, 1962a, 1962b, 1962c and 1962d), with aerial photo-interpretation digitized and adjusted to contour lines and hydrography in a digital environment. These landforms comprise morphological area types, resulting in polygons of convex tops, flattened watersheds, smooth to dissected slopes, fluvial valleys and floodplains. Symbols of morphological elements were chosen from adaptations of several authors (Doornkamp & King, 1971; Savigear, 1965; Tricart, 1965; Troppmair, 1970; Troppmair & Mnich, 1969), to describe valley limits, valleys, changes and breaks of slope, topographic divisors and slope shapes;
f.	Pedology (Plate 7): the identification of soil units was performed after laboratory analysis results (granulometry, chemistry and mineralogy) and field surveys (soil coring and morphological analysis of soil profiles). Four units were defined: Red Oxysols, Cambisols (evolving to Bw Horizon formation), Cambisols and Gleysols; soil boundaries followed the distribution of sample points collected for laboratory analysis, contour lines and field information. Colors and nomenclature were adapted from the legend of IBGE (2007);
g.	Geomorphology (Plates 8, 9, 10, 13 and 14): the geomorphological map was created by overlaying digitized information from the morphological and geological maps, using an integrated legend based on Ross (1991, 1992, 1994) aiming to rank the data and establish a hierarchy. Six Taxons were created according to the geomorphological context of the study area, from macro to micro units: 1. Morphostructure, with differentiation of relief genesis due to endogenous processes and division of crystalline and sedimentary substratum; 2. Morphosculpture, where relief genesis is classified based on dominant substratum and exogenous processes that shape the landforms; 3. Landforms, where relief forms are divided based upon occurrence and dominant substratum; 4. Morphology elements, classified by symbols indicating their occurrence in landforms and dominant substratum; 5. Morphometrical, pedological and geological information (altimetric intervals, slope, soil and lithological units), divided according to the taxons previously noted (unless geology, not divided for the third and fourth Taxon); 6. Relief feature information related to surface processes (dominance of denudation or accumulation, gravitational movements and deposits or altered materials exposed to the surface). Five geological profiles show bedrock transitions. Four small maps show morphostructural division, landform classification, hypsometry and slope. There are also symbols indicating geological data and soil samples collected for laboratory analysis. Some explanatory notes can be also found;
h.	Geomorphopedology (Plates 11 and 12): all the other maps were digitally combined by overlaying their data, presenting landforms, altimetric intervals, slope, geological substratum and soils, with soil and bedrock transitions represented through slope by schematic profiles (Villela, 2011).

4. Results

A brief characterization of the geology, geomorphology, soil and geomorphopedology of the study area is presented here.

4.1. Geology

The study area is located in the eastern border of the Parana Sedimentary Basin. This is the best example of geological and glacial processes that occurred during the existence of Gondwana supercontinent in Braziĺs territory (Bigarella, Becker, & Pinto, 1967). The stratigraphic column here is composed of: metarhythmites, quartzites and phyllites of the Sao Roque Group (Neoproterozoic), biotite granites of the Sorocaba Massif (Cambrian), fine sandstones and clayey siltstones of the Tubarao Group (Neopaleozoic) and alluvial deposits from the Quaternary (Jundsondas, 2012).

As a result of the transition from crystalline basement to sedimentary cover, the base of the stratigraphic column is filled with Neoproterozoic metasediments of the Sao Roque Group (Almeida et al., 1981), composed of fine granulated metarhythmites with well-developed foliation, fine to medium granulation quartzites of massive structure and graphitous phyllites of well-developed foliation. These units underwent regional low grade metamorphism (Silva, 1997) and now have generally subvertical structure conditioning the drainage system, often determining the position of photolineaments in orthogonal planes.

Cambrian plutonic injections compose the granite outcrops in the area, where only boulders may remain on the surface. They are porphyritic rocks with coarse granulation and feldspar fenocrystals, belonging to the Sorocaba Massif, a batholith originating from post-orogenetic events (Godoy, 1989). This granitic body intruded through the metasediments and can be found sustaining the steeper and rounder interfluves of the study area.

Above the crystalline bedrock, there are sediments. Due to its morphological location in the border of the Parana Basin, the area had significant glacial continental sedimentation during the Carboniferous and Permian periods (Ab'Sáber, 1964; Almeida, 1964; Bigarella, Salamuni, & Marques Filho, 1961; Rocha-Campos, 1967, 2000a and 2000b; Castro, 2004), resulting in the presence of glacial deposits of fine texture on the top of the stratigraphy. This is the Itarare Formation (Tubarao Group), which corresponds to the Paleozoic deposition phase of the Parana Basin (Almeida et al., 1981). It is composed of fine matrix and cemented sandstones containing polished diamictites, alternating with bigger pebble layers and well-developed crossbedding. On top of them, there are clayey and fractured siltstones, with massive structure and plain-parallel stratification, with fine sandy levels and occasionally the presence of ripple marks. Finally, along the fluvial plains, Quaternary silt deposits may appear mostly restricted to flood prone areas.

4.2. Geomorphology

The research area is completely within the Peripheral Depression of Sao Paulo, located in the Tiete River Hydrographic Basin. This was adjusted by Cenozoic tectonic events with the drainage system cutting the smooth hill landform types and developing a dendritic to parallel pattern.

Looking at hypsometry, slope and morphology of the study area, it is noticeable that the flattest or slightly convex tops dominate the smooth hill landform types, with low slopes up to 5%. The most convex and steepest landforms are in the Southeastern area (metarhythmites), where the morphology is similar to slope sectors in the Northern area, resulting in small spurs. In many cases the most elevated areas are linked to the valley bottoms by flat surfaces, sometimes more convex. The highest sectors, up to 680 m, present convex tops and generally moderate slopes, going from convex to flat levels between 680 and 645 m range. These levels have low slope and often correspond to sub-basin water divides. Downwards, the slopes start to increase to more than 20%, followed by breaks of slope and second-order streams in the range 645 to 615 m a. s. l. This shows the structural control of drainage, with some interfluve orientations (low and steep hills of convex-concave or convex-straight slope sectors) controlled by metamorphic foliation. Below 615 m, in the floodplain sectors, slope decreases to the local base level, the Ipaneminha River, at 595 m.

The altitude and slope ranges reflect the division of landforms into smoother or rougher forms, although generally the relief is dissected into a smooth broad hill pattern. Five landform units are considered:

•	Convex tops underlain by sedimentary bedrock (generally above 660 m) or crystalline lithology (up to ∼630 m);
•	Flattened watershed, characterized by flat plateau areas slightly convex at its lateral limits. There is also an interfluvial depression (pseudo-karst feature) described as a ‘possible flooding terrain’;
•	Smooth to dissected slope sectors. When associated with other units, the hill landform type has low to medium inclinations (6–19%) on sedimentary bedrock and steeper slopes (20–30%) in the crystalline substratum. Convex forms are dominant;
•	Fluvial valleys composed of amphitheaters in the headwaters and cavities in open terrain cut the slope sectors. In contact with river catchments, these fluvial valleys acquire more gradient, particularly in the crystalline substratum;
•	Floodplains, with flat valley bottoms narrower than 200 m wide.

These landforms have varied morphological elements displaying some structural or genetic relief factors, as follows:

•	Valley limits indicate structural drainage control when slopes are straight. Also there is a predominance of erosional processes with concave or convex shapes;
•	Valleys with convex slopes indicate homogeneous bedrock and similar erosion processes on both sides;
•	Valleys with concave slopes can show youth in erosion processes and headwater position;
•	Asymmetrical valleys represent subsequent streams, lithological contacts or different erosion resistances of bedrock;
•	Flat bottom valleys indicate accumulation processes and the presence of floodplains;
•	Wide open valleys suggest maturity in the morphosculptural processes;
•	‘V’ shaped valleys can indicate geological structural control or youth in the morphosculptural processes;
•	Convex and concave changes of slope indicate smooth discontinuities in the slopes, decreasing their inclination;
•	Convex and concave breaks of slope indicate more abrupt discontinuities in the slopes, increasing their inclination;
•	Saddles represent topographical gaps between convex tops, even for smoother ones;
•	Asymmetric and symmetric crests represent the alignment of some water divides; the former on a crystalline substratum and the latter on sedimentary bedrock;
•	Convex, concave and straight sectors of slope indicate morphodynamic processes. Convex and straight sectors indicate runoff dispersion, whereas concave sectors represent runoff concentration.

There is an anomalous situation in the flattened watershed area, where the ‘possible flooding terrain’ is surrounded by a concave break of slope and a saddle. Its occurrence seems to be associated with subsurface processes.

The geological profiles on the geomorphological map display the differences between the morphology and the substratum: sedimentary cover underlies flatter relief features, and crystalline rocks are linked to convex and steeper slopes. These subdivisions adopt the taxonomic hierarchy in the main legend, as follows:

•	1st Taxon – Morphostructural Unit: corresponds to the contact between the Atlantic Plateau and the Parana Sedimentary Basin, with genesis relief division according to sedimentary or crystalline substratum;
•	2nd Taxon – Morphosculptural Unit: corresponds only to the location of the study area in the Peripheral Depression of Sao Paulo;
•	3rd Taxon – Landforms: divided by type according to the genesis of geologic substratum. In the sedimentary substratum, there are convex tops, flattened watersheds, smooth slope sectors and fluvial valleys. In the crystalline substratum, there are convex tops, dissected slope sectors, fluvial valleys and floodplains;
•	4th Taxon – Morphological Elements: describes relief features according to their occurrence in the landforms;
•	5th Taxon: divided by landforms, this defines the predominant characteristics concerning altimetric interval, slope, soil and geology;
•	6th Taxon: describes surface processes related to relief feature information.

It is essential to understand the legend adopted to analyze the geomorphological map. Nevertheless, this map provides a synthesis of all landscape factors exposed so far.

4.3. Soils

Associated factors to landscape such as bedrock and tropical climate have produced varied soil development in the study area.

A general division can be established between sedimentary terrain and crystalline lithologies. This variation is visible at specific altitude intervals, with dissection between flattened watershed areas and the steeper sectors of slopes. Generally, on summit levels and flattened surfaces, from 715 to 650 m a. s. l., there are Red Oxysols of clayey texture, with moderate A and Bw horizons. In the smooth slope sectors, between 650 and 630 m, there are Cambisols evolving to Bw Horizon conditions, with clayey texture. Downwards, below 630 m a. s. l. Cambisols appear in the steeper slope sectors, linked to the substratum characteristics.

Additionally, there are fluvial plains restricted to open valleys with flat floored forms or wide open floodplains. In these fluvial plains it is possible to find hydromorphic soils of clayey texture (Gleysols), with an accumulation of organic matter and a reduction of chemical processes display in their horizons. Gleysols also occur in the interfluvial depression.

Laboratory analysis (based on texture, silt and clay relationship, fertility, Ki and Kr relationships and mineralogy of clay minerals) showed that the Red Oxysols are mature from a weathering perspective, indicating processes of monossialitization and formation of kaolinite. However, both kaolinite and gibbsite were identified in the mineralogical analysis. Due to this fact, a slight process of more advanced hydrolysis is also seen in these Oxysols, with gibbsite formation in the latossolic horizon and some eutrophic conditions.

In the smooth slope sectors, there is kaolinite formation and monossialitization, with the B horizons of these soils being thin; nevertheless, morphological and laboratory analyses show that illuviation conditions are thickening these soils. Compared to typical Cambisols found in steeper slope sectors (laboratory analyses indicate a mixture of 1:1 and 2:1 clay minerals), a different condition is displayed. Therefore, the smooth slope sector soils are classified as Cambisols evolving to Bw Horizon formation.

Consequently, four types of soils can be described in the study area:

1.	Red Oxysols, eutrophic, typically clayey, overlapping the flattened surface supported by silty-clay Paleozoic sediments, with a predominance of more advanced hydrolysis processes, being geochemically stable, old and thick (4 m deep or more);
2.	Cambisols evolving to Bw Horizon formation, whose smooth slope sectors are in the transition between a sedimentary and crystalline substratum. The pedogenesis process is characterized by partial hydrolysis (monossialitization) in its formation. As a result, the soil is partially weathered and thinner than the Oxysols (thickness unknown);
3.	Cambisols occurring in steeper slope sectors are related to a crystalline substratum, under bissialitization (occurrence of clays 1:1 and 2:1). Therefore, the soil is less weathered and thinner (2 m deep or less);
4.	Gleysols with clayey and hydromorphic conditions, with a depth of at least 1 m.

4.4. Geomorphopedological compartments

Combining all the maps shown previously, it was possible to identify four main geomorphopedological regions in the study area. They are a function of lithology, landforms and pedological cover, as follows:

•	Unit I: corresponds to the flattened watershed surface (Tp), with a predominance of slopes from 0 to 5%, situated above 640 m a. s. l., with some convex tops (Tc). Both landform types are supported by Paleozoic sediments composed of fine sandstones (CPia) and clayey siltstones (CPib). Soils associated with this substratum are typically Red Oxysols (LV) that may change to Cambisols evolving to Bw Horizon formation (CX Bw) when supported by crystalline lithologies (see schematic-related profile);
•	Unit II: it is characterized by an occurrence of convex tops (Tc), flattened watersheds (Tp), smooth slope sectors (Cad) and fluvial valleys (Vf), with a transition to more dissected slope sectors. The unit is 630–716 m a. s. l., with slopes of 20% or above only in restricted areas. The lithologies include fine sandstones (CPia) and clayey siltstones (CPib), changing to Neoproterozoic metasediments as metarhythmites (PSsX), quartzites (PSsQ) and phyllites (PSsF), or even Cambrian granites (εSo). Soils are Cambisols evolving to Bw Horizon formation (CX Bw) or Cambisols (CX), with Gleysols (G) at the foot of the slopes as indicated in the schematic profile;
•	Unit III: it is located between altitude intervals of 595–694 m a. s. l., comprising landforms of convex tops (Tc), flattened watersheds (Tp), dissected slope sectors (Cad) and fluvial valleys (Vf), with slopes up to 19% and at more restricted sectors above 20%. The lithology corresponds to metasediments (PSsX, PSsQ and PSsF) and granites (εSo), while the pedological cover is dominated by Cambisols (CX), as shown in the schematic profile for Unit III;
•	Unit IV: involves sections of fluvial valleys (Vf) and floodplains (Pf), between 595 and 680 m a. s. l., with dominant slopes below 20% but sometimes steeper at the beginning of fluvial valleys, when at higher altitudes. These landforms are characterized by Paleozoic sediments (CPia and CPib), metasediments (PSsX, PSsQ and PSsF) and granites (εSo). Alluvial deposits (Qa) are restricted to floodplains. Cambisols evolving to Bw Horizon formation (CX Bw) are associated with Paleozoic sediments; whereas, Cambisols (CX) are associated with metasediments and granites. Gleysols occur at the contact of valleys with floodplains (see Unit IV schematic profile).

As fluvial valleys cross all types of lithologies, landforms and soils, it was preferred to keep their representation and soil transitions with the same hatchured pattern. For the remainder of the geomorphopedological map, hatchures of different colors are used to identify landforms, letters indicate geology and solid colors represent soils. Rock-soil-relief relationship can be identified by overlaying the three representations.

5. Conclusions

Conclusions are presented based on the mapping and analysis of the geological substratum, relief forms and soils in the study area, as follows:

•	According to this investigation method, landforms in the study area are controlled by lithostructure. Lithology characterizes sedimentary and crystalline geologic formations, in somewhat continuous units. This corresponds to the flattened slope shapes in the domain of Paleozoic sediments and more dissected landforms in the domain of crystalline basement. The lithostructure control is also remarkable in the hydrography, since geological lineaments produce orthogonal planes in the drainage system;
•	Existing landforms are the product of a combination of factors. Flattened relief forms appear to be related to the plain-parallel stratified sedimentary rocks composing wide and gentle topographic mosaics; whereas, the most dissected landforms may be related to metamorphic foliation that facilitates water infiltration and erosion. Opposite behavior can be found in the granite batholith of the Sorocaba Massif: the absence of fractures in the igneous rock inhibits infiltration. Greater resistance to erosion exposes boulders from smooth to dissected slope sectors. This clearly characterizes differential erosion when compared to terrain underlain by metasediments;
•	The relief-rock-soil relationship may be characterized by geomorphopedological mapping (Castro & Salomão, 2000; Tricart & Kilian, 1982). Here lithostructure influence is emphasized by investigation based on fieldwork and desk analysis;
•	Reinforcing the perspective that relief is controlled by lithostructure, it is noted that thinner soils have parent material corresponding to metasediments (phyllites, metarhythmites and quartzites) over the roughest topography. Nevertheless, thicker soils have parent material corresponding to the silty-clay Paleozoic sediments on flat relief surfaces;
•	Due to advanced hydrolysis conditions shown by the occurrence of gibbsite and clay-mineral formation associated with flattened landforms, it is possible that there are geochemical erosion processes in the surface formation. Consequently, there is lowering of topography by processes such as leaching and mineral dissolution. The study area flat surface may be an ‘etched’ surface or an ‘etched plain’ (Wayland, 1933), corresponding to an ancient geomorphic surface covered by Oxysols (Wambeke, 1992);
•	Furthermore, the presence of an interfluvial depression in the flattened watershed area, forming a pseudo-karst feature, implies dissolution at the top of the stratigraphy (clayey siltstones), as well as downgrading of the topography;
•	Finally, the batholith outcrops of the Sorocaba Massif (base of the stratigraphic column) reveal truncation of an old exposed surface with more recently eroded layers (Paleozoic sediments, at the top of stratigraphic column). They make up the scenery of etchplanation with double-planation surfaces, as indicated by (Büdel 1957, 1982). Therefore, the landforms sustained by sedimentary lithology and covered by thick clayey soils may correspond to a wash surface, part of the geochemical maturity process. The exposure of rougher landforms sustained by granites that display the rock basement could be an old and exhumed basal weathering surface.

Software

All the maps were produced using MapInfo Professional v9.0.

Acknowledgements

We are grateful to Professor Lylian Zulma Doris Coltrinari and Dr. Marisa de Souto Matos Fierz, from University of Sao Paulo, for their support on geomorphological mapping. In the same way we would like to thank the geologist Leticia Vicente for her fundamental help in surveying and geological mapping. Finally, we would also like to thank the referee's peer review and the enlightening comments and suggestions of the Primary Editor, Dr. Paolo Paron, and the Editor in Chief, Dr. Mike Smith.