Laterite preservation and soil distribut (1)

(1)

Santana Santana

BORBOREMA BORBOREMA

Crato Crato ARARIPE

ARARIPE Cam

Campospos S Salesales

SERRA SERRA DAS DAS MATAS MATAS

P PEEREIREIRORO

Fortaleza Fortaleza

Jo

Jo‹‹o Pessoao Pessoa Sobral

Sobral

Mossoro Mossoro

CHAPADA DO APODI CHAPADA DO APODI MERUOCA

MERUOCA

Crateus Crateus

RRiioo

AAccaa

rraauu

R Riioo

PPiirr aann hhaa

ss

Fortaleza

Natal

Recife

Paracuru Camocim

Parnaiba

Macau

Santana

João Pessoa

Patos Souza

Crato Campos

Sales

Quixadá

Exu Iguatu Crateus

Sobral

Campina Grande Apodi

Aracati

Mossoro

Rio

Poti

Rio

Pir an ha

s Rio

Jagu

arib

e

40

°W

10°S

3 500

3 000

4 00₀

2 000

AL BA

SE PE

PA RN CE

PI

MERUOCA

IBIAPABA

IRAUÇUBA

CHAPADA DO APODI

MARTINS PEREIRO

ARARIPE _BORBOREMA

SERRA DAS MATAS

BATURITÉ

São Fra_ncisc o

Par

naib a

Rio Aca

rau

A A

tt ll aa n n

tt ii cc

O O cc

e e aa

nn

A t l a

n t i c

O c

e a

n

0 100 200 km

SOUTH AMERICA 1020

1099

1124

1002 1115

744

1197

1185 State border

Main cities

Altitudes (m) Main rivers

0 - 200 m

200 - 400 m

400 - 600 m

600 - 800 m

800 - 1 000 m

1 000 - 1 200 m

Fig. 1 - Location map

of the study area.

Onhsore topography from

the SRTM GTOPO30

Study฀area

Fig. 2 - Simplified geological map of the study area

(source: CPRM, 2003)

0 10 20

RRiiaacc hhoo฀฀฀฀฀฀฀฀฀฀

฀฀฀฀฀฀฀฀฀฀฀฀฀฀ CCoonncc

ee฀฀iiççaaoo

RRiioo฀฀ ฀฀฀฀฀฀฀฀฀฀฀฀฀฀

฀฀฀฀฀฀฀฀฀฀dd ooss฀฀฀฀฀฀

฀฀฀฀฀฀฀฀฀฀฀฀ ฀฀฀฀BBaass

ttiiooeess

Campos฀Sales Campos฀Sales

Araripe

Precambrian

basement Granitoïds

Cenomanian sandstones (Exu฀Formation) kilometres

Campos฀Sales

Albian฀lacustrine and฀marine฀deposits (Santana฀Formation)

Fig. 3 - Geomorphological map of the study area

฀฀฀฀฀฀฀฀฀฀฀฀฀฀CC oonnccee

฀฀iiççaaoo

RRiioo฀฀ ฀฀฀฀฀฀฀฀฀฀฀฀฀฀

฀฀฀฀฀฀฀฀฀฀dd ooss฀฀฀฀฀฀

฀฀฀฀฀฀฀฀฀฀฀฀฀฀฀฀BB aassttiioo

eess Campos฀Sales

Araripe

300 - 400 m 400 - 500 m 500 - 600 m 600 - 700 m 700 - 800 m 800 - 900 m

฀฀฀Topography

฀฀90฀m฀SRTM฀DEM

altitudes฀in฀metres

Geomorphology Cenomanian

structural฀surface

Low฀planation฀surface (”Sertaneja”฀surface) Exhumed฀Pre-Cenomanian

planation฀surface 1-฀Degraded 1

Laterite preservation and soil distribution in the Araripe-Campos Sales area,

Northeastern Brazil: consequences of uplift, erosion and climatic change

F. BETARD

(1) *

, J-P. PEULVAST

(2)

, V. CLAUDINO SALES

(3)

(1)

_{DEPAM, Université de Paris-Sorbonne, Paris, France}

(2)

_{IDES, CNRS, Université de Paris-Sud, Orsay, and DEPAM, Université de Paris-Sorbonne, Paris, France}

(3)

_{Universidade Federal do Ceará, Campus do Pici, Fortaleza, Brazil}

*

_{corresponding author e-mail : francois.betard@paris4.sorbonne.fr}

1. Introduction

2. Physical setting of the study area

The existence of laterites in semi-arid Brazilian “Nordeste” is a phenomenon poorly described in the literature, probably because of their

weak occurrence in the landscapes by comparison with other tropical shield regions (e.g. West Africa, Western peninsular India). However,

remnants of lateritic crusts have been recognized in a few areas, on small mesas capped by the Serra do Martins sandstones (Pereiro, Martins:

Fig. 1) or on the Campanian limestones and younger sediments of the Chapada do Apodi (Peulvast & Claudino Sales, 2004). One of the most

extensive areas of lateritization in NE Brazil is found in the Araripe-Campos Sales area (southwest Ceará). The objectives of the current work

are: (1) to precise the distribution and characteristics of laterites and soils in the selected area, and (2) to explore the laterite evolution and soil

development in terms of morphotectonic models because of the fundamental controls exerted by tectonics, erosion and changing climate on

pedogeomorphic landscapes at the Cenozoic time-scale.

<฀550฀mm 550-650฀฀mm 650-750฀฀mm 750-850฀฀mm

฀฀฀฀฀฀฀฀฀฀฀฀฀฀CC oonnccee

฀฀iiççaaoo

RRiioo฀฀ ฀฀฀฀฀฀฀฀฀฀฀฀฀฀

฀฀฀฀฀฀฀฀฀฀dd ooss฀฀฀฀฀฀

฀฀฀฀฀฀฀฀฀฀฀฀ ฀฀฀฀BBaass

ttiiooeess Campos฀Sales

Araripe 550

650

750

0 10 20 kilometres

Fig. 4 - Climatic map of the study area showing mean annual rainfall (source: IPLANCE, 1995) Geology

The study area geologically corresponds to a contact region between a Precambrian basement and a Mesozoic sedimentary cover. The basement is composed of monotonous gneisses and migmatites, with a wide strip of granitic terranes (Fig. 2). Its structural framework is organized around a NE-trending system of faults branching on large Proterozoic

shear zones formed during the

Brasiliano/Panafrican orogeny. Remnants of a Mesozoic sedimentary cover lie unconformably on the basement to the south (Araripe Basin). The study region only comprises the northwestern edge of the Araripe Basin, a sedimentary basin centred on grabens formed during Early Cretaceous intracontinental rifting. The final infill of the post-rift sequence is represented by the extensive fluvial sandstones of the Exu Formation of Cenomanian age, which directly overlap the basement in the study area.

Geomorphology

Rich in morphostratigraphic

markers, the relief of the study area clearly shows the existence of three main morphological storeys (Fig. 3) : (1) a

culminating structural surface of

Cenomanian age (800-900 m)

corresponding to the top of the post-rift Araripe series (sandstones of the Exu Formation), (2) a high exhumed Pre-Cenomanian planation surface (600-650 m) extending on the gneissic and granitic rocks of the basement, and (3) a low planation surface developed below by cyclic entrenchment in the same crystalline basement (300-400 m), and connected to the low “Sertaneja” pediplain. On the basis of this morphostratigraphic scheme, we obtain some precious dated reference levels that we will use for defining the conditions of laterite evolution and soil development in the context of the regional morphotectonic evolution.

Modern climate and vegetation

The study area is characterized by a hot, semi-arid climate, with a dry season of 6-8 months (Nimer, 1977).

Rainfall is less than 850 mm year-1

(locally <550 mm year-1; Fig. 4) and is

concentrated in the late summer (March-April). Mean temperature of the coldest month is higher than 20°C and mean annual temperatures ranges from 22 to 24°C. The present rainfall regime is mainly related to the annual movement of the intertropical convergence zone (ITCZ) and the rainy season occurs when the ITCZ moves over NE Brazil. Climatic conditions of the region have only allowed the development of a xerophytic vegetation type, known as the “caatinga” vegetation in NE Brazil. The study area is

dominated by arboreal caatinga,

containing a tree layer 8-10 m in height, a perennial spiny scrub layer and a seasonal herb layer dominated by grasses.

2,57 2,57 84

84 44,1

44,1 19,4

19,4 0,26

0,26 0,15

0,15 9,2

9,2 6,7

6,7 44

44 0,30

0,30 5,4

5,4 47

47--7070

2,66 2,66 85

85 61,8

61,8 13,6

13,6 0,30

0,30 0,13

0,13 6,5

6,5 4,7

4,7 22

22 0,18

0,18 5,3

5,3 70

70--110110

2,04 2,04 81

81 20,8

20,8 7,5

7,5 0,08

0,08 0,11

0,11 3,2

3,2 2,7

2,7 36

36 0,21

0,21 6,2

6,2 106

106--130+130+

2,00 2,00 77

77 22,9

22,9 7,8

7,8 0,08

0,08 0,09

0,09 2,9

2,9 2,9

2,9 34

34 0,22

0,22 6,2

6,2 79

79--106106

2,03 2,03 73

73 17,4

17,4 7,5

7,5 0,06

0,06 0,11

0,11 1,8

1,8 3,5

3,5 43

43 0,25

0,25 5,9

5,9 30

30--7979

2,08 2,08 65

65 21,9

21,9 7,9

7,9 0,05

0,05 0,29

0,29 1,3

1,3 3,5

3,5 36

36 0,56

0,56 5,7

5,7 15

15--3030

2,18 2,18 83

83 50,0

50,0 6,5

6,5 0,05

0,05 0,05

0,05 1,2

1,2 4,1

4,1 13

13 0,96

0,96 6,6

6,6 0

0--1515 CE 53

CE 53 --PodzóPodzólicolicoVermelho-Vermelho-AmareloAmareloEutrEutróóficofico((““FersialliticFersiallitic””soilsoil; ; Lixisol)Lixisol)

P 19

P 19 ––Bruno nBruno nããoocácálcicolcico((“Fersiallitic“Fersiallitic””soilsoil; ; Luvisol)Luvisol) CE 16

CE 16 --LatossoloLatossoloVermelho-Vermelho-AmareloAmareloáálicolico, , concrecionconcrecionááriorio((FerralliticFerralliticsoilsoil, , withwitha a lateritizedlateritizedhorizon)horizon) CE 04

CE 04 --LatossoloLatossoloVermelho-Vermelho-AmareloAmareloáálicolico((FerralliticFerralliticsoilsoil, non , non induratedindurated; ; Ferralsol)Ferralsol)

82 82 82 82 81 81 81 81 33 33 13 13 11 11 12 12 35 35 11 11 7 7 5 5 4 4 3 3 7 7 % % Base Base satsat..

2,55 2,55 2,64 2,64 2,63 2,63 2,50 2,50 1,69 1,69 1,90 1,90 2;28 2;28 2,42 2,42 2,64 2,64 1,69 1,69 1,72 1,72 1,75 1,75 1,75 1,75 1,68 1,68 1,74 1,74 Ki Ki

0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2

CEC CEC Exchangeable

ExchangeableBasesBases

(1) in

(1) in cmolcmol(+)/kg (+)/kg ofoffine fine earthearth, , andand(2) in (2) in cmolcmol(+)/kg (+)/kg ofofclayclay source: source: RadambrasilRadambrasil, 1982, 1982 52,5

52,5 2,1

2,1 0,07

0,07 0,05

0,05 0,6

0,6 0,6

0,6 4

4 0,21

0,21 5,2

5,2 90

90--110+110+

53,7 53,7 10,2

10,2 0,06

0,06 0,22

0,22 2,4

2,4 5,6

5,6 19

19 0,61

0,61 5,5

5,5 0

0--77

47,1 47,1 11,3

11,3 0,10

0,10 0,13

0,13 3,4

3,4 5,5

5,5 24

24 0,48

0,48 5,6

5,6 7

7--2020

38,2 38,2 18,7

18,7 0,14

0,14 0,14

0,14 7,5

7,5 7,6

7,6 49

49 0,42

0,42 5,3

5,3 20

20--3333

39,6 39,6 18,2

18,2 0,19

0,19 0,14

0,14 7,7

7,7 7,0

7,0 46

46 0,44

0,44 5,4

5,4 33

33--4747

6,4 6,4 2,7

2,7 0,08

0,08 0,02

0,02 42

42 0,38

0,38 5,3

5,3 133

133--168+168+

0,3 0,3 0,2 0,2 0,3 0,3 0,4

0,4 0,270,27 0,030,03 4,64,6 32,832,8

0,9 0,9 14

14 0,86

0,86 5,3

5,3 0

0--1010

20,0 20,0 3,4

3,4 0,02

0,02 0,11

0,11 0,3

0,3 17

17 0,58

0,58 4,6

4,6 10

10--1919

11,6 11,6 2,8

2,8 0,03

0,03 0,06

0,06 0,2

0,2 24

24 0,33

0,33 4,3

4,3 19

19--6161

9,4 9,4 3,0

3,0 0,04

0,04 0,08

0,08 0,3

0,3 32

32 0,41

0,41 4,4

4,4 61

61--9090

10,2 10,2 4,3

4,3 0,06

0,06 0,01

0,01 42

42 0,56

0,56 5,1

5,1 95

95--133133

15,2 15,2 6,1

6,1 0,07

0,07 0,02

0,02 40

40 0,76

0,76 5,1

5,1 65

65--9595

21,8 21,8 8,3

8,3 0,06

0,06 0,02

0,02 38

38 1,18

1,18 4,9

4,9 38

38--6565

29,7 29,7 11,3

11,3 0,05

0,05 0,03

0,03 38

38 1,88

1,88 4,8

4,8 13

13--3838

41,6 41,6 15,8

15,8 0,06

0,06 0,06

0,06 0,3

0,3 0,7

0,7 38

38 3,04

3,04 4,6

4,6 0

0--1313

(2) (2) (1)

(1) Na

Na K

K Mg

Mg Ca

Ca %

% %

% H

H₂₂OO cm

cm

Clay Clay Org

Org. C. C pH

pH Depth

Depth

2,57 2,57 84

84 44,1

44,1 19,4

19,4 0,26

0,26 0,15

0,15 9,2

9,2 6,7

6,7 44

44 0,30

0,30 5,4

5,4 47

47--7070

2,66 2,66 85

85 61,8

61,8 13,6

13,6 0,30

0,30 0,13

0,13 6,5

6,5 4,7

4,7 22

22 0,18

0,18 5,3

5,3 70

70--110110

2,04 2,04 81

81 20,8

20,8 7,5

7,5 0,08

0,08 0,11

0,11 3,2

3,2 2,7

2,7 36

36 0,21

0,21 6,2

6,2 106

106--130+130+

2,00 2,00 77

77 22,9

22,9 7,8

7,8 0,08

0,08 0,09

0,09 2,9

2,9 2,9

2,9 34

34 0,22

0,22 6,2

6,2 79

79--106106

2,03 2,03 73

73 17,4

17,4 7,5

7,5 0,06

0,06 0,11

0,11 1,8

1,8 3,5

3,5 43

43 0,25

0,25 5,9

5,9 30

30--7979

2,08 2,08 65

65 21,9

21,9 7,9

7,9 0,05

0,05 0,29

0,29 1,3

1,3 3,5

3,5 36

36 0,56

0,56 5,7

5,7 15

15--3030

2,18 2,18 83

83 50,0

50,0 6,5

6,5 0,05

0,05 0,05

0,05 1,2

1,2 4,1

4,1 13

13 0,96

0,96 6,6

6,6 0

0--1515 CE 53

CE 53 --PodzóPodzólicolicoVermelho-Vermelho-AmareloAmareloEutrEutróóficofico((““FersialliticFersiallitic””soilsoil; ; Lixisol)Lixisol)

P 19

P 19 ––Bruno nBruno nããoocácálcicolcico((“Fersiallitic“Fersiallitic””soilsoil; ; Luvisol)Luvisol) CE 16

CE 16 --LatossoloLatossoloVermelho-Vermelho-AmareloAmareloáálicolico, , concrecionconcrecionááriorio((FerralliticFerralliticsoilsoil, , withwitha a lateritizedlateritizedhorizon)horizon) CE 04

CE 04 --LatossoloLatossoloVermelho-Vermelho-AmareloAmareloáálicolico((FerralliticFerralliticsoilsoil, non , non induratedindurated; ; Ferralsol)Ferralsol)

82 82 82 82 81 81 81 81 33 33 13 13 11 11 12 12 35 35 11 11 7 7 5 5 4 4 3 3 7 7 % % Base Base satsat..

2,55 2,55 2,64 2,64 2,63 2,63 2,50 2,50 1,69 1,69 1,90 1,90 2;28 2;28 2,42 2,42 2,64 2,64 1,69 1,69 1,72 1,72 1,75 1,75 1,75 1,75 1,68 1,68 1,74 1,74 Ki Ki

0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2

CEC CEC Exchangeable

ExchangeableBasesBases

(1) in

(1) in cmolcmol(+)/kg (+)/kg ofoffine fine earthearth, , andand(2) in (2) in cmolcmol(+)/kg (+)/kg ofofclayclay source: source: RadambrasilRadambrasil, 1982, 1982 52,5

52,5 2,1

2,1 0,07

0,07 0,05

0,05 0,6

0,6 0,6

0,6 4

4 0,21

0,21 5,2

5,2 90

90--110+110+

53,7 53,7 10,2

10,2 0,06

0,06 0,22

0,22 2,4

2,4 5,6

5,6 19

19 0,61

0,61 5,5

5,5 0

0--77

47,1 47,1 11,3

11,3 0,10

0,10 0,13

0,13 3,4

3,4 5,5

5,5 24

24 0,48

0,48 5,6

5,6 7

7--2020

38,2 38,2 18,7

18,7 0,14

0,14 0,14

0,14 7,5

7,5 7,6

7,6 49

49 0,42

0,42 5,3

5,3 20

20--3333

39,6 39,6 18,2

18,2 0,19

0,19 0,14

0,14 7,7

7,7 7,0

7,0 46

46 0,44

0,44 5,4

5,4 33

33--4747

6,4 6,4 2,7

2,7 0,08

0,08 0,02

0,02 42

42 0,38

0,38 5,3

5,3 133

133--168+168+

0,3 0,3 0,2 0,2 0,3 0,3 0,4

0,4 0,270,27 0,030,03 4,64,6 32,832,8

0,9 0,9 14

14 0,86

0,86 5,3

5,3 0

0--1010

20,0 20,0 3,4

3,4 0,02

0,02 0,11

0,11 0,3

0,3 17

17 0,58

0,58 4,6

4,6 10

10--1919

11,6 11,6 2,8

2,8 0,03

0,03 0,06

0,06 0,2

0,2 24

24 0,33

0,33 4,3

4,3 19

19--6161

9,4 9,4 3,0

3,0 0,04

0,04 0,08

0,08 0,3

0,3 32

32 0,41

0,41 4,4

4,4 61

61--9090

10,2 10,2 4,3

4,3 0,06

0,06 0,01

0,01 42

42 0,56

0,56 5,1

5,1 95

95--133133

15,2 15,2 6,1

6,1 0,07

0,07 0,02

0,02 40

40 0,76

0,76 5,1

5,1 65

65--9595

21,8 21,8 8,3

8,3 0,06

0,06 0,02

0,02 38

38 1,18

1,18 4,9

4,9 38

38--6565

29,7 29,7 11,3

11,3 0,05

0,05 0,03

0,03 38

38 1,88

1,88 4,8

4,8 13

13--3838

41,6 41,6 15,8

15,8 0,06

0,06 0,06

0,06 0,3

0,3 0,7

0,7 38

38 3,04

3,04 4,6

4,6 0

0--1313

(2) (2) (1)

(1) Na

Na K

K Mg

Mg Ca

Ca %

% %

% H

H₂₂OO cm

cm

Clay Clay Org

Org. C. C pH

pH Depth

Depth

Fig. 6 - Analytical characteristics of soil pedons represented in the study area

6. References

BOURGEON G. & GUNNELL Y., 1998 – Rôle du régime tectonique et du taux de dénudation sur la répartition géographique et les propriétés des sols tropicaux. CRAS,Paris, Sér. IIa, 326, 167-172.

CPRM, 2003 – Atlas digital de geologia e recursos minerais do Ceará. Mapas na escala 1:500,000. Serviço Geológico do Brasil, CD Rom.

GUICHARD E, 1970 – Les sols du Bassin du Rio Jaguaribe. Mém. ORSTOM, Paris, 40, 146 p.

IPLANCE, 1997 – Atlas do Ceará, Fortaleza, 64 p.

ISSS Working Group RB, 1998 – World Reference Base for Soil Resources: Atlas (Bridges E.M., Batjes N.H., Nachtergaele F.O., Eds.). ISRIC-FAO-ISSS-Acco, Leuven.

NIMER E., 1977 – Clima. In: IBGE: Geografia do Brasil. Região Nordeste. Vol. 2, Fundação IBGE, Rio de Janeiro, 47-84.

PEULVAST J-P. & CLAUDINO SALES V., 2004 – Stepped surfaces and palaeolandforms in the northern Brazilian “Nordeste”: constraints on models of morphotectonic evolution. Geomorphology,

62, 89-122.

PEULVAST J-P., CLAUDINO SALES V. & BETARD F., submitted – Reconstructing the morphotectonic evolution of passive margins: a morphogenetic study of the northern Brazilian “Nordeste”.

Submitted to Basin Research.

PROJETO RADAMBRASIL, 1982 – Levantamento integrado dos recursos naturais do Brasil. Folha Jaguaribe-Natal. Ministério das Minas e Energia-MME, Rio de Janeiro, 740 p.

TARDY Y., 1994 – Pétrologie des latérites et des sols tropicaux, Masson, Paris, 461 p.

TARDY Y. & ROQUIN C., 1998 – Dérive des continents. Paléoclimats et altérations tropicales, Editions du BRGM, Orléans, 473 p.

WIDDOWSON M. & GUNNELL Y., 1999 – Lateritization, geomorphology and geodynamics of a passive continental margin : the Konkan and Kanara coastal lowlands of western peninsular India. In:

M. Thiry & R. Simon-Coinçon (eds.): Palaeoweathering, palaeosurfaces and related continental deposits. Spec. Publ. Int. Assoc. Sedim., 27, 245-274.

5. Conclusions

Fig. 5 - Soils and weathering patterns in the study area

Ferrallitic soils

(Ferralsols) “Fersiallitic” soils(Lixisols, Luvisols) Area of lateritization(carapace or cuirasse)

Exposed weathering front of the laterite profile

kilometres

Campos฀Sales Campos฀Sales

Araripe Araripe Riac

ho฀฀฀฀

฀฀฀฀฀฀฀฀

฀฀฀฀Co nceiç

ao

Rio฀฀฀฀฀฀฀฀ dos฀฀฀฀

฀฀฀฀฀฀B

as

ti

oes

Araripe Campos฀Sales

0 10 20

3. Laterite and soil distribution

Data acquisition and terminology

Data on soils were collected from different pedological works and maps, including the Radambrasil project (1982) and a study from Guichard (1970) on the soils of the Rio Jaguaribe river catchment. Laterite mapping has been completed using a combination of extensive field reconnaissance and satellite Landsat imagery and DEM processing techniques.

The term laterite is very confusing. This problem of terminology has been widely debated and the rationalisation of its use still remains difficult. In their recent work in western peninsular India, Widdowson & Gunnell (1999) used the term laterite in a generic

fashion to describe both relatively soft accumulation of kaolinite, goethite and hematite, and defined by Tardy (1994) as carapace,

and the harder cuirasse which represents the indurated end member of lateritization. In order to facilitate the comparisons with South

India and to simplify its usage in the following sections, we use the term laterite in the same sense as that used by Widdowson & Gunnell.

Another way of confusion concerns the numerous systems of soil classification. Although we could make use of the former Brazilian classification presented in the detailed Radambrasil project (1982), we follow the French system of soil classification used by Guichard (1970) because it seems especially well suited for geomorphological purposes and because the names of the soils well describe their material composition and maturity. However, systematic reference to the international WRB classification (ISSS Working Group, 1998) is made for better appreciation and comparison.

Distribution and characteristics of laterites and soils :

Laterite and soil distribution widely reflects the present morphological pattern described above. On the top of the Chapada do Araripe (800-900 m), corresponding to the culminating structural surface formed by the Cenomanian Exu sandstones, only non-indurated ferrallitic soils occur (ferralsols: ISSS Working Group, 1998; profile CE04: Fig. 6). The clay fraction of these soils reveals a high percentage of iron and aluminium oxides (hematite, goethite and gibbsite) near 1/1 layer clay minerals (kaolinite). It explains the low cation exchange capacity of the B horizons (“ferrallic” horizon) and the very low base saturation. Because ferrallitic soils generally develop under conditions of humid tropical climate and because of the considerable thickness of the saprolite (several tens of metres), present bioclimatic pattern cannot explain their entire formation, so they must be considered as paleosols.

Most of the high, exhumed Pre-Cenomanian paleosurface (600-650 m) is covered by ferrallitic soils that present a marked

lateritized horizon, of carapace- or cuirasse-type (profile CE16: Fig. 6), delimiting a series of laterite-capped plateaus easily

discerned on satellite Landsat and Radar imagery. These laterites directly developed from the Precambrian crystalline rocks of the basement and may be considered as the equivalents of the “autochtonous laterites” of

Widdowson & Gunnell, involving in situ development of the lateritic profile. This was verified in the field by the observation of preserved quartz dykes through the laterite carapace or cuirasse and/or the underlying saprolite

in which structure of parent crystalline rocks are also easily recognized (Fig. 7). In other words, there is no evidence of transported materials in the laterite profiles, contrary to that suggested in existing geological maps where laterite cappings have been treated and mapped as extensive colluvio-alluvial sediments of Plio-Quaternary age, lateritized or not (CPRM, 2003).

The area of lateritization is now largely eroded and dissected by the present entrenched drainage system and the ancient weathering front is often exposed (“exhumed etch surface”). Only “fersiallitic” soils of deep red colour (Guichard, 1970; Lixisols, Luvisols; profiles CE 53 and P 19: Fig. 6) developed on the exhumed etch surface, associated with sparse tors and a shallow bisialltic grus. The low regional planation level (300-400 m) is characterized by the same relatively immature soils (“fersialltic” soils, often associated with lithic soils) by comparison with the deep ferralltic soils inherited of older humid climates.

Fig. 7 - Laterite profile north-east of Campos Sales Cenomanian฀structural

฀฀฀฀฀฀฀฀฀฀฀฀฀surface Exposed฀weathering฀front

฀฀฀฀฀฀฀฀฀฀("etch฀surface")

Indurated฀laterite of฀cuirasse-type

Altered฀gneisses ฀฀฀฀฀฀฀(saprolite)

4. Laterite evolution and soil development: a morphotectonic-paleoclimatic coupling

Reconstruction of long-term pedogeomorphic evolution :

The Araripe region during Albian and Cenomanian consists in a low altitude landscape of lakes and lagoons surrounding by low hills under warm, tropical conditions with alternating humid-dry cycles. The deposition of the thin (100 m or less) but extensive sandstone layers of the Exu Formation (Cenomanian) represents the rapid westward progradation of a fluvial system of meandering and braided streams, with a source area in the Borborema region to the east more vigorousely uplifted. The subsequent lateritization of the exhumed Pre-Cenomanian paleosurface implies the rapid exhumation of the basement, probably as soon as Late Cretaceous or Early Tertiary (Fig. 8A).

Humid conditions seem to have prevailed during Early Tertiary, involving kaolinitization process and ferrallitic pedogenesis whereas lateritization probably occurred during a long period of seasonally dry tropical climate (Eocene ?) (B). Because the geochemical evolution of laterites implies slow, but non negligible surface lowering, the present lateritized surface does not exactly correspond to the initial exhumed Pre-Cenomanian paleosurface.

During the last stages of the pedogeomorphic history (C), the stripping of the inherited kaolinitic mantles, related to drainage incision and regressive erosion initiated from the early planated coastal zone to the north, controlled the development of a primary pedogenesis from a refreshed bedrock in a context of semi-arid climate (“fersiallitic” pedogenesis). As such, it is similar to large tracts of the uplifted Karnataka plateau of semi-arid India, widely blanketed by soils of fersiallitic properties and where laterite-capped mesas are particularly rare (Bourgeon & Gunnell, 1998).

Available constraints on uplift and erosion :

Indications on uplift magnitude and denudation depth may be inferred from deformed sedimentary strata of the Araripe Basin. The total amplitude of crustal uplift can be obtained from the position of Albian marine layers of the Santana Formation found in the study area up to 700 m above the present sea level. Since the Albian sea level rose +150 to +220 m, a minimal post-Albian crustal uplift of 500 m may be deduced.

The return of continental conditions in the Cenomanian with the deposition of the Exu sandstones in a context of global eustatic rising movement suggest that the regional uplift was amorced as soon as the Albian-Cenomanian and prolonged during the Cenozoic at a mean rate

<10 mm.kyr-1(if averaged on post-Albian times: Peulvast et al., submitted).

As the Araripe Basin results from a topographic inversion of the Cenomanian Exu sandstones of fluvial origin and never buried by younger sediments, post-Cenomanian erosion

probably does not exceed the total value of topographic inversion (ca 500 m between the top of

the Exu Sandstones and the low planation level). Close to long term denudation rates, this low value is similar to that of vertical movements. The development of a low planation surface by cyclic entrenchment and the drainage incision probably implies higher denudation rates concentrated both in time and space.

Paleoclimatic evidences :

Since laterites are considered as indicators of seasonnally dry tropical environnments, their presence in the landscape is very useful for paleoclimatic reconstructions (Tardy & Roquin, 1998). According to these autors, conditions favourable to laterite formation in Northeastern Brazil would have occurred during Early Tertiary (Eocene) whereas later and drier periods (Neogene and Quaternary) seem to correspond to predominant semi-arid conditions (particularly since the Miocene, with the deposition of the clastic Barreiras sediments on the coastal strip).

Late฀Cretaceous฀-฀Early฀Tertiary:฀Sedimentation฀of฀the฀Exu฀sandstones฀(Cenomanian)฀and฀then฀exhumation฀of฀the฀basement

Early฀Tertiary:฀Lateritization฀process,฀ferrallitic฀pedogenesis฀and฀slow฀rates฀of฀surface฀lowering

Late฀Tertiary฀-฀Present:฀Drainage฀incision,฀exhumation฀of฀the฀lateritic฀weathering฀front฀and฀"fersiallitic"฀pedogenesis

HUMID

SEMI-ARID

Cenomanian฀surface

?

Exhumed฀Pre-Cenomanian฀planation฀surface

UPLIFT

? Exhumed฀Pre-Cenomanian฀planation฀surface

UPLIFT Exu฀sandstones

Precambrian฀granito-gneissic฀basement

non-indurated

ferrallitic฀soils _{Lateritized฀surface฀derived฀from฀the}

exhumed฀Pre-Cenomanian฀paleosurface

UPLIFT 800

600

400

200

0 m

REGRESSIVE EROSION Fluvial

knickpoints Lateritic฀plateau

฀฀฀฀฀฀remnants _{Etch฀surface}

฀฀฀฀฀฀฀Bisiallitic฀grus and฀"fersiallitic"฀soils

S N

Geochemical lowering

Fig. 8 - Reconstruction of the pedogeomorphic history: a morphotectonic-paleoclimatic coupling

If lithology appears as a discriminating factor to explain some differences in soil characteristics (between non-indurated ferralltic soils on sandstones and ferrallitic soils with a lateritized horizon on Fe-rich crystalline rocks of the basement), it is insufficient to explain the major differences observed in the same crystalline basement (between ferrallitic and “fersiallitic” soils) in a context of present homogenous semi-arid climate. Considering a morphotectonic model that combines the effects of uplift, erosion and climatic change at the Cenozoic time-scale accounts adequately for the development of the regional pedogeomorphic landscape. By taking into account uplift and denudational history, our conclusions at regional-scale are similar to that proposed by Bourgeon & Gunnell (1998) to explain the differences in soil cover at continental-scale between South India and West Africa.