Araçatuba Formation: Palustrine Deposits from the Initial Sedimentation Phase of the Bauru Basin
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Anais da Academia Brasileira de Ciências (2003) 75(2): 173-187 (Annals of the Brazilian Academy of Sciences) ISSN 0001-3765 www.scielo.br/aabc Araçatuba Formation: palustrine deposits from the initial sedimentation phase of the Bauru Basin LUIZ A. FERNANDES1, PAULO C. F. GIANNINI2 and ANA MARIA GÓES3 1Universidade Federal do Paraná – UFPR, Setor de Ciências da Terra, Departamento de Geologia Centro Politécnico, Jardim das Américas, Caixa Postal 19001, 81531-990 Curitiba, PR, Brasil 2Universidade de São Paulo – USP, Instituto de Geociências, Departamento de Geologia Sedimentar e Ambiental Butantã, 05422-970 São Paulo, SP, Brasil 3Universidade Federal do Pará – UFPA, Campus Universitário, Centro de Geociências Caixa Postal 1611, 66075-900 Belém, PA, Brasil Manuscript received on October 4, 2001; accepted for publication on October 25, 2002; presented by Moacyr V. Coutinho ABSTRACT The Bauru Basin (Upper Cretaceous) accumulated an essentially sandy continental sedimentary sequence. In a first desertic phase the basaltic substratum was covered by a widespread and homogeneous aeolian sand unit with minor loess intercalations. The substratum relief favored the formation of an endorheic drainage system under semi-arid climate, a process that started the development of the Araçatuba Paleoswamp. The palustrine deposits (Araçatuba Formation) comprise siltstone and tipically greenish gray narrow tabular strata of sandstone cemented by carbonate. Moulds and gypsite and dolomite pseudomorphs were identified. The moulds seem to be genetically associated with desiccation cracks, root marks and climbing ripple lamination levels, that, on the whole, indicate calm shallow saline waters undergoing phases of subaerial exposition. At the boundaries of the study area, sand units may exhibit sigmoidal features and convolute bedding structure, which is characteristic of marginal deltaic deposits. TheAraçatuba Formation is enclosed in and later overlaid by the aeolian deposits of the Vale do Rio do Peixe Formation. Key words: Araçatuba, Cretaceous, Bauru, swamp, palustrine, continental. INTRODUCTION Platform southern-central part due to the accumu- lation of an almost 2000m thick layers of basaltic TheAraçatuba Formation spreads over the mid-west lava. This intracratonic basin was formed during of São Paulo State, along the valleys of the Tietê, the Upper Cretaceous, after the split of the Gond- Aguapeí and Feio rivers, and their tributaries, in the wana Continent. Once the Lower Cretaceous vol- Araçatuba region (Figure 1). This formation be- canic activities came to an end, the area where the longs to the lower part of the Bauru Group, which, basin would ultimately set remained exposed until, together with the Caiuá Group, forms the Upper during the Upper Cretaceous, a relative subsidence Cretaceous Sedimentary Sequence (Figure 2). This dropped the base level and allowed the accumulation succession filled the Bauru Basin, a depression de- and preservation of interior sediments. The basin veloped by the subsidence of the South American was filled with an essentially sandy sedimentary suc- cession, with an average a thickness of 100 to 200 Correspondence to: Luiz A. Fernandes E-mail: [email protected] meters, which is currently preserved and spread out An Acad Bras Cienc (2003) 75 (2) 174 LUIZ A. FERNANDES, PAULO C.F. GIANNINI and ANA MARIA GÓES in an area of roughly 370,000 km2. The maximum Bauru Group, brought back the designation Ara- thickness recorded for the sequence is 320 meters çatuba and subdivided the group into six forma- (Mezzalira et al. 1981). tions: Caiuá, Santo Anastácio, Araçatuba, São José The basement consists of Lower Cretaceous do Rio Preto, Uberaba and Marília. Fernandes and Serra Geral Formation volcanic rocks (mainly Coimbra (1994, 1996) divided the ‘‘Neocretaceous basalts), from which the sedimentary succession is suprabasaltic sequence’’ into two partially coeval separated by an erosive regional surface (non- groups: Caiuá (Paraná River, Goio Erê and Santo conformity). Its upper erosive limit is demarcated Anastácio formations) and Bauru (Uberaba, Ada- by the Sul-Americana (King 1956) / Japi Surface mantina and Marília formations; besides the Taiúva (Almeida 1964) or by the posterior dissection of this Analcimites). surface. The Araçatuba Formation (Fernandes and Coimbra 1998, 2000a), conception adopted here, corresponds partially to the Araçatuba Lithofacies THE ARAÇATUBA UNIT of Suguio et al. (1977), and also partially to the The knowledge regarding the sedimentary succes- Araçatuba Formation of Suguio (1981), Araçatuba sion which composed the sedimentary filling of the Formation of Zaine et al. (1980, unpublished); and Bauru Basin evolved in four main phases as listed Adamantina Formation’s KaIII unit, of Almeida et by Fernandes (1998): 1) pioneer phase (1905 to al. (1980). the early forties); 2) characterization phase (fifties and sixties); 3) lithostratigraphic regional mapping phase (seventies and early eighties); and 4) current CHARACTERIZATION OF THE ARAÇATUBA FORMATION phase. The name Araçatuba that appeared during the The Araçatuba unit presents a maximum thickness regional cartography (phase 3) was previously used of 70 meters (Brandt Neto 1977, Zaine et al. 1980, by Suguio et al. (1977) to designate a lithofacies unpublished), and is well exposed on cuts of the (facies association) in the former Bauru Formation. Marechal Rondon highway (SP 300) in the Ara- Zaine et al. (1980, Paulipetro Report BP-014/80, çatuba region. The unit is composed by siltstones unpublished) singled out a unit which they called and very fine sandstones, exhibiting a characteristic Araçatuba Formation, in an unpublished report of greenish-gray color. These are arranged in tabular regional mapping. The author, however, did not for- beds of massive aspect and are usually centimeters mally define the unit. Soares et al. (1979) outlined to decimeters thick, but may reach 1 meter (Fig- a proposal for the São Paulo State Cretaceous units ure 3a and b). At the boundaries of the occurrence where the Bauru unit was introduced as a group com- area, the sandy strata may display sigmoidal shapes posed of four formations corresponding to the fol- (Figure 3c). The strata makeup is composed of sub- lowing units: a) Caiuá Formation; b) Santo Anastá- metric overlapping sandy bodies, in vertical succes- cio Facies; c) the assemblage: Lithofacies Ara- sions that go up to 7 meters. Boundaries and/or çatuba-Taciba Facies-Ubirajara Facies; d) Marília internal low angle sigmoidal bedding can also be Lithofacies or Formation. Soares et al. (1980) for- observed (e.g. highway BR 153, km 112.5). The mally defined the Bauru Group as composed of the sigmoidal features correspond to approximate lon- Caiuá, Santo Anastácio, Adamantina and Marília gitudinal sections of amalgamated lobes. formations. Almeida et al. (1980) adopted the Soa- Carbonatic cement is a frequent component of res et al. (1979) proposition and divided the Ada- the sandstone in the Araçatuba Formation and oc- mantina Formation into five units for mapping pur- curs frequently as horizontal tabular crusts running poses. Suguio (1981), when covering the paleo- parallel to the rock-bedding. Despite the general environmental and paleogeographic aspects of the massive appearance, the layers can show discreet An Acad Bras Cienc (2003) 75 (2) ARAÇATUBA PALEOSWAMP (BAURU BASIN, BRAZIL) 175 Fig. 1 – Geologic map of the study region and localization of the Bauru Basin (after Fernandes 1998). normal grading to the top and parallel bedding at domorphs were observed (Figure 3d, e and f). Mud the top. Molds and radial-fibrous pseudomorphic cracks, root casts and dish structures are rather com- gypsite crystals as well as probable dolomite pseu- mon in the unit (Figure 4a and b). An Acad Bras Cienc (2003) 75 (2) 176 LUIZ A. FERNANDES, PAULO C.F. GIANNINI and ANA MARIA GÓES Fig. 2 – Stratigraphic chart of the Bauru Basin (after Fernandes and Coimbra 2000a). Saline crystal molds of the ‘‘desert roses’’ type ment, 2.2 km away from the SP 272, on the road have been described by Mezzalira (1974) on core that links it to Tarabaí. samples from holes drilled at Santo Anastácio and Illite and smectite are the most frequent clay Mirante do Paranapanema. Etchebehere et al. minerals in the Bauru Basin. Kaolinite, mixed layer (1991) also describe several millimetric rhomb- clays (illite-smectite) and chlorite appear less fre- shaped molds and some centimetric radial fibrous quently. Most of the identified illite is apparently structures in the Tietê River valley, east of Araçatu- of clastic origin, although sometimes it may occur ba. They were interpreted as probable gypsite crys- as an authigenic mineral. Smectite often appears as tal molds. Fernandes (1998) reported the finding pore-lining cement, and may be replaced by authi- of radial fibrous crystal molds with diameters rang- genic kaolinite during surface processes (Fernandes ing from 5 to 10 centimeters, in road cuts of the SP et al. 1994). Corrensite has also been found in 300 (km 548.5; Araçatuba), SP 461 (km 40.1; Brejo the Araçatuba Formation in pore lining (Figure 4c). Alegre) and SP 304 (close to km 412) highways. These secondary clay minerals were formed proba- In the last one crystal molds occur one centimeter bly between the sedimentation and initial diagenetic above root casts, a fact that suggests phases expo- processes under conditions of little water circu- sure. At the entrance to Brejo Alegre, laminated lation and/or a semi-arid climate. pelites contain crystal